Influence of sorption to dissolved humic substances on transformation reactions of hydrophobic organic compounds in water. Part II: Hydrolysis reactions Original Research Article

The effect of dissolved humic acid (HA) on two types of hydrolysis reactions was investigated: (I) dehydrochlorination of γ-hexachlorocyclohexane (HCH) and 1,1,2,2-tetrachloroethane (TeCA) as a reaction involving hydroxide ions (OH−) and (II) hydrolysis of 1-octyl acetate (OA) which is catalyzed by H+ at the applied pH value (pH 4.5). The rate of TeCA hydrolysis was not affected by addition of 2 g l−1 of HA at pH 10 (k′ = 0.33 h−1) but HCH hydrolysis was significantly inhibited (k′ = 4.6 × 10−3 h−1 without HA and 2.8 × 10−3 h−1 at 2 g l−1 HA). HCH is sorbed by 51% whereas TeCA sorption is insignificant at this HA concentration. Sorbed HCH molecules are effectively protected due to electrostatic repulsion of OH− by the net negative charge of the HA molecules. In contrast, OA hydrolysis at pH 4.5 (k′ = 1.6 × 10−5 h−1) was drastically accelerated after addition of 2 g l−1 HA (k′ = 1.1 × 10−3 h−1). The ratio of the pseudo-first-order rate constants of the sorbed and the freely dissolved ester fraction is about 70. H+ accumulation in the microenvironment of the negatively charged HA molecules was suggested to contribute to the higher reaction rate for the sorbed fraction in case of this H+-catalyzed reaction. Analogous effects from anionic surfactants are known as micellar catalysis.