title: Arsenic sorption to nanoparticulate mackinawite (FeS): An examination of phosphate competition.
abstract: Nanoparticulate mackinawite (FeS) can be an important host-phase for arsenic (As) in sulfidic, subsurface environments. Although not previously investigated, phosphate (PO43-) may compete with As for available sorption sites on FeS, thereby enhancing As mobility in FeS-bearing soils, sediments and groundwater systems. In this study, we examine the effect of PO43-on sorption of arsenate (As(V)) and arsenite (As(III)) to nanoparticulate FeS at pH 6, 7 and 9. Results show that PO43-(at 0.01-1.0 mM P) did not significantly affect sorption of either As(V) or As(III) to nanoparticulate FeS at initial aqueous As concentrations ranging from 0.01 to 1.0 mM. At pH 9 and 7, sorption of both As(III) and As(V) to nanoparticulate FeS was similar, with distribution coefficient (Kd) values spanning 0.76-15 L g-1(which corresponds to removal of 87-98% of initial aqueous As(III) and As(V) concentrations). Conversely, at pH 6, the sorption of As(III) was characterized by substantially higher Kdvalues (6.3-93.4 L g-1) than those for As(V) (Kd= 0.21-0.96 L g-1). Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that up to 52% of the added As(V) was reduced to As(III) in As(V) sorption experiments, as well as the formation of minor amounts of an As2S3-like species. In As(III) sorption experiments, XANES spectroscopy also demonstrated the formation of an As2S3-like species and the partial oxidation of As(III) to As(V) (despite the strictly O2-free experimental conditions). Overall, the XANES data indicate that As sorption to nanoparticulate FeS involves several redox transformations and various sorbed species, which display a complex dependency on pH and As loading but that are not influenced by the co-occurrence of PO43-. This study shows that nanoparticulate FeS can help to immobilize As(III) and As(V) in sulfidic subsurface environments where As co-exists with PO43-.
authors.affiliation: Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan; Southern Cross GeoScience, Southern Cross University, Lismore 2480, NSW, Australia; MARUM and Department of Geosciences, University of Bremen, Bremen D-28359, Germany. Electronic address: nabeelkniazi@gmail.com.
authors.forename: Nabeel Khan
authors.initials: NK
authors.lastname: Niazi
authors.affiliation: Southern Cross GeoScience, Southern Cross University, Lismore 2480, NSW, Australia.
authors.forename: Edward D
authors.initials: ED
authors.lastname: Burton
mesh_terms: D000327:Adsorption; D001151:Arsenic / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D019015:Geologic Sediments / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D053758:Nanoparticles; D010084:Oxidation-Reduction; D010710:Phosphates / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005296:Ferrous Compounds; D010710:Phosphates; D014874:Water Pollutants, Chemical; D001151:Arsenic; C022597:ferrous sulfide
keywords: Arsenic removal; Contamination; Groundwater; Reduced aqueous environments; Wastewater