Adsorptive removal of methylene blue and pharmaceutical effluents using biosynthesized zinc oxide nanoparticles

Abstract

Zinc oxide nanoparticles (ZnO-NPs) were biosynthesized using Ocimum basilicum leaf extract via an eco-friendly, plant-mediated approach and applied for the adsorption of methylene blue dye and pharmaceutical effluent. The synthesized ZnO NPs were characterized using various techniques, including UV–Vis spectroscopy, X-ray diffraction transmission electron microscopy and energy-dispersive X-ray analysis. The characterization results demonstrated that round-shaped ZnO NPs were prepared with particle sizes of 10.24 nm. Key phytochemicals identified by GC–MS that participate in nanoparticle synthesis include hexanal, 1-octen-3-ol, eucalyptol, fenchone, benzenemethanol, octanal, and 9-Octadecenoic acid. The effluent decolorization efficiency decreased from 72.00 to 44.88%, whereas that of the dye dropped from 75.11 to 53.33%. When the contact time was increased from 5 to 100 min, the effluent decolorization efficiency increased from 46.67 to 76.89%, while that of the dye rose from 48.44 to 82.26%. A positive enthalpy change (ΔH = 14.76 and 23.28 kJ/mol for MB and pharmaceutical effluent) indicates that the reaction is endothermic. This indicates that the reaction requires heat input to proceed. This study provides a low-cost and sustainable approach for the remediation of pharmaceutical wastewater via the use of green-synthesized ZnO-NPs. The novelty of the work lies in the combined treatment of methylene blue and pharmaceutical effluent using Ocimum basilicum-mediated ZnO nanoparticles, which has not been previously reported.

Introduction

The escalating concerns regarding environmental pollution pose significant threats to public health, biodiversity, and the overall ecological balance. Industrial and domestic wastewater, including hospital discharges, mainly contain residual analgesics, antibiotics, hormones, and heavy metals which are the primary sources of pharmaceutical effluents^1,2. The chemical stability and biological activeness of these compounds make them resistant to conventional treatments of wastewater and they are capable of causing endocrine disruption, antibiotic resistance, and aquatic toxicity^1,2,3. Methylene blue (MB) on the other hand, is widely utilized cationic dye in pharmaceutical, and textile industries, causing serious hazards in the environment even at low concentrations because of its mutagenic and cytotoxic effects on aquatic organisms and humans^4,5. The release of pharmaceutical residues and coloured dyes into aquatic systems has caused the development of advanced adsorbent materials, most especially nanoparticles of metal oxide obtained from green routes^6,7. Conventional remediation methods like coagulation, ion exchange, chemical precipitation, membrane processes, solvent extraction, filtering, sedimentation often fall short due to their high costs, inefficiency, and potential environmental harm^8–10. This scenario necessitates the exploration of advanced materials and technologies that can effectively and sustainably address these challenges.

Numerous sectors have been transformed by nanotechnology, biological sciences and environmental management are two fields where its applications are becoming increasingly prevalent. The synthesis and application of nanoparticles is one of the most exciting aspects of nanotechnology¹¹. Nanoparticles are widely used in wastewater treatment due to their distinctive physicochemical characteristics, such as high surface area, antibacterial activity, and photocatalytic effectiveness. Green synthesis methods are a desirable substitute for traditional synthesis methods in the production of multifunctional nanoparticles. Nayak et al.¹² reported the synthesis of silver nanoflowers using Pongamia pinnata seed cake extract (PSCAgNPs) within 24 h and deployed it for the removal of malachite green (MLG) dye which adsorbed 90.01 ± 0.17% of 50 mg/L. The biosynthesized of iron-oxide nanoparticles using the fruit-waste extracts was reported by Ahmad et al.⁶ and the results achieved excellent solar-driven removal of organics. In a similar way¹³ reported that plant-mediated CuO nanoparticles demonstrated great potency for the degradation of organic dyes in wastewater. The selective visual detection of nickel and cadmium ions by small-sized gold nanospheres and nanotriangles (CPAuNPs) synthesized from Cyclea peltata extract was investigated by Nayak et al.¹⁴. In another study by Nayak et al.¹⁵, gold nanospheres were fabricated from leaf extract of Cyclea peltata (CPAuNPs) in 5 h and used to removed methyl green dye and achieved 86.02 ± 0.12% of 50 mg/L MG. In another study, microwave-assisted green synthesized Mn₂O₃ nanoparticles eliminate about 97% of ciprofloxacin and ibuprofen under sunlight (Shameem et al.¹⁶.

Zinc oxide nanoparticles (ZnO-NPs) in particular have attracted interest and more studies are now being focused on using biological origins to create nanoparticles^17,18. By using natural organisms like fungi, bacteria, and plants as bioreductants, ZnO-NPs can be biosynthesised into nanoparticles that are more biocompatible and less dependent on hazardous chemicals. Additionally, the biological method frequently produces ZnO NPs with improved activity, which makes them useful in a variety of applications like drug delivery, antibacterial treatments, and as considered in this study, for water purification¹⁹. The method is equally simple to operate, thus eliminating complexes procedures. ZnO-NPs act as potent adsorbent capable of removing a wide range of pollutants, including dyes, pharmaceuticals, and heavy metals which makes them very valuable in effluent treatment. Zinc oxide nanoparticles have been used in removing several pollutants, offering an environment-friendly solution to industrial effluent management^20,21. For instance, pure ZnO nanoparticles (NPs) and Au-decorated ZnO (Au-ZnO) heterostructures were synthesized by Ahmad et al.²² via a green method using Carya illinoinensis (pecan nut) leaf extract as a reducing agent for the removal of Rhodamine B. The study of Dhivya et al.²³ reports the eco-friendly synthesis of pure ZnO and Mn-doped ZnO nanoparticles using Cassia angustifolia seed extract for methylene blue (MB) dye removal. The study on the removal of heavy metals by using magnetically reusable Fe₃O₄@ZnO nanocomposites from Andrographis paniculata leaf extract was reported by Manojkumar et al.²⁴. The adsorption experiments demonstrated high removal efficiencies of 99.81% for Cd(II), 99.76% for Pb(II), and 98.1% for Cr(IV) and it was also noted further that the magnetic properties of Fe₃O₄ allowed for easy recovery and reuse of the nanocomposites, highlighting their potential as environmentally friendly and effective adsorbents for wastewater treatment.

Nasrollahzadeh et al.²⁵ reviews the use of environmentally friendly, biogenic nanomaterials in wastewater treatment and highlighted the advantages of these green-synthesized nanocatalysts, such as high surface area, chemical reactivity, and cost-effectiveness, in removing organic and inorganic pollutants from water. The review also addresses challenges related to toxicity, biosafety, and the need for sustainable synthesis methods. In the study by Nguyen-Hong et al.²⁶ green synthesis method was employed to produce cerium-doped zinc oxide (Ce-doped ZnO) nanoparticles utilizing Hedyotis capitellata leaf extract and it was observed that the synthesized Ce-doped ZnO nanoparticles demonstrated enhanced removal activity towards methyl orange dye elimination.

