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
journal contribution
posted on 2019-01-24, 00:00 authored by Wei Zhao, Zicheng Liu, Yuan Yuan, Fuqiang Liu, Changqing Zhu, Chen Ling, Aimin LiThe
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.
