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The adsorption of substances on soil particles

The overall effect of this complex of factors is particularly important in the case of pesticides, since their mobility in the environment and their biological effects are connected with their adsorption. The general principles Involved in the study of pesticide adsorption are, however, valid in general and they are also relevant to the remaining types of chemical substances. [Pg.675]

The low or high adsorption of substances is governed by their chemical and physical characteristics. They are dependent on the molecular structure and on derived characteristics, such as the formula, weight, possibility of ionization, solubility in water, lipophilic character, polarity and volatility. The effects of particular characteristics are now discussed in more detail. [Pg.675]

A high mobility is also characteristic of substances which are non-ionized in aqueous solutions, since they are not adsorbed on soil particles. These include most chlorinated hydrocarbons, organophosphates, carbamates, as well as derivatives of urea, phenylamides, different esters, etc. A reasonable solubility in water is the only condition for their good mobility. [Pg.675]

It should be noted that although the relative solubility in water can determine to a remarkable extent the mobility and bioactivity of the various substances, comparisons should be restricted to chemical substances with similar properties. Take, for example, paraquat, which can yield even a 70% solution, whereas the concentration of 2,4-D cannot exceed 650 ppm. The mobility of these two substances in the soil cannot, however, be estimated correctly from these data since paraquat is also present in its cationic form in water and it is strongly adsorbed by the soil, whereas 2,4-D exhibits anionic characteristics and it is only slightly adsorbed. In contrast to this, it is possible to compare the adsorption of chemically similar substances, such as phenurone and diurone. Those are both derivatives of phenylurea. Phenurone solubility in water is as large as 2900 ppm and it is much more mobile in water than diurone, which has a solubility of only 42 ppm. [Pg.676]

Polarity. Polar substances are more strongly bound by soil particles than non-polar ones. Once again, only substances with similar chemical characteristics can be compared. For example 7-BHC (lindane), with a dipole of p = 0.95 X 10 cm (i.e. 2.84 Debye units) is adsorbed more strongly than its isomer /3-BHC with a dipole p = 0. [Pg.676]


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