Sediment sorbed pesticides

Another important consideration in investigation of the reaction of sorbed pesticides is the nature of the sorption process itself. Sorption/desorption kinetics and the physicochemical characteristics of the pesticide molecules in the sediment-sorbed state can be expected to influence the kinetic observations made in experimental systems. 

Unfortunately, our present understanding of sorption kinetics is inadequate to allow unambiguous representation of the sorption-desorption process. Clearly the states of sorbed pesticides include fractions which vary in their lability with respect to desorption (9. 10, 21). The fraction of the sorbed molecules in relatively labile and non-labile states is a function of the nature of the pesticide and sediment and the time of contact between the sediment and pesticide solution. 

This model, in light of the discussion above, is clearly not representative of all of the kinetic processes which are occurring in sediment/water systems containing hydrophobic pesticides. However, it does include at least the more labile fraction of the sorbed pesticide in the overall kinetic model. Complications due to the inadequacy of this representation will be illustrated and discussed below. 

For many modeling purposes, Nhas been assumed to be 1 (42), resulting in a simplified equation, S = C, where is the linear distribution coefficient. This assumption usually works for hydrophobic polycycHc aromatic compounds sorbed on sediments, if the equdibrium solution concentration is <10 M (43). For many pesticides, the error introduced by the assumption of linearity depends on the deviation from linearity. 

The hydrolysis of pesticides which are sorbed to sterilized natural sediments has been investigated in aqueous systems at acid, neutral and alkaline pH s. The results show that the rate constants of pH independent ("neutral") hydrolyses are the same within experimental uncertainties as the corresponding rate constants for dissolved aqueous phase pesticides. Base-catalyzed rates, on the other hand, are substantially retarded by sorption and acid-catalyzed rates are substantially enhanced. A large body of evidence will be presented which substantiates these conclusions for a variety of pesticide types sorbed to several well-characterized sediments. The significance of our results for the evaluation of the effects of sorption on the degradation of pesticides in waste treatment systems and natural water bodies will also be discussed. 

Since many pesticides are compounds of low water solubility, their form in aquatic systems is often dominated not by material in aqueous solution, but rather by material sorbed to suspended or bottom sediments ( ). Thus, an understanding of the hydrolytic reactions of pesticides which are sorbed to... 

It is the purpose of this article to summarize the present status of our understanding of the factors governing the rates of hydrolysis of pesticides which are sorbed to sediments. The work reported herein deals specifically with abiotic hydrolysis reactions, which for some pesticides, may be as important or more important than biologically mediated hydrolysis reactions (], . ... 

This review, then, reports results of experiments which provide information that can be used to test the hypothesis that hydrolysis reactions proceed at substantially reduced rates when the molecules undergoing hydrolysis are sorbed to sediments. Results are reported for a variety of pesticides and for model compounds that are similar in structural features to pesticides. Included are neutral, base-catalyzed and, to a limited extent, acid-catalyzed hydrolysis reactions. 

Diazinon released to water from both point and nonpoint sources may be emitted to the atmosphere by volatilization, sorbed to soils and sediments, or accumulated in aquatic organisms. While evaporation may not be expected to be significant based upon the Henry s law constant (see Table 3-2), volatilization of diazinon can be an important transport process. Sanders and Seiber (1983) reported that 17% of the diazinon added to a model pond volatilized in 24 hours. Diazinon released to water also may be adsorbed moderately by soils and sediments based on its organic carbon partition coefficient (K00) values measured in soil (Sharom et al. 1980a). Because this pesticide is only moderately adsorbed by some soils, leaching into groundwater can occur. 

Many processes are operative in the environment that contribute to the regional elimination of a contaminant by altering its distribution. Contaminants with sufficiently high vapor pressure can evaporate from contaminated terrestrial or aquatic compartments and be transferred through the atmosphere to new locations. Such processes of global distillation are considered largely responsible for the worldwide distribution of relatively volatile organochlorine pesticides such as lindane and hexachlorobenzene. Entrainment by wind and upper atmospheric currents of contaminant particles or dust onto which the contaminants are sorbed also contribute to contaminant redistribution. Sorption of contaminant to suspended solids in an aquatic environment with commensurate sedimentation can result with the removal of contaminants from the water... 

While a -HCH is one of the most frequently studied chiral legacy pesticides in air and water, only limited measurements have been made of its enantiomers in soils and sediments, given its relatively low propensity to sorb to natural organic matter compared to other POPs (log Kg of 3.9 [32]). A depletion of (+)-a-HCH (EF = 0.39) was observed in surficial sediments of the North water Polynya [137]. This value was more nonracemic than the water column composition (EF = 0.45)[137],suggestingeitherpreferentialmicrobialdegradationof(+)-a-HCHasit descended in the water column [127] and/or further degradation once deposited in sediments. 

While much of the preceding discussion has focused on the effect of water-sohd partitioning on the movement of pesticide compounds below the land surface, such partitioning may also influence the transport of these chemicals in surface waters (Figure 4). Compounds exhibiting a pronounced affinity for natural sediments— either because of hydrophobicity, low water solubility, or other chemical characteristics—are transported primarily with suspended sediments in surface waters, rather than in the aqueous phase (Wauchope, 1978). The deposition of sediments to which persistent pesticide compounds are sorbed leads to substantial increases in the residence time of these compounds in aquatic ecosystems. For this reason, detailed analyses of sediment cores obtained from reservoirs around the country have proved to be useful for observing long-term trends in the concentrations of OCs in aquatic environments over several decades (Van Metre et al., 1998). 

This example is a nonionic, moderately polar suite of pesticides. The compounds shown in Figure 7.12 are commonly used in the United States and occur in water at micrograms per liter concentrations. They have water solubilities of 1() to 500 mg/L, which makes them moderately polar substances and easily isolated by trace enrichment using C-18. Filtration of the water sample removes the particulate matter, but not the herbicides. They are present chiefly in the water phase, with less than 1% sorbed to the suspended sediments for most nonionic compounds (Squillace and Thurman, 1992). 

The extent of arsenic sorption in natural waters will be influenced by many factors, relating to both the sorbent and the water composition. As(V) and As(III) have different affinities for various sorbent phases that may be present in sediment, soils, and aquifers. Thus the redox speciation of arsenic and the characteristics of available sorbents will strongly affect the extent of arsenic sorption as will the pH and concenPations of co-occurring inorganic and organic solutes in the aqueous phase. Since sorption is a surface phenomenon and is limited by the availability of surface sites on the sorbing phase(s), the extent of competition between arsenic and other sorbates will depend not only on the affinity of each sorbate for the surface but also on their concentrations relative to each other and to the surface site concentration. Elevated concenPations of phosphate have been used to desorb arsenic from clays (51) and from soils contaminated with arsenical pesticides (113). 

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