Chromium removal using magnetic biochar derived from herb-residue

Magnetic biochar prepared with Astragalus membranaceus residue according to the Fe2+/Fe3+ co-precipitation method was used as an adsorbent for Cr(VI) in aqueous solutions during batch experiments. Scanning electron microscopy and energy dispersive spectroscopy results proved that the adsorbent contained substantial amounts of iron oxide and confirmed that magnetic biochar was prepared. The surface area decreased from 111.48 to 59.34 m2/g after the adsorbent became magnetic. X-ray diffraction analysis and X-ray photoelectron spectroscopy studies further demonstrated the existence of iron oxide in the magnetic biochar and showed that oxygen-containing groups decreased after adsorption. The maximum Cr(VI) adsorption occurred at pH 2 (23.85 ± 0.23 mg/g). The adsorption data were described well by a pseudo-second-order model and the Langmuir isotherm model (R2 = 0.994 and R2 = 0.993, respectively). The intraparticle diffusion model results indicated that intraparticle diffusion is not the only rate-limiting step. Together, these results proved that magnetic biochar could be separated easily from water with external magnetic fields and that such a material could be used as a cost-effective adsorbent in heavy metal removal applications.