Synthesis of Atmospherically Stable Zero-Valent Iron Nanoparticles (nZVI) for the Efﬁcient Catalytic Treatment of High-Strength Domestic Wastewater

: Here, we report the fabrication of nZVI by the wet chemical technique in the presence of ethanol using ferric iron and sodium borohydride as the reducing agents under ambient conditions. The obtained nZVI particles are mainly in a zero-valent oxidation state and do not undergo signiﬁcant oxidation for several weeks. The structural and morphological parameters of nZVI were investigated by using UV, XRD, SEM, EDX, TEM, and DLS analysis. The optical nature, bandgap energy, and absorption edge were all revealed by the UV–visible spectrum. The phase development and crystallinity of nZVI particles were shown by the XRD pattern. The morphological investigation revealed that the nanoparticles were spherical with an average size of 34–110 nm by using ImageJ software, and the elemental analysis was analyzed using EDX. Furthermore, the catalytic treatment performance of domestic wastewater was evaluated in terms of pH, COD (chemical oxygen demand) solubilization, total solids (TS), volatile solids (VS), phosphorous, and total nitrogen (TN) reduction under aerobic and anaerobic operating conditions. The efﬂuent was subjected to a process evaluation with a different range (100–500 mg/L) of nZVI dosages. The COD solubilization and suspended solids reduction were signiﬁcantly improved in the anaerobic condition in comparison to the aerobic condition. Furthermore, the effect of nZVI on phosphorous (PO 43 − ) reduction was enhanced by the electrons of iron ions. The high concentration of nZVI dosing has a positive impact on COD solubilization and phosphorous removal regardless of the aeration condition with 400 mg/L of nZVI dosage.


Introduction
Currently, wastewater is one of the most serious concerns of the world, caused by industrial/domestic waste or human-made sources [1,2]. Pollutants are discharged into the water by the industry without any prior treatment, resulting in significant water pollution, environmental and human health risks [3,4]. Until now, the primary sources of water contamination have been industrial and residential waste, mining, oil spills, nuclear waste, and pesticides [5][6][7][8]. Domestic and municipal wastewater is generally 99% water and 1% suspended, colloidal, and dissolved particles, predominantly organic since it includes

