Assessment of hematite nanoparticles as adsorbent for arsenic removal

Abstract

Widespread presence of elevated levels of arsenic (As) in groundwater is a major public health concern in Bangladesh. Although a number ofmcthods are presently used for removal of As from groundwater, they all suffer from certain drU\vbacks. Technologies based on nanoparticles (typically 1-100 11m in size) have received significant attention in recent times regarding their prospective lise in groundwater treatment (e.g., for As removal). However, because their minuscule size, nanoparticles cannot be used directly in an As removal system. For developing such a system, it is extremely important to characterize the nunoparticle itself (e.g. size, aggregation characteristics. etc.) and also to assess their adsorption characteristics. In this study, adsorption characteristics of arsenic on commercially procured hematite nanoparticles (HNPs) have been assessed in laboratory batch experiments. Characterization tests of the commercially procured HNPs were carried out to assess the structure, composition, particle size, distribution, specific surface area and surface charge. The XRD analysis confirmed structure of the a-Fe20) (Hematite) nanoparticles and absence of significant impurity. The specific surface area of the nanoparticles found from the BET test (13.81l1~/g) \vas well below the manufactured-rep0l1ed value of 50rn2/g. Potentiometric titration revealed the pH of zero surfltce charge (i.e., pHpzC>of IINPs to be around pH 6.5. which is an important parameter in assessing the adsorption characteristics. An important tinding of the DLS test was that the hydrodynamic radius (I<h) of the HNPs in aqueous suspension was ovcr four times the actual size of the particles. Batch adsorption tests werc carried out under different conditions (e.g. varying pH. presence of competing anions) in order to evaluate the suitability of HNPs for use in As removal. Effect of pH on adsorption of both arsenite and arsenate appear to follow the same trend. Adsorption of arsenite was more than that of arsenate for pl-I>5.0 under similar conditions. For pH<5.0 the adsorption of arsenite and arsenate \vas almost similar. \Vith increasing pH there was an initial decrease of arsenate adsorption onto hematite nanoparticles with the maximum adsorption of 0.065 mol/kg occurring at around pH 4.0 and minimum adsorption of 0.040 mol/kg at around pH 7.0. As pH increased further, adsorption of arsenate increased gradually to about 0.058 Illollkg at around pH 10.0. Similar but less pronounced trend was observed for the adsorption of arsenite on HNPs. The effect of pH on adsorption of As could be explained by the aggregation characteristics of HNPs and charge on the aqueous arsenic species. Adsorption isotherms showed that the maxilllulll As adsorption capacity (590~1I1101/gat pH 9.3) was more than that of arsenate (435~111l01/gat pH 6.1). However. at lower concentration of As «I71.1I1101/L), the adsorption of arsenate was more than that of arsenite. Significant reduction in the adsorption of both arsenite and arsenate on HNPs was observed in the presence of co~cxisting phosphate (PO/) Adsorption of As from natural groundwater on the HNPs was also found to be lower than that from aqueous solution containing inditTerent electrolyte NaNO). These results suggest significant effect of competing ions and aggregation properties of HNPs on As adsorption. The effect of dispersion of the HNPs on adsorption was found to be significant. Kinelics experiments showed that As adsorption reached equilibrium within 24 hours, with majority of adsorption taking place within the !irst few hours. The results of arsenic adsorption on HNPs indicates that along with other adsorbents (e.g., MnO" Geothite, Alumina, Fe(OH)" TiO,), the HNPs could be used as an effective adsorbent for As removal. The higher arsenite adsorption capacity of HNPs could potentially be used in removing As from groundwater (without the necessity of an oxidizing agent). where it is present primarily as arsenite. HNPs could be used for both in-situ and ex-situ treatment of groundwater; hO\vcver, such treatment systems must be carefully designed keeping in mind the adsorption as well as aggregation characteristics of the nanoparticles.