Kinetics, equilibrium and thermodynamics studies on biosorption of Rhodamine B from aqueous solution by earthworm manure derived biochar

Earthworm manure (EM) largely produced as vermicomposting is being widely disposed as organic solid waste. In this study, EM was pyrolyzed at 400–600 °C to prepare a series of biochar samples (EMCs). The physicochemical properties and adsorption capability to Rhodamine B (RB) using the EMCs as well as the underlying adsorption mechanism were thoroughly investigated. N₂ adsorption/desorption isotherms and scanning electron microscopy (SEM) analysis indicated that the specific surface area and porosity structure of the EMCs were significantly promoted compared with the raw EM. Also, abundant oxygen-containing surface functional groups like hydroxyl (-OH) and carboxyl (-COOH) were retained and aromatic C=C groups were largely generated in the EMCs after pyrolysis process, as evidenced by Fourier transform infrared spectroscopy (FTIR). The adsorption results showed that the EMCs were effective for RB adsorption. The equilibrium adsorption data could be well fitted by Langmuir isotherm model with a maximum adsorption capacity of 14.49–21.60 mg g⁻¹, whereas the adsorption kinetics followed the pseudo-second-order model. Thermodynamic analysis revealed that the adsorption was assigned to a spontaneous and endothermic process. Ion exchange, hydrogen bonding, electrostatic interaction and π-π stacking interaction driven by oxygen-containing surface functional groups and aromatic C=C of EMCs were found to be responsible for RB adsorption.