Study On Hydrolytic And Photolytic Behavior Of Acetamiprid In Environment

Acetamiprid belongs to a new-style insecticide known as chloro-neontictinoids. It was regarded as an important substitute of organophosphorus pesticides, and has beed extensively used worldwide.To our knowledge, no reports have been found to investigate the environmental behavior of acetamiprid up to now.Thus,information of the transformation of acetamiprid in environment is indispensable for an ecotoxicological evaluation of it.The hydrolysis and photolysis of acetamiprid were investigated thoroughly in this dissertation.The hydrolysis of acetamiprid was studied firstly in this dissertation.The results indicated that the hydrolysis of acetamiprid belongs to alkaline hydrolysis and the hydrolysis of acetamiprid fits the pseudo-first-order kinetics well.Two hydrolytic product, N-[(6-chloro-3-pyridyl)methyl]-N-methylacetamide and N-methyl(6-chloro -3-pyridyl) methylamine were identified by HPLC-MS. The hydrolytic rate of acetamiprid was increased with the increases of the values of pH and temperatures of the reaction medium.The presence of Cu²⁺,Pb²⁺ and Fe(OH)₃ could promote the hydrolysis of acetamiprid and humic acid could inhibit the hydrolysis of acetamiprid.The mechanism of the promoting effect of Cu²⁺and Pb²⁺ was suggested as followed:(1) Cu²⁺ and Pb²⁺ were coordinated with the atom of N of the group of C=N in the acetamiprid molecule and leads to withdrawing electron density away from the C and generating a more reactive electrophile, which made the bond of C-N more easily broken when attacked by OH-.(2) Cu²⁺and Pb²⁺ could induce the deprotonation of coordinated water molecules, therby generating metal hydroxo species, which is more stronger than OH-.The promoting mechanism of Fe(OH)₃ was suggested as followed:(1) Fe(OH)₃ was coordinated with the atom of N of the group of C=N in the acetamiprid molecule,and leads to withdrawing electron density away from the C atom and generating a more easily broken when attacked by OH-.(2) Fe(OH)₃ takes positron and has strong absorption ability,which made it can absorb acetamiprid molecule and OH- to form a environment with high concentration of reactants around the Fe(OH)₃,more quick hydrolysis of acetamiprid.The mechanisms of inhibting effect of humic acid on acetamiprid hydrolysis were proposed as followed:(1)Humic acid owns some acidic groups which could lower the pH values of the reaction medium and then inhibit the alkaline catalyzed hydrolysis of acetamiprid;(2)Humic acid could absorb acetamiprid and decrease the activity of acetamiprid,which inhibit the hydrolysis of acetamiprid.The photolysis of acetamiprid was also studied in this dissertation. Acetamiprid kept stable when xenon lamp was served as light source,but degraded quickly when exposed to the irradiation of medium-pressure or low-pressure mercury lamp,and the rate constant increased significantly with the increase of light intensity,which was due to acetamiprid could't absorb the irradiation of xenon lamp,but could absorb the irradiation of low-pressure and medium-pressure mercury lamp effectively.The photolysis of acetamiprid fits the pseudo-first-order kinetics well. N₂ -cyano-N1-methyl-N1((3-oxo-2-azabicyclo[2,2,0]hex-5-en-6)methyl)acetamidine was identified as photolytic product of acetamiprid by use of HPLC-MS.Effects of temperature and the values of pH were faint, the photolytic rate of acetamiprid increased slightly with the increases of temperature. Acetamiprid degraded rapidly in acidic medium. Dissolved oxygen could increase slightly the photolytic rate constant of acetamiprid. Fe³⁺, NO₃^-, humic acid and surfactant could inhibit the photolysis of acetamiprid. H2O₂ and TiO₂ could stimulate the photolysis of acetamiprid. The mechanisms of the inhibiting effect of Fe³⁺, NO₃^- and humic acid on photolysis of acetamiprid were all due to the competitive absorption of light between Fe³⁺, NO₃^- and humic acid with acetamiprid.The balance between the competitive absorption of light,the combination of the surfactant's active groups with the acetamiprid molecule,and the relatively high concentration of acetamiprid in the hydrophobic environment formed by micelles of surfactant resulted in the inhibiting effect of surfactant on photolysis of acetamiprid. The OH·generated by the irradiation on H₂O₂ and TiO₂ could promote the photolysis of acetamiprid.The photolytic rate constant of acetamiprid followed the sequence: acetone>water>methanol> acetonitrile>isopropyl.