Synthesis of Magnetic Fe3O4 Nano Hollow Spheres for Industrial TNT Wastewater Treatment

The aim of the present work was to synthesize magnetite (Fe3O4) nano hollow spheres (NHS) via simple, one-pot, template-free, hydrothermal method. The structural, morphological, and surface analysis of Fe3O4 NHS were studied by scanning electron microscopy (SEM), x-ray diffraction technique (XRD), Fourier transform infrared spectroscopy FTIR and burner-Emmett-teller (BET). The as obtained magnetic (Fe3O4) NHS were used as an adsorbent for treating industrial trinitrotoluene (TNT) wastewater to reduce its Chemical Oxygen Demand (COD) values. Adsorption capacity (Qe) of the NHS obtained is 70 mg/g, confirming the attractive forces present between adsorbent (Fe3O4 NHS) and adsorbate (TNT wastewater). COD value of TNT wastewater was reduced to >92% in 2 h at room temperature. The adsorption capacity of Fe3O4 NHS was observed as a function of time, initial concentration, pH, and temperature. The applied Fe3O4 NHS was recovered for reuse by simply manipulating its magnetic properties with slight shift in pH of the solution. A modest decrease in Qe (5.0–15.1%) was observed after each cycle. The novel Fe3O4 NHS could be an excellent candidate for treating wastewater generated by the intermediate processes during cyclonite, cyclotetramethylene-tetranitramine (HMX), nitroglycerin (NG) production and other various environmental pollutants/species.


Introduction
2,4,6-Trinitrotoluene (TNT) is a versatile aromatic compound used in drugs, herbicides, insecticides, dyes, polyurethane foams, and fungicides [1][2][3]. It is one of the most conventional explosives in use since the late 19th century, known for its insensitivity to shock and friction. Its influence is so pervasive that the standard unit for measuring the energy released after a detonation is a "ton of TNT," equaling 4.184 gigajoules. This metric aids in measuring the strength of bombs, detonation velocities and penetration power of other explosives [4][5][6].
When synthesizing TNT, the washing step during manufacturing produces waste products that end up in the surrounding environment, both in soil and particularly in water streams. These include dissolved species such as sulfates, mono nitro toluene (MNT), di nitro toluene (DNT), dinitro toluene sulfonate (DNTS) and several other derivatives of nitrobenzene (NB) [7]. These nitrogenous compounds exist in TNT wastewater in different ology, medicine, degradation of dyes, drug delivery and remediation of industrial polluted water [29][30][31][32].
Due to the adsorption, degradation, and magnetic properties of Fe 3 O 4 , it has become a useful tool for wastewater/effluent treatment [33,34]. Many researchers have utilized Fe 3 O 4 to adsorb dissolved species in industrial wastewater [23]. Nagi et al. applied nano-spherical quantum dots of Fe 3 O 4 to remove heavy metals such as Cr, Co, and pesticides [35]. Additionally, Elhassan et al. exploited the adsorption behavior of magnetite along with other nano metal oxides with components such as Cu 2+ , Pb 2+, Cr +4 , Cd 2+ and Ni 2+ , to remove atrazine and bisphenol-A from wastewater [36]. Ali Nematollah Zadeh et al. also reported the application of modified (Fe 3 O 4 ) nanoparticles for the adsorption of nitro benzene (NB), with a reported adsorption capacity of 66.72 mg g −1 [37]. Modified Fe 3 O 4 nanoparticles were used by Meiling et al. for the detection of heavy metal ions in water and successful detection of Cu 2+ , Cd 2 , Zn 2+ and Hg 2+ were carried out in an aqueous solution and applied further for purification of water [38]. The structural and magnetic properties of ZnxFe 3 −xO 4 nano hollow spheres were also investigated by Priyanka Saha et al., in which Zn was doped in magnetic nano hollow spheres (NHS) and tested successfully for biomedical applications [39]. Mahmood Iram et al. synthesized Fe 3 O 4 (NHS) through the hydrothermal method, which could then be employed for successful adsorption of Natural Red Dye [26]. Xiang Wang et al. reported the magnetic nanocomposites of Triethylenetetramine-modified Fe 3 O 4 /SiO 2 /CS-TETA for adsorption of Cr (VI). These researchers observed an adsorption capacity for Cr (VI) ions as high as 254.6 mg g −1 along with remarkable adsorption equilibrium times as less as 15 min [40]. However, magnetite has never been tested on TNT wastewater adsorption, to the best of our knowledge so far.
The novelty of the present work is the synthesis of the α-Fe 2 O 3 (hematite, rhombohedral crystal structure, R 3 c) and Fe 3 O 4 (magnetite, face-centered cubic crystal (FCC) structure, Fm3 m) through a hydrothermal, template-free method, using the same precursors while exploring modifications in temperature and calcination time. The economic factor in this work is the reuse of the applied catalyst (Fe 3 O 4 NHS), after recovering it by simply altering the pH of the solution and using its magnetic properties. Fe 3 O 4 (NHS) was employed as an adsorbent to investigate the adsorption efficacy and reduction in COD values of TNT wastewater. The mechanism for the removal of nitro bodies, sulfates, and other derivatives of nitro benzene present in the sample proceeds via an ion-exchange mechanism among positively-charged TNT water molecules and OH-at the Fe 3 O 4 catalyst surface. The fine control over phase purity and crystallinity of the iron oxides during synthesis are the major challenges with the method employed. However, our method for synthesizing iron oxides in an aqueous solution does have the advantage of reduced cost, being environmentally friendly and associated minimum chemical waste and energy consumption.

