Comparative study of calcium alginate, ball-milled biochar, and their composites on aqueous methylene blue adsorption

In this work, a novel composite, ball-milled biochar (BMB) encapsulated in calcium-alginate (CA) beads (CA-BMB), was synthesized as an alternative adsorbent for the removal of methylene blue (MB) from an aqueous solution. Sorption performance was compared among CA, BMB, and CA-BMB composite with batch adsorption experiments. With 25% BMB and 75% alginate, the new composite resembled CA in MB adsorption. With an initial MB concentration of 50 mg L^-1, kinetics studies showed that 74% MB removal by CA-BMB was achieved within 8 h, followed by slow kinetics reaching 91% removal in 16 h. The adsorption kinetics was well explained by the Ritchie's kinetic model, indicative of energetically heterogeneous solid surface of the composite. Adsorption isotherms of BMB, CA, and CA-BMB can all be fitted with the Langmuir models; the adsorption capacity of CA-BMB (1210.7 mg g^-1) was close to that of CA (1282.2 mg g^-1) and much higher than that of BMB alone (184.1 mg g^-1). The outstanding adsorption performance suggested that CA-BMB can serve as a low-cost and eco-friendly adsorbent for MB removal from an aqueous solution.