Basil (Ocimum basilicum) is a plant that can be found in Nigeria, and is also known as scent leaf, Efirin, Nchanwu, and Baidoya as local names in Nigeria. It is a member of the mint family and is grown for its aromatic behaviour and used for numerous medical applications and it is a medicinal and aromatic plant with a rich phytochemical profile that enables it acts as an effective bioreductant and stabilizer for green synthesis of nanoparticle^27–29. The extracts of its leaf contain several essential-oil components such as leucalyptol/1,8-cineole, inalool, eugenol, estragole as well as flavonoids and polyphenolics including luteolin, linalool, apigenin, quercetin and phenolic acids such as caffeic and rosmarinic acids^27–29. These phytoconstituents bear functional groups such as–COOH, –OH, –C = C, –C = O and among others which are capable of coordinating metal ions and donating electrons, thus facilitating the reduction of Zn²⁺ to ZnO and at same time acting as capping agents which limit aggregation and particle growth³⁰. The phytochemical-capped ZnO nanoparticles formed often exhibit controlled particle morphology, enhanced colloidal stability, and improved functional properties (like antimicrobial and antioxidant activity), which enhances their suitability for the remediation of the contaminants like pharmaceutical effluent and dye treatment³⁰.

Although several studies have reported the biosynthesis of ZnO nanoparticles using Ocimum basilicum extract, most have focused primarily on their antimicrobial or photocatalytic applications. However, limited research has explored their adsorptive efficiency toward real pharmaceutical effluents, which contain complex mixtures of organic and inorganic pollutants. The novelty of the present study lies in its combined investigation of pharmaceutical effluent and methylene blue adsorption using O. basilicum-derived ZnO nanoparticles, with detailed evaluation of kinetic, thermodynamic, and mechanistic parameters. Furthermore, this work provides a direct correlation between GC–MS-identified phytochemicals in basil extract and their roles in nanoparticle formation and performance, offering new insight into how natural biomolecules influence adsorption efficiency. This integrated approach not only extends the application of biosynthesized ZnO-NPs beyond model dyes but also demonstrates their real-world potential for sustainable wastewater remediation.

This article explored the functionalities of O. basilicum for the production of ZnO nanoparticles, focusing on their adsorptive activities for the remediation of pharmaceutical effluent.

Materials and methods

Materials

All the chemicals used in this work zinc chloride hexahydrate (ZnCl₂·6H₂O), hydrochloric acid (HCl, 37%) and sodium hydroxide (NaOH, 98%), methylene blue (C₁₆H₁₈ClN₃S), and potassium bromide (KBr, 99%) were obtained from Sigma, India. Since they are of good analytical grade, they were used without further purification.

Effluent collection and physiochemical analysis

The effluent sample was collected from the effluent reservoir of a pharmaceutical company located in Sango Ota, Ogun State, Nigeria. The sample was divided into two bottles and refrigerated for preservation until analysis. The parameters measured included temperature, electrical conductivity (EC), pH, total dissolved solids (TDS), total suspended solids (TSS), chlorides, nitrates, sulfates and some selected heavy metals. These measurements were conducted following standard methods outlined by USEPA³¹ and APHA³². The effluent before and after treatment were analyzed.

Plant extraction and phytochemicals screening

Basil (Ocimum basilicum) leaves were sampled in large quantities from a farm location near Ifo, Ogun State, Nigeria. The O. basilicum leaves were dusted, and washed with tap water to eliminate impurities and thereafter air-dried in the laboratory. Afterward, they were pulverized using a mortar and pestle and sieved to obtain a fine powder. For extract preparation, 5% concentration of dried O. basilicum was prepared in distilled water. Accordingly, 5 g of dried O. basilicum was weighed and boiled in 100 mL of distilled water using a 250 mL round-bottom flask on a heating mantle for approximately 15 min under reflux. Once cooled to room temperature, the mixture was filtered and centrifuged at 2000 rpm for 15 min to remove leaf microparticles. The resulting clear solution was transferred to a polypropylene tube, refrigerated, and stored for use in the production of zinc oxide nanoparticles. Figure 1 shows the images of (a) O. basilicum leaves and (b) dried leaves of O. basilicum. The extract was characterized by employing gas chromatography-mass spectrometry (GC–MS, 8860A gas chromatograph, Agilent Technologies) to identify various bioactive compounds.

Fig. 1.

Images of (a) O. basilicum leaves, (b) dried of leaves O. basilicum and (c) GC–MS of O. basilicum extract.

Biosynthesis of ZnO-NPs

The green synthesis of zinc oxide nanoparticles (ZnO-NPs) was done with the use of the aqueous extract of O. basilicum leaves which serves as both the reducing and stabilizing agent. Briefly, 20 mL of the extract prepared from the plant was reacted with 15 mL of distilled water and 2 mL of 1 M zinc chloride hexahydrate solution in a 100 mL volumetric flask. The reaction mixture was placed under continuous magnetic stirring (600 rpm) for 4 h at temperature of 70 ± 2 °C to ensure uniform mixing and enhance the reduction of Zn²⁺ ions. As the reaction proceeds, a distinct colour change from light yellow to milky white was observed which indicate the formation of zinc hydroxide intermediates and thereafter conversion into zinc oxide nanoparticles. After the reaction has proceeded for 4 h, the reaction mixture was removed from the magnetic stirrer, cooled to room temperature and allowed to settle, with the appearance of a milky-white precipitate formed at the bottom of the flask, indicating the formation of zinc nanoparticles. The milky-white precipitate formed was centrifuged at 4000 rpm for 10 min, washed with distilled water to eliminate unbound phytochemicals, and then dried at 80 °C for 6 h.

ZnO-NPs characterizations

The characterization of zinc oxide nanoparticles (ZnO-NPs) involved several analytical techniques. UV–Visible spectroscopy of the ZnO nanoparticles were obtained using a UV–Visible spectrophotometer (Shimadzu UV-1800, Japan) in the wavelength range of 200–800 nm with a resolution of 1 nm and distilled water was used as the reference. To identify the chemical bonds within the ZnO-NPs, samples were mixed with potassium bromide (KBr) and ground into a homogeneous powder. This mixture was then compressed under a pressure of 7 tons to form pellets. The FT-IR spectra were recorded using a PerkinElmer 3000 MX spectrometer (USA), covering the range from 4000 to 400 cm⁻¹. To determine the crystal structure of the ZnO-NPs, X-ray Diffraction (XRD) analysis was performed using (Bruker D8 Advance, Germany) equipped with Cu-Kα radiation (λ = 1.5406 Å) operated at 40 kV and 30 mA. The diffraction pattern was collected in the range of 2θ equals 20°–80° at a scan rate of 2°/min. The elemental composition of the ZnO-NPs were examined using a Zeiss EVO 18 scanning electron microscope (Carl Zeiss, Germany)) equipped with EDX capabilities and operating at an accelerating voltage of 15 kV under high-vacuum mode. Samples were mounted on carbon coated with a thin layer of gold using a sputter coater prior to analysis. For transmission electron microscopy (TEM) imaging, the prepared adsorbent powder was dispersed in distilled water and sonicated for 15 min to achieve a uniform suspension. A drop of this suspension was then placed onto a copper grid and allowed to dry before observation with a TEM (Tecnai 20 G2 FEI, Netherlands). A Zetasizer Nano ZS90 (Malvern, UK) was used to measured the pH at point of zero charge (pHpzc).