Materials
All chemical products were purchased from Sigma Aldrich, Germany supplied by Pro-Marketing Company Islamabad, Pakistan. All the precursors used in this work were of analytical grade with high purity (≥98%) including cetyl trimethyl ammonium bromide [ 6 ], 3.5 g of (NH 4 ) 2 S 2 O 8 , and 0.05 g of 0.02 M NaH 2 PO 4 solution were dissolved in 250 mL de-ionized water. The solution was magnetically stirred at room temperature in the presence of nitrogen gas until the solution turned visibly yellow in color. The solution was then poured into a 250 mL Teflon cup, sealed properly in an autoclave, and kept for calcination in an oven for 2 h, 6 h, and 8 h at a temperature of 120 • C, 160 • C and 180 • C, respectively. The impact of these process parameters on the shape and morphologies of the catalyst was determined. After the completion of hydrothermal treatment, the solution was centrifuged at 4000 rpm for 30 min and washed three times with de-ionized water and ethanol systematically. The dark brown solution was filtered and then dried in a vacuum oven overnight at 45 • C ( Figure 1).

Adsorbent
For the synthesis of Fe3O4 (NHS), 0.05 g of CTAB, 1.25 g of [K3 Fe (CN)6], 3.5 g of (NH4)2S2O8, and 0.05 g of 0.02 M NaH2PO4 solution were dissolved in 250 mL de-ionized water. The solution was magnetically stirred at room temperature in the presence of nitrogen gas until the solution turned visibly yellow in color. The solution was then poured into a 250 mL Teflon cup, sealed properly in an autoclave, and kept for calcination in an oven for 2 h, 6 h, and 8 h at a temperature of 120 C, 160 C and 180 C, respectively. The impact of these process parameters on the shape and morphologies of the catalyst was determined. After the completion of hydrothermal treatment, the solution was centrifuged at 4000 rpm for 30 min and washed three times with de-ionized water and ethanol systematically. The dark brown solution was filtered and then dried in a vacuum oven overnight at 45 °C ( Figure 1).

Scanning Electron Microscopy (SEM)
Surface morphologies of the as-obtained products after varying time and temperature conditions were studied using Scanning Electron Microscopy (Modal JSM 6490LA, JEOL, Tokyo, Japan) at 20 kV.

X-ray Diffraction (XRD)
Structural analysis of the samples was performed with an X-ray diffractometer (Model: X' TRA48 Thermo ARL, Tokyo, Japan using Cu Kα radiation (k ¼ 0.15406 nm), operating at 40 mA and 45 kV. The radial scans were performed in reflection scanning mode with 2θ values ranging from 5 to 80 and at a scanning rate of 1 min −1 . The patterns were evaluated, carefully examined, and reconfirmed with the records from the International Centre for Diffraction Data (ICDD) to verify the identity of the products.

Brunner-Emmet-Teller (BET)
BET adsorption was performed using a Surface Area and Porosity Analyzer (Model: Micromeritics Gemini VII, Norcross, GA, USA) for analyzing porosity and surface area of the synthesized Fe3O4 NHS.

