The Effects And Interaction Mechanism Between Cd²⁺ And Electrochemically Active Biofilms

Electroactive biofilms（EABs）is formed by microorganisms with special intracellular and extracellular electron exchange capabilities,which is a very important part of bioelectrochemical systems（BES）.BES technology is an innovative wastewater treatment technology that can generate energy while removing pollutants,and has broad application prospects.However,when applied to the treatment of wastewater containing both heavy metals and organic matter,EABs is often disturbed by pollutants,which affect its growth and extracellular electron transfer,so that the effect of heavy metals on EABs during BES operation will affect the migration of heavy metals.Therefore,in this study,a two-chamber bioelectrochemical system was constructed,and Cd²⁺with low redox potential was adding in the anodic chamber to research the effects of Cd²⁺on the electrical activity and morphological structure of EABs,and the mechanism of action during the short-term and long-term operation of BES were discussed in depth,which provided a method for optimizing BES performance by heavy metals acting on EABs.The specific results are described below.The electrical activity of EABs is shown by the power generation performance of BESs,when the concentration of Cd²⁺is 2 mg/L,The voltage and power density of BESs are not inhibited,When the concentration of Cd²⁺is increased from 5 mg/L to 20 mg/L,the power density is reduced from 0.34±0.01 W/m² to 0.17±0 W/m²,even at 30 mg/L,there is almost no power output.The results of cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）showed that the high concentration of Cd²⁺over 5 mg/L inhibit the secretion of cytochrome in the outer membrane,and thus reduce the electron transfer between EABs and the anode surface.The electroactivity of EABs is observed by scanning electron microscopy（SEM）and confocal laser scanning microscopy（CLSM）analysis,when20 mg/L and 30 mg/L Cd²⁺were added to the anodic chamber of BESs,the original dense biofilm on the anode surface became sparse with the increase of the Cd²⁺concentration.The number of green alive cells was decreased.The icrobial community analysis results indicated that,when the concentration of Cd²⁺was increased to 20 mg/L,the relative abundance of Geobacter decreased to 28.51%,while the relative abundance of Stenotrophomonas and Azospira increased.When subjected to Cd²⁺,the migration and transformation of Cd²⁺in BESs is also affected by the concentration of Cd²⁺,The results showed that the removal rate of Cd²⁺reach more than 98%when the concentration of Cd²⁺was 2 mg/L,but decreased to80.5±0.01%when concentration was increased to 30 mg/L.The X-ray photoelectron spectroscopy（XPS）,which showed that Cd²⁺in anode could be removed by binding carboxyl,ether and alcohol hydroxyl groups on the cell surface under the biosorption of microorganisms.The effect of Cd²⁺during the long-time operation on EABs and the morphology and structure of EAB_S was studied with the concentration gradient increasing interval of 2 mg/L and 5 mg/L.During the operation of BESs,the electricity production performance is inhibited with the increase of the concentration of Cd²⁺added.When the output voltage continues to drop below 0.1 V,the BESs in the concentration range of 2 mg/L are up to 140days.which is 40 days longer than that of BES with 5 mg/L concentration increasing interval.In addition to voltage analysis,the study also found that the maximum power density of BESs with 2 mg/L concentration increasing interval and 5 mg/L concentration increasing interval are 0.98 W/m² and 0.48 W/m²,respectively.However,with the increase of Cd²⁺concentration reaches 50 mg/L and 40 mg/L,respectively when the power density decreases to below 0.1 W/m².In addition,the increasing relative abundance of Stenotrophomonas and Geobacter of the main dominant bacteria,which also indicated that the existence of heavy metal resistant bacteria has been screened after a long period of acclimation.At the early stage of BESs operation under the two increasing concentrations,the removal rate of Cd²⁺reach more than 90%,but after the Cd²⁺concentration adding over 35mg/L,compared with the removal rate of Cd²⁺with 5 mg/L concentration increasing interval decreased to 72.53%,the removal rate of Cd²⁺with 2 mg/L increasing interval is more stable due to the longer operation time,and almost all of the removal rate can reach about 90%after one day of adding Cd²⁺.The BESs anode and biomembrane after long-term operation can be further studied and analyzed by scanning electron microscope（SEM）.The results showed that Cd²⁺could be adsorbed on the electrode or biofilm and formated precipitates.