Removal of metal ions using lignite in aqueous solution—Low cost biosorbents

Turkish lignite can be used as a new adsorption material for removing some toxic metals from aqueous solution. The adsorption of lignite (brown young coals) to remove copper (Cu2+), lead (Pb2+), and nickel (Ni2+) from aqueous solutions was studied as a function of pH, contact time, metal concentration and temperature. Adsorption equilibrium was achieved between 40 and 70 min for all studied cations except Pb2+, which is between 10 and 30 min. The adsorption capacities are 17.8 mg/g for Cu2+, 56.7 mg/g for Pb2+, 13.0 mg/g for Ni2+ for BC1 (Ilgın lignite) and 18.9 mg/g for Cu2+, 68.5 mg/g for Pb2+, 12.0 mg/g for Ni2+ for BC2 (Beysehir lignite) and 7.2 mg/g for Cu2+, 62.3 mg/g for Pb2+, 5.4 mg/g for Ni2+ for AC (activated carbon). More than 67% of studied cations were removed by BC1 and 60% BC2, respectively from aqueous solution in single step. Whereas about 30% of studied cations except Pb2+, which is 90%, were removed by activated carbon. Effective removal of metal ions was demonstrated at pH values of 3.8–5.5. The adsorption isotherms were measured at 20 °C, using adsorptive solutions at the optimum pH value to determine the adsorption capacity. The Langmuir adsorption isotherm was used to describe observed sorption phenomena. The rise in temperature caused a slight decrease in the value of the equilibrium constant (Kc) for the sorption of metal ions. The mechanism for cations removal by the lignite includes ion exchange, complexation and sorption. The process is very efficient especially in the case of low concentrations of pollutants in aqueous solution, where common methods are either economically unfavorable or technically complicated.