Optimization Of Extraction Condition Of Extracellular Polymeric Substances From Activated Sludge And Its Adsorption Ability And Mechanism For Lead Ions

Extracellular polymeric substances (EPS) is secreted by microbes and the third important component besides water and cells in activated sludge. EPS could be divided into two types according to the adhesive capacity difference to the microbial cells, i.e. loosely bound EPS (LB) and tightly bound EPS (TB). The main components of EPS include macromolecules such as polysaccharides, proteins and nucleic acids. These macromolecules contain a large number of negatively charged functional groups, which have a strong adsorption capacity to heavy metal ions. Therefore, EPS as a new type of bioabsorbent has attracted more and more attention to remove heavy metal ions from wastewater in recent years. In this study, the extraction conditions of EPS from activated sludge were optimized and the adsorption capacity of EPS for lead ions was investigated. Besides, the adsorption mechanism of LB and TB were elucidated by various characterization approaches such as IR, XRD, and XPS.The effects of various extraction parameters such as pH, temperature, ultrasonic frequency and ultrasonic time on EPS content from activated sludge were optimization by response surface methodology to determine the extraction condition. Results indicated that the optimum extraction conditions were pH of7.1, temperature of57℃, ultrasonic frequency of37W, and ultrasonic duration of3min. Under these optimized conditions, the extraction amount of EPS was153.446mg/g VSS. IR spectra suggested that EPS contains functional groups such as carboxyl group and hydroxyl group, et al.The adsorption properties on lead ions were studied using EPS as adsorbent, which was extracted from activated sludge. The parameters affected the adsorption rate were investigated and optimized via a response surface method,such as solution pH,dosage of EPS and temperature.The thermodynamic and kinetic absorption characteristics of EPS for lead ions were also discussed.Results showed that the optimal adsorption condition was temperature35℃, pH4.2and m(EPS):m(Pb2+)was2.5:1.Under the optimum condition,the actual adsorption efficiency of lead ions by EPS could reach89.16%.The absorption isotherm of lead ions by EPS could be described by both Langmuir and Freundlich model,but the Langmuir model was more accurate than the Freundlich model.According to the Langmuir equation, the maximum adsorption amount at the temperature of20℃、30℃and40℃can be expected to reach0.9229mg.mg-1、1.0129mg.mg-11.119mg.mg-1when adsorption equilibrium, respectively. In addition, the adsorption process fitted well with pseudo-second order equations.The adsorption equilibrium could be reached at the time of240min with the maximum theoretical absorption amount of0.45mg·mg-1EPS include two different types:loosely bound EPS(LB) and tightly bound EPS(TB),and the concentration of TB was higher than LB.The protein/polysaccharides of LB and TB were1.41and1.71,respectively.The functional groups C-OH,C-O-C,C-N,N-H of LB and C-OH,C-O-C,C-N,N-H of TB provide the most adsorption sites for Pb2+.LB and TB binds lead ions in disorder way,and lead ions was not destroy their molecular activity ability and polymer chain.XPS spectra showed that C,N and O are the dominant elements in LB and TB,and these elements were all involved in lead ions adsorption process.The surfaces of LB,LB-Pb2+and TB,TB-Pb2+are similar.