Photocatalytic Degradation Of Thidiazuron And Quinclorac And Investigation Of Their Degradation Mechanisms And Toxicity Of Products

Over the past years,due to the extensive use of herbicides in agricultural production,there are high pesticide residues detected in surface water,soil,cotton seed,fruits as well as vegetables.Among many water treatment technologies,photocatalysis is an excellent green and environment-friendly technology.In the presence of photocatalyst and light irradiation,abundant active species are in situ produced to attack and oxidize the pollutants to water,CO₂ and non-toxic small molecules.Traditional metal-Salen and nobel metal nanoparticals loaded in semiconductors have excellent stability and strong light absorption ability from the UV to visible-light regions.Furthermore,compared with other photocatalysts,they exhibit the merits of simple operation,high degradation rate,easy recycling and low cost in degradation of pesticide residues.So this thesis focuses on the synthesis of photocatalysts that are responsive to visible light,and their application to the photodegradation of two persistent herbicides——thidiazuron?TDZ?and quinclorac?QNC?.This dissertation includes the following four parts:?1?Highly efficient degradation of TDZ by Ag/AgCl–activated carbon composites under LED light irradiation.The Ag/AgCl–AC composite with an optimal ratio of 2:1?Ag@AC?exhibited a better photocatalytic performance,the degradation rate reached 91%.When the pH was in the range of 7-8,TDZ could be adsorbed sufficiently into the AC,which further prompted to the completion of photocatalytic degradation.Trapping experiments proved that superoxide radicals and holes played a principal role in the degradation.The main products detected after photocatalytic degradation confirmed that the cleavage of TDZ molecules involved hydroxylation of the aromatic ring and oxidation of the N=N as well as sulfur atom in the five-membered heterocycle,followed by transformation into low-molecular-weight molecules,CO₂,and H₂O.The toxicity evaluation experiment also showed that the toxicity of the degradation products was less than that of TDZ solution.The activity of the catalyst was stable and the photocatalytic degradation rates were better maintained in four cycles.?2?Degradation of TDZ in different water matrix in the presence of sunlight and Ag/AgCl-AC?2:1?was extensively explored.In the Yangtze River water,the best degradation rate was reached at pH=6.Sodium humate and fulvic acid had different little effects on the photocatalytic degradation of TDZ when they are in small concentrations.But the inhibitory effect would became stronger when increasing the sodium humate concentration.On the contrary,as increasing the concentration of fulvic acid to 12mg/L,the degradation of TDZ would be accelerated firstly and inhibited at last.The common inorganic anions(SO₄^2-,Cl^-and HCO₃^-)and inorganic cations(Ca²⁺,Cu²⁺and Mg²⁺)all showed inhibitory effects.Trapping experiments and UHPLC-MS detection showed that the superoxide radicals and holes were dominant species in TDZ degradation because they could oxidize N=N and five-membered heterocycle in TDZ molecules,finally transformed the fragments into CO₂,SO₂ and H₂O,etc..The toxicity of the degradation products was less than that of TDZ also proved the degradation was more thoroughly under the sunlight.?3?Preparation and characterization of AgCl/AgI and the photocatalytic degradation of QNC were studied under sunlight.AgCl/Ag/AgI and AgCl/AgI were firstly prepared and characterized by XRD,SEM,HRTEM and UV-Vis spectrometer.As the ratio of AgCl to AgI was 5:2,the degradation rate arrived the highest with AgCl/AgI.The UHPLC-MS results displayed that QNC had underwent the dechlorination,N=C rupture and ring opening,and was further oxidized to CO₂,H₂O and HNO₃.Meanwhile,the toxicity test before and after degradation indicated the toxicity of the degradation products was less than that of QNC solution.The activity of AgCl/AgI was gradually reduced in four reusing experiments because of the increasing of combination between the light induced electrons and the excessive holes.?4?Photocatalytic degradation of QNC with biomimetic Mn???salen under sunlight.?R,R?-(N,N-bis?3,5,3',5'-tetra-t-butylsalicylidene?cyclohexane acetate Mn???displayed the best performance in QNC degradation experiments.The main products resulted from the broken of N=C on the pyridine ring in the QNC molecule and the hydroxylation on the benzoic ring.Toxicity valuation experiment indicated that the toxicity of the degradation products was less than that of QNC solution.But,compared with AgCl/AgI,although the catalytic activity of Mn???salen was maintained better,its degradation products were more complicated.