Immobilisation of palladium nanostructures in polyethersulfone beads: recyclable catalyst for chromium(VI) remediation

Abstract

Environmental contextChromium, a carcinogenic metal present in the wastewater of several industries, is currently removed by treatment with large amounts of chemicals and expensive nano-catalysts. We have immobilised a nano-catalyst in tiny polymeric balls that are highly efficient at capturing chromium, and are easy to isolate for multiple reuse. Using our methodology, consumption of chemicals for removing chromium from wastewater is reduced by 97%. AbstractWe have synthesised and immobilised palladium nanostructures in porous polyethersulfone beads for the first time and demonstrated their catalytic application for the reductive transformation of toxic CrVI to nontoxic CrIII by formic acid at 25°C. The reduction of CrVI using palladium-polyethersulfone composite beads (Pd-PES), with a minimal Pd loading of 0.4 wt%, is found to be 98% with excellent operational stability retained up to 100 consecutive reaction cycles. Pseudo-first-order rate constant kapp for the 1st and 100th catalytic cycles is 0.167 and 0.158min−1 respectively. Pd-PES beads having a diameter of 2mm are easy to isolate post reduction by simple mesh filtration and can be re-used consecutively without any treatment. Owing to the high catalytic stability of the Pd nanostructures inside the beads, and the good mechanical and thermal stability of polyethersulfone, these beads can withstand rigorous treatment like mechanical stirring and elevated temperature, which renders them as highly reusable and as promising metal-polymer composite for practical application in CrVI remediation. For large scale application of this catalyst, we have demonstrated a methodology which reduces the consumption of formic acid by 98% in chromium remediation technology.

Application of polymer-coated Macadamia integrifolia nutshell biomass impregnated with palladium for chromium(VI) remediation

“…3 . As shown by the Cr(VI)] t / [Cr(VI)] 0 versus t plots, and in accordance with production of the lowest values of χ 2 plus the highest R 2 values in the 0.9986 to 0.9999 range (Table 2 ), the kinetic data were well simulated by the pseudo-first-order kinetic model, which matched previous reports 17 , 43 , 48 , 55 , 56 . Therefore, the rate constant of the pseudo-first-order model equation was used as the apparent rate constant ( k app ) for evaluation of the thermodynamics of the Cr(VI) conversion process.…”

“…To gain advantage from the extended period of exposure of the Pd@Polym-MNS to the acidic solution, an aliquot of the final solution was analyzed by ICP-OES whereupon no Pd was detected indicating that there was no leaching in the acidic conditions that prevailed during the Cr(VI) conversion process. The absence of Pd leaching in similar applications has previously been reported 17 , 55 , 65 , 66 .…”

“…On account of the Cr(VI) ions undergoing redox reaction with adsorbed hydrogen atoms from the degradation of HCOOH on the Pd@Polym-MNS surface as illustrated in Fig. 2 followed by the desorption of the nascent Cr(III) ions, which regenerated the Cr(VI) adsorption sites for further uptake of Cr(VI) ions 17 , the extent Cr(VI) adsorption from the solution containing HCOOH was higher. Consequently, the rate at which the intensity of the characteristic peak decreased was higher when the HCOOH-containing solution was treated with the Pd@Polym-MNS.…”

“…The reduction of the Cr(VI) in, for example, aqueous hydrogen chromate (HCrO 4 – ) ions by formic acid (HCOOH), which proceeds as per Eq. ( 1 ) 16 , 17 , is associated with positive electrode potentials indicating thermodynamic feasibility. However, the Cr(VI)-HCOOH redox reactions are still hampered by high activation barriers, which necessitate the use of a catalyst 18 .…”

“…Even though the use of nanoparticles results in enhanced catalysis, the recovery of the nanoparticles for reuse is often difficult and poses the risk of release into the environment together with the treated water. Therefore, the catalytic nanoparticles used in the HCOOH-mediated reduction of Cr(VI) have been immobilized on powders, granules, and films 16 , 17 , 41 – 47 . Despite several materials being used as supports for catalytic nanoparticles used in the HCOOH-mediated reduction of Cr(VI), the use of lignocellulosic material supports is scarcely reported in the literature.…”