腐植酸之化學組成與分子大小對Cr(VI)還原之影響

Abstract

Humic acids (HAs) are one of the major components in soils which influence greatly the behavior and fate of heavy metals in the environment. Due to the high chemical and structural heterogeneities of HAs, of which domains in humic substances involved in Cr (VI) removal are barely understood. Thus, in the study, a repetitive extract and ultrafiltration techniques were used to fractionate HA into several fractions containing various proportions of aliphatic and aromatic carbons and molecular weights. Three fractions (denoted as F1, F2, and F3) and five molecular weights (MWs) of HAs (Bulk, >100kD, 50-100kD, <10kD) were obtained by a repetitive extraction method and by using a hollow-fiber tangential flow filtration (TFF) system, respectively, from a Yangming Mountain peat soil, Taiwan. In the study, the chemical properties of each HA fraction were characterized by FTIR and 13C-NMR spectroscopy before and after reaction with Cr(VI). Spectral analyses found that the contents of aromatic and oxygen-containing C decreased with a progressive extraction, and lower molecular weights of HAs contained more aromatic carbons and carboxyl groups. The interactions of each HA fractions with Cr(VI) showed that Cr (VI) removal could be enhanced under illumination, and the removal was attributed to Cr(VI) reduction. The redox product, i.e, Cr(III), was either adsorbed on residual HAs or released into the solution. A higher aromatic C content and a smaller molecular weight of humic acid exhibited a greater efficiency for Cr(VI) reduction, and the 13C NMR and FTIR results indicated that the carboxyl and phenolic groups enriched in these domains of HAs were responsible for the enhancement of Cr (VI) reduction. Time-dependent FTIR and 13C NMR spectra suggested that the activated sites on HA were rapidly consumed or dissolved into the solutions upon Cr(VI) oxidation. Thus, Cr(VI) may react directly with dissolved organic C (DOC) in the solution instead of on the surfaces of HA fractions. In conclusion, the O-containing groups of HAs were dominant sites for Cr(VI) reduction; however, the redox reactions proceeded very slowly unless the light participated to the reaction. With an increase of DOC concentrations upon Cr(VI) interactions with HAs, this study highlighted the possibilities that DOC derived from HAs may react with Cr(VI) much longer to complement the slow reactions between these two reactants. Thus, HAs may still have a profound effect to Cr(VI) reduction in a soil system.