Organic soil components, sorption

Competitive sorption of trace elements to organic soil components has also been studied. Kinniburgh et al. (1996) demonstrated that Cd sorption on a humic acid was reduced by Ca, but in contrast, Cu sorption was poorly reduced. Mandel et al. (2000) showed clear competitive effects of Ca and Mg on Ni sorption to a soil fulvic acid. Many studies have showed evidence that there may be differences in competition between selected trace elements depending on the functional group composition of the humic substances (Kretzschmar and Voegelin, 2001). 

McCall and Agin (1985) speculated that the rapid sorption-desorption of picloram on soils was associated with external sites of organic matter, whereas the slower kinetics could be ascribed to particle diffusion (PD) in organic or inorganic soil components. The amount of picloram associated with the external sites remained nearly constant with time, while that associated with internal sites increased greatly over time. 

Although most nonionic organic chemicals are subject to low energy bonding mechanisms, sorption of phenyl- and other substituted-urea pesticides such as diuron to soil or soil components has been attributed to a variety of mechanisms, depending on the sorbent. The mechanisms include hydrophobic interactions, cation bridging, van der Waals forces, and charge-transfer complexes. 

The aim of this chapter is to provide the current state of knowledge on the factors that affect the mobility of trace elements in soil environments. Special attention is given to the influence of inorganic and organic ligands, including nutrients and root exudates, on the sorption—desorption processes of trace elements in cationic and anionic forms on/from soil components and soils. 

Sorption of trace elements onto soil components is greatly affected by pH, ionic factors, nature of the sorbents, redox reactions, and so on, but the sorption of elements in cationic form differs greatly from that of elements in anionic form. The presence of organic and inorganic ligands (including nutrients) in soil environments has a very important role in the sorption-desorption processes of trace elements. 

In contrast to sorption studies, relatively little information is available on the desorption of trace elements from soils or soil components as affected by organic and inorganic ligands. Desorption studies have showed biphasic reaction processes for sorption and desorption of trace elements (Sparks, 1990) a fast reaction followed by a slow reaction. 

Table 10.6. Sorption Affinity of Organics for Soil Components...

The extent to which an organic compound partitions out of water onto soil is determined by physical-chemical properties of both the soil and the compound. The soil s organic matter content is the single best characteristic for estimating the amount of soil adsorption of pesticides and other organic molecules. The partition, or sorption, coefficient of the organic molecule Koc (equal to ATp/SOM) is rather independent of soil type. This suggests that SOM is the principal soil component responsible for pesticide sorption and that the role of SOM is similar in different soils. 

Letunova et al. (1987) calculated that the microorganism biomass contains from 0.012 to 3.24% of the iodine present in surface soil layers, though some fungi that occur in soils are known to accumulate much higher amounts of iodine. Soil acids favor iodine sorption by soil components such as organic matter, hydrous oxides of iron and aluminum. However, liming is known to reduce the solubility of iodides, iodates and iodine in soils and thus also to reduce iodine bioavailability. 

Trace elanents in cationic and anionic form show a different adsorption capacity onto metal oxides, organic matter, and organomineral complexes. Heavy metals also compete for sorption sites on these soil components. However, we found that Cu inhibits the sorption of Zn more on ferrihydrite than on an organomineral complex or a humic acid-like material. 

The mobility of contaminants and their subsequent removal depend also on their sorption capacity to soil components. Therefore, soil properties, particularly those related to the presence of mineral colloids (e.g. montmorillonite, chlorite, kaolinite) and organic matter, additionally challenge the remediation of mixed contaminated soils. 

Because of high variabilities of the soil composition, the uptake of organic compounds by soil depends critically on the type and the content of soil components, as well as the media to which the soil is exposed. Sorption of a compound on dry or partially hydrated soils would certainly be different from that on wet... 

The sorption to soil components is a determinant factor for the mobility of contaminants, accounting for their distribution among soil, sediment and water phases. The extent to which chemicals partition between the solid and solution phases in soil, or between water and sediment in aquatic ecosystems, determines the likelihood of the contaminants leaching through the soil or being immobile. The soil sorption hence influences the elution of compounds into groundwater bodies as well as their availability for transformation by soil microbes, their volatilization from soil surfaces and their bioavailability for exposed organisms. 

The sorption of non-polar substances, generally to the organic matter of the soil, can be regarded as a distribution process between the polar phase of the soil water and the organic phase of the soil components. The equilibrium constant of this partitioning between solid and solution phases constitutes the adsorption coefficient for soil and sediment ... 

Suspended organic matter components of soil and sediment (e.g. humic acids) may result in decreased apparent toxicity by serving as a sink compartment for the toxicants due to sorption processes. 

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