Co-production of phenolic-rich bio-oil and magnetic biochar for phosphate removal via bauxite-residue-catalysed microwave pyrolysis of switchgrass

Bauxite residue (BR) was tested as a low-cost microwave absorber and catalyst for microwave-assisted pyrolysis for producing phenolic-rich bio-oil, along with magnetic biochar that can be used as an adsorbent for capturing excess nutrients, such as phosphates, from eutrophic water or wastewater. Mixing BR alone with switchgrass (SG) did not enhance SG microwave absorption, while mixing 10 wt% clinoptilolite and 20 wt% BR (10Clino–20BR) with SG increased the heating rate by 382% in comparison with 10Clino alone, due to the formation of good microwave absorbers (i.e. maghemite and magnetite) on the biochar surfaces, confirming their synergistic effects. The addition of BR reduced the acidity of the bio-oil by up to 47%, and increased the production of aromatics and alkylated phenols by up to 180% and 319%, respectively, compared with 10Clino alone. Biochar produced from 10Clino–20BR had magnetic properties, which are very important for facilitating the separation of spent biochar from P-contaminated aqueous solutions in industrial applications. Using the magnetic biochar for phosphorus adsorption, the highest adsorption capacity of 64.4 mg PO43−/g (21 mg PO43--P/g) was measured for the biochar produced using 10Clino–20BR at pH 4, and the adsorption isotherm was fitted by the Freundlich model very well. The magnetic biochar showed a high adsorption capacity for phosphate removal. In addition, the P-loaded biochar could be marketed as a balanced fertilizer to increase soil fertility, converting BR into a valuable product, instead of a waste material.