Insight into Cu(II) Adsorption on Polyamine Resin in the Presence of HEDP by Tracking the Evolution of Amino Groups and Cu(II)–HEDP Complexes

The presence of low-molecular-weight organic acids (LMWOAs) dramatically influences the species and adsorption behavior of heavy metal ions (HMIs). The effect of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) on the adsorption of Cu(II) on polyamine resin (PAMD) was investigated by tracking the evolution of amino groups on the PAMD surface and species of Cu(II)–HEDP complexes. First, a simplified proton consumption model was developed to quantify the chemical states of the surface amino groups, which were classified into four categories of different contents and acidity coefficients (pKa) as 2.78 mmol/g (pKa1 = 3.00; defined as type A), 4.44 mmol/g (pKa2 = 6.46; type B), 3.28 mmol/g (pKa3 = 8.64; type C), and 7.40 mmol/g (pKa4 = 10.83; type D). Then, based on theoretical calculations and potentiometric titration, the optimum structure of the Cu(II)–HEDP complexes in the bulk solution were determined to be hexacyclic [Cu(II)L]2–, [Cu(II)HL]−, and [Cu(II)H2L]0 with stability constants of 12.64, 7.07, and 3.80, respectively. When B-type amino group has not been deprotonated, the adsorption mechanism of Cu(II) involved coordination between Cu(II)–HEDP complexes and the deprotonated A-type amino group to form ternary complexes of Cu(II)(R-NH2)2L rather than electrostatic interaction between Cu(II)–HEDP complexes and protonated amino groups. With increasing deprotonation degree of the B-type amino groups, the complexing affinity of PAMD toward Cu(II) increased, resulting in that the ligand competition between HEDP in the liquid phase and deprotonated B-type amino groups on the surface of PAMD, ultimately achieved ligand substitution to form binary complex Cu(II)(R–NH–CH2–CH2–NH2)2. This evolution process provides important guidelines for the development of novel chelate adsorbents resistant to interference by LMWOAs. The enhanced adsorption affinity for HMIs by PAMD can be achieved by reducing the heavy metal complex stability with LMWOAs.