Adsorption Efficiency And Mechanism Of Heavy Metals In Soil By Extracelluar Polymeric Substances From Activated Sludge

Heavy metal contamination in soils is harmful to eco-environments and human being. Existing technologies for remediation of heavy-metal contaminated soil are limited by high-cost and hard-operated. It is necessary to find a new high-efficiency and cost effective technology for romoving heavy metals from soils.Many researches show that Extracellular polymeric substances (EPS) of microorganism have great ability to adsorption of metals. EPS are made up of protein, polysaccharide, nucleic acid and lipids primarily. EPS are the third basis of activated sludge besides cells and water. It contributes to 50⁹0% of organic substances and 15% of dehydrated sludge.In this paper, EPS from two kinds of sludges were used as adsorbant. The efficicency and mechanisms were studied. Influences of pH, EPS dosage, and adsorption duration on heavy metal removal from soil were investigated. Heavy metal removal efficiency at in- and ex- situ was tested and dynamics of adsorption process was also ananlyzed. Mechanisms of EPS adsorpting heavy metals were studied using modern analsis methods. The results show that, EPS have good ability in adsorpting Pb²⁺, Cd²⁺ and Zn²⁺ and EPSâ… are more efficient than EPSâ…¡. At Pb²⁺, Cd²⁺ and Zn²⁺ intial level of 1000 mg/kg, 500 mg/kg and 150 mg/kg and EPS dosage at 20mg/kg, the most proper pH is found to be 7-8, at this condition, heavy metal is removed by about 72%, 78% and 79% using EPSâ… and 70%, 75% and 80% using EPSâ…¡for the above 3 levels; the most efficient removal of heavy metals is gotten at dosage of EPSâ… at 20mg/g, at which Pb²⁺ and Zn²⁺ are removed by more than 47%, Cd ²⁺ by more 46%. For EPSâ…¡, the dosage is found out to be 20mg/g for the highest heavy metal removal with Pb, Cd and Zn removal at more than 40%; the economic adsorption time is 8 h for adsorpting Pb and Zn and 5h for Cd by EPSâ… and EPSâ…¡. The first order model fit the first 2 hours'adsorption process. Order of adsorption rate by EPSâ… is Cd>Zn>Pb and Zn>Cd >Pb by EPSâ…¡. Adsorption rate of Cd and Pb by EPSâ… is larger than those by EPSâ…¡, while for Zn, the order is in a reversed way. After EPS treatment, pH and TOC content of soil declin and surface area increases a little.Carboxyl, amidogen, carboxyl and C-O-C groups provide the most adsorption sites for Pb²⁺, Cd²⁺ and Zn²⁺. Amidogen seems more efficient in Pb²⁺ adsorption than Cd²⁺ and Zn²⁺ adsorption. XPS spectra show that the adsorpted Cd is oxided Cd or its compound. Surfaces of EPS-Pb and EPS-Zn are similar. EPS binds Pb²⁺ in disorder way while ordered for Cd²⁺ and Zn²⁺. Pb²⁺ is bond on the interface EPS, while Cd²⁺ shows great tendency to react with EPS on the surface. Ion exchange is more important for Pb²⁺ binding on EPS than Cd²⁺ and Zn²⁺. Pb²⁺ adsorbing on EPS contains more electrovalent bond than Cd²⁺ and Zn²⁺ situation. EPS has varied adsorbing sites, which show different appetency to different metals.This paper shows that ustilization of EPS form activated sludge as adsorbanat show a great promise and is worth to be recearched further. Extraction of EPS form waste activated sludge for heavy metal removal is meaningful as it can also bring reduce in mass and volume in waste wactivated sludge.