Impacts of municipal wastewater oxidative treatments: changes in metal physical speciation and bioavailability.

The environmental repercussions of the discharge of disinfected effluents are still poorly understood. This study assessed the impact of ozonation and UV oxidative treatment processes on metal forms - particulate, colloidal and permeable fractions - and bioavailability in disinfected wastewaters. In addition to wastewater analyses, mussels were placed in continuous flow-through aquaria and exposed for 4wk to wastewater, then metals in their tissues were analysed in parallel with exposure biomarkers. Metal size distribution was affected by oxidative processes; results showed that ozonation treatment generally increases the permeable fraction of some metals, particularly Cd and Cu, in treated waters, whereas UV treatment fosters the formation of permeable Zn. Ozone treatment of wastewater generally increased the bioavailability of specific metals. Metal bioaccumulation was in most cases significantly higher in mussels exposed to ozone-treated effluent compared to the UV treatment: 58%, 32%, 42% and 47% higher, respectively, for Ag, Cd, Cr and Cu. Physical metal speciation in these wastewaters comparatively measured the permeable fraction of metals to relate them to the bioaccumulation results for the exposed mussels. The levels of lipid peroxidation were significantly increased in gills but not in the digestive gland. The levels of metallothionein in the digestive gland were also significantly reduced suggest decreased input of particulate metals. Results of bioaccumulation in mussels suggested that metal bioavailability can be modified by the different oxidative processes. Despite this disadvantage, ozonation still represents a great choice of treatment considering the overall environmental benefits.