Effects Of Different Drving-rewetting Cycles On Adsorption And Tolerance Of Cadmium And Lead In Biofilm

Mining,metal smelting,electroplating and other industrial activities increase the level of heavy metals in the environment.Among them,heavy metals Cd²⁺and Pb²⁺have caused serious harm to the ecological environment and human health.Biofilm can affect the fate and bioavailability of heavy metals in the environment through surface adsorption,absorption and redox reaction.The increase of seasonal rivers leads to the increase of drying-rewetting（D-RW）frequency,which leads to the change of river hydrology,and then affects the interaction between biofilm and heavy metal ions.The changes of adsorption and tolerance of biofilm to heavy metals after D-RW are not clear.Therefore,in this study,D-RW were combined with the heavy metal tolerance and adsorption of biofilm,in order to explore the tolerance and adsorption behavior of biofilm to heavy metals after D-RW.In this study,biofilms and water samples were collected from the natural river,and biofilm was cultivated in four rotating annular reactors.By changing the cycles of drying-rewetting stress,the control biofilms did not do any treatment,and the other three reactors were cultivated to one（C1）,three（C3）and five（C5）cycles of drying-rewetting stress,the tolerance and adsorption behavior of biofilms to heavy metals after drying-rewetting were explored.The specific contents are as follows:（1）the effects of biofilms on heavy metals Cd²⁺and Pb²⁺were explored by using kinetics,isotherm and competitive adsorption model;（2）The changes of biofilm tolerance were further explored by measuringβ-glucosidase activity;（3）Combined with the composition of biofilm,surface morphological characteristics,changes of population structure and spectral analysis,the possible tolerance and adsorption mechanism of biofilm were put forward.The results are as follows:（1）After drying-rewetting,the biofilm thickness decreased due to the death of some microorganisms and the shedding during rewetting.The content of extracellular polymer（EPS）in biofilm decreased first,then increased and finally decreased.In addition,the different bacterial community structure,the change of function and the decrease of community diversity were observed in the biofilm after D-RW.（2）The adsorption kinetic model shows that the process of promoting the attachment of Cd²⁺and Pb²⁺to the biofilm surface involves the exchange or sharing of valence electrons.The adsorption of Cd²⁺by biofilm conforms to Freundlich model,higher K_F and n values indicated that the control biofilms showed higher affinity for Cd²⁺,while the adsorption of Pb²⁺generally conforms to Langmuir mode,and smaller R_L values indicated that C1 biofilms showed higher affinity for Pb²⁺.The competitive Langmuir model showed that the synergistic effects of C3 and C1 biofilms were observed in binary-metal system,respectively,while there were antagonistic effects in other biofilms.The results indicated that there was a competitive relationship between Cd²⁺and Pb²⁺,and D-RW would change the competitive relationship between Cd²⁺and Pb²⁺.During the adsorption of Cd²⁺and Pb²⁺on biofilms,functional groups including-NH,-COOH,-OH might bind to metal ions through coordination and complexation.（3）After different D-RW cycles,the tolerance of biofilm to Cd²⁺increased,the tolerance to Pb²⁺decreased first and then increased,and the adsorption capacity of biofilm showed a similar change trend.In addition,the changes of microbial population in biofilms and Pearson’s correlation analysis showed that D-RW could induce the changes of EPS content and microbial community structure in biofilm,and then cause the changes of adsorption capacity and tolerance of biofilm to Cd²⁺and Pb²⁺.In conclusion,different D-RW cycles can induce biofilm to form different drought-tolerance strategies.The thickness,EPS content,population structure and function of biofilms changed,the competitive interaction between the two metals and the affinity of the two metal binding sites changed after D-RW.In addition,D-RW caused changes in EPS content and microbial community structure in biofilm,and then caused changes in the adsorption capacity and tolerance of biofilm to Cd²⁺and Pb²⁺.