Mussel-inspired functionalization of biological calcium carbonate for improving Eu(III) adsorption and the related mechanisms

The high value rare earth elements (REEs) recycling from wastewater remains a great challenge. In the present work, biological calcium carbonate (bio-CaCO3) was functionalized by mussel-inspired polydopamine (PDA) chemistry to improve its Eu(III) adsorption in aqueous solutions. The functional groups of the organic matters in bio-CaCO3 that served as both reductant and coupling agent were found to be able to effectively inhibit the auto-oxidation of PDA in alkaline environments, rendering a uniform and tight PDA coating on bio-CaCO3. The batch experiments indicated that the mussel-inspired PDA functionalized bio-CaCO3 (bio-PDA) exhibited superior Eu (III) adsorption capacity in aqueous solutions. The adsorption isotherm was fitted well with the Langmuir model, and the maximum adsorption capacity was determined to be 151.52 mg/g. The adsorption kinetics followed pseudo-second-order model, suggesting that the adsorption was chemisorption. Based on the scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and X-ray photoelectron spectroscopic (XPS) analyses, the possible Eu(III) adsorption mechanism of bio-PDA was proposed as the coordination of Eu(III) with the abundant catechol and amine/imine groups on the PDA coating. Our work has demonstrated great application potentials of bio-PDA as an efficient Eu(III) adsorbent and provided new insights into the interaction mechanism between PDA and Eu(III).