Performance of CO₂ and Fe-modified lignin char on arsenic (V) removal from water

Biochar was produced by the pyrolysis of Kraft lignin at 600 °C followed by modification with CO₂ at 700 and 800 °C and impregnation with FeOx. The physicochemical properties and arsenic (V) adsorption performance of biochar were evaluated. The characteristics of the lignin biochar before and after CO₂ modification and FeOx impregnation were analyzed using the following methods: proximate and ultimate analysis, specific surface area (Brunauer–Emmett–Teller (BET) surface area), porosity, scanning electron microscopy and energy dispersive spectroscopy mapping, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The specific surface area and porosity of biochar were improved significantly after CO₂ modification. However, impregnation of FeOx in CO₂-modified biochar showed a 50%–60% decrease of BET surface area and porosity due to pore blocking of FeOx. The batch adsorption of arsenic (V) showed that FeOx-LC-800 (FeOx impregnation lignin char modified with CO₂ at 800 °C) had the highest adsorption efficiency among the biochars tested because of its highest Fe–O intensity and large surface area. The Langmuir adsorption model was suitable for the curve fitting arsenic (V) adsorption. The theoretical equilibrium adsorption amount (q_e) was calculated to be 6.8 mg/g using a pseudo-second-order kinetic model.