sustainability Article Reduction and Degradation of Paraoxon in Water Using Zero-Valent Iron Nanoparticles Veronica A. Okello 1, * , Isaac O. K’Owino 2 , Kevin Masika 2 and Victor O. Shikuku 3, * 1 2 3 * Citation: Okello, V.A.; K’Owino, I.O.; Masika, K.; Shikuku, V.O. Reduction and Degradation of Paraoxon in Water Using Zero-Valent Iron Nanoparticles. Sustainability 2022, 14, 9451. https://doi.org/10.3390/ Department of Physical Sciences, Machakos University, Machakos P.O. Box 136-90100, Kenya Department of Pure and Applied Chemistry, Masinde Muliro University of Science and Technology, Kakamega P.O. Box 190-50100, Kenya; ikowino@mmust.ac.ke (I.O.K.); kevin.masika@gmail.com (K.M.) Department of Physical Sciences, Kaimosi Friends University, Kaimosi P.O. Box 385-50309, Kenya Correspondence: vokello@mksu.ac.ke (V.A.O.); vshikuku@kafuco.ac.ke (V.O.S.) Abstract: Paraoxon is an emerging organophosphate pollutant that is commonly used as a pesticide and a drug, hence increasing the risk of contamination of water supplies. Its intensive use for vector control has led to pollutions in soil and water. Paraoxon is very toxic, with an LD50 of 2 to 30 mg/kg in rats. It can be metabolized in the body from parathion; thus, exposure can lead to serious health effects. In this study, zero valent iron (Fe◦ /ZVI NPs) nanoparticles were synthesized and investigated for the degradation of Paraoxon, a chemical warfare agent and insecticide, in an aqueous solution. The effects of solution pH, initial pollutant concentration, ZVI NPs dosage and contact time on mineralization efficiency were examined. Batch experiments demonstrated that 15 mg L−1 of Paraoxon was mineralized at degradation efficiencies of 75.9%, 63.9% and 48.9% after three-hour treatment with 6.0, 4.0 and 2.0% w/v Fe◦ , respectively. The calculated kinetic rate constant kobs was 0.4791 h−1 , 0.4519 h−1 and 0.4175 h−1 after treating 10, 15 and 20 mg L−1 of Paraoxon solution with 6.0% w/v Fe, respectively. The degradation dynamics were described by the first-order kinetic law as evidenced by rate constants independent of the initial Paraoxon concentration. The degradation efficiency was strongly dependent on pH, increasing with a decrease in pH, with maximum removal at pH 4. p-nitrophenol was detected as a degradation product, suggesting cleavage of the O-P bond and hydrolysis as possible reaction processes. This study showed that Fe◦ particles have the potential for degrading Paraoxon. Keywords: water remediation; reduction; degradation; Paraoxon; zero-valent iron; nanoparticles su14159451 Academic Editor: Alessio Siciliano Received: 31 March 2022 Accepted: 25 July 2022 Published: 2 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1. Introduction Emerging pollutants have increased the concern of all stakeholders over the world due to their harmful potential state and ubiquitous presence in water, soil and air. Organophosphate compounds, in particular, comprise mainly of a very large class of chemical pesticides that are extensively used in agriculture worldwide [1]. These compounds comprise of methyl parathion, parathion, chlorpyrifos, malathion and Paraoxon [1,2]. The organophosphate compounds such as tabun, sarin, soman and cyclohexyl sarin have also been utilized as chemical-warfare agents (CWAs) in countries in the Western and Eastern bloc [3]. For example, Paraoxon, a renowned chemical warfare agent, has been widely utilized in South Africa as a biological nerve agent and is also used as an ophthalmological drug against glaucoma [4]. These compounds, when used, act by inactivating acetylcholinesterase, an essential ingredient for nerve function in insects, animals and humans; hence, they can also be used as a pesticide [5]. Unfortunately, Paraoxon is very unstable and is extremely toxic, and therefore, it should be removed from drinking water, as exposure can lead to vomiting, poor visions, convulsions, dyspnoea, lung oedema and even death. Moreover, with the dwindling amounts of fresh water available for human/animal consumption and agricultural use, the removal of toxins from these water bodies is of utmost importance. Sustainability 2022, 14, 9451. https://doi.org/10.3390/su14159451 https://www.mdpi.com/journal/sustainability Sustainability 2022, 14, 9451 2 of 13 There are a number of methods that have been used for the degradation of organophosphates from water with appreciable success [6–13]. Ghosh et al. [13] studied the hydrolysis of carboxylic esters such as Paraoxon using N- hydroxylamines and N- hydroxy amides in surfactants such as cationic micellar media. The degradation rate of substrates is enhanced by the presence of CTA (Cetyltrimethylammonium) surfactants in the case of both nucleophiles, N-hydroxysuccinimide and N-hydroxyphthalimide. Given their ability to support the rate enhancement of esterolytic reactions, they also provide the best means of solubilizing hydrophobic compounds in aqueous media. Therefore, the hydrolysis process is enabled, since N-hydroxy