Flocculation into, and resuspension of, engineered nanomaterials from coagulated sludges

Abstract

Engineered nanomaterials (ENMs), like nano-silver (nAg), cerium dioxide (CeO2) and titanium dioxide (TiO2), are garnering growing attention from an array of industries due to their unique attributes. As their commercial production has grown, so too has their environmental footprint. Ingestion of water containing ENMs has been identified as a primary human exposure route. This makes understanding ENM removal, and its permanence, in conventional water treatment systems imperative. In this study, the removal of three ENMs (nAg, CeO2, TiO2) by flocculation and clarification in complex media was evaluated using environmentally relevant concentrations. ENM homoaggregation and heteroaggregation with aluminum sulfate, natural organic matter and clay were characterized using a combination of light scattering and nanoparticle tracking analysis. This approach provided an unprecedented assessment of ENM aggregation and concentrations persisting in clarified waters distinct from previous works using more gross measures of ENM characteristics/concentrations. Under optimum conditions the mass removal of all ENMs was ≥84%; however, the particle concentrations in the clarified water were >107 particles/mL. This corresponded to particle surface area values of 3.3, 25, and 2.0 cm2/L, respectively for the CeO2, citrate-nAg, and TiO2. Only the nAg showed a tendency to be released from the coagulated sludge through chemical and/or physical erosion processes suggesting that sludges containing ENMs may serve as point sources of ENM pollution during land application.