The enhancement of reactive red 24 adsorption from aqueous solution using agricultural waste-derived biochar modified with ZnO nanoparticles

Abstract

In this study, two types of agricultural wastes, sugarcane bagasse (SB) and cassava root husks (CRHs), were used to fabricate biochars. The pristine biochars derived from SB and CRHs (SBB and CRHB, respectively) were modified using ZnO nanoparticles to generate modified biochars (SBB-ZnO and CRHB-ZnO, respectively) for the removal of Reactive Red 24 (RR24) from stimulated wastewater. Batch experiments were performed to evaluate the effects of ZnO nanoparticles' loading ratio, solution pH, contact time, and initial RR24 concentration on the RR24 adsorption capacity of biochars. The RR24 adsorption isotherm and kinetic data on SBB, SBB-ZnO3, CRHB, and CRHB-ZnO3 were analyzed. Results indicate that SB- and CRH-derived biochars with a ZnO nanoparticle loading ratio of 3 wt% could generate maximum adsorption capacities of RR24 thanks to the double growth on the BET surface of modified biochars. The RR24 adsorption capacities of CRHB-ZnO3 and SBB-ZnO3 reached 81.04 and 105.24 mg g^-1, respectively, which were much higher than those of pristine CRHB and SBB (66.19 and 76.14, respectively) at an initial RR24 concentration of 250 mg L^-1, pH 3, and contact time of 60 min. The adsorption of RR24 onto biochars agreed well with the pseudo-first-order model and the Langmuir isotherm. The RR24 adsorption capacity on modified biochars, which were reused after five adsorption-desorption cycles showed no insignificant drop. The main adsorption mechanisms of RR24 onto biochars were controlled by electrostatic interactions between biochars' surface positively charged functional groups with azo dye anions, pore filling, hydrogen bonding formation, and π-π interaction.

Fig. 1. Effect of ZnO nanoparticle loading ratio on the RR24 adsorption capacities (q) of (a) SBB and (b) CRHB at the experimental conditions: initial RR24 concentration of 200 mg L⁻¹, adsorbent dose of 1.0 g L⁻¹, pH of 4, room temperature (25 ± 2 °C).

Fig. 2. Effect of the solution pH on the (a) RR24 adsorption onto various biochars and (b) pH_PZC value of the biochars at experimental conditions: initial RR24 concentration of 200 mg L⁻¹, adsorbent dose of 1.0 g L⁻¹, and the contact time of 60 min at room temperature (25 ± 2 °C).

Fig. 3. Effects of contact time and RR24 adsorption kinetics onto different biochars: (a) pseudo-first-order and (b) pseudo-second-order model at experimental conditions: contact time of 120 min, initial RR24 concentration of 200 mg L⁻¹, adsorbent dose of 1.0 g L⁻¹, and pH of 3.

Fig. 4. RR24 adsorption isotherms onto the biochars: (a) Langmuir model; (b) Freundlich model, and (c) Sips model.

Fig. 5. Reusability of modified biochars for the removal of RR24 through five adsorption–desorption cycles.

Fig. 6. SEM of (a) pristine SBB and (b) SBB-ZnO3 before and (c) SBB-ZnO3 after RR24 adsorption; EDX of (d) pristine SBB and (e) SBB-ZnO3 before and (f) SBB-ZnO3 after RR24 adsorption.

Fig. 7. SEM of (a) pristine CRHB and (b) CRHB-ZnO3 before and (c) CRHB-ZnO3 after RR24 adsorption; and EDX of (d) pristine CRHB and (e) CRHB-ZnO3 before and (f) CRHB-ZnO3 after RR24 adsorption.

Fig. 8. FTIR of pristine SBB and SBB-ZnO3 before and after RR24 adsorption (a) and (b) pristine CRHB and CRHB-ZnO3 before and after RR24 adsorption.

Fig. 9. XPS of (a) pristine SBB and SBB-ZnO3 and (b) pristine CRHB and CRHB-ZnO3.