Humic substances mineralization: the variation of pH, electrical conductivity and optical density

CUNHA-SANTINO¹, M.B. & BIANCHINI-JÚNIOR¹, I.

¹ Pós-Graduação em Ecologia e Recursos Naturais, Universidade Federal de São Carlos, Rodovia Washington Luis, km 235, CEP 13565-905, Caixa Postal 676. São Carlos, SP, Brasil. e-mail: 1 mbcunha@cosmo.com.br; ² irineu@power.ufscar.br

ABSTRACT: The variation of pH, electrical conductivity and optical density was described during the mineralization of humic substances (humic acid: HA and fulvic acid: FA) extracted from different sources: sediment, dissolved organic matter from 120 days of decomposition of aquatic macrophytes (Scirpus cubensis and Cabomba piauhyensis) and from water column of Infernão lagoon (21o 33’ to 21o 37’S and 47o 45’ to 47o 51’W). The assays were incubated in the dark under aerobic and anaerobic conditions at 21.0°C. The initial and final dissolved organic carbon concentrations from FA or HA incubations were measured. The pH, electrical conductivity and optical density values were registered during 95 days. It was possible to verify the presence of a buffer system generated by the humic substances. The decreases of the values of electric conductivity were registered in the two experimental conditions (aerobic and anaerobic), probably because of the prevalence of the biological assimilation (immobilization) in detriment to the ion dissolution. The increase on the optical density values was drive by the elevation of the pH values; the subsequent decrease on the optical density values occurred due to the mineralization of the chromophores groups of FA and HA molecules. The global heterotrophic uptake of carbon was also attributed to the decolorization of the substrata, being observed a positive correlation only for FA. In Infernão lagoon during FA and HA mineralization, the anaerobic process is responsible to 30% of the decolorization of these substances.

Key-words: pH, electrical conductivity, optical density, humic acid, fulvic acid, aerobic and anaerobic mineralization.