The potential of transforming rice straw (Oryza sativa) and golden shower (Cassia fistula) seed waste into high-efficiency biochar by atmospheric pressure microwave plasma

Abstract

Biochars were successfully produced from rice straw and golden shower seed wastes and used to obtain agricultural waste-derived and low-cost adsorbents to remove methylene blue (MB) dye pollutants. The biochars were prepared using atmospheric pressure microwave plasma in a nitrogen environment at three individual flow rates of 7, 9, and 11 L/min for 5 min via pyrolysis. After pyrolysis, RS biochar’s surface area and pore volume increased from 1.224 m2/g to 13.796 m2/g, 11.995 m2/g, 6.865 m2/g, and 0.006 cm3/g to 0.804 cm3/g, 0.168 cm3/g, 0.08 cm3/g, respectively, then GS increased from 11.874 m2/g to 40.423 m2/g, 45.731 m2/g, 73.067 m2/g, and 0.006 cm3/g to 0.181 cm3/g, 0.277 cm3/g, 4.227 cm3/g, respectively. FTIR results showed O–H stretching in hydroxyl groups, CO functional group, C-H bending of CH3, and C-H vibration aromatic compounds, essential for MB adsorption. The golden shower seed biochar activated by plasma gas flowrate 11 l/m (GS-11) and rice straw biochar activated by plasma gas flowrate 7 l/m (RS-7) had the highest adsorption capacity among all biochars, 344.82 mg/g, and 188.32 mg/g, respectively, which was 10.6 and 2.07 times higher than other golden shower and rice straw biochar from the previous research by much shorter pyrolysis duration of 24 and 48 times, respectively. In addition, both biochars had higher adsorption capacity than previous biochars from different biomass and pyrolysis methods even though they applied 5 times lower in dosage, 3 times higher on initial MB concentration, and 36–144 times shorter pyrolysis time. The equilibrium data fit well with the Langmuir model, and the adsorption kinetics followed pseudo-second-order kinetics. According to the current study, the atmospheric pressure microwave plasma can generate high-quality biochars quickly and be promoted as a recycling technology method. Then, the product biochars can reach high adsorption capacity with a low dosage, high initial MB concentration, low energy consumption, and no chemical added. Furthermore, the product can enhance the value of rice straw and golden shower seed waste, support a circular economy approach, reduce the waste released to the environment, and reduce air pollution generated.