title: Influences of humic acid, bicarbonate and calcium on Cr(VI) reductive removal by zero-valent iron.
abstract: The influences of various geochemical constituents, such as humic acid, HCO(3)(-), and Ca(2+), on Cr(VI) removal by zero-valent iron (Fe(0)) were investigated in a batch setting. The collective impacts of humic acid, HCO(3)(-), and Ca(2+) on the Cr(VI) reduction process by Fe(0) appeared to significantly differ from their individual impacts. Humic acid introduced a marginal influence on Fe(0) reactivity toward Cr(VI) reduction, whereas HCO(3)(-) greatly enhanced Cr(VI) removal by maintaining the solution pH near neutral. The Cr(VI) reduction rate constants (k(obs)) were increased by 37.8% and 78.3%, respectively, with 2 mM and 6 mM HCO(3)(-) in solutions where humic acid and Ca(2+) were absent. Singly present Ca(2+) did not show a significant impact to Cr(VI) reduction. However, probably due to the formation of passivating CaCO(3), further addition of Ca(2+) to HCO(3)(-) containing solutions resulted in a decrease of k(obs) compared to solutions containing HCO(3)(-) alone. Ca(2+) enhanced humic acid adsorption led to a minor decrease of Cr(VI) reduction rates. In Ca(2+)-free solutions, humic acid increased the amount of total dissolved iron to 25 mg/l due to the formation of soluble Fe-humate complexes and stably dispersed fine Fe (oxy)hydroxide colloids, which appeared to suppress iron precipitation. In contrast, the coexistence of humic acid and Ca(2+) significantly promoted the aggregation of Fe (oxy)hydroxides, with which humic acid co-aggregated and co-precipitated. These aggregates would progressively be deposited on Fe(0) surfaces and impose long-term impacts on the permeability of PRBs.
authors.affiliation: Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong.
authors.forename: Tongzhou
authors.initials: T
authors.lastname: Liu
authors.forename: Pinhua
authors.initials: P
authors.lastname: Rao
authors.forename: Irene M C
authors.initials: IM
authors.lastname: Lo
mesh_terms: D001639:Bicarbonates / Q000737:chemistry*; D002119:Calcium Carbonate / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D006812:Humic Substances; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D013499:Surface Properties; D014874:Water Pollutants, Chemical / Q000737:chemistry*; D018508:Water Purification / Q000379:methods
chemical_list: D001639:Bicarbonates; D006812:Humic Substances; D014874:Water Pollutants, Chemical; D002857:Chromium; D007501:Iron; D002119:Calcium Carbonate