Methylene blue solution preparation

Methylene blue stock solution (250 mg/L) was made by dissolving 0.25 mg quantity of methylene blue powder in 250 mL of distilled water. Distilled water and dilution principle were used to dilute this stock solution to obtain working concentrations ranging from 5 to 80 mg/L for adsorption studies. The MB solutions were prepared freshly prior each experiment and kept in amber bottles to prevent light-induced degradation.

Effluent treatment with ZnO-NPs

An aliquot of 15 mL of the effluent or methylene blue was transferred into a conical flask, and its pH was adjusted using 5% HCl or NaOH solutions. After stirring for 10 min, 10 mg of the ZnO-NPs was added, and the mixture was placed on an orbital shaker to equilibrate for 100 min. At predetermined intervals, samples were withdrawn and analyzed using a UV–Vis spectrophotometer (Shimadzu UV-3600 UV–Vis-NIR spectrophotometer). The decoloration efficiency (DE) was calculated using Eq. 1:

1

where C₀ and C_e represent the initial and final concentrations of the contaminants at any reaction time t, respectively. Each experiment was performed in triplicate, and the average values were reported. The procedure was repeated with varying mixtures of ZnO-NPs and the effluent under different experimental conditions in the range of: effluent concentration 5 to 30 mg/L, contact time of 5 to 100 min, pH of 2 to 10, and temperature of 25 to 85 °C.

Desorption/Regeneration study

Adorption-desorption experiments were performed using the exhausted adsorbent. After each equilibrium experiment was attained, the ZnO-NPs was separated, and desorption was performed using 0.5 M acetic acid as the eluting agent. The desorption experiment was then carried for 60 min and samples were taken for UV–Vis spectrophotometer analysis. The process was done five times, and the desorption percentage (DP) was estimated via Eq. 2

2

Where the effluent/MB amount adsorbed and desorbed are denoted as AA and AD respectively.

Results discussion

Physicochemical and heavy metals assay

The treatment of pharmaceutical effluent with ZnO nanoparticles (ZnO-NPs) leads to significant physicochemical changes, indicating the effectiveness of ZnO-NPs in reducing the effluent’s pollution load as shown in Table 1. The conductivity of the raw effluent increased from 56 to 61 μS/cm after treatment. This increase may be as a result of the partial mineralization of complex organic pollutants, which generates smaller ionic species, as evidence by the reduction in BOD and COD values. Moreover, during the treatment, the release of Zn2⁺ ions from the nanoparticle surface process could further increase the ionic load of the solution thereby contributing to the rise in the conductivity³³.

Table 1.

Physicochemical analysis of pharmaceutical effluent.

Parameters	Raw effluent	Treated effluent with ZnO-NPs
Conductivity	56 μs/cm	61 μs/cm
Total dissolved solids	350 mg/L	206 mg/L
pH	5.50	7.20
TSS	0.25 mg/L	0.14 mg/L
BOD	2353.01 ± 63 mg L−1	1022.25 ± 52 mg/ L
COD	350.84 ± 48 mg L−1	152.18 ± 63 mg/ L
Sulfates	46.04 ± 22 mg L−1	19.82 ± 03 mg/ L
Chlorides	13.06 ± 63 mg L−1	6.02 ± 05 mg/ L
Cu	1.24	ND
Co	0.16	0.08
Cd	0.13	0.06
Ni	ND	ND
Pb	0.06	ND
Mn	ND	ND
Cr	1.37	0.13
Zn	2.71	0.90
Fe	2.43	0.80

ND = not detected.

The values of total dissolved solids (TDS) in the raw effluent and after treatment with ZnO-NPs are 350 mg/L and 206 mg/L and this decrease in TDS after treatment reflects the breakdown of complex organic and inorganic compounds into smaller soluble particles and most likely, their removal from the effluent. The value of the pH of the raw effluent increased from 5.5 (acidic) to 7.2 (neutral) in the treated sample as the ZnO-NPs can neutralize the acidic nature of the effluent by adsorbing acidic species and generating hydroxyl radicals during the adsorption process. The total suspended solids (TSS) reduced from 0.215 mg/L in the raw effluent to 0.14 mg/L after treatment as the ZnO-NPs could facilitate coagulation and sedimentation of suspended particles, effectively reducing TSS. The contents of the biochemical oxygen demand (BOD) reduced from 2353.01 ± 63 mg L⁻¹ in the raw effluent to a value of 1022.25 ± 52 mg L⁻¹ in the treatment sample and this reduction in BOD indicates the effective removal of organic pollutants by ZnO-NPs. BOD measures the oxygen required by microorganisms to break down organic matter, and its reduction reflects the diminished presence of biodegradable organic pollutants. In a similar way, the chemical oxygen demand (COD) contents of the raw effluent got reduced from 350.84 ± 48 mg L⁻¹ to 152.18 ± 63 mg L⁻¹ after treatment as the ZnO-NPs is able to break down the organic compounds in the effluent through oxidation, resulting in a notable decrease in COD. The values of the sulfates and chlorides found in the raw effluent are 46.04 ± 22 mg L⁻¹and 13.06 ± 63 mg L⁻¹ respectively. However, after treatment with ZnO-NPs, the values reduced to 19.82 ± 3 mg L⁻¹ and 6.02 ± 5 mg L⁻¹ respectively. The ZnO-NPs are capable to adsorb and degrade sulfur- and chlorine-containing compounds during treatment and this reduces the concentrations of sulfates and chlorides in the effluent.

Heavy metals

In the raw effluent, high concentrations of Cu (1.24 mg/L), Co (0.16 mg/L), Cd (0.13 mg/L), Cr (1.37 mg/L), Zn (2.71 mg/L), and Fe (2.43 mg/L) were observed (Table 1). Whereas, after treatment with ZnO-NPs, heavy metals like Cu, Pb, Mn, and Ni were not detectable; while Co, Cd, Cr, Zn, and Fe show marked decreases (e.g., Cr drops to 0.13 mg/L, Zn to 0.90 mg/L, and Fe to 0.80 mg/L). The ZnO-NPs have a strong adsorptive capacity for heavy metals due to their large surface area and high reactivity. Additionally, heavy metals may form insoluble complexes or precipitate during treatment, thereby reducing their concentration in the effluent.

