Adsorption mechanisms and optimal production of magnetic biochar composites from red mud and soft wood biomass

We investigated the adsorption capacity of four composites generated by co-pyrolyzing red mud, an iron rich waste product from the production of alumina, together with Canadian soft wood biomasses, including Douglas fir, pine wood, red cedar and hemlock. Pyrolysis conditions included manually mixing of both biomass and red mud together before pyrolyzing at 800 °C for 3 h, with a ramp rate of 10 °C/min and a flow rate of 2 mL/min of CO2. Differences in the surface functional groups and aromaticity were observed in the biochars produced from these biomasses. It was found that oxygen containing functional groups had a greater impact on π-π electron-donor acceptor interactions, whereas greater amounts of carbonyl groups increased the adsorption of compounds with electron donating groups over compounds with electron withdrawing groups. Furthermore, optimal ratios of biomass to red mud were investigated for co-pyrolysis to achieve both strong adsorptive and magnetic properties. A greater biomass content during co-pyrolysis showed greater adsorptive properties, while only a small amount of red mud, as low as 10 wt%, was sufficient to provide the desired magnetic properties. This work was able to successfully produce an adsorbent capable of adsorbing up to 32.8 mg/g of acetaminophen by co-pyrolyzing red mud and pine wood biomass at a 9:1 ratio. The resulting composite also maintained magnetic properties, allowing the adsorbent to be easily removed from water after adsorption.