The Interaction Influence Between Biofilm And Its Extracellular Polymers By Cu²⁺

Recently, wastewater polluted with heavy metals has drawn much attention due to its non-biodegradable, effect of bioconcentration in the top of the food chain, great and everlasting threat to the environment. Traditional methods of the treatment of heavy metal wastewater include chemical neutralization, electrolytic method and adsorption. However, these methods have shortcomings including a large amount of sediments, high pharmaceutical dosage and pH value of the effluent which may cause serious secondary pollution easily. These make it necessary to seek high-efficiency, economic technical methods without secondary pollution, and biomass adsorbents-a hotspot of the current research-just meet the above requirements.Biofilm is the aggregate of microorganisms, of which the special structure makes itself a buffer zone of the harsh environment and a greenhouse for the microorganisms. Extracellular polymeric substances (EPS), widely existing extracellular macromolecules secretions in microorganisms under certain environmental condition, can maintain the integrity of the structure and function of the biofilm and play an important role in wastewater treatment. They function as the key components to the surface properties of biofilm as they are critical to the microbial morphology and the attachment, composition, performance of the microbial aggregates. Heavy metals existing in wastewater can:1) react with the proteins from EPS thus resulting in the changes of the compositions and microstructures of EPS which will affect the mass transfer channels and the buffering effect of the biofilm,2) change the microstructures of the biofilm in a short time and influence the stability and mass transfer property of the biofilm system consequently by giving an impetus to yielding much EPS by microorganisms.On that account, the effect of Cu2+on the micromorphology, composition and adsorption of biofilm and EPS, as well as the action mechanisms of Cu2+with biofilm and EPS, was systematically investigated in this study. The microstructures of biofilm and EPS were also studied here. The results show that:1) The cultivation of biofilm was conducted in a self-designed reactor following an intermittent operation method. The COD, NH3-N in the reactor and the EPS content were daily monitored. The results show that the biofilm is successfully formed in the eighth day, and there are various microbes in the reactor. Glass carriers are of cheap price, easy availability, good performance, long service life. Therefore they are the best choice to cultivate biofilm. 2) When pH value varies between5and6, and the temperature is20℃, the performance of biofilm on the adsorption of Cu2+is the best. And the equilibrium of adsorption for Cu2+by biofilm is reached in40minutes. When the initial metal ion concentration of Cu2+is0-20mg/L, the removal rate of Cu2+through biofilm sorption is the best. But higher concentration of Cu2+will poison the biofilm and the process of adsorption will be affected. Ionic strength has a strongly influence on the adsorption of Cu2+. In coexisting systems there exist competitive adsorption, and the adsorption capacity of heavy metal cations by biofilm follows the order of Pb2+>Cd2+Cu2+. Pb2+has a greater influence on the the absorption of Cu2+by biofilm compared to Cd2+. By the FTIR spectra of biofilm, the main adsorption functional groups are hydroxyl and amine. From the SEM results, we find that Cu2+stimulates the biofilm to secrete more extracellular polymers.3) The resistance to Cu2+of biofilm is better than that of activated sludge. When Cu2+is brought in, biofilm will react to the change by starting a series of adaptation and detoxification mechanisms. But the tolerance range is very low. To be specific, only when the concentration of Cu2+is5mg/L or lower, biofilm microorganisms will secrete more EPS to protect themselves. Cu2+can inhibit the biofilm activity by inhibiting the PS in biofilm. Conversely, the resistance of Cu+by biofilm can achieved by secreting more PN. The more EPS content biofilm secretes, the greater the adsorption capacity of Cu+by biofilm will be.4) When the value of C/N ratio is12, the biofilm grows best. Higher C/N ratios will do harm to the growth of biofilm. As the C/N ratio increases, PN/PS ratio declines showing an opposite tendency. The smaller PN/PS value is, the higher the amount of adsorption of Cu2+by biofilm will be, while the relationship of PN/PS value and the removal rate of Cu2+by biofilm doesn’t follow this rule-the removal rate of Cu2+doesn’t strictly increase or decrease as PN/PS ratio changes. The ion exchange plays a main role in the process of the adsorption of Cu+by biofilm. What’s more, Cu2+is easier to exchange with Ca2+and Mg2+. The hydroxyl, carboxyl and amino in EPS are the main functional groups for the adsorption of Cu+. SEM image shows that the biofilm can grow better only when the C/N ratio is suitable.