Soil sorption from water kinetics

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Fig. 6 The distribution of 3-chlorophenol in the pore water of a model soil column from Na bentonite irreversibly precharged by the pollutant for the case of sorption equilibrium and nonequilibrium ( kinetics ) after 1.5 days...

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

Kinetic data for the hydrophobic halocarbon, DDE, provide another example of a photoreaction rate enhancement attributable to sorption on NOM. The direct photoreaction rates of DDE, corrected for light attenuation effects, are increased in filtered natural water samples (0.2 pm) or a solution of a soil fulvic acid (Contech) with high DOC (Table II). As shown in Table II, the degree of rate enhancement is approximately equal to the enhancement in direct photoreaction rates in going from an aqueous to hydrocarbon solution (Table I) (38). 

While the occurrence of most processes affecting organophosphorus insecticide concentrations in surface runoff are well documented, many are relatively poorly understood on a mechanistic level. Key processes requiring further study include sorption to plant and soil surfaces, and transfer from soil water into overland flow. Sorption to foliar surfaces determines the amount of applied insecticide available for washoff onto the soil surface or into surface runoff The kinetics of desorption from plant cuticular materials and the effect of adjuvants on foliar sorption processes need additional research. 

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