In scaled-up applications, the insoluble ZnO–metal complexes formed during treatment can be readily separated from the treated effluent by sedimentation or filtration. These solids can subsequently be regenerated via mild acid washing or thermal desorption to recover ZnO nanoparticles, or immobilized in stable matrices for safe disposal. Such post-treatment handling ensures that the biosynthesized ZnO nanoparticle process remains sustainable and compliant with industrial wastewater management practices. The various prominent phytochemicals and the retention rates from the GCMS analysis of O. basilicum extract which is shown in Fig. 1c gave the following biomolecules: Hexanal (3.485), Benzene (4.296), p-Xylene (4.586), 1-Octen-3-ol (5.588), 5-Hepten-2-one (5.700), Eucalyptol (6.321), Ethyl 2-(5-methyl-5-vinyltetrahydrofuran-2-yl)propan-2-yl carbonate (6.807), Fenchone (7.040), Fenchol (7.337), Bicyclo[2.2.1]heptan-2-one (7.742), Benzenemethanol (8.157), 2,3-Dehydro-1,8-cineole (9.683), 4-tert-Butylcyclohexyl acetate (10.172), Isomethyl ionone (11.313), Octanal (13.706), Metaraminol (15.269), Propanamide (16.585), and 9-Octadecenoic acid (16.798). The gas GC–MS analysis of basil extract revealed a complex mixture of phytochemicals, including aldehydes, aromatic compounds, alcohols, ketones, and terpenes. These compounds could play a significant role in the green synthesis of zinc nanoparticles by acting as reducing and stabilizing agents¹⁰. Some of the key phytochemicals as identified by the GC–MS that can participate in the nanoparticle’s synthesis are hexanal, 1-Octen-3-ol, eucalyptol, fenchone, benzenemethanol, octanal, and 9-Octadecenoic acid. During the green synthesis process, these phytochemicals interact with zinc salts, leading to the formation of ZnNPs. Compounds like hexanal and octanal can reduce zinc ions (Zn²⁺) to elemental zinc (Zn⁰), while terpenes and alcohols such as eucalyptol and fenchol could stabilize the nanoparticles by capping their surfaces. This dual functionality ensures the production of stable ZnNPs with controlled sizes and shapes. Previous study by Zeynep et al.³⁴ has demonstrated the efficacy of basil extracts in synthesizing ZnNPs, highlighting the importance of its phytochemical composition in the reduction and stabilization processes.

Proposed ZnO nanoparticle formation mechanism

The green synthesis of ZnO nanoparticles with O. basilicum extract involves three main steps viz: reduction, nucleation, and stabilization. Zinc chloride supplies Zn²⁺ ions that interact with phytochemicals available in the basil extract. Compounds such as eucalyptol, hexanal, octanal, and fenchone identified by GC–MS analysis, act as reducing and capping agents^18,35,³⁶. First, the carbonyl and hydroxyl groups from these phytochemicals reduce Zn²⁺ ions to zinc hydroxide (Zn(OH)₂) according to Eq. 3:

3

After heating and stirring, the zinc hydroxide formed above undergoes dehydration producing ZnO nuclei as indicated in Eq. 4:

4

The ZnO nuclei formed thereafter aggregate and grow into nanocrystals, while phytochemicals like phenolic, alcohols, and terpenes compounds adsorb onto their surfaces, prevent their agglomeration and controls the particle size.

Characterization of synthesized zinc oxide nanoparticles

The UV–Vis absorption spectra of both the plant extract and the synthesized zinc oxide nanoparticles (ZnO NPs) is shown in Fig. 2a. The UV–Vis spectra of the plant extract reveals that the peak observed at 270 nm is an indicative of the presence of various phytochemicals, such as flavonoids, phenolic acids, and other aromatic compounds. These compounds typically exhibit strong absorbance in the UV region due to their conjugated π-electron systems. The specific absorption at this wavelength suggests the presence of biomolecules capable of acting as reducing and stabilizing agents during the synthesis of ZnO NPs. The shift in the absorption peak to 300 nm for the ZnO NPs synthesized using the plant extract indicates the successful formation of nanoparticles. ZnO NPs are known to exhibit characteristic absorption in the UV region, typically between 300 and 380 nm, depending on particle size and morphology³⁷. The absorption at 300 nm aligns with reported values for ZnO NPs, confirming their synthesis³⁷. The ZnO nanoparticles demonstrated a characteristic UV–Vis absorption peak observed at 300 nm, as against typical 350–370 nm often reported for ZnO. This shift is an indication of the formation of very small ZnO nanoparticles with strong quantum confinement effects. The reduction in particle size elongates the band gap energy, leading to a higher-energy (shorter wavelength) absorption which could be associated with the presence of phytochemicals in the O. basilicum extract acting as capping agents, and modifying surface states. Related blue shifts have been documented in other similar works on the biosynthesized ZnO nanoparticle, where organic capping layers and smaller particle sizes caused increased band gap energies^35,37,³⁶. The red shift from 270 (plant extract) to 300 nm (ZnO NPs) suggests an interaction between the plant biomolecules and zinc ions, leading to the formation of ZnO NPs. This shift is indicative of the successful reduction of zinc ions facilitated by the phytochemicals present in the extract^38,39. Song et al.³⁸ reported a strong absorption peak at 362 nm extending considerably into the UV range, whereas an excitonic absorption peak at approximately 258 nm was reported for ZnO nanoparticles by Talam et al.³⁹.

Fig. 2.

(a) UV analysis of the Ocimum basilicum extract, (b) XRD pattern showing characteristic diffraction peaks at different 2θ and (c) FTIR spectrum showing characteristic absorption bands of the biosynthesized ZnO nanoparticles.

