Adsorption, separation, and thermochemical data on the herbicide picloram anchored on silica gel and its cation interaction behavior

The herbicide 4-amino-3,5,6-trichloropicolinic acid (picloram), chemically anchored on silica gel surface (SiPi), has been used for divalent cation M2+ (Cu, Ni, Zn, and Cd) adsorption from aqueous solutions at room temperature. The series of adsorption isotherms were adjusted to a modified Langmuir equation from data obtained by suspending the solid with MCl2 solutions, which gave the maximum number of moles adsorbed as 9.27, 7.54. 5.12, and 1.54 x 10(-4) mol g(-1) for Cu, Ni, Zn, and Cd, respectively. The minimum cation retention capacity from aqueous solution was observed at pH 1, increasing up to pH 4, and was maintained constant at pH 5 for all cations. The maximum retention capacity followed the sequence Cu > Ni > Zn > Cd. SiPi-M (Cu, Ni) interactions presented endothermic enthalpic values, which contrasted with exothermic values for SiPi-M (Zn, Cd) interactions. The anchored herbicide was also used to separate the cations when loaded in a column. Their resolutions were determined as the ability of this surface to separate cations, to give Rs(Cd-Zn) = 2.33, Rs(Cd-Ni) = 3.16, Rs(Cd-Cu) = 7.21, Rs(Zn-Ni) = 1.31. Rs(Zn-Cu) = 2.55, and Rs(Ni-Cu) = 0.72. (C) 2003 Elsevier Inc. All rights reserved.