Fourier Transform Infrared Spectroscopy (FTIR)
The functional groups in the synthesized samples were investigated through Fourier Transform Infrared Spectroscopy (FTIR, Model Nicolet 6700, Thermo Scientific, Waltham, MA, USA). Samples were shaped into pellets interspersed with KBr powder, and the respective spectra were obtained using attenuated total reflectance mode in the range of

Scanning Electron Microscopy (SEM)
Surface morphologies of the as-obtained products after varying time and temperature conditions were studied using Scanning Electron Microscopy (Modal JSM 6490LA, JEOL, Tokyo, Japan) at 20 kV.

X-Ray Diffraction (XRD)
Structural analysis of the samples was performed with an X-ray diffractometer (Model: X' TRA48 Thermo ARL, Tokyo, Japan using Cu Kα radiation (k 1 4 0.15406 nm), operating at 40 mA and 45 kV. The radial scans were performed in reflection scanning mode with 2θ values ranging from 5 to 80 and at a scanning rate of 1 min −1 . The patterns were evaluated, carefully examined, and reconfirmed with the records from the International Centre for Diffraction Data (ICDD) to verify the identity of the products.

Brunner-Emmet-Teller (BET)
BET adsorption was performed using a Surface Area and Porosity Analyzer (Model: Micromeritics Gemini VII, Norcross, GA, USA) for analyzing porosity and surface area of the synthesized Fe 3 O 4 NHS.

Fourier Transform Infrared Spectroscopy (FTIR)
The functional groups in the synthesized samples were investigated through Fourier Transform Infrared Spectroscopy (FTIR, Model Nicolet 6700, Thermo Scientific, Waltham, MA, USA). Samples were shaped into pellets interspersed with KBr powder, and the respective spectra were obtained using attenuated total reflectance mode in the range of 4000 to 400 cm −1 with a resolution of 6 cm −1 . An average of 32 scans are reported for each sample. coded as B, T, E and M, respectively) were prepared for analysis. The performance of (Fe 3 O 4 ) NHS was investigated by varying concentration, weight of the adsorbent applied, contact time and temperature.

Chemical Oxygen Demand (COD) Determination
The COD of the TNT wastewater samples was determined by the standard method (Merck Method) [41]. For this, 50 mL of the sample was placed in a 500 mL conical flask with 50 mL distilled water, 25 mL Potassium Dichromate solution, 1.0 g silver sulfate and 2.0 g of mercury (II) sulfate. Approximately 75 mL of concentrated sulfuric acid was added dropwise under continuous stirring. The mixture was boiled over the sand bath for 2 h under reflux and then cooled for 30 min. The obtained mixture was then treated with 0.25 M ammonium iron (II) sulfate solution until the color changed from bluish-green to reddish-brown. Under the same conditions the blank sample was also determined using 50 mL distilled water instead of the TNT red water. COD values were calculated using the following formula.
Here, A is the mL ammonium iron (II) sulfate solution titrated with blank (solvent), B is the mL ammonium iron (II) sulfate solution titrated with the sample (TNT wastewater), C is the molarity of ammonium iron (II) sulfate solution, f is the titer molarities (1 M) (from MERCK table), and D is the mL effluent sample (TNT wastewater) used.
The adsorption capacity (Qe) and efficiency (η) of Fe 3 O 4 NHS were determined by the following formulae.
Here, (Qe) and (η) are the adsorption capacity and efficiency of the Fe 3 O 4 NHS respectively, (COD)i is the initial COD of TNT wastewater, (COD)e is the value of COD at equilibrium, V is the volume of TNT wastewater used and W is the weight of Fe 3 O 4 NHS applied.

