Characteristic Of Dissolved Organic Matter Retention On Ferrallitic Soils And Its Effect To 2-Methyl-4-Chlorophenoxyacetic Acid Sorption

Dissolved organic matter(DOM)extensive in environment has an important effect on transportation,transformation and biogeochemistry of pesticides due to the high chemistries and biological activities,has become the research focus center of environmental science.In this study,Chemical structures of DOM from different sources was investigated by UV-Vis,Fourier transform infrared(FTIR)and three-dimensional excitation emission matrix fluorescence(3D-EEM)spectroscopies,and its sorption behavior on ferralsol(or iron oxide)was evaluated by batch experiments.Moreover,the effect and its mechanism on 4-chloro-2-methylphenoxyacetic acid(MCPA)sorption on ferralsol were researched.The results provided above will contribute to a better understanding of DOM sorption and its impacts on the distribution of pesticide in soil-water system,and supported management of agricultural and remediation of pesticide.The main conclusions are shown as follow:(1)Chemical structures of DOM was various from different sources.The results shown that compost DOM(CC)had more humic acid-like matter,monohydroxyp-henyl substance and the function groups of C=O,C=C,CHX,N-O,C-O,-NH2 with high aromaticity and molecular.Soil DOM(SPd)contained more aliphatic nitriles and a little NH4+,-OH,-COOH,C-O but lower hydrophobic.Plant residues DOM had more fulvic acid-like substance,aliphatic hydrocarbons and protein-like substance with-OH,-CHX,N-H and C-O,which the hydrophily was highest.(2)Content of polarity fraction in DOM was investigated by XAD-8 resin.The results suggested that hydrophobic acids(HOA)and hydrophillic matter(HIM)were the main components of CC(45.2%—49.6%)and SPd(or RS)(46.2%—52.2%),respectively,while the contents of acid-insoluble matter(AIM)and hydrophobic bases(HOB)was lower.Moreover,the main components of hydrophobic neutrals(HON)was aliphatic hydrocarbons,humic acid-like substance and the function groups of O-H?C=C?CHx?NH2.HOB was an aromatic matter with aromatic amine and fulvic acid-like substance,contained O-H,C=C,C-O,NH2,CHX.HOA was an organic matter contained larger number of functional group,such as O-H,CHX,COOH,NH2,C=C,and fulvic acid-like substance was also observed in it.HIM was a hydrotropic matter with more fulvic acid-like substance.AIM was an aromatic substances contained aliphatic hydrocarb-ons,humic acid-like substance and the function groups of C=C,NH2 and O-H.(3)The kinetics of DOM sorption on ferrallitic soils could be divided into the two fast and slow reactions stages:.It could be better fitted by Pseudo-frist order model,while the sorption isotherms could be better fitted by Freundlich equation.These results indicated DOM sorption on ferrallitic soils was a heterogeneous multilayer sorption caused by physical force.The sorption capacity of DOM decreas-ed in order Rhodic ferralsol>Haplic ferralsol>Paddy soil,positively correlated with CEC,free iron oxide,free aluminium oxide and clay content,while negatively correl-ated with sand content.Moreover,DOM sorption was concerned with the character of DOM,which correlated with the contents of HON and AIM due to the chain structure of aliphatic alkanes,aromaticity,molecular weight and hydrophobicity.(4)The characteristic of DOM sorption on iron oxide was studied to reveal its role on the ferrallitic soils.The results shown that the process of DOM sorption on iron oxides was a heterogeneous multilayer sorption caused by physical force,positively correlated with the total pore volume.This results of DOM sorption decreased in order hematite>ferrihydrite>goethite.The characteristics of DOM and iron oxides was analysed by UV-Vis,FTIR,3D-EEM and BET after sorption balance.These results indicated the mechanism of DOM was varied from different iron oxides,which the sorption mechanism of hematite,ferrihydrite and goethite were pore filling effect,surface sorption and ligand exchange force,respectively.Moreover,DOM sorption was related to the characteristic of DOM fractions,while HON and AIM was preferentially sorbed,caused by higher aromaticity,hydrophobicity and molecular weight.In addition,the blocked micropore of mineral was observed after sorption balance,which may inhibit the pesticides sorption on it.(5)MCPA sorption on ferrallitic soils and effect of DOM were investigated.The results shown that sorption kinetics of MCPA can be divided into fast sorption followed a slow reaction,which could be better fitted by pseudo-first-order model.Moreover,The MCPA sorption on ferrallitic soils was a spontaneous exothermic process caused by physical force with heterogeneous multilayer sorption,which decreased in order Paddy soil>Rhodic ferralsol>Haplic ferralsol,mostly correlated with soil pH,and the Freundlich coefficient values(Kf)was less than 5,indicated the potential environmental risks of groundwater.In the presence of DOM,MCPA sorption decreased as follow compost DOM,soil DOM,straw DOM,negatively correlated with hydrophobicity,and the hydrophobic neutrals and acid-insoluble matter were key fractions of it,which could enhance the solubility of MCPA and clog soil aperture with hydrophobic forces,shared electrons and Van der Wals forces.