Adsorption of phorate, an organophosphorus pesticide, on vertisol.

Adsorption of phorate, an organophosphorus pesticide, on a vertisol soil was studied. The resulting data were well described by Freundlich and Langmuir adsorption isotherms. Adsorption was fast and the equilibrium was established within 8 h, which is comparatively less than reported previously. The mechanism of interaction between phorate and clay and humic acid extracted from the same soil was studied by Fourier-transform infrared (FTIR) spectroscopy. FTIR results suggested the formation of hydrogen bonds between carboxylic acid groups present in humic acid and appropriate electrophilic hydrogen atoms present in phorate. Also there is an indication of involvement of -P-O- group of phorate in the interaction with humic acid. However, the binding of phorate with clay minerals involves van der Waal forces of attraction.