amides have structural similarity with hydroxamic acids with N-OH as functional groups [6]. However, the nucleophile reactivity in micelle depends on the fixation of the substrate and the reaction with anionic nucleophiles; thus, it is not an effective method [7]. Further studies by Chi et al. [8] used 4-nitrophenol surface-modified multiwalled carbon nanotubes for the sequestration of Paraoxon. The selective binding sites on the enzyme are made during polymer synthesis by the addition of a specific molecule as the template. However, this technique seemed to involve high capital investment to develop a highly selective enzyme, such as phosphotriesterase-immobilized polymers, thus limiting its application in the large-scale sequestration of Paraoxon. The molecular dynamics applying the combined quantum mechanical and molecular mechanical (QM/MM) potentials were also studied to determine the hydrolysis reaction mechanism of Paraoxon by phosphotriesterase (PTE) [9]. In other studies, microbial degradation approaches were considered to offer reliable and inexpensive ways to safely eliminate Paraoxon from both terrestrial and aquatic environments [10]. These frontiers have resulted in the identification of bacterial strains and enzymes that are capable of detoxifying organophosphate pesticides. The study by Wu et al. [10] noted that the immobilized organophosphorus hydrolase (OPH) enzyme isolated from Pseudomonas diminuta MG and Flavobacterium sp. ATCC 27551 was capable of degrading Paraoxon. Catalysis of this Paraoxon is by hydrolyzing phosphorus ester bonds, such as P-O and P-S bonds, through a hydrolytic mechanism that involves adding an activated water molecule at the phosphorus center [11]. More so, E. coli has been utilized to mineralize Paraoxon by co-expression of organophosphorus hydrolase (OPH) under trc promoter and Vitreoscilla hemoglobin (VHb) under the O2 -dependent nar promoter [12]. Additionally, a strain of Pseudomonas putida was constructed, which was seen to efficiently degrade Paraoxon and utilize it as an energy source. This strain was catalyzed with the pnp operon from Pseudomonas sp. strain ENV2030, which incorporated enzymes that convert p-nitrophenol into β-ketoadipate. Due to the large extent of the polluted environment, chemical and enzyme preparation strategies seemed inappropriate, as they are uneconomical. Phytoremediation has also been proposed as a potentially cheap, efficient and an environmentally benign approach to be used for degradation of organophosphates such as Paraoxon and methyl parathion. Presently, there is a growing interest in the use of nanoparticles in the degradation of toxic organic compounds from the environment [14]. Prasad et al. [15] reported the photocatalytic destruction of Paraoxon-ethyl in aqueous solution using titania nanoparticulate film. The study showed that titania nanoparticles degrade Paraoxon-ethyl to form silylated products, which include p-nitrophenol, O,Odiethyl phosphonic acid, O-ethyl diphosphonic acid and phosphoric acid as by-products. These products form due to the cleavage of P–O–C bonds and oxidation of the P atom. Despite its effectiveness, recovery of the spent TiO2 photocatalyst from solutions is costly and technologically prohibitive. Elsewhere, Xiudong et al. [16] used nanocrystalline magnesia powder (nano-MgO) on the degradation of Paraoxon. However, the catalytic activity relies on the availability of light. The two micellized ion complexes, Co(II) and Cr(III), possessed fine catalytic activity in mineralizing Paraoxon. These metal ions polarize the P = O group, since micellization facilitates reduction and also promotes the electrophilicity of the metal towards the micelle- Sustainability 2022, 14, 9451 3 of 13 bound substrate [6]. Bromberg and co-workers [17] reported the use of functionalized silver and cobalt nanoparticles for Paraoxon hydrolysis. In other studies, the use of core/shell molecular imprinting microparticles fabricated using reversible addition-fragmentation transfer polymerization (RAFT) technology was developed for the degradation of Paraoxon. However, this technology requires the use of phosphotriesterase to catalyze the hydrolysis reaction for the degradation of Paraoxon [18]. Furthermore, these methods are very expensive to use, and recovery of the complexes from the environment remains a challenge. Therefore, there is a need to develop other suitable and convenient methods to completely eradicate the toxic Paraoxon pesticide from the environment. With the limited studies on the degradation of Paraoxon reported in literature and the postulated limitations, alternative treatment procedures should thus be developed. Therefore, we hereby report the possible application of zero-valent iron nanoparticles (ZVI NPs) in the degradation of the toxic organophosphorus compound, Paraoxon, in aqueous media into non-toxic materials under room temperature batch experiment conditions. Moreover, to the best of our knowledge, no study has