Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species

Uncertainty still exists regarding the role(s) of natural organic matter in the reduction of chemicals in anoxic environments. This work studied the effect of Suwannee river humic acid (SRHA) on the reduction of nitrobenzenes in goethite suspensions by Fe(II) species. The pseudo-first-order rate constant for the reduction of p-cyanonitrobenzene (k_CNNB) was different for the first 3 half-lives in systems where Fe(II)_aq and dissolved SRHA were equilibrated in reverse orders with goethite in suspensions. k_CNNB and the reduction capacity of the system having SRHA added after Fe(II)_aq was equilibrated with goethite was lower than that of the system for which the components were added in the reverse order. SRHA decreased the reduction capacity of the former system by oxidizing and/or complexing the surface-associated Fe(II), Fe(II)_surf, and/or hindering the access of CNNB to Fe(II)_surf. The log k_CNNB increased linearly with increasing concentrations of Fe(II)_aq, which decreased as a result of increasing concentrations of SRHA in the system. Different k_CNNBs were observed for systems in which Fe(II)_aq was equilibrated with goethite/SRHA suspensions for 24 and 48 h, suggesting sorbed SRHA oxidized and/or complexed Fe(II)_aq. Findings suggest the concentration of Fe(II)_aq and accessible Fe(II)_surf will influence the reduction rates of nitroaromatics in anoxic environments.