INFRARED SPECTRA OF HUMIC AND FULVIC ACIDS, CONTAINING SILICA, METAL IONS, AND HYGROSCOPIC MOISTURE1

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Abstract
The effect of metal composition, hygroscopic moisture, and SiO2 concentration on infrared spectra of humic compounds was investigated for a definite assessment of the absorption bands between 1100–1000 cm-1 and at 3400 cm-1 as artifacts or as true diagnostic features. Humic acid (HA) and fulvic acid (FA) were extracted from the surface soil of a Typic Hapludult, Rhodic Paleudult, Udic Argiustoll, and an unclassified Spodosol. Ash and SiO2 contents of HA and FA were analyzed gravimetrically, whereas Al, Fe, Zn, Cu, Mn, Na, K, Ca, Mg, and P were determined by atomic absorption and/or plasma spectroscopy. Characterization of the humic compounds was carried out by infrared spectroscopy using the micro-KBr pellet technique, and samples dried for 48 h over CaCl2. The results indicated that both FA and HA had infrared spectra with a strong absorption at 3400 cm-1. Only trace amount or no SiO2 was detected in FA, that exhibited infrared absorption at 1020 cm-1, indicating that this absorption could not have been attributed to the presence of silica impurities. When present, silica was observed to precipitate together with HA after acidification with HCI. This observation was compatible with the fact that silica was insoluble in an acid medium, whereas FA was defined as that humic fraction soluble in acid. HA and FA were also low in metals and P content. The low Na concentration seemed to disagree with the general belief that NaOH extraction yielded Na-humates. The elements retained by either FA or HA were apparently held by bonds stronger than those exerted by adsorption processes since the elements did not possess a behavior similar to that of adsorbed or free ions.

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