In this work, samples of humic acids (HA) extracted from six lake sediments were investigated. HA were isolated with method of two-stage successive extraction (A) – extraction with an neutral solution 0.1 mol L^-1 Na₄P₂O₇ and (B) – extraction with an alkaline solution 0.1 mol L^-1 NaOH). It is observed that HA-B are characterized by relatively higher H and O contents than it is observed in HA-A. Also the stable fraction of HA-B are characterized by higher oxidation degree than HA-A fraction loosely associated with mineral part. Analysis of optical parameters showed that the labile fractions of HA-A are characterized by a higher capacity to absorb the UV-VIS radiation than the stable HA-B, as indicated by higher specific absorbance coefficients z₂₈₀ and ₆₀₀. The results indicated that the HA-B extracted by alkaline solution are a more complex structure, larger molecular size and molecular weight and higher condensed of aromatic nuclei than HA-A separated by neutral pyrophosphate solution.

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