Carbon-supported metal catalysts for the capture/degradation of toxic gases

Abstract

In the current global scenario, the potential use of toxic industrial gases (TICs), chemical warfare agents (CWAs), and their combination poses a significant threat, particularly by the state actors, raising serious concerns for the global community. This study focuses on the design, material composition, and performance evaluation of adsorbent materials capable of providing simultaneous protection against both CWAs and TICs. Herein, materials were developed by incorporating various surface functionalities along with introduction of catalytic centers in the carbon matrix. To address the vast spectra of CWA and TICs, three distinct adsorbent materials were prepared; the first comprising of catalytic centers of Cu, Ag, Zn, Mo and the organic amine TEDA (triethylenediamine) to facilitate chemisorption and neutralization; the second was acid treated carbon to enrich the surface functionalities along with CuCl₂ and KI, and the third comprised of CuCl₂, NaOH and KI. To confirm the modification, adsorbent materials were characterized utilizing SEM, EDS, elemental mapping, FT-IR, TGA, PXRD, BET analysis and pore size distribution analysis. A final composite formulation, optimized by mixing the adsorbents in appropriate ratios was evaluated against CWAs simulant CCl₄, and TICs (NH₃, SO₂, Cl₂, H₂S) and blood agents HCN and found to be effective against broad-spectrum efficacy as per EN 14387 standard. This study will pave the way for the development of next-generation adsorbent materials that provide a unified and practical solution for respiratory protection against a wide spectrum of toxicants, specifically designed for first responders during chemical emergencies.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-50067-3.

Introduction

In the modern geopolitical landscape, military conflicts and unconventional warfare continue to pose significant challenges to global peace and stability, particularly through the use of chemical warfare agents (CWAs) and toxic industrial chemicals (TICs). Beyond warfare, rapid industrialization, fossil fuel combustion, and human activities have lead to substantial emissions of toxic pollutants, contributing to climate change, ozone layer depletion, ecological damage and severe health risks¹ The accidental or deliberate release of toxic chemicals into the atmosphere from both industrial and defense scenarios poses a serious risk to civilians, industrial workers, first responders, and military personnel. Noxious air toxicants including CWAs and TICs² are especially hazardous due to their high toxicity and potential for widespread environmental dispersion³, contributes to the dissemination of harmful substances into the environment. TICs are typically defined as chemicals with an annual production exceeding 30 tons and an LCt₅₀ value below 100,000 mg·min/m3 in mammals; common examples include CO, H₂S, NOₓ, NH₃, SOₓ, etc^4,5. Historical events like in World War I, the use of chlorine and bromine, and the 1984 Bhopal gas tragedy, demonstrate the acute and chronic health impacts of such gases^6,7. Toxic gases are categorized as ABEK gases: A (Organic vapors having boiling point > 65 °C), B (Inorganic gases and vapors e.g., chlorine, hydrogen sulphide and hydrogen cyanide), E (Acidic gases such as sulfur dioxide and hydrogen chloride) and K (ammonia or its organic derivative). Theese pollutants primarily originates from anthropogenic sources for instance, fossil fuel combustion releases SO₂ causing acid rain and smog; H₂S is toxic and corrosive; NH₃ is widely used industrial applications.

CWAs are among the most lethal volatile organic compounds (VOCs) and remain a global security threat despite regulation under the Chemical Weapons Convention (1997). As weapons of mass destruction, CWAs are extremely toxic and can be dispersed as gases, aerosols, or liquids. Based on volatility, CWAs are classified as persistent agents (e.g., nerve and blister agents) and non-persistent agents (e.g., blood and choking agents)⁸. CWAs have been repeatedly used in conflicts and terrorist attacks, from Southeast Asia and Afghanistan (mycotoxins) to Iraq-Iran war (mustard gas, tabun), and the 1995 sarin attack in Japan⁹. More recent incidents include chlorine and nerve agent use in Middle East, the 2017 VX assassination in Malaysia, the 2018 Novichok attack in the UK, and reported CS use in the Russia–Ukraine conflict (2024)^10,11 underscore the persistent threat and the urgent need for effective countermeasure and protective technologies.

Current research efforts focuses on development of advanced materials capable to provide simultaneous protection against CWAs and TICs. Activated carbon and other carbonaceous materials remain the most widely used adsorbents for the capturing of harmful gases due to their high surface area, microporosity, and tunable surface chemistry through acid base treatment or metal impregnation^12,13. The versatility of carbon-based materials confer a high degree of adsorptive performance, making them indispensable for the first responders during CWAs and TICs incidents. Early developments such as ASC whetlerite carbon impregnated with Cu, Ag, and Cr, offers effective protection against CWAs through physisorption and chemisorption^14,15. However, the presence of carcinogenic Cr(VI), raises serious health concerns^16,17, promoting the development of alternative carbon using metals Cu, Ag, Ni, Zn, Mo, and V^18,19. Beyond carbon-based materials, emerging materials such as metal–organic frameworks (MOFs) are promising alternatives for the removal of CWAs^20–23 and TICs owing to their exceptionally high surface area, tunable pore sizes²⁴, defect engineering^25–27 and the versatility of post-synthetic modification²⁸. Despite these advantages, their limited thermal and moisture stability restricts its practical utility in air filtration^29–32. Similarly, the use of highly microporous materials zeolites in air filtration^33,34, often suffer structural instability under harsh environment³⁵, restricting its industrial applicability. In recent years, ionic liquids (ILs) have gained prominence as versatile material³⁶ for gas separation³⁷, green solvents³⁸ and as a catalyst³⁹ however, high viscosity⁴⁰ and cost hinder their direct use. Supported ionic liquid phase (SILPs) on activated carbon^41–44 and silica^45,46 improve adsorption efficiency⁴⁷, but remain ineffective at high toxicant concentrations^41,47. To address these aforementioned limitations and to provide a unified solution capable of simultaneously combating CWAs and TICs, our lab working on development of functional materials by incorporating diverse metal salts and organic compounds in the carbon substrates. We develop materials with tailored surface charge and pore architecture^48–50, where acid functionalization of carbon introduces carboxylic and phenolic groups to enhance adsorption. The development of multifunctional materials capable of simultaneously mitigating CWAs and TICs involves scientific and technical challenges. One major challenge is associated with optimized adsorption of chemically diverse toxic agents, especially acidic and basic gases on a single material without compromising selectivity or adsorption capacity. Furthermore, the presence of multiple active sites may result in the competitive adsorption or non-compatibility of metals which leads to deactivation and can diminish overall performance and stability under operational conditions. These challenges are addressed through rational surface engineering and controlled impregnation strategies. Acid functionalization introduces oxygen containing functional groups to enhance interaction with basic gases, while selective metal centers and metal oxides provide Lewis acidic and basic sites for toxic gases capture followed by degradation.

In this study, we demonstrate the enhanced adsorption and mass transfer of CWAs and TICs utilizing developed hybrid materials through the integration of catalytic centres and the tailored carbon surface chemistry. A key challenge lies in the development of a single adsorbent system for a respirator capable of addressing the diverse range of CWAs and TICs. To achieve broad spectrum performance, multiple selective adsorbents with various functionalization are integrated into a single layer configuration, eliminating the need for multilayer systems. Such modifications facilitate stronger host–guest interactions through mechanisms including adsorption, coordination bonding, acid–base or electrostatic interactions, π-complex formation, and hydrogen bonding. This approach simplifies respirator fabrication, enhances manufacturing efficiency, and offers broad-spectrum protection against toxic agents. The performance of developed materials was evaluated through breakthrough time (BTT) studies against CWAs and TICs, confirming the suitability of the hybrid composition for the simultaneous removal of both classes of toxic agents. Comprehensive characterization was conducted using Fourier transform Infrared (FT-IR) spectroscopy, Thermogravimetric analysis (TGA), Powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET) surface area, Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS), and elemental mapping. The chemical structure of TICs and CWAs simulant used in breakthrough studies are presented in Fig. 1.

Fig. 1.

Structure of TICs (hydrogen sulphide, hydrogen cyanide, chlorine, cyanogen chloride, sulphur dioxide and ammonia) and CWA simulant (carbon tetrachloride) along with their proposed removal mechanism and the corresponding metal impregnants responsible for their capture.

Materials and methods

Chemicals

Basic copper carbonate (CuCO₃.Cu(OH)₂), basic zinc carbonate (Zn₅(CO₃)₂(OH)₆), ammonium molybdate (NH₄Mo₆O₂₄.7H₂O), liquor ammonia, triethylenediamine (TEDA, C₆H₁₂N₂) or DABCO (1,4-diazabicyclo[2.2.2]octane) were supplied by Sigma Aldrich, St. Louis, USA. Silver nitrate (AgNO₃), cupric chloride dihydrate (CuCl₂.2H₂O), sodium hydroxide (NaOH), and potassium iodide (KI), 70% nitric acid (HNO₃) were purchased from Merck, Mumbai, India. All the chemicals used without further any purification or treatment. Milli-Q water was obtained from Milli-Q water filtration system (Millipore). Granular activated carbon was purchased from M/s Suracsh Filters Pvt Ltd, Cochin India. The certified cylinders of TICs Cl₂ (50000 ppm, balance N₂), SO₂ (50000ppm, balance N₂), NH₃(50000 ppm, balance N₂) and H₂S (50000 ppm, balance N₂) were purchased from M/s Gwalior chemical and gases, Gwalior, India.