Scanning Electron Microscopy (SEM)
The shape and morphology of the synthesized Fe 3 O 4 NHS were investigated via SEM ( Figure 2). A compact morphology with an average size varying from 11 to 112 nm formed after 2 h of calcination at 120 • C in an autoclave ( Figure 2a). On the other hand, for our next case as the reaction progressed for 6 h at 160 • C, pores formed on the surface of the nanoparticles, increasing the surface area-to-volume ratio, shown in (Figure 2b). Calcination treatment of 8 h at 180 • C resulted in the formation of an intense porous outer surface as shown in (Figure 2c). The porous architecture of the Fe 3 O 4 NHS enhances surface adsorption properties due to the large available surface area, thus making these NHS effective dsorbent for our targeted adsorption [30,42].
With this template-free synthesis, we concluded that the pore size, shape, volume, and surface morphologies of Fe 3 O 4 NHS depend on the calcination time and temperature. In this work, our results show the ideal temperature and calcination time in an autoclave were (180 • C and 8 h). The concentration of Na 2 H 2 PO 4 employed here plays an important role in the synthesis of the cavities inside the Fe 3 O 4 NHS. This choice was motivated by prior studies indicating that with higher concentrations of Na 2 H 2 PO 4 , the acidity of the solution increases, leading to an uncontrolled rate of ionization that may destroy the magnetite structure [32]. The ionization of K 3 Fe (CN) 6 leads to the formation of hollow spheres of Fe 3 O 4 upon addition of Na 2 H 2 PO 4 , which brings high surface energies and associated high stability to the product [21]. Ostwald ripening is the proposed mechanism for the synthesis of these Fe 3 O 4 NHS through the one-pot, template-free, hydrothermal method in an aqueous solution [43]. Here, nucleation and growth dominate, with large particles growing larger due to the instability of the high surface-to-volume ratio associated with small particles. Various factors affecting the Ostwald ripening process include particle size, solubility, surface energy and dissolution [44]. In this work, we employ CTAB as a dispersant to maximize the yield of Fe 3 O 4 NHS. With this template-free synthesis, we concluded that the pore size, shape, volume, and surface morphologies of Fe3O4 NHS depend on the calcination time and temperature. In this work, our results show the ideal temperature and calcination time in an autoclave were (180 °C and 8 h). The concentration of Na2H2PO4 employed here plays an important role in the synthesis of the cavities inside the Fe3O4 NHS. This choice was motivated by prior studies indicating that with higher concentrations of Na2H2PO4, the acidity of the solution increases, leading to an uncontrolled rate of ionization that may destroy the magnetite structure [32]. The ionization of K3Fe (CN)6 leads to the formation of hollow spheres of Fe3O4 upon addition of Na2H2PO4, which brings high surface energies and associated high stability to the product [21]. Ostwald ripening is the proposed mechanism for the synthesis of these Fe3O4 NHS through the one-pot, template-free, hydrothermal method in an aqueous solution [43]. Here, nucleation and growth dominate, with large particles growing larger due to the instability of the high surface-to-volume ratio associated with small particles. Various factors affecting the Ostwald ripening process include particle size, solubility, surface energy and dissolution [44]. In this work, we employ CTAB as a dispersant to maximize the yield of Fe3O4 NHS.

X-ray Diffraction Spectroscopy (XRD)
XRD analyses were carried out to confirm the phase purity of the as-obtained samples and identify the iron oxide from the various phases possible. For this purpose, reaction conditions and calcination times were optimized as discussed earlier, providing both desired α-Fe2O3 (hematite, rhombohedral crystal structure, R3 c) and Fe3O4 (magnetite, face-centered cubic (FCC) crystal structure, Fm3 m) products as shown in Figures 3 and  4. The XRD data obtained were checked against standardized records from the International Centre for Diffraction Data (ICDD) to verify the identity of the products.

X-Ray Diffraction Spectroscopy (XRD)
XRD analyses were carried out to confirm the phase purity of the as-obtained samples and identify the iron oxide from the various phases possible. For this purpose, reaction conditions and calcination times were optimized as discussed earlier, providing both desired α-Fe 2 O 3 (hematite, rhombohedral crystal structure, R3 c) and Fe 3 O 4 (magnetite, face-centered cubic (FCC) crystal structure, Fm3 m) products as shown in Figures 3 and 4. The XRD data obtained were checked against standardized records from the International Centre for Diffraction Data (ICDD) to verify the identity of the products.

Brunner-Emmett-Teller (BET) Adsorption Method
The adsorption behavior of Fe3O4 NHS over nitrogen gas was measured, as shown in Figure 5 to determine the efficacy of this adsorbent for TNT wastewater treatment. The BET graph shows the relationship between the adsorption of N2 gas (1/[X (P 0 /P)−1]) and the relative pressure (P/P 0 ) applied, showing a positive linear behavior which indicates an improved rate of adsorption with increasing relative pressure. The results we obtained from the BET tests are encouraging and thus provide us with the required data for a Fe3O4 NHS to be applied on TNT wastewater as an adsorbent tool [45,46].

