Performance and mechanism of Pb²⁺ and Cd²⁺ ions’ adsorption via modified antibiotic residue-based hydrochar

Abstract

This study investigated the hydrochar-based porous carbon prepared by combining the technical route of hydrothermal carbonization (HTC) + chemical activation. The hydrochar morphology was adjusted by changing the activation reaction conditions and adding metal salts. Experiments showed that the activation of KHCO₃ significantly increased the specific surface area and pore size of the hydrochar. Besides, oxygen-rich groups on the surface of the activated hydrochar interacted with heavy metal ions to achieve efficient adsorption. The activated hydrothermal carbon adsorption capacity for Pb²⁺ and Cd²⁺ ions reached 289 and 186 mg/g, respectively. The adsorption mechanism study indicated that the adsorption of Pb²⁺ and Cd²⁺ was related to electrostatic attraction, ion exchange, and complexation reactions. The “HTC + chemical activation” technology was environmentally friendly and effectively implemented antibiotic residues. Carbon materials with high adsorption capacity can be prepared so that biomass resources can be utilized with excessive value, as a consequence presenting technical assistance for the comprehensive disposal of organic waste in the pharmaceutical industry and establishing a green and clean production system.

Highlights

• •The high levels of residual Avermectin were almost completely removed by hydrothermal carbonization.
• •Hydrothermal carbonization effectively improves the performance of carbon materials.
• •KHCO₃ is an environmentally friendly activation reagent.
• •High-performance adsorption carbon materials can be prepared after “hydrothermal carbonization + chemical activation".
• •The maximum adsorption capacity of activated hydrochar adsorb Pb²⁺, Cd²⁺ is 289.35 mg/g, 186 mg/g, respectively.

1. Introduction

Antibiotics and their byproducts are increasingly produced worldwide. In particular, China is one of the world's largest producers and exporters of antibiotics, with an annual production of 248,000 tons [1]. During the fermentation process of antibiotic production, a large amount of antibiotic mycelium residue is generated [2], including mycelium, residual substrates, intermediate metabolites, and antibiotics remaining due to incomplete extraction [3]. Improper handling of antibiotic residues can bring severe environmental pollution, even lead to food chain pollution, and eventually endanger human health [4]. Antibiotic residues have been classified as hazardous wastes in HW02 at 271-002-02 and 276-002-02 in the 2021 National Hazardous Waste Directory [5]. Therefore, the choice of treatment of antibiotic residue should consider both harmless performance and maximum recovery of biological resources to compensate for the cost of treatment.

Incineration is considered feasible in practical applications among the various disposal methods because of its simple and large-scale operations [6]. However, the high moisture content required much energy during pyrolysis, leading to a significant increase in the cost of this approach. In addition, exhaust gases such as dioxins, nitrogen dioxide, and carbon dioxide from antibiotic residues are also prone to secondary pollution, making them a crucial factor in environmental challenges [7]. Pyrolysis of antibiotic residue can produce bio-oil, but the chemical composition is complex, and the calorific value is low (13–18 MJ/kg), which is not conducive to broad application. In addition, the antibiotic residue must be dried and pretreated before pyrolysis, consuming more energy and increasing costs. Acidolysis/alkali lysis and ionizing radiation in the reaction process due to the need for a large amount of electrolyte and low degradation rate affect its promotion and application. Treatment of antibiotic residues by composting technology degrades residual antibiotics effectively, but there is a risk of increased antibiotic resistance genes [8,9]. Anaerobic digestion treatment technology converts the organic matter in antibiotic residue into high-quality biogas. Still, the technical cost is high, and the treatment time is extended, which is not conducive to large-scale utilization. Hydrothermal treatment of biomass is considered a promising method to reduce greenhouse gases such as carbon dioxide and methane emitted into the atmosphere [10,11]. Compared with other treatment methods, water is used as a reactant, making the high moisture content of antibiotic residue an advantage, eliminating the dehydration pretreatment step, and reducing the cost of equipment investment [12]. Recently, the hydrothermal treatment of antibiotic residue has been used to meet the requirements of harmlessness, which can effectively eliminate residual antibiotics [[13], [14], [15], [16]]. HTC converts high-moisture-content feedstocks into biochar over a temperature range of 180–260 °C without predrying [17,18]. Biochars produced from HTC are commonly referred to as hydrochar, and the surfaces of hydrochars are rich in oxygen-containing functional groups, contributing to their ability to adsorb pollutants [19]. Increasing the reaction temperature and retention time increases the pore structure of hydrochar, thereby increasing the potential of hydrochar for application as an adsorbent [20]. Hydrothermal treatment aims to convert waste biomass into a material with high carbon content and a surface rich in oxygen-containing functional groups. This allows for further carbonization or chemical activation of the material after hydrothermal treatment [21]. Chemical activation is using chemical activators to modify the biochar material to increase the specific surface area and porous structure, which improves the adsorption capacity for pollutants. Chemical activators contain strong acids, strong bases, and metal salts, which will cause significant overdose hazards and problems such as secondary pollution and poisonous gases. KHCO₃ is a weakly alkaline and slightly corrosive substance that is stable in the air. KHCO₃ can substitute for strong alkaline activators, reducing the number of chemical activators used, and has twin benefits for the economy and environment.

