Sorption properties of low calorific value Greek lignites: Removal of lead, cadmium, zinc and copper ions from aqueous solutions

The removal of metal ions (Pb, Cd, Zn, and Cu) from spiked aqueous solutions using four lignite samples (TH2, TH7, MT2, and MT8) of different quality, from different areas in Greece, was investigated. Cation exchange capacity, humic and fulvic acid content, and the BET specific surface area of the samples were determined, proximate and ultimate analyses were conducted and the mineralogy of their low temperature ash was studied. Equilibrium and kinetic studies were performed in batch conditions. Competitive adsorption of the four elements examined was also investigated. It was observed that the four lignite samples were considerably effective in removing Pb, Cd, Zn, and Cu ions from aqueous solutions, with the sample MT2 being the most effective. Among the elements, Pb appeared to have the strongest affinity based on a mass uptake by lignite samples. The same behaviour was observed during the competitive adsorption experiments. Kinetic experiments proved that, in all cases, equilibrium was achieved within 45 min. Sorption isotherm studies were conducted by varying the initial concentration of the elements. MATLAB software was used to fit experimental data to Langmuir and Freundlich equations. The data were better fitted to the Langmuir equation. Attempt was made to correlate the adsorption behaviour of the lignite samples with the mineralogy of their low temperature ashes and their content in humic and fulvic acids.