| Summary: | 博士 === 國立中興大學 === 土壤環境科學系所 === 102 === Most studies of humic substances (HSs) been carried out on extracts of soils in dilute sodium hydroxide solutions, i.e., the isolation of the International Humic Substances Society (IHSS). However, humin, is insoluble in aqueous base. Recently, a sequential exhaustive extraction (SEE) process has been shown to be capable of isolating and separating the major components of the classically defined HSs from soils of the temperate and tropical regions. The SEE system was used to isolate the HA/FA and humin fractions from a subtropical volcanic Taiwanese soil. Increases in
the aliphatic relative to aromatic carbon contents were observed for both the HA and FA fractions when the pH values of the extraction media were increased. HAs and FAs isolated using the SEE method have spectroscopic profiles similar to those from the IHSS isolate; however, the cumulative extraction efficiency (%) of the SEE method (65%) for the volcanic soil was much higher than for the traditional IHSS
method (33%). When the residual volcanic soil, following extractions once, three, and eight times with 0.1 M NaOH were then extracted with dimethylsulphoxide (DMSO) plus concentrated sulphuric acid (the final solvent in the SEE sequence), it was seen that the content of crystalline polymethylene hydrocarbon (33 ppm 13 C NMR resonance in the humin (or DMSO-acid)) extract increased relative to the amorphous methylene (30 ppm). That highlights the difficulty in dissolving the more highly ordered hydrocarbon structures that would be expected to have closer associations with the
mineral colloids.In addition,the interactions of HSs with
clay minerals commonly occur in soil which may lead to the formations of soil aggregates, forming an organo-clay matrix. Quinones, naturally abiotic polyphenol polymerization reaction catalyzed by MnO2, have been considered as precursor of humification in soil. Clarifying the reactive mechanisms of quinones (HQs) and HSs with widely distributed Fe(oxy)hydroxides can provide insight into the effects of the inorganic minerals on the humification of organic components in soil and the
important physico-chemical reactions of environmental processes. The organo-clay of HA and HQs adsorbed on Fe(oxy)hydroxides followed the order of HA > HQ-20 > HQ-7 > HQ-1. Adsorption results indicated that low pH was favorable for sorption of HA/FA on soil minerals, and OH-concentrations affected dramatically the sorption behaviors of the organic acids with a pH larger than the ZPC of the colloid adsorbents.Sorption of HA/FA on soil minerals followed generally the orderof ferrihydrite > goethite > kaolinite ≒ montmorillonite. The FTIR absorption band at 1720 cm-1 disappeared while the bands at 1609 and 1575 cm-1 broaden and increased in intensity, demonstrating that the formation of a strong complex through a ligand exchange of the carboxylic groups of HA and quinones with OH on the
surfaces of Fe(oxy)hydroxides was the major reaction involved in the sorption process. The hollow-fiber filtration system could further fractionate efficiently HA into three categories of < 50, 50-100, and >100 kD HA, and high molecular weight (MWs) (i.e., > 100 kD HA) of HA exhibited a preferential sorption behavior on Fe(oxy)hydroxides. The preferential sorption of larger HA molecules may be attributed to the presence of higher contents of (1) aromatic carbon (helpful for increasing the hydrophobic interactions); (2) carboxylic group (the major reactive sites); and (3) carbon mass in their structures.
Key Words:
humin extraction; sequential exhaustive extraction; quinones; polyphenol polymerization; Fe(oxy)hydroxides; adsorption
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