Evaluation of nitrate and phosphate adsorption on Al-modified biochar: Influence of Al content

Biochars with different Al contents (i.e., 5, 10, 15, and 20 wt%) were prepared to evaluate their adsorption capacities for nitrate (NO₃^-) and phosphate (PO₄^3-) from eutrophic water. Several techniques, including N₂ adsorption-desorption, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectrometry, were applied to characterize the physical-chemical properties of the biochars. We found that the NO₃^- and PO₄^3- adsorptions significantly improved on the Al-modified biochars because of their multifunctional and surface charge properties. In single-solute adsorption, 15 Al/BC and 20 Al/BC exhibited optimal NO₃^- and PO₄^3- adsorption capacities, respectively. In bi-solute coadsorption, the PO₄^3- adsorption on the biochar was less affected with the coexistence of NO₃^-, whereas the coexistence of PO₄^3- had a significant impact on the NO₃^- adsorption. The optimal solution pH for NO₃^- adsorption was 6, and pH < 6 was advantageous to PO₄^3- adsorption. In the kinetic study, the pseudo-second-order model could describe the NO₃^- and PO₄^3- adsorptions on biochar well, indicating that chemical adsorption was the main adsorption mechanism. The Langmuir-Freundlich model agreed well with the NO₃^- and PO₄^3- adsorptions on the biochars, and the maximum adsorption capacities for NO₃^- and PO₄^3- reached 89.58 mg/g and 57.49 mg/g, respectively. Therefore, the Al-modified biochar was a good choice for the remediation of eutrophic water.