Brunner-Emmett-Teller (BET) Adsorption Method
The adsorption behavior of Fe 3 O 4 NHS over nitrogen gas was measured, as shown in Figure 5 to determine the efficacy of this adsorbent for TNT wastewater treatment. The BET graph shows the relationship between the adsorption of N 2 gas (1/[X (P 0 /P)−1]) and the relative pressure (P/P 0 ) applied, showing a positive linear behavior which indicates an improved rate of adsorption with increasing relative pressure. The results we obtained from the BET tests are encouraging and thus provide us with the required data for a Fe 3 O 4 NHS to be applied on TNT wastewater as an adsorbent tool [45,46]. The surface area and pore size of the Fe3O4 NHS were greater than that of α-Fe2O3. Fe3O4 NHS produced under the conditions (8 h at 180 C) had a BET surface area of 66.057 m 2 /g, with a calculated Langmuir surface area of 650.288 m 2 /g, and a cumulative surface area of 144.096 m 2 /g. The average pore volume calculated is 0.225 cm 3 /g and the pore size is 136.429 A°. The tabulate data for BET isotherm and BET surface area are shown in Tables 1 and 2, respectively. Table 1. BET isotherm tabular report for N2 adsorption over the surface of Fe3O4 NHS at evacuation rate of 1000.0 mmHg/min and saturation pressure of 760.0 (mmHg).  The surface area of the adsorbent obtained here is due to the face-centered cubic (FCC) interstitial spaces between the adjacent Fe 3 O 4 NHS. However, fine control over the size and pore volume of Fe 3 O 4 NHS thus remains a challenge using this synthetic route. The rate of ionization of [Fe (CN)6−3] upon addition of [H 2 PO 4 −] plays an important role in managing the overall acidity of the ongoing reaction. Ultimately, the hollow porous architectures formed here consist of both micro and nano-sized spheres. Despite these variations in particle size, the mesoporous architecture leads to enhance photocatalytic activity and effective adsorption of organic species from TNT wastewater and straightforward recovery via a simple magnetic separation method.

Fourier Transform Infrared Spectroscopy (FTIR)
FTIR characterization of the Fe 3 O 4 NHS before and after its interaction with TNT wastewater is reported in Figure 6. The stretching and bending vibrations of Fe 3 O 4 NHS closely resemble the standard spectrum reported in prior works [20,34]. The adsorption peak present at 581 cm −1 refers to the characteristic peak of (Fe-O). Hydroxyl group (O-H) bending and stretching vibrations are observed at 1627 cm −1 and 3417 cm −1 , respectively, in the neat Fe 3 O 4 NHS sample spectra before adsorption. FTIR spectra were also collected after applying the Fe 3 O 4 NHS to the TNT wastewater sample (Figure 7, black spectra). The result clearly shows characteristic peaks of some nitro and sulfate groups which were adsorbed to the surface of the Fe 3 O 4 NHS Fe 3 O 4 NHS. Specifically, peaks at 1548 cm −1 and 1370 cm −1 are attributed to the asymmetric and symmetric vibrations of the nitro groups adsorbed by the Fe 3 O 4 NHS [47]. Similarly, the asymmetric and symmetric stretching of sulfonates groups are observed at 1221 cm −1 and 1046 cm −1 , respectively [48]. Additionally, stretching vibration of (C-N) bond at 840 cm −1 and scissoring vibration of nitro groups at 735 cm −1 are observed. Furthermore, the bending vibration of (C-N = O) group is also observed at 630 cm −1 [21]. With the above observations, it is confirmed that the Fe 3 O 4 NHS applied on TNT wastewater has adsorbed the derivatives of nitrobenzene including 2,4-DNT-3-SO-3 and 2,4-DNT-5-SO-3 from the TNT wastewater and thus decreased the COD values, as per our expectations.
stretching of sulfonates groups are observed at 1221 cm −1 and 1046 cm −1 , respectively [48]. Additionally, stretching vibration of (C-N) bond at 840 cm −1 and scissoring vibration of nitro groups at 735 cm −1 are observed. Furthermore, the bending vibration of (C-N = O) group is also observed at 630 cm −1 [21]. With the above observations, it is confirmed that the Fe3O4 NHS applied on TNT wastewater has adsorbed the derivatives of nitrobenzene including 2, 4-DNT-3-SO-3 and 2, 4-DNT-5-SO-3 from the TNT wastewater and thus decreased the COD values, as per our expectations. Figure 6. FTIR spectra of Fe3O4 NHS before and after its application on TNT wastewater for required adsorption.