Characterization Results of nZVI
The UV-Visible spectrum of the nZVI particle was depicted in Figure 1a. The activation of surface plasmon vibrations in the nZVI solution results in two absorption peaks at 216 and 264 nm, which are identical to the UV-visible spectrum of metallic iron and indicate the fabrication of nZVI particles. The optical bandgap (E g ) of the synthesized samples were determined using the Tauc plot, hν vs. (αhν) 2 , as indicated in Figure 1b  where k is a constant, hν is the photon energy, E g the bandgap energy and the absorption coefficient (α). The n factor equals 1/2 or 2 for the indirect and the direct band gaps, respectively, depending on the nature of the electric transition. The absorption peaks obtained at 216 and 264 nm are assigned to a characteristic peak of monodispersed iron nanoparticles in zero valent states [31]. The bandgap energy of synthesized nZVI particles was calculated to be 3.9 eV. This value is slightly higher to bulk iron, i.e., Eg = 1.5 eV [35]. These results further confirmed the formation of nanosized nZVI particles.
Catalysts 2022, 12, x FOR PEER REVIEW 3 of 16 were determined using the Tauc plot, hν vs. (αhν) 2 , as indicated in Figure 1b using the following Equation (1): where k is a constant, hν is the photon energy, Eg the bandgap energy and the absorption coefficient (α). The n factor equals 1/2 or 2 for the indirect and the direct band gaps, respectively, depending on the nature of the electric transition. The absorption peaks obtained at 216 and 264 nm are assigned to a characteristic peak of monodispersed iron nanoparticles in zero valent states [31]. The bandgap energy of synthesized nZVI particles was calculated to be 3.9 eV. This value is slightly higher to bulk iron, i.e., Eg = 1.5 eV [35]. These results further confirmed the formation of nanosized nZVI particles. XRD analysis was used to determine the crystallinity of freshly prepared nZVI particles, as shown in Figure 2. XRD analysis results can be used to calculate the average particle size of synthesized nanoparticles. The highest peak of the XRD spectrum indicates an intensity at 2θ = 45.7° demonstrated the (110) and (200) plane of nZVI and showed the crystalline phases of particles. The peaks of the diffraction patterns revealed a crystalline structure for the nZVI particles, and Miller indices were calculated using JCPDF cards 06-0696 [13]. The average crystalline size was found to be approximately 26.8 nm by using the Debay-Scherrer formula D = 0.9λ ∕ β cos θ (2) where β is the full-width half maxima of the peak, θ is the angle, λ is the x-ray wavelength of XRD patterns. XRD analysis was used to determine the crystallinity of freshly prepared nZVI particles, as shown in Figure 2. XRD analysis results can be used to calculate the average particle size of synthesized nanoparticles. The highest peak of the XRD spectrum indicates an intensity at 2θ = 45.7 • demonstrated the (110) and (200) plane of nZVI and showed the crystalline phases of particles. The peaks of the diffraction patterns revealed a crystalline structure for the nZVI particles, and Miller indices were calculated using JCPDF cards 06-0696 [13]. The average crystalline size was found to be approximately 26.8 nm by using the Debay-Scherrer formula where β is the full-width half maxima of the peak, θ is the angle, λ is the X-ray wavelength of XRD patterns. SEM, TEM, and EDX analysis were used to investigate the typical morphology, size characteristics of nanoparticles, and elemental distribution analysis of each nZVI sample. Figure 3 shows the morphology, particle size, and elemental compositions of the synthesized nZVI, which has a dense porous surface and spherical, polydisperse structures with different processing adsorption capacity sizes. The result shows that some agglomerations of particles were found, which are represented by spherical shape and irregular size with different void spaces. Because of van der Waal's forces and magnetic attraction between Fe 0 particles, a cluster of structures was observed at 20,000 × resolution in nZVI. Although the SEM micrographs provide direct visual evidence of particle size, due to the small number of iron particles examined, it is unable to estimate the size with high statistical certainty. The surface composition was investigated using an EDX coupled to an SEM instrument. Figure 3b shows the elemental distribution of the nZVI particles as well as the percentage of the elements present. The averages of three experimental replications are shown in the bar graph. It can also provide qualitative and quantitative data on the elements that contribute to nanoparticles synthesis. The strongest peak in the EDX spectra of nZVI indicates the existence of Fe 0 particles due to elemental iron absorption. Figure 3b shows that Fe was the most abundant element on the surface of nZVI, accounting for 68.11 weight percent, followed by O, which contributed 31.89 weight percent. The elemental mapping of EDX micrographs and the surface of nZVI particles, which is mostly constituent of iron (Fe), is shown in Figure 3c.
The nanoscale nZVI particles had a chain-like aggregated structure with particle diameters up to 110 nm by using ImageJ software, indicating that they were synthesized at the nanoscale. In the TEM micrographs, a single particle appears to be constituted of a dense core surrounded by a thin shell with much less contrast than the inner core. The nZVI nanoparticles have a chain-like structure and are covered on the surface by a passivation layer, as revealed by their natural magnetism and electrostatic interactions ( Figure 4). The TEM image of the nZVI nanoparticles shows a helical shape with numerous pores that supports the adsorption or removal process. According to the graph, the iron content assisted is proportional to the volume of solution introduced into the method during the reduction process.
The hydrodynamic diameter of nZVI particles was precisely determined using particle size analysis ( Figure 5). The particle sizes were measured to be 28.2 nm, and the particle size distribution curve for nZVI showed an intensity of 12.7%, which might indicate some agglomerations of nZVI particles in the solvent. However, chemically synthesized nanoparticles were agglomerated up to 2.3 µm [36]. SEM, TEM, and EDX analysis were used to investigate the typical morphology, s characteristics of nanoparticles, and elemental distribution analysis of each nZVI samp Figure 3 shows the morphology, particle size, and elemental compositions of the synth sized nZVI, which has a dense porous surface and spherical, polydisperse structures w different processing adsorption capacity sizes. The result shows that some agglomeratio   The nanoscale nZVI particles had a chain-like aggregated structure with particle diameters up to 110 nm by using ImageJ software, indicating that they were synthesized at the nanoscale. In the TEM micrographs, a single particle appears to be constituted of a dense core surrounded by a thin shell with much less contrast than the inner core. The nZVI nanoparticles have a chain-like structure and are covered on the surface by a passivation layer, as revealed by their natural magnetism and electrostatic interactions (Figure 4). The TEM image of the nZVI nanoparticles shows a helical shape with numerous pores that supports the adsorption or removal process. According to the graph, the iron content assisted is proportional to the volume of solution introduced into the method during the reduction process.   The hydrodynamic diameter of nZVI particles was precisely determined using particle size analysis ( Figure 5). The particle sizes were measured to be 28.2 nm, and the particle size distribution curve for nZVI showed an intensity of 12.7%, which might indicate some agglomerations of nZVI particles in the solvent. However, chemically synthesized nanoparticles were agglomerated up to 2.3 µm [36].

