Study On The Performance Of Carbonate-mineralized Bacteria Combined With Eggshell On Remediation Of Pb And Cd Contamination

Cadmium and lead are the widespread metals around the globe and toxic to various organisms.However,the amendments that can efficiently remediate Cd and Pb polluted soils and water are relatively rare.The use of microbial induced carbonate precipitation to remediate heavy metal contaminated environment attracted more and more attention in these years.Microbially induced carbonate precipitation(MICP)is a advanced bioremediation technique for removing heavy metals(HMs)from polluted water and soil.In this study,metal tolerant urease-producing bacterial isolates were isolated from HMs contaminated soil.The isolates namely UR1,UR16,UR20 and UR21 were further tested for their ability to hydrolyze urea and calcite production.The 16S rRNA gene was sequenced and compared with the known sequence in the NCBI database.The results revealed that the isolates UR1,UR 16,and UR21 are closely related to one another and correspond to the genus of Bacillus sp.and the isolate UR20 belongs to the genus Citrobacter sp.Among the selected bacterial isolates,UR20 exhibited a high degree of resistance to Pb and the MIC value was 1400 mg L-1,followed by the isolate UR21 showed the MIC value of 1300 mg L-1.Whereas,the MIC value for the Cd was 500 mg L-1 and 400 mg L-1 for UR20 and UR21,respectively.In addition,the effectiveness of these isolates for immobilizing Pb and Cd in water was investigated.Through MICP,we discovered that UR21 had the greatest removal efficiencies of Pb(81.9%)and Cd(65.0%)in solution within 72h.The morphology and presence of PbCO3 and CdCO3 crystals in the precipitates were verified by scanning electron microscopy-energy dispersive xray(SEM-EDS)and x-ray diffraction(XRD)analyses.The bacterial strain UR21 was then combined with eggshell waste(ES)to analyze the impact of MICP on soil.The combined application of eggshell and UR21 lowered the fraction of soluble-exchangeable Pb and Cd in the soil,resulting an increase in carbonated bound Pb and Cd in the soil.When compared to the control,the DTPA extractable Pb and Cd were decreased by 29.2%and 25.2%,respectively,following the treatment of UR21+urea+ES.Furthermore,the use of UR21 and ES resulted in a substantial improvement in soil pH and cation exchange capacity.MICP also enhanced the enzymatic activity of the soil.Furthermore,a pot experiment was designed to examine the interactive effect of UR21 and eggshell(ES)on growth,physiological parameters,antioxidant performance,quality indexes and Cd accumulation of pakchoi(Brassica chinensis L.)grown in cadmium contaminated soil,the soil enzyme activity were also assessed.The application of UR21 together with ES significantly increased the plant biomass,photosynthetic pigment content and the activities of antioxidant enzymes superoxide dismutase(SOD)and peroxidase(POD)in the shoots and roots of the pakchoi plants,which assist to mitigate the oxidative damage caused by Cd stress.The outcome revealed that the highest increment of fresh and dry mass in shoots and roots obtained under ES+UR21 treatment which increased 57.70%,24.19%and 61.54%and 72.73%,respectively as compared to the Cd stressed plants.Similarly,the maximum increase of the plants height and root length was 13.08%and 12.72%under the treatment of ES as compared to the Cd treated plants.The quality index of the plant such as soluble sugar and soluble protein content was also improved with the application of UR21 and eggshell.The ES+UR21 treatment significantly declined the content of Cd in shoots and roots of the pakchoi plant by 26.96%and 42.91%,respectively as compared to Cd stressed plants.The enzymatic activity of sucrose was increased by 9.33%and the urease activity by 2.39%by the application of ES+UR21.Through a conjunction of metal-resistant ureolytic bacterial strains and eggshell waste,our findings may give a unique perspective for an eco-friendly and sustainable strategy to remediating heavy metal polluted environments.