Soil humic substances were extracted with dipolar aprotic solvents in mixture with dilute HCl, by an intermolecular H-bond-disruption mechanism, and their chemical and physical-chemical properties were compared with humic acids obtained with NaOH and Na4P2O7 solutions. The effects of the HCl-HF purification treatment on the properties of extracts were also evaluated.
Dipolar aprotic mixtures revealed relatively low extracting efficiencies, but their extracts had a high degree of purity. Functional group values, elemental composition, and light absorbance were directly related to degree of purity of extracts. Functional group content generally increased with purification except for total carbonyl groups in extracts of dipolar aprotics. Values for C/N ratios showed that carbon content decreased with purification in extracts of inorganic solutions, whereas a loss of nitrogen was suffered by extracts of dipolar aprotic mixture. These extractants gave, as shown by gel permeation chromatography, humic materials of low and intermediate molecular weight, compared with inorganic solutions. Purification increased the fraction of High molecular weight in the extracts of NaOH, Na4P2O7, and acetone, suggesting a modification of chemical structure through condensation reactions. Infrared spectra revealed the presence of strongly bound aliphatic and nitrogen-containing materials in the extracts of organic mixtures and confirmed that alteration due to purification mainly affected the extracts of inorganic solutions.
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