Effects of nZVI Particles on COD Solubilization
The effect of different nZVI dosages on COD solubilization in aerobic and anaerobic conditions was investigated ( Figure 6). The results show that significant COD solubilization was achieved within 60-120 min of treatment. The COD solubilization was reduced from 680 mg/L to 110, 104, 102, 98, 105 mg/L and 76, 72, 71, 71, 71 mg/L under aerobic and anaerobic conditions, respectively, when 100, 200, 300, 400, and 500 mg/L nZVI dose was  The hydrodynamic diameter of nZVI particles was precisely determined using particle size analysis ( Figure 5). The particle sizes were measured to be 28.2 nm, and the particle size distribution curve for nZVI showed an intensity of 12.7%, which might indicate some agglomerations of nZVI particles in the solvent. However, chemically synthesized nanoparticles were agglomerated up to 2.3 µm [36].

Effects of nZVI Particles on COD Solubilization
The effect of different nZVI dosages on COD solubilization in aerobic and anaerobic conditions was investigated ( Figure 6). The results show that significant COD solubilization was achieved within 60-120 min of treatment. The COD solubilization was reduced from 680 mg/L to 110, 104, 102, 98, 105 mg/L and 76, 72, 71, 71, 71 mg/L under aerobic and anaerobic conditions, respectively, when 100, 200, 300, 400, and 500 mg/L nZVI dose was