Due to its long-term accumulation and inability to degrade, heavy metal contamination in the aquatic environment is a global hazard to population health. The two heavy metal ions with the most harmful effect on human health are Pb²⁺ and Cd²⁺, which are still found in industrial waste streams in high residual amounts. Various physicochemical methods, such as membrane filtration, flocculation, and chemical oxidation, are used to remove heavy metal ions. These methods have a high removal efficiency but also suffer from complicated treatment processes and high energy consumption. Solid materials are used for adsorption to remove heavy metals and other pollutants in waste streams. It has the benefits of being inexpensive, not creating any byproducts, and performing excellently in terms of adsorption [22], which has led to an increase in the number of academics focusing on adsorption methods and prompted extensive research on the use of biochar or modified biochar with a large specific surface area, multilevel pore structure and rich oxygen-containing functional groups for heavy metal adsorption [[23], [24], [25], [26]]. The chemical structure of hydrochar includes many aromatic structures containing many oxygen-containing energy groups, which allows hydrochar to further function by adsorption, catalysis, and activated carbon coincidence. Specifically, among hydrochar, there are many oxygen-containing energy groups and lower carbon content, making it more popular with cheaper adsorption materials, which also allows environmental pollutants to be reduced.

Due to the hazards associated with antibiotic residues and considering the large amount of organic matter, it is possible to realize their resourceful use. High-performance hydrothermal carbon can be prepared by hydrothermal carbonization and activated using KHCO₃ for the adsorption of heavy metal ions in wastewater to treat waste with waste. Xia et al. [27] used pine wood chips as raw material to prepare hydrochar material by a hydrothermal carbonization method and conducted Pb²⁺ adsorption experiments on virgin hydrochar modified with H₂O₂. The results showed that the maximum adsorption capacity of modified hydrochar for Pb²⁺ was 92.80 mg g⁻¹ at pH 5 = 0 and T = 298 K, exceeding that of hydrochar (2.20 mg g⁻¹) by 42 times. Bashir et al. [28] prepared hydrochar from straw and modified it by impregnation with 3.5 mol/L KOH. The cadmium adsorption capacity of the modified straw hydrochar increased from 12.1 to 41.9 mg g⁻¹. In summary, hydrochar and modified hydrochar can adsorb heavy metal ions from aqueous solutions.

In this paper, hydrothermal carbonization of antibiotic residues was investigated, and the resulting hydrochar was evaluated. The hydrochar samples with the highest carbon content were selected for KHCO₃ chemical activation to ensure no antibiotic residue. The activated hydrochar was characterized to analyze the effect of activation conditions on the hydrochar. Furthermore, the adsorption performance of activated hydrochar on Pb²⁺ and Cd²⁺ was researched. The results of this study provide a new method for the utilization of antibiotic residues and the control of heavy metal pollution in the aqueous environment. Preparing biomass charcoal from hazardous waste by hydrothermal carbonization has significant environmental, economic, and social benefits, as it solves the environmental pollution problem of hazardous waste and obtains high-value-added functional charcoal materials and chemicals. The decomposition pathway in the hydrothermal carbonization treatment of antibiotic residues was not studied in depth in this study and will be the focus of future research.

2. Materials and methods

2.1. Chemicals

Avermectin residue was obtained from a domestic pharmaceutical company, and the chemical reagents were above 99% analytical purity. KHCO₃ (CAS: 198-14-6), PbCl₂ (CAS: 7758-95-4), and CdCl₂ (CAS: 7790-78-5) were purchased from Shanghai Yi'en Chemical Technology Co., Ltd.

2.2. Preparation of hydrochar

The hydrochar was prepared from the avermectin residue, which was dried in a blast dryer at 105 °C for 24 h to constant weight, passed through a sixty mesh sieve, and set aside. The hydrothermal carbonization of avermectin residue was carried out in a high-pressure hydrothermal kettle. First, the installed hydrothermal kettle was purged by nitrogen for 2–3 min and then heated to the specified temperature (180, 200, 220, and 240 °C) at a heating rate of 5 °C/min, with an electromagnetic stirring speed of 400 r/min and holding times of 30, 60, 90, and 120 min. The result was filtered to separate the solid from liquid after the hydrothermal kettle was cooled to room temperature, and it was then dried in an oven to a constant weight. The solid phase product was obtained as hydrochar, recorded as X–Y (X represents the reaction temperature, and Y represents the holding time).

