Experimental and modeling investigations of ball-milled biochar for the removal of aqueous methylene blue

Abstract

Novel adsorbents were produced through the grinding of biochars of different feedstock and pyrolysis temperature in a planetary ball mill. The removal effectiveness and mechanisms of aqueous methylene blue by these adsorbents were examined. Ball milled sugarcane bagasse biomass pyrolyzed at 450 °C (BMBG450) had the highest methylene blue sorption capacity thus was chosen for further study. Compared to unmilled bagasse biochar (BG450), BMBG450 had greater specific surface area, larger pore volume, smaller hydrodynamic radius, stronger negative zeta potential (about 1.6-fold increase), and more oxygen-containing functional groups (1.05 mmol/g increase). These characteristics resulted in much greater methylene blue removal capacity (354 mg/g Langmuir maximum adsorption capacity vs. 17.2 mg/g of original BG450). The increased in BM–biochar’s external and internal surface areas exposed its graphitic structure and oxygen-containing functional groups, thus enhancing methylene blue adsorption via π–π interaction and electrostatic attraction. Experimental and modeling results suggest external mass transfer as the rate-limiting adsorption step for BG450 and surface diffusion as the rate-limiting adsorption step for BMBG450. BM–biochars had a lower equilibrium dosage and shorter reactor detention time when applied in a completely mixed flow reactor (CMFR).