The impact of heavy metals on the aerobic biodegradation of 1,2-dichloroethane in soil.
1,2-Dichloroethane (1,2-DCA), a short chain chlorinated aliphatic compound, is one of the most hazardous toxic pollutant of soil and groundwater, with an annual production in excess of 5.44 × 109 kg. The major concern over soil contamination with 1,2-DCA stems largely from health risks. Owing to their toxicity, persistence and potential for bioaccumulation, there is a growing interest in technologies for their removal. Many sites are, however, co-contaminated with a complex mixture of 1,2-DCA and heavy metal contaminants. Co-contaminated environments are considered difficult to remediate because of the mixed nature of the contaminants and the fact that the two components often must be treated differently. Therefore, the objective of this study was to evaluate the aerobic biodegradation of 1,2-DCA by autochthonous microorganisms in soil co-contaminated with 1,2-DCA and heavy metals, namely; arsenic (As3+), cadmium (Cd2+), mercury (Hg2+) and lead (Pb2+), via a direct and quantitative measurement of the inhibitory effects of heavy metals in a microcosm setting. Effects of various metal concentrations and their combinations were evaluated based on the following: (i) degradation rate constants; (ii) estimated minimal inhibitory concentrations (MICs) of metals; (iii) concentrations of heavy metals that caused biodegradation half-life doublings (HLDs); and (iv) heavy metal concentrations that caused a significant effect on biodegradation (> 10% increase in t½ of 1,2-DCA). The effects of biostimulation, bioaugmentation and the addition of treatment additives on the biodegradation process were evaluated. The presence of heavy metals was observed to have a negative impact on the biodegradation of 1,2-DCA in both clay and loam soil samples, with the toxic effect being more pronounced in loam soil for all heavy metal concentrations except for Hg2+, after 15 days. Heavy metal concentrations of 75 mg/kg As3+, 840 mg/kg Hg2+, and 420 mg/kg Pb2+, resulted in 34.24%, 40.64%, and 45.94% increases in the t½ of 1,2-DCA, respectively, in loam soil compared to clay soil. Moreover, the combination of four heavy metals in loam soil resulted in 6.26% less degradation of 1,2-DCA compared to clay soil, after 15 days. Generally, more than 127.5 mg/kg Cd2+, 840 mg/kg Hg2+ and 420 mg/kg of Pb2+ was able to cause a > 10% increase in the t½ of 1,2-DCA in clay soil, while less than 75 mg/kg was required for As3+. An increased reduction in 1,2-DCA degradation was observed with increasing concentration of the heavy metals. In clay soil, a dose-dependant relationship between k1 and metal ion concentrations in which k1 decreased with higher initial metal concentrations was observed for all the heavy metals tested except Hg2+. Ammonium nitrate-extractable fractions of bioavailable As3+ and Cd2+ concentrations varied greatly, with approximately < 2.73% and < 0.62% of the total metal added to the system being bioavailable, respectively. Although bioavailable heavy metal fractions were lower than the total metal concentration added to the system, indigenous microorganisms were sensitive to the heavy metals. Biostimulation, bioaugmentation and amendment with treatment additives were all effective in enhancing the biodegradation of 1,2-DCA in the co-contaminated soil. In particular, biostimulation with fertilizer, dual-bioaugmentation and amendment with CaCO3 were most efficient in enhancing 1,2-DCA degradation resulting in 41.93%, 59.95% and 51.32% increases in the degradation rate constant of 1,2-DCA in the As3+ co-contaminated soil, respectively, after 20 days. Among all the treatments, dualbioaugmentation produced the highest 1,2-DCA degrading population of up to 453.33 × 107 cfu/ml in the Cd2+ co-contaminated soil. On comparison of the As3+ and Cd2+ co-contaminated soil undergoing either biostimulation or dual-bioaugmentation, similarity in the denaturing gradient gel electrophoresis (DGGE) banding patterns was observed. However, the banding patterns for the different bioremediation options demonstrated a difference in bacterial diversity between the fertilized and dual-bioaugmented samples. DGGE profiles also indicate that while numerous bands were common in the fertilized co-contaminated soils, there were also changes in the presence and intensity of bands due to treatment and temporal effects. Dehydrogenase and urease activities provided a more accurate assessment of the negative impact of heavy metals on the indigenous soil microorganisms, resulting in up to 87.26% and 69.58% decreases in activities, respectively. In both the biostimulated and bioaugmented soil microcosms, dehydrogenase activity appeared biphasic with an initial decrease followed by an increase in the treated soils over time. Results from this study provide relevant information on some alterations that could be introduced to overcome a critical bottle-neck of the application of bioremediation technology. In conclusion, the bioremediation strategies adopted in this study may be used as a rational methodology for remediation of sites co-contaminated with 1,2-DCA and heavy metals, subject to a thorough understanding of the microbial ecology and physico-chemical parameters of the site.