This work investigates the production of biochars from phosphoric acid-treated spent-tea residue (STR) by pyrolysis at different temperatures. The biochar morphology and chemistry were characterized via SEM, FT-IR, XRD and BET analyses. Results showed that an increase in the pyrolysis temperature could enhance the porous-structure and increase the surface area, whereas it might deteriorate the surface-functional groups to some extent. The produced biochar samples were employed for the adsorption of methylene blue (MB) from water. The batch adsorption process was investigated via a data-based multivariate optimization approach, Pareto sensitivity analysis and response surface methodology (RSM) for the intensification of the process. The STR biochar produced at the pyrolysis temperature of 400 ◦C offered the maximum MB removal of 99.26% at the optimal operating conditions of pH = 9.65, initial dye concentration = 81.24 mg/L, adsorbent dosage = 0.26 g and contact time = 97.92 min. Pareto sensitivity analysis revealed that the MB adsorption on STR-derived biochar is strongly influenced by pH and contact time. The isothermal and kinetic studies indicated that the adsorption of MB on STR biochar is well described by the Langmuir isotherm and pseudo-second order model, respectively. The adsorption capacity of the optimal biochar was obtained to be 91.13 mg/g.