Surface functionalization and impregnation of granular activated carbon

GAC was first dried at 120 °C for 6 hours to remove residual moisture and other impurities, thereby ensuring complete activation and maximizing the surface area without altering its effective weight^36,47. The bulk density of blank GAC was 0.40–0.41 g/cm³, with a pH of 9.5 and a moisture content of approximately 6%. GACs were impregnated with metal salts^45,51 using the incipient wetness method in a multistep process to minimize surface coatings and ensure the impregnate to reaches the pore of the carbon. In this study, three distinct types of impregnated granular activated carbons (IGACs) adsorbent materials were prepared.

For the synthesis of the first adsorbent material (IGAC 1), GAC was impregnated sequentially with 6% (w/w) each of basic cupric carbonate, ammonium molybdate, and basic zinc carbonate^18,52. The solution of the above metal salts was prepared in a liquor ammonia solvent and ammonium carbonate was also added to increase the solubility. After impregnation, the metal oxide infused carbon was dried in an oven at 110ºC for 6 hr. After complete drying, the materials were treated with a 0.2% (w/w) aqueous solution of silver nitrate followed by drying as stated above. Finally, a 6% (w/w) aqueous solution of triethylenediamine (TEDA)^53–56 was added to the metal oxide infused carbon, followed by drying at 80ºC to ensure effective loading, as TEDA sublimes at higher temperature. Finally IGAC 1formulation contain Cu, Mo, Zn, Ag and TEDA in GAC.

To synthesize the second adsorbent material (IGAC 2), 50 grams of GAC were acidified using 250 mL of a 6M nitric acid solution⁵⁷. This acidification was carried out in two-neck round-bottom flasks, where the GAC was refluxed and then charged with nitric acid dropwise at 110℃ for 6 hours. The resulting acidified granular activated carbon (AGAC) was thoroughly washed with a sufficient amount of Milli-Q water until neutral pH (pH 7) was achieved, followed by overnight drying in a hot air oven at 110°C. Subsequently, AGAC was impregnated with an aqueous solution of 15% (w/w) cupric chloride dihydrate⁵⁸ and dried at 110 °C for 4 hr. This was followed by impregnation with 3% (w/w) potassium iodide³⁶, followed by drying overnight at 110℃. The material was cooled to room temperature in a desiccator and finally stored in sealed bottles for further use. Finally IGAC 2, formulation contain CuCl₂, and KI in Acidified GAC.

To synthesize the third adsorbent material (IGAC 3), GAC was treated with 10% (w/w) cupric chloride dihydrate⁵⁸, followed by drying at 110℃ for 12 h. The dried material was further treated with 5% (w/w) sodium hydroxide⁴⁹ and 3% (w/w) potassium iodide³⁶, followed by drying at 110℃ for 12 hr. After the drying process, the resulting material was cooled to ambient temperature in a desiccator and thereafter kept in sealed bottles until further use. Finally IGAC 3, formulation contain CuCl₂, NaOH and KI in GAC.

Experimental conditions for freakthrough time (BTT) experiments

Breakthrough time experiments were performed utilizing a customized test assembly operating at room temperature and ambient pressure under continuous flow conditions. Fig. 2 provides a pictorial representation and complete overview of the BTT test setup. The test setup consisted of a zero-air cylinder for the dilution of TICs, a corresponding TIC cylinder, flow meters to regulate the respective flows, a water bubbler to control relative humidity (RH), a mixing chamber for dilution, a glass column to contain the packed sample bed, and a detector to measure the breakthrough concentration at the glass column outlet. Breakthrough times for samples labelled as materials 1–10 were determined using electrochemical detectors. Each material was individually tested in a glass column condition (height = 12 cm, internal diameter = 2.0 cm) and exposed to a 5000 ppm (50,000 ppm diluted with air) concentration of the sulfur dioxide (SO₂), ammonia (NH₃), hydrogen sulfide (H₂S), and chlorine (Cl₂) gases at an inlet flow rate of 1.2 LPM. 5000 ppm of HCN and CNCl were generated and exposed to samples. The breakthrough concentrations for all gases were measured at their respective threshold limiting values (TLVs): 5 ppm for SO₂, 25 ppm for NH₃, 10 ppm for H₂S, 0.5 ppm for Cl₂, 10 ppm for HCN, 2 ppm for CNCl, and 10 ppm for CCl₄. These concentrations were measured using an electrochemical detector at the outlet. Table 1 summarizes the nomenclature, colour coding, filter type, concentration, and application details of ABEK gases and their corresponding filters.

Fig. 2.

Schematic representation of the experimental setup test rig for TICs challenge.

Table 1.

Nomenclature, colour, type, concentration and minimum breakthrough time of ABEK agents and filters.

Colour	Type	Concentration	Test gases	Minimum breakthrough time (min)
Brown	A (Organic gases and vapors boiling point > 65 °C)	5000 ppm	CCl4	40
Gray	B Inorganic gases and vapors	5000 ppm	Cl2 H2S	20 40
Yellow	E acidic gas and vapors	5000 ppm	SO2	20
Green	K Ammonia and its organic derivatives	5000 ppm	NH3	40

CCl₄ breakthrough time test for IGAC

To evaluate performance against CWAs, simulant carbon tetra chloride (CCl₄) vapor was generated. CCl₄ is commonly used as a simulant for CWAs due to its physicochemical similarity to many CWAs and its relative safety for laboratory handling. It is widely recognized in national and international testing standards (e.g., ISO, DIN, and NATO protocols) as a benchmark simulant for evaluating the adsorption capacity and breakthrough times of gas mask canisters. In particular, ASTM D3467: Standard Test Method for CCl₄ activity of activated carbon provides a standardized method to assess the adsorption performance of activated carbon, which correlates with protection against volatile toxic compounds. Fig. 3 presents the schematic test setup for CCl₄ vapour breakthrough measurements of developed adsorbent materials.

Fig. 3.

Schematic representation of the experimental setup test rig for CWA simulant CCl₄ challenge.

As depicted in the Fig. 3, air from pressure pump was regulated by a flowmeter and passed through a water bubbler to maintain the desired RH. CCl₄ was fed into the mixing chamber using a syringe pump, and the generated vapor was then directed to the mixing chamber at the required flow rate via a flowmeter with a control valve. The target concentration of CCl₄ vapour was achieved by injecting a controlled amount of CCl₄ using an automated syringe pump into the mixing chamber, which was maintained at 80°C.The RH of the mixed air-vapor mixture was continuously monitored using a calibrated RH sensor to ensure consistent environmental conditions. The synthesized material was packed in a glass column and exposed to the CCl₄ under aforementioned experimental conditions. Breakthrough time and breakthrough concentration (10 ppm) of CCl₄ were monitored at the outlet of the test canister with the VOC monitor equipped with an 11.7 eV photo ionization detector. The developed adsorbent materials were evaluated at a flow rate of 1.2 lpm of a CCl₄-air mixture containing 5000 ppm CCl₄ concentration, at room temperature and 75 ± 5% RH.

Instrumentation

Comprehensive characterization of the samples was performed using multiple analytical techniques to elucidate their structural, surface, and morphological properties. Powder X-Ray Diffraction (XRD) patterns were recorded with a Rikagu Mini Flex 600 instrument, Akishima-shi, Tokyo, Japan to assess the crystalline structure. Thermogravimetric analysis (TGA) was carried out in a nitrogen atmosphere using a Perkin Elmer Pyris 1, Shelton, CT, USA. apparatus at a heating ramp of 10⁰C per min from 30⁰C to 800⁰C to determine the thermal stability. Surface functional groups were investigated using Bruker alpha II infrared spectrometer, Massachusetts, USA. The BET specific surface area of both un-impregnated and impregnated activated carbons was measured in N₂ at −196⁰C (77 K) up to relative pressure of 1 bar, employing a Quantachrome Autosorb iQ TPX surface area analyzer, Florida, USA. Pore size distribution was measured by BJH and HK methods. Prior to above analysis, all the samples were outgassed at 120⁰C for 10 hours to ensure removal of any trapped impurities. Morphological characterization of synthesized materials was performed by Field Emission Scanning Electron Microscopy (FESEM) using Zeiss Sigma 500, Oberkochen, Germany, while energy dispersive X-ray spectroscopy (EDS) and elemental mapping were carried out by employing Oxford’s X-max 80 EDS system to confirm the uniform dispersion of impregnants within the matrix.

Results and discussion

Material characterization

SEM-EDX analysis

Micrographs were captured using field emission scanning electron microscopy (FESEM) to investigate the morphological changes of GAC before and after incorporation, as illustrated in Fig. 4. The scanning electron micrographs clearly demonstrate that pristine GAC (Fig. 4A) features a rough surface texture characterized by open pores^36,59. Following impregnation, the micrograph of the first adsorbent material (IGAC 1) (Fig. 4B) shows a surface populated with various metal species, including distinct morphologies, such as agglomerated globular-shaped particles resulting from the in-situ formation of zinc (II) oxide^60–62 and copper (II) oxide^63,64.

Fig. 4.

SEM images for (A) GAC, (B) IGAC 1, (C)IGAC 2, (D) IGAC 3.