reported on the degradation of Paraoxon using ZVI NPs. These particles were selected because they have been preferably used to transform a plethora of environmental pollutants, such as chlorinated hydrocarbons and pesticides, etc., under reduction reactions or sorption processes. This is due to their increased surface area, fast reaction, low cost and availability [19]. The ZVI NPs are excellent electron donors and have a high potential to reduce various contaminants. This study, therefore, evaluated the capability of ZVI NPs to degrade Paraoxon in water as a function of the Paraoxon initial concentration, ZVI NPs dosage, solution pH and contact time. The results indicate a great potential in the use of ZVI NPs for the degradation of Paraoxon in water. 2. Materials and Methods Paraoxon Standard was obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany). Acetonitrile (HPLC grade, 98.0% purity), Sodium Borohydride, Ethanol and Ferric Chloride Hexahydrate were all analytical grade, obtained from Kobian Company Ltd. A stock solution of 1000 mg L−1 Paraoxon in acetonitrile was prepared. Solutions of varying concentrations (1, 5, 10, 15, 20 and 25 mg L−1 ) were prepared by serial dilution of the stock solution using acetonitrile. The effect of Fe◦ dosage was evaluated using 2.0, 4.0, 6.0, 8.0 and 10.0% w/v dosages for a fixed initial Paraoxon concentration. All aqueous solutions were made with ultra-high-purity water, obtained using an ultrapure water system Milli-Q Plus (Merck Millipore Co., Darmstadt, Germany). 2.1. Preparation of ZVI NPs The preparation of solutions involved dissolving 9.4575 g of sodium borohydride (NaBH4 ) in 0.1 M NaOH solution and then 12.1635 g of FeCl3 ·6H2 O into 100 mL of ethanol/water mixture [20]. The stabilized ZVI NPs were prepared adding 0.25 M NaBH4 aqueous solution dropwise to a Neon gas-purged 0.045 M FeCl3 ·6H2 O aqueous solution in the ratio of 1:1 at 23 ◦ C with magnetic stirring. The solution was magnetically stirred for 20 min, then centrifuged for 2 min, and the supernatant solution was replaced with acetone. The ZVI NPs were finally washed with 25 mL of absolute ethanol, then stored in a refrigerator at <4 ◦ C. 2.2. Characterization of Zero Valent Iron Nanoparticles 2.2.1. Transmission Electron Microscopy (TEM) Analysis The images (size and shape) of iron nanoparticles were obtained using 4q TEM JEOL equipment equipped with a LaB6 model filament set at an acceleration of 200 kV and beam current of 102 µA. Sustainability 2022, 14, 9451 4 of 13 2.2.2. Scanning Electron Microscopy (SEM) Analysis The Scanning Electron Microscopy (SEM) analysis was done using the Philips XL30 SEM machine (F.E.I. Company, Hillboro, OR, USA) to inspect the surface morphology. The sample surface images were taken at different magnifications. 2.3. Batch Experiments Here, separate 50 mL of 15 mg L−1 Paraoxon solutions were mixed with 2.0, 4.0 and 6.0% w/v of ZVI NPs (Fe◦ ). For study of the effect of pH on Paraoxon destruction, the pH of Paraoxon solutions containing the nanoparticles were adjusted to 10, 7 and 4. The effect of initial Paraoxon concentration (10, 15 and 20 mg L−1 ) on degradation rate was studied using 6.0% w/v Fe◦ dosage at 22 ◦ C. At pre-determined time intervals, 2 mL of aliquot was withdrawn from the reaction vessel, centrifuged at 3000 rpm and analyzed for the residual Paraoxon concentration using a Shimadzu UV-Vis Spectrophotometer (Model 1601 by ThermoFisher Scientific, Randburg, South Africa) at a wavelength of 280 nm. The Paraoxon degradation kinetics were modeled with a first-order rate equation (Equation (1)) with the residual concentration of Paraoxon measurements made at predetermined time intervals for 3 h. d[ paraoxon] = −k obs [ paraoxon] dt (1) where [paraoxon] is the concentration of Paraoxon in the aqueous solution (mg L−1 ), t is time t (min) and kobs is the first-order rate constant (min−1 ). 2.4. Analytical Methods A chromatographic analysis was performed using an HPLC system, Shimadzu LC20AT, equipped with an SPD-20A Shimadzu prominence UV/visible detector, degasser (DGU-20A prominence, Shimadzu) and Phenomenex 00 G-4420-E0 (250 £ 4.60 mm, 5 microns, HyperClone 5u BDS C-18 130A) columns. The mobile phase was Acetonitrile:water (1:1) at a flow of 0.7 mL/min, detector wavelength of 254 nm and an injection volume of 20 µL. The equipment was purchased from LAB2, Berkel en Rodenrijs, The Netherlands. Quality Control For quality control, reagents and materials used were either HPLC grade or analytical grade, unless stated otherwise. Data were obtained from triplicate analyses and processed using Microsoft Excel (5.0/95 workbook by Microsoft Office, TDK solutions Ltd, Nairobi, Kenya) and OriginPro 9.0 software (OriginLab Corporation, Northampton, MA, USA). 