Selective adsorption of organic pigments on inorganically modified mesoporous biochar and its mechanism based on molecular structure.

The treatment of organic pigments has gained significant attention worldwide owing to the large amounts of pollutants emitted during the process. The main purpose of this study is to prepare an environmentally friendly, low-cost adsorbent with high efficiency with selective adsorption for water purification. An inorganically modified mesoporous biochar derived from sorghum straw was synthesized in one step, and three typical organic pigments including methyl blue (MB), acid orange 7 (AO7), and alizarin red (AR) were selected as cationic, azo, and anionic pigments, respectively. The characterization results demonstrated that Fe3O4 particles successfully attached to the surface of biochar after modification. Although the enhanced adsorption behaviors of the three pigments on the modified biochar were described effectively by the Langmuir isotherm, the as-prepared materials showed a better selective adsorption effect on the cationic pigments. Moreover, the adsorption processes of all targeted pigments were endothermic and governed by entropy. Smaller molecular dimensions, lighter molecular weight, and the low molecular electrostatic potential of MB are responsible for its selective adsorption; however, the enhanced adsorption affinity is attributed to the hydrogen bond and n-π interaction with the benzene ring of the pigment molecules.