Fungal-Mediated Calcite Precipitation And Its Role In Heavy Metals Remediation From Soil

The main drawbacks emerging from traditional treatment approaches for heavy metal decontaminations,such as high energy consumption,high economic cost and secondary pollution,have inevitably drawn the attention to novel alternatives of eco-friendly and safe.Hence,the microbially induced calcite precipitation(MICP)technology,as a novel in-situ bioremediation technology,was increasingly valued by researchers especially due to its high effectiveness and environmental friendliness.Although MICP is extensively studied using bacteria for the immobilization of heavy metals,this technology is still not completely characterized for the remediation of metals from solution and soil using fungi.To deepen the knowledge about the possible role of filamentous fungi and mechanisms involved in fungal mediated calcite precipitation process for heavy metal immobilization,both shake flask and soil experiments were carried out in this research using an isolated ureolytically fungal strain CS1,identified as Penicilium chrysogenum on molecular basis.This fungal strain was utilized to remediate Cr(?)(chromate)and Pb(lead)from contaminated environment.Fungal strain CS1,was resistant for up to 150 mg/L of Cr(?)and 400 mg/L of Pb,was employed in bioremediation experiment firstly at shake flask level after optimization of medium and condition factors for fungal growth and urease activity.Results showed that fungal uptake of Cr and Pb with MICP treatment improved by 32%-42.2%and 29.9%-61.4%,respectively.It proved that MICP treatment with P.chrysogenum CS1 was able to promote removal rate of both Cr(?)and Pb significantly.The possible mechanisms for removal of Cr(?)and Pb was further studied from morphology and mineralogy using SEM,FTIR,and XRD.Results indicated that both biosorption and bioprecipitation were involved during fungal MICP process for heavy metals removal,where biosorption was regarded as a pre-step for bioprecipitation.In contrast to the direct formation and deposition of Pb as lead carbonates,Cr(?)was immobilized in a co-precipitation way where CrO4-was incorporated into calcite lattice and finally formed chromium contained carbonates of a mixed solid phase.Besides,morphology analyses showed that the fungal mycelia may have a superiority over bacteria that it was acting as "bio-glue" to form a thick calcareous shell,providing numerous substrates and nucleation sites for biomineral precipitation,and capturing those biominerals inside the filamentous structure from releasing again.Further,MICP supported P.chrysogenum CS1 was employed in heavy metals bioremediation from soil matrix.The results indicated that most of mobile Cr and Pb was reduced from exchangeable fraction to carbonate bound fraction,by which the bioavailability and toxicity of heavy metals was substantially diminished after MICP treatment with P.chrysogenum CS1.The bioremediation efficiency slightly decreased in respect to deeper level in soil,but the variance was small.It confirmed that MICP treatment with P.chrysogenum CS1 was considerably effective and operative in in-situ bioremediation of both Pb and Cr from soil matrix.Despite a minor decrease in soil porosity after MICP treatment,it didn't cause calcareous hardening problem and the multiporous structures of soil was reserved largely for plants and microorganisms growth.This research is probably the first study where fungal mediated calcite precipitation was completely characterized in remediation of heavy metals.