title: Chromium(VI) reduction kinetics by zero-valent iron in moderately hard water with humic acid: iron dissolution and humic acid adsorption.
abstract: In zerovalent iron treatment systems, the presence of multiple solution components may impose combined effects that differ from corresponding individual effects. The copresence of humic acid and hardness (Ca2+/Mg2+) was found to influence Cr(VI) reduction by Feo and iron dissolution in a way different from their respective presence in batch kinetics experiments with synthetic groundwater at initial pH 6 and 9.5. Cr(VI) reduction rate constants (k(obs)) were slightly inhibited by humic acid adsorption on iron filings (decreases of 7-9% and 10-12% in the presence of humic acid alone and together with hardness, respectively). The total amount of dissolved Fe steadily increased to 25 mg L(-1) in the presence of humic acid alone because the formation of soluble Fe-humate complexes appeared to suppress iron precipitation. Substantial amounts of soluble and colloidal Fe-humate complexes in groundwater may arouse aesthetic and safety concerns in groundwater use. In contrast, the coexistence of humic acid and Ca2+/Mg2+ significantly promoted aggregation of humic acid and metal hydrolyzed species, as indicated by XPS and TEM analyses, which remained nondissolved (>0.45 microm) in solution. These metal-humate aggregates may impose long-term impacts on PRBs in subsurface settings.
authors.affiliation: Department of Civil Engineering, The Hong Kong University of Science and Technology, Hong Kong, China.
authors.forename: Tongzhou
authors.initials: T
authors.lastname: Liu
authors.forename: Daniel C W
authors.initials: DC
authors.lastname: Tsang
authors.forename: Irene M C
authors.initials: IM
authors.lastname: Lo
mesh_terms: D000327:Adsorption; D002857:Chromium / Q000737:chemistry*; D006812:Humic Substances; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D012995:Solubility; D014867:Water / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000737:chemistry*; D018508:Water Purification
chemical_list: D006812:Humic Substances; D014874:Water Pollutants, Chemical; D014867:Water; D002857:Chromium; C074702:chromium hexavalent ion; D007501:Iron