2.3. Instrumental analysis of hydrochar

Avermectin residue was determined using high-performance liquid chromatography (HPLC, Waters e2695. USA), Brunauer‒Emmett‒Teller (BET) surface area, pore volume, and pore size of hydrochar by Kubo-X1000. The elemental contents of C, H, O, and N were determined by an organic elemental analyzer (Elementar Varioel III. Germany) for analysis. A Fourier transform infrared spectrometer (FT-IR, Thermo Scientific Nicolet iS20, USA) analyzed the surface functional groups of biochar. A scanning electron microscope was used to explore the microstructure and surface morphology of the hydrochar.

2.4. Chemical activation of hydrochar

The activation process of hydrochar is shown in Fig. 1. A certain mass of 180–120 (highest carbon content hydrochar material) hydrochar and KHCO₃ (KHCO₃/hydrochar = 2, 4, 6) were weighed and mixed in a mortar and pestle. After the mixture was transferred to a corundum boat and placed in a tube furnace, it was heated to the specified temperatures (600, 700, and 800 °C) at a rate of 5 °C/min under a nitrogen flow rate of 100 mL/min and maintained for 1 h. When the tube furnace cooled to room temperature, the resulting product was washed with deionized water by centrifugation several times to neutral and finally dried in a blast dryer at 80 °C for 24 h. The final activated hydrochar was labeled KHC-T-Z, where T denotes activation temperature, and Z is KHCO₃/hydrochar weight ratio.

Fig. 1.

The activation process of hydrochar.

2.5. Instrumental analysis of activated hydrochar

The Brunauer‒Emmett‒Teller (BET) surface area, pore volume, and pore size of activated hydrochar were determined by Kubo-X1000. Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific Nicolet iS20, USA) analyzed the surface functional groups of the biochar. A scanning electron microscope was used to study the microstructure and surface morphology of activated hydrochar. The physical phase composition of the hydrothermally activated carbon samples was assessed using X-ray diffraction (XRD, Nihon Rei Ultima IV, Japan).

2.6. Batch adsorption studies

A specific mass (10, 20, 50, and 100 mg) of activated hydrochar sample was weighed in a 250 mL beaker, added to 50 mL of simulated wastewater containing Pb²⁺ or Cd²⁺ at specific concentrations (10, 50, 100, 200, 300, and 500 mg/L), placed in a constant temperature oscillator, shaken at 150 rpm at 25 °C, and filtered through a 0.22 μm membrane. The filtrate was taken for the determination of Pb²⁺ and Cd²⁺ concentrations by inductively coupled plasma emission spectroscopy (ICP‒MS, NexION 1000, China), and the adsorption capacity of heavy metal ions (q_t) were calculated using Eq. (1) and removal efficiency (Re%) were calculated using Eq. (2).

$$q_t=\frac{C_0-C_t}{m}\times V$$ (1)

$$Re(\%)=\frac{C_0-C_t}{C_0}\times 100\%$$ (2)

where q_t is the adsorption amount of heavy metals on hydrothermally activated carbon at time t, mg/g; C₀ and C_t are the initial concentration of heavy metal ions and that at time t, mg/L; m is the hydrochar mass, mg; V is the solution volume, mL.

This study used quasi-first-order models Eq. (3) and quasi-second-order models Eq. (4) to simulate hydrogenation kinetics on Pb²⁺ and Cd²⁺.

Quasi-first-order model:

$$q_t=q_e\left(1-e^{-k_{1}t}\right)$$ (3)

Quasi-second-order model:

$$q_t=\frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}$$ (4)

where q_t is the amount of adsorption at time t, mg/g; K₁ and K₂ are the rate constants for quasi-first- and quasi-second-order models, with units of min⁻¹ and g mg⁻¹min⁻¹, respectively.

The sorption isotherms for Pb²⁺/Cd²⁺ (10–500 mg/L) were also determined at different initial concentrations. Two well-known isotherm models, the Langmuir and Freundlich models, were utilized and subsequently used to fit the absorption data. Specifically, the Langmuir model represented by Eq. (5) considers the removal of monolayers with active adsorption sites in homogeneous media. The Freundlich model described by Eq. (6) is commonly used to describe multilayer binding processes on nonhomogeneous surfaces:

$$q_t=\frac{q_e{k}_L{C}_e}{1+k_{L}Ce}$$ (5)

$$q_e=k_{F}C{e}^{1/n}$$ (6)

where q_e is the maximum sorption of Pb and Cd by KHC, mg/g; K_L is the energy constant, mg⁻¹; K_F is the sorption correlation coefficient.

3. Results and discussion

3.1. Characterization of the hydrochar

3.1.1. Physicochemical properties of the hydrochar

The physicochemical properties of the hydrochar are summarized in Table 1, namely, C, H, O, N, ash, yield, etc. According to the data, the carbon content of hydrochar decreased gradually with temperature because the carbon distribution in the product gradually shifted to the gas and liquid phases with a further increase in hydrothermal temperature. Increasing the reaction temperature provided more complete biomass decomposition while reducing the carbon content of the hydrochar. The production of hydrochar was greatly affected by temperature; the temperature increased from 180 °C to 240 °C, and the hydrochar decreased from 70.97 to 56.11%. The high temperature will lead to aggravation of the dehydration reaction, the degree of liquefaction and gasification of biomass will go further, and it will be carried out with the carbonization reaction simultaneously, resulting in the reduced yield of hydrochar.