Ultraviolet/Visible Spectrophotometer (UV-Vis)
UV/Visible absorbance for the TNT solutions shows the trend of increase in absorbance with increasing TNT concentration in different solvents (benzene, toluene, ethanol, and methanol coded as B, T, E and M, respectively) as shown in Figure 7a. This illustrates how the quantity of dissolved TNT increases the UV/visible absorbance of the sample, following the Beer-Lambert law [49]. The performance of the synthesized Fe3O4 NHS in terms of decreasing UV/visible absorbance is shown in Figure 7b. Here, the gram amount of the adsorbent added to different TNT solutions was varied. Continuous, magnetic stirring (350 rpm) was employed to expose the high surface area of the Fe3O4 NHS to TNT molecules present in solution and thus facilitate their adsorption at room temperature (25 °C).
Within the limits associated with the volatilities and temperature limits of the organic solvents used here, the TNT solutions were exposed to heat to determine the influence of temperature on UV/visible absorbance (Figure 8a). It was observed that with increasing temperature, the kinetic energy also increases, maximizing the accommodation of nitro bodies over the surface area of the adsorbent which leads to a decrease in UV/visible absorbance (Figure 8a).

Ultraviolet/Visible Spectrophotometer (UV-Vis)
UV/Visible absorbance for the TNT solutions shows the trend of increase in absorbance with increasing TNT concentration in different solvents (benzene, toluene, ethanol, and methanol coded as B, T, E and M, respectively) as shown in Figure 7a. This illustrates how the quantity of dissolved TNT increases the UV/visible absorbance of the sample, following the Beer-Lambert law [49]. The performance of the synthesized Fe 3 O 4 NHS in terms of decreasing UV/visible absorbance is shown in Figure 7b. Here, the gram amount of the adsorbent added to different TNT solutions was varied. Continuous, magnetic stirring (350 rpm) was employed to expose the high surface area of the Fe 3 O 4 NHS to TNT molecules present in solution and thus facilitate their adsorption at room temperature (25 • C).
Within the limits associated with the volatilities and temperature limits of the organic solvents used here, the TNT solutions were exposed to heat to determine the influence of temperature on UV/visible absorbance (Figure 8a). It was observed that with increasing temperature, the kinetic energy also increases, maximizing the accommodation of nitro bodies over the surface area of the adsorbent which leads to a decrease in UV/visible absorbance (Figure 8a). ring (350 rpm) was employed to expose the high surface area of the Fe3O4 NHS to TNT molecules present in solution and thus facilitate their adsorption at room temperature (25 °C).
Within the limits associated with the volatilities and temperature limits of the organic solvents used here, the TNT solutions were exposed to heat to determine the influence of temperature on UV/visible absorbance (Figure 8a). It was observed that with increasing temperature, the kinetic energy also increases, maximizing the accommodation of nitro bodies over the surface area of the adsorbent which leads to a decrease in UV/visible absorbance (Figure 8a).  Contact time between the adsorbate and adsorbent is a vital part in determining the overall efficiency of the adsorbent applied for adsorption in any industrial process. This important process parameter was also measured for Fe 3 O 4 NHS synthesized here, by increasing contact time up to 2 h. while maintaining a constant amount of adsorbent (1.0 g). The influence of contact time on UV/Visible data is shown in (Figure 8b). Increasing the contact time between the Fe 3 O 4 NHS and TNT solution ultimately provided the adsorbent enough time to impregnate its surface and active sites with adsorbate molecules and thus results in optimized adsorption.

Chemical Oxygen Demand (COD)
A COD test was carried out using the MERCK method for TNT wastewater samples provided by Pakistan Ordnance Factories (POFs) Wah Cantt, Pakistan [41]. To begin, 1.0 g of Fe 3 O 4 NHS was added to 50 mL of the TNT wastewater samples for 2 h. The initial COD of this sample was calculated as 600 mg/L using (Equation (1)). A linear decline in COD values was observed with increasing amount of Fe 3 O 4 NHS, until 4.0g of the adsorbent was consumed in TNT wastewater. With subsequent addition of adsorbent (5.0 g and beyond), the COD values plateaued at approximately 80 % decrease from initial COD values (Figure 9). This behavior revealed the amount of Fe 3 O 4 NHS necessary for removing hazardous chemicals from the water stream to balance extraction efficiency and cost effectiveness. The schematic representation of the decrease of the COD values and change in color upon addition of Fe 3 O 4 NHS is shown in (Figure 10).

