Exploring zero-valent iron nanoparticles (nZVI) for Bisphenol A removal: Experimental investigations and Monte-Carlo simulation insights

Globally, water pollution is a significant environmental issue, particularly regarding water supply since some pollutants such as Bisphenol-A (BPA) have not been considered for removal in many developing countries (Mashabi et al. 2022; Elwakeel et al. 2023). BPA has been detected in several environmental samples, including groundwater, surface water, sediment, sludge, and agricultural soil, largely due to industrial wastewater discharges and improper waste disposal. Its significant effects on the endocrine system and the central nervous system have serious concerns (Mpatani et al. 2021, 2022; Ahmad et al. 2022; Al-Qadri et al. 2022). As a result of indiscriminate disposal of personal care products, plastics, and food cans, among others into water bodies, BPA is released into the environment, resulting in adverse effects on human health and the ecosystem. BPA is a classified carcinogen known for its hormone-disrupting and estrogenic activities, causing hormonal imbalance, high blood pressure, cardiovascular diseases, and breast cancer (Bhatia & Datta 2019; Cao et al. 2022; Chandrasekar et al. 2022; Ding et al. 2022; Porcar-Santos et al. 2022). In addition, BPA has been shown to cause metabolic disorders, kidney and liver diseases, and prostate cancer in humans (G'omez-Toledano 2022; Molangiri et al. 2022). More particularly, BPA can exacerbate its effects in ecosystems via bioaccumulation and biomagnification (Liu et al. 2021; Torres-García et al. 2022).

It is therefore inevitable to remove BPA from wastewater before its discharge into the environment. There have been various methods for removing BPA from wastewater, including membrane separation, enzymatic degradation, chemical oxidation, microbial degradation, adsorption, ozonization, reverse osmosis, and photo-oxidation (Mpatani et al. 2021; Cao et al. 2022; Torres-García et al. 2022; Yan et al. 2022). Due to its cost-effectiveness, efficiency, simplicity, and convenience, adsorption remains the most efficient, effective, and safest approach for removing emerging pollutants. Various adsorbents both expensive and cost-effective have been deployed for BPA remediation including tripartite magnetic montmorillonite composites, fly ash-derived zeolite modified by β-cyclodextrin, graphene oxide functionalized with amine molecules, Fe₃O₄ loaded eggshell, mineralized sawdust, powdered activated carbon, ultra-high surface area porous activated carbon derived from asphalt and metal-organic frameworks (Bandura et al. 2021; Konzen et al. 2021; Mpatani et al. 2021; Al-Qadri et al. 2022; Chandrasekar et al. 2022; Okon et al. 2022; Yan et al. 2022).

Among the adsorbents previously applied for BPA removal are nanomaterials owing to their large surface area, adaptive characteristics, vast morphology, and reactive surfaces. Nanomaterials as adsorbents for water treatment have been widely applied because they play a significant role in environmental pollution treatment, particularly water pollution (Dehghani et al. 2020; Bucur et al. 2022). Among the nanomaterials previously applied are silica, nanobentonite intercalated with magnetite and sodium alginate, magnetic nanoparticles functionalized with 2-vinylpyridine, amine-modified magnetic multiwalled carbon nanotubes, polymer-based sugarcane bagasse impregnated with silver nanoparticles, multiwalled carbon nanotubes coated with CoFe₂O₄ nanoparticles, and sugarcane waste ash mesoporous silica nanoparticles (Bhatia & Datta 2019; Rovani et al. 2020; Al-Musawi et al. 2022; Bucur et al. 2022; El-Sharkawy et al. 2022; Mpatani et al. 2022). Due to its unparalleled properties, zerovalent iron nanoparticles (ZVI) are one of the most important categories of iron-based nanomaterials. The environmental remediation advantages of nZVI are their large surface areas, excellent pollutant degradation ability and affordability. Moreover, some studies have applied nZVI in combination with graphene, chitosan metals such as CoFe₂O₄, ZnFe₂O₄, and nZVI-GO-Cu (Dehghani et al. 2020; Rani et al. 2020; Yousefinia et al. 2021).

