Ag(I) removal and recovery from wastewater adopting NH2-MIL-125 as efficient adsorbent: A 3Rs (reduce, recycle and reuse) approach and practice

In this work, NH2-MIL-125(Ti) were adopted to eliminate Ag(I) from the simulated silver-plating wastewater. The results revealed that NH2-MIL-125 exhibited superior adsorption performance toward Ag(I) ions with uptake capacity of 192.5 mg·g−1 to that of MIL-125 (139.8 mg·g−1) within 60 min. The isotherm and kinetic data were exactly fitted to both Langmuir and Pseudo-second-order models. The thermodynamic parameters like enthalpy change (ΔHo), entropy change (ΔSo) and Gibbs free energy change (ΔGo) confirmed that the adsorption process was spontaneous, exothermic and disordered. As well, the influencing parameters of the adsorption process like pH, adsorbent dose and foreign metal ions were examined. The fixed-bed column filled with NH2-MIL-125 powder immobilized onto cotton fiber could continuously adsorb Ag(I), which offered the possibility of achieving potential large-scale applications. The possible adsorption mechanism of NH2-MIL-125 toward Ag(I) primarily involved the electrostatic adsorption and coordinative interactions, which was further affirmed by the density functional theory (DFT) calculations. In addition, the used NH2-MIL-125 saturated with Ag(I) ions could be either desorbed to release the Ag(I) for NH2-MIL-125 re-generation or further calcinated into Ag/C/TiO2 photocatalyst to accomplish photocatalytic degradation toward organic pollutants like methylene blue (MB) and phenol. In this work, the 3Rs (reduce, recycle and reuse) approach was practiced, accomplishing that one stone killed three birds (pollutant elimination, resource recovery and resource utilization).