3. Results and Discussion 3.1. Characterization of Zero Valent Iron Nanoparticles 3.1.1. Transmission Electron Microscopy (TEM) Generally, the removal of Paraoxon and/or other pollutants from water is highly dependent on the shape and size of the nanoparticle used, hence the need to perform morphological characterization of the ZVI NPs. Figure 1 displays the sizes of the iron nanoparticles after being characterized using TEM. The cumulative size distribution average of the nanoparticles confirmed that the NPs with less than 100-nm diameters constituted more than 80.0%, while particles with less than 60 nm size accounted for less than 50.0%. The fabricated nanoparticles showed chain-like hollow structures, the majority of which ranged between 20 nm–90 nm diameter. It is believed that these aggregated to remain in the most thermodynamically stable state [21]. Sustainability 2022, 14, 9451 of the nanoparticles confirmed that the NPs with less than 100-nm diameters constituted more than 80.0%, while particles with less than 60 nm size accounted for less than 50.0%. The fabricated nanoparticles showed chain-like hollow structures, the majority of which 5 of 13 ranged between 20 nm–90 nm diameter. It is believed that these aggregated to remain in the most thermodynamically stable state [21]. Figure 1. 1. TEM freshly prepared ZVIZVI NPsNPs (100(100 nm) nm) showing a network of chain-like hollow Figure TEMimages imagesofof freshly prepared showing a network of chain-like structures. hollow structures. 3.1.2. Scanning Scanning Electron Electron Microscopy Microscopy(SEM) (SEM) 3.1.2. The The SEM SEM micrograph micrograph for for the the particles particles after after contact contact with with Paraoxon Paraoxon is is presented presented in in Figure Figure 2. 2. It depicts depicts the the formation formation of of large large agglomerates agglomerates with with highly highly irregular irregular surfaces, surfaces, containing containing large large and and small small intertwined intertwined agglomerates agglomerates [22]. [22]. The The surface surface morphology morphology of of freshly freshly prepared preparedZVI ZVINPs NPschanged changedgreatly greatlyafter afterthe theinteraction interactionwith withParaoxon Paraoxoninto into white white islands latter was attributed to the dissociative adsorption islandsof ofporous porousspherical sphericalparticles. particles.The The latter was attributed to the dissociative adsorpof water and and Paraoxon intointo the the hollow porous sites, which served tion of water Paraoxon hollow porous sites, which servedasasreaction reactionsites sitesfor for 0 the the ensuing ensuing reduction. reduction. In this case, case, the the iron iron surface surface Fe Fe0 released electrons to the water, resulting resultingto tothe the formation formation of of surface-fixed surface-fixed hydroxyl hydroxylmoieties moieties that that created created highly highly reducing reducing conditions for Paraoxon, as indicated in the following equations. Equation conditions for Paraoxon, as indicated in the following equations. Equation (2) (2) indicates indicates the thereaction reactionof ofZVI ZVI NPs NPs with with dissolved dissolved oxygen oxygen in in water water to to form form hydrogen hydrogen peroxide, peroxide, which which was wassubsequently subsequently reduced reduced to to water water (Equation (Equation (3)) (3)) or or to to hydroxyl hydroxyl radicals radicals (Equation (Equation (4)). (4)). The Thelatter latterreaction reactionisisresponsible responsiblefor forthe thedegradation degradationof ofParaoxon, Paraoxon,as asshown shownininScheme Scheme1.1. Sustainability 2022, 14, x FOR PEER REVIEW 6 of 13 Fe◦ + O2 + 2H+ → Fe2+ + H2 O2 (2) Fe◦ + H2 O2 + 2H+ → Fe2+ + 2H2 O (3) Fe2+ + H2 O2 → Fe3+ +·OH + OH− (4) Scheme 1. Degradation mechanism of Paraoxon (1) by the hydroxyl radicals to form p-nitrophenol (2) and O,O-diethyl phosphate (3). Scheme 1. Degradation mechanism of Paraoxon (1) by the hydroxyl radicals to form p-nitrophenol (2) and O,O-diethyl phosphate (3). Fe° + O2 + 2H+ → Fe2+ + H2O2 (2) Fe° + H2O2 + 2H+ → Fe2+ + 2H2O (3) Fe2+ + H2O2 → Fe3+ +·OH + OH− (4) (2) and O,O-diethyl phosphate (3). Fe° + O2 + 2H+ → Fe2+ + H2O2 Fe° + H2O2 + 2H+ → Fe2+ + 2H2O Sustainability 2022, 14, 9451 Fe2+ + H2O2 → Fe3+ +·OH + OH− (2) (3) 6 of 13 (4) Figure Figure 2. 2. The The SEM SEM images images of of Fe° Fe◦ nanoparticles nanoparticles after after incubation incubation with with Paraoxon. Paraoxon. 3.1.3. Energy Dispersive X-ray Analysis (EDXA) 3.1.3. Energy Dispersive X-ray Analysis (EDXA) EDXA is an X-ray technique that is used in the identification of the elemental comEDXA is an X-ray technique that is used in the identification of the elemental composiposition of materials. The EDXA image obtained before subjecting the particles to tion of materials. The EDXA image obtained before subjecting the particles to Paraoxon Sustainability 2022, 14, x FOR PEER REVIEW 7 ofis 13 Paraoxon is as shown in Figure S1. The characteristic broad peaks in Figure S1 indicated as shown in Figure S1. The characteristic broad peaks in Figure S1 indicated a sufficient a sufficient presence of zero-valent iron in the sample. EDXA spectrum Figure 3a,b shows presence