Studies on the adsorption of arsenic on Cali[6]arene.

p-tert-butyl calix[6]arene (PTC6) was synthesized and characterized by solid- and liquid-state NMR and LC-MS
techniques. The adsorption of arsenite and arsenate on calix[6]arene under different pH conditions and adsorbate
doses was studied. The maximum adsorption of arsenic species on calix[6]arene was observed around neutral pH and
the adsorption density of As (III) was higher than that of As (V). The adsorption of neutral H3AsO3 and negatively
charged H2AsOS4 molecules on calix[6]arene was attributed to the condensation reaction between hydroxyl groups of
PTC6 and arsenic species. The complexation of arsenite with phenolic oxygen was confirmed by solid-state 13C NMR
CP-MAS. Exo attack mechanism was proposed to describe the interaction of arsenous and arsenic acid molecules with
PTC6. The specific interaction between calix[6]arene and arsenic species was further substantiated by zeta-potential
(z-potential) measurements and free energy of adsorption. The free energy of adsorption (Gads) estimated from
Stern–Grahame equation was found to be 25 kJ/mole for As (III) and 19 kJ/mole for As (V).