Ion imprinted polymer/metal-organic framework composites as efficient adsorbents for selective recognition and separation of selected platinum group metals from wastewater

Abstract

Platinum group metal (PGMs) are the most extracted non-renewable mineral resources in South African due to their high demand for various applications. During mineral processing some of these valuable metals are lost into the generated toxic which is excessively released into the environment. Adsorption technology offers advantages of ease operation, high effectiveness and opportunity to incorporate other techniques such as ion-imprinting method. Metal-organic frameworks have been explored as adsorbent due to their interesting properties such tuneable surface functionality and ability to host guest molecules. In this work, metal organic framework, MIL-101(Cr), (MIL= Matérial Institut Lavoisier) prepared from polyethylene-terephlalate waste was initially functionalised with ethylenediamine (ED) to form MIL-101(Cr)/ED for the adsorption of palladium (Pd) ions from aqueous solution. The successful incorporation of ED was confirmed by Fourier transform infrared spectroscopy with N-H vibrations at 3300 -3000 cm-1. The MIL-101(Cr)/ED achieved a maximum adsorption capacity (qm) of 454.2 mg/g for the uptake of Pd(II) ions in a pH =3.0 and 0.03 g of the adsorbent dose. MIL-101(Cr)/ED adsorbent was selective and fast towards the adsorption of Pd(II) ions. However, the regeneration and reusability of the adsorbent was still a challenge. The MIL-101(Cr)/ED was further grafting with glutaraldehyde (GA) to enhance the regeneration and reusability. This functionalisation was noted by the appearance 1554 and 1052 cm-1 for N-H bend and C-N stretch in the MIL-101(Cr)/ED-GA due to the amide bond formation. The efficiency of MIL-101(Cr)/ED-GA was tested for Pd(II) and platinum (Pt(IV)) ions. The data showed a good fit to the Langmuir isotherm and pseudo-second order (PSO) kinetic models. The estimated qm values were 414.4 for Pt(IV) and 308.3 mg/g for Pd(II) ions. The adsorption rate of the MIL-101(Cr)/ED-GA adsorbent was very rapid towards Pd(II) ions intake than Pt(IV) ions and resulted in higher selectivity of Pd(II) ions over Pt(IV) ions. Adsorption-desorption cycles for the reusability of MIL-101(Cr)/ED-GA in removing both metal ions were stable for two successive cycles and started to decrease in the third cycle. The reusability of MIL-101(Cr)/ED-GA showed a significant drop in removal efficiency (%R) towards Pd(II) ions from 84% in the first cycle to 73% in the fourth and fifth cycle. The regeneration and reusability of the MIL-101(Cr)/ED-GA was improved by ion-imprinting Pd(II) ions on a glycylglycine (glygly) functional monomer and formed an ion specific IPMIL-101(Cr) composite. The optimum adsorption conditions for the uptake of Pd(II) ions were achieved at 0.03 g of the IP-MIL-101(Cr) adsorbent and pH=2.0. This suggested electrostatic interaction between the protonated nitrogen (N) groups on the adsorbent surface and the Pd(II) ions complexes. The Langmuir isotherm was the well fitted model and estimated the qm of 195.3 mg/g for the adsorption of Pd(II) ions IPMIL-101(Cr) composite which was higher than 177.7 mg/g of the its non-imprinted counterpart (NIMIL-101(Cr). Kinetics data further confirmed the improved rapid adsorption rate of Pd(II) by the IPMIL-101(Cr) which obeyed the PSO kinetic model. The ion specific IPMIL-101(Cr) adsorbent was re-generable for five consecutive cycle without loss in the removal efficiency and adsorption capacity. The Pt(IV) ion imprinted IIGlyMIL-101(Cr) composite, which was prepared using the same glygly functional monomer showed enhanced performance in the adsorption of targeted Pt(IV) ions from aqueous solution. In an initial solution pH of 3.0, 0.02 g of the IIGlyMIL-101(Cr) adsorbent was able to recover 85% of the Pt(IV) ions at 25 oC. The qm value were obtained from the Langmuir isotherm model to be 531.3 mg/g for the IIGlyMIL-101(Cr) adsorbent in the intake of Pt(IV) ions, which was higher than 296.4 mg/g of the non-imprinted counterpart (NIGlyMIL-101(Cr)) composite. Kinetics data revealed fast adsorption of Pt(IV) ions by IIGlyMIL-101(Cr) which equilibrated within 30 minutes and fitted well with PSO kinetic model. The selectivity towards Pt(IV) ions intake by the IIGlyMIL-101(Cr) composite was also improved with an efficiency of 80% in comparison to the non-imprinted adsorbent which was 50% in the presence of various anions. Moreover, the IIGlyMIL-101(Cr) composite was able to maintained 88% removal efficiency after five subsequent cycles, suggesting improved stability of the adsorption active sites. The results observed in this study proved the efficiency of ion-imprinted MOF composites and their potential application in the recovery of PGMs form industrial wastewater should be considered.