Facile One-Step Pyrolysis of ZnO/Biochar Nanocomposite for Highly Efficient Removal of Methylene Blue Dye from Aqueous Solution

Abstract

In this work, we developed a facile one-step pyrolysis method for preparing porous ZnO/biochar nanocomposites (ZBCs) with a large surface area to enhance the removal efficiency of dye from aqueous solution. Peanut shells were pyrolyzed under oxygen-limited conditions with a molten salt ZnCl₂, which played the roles of the activating agent and precursor for the formation of nanoparticles. The effects of the mass ratio between the molten salt ZnCl₂ and peanut shells as well as pyrolysis temperature on the formation of ZBCs were investigated. Characterization results revealed that the as-synthesized ZBCs exhibited a highly porous structure with a specific surface area of 832.12 m²/g, suggesting a good adsorbent for efficient removal of methylene blue (MB). The maximum adsorption capacity of ZBCs on MB was 826.44 mg/g, which surpassed recently reported adsorbents. The formation mechanism of ZnO nanoparticles on the biochar surface was due to ZnCl₂ vaporization and reaction with water molecules extracted from the lignocellulosic structures. This study provides a basis for developing a simple and large-scale synthesis method for wastewater with a high adsorption capacity.

Figure 1

XRD patterns of samples obtained at different (a) mass ratios of ZnCl₂ and peanut shells; and (b) pyrolysis temperatures. The crystal structure of graphitic carbon is labeled GC.

Figure 2

Raman spectra of samples obtained at different (a) mass ratios of ZnCl₂ and peanut shells and (b) pyrolysis temperatures.

Figure 3

FTIR spectra of samples obtained at different (a) mass ratios of ZnCl₂ and peanut shells and (b) pyrolysis temperatures.

Figure 4

FE-SEM images of BC, ZBC-2, ZBC-3, ZBC-600, and ZBC-700.

Figure 5

(a) N₂ isotherms at −196 °C for the ZBCs, and biochar; (b) magnified view of biochar.

Figure 6

Pore size distribution curves of the biochar, and ZBCs.

Scheme 1. Illustration of the Formation of ZnO/Biochar Nanocomposites

Figure 7

Removal efficiency and adsorption capacity of (a) samples synthesized at different (a) mass ratios of ZnCl₂ and peanut shells and (b) pyrolysis temperatures.

Figure 8

(a) Effect of pH on MB adsorption at an initial concentration of 200 mg/L, an adsorbent dose of 25 mg ZBC-3/25 mL solution, a contact time of 120 min, a temperature of 30 °C, and (b) pH_PZC of ZBC-3.

Figure 9

(a) Effect of initial concentrations on MB adsorption by ZnO/biochar; (b) experimental data fitted with the Langmuir model.

Figure 10

(a) Effect of contact time on MB adsorption onto ZnO/biochar at initial MB concentration of 200 mg/L; (b) the fitted results according to the pseudo-second order kinetics.

Figure 11

(a,b; c,d) TEM images of the ZnO/biochar before and after MB adsorption, respectively; (e) FTIR spectra; and (f) proposed mechanism for MB adsorption on ZnO/biochar.