Evaluation of the possibilities of using humic acids obtained from lignite in modern water treatment

The aim of this study was to evaluate the possibilities of using humic acids obtained from lignite in modern water treatment. The humic acids (HAs) were extracted from polish deposit of lignite - Sieniawa Lubuska by alkaline extraction using for this purpose following extractants: 0.1 M NaOH - HAs SN0.1 and 0.1 M Na4P2O7 - HAs SP0.1. In order to assess the impact of extractant on the sorption properties of the examined humic acids, their specific surface area and the content of active functional groups were determined. Thermo-oxidative behavior of examined humic substances was investigated employing the thermal analysis. The humic acids obtained from lignite were characterized also by C-13 NMR spectroscopy. In addition to demonstrate the potential of examined humic acids as a low-cost reactive barrier material for environmental protection applications of toxic metals, sorption of Cr(VI) on the HAs was carried out using a batch method. The structure of obtained humic acids was undoubtedly influenced by the type of extractants used for their isolation. In structure of humic acids derived from lignite the aliphatic connections were dominated. Their participation in HAs obtained by using 0.1 M NaOH amounts to 78.9% and 58.5% for sample obtained using 0.1 M Na4P2O7. The highest content of all acidic functional groups - referred to total acidity (TA) is characterized by HAs obtained by extraction with 0.1 M Na4P2O7 and it was 7.69 mmol/g. In lignite humic acids the carboxylic groups, assigned to peak in the range of 160-190 ppm of C-13 NMR spectra constituted for HAs SP0.1 - 3.54% and 2.40% for HAs SN0.1. The specific surface area for all humic acids samples was high from 370.0 to 394.4 m(2)/g and correlate with the results of humic acids acidity. The sorption of Cr(VI) on the surface of HAs depends strongly on the pH and Cr(VI) sorption increased with a decreasing pH. It was observed that the removal efficiency of Cr(VI) was the highest for SP0.1 and decreased from 79.8% to 66.5% with the rise of pH from 2 to 5. Therefore and because of the highly developed specific surface area of examined humic acids as well as the presence of hydrophilic functional groups resulting in their specific sorption and ion exchange properties can be an attractive and inexpensive option in the water treatment.