DISSOLUTION PROCESS OF AS-MINERALS IN GROUND WATER: AN ELECTRON MICROSCOPIC STUDY

Arsenic–resistant microorganisms were observed in a simple laboratory experiment for which Arsenic-minerals (As-minerals) and Fe-rich groundwater were used. The experiment was designed to elucidate the characteristics of microbes in groundwater where As-minerals were dissolved. The experiment was conducted with 7 standard As-minerals in bacteria-free groundwater. In the initial stage, 1 gm of each mineral and 80 ml of groundwater was taken into the experimental vessels and was stored for one year without any supplementary nutrients. Optical microscopic observations and flurometric enumeration revealed the abundance of As-resistant microorganisms on the surface of most of As-mineral precipitations, which was found at the bottom of the experimental vessels both in high and low pH with oxidative and reductive conditions. They were mostly bacillus, coccus and filamentous types of microbes presumably dissoluting the mineral surface, evidenced by scanning electron microscopy. Furthermore, DAPI stained epifluorescence micrograph confirmed the presence of DNA in the active bacterial community that was indicated by blue pigmentation in high and low pH with oxidative conditions. However, abundance of bacteria was observed on the surface of realgar (As₄S₄) precipitations when pH was elevated in a relatively less oxidative condition. Consecutively, CFDA staining method revealed the occurrence of enzymatically active bacteria on the surface of As-minerals precipitated in the experimental vessels. Based on the results it can be concluded that bacillus, cocci, and filamentous types of microbes enable to survive in As-rich aquatic environment, where As dissolution might be occurred by the process of weathering, subsequently harvesting their energy from the dissolved solutions of As-minerals with the modification of its chemical parameters.