Table 1.

Chemical analysis of hydrochar under various hydrothermal conditions.

Sample	Elements/%				FIX Carbon/%	Ash/%	Yield/%
	C	H	O	N
Raw Material	25.36	3.82	66.42	4.40	10.23	42.51
180–120	26.11	3.64	67.95	2.30	10.25	56.74	70.97
200–120	24.59	3.17	70.37	1.87	9.53	60.02	66.20
220–120	22.80	2.95	72.53	1.72	10.07	63.65	64.21
240–120	17.81	2.21	78.38	1.60	9.39	72.11	56.11

3.1.2. Hydrochar environmental friendliness check

The content of avermectin under different hydrothermal treatment conditions of residual avermectin is shown in Fig. 2. For residual avermectin in the same reaction time with the increase of temperature decreases, the remnant of avermectin maintained at 180 °C for 30 min was 38.02 mg/kg, and the degradation rate reached 98.68%. As the temperature continued to increase, the remnant of avermectin was lower than the detection limit of 0.07 mg/kg at 200 °C, and no residual avermectin was detected when the reaction temperature and the reaction time were both increased. The experimental results show that hydrothermal carbonization treatment of avermectin residues can effectively eliminate residual antibiotics and realize the harmless treatment of antibiotic residues, which decreases the hidden danger for the next step of resource utilization.

Fig. 2.

Avermectin content under different hydrothermal conditions.

3.1.3. SEM of hydrochar and activated hydrochar

SEM observations of the raw material, hydrochar, and activated hydrochar are shown in Fig. 3. After HTC, the pore channels are clear and free of debris filling the channels and exhibit a multistage pore structure, which is consistent with the pore size distribution results described below. However, as the temperature rises, the char surface pores become disordered, and pore collapse also occurs, caused by the high temperature causing the further reaction of antibiotic residues (shown in Fig. 3(a–e)). Hydrothermal treatment can generate special rough surfaces, functional groups, and carbon content, facilitating subsequent activation reactions and increasing surface area and porosity. However, the surface morphology of adsorbed hydrochar shows well-developed and ordered pore structures, similar to the 3D honeycomb structure of its surface, due to the thermochemical reaction between the components of antibiotic residue caused by KHCO₃ activation. The gases generated during these reactions can create mesopores and macropores, and the embedded K and K₂O in biochar can create more micropores and mesopores [29]. As the activation temperature increased, more micropores were generated on the char surface, showing the best pore structure at 700 °C. As the temperature increased further, part of the pore channels on the surface of the activated char corroded and collapsed, causing blockage of the micropores and mesopores (shown in Fig. 3(f–h)). KHCO₃ activation can form a more developed pore structure in the biochar, indicating its high potential for contaminant removal.

Fig. .3.

SEM/mapping images of raw material (a), hydrochar (b–e), activated hydrochar (f–h), and after adsorption of activated hydrochar (i–l).

3.1.4. N₂ adsorption and desorption isotherms of hydrochar and activated hydrochar

The specific surface area and pore structure of biochar were studied by N₂ adsorption and desorption isotherms. It is clear from the adsorption isotherm curves that the specific surface area increased as the activation temperature and alkali-carbon ratio increased, as shown in Fig. 4(a), but that the specific surface area also decreased at high activation temperatures and alkali-carbon ratios (Fig. 4(b)), likely because the high temperature collapsed the surface pores of biochar and the high alkali-carbon ratio corroded the surface pores of carbon and blocked the micropores, which was consistent with the SEM results. As shown in Fig. 4(c), the raw material's adsorption isotherm almost perfectly matched the X-axis. The N₂ adsorption was incredibly low, demonstrating that the raw material's pores were highly underdeveloped. All mixed I/IV type adsorption isotherms of hydrochar and activated carbon exist. Their steep adsorption curves at low relative pressures of 0–0.1 may be related to the microporous structure. At relative pressures of 0.45–0.9, the type IV hysteresis curves indicated a certain amount of mesoporous structure, and at relative pressures of 0.9–1.0, the adsorption tended to increase rapidly upward, inferring a small amount of large pore size mesopores or small pore size macropores on the surface of the carbon materials. These findings are in line with the pore size distribution. The pore size distribution diagram in Fig. 4(d) shows that the hydrochar activated by KHCO₃ has a multilevel pore structure with microporous, mesoporous, and mac pores, the bulk of which are microporous. Furthermore, the multilayer pore structure enhances the carbon material's capacity for adsorption [30]. The surface area and pore volume parameters of biochar are summarized in Table 2. The specific surface area of KHC (KHC stands for 700-4 activated hydrochars) after activation reached 313 m² g^-1, which was 43.39 times higher than that of hydrochar (hydrochar stands for 180-120 hydrochars). In addition, the total pore volume of KHC also increased 6.55 times compared with that of hydrochar, and the larger specific surface area and developed pore structure could improve the application potential of KHC. Therefore, KHCO₃ was successful and effective in the activation of biochar. Hydrothermal treatment can increase the specific surface area and pore volume of biochar, which is beneficial to the formation of microporous structures. KHCO₃ activation significantly improves the specific surface area of hydrochar and forms more microporous structures in hydrochar.

