The Study On The Reclamation Of The Glyphosate Neutralization Liquor By Bipolar Membrane Electrodialysis

Glyphosate is a kind of efficient,widespread and low toxicity pesticide. Due to the expanded planting of genetically modified crops in recent years, glyphosate is with a rapid growth in sales, and now it has become to one of the most needed pesticide in the world. According to related statistics,China is the biggest glyphosate-manufacturing and glyphosate-exporting country, and more than70%glyphosate is produced by means of the glycine-dimethylphosphit (DMP) process because of the abundance of the glycine, one of the key raw materials, in china. A great deal of alkaline glyphosate neutralization liquor with high salinity is generated during the production. It is not only a pollution problem but also a resource waste if the neutralization liquor is discharged directly.In this work, a environment friendly zero discharge strategy that the bipolar membrane electrodialysis(BMED) is employed for the reclamation of the neutralization liquor, including desalinating glyphosate liquor for the subsequent concentration and recycling HCl and NaOH for the glyphosate production, is put forward.A lab-scale bipolar membrane electrodialysis study was first carried out to explore the feasibility of using this method to reclamation of the neutralization liquor. This lab-scale study based on the simulated feed solution of15%NaCl,1.2%glyphosate, and the pH is about11. the membrane stacks is assembled by Ger./domestic bipolar membrane(BPM), domestic heterogeneous cation exchange membrane (CEM) and domestic heterogeneous anion exchange membrane(AEM). The corresponding concentration of hydrochloric acid and sodium hydroxide is determined by titrations with standard sodium hydroxide solution and standard hydrochloric acid solution using phenolphthalein and methyl orange as indicators, respectively. The conductivity and pH of the neutralization liquor was monitored by pH meter and conductivity meter. The alteration of the glyphosate concentration in each compartment is measured with a spectrophotometer at242nm according to the Chinese Standard Method (GB12686-2004). The result proves this method is feasible, the desalination of the glyphosate neutralization liquor and salt conversion can be carried out simultaneously by the BMED process, and almost no glyphosate leakage from the feed solution to acid and base compartments can be found. Based on these,the product concentration, current efficiency, energy consumption, economic evaluation, etc. at different current densities ranging from50mA/cm2to110mA/cm2are investigated. The lab-scale study indicates an acceptable current efficiency and energy consumption. For example, the current efficiency and energy consumption can reach63%and10.5kWh/kg acid, respectively, at the70mA/cm2. Furthermore, a preliminary economic feasibility is evaluated through the investigation on the process economics on the basis of the experimental results. And then we experimented with membrane stack assembled by Germany bipolar membranes at the same conditions, the results show that during the experimental period, there had no big difference on current efficiency and energy consumption between these two stacks.The pilot-scale bipolar electrodialysis was carried out then, current density, initial concentration of acid and alkali, initial salt concentration of feed liquid and flow were designed as four independent factors to handle their influences on the BMED process to reclamation of the neutralization liquor, every factor has5levels, using the Central Composite Design(CCD) mothed of response surface method to design experiments.Results show that current density, initial concentration of acid and alkali, initial salt concentration of feed liquid, interaction between current density and the initial salt concentration of feed liquid have significant influences on the process costs. Flow and other interactions between factors have no obvious influence. By optimizing the process cost,the minimum process cost can be achieved at a current density of87.46mA/cm2, initial salt concentration of feed liquid of14g/100mL, initial concentration of acid and alkali of0.08mol/L and flow of40L/h.The predicted value is0.72$/kg of process cost and the actual experimental value according to the value at the current density of90mA/cm2is0.70$/kg, indicating that the optimum condition is appropriate.Considering from the process cost, domestic membranes have lower prices. So we had experiments using domestic BPM and homogeneous/heterogeneous CEM at a current density of50mA/cm2, initial salt concentration of feed liquid of14g/100mL, initial concentration of acid and alkali of0.04mol/L and flow of40L/h. The results show that there are no big difference on energy consumption between Ger. and domestic membranes. But because of the big difference on the price of the membranes, domestic membranes have a advantage on the process cost. But due to domestic BPM has not been commercialized at a large scale, we just made a preliminary exploration.