Study on the adsorption of dyestuffs with different properties by sludge-rice husk biochar: Adsorption capacity, isotherm, kinetic, thermodynamics and mechanism

Abstract The adsorption capacity, isotherm, kinetics, thermodynamics and mechanism of four dyes with different properties on a novel economical sludge-rice husk biochar (SRHB) were determined. The sludge and rice husk with ratio of 1:1 were pyrolyzed at 500 °C, for 2 h in a tubular furnace. The SRHB was characterized using an element analyzer, Brunauer-Emmett-Teller (BET) method, X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential measurement and X-ray photoelectron spectroscopy. Results of the adsorption experiments revealed that the adsorption capacities of these dyes on the SRHB followed the order: Direct Red 4BS (DR) > Acid Orange II (AO) > React Blue 19 (RB) > Methylene Blue (MB). Adsorption data of DR, RB, AO, MB all fitted both the Langmuir isotherm and Freundlich isotherm well, Temkin and D-R isotherm models verified the result. Kinetic data of the four dyes showed good correlation with the pseudo-second-order model. The adsorption of DR and AO onto SRHB were exothermic, while that of RB and MB were endothermic. Dyestuffs sorption on the SRHB involved surface participation, electrostatic interaction, π-π interactions, hydrogen bond. However the main adsorption mechanisms of the different dyes onto the SRHB varied. Therefore, the SRHB could be an effective adsorbent for removing dyestuffs from wastewater, and it is most effectively for direct dye adsorption, followed by acid and reactive dyes. However, the SRHB was apparently not effective for cationic dyes.