| As the world population increasing quickly and economy growing fast, the problem of environmental pollution has become increasingly worse. In recent years, many municipal sewage treatment plants have been built, which effectively alleviate the pollution of water environment caused by the discharge of municipal sewage and industrial wastewater. At the same time, a large amount of sewage sludge produced during the process of sewage treatment may result in second pollution to the environment. Since there are high quality of nutrition elements such as nitrogen, phosphorus and kalium, the agricultural utilization of sewage sludge is become an important topics in nowadays research. However, the heavy metals in sewage sludge are the key factor which will restrict the utilization of sewage sludge in agriculture. At the same time, in the last decades, large amounts of polluted wastes associated with industrial, mining, agricultural, and chemical activities have been produced. Electroplating sludge is a heavy metals-bearing byproduct that comes from the electroplating industry’s activities. Some of it consists of multiple metals such as Cu, Ni, Zn, Cr, and Pb, etc. It is a discharged residue after chemical precipitation of heavy metals from acidic or alkaline solutions as well as rinse waters generated by the electroplating processes. The sludge is categorized as hazardous waste. If disposed improperly, these metals may:1) cause serious environmental damages polluting surface and ground water, and soil; 2) be transferred into human body via the food chain. In China alone, every year more than 100,000 tons of valuable heavy metals in the form of electroplating sludge is wasted. In order to have both sound environment protection and sustainable development that highly emphasizes resources reuse, it is of great importance to maximize the recovery and recycling of heavy metals economically from electroplating sludge. Many studies have been made to decrease and reduce the poisonous effect of the heavy metals in sewage sludge. At the present time, to remove heavy metals from sewage sludge, chemical extraction and bioleaching treatment were proposed. Electrokinetic (EK) technology is considered as one of the most promising methods to remove heavy metals from the matter with low hydraulic permeability including contaminated soils and sludges. The advantages of electrokinetic technology used for treatment of sewage sludge are the simultaneous removal of different heavy metals, high efficiency of removal and short time of the process. Heavy metals approaching electrodes could be removed or reclaimed by different methods, e.g., by adsorption, pumping out, precipitation or ion-selective membrane. In this research, the sewage sludge from the sewage treatment plant, Hangzhou and the electroplating sludge from an electroplate factory in Hangzhou city were taken and studied. The research’s aim is to find an economical and effective method to reduce or remove the poisonous heavy metals from sewage sludge and electroplating sludge by bioleaching and electrokinetic removal technology respectively, adding several activated reagents as processing fluids in the electrode chambers.1. In view of reducing the total treatment volume of sludge and achieving high removal efficency of heavy metals, when we bioleaching heavy metals from sludge by thiobacillus ferrooxidans,10% inoculums and 10g.L-1(w/v) ferrous sulfate as energy substrate is tested as the best reaction parameter.2. The initial sludge pH affected bioleaching efficency largely, and the effects on removal of heavy metals followed as Zn>Cu; The bioleaching efficency showed decreasing trend with the increase of sludge solid concentration, and the startup time of heavy metals dissolved from sludge increased with the solid concentration of sludge, and the startup time followed as Cu>Zn, However, the effects of solid concentration on removal of heavy metals is very small when the bioleaching time was prolonged, so in the practical run, we can enhance the solid concentration of sludge to reduce the cost; There was much discrepancy in effect of different organic compound addition on bioleaching, where the effect extent followed as Cu>Zn. The effect of the alcohol compound and the two carboxyl acid on bioleaching was low. The negative influence on bioleaching was decreased with the increase of carbon chain length of single carboxyl organic acid. The negative influence on bioleaching was increased with the increase of carboxyl number of aroma organic acid, and the negative effect was lower than that of organic acid with low molecular weight; The sludge treated with anaerobic hydrolyzed acidification declined the removal efficency of Cu, but the effect was small to that of Zn, due to neutralize the decrease of pH and the increase of Eh.3. After bioleaching, the content of heavy metals in sludge decrease sharply, the fractions