Infrared Spectra of Humic Acid and Metal Humates Precipitated from Groundwater

Abstract

Brown groundwater from Santa Barbara, Iloilo, Philippines was treated with concentrated hydrochloric acid resulting in the precipitation of dark brown substances. The precipitate was analysed under an FT-IR spectrophotometer with absorption wavelengths revealing the 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: Fe 3+ , Cr 3+ , Ni 2+ , Cu 2+ , Cd 2+ and Pb 2+ , resulting in the precipitation of light 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 Fe 3+ , Cr 3+ , Ni 2+ and Cu 2+ metalhumatecomplexes 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 -1 in 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 Cd 2+ and Pb 2+ metal-humatecomplexes. However, it was determined that the binding sites for Cd 2+ and Pb 2+ were limited to the carboxyl functional group, becausethe absorption band relating to the carbonyl oxygen (C=O) was still observed in their IR spectra. KeywordsHumic Acid; Humic Substances; Infrared Spectroscopy; Metal Humates