Effects of nZVI Particles on COD Solubilization
The effect of different nZVI dosages on COD solubilization in aerobic and anaerobic conditions was investigated ( Figure 6). The results show that significant COD solubilization was achieved within 60-120 min of treatment. The COD solubilization was reduced from 680 mg/L to 110, 104, 102, 98, 105 mg/L and 76, 72, 71, 71, 71 mg/L under aerobic and anaerobic conditions, respectively, when 100, 200, 300, 400, and 500 mg/L nZVI dose was added. In both treatment conditions, COD solubilization increased linearly with increasing nZVI dosage concentration. After each 100 mg/L nZVI dosages, the COD concentration in both conditions decreased by 110 and 76 mg/L, respectively. With increasing doses of nZVI, COD solubilization increased gradually to 98 and 71 mg/L, respectively. A comparable improvement was observed with higher catalyst dosage (200, 300, 400, and 500 mg/L) ( Figure 6a). The concentration of nZVI was found to have a strong positive relationship with COD solubilization. Maximum COD solubilization at 400 mg/L nZVI doses was 94 and 71 mg/L in both treatment conditions. The COD solubilization increased gradually up to 120 h, after which the solubilization was delayed and showed a minor increase. Reduced solubilization could be attributed to the formation of refractory compounds and their associated resistance to degradation After 120 h of exposure to 400 mg/L nZVI nanoparticles, the degree of COD solubilization reached 92%. After 120 h of treatment, the maximum reduction in COD concertation by nZVI catalyst was achieved under anaerobic conditions ( Figure 6b). It was revealed that anaerobic conditions, when compared to aerobic conditions, can accelerate COD removal unless 300 mg/L of nZVI is added. However, no major difference was observed in COD solubilization after 400 mg/L nZVI dosages in both aerobic and anaerobic conditions. As a result, the optimal dose of nZVI was determined to be 400 mg/L. Regardless of the nanoparticles or concentration doses, the equilibrium value was achieved after 80-100 h of exposure in the presence of oxygen. The hydroxyl radical has the potential to degrade complex floc structures, releasing intracellular organic matter into the soluble phase of the wastewater and increasing COD solubilization [37][38][39]. This could be due to the highly reactive and non-selective OH 0 radical produced during the oxidation of the carbohydrate, protein, and other organic compounds released by the flocs [18,40,41]. The results of the current study were comparable to another finding in terms of COD solubilization. Martins et al. and Zhou et al. achieved COD solubilization ranging from 31.4 to 50.7% at pH 2 and nZVI doses ranging from 500 to 750 mg/L [42,43]. This could be attributed to the complete mineralization of the organic matters as a result of a high dose of nZVI and H 2 O 2 . However, in comparison to the current study, the result of Sahinkaya et al. was found to be 22.2% at pH 3 and nZVI dosages of 4 g/kg TS [44]. Finally, it can be stated that COD solubilization can be improved by using nZVI with anaerobic conditions. The decrease in COD concentration indicates floc disruption, cell lysis, and the release of intracellular organic materials into the liquid phase. added. In both treatment conditions, COD solubilization increased linearly with increasing nZVI dosage concentration. After each 100 mg/L nZVI dosages, the COD concentration in both conditions decreased by 110 and 76 mg/L, respectively. With increasing doses of nZVI, COD solubilization increased gradually to 98 and 71 mg/L, respectively. A comparable improvement was observed with higher catalyst dosage (200, 300, 400, and 500 mg/L) (Figure 6a). The concentration of nZVI was found to have a strong positive relationship with COD solubilization. Maximum COD solubilization at 400 mg/L nZVI doses was 94 and 71 mg/L in both treatment conditions. The COD solubilization increased gradually up to 120 h, after which the solubilization was delayed and showed a minor increase. Reduced solubilization could be attributed to the formation of refractory compounds and their associated resistance to degradation After 120 h of exposure to 400 mg/L nZVI nanoparticles, the degree of COD solubilization reached 92%. After 120 h of treatment, the maximum reduction in COD concertation by nZVI catalyst was achieved under anaerobic conditions (Figure 6b). It was revealed that anaerobic conditions, when compared to aerobic conditions, can accelerate COD removal unless 300 mg/L of nZVI is added. However, no major difference was observed in COD solubilization after 400 mg/L nZVI dosages in both aerobic and anaerobic conditions. As a result, the optimal dose of nZVI was determined to be 400 mg/L. Regardless of the nanoparticles or concentration doses, the equilibrium value was achieved after 80-100 h of exposure in the presence of oxygen. The hydroxyl radical has the potential to degrade complex floc structures, releasing intracellular organic matter into the soluble phase of the wastewater and increasing COD solubilization [37][38][39]. This could be due to the highly reactive and non-selective OH 0 radical produced during the oxidation of the carbohydrate, protein, and other organic compounds released by the flocs [18,40,41].  [44]. Finally, it can be stated that COD solubilization can be improved by using nZVI with anaerobic conditions. The decrease in COD concentration indicates floc disruption, cell lysis, and the release of intracellular organic materials into the liquid phase.

