| Dithiamon was developed by Merck KGaA in the 1960s and has many advantages such as broad spectrum and low toxicity. Besides, the ability of injurious insects to resist dithianon treatment is low. Therefore, it has been widely used as protective fungicide, and has a significant effect on prevention and treatment of a variety of fungal diseases, in particular on control of the anthracnose disease. In the industrial manufacturing process of dithiamon, a large amount of wastewater is produced containing sodium acetate (-76.6%), naphthalene, anthraquinone and pigment. It is difficult to handle the waste residue properly because of its complex compositions. The common treatment method is burning or landfill at present, which will cause air, soil or water pollution directly or indirectly. Besides, it is an enormous waste of resources when the waste residue containing-76.6 wt% CH3COONa is directly discharged. For the waste residue, CH3COONa is the main component and the other impurities are uncharged, and hence membrance sparation technologies including diffusion dialysis (DD), conventional electrodialysis (ED) and bipolar membrane electrodialysis (BMED) are feasible for recovering CH3COONa. In DD process with cation exchange membrane, for instance, Na+ migrates from feed compartment into water compartment through cation exchange membrane, and CH3COO- is also migrated into water compartment to keep electric neutrality of the solution. Then purified CH3COONa can be obtained in water compartment. In conventional ED, Na+ migrates to the cathode under electric field and hence can pass through cation exchange membrane into the recovery compartment, while CH3COO- moves to the opposite direction into the recovery compartment. Accordingly, purified CH3COONa can be obtained in recovery compartment by conventional ED. As for BMED process for treating the sodium acetate waste residue, Na+ transports through cation exchange membrane into base compartment under external electric field, and then combines with OH- yielded by bipolar membrane. Simultaneously, CH3COO- combines with H+ released by bipolar membrane in acid compartment. That is, we can get two kinds of important chemical material (CH3COOH and NaOH) at the same time through BMED. This paper is divided into five chapters, and the main contents are as follows:Chapter 1 is the introductory part explaining the sources and composition of sodium acetate waste residue, the categoris and shortcomings of existing treatment methods and shortcomings. Then the feasibility of membrane separation technology is discussed according to analysis of the waste residue compositions. At last, the membrane separation technologies frequently used at present are introduced in details and the background, meanings and main contents of the these are presented.In chapter 2, sodium acetate (CH3COONa) waste residue is firstly tested by diffusion dialysis (DD) to investigate the permeability of different commercial ion exchange membranes. Selemion CMV and AMV membranes show low permeability to the organic impurities of the waste residue, and hence are chosen for the following bipolar membrane electrodialysis (BMED) experiments. The BMED firstly uses a BP-A-C-BP configuration to select an optimized current density of-50 mA/cm2, the feed concentration is fixed at 1.49 mol/L. Afterwards, four membrane stack configurations are compared including BP-A-C-BP, C-BP-A-C, BP-C-BP and BP-A-BP under 50 mA/cm2. In consideration of the current efficiency, energy consumption, and concentrations of the produced acid/base, C-BP-A-C is a favorable configuration. Besides, the addition of a strong acid 001*7 type of cation-exchange resin into acid compartment can substantially decrease the voltage drop across the stack and reduce the energy consumption. Under the optimized conditions, namely, C-BP-A-C configuration at 50 mA/cm2 with 001*7 resin in acid compartment, high output (0.491 mol/L CH3COOH and 0.556 mol/L NaOH), high current efficiency (87.7% for CH3COOH and 99.0% for NaOH) can be achieved and the recovery rate of sodium acetate is 45.4%. But the energy consumption is relatively high, i. e.22.3 kW h/kg CH3COOH and 29.7 kW h/kg NaOH. Meanwhile, the purities of acid and base are relatively low, as reflected by the TOC value of the impurities (1.61 g/L and 0.16 g/L in acid and base compartment respectively). Overall, this chapter illustrates the feasibility and practical significance of BMED process for treating sodium acetate waste residue, including reducing soil and water pollution and producing valuable products in dithiamon industries. Nevertheless, the energy consumption needs to be decreased and the products’ purities should be enhanced, for which the following researches are carried out.In chapter 3 and chapter 4, we make attempts to couple different membrane processes to solve the problems in chapter 2 and get more excellent results, including the coupling DD and BMED in Chapter 3 and the coupling of ED with BMED in Chapter 4. After the optimization of the experimental conditions, the feed concentration is fixed at 1.49 mol/L, the DD-BMED can have outputs of 0.381 mol/L CH3COOH and 0.431 mol/L NaOH, and current efficiency of 78.3% for CH3COOH and 90.3% for NaOH. The energy consumption is significantly decreased to 3.1 kW h/kg CH3COOH and 4.1 kW h/kg NaOH. The ED-BMED also has a high output (0.46 mol/L CH3COOH and 0.50 mol/L NaOH) and current efficiency (80.6% for CH3COOH, and 89.5% for NaOH), relatively low energy consumption(11.8 kW h/kg CH3COOH and 12.7 kW h/kg NaOH). Besides, the products have higher purity than that in our previous work (Chapter 2). For ED-BMED, for instance, the TOC values of the impurities are 0.42 g/L and 0.03 g/L in acid and base compartment, respectively. In summary, the coupling processes of DD-BMED and ED-BMED for treating sodium acetate waste residue has significant advantages than that of solely BMED process.Chapter 5 is a summary of the whole thesis. It can be concluded from the laboratory-scale experiments that the waste residue can be treated well by BMED, and the coupling processes of DD-BMED and ED-BMED. By optimizing the experimental conditions, we can get excellent treatment results. However, lots of in-depth exploration and pilot-scale test are needed for real industrialization, including reduction of the energy consumption, improvement of the membranes’ life-time and the running stability, and optimization of the membrane processes. |
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