Salt effects on the adsorption of a pesticide on modified bentonites

Adsorption of the herbicide linuron on organo-bentonites was measured in the presence of several salts. The bentonites had been modified by cation exchange with hexadecylpyridinium and hexadecyl tributylphosphonium ions. The organo-bentonites adsorbed distinctly higher amounts of linuron than calcium bentonite and desorption was also reduced. The binding coefficient K (derived from the Freundlich adsorption isotherm) was several times higher than for calcium bentonite. The adsorption isotherms of linuron on the organo-bentonites from salt-free aqueous solutions were of S-type but of J-type from saline water. In the presence of many salts, the amount of linuron adsorbed decreased with increasing salt concentration. These salts reduced the linuron adsorption (at a salt concentration of 20 g/L) in the order NaClO4 approximate to KClO4 < NaCl < NaBr < RbCl < CsCl. Only NaI, CsBr, and CsI increased the adsorption of linuron in comparison to salt-free solutions. Desorption of linuron was also reduced in saline (NaCl) solutions. The salt effect may have environmental importance in developing environmentally friendly formulations of herbicides with reduced mobility in soil and enhanced ground water protection.