Effects Of Low Molecular Weight Organic Acids On The Dissolution Of Polycyclic Aromatic Hydrocarbons In Soil Organic Matter Fractions And Their Mechanisms

Polycyclic aromatic hydrocarbons（PAHs）,as a kind of hydrophobic organic pollutants,have the"three-factors"effect,will cause great harm to human health.PAHs have a high octanol-water partition coefficient,and their solubility is small,so they are easy to be enriched in soil and sediment,and difficult to be degraded.Soil organic matter（SOM）is the main factor affecting the environmental behavior of PAHs in the soil environment.PAHs will be adsorptive on the solid SOM,and its extractability and bioavailability decrease with aging.Low molecular weight organic acids（LMWOAs）exist widely in soil,which can promote the transformation of solid SOM into dissolved organic matter（DOM）by destroying SOM and aggregate structure,and then affect the environmental behavior of PAHs in soil.SOM can be divided into different SOM fractions according to physical and chemical methods,and the chemical structure characteristics of different SOM fractions and their combination with soil minerals are different.Therefore,the effects and mechanisms of LMWOAs on the dissolution of PAHs in different SOM fractions should also be different.However,at present,there are few reports about this aspect of research.Therefore,in this paper,four kinds of field PAHs-contaminated soils were used to divide SOM into light fraction（LF）and heavy fraction（HF）by density grouping,and then the HF was divided into coarse mineral associated organic matter（CMAOM,>53μm）and fine mineral associated organic matter（FMAOM,<53μm）.Firstly,the distribution characteristics of PAHs in different SOM fractions were studied.Secondly,the difference in the contents of PAHs dissolved by six LMWOAs with different structures（formic acid,acetic acid,citric acid,oxalic acid,benzoic acid and phthalic acid）in different SOM fractions was studied,Thirdly,LMWOAs with different dissolution capacities were selected to dissoved PAHs of SOM fractions sequentially and continuously.Combined with the dissolution of DOM and metal ions in different SOM fractions under the action of LMWOAs and the change of solution p H before and after the action of LMWOAs,the influence mechanism of LMWOAs on the dissolution of PAHs in SOM fractions was clarified.The results are as follows:1.The distribution characteristics of PAHs in four field polluted soils,LF,CMAOM and FMAOM,were studied,and the environmental risk of PAHs in the bulk soil and each fraction was evaluated by using the toxic equivalent concentration of benzo[a]pyrene（Ba P）TEQ_{Ba P}.The results showed that the content of PAHs in the four soils ranged from3776.8 to 16597μg/kg,and the PAHs were mainly composed of 4-ring PAHs,accounting for 44.2%-53.4%of the total PAHs in the soil.w The content of PAHs in LF was significantly higher than that in CMAOM and FMAOM,but the composition of PAHs in the three fractions was similar,and they were all dominated by 4-ring PAHs,accounting for 42.94%-53.38%,indicated that there was no selectivity of PAHs with different ring numbers among the distribution of different organic matter fractions in soil.The total toxic equivalent concentrations of the 15 PAHs in the four types of field contaminated soils ranged from 401 to 1533μg/kg.The toxic equivalent concentrations of PAHs in LF ranged from 13317 to 31010μg/kg,which were significantly higher than those of CMAOM（428～1539μg/kg）and FMAOM（89～908μg/kg）（p<0.01）.2.The dissolution of PAHs in the whole black soil and its organic matter fractions by six LMWOAs with different structures was studied.The dissolution of PAHs has a promoting effect,among which benzoic acid has the strongest promoting effect;while formic acid and acetic acid have the weakest dissolution ability,they have an inhibitory effect on the dissolution of PAHs in the bulk soil and CMAOM fractions.The total dissolved contents of PAHs in CMAOM by six LMWOAs was 1310±160～2282±118μg/kg,which was higher than that of LF（434±25～2136±692μg/kg）and significantly higher than that of FMAOM（154±8～491±3μg/kg）.The contents of PAHs in LF was significantly higher than that in CMAOM,but the number of PAHs dissolved under the action of organic acids was significantly lower than that in CMAOM,indicating that the composition and structural characteristics of organic matter in different soil fractions would affect the dissolution of their bound PAHs,and the environmental risk of PAHs also different.Therefore,when assessing the environmental risks of PAHs-contaminated soils,the distribution characteristics of PAHs in different soil fractions and the environmental risks of PAHs combined with different fractions should be considered.3.In order to explore the mechanism of different organic acids on the dissolution of PAHs in soil,formic acid,oxalic acid and benzoic acid with obvious differences in dissolution ability were selected.Continuous dissolution experiments of PAHs in the bulk black soil and CMAOM and FMAOM fractions were carried out with Na Cl solution,formic acid,oxalic acid and benzoic acid in the order of gradually increasing dissolution ability.The results showed that the contents of PAHs dissolved in the bulk soil in the above order were 1257±88μg/kg,1261±127μg/kg,433±41μg/kg,786±31μg/kg,respectively;The PAHs dissolved in the CMAOM were 1469±68μg/kg,1547±7μg/kg,473±72μg/kg,1090±70μg/kg,respectively;The PAHs dissolved in the FMAOM were813±67μg/kg,228±13μg/kg,219±1μg/kg,501±6μg/kg,respectively.From the amount of PAHs dissolved in sequence by different acids and the concentration of DOM and metal cations（calcium,magnesium,iron,aluminum）,it can be preliminarily inferred that the possible mechanisms that formic acid,oxalic acid and benzoic acid promote the dissolution of PAHs in soil are mainly the destruction of negative charge assisted hydrogen bond in organic matter,the destruction of cationic bridge bond between organic matter and mineral by chelating metal ions,and the desorption of PAHs on organic matter by van der Waals force.In summary,this study preliminarily explored the distribution of PAHs on different organic matter fractions,the effect of LMWOAs on the dissolution of PAHs in different organic matter fractions,and the mechanism of dissolution of PAHs in soil by different LMWOAs.The results can provide a theoretical basis for the remediation and treatment of PAHs contaminated soil.