Sorption/desorption organic contaminants

An important and recently reported issue, namely slow sorption/desorption rates, their causes at the intra-particle level of various solid phases, and how these phenomena relate to contaminant transport, bio availability, and remediation, is also discussed and evaluated. A case study showing the environmental impact of solid waste materials which are mainly complex organic mixtures and/or their reuse/recycling as highway construction and repair materials is presented and evaluated from the point of view of sorption/desorption behavior and data modeling. 

The bio availability of organic compounds in soils/sediments to microbes, plants, and animals is important from the perspective of remediation and risk assessment. Cleanup technology (ex situ or in situ) of contaminated soils and bottom sediments requires mass transport of contaminants through the solid materials, which in turn depends on sorption/desorption kinetics. 

The bioavailability of contaminants to wildlife and humans is also an area of critical importance, where contaminants can be taken up in pore water and by dermal contact, particle ingestion, or particle inhalation. The dynamics of sorption/desorption are not currently incorporated into exposure and risk assessment models for organic compounds, where availability, in most cases, is assumed to be 100% [224]. Recently, the following have been demonstrated and reported ... 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

Fig. 8.47 Phenanthrene adsorption-desorption hysteresis observed for Lachine and Chelsa humic acid aggregate the hysteresis index is given by n. Reprinted from Huang W, Peng, P. Yu, Z. and Fu J (2003) Effects of organic matter heterogeneity on sorption and desorption of organic contaminants by soils and sediments. Appl Geochem 18 955-972. Copyright 2003 with permission of Elsevier...

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. 

Wolfe (1989) suggested a model to describe abiotic reduction in sediments, where a nonreactive sorptive site and an independent reactive sorptive site are considered. The nonreactive sorptive sink is consistent with partitioning of the contaminant to the organic carbon matrix of the solids. The model is described by Fig. 13.5 where P S is the compound at the reactive sorbed site P is the compound in the aqueous phase S and S are the sediments, P S is the compound in the nonreactive sink k, k , k , and k are the sorption-desorption rate constants, and k, k, and k are the respective reaction rate constants. If the reaction constants k and k are neglected, two rate-limiting situations are observed transport to the reactive site and reduction at the reactive site. The available kinetic data, however, do not allow one to distinguish between the two mechanisms. 

A potential limitation of surfactant-enhanced desorption is the observation that sorbed surfactant molecules can increase the sorption of hydrophobic organic contaminants (Edwards et al. 1994 Sun et al. 1995 Ko et al. 1998). Sun et al. (1995) reported that the nonionic surfactant Triton X-100 increased the sorption of p,p -DDT, 2,2 ,4,4 ,5,5 -PCB, and 1,2,4-trichlorobenzene to a soil (joc= 0.001) at concentrations below CMC. At concentrations above CMC, the distribution coefficients (Kp) of the DDT and PCB studied were reduced to levels below their respective values in pure water. However, at a surfactant concentration of five times CMC, the Kp of 1,2,4-trichlorobenzene was still a factor of three higher than Kp in pure water. Edwards et al. (1994) and Ko et al. (Ko et al. 1998) reported similar results for different groups of surfactants. 

The type of soil also plays an important role in determining how readily contaminants will desorb when treated with thermal desorption. Studies conducted on harbor sediment indicate organic contaminants exhibit preferential sorption to coal and coal-derived particles over sand, silt, and clays. Researchers found that while coal/wood derived particles only accounted for 5%i of particles in the test soil, 62% of total PAHs were sorbed to these particles. Successive extractions were conducted and revealed that only 8%i of the PAHs sorbed to the coal/wood-derived... 

The CSIA concept relies on the presumption that only biodegradation significantly alters the isotope composition of contaminants in the aquifer. Other processes such as dilution, evaporation and sorption-desorption that are also involved in the attenuation of contaminants are considered not to affect their isotope composition to a significant extent. Smallwood et al. [17] found that equilibrium partitioning of MTBE from an organic phase (e.g. spilt gasoline)... 

This paper summarizes the types of compounds found by our technique while biomonitoring for a variety of volatile and semi-volatile organic contaminant residues. Briefly, hive atmospheres were drawn through multibed sorption traps and subsequently analyzed by thermal desorption/gas chromatography/mass spectrometry (TD/GC/MS). 

A Three-Domain Model for Sorption and Desorption of Organic Contaminants by Soils and Sediments... 

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