Adsorption, preconcentration and separation of cations on silica gel chemically modified with the herbicide 2,4-dichlorophenoxyacetic acid

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), chemically anchored on a silica gel surface, has been used for Cu(II), Ni(II), Zn(II) and Cd(II) adsorption from aqueous and ethanolic 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 (M=Cu, Ni, Zn and Cd), which gave the maximum number of moles adsorbed as 0.44, 0.32, 0.27 and 0.14mmolg-1 in water and 0.75, 0.52, 0.47 and 0.38mmolg-1 in ethanol for Cu, Ni, Zn, Cd, respectively. The retention of cations from aqueous solution was null at pH<=3, however, the adsorption increased above this value up to pH 7. The maximum retention capacity followed the sequence Cu>Ni>Zn>Cd in both solvents and presented a non-linear correlation with enthalpy of hydration and also with charge/radius of each cation. The modified silica was applied for preconcentration and quantification of trace level metal ions present in water and ethanol samples. The immobilized surface was also used as a separation column for these four cations. Column resolution was determined for all cations as the ability of this surface to separate metal ions, determined as RsCd---Zn=0.54, RsCd---Ni=0.64, RsCd--Cu=1.62, RsZn---Ni=0.28, RsZn---Cu=1.26 and RsNi---Cu=0.68.