Production and characterization of magnetic Biochar derived from pyrolysis of waste areca nut husk for removal of methylene blue dye from wastewater

Abstract

The textile industry causes lots of pollution due to its discharge of untreated coloured effluents into water bodies, impacting the environment. The present study includes a slow pyrolysis technique to produce magnetic biochar derived from waste areca nut husk (ANH)) biomass to adsorb methylene blue dye. The biochar and biomass were characterised via proximate analysis, ultimate analysis, bulk density, heating value, extractive content, biochemical analysis, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), SEM, BET surface area, pH, water holding capacity (WHC) and X-ray diffraction (XRD). A semi-batch reactor was used to produce biochar (ANHB) at 600 and 800 ^oC at 10 ^oC min^{- 1} heating rate and 45 min holding time in an inert atmosphere. The produced biochar was magnetised by blending aqueous biochar suspensions with aqueous Fe³⁺/Fe²⁺ solutions. Further, magnetised biochar is employed to eliminate methylene blue (MB) dyes at different pHs, contact times, temperatures, dosages and concentrations. Biochar derived at 800 ^oC (ANHB800) gave increased carbon content (62.93%), heating value (33.02 MJ/kg), and BET surface area (112 m²/g) over biochar derived at 600 ^oC. The results of the acid treatment biochar (ANHBA800) demonstrated that 5M H₂SO₄ causes a BET surface area increase (265 m²/g) and a ash content decrease (9.96%). However, when magnetic biochar was produced at 800 ^oC it shows an additional increase in BET surface area upto 385 m²/g. The MB dye absorption analysis confirmed 85.47% adsorption at 0.3 g/l dosage, 100 ppm concentration, 30 ^oC, 60 min contact time, and pH 7. The adsorption capacity was 785.34 mg/g when fit by the Langmuir isotherm model. Magnetic nanoparticles enhance active sites, electrostatic interactions, and recovery, improving efficiency, cost-effectiveness, and sustainability in dye removal. The adsorption kinetics results suggested that the pseudo-second-order model best explains the experimental data with an R² value of 0.994. Additionally, the adsorption isotherm studies were best fitted by the Langmuir model adsorption conforming monolayer adsorption of MB on biochar surface.

Keywords: Adsorption; Biomass; Characterisation; Magnetic Biochar; Pyrolysis; Wastewater.

Fig. 1

Schematic presentation of the experimental setup.

Fig. 2

General scheme for the magnetic biochar preparation.

Fig. 3

(a) TGA profile of Areca nut husk at a heating rate of 10 ℃ min⁻¹, and (b) FTIR profile of Areca nut husk (ANH).

Fig. 4

(a) TG profile of ANHB and (b) DTG profile of ANHB at 600 and 800℃.

Fig. 5

(a) FTIR analysis of ANHB at 600 and 800 °C, (b) FTIR analysis of acid-treated, magnetic biochar and biochar after adsorption, and (c) XRD analysis of raw, acid-modified and magnetic biochar at 600 and 800 °C.

Fig. 6

SEM analysis of ANHB (a) 600 ℃ (b) 800 ℃ (c) acid modified 600 ℃ (d) acid modified 800 ℃, (e) Magnetic biochar at 600 °C and (f) magnetic biochar at 800 °C. (g). The magnetic moment of magnetic biochar derived at 600 and 800 °C at 30 °C.

Fig. 7

(a) Effect of biochar dosage at constant pH of 7, concentration and temp At pH 7, t = 60 min, C₀ = 100 ppm, T = 30 °C. (b) Effect of initial dye concentration at constant pH = 7, 0.3 g/L, t = 60 min and T = 30 °C. (c) Effect of temperature at pH 7, t = 60 min, C₀ = 100 ppm, T = 30 °C.

Fig. 8

(a) Effect of contact time at pH 7, t = 60 min, C₀ = 100 ppm, T = 30 °C. (b) Effect of pH on the removal of MB dyes at 100 ppm concentration, T = 30 °C and 60 min contact time.

Fig. 9

The kinetics model plots of the adsorption experiment of MB pH 7, t = 60 min, C₀ = 100 ppm, T = 30 °C.

Fig. 10

Adsorption Isotherm plot of the experiment, at pH 7, t = 60 min, C₀ = 100 ppm, T = 30 ^oC.

Fig. 11

The adsorption mechanism for methylene blue (MB) dye onto biochar.

Fig. 12

Desorption study bio-adsorbents at t = 60 min, C₀ = 100 ppm, T = 30 ^oC.