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.