To ascertain the effect of the incorporation of calcium to the HA, i.r. spectra of the original HA and the calcium humates complexes obtained at pH 5 and 7 were obtained, see Figure I.
The spectra differed from each other mainly in the 1700 to 1500 cm-' region. The band at 1725 cm-' (carbonyl from carboxylic acid) of the HA decreased in the Cahumate flocculated at pH 5 and it is completely absent in the Ca-humate flocculated at pH 7. Simultaneously the intensity of the 1400 cm-' and 1600 cm-1 bands characteristics of the carboxylate groups increased while the 2920 cm-' band (aliphatic C-H) decreased.
According to Vinkler4 the COO-band of the carboxylate group in mostly ionic bonds (i.e., bonds between alkaline metal ions and carboxylate groups) can be detected in the wavenumber range 1630-1573 cm-' and the absorption of the group coordinated in a covalent bond between 1650 and 1620 cm-'. Hence, the displacement of the band from 1620 cm-' to 1580 cm-' indicated that the formation of Ca-humate is mostly ionic. The binding of metals to humic substances by carboxylate groups has been reported previously by Schnitzer and Skinner' working with humic substances from soils.