Preparation Of KOH Activated Silkworm Excrement-based Micropore Carbon And Study On Its Absorption And Sustained/Controlled Release Properties Against Thiamethoxam

Absorption-type slow release agents for pesticides have been favored by people for their advantages of low preparation cost,simple preparation process and short preparation cycles.However,these slow release agents exhibit limited drug-loading capacity and release control effect,which seriously hampers their market application.This paper discusses the absorption and release control properties of high specific surface area microporous charcoal（hereinafter referred to as SMAC）,which was made from high-temperature carbonized and KOH activated silkworm excrement as the carbon-based raw material,to the nicotinic pesticide Thiamethoxam.The SMAC was surface-modified by loading metal cations,anions and surface oxidants to adjust the Thiamethoxam absorption property of the SMAC and thus enable the SMAC to regulate drug loading capacity and drug release kinetics.The findings are:At 850℃ carbonized and 800 ℃ activated,the prepared SMAC exhibited superior absorption and release control properties to Thiamethoxam.The characterization results showed the SMAC had a specific surface area of 1214 m2/g,an average pore size of 2.21 nm and a microporosity of 52.3%.The microporous structure of SMAC is suitable for the rapid adsorption of the Thiamethoxam,the adsorption kinetic curve of SMAC showed that the adsorption in the vicinity of 10 h gradually tends to balance,The maximum Thiamethoxam absorption capacity of SMAC was about 619.2 mg/g.The adsorption kinetics curve could be divided into fast adsorption stage and slow adsorption stages,and the two stages were fitted by the pseudo first-order kinetic equation.The kinetic curve of release control properties of SMAC to Thiamethoxam showed that release process was split into the burst release phase and the smooth release phase Thiamethoxam release rate was 20.5%in the burst release phase（in 6 days）and no more than 40%in the two phases（in 40 days）.The burst release phase and the smooth release phase of Thiamethoxam with SMAC were fitted by pseudo first-order kinetics.By the rate constant k1,Release rate of the burst release phase was 3.5 times than the smooth release phase.The SMAC was modified by loading cations(Fe³⁺,Cu²⁺,Ag+),anions（NO3-,SO42-,Cl-）and surface oxidants（HNO3,H2O2,H3PO4）respectively and the resultant products were labeled SMAC-Fe,SMAC-Cu,SMAC-Ag,SMAC-Fe,SMAC-S,SMAC-Cl,SMAC-HN,SMAC-HO and SMAC-HP.These modifiers could weaken the acting force on Thiamethoxam and thus control the release of it.The characterization results showed the specific surface area of all modified SMAC was slightly reduced,the metal cations and the anions of the modified SMAC were evenly distributed on the surface,and the acidic functional groups on the surface of the modified SMAC were significantly increased.Due to the reduction of the specific surface area of the modified SMAC and its surface functional groups,the force reduced between SMAC and Thiamethoxam,the adsorption rate of the modified material was decreased,but the release rate was accelerated.The burst release phase and the smooth release phase of Thiamethoxam with modifiers were fitted by pseudo first-order kinetics,To compare the release rate from various kinds of modified materials by the rate constant k1,The findings are:（1）The Thiamethoxam release rate from the burst release phase and the smooth release phase of metal cation modified SMAC both rank as:SMAC-Fe>SMAC-Cu>SMAC-Ag>SMAC,This is because the Fe³⁺,Cu²⁺ and Ag+belong to hard acid,border acid,and soft acid.thiamethoxam molecules have abundant soft alkali functional group,and the soft alkali have Stronger binding force with soft acid than hard acid,The release rate rule accord with HSAB theory.Comparison with SMAC,the release rate was accelerated by 20.2-36.4%in the burst release phase,and the release rate was accelerated by 11.5-18.8%in the smooth release phase.（2）The Thiamethoxam release rate from the burst release phase and the smooth release phase of anion modified SMAC both rank as:SMAC-Fe>SMAC-S>SMAC-C1>SMAC,The electron withdrawing ability of material,witch was modified with three kinds of anion,is as SMAC-Fe>SMAC-S>SMAC-Cl.This is that thiamethoxam has abundant strong electron withdrawing groups,it had weak binding force between strong electron withdrawing groups,The release rate rule accord with the push-pull electron theory.Comparison with SMAC,the release rate was accelerated by 13.9-36.4%in the burst release phase,and the release rate was accelerated by 12.5-18.8%in the smooth release phase.（3）Thiamethoxam release rate from the burst release phase and the smooth release phase of oxidation modified SMAC both rank as:SMAC-HN>SMAC-HO>SMAC-HP>SMAC,it corresponds to the oxidizabilities of these oxidants.by the rate constant k1,the release rate was accelerated by 18.4-72.8%in the burst release phase,and the release rate was accelerated by 6.7-25.2%in the smooth release phase.