Effects of solution chemistry on the dechlorination of 1,2,3-trichloropropane by zero-valent zinc

The reactivity of zerovalent zinc (ZVZ) toward 1,2,3-trichloropropane (TCP) was evaluated under a variety of solution conditions, including deionized water, groundwater, and artificial groundwater, over a pH range of about 6.5-12. In deionized water, first-order rate constants for TCP disappearance (k(obs)) exhibit a broad minimum between pH 8 and 10, with increasing k(obs) observed at lower and higher pH. The similarity between this trend and zinc oxide (ZnO) solubility behavior suggests pH related changes to the ZnO surface layer strongly influence ZVZ reactivity. Values of k(obs) measured in acidic groundwater are similar to those measured in DI water, whereas values measured in alkaline groundwater are much smaller (>1 order of magnitude at pH values >10). Characterization of the surfaces of ZVZ exposed to deionized water, acidic groundwater, and alkaline groundwater suggests that the slower rates obtained in alkaline groundwater are related to the presence of a morphologically distinct surface film that passivates the ZVZ surface. TCP degradation rates in artificial groundwater containing individual solutes present in groundwater suggest that silicate anions contribute to the formation of this passivating film.