The X-ray diffraction (XRD) analysis of zinc nanoparticles (Fig. 2b) reveals distinct peaks at 2θ values of 31.02°, 34.04°, 36.21°, 57.03°, 63.03°, 67.03°, and 68.71°, and 70.02° corresponding to specific crystallographic reflection for: 100, 002, 101, 102, 110, 103, 112, and 201 planes as identified from JCPDS card no.00–036-1451. These reflections are characteristic of the hexagonal wurtzite structure of ZnO nanoparticles. The FT-IR spectroscopy analysis of zinc oxide nanoparticles synthesized (Fig. 2c) reveals several characteristic absorption bands. The band seen at 3532 cm⁻¹ is typically associated with the O–H/N–H stretching vibrations of hydroxyl or amine groups, indicating the involvement of plant-based polyphenols or flavonoids, amine³⁵. The presence of this band implies that nitrogen-containing compounds or aromatic structures from the plant extract interacted with the nanoparticle surface, possibly aiding in capping and stabilization³⁵. The band observed at 1094 cm⁻¹ is attributed to the C–O stretching vibrations of alcohols, ethers, or carboxylic acids. Such functional groups are prevalent in plant metabolites and play a crucial role in the bioreduction process during nanoparticle synthesis³⁵. The peak which is often linked to the bending vibrations of C–H bonds in alkenes, was noticed at 965 cm⁻¹ indicating the presence of unsaturated compounds from the plant extract on the nanoparticle surface. The peaks seen at 650 cm⁻¹, 550 cm⁻¹, and 500 cm⁻¹ are characteristic of Zn–O stretching vibrations, confirming the formation of ZnO nanoparticles³⁵,³⁶. The presence of these bands confirms the successful synthesis of ZnO nanoparticles through the green synthesis approach.

The TEM analysis of ZnO nanoparticles synthesized using O. basilicum extract (Fig. 3a) reveals a predominantly round morphology with an average particle size of approximately 10.24 nm. The spherical shape of the ZnO nanoparticles is advantageous for various applications, including catalysis, drug delivery, and antimicrobial activities, due to the uniform surface area and reduced agglomeration tendencies. The nanoscale dimension of 10.24 nm indicates a high surface-to-volume ratio, enhancing the reactivity and interaction potential of the nanoparticles. Such small-sized nanoparticles are particularly effective in applications requiring high surface activity. A study reported rod-shaped ZnO nanoparticles with sizes ranging from 100 to 200 nm⁴⁰. The EDS analysis of ZnO-NPs synthesized using O. basilicum leaf extract (Fig. 3b) reveals the elemental composition to be Zinc (54.60%), Oxygen (42.20%), and Carbon (4.20%). The predominant presence of zinc and oxygen confirms the successful formation of ZnO nanoparticles. The stoichiometric ratio of Zn to O is approximately 1:0.77, which is close to the expected 1:1 ratio for ZnO, considering minor experimental deviations. The detection of carbon at 4.20% is indicative of organic residues from the O. basilicum extract used in the green synthesis process. These organic molecules, such as polyphenols, flavonoids, and other phytochemicals, act as reducing and stabilizing agents during nanoparticle formation. The residual carbon suggests that some of these organic compounds remain attached to the surface of the ZnO nanoparticles, providing a capping layer that enhances stability and prevents agglomeration.

Fig. 3.

(a) TEM micrograph of ZnO-NPs, and (b) elemental composition of synthesized ZnO-NPs.

Decolorization studies

The image showed in Fig. 4a represents the untreated (raw) wastewater before any treatment which is a dark brown-colored. This is an indication of the likely presence of high concentrations of organic contaminants or dye molecules in the effluent.

Fig. 4.

Picture sowing effluent before (a) and after treatment (b) with ZnO-NPs.

On the other hand, Fig. 4b contains the treated effluent with zinc oxide nanoparticles (ZnO-NPs) showing a much lighter, almost transparent liquid. The observed clear difference in color between the untreated and treated sample reflect a significant improvement in the quality of the effluent after treatment using ZnO-NPs. The reduction in the effluent color after treatment indicates the removal of chromophoric substances.

Effect of the effluent concentration

The effect of contaminant concentration on the removal efficiency of the synthesized catalyst was evaluated, and the results are presented in Fig. 5a. The absorbance of the treated effluent increased from 0.063 to 0.25 as the contaminant concentration rose from 5 to 30 mg/L. Conversely, the percentage of decolorization decreased from 72.00 to 44.88% over the same range. At lower contaminant concentrations, the catalyst exhibited better performance due to the reduced number of contaminant molecules in the solution, resulting in less competition for the active sites of the catalyst. But as the contaminant concentration increased, the number of competing molecules also rose, leading to reduced catalyst efficiency since the quantity of the catalyst remained constant despite the higher contaminant load¹⁰. The effect of dye concentration on the adsorption efficiency of the catalyst was evaluated, and the results are shown in Fig. 5b. It was observed that the absorbance of the dye increased from 0.056 to 0.105 as the dye concentration was raised from 5 to 80 mg/L. However, the percentage of decolorization decreased from 75.11% to 53.33% as the dye concentration increased from 5 to 80 mg/L. At lower dye concentrations, the catalyst performed better due to fewer methylene blue molecules competing for the catalyst’s active sites. However, at higher dye concentrations, the increased number of methylene blue molecules led to greater competition for the catalyst, and since the quantity of the catalyst remained constant, its efficiency was reduced. In a study conducted by Mohamed et al. ⁴¹, it was observed that the photodegradation efficiency of methylene blue by Fe/ZnO/SiO₂ nanoparticles decreases as its initial concentration increases. But at lower concentrations of 50 ppm, the efficiency reaches 100%, while at higher concentrations (200 ppm), it drops to 65%⁴¹. This they attributed to several factors such as increased adsorption of organic substances on the catalyst surface, reducing available active sites for hydroxyl radical formation; reduced photon penetration due to higher dye concentration; and limited production of reactive species⁴¹.

Fig. 5.

Effect of concentration of (a) effluent, and (b) MB on the removal efficiency of ZnO-NPs (% EDC = percentage effluent decolorization, %DDC = percentage dye decolorization).

Study on contact time

The effect of contact time on the adsorption efficiency of the catalyst was studied for both the effluent and methylene blue dye, with the results presented in Fig. 6a, b, respectively. In Fig. 6a, the absorbance of the effluent decreased from 0.120 to 0.052, while the absorbance of the dye decreased from 0.116 to 0.039 as the contact time increased from 5 to 100 minutes. Similarly, Fig. 6b shows that the percentage decolorization of the effluent increased from 46.67 to 76.89%, whereas the decolorization of the dye rose from 48.44 to 82.26% over the same time period. These results indicate that the decolorization becomes more efficient with increasing contact time, leading to enhanced decolorization in both the effluent and dye. A study by Mohamed et al.⁴¹. investigated the photocatalytic degradation of MB dye using Fe/ZnO/SiO₂ catalysts under UV light and the results showed that the degradation efficiency increased with longer contact times, achieving significant decolorization within 120 minutes. Imohiosen et al.¹⁰ examined that the effect of contact time on decolorization efficiency pharmaceutic effluent with two catalysts and the results show that when hydrogen peroxide was used, absorbance decreased from 0.098 to 0.048 as contact time increased from 10 to 60 minutes, and decolorization rose from 70.39 to 83.49%. However, with H₂O₂@nZVI, absorbance dropped from 0.049 to 0.034, and decolorization increased from 85.19 to 89.73% and the initial rapid reaction seen was due to the abundance of hydroxyl radicals attacking the effluent. The results from the work presented by Kalpesh and Vinod⁴² indicates that increasing the catalyst’s contact time enhances MB degradation as the study shows a rapid degradation rate from 0 to 140 minutes which they claimed was due to the abundance of active sites. The work further shows that after 140 minutes, the reaction reaches equilibrium, as repulsion between dye particles and the catalyst surface slows the degradation rate⁴².

