Effects Of Dissolved Organic Fractions From Humus Soil On Sorption Of Benzotriazole To Brown Soil And Black Soil

Dissolved organic matter(DOM)affects sorption behavior of organic pollutants in soils.In order to understand the effects of DOM on sorption of benzotriazole(BTA)in soils,bulk DOM(marked as DOMbulk)was extracted from natural humus soil and divided into five individual fractions based on their hydrophobicity-hydrophilicity.The isolated DOM fractions were characterized by FTIR,elemental analysis,UV spectrophotometer and potentiometric titration.Batch experiments were conducted to investigate the effects of different fractions of DOM on sorption of benzotriazole(BTA)in two Chinese reference soils(i.e.,brown soil and black soil).Main results are as follows:(1)Proportions of hydrophobic acid(HOA),hydrophobic neutral(HON),hydrophilic acid(HIA),hydrophilic base(HIB)and hydrophilic neutral(HIN)in DOMbulk were 61%,17%,6%,2% and 14%(in weight),respectively.Hydrophobic fractions(i.e.,HOA and HON)had higher E4/E6 values,but lower(O+N)/C values than other fractions.This demonstrated the higher molecular weight,more complex molecular structure and lower polarity of hydrophobic fractions.Among all fractions,HIB had the most hydroxy groups,and HIA had the most carboxy groups.(2)Sorption affinities of BTA and DOM to black soil were greater than those in brown soil.The reason is that organic carbon(OC)content of black soil is higher than that of brown soil.BTA sorption in soil depends on its OC content.Brown soil had a higher normalized distribution coefficient(Koc)of sorbed DOMbulk than black soil.The reason is that brown soil had a greater proportion of clay,which plays important roles in sorption of DOM fractions.HIN and HOA had similar polarity.Dissociation of carboxy group of HOA made it negatively charged.Repulsive interaction of soil with HOA weakened the soil sorption of HOA than HIN.The reasons of the weakest HON sorption to soils were enhance the dissolution of SOM caused by HON and/or inhibition of SOM for sorption of hydrophobic DOM to soil.(3)Dialysis-bag experiments demonstrated that binding coefficient between each isolated DOM fraction and BTA in solution was very weak.The reason is that the binding sites of DOM are occupied by water molecules in solution.Among the isolated DOM fractions,HIN showed the strongest sorption affinity in both soils.Sorption of HIN increased OC content and consequently produced new sorption sites in soils.The enhanced sorption effect from new sorption sites was greater than that from competitive sorption between BTA and HIN.On a whole,HIN enhanced sorption of BTA in soil.HOA and HON inhibited the sorption of BTA in soils.Because the sorption of hydrophobic fractions,especially HON,in soils was less than that of other fractions,competitive sorption between hydrophobic fractions and BTA was the main mechanism.DOMbulk that consists of 78% hydrophobic fractions had similar effects with hydrophobic fractions on sorption of BTA in two soils.The effects of DOMbulk on contaminant sorption can be predicted by superposition of effects of different DOM fractions.