Organic pesticides from water, sorption

Data on sorption of organic chemicals on dry and partially hydrated soils and of ionic organic compounds from water are limited. Consequently, the present discussion will emphasize the soil sorption of nonionic organic compounds from water. Moreover, the characteristic behavior of nonionic organic chemicals between soil and water is directly applicable in interpreting the fate of the majority of pesticides and industrial pollutants in aquatic systems. 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the sod—water or sod—air interface, resulting in molecular layers on the surface of sod particles. Experimentally, sorption is characterized by the loss of pesticide from the sod solution, making it almost impossible to distinguish between sorption in which molecular layers form on sod particle surfaces, precipitation in which either a separate soHd phase forms on soHd surfaces, covalent bonding with the sod particle surface, or absorption into sod particles or organisms. Sorption is generally considered a reversible equdibrium process. 

Several studies have shown that sorption of various organic compounds on solid phases could be depicted as an accumulation at hydrophobic sites at the OM/water interface in a way similar to surface active agents. In addition Hansch s constants [19,199-201], derived from the partition distribution between 1-octanol and water, expressed this behavior better than other parameters. Excellent linear correlations between Koc and Kow were found for a variety of nonpolar organic compounds, including various pesticides, phenols, PCBs, PAHs, and halogenated alkenes and benzenes, and various soils and sediments that were investigated for sorption [19,76,80,199-201]. 

The fate of pesticides and organic pollutants in natural waters and in soils is strongly dependent on their sorptive behavior (Karickhoff, 1980). Sorption affects not only physical transport of these materials but also their degradation. It is also important to note that the chemical reactivity of pollutants in a sorbed state may be different from their behavior in aqueous solution. Karickhoff (1980) notes that sorbents such as inorganic and organic soil constituents may affect solution-phase processes by changing the solution-phase pollutant concentration or by affecting the release of pollutants into the solution phase. 

Seed kernels of M. oleifera are known to remove lead, iron and cadmium ions from contaminated water by a biosorption process through the metal -protein interactions(2d,5i/ M. oleifera pods (MOP) shows sorption potential for the removal of organics (e.g. benzene, toluene, ethylbenzene and cumene (BTEC)) from aqueous media and methyl parathion pesticide (MP) from surface and ground waters (68,69). M. oleifera alone or in combination can effectively be used and replace alum for dewatering of chemical sludge. In comparison to alum, M. oleifera shows comparable conditioning effects as alum 105). 

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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