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 (PO43−) 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 PO43− 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 PO43− adsorption capacities, respectively. In bi-solute coadsorption, the PO43− adsorption on the biochar was less affected with the coexistence of NO3−, whereas the coexistence of PO43− had a significant impact on the NO3− adsorption. The optimal solution pH for NO3− adsorption was 6, and pH < 6 was advantageous to PO43− adsorption. In the kinetic study, the pseudo-second-order model could describe the NO3− and PO43− adsorptions on biochar well, indicating that chemical adsorption was the main adsorption mechanism. The Langmuir–Freundlich model agreed well with the NO3− and PO43− adsorptions on the biochars, and the maximum adsorption capacities for NO3− and PO43− 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.