Fig. 6.

Plot of (a) absorbance and (b) decolorization efficiency of ZnO-NPs for dye and effluent under different contact time.

Study on pH

The color seen in the pharmaceutical effluent is primarily due to the presence of residual aromatic drug intermediates, synthetic dyes, and other organic molecules containing chromophoric functional groups like–C=C–, –C=O, –NO₂ and –N=N–. These compounds show visible absorption arising from n–π* and π–π* electronic transitions. The degree of color removal was found to be strongly dependent on the pH of the solution. In acidic medium, the ZnO surface possess a positive charge and shows higher affinity toward the dye molecules that is negatively charged and colored organic ions, enhancing decolorization efficiency. On the other hand, at alkaline medium, electrostatic repulsion between ZnO and anionic chromophores and surface deprotonation reduce adsorption efficiency¹⁸,Karam et al.³⁶. In Fig. 7a, the absorbance of the dye increased from 0.101 to 0.128, while the absorbance of the effluent increased from 0.064 to 0.116 as the pH was raised from 2 to 10. Similarly, Figure 7b shows that the percentage decolorization of the dye decreased from 55.11% to 43.11%, and that of the effluent decreased from 71.56% to 48.44% as the pH increased from 2 to 10. The observed pH-dependent as it relates to the adsorption efficiency can be further described by the surface charge behavior of the synthesized ZnO nanoparticles. The estimated point of zero charge (pHpzc) of the ZnO-NPs 8.5 and when the pH values is below the pHpzc, the nanoparticle surface is positively charged, enhancing electrostatic attraction with negatively charged effluent and dye constituents, whereas at pH values above this point, surface deprotonation leads to electrostatic repulsion, thereby hindering adsorption efficiency³⁵,³⁶.

Fig. 7.

Plot of (a) absorbance and (b) decolorization efficiency of ZnO-NPs for dye and effluent under different pH.

For instance, a study on Rhodamine B dye removal by Guo et al.⁴³ reported that the removal efficiency improved with increasing pH, reaching up to 96% at pH 11. The efficiency of Fe/ZnO/SiO₂ nanoparticles towards the removal of methylene blue decreases as pH increases and it was observed that at pH 2, efficiency is highest at 99.9% and remains nearly unchanged at pH 4. However, efficiency drops to 96.1% at pH 7 and further declines to 86% at pH 9. They attributed this to the change in ZnO surface charge; at low pH, ZnO is positively charged, promoting adsorption of the negatively charged methylene blue, but that as the pH increases, ZnO becomes negatively charged, leading to electrostatic repulsion with the anionic dye, causing decline in photocatalytic activity. The role of pH on degradation efficiency of pharmaceutics effluent was tested with two catalysts by Imohiosen et al.¹⁰ and the results shows that reaction was more effective in acidic conditions, as iron dissolved better in lower pH, aiding the degradation process. Kalpesh and Vinod⁴² noted that dye removal efficiency increased as pH decreased, reaching its highest levels in acidic conditions. They opined that at low pH, the catalyst surfaces became positively charged due to protonation, leading to stronger electrostatic attraction of dye cations and enhanced oxidation and that in acidic pH, positive holes were the dominant oxidation species, while hydroxyl radicals were more prevalent at neutral or alkaline pH.

Study on temperature

The effect of temperature on the decolorization efficiency of ZnO-NPs towards the two contaminants was investigated, and the results are presented in Figs. 8a and b, respectively. In Fig. 8a, the absorbance of the effluent decreased from 0.116 to 0.105, while the absorbance of the dye increased from 0.104 to 0.122 as the temperature was raised from 25 to 85 °C. In Fig. 8b, the percentage decolorization of the effluent increased from 48.44 to 53.33%, while the percentage decolorization of the dye decreased from 53.78 to 45.78% over the same temperature range. It was observed that the reaction in the dye increased with temperature, but began to decline at 55 °C. Similarly, the reaction in the effluent showed an increase with temperature, but started to decline at 65 °C. These results suggest that the catalyst is more effective in treating the dye at lower temperatures compared to the effluent. Imohiosen et al.¹⁰ observed that the decolorization efficiency of pharmaceutical effluents with H₂O₂ alone improved from 69.17 to 83.08%, whereas with H₂O₂@nZVI, it declined from 81.95 to 53.01% as temperature increased from 25 to 65 °C and it was suggested that the higher temperatures enhance hydrogen peroxide-catalyst collisions, increasing hydroxyl radical production and reaction efficiency.

Fig. 8.

Plot of absorbance and decolorization efficiency of ZnO-NPs for dye and effluent under different temperature.

Comparative analysis with previous studies

In other to assess the performance of the ZnO-NPs biosynthesized in this study, a comparison was made with previously reported ZnO-based materials used for pharmaceutical pollutant or dyes removal (Table 2). The ZnO-NPs biosynthesized in this study achieved 82.26% removal of methylene blue within 100 min under mild, non-irradiated conditions. On the other hand, Dhivya et al. ²³ reported 95% degradation of methylene blue using Mn-doped ZnO under UV-assisted photocatalytic conditions, that involved a more complex synthesis route and additional energy input. Similarly, Abomuti et al.³⁵ using Salvia officinalis-mediated ZnO attained 91% dye degradation, however, their process depends on light-driven photocatalysis rather than simple adsorption. Though these photocatalytic systems showed slightly higher removal efficiencies, the current study which employed purely adsorptive process is distinguished, eco-friendly, and simultaneous treatment of both complex pharmaceutical effluent and methylene blue that better mimic real-world wastewater challenges. The significant contribution of surface functional groups identified via FTIR (–OH, -N–H, and C–O) as well as the strong surface activity of the basil-mediated ZnO nanoparticles contributed to the adsorption capacity is reflected in the comparable removal efficiency achieved without external irradiation.

Table 2.

Comparative performance of ZnO nanoparticles with other adsorbents.