of zero-valent iron in the sample. EDXA spectrum Figure 3a,b shows the elemental the elemental of incubation ZVI NPs after Paraoxon. Theshows result that analysis analysis of ZVIanalysis NPs after withincubation Paraoxon. with The result analysis ZVI uranium, 0.1%NPs of tin, 0.1% of lead, 0.2% of thorium and 0.1% of0.5% aluminum. These results shows that ZVI consist 76.2% of iron, 22.7% of oxygen, of silicon, 0.2% of NPs consist of 76.2% of iron, 22.7% of oxygen, 0.5% of silicon, 0.2% of uranium, 0.1% of tin, confirmed the presence of different metal elements in the synthesized ZVI NPs with high 0.1% of lead, 0.2% of thorium and 0.1% of aluminum. These results confirmed the presence amounts of Fe (76.2%). of different metal elements in the synthesized ZVI NPs with high amounts of Fe (76.2%). (a) (b) Figure Energy Dispersive X-ray Analysis (EDXA) image of zero-valent iron after NPs incubation after incubaFigure 3. 3. (a)(a) Energy Dispersive X-ray Analysis (EDXA) image of zero-valent iron NPs tion with Paraoxon. (b) SEM image showing the crystallinity of ZVI NPs. with Paraoxon. (b) SEM image showing the crystallinity of ZVI NPs. 3.2. Degradation kinetics of Paraoxon 3.2.1. Effect of Time on the Degradation of Paraoxon The rate of disappearance of Paraoxon with time, in the presence and absence of Fe° NPs, was monitored over a three-hour period. The results, shown in Figure 4, demon- (a) Sustainability 2022, 14, 9451 (b) Figure 3. (a) Energy Dispersive X-ray Analysis (EDXA) image of zero-valent iron NPs after incubation with Paraoxon. (b) SEM image showing the crystallinity of ZVI NPs. 7 of 13 3.2. Degradation kinetics of Paraoxon 3.2.1. of Time on the of Paraoxon 3.2. Effect Degradation kinetics of Degradation Paraoxon The rate of Time disappearance of Paraoxon time, in the presence and absence of Fe° 3.2.1. Effect on the Degradation of with Paraoxon NPs, was monitored over a three-hour period. The results, in Figure 4, demonThe rate of disappearance of Paraoxon with time, in theshown presence and absence of Fe◦ strated that 63.5% mineralization of Paraoxon was attained in 3 h in the presence of Fe° NPs, was monitored over a three-hour period. The results, shown in Figure 4, demonstrated ◦ NPs 2.0% w/v dosage. Contrarily, without Fe° NPs in (control), nopresence appreciable thatat63.5% mineralization of Paraoxon was attained 3 h in the of Fedegradation NPs at 2.0% (<2.0%) of Paraoxon was realized, the effectiveness of Fe° NPs in the sequestraw/v dosage. Contrarily, withoutindicating Fe◦ NPs (control), no appreciable degradation (<2.0%) ◦ tion of Paraoxon under these conditions. of Paraoxon was realized, indicating the effectiveness of Fe NPs in the sequestration of Paraoxon under these conditions. Figure Percentdegradation degradationofofParaoxon Paraoxonwith withtime time(C (Coo==1515mg mgLL−1−, 1dosage , dosage 2.0% w/v ZVI NPs). Figure 4.4.Percent == 2.0% w/v ZVI NPs). The errorbars barsreflect reflectthe thestandard standarddeviation deviationofofthree threerepeat repeatexperiments. experiments. The error 3.2.2. Effect of Nano Zero Valent Iron (Fe◦ ) Dosage on Degradation Efficiency The study on the effect of Fe◦ dosage on degradation efficiency was evaluated. From Figure 5, an increase in Fe◦ dosage, from 2.0% w/v to 6.0% w/v, significantly ameliorated the degradation efficiency of Paraoxon from 63.5% to 85.8%, respectively. The rapid initial degradation rate of Paraoxon was attributed to the presence of a large number of energetically favorable catalytic sites. However, beyond 6.0% w/v dosage, there was no appreciable change in percent degradation due to overlapping of the active sites as a result of agglomeration of the ZVI particles. A dosage of 6.0% w/v was realized as the optimum. The change in the concentration of Paraoxon is attributed to both degradations to other metabolites and adsorption. A mass balance analysis was beyond the scope of the present work. The zero valent iron nanoparticles reacted with oxygen to form Fe2+ and O2 2− . The O2 2− formed reacts with hydrogen to form hydrogen peroxide, which then reacts with Fe2+ to form Fe3+ and hydroxyl radicals, which are highly reactive oxidants. The hydroxyl radicals initiate a nucleophilic attack on the phosphorus center by cleaving the P-O-C bond to produce 4-nitrophenol as a major by-product, which is eventually mineralized through further oxidation. The Fe3+ generated will be precipitated out as hydroxides [19]. See Equations (1)–(3) in Section 3.1.2. Sustainability 2022, 14, 9451 Figure 5, an increase in Fe° dosage, from 2.0% w/v to 6.0% w/v, significantly ameliorated the degradation efficiency of Paraoxon from 63.5% to 85.8%, respectively. The rapid initial degradation rate of Paraoxon was attributed to the presence of a large number of energetically favorable catalytic sites. However, beyond 6.0% w/v dosage, there was no appreciaof 13 ble change in percent degradation due to overlapping of the active sites as a result of 8agglomeration of the ZVI particles. A dosage of 6.0% w/v was realized as the optimum. Figure Effect ZVI NPs dosage Paraoxon degradation. The error bars reflect standard Figure 5. 