Remediation of high concentration of Ni²⁺ and Pb²⁺ contaminated water by bacteria immobilized in calcium alginate beads

Heavy metal pollution is a global environmental issue. Nickel (Ni) and lead (Pb) are commonly found in polluted environments, and remediation of high concentrations of heavy metals has always been a challenge. Microbially induced carbonate precipitation (MICP) has been used for the remediation of contaminated water, while the enzyme activity was susceptible to the toxicity of high concentrations of heavy metals, resulting in a limited removal rate. This study innovatively employed calcium alginate immobilized bacteria for remediation of high concentrations of Ni²⁺ and Pb²⁺. Results indicated that calcium alginate beads prepared by 1% sodium alginate, 0.1 mol/L CaCl₂, and 12 h curing time exhibited the highest urease activity and were used for following use. With the presence of 50 mmol/L of Ni²⁺ in the contaminated water, the removal rate was 71.5% when using calcium alginate immobilized bacteria, which was 60% higher than that when using free bacteria (44.7%). With the presence of 50 mmol/L of Pb²⁺ in the comminated water, the removal rate was 99.6% when using calcium alginate immobilized bacteria, which was about 6 times of that when using free bacteria (15.4%). With the presence of mixed comminated water containing 50 mmol/L Ni²⁺ and 50 mmol/L Pb²⁺, the removal rates of Ni²⁺ and Pb²⁺ when using calcium alginate immobilized bacteria were around 9 times and 2 times higher than those when using free bacteria, respectively. Additionally, it has been observed that the presence of enormous precipitates on the surface of the calcium alginate beads, which were identified as NiCO₃(OH)₂ and PbCO₃ precipitates. This concludes that calcium alginate immobilized bacteria has significant advantages in removing high concentrations of Ni²⁺ and Pb²⁺, indicating great potential for widespread application in contaminated water remediation. However, the long-term stability of the immobilized beads and their performance in continuous-flow or real-field applications require further investigation.