Effect And Mechanism Of Low Molecular Weight Organic Acids On The Dissolution And Adsorption Of Polycyclic Aromatic Hydrocarbons In Soil

Polycyclic aromatic hydrocarbons（PAHs）,as a type of hydrophobic organic pollutants with a"three-factor"effect,can stably exist in the soil for a long time and are difficult to be degraded.The adsorption-desorption of PAHs and other environmental behaviors depend on their interactions with different components of the soil.Low molecular weight organic acids（LMWOAs）are ubiquitous in soil.They can interact with solid soil organic matter（SOM）and minerals,and promote the dissolution of SOM into dissolved organic matter by destroying the structure of SOM and soil aggregates（DOM）,which in turn affects the environmental behavior of PAHs in the soil.However,the mechanism of the influence of LMWOAs on the environmental behavior of PAHs in soil has not yet been fully clarified.Therefore,this study selected LMWOAs（formic acid,acetic acid,citric acid,and oxalic acid）with different structural characteristics and long-term contaminated soils（black soil,50.04 g/kg OC;paddy soil,21.69 g/kg OC）and field PAHs with different organic content.Relatively clean soil（fluvo-aquic soil,43.50g/kg OC;black soil,10.76 g/kg OC）,compare and study the difference of the dissolution of PAHs in the field contaminated by PAHs by different LMWOAs,and the effect of LMWOAs treatment on the adsorption of PAHs in relatively clean soil.And in-depth analysis of DOM and metal ion dissolution in the soil under the influence of LMWOAs,as well as changes in soil specific surface area and pore distribution,etc.,to clarify the impact mechanism of LMWOAs on the environmental behavior of PAHs in the soil.The results of the study are as follows:（1）The results of dissolution of PAHs in field contaminated soil by different LMWOAs showed that compared with the control（Na Cl solution）,the four LMWOAs all promoted the dissolution of PAHs in black soil,and the dissolution of 2～3 ring PAHs accounted for a larger proportion.Dissolved 62.0±3.7%～69.2±4.6%of the total amount of PAHs.When the organic acid concentration is 200 and 500 mmol/L,compared with the control,the incremental ratio of formic acid to the dissolution of PAHs in black soil is18.5±3.5%and 34.2±4.3%,which are lower than those of acetic acid at the same concentration（51.3±5.3%and 55.8±3.5%）and citric acid（44.5±4.7%and 53.7±6.3%）are also lower than oxalic acid（47.2±4.7%）at a concentration of 200 mmol/L.When the organic acid concentration is 800 and 1000 mmol/L,the incremental ratios of formic acid,acetic acid and citric acid to the dissolution of PAHs in black soil are in the same order,all of which are citric acid（88.9±6.7%and 76.1±4.9%）.)>Acetic acid（68.7±7.5%and46.6±2.7%）>Formic acid（29.5±3.7%and 11.9±5.9%）.At the concentration of 200～800mmol/L,the promoting ability of citric acid on the dissolution of PAHs in black soil tends to increase with the increase of its concentration,while changes in the concentration of formic acid,acetic acid and oxalic acid have no effect on the dissolution of PAHs in black soil.Significant（p>0.05）.The influence of LMWOAs on the dissolution of PAHs in paddy soil is more complicated.Formic acid and acetic acid can inhibit the dissolution of PAHs in paddy soil at all concentrations;oxalic acid can slightly promote the dissolution of PAHs in paddy soil at 200 mmol/L.When it is less than 200 mmol/L,it has an inhibitory effect;citric acid also inhibits the dissolution of PAHs in paddy soil at 200 and500 mmol/L,but when the concentration is 800 and 1000 mmol/L,it turns into an accelerating effect.The results of the correlation analysis show that under the action of citric acid and oxalic acid,the dissolution of iron and aluminum ions in black soil has a very significant positive correlation with the dissolution of PAHs,the increase ratio of PAHs dissolution,and the dissolution of DOM（p<0.01）.It shows that citric acid and oxalic acid break the cationic bridges between organic matter and minerals by chelating with metal ions,and increase the dissolution of organic matter,accompanied by the dissolution of PAHs.In black soil,the reduction of hydrogen ion concentration in the solution under the action of formic acid,acetic acid,citric acid and oxalic acid was significantly positively correlated with the amount of DOM dissolved in the soil and the increase ratio of the dissolution of PAHs in the soil（p<0.05）).This may be due to the destruction of the negative charge auxiliary hydrogen bonds in SOM by LMWOAs,which reduces the degree of polymerization of SOM and promotes the dissolution of PAHs in the soil.（2）Studies on the adsorption of phenanthrene on fluvo-aquic soil and black soil under LMWOAs treatment show that the non-linearity of phenanthrene adsorption isotherm is higher in fluvo-aquic soil and black soil treated with formic acid,acetic acid,citric acid and oxalic acid after removing DOM dissolved from the soil.To preserve the soil of DOM.In the fluvo-aquic soil,the N value range of the Freundlich equation fitted to the phenanthrene adsorption data of the soil without DOM is 0.426～0.551,while the N value range of the soil retaining DOM is 0.719～0.873;in the black soil,the phenanthrene adsorption data of the soil without DOM is The range of N value fitted by Freundlich equation is 0.403～0.457,while the range of N value of preserved DOM soil is0.777～0.971.Among them,the adsorption isotherm of phenanthrene in soils that retain DOM under citric acid treatment has the highest linearity,and the N values in fluvo-aquic soil and black soil are 0.873±0.045 and 0.971±0.088,respectively.The SOM structure of the soil that retains DOM is looser and has a larger N value;while the soil that removes DOM has a relatively high degree of aggregation of the remaining SOM and a smaller N value.In the fluvo-aquic soil and black soil treated with organic acid,the adsorption capacity（K_f）of soil for removing and retaining DOM for phenanthrene had no significant difference among the four acids（p>0.05）.Compared with the control（Na N₃ solution）,the adsorption capacity（K_d）of the fluvo-aquic soil that removes and retains DOM increases at any phenanthrene concentration under the treatment of four organic acids,and the K_d of phenanthrene adsorption to the black soil that retains DOM also increases significantly.In addition,phenanthrene had the largest K_d value in the DOM-preserved black soil and fluvo-aquic soil treated with citric acid.This may be due to the fact that citric acid,as a tricarboxylic acid,can deprive the metal cations in the soil to form chelates,destroy the soil structure,increase the adsorption sites in the soil,and improve the ability of the soil to adsorb phenanthrene.The K_d of phenanthrene adsorption on the fluvo-aquic soil and black soil that retain and remove DOM under the treatment of formic acid and acetic acid also increases.In addition,compared with the soil with DOM removed,the adsorption isotherm of phenanthrene on soil with DOM is more linear,and the N value is closer to 1.This may be because organic acids destroy the（-）CAHB inside SOM through competition,reduce the degree of SOM polymerization and promote the dissolution of DOM.After the depolymerization of the SOM structure,the number of adsorption sites increases,resulting in the enhancement of the soil’s adsorption capacity for phenanthrene.In addition,under the action of citric acid,the specific surface area and micropore volume of the DOM-removed soil in the fluvo-aquic soil and the black soil are greater than those in the DOM-preserving soil under the same organic acid treatment.It shows that the pore filling mechanism of DOM-removed soil has a greater contribution,which is consistent with the result that the adsorption isotherm of phenanthrene by DOM-removed soil is smaller.