Insights into the pH-Dependent Adsorption Behavior of Ionic Dyes on Phosphoric Acid-Activated Biochar

Activated biochar is a promising porous carbonaceous adsorbent material for organic pollutant removal, but it remains challenging to obtain high porosity and aromaticity through a simple and low-cost synthetic method. The common adsorption mechanisms of organic dyes on activated biochar should be further investigated in order to guide the synthesis of high-efficiency adsorbent materials. Here, we proposed a high-yield (up to 40 wt %) synthetic method of phosphoric acid-activated biochar from pomelo peel (PPC) with a high specific area of 877.3 m²/g through a facile thermal treatment at a relatively low temperature (250 °C). The specific activation mechanism of H₃PO₄ in the preparation of the adsorbent was investigated by a range of experiments and characterizations. The kinetic and isotherm experiments are also conducted to evaluate its dye adsorption behavior. According to the adsorption experiment results, PPC exhibits high saturated adsorption capacities for methyl orange (MO, 239.1 mg/g), rhodamine B (RhB, 2821.8 mg/g), methylene blue (MB, 580.5 mg/g), and crystal violate (CV, 396.6 mg/g) according to the Langmuir model. The maximum initial concentration of each dye solution for acquiring 90% removal efficiency is estimated to be 234.55 ppm (MO), 2943.8 ppm (RhB), 633.8 ppm (MB), and 423.6 ppm (CV) at 298 K with an adsorbent dosage of 1 g/L. The characterization results also indicate PPC has a complex synergetic mechanism for ionic dye adsorption behavior. This provides perspectives regarding PPC as a promising biochar adsorbent from biomass waste, which is probably useful for high-efficiency dye removal in water treatment.