Biosorption of lead, hexavalent chrome and cadmium from aqueous solution by torrefied biomass

Within the scope of eco-friendly green technology, the interest in alternative biosorbents in heavy metal removal from water is increasing. However, chemical- or thermal- activation pretreatments are needed for biomass to have good biosorbent properties. The thermal processes such as carbonization and pyrolysis, which are frequently applied for activation, cause high energy consumption and lead to severe degradation in biomass structure and in surface functional groups. The importance of this study stems from the torrefaction of various biomass species at low temperatures for the purpose of biosorbent preparation and the testing of the biosorbents’ heavy metal removal capacity. Thus, biomasses such as tea production waste, corn stalk, pine cone, brown seaweed, rhododendron, and grape seed were torrefied at temperatures of 150–300 °C and the obtained biosorbents were used to remove Pb(II), Cr(VI), and Cd(II) from aqueous solution where concentrations of each metal were 1000 ppm. Effects of the torrefaction temperature, solution temperature, and solution pH on metal uptake were studied by batch adsorption tests. It was determined that the biomasses torrefied at 150 °C has the optimum metal removal capacity. When torrefied corn stalk was used, 17.9 mg/g for Pb(II) and 16.8 mg/g Cr(VI) could be separated from solution, while the maximum Cd(II) uptake was 20.8 mg/g for torrefied tea production waste. It was concluded that the heavy metal removal capacity of biosorbents prepared by low-temperature torrefaction is at non-negligible levels.