Organosiloxanes in the Photovoltaic Production Industry: Their Sources, Mass Loads, and Actual Contribution to Mercury Methylation in Wastewater

Abstract

This study systematically investigated profiles of siloxanes in the photovoltaic (PV) production industry of China and their influences on methylmercury (MeHg) generation. The ubiquitous distribution of 21 siloxanes in PV production additives/accessories [<LOD-626 μg/g, detection frequency (df) = 6.7–100%, n = 45] indicated their potential environmental emission, which was confirmed by wide detection (<LOD-14.6 mg/L, df = 30–100%, n = 10) of siloxanes and their hydrolysis product [Me₂Si(OH)₂] in wastewaters from ten PV production facilities. Overall, annual siloxanes usage (59–463 thousand tonnes) during PV production accounted for 3.9–30% of their total consumption in China. Annual mass loads (389 tonnes) in PV production wastewater were 1.7 times those (from personal care products) in national domestic wastewater. In MeHg-containing (4.35–185 ng/L) wastewaters of PV production, concentrations of ∑siloxanes were positively correlated (R² = 0.44, p < 0.05) with MeHg/THg ratios, suggesting potential contribution of Si–CH₃ bonds to mercury methylation. Laboratory experiments in simulated PV wastewater indicated that compared with dimethylsiloxanes, other methylsiloxanes and methylsilanol containing electrophilic groups (phenyl, vinyl, and hydroxyl) would lead to 1.3–7.1 times larger abiotic mercury methylation fractions (0.14–4.0% after 18 h incubation) by accelerating the cleavage of Si–CH₃. In addition, both hydrolysis of Si–O bonds (in acidic conditions) and generation of Si–F bonds (in F^–-rich wastewater) could accelerate mercury methylation by introducing electrophilic groups.