Abstract: The objective of this study was to clarify the optimal cementing rounds of microbially induced carbonate precipitation (MICP) technique in the remediation of heavy metal contaminated soil in northwest China. As with strong evaporation and high wind erosion in northwest China, we selected a copper and silver mine in Zhongwei City of Ningxia Hui Autonomous Region as the research area to examine the effects of bacterial dose and spraying frequency of Lysinibacillus fusiformis on inducing calcium carbonate precipitation. We used the cementing solution made by spindle-type L. fusiformis solution (OD₆₀₀=2.02), urea, and calcium chloride (cemented 1 to 6 times, respectively, denoted as T1 to T6, 140 mL for each time), to explore the effects of different cementation rounds on heavy metal content, occurrence valence, and physicochemical properties of the tailings soil in the study area. The crystal types of calcium carbonate in the tailings soil under different cementation rounds were analyzed by XRD. The results showed that there were no significant differences in Cr, As, and Hg contents in the T4-T6 tailings soil (P>0.05). At T6, the conductivity, ammonium ions, and precipitation of calcium carbonate were the largest (6271.67 μS·cm^-1, 15.45%, and 0.09 μg·g^-1, respectively), and pH was the lowest. At T4, Cu was negatively correlated with reducible Cr, As, and oxidizable Hg (P<0.01), while Cr was positively correlated with reducible Cu and weak acid-extractable Hg (P<0.05). The pH of the tailings soil and the total amount of heavy metals affected the occurrence forms of heavy metals. In summary, our results indicated that the MICP technique with four cementation rounds performed better, providing a theoretical basis for effectively repairing soil pollution by MICP technique in arid areas.

Key words: microbially induced carbonate precipitation (MICP), arid region, cementation round, pollution remediation.