Abstract
The purpose of this study was to elucidate the characteristics and mechanisms of adsorption and desorption for heavy metals by micro and nano-sized biogenic CaCO3 induced by Bacillus subtilis, and the pH effect on adsorption was investigated. The results showed that the adsorption characteristics of Cd2+ and Pb2+ are well described by the Langmuir adsorption isothermal equation, and the maximum adsorption amounts for Cd2+ and Pb2+ were 94.340 and 416.667 mg/g, respectively. The maximum removal efficiencies were 97% for Cd2+, 100% for Pb2+, and the desorption rate was smaller than 3%. Further experiments revealed that the biogenic CaCO3 could maintain its high adsorption capability for heavy metals within wide pH ranges (3–8). The FTIR and XRD results showed that, after the biogenic CaCO3 adsorbed Cd2+ or Pb2+, it did not produce a new phase, which indicated that biogenic CaCO3 and heavy metal ions were governed by a physical adsorption process, and the high adsorptive capacity of biogenic CaCO3 for Cd2+ and Pb2+ were mainly attributed to its large total specific surface area. The findings could improve the state of knowledge about biogenic CaCO3 formation in the environment and its potential roles in the biogeochemical cycles of heavy metals.
Highlights
The main current environmental problems are the increasing atmospheric greenhouse effect and environmental pollution of large areas
This research on the adsorption of heavy metals with biogenic CaCO3 induced by the strain is the first report, and the maximum adsorption capacities of biogenic CaCO3 for heavy metals are apparently higher than that of natural calcite (p < 0.01), which suggests a considerable potential to immobilize or passivate heavy metals in contaminated soil
We found that the pH value of the adsorption system was increased after adding biogenic CaCO3, and the final pH value after adsorption is around 8.61 (Figure 6B), which should be attributable by the biogenic CaCO3 and alkaline metabolites produced by B. subtilis
Summary
The main current environmental problems are the increasing atmospheric greenhouse effect and environmental pollution of large areas. The increase of population and industrial or agricultural production makes such environmental issues more prominent. Heavy metal pollution in water and soil is already a global problem, and it is especially serious in soils (Zhao et al, 2014a). The proportion of heavy metal pollution has exceeded 2.5% by land area, covering 2.32 million hectares in China, and the exceedances of permissible threshold values for Cd, Hg, As, Cu, Pb, Cr, Zn, and Ni are 7, 1.6, 2.7, 2.1, 1.5, 1.1, 0.9, and 4.8%, respectively (The Ministry of Environmental Protection, 2014; Zhao et al, 2014a; The Ministry of Land and Resources, 2015)
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