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 (NO3-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 FeCl3 solution, AlCl3 solution, and FeCl3 and AlCl3 mixed solution, respectively. These composites were used as sorbents to remove NO3-N from pretreated REW (PREW). The influence of adsorbent dosage, pH, contact time, and temperature on the NO3-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 NO3-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, Fe2O3, Fe2+, and Al2O3 mixture on the surface of Fe-Al/CSHB, and Al element existed on the surface of Al/CSTB in the form of Al2O3. The pseudo-second-order model depicted NO3-N adsorption kinetics of Fe-Al/CSHB and Al/CSTB well. The electrostatic attraction may be the primary mechanism of NO3-N adsorption by Fe-Al/CSHB on account of the preferential complexation of Ca2+ with -OH (or Fe-O) functional group. Al/CSTB removed NO3-N by ligand exchange.

Automated summary

In this study, Fe-Al/CSHB and Al/CSTB composites were prepared by impregnation method as sorbents for the removal of NO3-N in rare earth wastewater. The effects of adsorbent dosage, pH, contact time, and temperature on the adsorption performance were investigated. The results showed that Fe-Al/CSHB and Al/CSTB exhibited good adsorption performance for NO3-N at pH 1. The adsorption capacity of Fe-Al/CSHB decreased with increasing temperature, while that of Al/CSTB increased. Characterization analysis revealed the formation of FeOOH, Fe2O3, Fe2+, and Al2O3 mixture on the surface of Fe-Al/CSHB, and the presence of Al2O3 on the surface of Al/CSTB.