Enhanced adsorption of Eu(III) from wastewater using Solidago canadensis-derived biochar functionalized by Ca/Al-LDH and hydroxyapatite

Highlights: • The invasive plant (Solidago canadensis) was used to prepare biochar. • CaAl hydrotalcite or hydroxyapatite was used to decorate the biochar. • Both CaAl hydrotalcite or hydroxyapatite enhanced Eu(III) adsorption on biochar. • The adsorption mechanism involved complexation, ion exchange, and precipitation. In this paper, CaAl hydrotalcite or hydroxyapatite decorated invasive plant (Solidago canadensis)-derived biochar were synthesized by co-precipitation and high temperature pyrolysis method. The three composites and bare biochar were characterized by Scanning electron microscopy (SEM), X-ray spectroscopy (EDX), the Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) techniques, and employed to remove Eu(III) from wastewater under a series of environmental conditions. The results indicated that the Eu(III) adsorption on BC@LDH@HAP was higher than that on BC, BC@LDH, and BC@HAP, and belonged to multi-layer adsorption, which was different from the monolayer adsorption of biochar. The adsorption could be fitted best by Langmuir model and the maximum adsorption capacity of Eu(III) on BC@LDH@HAP was up to 714 mg/g at pH ~ 6. The characterization results suggested that this excellent adsorption of BC@LDH@HAP arose from the abundant P-, C-, O-containing functional groups rather than larger specific surface area. The spectra of XPS, FT-IR, and XRD uncovered that the adsorption mechanisms of Eu(III) on BC@LDH@HAP involved surface complexation, ion exchange, and precipitation. Moreover, the absorbed Eu(III) was formed into two precipitate induced by P-containing groups and carbonate groups. This research confirmed BC@LDH@HAP is a promising material for the removal of Eu(III) in wastewater.