Effect Of Organic Substrate On The Adsorption Of Copper And Zinc In Water By Extracellular Polymeric Substances From Activated Sludge

With the development of industry and the increase of population, the problem ofheavy metal pollution in water becomes more and more serious, which is harmful toeco-environments and human being. There are many methods to treat heavy metal pollutedwater. Compared with traditional method to treat heavy metal pollution, biosorption hasmany advantages. Especially when it is used to deal with wastewater containing low-levelmetals, the efficiency is satifying.EPS is a sticky substance result from bacterial excretion mainly composed ofpolysaccharide, protein, nucleic acid and lipids. EPS contains many ionizable functionalgroups such as carboxylic, phosphoric, amino, and hydroxylic, which endouws EPS theability to combine with metals. Many researches show that EPS have great ability toadsorption of metals. EPS is the key substances that hinder the excess sludge volumereduction. Reduction in volume and mass of excess sludge by extraction of EPS enlightensthe meaning of using EPS in heavy metal contamination control.Three activated sludges have been studied in this paper, activated sludge I and II wereseeded with activated sludge III collected from a full-scale municipal wastewater plantcultivated with synthetic wastewater that contained different carbon source glucose andpeptone, respectively. There are two researchful directions. On the one hand, three sludgesare taken as research object, flocculation and settlement performance of activated sludge,the form of sludge, the optimal EPS extraction parameters, the composition of EPS,functional groups within EPS were investigated. On the other hand, study EPS I, EPS II,EPS III as adsorbents, which is given by best extracted way, adsorption removal effectversus heavy metal ion that includes Cu²⁺and Zn²⁺. This paper focused on the influences ofpH value, dosage of EPS, adsorption temperature, initial concentration of metal ions,adsorption time and co-existed ions on adsorption ability. The mechanism of adsorption ofCu²⁺and Zn²⁺by three kinds of EPS is analysed by the investigation of kinetic and thermodynamic characteristics of Cu²⁺and Zn²⁺adsorbed by EPS.The experimental results demonstrate that flocculation and settlement of threeactivated sludges are satisfying. The settling velocity（SV） of three sludges are15%-50%,sludge volume index （SVI） are80-120ml/g, mixed liquor suspended solids （MLSS） are2000-3000mg/L. Furthermore, activated sludge I has the worst flocculation and settlementperformance, while activated sludge III has the best. The optimal extraction parameters ofheating extraction is80℃and35min, which extracts the most EPS and the level ofdestruction on cell is the smallest. During a culture cycle, order of polysaccharide, proteinand EPS generate speed is: activated sludge I> activated sludge III> activated sludge II;EPSs are high in polysaccharide but low in protein. Polysaccharide contributes to nearly50%of EPS. Order of the amount of polysaccharide is: EPS I> EPS II> EPS III, order ofthe amount of protein is: EPS II> EPS III> EPS I; The ratio of polysaccharide to proteinof three EPSs are21.3、4.8、6.4, respectively; The ratio of C to N of three EPSs are8.5、5.5、6.1, respectively; The IR spectrum show the presence of the functional grounps ofthree EPS, they are exceedingly similar. The main chemical groups in three EPSs arecarboxyl, alcohol, peptide bonds in proteins and so on. The peaks of chemical groups inpolysaccharide and protein are strong. EPS is efficient in Cu²⁺and Zn²⁺adsorption. Theoptimal pH value is6. The optimal dosage of three EPS is350mg/L,300mg/L and350mg/L, respectively. The adsorption processes of Cu²⁺and Zn²⁺with EPS all haven twosteps: fast adsorption and slow adsorption, and Cu²⁺is faster than Zn²⁺to approachadsorption equilibrium. Freundlich isotherm equations could describe the thermodynamicadsorption processes of Cu²⁺, but Langmuir isotherm equation is more desirable for thedescription of the thermodynamic adsorption processes of Zn²⁺. The adsorption ability ofZn²⁺with EPS is better than Cu²⁺. There are many kinds of adsorption ways between twometal ions and three EPSs. Pseudo-second-order kinetic could describe the kineticsadsorption processes of Cu²⁺and Zn²⁺well. Two metal ions adsorbe with EPSs by covalentbond or ion exchange. From pseudo-second-order kinetics, the maximum adsorptionreaches52mg/g of Cu²⁺and54mg/g of Zn²⁺by EPS I,40mg/g of Cu²⁺and48mg/g ofZn²⁺by EPS II,47mg/g of Cu²⁺and49mg/g of Zn²⁺by EPS III. FTIR analyses revealedthat the main chemical groups involved in the interactions between three EPSs and twometal ions were apparently alcohol, amino, amide groups, carboxyl and C-O-C groups etal.This paper shows that the adsorptive capacity of EPS from activated sludge is fine andis worth to be recearched further. Recearch the effect of organic substrate on the adsorptionof heavy metals by EPS contributes to the understanding of adsorption mechanisms.Extraction of EPS from excess sludge for heavy metal removal is meaningful as it can alsobring reduce in mass and volume in excess sludge.