Sorption processes, contaminated soils

Phenanthrene dissolved in Triton X-100 solution was separated by sorption with three GACs with different particle size (4 12,12 20, and 20 40 mesh). The highest adsorption selectivity was obtained with the 20 40 mesh over a wide concentration range of phenanthrene and Triton X-100. The results demonstrate that the selective adsorption is potentially effective to reuse surfactants in a soil-washing process for the remediation of contaminated soils. 

We begin with a discussion of the most common minerals present in Earth s crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is (1) a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH (2) the nature of the electrical double layer and how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid and (3) dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. We finish with a discussion of some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces. 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

Phosphates — if of organic origin — have an important indicator value as the soil complex retains them strongly by both sorption processes and chemical reactions. Phosphates of inorganic origin (e.g. industrial fertilizers, corrosion inhibitors), or those which were formed by plant degradation are not significant as indicators of faecal contamination [27-32]. 

Ever since the first landfills became operative in the UK in 1912 (Hasan 1995), leachate has been an environmental problem. These landfills exploited soils and rock formations rich in clay, relying upon their natural characteristics for the containment of contaminants present in the leachates. Due to the low permeability of the clay minerals they contain (1 x 10 and 1 x 10 cm/s), formations such as the Oxford Clay in the UK and Fort Benton in the USA were the primary target of landfill engineers. It was hoped that clay minerals present in situ in rock formations underlying the chosen landfill sites would reduce the flow of leachate. In addition, the clay formations were relied on to attenuate the potentially harmful substances contained in the leachates by a variety of sorption processes absorption, adsorption and cation exchange. 

Adsorption refers to the attachment of a chemical to the surface of a soil particle absorption infers incorporation of a chemical into the structure of the soil particle. As a practical matter, it is difficult to determine the contribution of each process thus, they are usually combined into the single term sorption. In soil, this process is exhibited when a molecule of gas, free liquid, or contaminants dissolved in water is attached to the surface of an individual soil particle (often in the form of organic carbon). This surface attachment can be by any of three general processes ... 

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... 

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