Effect of pH and Initial Adsorbate Concentration
The pH and concentration of the adsorbate (TNT wastewater) are the two important rate controlling parameters in calculating the overall adsorption efficiency of the applied adsorbent. To determine the optimum values of these parameters, a series of experiments were carried out varying pH and concentration values of the TNT wastewater, as shown in Figure 11a,b, respectively. It was observed that the applied Fe 3 O 4 NHS works best at pH range 6-7 and no significant change was noted beyond pH 6.5. In a relatively more acidic environment, an excess of H+ ions compete with positive cations offered by the adsorbate, and thus decreases the adsorption of TNT wastewater. Figure 11a shows the extent of adsorption is minimum at pH 4 and increases with pH of the adsorbate until it reaches its maximum value of adsorption at pH 6.5, where the adsorbent performs well. g of Fe3O4 NHS was added to 50 mL of the TNT wastewater samples for 2 hrs COD of this sample was calculated as 600 mg/L using (Equation (1)). A linear COD values was observed with increasing amount of Fe3O4 NHS, until 4.0g of bent was consumed in TNT wastewater. With subsequent addition of adsorben beyond), the COD values plateaued at approximately 80 % decrease from i values (Figure 9). This behavior revealed the amount of Fe3O4 NHS nec removing hazardous chemicals from the water stream to balance extraction eff cost effectiveness. The schematic representation of the decrease of the COD change in color upon addition of Fe3O4 NHS is shown in (Figure 10).   provided by Pakistan Ordnance Factories (POFs) Wah Cantt, Pakistan [41]. To begin, 1 g of Fe3O4 NHS was added to 50 mL of the TNT wastewater samples for 2 hrs. The init COD of this sample was calculated as 600 mg/L using (Equation (1)). A linear decline COD values was observed with increasing amount of Fe3O4 NHS, until 4.0g of the adso bent was consumed in TNT wastewater. With subsequent addition of adsorbent (5.0 g a beyond), the COD values plateaued at approximately 80 % decrease from initial CO values (Figure 9). This behavior revealed the amount of Fe3O4 NHS necessary f removing hazardous chemicals from the water stream to balance extraction efficiency a cost effectiveness. The schematic representation of the decrease of the COD values a change in color upon addition of Fe3O4 NHS is shown in (Figure 10).

Adsorption Behavior
The overall adsorption capacity (Qe) of Fe 3 O 4 NHS increased from 38 mg/g to 70 mg/g, when increasing the Fe 3 O 4 NHS dose from 1.0 g to 3.0 g respectively, as shown in Figure 12. The initial adsorption of adsorbate is much faster, indicating that the adsorption rate increases with an increasing adsorbent dose until it reaches its optimum value of 70 mg/g, where equilibrium is established and a descending trend in adsorption is observed. This decrease in adsorption capacity is due to the occupied active sites over the surface of the Fe 3 O 4 NHS. In contrast, the COD values of TNT wastewater decreased up to 92% in a gradual and steady fashion. Apart from the active sites' chemistry, there exist other factors contributing to and facilitating this adsorption process. The inner and outer surface of the Fe 3 O 4 NHS accommodates a large number of hydroxide (OH−) groups, creating a negative charge on its surface and increasing electrostatic forces, resulting in an increase in adsorption capacity of the adsorbent. It is also concluded that the negatively charged adsorbent (Fe 3 O 4 ) possessed weakly attractive Van der Waals forces with TNT wastewater (positively charged), which is proposed as one of the dominant adsorption mechanisms. For calculating the adsorption efficiency (η) using (Equation (3)), Fe 3 O 4 NHS was regained after changing the pH of the adsorbate solution and reapplying the adsorbent to the TNT wastewater. A decrease in (η) of Fe 3 O 4 NHS after each cycle is shown in Figure 13. This decrease in adsorption efficiency (η) of Fe 3 O 4 NHS was 15.12% after the 5th adsorption cycle.

