Gadolinium doping-induced electronic structure optimization of MIL-101-NH2: Efficient adsorption of arsenic (V) and phosphorus and electrochemical regeneration

Abstract

The high concentration of acids/bases used to regenerate MOF adsorbents can compromise their structural integrity and may generate additional environmental pollution. The regeneration challenges of MOF adsorbents limit their use in environmental applications. Here, the electronic structure of the iron-based metal–organic framework MIL-101-NH2 was optimized by gadolinium (Gd) doping to provide oxygen vacancies and enhance the electrochemical activity for the selective removal of arsenic(V) and phosphorus from water. 0.75GF-MILN, which was prepared using a gadolinium to iron molar ratio of 0.75, showed a maximum arsenic(V) adsorption capacity of 220.7 mg g−1 and maximum phosphorus adsorption capacity of 112.8 mg P g−1. Increasing the temperature was conducive to the adsorption of arsenic(V) and phosphorus by 0.75GF-MILN, which also maintained a high uptake capacity for arsenic(V) and phosphorus over the pH range of 4.0–9.0. The electro-assisted desorption of arsenic(V) and phosphorus from 0.75GF-MILN was achieved at −3 V. Moreover, 0.75GF-MILN still maintained 99 % arsenic(V) and phosphorus removal efficiencies from real water samples even after four cycles of electro-assisted adsorption–desorption. Density functional theory calculations and electrochemical tests showed that gadolinium doping optimizes the electron structure of MIL-101-NH2, leading to improved electron mobility. This is beneficial for electrochemical elution. Unlike other rare earth-doped MOF materials, arsenic can be desorbed from 0.75GF-MILN using a capacitive deionization process, avoiding the destruction of the MOF via exposure to a highly concentrated acid-base desorption solution. This study provides a design strategy for constructing adsorbents suitable for electrochemical elution to co-remove arsenic(V) and phosphorus, demonstrating the practicality of using rare earth-doped MOF materials in the field of water treatment.