Preparation and characterisation of chitosan-Hura crepitans pod biochar and evaluation of its fuel properties and adsorption capacity for methylene blue remediation

Towards the beneficiation of agricultural waste for water treatment and energy, Hura crepitans pods (HC), pyrolysed at 500, 600 and 700 °C with holding times of 30 and 60 min, were modified with chitosan and used for the adsorption of methylene blue. The biochar was characterised using pH of point of zero charge (pHpzc), elemental analysis, BET, FTIR spectroscopy, XRD and SEM–EDX. The pHpzc of HC was 9.2 and elemental analysis showed that the % carbon of the biochar was higher than that of the feedstock. The higher heating value of HC biochar was greater than 20 MJ kg⁻¹; projecting it as alternative solid-fuel source. The biochar appeared in the region for anthracite on van Krevelen diagram. Modification of the biochar with chitosan decreased the BET surface area. The FTIR spectra showed distinctive functional groups responsible for adsorption, XRD depicted the amorphous nature of the biochar. The adsorption capacity of HC biochar increased as the pyrolytic temperature and holding time increased from 500 to 700 °C and 30 to 60 min, respectively. Langmuir isotherm and pseudo-second order kinetic models gave the best fit to the experimental data. The monolayer adsorption capacity of HC7B biochar was 48.78 mg g⁻¹. Thermodynamic parameters showed that the adsorption process was endothermic, disorderly and spontaneous. The mechanism of adsorption was mainly by non-electrostatic interaction such as π-π interaction. The highest percentage desorption was achieved with 0.1 M H₂SO₄ and HC biochar was successfully regenerated after 5 cycles. Hura crepitans pod biochar was effective in the adsorption of MB from aqueous solution and modification with chitosan improved its adsorption capacity.