Sorption of chlorimuron-ethyl on montmorillonite clays: effects of exchangeable cations,pH, and ionic strength

Sorption interaction of chlorimuron-ethyl with montmorillonite clays was investigated under varied types of exchangeable cation, pH, and ionic strength conditions. Chlorimuron-ethyl sorption on bentonites exhibited pronounced cation dependency, and the sorption ability increased as the sequence Ca²⁺- < Na⁺- < Al³⁺- < Fe³⁺-bentonite, due to different sorption mechanisms, whereas the cation dependency was influenced by the clay type and much weaker for montmorillonites. The decrease of pH at the range of 4.0–6.0 prominently increased sorption of chlorimuron-ethyl on all cation-exchanged montmorillonite clays, and nearly a neglected sorption (about 2 %) can be observed at pH over 7.0. In the presence of CaCl₂, sorption of chlorimuron-ethyl on Fe³⁺-bentonite was promoted because of complexion of Ca²⁺ and the surface of Fe³⁺-bentonite. However, as the concentration of CaCl₂ increased, chlorimuron-ethyl sorption on Ca²⁺- and Fe³⁺-exchanged bentonite decreased, suggesting that Ca bridging was not the prevailing mechanism for sorption of chlorimuron-ethyl on these clays. Furthermore, chlorimuron-ethyl sorption was relatively sensitive to pH, and the change of pH may obscure effect of other factors on the sorption, so it was quite necessary to control pH at a constant value when the effect of other factor was being studied.