Rice husk silica derived MICROSCAFS® for a green solar-driven photodegradation of minocycline in aqueous media

The increasing consumption of antibiotics is causing the rise of multidrug-resistant bacterial strains. Therefore, the removal of these molecules from water is highly important and requires the development of high-performance systems, whose production from biobased resources, especially residues, would greatly enhance their sustainability value. This work focuses on the development of TiO2-containing porous silica microspheres, MICROSCAFS®, using rice husk as silica source. The MICROSCAFS® are employed in the adsorption and solar-driven photodegradation of minocycline, a ubiquitous antibiotic for various therapies. The biobased content of the MICROSCAFS® is increased by substituting tetraethyl orthosilicate, commonly employed for their preparation, with rice husk-extracted silica. The incorporation of biogenic silica alters the structure of MICROSCAFS®, that display a hollow structure with high surface porosity. Furthermore, upon high temperature treatment, a significant increase in surface area, up to ca. 92 m2∙g−1 is observed, leading to an adsorption capacity of 80.81 mg∙g−1. The adsorption mechanism is best described by the Freundlich model, implying that adsorbate multilayers are formed during the process. The presence of 3.4 nm anatase crystallites confers an efficient photocatalytic ability (k = 7.6∙10−2 mg∙L−1∙min−1) to the biobased MICROSCAFS®, yielding a complete removal of minocycline (10 mg∙L−1 in simple water matrices and adsorbent dosage of 50 mg) via a 45-minute irradiation in solar-simulated conditions (1 sun) compared to 165 min needed for fossil-based MICROSCAFS®. Zero-order kinetics best fit the process, the rate limiting step being the photodegradation reaction. Lastly, the high rate of photodegradation is maintained over 4 cycles, demonstrating the high system stability.