| Reductive dechlorination of carbon tetrachloride (CT) by the mineral magnetite (Fe3O4) was examined in batch reactors under anaerobic environments. The dechlorination rates and product distributions were influenced by pH, NaCl concentrations, and the type of pH buffer.; Three buffers were examined including; Tris(hydroxymethyl)aminomethane (TRIS), N,N,N',N'-tetraethylethylenediamine (TEEN), and triethylamine (TEA). Relative to buffer free samples, the reaction rates were enhanced by TRIS, but not by TEEN or TEA. TRIS rate enhancement was attributed to surface-buffer complexation. Production of the toxic reaction product chloroform (CF) was enhanced 29% by TEEN buffer but only by 6 and 4% by TRIS and TEA respectively. Interaction between radical intermediates and potential hydrogen or proton donors was suspected and tracked with deuterated compounds. Water hydrating the solid surface was the most common source of hydrogen or deuterium in CF. Buffers that interacted with radical intermediates formed surface complexes and contained hydrogen atoms or protons in co-ordinations that were energetically favorable for abstraction. None of the buffers enhanced Fe(II)aq solubility; therefore, iron solubility is a poor screening tool for buffer interaction with reductive dechlorination reactions.; The reaction rates were pH dependent and increased by a factor of 16 between pH 6 and 10 as reactive surface species were created through deprotonation of surface sites. Measurements of Fe(II)aq and total iron showed that Fe(II) was desorbed under mildly acidic conditions where proton sorption displaced reactive surface sites. Increases in pH were positively correlated to CF production and inversely correlated to CO production which is consistent with a reduced ability of the deprotonated surface to stabilize a trichlorocarbanion intermediate.; The influence of aggregation on reaction kinetics was examined by monitoring CT dechlorination rates and magnetite settling behavior in response to increasing the NaCl concentration at three constant pH values. The reaction kinetics and products were consistent with sodium sorption shifting the point of zero charge of magnetite to higher pH values and not with aggregation related mass transfer limitations.; Cumulatively, these results indicate that varying geochemical conditions influence reaction kinetics and product pathways and must be accounted for to accurately predict the fate of CT dechlorinated by magnetite. |