Material	Pollutant	Process Type	Efficiency (%)	Contact Time (min)	Special Conditions	Reference
Mn–ZnO NPs (Mn-doped)	Methylene blue	Photocatalytic	95	120	UV light, doped catalyst	23
Salvia officinalis ZnO	Rhodamine B	Photocatalytic	91	90	Visible light irradiation	35
Cyclea peltata Au NPs	Methyl green	Adsorptive	86	120	Ambient	15
			70.3
Carya illinoinensis leaf extract-ZnO-NPs	Rhodamine B	photocatalytic	95%			22
Phoenix dactylifera waste-ZnO-NPs	Congo red	photocatalytic	88%			18
Basil-mediated ZnO (this study)	MB & pharmaceutical effluent	Adsorptive (no light)	82.26 (MB) / 76.89 (effluent)	100	Ambient, eco-synthesized	Present study

Analysis of the kinetic data

The analysis of the kinetic data provides valuable insights into the adsorption mechanism of methylene blue (MB) and contaminants in pharmaceutical effluent and methylene blue (MB) onto ZnO-NPs. The comparison between the pseudo-first-order and pseudo-second-order models helps determine whether the degradation process is primarily governed by physisorption (physical interactions) or chemisorption (chemical interactions)⁴⁴. The pseudo-first-order model typically describes a process driven by physical adsorption (van der Waals forces), where the rate of adsorption depends mainly on the difference in contaminant concentration over time. The pseudo-second-order model, on the other hand, suggests that the adsorption process involves chemical interactions, such as covalent bonding, ion exchange, or surface complexation, between ZnO-NPs and the contaminants⁴⁵. To elucidate the mechanism of pharmaceutical effluent and MB adsorption onto ZnO-NPs, experimental kinetic data were analyzed using the pseudo-first-order and pseudo-second-order models, as described by the Eqs. 5 and 6 below⁴⁵:

5

6

Here, C₀ and C_t represent the initial and equilibrium concentrations of the contaminants in mg/L, while k₁ and k₂ are the rate constants for the first- and second-order reactions, respectively. As shown in Fig. 9a and b, the pseudo-second-order model showed a significantly better fit to the experimental data, with higher correlation coefficient (R²) values (0.986 and 0.987 for pharmaceutical effluent and MB, respectively) than the pseudo-first-order model (0.955 and 0.958). A better fit to the second-order model suggests that the degradation process is more dependent on the available active sites on ZnO-NPs rather than just the concentration gradient of the contaminants. Since the adsorption of pharmaceutical effluent and MB onto ZnO-NPs follows the pseudo-second-order kinetics, this implies that chemical bonding or electron transfer mechanisms play a significant role in the removal process. The result from this study is in accordance with the view of Fan et al.⁴⁶ who observed that the uptake of methylene blue on zinc oxide nanoparticles followed the pseudo-second-order kinetics. Similarly, Shahin and Chinenye⁴⁷ also observed that with higher values of R² and lower values of RMSE, SD, pseudo-second-order kinetic model best explained the degradation process of methylene blue by zinc oxide nanoparticles. The pharmaceutical effluents decolorization was reported by Imohiosen et al.¹⁰ to have exhibited the characteristic of a second-order kinetics reaction.

Fig. 9.

Plots of (a) first-order, (b) second-order, and (c) thermodynamic for adsorption pharmaceutical effluent and methylene blue by ZnO-NPs.

Isotherms

The Langmuir and Freundlich isotherm parameters data for the adsorption of pharmaceutical effluents and methylene blue dye by ZnO nanoparticles (ZnO-NPs) are presented in Table 3 as deduced from Fig. 10. The mathematical expressions for these two models are given in Eqs. 7 and 8 below^48,49:

7

8

Table 3.

Isotherm data for effluent and MB adsorption by ZnO-NPs.

Isotherms	Parameters	Effluent	Dye
Langmuir	qm(mg/g)	32.4	41.3
	RL (mg/L)	0.45	0.21
	R2	0.954	0.967
Freundlich	KF (mg/g)	25.2	32.1
	1/n	0.26	0.14
	R2	0.989	0.995

Fig. 10.

Plots of (a) Langmuir, and (b) Freundlich model for adsorption pharmaceutical effluent and methylene blue by ZnO-NPs.

The separation factor R_L which predicts the favorability of adsorption is given in Eq. 9 as:

9

where the maximum adsorption capacity is given as q_max (mg/g), Langmuir and Freundlich constants as K_L (L/mg) and K_F (mg/g), equilibrium concentration and the amount adsorbed at equilibrium are denoted as C_e (mg/L) and q_e (mg/g) respectively. Their variable values provided in Table 3 and Fig. 10 shows their plots.

The Langmuir model assumes a finite number of identical sites with a monolayer adsorption onto a homogeneous surface^48,49. The maximum adsorption capacity (q_max) obtained for effluent and methylene blue are 32.4 mg/g and 41.3 mg/g respectively. This suggest that ZnO-NPs have a higher adsorption capacity for the dye when compared with the pharmaceutical mixture which is more complex, compare to methylene blue’s smaller, and more uniform molecular structure. The separation factor (R_L) values of 0.45 and 0.21 were found for the effluent and methylene blue with both falling between 0 and 1. This is an indication of favorable adsorption process in both contaminants with methylene blue (lower R_L value) demonstrating a stronger^48,49. This was further corroborated by the value deuced from the correlation coefficient (R²) values of 0.954 and 0.967 for the effluent and the dye respectively, suggesting a good fit to the Langmuir model.

The Freundlich model assumes varying affinities of the adsorbent sites and with multilayer adsorption on heterogeneous surfaces. The Freundlich constant (K_F) values, obtained for the effluent and methylene blue are 25.2 and 32.1 respectively. Higher K_F for the dye further indicates better affinity for methylene blue molecules by ZnO-NPs compare with effluent molecules. The favorability of the adsorption process was further confirmed with 1/n values of 0.26 and 0.14 for effluent and methylene respectively suggesting a stronger interaction between the dye molecules and adsorbent. Deducing from the values of R² (0.989 for the effluent and 0.995 for the dye), the Freundlich model fits the experimental data better than the Langmuir model. This suggests that adsorption took place on a heterogeneous surface with possible multilayer formation.

Thermodynamics evaluations

The thermodynamic results provided from the biosynthesis of ZnO NPs using basil leaf extract for treatment of MB and pharmaceutical effluent in shown in Fig. 9c and the parameters listed in Table 4. Equations 10 and 11 were used to deduced the thermodynamic parameters^50,51:

10

11

Table 4.

Thermodynamic constants for degrading pharmaceutical effluent and methylene blue by ZnO-NPs.

