Title:Chromium Adsorption from Aqueous Solution onto Dowex Retardion
11A8 and Amberlite IRA 743 Free Base: An Insight into the Mechanism
Volume: 18
Issue: 3
Author(s): Swastika Gogoi and Monali Dutta Saikia*
Affiliation:
- Department of Chemistry, Arya Vidyapeeth College, Guwahati 781 016, Assam,India
Keywords:
Adsorption, adsorption-coupled reduction, ion exchange resin, carboxylic group, liu isotherm, quaternary ammonium group.
Abstract: Background: The presence of heavy metal contaminants such as chromium, lead, mercury, cadmium,
arsenic, nickel, and copper has become a major issue for human health. Chromium is extremely toxic to
living organisms as it acts as a carcinogen and mutagen. The 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 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 the liver, kidney ulceration, damage to nerve tissue,
and long-term exposure above the maximum contaminated level even leads 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 are adsorbed by these two ion exchange resins and will be
extended further to investigate the uptake mechanism of other metal ions in future research.
Methods: Equilibrium isotherms were obtained by reacting 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 the metal ions
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 the 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= VΔC/W,
where ΔC 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 adsorption occurred on 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 was 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 an
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 ion-exchange resins is possible at basic pH and can be reused. However, the application of these two ionexchange
resins using real effluent is under consideration.
