Wastewater treatment plant discharged phosphorus impacts the release of arsenic from arsenic-enriched streambed sediment

Phosphate competition with arsenic is one of the leading causes of As release from sediments into freshwaters. An important P source to freshwaters is wastewater treatment plants (WWTP), estimated to contribute 25–45% of all P in surface waters.

A stream surrounded by soils and sediments with naturally elevated concentrations of As (> 200 mg/kg) and continuous entry of small capacity WWTP discharge was studied. The methods used were XRD, EPMA, bulk analyses, single extractions, and batch leaching experiments. Since 2013, the WWTP effluent supplies 7–23 mg/l of PO₄ into the stream and an increased concentration of As (150–180 µg/l) due to everyday usage of As‑enriched wells water in the households. This study revealed that the fractionation of As and P in sediments changed due to exposure to treated wastewater. The adsorbed As fraction decreased by 9 %, whereas the adsorbed P fraction increased by 9 % in the downstream samples. As a result, the P‑retention capacity of the sediment decreased in the downstream samples from 16 % to 10–12 %. These findings are supported by a mineralogical study, which showed that P and As distribution within the Fe (hydr)oxides differed significantly between the samples taken upstream and downstream of the effluent discharge point. The samples upstream showed higher As and lower P median concentration (1.3 wt % of As₂O₅ and 0.8 of P₂O₅ wt %, respectively), while the opposite behavior was observed downstream: As 0.7 wt % of As₂O₅ and P 1.6 wt % of P₂O₅. These findings indicate that elevated phosphate is replaced by arsenate in the Fe (hydr)oxides, and the As is mobilized into the aqueous phase. Moreover, a detailed mineralogical investigation of samples exposed to the P-enriched effluent showed newly created Fe (hydr)oxide coatings significantly enriched in P (< 18.2 wt % of P₂O₅), Ca (< 10.9 wt % CaO) while depleted in As (< 3.3 wt % As₂O₅).

Our results showed that local sources of phosphate, such as WWTP, in areas with elevated concentrations of As can significantly impact As behavior and may be responsible for elevated concentrations of As in surface waters.

Acknowledgments: This research was supported by the Grant Agency of Charles University (GAUK no. 790120), Czech Science Foundation (GAČR no. 22-27939S), and the Center for Geosphere Dynamics (UNCE/SCI/006).