Process engineering to produce defined hierarchical structures of diatoms for wastewater polishing and micropollutant removal
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI-COLUMBIA AT REQUEST OF AUTHOR.] Diatoms are unicellular algae, which are among the most common types of phytoplankton. They are widespread in fresh water, oceans and soils with different sizes and morphologies. The diatom cell wall is composed of highly ordered 3D nano-porous silica, or so-called frustule. The abundance and unique cell wall structure of diatoms make them useful in biological research and promote their broad applications in the fields of biology, ecology, environment sciences, and especially nanotechnology, as diatom frustule is ideal for fabrication of new nanomaterials and devices. This study first aimed to determine how solids retention time (SRTs) would affect the diatom micro- and nanostructures, as well as removal efficiency of nutrients in a membrane bioreactor (MBR). Second, we determined the changes in morphology and nanostructure of diatoms at different hydraulic retention times (HRTs ranging from 3 h to 24 h). We also developed a biological and environmental friendly production method to incorporate different sources of titania into the porous 3D silica frustule of diatoms for the photo-degradation of pharmaceutical and personal care products (PPCPs). The photocatalytic activity of the produced silica-titania frustule increased with the decrease in HRT. These new nano-materials with silica-titania frustule had excellent photocatalytic properties, including high porosity, high surface area, and reliable immobilization of TiO2 into the ordered silicon frustule. Finally, we investigate the effect of natural organic matter (NOM) on the photo-degradation of PPCPs by diatom incorporated with titanium. NOM and PPCPs are known to compete for adsorption sites on catalytic materials, which results in decreasing of the degradation rate of PPCPs. In this study, three selected PPCPs, Triclosan, Bisphenol A, and DEET were used to determine the effects of NOM concentration on photo-degradation by nano-TiO[subscript 2] (P25) or titanium-incorporated diatoms. A total of 7.2 [plus/minus] 1.4% wt titanium was incorporated in the frustule without a significant change in morphology, nanostructure of diatoms and their activity. As the concentration of humic acid increased to 10 mg L[superscript -1] diatom incorporated with titanium had 2 fold higher photocatalytic activity in degrading triclosan and 4 fold higher photocatalytic activities in degrading bisphenol A and DEET than P25. PPCPs can be selectively removed by diatoms incorporated with TiO2 in the present of NOM, - most likely because sub-10 nm pores of original 3D hierarchical structure of diatom frustules serve as a pretreatment unit in membrane filtration by efficiently rejecting large NOM molecules while let PPCPs (with their molecular weight of typically < 600 g/mol) pass through the nanopores.
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