Water-modification of micro- and nano-adsorbents: A strategy for enhancing biodiesel refining efficiency

In this study, we compared nanometer-scale (nm) silica (SiO2) and alumina (Al2O3) with conventional micrometer-scale (μm) adsorbents (SiO2 and Al2O3) to determine their utility in refining crude fatty acid methyl esters (FAME). Unlike their μm counterparts, hydrated SiO2 and Al2O3 nm-adsorbents did not retain water after utilization in FAME refining. However, hydrated adsorbents significantly improved performance of μm-scale SiO2 and Al2O3 adsorbents for FAME refining. Structurally, the μm-adsorbents are porous multi-layered structures, with FAME adhering to the surface or entrained between layers. Meanwhile, nm-adsorbents were aggregated. As such, FAME loss associated with nm-SiO2 adsorbent was 50.5 % higher than μm-SiO2 adsorbent. In addition, the electrical charge density of the adsorbents influenced their performance in FAME refining. Finally, conventional separation methods (e.g., gravitational setting) were impractical for separating spent nm-adsorbents from FAME; although, the use of an electrostatic field enhanced the removal of nm-adsorbents from solution. Altogether, this study examines the application of nm-adsorbents, compared to μm-adsorbents, and highlights potential increased efficiency in refining biodiesel, such as potentially reducing the need for extensive wet washing processes. Although new separation technologies may be required in recovering spent nm-adsorbents, their application could enhance the sustainability and cost efficiency of biodiesel production.