Additionally, the presence of needle shaped structures confirms the in-situformation of molybdenum (VI) oxide, hence demonstrating successful metal incorporation into the carbon matrix^65,66. In the second adsorbent material (IGAC 2), the micrograph (Fig. 4C) reveals a uniform distribution of spherical cupric chloride and potassium iodide particles without aggregation on the surface of acidified GAC, which can be ascribed to the interaction between the acid-functionalized carbon surface and the impregnated metal salts⁶⁷. In the case of the third adsorbent material (IGAC 3) (Fig. 4D), the micrograph reveals homogeneously dispersed spherical cupric chloride particles accompanied by localized copper (II) oxide aggregates, formed in-situ through the interaction of cupric chloride with sodium hydroxide. These findings confirm that impregnating GAC with metal salts significantly alters its structure, leading to the in-situ formation of metal oxides within the carbon matrices. The EDX analysis was conducted to compare the elemental compositions of the synthesized materials with pristine GAC and to confirm the incorporation of metal oxides into the carbon matrix. The EDX spectrum of GAC (Fig. 5A) shows 94% carbon and 6% oxygen. In contrast, the elemental profile of IGAC 1 (Fig. 5B) reveals the presence of 70% carbon, 10% oxygen, 6.9% nitrogen, and also contain elements Cu, Zn, Mo, and Ag metals, which are absent in GAC, indicating successful incorporation of this metal oxide into the carbon matrix. The EDX spectrum of IGAC 2 (Fig. 5C) exhibit carbon (77%), oxygen 13%, Cu (5.9%), Cl (4.6%), Na (0.7%), K (0.2%) and I (0.1%), confirming the presence of CuCl₂ and KI, with the enhanced oxygen functionalities when compared to GAC and IGAC 1, generated during the acidification of carbon surface. IGAC 3 (Fig. 5D) shows a similar profile with carbon (78.5%), oxygen (8%), Cu (6.3%), Cl (5.2%), K (0.8%), and I (0.6%), though a lower oxygen percentage suggests fewer oxygen functionalities than in IGAC 2. The presence of Cu, Cl, K and I confirm the successful incorporation of CuCl₂ and KI into the carbon matrix. Overall, these elemental profiles of IGAC 1, IGAC 2, and IGAC 3 confirm the successful impregnation of metals in to GAC matrix. In order to further confirm the metal percentage content of IGAC 1, IGAC 2 and IGAC 3, the samples were subjected to ICP-OES analysis in triplicate manner and the results are presented in Table S2. Furthermore, EDX color mapping was carried out to determine the elemental combination (dispersion of all metallic and non-metallic impregnates) and their occurrence in the composite matrix (Fig. 6, left to right). The color micrographs of pristine GAC (Fig. 6A) and other metal functionalized carbon show distinct constituents in the carbon matrix. As can be seen from Fig. 6A, a pronounced dispersion of carbon and oxygen confirms the pristine GAC. Color scans of IGAC 1 (Fig. 6B) reveal the significant and uniform dispersion of C, O, N, Cu, Zn, and Mo, in contrast to silver, which exhibits minimal impregnation and limited distribution.

Fig. 5.

Elemental mapping of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

Fig. 6.

EDX analysis of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

The color mapping of IGAC1 is different than pristine GAC, where only C and O is present, which confirm the modification of IGAC1. In Fig. 6C, IGAC 2 displays the distinct presence of C, O, Cu, Cl, N, K and I, confirming successful functionalization of carbon matrix with CuCl₂ and KI. IGAC 3 (Fig. 6D) demonstrate the presence of C, O, Cu, Cl, Na, K and I element, where Cu and Cl originate from CuCl₂, Na from NaOH, and K and I from KI impregnation. The greater dispersion of CuCl₂ in the mapping demonstrates that CuCl₂ is dispersed more extensively throughout the carbon matrix than potassium iodide, attributing a lower impregnation percentage of KI. Furthermore, IGAC 3 exhibits a uniform dispersion of Na along with C, O, Cu, Cl, Na, K and I element, confirming the additional impregnation of sodium hydroxide compared to IGAC 2.

FT-IR analysis

The FTIR analysis (Fig. 7) was employed to identify the changes observed in the surface functionalities of GAC after impregnation. The FTIR spectrum of GAC (Fig. 7A) exhibits minimal functional groups on the surface, with only a few weak peaks present. A broad peak at 3422 cm^–1indicates O-H stretching due to moisture adsorption⁶⁸. Further, a weak peak at 2983 cm^–1 is associated with CH₂ groups in the graphitic structure⁶⁹, while a tiny peak at 2315 cm^–1 corresponds to CO₂ adsorption within the carbon matrix⁵⁹. Additionally, peaks at 1523 cm^–1 and 1116 cm⁻¹ were recorded, attributed to C=C bonds and C-O bonds, respectively⁷⁰. Apart from peaks corresponding to GAC, some new peaks were observed in IGACs after impregnation. The FTIR spectrum of IGAC 1 (Fig. 7B) exhibited additional peaks compared to GAC attributed to the in-situ formation of Copper (II) oxide, zinc oxide, and molybdenum oxide. The peaks at 529 cm^–1 and 645 cm^–1correspond to the Cu-O bond^63,71, while the weak peak at 703 cm^–1 is associated with the Zn-O bond^72,73. Additionally, three peaks at 608 cm^–1, 824 cm^–1, and 1025 cm^–1 represent Mo-O-Mo bending vibrations, Mo-O-Mo stretching, and Mo=O asymmetric stretching, respectively⁷⁴. An additional peak at 1386 cm^–1 indicates the presence of a C-N bond in TEDA. The FTIR spectrum of IGAC 2 (Fig. 7C) displays additional peaks at 540 cm^–1 and 1715 cm^–1, indicating the presence of a Cu-Cl bond and a C=O bond, respectively. The absence of C=O peaks in the FTIR spectrum of IGAC 3 (Fig. 7D) confirmed the acidification in IGAC 2. Additionally, a peak at 518 cm^–1 signifies the presence of a Cu-O bond, corroborating the partial reaction between sodium hydroxide and cupric chloride. A further peak at 615 cm⁻¹confirms the presence of the Cu-Cl bond⁷⁵. This analysis confirms the change in surface functionality in GAC after impregnation.

Fig. 7.

FTIR analysis of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

TGA analysis

Thermogravimetric analysis (Fig. 8) was conducted on GAC, IGAC 1, IGAC 2, and IGAC 3 to explore their stabilities and the effects of acidification as well as impregnation on these materials. TGA thermograms of the materials were recorded from 30℃ to 800℃ at a ramp rate of 10℃/min under an inert nitrogen gas atmosphere. The thermogram of GAC (Fig. 8A) indicates a weight loss of 5–6% between 80–120℃, attributed to the removal of air and moisture gained from the atmosphere. No substantial weight loss was observed up to 500℃. In the subsequent stage, between 500 and 800℃, a gradual weight loss of 60% was observed, indicating the collapse of the graphitic layered structure of GAC^36,59. In addition to the 4–6% weight loss at 80–120℃, attributed to moisture in IGAC 1 (Fig. 8B), and further 5–7% weight reduction occurs due to the elimination of highly volatile TEDA at 150–200℃. Subsequently, a gradual weight loss of 65% is observed between 300 and 780℃, corresponding to the breakdown of graphitic material. No further weight loss was observed due to the presence of highly stable metal oxides such as CuO, ZnO, and MoO₃^76–83. The thermograms of IGAC 2 (Fig. 8C) and IGAC 3 (Fig. 8D) indicate a weight loss of 6–8% at 80–130℃, confirming the presence of moisture and environmental gases. Furthermore, IGAC 2 exhibited a significant weight loss of up to 80% between 250 and 500℃, attributed to the elimination of oxygen functionalities from the surface of acidified carbon and the breakdown of the graphitic carbon network. Acidification evidently diminished the strength of carbon, with an additional 5% weight loss at 500–800℃ attributed to the decomposition of cupric chloride into cuprous chloride and the release of chlorine gas. The thermogram of IGAC 3 indicated a 75% weight reduction at 320–720℃, confirming the destruction of the graphitic network, while a further 8% weight loss corroborated the removal of chlorine gas from cupric chloride, resulting in the formation of cuprous chloride^84,85. All IGACs exhibited a reduction in stability following impregnation compared to GAC. Additionally, the presence of copper salts in all IGACs, known for their high heat storage capacity, contributes to weakening of the graphitic network⁸⁶.

Fig. 8.

TGA curves of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

PXRD analysis

PXRD analysis (Fig. 9) was conducted to investigate the changes in GAC following the incorporation of metal salts to form IGACs. Both the diffractogram of pristine GAC (Fig. 9A) and IGACs (Fig. 9B-D) exhibited two broad peaks at approximately 24.50° and 43.15°. These features confirm the presence of the 002 planes perpendicular to to the graphitic layers, indicating the highly amorphous nature of GAC and IGACs⁸⁷. The diffractogram of IGAC 1 (Fig. 9B) exhibited several prominent peaks, which displayed slight shifts attributed to the presence of various impregnants within the carbon matrix. The distinct peaks observed at 33.30° (110), 37.87° (111), 38.20° (200), 49.49° (202), 62.72° (220), and 66.69° (311) confirm the in-situtransformation of basic copper carbonate into copper (II) oxide^88,89. In a similar manner, the presence of silver nitrate was validated through peaks observed at 38.20° (111) merged with CuO, 44.47° (200), 65.33° (220), and 78.40° (311)^90,91.

Fig. 9.

