Synchrotron-based measurement of As speciation by XANES
confirmed that most of the As was found in the solid phase in the
form of As(V) after incubation (Fig. 3). The ratio of As(III) to total As
after incubation decreased in the following order: GAB (27%)>
SBAG (6.1%) > BAG (4.7%) (Table 1). For group BAG, after a 1-h incubation
of bacteria and As, most of As(III) was oxidized to As(V),
and then adsorption occurred (Fig. 4(b)). However, for group GAB,
after a 1-h incubation of goethite and As(III), most of the As(III) was
adsorbed on the mineral surface, and then oxidation occurred there
(Fig. 4(c)). Thus, the oxidation of As(III) primarily occurred in the
aqueous phase for group BAG and in the solid phase for group GAB.
Based on these results, 73% of As(III) in solid phase was oxidized in
9 h (1.3 mmol h1). The calculated oxidation rate was both around
1.3 mmol h1 according to the proportion of As(V) in solid phase at
8 h and 10 h, which is 65.7% and 82.4%, respectively. However, in
the absence of minerals, As(III) oxidation could be almost
completed in 10 min in the aqueous phase (96.2 mmol h1) (Fig. S2).
Therefore, the oxidation rate is approximately 75 times faster in the
aqueous phase than in solid phase by the As(III)-oxidizing bacterium,
Pseudomonas sp. HN-1, indicating that the presence of minerals
hindered microbial oxidation. For group SBAG, the proportion
of As(III)/As(V) after incubationwas closer to that of BAG than GAB.