| Sediment pollution is quite a serious problem in the whole world. The main sources of pollutants in lakes include external loading (such as rainfall, runoff, soil leaching, industrial and municipal effluents) and internal loading. When the external source pollution got controlled, a variety of pollutants in the sediment will release to aquatic environment through exchange, which is the major source of polluting overlying water.The main pollutants in sediment from Wuhan East Lake are nutrients (nitrogen and phosphorus), heavy metals and refractory organic matter. By applying three ways of single technology including chemical passivating treatment with salt, sediment bioremediation and electrokinetic (EK) remediation, our research has investigated the effect of pollutants control from sediment. As phosphorus has long been considered a key factor affecting the process of eutrophication, sediment was treated with salts to reduce phosphorus release. In this paper, the effects of sediment bioremediation with bacillus probiotics and multiple enzymes were examined. According to the characteristics of chromium and linear alkylbenzene sulfate (LAS), EK remediation was studied on the migration and removal of both pollutants. For the first time, hybrid technology of electrikinetic bioremediation was adopted to restore LAS in sediment; Meanwhile, another hybrid technology of bio-leaching electrokinetic remediation was conducted to process chromium in the sediment. The results presented in this paper will be a good help for the development of pollution control of lake sediment both in practice and theory. The main research results of this paper include:1. Factors of phosphorus release from sediment and the characteristics of phosphorus sorption.The effects of marked environmental factors (temperature, DO, pH and stir) on phosphorus release from sediment were analyzed and model of sediment oxygen demand in Wuhan East Lake was proposed at laboratory simulated test. The rate of average TP effluent under aerobic condition was only 9.1 percent of that under anoxic condition. At the lower or higher pH (pH<6 or pH>9.5), the phosphorus release from the sediment speeded up greatly. Temperature increasing and disturbance of water could cause the release of P from sediment into overlying water. When TP concentrationin overlying water was high, phosphorus in water would be absorbed by sediments. By orthogonal test, the significant value of each affecting factor was analyzed and the order of marked factors was DO> Temperature> pH> Stir. The phosphate sorption isotherms of the sediment in Wuhan East Lake fit Langmuir equation. The greatest phosphate adsorption amount (Smax) and adsorption constant (k) under aerobic condition were much higher than those under anoxic condition, furthermore equilibrium phosphate concentration (EPCo) under anaerobic conditions was significantly higher than that under aerobic conditions. EPCo of strong disturbance sediments was increased obviously compared with that of weak disturbance sediments. The value of Smax decreased significantly after sediment was strongly disturbed.2. Effects of chemical passivating treatment on phosphorus release in Wuhan East LakeThe forms of phosphorus in the sediments of Wuhan East Lake were investigated by sequential chemical extraction. The results showed the order of TP fractional composition in sediment is TP> PFe> Porg> PAl> PCa. The facts, which organic phosphorus, Ca and Fe binding phosphorus in sediments were well related to the quantity of phosphorus released from the sediments, implied that they played importmant roles in internal phosphorus release. We found that the addition of salts (ferrous and calcium, and especially aluminum and nitrate) into the sediment could inhibit the transfer of phosphorus in sediments. The composition of sediment in terms of its various forms of phosphorus was changed after chemical passivating treatment. Only treatment with Al2(SO4)3 resulted in negative growth of P in the water phase. Chemical passivating treatment with salts, especially aluminum and ferric chloride under anoxic condition resulted in a high capacity for inhibition of endogenous phosphorus release and increased sediment stability.3. Study on biodegradation characteristics of LAS and organic phosphorus from sedimentThrough microscopic examination, we found the dominant species of planktonic algae in Wuhan East Lake were cyanophyta and chlorophyta, in combination with other biochemical parameters, indicating that the level of eutrophication was already high. Altuough both bacillus probiotics and multiple enzymes could enhance capacity the of sediment biodegradation, but the effects of bacillus probiotics was better than that of multiple enzymes. The effect of adding mixture of bacillus probiotics and multiple enzymes was better than that of just adding bacillus probiotics. After the sediments were treated by the mixture, the value of G, heterotrophic bacteria, DHA, PA increased obviously, and