XRD pattern of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

Furthermore, the in-situ deposition of zinc (II) oxide was confirmed by the presence of significantly intense peaks at 31.00° (100), 34.40° (002), 37.05° (101), and 46.66° (102)^61,72,73. Furthermore, the in-situ synthesis of molybdenum trioxide was confirmed in IGAC 1, as indicated by the peak observed at 14.61°, attributed to the intermediate formed during the decomposition of ammonium molybdate⁹². Additionally, diffraction peaks observed at 12.70° (020), 22.58° (110), 25.70° (040), 28.33° (021), and 33.30° (111) were found to merge with the CuO peak, while the peak at 35.50° (041) overlapped with the ZnO peak. The peak at 38.90° (060) merged with the AgNO₃ peak, and the peak at 46.66° (210) also merged with the ZnO peak. Furthermore, the peak at 49.49° (002) merged with the CuO peak, followed by peaks at 54.06° (211), 57.90° (042), 59.96° (081), and merged with the CuO peak at 66.69° (152), and the peak at 70.05° (261)⁹³. Finally, the peaks at 17.42°, 19.25° and 37.87° confirmed the impregnation of TEDA⁹⁴. Three peaks were observed in the diffractogram of IGAC 2 (Fig. 9C). The initial peak at 13.99° indicated the formation of carbon-supported cupric acetate because the acetate type group has strong coordination ability with Cu (II) ion, confirming the presence of carboxylic acid functional groups on the surface of GAC^95–97. The second peak at 16.79° (001) exhibited a distinct sharp peak indicative of CuCl₂, while the third peak observed at 18.45° validated the presence of the hydrated form of cupric chloride, thereby confirming the impregnation of CuCl₂.H₂O⁹⁸. The diffractogram of IGAC 3 (Fig. 9D) revealed several peaks in addition to GAC. In alignment with IGAC 2, the peaks observed at 16.15° (100) and 18.50° corroborate the presence of cupric chloride and its hydrated form, respectively⁹⁸. Subsequently, additional peaks were identified at 31.89° (110), 35.50° (111), 39.40° (200), 49.43° (202), 52.74° (113), 61.50° (220), and 67.49° (311), attributed to the formation of copper (II) oxide, resulting from the reaction of impregnated cupric chloride with sodium hydroxide within the carbon matrix⁹⁹. This PXRD study confirms the modification of the carbon matrix utilizing different metal oxides.

BET analysis

The BET isotherms (Fig. 10) for GAC, IGAC 1, IGAC 2, and IGAC 3 were measured to determine the BET surface area, micropore surface area (S_micro), and mesopore surface area (S_meso). Fig. 10 A-D illustrates that at relative pressures ranging from 0.05 to 0.30⁵⁹, a steep line is observed, confirming a highly microporous specific surface area. The maximum nitrogen adsorption volume, and the presence of a type IV hysteresis loop further indicate the mesoporous characteristics^87,100. The BET surface area of GAC, as shown in Fig. 10A, was measured at 1687.55 m²/g, indicating a highly microporous structure (Table 2). After impregnation, IGAC 1 (Fig. 10B) exhibits a BET surface area of 836.56 m²/g, approximately 50% lower than that of the control GAC. The microporous and mesoporous surface areas decreased to 48.23% and 59.16%, respectively. After acidification and impregnation of GAC for the development of IGAC 2 (Fig. 10C), the surface area reduced to 632.66 m²/g, accounting for 37.49% of the original GAC surface area. This significant reduction in surface area is primarily attributed to the acid treatment, which introduces various oxygen-containing functional groups such as carboxylic, phenolic, and epoxy groups.

Fig. 10.

Nitrogen adsorption/desorption isotherm of (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

Table 2.

BET surface area of GAC and IGACs.

Adsorbent materials	BET surface area (m2/g)	Surface area (Smicro) (m2/g)	Surface area (Smeso) (m2/g)
GAC	1687.55	1483.50	204.05
IGAC 1	836.56	715.84	120.72
IGAC 2	632.66	534.69	97.97
IGAC 3	1131.99	1072.79	59.19

Furthermore, the subsequent impregnation in the acidified GAC contributed to additional surface blockage, resulting in a further decrease in surface area. Correspondingly, the micropore and mesopore volumes declined to 36% and 48%, respectively, relative to pristine GAC. In case of IGAC 3 (Fig. 10D) the surface area was found to be 1132 m²/g, corresponds to 67% surface area of GAC. A reduction in micropores and mesopores from the GAC was observed to be 28% and 29%, respectively in IGAC 3 (Table 2).

The relatively higher surface area of IGAC 3, can be explained by its lower metal impregnation and lower surface modification compared to other IGACs. The overall reduced surface area of all the IGAC samples provides key evidence for the successful metal impregnation into a carbon matrix as the metal particles occupy available pore spaces and effectively block or fill the mesopores and micropores of pristine GAC. Porosity, pore volume, and pore size distribution are vital characteristics of porous materials that directly influence the impregnation capacity, physisorption and chemisorption processes. The total pore volume (micro and mesopore) and pore volume measurement obtained using the MP method, HK method, and BJH method are presented in Table 3.

Table 3.

PSD & pore volume analysis of GAC and IGACs.

Adsorbent materials	Total pore volume e−1 (cc/g)	V-t method pore volume (cc/g)	HK method pore volume (cc/g)	BJH method pore volume (cc/g)
GAC	8.105	0.628	0.693	0.108
IGAC 1	4.381	0.305	0.344	0.084
IGAC 2	3.244	0.233	0.262	0.055
IGAC 3	4.973	0.452	0.464	0.023

Pore size distribution (PSD) of GAC and IGACs are shown in Fig. 11^101,102. The micropore size distribution of GAC, IGAC 1, IGAC 2, and IGAC 3 was determined via the HK method (Fig. 11A), revealing a consistent micropore size distribution measured by half pore width ranging from 2 to 4 angstrom across all samples. This distribution was uniform among the materials; however, the total pore volume was observed to decrease with higher impregnation levels. The mesopore distribution of all aforementioned materials was examined using the BJH method (Fig. 11B), which indicates that the maximum mesopore distribution ranges from 18 to 21 angstrom over the material. The volume of mesopores diminished in accordance with the quantity of impregnants inside the carbon matrix.

Fig. 11.

Pore size distribution (PSD) curves: Micropore A: (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3 and Mesopore B: (A) GAC, (B) IGAC 1, (C) IGAC 2, (D) IGAC 3.

Rational cesign and optimization of functionalized adsorbent systems for TICs and CWA simulant removal

In accordance with international standard EN14387:2004 and IS 15323:2003, several combinations of composite materials were studied to get the final composition of materials that offer effective respiratory protection against CWA simulant (CCl₄), blood agents (HCN & CNCl), and TICs (NH₃, SO₂, H₂S, and Cl₂) and to understand the role of each component in the hybrid materials. The optimization procedure involved altering the bed height, variation in metal salts, and varying the degree of impregnation across different combinations of IGACs. Throughout the optimization process, ten materials with varying metal compositions were formulated as shown in Table 4, and their efficacy was evaluated by breakthrough time studies against 5000 ppm of ABEK gases (NH₃, SO₂, H₂S, and Cl₂) and a CWA simulant (CCl₄). Successive materials showed incremental improvement in BTT tests.

Table 4.

Various material formulations (1–10), carbon bed heights, and corresponding breakthrough times (BTT) against toxic gases.

S. N.	Compositions	Formulations	Bed height (cm)	BTT(min)
1.	Material 1	GAC	3.5	NH3: 1.5–2.5 SO2: 05–06 H2S: 12–15 Cl2: 02–03
2.	Material 2	15%CuCl2@AGAC	3.5	NH3: 136–140 SO2: 04–05 H2S: 15–17 Cl2: 02–03
3.	Material 3	6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC	2.5 + 1.0 = 3.5	NH3: 31–35 SO2: 12–15 H2S: 20–23 Cl2: 04–05
4.	Material 4	6%Cu,6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC	2.5 + 1.0 = 3.5	NH3: 30–32 SO2: 10–13 H2S :30–33 Cl2: 05–06
5.	Material 5	6%Cu,3%Mo,6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC	2.5 + 1.0 = 3.5	NH3: 45–48 SO2: 12–14 H2S: 47–50 Cl2: 07–08
6.	Material 6	6%Cu,3%Mo,6%Zn,0.02%Ag,6%TEDA@GAC	3.5	NH3: 18–19 SO2: 12–15 H2S: 56–60 Cl2: 9–11
7.	Material 7	6%Cu,3%Mo,6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC + 10%CuCl2,5%NaOH@GAC	2.5 + 1.0 + 0.5 = 4.0	NH3: 45–46 SO2: 19–21 H2S: 45–50 Cl2: 23–25
8.	Material 8	6%Cu,6%Mo,6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC + 10%CuCl2,5%NaOH@GAC	2.5 + 1.0 + 0.5 = 4.0	NH3: 54–57 SO2: 20–22 H2S: 50–56 Cl2: 31–35
9.	Material 9	6%Cu,6%Mo,6%Zn,0.02%Ag,6%TEDA@GAC + 15%CuCl2@AGAC + 10%CuCl2,5%NaOH,3%KI@GAC	2.5 + 1.0 + 0.5 = 4.0	NH3: 61–66 SO2: 21–23 H2S :63–69 Cl2: 29–31
10.	Material 10	6%Cu,6%Mo,6%Zn,0.02%Ag,6%TEDA@GAC (IGAC 1) + 15%CuCl2, 3%KI@AGAC (IGAC 2) + 10%CuCl2,5%NaOH,3%KI@GAC (IGAC 3)	2.5 + 1.0 + 0.5 = 4.0	NH3: 55–58 SO2: 23–25 H2S: 45–47 Cl2: 24–25 HCN: 28–29 CNCl: 42–45 CCl4: 44–45

Figure 12 and Table 4 illustrate the breakthrough times graph for GAC and various material having a different composition of metal salts challenged with a concentration of 5000 ppm of TICs, at a flow rate of 1.2 LPM. The breakthrough time for TICs was determined until the downstream concentrations of NH₃, SO₂, H₂S, and Cl₂ attained 25 ppm, 0.5 ppm, 10 ppm, and 0.05 ppm (in accordance with the threshold limiting value), respectively, and were identified using an electrochemical detector. Fig. 12A illustrates that BTT of Material-1 comprising GAC alone with a bed height of 3.5 cm, exhibited minimal protection with BTT of approximately 1.5–2.5 minutes for NH₃, 5–6 minutes for SO₂, 12–15 minutes for H₂S, and 2–3 minutes for Cl_2, demonstrating that TICs are carbon breaking agents. The Material-2 (Fig. 12B) consists of CuCl₂@AGAC) with a bed height of 3.5 cm, demonstrating significant protection with BTT of 136–140 minutes against NH₃. But limited protection offers against SO₂ with a BTT of 4–5 minutes, 15–17 minutes against H₂S, and 2-3 minutes against Cl_2, highlighting selective NH₃ removal.