Fig. 4.

N₂ adsorption/desorption isotherms and pore size distribution curves for different temperatures (600 °C 700 °C 800 °C) of activated hydrochar (a) N₂ adsorption/desorption isotherms and pore size distribution curves for different alkali-to-carbon ratios of activated hydrochar (b), N₂ adsorption/desorption isotherms curves for raw, hydrochar, and activated hydrochar (c), and pore size distribution curves for raw, hydrochar, and activated hydrochar (d).

Table 2.

Pore parameters of raw material, hydrochar (HC), and KHC.

Sample	BET surface area (m2g−1)	Pore Volume (cm3g−1)	Pore Size (nm)
Raw	1.23	0.0064	10.44
HC	7.22	0.0550	15.30
600–4	111.43	0.0955	1.71
700–4	313.24	0.3600	2.31
800–4	140.33	0.1962	2.8
700–2	272.29	0.3353	2.46
700–6	115.88	0.2266	3.91

3.1.5. XRD and FTIR analysis

Fig. 5(a) shows a more obvious diffraction peak near 22.14° for both raw and hydrochar cases, while the broad peak near 22.14° corresponds to the (002) crystal plane of graphite crystals, and the broader diffraction peak in the Fig indicates that both samples are amorphous carbon [31]; meanwhile, hydrochar has almost no pronounced small and sharp diffraction peaks, indicating that the surface of hydrochar contains trace metal impurities, which also suggests that hydrothermal carbonization can effectively remove the heavy metal elements from the antibiotic residue. From Fig. 5(b), it can be seen that the composition of the hydrochar after activation by KHCO₃ at 600 °C is mainly CaCO₃. With the temperature rise to 700 °C, the composition of the activated carbon is CaCO₃ and Ca₅Si₂O₇(CO₃)₂, which shows that the phenomenon of partial sintering starts at high temperature so that the components in the hydrochar react with each other to form Ca₅Si₂O₇(CO₃)₂ [32]. In contrast, the diffraction peak of Ca₅Si₂O₇(CO₃)₂ disappears at 800 °C, certifying the decomposition of Ca₅Si₂O₇(CO₃)₂ at a high temperature of 800 °C. Changing the alkali-to-carbon ratio at the same temperature, the peak shape becomes increasingly sharper, indicating that the degree of crystallization gradually increases. Still, the intensity of the diffraction peak slightly decreases with a further increase in the alkali-carbon ratio to six, probably due to the corrosion of the crystal structure by the excessive KHCO₃. The diffraction peak intensity of hydrochar activated by KHCO₃ increased and narrowed, suggesting a more ordered crystal structure formed by KHCO₃ activation treatment [33,34]. Therefore, KHC can be used as a potential environmentally friendly adsorbent for wastewater treatment.

Fig. 5.

XRD patterns of raw, hydrochar (a), and activated hydrochar (b); FT-IR spectra of activated hydrochar (c), raw, hydrochar, and activated hydrochar (d).

The type and quantity of oxygen-containing functional groups on the sorbent significantly impact how well it performs as an adsorbent [35]. The observations of the FTIR analysis, which was performed to analyze the functional groups on the carbon surface, are shown in Fig. 5. With increased activation temperature, as seen in Fig. 5(c), the absorption peaks of oxygen-containing functional groups gradually improved, suggesting that these groups multiplied sequentially; however, the opposite trend was observed with increasing alkali-to-carbon ratio. The surface groups were well preserved during the hydrothermal carbonization, according to FTIR measurement of the surface following hydrothermal treatment (Fig. 5(d)). The characteristic peak at 3278 cm⁻¹ is the stretching vibration peak of –OH, while that at 2926 cm⁻¹ indicates the stretching vibration peak of aliphatic C–H. The absorption peak near 1664 cm⁻¹ is mainly the -C-O- stretching vibration due to the absorption of lipids, and carboxylic acids and the stretching vibration peak of -C-O-C is near the characteristic peak at 1058 cm⁻¹. Meanwhile, the hydrochar activated by KHCO₃ at 1008 cm⁻¹ formed a more pronounced -C-O stretching vibration at 1008 cm-1. Moreover, the –C O stretching vibration peak near 3589 cm⁻¹ and the –OH wobble vibration peak near 690 cm⁻¹ were produced in the KHCO₃-activated hydrochar compared to the inactivated hydrochar, indicating that the KHCO₃-activated hydrochar not only preserved the oxygen-containing functional groups on the surface of the original hydrochar but also formed other oxygen-containing functional groups.