the content of organic matter and organic phosphorus decreased accordingly. Due to too high content of LAS in sediment, the purpose of LAS degradation by directly adding bacillus probiotics into sediment was not obtained. After bacilli had been cultured in simulated LAS wastewater, LAS degrading strains were available.4. Study on the electrokinetic remediation of LAS and chromium polluted lake sediment at laboratory scale.The feasibility of the remediation of chromium-polluted sediment by electrokinetic technology was studied. When EK experiment was operated for 48 hours at 20V direct current power, the removal rate of Cr(III) and Cr(VI) was 57.8% and 17.8% respectively. The results showed that the chromium in lake sediment could be effectively removed by EK remediation method. The influence of the applied voltage and the operating time on the removal efficiency of chromium, complex types and the electric field distribution on the removal efficiency of chromium, as well as the migration and distribution of chromium in sediment were investigated. We found that complex type had crucial effect on the removal efficiencies of chromium. The removal rate of the total Cr and Cr(Ⅲ) increased 75% and nearly 300% after adding the citric acid on cathode. Secondly the applied voltage gradient had important influence on the efficiencies and the optimum voltage gradient was 1V/cm in the test; Operating time and electric field distribution affected the efficiencies in some degree. After the EK remediation system had been operated for 48 hours, the removal rate of the total Cr increased slowly. In the comparison of the uniform and the non-uniform electric fields, the effective area of the former was wider, but close to the latter around the area of the electrode.Electro-kinetic process stimulated the migration and enrichment of LAS in the sediments, and also led to significant changes of pH value nearby the cathode and anode. The effects of LAS removal depended on the reaction time, electrolysis voltage, initial pH and sediment moisture content. The effect of sediment moisture content was obvious on EK remediation in the case of a low water content because the efficiency of electroosmosis and electromigration was small, while the effect of moisture content was little in the case of saturated sediment moisture. By orthogonal test on LAS electrokinetic remediation, the significant value of each affecting factor was analyzed and the order of marked factors was cell voltage> distance between electrodes> reaction time> initial time. The tests indicated the removal efficiency of LAS in 500 mL sediment samples after EK remediation for 6h could be up to 41.4% under the optimum conditions of initial pH7, voltage 10V, electrodes distance 10cm.5. Study on hybrid technique of electrikinetic bioremediation and bio-leaching EK remediation.Firstly, effects of EK transport on the vulnerability of bacteria were studied in this part. Because strong electric field for inactivation of microorganisms had the inhibiting effects of electrochemical products generated at the electrodes, dehydrogenase activity in sediment under weak electric field was higher than that under stronger electric field, and the electric current under weak electric field could activate the activity of bacteria. We found electric field could inhibit the growth of heterotrophic bacteria nearby cathode, and electric field polarity reversal might be an effective way to minimize such effects. Secondly, the test proved EK technology had the ability to introduce nitrogen compounds into the sediment, with the nitrate showing greater mobility than the ammonium. The mass of ammonium, as well as moving, had decreased in the system during the tests, due to chemical reactions. The nitrate showed a less reactive behaviour, remaining in the reactor and only moving from anode to cathode. Meanwhile, the injectionof phosphorus did not prove to be successful. The phosphate concentration declined in the catholyte where it was placed, but it did not travel through the sediment. The results that LAS removal rate of sediment treated by EK bioremediation, EK remediation and biological remediation was 60.5%,41.4%,25.8%, respectively, indicating that EK technology can enhance the process of sediment microbial remediation. Finally, chromium in sediment was treated by bioleaching electroremediation technology. The speciation of chromium had significant changes during EK remediation. The leaching rate of chromium in sediment was up to 25.1%, which was closely related to pH and ORP of sediment. After bioleaching-electrokinetic remediation, the average removal rate of total chromium was 53.4%. In the comparison of bioleaching-electrokinetic and electrokinetic remediation, the removal rate of total chromium of the former was nearly 21 percent higher than that of the latter. Therefore, the method of bioleaching electrokinetic remediation is suitable for heavy metal pollution control in sediment. |
PDF Full Text Request