Fig. 12.

Optimization of various compositions against TICs or ABEK-2 gases :(A) Material 1, (B)Material 2, (C) Material 3, (D) Material 4, (E) Material 5, (F) Material 6, (G) Material 7, (H) Material 8, (I) Material 9 and (J) Material 10.

The Material-3 (Fig. 12C) consists of Zn, Ag, TEDA@GAC (2.5 cm bed height), and CuCl₂@AGAC (1 cm bed height), resulting in a total bed height of 3.5 cm. This configuration showed a BTT of 31–35 minutes for NH₃, a slightly increased BTT of 12–15 minutes for SO₂ compared to composition 1 and 2, 20–23 minutes for H₂S, and a no significant protection was observed for Cl₂ with a BTT of 4–5 minutes. The Material-4 (Fig. 12D) consists of Cu, Zn, Ag, TEDA@GAC (2.5 cm bed height) and CuCl₂@AGAC (1 cm bed height), achieving a combined bed height of 3.5 cm. The BTT recorded was 30–32 minutes for NH₃, 10–13 minutes for SO₂, while 30–33 minutes for H₂S, and 5–6 minutes was recorded for Cl₂. The increased H₂S BTT observed in material 4 is due to the presence of Cu, which chemisorb the H₂S. Copper species (mainly CuO or Cu^2⁺) on the carbon surface interact strongly with H₂S, facilitating a chemical reaction rather than just physical adsorption. This reaction leads to the formation of copper sulfide (CuS or Cu₂S), which effectively removes H₂S from the gas stream and enhances the adsorption capacity of the material. The introduction of Mo in material-4 (Fig. 12E) to formulate material-5, containing Mo, Cu, Zn, Ag, TEDA@GAC (2.5 cm bed height), and CuCl₂@AGAC (1 cm bed height) with a bed height of 3.5 cm notably enhanced broad spectrum protection against TICs. Experimentally, the measured BTT were 45–48 minutes for NH₃, 12–14 minutes for SO₂, 47–50 minutes for H₂S, and 7–8 minutes for Cl₂. The reason for this high BTT in presence of Mo along with other metals as reported above is due to fact that molybdenum oxides facilitate redox and acid-base reactions, enabling the capture of both acidic and basic gases by providing multifunctional active sites, particularly with increased adsorption for polar gases such as NH₃ and H₂S gas. This synergizes with copper, zinc, and silver impregnation to enhance adsorption capacities for a diverse range of TICs through both physisorption and chemisorption processes¹⁰³.

In order to evaluate the specific contribution of CuCl₂@AGAC (1 cm bed height) in the removal efficiency of TICs, the Mo, Cu, Zn, Ag, TEDA@GAC composition was evaluated at 3.5 cm bed height without the presence of CuCl₂@AGAC named as material 6. This material was subsequently challenged with TICs. The BTT observed for this configuration was 18–19 minutes for NH₃, 12–15 minutes for SO₂, 56–60 minutes for H₂S, and 9–11 minutes for Cl₂. As can be seen from the above data, the BTT time for ammonia is drastically reduced to 18 min, compared with 45 min observed for material 4 that included both CuCl₂ and AGAC. This marked decrease in NH₃ BTT emphasizes the critical role of CuCl₂ and acidified carbon in the removal of ammonia. Despite this reduction in NH₃ protection, the absence of CuCl₂@AGAC did not substantially diminish the BTT for SO₂, H₂S, and Cl₂, demonstrating that these other reactive phases and functionalities in the composite maintain adequate sorptive capabilities for the majority of tested TICs. However, the materials 1–6 do not offer sufficient protection against all TICs; therefore, the bed height of final composition was increased from 3.5 cm to 4 cm bed height to ensure comprehensive protection as per IS 15323:2003. In material-7, one additional layer of CuCl₂-NaOH@GAC (0.5 cm bed height) was incorporated along with the same as material-5, consisting of Mo, Cu, Zn, Ag, TEDA@GAC (2.5 cm bed height), and CuCl₂@AGAC (1 cm bed height), resulting in a total bed height of 4.0 cm, and subsequently challenged with TICs. After incorporation of CuCl₂-NaOH@GAC (0.5 cm), it was found that 45–46 min BTT for NH₃, 19–21 minutes for SO₂, 45–50 minutes for H₂S, and 23–25 minutes for Cl₂. The BTT results indicate that material-7 offers adequate protection against NH₃, H₂S, and Cl₂, but is marginally effective in the case of SO₂. In material-8, the molybdenum content increased from 3% to 6% while other component remained constant as in material-7. The BTTs observed for NH₃ was 54–57 minutes, 20–22 minutes for SO₂, 50–56 minutes for H₂S, and 31–35 minutes for Cl₂. However, the BTT for SO₂ remains on the boundary line, indicating that additional time is necessary for safety considerations. To improve performance against SO₂ gas, a 3% KI was impregnated into CuCl₂/NaOH@GAC and formulated material-9: Mo/Cu/Zn/Ag/TEDA@GAC (2.5 cm bed height), CuCl₂@AGAC (1 cm bed height), and CuCl₂, NaOH, KI@GAC (0.5 cm bed height), with a total bed height of 4 cm. Above formulation was tested against the aforementioned TICs, yielding protection times of 61–66 minutes for NH₃, 21–23 minutes for SO₂, 63–69 minutes for H₂S, and 29–31 minutes for Cl₂. The inclusion of KI demonstrates a marginal positive effect on SO₂ gas.

To achieve a 3-5minute safety window for SO₂ gas, 3% KI was also introduced into CuCl₂@AGAC composition, resulting in a new material composition of Mo/Cu/Zn/Ag/TEDA@GAC (IGAC1; 2.5 cm bed height), CuCl₂/KI@AGAC (IGAC2; 1 cm bed height), and CuCl₂/NaOH/KI@GAC (IGAC3; 0.5 cm bed height) designated as material 10. This formulation was tested against TICs and demonstrated a protection time of 55–58 minutes for NH₃, 23–25 minutes for SO₂, 45–47 minutes for H₂S, and 24–25 minutes for Cl₂. Further testing of Material 10, yielded a protection time of 28–29 minutes for HCN, 42–45 minutes for CNCl (blood agents) and 44–45 minutes for the CWA simulant (CCl₄) (Table 5), providing the necessary protection against the aforementioned CCl₄, HCN, CNCl and TICs. At a carbon bed height of 4 cm, the inhalation resistance was measured and found to be 4.70 mbar at a constant airflow of 95 L/min (Table S1). This value is significantly below the maximum permissible limit of 9.8 mbar specified in IS 15323:2003 and DIN EN 14387 for inhalation resistance at 95 L/min, thereby demonstrating compliance with the applicable standards.

Table 5.

Breakthrough concentration, required breakthrough time and observed breakthrough time.

TICs	Breakthrough concentration (ppm)	Required BTT (Minutes)	Observed BTT (Minutes)
CCl4	10	40	44–45
H2S	10	40	45–47
HCN	5	25	28–29
CNCl	2	30	42–45
Cl2	0.5	20	24–25
SO2	5	20	23–25
NH3	25	40	55–58

Breakthrough behaviour of TICs against material 10

Figure S1 illustrates the breakthrough curves of NH₃, SO₂, H₂S, and Cl₂ for the optimized material-10 which consists of IGAC 1, IGAC 2 and IGAC 3 with optimized bed heights. As can be seen from Figure S1, the NH₃ outlet concentration from material 10 remained negligible up to 49 min, reached 5 ppm at 50 minutes, 13 ppm at 55 minutes, and 25 ppm at 58 minutes, indicating the breakthrough concentration.

The NH₃ profile exhibits a gradual concentration rise followed by a sharp increase upon breakthrough time. For SO₂, the outlet concentration reached 0.7 ppm at 20 minutes and increased sharply to 5 ppm (breakthrough concentration) at 25 minutes and 10.8 ppm at 28 minutes, demonstrating a rapid increase in concentration post-breakthrough. In the case of H₂S, the outlet concentration gradually increased, at 41 minutes it reached 3 ppm, and it reached 10 ppm (breakthrough concentration) at 47 minutes. After attaining breakthrough concentration, it sharply reached to 18.6 ppm at 49 minutes. The breakthrough behavior of Cl₂ exhibited an even steeper trend, where the concentration increased from 0.5 ppm at 25 minutes to 22 ppm at 26 minutes. The breakthrough profile of Cl₂ displayed an extremely steep gradient, with the outlet concentration rising abruptly from 0.5 ppm at 25 minutes to 22 ppm at 26 minutes, indicating an almost instantaneous breakthrough and rapid saturation of the adsorbent bed. The performance of the developed material was systematically compared with previously reported studies on metal-impregnated activated carbons (Table S4). The results demonstrate that the present system exhibits superior breakthrough performance, particularly under high challenge conditions. Importantly, unlike others materials, it effectively enables the simultaneous removal of both CWAs and TICs, highlighting its potential as a universal adsorbent.

