Complex formation of calcium with humic acid and polyacrylic acid

A. Kirishima, K. Tanaka, Y. Niibori, O. Tochiyama

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)


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 (∼ 10-10 M) of Ca(II), the solvent extraction of 45Ca 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 βα have been obtained at pH 5 ∼ 7 in 0.1, 0.4 and 1.0 M NaCl, where βα is the apparent formation constants defined by βα = [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 βα of humate decreases from 2.19 ∼ 2.92 (depending on pH and ionic strength 1.0 < I < 0.4) at pCa = 10 to 1.98 ∼ 2.44 at pCa = 4, while the variation of pCa has no appreciable influence on the log βα of polyacrylate (1.36 ∼ 3.24 for I = 0.1 ∼ 1.0). For both humate and polyacrylate, log βα decreases linearly with log[Na+], where [Na+] is the bulk concentration of sodium ion. Their dependences of log βα on ionic strength are stronger than those of log β 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.

Original languageEnglish
Pages (from-to)555-561
Number of pages7
JournalRadiochimica Acta
Issue number9-11
Publication statusPublished - 2002


  • Calcium
  • Complex formation
  • Humic acid
  • Ionic strength
  • Polyacrylic acid
  • Polyelectrolyte effect


Dive into the research topics of 'Complex formation of calcium with humic acid and polyacrylic acid'. Together they form a unique fingerprint.

Cite this