| Mobilisation of solid phase arsenic under reducing conditions involves a combination of
microbial arsenate and iron reduction and is affected by secondary reactions of released
products. A series of model anoxic incubations were performed to understand the
concurrence between arsenate and ferrihydrite reduction by Shewanella putrefaciens strain
CN-32 at different concentrations of arsenate, ferrihydrite and lactate, and with given ΔGrxn
for arsenate and ferrihydrite reduction in non-growth conditions at pH 7. The reduction
kinetics of arsenate sorbed to ferrihydrite is predominately controlled by the availability of
dissolved arsenate, which is measured by the integral of dissolved arsenate concentrations
against incubation time and shown to correlate with the first order rate constants. Thus, the
mobilisation of adsorbed As(V) can be regarded as the rate determining step of microbial
reduction of As(V) sorbed to ferrihydrite. High lactate concentrations slightly slowed down
the rate of arsenate reduction due to the competition with arsenate for microbial
contact. Under all experimental conditions, simultaneous arsenate and ferrihydrite
reduction occurred following addition of S. putrefaciens inoculums and suggested no
apparent competition between these two enzymatic reductions. Ferrous ions released
from iron reduction might retard microbial arsenate reduction at high arsenate and
ferrihydrite concentrations due to formation of ferrous arsenate. At high arsenate to
ferrihydrite ratios, reductive dissolution of ferrihydrite shifted arsenate from sorption
to dissolution and hence accelerated arsenate reduction. Reductive dissolution of
ferrihydrite may cause additional releases of adsorbed As(V) into solution, which is
especially effective at high As(V) to ferrihydrite ratios. In comparison, formation
of Fe(II) secondary minerals during microbial Fe(III) reduction were responsible
for trapping solution As(V) in the systems with high ferrihydrite but low As(V)
concentrations. In summary, the interaction between microbial arsenate and ferrihydrite
reduction did not correlate with ΔGrxn, but instead was governed by geochemical and
microbial parameters, which may substantially influence the mobility of arsenic. |