Microwave Synthesis of Hierarchical BiOCl Microspheres as a Green Adsorbent for the pH-Dependent Adsorption of Methylene Blue

Flower-like BiOCl microspheres are synthesized by a microwave route, and their pH-dependent adsorption and recovery of methylene blue (MB) are studied. The as-synthesized BiOCl exhibits hierarchical structures with dominant {001} facets, a specific surface area of 36.39 m² g⁻¹, and mesoporous approximately 18 nm in diameter. Compared with BiOCl nanoplates, the adsorption of MB (20 mg L⁻¹, pH = 6.8) over the BiOCl microspheres increases from 21% to 98.2% after only 2 min using an adsorbent dosage of 1 g L⁻¹. The excellent adsorption capacity, i.e., a maximum capacity of 24.2 mg g⁻¹ and 26.2 mg g⁻¹ at pH = 6.8 and 3.5, respectively, is attributed to the negatively charged facets, high surface area, and unique defects/vacancies and micro/nanostructures of as-synthesized material. In addition, the adsorption of MB at pH = 11.9 is only approximately 1/14 of that at pH less than 6.8, indicating the pH-dependent adsorption behavior of the BiOCl. Thus, the BiOCl microspheres are regarded as a promising green pH-dependent adsorbent, and the adsorbed MB is recovered instead of degraded. The pH-dependent adsorption behavior for MB over the BiOCl is demonstrated to be closely related with not only the unique surface of the BiOCl microspheres but also the polarity of MB in different solution pH.