Potential And Mechanism Of Low Molecular Weight Organic Acids On Desorption Of Polycyclic Aromatic Hydrocarbons In Soil

Polycyclic aromatic hydrocarbons（PAHs）,as a type of hydrophobic organic pollutants with"three causing"effect,may diffuse into soil organic matter（SOM）or aggregates with the extension of time after entering the soil,resulting in the difficulty of remediation of long-term PAHs contaminated soil in the field.Low molecular weight organic acids can reduce the degree of SOM polymerization through physical and chemical effects,destroy the structure of soil aggregates,and make the locked PAHs in the soil easier to be released,so as to improve the bioavailability of PAHs and promote the degradation and repair of PAHs in the soil.However,there are few reports on the desorption kinetics,desorption potential and mechanism of PAHs in soil under the action of low molecular weight organic acids.Therefore,eight kinds of low molecular weight organic acids with different structures（formic acid,butyric acid,benzoic acid,salicylic acid,phthalic acid,oxalic acid,citric acid and succinic acid）and two kinds of field long-term PAHs contaminated soils with different properties（black soil:p H=5.31,organic carbon content 56.5 g/kg;lime soil:p H=7.52,organic carbon content 35.1 g/kg）were selected in this study.Through dynamic continuous desorption experiments（12,24,48,96,144,264,408,552 and 696 h）,the desorption potential of different organic acids on PAHs in soil was compared and studied,and the influence mechanism of low molecular weight organic acids on the desorption potential of PAHs in soil was clarified by analyzing the content and properties of DOC dissolved in soil under the action of organic acids,the types and content of metal cations,and the change of hydrogen ion content in solution.And the change of bioavailable PAHs content in soil before and after organic acid treatment was characterized by n-butanol extraction method,aims to explore the relationship between the desorption potential of low molecular weight organic acids on PAHs in soil and the bioavailability of PAHs.The results are as follows:（1）There were significant differences in the desorption of PAHs in soil by low molecular weight organic acids with different structures.After 696 hours of dynamic continuous desorption,compared with Na Cl control,the six organic acids promoted the total desorption of PAHs in black soil,and the increment ratio from large to small was benzoic acid（191%）>salicylic acid（44%）>phthalic acid（30%）>butyric acid（26%）>citric acid（18%）>oxalic acid（18%）,of which the desorption of 2-ring PAHs was the highest,accounting for 32%～52%of the total desorption.The eight selected organic acids promoted the desorption of PAHs in calcareous soil.Compared with Na Cl control,the desorption increment ratio of different organic acids to PAHs was benzoic acid（54%）>butyric acid（53%）>salicylic acid（43%）>citric acid（40%）>succinic acid（29%）>oxalic acid（26%）>phthalic acid（22%）>formic acid（14%）.The PAHs desorbed by salicylic acid,phthalic acid,oxalic acid and citric acid in calcareous soil are mainly 4-ring PAHs（40%～44%）;Among formic acid,butyric acid,benzoic acid and succinic acid,the desorption amount of 2-ring PAHs is the highest（44%～45%）.（2）The dynamic continuous desorption accumulation of PAHs in black soil and lime soil by different low molecular weight organic acids can be well fitted by the first-order kinetic model（R²value range is 0.8192～0.9948）.The comprehensive analysis results show that the desorption potential（P₀）of benzoic acid to field long-term PAHs contaminated soil is the highest,P₀value is 3098μg/kg,accounting for 24.3%of the total PAHs content of black soil,followed by butyric acid,phthalic acid,citric acid,salicylic acid,formic acid,oxalic acid and succinic acid.The proportion of P₀value to the total PAHs content of black soil ranges from 11%to 14%;The P₀value of PAHs desorption from lime soil by benzoic acid and butyric acid is 1321μg/kg and 1337μg/kg,accounting for 21%and 21%of the total PAHs in lime soil respectively.The P₀value of the desorption of PAHs by the other six organic acids in lime soil accounts for 17%～19%of the total PAHs.（3）The dynamic dissolution amount of DOC in black soil and lime soil by different low molecular weight organic acids can also be well fitted by the first-order kinetic model（R²value range is 0.5008～0.9837）,and the DOC dissolution potential（C₀）range is89～8256 mg·L^-1,but there is no significant correlation between the C₀value of DOC dissolution by different low molecular weight organic acids and the P₀value of PAHs desorption.The correlation analysis of metal cations released during dynamic desorption,PAHs desorption and DOC dissolution showed that there was a significant correlation between metal cations and PAHs desorption and DOC dissolution under the action of formic acid,oxalic acid,citric acid,butyric acid and succinic acid（P<0.05）.There was no significant correlation between the metal cation dissolution of benzoic acid,salicylic acid and phthalic acid and the desorption amount of PAHs.Only under the action of phthalic acid on black soil,there was a significant negative correlation between PAHs desorption and DOC dissolution（P<0.05）.（4）The extractable amount（acetone and dichloromethane extraction）and available content（n-butanol）of PAHs in soil changed significantly before and after the action of low molecular weight organic acids.Oxalic acid improved the extractability of PAHs in black soil,and the total extractable amount of PAHs increased by 26%compared with that in original soil.benzoic acid inhibited the extractability of PAHs in black soil,and the total extractable amount of PAHs decreased by 18%.Salicylic acid also inhibited the extractability of PAHs in lime soil,and the total extractable amount of PAHs decreased by12%.The bioavailability content of PAHs in the two soils treated with oxalic acid is the highest,and the dissolution of DOC in the soil by oxalic acid is higher,indicating that oxalic acid destroys the polymerization structure of SOM and enhances the bioavailability of PAHs in the soil.Before the action of organic acids,the P₀values of butyric acid,benzoic acid,salicylic acid and oxalic acid on the desorption of PAHs in black soil were significantly lower than the content of available PAHs extracted by n-butanol,accounting for 25%,46%,20%and 20%of the content of bioavailable PAHs in black soil respectively.The P₀value of organic acid desorption of PAHs in lime soil is 1.5～1.7 times higher than the bioavailable PAHs content extracted by n-butanol,and the P₀value of oxalic acid desorption potential of PAHs in lime soil is close to the bioavailable PAHs content extracted by n-butanol.After treatment with butyric acid,benzoic acid,salicylic acid and oxalic acid,the content of available PAHs extracted with n-butanol was 36%,103%,12%and 115%higher than P₀respectively,indicating that the bioavailability of organic acid on lime soil can promote the transformation of more locked PAHs into bioavailable form.