Iron-aluminum and aluminum-single impregnated biochar composite for nitrate adsorption in rare earth wastewater: behavior and mechanism

The rare earth wastewater (REW) contains nitrate-nitrogen (NO 3 -N) and a large amount of metal ions. The commercial coconut shell biochar (CSHB), corn straw biochar (CSTB), and sawdust biochar (SDB) composites were fabricated by impregnating FeCl 3 solution, AlCl 3 solution, and FeCl 3 and AlCl 3 mixed solution, respectively. These composites were used as sorbents to remove NO 3 -N from pretreated REW (PREW). The influence of adsorbent dosage, pH, contact time, and temperature on the NO 3 -N removal was examined. Results revealed that Fe–Al impregnated CSHB (Fe–Al/CSHB) and Al-single impregnated CSTB (Al/CSTB) had good performance of NO 3 -N at pH 1, with equilibrium adsorption capacity of 1.32 and 0.86 mg g −1 at dosage of 5.3 and 8.0 g L −1 , respectively. The adsorption capacity of Fe-Al/CSHB decreased and that of Al/CSTB increased with increasing of temperature. Characterization analysis of the composites indicated that the preparation process induced a layer of FeOOH, Fe 2 O 3 , Fe 2+ , and Al 2 O 3 mixture on the surface of Fe–Al/CSHB, and Al element existed on the surface of Al/CSTB in the form of Al 2 O 3 . The pseudo-second-order model depicted NO 3 -N adsorption kinetics of Fe-Al/CSHB and Al/CSTB well. The electrostatic attraction may be the primary mechanism of NO 3 -N adsorption by Fe-Al/CSHB on account of the preferential complexation of Ca 2+ with –OH (or Fe–O) functional group. Al/CSTB removed NO 3 -N by ligand exchange.