Bioprecipitation of heavy metals and\ud radionuclides with calcium carbonate in\ud aqueous solutions and particulate media
The possibility of utilisation of microbially induced calcium carbonate precipitation\ud as a potential remedial technology for sequestering divalent metallic contaminants in\ud subsurface environment was explored. Contamination by heavy metals and\ud radionuclides is a significant environmental problem. Incorporating metals in calcium\ud carbonate minerals is a mechanism that may offer long-term removal of metallic\ud cations. Stimulating native ureolytic bacteria by supplying required nutrient and\ud chemicals to create alkaline conditions suitable for calcium carbonate precipitation\ud may accelerate biomineralisation processes in polluted soil. This study reports the\ud ability of a bacterium in soils, Sporosarcina pasteurii, to remediate range of heavy\ud metal and radionuclide (by using non-radioactive proxies) concentrations in aqueous\ud solutions and particulate media by inducing urea hydrolysis and calcium carbonate\ud precipitation. However, bacterial activity is limited by heavy metals toxicity and\ud availability, hence, initially the minimal inhibitory concentration (MIC) of metals to\ud bacterial activity was estimated, without the presence of a metal precipitation\ud mechanism (only metals ions and nutrient broth). Metals inhibited bacteria in the\ud following order Cd >Zn >Cu >Pb >Sr. The bacterial cells showed an improved growth\ud and metal toxicity resistance in presence of urea containing medium over their\ud presence in urea-free medium. Cd, Pb and Sr were fully removed, whilst Zn and Cu\ud were partially removed by this technique. Removal of heavy metals was strongly\ud correlated with full removal of calcium and high pH increase then white precipitate\ud was produced, suggesting that urea hydrolysis by bacterial cells did play a role.\ud Finally, it was explored the effects of ground conditions (transport and availability of\ud heavy metals, preferential flow and heterogeneous hydraulic conductivity) on\ud biomineralisation process in porous media in different sand fractions (fine, medium\ud and coarse). High removal rates were achieved in porous media especially for\ud cadmium and strontium. It is resulted that the heterogeneity of hydraulic conductivity\ud plays a main role in distributing bacterial activity and subsequent precipitation\ud processes in porous media. XRF, XRD, SEM and EDX spectrum tests for sand\ud samples showed the presence of calcite and aragonite in precipitation crystals. The\ud gained results demonstrate that microbially induce calcium carbonate technique is a\ud means of sequestration of soluble heavy metals via co-precipitation with calcium\ud carbonate precipitation that can be useful for divalent heavy metal and radionuclides\ud bioremediation.