Mechanisms Of Rate Limited Desorption

Because of the important role sorption and desorption have on the transport and remediation of organic compounds, extensive research has been conducted to elucidate the cause of rate-limited sorption and desorption. Most researchers believe that diffusional resistance encountered within the soil matrix is responsible for observed kinetic behavior ((Pignatello and Xing 1996), and references cited therein). Intraparticle pore diffusion (Ball and Roberts 1991; Farrell and Reinhard 1994; Harmon and Roberts 1994; Cunningham et al. 1997) and intra-organic matter diffusion (Brusseau et al. 1991a; Carroll et al. 1994; Huang and Weber 1997; Kan et al. 1997; Weber and Young 1997; Deitsch and Smith 1999) are often cited as rate-limited diffusion mechanisms. Intraparticle pore diffusion attributes the slow rate of desorption to restrictions encountered by a solute as it diffuses through a network of interconnected mesopores and micropores. Diffusive restrictions are a result of the tortuous path that the pore network creates, as well as the severe constriction of diffusive pathways within micropores. Intra-organic matter diffusions attributes the rate-limited desorption to diffusive restrictions encountered by the solute as it diffuses into and out the soil organic matter phase. Depending upon the soil characteristics, it is possible that either intra-particle pore diffusion or intra-organic matter diffusion may be the predominant rate-limiting mechanism.

In support of the intra-organic matter diffusion mechanism, the amorphous/glassy two-domain model is usually invoked to describe the structure of soil organic matter (Carroll et al. 1994; Huang and Weber 1997; LeBoeuf and Weber 1997; Weber and Young 1997; Xing and Pignatello 1997; Deitsch and Smith 1999). The soil organic matter is envisioned to be composed of two regions: an amorphous region that is capable of conformational changes in response to environmental changes (Carroll et al. 1994; LeBoeuf and Weber 1997), and a "more condensed, highly cross-linked" region that is "glassy" in nature (Carroll et al. 1994; LeBoeuf and Weber 1997). Typically, the building blocks for the various components of soil organic matter (i.e., humic and fulvic acids, humins, etc.) are believed to be aromatic nuclei that are connected by aliphatic chains. Young and Weber (1995) state that "the diagenetic process results in the removal of oxygen-containing functional groups, increases in molecular weight, and increases in the fraction of carbon present in condensed aromatic nuclei compared to those present in aliphatic linkages." Thus, diagenesis transforms soil organic matter from an amorphous state to a condensed, crystalline state.

There is a growing body of experimental evidence that suggests that the rate-limited sorption and desorption of hydrophobic organic contaminants may be partially attributable to extremely slow diffusion of the solute through the condensed regions of the soil organic matter (Carroll et al. 1994; Young and Weber 1995; Huang and Weber 1997; LeBoeuf and Weber 1997; Xing and Pignatello 1997). In support of this concept, one study showed that solute diffusion coefficients in synthetic polymers were consistently two to three orders of magnitude lower in the polymer's glassy state than in its amorphous state (Berens 1989). In addition, it is believed that conformational changes in the soil organic matter after sorption has occurred may be a cause rate-limited desorption (Kan et al. 1994; Kan et al. 1997; Deitsch and Smith 1999). In addition, a fraction of the solute that is sorbed to soil appears to be irreversibly bound.

0 0

Post a comment