Effect of Complexing Ligands on the Adsorption of Cu(II) onto the Silica Gel Surface: I. Adsorption of Ligands

Abstract

The adsorption of several ligands on silica gel was investigated in aqueous solutions. The ligands used were 2,2′,6′,2″-terpyridine, pyridine, 3,4-lutidine, 2-aminomethyl pyridine, 2-pyridine methanol, picolinic acid, salicylic acid, and 5-sulfosalicylic acid. The adsorption behaviors of these ligands were interpreted by means of three adsorption modes: ion exchange, hydrogen bonding, and hydrophobic interaction. For 2,2′,6′,2″-terpyridine, pyridine, and 3,4-lutidine, the adsorption maxima appeared near their respective pKa values and were found to be due mainly to ion exchange, whereas the adsorption of these ligands at low pH was strongly attributed to hydrophobic interaction. The adsorption of 2-aminomethyl pyridine increased with increasing pH over the entire pH range investigated and was due mainly to ion exchange. Picolinic acid was adsorbed mainly by hydrogen bonding either via pyridine N atoms at low pH or via carboxylic O atoms at high pH. 2-Pyridine methanol was adsorbed by hydrophobic interaction at low pH and by hydrogen bonding at high pH. The adsorptions of salicylic and 5-sulfosalicylic acid were very small over the entire pH ranges investigated. For the adsorption mechanism, the Stern model was used to fit adsorption data.