Comparison Of Adsorption Capacity And Adsorption Mechanism Of Three Microorganisms To Cadmium

Heavy metals pollute the environment and pose a threat to public health through drinking water and the food chain.Conventional physical and chemical treatment technologies are more expensive,are more demanding on operating conditions and it is difficult to treat lower concentrations of heavy metal wastewater.Microbial biosorption technology provides a more economical and competitive solution for the bioremediation of toxic substances?such as heavy metals?,and microbial genetic modification can make microorganisms modify in the direction of optimal adsorption.Based on the above purposes,it is very important to screen suitable strains and explore their adsorption mechanism.In this study,Escherichia coli,Bacillus subtilis and Saccharomyces cerevisiae were isolated and identified from uncontaminated soil.The tolerance of these three strains to cadmium(Cd²⁺)were studied,and their differences in Cd²⁺ adsorption were compared.Firstly,according to the “Bergey's Manual of Determinative Bacteriology” and “Fungus Identification Manual”,strains similar to the colony characteristics and morphology of Escherichia coli,Bacillus subtilis and Saccharomyces cerevisiae were screened.The strains were sequenced using 16 S r DNA or 26 S r DNA,respectively.The sequencing results were compared with the NCBI database by BLAST.The three strains used here had the highest similarity?more than 99%?with Escherichia coli,Bacillus subtilis and Saccharomyces cerevisiae.The phylogenetic analysis also showed that the three strains were most closely related to the published E.coli,B.subtilis and S.cerevisiae.Based on this,the tolerance of three strains to Cd²⁺ were studied.It was found that Escherichia coli is more tolerant and can grow at a concentration of 40 mg / L Cd²⁺,while Bacillus subtilis and Saccharomyces cerevisiae can only grow at a concentration of 20 mg / L Cd²⁺.Secondly,we evaluated the effects of exposure time and initial concentration on Cd²⁺ adsorption and found that a pseudo-second-order kinetic model is closer to describing the Cd²⁺ biosorption process of the three strains.Adsorption isotherms were used to reflect their biosorption capacity.The maximum biosorption capacity of the three strains calculated by the Langmuir model was 37.764,56.497,and 22.437 mg Cd / g biomass,respectively.In the bacterial group,the biosorption of Cd²⁺ mainly occurred on the cell wall,while the biosorption between yeast inside and outside the cell was not significantly different.We found that due to the structural difference,Bacillus subtilis had the best adsorption effect at higher concentrations,while the removal rate of Escherichia coli surface decreased at high concentrations,and the biosorption capacity of Saccharomyces cerevisiae remained low.Besides,Fourier transform infrared spectroscopy reflects the differences in the functional groups involved in biosorption of the three strains.The results showed that Bacillus subtilis had the most functional groups in the process of Cd²⁺ adsorption.Scanning electron microscopy and EDS analysis showed that the binding of Cd²⁺ to microorganisms mainly depended on the ion exchange mechanism.In addition,it was found that the surface of Escherichia coli and Saccharomyces cerevisiae became rough and wrinkled after Cd²⁺ adsorption,and extracellular polymers were produced on the surface of Bacillus subtilis,which is an important site for Cd²⁺ binding.In briefly,among the three strains Escherichia coli,Bacillus subtilis and Saccharomyces cerevisiae isolated from the same site,Escherichia coli has higher Cd²⁺ tolerance.Different strains have different Cd²⁺ adsorption characteristics and adsorption capacity due to their structural differences,but the ion exchange mechanism is the main mechanism for three strains to adsorb Cd²⁺.Among them,Bacillus subtilis had a stronger adsorption capacity for Cd,reaching 56.497 mg / g.This is because more types of functional groups are involved in surface adsorption,and extracellularpolymers are generated during the contact of Cd²⁺,which enhances the extracellular adsorption capacity.