5. Effect of of ZVI NPs dosage onon Paraoxon degradation. The error bars reflect thethe standard deviation of of three repeat experiments. deviation three repeat experiments. 3.2.3. of pH on Paraoxon degradation TheEffect change in the concentration of Paraoxon is attributed to both degradations to The effect of solution pH on Paraoxon degradation was beyond examined pH 4, and other metabolites and adsorption. A mass balance analysis was theatscope of7the 2+ 10, holding other conditions constant. The observed degradation efficiency of Paraoxon present work. The zero valent iron nanoparticles reacted with oxygen to form Fe and 2−. The O22− significantly (p <with 0.05)hydrogen with a decrease solution pH with a which maximum O2increased formed reacts to forminhydrogen peroxide, then removal reacts Sustainability 2022, 14, x FOR PEER REVIEW 9 ofhy13 at pH (Figure indicates that the efficiency of zero valentoxidants. iron nanoparticles with Fe2+4 to form 6). Fe3+This andtrend hydroxyl radicals, which are highly reactive The + to degrade Paraoxon influenced by Hon ions. Similar observation was reported droxyl radicals initiate awas nucleophilic attack the phosphorus center by cleaving the P-by Li et al. [23] for the degradation of acid orange 7 dye using nanoscale zero valent iron. O-C bond to produce 4-nitrophenol as a major by-product, which is eventually mineral- ized through further oxidation. The Fe3+ generated will be precipitated out as hydroxides [19]. See Equations (1)–(3) in Section 3.1.2. 3.2.3. Effect of pH on Paraoxon degradation The effect of solution pH on Paraoxon degradation was examined at pH 4, 7 and 10, holding other conditions constant. The observed degradation efficiency of Paraoxon increased significantly (p < 0.05) with a decrease in solution pH with a maximum removal at pH 4 (Figure 6). This trend indicates that the efficiency of zero valent iron nanoparticles to degrade Paraoxon was influenced by H+ ions. Similar observation was reported by Li et al. [23] for the degradation of acid orange 7 dye using nanoscale zero valent iron. Figure 6. Effect of pH on Paraoxon degradation. The error bars reflect the standard deviation of three Figure 6. Effect of pH on Paraoxon degradation. The error bars reflect the standard deviation of three repeat experiments. repeat experiments. 3.2.4. Effect of Nano Zero Valent Iron (Fe°) Dosage on Degradation Kinetics The Paraoxon degradation kinetics were modeled with a first-order rate equation (see Equation (1)), since degradation processes are typically first-order reactions. The suitability of the model was inspected using the coefficient of determination (R2) values. The rate constants (kobs) were determined by linear regression of the experimental data following the first-order rate equation. The value of kobs, calculated from Figure 7, was Sustainability 2022, 14, 9451 Figure 6. Effect of pH on Paraoxon degradation. The error bars reflect the standard deviation of three 9 of 13 repeat experiments. 3.2.4. Effect of Nano Zero Valent Iron (Fe°) Dosage on Degradation Kinetics ◦ ) Dosage on Degradation Kinetics 3.2.4. Effect of Nano Zero Valentkinetics Iron (Fewere The Paraoxon degradation modeled with a first-order rate equation The Paraoxon degradation kinetics were modeled withfirst-order a first-order rate equation (see (see Equation (1)), since degradation processes are typically reactions. The suitEquation (1)), since was degradation processes are typically of first-order reactions. suitability ability of the model inspected using the coefficient determination (R2)The values. 2 ) values. of the model was inspected coefficient determination (Rof The rate constants (kobs) using were the determined byoflinear regression the experimental The rate constants (k ) were determined by linear regression of the experimental data obs rate equation. The value of kobs, calculated from data following the first-order Figure 7, was following the first-order rate equation. The value of k , calculated from Figure 7, −1 −1 −1 obs and 6.0% w/v of ZVI NPs, was 0.2245 h , 0.3354 h and 0.4800 h for 2.0% w/v, 4.0% w/v re−1 , 0.3354 h−1 and 0.4800 h−1 for 2.0% w/v, 4.0% w/v and 6.0% w/v of ZVI NPs, 0.2245 h spectively. This proved that the rate of degradation was enhanced with the increased ZVI respectively. This proved thatthe theobserved rate of degradation enhanced with the increaseddisZVI NPs dosage consistent with increased was removal efficiency previously NPs dosage consistent with the increased efficiencythat previously discussed. cussed. The high coefficients of observed determination (R2)removal values indicate the degradation 2 ) values indicate that the degradation kinetics The high coefficients of determination (R kinetics followed the first-order kinetic law. followed the first-order kinetic law. Figure7.7.Paraoxon Paraoxondegradation