In summary, activated carbon with an activation temperature of 700 °C and an alkali-to-carbon ratio of four was used as the carbon material (expressed as KHC) for the next batch of adsorption experiments and compared with 180–120 hydrochar (expressed as HC). Each experiment set was repeated thrice.

3.2. Adsorption studies

3.2.1. Effect of the initial pH

Because it influences both the density of the material's surface charge and the morphology of the heavy metals, the initial pH is crucial to the adsorption process. Depending on the pH and the initial metal ion concentration, Pb²⁺ and Cd²⁺ can exist in aqueous solutions in several ways. They may be in equilibrium with the solvent compound mass as free ions at low pH. However, at high pH, they tend to form precipitates due to the presence of OH⁻ groups [36]. Therefore, the adsorption experiments were investigated only at pH ⩽ 7 in this study. The experiment was performed at room temperature (22 $\pm$ 1 °C) using 200 mg/L initial ion concentrations and 50 mg of adsorbent mass for 12 h. As shown in Fig. 6(a) and (b), when the pH is low, there is a significant concentration of H⁺ in the solution, which competes with heavy metal ions for adsorption sites and prevents the dissociation of the active groups, leading to a limited adsorption capacity. As the pH increases from 3.0 to 6.0, the adsorption of Pb²⁺ by KHC increases from 87 to 214.5 mg/g, Cd²⁺ increases from 64 to 78 mg/g, the adsorption of Pb²⁺ increases from 3 to 34 mg/g and Cd²⁺ increases from 4 to 17 mg/g by hydrochar. Further analysis showed that the adsorption capacity increases as the H⁺ concentration drops, the competition between H⁺ and heavy metal ions decreases, and more active groups and sites are exposed. Additionally, once the pH falls below a specific point, the amount of H⁺ in the solution lowers even more, the adsorbent's surface has fewer adsorption active sites, and the adsorption capacity is diminished; when the adsorption capacity of the KHC, as well as the hydrochar, reaches the maximum at pH = 6, which are 214.6, 78, 34, and 17 mg/g, respectively. Therefore, considering the precipitation effect of heavy metals, pH = 6 was selected as the optimal pH for the subsequent intermittent adsorption experiments to ensure a better adsorption effect.

Fig. 6.

The effect of solution pH on lead adsorption performance ((a) Pb²⁺ adsorption capacity, (b) Cd²⁺ adsorption capacity; T = 298 K pH = 2–7).

3.2.2. Effect of adsorbent quality

The effect of adsorbent quality on the adsorption capacity of Pb²⁺ and Cd²⁺ ions and the removal efficiency was studied at pH = 6 and T = 25 °C. As shown in Fig. 7 (a,b), the Pb²⁺ and Cd²⁺ adsorption capacity by KHC gradually decreased with increasing adsorbent quality. This may be because heavy metal ions are in less contact with the unit mass of the adsorbent. The exposed active sites on the adsorbent no longer contribute to the adsorption process [37]. The increase in adsorbent quality from 10 mg to 100 mg resulted in a sharp increase in the removal of Pb²⁺ by KHC and hydrochar, a result that may be attributed to the larger adsorbent surface area and pore volume available at higher adsorbent amounts, which provided more functional groups and active adsorption sites, resulting in higher metal ion removal [38]. When the quality was more than 100 mg, there was no further change in the removal performance of metal ions. Considering the adsorption effect and the convenience of the subsequent experiments, 100 mg was selected as the optimal dosage for the subsequent experiments.

Fig. 7.

The adsorbent quality on lead adsorption performance ((a) Pb²⁺ adsorption capacity, (b) Cd²⁺ adsorption capacity; pH = 6 and T = 298 K adsorbent quantity = 10–200 mg).

3.2.3. Effect of initial concentration

Fig. 8 shows the effect of the initial Pb²⁺ and Cd²⁺ concentrations on the adsorption capacity. As the initial concentration reached 100 mg/L, the adsorption capacity of KHC and hydrochar for these two metal ions increased in the solution. The removal efficiency of Pb²⁺ and Cd²⁺ reached maximum values of 99.86 (Fig. 8(a)) and 58.9% (Fig. 8(b)) versus 92.3 and 45.65%, respectively. This is attributed to the high contact probability between KHC, hydrochar, and metal ions (Pb²⁺ and Cd²⁺). The lower metal concentration causes complexation on the adsorbent surface, which also increases the availability of adsorption sites on KHC and hydrochar [39]. Moreover, the metal ion removal rate slightly decreased with a further increase in the initial concentration. Due to the saturation of the active functional groups present on the surface of the adsorbent, the available active sites have been exhausted [40]. This is consistent with past research showing that as the metal ion concentration increases, the extent of adsorption drops obtained in experiments on the adsorption of Pb²⁺ and Cd²⁺ by hydrochar prepared from Prosopis africana shells by Sunday et al. [36].

Fig. 8.

The ion concentration of the solution on lead adsorption performance (pH = 6 and T = 298 K adsorbent quantity = 100 mg; Ce = 10–500 mg/L).

