Adsorption characteristics of methylene blue by biochar prepared using sheep, rabbit and pig manure

Sheep manure biochar (SMB500), rabbit faeces biochar (RFB500) and pig manure biochar (PMB500) prepared by controlled thermal decomposition at 500 °C were used to adsorb methylene blue (MB) in water. Elemental analysis, BET and SEM results showed that the specific surface area, total pore volume and average pore diameter of SMB500 were 7.59, 4.20 and 1.16 times greater than those of RFB500, which were also 12.02, 6.88 and 1.37 times greater than those of PMB500, respectively. SMB500 had stronger stability and aromaticity. When the initial concentration of MB was 50 mg L−1 and pH was 11, adsorption achieved equilibrium at approximately 210 min. The adsorption followed pseudo-second-order kinetics (R2 > 0.96), indicating that liquid film diffusion, surface adsorption and intraparticle diffusion all contributed to the adsorption rate. The results of isothermal adsorption showed that the adsorption performance of SMB500 was more consistent with a Freundlich model, whereas the performance of RFB500 and PMB500 was more consistent with a Langmuir model with a maximum adsorption capacity of 53.68 to 238.31 mg g−1. Thermodynamic and FTIR studies showed that the adsorption of MB on SMB500, RFB500 and PMB500 was spontaneous and endothermic, and hydrogen bonds and π-π bonds were closely related to the adsorption process. The results of regeneration show that the optimal number of cycles for SMB500, RFB500 and PMB500 are 8, 5 and 3, respectively.