Adsorption Behavior Between Heavy Metals And Extracellular Polymeric Substances Extracted From Activated Sludge

Extracellular polymeric substances(EPS)is an important component of activated sludge,and it has excellent flocculation and adsorption performance in the field of wastewater treatment.There are many functional groups on EPS,which can provide adsorption sites for metal ions.People often apply this characteristic to treat heavy metal wastewater.With the hope of providing some theoretical basis for the control of heavy metal contamination by EPS,and take the possibility that heavy metals would accumulate during the treatment process into consideration,the behavior of activated sludge EPS adsorbing heavy metals was studied.The mechanisms of adsorption process were also investigated with the aid of modern analytical technique.The research started with the extraction of activated sludge EPS from aeration tank.Five commonly extraction methods were selected to study their effectiveness in extracting EPS from activated sludge.The results showed that,the efficiency of extraction methods,in decreasing order was:NaOH>CER?heating>H2SO4?ultrasoniccation.The results from combination methods extraction experiment indicated that,combination methods provided more EPS not only than every single method,but also than the superposition of single method;ultrasonication could increase the extraction amount of polysaccharide significantly,while NaOH was the key to protein extraction.The pH value was the most important factor in alkali-heat method,as it influenced the extraction amount of carbohydrates and proteins;optimum conditions were as follows:pH 13,80? and 10min;the highest extraction values in terms of proteins(29.55mg·g-1 VSS)and carbohydrates(33.37mg·g-1 VSS)were achieved.Considering the toxicity and universality of heavy metals,Cu2+,Zn2+ and Pb2+were selected as research objects.The effect of pH value,EPS dosage,contact time and initial concentration of metal ions on adsorption performance was investigated,and the kinetics and thermodynamics of adsorption process were also studied simultaneously.The result showed that,pH had a significant effect on heavy metal adsorption onto EPS,the applicable pH range was 5-6;m(M2+):m(EPS)=1:2 was suggested to be the best dosage.Zn2+ and Pb2+ completed rapid phase within 10 min,while Cu2+ spent 20 min;all the metal ions came to equilibrium phase after 60 min;the adsor;ption followed second-order kinetic equation.The affinity of EPS to metal ions,in decreasing order was:Zn2+>Cu2+>Pb2+.The adsorption mechanism was investigated with the aid of FTIR,TEM and EDX analysis.The FTIR spectrum showed that,the mechanisms of Zn2+ and Pb2+adsorption onto EPS were similar,and carboxyl,hydroxyl and phosphate groups played an important role in adsorption;the mechanism of Cu2+ was more complicated,in addition to above function groups,amino and methylene were also involved in adsorption process.TEM analysis showed that,there were a lot of black spots gathered at the edge or in the interior of EPS disorderly;distinct lattice fringes were observed under high power microscopic.Furthermore,EXD analysis suggested the release of Ca and Mg was very significant,indicating the involvement of ion exchange mechanism for the adsorption of metal ions by EPS.XRD analysis showed a new peak appeared at 20?29.40°,then an assumption was made that,phosphate crystal formed during the adsorption process.In conclusion,maximum extraction amount could be obtained by mean of ultrasonication +NaOH+heating.NaOH was the key to protein extraction,while ultrasonication made contribution to polysaccharide extraction.The pH value,EPS dosage,contact time and initial concentration of metal ions had significant effect on the adsorption of metal ions onto EPS.The adsorption followed second-order kinetic equation.The machenisms of Zn2+ and Pb2+ adsorbing onto EPS were similar,while that of Cu2+ was more complicated.Ion exchange was involved in the adsorption mechanism,and hydrated phosphate formed during the adsorption process.