Carbonate and bicarbonate ions impacts on the reactivity of ferrate(VI) for 3,4-dichlorophenol removal.

Abstract

Carbonate and bicarbonate ions are common constituents found in wastewater and natural water matrices, and their impacts on the reactivity of ferrate(VI) (Fe(VI)) with 3,4-dichlorophenol (3,4-DCP) were investigated by determining second-order rate constants of 3,4-DCP removal by Fe(VI) in the presence of CO32- and/or HCO3-. The second-order rate constants decreased from 41.75 to 7.04M-1s-1 with an increase of [CO32-] from 0 to 2.0mM, indicating that CO32- exhibits an inhibitory effect on 3,4-DCP removal kinetics, and experiments on pH effect, radical quenching, and Fe(VI) stability were conducted to explore possible reasons for its effect. Under identical pH conditions, the rate constant in NaOH medium was always higher than in Na2CO3 medium, suggesting that the inhibitory effect partially comes from an increase in alkalinity. Furthermore, the scavenging of hydroxyl radical by carbonate ion also contributed to the inhibitory effect of CO32-. On the other hand, the enhancement effect of CO32- depending on the increase in Fe(VI) stability was found, but did not exceed its inhibitory effect. In addition, 3,4-DCP removal kinetics was not affected by HCO3-, while synergistically inhibited by CO32-/HCO3-. Moreover, 3,4-DCP removal efficiency was substantially suppressed in the presence of CO32-, while the slight enhancement effect of HCO3- and the synergistic inhibitory effect of CO32-/HCO3- were observed. The experimental results clearly demonstrated that carbonate and bicarbonate ions play an important role in the process of 3,4-DCP removal by Fe(VI) and should not be considered only as scavengers.