Reaction Rates and Mechanisms of Organic Pollutant Reactions

TABLE 6.5 Grouping of Pesticides Based on Their Persistence in Soils under Laboratory Incubation Conditions  

Nonpersistent, tt/2 20 days Moderately persistent, 20 r1/2 100 days Persistent, h/2 100 days 

Captan Parathion Diazinon Fonofos Heptachlor PCP Lindane 

Besides pesticides, toxic organic substances are of great concern as we attempt to preserve the quality of our environment. Many of these substances have been deposited into aquatic and soil environments. In addition to understanding the equilibrium aspects of these pollutants in soils and sediments, it is imperative that there be an understanding of the rates and mechanisms of retention and mobility. Unfortunately, few of these studies have appeared in the scientific literature. This is most certainly an area of research in the soil and environmental sciences that needs extensive investigation. 

Karickhoff (1980) and Karickhoff et al. (1979) have studied sorption and desorption kinetics of hydrophobic pollutants on sediments. Sorption kinetics of pyrene, phenanthrene, and naphthalene on sediments showed an initial rapid increase in sorption with time (5-15 min) followed by a slow approach to equilibrium (Fig. 6.7). This same type of behavior was observed for pesticide sorption on soils and soil constituents and suggests rapid sorption on readily available sites followed by tortuous diffusion-controlled reactions. Karickhoff et al. (1979) modeled sorption of the hydrophobic aromatic hydrocarbons on the sediments using a two-stage kinetic process. The chemicals were fractionated into a labile state (equilibrium occurring in 1 h) and a nonlabile state. 

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