3.2.4. Adsorption kinetics and adsorption isotherms

Adsorption kinetics primarily describes how quickly an adsorbent absorbs a solution. At pH = 6 and T = 298 K, the kinetics of Pb²⁺ (200 mg/L) and Cd²⁺ (200 mg/L) adsorption by KHC materials were investigated (see Fig. 9 (a, b)). According to the results of the fitted data, the correlation coefficients R² for the quasi-first-order and quasi-second-order adsorption kinetic data are at levels that are statistically significant. Table 3 displays the results of the kinetic parameter fitting. The quasi-second-order kinetic models were more appropriate for KHC–Pb (R² = 0.99) and KHC–Cd (0.96) than the quasi-first-order kinetic models (0.95 and 0.93), showing that chemisorption predominated in the sorption of Pb²⁺ and Cd²⁺ by KHC [41,42].

Fig. 9.

Kinetic fitting results for (a) Pb(II) and (b) Cd(II) uptake onto KHC; Nonlinear fitting curves from Langmuir and Freundlich isotherm models for temperature-dependent adsorption of (c) Pb(II) and d) Cd(II) by KHC (pH = 6; T = 298 K; t = 10–240 min; Ce = 10–500 mg/g).

Table 3.

Adsorption kinetic fitting parameters of the KHC sample.

KHC	Quasi-first-order kinetic model			Quasi-second-order kinetic model
	qe (mg/g)	KL (h−1)	R2	qe (mg/g)	KL (h−1)	R2
KHC–Pb	203.183	0.039	0.95	228.76	0.022	0.99
KHC–Cd	126.649	0.024	0.93	148.25	0.020	0.96
	Langmuir			Freundlich
	Qm (mg/g)	KL (L/mg)	R2	KF	n	R2
298 K–Pb	289.35	190	0.98	5.75	0.62	0.96
298 K–Cd	186	118	0.99	8.87	0.47	0.93

Freundlich isotherms are more relevant than Langmuir isotherms regarding nonhomogeneous surfaces and multilayer adsorption. Langmuir isotherms characterize monolayer adsorption on homogeneous surfaces [43]. The Langmuir and Freundlich models were utilized to observe the nonlinear fitting of the adsorption isotherms of KHC materials for Pb²⁺ and Cd²⁺, as shown in Fig. 9(c and d). The correlation coefficients obtained from the fitting are displayed in Table 3. The findings demonstrate that monolayer and multilayer adsorption coexisted in the experiments, supporting the fit of the Langmuir and Freundlich curves' strong consensus with the experimental data. The Langmuir isotherm can more accurately explain how Pb²⁺ and Cd²⁺ are adsorbed by KHC, according to the correlation coefficient (R²). Furthermore, we may infer from the isotherm that the maximal Pb²⁺ and Cd²⁺ adsorption by KHC is 289.35 and 186 mg/g, respectively, outperforming many adsorbents suggested in previous works. (shown in Table 4), KHC can be used as an effective adsorbent for Pb²⁺ and Cd²⁺ ions.

Table 4.

Summary of various adsorbents for Pb²⁺/Cd²⁺ adsorption capacities.

Adsorbent	Adsorption capacities (mg/g)		References
	Pb2+	Cd2+
NaOH activated carbon from canola wastes	44.248	52.356	[44]
H2O2 modified hydrochar	92.80		[27]
Amino-functionalized activated carbon	142.03	79.20	[45]
Thiol-functionalized activated carbon	232.02	130.05
Astragalus residue by KOH		116.96	[46]
Astragalus residue by KOH and modified with KMnO4		217.00
Activated carbon prepared from Terminalia catappa leaf samples	27.5	42.5	[47]
Sulfide-modified magnetic hydrochar	62.49	149.33	[10]
Microwave-assisted hydrothermal carbonization of Prosopis africana shell	45.3	38.3	[36]
Hydrochar prepared from antibiotic residue	58.9	26	This study
KHCO3 activated hydrochar	289.35	186	This study

3.2.5. Adsorption mechanism

Pb²⁺ and Cd²⁺ ions were consistently dispersed on the surface of the adsorbed KHC, which is visible from the SEM mapping of the adsorbed KHC (see Fig. 3(i–l)). This demonstrates that the KHC was successful in adsorbing Pb²⁺ and Cd²⁺. Analysis was carried out utilizing FT-IR and XPS to comprehend which oxygen-containing functional group impacts the adsorption capacity of the adsorbed Pb²⁺ and Cd²⁺. After adsorption, an apparent peak at 1404.8 cm⁻¹ was observed in the FT-IR spectrum (see Fig. 10 (a)) due to the symmetric stretching of the Pb–O/Cd–O bond created by the complexation of the Pb²⁺/Cd²⁺ with –COOH during adsorption [48]. Pb²⁺ and Cd²⁺ adsorption resulted in the considerable reduction of various peaks, including those expressing C–C and C–O stretching vibrations at 694.25 and 1012.44 cm⁻¹, respectively. In contrast to Pb²⁺, which remained essentially unaltered during Cd²⁺ adsorption, the C–C content of the aromatic compound declined, showing that π-π interactions were involved in the process. The C–O peaks also decreased to variable degrees throughout the adsorption of both ions, indicating that the complexation of oxygen-containing functional groups was also involved.