Effect of pH and initial adsorbate concentration
The pH and concentration of the adsorbate (TNT wastewater) are the two important rate controlling parameters in calculating the overall adsorption efficiency of the applied adsorbent. To determine the optimum values of these parameters, a series of experiments were carried out varying pH and concentration values of the TNT wastewater, as shown in Figure 11a,b, respectively. It was observed that the applied Fe3O4 NHS works best at pH range 6-7 and no significant change was noted beyond pH 6.5. In a relatively more acidic environment, an excess of H+ ions compete with positive cations offered by the adsorbate, and thus decreases the adsorption of TNT wastewater. Figure 11a shows the extent of adsorption is minimum at pH 4 and increases with pH of the adsorbate until it reaches its maximum value of adsorption at pH 6.5, where the adsorbent performs well. Figure 11b shows the effect of the initial concentration of TNT wastewater against the constant weight of Fe3O4 NHS (1.0 g). At high concentrations of TNT wastewater, the rate of adsorption increases as the available active sites of Fe3O4 NHS are surrounded by the adsorbate cations due to the electrostatic interactions.

Adsorption Behavior
The overall adsorption capacity (Qe) of Fe3O4 NHS increased from 38 mg/g to 70 mg/g, when increasing the Fe3O4 NHS dose from 1.0 g to 3.0 g respectively, as shown in Figure 12. The initial adsorption of adsorbate is much faster, indicating that the adsorption rate increases with an increasing adsorbent dose until it reaches its optimum value of 70 mg/g, where equilibrium is established and a descending trend in adsorption is observed. This decrease in adsorption capacity is due to the occupied active sites over the surface of the Fe3O4 NHS. In contrast, the COD values of TNT wastewater decreased up to 92% in a gradual and steady fashion. Apart from the active sites' chemistry, there exist other factors contributing to and facilitating this adsorption process. The inner and outer surface of the Fe3O4 NHS accommodates a large number of hydroxide (OH−) groups, creating a negative charge on its surface and increasing electrostatic forces, resulting in an increase in adsorption capacity of the adsorbent. It is also concluded that the negatively charged adsorbent (Fe3O4) possessed weakly attractive Van der Waals forces with TNT wastewater (positively charged), which is proposed as one of the dominant adsorption mechanisms. For calculating the adsorption efficiency (η) using (Equation (3)), Fe3O4 NHS was regained after changing the pH of the adsorbate solution and reapplying the adsorbent to the TNT wastewater. A decrease in (η) of Fe3O4 NHS after each cycle is shown in Figure 13. This decrease in adsorption efficiency (η) of Fe3O4 NHS was 15.12% after the 5th adsorption cycle.

Conclusions
This research work demonstrates a one-pot, hydrothermal, template-free method for the successful synthesis of α-Fe 2 O 3 (hematite) and Fe 3 O 4 (magnetite). Fe 3 O 4 NHS were further used for the treatment of TNT wastewater. The variation in shape, size, morphology, porosity, and surface area was observed upon variation in temperature and time of the calcination in an autoclave. The increased surface area and high porosity associated with the Fe 3 O 4 NHS yielded impressive results with regards to adsorption of TNT and other associated species with TNT wastewater. UV/visible spectroscopy results have confirmed the quick adsorption action of Fe 3 O 4 NHS. The adsorbent Fe 3 O 4 NHS was also tested as a function of contact time, dose, and temperature. In an industry where adsorption of different hazardous nitro-bodies like in TNT effluents is required, these magnetic NHS could have a significant impact. The synthesized Fe 3 O 4 NHS effectively adsorbed nitrobodies from the provided TNT effluent sample and decreased its COD values by 92 %, providing a safe environment for living and marine life in the aqueous environment. Better adsorption and recyclability in a shorter time-period gives NHS the benefits of increased efficiency and makes it a more economical option. The template-free hydrothermal synthesis, practical scale up options, and ease at which it can be employed, gives the advantage of applicability on an industrial scale. Considering all these advantages, this process is recommended for treating any industrial effluents generated from the production of cyclonite, cyclotetramethylene-tetranitramine (HMX), nitroglycerin (NG) production plants and other various environmental pollutants/species, which are hazardous for our environment and marine life. Data Availability Statement: All the data will be available to the readers.