Trace Cr(VI) Removal: Evidence of Redox-Active Ion Exchange by a Weak-Base Anion Exchanger

Abstract

The presence of Cr(VI) in water, even in trace concentrations, is toxic to numerous animals, plants, and microorganisms. Remediation efforts require selective removal of trace concentrations of Cr(VI) anions from the background of other competing ions present in concentrations at least 2 orders of magnitude greater than Cr(VI). The objective of this study was to evaluate the trace Cr(VI) removal capacities of commonly available anion exchangers. Batch and column studies revealed that Duolite A7 showed extraordinarily high Cr(VI) removal capacity compared to other anion exchangers. For a synthetic influent solution containing trace Cr(VI) at pH 5, Duolite A7 removed Cr(VI) for 30 000 bed volumes (BVs), while the columns having other anion exchangers reached breakthrough almost instantaneously less than 1000 BVs. Equilibrium and kinetic studies were performed on Duolite A7 at varying pH and contact time, coupled with characterization studies on the raw and exhausted ion exchanger to elucidate the mechanism behind such a significantly higher Cr(VI) removal. Based on the results, it was concluded that there was an occurrence of redox reaction within the resin in addition to ion exchange because of the unique resin composition. Duolite A7 is a weak-base anion exchange resin with a phenol formaldehyde matrix, and both secondary and tertiary amine functional groups. The redox reactions resulted in the perpetual regeneration of ion exchange sites and 30× greater Cr(VI) removal capacity of Duolite A7. This finding may pave the way to design a new class of redoxactive ion exchangers for various water and wastewater treatment applications.

Guanidinium-Based Ionic Covalent-Organic Nanosheets for Sequestration of Cr(VI) and As(V) Oxoanions in Water

“…The Q e Cr /(IEC/2) value slightly exceeded unity in some iCONs (e.g., 122% for II, 107% for VI, and 110% for VII) possibly due to the concurring non-ion-exchange adsorption pathways. 32,59 The pore size appeared to have some impact on the Cr uptake efficiency as iCON I with the smallest pore size showed a slightly lower Q e Cr /(IEC/2) value of 80%. Interestingly, the Q e Cr /(IEC/2) values were comparable between iCONs with syn-(III) and anti-hydrogen bonding conformations (II) at the guanidinium sites, suggesting that the parallel bidentate hydrogen bonding interactions with tetrahedral oxoanion are not essential to chromate adsorption.…”

Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process

“…It is worth mentioning that the Cr(VI)/Cr(III) redox process also occurred, though to a much smaller extent, in iCON-OH 0 presumably due to the presence of unreacted amine and aldehyde groups. [10,22] Further analysis with iCON-OH 1-2 revealed that the Q m /(IEC/2) values in chromate adsorption linearly increase with the number of hydroxyl groups on the aromatic linker (Figure 3e), indicating a phenol-mediated redox conversion. [11] The phenol-mediated Cr(VI)/Cr(III) redox process in iCON-OH 3 as well as in iCON-OH 1-2 was clearly accelerated by the ion exchange process as the control neutral hydroxylated CON-OH 0-3 showed insignificant redox-based adsorption of chromate at the neutral pH (Figure 3f).…”

Mechanically robust sodium alginate/cellulose nanofibers/polyethyleneimine composite aerogel for effective removal of hexavalent chromium and anionic dyes

“…[ 1 ] Organic dyes (such as Congo Red [CR] and Methyl orange [MO]) are widely used in the textile industry, and the dye pollutants can induce water quality deterioration and adversely affect human health. [ 2 ] Nowadays, the standard methods for wastewater treatment include adsorption, [ 3 ] photocatalysis reduction, [ 4 ] membrane separation, [ 5,6 ] ion exchange, [ 7 ] and electrochemical methods [ 1 ] that have been used to remove pollutants from wastewater. [ 8 ] Among them, adsorption is regarded as the most promising method because of its low pollution and economic performance.…”