Role And Mechanism Of Low-molecular-weight Organic Acids On The Release Of Polycyclic Aromatic Hydrocarbons From Soils

Polycyclic aromatic hydrocarbons（PAHs）,as a class of hydrophobic organic pollutants with carcinogenicity-mutagenicity-teratogenicity,may diffuse into soil organic matter（SOM）or aggregates as time goes.The difficulties was occurred in the field long-term PAHs contaminated soil remediation.Low-molecular-weight organic acids can enhance the solubility of SOM through physical and chemical interactions,promote the release of dissolved organic matter（DOM）,disrupt soil aggregate structure,and make it easier for locked PAHs in the soil to be released,thereby improving the bioaccessibility of PAHs and promoting the biodegradation and remediation of PAHs in natural soil.Kinds of low molecular weight organic acids were detected in soil with a wide range of concentration variations.However,current researches on PAHs release by organic acids were mainly focused on monocarboxylic or dicarboxylic acids,while the effects and mechanisms of low-molecular-weight organic acids with different structures,such as monocarboxylic acids and aromatic acids,on the release of PAHs are not yet clear;In addition,changes in soil environmental conditions（pH,the concentration and coexisting state of organic acids）can affect the morphological distribution of organic acids and their interactions with the soil,thereby having an impact on the release of PAHs.However,there are currently few reports on the laws and mechanisms in this respect.In order to reveal the impact of low-molecular-weight organic acids with different molecular structures on PAHs release from soils and its mechanism,nine low molecular weight organic acids（formic acid,butyric acid,benzoic acid,salicylic acid,phthalic acid,oxalic acid,citric acid,succinic acid and EDTA）and two filed long-term PAHs contaminated soils with different properties（black soil:pH=5.21,the content of organic carbon was 56.5 g/kg;lime soil:pH=7.52,the content of organic carbon was 35.1 g/kg）were selected to study.The dynamic changes in PAHs release from soils with different structural organic acids were also studied,as well as the release of PAHs in the function of the different concentrations（10 m M,50 m M,100 m M,200 m M）,pH（3,4,5,6,7,8,9,10）,and the mixed of organic acids.The degree of SOM dissociation was exhibited by dissolved organic carbon（DOC）release and four metal cations of aluminum,calcium,iron,and magnesium release from soils.The fluorescence characteristics of soil solution represent the dissolved DOM fluorescence composition,and combined with soil solid-phase characterization（Fourier transform infrared spectroscopy,X-ray diffraction,soil particle size distribution and pore distribution changes）,the influence of the chemical structure of low-molecular-weight organic acids on the kinetic characteristics of PAHs release in soil was clarified.The effective concentrations of PAHs dissolution in soil with different structural organic acids were studied.The change pattern of organic acids on PAHs release from soils in the different pH conditions of the solid-liquid system were explored.And the release characteristics of PAHs from soils in the function of multiple organic acids coexistence were clarified.Main conclusions as follow:（1）The study on the dynamic changes of PAHs release from soils under the action of low molecular weight organic acids showed that salicylic acid,benzoic acid,butyric acid,and phthalic acid with higher octanol water partition coefficient(logK_ow)have the strongest ability to release PAHs from Phaeozems.The maximum amount of PAHs dissolved in equilibrium(PAH₀,μg·kg^-1)compared with NaCl control,PAH₀increased by311.0%,140.8%,74.8%,and 48.9%,respectively.In Calcareous soil,salicylic acid and citric acid which has strong metal chelating ability,had the strongest ability to release PAHs,with an increase of 69%and 61%compared to the NaCl control,respectively.In Phaeozems,the PAH₀ value under the action of different organic acids had a significant positive correlation with the DOC aromatic index(SUVA₂₅₄)value and logK_ow value（p<0.05）,indicated that organic acids with hydrophobic groups（benzene ring and fatty carbon chain）such as salicylic acid,benzoic acid,butyric acid and phthalate promoted the PAHs release through hydrophobic effect or competitive adsorption.In Calcareous soil,there was a significant positive correlation between the PAH₀ value and the amount of DOC（p<0.05）,while there was no significant correlation between the PAH₀ value and the SUVA₂₅₄ value of soil solution and logK_ow value of organic acids.It indicated that organic acids in Calcareous soil mainly promoted the release of soil DOC,accompanied with PAHs.