Phosphoric acid-activated bamboo hydrochar for methylene blue adsorption: isotherm and kinetic studies

Bamboo-based activated carbon was produced by using two different methods: single-stage (direct activation) and two-stage method (HTC pre-treatment and activation) by using H3PO4 as the activator. This study aims to analyse and compare the physicochemical characteristics and the adsorption performance of bamboo-based activated carbon derived from these two methods. In addition, the effect of different HTC feedwater on the hydrochar properties was also studied for the two-stage method by varying the HTC feedwater (0.5 M HCl, 0.5 M NaOH, and distilled water). The results showed that acidic feedwater could produce hydrochar with a relatively large surface area and good adsorption capacity for methylene blue. As such, the hydrochar produced using acidic (0.5 M HCl) feedwater HTC (HCA) was chosen to be activated. The performance of BAH in removing methylene blue was compared with bamboo-activated carbon (BAC) which was produced through direct activation. BAH with a surface area of 1798 m(2) g(-1) showed an outstanding maximum removal capacity of methylene blue at 558 mg g(-1), which is approximately twice the removal performance of BAC with a surface area of 1278 m(2) g(-1). The equilibrium adsorption for both adsorbents was best fitted with Langmuir isotherm, while the kinetics were best described through a pseudo-second-order model. The intraparticle diffusion model showed that the adsorption is not solely limited to it, and other mechanisms might influence the adsorption behaviour. The multilinearity of the intraparticle diffusion model is explained by the boundary layer diffusion that usually gives the first portion of the plot, while the remaining linear portion can be described through intraparticle diffusion. In conclusion, HTC-pretreated activated bamboo is an effective adsorbent for methylene blue removal in an aqueous solution, and the finding of this study can help in devising a suitable activation strategy for biomass-based activated carbon.