degradationfirst-order first-orderlinear linearregression regressionplots. plots. Figure 3.2.5. Effect of Paraoxon Initial Concentration on Degradation Kinetics The effect of initial Paraoxon concentration (10, 15, and 20 mg L−1 ) on the degradation kinetics was investigated at a fixed Fe◦ dosage (6.0% w/v) and also to verify the validity of the previously affirmed conclusion that degradation of Paraoxon in the presence of Fe◦ is a first-order reaction. The experimental data showed high linearity when tested against the first-order kinetics equation (Figure 8). The corresponding calculated kobs values were 0.0074 h−1 , 0.0073 h−1 and 0.0069 h−1 for initial concentrations of 10, 15 and 20 mgL−1 , respectively. These values denote that the degradation half-life of Paraoxon is independent of the initial concentration. This confirms the degradation dynamics for the first-order kinetics. Sustainability 2022, 14, 9451 a first-order reaction. The experimental data showed high linearity when tested against the first-order kinetics equation (Figure 8). The corresponding calculated kobs values were 0.0074 h−1, 0.0073 h−1 and 0.0069 h−1 for initial concentrations of 10, 15 and 20 mgL−1, respectively. These values denote that the degradation half-life of Paraoxon is independent of the initial concentration. This confirms the degradation dynamics for the first-order kinet10 of 13 ics. Figure First-orderlinear linearregressions regressions plots plots for concentrations (dose = 6.0% Figure 8.8.First-order for Paraoxon Paraoxonatatdifferent differentinitial initial concentrations (dose = w/v ZVI NPs). 6.0% w/v ZVI NPs). 3.3. Degradation Products of Paraoxon 3.3. Degradation Products of Paraoxon As aforementioned, ZVI NPs have been used extensively for the remediation of conAs aforementioned, ZVI NPs have been used extensively for the remediation of contaminated soil and groundwater. This is due to their large active surface area; hence, taminated soil groundwater. Thisreductants is due to their large active surface area; hence, they they serve asand strong and effective for environmental pollutants. We further, serve as strong and effective reductants for environmental pollutants. We further, invesinvestigated this fact using HPLC, in which the reduction in Paraoxon peak was monitored tigated this fact HPLC, in which the reduction in incubation Paraoxon peak was monitored with time. The using residual Paraoxon and metabolites after of Paraoxon with ZVI with time. The residual Paraoxon and metabolites after incubation of Paraoxon with ZVI NPs for 0 to 3 h was determined by noting their respective retention time. Paraoxon was NPs for 0 to was determined by noting their respective retention Paraoxon was detected at3ahretention time of 24.1 min and a degradation producttime. at 10.6 min. Qualitadetected at a retention time 24.1 min and a degradation product at 10.6 min. Qualitatively, chromatogram peaksoffor Paraoxon and p-nitrophenol standards coincided with the tively, chromatogram peaks for Paraoxon and p-nitrophenol standards coincided with the retention times for those of Paraoxon, and the metabolite peak formed after the degradation retention for those and the metabolite peak formed after in theFigure degradaprocess, times suggesting that of theParaoxon, by-product formed was p-nitrophenol. As seen 9, the tion process, suggesting that the by-product formed was p-nitrophenol. As seen in Figure emergence of the metabolite peak, p-nitrophenol, is followed by a consistent increase of 9,this the peak emergence of the peak, p-nitrophenol, is followed with time andmetabolite concomitant decrease in the Paraoxon peak. by a consistent increase The of this peak with time and concomitant decrease in the Paraoxon peak.spectroscopy findings in Figure 9 were complemented further by Uv-Visible The findings in Figure 9 were complemented further by Uv-Visible spectroscopy re-in results (Figure S2). In this case, the addition of ZVI NPs to Paraoxon caused a decrease sults S2).peak In this case, the of ZVI NPs to Paraoxon caused a decrease in the (Figure absorption intensity at addition 270 nm with time, with a concomitant gradual increase the peaktime intensity at 270 nmobserved with time, concomitant gradual increase in inabsorption intensity with of a new band atwith ~310anm. The decrease in the Paraoxon intensity with time a new bandasobserved nm. The decrease in theand Paraoxon peakof peak intensity at of 270 nm was a result at of~310 cleavage of the O-P bond formation intensity at 270 nm was as1).a This result of cleavageby ofan the O-P bond andobserved formation new new product(s) (Scheme is supported isosbestic point at of ~295 nm (Figure S2) that gives credence to a new species forming during the interaction between the Paraoxon and the ZVI NPs. Sustainability 2022, 14, 9451 cleavage of the O-P bond to p-nitrophenol (2) and O,O-diethyl phosphate (3). This proposed mechanism is premised on a number of studies [24,25] that have proven that compounds 