Treatment additives reduced arsenic and cadmium bioavailability and increased 1,2-dichloroethane biodegradation and microbial enzyme activities in co-contaminated soil

Abstract

Bioremediation of co-contaminated environments is difficult because of the mixed nature of the contaminants and the fact that the two components often must be treated differently. This study investigated the use of inorganic treatment additives, namely calcium carbonate (CaCO3), gypsum (CaSO4·2H2O), and disodium phosphate (Na2HPO4) to improve remediation of soil co-contaminated with 1,2-dichloroethane (1,2-DCA) and arsenic or cadmium. The soil samples were collected from a specific site in the Westville area in Durban, KwaZulu-Natal, South Africa. Microcosms were set up by artificially co-contaminating the soil sample (100 g mixed with 75 ml of synthetic groundwater in sterile screw-capped 250-ml serum bottles) with 1,2-DCA + risk elements; As3+ (150 mg/kg); or Cd2+ (170 mg/kg). Thereafter, each microcosm was amended with either 5 g CaCO3, 2 g CaSO4·2H2O, or 1.12 g Na2HPO4 + 0.046 g NaCl, separately. The samples were analyzed for the degradation of 1,2-DCA using GC–MS, while total 1,2-DCA degrading bacterial populations were determined at different sampling times using a standard spread plate technique. Soil dehydrogenase and urease activities were also monitored during the experimental period using standard enzyme assays. Addition of CaCO3 resulted in an approximately 2-fold increase in 1,2-DCA degradation in both the As3+ and the Cd2+ co-contaminated soil as compared to the co-contaminated soil without CaCO3. All the treatment additives were more effective in the As3+ co-contaminated soil resulting in 11.19, 9.25, and 5.63% increase in 1,2-DCA degradation in the presence of CaCO3, Na2HPO4 + NaCl, and CaSO4·2H2O, respectively, compared to the Cd2+ co-contaminated soil. The total 1,2-DCA degrading bacterial population increased in treated soils over time. Overall, soil dehydrogenase and urease activities were lower in the heavy metal co-contaminated samples compared to the treated soil. The inhibitory effect of heavy metal was less in As3+ co-contaminated soil for both CaCO3- and Na2HPO4 + NaCl-treated soil, with up to 7.92% increase in dehydrogenase activity obtained compared to soil co-contaminated with Cd2+. Results from this study indicate that treatment additives can be used to reduce bioavailable fractions of risk elements in the soil matrices, thereby limiting the toxicity of these risk elements to 1,2-DCA degrading microorganisms. Thus, this approach can be applied to enhance organic compound degradation in co-contaminated soil environments.