Rapid removal of salicylic acid, 4-nitroaniline, benzoic acid and phthalic acid from wastewater using magnetized fast pyrolysis biochar from waste Douglas fir

Biochar was produced from the fast pyrolysis of Douglas fir (DFBC). Magnetic biochar (MDFBC) was prepared by magnetite (Fe3O4) precipitation onto the biochar's surface from an aqueous Fe3+/Fe2+ solution upon NaOH treatment. The resulting MDFBC was used to remove 4-nitroaniline (4NA), salicylic acid (SA), benzoic acid (BA) and phthalic acid (PA) from water. The surface chemistry and composition of the MDFBC were examined by SEM, SEM-EDX, TEM, PZC, XPS, XRD, elemental analysis, and surface area measurements. Batch sorption studies were carried out from pH 2 to 10 and adsorbate concentrations from 25 to 500 mg/L at 15, 25, 35 and 45 °C. MDFBC suspensions in the contaminated solutions were vortexed for two min and then magnetically removed. Remediated solutions were then analyzed using UV-Visible spectroscopy. The amounts of 4NA, SA, BA and PA adsorbed onto MDFBC was higher at low pH values and decreased with increasing pH. MDFBC sorption at 15, 25, 35 and 45 °C was evaluated using the Langmuir, Freundlich, Sips, Redlich-Peterson, and Toth adsorption isotherm models. Langmuir adsorption capacities at pH 5 and 45 °C for 4NA, SA, BA and PA were 114, 109, 90 and 86 mg/g, respectively. Thrice recycled MDFBC using extraction with methanol (5 × 10 mL) and water (10 mL) had 90% of the original adsorption capacities for 4NA, SA, BA and PA, respectively. The adsorption kinetics of MDFBC and DFBC were far faster than four other biochars (mixed feed, magnetized mixed feed, pinewood and magnetized switchgrass) and also faster than a commercial activated carbon. Equilibrium for MDFBC and its DFBC precursor was reached within 2 min, while the other biochars took between 8 and 20 h. The fast adsorption kinetics and high adsorption capacities of MDFBC could be advantageously employed in filtration devices, columns, or as shown here, in batch operations with stirring to speed adsorption, followed by magnetic separation of the adsorbent for regeneration.