Effect of humidity on breakthrough behaviour of material 10

After optimizing the metal loading in GAC, the effect of relative humidity (RH) on the performance of Material 10 was evaluated in terms of breakthrough behavior for CCl₄ and various TICs. The RH was varied from 25% to 85% under the aforementioned test conditions, and the results are presented in Fig. S2. As shown in the Figure S2 for CCl₄, the BTT decreased with increasing RH (25% to 85%), which can be attributed to competitive adsorption between water vapor and CCl₄ molecules for the available micropores. The BTT decreased from 60 min at 25% RH to 31 min at 85% RH. In the case of H₂S, humidity significantly influenced the removal performance. At moderate RH, moisture facilitates the dissociation of H₂S and enhances its interaction with active metal sites, improving removal efficiency. The BTT was increased from 32 min (25% RH) to 45 min (65% RH) followed by a decline to 39 min at 85% RH due to an excessive moisture competing for access to catalytic sites and partially blocking pores which leads to a reduced performance. For HCN and CNCl, moderate humidity enhances removal by promoting chemisorption at active sites (Cu, Zn, Ag) and facilitating hydrolysis. For HCN, CuO, ZnO and NaOH play key roles with BTT increasing from 15 min to 28 min at 65% RH, followed by a decrease to 20 min at 85% RH. TEDA plays a crucial role in CNCl removal by acting as a nucleophilic base, facilitating catalytic transformation via hydrolysis and complex formation. The BTT for CNCl increased from 25 min to a maximum of 42 min at 65% RH, then slightly decreased to 39 min at higher RH due to competitive adsorption by water. Chlorine (Cl₂) removal is also enhanced at moderate RH due to hydrolysis forming HCl and HOCl, which are more readily captured by metal oxides (CuO, ZnO, MoO₃) via chemisorption and acid–base interactions. The BTT increased from 12 min to 24 min at 65% RH, followed by a decrease to 20 min at 85% RH due to excessive moisture covering active sites and reducing accessibility. For SO₂, the BTT increased with RH up to 65%, attributed to the formation of sulfurous and sulfuric acids, creating an acidic microenvironment. This promotes in-situ generation of iodine (I₂) from iodide species, which further enhances SO₂ removal. The BTT increased from 12 min (25% RH) to 23 min (65% RH), then decreased to 19 min at 85% RH due to excessive moisture neutralizing active sites and hindering accessibility to CuO and ZnO. For NH₃, humidity improves removal through acid–base interactions with acidified carbon (–COOH, –OH groups) and formation of NH₄⁺. CuCl₂ and Cu species enhance removal via coordination (ammine complex formation), while MoO₃ provides additional acidic and redox-active sites. The BTT increased from 27 min (25% RH) to 55 min (65% RH), then decreased to 49 min at 85% RH due to active site competition and pore blockage at high humidity.

In view of the above, an optimum RH of around 65% is observed at which the synergistic effects of moisture, metal impregnants, and surface functionalities lead to maximum removal efficiency. At higher humidity levels, a slight decline in performance is noted due to competitive adsorption and partial pore saturation.

pH analysis of optimized materials before and after exposure

To further understand the mechanism of CWAs and TICs removal by the hybrid material system (material 10), the pH analysis was also carried out for GAC, IGAC 1, IGAC 2, IGAC 3 and material 10, which consists of IGAC1, 2 and 3 in a 5:2:1 ratio. The pH of the system can affect the ionization states of both the adsorbate and adsorbent, thereby affecting the adsorption capacity. The pH analysis of optimized materials before and after exposure provides key insights into their surface chemistry and interaction with toxic gas. pH changes during the removal of TICs reveal the change in surface acidity or basicity, which are closely related to adsorption capacities and regeneration properties. Before exposure, optimized materials are characterized by a specific surface acidity or basicity tailored to maximize the capture of acidic or basic toxic gases, such as sulfur dioxide or ammonia.To evaluate the change in pH before and after exposure, 9.1 g of each of the aforementioned compositions was separately added to 500 ml of deionized water and stirred for 8 hours⁵⁸. A calibrated pH meter was utilized to precisely measure the pH of each material both before and after exposure to gases. Table 6 presents the measured pH values of all the compositions. Impregnation with different metals Cu, Mo, and Ag caused a progressive decrease in the pH of the blank carbon. As presented in Table 6, the three individual components exhibited distinct pH values. IGAC 1 showed a near-neutral pH of 7.6. The second bed, containing KI and a higher proportion of CuCl₂, exhibited a markedly acidic pH of 3.09, attributed to the acidic nature of CuCl₂ and due to the presence of more acidic groups on the carbon surface during the acidification process. The IGAC3, comprising NaOH, CuCl₂, and KI, demonstrated a pH of 4.09. Although the high CuCl₂ content imparted acidity, the presence of NaOH counteracted the effect to some extent, preventing the pH from dropping as low as that of the second composition. Upon mixing all three components, i.e., IGAC 1, 2 and 3, the combined system showed an overall pH of 5.06, which was lower than the blank due to the contribution of the various metal salts and their acidic nature. Exposure of the system to CCl₄ did not result in any significant pH change, as CCl₄ is an organic compound and chemically neutral in aqueous medium. In contrast, exposure to acidic gases such as SO₂, Cl₂, and H₂S led to a noticeable decrease in pH, the extent of which correlated with the intrinsic acidity of the challenge gases. The order of acidity observed was SO₂ > Cl₂ > H₂S. This can be explained by the fact that SO₂, being an acidic oxide, readily forms sulfurous acid in water; H₂S behaves as a weak diprotic acid; whereas Cl₂ generates HCl and hypochlorous acid upon dissolution in water, thereby contributing to acidity. Consequently, the system exhibited the greatest pH reduction upon SO₂ exposure, followed by Cl₂ and then H₂S. In contrast, exposure to NH₃, a basic gas, resulted in an increase in pH from 5.50 to 6.58, confirming its neutralization of the acidic components of the system. These findings underscore the critical role of pH behavior in understanding and optimizing the removal performance of the materials toward toxic gases.

Table 6.

pH analysis of GAC and IGACs.

S. No.	Adsorbents before and after exposure	pH
1.	GAC	9.50
2.	AGAC	6.65
3.	6%Cu, 6%Mo, 6%Zn, 0.02%Ag, 6%TEDA@GAC (IGAC 1)	7.65
4.	15%Cu, 3%KI@AGAC (IGAC 2)	3.09
5.	10%Cu, 5%NaOH, 3%KI@GAC (IGAC 3)	4.15
6.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3)	5.06
7.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after CCl4 EXP.	5.10
8.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after H2S EXP.	4.96
9.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after HCN EXP.	5.86
10.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after CNCl EXP.	5.33
11.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after Cl2 EXP.	4.28
12.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after SO2 EXP.	4.15
13.	Material 10 (IGAC 1 + IGAC 2 + IGAC 3) after NH3 EXP.	6.58

Adsorption capacities of CWA simulant CCl₄, blood agents, and TICs

The adsorption capacity for material 10, following exposure to all CWA simulants CCl₄ and blood agent HCN, as well as TICs, was calculated in milligrams per gram of adsorbent material at room temperature and relative humidity (65±5) %. This was achieved by converting gas concentrations from ppm to mg/l, multiplying by the gas flow rate (1.2 LPM), the breakthrough time (in minutes), and dividing by the weight of the adsorbent material utilized in each BTT experiment³⁶. Based on the experimental data, the sequence of adsorption capacity for CWA simulant (CCl₄), blood agents (HCN & CNCl) and TICs is as follows: CCl₄ (187.63mg/g)> CNCl (74.75 mg/g)> Cl₂ (47.98 mg/g)> SO₂ (43.23 mg/g)> H₂S (43.19mg/g) > HCN (30.12 mg/g)> NH₃ (26.51 mg/g). The variation in adsorption capacities among different gases is governed by both physical and chemical factors associated with the adsorbent and the adsorbate. The results indicate that CWA simulant CCl₄ exhibits the highest adsorption capacity among all the gases tested. This superior uptake is primarily attributed to its non-polar structure and strong physisorption within IGAC’s microporous network. In contrast, blood agents like HCN and NH₃ exhibited the lowest capacities, likely due to weaker interactions and limited availability of specific functional groups/catalytic centers on the adsorbent surface for chemisorption. The large surface area and uniform pore size in the adsorbent facilitate efficient packing and retention of such molecules, resulting in higher mg/g values. In contrast, gases like NH₃ and HCN have lower adsorption capacities because their chemical properties such as polarity, molecular size, and affinity for the existing functional groups do not favor strong physical adsorption and limited specific chemisorption interactions with the impregnated active sites. The moderate adsorption capacities of CNCl (74.75 mg/g), Cl₂ (47.98 mg/g), SO₂ (43.23 mg/g), and H₂S (43.19 mg/g) indicate that effective capture is facilitated by the synergistic interplay between porosity and active chemical sites, such as impregnated metal oxides and alkaline additives. In contrast, blood agents such as HCN exhibited comparatively lower adsorption capacity (30.12 mg/g), which may be attributed to differences in physicochemical interactions and limited chemisorption sites. Ammonia (NH₃) showed the lowest capacity (26.51 mg/g), reflecting the composition’s limited affinity, possibly due to the absence of highly specific functional groups for NH₃ capture. This progression underscores the dual role of IGAC materials: maximizing physical adsorption for highly volatile species (such as CCl₄) while leveraging tailored impregnates for enhanced binding and neutralization of TICs and blood agents. Overall, these findings reveal the versatile and tunable nature of IGAC adsorbents for comprehensive protection against a wide chemical threat spectrum, highlighting their suitability for the development of advanced respiratory filtration systems for CWAs and TICs.

