Removal of TiO₂ nanoparticles by porous media: Effect of filtration media and water chemistry

The use of nanoparticles in manufacturing as well as in commercial products continues to rise despite concerns over the environmental release and potentially negative ecological and health effects. Some aqueous waste streams carry a large fraction of released nanoparticles and thus should be targeted for treatment. Conventional porous media filtration has focused on sand as the bed material with discouraging results. This study investigated the effectiveness of three different bed materials, namely, sand, activated carbon, and diatomaceous earth, on the removal of nano-TiO₂ from aqueous streams. Additionally, the impact of solution chemistry (a commercial dispersant and the two organic compounds lysozyme and glycine) on nanoparticle retention by the various bed materials was evaluated. Diatomaceous earth displayed great promise in nanoparticle capture, providing full retention of a 50mg TiO₂L^-1 stream for the 30 bed volumes tested as compared to zero and only 20% TiO₂ capture for sand and activated carbon, respectively. Batch isotherms showed that diatomaceous earth, with specific loading capacities exceeding 25mg TiO₂ gmedium-1, has a high affinity for nano-TiO₂. This loading capacity is 20- and 1000-fold higher compared to activated carbon and sand, respectively. The solution contaminants investigated had varying effects on nano-TiO₂ retention depending on the bed material, indicating the need for investigation of co-contaminants and their role on nanoparticle filtration. This study demonstrates the superiority of diatomaceous earth as a filtration material compared to conventional sand and indicates its suitability as a new material for the removal of nanoparticles in porous media filtration.