Study of As(III) and As(V) oxoanion adsorption onto single and mixed ferrite and hausmannite nanomaterials
Introduction
Arsenic is an element that is ubiquitous throughout the world: found in the earth's crust, in both surface and groundwater, and within in the human body [1], [2]. The toxic effects of arsenic in humans come from the ingestion of arsenic contaminated food and water. However, in general the inorganic compounds of are arsenic more toxic than the organic arsenicals and are common contaminates in drinking water [1]. The As(III) (arsenite) compounds are much more toxic than the As(V) (arsenate) compounds [2]. Arsenic has been linked to variety of health effects when ingested in small consistent dosages through combined food or drinking water [1], [2]. The effects of As include abnormal skin conditions, gastrointestinal problems, neurological effects, and diabetes [1], [2], [3], [4], [5]. Furthermore, links between arsenic exposure and several types of cancer have been established, which includes: lung, skin, kidney, liver, and prostate [4]. Due to the numerous health risks the Environmental Protection Agency has set the MCL of arsenic in drinking water from 0.050 to 0.010 ppm in an effort to reduce number the health effects caused by the long-term ingestion of arsenic in the US population [2], [6].
There are several methods to remove arsenic from drinking water, which include precipitation, ion exchange, membrane process, coagulation, and adsorption [7], [8], [9], [10], [11], [12]. In general technologies to remove arsenic from drinking water are generally non-specific and expensive to water treatment plants. However, nano-adsorbents may provide a more cost effective technology for the removal of As(III) and As(V) from contaminated water [10], [11], [13], [14], [15], [16], [17]. Nanomaterials are a promising emerging technology with many different applications due to their enhanced reactivity and high surface area to volume ratio. Adsorbents have been studied for the remediation/removal of many different ions from aqueous solution. More recently, nanomaterials have been investigated for the removal of inorganic contaminates from aqueous solution, including the inorganic forms of arsenic. Adsorbents such as activated alumina, clay based materials, red mud (the waste from aluminum processing), Al-WTR (water treatment residuals) Fe-WTR, iron oxide materials, manganese oxide nanomaterials, granular ferric oxide, as well as metal sulfide nanomaterials [14], [15], [16], [17], [18], [19].
Studies investigating the adsorption of As(III) and As(V) using activated alumina have shown the effect of pH, surface oxidation, and competing ions [6]. It has been shown that between pH 7 and 8 activated alumina has a net positive charge, which showed a preference for the adsorption of anions from solution including arsenic. Acidic pHs are generally considered optimum for arsenic removal with activated alumina. Genc-Fuhrman et al. found arsenic adsorption using activated red mud was effective for As(V) adsorption. The optimum pH for As(V) adsorption was 4.5 with a removal of approximately 100%. In addition, the desorption of As(V) was found to be optimum pH 11.6 with a maximum desorption of 40%. In contrast, the optimum pH for As(III) binding was found to be 8.5, and the removal efficiency was dependent on the initial As(III) concentration [20]. In a similar study Altundogan et al. also investigated the application of activated red mud on arsenic removal [21]. Altundogan et al. showed the optimum binding pH range for As(III) was from 5.8 to .5 and the optimum pH range for As(V) binding was from 1.8 to 3.5; with a maximum removal of As(V) was 96.52% and 87.54% for As(III) [21].
Adsorption techniques using nanoparticles have shown promise as being an effective technique to remove ions from water. Luther et al. showed that the adsorption of As(III) to Fe2O3 and Fe3O4 nanomaterials was 1.250 mg/g and 8.196 mg/g after 1 h of contact time, respectively [22]. However, at a contact time of 24 h the 20 mg/g for Fe2O3 and 5.680 mg/g for Fe3O4 were observed for As(III) binding to the nanomaterials [22]. The binding capacities for As(V) were lower in magnitude at both the 1 h and 24 hour contact times. The Fe2O3 nanomaterials had similar capacities of 4.6 mg/g and 4.9 mg/g for the 1 h and 24 hour contact times for As(V) binding, respectively. Whereas, the Fe3O4 nanomaterial had capacities of 6.7 and 4.8 for the 1 h and 24 hour contact times, respectively for As(V) binding [22]. Parsons et al. have investigated the binding of As(III) and As(V) binding to Mn3O4, a MnFe2O4 and Fe3O4 nanomaterials [11]. In this study the maximum binding capacity for the Fe3O4 was 0.0322 mg/g and 1.575 mg/g for the As(III) and As(V), respectively [11]. The binding capacity of the MnFe2O4 nanomaterial had a binding capacity of 0.718 and 2.212 mg/g for As(III) and As(V) respectively. The Mn3O4 nanomaterial had a binding capacity of 0.0089 and 0.211 for the As(III) and As(V), respectively [11]. In addition, at the concentrations used the pH dependency of the arsenic binding was pH dependent increasing from pH 2 to pH 6. Al-WTRs have been shown to have varied between capacities for As(III) and As(V) of 1.8–15 mg/g for As(V) and between 7.500– 15 mg/g for As(III) after 48 h of equilibrium with a pH range from 6 to 6.5 [17]. Laterite iron concretions have been shown to have As sorption capacities of 909 μg/g and 714 μg/g for As(III) and (V), respectively at pH 7 [23].
In the present study the adsorption of arsenic(III) and arsenic(V) on to single and mixed phase ferrite and hausmannite nanomaterials was investigated. The nanomaterials investigated were synthesized through a precipitation process and subsequently characterized using XRD for phase and average grain size of the material. Batch studies were performed to determine the effect of pH and the effect of interfering ions on the adsorption of both As(III) and As(V) onto the different metal oxide nanomaterials. In addition, the binding capacities for the different materials were determined using isotherm studies, which were found to follow the Langmuir isotherm.
Section snippets
Synthesis of the nanoadsorbents
The synthesis of the Fe3O4 nanomaterial a 1.0 L of metal ion solution containing 30.0 mM of Fe(II) (from FeCl2), was prepared. For the manganese substituted nanomaterials a specific percentage of the Iron(II) was substituted with manganese(II) (from MnCl2). The solution for the 25% Mn–75% Fe consisted of 7.5 mM Mn2 + and 22.5 mM Fe2 +. The solution for the synthesis of the 50% Mn–50% Fe, contained 15 mM Mn2 + and 15 mM Fe2 +. The 75% Mn–25% Fe was synthesized from a solution containing 22.5 mM Mn2 + and 7.5
XRD results
Fig. 1 shows the diffraction patterns obtained for the synthesized nanomaterials after drying. In addition, the refined lattice parameters for the substitution of the Mn into the Fe3O4 lattices from the fitting are shown in Table 2. The diffraction patterns were fitted for both phases of the material using the Le Bail fitting procedure in the Fullproff software [24]. As can be seen in the fitting shown in Fig. 1A the Fe3O4 sample diffraction patterns match very well with the diffraction pattern
Acknowledgments
Authors would like to thank the NIH UTPA RISE program (grant number 1R25GM100866-01) and the HHMI (grant number 52007568). The Authors acknowledge financial support from the Welch Foundation for supporting the Department of Chemistry (grant number GB-0017) and UTPA for sponsoring this research project.
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