Plausible mechanisms of ABEK gases and CWA simulant (CCl₄) removal

Mechanism of NH₃ removal

The NH₃ removal process involves several reactions that are influenced by the impregnants present in the IGACs. The presence of moisture in the air stream is an important ingredient in the NH₃ removal process, through a dissolution process that allows the retention of NH₃ in water, attributed to its high solubility and it reacts with NH₃ to form NH₄OH^104,105. Material 1 does not exhibit chemical protection against any TICs, as evident from Table 4, indicating that effective protection is achieved primarily through only chemisorption. Material 2, which contains CuCl₂ supported on acidified carbon, shows a significant increase in NH₃ BTT from 2.0 min to 136 min. This enhancement is attributed to the combined effect of CuCl₂ and the acidic functional groups generated during carbon acidification, which promote acid–base interactions with NH₃.

Similar behavior is observed for Materials 3 and 4; however, the BTT decreases from ~140 min to ~35 min due to the reduction in bed height of CuCl₂ from 3.5 cm to 1.0 cm. Although Material 3 contains additional Cu (6% as CuO) and material 4 contains additional Zn (6% as ZnO), no further significant improvement in NH₃ BTT is observed, indicating that these oxides do not contribute significantly to NH₃ removal. Material 5 exhibits enhanced BTT compared to Material 4, which can be attributed to the presence of Mo, as all other components are identical. Material 6 provides limited ammonia protection (18–19 min) due to the absence of CuCl₂ supported on acidified carbon. These results further confirm that the combination of CuCl₂ and acidified carbon is primarily responsible for NH₃ chemisorption. In Materials 7–9, the presence of CuCl₂ supported on acidified carbon with an increased bed height of 1.5 cm leads to a further enhancement in BTT. The material 10 comprises of IGAC1, IGAC 2 and IGAC 3. The IGAC 1 contains metal oxides such as CuO, and MoO₃, which react with NH₃ to produce Cu metal, and (NH₄)₂MoO₄, respectively, as shown in the scheme 1^106,107. However, IGAC 2 contains acidified carbon having oxygen-containing functional groups such as carboxyl, epoxy, phenolic, and lactones on the surface. These functionalities produce Bronsted acid sites, which play a significant role in the removal of basic NH₃ gas through acid-base interactions^57,104. In this process, proton released from carboxylic acid groups enables formation of ammonium ion and carboxyl group. Similarly, epoxy group reacts with NH₃ to yield hydroxyl and amine functionalities^108,109. Furthermore, the presence of CuCl₂ in IGAC 2 and IGAC 3 effectively removes NH₃ by the formation of Cu-complexes Cu(NH₃)₂Cl₂. Water further interact with the Cu-complexes to stabilize the Lewis acid sites present on the copper atom⁵⁸. All the necessary reactions are given in the scheme 1.

Scheme 1.

Schematic representation showing plausible mechanism of NH₃ removal.

Mechanism of SO₂ removal

The SO₂ removal process significantly relies on moisture in the air stream, which acts as a critical component in SO₂ removal. SO₂ interacts with H₂O to produce H₂SO₃, and in the presence of oxygen, forms H₂SO₄, which is efficiently absorbed by the microporous IGACs^110,111. Material 1 does not exhibit chemical protection against SO₂, as shown in Table 4, indicating that SO₂ removal occurs predominantly via chemisorption. Material 2, containing CuCl₂ supported on acidified carbon, shows protection comparable to Material 1, suggesting a negligible role of CuCl₂ and acidified carbon in SO₂ removal. Similarly, Materials 3−6 do not contribute significantly to SO₂ removal. In contrast, Material 7 demonstrates enhanced SO₂ breakthrough time (19–21 min) due to the combined effect of CuO and NaOH as shown in scheme 2. Material 8 does not show a significant improvement as there is only increase of Mo content compared to material 7. Materials 9 and 10 exhibit notable SO₂ removal, with BTT values increased to 23–25 min. The material 10 comprises of combination of IGAC 1, IGAC2 and IGAC 3. In IGAC 1, impregnated CuO enables the formation of CuSO₄, while ZnO reacts with SO₂ and O₂ to yield ZnSO₄¹⁹. Additionally, IGAC 3 contain CuCl₂, NaOH and KI. In the presence of moisture KI produces in-situ iodine within the carbon matrix, which further reacts with SO₂ under moisture and finally forms H₂SO₄and HI^36,112. The presence of NaOH in IGAC 3 initiates the formation of sodium bisulfite upon reaction with SO₂, which subsequently converts to sodium sulfite^113,114. All relevant reactions are illustrated in Scheme 2:

Scheme 2.

Schematic representation showing plausible mechanism of SO₂ removal.

Mechanism of H₂S removal

H₂S gas is an acidic inorganic gas that that exhibits a strong tendency to interact with alkaline materials and metal salts through acid-base interaction. For protection against H₂S, no single impregnant appears solely responsible; rather, it results from the synergistic contribution of multiple reactive species. Among the evaluated species, CuO, MoO₃, and ZnO all participates for the removal of H₂S. However, reactivity of H₂S toward CuCl₂ is relatively low due to acidic nature of both the species. Consequently, Material 2 containing 15% CuCl₂ provide a limited protection time of 15–17 min whereas material 3 with 6% ZnO exhibits a higher protection time of 20–23 min. Furthermore, addition of Cu and Mo species in Material 4−6 has significantly enhanced the protection times of 30–33 min, 47–50 min and 56–60 min, respectively. These results confirm the active participation of above active species for the enhanced removal of H₂S and support the proposed reaction mechanism. Material 7, exhibits reduced protection due to the presence of CuCl₂ and acidified carbon, however, in material 8 and 9, the protection time increases markedly to 50–56 min, which can be attributed to the higher Mo conc and the presence of basic medium. The material 10, comprises of IGAC 1 which contains reactive CuO, MoO₃, and ZnO which react with H₂S gas. In brief, CuO reacts with moisture to form Cu(OH)₂, which subsequently react with H₂S and form CuS, while MoO₃, and ZnO forms MoS₃, and ZnS respectively^115–119. In IGAC 3, the presence of CuCl₂ in some extent facilitates its interaction with H₂S in presence of sodium hydroxide resulting in the formation of CuS^119,120. When H₂S gas partially interacts with NaOH in IGAC 3, NaSH and H₂O are produces. The generated NaSH subsequently reacts with the atmospheric O₂, yielding NaOH and elemental sulfur. Upon complete reaction with NaOH, H₂S forms Na₂S and H₂O^68,110,121. Additionally, the adsorption of moisture and CO₂ from the air leads to the formation of NaHCO₃ and Na₂CO₃. The in-situ produced Na₂CO₃ further reacts with H₂S gas, forming NaSH and NaHCO₃¹²². The detailed reaction mechanisms are illustrated in Scheme 3:

Scheme 3.

Schematic representation showing plausible mechanism of H₂S removal.

Mechanism of Cl₂ removal

Initially, Cl₂ gas interacts with the atmospheric moisture to form HCl and HOCl. The generated HOCl subsequently participates in the slow oxidation of IGACs, leading to the formation of a carbonyl group on the carbon surface. As can be seen from the Table 4, material 1 and 2 do not provide any protection against Cl₂, indicating that CuCl₂ along with acidified carbon does not contribute to the chlorine removal. Material 3−6 containing ZnO MoO₃ and TEDA (Material 4−6), exhibit only limited protection performance up to 9–11 min. In contrast, Material 7, which contain NaOH, shows a significant increase in BTT to 25 min. In material 8, it was increased to 35 min due to cumulative effect of M_OO₃ and NaOH. However, addition of KI reduced Cl₂ BTT. The Material 10, exhibits a Cl₂ BTT of 24–25 min. HCl generated from Cl₂ in the presence of atmospheric moisture, interacts with NaOH present in IGAC 3, producing NaCl and H₂O, while HOCl also reacts with NaOH to generate NaOCl and H₂O. The in-situ water generated during these processes enhances the removal of chlorine gas^123,124. All metal oxides IGAC 1 such as CuO, MoO₃, and ZnO react with HCl produced in the carbon matrix to form CuCl₂, MoO₂Cl₂, and ZnCl₂their respective chlorides^125,126. Detailed reactions given in scheme 4 below:

Scheme 4.

Schematic representation showing plausible mechanism of Cl₂ removal.

