Research On Biosorption Characteristics Of Cadmium And Lead By Extracellular Polymeric Substances From Enterobacter Sp.

Extracellular polymeric substances(EPS)are metabolites secreted outside the cell by microorganisms during the process of growth and metabolism,which are a complex mixture of macromolecular substance that are easily separated from microorganisms.Their excellent metal-adsorption properties have been widely reported.Different sources will lead to different structures and components of EPS,as a result,the adsorption properties and action mechanism of EPS for heavy metals(HMs)may be different.In this paper,S-EPS(Soluble EPS),LB-EPS(Loosely Bound EPS)and TB-EPS(Tightly Bound EPS),secreted by Enterobacter sp.FM-1 isolated from HM-contaminated soil in our previous study,were taken as research objects.The effects of different external conditions on the secretion characteristics of different EPS and the adsorption characteristics in Cd²⁺and Pb²⁺coexisting solution were discussed.In addition,the effects of different EPS on soil physical and chemical properties and microbial community were studied.The main research results are as follows:(1)Among the three types of EPS,the yield of S-EPS reached a maximum,followed by LB-EPS and TB-EPS reached a minimum.At p H 7.0,S-EPS reached a maximum yield with a value of 43.19 mg/L.The sum of the three types of EPS reached the maximum yield of 63.11 mg/L at 30?.When the strain was cultured for 24 h,the yield of different EPS reached the maximum.In a word,the best extraction conditions of EPS are at the culture time of 24 h,temperature of 30?and p H value of 7.0.(2)In the coexistence solution of Cd²⁺and Pb²⁺,p H value of 6.0 was the most suitable for the adsorption and removal of Cd²⁺and Pb²⁺by LB-EPS and TB-EPS.At 30?,S-EPS?LB-EPS and TB-EPS reached the maximum adsorption capacity of Cd²⁺which was 10.25mg/g,6.76 mg/g and 7.00 mg/g.The adsorption can basically reach the adsorption equilibrium at about 120 min,which was the optimal adsorption time.Generally speaking,the best adsorption conditions for EPS are the adsorption time of 120 min,temperature of30?and p H value of 6.0.(3)For different EPS,Langmuir model fitted the equilibrium data better than the Freundlich isotherm.LB-EPS had the theoretical maximum adsorption capacity for Cd²⁺,which is 42.75 mg/g;S-EPS had the largest adsorption capacity for Pb²⁺,which reached139.33 mg/g.The adsorption of Cd²⁺and Pb²⁺by different EPS is a spontaneous reaction process(?G<0).Among them,for Cd²⁺adsorption,the adsorption process of different EPS is exothermic reaction(?H<0)and which is endothermic reaction(?H>0)for Pb²⁺adsorption.The positive?S value suggested an increase in the randomness during the biosorption process.The adsorption kinetic data of different EPS for Cd²⁺and Pb²⁺fitted better with the quasi-second-order kinetic model,which indicated that chemisorption dominated the adsorption process.(4)From the EDS spectrum,it can be seen that the content of elements as C and N significantly decreased,Na decreased or even disappeared and Cd and Pb increased after the adsorption of different EPS.Meanwhile,the zeta potential showed that the surfaces of different EPS were negatively charged.After adsorbing Cd²⁺and Pb²⁺,the zeta potential of different EPS increased and the negative charge on the EPS surface decreased,indicating that a large number of negatively charged C-containing and N-containing functional groups participated in biosorption which taken place by chemical ion-exchange.Combined with FTIR and XPS spectrum,it can be speculated that the functional groups in EPS participated in the adsorption are mainly C-(C,H),-OH,C=O and C-O in polysaccharides and C=O and N-H in proteins.The 3D-EEM spectrum indicated that tryptophan-like proteins,humic acid-like substances and tyrosine-like proteins play an important role in the adsorption,among which,tyrosine-like substances were important complex substances for EPS binding to Cd²⁺.(5)The addition pattern of different EPS increased the contents of bio-Cd,bio-Pb and bio-Zn in the soil.In non-contaminated soil(NC soil)and lower contaminated soil(LC soil),the addition pattern of FM-1,LB-EPS and TB-EPS could increase the contents of EPS-polysaccharide and DNA by 8.49%-92.01%and 37.57-318.93%.(6)The addition pattern of different EPS promoted the soil urease,sucrase and catalase activities compared to control in 42 days,which increased by 13.96-198.83%,22.08%-413.21%and 18.35%-112.12%,respectively.It also increased the content of soil organic matter(SOM),ammonia(NH₄⁺-N)and available phosphorus(AP),SOM increased by 0.66-2.01 fold in LC soil and 2.44-5.28 fold in high contaminated soil(HC soil).The AP content in the three types of soil increased ranging from 10.81%to 161.52%.The highest NH₄⁺-N content was found in different soils under the S-EPS addition pattern,which was 83.86mg/kg,81.47 mg/kg and 36.59 mg/kg,respectively.(7)The addition pattern of different EPS resulted in the increase of the relative abundance of Proteobacteria,Gemmatimonadetes and Bacteroidetes,made the Alphaproteobacteria,Gammaproteobacteria and Bacteroidia increased by 0.20-10.12%,1.50%-7.98%and 0.53%-16.14%,respectively.In LC soil,it significantly increased the Shannon index,which was 1.13,1.15 and 1.13 times higher than control,respectively.For fungi,the addition pattern of different EPS increased the relative abundance of Ascomycetes,and increased by 12.90%-27.69%in LC soil,besides,the Chao 1 index significantly increased while the Shannon index decreased.Therefore,the addition pattern of EPS changed the composition of soil microbial community and increased the abundance of microorganisms with high tolerance to HMs.