Binding and removal of octahedral, tetrahedral, square planar and linear anions in water by means of activated carbon functionalized with a pyrimidine-based anion receptor

Abstract

Binding of S₂O₃²⁻, SeO₄²⁻, Pt(CN)₄²⁻, Co(CN)₆³⁻, Au(S₂O₃)₂³⁻ and Fe(CN)₆⁴⁻ anions by the protonated (positively charged) forms of tren (tris(2-aminoethyl)amine) and of the tren-derivative (HL) containing a pyrimidine residue was studied by means of potentiometric measurements in 0.1 M NMe₄Cl solutions at 298.1 ± 0.1 K. Both ligands form stable complexes with these anions which appear to be mostly stabilized by electrostatic forces. In the case of HL, an anion–π interaction with the pyrimidine residue of the ligand also affords a significant contribution to complex stability. Some shape preference for tetrahedral and octahedral anions over square planar ones is observed. A hybrid AC/HL material obtained by adsorption of HL on commercial activated carbon (AC) was used to study the extraction of these anions from water. AC/HL shows enhanced adsorption capacity toward all the anions studied with respect to AC. This behavior is ascribed to the stronger interaction of anions with the HL function of AC/HL than with the Cπ-H₃O⁺ sites of unfunctionalized AC. Of special interest is the enhancement of the adsorption capacities found for Au(S₂O₃)₂³⁻ and Pt(CN)₄²⁻, two anions of great relevance for the extraction of platinum and gold from ores and from metallic wastes.