Hence, this study investigated the effective removal of BPA in simulated wastewater using nZVI synthesized with Terminalia catappa extract. This study explored quantum chemical analysis and Monte-Carlo (MC) simulation to provide unique insights into the stable surface interactions between BPA and nZVI. Molecular dynamics calculations further elucidated the adsorption mechanisms of BPA on nZVI, adding considerable new understanding to the effective removal of BPA.

Analar grade Bisphenol-A (BPA), 98% pure with the molecular formula (C₁₅H₁₆O₂), molecular mass 228.29 gmol⁻¹, maximum absorption at 280 nm, and pKa of 9.6 were purchased from Loba Chemie, India. A standard stock solution of 1,000 μg mL⁻¹ BPA and other concentrations ranging from 10 to 50 mg L⁻¹ of BPA were prepared and measured at 280 nm. The measurements were further confirmed with Agilent HPLC 1100 Series.

An Agilent HPLC 1100 Series (Germany) with a binary pump, an auto liquid sampler (ALS), a thermostat column compartment, a multiwavelength UV/Vis detector (MWD), and an online degasser was used to analyze the BPA standard solutions and residual concentrations after adsorption. For the standard stock solution analysis, 20 mL of LiChrosolv Methanol (LC Grade, Germany) was added into a flask containing 25.7 mg BPA and agitated for 2 min before filling to the 1 L mark with the solvent. The BPA samples were transferred into HPLC vials with a stationary phase consisting of a Column (Waters Xbridge C18 (150 × 4.6 mm I.D., 3.5 um), mobile phases A and B were 40% water and 60% methanol, respectively, and run for 7 min at a flow rate of 1 mL min⁻¹ and at a wavelength of 230 nm. The R² for the calibration curves of standard concentrations analyzed was 0.9979. Residual BPA concentration in the adsorption experiment was similarly run and analyzed.

As reported by Azeez et al. (2022a), pH_pzc of nZVI was determined by adding 0.1 g of nZVI to 50 mL of 0.1 M NaCl and adjusting the pH between 2 and 12 with 0.1 M NaOH or 0.1 M HCl. The final pH values were determined after agitating the solutions for 24 h on an orbital shaker (Stuart SSLI, Barlword Scientific Ltd, Britain). The pH_pzc was calculated by plotting the final pH against the initial pH and the intersection point indicates pH_pzc.

The adsorption study was carried out at the Department of Pure and Applied Chemistry Laboratory, Osun State University, Nigeria.

To explain the adsorption of BPA by nZVI from a quantum chemical viewpoint and determine the electronic properties of the adsorbate (BPA) that might be responsible for interaction with the nanoparticles, the frontier molecular orbitals (HOMO and LUMO) electron density isosurfaces of BPA were obtained by density functional theory (DFT) calculation using the popularly known B3LYP/6-31 + G(d,p) model which has been adjudged sufficient for accurate determination of molecular geometries (Parr 1980; Becke 1983; Olasunkanmi et al. 2020, 2021; Waziri et al. 2023). The molecular structure of BPA was carefully constructed on GaussView 5.0 modeling and visualization tool (Dennington & Millam 2009) and then subjected to an unconstrained optimization with frequency calculation to obtain its ground state geometry and minimum energy. The obtained geometry was confirmed to be authentic by the absence of an imaginary value in its vibrational frequency data. All calculations were carried out using the Gaussian 09 program (Frisch et al. 2009).

An MC simulation visualizes the adsorption of a molecule (adsorbate) on a substrate (adsorbent) by using molecular dynamics calculations (Rahmati & Modarress 2009; Basiony et al. 2019; Olasunkanmi et al. 2020, 2021). Additionally, it provides a clear understanding of what changes may occur in the molecular configuration when an adsorbent interacts with a molecule (adsorbate).

Simulation of the adsorption of BPA from water by synthesized nZVI was conducted using the Monte-Carlo method with a single BPA molecule competing with 50 H₂O molecules for surface space. This simulation was carried out using the adsorption locator module of Acceryls Material Studio 2013 (Akkermans et al. 2013). As the synthesized nZVI is mainly made up of iron, the adsorption site is represented by cleaved surfaces of iron, in accordance with cleavage planes revealed by XRD measurements (h k l). Having built a well-constructed surface, the DFT-optimized BPA structure was then mounted and allowed to relax into a more favorable configuration within a 10 Å distance. The simulation was then performed at a fine accuracy level using a COMPASS force field with a smart algorithm. This was done by running a simulated annealing task in five cycles where each cycle consisted of 50,000 steps.

