Immobilization of heavy metal contaminated mine wastes using Canavalia ensiformis extract

• Canavalia ensiformis extract was used to produce CaCO3 in mine wastes.
• The extract catalyzed urea hydrolysis and decreased metal amounts in leachates.
• Microbial populations were considerably increased after biocatalyst treatment.
• Canavalia ensiformis extracts may assist in the stabilization of heavy metals.

This study examined the ability of crude extracts of Canavalia ensiformis to catalyze the precipitation of calcium carbonate (CaCO3) in columns packed with heavy metal contaminated mine waste collected from an abandoned mine site, and examined the effect of CaCO3 precipitates on the leaching of heavy metals out of such waste. X-ray diffraction and scanning electron microscopy were employed to confirm CaCO3 precipitation and to characterize the morphology of the resulting material. Urease in the C. ensiformis crude extracts catalyzed the hydrolysis of urea, leading to the formation of CaCO3 precipitates that formed bridges between the particles in the mine waste. In column experiments, the amounts of the heavy metals As, Mn, Zn, Pb, Cr, and Cu in leachates from the mine waste were reduced by 31.7%, 65.7%, 52.3%, 53.8%, 55.2%, and 49.0%, respectively, when the waste was treated with C. ensiformis crude extract. This reduction can be attributed to immobilization of heavy metals within the mine waste as a result of CaCO3 precipitation. Comparison of the microbial communities in mine waste columns that were untreated or incubated for 2 weeks with C. ensiformis crude extract and purified urease, using PCR-DGGE of 16S rDNA, showed that a greater diversity of microorganisms was present in the columns treated with C. ensiformis crude extract and purified urease. These findings suggest that crude extracts of C. ensiformis may be used to stabilize and immobilize heavy metals in contaminated mine waste to prevent further dispersion to the surrounding environment.