title: Transport, retention, and long-term release behavior of ZnO nanoparticle aggregates in saturated quartz sand: Role of solution pH and biofilm coating.
abstract: The transport, retention, and long-term release of zinc oxide nanoparticle aggregates (denoted below as 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. The enhanced NPs retention at high IS was attributed to more favorable NP-silica and NP-NP interactions; this was consistent with the interaction energy calculations. Meanwhile, the greater NPs retention in the presence of biofilm was attributed to larger roughness heights which alter the mass transfer rate, the interaction energy profile, and lever arms associated with the torque balance; e.g., scanning electron and atomic force microscopy was used to determine roughness heights of 33.4 nm and 97.8 nm for bare sand and biofilm-coated sand, respectively. Interactions between NPs and extracellular polymeric substances may have also contributed to enhanced NP retention in biofilm-coated sand at low IS. The long-term release of retained ZnO-NPs was subsequently investigated by continuously injecting NP-free 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. Specifically, 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: (i) the solubility of ZnO-NPs increases with decreasing pH; and (ii) 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 Zn(2+) due to the strong Zn(2+) re-adsorption to the biofilm. Our study indicates that dissolution will eventually lead to the complete removal of retained ZnO-NPs and the transport of toxic Zn(2+) ions in groundwater environments with pH ranges of 5-9.
authors.affiliation: Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea.
authors.forename: Yosep
authors.initials: Y
authors.lastname: Han
authors.affiliation: Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea.
authors.forename: Gukhwa
authors.initials: G
authors.lastname: Hwang
authors.affiliation: Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea.
authors.forename: Donghyun
authors.initials: D
authors.lastname: Kim
authors.affiliation: USDA, ARS, US Salinity Laboratory, Riverside, CA 92507, USA.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Bradford
authors.affiliation: Risk Assessment Division, National Institute of Environmental Research, Hwangyeong-ro 42, Seo-gu, Incheon 404-708, Republic of Korea.
authors.forename: Byoungcheun
authors.initials: B
authors.lastname: Lee
authors.affiliation: Risk Assessment Division, National Institute of Environmental Research, Hwangyeong-ro 42, Seo-gu, Incheon 404-708, Republic of Korea.
authors.forename: Igchun
authors.initials: I
authors.lastname: Eom
authors.affiliation: Risk Assessment Division, National Institute of Environmental Research, Hwangyeong-ro 42, Seo-gu, Incheon 404-708, Republic of Korea.
authors.forename: Pil Je
authors.initials: PJ
authors.lastname: Kim
authors.affiliation: Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea.
authors.forename: Siyoung Q
authors.initials: SQ
authors.lastname: Choi
authors.affiliation: Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea. Electronic address: kshjkim@jbnu.ac.kr.
authors.forename: Hyunjung
authors.initials: H
authors.lastname: Kim
mesh_terms: D018441:Biofilms; D004573:Electrolytes; D060587:Groundwater; D006863:Hydrogen-Ion Concentration; D007477:Ions; D053768:Metal Nanoparticles / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D009994:Osmolar Concentration; D011108:Polymers; D016062:Porosity; D016958:Pseudomonas putida; D011791:Quartz / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry; D012995:Solubility; D019415:Torque; D014874:Water Pollutants, Chemical / Q000302:isolation & purification; D018508:Water Purification / Q000379:methods*; D015032:Zinc; D015034:Zinc Oxide / Q000737:chemistry*
chemical_list: D004573:Electrolytes; D007477:Ions; D011108:Polymers; D014874:Water Pollutants, Chemical; D011791:Quartz; D012822:Silicon Dioxide; D015032:Zinc; D015034:Zinc Oxide
keywords: Biofilm; Long-term release; Saturated quartz sand; Solution pH; Transport and retention; ZnO nanoparticle aggregates