Influence of epipsammic biofilm on the biogeochemistry of arsenic in freshwater environments

The influence of epipsammic biofilm developed on riverbed sediment on the sorption, uptake, mobility and transformation of AsV was studied. Native biofilm was incubated on sediment samples at microcosm level. Once the biofilm had developed, 500 µg L−1 AsV was spiked in two systems designated BAS and BASP, without P and with equimolar As:P concentration ratio, respectively, and compared with identical control (sterilized) systems (CAS and CASP). The evolution and speciation of arsenic (As) concentrations in the overlying water were followed during two additional weeks. The biofilm enhanced removal of AsV from the water up to 91 % of its initial concentration, while only ~70 % removal was attained in CAS. Presence of equimolar P concentration enhanced the amount of As removal up to 97 % in BASP, but had no effect in CASP. In the systems with biofilm, As was mostly (~97 %) in AsV form, whilst AsIII only accounted for ~1 % of total aqueous As. The organic species, monomethylarsonic acid (MMAV) and dimethylarsinic acid (DMAV), represented 0.6 and 0.7 % of total As, respectively. In contrast, in the systems devoid of biofilm, AsIII accounted for up to 39 % of aqueous As, whereas methylated aqueous species were negligible. The distribution of As in the biofilm showed that ~71 % of the retained As was extracellular, most (>99.5 %) in the form of AsV. Volatile As forms were only detected in the systems incorporating biofilm. It is concluded that biofilm covering sediments increases As retention, inhibits reduction of AsV to AsIII and methylates inorganic As, thus playing a key role in the biogeochemistry of As in river environments.