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

摘要

The effect of dissolved humic acid (HA) on two types of hydrolysis reactions was investigated: (I) dehydrochlorination of γ-hexachlo-rocyclohexane (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.