Nano-hydroxyapatite-assisted enzyme-induced carbonate precipitation enhances Pb-contaminated aqueous solution and loess remediation.

Intensive agricultural activities could cause lead (Pb) bioaccumulation, threatening human health. Although the enzyme-induced carbonate precipitation (EICP) technology has been applied to tackle the aforesaid problem, the urease may denature or even lose its activity when subjected to a significant Pb2+ toxicity effect. To this end, the nano-hydroxyapatite (nHAP)-assisted EICP was proposed to reduce the mobility of Pb2+. Results indicated that a below 30% immobilization efficiency at 60 mM Pb2+ was attained under EICP. nHAP adsorbed the majority of Pb2+, preventing Pb2+ attachment to urease. Further, hydroxylphosphohedyphane or hydroxylpyromorphite was formed at 60 mM Pb2+, followed by the formation of cerussite, allowing hydroxylphosphohedyphane or hydroxylpyromorphite to be wrapped by cerussite. By contrast, carbonate-bearing hydroxylpyromorphite of higher stability (Pb10(PO4)6CO3) was developed at 20 mM Pb2+ as CO3 2- substituted the hydroxyl group in hydroxylpyromorphite. Moreover, nHAP helped EICP to form nucleated minerals. As a result, the EICP-nHAP technology raised the immobilization efficiency at 60 mM Pb2+ up to 70%. The findings highlight the potential of applying the EICP-nHAP technology to Pb-containing water bodies remediation.

Struvite and ethylenediaminedisuccinic acid (EDDS) enhance electrokinetic-biological permeable reactive barrier removal of copper and lead from contaminated loess.

Removal of heavy metals using the electrokinetic (EK) remediation technology is restricted by soils containing a fraction of clay particles above 12%. Furthermore, it is also affected by hydroxide precipitation (focusing phenomenon) close to the cathode. A modified EK reactor containing a permeable reactive barrier (PRB) was proposed herein where the enzyme-induced carbonate precipitation (EICP) treatment was incorporated into the PRB. Despite that, NH4+-N pollution induced by the urea hydrolysis resulting from the EICP treatment causes serious threats to surrounding environments and human health. There were four types of tests applied to the present work, including CP, TS1, TS2, and TS3 tests. CP test neglected the bio-PRB, while TS1 test considered the bio-PRB. TS2 test based on TS1 test tackled NH4+-N pollution using the struvite precipitation technology. TS3 test based on TS2 test applied EDDS to enhance the removal of Cu and Pb. In CP test, the removal efficiency applied to Cu and Pb removals was as low as approximately 10%, presumably due to the focusing phenomenon. The removal efficiency was elevated to approximately 24% when the bio-PRB and the electrolyte reservoir were involved in TS1 test. TS2 test indicated that the rate of struvite precipitation was 40 times faster than the ureolysis rate, meaning that the struvite precipitate had sequestered NH4+ before it started threatening surrounding environments. The chelation between Cu2+ and EDDS took place when EDDS played a part in TS3 test. It made Cu2+ negatively surface charged by transforming Cu2+ into EDDSCu2-. The chelation caused those left in S4 and S4 to migrate toward the bio-PRB, whereas it also caused those left in S1 and S2 to migrate toward the anode. Due to this reason, the fraction of Cu2+ removed by the bio-PRB and the electrolyte reservoir is raised to 32% and 26% respectively, and the fraction of remaining Cu was reduced to 41%. Also, the removal efficiency applied to Pb removal was raised to 50%. Results demonstrate the potential of struvite and EDDS-assisted EK-PRB technology as a cleanup method for Cu- and Pb-contaminated loess.

Immobilization of Pb(II) by Bacillus megaterium-based microbial-induced phosphate precipitation (MIPP) considering bacterial phosphorolysis ability and Ca-mediated alleviation of lead toxicity.

Inappropriate handling of lead (Pb)-containing wastewater that is produced as a result of smelting activities threatens the surrounding environment and human health. The microbial-induced phosphate precipitation (MIPP) technology was applied to immobilize Pb2+ in an aqueous solution considering bacterial phosphorolysis ability and Ca-mediated alleviation of lead toxicity. Pb immobilization was accompanied by sample characterization in order to explore the inherent mechanism that affected the immobilization efficiency. Results showed that Ca2+ use elevated the immobilization efficiency through the prevention of bacterial physisorption and chemisorption, an enhancement to the phosphatase activity and the degree of SGP hydrolysis, and the provision of nucleation sites for Pb2+ to attach. The formation of the Pb-GP complex helped the bacteria to maintain its activity at the commencement of catalyzing SGP hydrolysis. The nucleated minerals that were precipitated in a columnar shape through a directional stacking manner under MIPP featured higher chemical stability compared to non-nucleated minerals. As a result, there were three pathways, namely, bacterial physisorption, bacterial chemisorption, and substrate chelation, applied for Pb immobilization. The immobilization efficiency of 99.6% is achieved by precipitating bioprecipitates including Pb5(PO4)3Cl, Pb10(PO4)6Cl2, and Ca2Pb3(PO4)3Cl. The findings accentuate the potential of applying the MIPP technology to Pb-containing wastewater remediation.

Deterioration phenomenon of Pb-contaminated aqueous solution remediation and enhancement mechanism of nano-hydroxyapatite-assisted biomineralization.

Modern metallurgical and smelting activities discharge the lead-containing wastewater, causing serious threats to human health. Bacteria and urease applied to microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP) are denatured under high Pb2+ concentration. The nano-hydroxyapatite (nHAP)-assisted biomineralization technology was applied in this study for Pb immobilization. Results showed that the extracellular polymers and cell membranes failed to secure the urease activity when subjected to 60 mM Pb2+. The immobilization efficiency dropped to below 50% under MICP, whereas it due to a lack of extracellular polymers and cell membranes dropped to below 30% under EICP. nHAP prevented the attachment of Pb2+ either through competing with bacteria and urease or promoting Ca2+/Pb2+ ion exchange. Furthermore, CO32- from ureolysis replaced the hydroxyl (-OH) in hydroxylpyromorphite to encourage the formation of carbonate-bearing hydroxylpyromorphite of higher stability (Pb10(PO4)6CO3). Moreover, nHAP application overcame an inability to provide nucleation sites by urease. As a result, the immobilization efficiency, when subjected to 60 mM Pb2+, elevated to above 80% under MICP-nHAP and to some 70% under EICP-nHAP. The findings highlight the potential of applying the nHAP-assisted biomineralization technology to Pb-containing water bodies remediation.