Research On Extraction Processes And Mechanism Of Several Heavy Metal Ions From Sewage Sludge With Poly-epoxy-succinic Acid

The retained heavy metals in sewage sludge will pose the risk of human health and phytotoxicity from land application, which restricts significantly the reuse of sewage sludge. Therefore, how to remove the heavy metals from soil and sewage sludge with the green chemical theory has attracted more attention in recent years. Focussing on the recent development trend of green chemistry and the characteristic for sewage sludge containing higher concentration heavy metal, the extraction efficiencies for poly-epoxy-succinic acid （PESA） as a novel biodegradable chelant to remove heavy metals from sewage aludge based on chemical extraction technology were studied in this paper. The complexation mechanism between PESA and several heavy metal cations were studied by means of acid-base potentionmetric titration. The migration of heavy metals in the actual polluted soil sampled nearby Pb-Zn ore washing plant was investigated based on the condition of soil near ore plant contaminated widely by heavy metals. Under the support of the Natural Science Foundation of China, the purpose of this research is to provide academic suggestion and technical guidance for the development of green remediation technology of soils and sewage sludge contaminated with heavy metals.This paper consists of four parts: （ I ） Study on the complexation mechanism between PESA and heavy metal cations. （II） Study on the separation of heavy meatls from sewage sludge with PESA. （III） Study on the comparison of extraction efficiency for heavy metals from sewage sludge with EDDS, EDTA and PESA. （IV） Study on the migration of heavy metals in the actual polluted soil.（I） Study on the complexation mechanism between PESA and heavy metal cations. Based on the acid-base potentionmetric titration and the advanced BEST computer program developed by A. E Martell et al, the deprotonnation constant of PESA and the complex formation constants of PESA with six heavy metal cations (Zn²⁺, Pb²⁺, Cd²⁺, Ni²⁺, Cr³⁺, Cu²⁺) were determined. The distribution curves of the species in aqueous solution at different pH were also given out. It was demonstrated that the H₂L model with five structure unit to describe the coordination chemistry of PESA, which was set up according to the experimental data, is reasonable and the contants are also accurate. The experimental results confirmed that the models were feasible to match the deprotonation of PESA and the conplexation between PESA and heavy metal cations. The chelation abilities of PESA with heavy metal cations are good. Considering the main complex ML, the order of complexation tendency between heavy metal cations and PESA is Cu²⁺>Cr³⁺>Cd²⁺>Zn²⁺>Ni²⁺>Pb²⁺.（II） Study on the separation of heavy meatls from sewage sludge with PESA. Based on chemical extraction technology, PESA was used to remove the heavy metals from the sewage sludge. For the sewage sludge from Shanghai Taopu Municipal Wastewater Plant, the influence factors including system pH, concentration of PESA, extraction time, extraction temperature, heavy metal species distribution, H₂O₂ and H₃PO₄ addition on the extraction efficiency were investigated. It was found that PESA could extract target heavy metals from sludge effectively. The total extraction efficiency for all of the target heavy metals is good when the molar ratio of PESA to total heavy metals was 2:1 and system pH was 4. The order for heavy metals extracted with PESA is Cd>Zn>Pb>Ni>Cu>Cr, and the extraction efficiency is 78%, 73 %, 12%, 60%, 54% and 36% respectively. The results showed that the complex formation constants of PESA with heavy metal cations, concentration of PESA, system pH, extraction time, extraction temperature, heavy metal species distribution, H₂O₂ and H₃PO₄ addition have a significant effect on the extraction efficiencies. The BCR （European Community Bureau of Reference） analysis of species distribution showed that the extracted heavy metals mainly came from the following species, water soluble, acid soluble, reducible and oxidizable fractions. H₂O₂ addition could remove Ni, Cu and Zn from sludge effectively, and the H₂O₂-PESA associated system could promote the separation of Cu obviously. H₃PO₄ addition could remove Zn, Cd and Ni from sludge effectively, and the H₃PO₄-PESA associated system could promote the separation of Pb apparently.（III） Study on the comparison of extraction efficiency for heavy metals from sewage sludge with EDDS, EDTA and PESA. Under conditions of molar ratio of chelat and the sum of heavy metals was 1:1 or 10:1 with system pH adjusted, the extraction efficiencies for heavy metal extracted with EDDS, EDTA and PESA were compared. The result revealed that EDTA had higher extraction efficiency for Cd, Zn, Pb, Cu and Ni within experiment pH range, and the order for heavy meatals extracted with EDTA is Cd>Zn>Pb>Cu>Ni>Cr when molar ratio of EDTA and sum of heavy metals was 10:1. EDDS had higher extraction efficiency for Cu, Cd, Zn, Pb and Ni under neutral or alkali condition, and the order for heavy meatals extracted with EDDS is Cu>Cd>Zn>Pb>Ni>Cr when molar ratio of EDDS and sum of heavy metals was 10:1. PESA had higher extraction efficiency for Cd, Zn, Pb, Ni, Cu and Cr under acidic condition, and the order for heavy meatals extracted with PESA is Cd>Zn>Pb>Ni>Cu>Cr when molar ratio of PESA and sum of heavy metals was 10:1. Under conditions of pH=4 and ratio 10:1, PESA gave comparable better extraction efficiency than EDDS and EDTA in extracting Cd, Zn or Pb. PESA gave better performance than EDDS and EDTA for Cr extraction, EDTA and EDDS better than PESA for Cu, and EDTA better than EDDS and PESA for Pb. In comparison with EDTA or EDDS, PESA gave potential alternative performance in extracting heavy metals such as Cd, Zn, Pb or Cr, in addition to the other advantages, such as ready biodegradability and nitrogen- and phophorus-free components.（IV） Study on the migration of heavy metals in the actual polluted soil. The soil column leaching model was used to study the leaching process and migration of heavy metals with the washing of simulated acid rain. The pH of the simulated acid rain was controlled in three levels of 2.5, 3.6 and 5.6. The investigation revealed that, with the pH decrement of the acid rain, the conductivity of the soil leachate increased gradually while the pH of the leachate decreased under the same conditions. With the pH increment of the acid rain, the total content of all the five heavy metals in the leachate showed the rising tendence. However, the results also showed that different kind of heavy metals had their own leaching characteristics. The release of zinc, nickel and copper from the polluted soil increased rapidly in the first stage and then maintained a relative stable state. For the release process of chromium or lead, it can be divided into two stages of fast leaching and slow leaching. The order of the total released amount of five heavy metals was Zn>Cu> Cr>Ni>Pb under the same condition. The results revealed that Zn, Pb, Cu, Cr or Ni in the polluted soil might migrate to deeper soil when pH of the simulated acid rain was 3.6, and the translation ability for Zn or Ni was greater than Cu, Pb or Cr. Under the same acid rain intensity, the heavy metals migration in soil followed the sequence of Zn>Ni>Cu>Pb>Cr, and the heavy metals content in the same soil depths was in the order of Zn>Pb>Cu>Cr>Ni.