Effects of nZVI Particles on Solid Reduction
The suspended solids reduction is an important indicator for wastewater treatment. The effect of different nZVI dosages on suspended solid reduction was depicted in Figure 7. The concentration of suspended solids in the wastewater decreased with increasing catalyst dosage and treatment time. The initial suspended solid concentration was 332 mg/L, which rapidly decreased to 20 mg/L after the first 60-120 min of aerobic and anaerobic treatment, with SS reductions of 86 and 92%, respectively. In 120 min, a maximum suspended solid re- duction was observed with 400 mg/L of nZVI catalyst dosage in both treatment conditions. When compared to 300 mg/L of nZVI in aerobic conditions, the nZVI dosage of 500 mg/L showed a slight increase (almost sustained) in suspended solid reduction after 120 h of treatment (Figure 7a). In anaerobic conditions, 300 mg/L of nZVI dosage showed a significant reduction in solids after 120 h of treatment. The degree of suspended solid reduction was in the range of 48-92% for both treatment conditions. The maximum reduction in solids was obtained within 120 min of anaerobic treatment conditions (Figure 7b). Furthermore, in both treatment conditions, treatment times exceeding 80 h did not show a significant increase in suspended solid reduction. The reduction in suspended solids reveals wastewater stability and is used to assess process efficiency and stabilize wastewater. The breakdown of solid flocs and the disruption of EPS has increased by suspended solids reduction into the liquid medium while simultaneously reducing total and volatile solids. The VS/TS ratio of the sludge decreased from 71% to 26% after treatment with aerobic and anaerobic conditions, indicating mineral solubilization. The air acted over the flocs during the aerobic treatment, loosening them through EPS disruption. Further application of air destroyed the cell wall, allowing the intracellular organic material to be released [3,14,29]. The addition of nZVI particle scrap to wastewater resulted in a 48.27% increase in the rate of volatile solids reduction. During the anaerobic oxidation phase of wastewater treatment using nZVI particles, the ratio of the suspended solid decreased linearly, indicating an accumulation of inorganic components in the wastewater [45]. These findings are consistent with previous studies on other conventional Fenton processes [18,25,46]. According to Kim et al. and Pham et al. the Fenton process improved the dewaterability of wastewater by 49% to 74% solid reduction efficiency [47,48]. However, the anaerobic condition approach resulted in a significant reduction in suspended solids containing nZVI particles. This might be due to the accumulation of more refractory compounds in the wastewater.

Effects of nZVI Particles on Solid Reduction
The suspended solids reduction is an important indicator for wastewater treatment. The effect of different nZVI dosages on suspended solid reduction was depicted in Figure  7. The concentration of suspended solids in the wastewater decreased with increasing catalyst dosage and treatment time. The initial suspended solid concentration was 332 mg/L, which rapidly decreased to 20 mg/L after the first 60-120 min of aerobic and anaerobic treatment, with SS reductions of 86 and 92%, respectively. In 120 min, a maximum suspended solid reduction was observed with 400 mg/L of nZVI catalyst dosage in both treatment conditions. When compared to 300 mg/L of nZVI in aerobic conditions, the nZVI dosage of 500 mg/L showed a slight increase (almost sustained) in suspended solid reduction after 120 h of treatment (Figure 7a). In anaerobic conditions, 300 mg/L of nZVI dosage showed a significant reduction in solids after 120 h of treatment. The degree of suspended solid reduction was in the range of 48-92% for both treatment conditions. The maximum reduction in solids was obtained within 120 min of anaerobic treatment conditions ( Figure  7b). Furthermore, in both treatment conditions, treatment times exceeding 80 h did not show a significant increase in suspended solid reduction. The reduction in suspended solids reveals wastewater stability and is used to assess process efficiency and stabilize wastewater. The breakdown of solid flocs and the disruption of EPS has increased by suspended solids reduction into the liquid medium while simultaneously reducing total and volatile solids. The VS/TS ratio of the sludge decreased from 71% to 26% after treatment with aerobic and anaerobic conditions, indicating mineral solubilization. The air acted over the flocs during the aerobic treatment, loosening them through EPS disruption. Further application of air destroyed the cell wall, allowing the intracellular organic material to be released [3,14,29]. The addition of nZVI particle scrap to wastewater resulted in a 48.27% increase in the rate of volatile solids reduction. During the anaerobic oxidation phase of wastewater treatment using nZVI particles, the ratio of the suspended solid decreased linearly, indicating an accumulation of inorganic components in the wastewater [45]. These findings are consistent with previous studies on other conventional Fenton processes [18,25,46]. According to Kim et al. and Pham et al. the Fenton process improved the dewaterability of wastewater by 49 to 74% solid reduction efficiency [47,48]. However, the anaerobic condition approach resulted in a significant reduction in suspended solids containing nZVI particles. This might be due to the accumulation of more refractory compounds in the wastewater.

