Solution interaction of humic acids with calcium ions involves a two-phase system

Abstract

Humic acid particles, dissolved into a homogenous solution, interact with calcium ions via binding sites existing both within the particles and on their surfaces. This is shown by three series of studies in 0.1 M KC1 and 24 °C involving kinetic data, and two sets of steady-state data: pH titrations (Z-data) and data obtained using a calcium-ion-sensitive electrode. The steady-state was controlled via a redox electrode. The diffusion of calcium ions into the humic acid particles is substantial, and the intraparticle bound fraction constitutes 24-42 mol % of the total Ca2+ concentration (B) at B ≤ 1 mM. The electric field produced within the particles by the bivalent calcium ions leads to the release of up to two protons beyond those dissociating in the absence of calcium ions, yielding internal binding sites formally described as CaH-1A and CaH-2A (A is a humic acid ligand). The reactions occurring on the surfaces of the particles are consistent with the formation of a major complex, CaA, and a minor complex, Ca2A, with the apparent log β11 = 3.6 and log β21 = 6.7, respectively. The concentration of humic acids was 1-2 g 1-1, B varied from 0.13 to 1.73 mM and the pH from 2.5 to 7. Analysis of the acid-base data confirms our previous study in 0.1 M NaC1O4 medium [Acta Chem. Scand. 53 (1999) 172] yielding three apparent pK-values (5.79, 3.77, and 1,7), as well as the 'dimer' HA2 supposed to involve hydrogen bonds of the particles; the apparent log *β12 = 8.58.