Adsorption Performance And Mechanism Of Pseudomonas Putita EPS For Lead And Cadmium

The situation of heavy metal pollution is still severe,threatening ecological and environmental safety and people's health,and there is an urgent need to adopt effective means to manage heavy metal pollution.The application potential of extracellular polymeric substances(EPS)as novel heavy metal biosorbent has received wide attention,but the low yield of reported microbial EPS has limited the further application of this method.Pseudomonas putida(P.putida)is a common model organism that is easy to cultivate and has high EPS yields,making it an advantageous strain for biosorption studies.At the same time,there are still few studies on the stress culture of P.putida using heavy metals,and the mechanism of the changes in the content of chemical components in EPS before and after stress and the effect on the adsorption performance of heavy metals is still unclear.Therefore,further studies are necessary.In this thesis,the adsorption performance and action mechanism of P.putida EPS on lead and cadmium was studied.Different heavy metal ions(Pb(?),Cd(?),Pb(?)+Cd(?))were used to culture P.putida under stress,directional regulation of its EPS characteristics,and further improve the EPS production of P.putida,especially the protein content in EPS.At the same time,the adsorption performance of P.putida EPS on heavy metals Pb(?)and Cd(?)before and after stress was investigated,and P.putida and its EPS before and after stress were characterized and analyzed,and the influence mechanism of different stress methods on the growth and metabolism of P.putida was elucidated,so as to reveal the mechanism of heavy metal adsorption by P.putida EPS.Firstly,P.putida EPS yield achieved a maximum value of 131.55 mg/g at Pb(?)stress concentration of 10 mg/L,which was 25.35%higher than before the stress;protein content was 87.01 mg/g,which was 35.15%higher than before the stress,while the maximum adsorption capacity of Pb(?)was 1452.59 mg/g,which was 74.05%higher than before the stress.The analysis showed that Pb(?)stress could increase the content of amino acids and protein-like substances in P.putida EPS,thus improving the adsorption performance of P.putida EPS on Pb(?).And the enhancement of the intensity of the spectral peaks of the negative functional groups C=O,N-H,-COO-,C-N,and-OH and the higher electronegativity Zeta potential values also confirmed that Pb(?)stress improved the adsorption performance of P.putida EPS.Secondly,the stress culture of P.putida using Cd(?)showed that at the optimum stress concentration of Cd(?)(10 mg/L),the EPS yield,protein content and maximum adsorption of Cd(?)by P.putida were 117.20 mg/g,70.29 mg/g and 825.59 mg/g respectively,which were54.76%,73.47%and 125.60%respectively higher than before the stress.The XPS results indicated that the presence forms of C,N and O elements in EPS changed after Cd(?)stress on P.putida,and C elements combined with N and O elements to generate more C-O,C-N,C=O,C-NH/C-NH₂,reduce the protonation phenomenon in EPS and form more protein backbone,which can improve the metal coordination ability of EPS,and thus enhance the ability to Cd(?)adsorption ability.However,P.putida EPS yield,protein content,and fluorescence intensity of tyrosine and tryptophan fluorescence peaks of proteins appeared to be reduced after Cd(?)stress compared with Pb(?)stress.Therefore,the stress effect of Pb(?)on P.putida was greater than that of Cd(?).Finally,the biological dry weight of P.putida under Pb(?)and Cd(?)co-stress decreased from 1.29 g/L to 0.31 g/L.The maximum yield of EPS after stress was 100.02 mg/g,which was 32.07%higher than that before stress;the maximum protein content was 59.69 mg/g,which was 47.31%higher than that before stress.The adsorption performance of Pb(?)and Cd(?)by Pb+Cd EPS obtained after co-stressing was still better than Control EPS,with the adsorption amounts of 1077.75 mg/g and 462.10 mg/g,which were 30.76%and 48.25%higher than Control EPS,respectively.However,the effects of Pb(?)and Cd(?)co-stress were all smaller than those of Pb(?)and Cd(?)alone.The optimal adsorption conditions for the adsorption test were pH 5.0,adsorption temperature of 40?and adsorption time of 120 min.