Heavy metals bioremediation and water softening using ureolytic strains Metschnikowia pulcherrima and Raoultella planticola

BACKGROUND The recalcitrant nature of lead (Pb) and mercury (Hg) to degradation represents a real risk to living creatures and their surrounding ecosystems. Therefore, this study employed ureolytic fungal strain Metschnikowia pulcherrima (29A) and bacterial strain Raoultella planticola (VIP) in their removal using the promising CaCO3 bioprecipitation technique, for the first time. RESULTS Out of 50 isolates, strains 29A and VIP were selected based on their highest ureolytic activity, followed by minimum inhibitory concentration assay using 350 ppm of Pb2+ and Hg2+. Throughout 7 days of incubation at 30 degrees C and 150 rpm with 10(8) CFU mL(-1) inoculum, the maximum urease activity was 884 and 639 U mL(-1) for 29A and VIP at 24 and 30 h of incubation, respectively. Complete removal of Pb2+ was achieved at 42 h (29A) and 90 h (VIP), while Hg2+ was totally removed at 60 h (29A) and 102 h (VIP). Remarkable removal of Ca2+ (>95%) was achieved by the end of the experiments, which would address the hardness problem in water treatment processes. Further, energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the remediated precipitates. EDX profiles showed characteristic peaks of C, O and Ca besides Pb and Hg. SEM illustrated the presence of microbial imprints and calcinated cells in the remediated bioliths. However, XRD confirmed transformation of soluble metals to insoluble forms entrapped in calcite or vaterite lattice. CONCLUSIONS Such a bioremediation approach ensures the detoxification and sequestration of heavy metals in stable and durable matrices, hindering their leaching from carbonate complex trap to the environment. (c) 2021 Society of Chemical Industry (SCI).

Automated summary

This study successfully removed lead and mercury using fungal and bacterial strains with high ureolytic activity, and characterized the precipitates using EDX, SEM and XRD, achieving detoxification and sequestration of heavy metals.