Insights into the bioremediation potential of native Bacillus isolates and their consortia against Iron, Cadmium and Chromium at pH 5.0.

Due to high Sulphur content of coals in Meghalaya, India, coal mining in this region generates huge amount of acidic waste characterized by low pH and leaching of toxic heavy metals which is a major environmental problem. This study involves isolating native Bacillus species from samples of active and abandoned Rat-hole coal mines in order to mitigate these phenomena. These species were then evaluated for significant bioremediation potential in the form of heavy metal binding and adsorption, as well as raising acidic pH to pH 8.0 (basic) under in vitro conditions. Sixteen (16) isolates were initially screened to develop the microbial consortia and five isolates (05) were finally selected for this study (sites specific) based on highest heavy metal resistance, and the ability to reverse or increase acidic pH. The isolates demonstrated resistance to the heavy metals under investigation (iron, cadmium, and chromium) with minimum bactericidal concentrations ranging from 1600 mg/L to 2000 mg/L of Fe, 128 mg/L to 1024 mg/L of Cd, and 64 mg/L to 256 mg/L of Cr. The lowest acidic pH the isolates could grow in in vitro conditions was pH 5.0, at 37 °C, 150 rpm shaking with or without metal. On average, Bacillus sp. KH5M11 and Bacillus sp. KHCL13 have the better binding and removal percentage (%) of iron (99.89 ± 0.066 and 99.87 ± 0.098) respectively. Lysinibacillus sp. SK18-4 have better removal percentage (%) of Chromium (79.903 ± 17.702), followed by Bacillus sp. KHCL13 (76.14 ± 12.233). Bacillus sp. KHCL13 (97.715 ± 1.040) and Bacillus sp. KH5M11 (97.363 ± 1.260) have better removal percentage (%) of Cadmium, as compared to other isolates. Amongst the consortia, the removal percentage, does not show significant differences in heavy metal removals. The iron removal percentage (%) of Consortia-A22ED9 is 92.08 ± 10.281, and Consortia-B18-4M11 is 89.597 ± 10.829. The chromium removal percentage (%) of Consortia-A22ED9 is 74.15 ± 12.906 and Consortia-B18-4M11 is 77.467 ± 12.420. However, both the consortia have similar removal percentage (%) for cadmium. Consortia-A22ED9 removed 97.587 ± 0.958% and Consortia-B18-4M11 removed 97.673 ± 0.475% of Cd respectively. Metal binding was confirmed by FTIR and SEM-EDX analysis, where distinct peaks and clear spectral pointed out to a specific cell surface functional groups and heavy metals binding. In all cases, the average cells optical density (OD-600) for all five isolates was high, ranges from 1.99 to 2.3 for isolates and 2.2 to 2.3 for consortia while the average pH remains relatively above pH 5.0, ranging from pH 5.4 to pH 6.23 for isolates and pH 5.7 to pH 6.3 for consortia. The study concluded that the bioremediation potential of the isolates under said condition was commendable both as individual isolates and in consortia and the possible mechanisms of bioremediation is through adsorption and binding of heavy metal and not solely because of metals precipitation. We hope with minimum manipulation of coal mines acidic condition these Bacillus spp. may become employable in bioremediation of heavy metal and control of acidic pH generation. However, our study is limited only to the in vitro as we do not have chance to carry out field trial which would be the future aspects of these beautiful findings. Much more study may be done on this field for practical applications.