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Title: Transport, retention, and long-term release behavior of ZnO nanoparticle aggregates in saturated quartz sand: Role of solution pH and biofilm coating

Author
item HAN, YOSEP - Chonbuk National University
item HWANG, GUKKWA - Chonbuk National University
item KIM, DONGHYUN - Chonbuk National University
item Bradford, Scott
item LEE, BYOUNGCHEUM - National Institute Of Environmental Research
item EOM, IGCHUN - National Institute Of Environmental Research
item JE KIM, PIL - National Institute Of Environmental Research
item CHOI, SIYOUNG - Korean Advanced Institute Of Science And Technology (KAIST)
item KIM, HYUNJUN - Chonbuk National University

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2015
Publication Date: 12/17/2015
Citation: Han, Y., Hwang, G., Kim, D., Bradford, S.A., Lee, B., Eom, I., Je Kim, P., Choi, S.Q., Kim, H. 2015. Transport, retention, and long-term release behavior of ZnO nanoparticle aggregates in saturated quartz sand: Role of solution pH and biofilm coating. Water Research. 90:247-257. doi: 10.1016/j.watres.2015.12.009.

Interpretive Summary: Zinc oxide nanoparticles (ZnO NPs) are increasingly being employed in a wide variety of commercial products and industrial applications, and knowledge of their subsurface transport and fate is needed to assess potential health risks to ecosystems and humans. The objective of this study was to study the transport and dissolution behavior of ZnO NPs under various solution ionic strength and pH conditions on sand in the presence and absence of bacterial biofilm coatings. Results indicate that the long-term fate of ZnO NPs was controlled by dissolution, which was enhanced at lower pH conditions and in the absence of biofilm. This information will be of interest to scientists and engineers concerned with predicting the fate of ZnO NPs in soil and groundwater environments.

Technical Abstract: The transport, retention, and long-term fate of zinc oxide nanoparticles (ZnO-NPs) were investigated in saturated, bare and biofilm (Pseudomonas putida) coated sand packed columns. Almost complete retention of ZnO-NPs occurred in bare and biofilm coated sand when the influent solution pH was 9 and the ionic strength (IS) was 0.1 or 10 mM NaCl, and the retention profiles were always hyper-exponential. Increasing the solution IS and biofilm coating produced enhanced retention of ZnO-NPs near the column inlet, and this was attributed to more favorable NPs-NPs interaction and greater surface roughness. This was supported by interaction energy calculations and electron microscopic observations. The long-term fate of retained ZnO-NPs was subsequently investigated by continuously injecting pure solution at pH 6, 9, or 10 and keeping the IS constant at 10 mM. The amount and rate of retained ZnO-NP removal was strongly dependent on the solution pH, with increasing peak effluent concentrations and faster rates of removal at a lower pH. Almost complete removal of retained ZnO-NPs was observed after 627 pore volumes when the solution pH was 6, whereas much less Zn was recovered when the eluting solution pH was buffered to pH=9 and especially 10. This long-term removal was attributed to pH-dependent dissolution of retained ZnO-NPs because the solubility of ZnO-NPs increases with decreasing pH and ZnO-NPs were not detected in the effluent. The presence of biofilm also decreased the initial rate and amount of dissolution and the subsequent transport of Zn2+, presumably due to chemical interactions with the biofilm. Our study indicates that dissolution will eventually lead to the complete removal of retained ZnO-NPs and the transport of toxic Zn2+ ions in groundwater environments with pH ranges of 5 to 9.