Degradation Mechanism And Toxicity Evaluation Of Malathion By UV Advanced Oxidation Process And Oxidation-hydrolysis Coupling Process

Malathion is an organophosphorus pesticide with a high frequency of detection in water,which is less efficient in conventional drinking water treatment and can be converted into malaoxon in the process of disinfection.Compared with malathion,malaoxon has higher biological toxicity.Therefore,how to effectively degrade and remove malathion and its oxidation products in water is of great significance to ensure the safety of drinking water.In this paper,we first discuss the degradation of malathion and the change of toxicity in the advanced oxidation process（UV-AOPs）based on ultraviolet light,and then propose a new oxidation-hydrolysis coupling technology to degrade and detoxify malathion..The main research results of this paper are as follows（1）The degradation of malathion by four processes,UV,UV/H₂O₂,UV/Ti O₂and UV/Fenton,was studied experimentally.The degradation products,degradation pathways and degradation kinetics under the four processes were deduced.The toxicity of the solution after the reaction was evaluated using the luminescent bacteria Vibrio fischeri and the acetylcholinesterase toxicity test technique（ACh E）.It was found that the degradation of malathion followed pseudo-first-order kinetics,and the reaction rate constant of UV-AOPs（101.4-142.6 cm²/m J）was significantly higher than that of the UV alone process（6.5 cm²/m J）.After identifying the main intermediate degradation products based on MS and MS/MS mass spectra,the possible transformation pathway of malathion was speculated.In the UV alone process,the initial conversion pathway of malathion is through the splitting of P-S bond.In the photocatalytic reaction,the main degradation pathway is the oxidation of P=S bond to P=O bond.The toxicity test results confirmed that the toxicity of the solution after UV-AOPs treatment increased sharply,indicating that the process produced a more toxic intermediate product.In contrast,no significant change or even lower toxicity was observed for the aqueous solution of malathion treated by UV treatment alone.（2）The degradation of malathion by oxidation-hydrolysis coupling technology was studied.The degradation products,degradation pathways and toxicity changes were identified,and the degradation effects of malathion under different chlorination and p H values were studied.It was found that chlorine can rapidly oxidize the P=S bond of malathion to P=O bond,thus forming its corresponding oxide malathion,which is rapidly hydrolyzed by the nucleophilic attack of OH^-under alkaline conditions.At the same p H,the degradation rate of malathion is 3-10 times higher than that of malathion.The toxicity test of ACh E showed that the toxicity of solution increased first and then decreased in the oxidative hydrolysis process,and the final toxicity of solution was less than the initial toxicity of malathion aqueous solution.