Transport and release of electron donors and alkalinity during reductive dechlorination by combined emulsified vegetable oil and colloidal Mg(OH)2: Laboratory sand column and microcosm tests

Abstract

Emulsified vegetable oil combined with colloidal Mg(OH)2 (EVO-CM) can slowly release electron donors and OH– into groundwater and is therefore regarded as a promising amendment for enhanced in situ treatment of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). However, its migration ability in different porous media and the simultaneous release of electron donor and pH buffer during enhanced reductive dechlorination (ERD) have never been evaluated in detail. In this study, EVO-CM with uniform drop size and desirable stability was prepared. Laboratory-scale column tests were conducted to investigate the transport and spatial distribution of the EVO-CM in different porous media. Batch microcosm experiments were carried out to study the dechlorination efficiency under different EVO:Mg(OH)2 ratios. Experimental results indicate that prepared EVO-CM emulsions can be transported effectively through different porous media with grain size ranging from 0.1–1.0 mm. The emulsified vegetable oil (EVO) and colloidal Mg(OH)2 showed synchronous movement through the porous media column systems, providing both electron donor and alkalinity at the same location. The retention degree of EVO-CM is greater in the finer grained media and decreases with increasing distance from the inlet. The injection of EVO-CM lead to a significant mobilization of TCE in the column. The reductive dechlorination of TCE in the microcosms was remarkably enhanced in the presence of EVO-CM. The coaddition of sufficient colloidal Mg(OH)2 effectively limits the deleterious pH decline caused by acid release. The microcosm achieves an optimum dechlorination efficiency when the EVO:Mg(OH)2 ratio is 1:1.