Characterization of arsenite (As(III)) and arsenate (As(V)) sorption on synthetic siderite spherules under anoxic conditions: Different sorption behaviors with crystal size and arsenic species

Arsenite (As(III)) and arsenate (As(V)) uptake by synthesized small- and large-sized siderites (S-siderite and L-siderite) and the effects of crystal size on arsenic sorption were investigated under extremely anoxic and neutral pH conditions. Both siderites exhibited spherical growth mechanism with an inverse relationship between crystal size and specific surface area (SSA). The maximum adsorption capacities normalized to SSA (q_m,nor) of S-siderite and L-siderite were 0.161 and 0.174 mg/m² for As(III), and 1.460 and 0.360 mg/m² for As(V), respectively, indicating that the sorption affinity of S-siderite depends more on the arsenic species (III and V). Extended X-ray absorption fine structure (EXAFS) revealed that without oxidation change, As(V) adsorbed on both siderites forms inner-sphere complexes through bidentate-binuclear corner-sharing. In contrast, outer-sphere and inner-sphere complexes are formed for As(III) adsorbed on these siderites. In addition, the highest sorption affinity for As(V) uptake by S-siderite is attributed to the precipitation of symplesite (Fe^II₃(As^VO₄)₂·8H₂O), whereas the lowest sorption affinity for As(III) uptake by S-siderite was due to bicarbonates generated by the faster dissolution of S-siderite competing for sorption sites. Our findings suggest that arsenic sorption behaviors and mechanisms are strongly dependent on the arsenic species and the crystal size of siderite.