Extraction and purification of humic acids from lignite wastes using alkaline treatment and membrane ultrafiltration

The cleaner production of high purity and safe humic acid (HA) with very low concentration of heavy metals leads to achieve a final product with potential economic value in agriculture, biomedicine, environment and so on. This can be obtained by environmental-friendly separation of low molecular weight fulvates and inorganic elements present in humic alkaline solutions. The aim of this study is the alkaline extraction of solutions containing humates and fulvates derived from lignite wastes followed by purification of humates using membrane processes. A polysulfone 5 kDa molecular weight cut-off membrane in a plate and frame module was used for ultrafiltration of lignite coals-derived alkaline extracts. The effects of operating parameters including transmembrane pressure (TMP) at four levels (1, 2, 3, and 4 bar), feed flow rate (FFR) at five levels (5,15, 25, 35, and 45 ml s(-1)) and processing temperature at three levels (35, 45, and 55 degrees C) were investigated on permeate flux of fulvate compounds, membrane fouling, purity percentage and ash contents of the HA product. Results showed that the highest permeate flux, the lowest membrane fouling, and the most desirable qualitative/quantitative properties of the purified HA was obtained at TMP of 3 bar, FFR of 45 ml s(-1) and temperature of 45 degrees C. The results of ICP-OES analysis showed very low concentrations of heavy metals and inorganic elements in the purified HA while the FT-IR results confirmed the ICP-OES findings and revealed the rich carboxylic and phenolic functional groups of the final product. (C) 2019 Elsevier Ltd. All rights reserved.