Immobilisation of Cd(II) on biogenic and abiotic calcium carbonate

Ubiquitous calcium carbonate (biogenic or abiotic) exerted an important impact on the migration and transformation of heavy metal in the environment. Cd(II) pollution, common in China, has attracted much attention due to its critical toxicity. The purpose of this study is to compare the differences in adsorption and desorption characteristics of Cd(II) between biogenic CaCO 3 (BCa) induced by Bacillus subtilis and abiotic CaCO 3 [AR (analytical reagent grade)-CaCO 3 and limestone]. The results show that the adsorption data of BCa and abiotic CaCO 3 for Cd(II) more closely followed the Langmuir model compared to the Freundlich model. The maximum adsorption capacity (obtained from Langmuir isotherm) of BCa for Cd(II) (172.41 mg/g) was significantly greater than that of abiotic CaCO 3 (AR-CaCO 3 : 6.31 mg/g, limestone: 21.01 mg/g), and its maximum desorption rate (1.48%) was significantly lower than that of abiotic CaCO 3 (AR-CaCO 3 : 74.30%, limestone: 5.65%). Furthermore, the adsorption process of BCa is a spontaneous endothermic reaction and obeys pseudo-second-order kinetics. The obvious advantages of BCa are that it is easily obtained by bacterial culture, and another is its stronger immobilisation of Cd(II) compared to abiotic CaCO 3 , giving a promising potential for heavy metal remediation.