Reasearch On The Removal Of Heavy Metals In Industrial Sludge By Biodegradable Chelating Agents IDS And GLDA

Sludge is a solid or semi-solid gel waste material produced in large quantities during wastewater treatment process. To date, achieving the minimization, stabilization, harmlessness and reclamation of sludge remain necessary condition for sustainable development. Land use is the main way to realize sludge reclamation and is also the direction of future development. Heavy metals such as Cd, Pb, Ni, Cr, Hg, As, Zn and Cu are not only the commonly detected contaminants in the sludge generated from smelting, battery, electroplating, leather, chemical industry and sewage treatment enterprise, but also the major pollution indicators of the World Sludge Land Use Standard System. Therefore, carrying out relevant studies on the harmless disposal techniques of sludge containing heavy metals is the focus and hot topic in the field of environmental pollution control. Such studies would be of great significance with respect to effectively removing heavy metals within the sludge, facilitating the realization of sludge reclamation, as well as ensuring the environmental security and ecosystem sustainability.The research object of this study is concerned with the sludge generated in a battery and a metal smelting wastewater treatment plant in Henan Province. On a basis of the current pollution status and practical treatments of industrial sludge containing heavy metals, this study centers on studying the extraction and removal process of heavy metals in sludge samples with the aid of biodegradable chelating agent IDS (tetrasodium iminodisuccinic) and GLDA (tetrasodium N,N-bis(carboxymethyl) glutamic acid). In this study, the technological conditions within the extraction process of heavy metals in sludge by using chelating agents GLDA/IDS were proposed. The effects of species distribution of heavy metals and co-existence of metal ions on the extraction efficiency were analyzed. The relationship between the stability constant/conditional stability constant of GLDA/IDS with heavy metals and the extraction efficiency was probed. In turn, this work is envisaged to provide fundamental basis for the application of biodegradable chelating agents in the treatment of sludges containing heavy metals. The thesis mainly encompasses the following four sections:1. Study on the extraction of heavy metals from the battery sludge with the aid of IDS:Our obtained results indicate that, under the conditions of molar ratio of IDS:M = 7:1 (M denotes the heavy metal, the same below), no adjustment of pH value (about 11.5) and extraction time of 24 h, the best extraction efficiency of Cd with the presence of IDS was 68%. H3PO4 has been proven to be beneficial to the extraction of heavy metals in sludge, the compound system consisting of IDS (molar ratio of IDS:M = 1:1) and H3PO4 (with a low concentration of 1.2%) had a positive effect on the extraction of Cu and Ni, where the extraction efficiency reached 90.7% and 91.3%, respectively. Our study also reveals that the extraction time, pH value of the system and the concentration of IDS had an appreciable influence in the extraction of heavy metals in sludge, other parameters such as stability constant and species distribution of heavy metals in sludge were also the important factors during the extraction process.2. Study on the extraction of heavy metals from the battery sludge with the aid of GLDA:The results indicate that GLDA was effective for the extraction of heavy metals in sludge such as Cd and Ni, however, a relatively low extraction efficiency of Zn was obtained, possibly due to the fact that Zn remained in the form of residue in the sludge sample (so that it was hard to be extracted). Under the conditions of molar ratio of GLDA:M = 7:1, pH = 4 at room temperature, the extraction efficiency of Cu, Ni and Cd in the sludge sample was 84%,82% and 89%, respectively. A series of parameters, such as extraction time, pH value of the system, the concentration of GLDA and the speciation of heavy metals etc. imposed a major effect on the extraction efficiency. Moreover, it has been found that there was a good correlation between the distribution variation of heavy metals prior to/after extraction and the extraction efficiency. The stability constant of metal-chelant was influenced by the pH values of system.3. Comparative study on the extraction of heavy metals from the battery sludge with the aid of IDS, GLDA and EDTA:EDTA was effective for heavy metal extraction in battery sludge in our experimental investigation. At the molar ratio of EDTA:M = 5:1, the extraction efficiency of Cd, Cu, Zn and Ni was 80%,60%,40% and 20%, respectively. Under the conditions of GLDA:M=5:1, pH = 1～10, the order of extraction efficiency by employing GLDA was Cd>Zn>Cu>Ni, with the corresponding extraction efficiency at 71%,27%,21% and 8%, respectively. Under the conditions of IDS:M = 1:1, pH<5, the extraction efficiency of Cu with the presence of IDS remained higher than 60%. At the molar ratio of IDS:M = 5:1 in an alkaline ambient, IDS had a better response to the Cd extraction, but was inferior to that of GLDA and EDTA in terms of the extraction of Ni and Zn. By comparing the extraction efficiencies of heavy metals from the battery sludge, we found that the extraction capacity of GLDA and EDTA was equally outstanding for Cd, both of which were superior to that of IDS; the extraction capacity of EDTA in terms of Ni and Zn was better than those of GLDA and IDS; for Cu, the capacity of IDS was greater than the other two.4. Comparative study on the extraction of heavy metals from the metal smelting sludge with the aid of IDS, GLDA and EDTA:Our results indicate that under the conditions of GLDA:M = 7:1, pH = 4 and extraction time of 24 h, the order of extraction performances with the aid of GLDA on heavy metals in the metal smelting sludge sample followed Cu>Cd>Ni>Zn>Pb>As, where the extraction efficiency was 65%, 64%,62%,46%,43% and 33%, respectively. The extraction capacity of GLDA on Cd, Ni, Cu and Zn is inconsistent with the stability constant of their chelate, the reason might lie in the fact that the extraction efficiency was partially governed by the speciation of heavy metals and co-existence of metal ions within the sludge. The extraction efficiencies of different types of heavy metals in sludge by employing IDS were all below 40%, which was inferior to that of GLDA. In contrast, the effectiveness of GLDA and IDS on the extraction of heavy metals in metal smelting sludge was worse than that for battery sludge, since it might be related to the difference in the species distribution of heavy metals for the two samples. At the molar ratio of EDTA:M = 5:1, the good extraction efficiency of Pb, Cd, Cu, Zn and Ni reached 60%,50%,50%,58% and 50%, respectively. By comparing the extraction efficiencies of heavy metals from the metal smelting sludge, we found that with respect to the heavy metals, the extraction capacity of GLDA and EDTA was equally great, both of which were superior to that of IDS.