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 PMB_500, 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.