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Background: The presence of heavy metal contaminants such as chromium, lead, mercury, cadmium, arsenic, nickel, and copper have become a major issue towards human health. Chromium is extremely toxic to living organisms as it acts as carcinogen and mutagen. High concentration of chromium may cause detrimental effects to human health in the long term. The mutagenic and carcinogenic properties, included Cr(VI) in the group “A” of human carcinogens. Cr(VI) can easily penetrate into the cell wall and exert its noxious effect due to its mobility in the environment. Cr(VI) is nearly 100 times more toxic than Cr(III). Cr(VI) causes skin and stomach irritation or ulceration, damage to liver, kidney ulceration, damage to nerve tissue, and long-term exposure above the maximum contaminated level even led to death. Therefore, it is essential to remove chromium from wastewater prior to its final discharge into the environment. This study attempts to explore the mechanism by which chromium ions had been adsorbed by these two ion exchange resins and will be extended further to investigate the uptake mechanism of other metal ions within future research. Methods: Equilibrium isotherms were obtained by contacting 20 mL of aqueous metal ion solution with different amounts of adsorbents in a shaker bath controlled at 25±0.5oC. The initial concentration of metal ion in the aqueous solution was varied between 40-100 mg L -1 . Equilibrium isotherms for the above metal ion were generated at pH 3, 4 and 5. The pH of the solution was varied between pH 3 to 5 using appropriate doses of buffer. Preliminary runs exhibited that the adsorption equilibrium was achieved after 1–1.30 h of contact time for both the tested resins. The adsorbents used were DOWEX and AMB resins. For estimation of adsorption enthalpy, adsorption equilibrium experiments were performed at temperatures 30, 40 and 55oC. The amount of metal ion adsorbed per unit mass of the adsorbent (mg g-1) was calculated as q= V∆C/W, where ∆C is the change in solute concentration (mg L -1 ), V is the solution volume (L) and W is the weight of the adsorbent (g). Experiments on adsorption kinetics were performed in a stirred constant volume vessel. The liquid volume was 100 cm3 with 10g of adsorbent sample. The initial concentration of metal ion was 80 mg L -1 at 25±0.5oC. The aqueous phase concentration was examined at equal time intervals till equilibration. Results: The electrostatic interaction of Cr(VI) with the positively charged nitrogen atom of the functional groups and chelation of Cr(III) with the electron donor groups were the possible mechanistic pathways through which the adsorption had occurred onto both the ion-exchange resins. Though electrostatic interaction was the predominant interaction in both the resins for the adsorption of anionic Cr(VI) species, but it had been observed that the mechanism of Cr(VI) adsorption was not only “anionic adsorption” but also the complexation of the reduced Cr(III) with the ammonium group of the resins. Thus, “adsorption- coupled reduction” was the main mechanism for the uptake of chromium ions. Conclusion: The present work demonstrated that both resins could effectively adsorb Cr(VI) ions from aqueous solution. More adsorption had taken place onto DOWEX compared to AMB. The adsorption characteristics of both the resins were studied under various equilibrium and thermodynamic conditions which proposed the spontaneous nature of the process. The adsorption capacities of both resins were influenced by the pH of the medium and exhibited high adsorption performances at pH 3. The mechanism of adsorption onto the two resins studied here was anionic adsorption of Cr (VI) and chelation of Cr (III) ion. The Cr(III) ions might have formed because of the reduction of Cr(VI) by the electron donor atoms present in the resins and interacted with the adsorbent surface. FTIR spectra also supported the interaction of chromium ions with functional groups present in the resin structures. Thus chromium uptake by DOWEX and AMB resins was mainly governed by “adsorption- coupled reduction”. Desorption studies revealed that regeneration of both the ionexchange resins are possible at basic pH and can be reused. However, the application of these two ion-exchange resins using real effluent is under consideration.

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