Arsenic mobility in the arsenic-contaminated Yangzonghai Lake in China

• Remediating an As-contaminated lake by FeCl3 may have potential ecological risks.
• Microbial activity promoted As release from sediment to water.
• Microbial activity was the main reason for secondary As pollution.
• Thermal stratification in summer enhanced the mobility and transformation of As.

Coagulation-precipitation techniques have been used successfully in the remediation of arsenic (As) contamination, but insufficient data exist to evaluate the side effects on lake environments (especially secondary pollution). Yangzonghai Lake, a deep lake located in southwest China that was treated with flocculants after an accident resulted in As-contamination, was selected as a case study. We attempted to elucidate how As migrates and transforms in the lake based on water simulation experiments. The results were expected to facilitate evaluation of the suitability and safety of the technology when used in a natural water body. The results showed that the As that had already been precipitated into the sediment by FeCl3 would be released again into the water body due to the increasing activity of anaerobic microorganisms, thereby causing secondary pollution. This phenomenon was especially evident during summer because water temperature stratification reduced the dissolved oxygen (DO) at the sediment surface and led to anaerobic conditions, which enhanced the anaerobic activity at the bottom of the lake. In summer, the concentration of As in the water column increased with increasing depth. In contrast, during winter, the concentration of As was quite similar at all depths of the lake because the water temperature was uniform during this period. As was released from sediments to the aqueous phase in the form of trivalent As [As(III)] upon anaerobic incubation and was oxidized gradually into pentavalent As [As(V)] by the higher DO in the upper layers of the lake water. Contrary to expectation, the disturbance (turnover in the fall) did not increase, but rather decreased the As concentration in the lake, which might result in further coagulation and precipitation through intensive mixture of the unsaturated flocculants from the sediments.