Fenuron sorption on homoionic natural and modified smectites

The adsorption isotherms of fenuron (1,1-dimethyl-3-phenylurea) on three smectites (SWy and SAz montmorillonites and SH hectorite) differing in their layer charge (SH<SWy<SAz) and saturated with several inorganic and organic cations were determined. The isotherms and sorption parameters from Freundlich equation indicate low adsorptivity on inorganic clays, but medium sorption in organoclays (OCls). Fenuron adsorption on homoionic smectites increases with decreasing layer charge and hydratation power of the inorganic exchangeable cation (except Fe3+), indicating that fenuron adsorbs as neutral molecule on uncharged siloxane surface by hydrophobic bonding, with some contribution of polar bond (fenuron C=O group and water associated to exchangeable cation). In the case of Fe3+-saturated smectite fenuron protonation, provided by the interlayer acidic environment, promotes further sorption of fenuron as cationic form. The sorption on organoclays is enhanced via hydrophobic interaction with organocations, which is favoured for high layer charge and basal spacing and organocation saturation close to CEC. Quaternary alkylamonium is more efficient in high layer charge smectite, whereas primary alkylammonium is more efficient in medium charge smectite. The low values of the maximum sorption obtained with homoionic inorganic and organic smectites (100 and 5000 mu mo/Kg) represent one fenuron molecule for each 2000-200 exchange sites and indicate that fenuron sorption is mainly associated to the outer exchange sites. This low adsorptivity of fenuron, as consequence of its high water affinity thigh water solubility) would suggest high mobility of fenuron in natural soil and water systems.