Reductive Dechlorination of Tetrachloroethylene by Fe(II) in Cement Slurries

Abstract

Fe(II) is a potentially effective reductant for treating chlorinated organics in soils by degradative solidification/stabilization (DS/S) process. DS/S is a modification of conventional S/S that promotes degradation of organic contaminants while immobilizing inorganic contaminants. In this study, degradation of tetrachloroethylene (PCE) by Fe(II) in the presence of cement hydration products was characterized by using batch slurry reactors. Cement has been found to catalyze or participate in the PCE degradation reactions over the pH range investigated (10.6−13.8), and the degradation kinetics can generally be described by a pseudo-first-order rate law. PCE degradation rate was greatest at pH ∼12.1 with a half-life of 4.1 days when [PCE]0 = 0.245 mM and [Fe(II)]0 = 39.2 mM. Under this condition, 98% of the PCE initially present in the system transformed to nonchlorinated products (acetylene, ethylene, ethane) with acetylene being predominant, implying that elimination pathways were favored in the cement systems at high pH. Production of chlorinated intermediates was minimal in Fe(II)/cement systems. Addition of Fe(III) to Fe(II)/cement systems increased PCE degradation rates by factors of ∼2 to 3, suggesting Fe(II)-Fe(III) (hydr)oxides might have been reactive agents. Three hypotheses for the reaction mechanisms are discussed.