Box–Behnken design to optimize Ni(II) adsorption using coffee husk-derived biochar compositing with MnFe 2 O 4

The main objective of this research was to optimize Ni(II) adsorption using coffee husk-derived biochar composited with MnFe 2 O 4 nanoparticles (MFO@BC). The MnFe 2 O 4 nanoparticles (MnFe 2 O 4 –NPs) were synthesized by the co-precipitate method. Then, the MFO@BC nanocomposite was formed simultaneously through co-precipitation and hydrothermal processes. Box–Behnken experimental design in response surface methodology (Design Expert 11, Stat-Ease, USA) was used to carry the Ni(II) adsorption optimization onto MFO@BC. The effects of initial Ni(II) concentration, solution pH, contact time, and adsorption material mass were chosen as independent variables for Ni(II) adsorption processes on MFO@BC. The results indicated that the loaded mass ratio of 2.5 MnFe 2 O 4 –NPs (w/w) on the coffee husk-derived biochar was the most suitable for Ni(II) adsorption onto MFO@BC. The result of ANOVA analysis, where p values of 0.0005 were significant for the quadratic polynomial model. In addition, the proposed model was fitted with the actual values with the high R 2 and R 2 –adjusted (R 2 adj ) values of 0.8867 and 0.7647, respectively. Three factors, including initial Ni(II) concentration, solution pH and adsorbent mass, had the main effect on Ni(II) adsorption capacity by MFO@BC. Optimum conditions for Ni(II) adsorption onto MFO@BC were obtained at solution pH 7, 50 min of contact time, initial Ni(II) concentration of 4.0 mg/L and 0.025 g adsorbent/25 mL. Under these conditions, the maximum adsorption capacity of Ni(II) onto MFO@BC reached 5.51 mg/g. Five cycles of adsorption–desorption did not substantially decrease the adsorption capacities of MFO@BC.