T (oC)	ΔG (kJ/mol)	ΔH ( kJ/mol)	ΔS (kJ/mol)	ΔG (kJ/mol)	ΔH ( kJ/mol)	ΔS (kJ/mol)
	Methylene blue			Pharmaceutical effluent
25	− 1.04			− 1.55
35	− 1.21			− 1.93
45	− 1.52			− 2.16
55	− 1.86	14.76	5.08	− 2.66	23.28	10.06
65	− 2.02			− 3.15

A negative ΔG for both pollutants indicates that the process of degradation using ZnO NPs is spontaneous and this implies that the nanoparticles effectively interact with the pollutant molecules, promoting a favorable reaction for the adsorption under the given conditions. For the enthalpy change, positive values of 14.76 and 23.28 kJ/mol for MB and pharmaceutical effluent implies that the reaction is endothermic, meaning it requires an input of heat to proceed^50,51. This indicates that the process is energetically favored at higher temperatures because increased thermal energy improves the diffusion of the molecules of the adsorbate and activates more surface binding sites. However, it does not mean that external heating is needed for the reaction to proceed. Adsorption occurred spontaneously at room temperature, as evidence by the negative ΔG° values obtained. The positive ΔH° only indicates that the system absorbs a small amount of heat from its surroundings to enhance the interaction between the ZnO nanoparticle surface and adsorbate molecules. In case of entropy change, positive values of 5.08 and 10.06 kJ/mol for MB and pharmaceutical effluent indicates that the system becomes more disordered during the process, which is generally associated with the breakdown of larger molecular structures (like dye molecules) into smaller fragments or ions. This disorder could result from the adsorption of dye molecules and pharmaceutical effluent onto the ZnO surface. Both processes show positive entropy changes, indicating an increase in disorder as the pollutants (MB and pharmaceutical compounds) are broken down into smaller, more disordered fragments.

The result is in agreement with the literature reports of Muhammad et al.⁵² where it was observed that the standard Gibbs free energy (ΔG) values were negative with the negativity increasing as temperature rises across different initial dye concentrations and that the adsorption process is spontaneous and becomes more favorable at higher temperatures. Additionally, they reported positive values of enthalpy (ΔH) and entropy (ΔS) within the temperature range of 35 to 55 °C suggesting that the adsorption of the dyes onto ZnO-NPs is endothermic and primarily governed by chemisorption. Also, Nayanathara et al⁵³ studied the thermodynamics of pharmaceutical adsorption onto ZnO-NPs which revealed the feasibility and endothermic nature of the adsorption process. In addition, El Jery et al.⁵⁴ confirmed via thermodynamic investigation that the adsorption of MB by activated carbon is spontaneous as revealed by the negative Gibbs free energy (ΔG°) values,

Possible adsorption mechanism

The uptake of MB and pharmaceutical effluent onto biosynthesized ZnO nanoparticles (ZnO-NPs) can be described based on the possible of the interactions of the pollutant molecules with the surface functional groups identified by FTIR analysis. The FTIR spectrum revealed characteristic absorption bands corresponding to O–H/N–H stretching, C–O stretching, C = C stretching of aromatic compounds) indicating the presence of hydroxyl, carbonyl, and ether groups on the nanoparticle surface. The phytochemicals in O. basilicum extract contain these functional groups which remained bound to the ZnO surface, and can serve as active sites for pollutant adsorption. The N–H and O–H groups on the surface of ZnO-NPs can form electrostatic interactions or hydrogen bonds with the polar functional groups of pharmaceutical or methylene blue contaminants. For instance, the sites which are negatively charged like –SO₃⁻, –COO⁻, or aromatic nitrogen atoms in MB can react with the ZnO–OH₂⁺ surface that is positively charged under acidic conditions (pH < pHpzc). But, in alkaline conditions, deprotonates of the surface of ZnO–O⁻, favours cationic species interactions through complexation, and electrostatic attraction.

The proposed interactions are given in Eqs. 12 and 13 as:

Hydrogen bonding:

12

Electrostatic interaction:

13

These interactions are in accordance with kinetics reports that predict a pseudo-second-order model, suggesting a chemisorption-controlled process involving surface functional groups. This is in agreement with the view of Fan et al.⁴⁶ where it was observed that the interaction between methylene blue (MB) and ZnO is primarily driven by ionic bonding, and that the negatively charged functional groups of MB (–SO₃⁻) interact with the positively charged Zn(OH)⁺ sites on the ZnO surface. Similarly, the adsorption of pharmaceutical and methylene blue compounds onto ZnO-NPs could proceed via hydrogen bonding mechanism, where the pharmaceutical and methylene blue compounds which contained functional groups such as hydroxyl (–OH), amino (–NH₂), or carbonyl (C = O) can form hydrogen bonds with surface hydroxyl groups on ZnO NPs^55,56. Figure 11 shows the schematic diagram for the adsorption mechanism of the process.

Fig. 11.

Schematic diagram for the adsorption mechanism of the process.

Desorption/Resusability study

The data representing the desorption efficiency of methylene blue dye and pharmaceutical effluent treated with ZnO-NPs in five cycles is depicted in Fig. 12. It was observed that the desorption efficiency decreases gradually for both samples, suggesting a reduction in the adsorption ability of ZnO-NPs to eliminate contaminants after repeated use, although this reduction is very minimal within the first-three cycles. At the initial stage, the effluent had a desorption percentage of 66.5%, which then reduced gradually to 57.8% by the fifth cycle. Likewise, the dye with higher desorption percentage of 74.5% in the first cycle, decreases to 66.2% by the fifth cycle. This trend indicates that though ZnO-NPs are effective in removing both contaminants, their efficiency reduces with repeated adsorption–desorption cycles. The gradual decline in desorption efficiency with successive cycles could be attributed to structural changes, surface saturation, or partial agglomeration of ZnO nanoparticles, which lessens the available number of active sites for adsorption and desorption.

Fig. 12.

Desorption study on the MB and effluent by ZnO-NPs.

Conclusion

This study successfully demonstrated the green synthesis of ZnO nanoparticles (ZnO-NPs) using basil leaf extract, confirming their potential for pharmaceutical effluent treatment. Characterization analyses revealed that the synthesized ZnO-NPs possess a uniform round shape with an average particle size of 10.24 nm. Key phytochemicals identified in the basil extract, including hexanal, eucalyptol, and 9-octadecenoic acid, play a crucial role in nanoparticle formation. The adsorption studies indicated that ZnO-NPs effectively remove pharmaceutical effluents and methylene blue dyes, with decolorization efficiency improving over extended contact time. The process exhibited endothermic behavior, requiring heat input to enhance adsorption efficiency most especially for the effluent treatment. These findings highlight the potential of biosynthesized ZnO-NPs as an eco-friendly and sustainable solution for wastewater treatment and environmental remediation.

Author contributions

Amr S. Abouzied: Validation, Project administration, Writing—review & editing. Edwin Andrew Ofudje: Conceptualization, Investigation, Supervision. Favour Abumere Imohiosen: ´ Investigation, Visualization, Writing—original draft. Khairia Mohammed Al-Ahmary: Writing—review & editing, Validation. Saedah R. Al-Mhyawi: Validation, Resources, Project administration, Hamad AlMohamadi: Writing—review & editing, Validation, Data Analysis. Ibtehaj F. Alshdoukhi and Jawza Sh Alnawmasi: Data Analysis, Methodology, Writing—review & editing. Tahani Mohammed Alrosaa: Funding. Resources.

Funding

This study was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2025R580), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

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.