（2）The research on the effect of low-molecular-weight organic acid concentration on PAHs release from soils indicated that the release amount of PAHs in Phaeozems and Calcareous soil showed an overall increasing trend with the increase of low-molecular-weight organic acid concentration,but the release ability of different organic acids on PAHs in soil varies with the concentration.10 m M formic acid,butyric acid,and oxalic acid had no significant differences in PAHs released from soils compared to the NaCl control,indicated that fatty acids with a concentration less than 10 m M may not have a promoting effect on PAHs release from soils.Benzoic acid,salicylic acid,and citric acid can significantly promote PAHs release from soils at every selected concentrations.Organic acids with strong chelating ability（citric acid,oxalic acid,and salicylic acid）had a certain release effect on the four metal cations of aluminum,calcium,iron,and magnesium,which have the dissociation ability of the organic matter of calcium and magnesium bonds and iron and aluminum bonds.monocarboxylic acid mainly dissociate organic matter bound by calcium and magnesium bonds in soils.（3）The results on the release effect of low molecular weight organic acids on PAHs in the function of different pH conditions indicated that the released amount of PAHs from Phaeozems and Calcareous soil decreases with the increase of pH value,while citric acid had the opposite trend with PAHs release.However,oxalic acid and formic acid did not exhibit a significant pattern in the release of PAHs from both two soils as a function of pH value.Under the action of different organic acids,the amount of 2-ring dissolved in both soils showed a negative correlation with the pH value of the soil solution endpoint（p<0.01）,and a significant positive correlation with the SUVA₂₅₄ value of DOC（p<0.01）;The dissolution of 5-ring PAHs in both soils under the action of all organic acids was positively correlated with the endpoint pH value of the soil solution（p<0.01）,and significantly positively correlated with the dissolution of DOC（p<0.01）.Under acidic conditions（pH 3-5）,hydrophobic interaction and competitive adsorption are the main mechanisms for organic acids to dissolve PAHs in soil.The logK_ow of low molecular weight PAHs is lower than that of high molecular weight PAHs,which had strong mobility in soil and was more easily dissolved by aromatic acids or linear fatty acids through hydrophobic interaction.The dissolution amount of low molecular weight PAHs is relatively high.The dissociation effect of organic acids on soil organic matter under alkaline conditions（pH 7-10）was the main mechanism for the dissolution of PAHs.High molecular weight PAHs were tightly bound to the soil solid phase,and with the increase of pH,the solubility of SOM increased under the action of organic acids such as citric acid with strong chelating ability.The dissolution amount of high molecular weight PAHs increased with the release of DOC.（4）The study on the effect of mixed low molecular weight organic acids on the PAHs release from soils showed that in Phaeozems,except for oxalic acid benzoic acid mixed acids,the treatment with benzoic acid mixed acids has a relatively high release rate of PAHs in soil,ranged from 288.2 to 459.6μg·kg^-1.In Calcareous soil,mixed acid treatments containing oxalic acid or citric acid have a higher leaching ability of PAHs,with a release amount range of 308.4 to 354.2μg·kg^-1.In Phaeozems,the mixed acid treatment containing oxalic acid had a higher dissolved amount of DOC in the soil,but it was not higher than the dissolved amount of DOC in Phaeozems treated with oxalic acid;The dissolved amount of DOC in Calcareous soil treated with mixed acid was significantly lower than that of citric acid.The release amount of DOC from soils under the coexistence of multiple organic acids mainly depended on the single acid with strong ability to dissolve DOC.The mixing of different organic acids had no significant synergistic effect on the release of DOC or PAHs.Under the coexistence of multiple organic acids with different structures,a single acid with strong ability to dissolve PAHs can play a dominant role in promoting PAHs release.To sum up,the mechanisms of low-molecular-weight organic acids with different molecular structures on the dissolution of PAHs in soil mainly include:（1）single ring aromatic carboxylic acids（benzoic acid,salicylic acid and phthalic acid）and single straight chain fatty acids（butyric acid）.Their own structures contain benzene rings and fatty carbon chains,which can effectively promote PAHs release from soil by competing with PAHs for adsorption sites or hydrophobic effects in soil;（2）Mono-and di-carboxylic acids（formic acid,succinic acid,citric acid,oxalic acid,and EDTA）release DOC by complexing metal cations in the soil,accompanied by the dissolution of PAHs.The research results contribute to a in-depth understanding of the mechanism of low-molecular-weight organic acids with different structures in PAHs release from soils.