Mechanism of HCN removal

Hydrogen cyanide is an inorganic gas that undergoes specific reactions with transition metal oxides in IGAC materials. As shown in Table S3, Material 1 provides minimal protection against HCN with a BTT of 1–2 min. Material 2 containing acidified carbon and CuCl₂, also shows negligible protection due to the acidic nature of the carbon, CuCl₂ and HCN. Material 3 exhibits a BTT of 18–20 min, attributed to the presence of Zn and TEDA, facilitating the formation of Cu(CN)₂ and H₂O. Material 4, containing 6% Cu (CuO), further enhances protection to 22–25 min by reacting with HCN to form Cu(CN)₂, (CN)₂ and H₂O. Material 5 and 6, which include Mo species, do not significantly contribute to HCN removal indicating that Mo does not contribute to HCN uptake. Material 7, incorporating NaOH which increases the BTT to 27–30 min by converting to HCN followed by (CN)₂. In material 8, increasing the Mo content from 3 to 6% while keeping other composition constant as material 7 does not affect the HCN BTT. Similarly, Material 9 shown no improvement in HCN removal upon the addition of KI. In material 10, containing IGAC 1, as shown in the scheme 5, CuO reacts with HCN to form Cu(CN)₂ and H₂O; with excess HCN, it additionally forms Cu(CN)₂, (CN)₂ and H₂O.Similarly, ZnO reacts with HCN to produce Zn(CN)₂ and H₂O. HCN also undergoes oxidation in air within the carbon matrix, generating cyanogen (CN)₂ and water. In IGAC 3, CuCl₂ in presence of base reacts with HCN to Cu(CN)₂, (CN)₂ and H₂O while NaOH present in IGAC 3 form NaCN after reaction with HCN^18,127.

Scheme 5.

Schematic representation showing plausible mechanism of HCN removal.

Mechanism of CNCl removal

Cyanogen chloride is a highly toxic, volatile inorganic gas and a pseudohalogen that exhibits unique reactivity with transition metals. As shown in Table S3, Material 1 and 2 do not provide any protection against CNCl. In contrast, Material 3 offers significant protection (25–28 min) due to the presence of TEDA and Zn (Scheme 6). TEDA functions primarily through nucleophilic reaction and catalyzed hydrolysis as shown in scheme 6. In the absence of TEDA, CNCl hydrolysis is very slow, however, in its presence TEDA effectively serves as a source of OH^- in the presence of water. After the reaction of CNCl with OH^- it forms cyanic acid which is unstable and rapidly hydrolyzes further to CO₂ and NH₃ (NH₄ ⁺ under acidic medium). Material 4, containing 6% Cu and Zn, further enhances protection time to 27–30 min due to the combined presence of TEDA with Cu²⁺ and Zn²⁺ species. TEDA enhances dispersion of basic Cu²⁺ and Zn²⁺ impregnant, enabling enhanced reaction with adsorbed CNCl. Material 5 which has a similar composition to Material 4 but includes Mo species which do not participate in CNCl removal and exhibits nearly same BTT as material 4. Material 6 also shows comparable BTT performance. In material 7, the introduction of NaOH enhances CNCl BTT to 40–42 min by facilitating base catalyzed CNCl hydrolysis. Material 8 does not result in a significant improvement in CNCl removal, as Mo species does not participate in the chemical reaction with CNCl. The presence of KI in material 9 and 10 also has no notable effect on CNCl BTT. The above results demonstrate that TEDA plays a crucial role for the removal of CNCl by promoting base-catalyzed hydrolysis on impregnated carbon under humid conditions¹²⁸.

Scheme 6.

Schematic representation showing plausible mechanism of CNCl removal.

FT-IR analysis after post exposure of TICs on material 10

After exposure to TICs, FTIR analysis was performed to elucidate the interaction mechanisms and the role of active species in Material 10 (Fig. S3).

Following Cl₂ exposure (Figure S3A), the FTIR spectrum reveals the formation of chlorinated and oxychlorinated species. A band at ~3424 cm^–1 corresponds to O–H stretching, indicating the formation of HOCl. The peak at ~637 cm^–1 is assigned to O–Cl vibrations. The formation of molybdenum oxychloride (MoO₂Cl₂) is confirmed by a characteristic Mo=O stretching band at ~1007 cm^–1. Additionally, a peak near ~1185 cm^–1 (C–O stretching) indicates the involvement of oxygen-containing surface groups in chlorine adsorption. These observations confirm that chlorine removal occurs via hydrolysis followed by chemisorption, leading to the formation of HOCl, metal chlorides (CuCl₂, ZnCl₂), and oxychloride species. After exposure to H₂S (Figure S3B), a broad band at 3401.54 cm^–1 is assigned to O–H stretching, arising from hydroxyl species such as Cu(OH)₂, NaOH, or water formed during the reaction. A peak at 1559 cm^–1 is attributed to C=O or surface-bound species, while bands in the range of 1000–1200 cm^–1 correspond to C–O stretching vibrations. The formation of metal sulfides is confirmed by characteristic Cu–S and Zn–S vibrations observed around 628 cm^–1. A weak band at ~854 cm^–1 is attributed to MoS₃, corresponding to polysulfide stretching modes.Following NH₃ exposure (Figure S3C), a peak at 3428.55 cm^–1 corresponds to N–H stretching vibrations. The interaction of NH₃ with surface –COOH groups leads to the formation of amide (–CONH₂) functionalities, evidenced by a C=O stretching peak at 1625.89 cm^–1 and an N–H bending peak at 1582.10 cm^–1. The C–N stretching vibration appears at 1219.64 cm^–1. Additionally, NH₃ coordination with Cu species leads to the formation of diamine cupric complexes, reflected in characteristic bands in the same region. The formation of ammonium molybdate is confirmed by the N–H bending peak at 1413.26 cm^–1 along with a Mo=O stretching band at 922.94 cm^–1.For SO₂ exposure (Figure S3D), the FTIR spectrum indicates the formation of sulfite and sulfate species. Strong bands at ~1232 cm^–1 correspond to asymmetric S=O stretching of sulfate (SO₄^2-), while peaks near 877.56 cm-1 are assigned to O–S–O bending modes. The presence of sulfite (SO₃^2-) or sulfurous acid (H₂SO₃) is indicated by bands around ~980 cm^–1. Broad bands in the 3200–3600 cm⁻¹ region and a peak near ~1570 cm^–1 correspond to O–H stretching and bending vibrations, confirming the role of moisture and hydrated species formation. The peak at 3432 cm^–1 further supports the presence of water generated during the reaction. These results confirm that SO₂ removal proceeds via moisture-assisted chemisorption involving sulfite and sulfate formation on CuO and ZnO impregnated carbon. These FTIR findings support the proposed reaction mechanism described in Section "Plausible mechanisms of ABEK gases and CWA simulant (CCl₄) removal".

Conclusions

This work demonstrates a hybrid adsorbent system capable of providing broad-spectrum protection against chemical warfare agent simulants and toxic industrial chemicals, addressing a critical technology gap in contemporary respiratory protection technologies, particularly in the context of asymmetric warfare. This study focuses on the design aspect, material composition and highlights the important role of each constituent for capturing and neutralizing toxic gases. By combining three tailored adsorbent materials containing Cu, Ag, Zn, Mo, TEDA, NaOH, and KI in an optimized ratio, the developed system offers effective adsorption and neutralization across diverse gas classes. The adsorption performance of developed material followed the order: CCl₄ (187.63mg/g)> CNCl (74.75 mg/g)> Cl₂ (47.98 mg/g)> SO₂ (43.23 mg/g)> H₂S (43.19mg/g) > HCN (30.12 mg/g)> NH₃ (26.51 mg/g). Overall, the adsorption capacities varied significantly among the tested gases due to differences in both adsorbate properties and adsorbent surface characteristics. CCl₄ showed the highest uptake, driven by its non-polar nature and strong physisorption within the microporous structure of IGAC. In contrast, blood agents such as HCN and NH₃ exhibited much lower adsorption capacities, likely because of weaker interactions and the limited presence of suitable functional or catalytic sites on the adsorbent surface. Under alkaline conditions, TEDA plays a crucial role in enhancing CNCl hydrolysis, particularly in the presence of Cu^2⁺and Zn^2⁺ species, extending protection times to 40–42 minutes and maximizing overall protection efficiency. The composite of IGAC-1, IGAC-2, and IGAC-3 exhibited a pH of 5.06, highlighting pH as a key parameter in optimizing removal efficiency toward toxic gases. The optimized composite adsorbent exhibited broad-spectrum efficacy against CCl₄ (44–45 min), TICs; NH₃ (55–58 min), SO₂ (23–25 min), Cl₂ (24–25 min), H₂S (45–47 min), HCN (28–29 min) and CNCl (42–45 min) satisfying EN 14387 requirements. The formulation met all EN 14387 performance criteria, and its low inhalation resistance (<4.70 mbar) ensures suitability for respiratory protection applications.This material is therefore suitable for use as a replacement adsorbent in respiratory protection applications. Overall, the results confirm that strategic functional group tailoring and metal integration can deliver efficient, dual-functional adsorbents with potential for lightweight, scalable, and regenerable respiratory protection systems, addressing emerging chemical threats in modern conflict scenarios.

Abbreviations

AGAC

Acidified granular activated carbon

BTT

Breakthrough time

BET surface area

Brunauer–Emmett–Teller surface area

CWAs

Chemical warfare agents

EDS (or EDX)

Energy-Dispersive Spectroscopy

FT-IR

Fourier transform Infrared Spectroscopy

GAC

Granular activated carbon

IGAC

Impregnated granular activated carbon

PSD

Pore size distribution

PXRD

Powder x ray diffraction

RH

Relative humidity

SEM

Scanning electron microscopy

TEDA

Triethylenediamine

TGA

Thermogravimetric analysis

TICs

Toxic industrial chemicals

Author contributions

Akash Verma: Methodology, Investigation, data analysis, characterization and writing Virendra V. Singh: Conceptualization, methodology, writing-review and editing and supervision L. K. Pandey: Review and Formal analysis. Vikas B. Thakare: Visualization, Review & editing, Resources. Sanjay Upadhyay: Review & editing, Resources.

Funding

The authors received no funding for this work.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.