CONTRIBUTIONS FROM INDIRECT AND DIRECT BIOLOGICAL OXIDATION OF ARSENITE BY A PHOTOSYNTHETIC BIOMAT

The oxidation of arsenite (As(III)) plays an important role in water treatment as well as natural cycling in the environment. In this study, we constructed a laboratory-scale open water wetland system inoculated with a benthic photosynthetic biomat harvested from the Prado Constructed Wetland complex in Corona, CA. These flow-through reactors mimic processes in the field-scale system under alternating 12-hour light and dark conditions controlled by commercial LED lights. Besides modest retardation, approximately 90% of introduced As(III) was oxidized during a 24-hour hydraulic residence time across influent ranges from 0.1 to 1 mg/L without the need for aggressive oxidants typically employed to achieve this conversion. When applied as a pretreatment strategy coupled with nanofiltration, this oxidation elevated the removal of arsenic in the form of arsenite from 65% to 96% at neutral pH with >99.9% at higher pHs achieved in association with photosynthetic processes. Similar gains are projected to be achieved for alternative treatment strategies such as iron-based coagulation. Batch experiments contrasting autoclaved and fresh biomat confirmed that biological activity stimulated arsenic oxidation. Under the light condition, the water column was supersaturated with dissolved oxygen (>20 mg/L O₂) resulting from photosynthesis. Metagenomic and metatranscriptomic sequencing data from the Prado Wetlands complex revealed the presence and expression of the heterotrophic arsenite oxidation gene aioB, which could provide a direct route for aerobic microbial oxidation. The oxygenated condition may also support an indirect route for biological oxidation through the formation of free radicals when oxygen reacts with sequestered iron (>18.6 g/kg) within the biomat. Anaerobic oxidation may also contribute given the stratified nature of the biomat that created potential anoxic microenvironments and the presence and expression of anaerobic oxidation gene arxA. Ongoing experiments are focused on parsing out the contributions between direct and indirect biological processes and exploring implications on seasonality, reliability and enhancement of this nature-based pretreatment system.