Organic compounds sorption mechanism

Jaynes WF, Vance GF (1999) Sorption of benzene, toluene, ethylbenzene and xylene (BTEX) compounds by hectorite clays exchanged with aromatic organic cations. Clays Clay Miner 47 358-365 Johnston CT, De Oliveira MF, Teppen BJ, Sheng G, Boyd SA (2001) Spectroscopic study of nitroaromatic-smectite sorption mechanisms. Environ Sci Technol 35 4767-4772... 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

What is a sorption isotherm Which types of sorption isotherms may be encountered when dealing with sorption of organic compounds to natural sorbents Does the shape of a sorption isotherm tell you anything about the sorption mechanism(s) If yes, what If no, why not ... 

Since the neutral compound is very polar, you do not expect it to be extensively absorbed into natural organic matter. Hence, you consider cation exchange to be the major sorption mechanism. Therefore ... 

Adsorption may occur in a combination of three possible mechanisms hydrophobic expulsion, electrostatic attraction, and complexation. Most nonpolar compounds, such as various organics, adsorb by this mechanism, and the degree of partitioning is correlated to the octanol/water partitioning coefficient, Kou, Polar substrates such as various metals sorb via electrostatic attraction and complexation. Table 13.1 shows the typical sorption mechanisms and typical examples. 

For neutral organic compounds, in soils having a low clay/organic carbon ratio, sorption coefficients tend to increase as the hydrophobicity of the compound increases. Aqueous solubility or octanol/ water partition coefficients often are used as indicators of a compound s hydrophobicity. An increase in polarity, number of functional groups, and ionic nature of the chemical will increase the number of potential sorption mechanisms for a given chemical (Garbarini and Lion, 1985). For ionizable compounds, pKa is of particular importance because it determines the dominant form of a chemical at the specific environmental pH. 

Cornelissen, G., Gustafsson, O., Bucheli, T.D., Jonker, M.T.O., Koelmans, A.A., van No-ort, P.C.M., 2005. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils Mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environ. Sci. Technol. 39, 6881-6895. 

Li J, Werth CJ (2001) Evaluating competitive sorption mechanisms of volatile organic compounds in soils and sediments using polymers and zeolites. Environ Sci Technol 35 568-574... 

Xing B, Pignatello JJ (1996) Time-dependent isotherm of organic compounds in soil organic matter implications for sorption mechanism. Environ Toxicol Chem 15(8) 1282-1288... 

Ongoing research is investigating mechanisms of nonionic surfactant sorption onto soil, nonionic surfactant solubilization of hydrophobic organic compounds (HOCs) from soil, and microbial degradation of HOCs in soil-aqueous systems with nonionic surfactants. The equilibrium solubilization of HOC from soil can be described by a physicochemical model with parameters obtained from independent experiments. The microbial degradation of phenanthrene in soil-aqueous systems is inhibited by addition of alkyl ethoxylate, alkylphenyl ethoxylate, and Tween-type surfactants at doses that result in micellization and solubilization of phenanthrene from soil. 

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