Effects Of Biochar On The Bioavailability Of Three Ionizable Pesticides In Soil And Estimation Of Chemical Methods

The adsorption and bioactivity?including toxicity or bioaccumulation?of ionizable pesticides could be affected by their speciation?i.e.,neutral and ionized forms?,and which makes their bioavailability more complicated than the neutral pesticides in bioavailability.In this paper,the effect of biochar and pH on the characteristics of adsorption in soil and bioavailability of the weak acid herbicide imazethapyr,2,4-D and the weakly alkaline fungicide imazalil to rice,were observed,and then its phytotoxicity or bioaccumulation were estimated by chemical methods.The results were as followed:?1?Effects of biochar and pH on adsorption of three ionizable pesticides were studied by the batch equilibrium method.The biochar content increased by 1%,the adsorption constant?Kd?of imazethapyr increased by 0.25,0.335 and 0.323 L/kg in pH 5.5,7 and 8.5 soil respectively;when increased by 1%and 2%,the K_d of 2,4-D increased from 7.52 to 28.01and 75.52 L/kg;the Kd of imazalil increased from 10.89 to 18.78 L/kg when 0.5%biochar added,which indicated that the adsorption capacity of the soil could be enhanced significantly by adding biochar.With the increase of pH,the adsorption capacity of soil decreased for acid imazethapyr and 2,4-D.The Kd of Imazethapyr in soil with pH 7.0 and 8.5was 60.3%and 34.9%of that in soil with pH 5.5.The adsorption capacity of biochar to basic imazalil increased first then decreased and increased slightly last?pH 2?11?,with the maximum adsorption amount in pH?pKa?5.8?.?2?In aqueous solution,the phytotoxicity of imazethapyr and 2,4-D to rice decreased with the increase of pH.In pH 8.0,the concentration for 50%inhibition(IC₅₀)of imazethapyr and2,4-D in was 3 times and double of in pH 5.0.However,in the soil,with the increase of pH,the phytotoxicity of imazethapyr and 2,4-D increased to rice.The IC₅₀0 of imazethapyr decreased from 1.33 to 0.932 and 0.619 mg/kg in the soil with pH of 5.5,7 and 8.5.When the pH increased from 5.5 to 8.5,the IC50 of 2,4-D decreased from 3.49 to 1.871 mg/kg.After addiction of biochar,the IC₅₀0 increased from 0.619-1.33 to 1.65-3.826 mg/kg,2,4-D from 1.871-3.49 to 7.16-72.83 mg/kg in different pH soils.Thus,the adsorption of soil and biochar was able to significantly reduce the phytotoxicity of imazethapyr and 2,4-D to rice.?3?Three different kinds of concentration:added concentration?CAC?,in situ pore water concentrations?CIPW?and concentration extracted by hollow fiber liquid phase microextraction?HF-LPME??CHL?,respectively,were used to estimate the phytotoxicity of imazethapyr and 2,4-D.The IC50 calculated from CAC ranged from 0.619?3.826 mg/kg for imazethapyr and 1.871 to 72.83 mg/kg for 2,4-D in the six tested soils and the coefficient of variation?CV?of IC50 was 65.61%and 129.99%respectively.When we calculated from the CIPW and CHL,the CV decreased to 23.51%and 36.23%for imazethapyr;40.21%and50.93%for 2,4-D respectively.Therefore,both the values of the IC₅₀0 calculated from the in situ pore water concentrations and concentration extracted by HF-LPME were more appropriate than added concentration to evaluate the phytotoxicity.?4?The bioaccumulation factor of imazalil in rice decreased from 14.52-22.6 in aqueous solution to 0.23-1.03 in soil.After 0.5%biochar added,the bioaccumulation concentration decreased from 2.81-4.12 mg/kg to 0.92-1.55 mg/kg for 0.5%.Meanwhile,the bioaccumulation in different pH soil was no significant difference.Thus,the adsorption of soil and biochar might be the main factor affecting rice bioaccumulation.When soil pH value increased from 5.5 to 8.5,the C_HLL of imazalil decreased from 23.61 to 4.77 mg/L in pure soil,and from 0.33 to 0.76 mg/L in 0.5%biochar-amended soil.In addition,better bioaccumulation simulation effect in the biochar-amended soil and the pH 8.5 soil by HF-LPME,and which has the potential to simulate bioaccumulation.