Rice husk bio-chars as adsorbent for methylene blue and ethinylestradiol from water

Biobased adsorbents are eco-friendly materials used for water and wastewater treatment and air cleaning. This research work explores the potential use of rice husk chars as a low-cost adsorbent for pollutants from water, such as methylene blue and ethinylestradiol. Rice husk chars are prepared by combustion of rice husks (RH-Char) or pre-treated rice husks (PT-Char). A third char (M-Char) supplied by a local company which uses rice husks as combustion material is also studied. The chars are characterized by field emission scanning electron microscopy (FE-SEM) in conjunction with energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), and zeta-potential measurements. The three bio-chars are tested as adsorbent for the removal of two wastewater contaminants: methylene blue (MB) and ethinylestradiol (EE2). According to the results, RH-Char and PT-Char present zeta-potential values near −52 mV, whereas M-Char shows a zeta-potential value of −32 mV. Both RH-Char and PTChar are rich in amorphous SiO2 and M-Char has crystalline SiO2 (cristobalite). The bio-chars remove MB and EE2 efficiently showing the following results for the adsorption capacity of MB (in μmol g−1): 769.2 (RH-Char), 41.2 (PT-Char), and 31.7 (M-Char). The adsorption capacity values for EE2 (in μmol g−1) are: 33.1 (RH-Char), 19.1 (PT-Char), and 16.9 (M-Char). The information gathered in the present work evidences the potential of rice husks bio-chars for bio-remediation and may in future contribute to the conversion of a side-stream from the rice industry into value-added materials.