| In recent years,with the rapid development of industrial and urban modernization,the problem of heavy metal water pollution is becoming more and more serious.Compared with traditional repair technology,biological adsorption,represented by microbiological method,has gained more and more attention because of its low price,high efficiency and environmental friendliness.In this study,the gram-negative bacteria Rhizobium sp.W33 and gram-positive bacteria Bacillus sp.H3 were selected as the tested strains,and the adsorption of lead and cadmium by two kinds of microbial adsorbents on lead and cadmium and their mechanism were compared and analyzed.The main results are as follows:1.The effects of pH,lead and cadmium concentrations on the growth of strains were measured.Orthogonal test was designed to investigate the optimal adsorption conditions.Compared to Cd,the strains had strong resistance to Pb.The optimal growth pH was within the 6-7 range.The Removal efficiency of Cd by Rhizobium sp.W33 and Bacillus sp.H3 was better under alkaline conditions,and the removal rates were 54%and 68%respectively.Under acidic conditions,the removal efficiency of Pb was better,and the removal rate was 81%and 89%respectively.Further orthogonal results showed that the concentration of heavy metals and pH were the significant factors affecting the removal of cadmium and lead.2.The adsorption capacity of lead and cadmium by the cells between the strains and different states(Growing,Grown,Dead)was compared.The results showed that the adsorption of lead and cadmium by the gram-negative bacteria Rhizobium sp.W33 was better than that of gram-positive bacteria Bacillus sp.H3.The maximum bioadsorption of lead and cadmium by Rhizobium sp.W33 live cells reached 62.41 mg g-1 and 36.98 mg g-1,respectively,the dead cells were 42.39 mg g-1,25.65 mg g-1 respectively;Bacillus sp.H3 live cells were 30.84 mg g-1,19.15 mg g-1,and the dead cells were 25.08 mg g-1 and 24.60 mg g-1,respectively.In addition,lead and cadmium were adsorbed through the way of extracellular adsorption,and the stains had good intracellular accumulation ability of lead.The results of the single and competitive adsorption of lead and cadmium by Rhizobium sp.W33 and Bacillus sp.H3 were analyzed.It was found that the affinity for lead was far greater than that of cadmium,and lead had a strong inhibitory effect on the adsorption of cadmium.The results of dynamic adsorption of growing cells showed that the second-order kinetics model could better fit the adsorption process of lead and cadmium.It showed that the adsorption rate was controlled by the mechanism of chemical adsorption,which involved the interaction between the surface functional groups of the bacteria and Cd and Pb.3.Under the induction of heavy metals,the changes of extracellular polysaccharide and protein contents of the strains showed that the total polysaccharide content of Rhizobium sp.W33 increased by 5%,the total protein content decreased by 12.6%after lead treatment,and the total protein content under cadmium treatment increased by about 7%.The total polysaccharide content of Bacillus sp.H3 increased 4.5%and 7.3%respecti-vely after lead and cadmium treatment,the total protein content increased by 1.37 times and 1.14 times respectively.SDS-PAGE results showed that Rhizobium sp.W33 and Bacillus sp.H3 induced two new bands in 30 kDa and 25 kDa,in addition,along with other bands of up-regulation or down-regulation.Using FTIR to analyze the adsorption related functional groups,the surface functional groups of the two strains were significantly different.In the adsorption process of live cells(Grown),-OH,-NH,-CN,-C=O,-CH2 functional groups mainly played a role,and there were phosphate groups involved in the process of dead cells adsorption.SEM analysis showed that compared with lead treatment,the morphology of Rhizobium sp.W33 and Bacillus sp.H3 changed seriously under cadmium treatment.The surface of Rhizobium sp.W33 became concave,and the surface of Bacillus sp.H3 became crumpled,some long rod like faults changed into short rods.EDS energy spectrum analysis showed that the particles on the surface of bacteria were lead and cadmium.TEM analysis showed that intracellular accumulation of lead was more evenly distributed in the cytoplasm. |
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