Adsorptive Removal of Arsenic by Synthetic Iron-loaded Goethite: Isotherms, Kinetics, and Mechanism

9 Arsenic contamination in the groundwater is a worldwide concern. Therefore, this study was designed to use synthetic 10 iron-loaded goethite to remove arsenic. Adsorption was significantly pH-dependent; hence, pH values between 5.0 11 and 7.0 resulted in the highest removal of arsenate and arsenite. Langmuir and Freundlich isotherms were almost 12 perfectly matched in terms of strong positive coefficient of determinati on “ R 2 ” arsenate – 0.941 and 0.992 and arsenite 13 – 0.945 and 0.993. The adsorption intensity “ n ” resulted as arsenate – 2.542 and arsenite – 2.707; besides separation 14 factor “ R L ” found as arsenate – 0.1 and arsenite – 0.5, respectively. However, both “n” and “R L ” leads to a favourable 15 adsorption process. Temkin isotherm yielded in equal binding energies “b t ” showing as 0.004 (J/μg) for both arsenate 16 and arsenite. Jovanovic monolayers isotherm was dominated by the Langmuir isotherm. This resulting in maximum 17 adsorption capacity “ Q max ” of arsenate – 1369.877 and arsenite – 1276.742 (μg/g) , which approaches to the saturated 18 binding sites. Kinetic data revealed that adsorption equilibrium was achieved in 240 – arsenate and 360 – arsenite 19 (minutes), respectively. Chemisorption was found effective with high “ R 2 ” values 0.981 – arsenate and 0.994 – 20 arsenite, respectively, with the best fitting of pseudo-second order. Moreover, Brunauer Emmett Teller (BET), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) 22 were used to determine the morphological content, surface area, crystalline structure, and chemical characteristics of 23 the adsorbent. It is anticipated that optimal arsenic removal was achieved by the porosity, chemical bindings, and surface binding sites of the adsorbent.


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Arsenic is a well-known carcinogenic agent found in water bodies worldwide, which may cause severe human health

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Moreover, arsenic oxidation states -3, 0, +3, and +5 often detected in the groundwater are typically organic 51 and inorganic speciation forms (Pokhrel and Viraraghavan 2006). Arsenate and arsenite are inorganic groundwater 52 pollutants, and both are highly pH and redox-dependent. (Singh et al. 2015). However, pH plays a key role; between 53 pH 3 and 9, arsenate species exist as H2AsO −4 and HAsO2 −4 , whereas arsenite exists as H3AsO3 in a neutral state

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Treatment of arsenic-contaminated groundwater is extremely important in order to provide safe drinking water.

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However, the available range of arsenic removal techniques such as adsorption, biological treatment, precipitation,

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from the contaminated groundwater. Thus, the objective of this research study was to use synthesised iron-loaded The material synthesis was carried out as follows: 120 ml (12.5% wt ammonia solution was added to 250 ml clean 84 water constant volume with concentrated ammonia water (mass fraction 25-28%). Then, 250 ml of 0.9 mol/L 85 FeSO47H2O solution was made; about 300 ml of sterile water was heated for several minutes and chilled to 86 deoxygenate, weighing 62.5523 g of (FeSO47H2O) that was dissolved in clean deoxygenated water to a volume of 87 250 ml. Then, 250 mL (0.9) mol/L (FeSO4H2O) solution was added to the 500 mL beaker; shaking was used to convert  10, 20, 30, 60, 120, 240, 360, 480, 960, and 1440) minutes, the impact of contact time was seen. Furthermore, the pH 99 impact was determined at pH values of 3, 5, 7, 9, and 11, which were changed with HCl and NaOH using a pH metre.

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Finally, suspensions were centrifuged and filtered through 0.45 mm filter to determine adsorption rate using Atomic 101 Fluorescence Spectrometry (AFS).

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The amount of material adsorbed (Qe) was determined using (Equation 1), and the adsorption efficiency was The equation parameters (Co and Ce) g/L denote the starting and equilibrium concentrations, respectively; hence, V 107 (L) and W (g) denote the volume of solution and adsorbent weight, respectively. Langmuir, Freundlich, Temkin, and Pseudo-second-order models yielded the kinetic parameters. Nonlinear regression was used to understand the isotherm 110 and kinetic models, respectively.

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Material morphology and microstructure were determined using scanning electron microscopy (HITACHI SU8010).

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In order to identify, the crystalline composition, surface area, chemical properties and functional groups of the 114 adsorbent material were determined using X-ray Diffraction analyser -Bruke D8 Advance (XRD), Brunauer Emmet

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The spectrum data from iron-loaded goethite provide evidence of the formation of inner-sphere complexes,   150 Q e = Q max . K L . C e 1 + K L . C e (3) This model explains the adsorption equilibrium to arsenate and arsenite (Figure 4-

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The adsorption capacity is represented by the Freundlich constant "KF" (μg/g), which is associated with temperature 164 and physical and chemical properties. Thus, the exponent "n" denotes a change in the adsorption intensity; also, the 165 value of "n" indicates whether a favourablen > 1 or unfavourablen < 1, adsorption process (Pham et al. 2020).

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( Table 1) and graphical depiction of (Figure 4-b, c) shows the isotherm parameters. The favourable adsorption has 167 been observed and indicates significant positive coefficients of determination "R 2 " 0.942 for arsenate and 0.957 for 168 arsenite. Besides, up to the mark adsorption intensity, "n" was also observed as 2.542 of arsenate and 2.707 of arsenite, 169 respectively.

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Whereas results revealed that this isotherm is not approaching the maximum saturation sites (Figure 4-b, c). Besides,

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the Langmuir isotherm resulted in high "Qmax" values, showing strong adsorption by approaching the maximum 182 binding sites of adsorbents.

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Kinetic study

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Adsorption kinetic provides information about the remove mechanism, pathways, and the rate of adsorption (Qin et

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The kinetic model helps in understanding the adsorption process, the determination of contact time for

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Whereas Qt and Qe denote the equilibrium time (t-minutes) and adsorption capacity (μg/g), respectively. While K1

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Furthermore, adsorption was strongly followed pseudo-second-order. This resulted in the high coefficient of 201 determination "R 2 " of arsenate and arsenite and showed most of the adsorption was achieved by the chemosorption.

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Factors affecting and the state of adsorption

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The batch experiment was performed in the pH range of 3 to 11; however, pH (5 and 7) resulted in maximum removal 204 of arsenate and arsenite. This study shows that increasing the pH from acidic to neutral results in efficient adsorption.

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Whereas the basic pH was found ineffective, this might occur due to the change in the contaminant's structure and 206 surface charge of the adsorbent. Alam reported that the lower pH is more favourable for the adsorption of anionic 207 speciation forms than higher pH due to more H + ions at lower pH and -OH ions at higher pH (Alam et al. 2018