Effects of nZVI Particles on Phosphorous Removal
The phosphorus concentration in the wastewater was chemically controlled through adsorption with nZVI particles. Although the phosphorus concentration in the batch experiment only decreased from 13.86 to 0.42 mg/L in the absence of air, the reduction in phosphorus in the aerobic condition was observed to be lower than in the anaerobic condition, as shown in Figure 8. Furthermore, the increasing dose of nZVI particles increased the phosphorus removal efficiency under anaerobic conditions, with phosphorus

Effects of nZVI Particles on Phosphorous Removal
The phosphorus concentration in the wastewater was chemically controlled through adsorption with nZVI particles. Although the phosphorus concentration in the batch experiment only decreased from 13.86 to 0.42 mg/L in the absence of air, the reduction in phosphorus in the aerobic condition was observed to be lower than in the anaerobic condition, as shown in Figure 8. Furthermore, the increasing dose of nZVI particles increased the phosphorus removal efficiency under anaerobic conditions, with phosphorus concentrations decreasing from 13.86 mg/L to 1.12, 0.86, 0.64, 0.42, and 1.20 mg/L when 100, 200, 300, 400, and 500 mg/L nZVI particles were added, respectively. However, the phosphorus content of wastewater samples was reduced to 1.12 mg/L after exposure to 100 mg/L of nZVI dosage. The results showed that using nZVI particles improved the phosphorus removal rate, and as the nZVI dose was increased, the phosphorus removal equilibration capacity gradually increased. Figure 8a shows that phosphorus adsorption on the nZVI surface is initially rapid, with more than 80% phosphorus absorption occurring within the first 120 min. Thereafter, phosphorus absorption gradually increased in both treatment conditions. The adsorption equilibrium was approached after 120 min under aerobic conditions and the phosphorus concentration remained almost sustained. While removal equilibrium under anaerobic conditions showed significant removal of the phosphorus after 120 min of treatment. The removal efficiency of phosphorus improved significantly when only 100 mg/L of nZVI was used, due to the complete dissolution of nZVI particles, which generated iron ions components. However, when nZVI dosages are above 400 mg/L, the rate of phosphorus removal appears to be slower. After 120 min, significant phosphorus removal to supports all nZVI dosages under both treatment conditions. However, a further increasing the nZVI dose to 500 mg/L did not improve phosphorus removal efficiency, whereas 400 mg/L nZVI particle dosages showed better phosphorus removal after 120 min and phosphorus concentrations were reduced to 0.42 mg/L under anaerobic conditions (Figure 8b). As a result, increasing the nZVI particle concentration as an adsorbent increased the surface area of the nZVI particles, which contains the iron ions electrons components, resulting in more vacant sites, which improved phosphorus adsorption [49,50]. However, the phosphorus content of wastewater samples was reduced to 1.12 mg/L after exposure to 100 mg/L of nZVI dosage. The results showed that using nZVI particles improved the phosphorus removal rate, and as the nZVI dose was increased, the phosphorus removal equilibration capacity gradually increased. Figure 8a shows that phosphorus adsorption on the nZVI surface is initially rapid, with more than 80% phosphorus absorption occurring within the first 120 min. Thereafter, phosphorus absorption gradually increased in both treatment conditions. The adsorption equilibrium was approached after 120 min under aerobic conditions and the phosphorus concentration remained almost sustained. While removal equilibrium under anaerobic conditions showed significant removal of the phosphorus after 120 min of treatment. The removal efficiency of phosphorus improved significantly when only 100 mg/L of nZVI was used, due to the complete dissolution of nZVI particles, which generated iron ions components. However, when nZVI dosages are above 400 mg/L, the rate of phosphorus removal appears to be slower. After 120 min, significant phosphorus removal to supports all nZVI dosages under both treatment conditions. However, a further increasing the nZVI dose to 500 mg/L did not improve phosphorus removal efficiency, whereas 400 mg/L nZVI particle dosages showed better phosphorus removal after 120 min and phosphorus concentrations were reduced to 0.42 mg/L under anaerobic conditions (Figure 8b). As a result, increasing the nZVI particle concentration as an adsorbent increased the surface area of the nZVI particles, which contains the iron ions electrons components, resulting in more vacant sites, which improved phosphorus adsorption [49,50].
Based on the observed phenomena and the literature, the high reactivity of nZVI nanoparticles can only be achieved in anaerobic conditions due to the interaction of the nZVI and oxygen. Iron ion components are associated with increased phosphorus adsorption rates from the wastewater [51,52]. Since nZVI particles have a larger surface area, they may have a larger active site on the surface of the nanoparticles, allowing phosphorus to be absorbed more easily. A schematic representation of the mechanism during the treatment process of domestic wastewater with nZVI particles is shown in Figure 9. Furthermore, the phosphorus concentration decreased as a result of the continuous release of Fe 2+ and/or Fe 3+ , which has a significant impact on the coagulation/precipitation process. [22,40].  Table 1 shows the Pearson correlation matrices for five variables. The correlation matrix result for aerobic treatment shows a positive correlation between COD and TSS (r 2 = 0.98) and VS (r 2 = 0.99). Likewise, COD negatively correlated with PO 4 3− (r 2 = −0.98) and TN (r 2 = −0.97) in aerobic treatment of condition. Similarly, the TSS shows a strong positive correlation with VS (r 2 = 0.98) and PO 4 3− (r 2 = 0.97) while TN shows a negative correlation with TSS (r 2 = −0.94) and PO 4 3− (r 2 = −0.99) in aerobic condition. The correlation matrices for the anaerobic treatment condition reveal that COD activity is positively correlated with TSS (r 2 = 0.97), PO 4 3− (r 2 = 0.98) and TN (r 2 = 0.96) but negatively correlated with VS (r 2 = −0.86). Likewise, the activity of TSS and PO 4 3− shows a positive correlation with VS (r 2 = 0.99) and TN (r 2 = 0.98) in anaerobic condition.

