Lead contamination fixation through CO2-based biomineralization

Abstract

Abstract Ureolytic microbially-induced carbonate precipitation and ureolytic enzymatically-induced carbonate precipitation have been proposed in recent years as emerging environmentally friendly techniques for heavy metal fixation. However, they rely on the hydrolysis of urea for generating carbonate ions needed to immobilize heavy metal cations. In this study, for the first time, a novel alternative is proposed, and its feasibility for lead removal in aqueous solutions is examined. The method relies on direct CO 2 biocementation. Where contaminated solutions are exposed to CO 2 , and CO 2 hydration and formation of carbonate ions is biologically facilitated through (I) microbial strains capable of producing carbonic anhydrase enzyme (i.e., microbially-facilitated carbonation, MFC), and (II) an enzymatic solution containing bovine CA enzyme (i.e., enzymatically-facilitated carbonation, EFC). The results of atomic absorption spectrophotometry indicate up to 99% Pb removal achieved in both MFC and EFC, comparable to the control ureolytic test (98%). However, the former (proposed) techniques are ammonium-free and more environmentally benign. Furthermore, they provide a dual environmental benefit: a method for carbon capture through biomineralization and its utilization for heavy metal removal. The use of Tris buffer considerably enhanced the efficiency of enzymatically-facilitated carbonation for Pb removal (i.e., increasing it from 78% to 99%). The results of microfabric analysis of precipitates (i.e., SEM imaging, XRD, FTIR, and Raman spectroscopy) confirmed the formation of calcium and lead carbonates such as calcite, cerussite, and hydrocerussite, indicating potential removal mechanisms such as lead carbonate precipitation as well as Pb trapping inside calcium carbonate crystals.