title: Effect of reduced humic acid on the transport of ferrihydrite nanoparticles under anoxic conditions.
abstract: Humic acid (HA) occurs ubiquitously in the subsurface environments and is well-known to play a critical role in the fate and transport of ferrihydrite nanoparticles (NPs) and NPs-associated contaminants. Under anoxic conditions, HA can readily be reduced by microorganisms or geochemical reducing species, and the mechanisms and kinetics of ferrihydrite reduction by reduced HA (HAred) are well-documented; however, the role of these redox reactions on the transport of ferrihydrite NPs is largely underestimated. This study provides new knowledge regarding the role of HA (both reduced HA (HAred) and oxidized HA (HAox)) of environmentally relevant concentrations (0-50 mg C/L) on the transport of ferrihydrite NPs in anoxic sand columns. Our findings show that, regardless of the redox state, the presence of a low concentration of HA (3 mg C/L) inhibited ferrihydrite NP's transport due to enhanced aggregation (and deposition) between positively charged ferrihydrite NPs and negatively charged HA molecules. In contrast, higher HA (both HAredand HAox) concentration (≥10 mg C/L) significantly enhanced the mobility of ferrihydrite NPs, primarily due to the enhanced electrostatic and steric stabilization originating from excessively adsorbed HA molecules. Interestingly, the transport of ferrihydrite NPs is substantially lower in the presence of HAredthan in the presence of HAox. This distinct effect (HAredvs. HAox) on the particle transport is attributed to the fact that reductive dissolution of ferrihydrite NPs occurs in the presence of HAred(ferrihydrite dissolves and thus total breakthrough decreases), but not in the presence of HAox. Furthermore, the abatement extent of ferrihydrite NPs transport triggered by the presence of HAredis dependent on dissolved HAredconcentration. Taken together, our findings provide direct, and much needed insights into the distinct roles of redox state of HA on the transport of redox-sensitive metal-bearing NPs in porous media.
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, PR China.
authors.forename: Peng
authors.initials: P
authors.lastname: Liao
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, 63130, United States.
authors.forename: Wenlu
authors.initials: W
authors.lastname: Li
authors.affiliation: Department of Plant and Soil Sciences, University of Delaware, Newark, DE, 19716, United States.
authors.forename: Dengjun
authors.initials: D
authors.lastname: Wang
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, 63130, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Jiang
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, 63130, United States.
authors.forename: Chao
authors.initials: C
authors.lastname: Pan
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, 63130, United States.
authors.forename: John D
authors.initials: JD
authors.lastname: Fortner
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, PR China. Electronic address: yuansonghu622@cug.edu.cn.
authors.forename: Songhu
authors.initials: S
authors.lastname: Yuan
mesh_terms: D000327:Adsorption; D006812:Humic Substances; D007700:Kinetics; D053758:Nanoparticles / Q000737:chemistry*; D016062:Porosity; D012822:Silicon Dioxide / Q000737:chemistry
chemical_list: D006812:Humic Substances; D012822:Silicon Dioxide
keywords: Ferrihydrite nanoparticles; Humic acid; Mechanism; Porous media; Redox state; Transport