Analysis of Variance (ANOVA) Test
The ANOVA is a statistical analysis method that distributes the total variability present within a data set. Tables exhibit the ANOVA tests of the entire systematic factor (variables) of COD solubilization from aerobic and anaerobic treatment conditions. The two-way ANOVA test of COD of aerobic treatment turned to be highly significant with p-values < 0.05 while in anaerobic treatment statistically nonsignificant with p-values > 0.05 (Table 2). Similarly, the ANOVA test of TSS reduction in both treatment conditions (aerobic and anaerobic) is statistically significant with p-values < 0.05 (Table 3). Likewise, the ANOVA test of phosphorous removal in aerobic treatment was found to be statistically significant; however, in anaerobic conditions, it was determined to be nonsignificant with p-values > 0.05 (Table 4).

Material and Methods
Ferric chloride solution (FeCl 3 ·6H 2 O), sodium borohydride (NaBH 4 , 98%) pure, and anhydrous ethanol (C 2 H 5 OH) were obtained from Merck KGaA, Mumbai, India. All the solutions were prepared with deionized water. All the chemicals used in the study were analytical grade and had not been purified further. A double distillation unit was used to make deionized water. Microporous membranes (Whatman cellulose nitrate filter paper 0.45 µm × 47 mm) were used in vacuum filtration.

Fabrication of nZVI
The nZVI nanoparticles were fabricated using the liquid-phase reduction method, which involved reducing the ferric chloride solution with NaBH 4 as the reducing agent. A volume ratio of 4/1 (v/v) ethanol/water mixture was vigorously reacted with sodium borohydride (0.1 M) solution and anhydrous ferric chloride (0.06 M). The borohydride solution was added dropwise into ferric chloride solution in a flask under constant stirring (300 rpm). After the initial drop of the borohydride solution, black particles appeared, and the remaining borohydride solution was added to accelerate the reduction reaction. The mixture was stirred for a few minutes before the nZVI particles were extracted from the liquid phase using vacuum filtration. The solid particles were washed twice with anhydrous ethanol to remove impurities. Finally, to avoid oxidation, fresh nZVI particles were stored in the drying chamber before use.

