Magnetic rice husk ash 'cleanser' as efficient methylene blue adsorbent

Burning of agricultural residues has been a prevalent cause of environmental concern especially in developing countries. In the Philippines, rice husks are usually burned in open air and either usually left in the field or rarely collected to be sold in the wet market as a cleanser. Hence in this study, an alternative use of this burning residue (rice husks ash, RHA) for removal of dyes from aqueous solutions was explored. Yet, its applications in real wastewater will be difficult in its pristine powdered form. Therefore, the RHA was functionalized with Fe<sub>3</sub>O<sub>4</sub> to become inherently magnetic (MRHA), providing relative ease in handling. The RHA used was obtained from the local markets of Davao City, Philippines. Systematic adsorption studies on the effects of contact time, solution pH, solid/liquid ratio, and feed concentration were conducted. MB adsorption was Langmuir-type and followed a pseudo-second-order rate model. Its adsorption rate is significantly fast (<i>t</i> = 10 min, <i>k</i><sub>2</sub> = 0.015 g mg<sup>-1</sup> min<sup>-1</sup>) with high <i>q<sub>m</sub></i> (150.5 mg g<sup>-1</sup>). MRHA was highly stable and reusable (> 10 cycles) demonstrating its high potential as an economical alternative for treatment of dye contaminated wastewater.