The energies of adsorption (E) are 3.548, 3.929, and 19.829 kJ mol⁻¹ at 303, 308, and 313 K, respectively, for the removal of BPA obtained from Dubinin–Radushkevich plots demonstrating that physisorption was the underlying mechanism at 303 and 308 K since E < 8 kJ mol⁻¹ at lower temperatures while it was chemisorption at 313 K because it falls between 8 < E < 16 kJ mol⁻¹ (Al-Musawi et al. 2022; Azeez et al. 2022a).

The thermodynamic parameters of BPA adsorption indicated an exothermic process (ΔH° = −32.419 kJ mol⁻¹), which is below 40 kJ mol⁻¹, suggesting physical adsorption (Azeez et al. 2022a, 2022b, 2023). Moreover, increased temperature resulted in an increased degree of disorderliness (+ΔS°) on the surface of mesoporous nZVI (Table 6). This is in agreement with the results of Al-Musawi et al. (2022). The ΔG° values for BPA removal vary between −34.851 and −35.009 kJ mol⁻¹ implying that the adsorption process was more spontaneous with an increase in temperature (Dehghani et al. 2020).

Since ΔN > 0, the adsorption of BPA onto nZVI involves the movement of electrons from BPA to nZVI while the opposite would have been the case if ΔN < 0 (Basiony et al. 2019).

The XRD data presented in Table 7 show a number of h k l combinations corresponding to different cleavage planes and particle sizes which are all within the nano range. Since each h k l combination represents a different surface morphology of the synthesized nZVI, it would be interesting to compare the different surfaces (i.e., h k l combinations) of the nZVI in terms of adsorption capacity as this will help to predict the h k l combination with the highest potential for the removal of BPA. To achieve this, the calculation was limited to only the h k l combinations that produce a uniform surface with a relatively large atomic density. These are 1 0 −1, 1 2 1, and 0 0 2. The 1 1 0 surface which is widely reported as the most stable Fe surface (Anderson & Mehandru 1984; Arya & Carter 2004; Guo et al. 2017; Basiony et al. 2019; Olasunkanmi et al. 2021) was also included for comparison.

Zerovalent iron nanoparticles (nZVI) mediated with T. catappa extract absorbed maximally at 373 nm, characteristic of iron nanoparticles with FTIR peaks indicating Fe–O, C = O, and -OH bands. Clustered-wired particles with a particle size of 91.15 ± 12.31 nm were observed in the SEM micrograph and EDX showed prominent Fe and FeO presence. For nZVI in this study, seven crystalline planes matching JCPDS card number 00-153-5451 were observed. The T. catappa-mediated nZVI had a surface area of 159.419 m² g⁻¹ and a pore size of 2.138 nm, suggesting a mesoporous surface. The pH_pzc of nZVI was 3.87, and maximum adsorption of BPA occurred at pH 2, decreasing with increasing pH. Other batch adsorption parameters significantly affected BPA removal. BPA adsorption was predicted by Freundlich isotherm at 303 K, and Langmuir isotherm at 308 and 313 K. In this study, BPA adsorption was favorable with n values > 1 and R_L < 1. The adsorption at 303 and 308 K was physisorption, whereas the adsorption at 313 K was chemisorption. It was found that pseudo-second-order kinetics was the most accurate for predicting the adsorption rate, whereas intraparticle diffusion, intramolecular diffusion, and adsorption were the mechanisms controlling the BPA removal. The adsorption was spontaneous as demonstrated by –ΔG°, decreasing with increasing temperature; exothermic with decreased order of disorderliness as shown by –ΔH° and ΔS°. The DFT and MSC studies revealed that the isopropyl moiety of BPA, the phenyl rings, and the hydroxyl substituents were involved in electrostatic interaction with nZVI with crystal plane 121 having the most negative ΔE_ads responsible for most BPA removal. The nZVI mediated with T. catappa is a more promising adsorbent for removing BPA due to its over 50% reusability and adsorption capacity.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.