Characterization of nZVI
Several techniques were used to examine the structural, morphological, and physical properties of prepared nZVI particles. The UV-Vis spectrum was used to monitor the reduction in nZVI from pure Fe +3 ions by sampling aliquots of Fe • nanoparticle solution and diluting the sample in distilled water. The peak wavelength between 200 and 800 nm was investigated using ultraviolet-visible spectroscopy (UV-Vis, Hitachi model-U3900, Tokyo, Japan). The surface chemistry and crystal phase compositions of the synthesized particles were determined using high-resolution X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan) with an operating 2θ (degree) angle scanning range of 20 • to 80 • at a speed of 10 • min −1 . A high-resolution transmission electron microscope (HR-TEM, Model F30 S-Twin, Technai, Japan) and field emission scanning electron microscope (FE-SEM, Model No. SIGMA VP, Zeiss, Germany) were used to investigate the particles surface morphology and shape with an accelerating voltage of 200 kV and 10-12.5 kV, respectively. The energydispersive X-ray spectroscopy (EDX) microanalysis equipped with SEM was used to analyze the composition and element distribution of prepared nanoparticles. Additionally, Dynamic Light Scattering was used to validate the particle size distribution profile (DLS, Model Zetasizer Nano ZS, Malvern, UK).

Batch Experiments
The wastewater sample was taken at the entrance to a domestic wastewater treatment plant in Okhla, New Delhi, India, which has a treatment capacity of 20 million gallons per day (MGD). The samples were collected from the inlet of WWTPs and stored at 4 • C in the laboratory after being pre-examined through a 2 mm mesh to remove larger particles. Table 5 summarizes the initial characteristics of various Physico-chemical parameters of raw wastewater. Batch studies were conducted to investigate the potential impact of nZVI nanoparticles on the wastewater treatment process, as well as the rates of COD solubilization, solid reduction, and phosphorous removal. To ensure the aerobic condition, the sample was continuously injected with oxygen for 30 min, and nitrogen gas was used to purge dissolved oxygen (DO) in the same way for the anaerobic condition. In this study, 300 mL of raw sample was used, and experiments were carried out under aerobic and anaerobic conditions to examine the effect of different nZVI particle dosages (100, 200, 300, and 500 mg/L). All experiments were vigorously shaken at 150 rpm at 37 • C using a reciprocating shaker (MaxQ-2508, Mumbai, India).

Analytical Methods
All the chemicals used in the experiment were analytical grade and had not been purified further. Standard Methods for the Examination of Water and Wastewater [53] were used to examine the samples. During the batch tests, 10 mL samples were extracted with a 20 mL syringe at specified predefined times of 0, 20, 40, 80, and 120 min. The entire investigation was carried out in triplicate. Water samples were filtered using a 0.45 µm disposable sterile syringe cellulose membrane filter (Merck KGaA, Mumbai, India). The standard method for water and wastewater examination was used to assess COD solubilization and solid reduction. The USEPA PhosVer (Ascorbic Acid) technique was used to determine phosphorus contents using a Hach spectrophotometer (DR 3900, Hach Company, Loveland, CO, USA). The concentration of DO was measured with a DO meter, and the pH was measured with a portable digital pH meter (Decibel Dynamics Ltd., Chandigarh, India).

Statistical Analysis
All analysis was carried out by using origin 2020 (Origin Lab, Northampton, MA, USA). Two-way analysis of variance (ANOVA), Pearson correlation coefficient, and Standard Deviation (SD) were used to examine the data.

Conclusions
The synthesis of nZVI was facilitated by infiltration of nZVI particles with a ferric chloride solution and reduction with sodium borohydride, as revealed by UV, XRD, SEM, EDS, and TEM studies. The results of this study showed that nZVI nanoparticles can successfully treat domestic wastewater with and without oxygen. The findings suggest that a high dose of nZVI particles added to wastewater improved COD, TSS, and Phosphorus removal efficiency throughout the degradation process. The nZVI can generate more ions under the anaerobic condition and play a crucial role in the removal of COD and phosphorous concentration during the treatment of domestic wastewater. The addition of 100 to 500 mg/L nZVI dose increased COD and TSS removal efficiency, with 400 mg/L nZVI providing the highest removal rate when compared to a higher dosage of catalyst concentrations. Furthermore, as compared to the aerobic condition, the anaerobic condition significantly increased the efficiency of phosphorus removal. The treated domestic wastewater can help fulfil the increasing demand for water supply. Overall, the ability to offer biological or chemical degradation of domestic wastewater will open plenty of new possibilities for better understanding and application of nZVI particles in wastewater treatment and resource recovery.