Complex formation of calcium with humic acid and polyacrylic acid

In order to understand the migration behavior of radionuclides in the underground, it is also important to estimate the effect of the competing cations originally present in the groundwater. In this connection, the complexation of Ca(II) with Aldrich humic acid has been examined. For the study at trace concentrations (similar to 10(-10) M) of Ca(II), the solvent extraction of Ca-45 with TTA and TOPO in cyclohexane has been used. At macro concentrations (10(-4) M) of Ca(II), the measurement of the free Ca2+ ion concentration with a calcium selective electrode has been conducted. To estimate the polyelectrolyte effect of humic acid separately from its heterogeneous composition effect, polyacrylic acid ([-CH2CH(COOH)-](n)) has been selected as a representative of the homogeneous polymeric weak acids and its complexation with Ca(II) has also been examined. The values of log beta(alpha) have been obtained at pH 5 similar to 7 in 0.1, 0.4 and 1.0 M NaCl, where beta(alpha) is the apparent formation constants defined by beta(alpha) = [ML]/([M][R]). In this definition, [ML] and [M] are the concentrations of bound and free Ca2+ respectively, [R] is the concentration of dissociated proton exchanging sites. log beta(alpha) of humate decreases from 2.19 similar to 2.92 (depending on pH and ionic strength 1.0 < I < 0.4) at pCa = 10 to 1.98 similar to 2.44 at pCa = 4, while the variation of pCa has no appreciable influence on the log beta(alpha) of polyacrylate (1.36 similar to 3.24 for I = 0.1 - 1.0). For both humate and polyacrylate, log beta(alpha) decreases linearly with log[Na+], where [Na+] is the bulk concentration of sodium ion. Their dependences of log beta(alpha) on ionic strength are stronger than those of log of monomeric carboxylates such as oxalate and EDTA, indicating the large electrostatic effect of crowded negative charges on the macromolecules of humic or polyacrylic acid.