of the residue metals in sludge mostly exist as stable fractions, so the bioavailability of heavy metals is very low; The losses of nitrogen,phosphorus, kalium and organic matter are 27.3%、70%、5.4% and 41.3% respectively,despite there have some losses, it can also meet the requirement of land utilization; The high content o ferric ions in the sludge after bioleaching is the essential element of plant growing, so it does not lead to the toxicity of land, and the dehydration sludge after bioleaching can produce organic ferric fertilizer; Consequently, the land utilization of this treated sludge is very secure.4. The combination of bioleaching and electrokinetic remediation has been proved to be an effective method to remove heavy metals from municipal sewage sludge. The results showed that using electrokinetic remediation for six days after bioleaching for four days, the contents of Cu and Zn in sewage sludge were 63.4 mg.kg-1 and 33.3 mg.kg-1, respectively, while using electrokinetic remediation and bioleaching simultaneity, the contents were 135.21mg.kg-1 and 82.34mg.kg-1 respectively, which both could meet the Chinese standard for land application of the heavy metals in sewage sludge. During the reaction process, both the organic sulfide fraction of Cu and the carbonate-bound and organic sulfide fractions of Zn in sludge mainly transformed to dissociative metals which could easily migrated to the cathode zone with the process of electrokinetic remediation. During the reaction process by using electrokinetic remediation and bioleaching simultaneity, ferrous ions of the FeSO4.7H2O migrated to the electrode chamber which would lead to the shortage of energy sources at the back stage and then influence the transform of organic sulfide fractions. Therefore, this study proved that using electrokinetic remediation for six days after bioleaching for four days is the better economical and feasible treatment.5. After the experiment with acidified electroplating sludge and pH adjustment at cathode chamber, the exchangeable, carbonate and organic sulfate fractions of heavy metals in initial electroplating sludge were converted to soluble fraction which improved the efficiency of heavy metals, the removal efficiencies of heavy metals were attained:74.02% for Zn,68.38% for Ni,39.22% for Cu,21.37% for Pb. However, even the carbonate and sulfate fractions of Cr in initial electroplating sludge were converted to soluble fractionation, this treatment is not beneficial to improve the removal efficiency of Cr. The removal efficiency of Cr decreased from 77.83% of control treatment to 40.65% of electroplating sludge acidified treatment, and furthermore, decreased to 12.80% of pH adjusted in cathode chamber of acidified electroplating sludge treatment.6. The removal efficiency of metals were in the range of 20-51%,26-65%,34-69% for EK experiments with tap water, SDS,and citric acid as electrode processing fluid respectively. It was shown that a best performance was found in EK-CA experiment; Results were also showed that the metal removal efficiency by EK process was:Cr>Zn>Ni>Cu>Pb; The results of sequential extraction analysis revealed that the binding form of metals with sludge after EK experiment was changed from the residual form, the most difficult extraction type, to the soluble, exchangeable, sorbed and sulfate forms, the easier extraction types; The overall cost of these treatments can be estimated to 0.14,0.19 and 0.57 Yuan per mol of metals removed for EK experiments with tap water, SDS, and citric acid as processing fluid respectively; Although the lowest cost was found in EK-TW experiment, but the lowest removal efficiency was a limit for this experiment, it was found that the EK-SDS experiment was the most cost-effective.7. We investigated the effect of pH value of cathode buffer solution on electrokinetic remediation efficiency of heavy metals in the electroplating sludge. After EK remediation, all experiment systems had different remediation efficiency, and the best remediation effect of Ni、Cu、Zn and Cr was observed at pH 3.0 of cathode buffer solution, and the removal rates of Ni、Cu、Zn and Cr was 70%、59%、30% and 29% respectively, but the best remediation effect of Pb was observed at pH 5.0, and the removal rates of it was 46%. In addition, the concentrations of Zn, Cu, Cr, Pb and Ni in the five sections decreased in different degree, and the distribution of Cu、Pb and Ni was homogeneous in sections, but the distribution of Cr and Zn was fluctuated severely. We also found that all forms of heavy metals were removed partly, and among which the decrease of carbonate bound fraction and residual fraction heavy metal was most significant, but the decrease of exchangeable fraction and Fe-Mn oxide fraction heavy metal was not distinct, and the decrease of organic sulfate fraction was moderate. |
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