Strategies for effective bioremediation of water co-contaminated with 1, 2-dichloroethane and heavy metals.
The production of 1,2-Dichloroethane (1,2-DCA) exceeds 5.44 billion kg per year, and is higher than that of any other industrial halogenated chemical. Improper disposal practices or accidental spills of this compound have made it a common contaminant of soil and groundwater. 1,2-DCA has been classified as a priority pollutant by the Environmental Protection Agency owing to its toxicity, persistence and bioaccumulation in the environment. It has also been shown to have mutagenic and potential carcinogenic effects on animals and humans. Bioremediation is emerging as a promising technology for the clean-up of sites contaminated with chlorinated hydrocarbons. However, sites co-contaminated with heavy metals and 1,2-DCA may pose a greater challenge for bioremediation, as the former pollutant could inhibit the activities of microbes involved in bioremediation. Therefore, this study was undertaken to quantitatively assess the effects of heavy metals on 1,2-DCA biodegradation and to investigate the use of biostimulation, bioaugmentation, dual bioaugmentation, and biosorption for remediation of water co-contaminated with 1,2-DCA and heavy metals in microcosms. The combined effect of 1,2-DCA and the respective heavy metals on the microbial population and diversity was also investigated. The minimum inhibitory concentrations (MICs) and concentrations of the heavy metals (arsenic, cadmium, mercury and lead) that caused half-life doubling (HLDs) of 1,2-DCA as well as the degradation rate coefficient (k1) and half-life (t1/2) of 1,2-DCA were measured in two different wastewater types. An increase in heavy metal concentration from 0.01–0.3 mM, resulted in a progressive increase in the t1/2 and relative t1/2 and a decrease in k1. The MICs and HLDs of the heavy metals were found to vary, depending on the heavy metals and wastewater type. In addition, the presence of heavy metals was shown to inhibit 1,2-DCA biodegradation in a dose-dependent manner, with the following order of decreasing inhibitory effect: Hg2+ > As3+ > Cd2+ > Pb2+. For the bioremediation experimental set-up, 150 ml wastewater was spiked with 1,2-DCA (2.5 mM) and the respective heavy metal in air-tight serum bottles (Wheaton). The bottles were biostimulated, bioaugmented, dual-bioaugmented or undergoing biosorption. The microcosms were incubated at 25 °C and the 1,2-DCA concentration was measured weekly. Co-contaminated water undergoing biostimulation, bioaugmentation and, in particular, dual bioaugmentation were observed to exhibit higher degradation of 1,2-DCA in the presence of the heavy metal, compared to co-contaminated water receiving none of the treatments. Dual bioaugmentation, proved to be most effective, resulting in up to 60% increase in 1,2-DCA degradation after 4 weeks, followed by bioaugmentation (55%) and biostimulation (51%). In addition, an increase in the total number of 1,2-DCA degrading bacterial population was observed in the bioaugmentated microcosms compared to those biostimulated, which corresponds to an increased 1,2-DCA degradation observed in the bioaugmentated co-contaminated microcosms. Dominant bacterial strains obtained from the co-contaminated microcosms were identified as members of the genera, Burkholderia, Pseudomonas, Bacillus, Enterobacter and Bradyrhizobiaceae, that have been previously reported to degrade 1,2-DCA and other chlorinated compounds. Some of these isolates also belong to genera that have been previously shown to be resistant to heavy metals. PCR-DGGE analysis revealed variations in microbial diversity over time in the different co-contaminated microcosms, whereby the number of bands was reduced, the intensity of certain bands increased, and new bands appeared. Agricultural biosorbents (AB) were found to adsorb heavy metals effectively when utilized at a concentration of 2.5%, with the level of biosorption found to be dependent on the type of AB as well on the type of heavy metal present. OP proved to be the most efficient biosorbent for the heavy metals tested, followed by CNF, and corn cobs (CC) least efficient; therefore CC was not used in further bioremediation experiments. Both orange peel (OP) and coconut fibre (CNF) were found to be excellent at removing heavy metals from co-contaminated microcosms, with OP removing 14.59, 74.79, 60.79 and 87.53% of As3+, Cd 2+, Hg2+ and Pb2+, respectively, while 10.03, 40.29, 68.47 and 70.00% of As3+, Cd2+, Hg2+ and Pb2+, respectively, was adsorbed by CNF. Consequently, a higher degradation of 1,2-DCA was observed in the presence of OP and CNF, compared to the untreated control. It can be concluded that the remediation approaches utilized in this study proved effective in the bioremediation of water co-contaminated with 1,2-DCA and heavy metals and may provide the foundation for new and innovative treatment strategies for co-contaminated sites.