Organocation speciation. I. A comparison of the interactions of methylene blue and paraquat with bentonite and humic acid

Abstract

A model System composed of bentonite, humic acid (HA), polystyrene sulphonic acid (PSSA), methylene blue (MB), and paraquat was used to characterize the possible interactions of organocations in natural waters. The adsorption and desorption studies with bentonite indicated that the clay had a strong preference for the organocations over alkali and alkaline earth cations. Paraquat saturated the bentonite up to the cation exchange capacity whereas methylene blue did not completely saturate the clay but did form methylene blue aggregates on the clay surface. The aggregates could be readily removed by washing with distilled water. The organocation interactions with the anionic polyelectrolytes were more susceptible to changes in pH or ionic strength. Paraquat could be completely desorbed from HA and PSSA by raising the solution ionic strength to 0.1. Methylene blue could not be completely removed from the humic acid but could be removed from PSSA by raising the ionic strength to 0.1. The studies indicate that a combination of dialysis separations, variation of ionic strength, and spectrophotometric analysis could be used to determine the free paraquat and the bound paraquat susceptible to changes in environmental levels of pH and ionic strength.