Influences of humic acid, bicarbonate and calcium on Cr(VI) reductive removal by zero-valent iron

The influences of various geochemical constituents, such as humic acid, HCO3−, and Ca2+, on Cr(VI) removal by zero-valent iron (Fe0) were investigated in a batch setting. The collective impacts of humic acid, HCO3−, and Ca2+ on the Cr(VI) reduction process by Fe0 appeared to significantly differ from their individual impacts. Humic acid introduced a marginal influence on Fe0 reactivity toward Cr(VI) reduction, whereas HCO3− greatly enhanced Cr(VI) removal by maintaining the solution pH near neutral. The Cr(VI) reduction rate constants (kobs) were increased by 37.8% and 78.3%, respectively, with 2 mM and 6 mM HCO3− in solutions where humic acid and Ca2+ were absent. Singly present Ca2+ did not show a significant impact to Cr(VI) reduction. However, probably due to the formation of passivating CaCO3, further addition of Ca2+ to HCO3− containing solutions resulted in a decrease of kobs compared to solutions containing HCO3− alone. Ca2+ enhanced humic acid adsorption led to a minor decrease of Cr(VI) reduction rates. In Ca2+-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 Ca2+ significantly promoted the aggregation of Fe (oxy)hydroxides, with which humic acid co-aggregated and co-precipitated. These aggregates would progressively be deposited on Fe0 surfaces and impose long-term impacts on the permeability of PRBs.