Effects of Dissolved Organic Matter Fractions Extracted from Humus Soil on Sorption of Benzotriazole in Brown Soil and Black Soil

Abstract

Dissolved organic matter (DOM) affects the sorption behavior of organic pollutants in soils. In this study, bulk DOM (marked as DOMbulk) was extracted from natural humus soil and divided into five individual fractions by XAD-8 resin and anion/cation exchange resins 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 (brown soil and black soil). 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%, respectively. Sorption affinity of BTA and DOM in black soil was greater than that in brown soil, which was attributed to the higher organic carbon (OC) content of black soil as compared to that of brown soil. Brown soil had higher normalized distribution coefficient Koc of sorbed DOM fractions than black soil, because brown soil has a greater proportion of clay and silt, which play important roles in sorption of DOM fractions. 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, which 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 the 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.