2 and 3 are products of hydroxyl degradation of Paraoxon. Thus, while the present study highlights the degradation of Paraoxon to p-nitrophenol by Fe° NPs, it highly possible that other degradation products, such as O,O-diethyl phosphate, undetected 11 of 13 herein by the HPLC method were formed. Figure 9. Chromatograms showing Paraoxon and its metabolite (p-nitrophenol) peak after various hours of incubation of ZVI NPs with 15 mg mL−1 Paraoxon. Figure 9. Chromatograms showing Paraoxon and its metabolite (p-nitrophenol) peak after various −1 Paraoxon. hoursThe of incubation of ZVI with 15 mg mechanism of NPs degradation ofmL Paraoxon is shown in Scheme 1. In this case, the hydroxyl radical and hydroxyl ions reacted with the Paraoxon (1) molecules to induce the 4. Conclusions cleavage of the O-P bond to p-nitrophenol (2) and O,O-diethyl phosphate (3). This proposed The degradation organophosphate-based chemical agent2 mechanism is premisedofonParaoxon, a numberan of studies [24,25] that have proven thatwarfare compounds and emerging environmental pollutant, was investigated using ZVI the NPs. The results and an 3 are products of hydroxyl degradation of Paraoxon. Thus, while present study demonstrated the ZVI NPs are capable of degrading by Paraoxon in it water. However, highlights the that degradation of Paraoxon to p-nitrophenol Fe◦ NPs, highly possible characterization of the products, ZVI NPs before after sorption of paraoxon shows that by these that other degradation such asand O,O-diethyl phosphate, undetected herein the HPLC methodundergo were formed. nanoparticles several surface changes after the sorption process, as indicated by the TEM and SEM images obtained. The effects of solution pH, initial pollutant concen4. Conclusions tration, ZVI dosage and contact time on mineralization efficiency were investigated. Batch The degradation of Paraoxon, an organophosphate-based chemicalefficiencies warfare agent experiments demonstrated that Paraoxon was mineralized at degradation that and an emergingon environmental pollutant, was investigated NPs. The degraresults were dependent treatment time, pH and amount of the ZVIusing NPs. ZVI The optimum demonstrated that the of degrading Paraoxon in The water. However, dation of Paraoxon wasZVI 6.0%NPs w/vare ZVIcapable NPs, with an efficiency of 75.9%. degradation characterization of the ZVI NPs before and after sorption of paraoxon shows that these of Paraoxon pesticide is strongly pH-dependent, with enhanced degradation efficiency nanoparticles undergo severaldegradation surface changes after the sorption process, indicatedby bythe the observed at pH 4. Paraoxon followed first-order kinetics,asindicated TEM and SEM images obtained. The effects of solution pH, initial pollutant concentration, ZVI dosage and contact time on mineralization efficiency were investigated. Batch experiments demonstrated that Paraoxon was mineralized at degradation efficiencies that were dependent on treatment time, pH and amount of the ZVI NPs. The optimum degradation of Paraoxon was 6.0% w/v ZVI NPs, with an efficiency of 75.9%. The degradation of Paraoxon pesticide is strongly pH-dependent, with enhanced degradation efficiency observed at pH 4. Paraoxon degradation followed first-order kinetics, indicated by the high coefficients of determination and independence of the rate constant to initial Paraoxon concentration. This study shows that ZVI NPs potentially offer an alternative method for the destruction of Paraoxon from the aqueous environment, subject to further investigations in real environmental water samples. Given the high value of nanoparticles and that nanosized materials can impact human health, in addition to presenting environmental, separation, Sustainability 2022, 14, 9451 12 of 13 recovery and reuse problems, further investigations on the possible recycling of the ZVI NPs are necessary. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/su14159451/s1, Figure S1: Energy Dispersive X-ray Analysis (EDXA) image of freshly prepared zero-valent iron NPs. Figure S2. Evolution of Paraoxon concentration with time after incubation of 15 mg L−1 of Paraoxon with 2% ZVI NPs for 3 h. Author Contributions: Conceptualization, V.A.O. and I.O.K.; methodology, V.A.O. and I.O.K.; software, V.A.O. and V.O.S.; validation, V.A.O., I.O.K. and V.O.S.; formal analysis, K.M.; investigation, K.M.; resources, V.A.O. and I.O.K.; data curation, V.O.S.; writing—original draft preparation, K.M.; writing—review and editing, was done by all the authors; visualization, V.A.O. and I.O.K.; supervision, V.A.O. and I.O.K.; project administration, V.A.O. and I.O.K.; funding acquisition, V.A.O. and I.O.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Commission of Science, Technology and Innovation, Kenya, under Grant number (NCST/5/003/3rd CALL MSc/026). The APC was not funded but was paid from personal finances. Institutional Review Board Statement: Not applicable. 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