Fig. 10.

FT-IR spectra of KHC and KHC after adsorption Pb²⁺ and Cd²⁺ (a), Adsorption performance of KHC after five cycles (adsorption conditions were pH = 6 and T = 298 K adsorbent quantity = 100 mg; Ce = 200 mg/L) (b).

XPS analysis was used to further study the mechanism of Pb²⁺ and Cd²⁺ adsorption by KHC as an adsorbent (see Fig. 11). Because of the appearance of new Pb 4f and Cd 3 d peaks in the complete spectrum following adsorption. In contrast, the original K 2p spectrum was attenuated, as illustrated in Fig. 11(a), and an ion exchange reaction may have occurred during the adsorption process [49]. In particular, as depicted in Fig. 11(b), the peaks at 138.8 and 143.6 eV are separated into discrete Pb 4f_7/2 and Pb 4f_5/2 peaks [50]. Additionally, the Cd 3d_5/2 and Cd 3d_3/2 peaks at 406.6 and 412.3 eV show that KHC adsorbs Pb and Cd. The C1 S spectra may be broken down into contributing peaks for C–C (284.8 eV), C–OH (286.2 eV), and C–COOH (289.3 eV). After adsorption, the C–C and C O concentrations were significantly reduced (Fig. 11(C)), showing that functional groups were involved in the ion adsorption process. Additionally, the C O content was significantly decreased, which may have been caused by the electron donor-acceptor (EDA) interaction between the ion and the KHC [51]. Furthermore, –OH increased significantly after adsorption, indicating that hydroxyl groups were involved in the adsorption process via hydrogen bonding [52]. Following Pb²⁺ adsorption, the C–O in the O1S spectrum (shown in Fig. 11(d)), which has a binding energy of 532.6 eV, shifts toward lower binding energies, most likely as a result of the formation of Pb–O complexes in which O moves closer to Pb²⁺ and the electron density of the neighboring oxygen and carbon atoms decreases [27].

Fig. 11.

(a) Total survey scans of XPS spectra; (b) high-resolution XPS spectra of Pb 4f/Cd 3 d; (c) C 1 s; (d) O 1 s.

To summarize, the binding mechanisms of KHC with Pb(II) and Cd(II) are relevant to chemisorption, complexation reactions, electrostatic attraction, ion exchange, and π-π interactions. Based on isotherm and kinetic analyses, the adsorption of Pb²⁺ and Cd²⁺ ions by KHC is based on chemisorption and contains both monolayer and multilayer adsorption. Among the different adsorption mechanisms, surface complexation is a dominant process that is actively involved in metal removal by KHC. By XPS analysis, new peaks appeared in the complete spectrum after adsorption, and the original spectrum was weakened so that ion exchange reactions may have occurred during the adsorption process. The C–C content of aromatic compounds decreases, suggesting that π-π interactions are involved in this process.

After adsorption, it is necessary to ensure the recycling performance of KHC for practical applications. In Fig. 10 (b), the Pb(II) adsorption of KHC still exceeded 67.5% after five adsorption cycles. The subtle decline in the uptake performance was attributed to the permanent occupation of some adsorptive sites by the undesorbed metal ions on the KHC surface [53]. Consequently, KHC has been confirmed as a reliable and efficient adsorbent for purifying Cd(II)/Pb(II)-polluted water.

4. Conclusions

High-performance adsorbent materials for the absorption of heavy metals in wastewater can be obtained by hydrothermal carbonization (HTC) + chemical activation of KHCO₃. The hydrochar produced by HTC at 180 °C for 120 min, with the following heat treatment at an activation temperature of 700 °C, produced KHC with an alkali-carbon ratio of 4, which can be used as an adsorption material for Pb²⁺ and Cd²⁺ in simulated wastewater. The maximum adsorption values of these two ions by KHC were 289.35 and 186 mg/g, respectively. The kinetic adsorption and isotherm experiments showed that the adsorption of KHC was consistent with the quasi-second-order kinetic model and the Langmuir model, respectively. Its adsorption mechanism comprised chemisorption, surface complexation, electrostatic interaction, and ion exchange. The low-cost carbon material extracted from hazardous waste obtained under mild and environmentally friendly experimental conditions can be used as an effective adsorbent to remove heavy metal ions from wastewater.

Author contribution statement

Bingtong Chen: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Haibin Guan; Suxiang Liu; Baofeng Zhao; Heming Zhang; Cunqing Zhong; Wenran Ding; Angang Song; Di Zhu: Conceived and designed the experiments.

Yue Zhang; Liangbei Liu: Performed the experiments.

Bari Wulan; Xiangyu Feng: Contributed reagents, materials, analysis tools or data.

Huan Li; Guofu Liu: Analyzed and interpreted the data.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.