Effect Of Low-Molecular-Weight Organic Acids And Black Carbon On Forms Of PAHs And Bound-PAH Residue Release In Soils

Polycyclic aromatic hydrocarbons?PAHs?are a group of persistent organic pollutants with teratogenic,carcinogenic and mutagenic toxicity.PAHs are stable and difficult to be degraded in soils.They have been found in soil with high concentrations all over the world.The forms of PAHs in soil with different bioavailability may change at certain conditions.Low-molecular-weight organic acid?LMWOAs?are primary components of plant root exudates in soil.Black carbon?BC?is highly aromatic polymer due to incomplete combustion of organisms and fossil fuels,and BC accounts for 60%of organic carbon content in soil.However,to our knowledge,little information is available on impacts of LMWOA and BC on PAHs forms in soil and soil different particle-size fractions.In this work,phenanthrene and pyrene were selected as representative PAHs to investigate the fractionation of the PAH forms in different particle-size fractions of soil,the effect of black carbon on the forms of PAHs in soil,and the effect of LMWOAs on the release of soil bound-PAH residues.The main results are shown as follows:?1?The forms of pyrene were fractionated in different particle-size fractions of yellow-brown soil aged for 4 weeks.Pyrene was found to dominantly present as desorbing fraction and non-desorbing fraction in test soil,Bound residues account for a very small proportion?0.33%?;and 6.00%,4.66%,34.68%,and 40.88%of pyrene was found in fine sand,silt,coarse clay,and fine clay,respectively.The concentrations of each form of pyrene in every particle-size fraction were in the order of non-desorbing fraction>desorbing fraction>bound residue,and non-desorbing and desorbing fraction were the dominant forms of pyrene in test particles?more than 98.82%?,while bound residue contributed less than 1.18%to the total contents of pyrene in each particles.Clays?coarse and fine clays?were the dominant sink of pyrene in soil irrespective of its desorbing fraction,non-desorbing fiaction,bound residue,and extractable fraction.?2?The forms of phenanthrene and pyrene were sequentially fractionated in the original and black carbon-added soils aged for 15,30,45 and 60 days.Black carbon?BC?contents in soil were 0.2%,0.5%and 1%.BC reduced the concentrations of desorbing fraction of phenanthrene and pyrene while enhanced the concentrations of non-desorbing fraction and bound residue in soil.BC reduced the bioavailability of test PAHs in soils,and the available residues of phenanthrene and pyrene reduced 9.66%and 27.90%with the BC addition,respectively.Overall,BC promoted the transfer of PAH forms from easy extraction and degradation residues to more stable and refractory residues in soil.?3?The release of bound residue of phenanthrene was investigated in BC-added soils with the addition of LMWOAs using a batch equilibrium method.Citric,oxalic and malic acids were the representatives of LMWOAs in rhizosphere soil.The three LMWOAs(0?100.0 mmol·L^-1)promoted the release of bound residues of phenanthrene in soi.The release amounts of bound resides of phenanthrene positively correlated to the concentrations of LMWOAs,and the influence levels of these three LMWOAs were in the order of citric acid>oxalic acid>malic acid.The release amounts of bound resides of phenanthrene decreased when BC presented in soil.The higher the soil BC content resulted in the less release of bound residues of phenanthrere in soil.In general,LMWOAs promoted the bioavailability of bound residues of phenanthrene in soils,while BC inhibited the activation of bound residues of phenanthrene by LMWOAs in soil environments.