17    COMPETITIVE ADSORPTION AND DESORPTION OF A
PHENOXY HERBICIDE AND A CHLOROPHENOL IN A
VOLCANIC SOIL
Sridhar Susarla, Graduate Student
S.M. Rao Bhamidimarn, Coordinator
Department of Biotechnology
Massey University
Palmerston North, New Zealand
G.V. Bhaskar, Postdoctoral Fellow
Department of Food Technology
Massey University
Palmerston North, New Zealand
INTRODUCTION
Widespread application of herbicide and pesticide chemicals in recent years has resulted in a
potential environment risk through accumulation of toxic chemical species in soils. The persistence
and migration of these species depend on their adsorption and desorption characteristics. In literature, the adsorption and desorption of pesticide chemicals in single component systems have been
reported extensively.1"3 However, only a few researchers have examined the competitive effects of
these chemicals in soils.
In the last decade several multisolute adsorption isotherms have been proposed4"7 to describe the
uptake of solutes that individually follow the Freundlich isotherm. A multicomponent adsorption
isotherm based on Ideal Adsorption Solution Theory (IAST) was developed by Radke and Prausnitz4
and later used by other workers5,8,9 to predict competitive adsorption in bisolute mixtures containing
phenolic and low molecular weight compounds. Digiano et al.8 and Yonge and Keinath9 have modified IAST to predict competitive adsorption of phenols onto activated carbon. Fritz and Schluender5
proposed an empirical bisolute adsorption isotherm model. However, the model requires six additional parameters to predict competitive adsorption from bisolute experiments. Murali and Aylmore,6
Bajracharya and Vigneswaran7 used a modified model of Fritz and Shluender5 to predict the adsorption of inorganic ions in soils.
Sheindorf et al.10 proposed a Freundlich-type multicomponent adsorption isotherm model based on
single component parameters and successfully applied to predict the competitive adsorption of p-
nitrophenol and p-bromophenol onto activated carbon. In this work, the model of Sheindorf et al.,0
is applied to describe the competitive adsorption and desorption of MCPA and PCOC in a volcanic
soil. The effect of solute concentrations on the competition coefficients is discussed.
THEORY
The sorption of organic chemicals in soils is generally described by a Freundlich isotherm,"
q = KC„1/n (1)
where K represents the capacity and 1/n the intensity of sorption. Sheindorf et al.10 extended the
single component Freundlich isotherm to bisolute systems assuming an exponential distribution of
energies. Their equations for a bisolute system are:
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