Development of a three-step approach to repurpose nickel-laterite mining waste into magnetite adsorbents for As(III) and As(V) removal: Synthesis, characterization and adsorption studies

Nickel (Ni)-laterite mining is a major industry in the Philippines but is generating large amounts of Ni-laterite mining waste (NMW) that threatens surrounding water bodies due to siltation. To repurpose this waste as magnetic iron (Fe)-based adsorbent, a three-step extraction-reduction-coprecipitation approach was developed, and the adsorption properties of synthesized product for arsenate (As(V)) and arsenite (As(III)) were investigated. The three-step approach extracted > 90% Fe from NMW and produced fine (<4 mu m), high-purity magnetite based on XPS, XRD, FTIR and SEM-EDS. NMW-derived magnetite also exhibited a positively charged surface between pH 5 and 8 (i.e., IEP of 8.5), which could be attributed to the incorporation of impurities like Mg, Al and Cr. The kinetic and isotherm adsorption results at pH 5 showed that NMW-derived magnetite had high maximum adsorption capacities (q(max)) for As(V) and As(III) estimated at 714 mg/g and 435 mg/g, respectively. The good fit of pseudo-second-order rate law to the kinetic results implied that chemisorption was a dominant removal mechanism for both As(V) and As(III), which likely occurred via inner sphere complexation based on the XPS results. NMW-derived magnetite could also act as an oxidant or reductant depending on the dominant As species in solution. When As(V) species dominated, they were reduced to As(III) via Fe(II)-surface species. Meanwhile, Fe(III)-surface species on the adsorbent oxidized As(III) to As(V) that improved As(III) removal from solutions. These results demonstrate the potential of NMW as raw material for magnetic adsorbent synthesis for environmental applications.