Infrared Spectra of Humic Acid and Metal Humates Precipitated from Groundwater

William A. Laride
Brown groundwater from Santa Barbara, Iloilo, Philippines was treated with concentrated hydrochloric acid resulting in the precipitation ofdark brown substances. The precipitate was analysed under anFT-IR spectrophotometer with absorption wavelengths revealingthe presence of alkene, aromatic, carbonyl, carboxyl, hydroxyl, phenol and polysaccharide functional groups. Comparison of the absorption wavelengths with previous studies confirmed that the precipitate was humic acid (HA). The same water sample was also saturated with selected metal ions: Fe3+, Cr3+, Ni2+, Cu2+, Cd2+ and Pb2+, resulting in the precipitation oflight to dark brown substances. When subjected to FT-IR analysis, the precipitate showed positive results for the formation of metal complexes with humic acid. The absorption wavelength revealedthat the active binding sites for Fe3+, Cr3+, Ni2+ and Cu2+metal-humatecomplexes are the oxygen atoms of the humic acid carbonyl and carboxylic acid functional groups. This was indicated by the reduction of the distinctive COO- symmetric stretching from 1712 cm-1 to 1600 cm-1in humic acid and the appearance of COO-antisymmetric stretching at 1400 cm-1 in the metal–humates. A similar lowering of the COO- symmetric stretch and the appearance of the COO-antisymmetric stretch were also observed for the Cd2+ and Pb2+metal-humatecomplexes. However, it was determined that the binding sites for Cd2+ and Pb2+were limited to the carboxyl functional group, because the absorption band relating to the carbonyl oxygen (C=O) was still observed in their IR spectra.
Humic acid; Humic substances; Infrared spectroscopy; Metalhumates
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