Natural organic matter -, Solid

Therefore, we may not be too surprised to find that nonionic chemicals show increasing solid-water distribution ratios for soils and sediments with increasing amounts of natural organic matter. This is illustrated for tetrachloromethane (carbon tetrachloride, CT) and 1,2-dichlorobenzene (DCB) when these two sorbates were examined for their solid-water distribution coefficients using a large number of soils and sediments (Fig. 9.7, Kile et al., 1995.)... 

Obviously, it is actually the total organic mass consisting of carbon, hydrogen, oxygen, nitrogen, etc. within the solid phase that acts to sorb the chemical of interest (i.e., the fom in kg om kg-1 solid). Natural organic matter is typically made up of about half carbon (40 to 60% carbon) hence, fom approximately equals 2-f and these two metrics are reasonably correlated. 

Let us now come back to the issue of linearity of the isotherm and dependency of Kld on the sorbate concentration. In numerous field studies in which both particle-associated and dissolved concentrations of PAHs are measured, apparent Kioc values are up to two orders of magnitude higher than one would have predicted from a simple absorption model (Gustafsson and Gschwend, 1999). If a natural soil or sediment matrix includes impenetrable hydrophobic solids on which the chemical of interest may absorb, the overall Kioc value must reflect both absorption into recent natural organic matter and absorption onto these surfaces. 

Let us evaluate some experimental data. To this end, we use a dual-mode model (Eq. 9-6). This model is a combination of a linear absorption (to represent the sorbate s mixing into natural organic matter) and a Freundlich equation (as seen for adsorption to hydrophobic surfaces or pores of solids like activated carbons) ... 

The value of the partition coefficient, Kip, is given by the product, focKioc, where/ic and Kjoc apply only to the natural organic matter into which the sorbate can penetrate. The value of Kif is less well understood, but recent observations suggest it should be related to the quantity of adsorbent present (e.g., the fraction of black carbon in a solid matrix, /bc) and the particular compound s black-carbon-normalized adsorption coefficient (e.g., Klhc). Typical values of the Freundlich exponent are near 0.7. Hence, in a first approximation the data should fit ... 

The amount of charges on particle surfaces depends on the mineralogy of the solid and the nature of the aqueous solution in which it occurs. Several important kinds of surfaces are common in the environment (Table 11.3). Here we especially consider (1) oxides or oxyhydroxides, (2) alumino-silicates or clay minerals, and (3) natural organic matter and other solids like carbonates. 

Doll, T. E., F. H. Frimmel, M. U. Kumke, and G. Ohlenbusch, Interaction between natural organic matter (NOM) and polycyclic aromatic compounds (PAC) - comparison of fluorescence quenching and solid phase micto extraction (SPME) , Fresenius J. Anal. Chem., 364, 313-319 (1999). 

Simpson, M. J., and Hatcher, P. G. (2004). Determination of black carbon in natural organic matter by chemical oxidation and solid-state 13C nuclear magnetic resonance spectroscopy. Org. Geochem. 35, 923-935. 

Smernik, R. J., and Oades, J. M. (2000a). The use of spin counting for determining quantitation in solid state C-13 NMR spectra of natural organic matter 1. Model systems and the effects of paramagnetic impurities. Geoderma 96,101-129. 

Peuravuori, J. and Pihlaja, K. (1997a) Isolation and characterization of natural organic matter from lake water comparison of isolation with solid adsorption and tangential membrane filtration. Environ. Int., 23, 441—451. 

Figure 1 illustrates the different phases available for organic chemicals in the environment. These may include the original HWM, as well as water, air, soil, and other organic chemicals present in liquid or solid phases. As previously mentioned, the HWM may be recycled-asphalt pavement or shredded tires. Both the HWM and soil can be very complex. HWM may consist of different polymers, rocks and mineral fragments, and various HOPs. Soils consist of different types and amounts of natural organic matter, black carbon, and minerals. As discussed below, carbonaceous materials typically have the greatest affinity for HOPs. 

Haack, 2001). Furthermore, Fe and Mn oxyhydr-oxides form surface coatings on other types of mineral surfaces such as clays, carbonates, and grains of feldspar and quartz. The three most common environmental solid substrates are Fe-oxides, Mn-oxides, and natural organic matter (NOM) (Warren and Haack, 2001). 

Sorption of VOCs involves the processes of adsorption and partitioning. Partitioning is the incorporation of the VOC into the natural organic matter associated with the solid and is analogous to the dissolution of an organic compound into an organic solvent. Adsorption is the formation of a chemical or physical bond between the VOC and the mineral surface of a solid particle (Rathbun, 1998). The equilibrium relation between aqueous and solid phase concentrations then is expressed as... 

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