Structure-tunable trivalent Fe-Al-based bimetallic organic frameworks for arsenic removal from contaminated water

Abstract

Arsenic pollution of groundwater and surface water has come to the attention of the public. To explore economically affordable and efficient adsorbents for the treatment of arsenic, bimetallic MOFs based on trivalent iron and aluminum were synthesized and used to capture arsenic from wastewater. Characterization methods including SEM, EDX, XRD, and N2 adsorption–desorption were applied to analyze the morphology and element content, crystal structure, porosity and specific surface area of the Fe/Al-BDC-NH2 materials. The microstructures of bimetallic MOFs were tuned through changing the ratio of Al and Fe in the precursor material. Adsorption results show that introducing aluminum into the iron-based material formed bimetallic Fe/Al-BDC-NH2 MOFs that had much better adsorption performance for arsenic than the monometallic Fe-BDC-NH2 or Al-BDC-NH2 MOF. The adsorption isotherm of bimetallic MOF materials for arsenic is in accordance with the Langmuir isotherm model, and the maximum adsorption capability of Fe/Al-BDC-NH2 (1:1) at 298 K was 146.8 mg g−1. The kinetics of arsenic adsorption follow the pseudo-second-order model. Also, the thermodynamics indicate that arsenic adsorption is endothermic. Fe/Al-BDC-NH2 (1:1) is suitable for removal of arsenic from wastewater in a relatively wide pH range (5.0–9.0). Within this pH range, Fe and Al are hardly dissolved in water after arsenic capture, and this indicates that Fe/Al-BDC-NH2 (1:1) is very stable and suitable for arsenic capture from water without generating secondary pollution. Furthermore, through FTIR and XPS analysis, ligand exchange was identified as the main adsorption mechanism of Fe/Al-BDC-NH2 (1:1) for arsenic.