Adsorption of anionic and cationic dyes by activated carbons, PVA hydrogels, and PVA/AC composite

The textural and adsorption characteristics of a series of activated carbons (ACs), porous poly(vinyl alcohol) (PVA) gels, and PVA/AC composites were studied using scanning electron microscopy, mercury porosimetry, adsorption of nitrogen (at 77.4 K), cationic methylene blue (MB), anionic methyl orange (MO), and Congo red (CR) from the aqueous solutions. Dye–PVA–AC–water interactions were modeled using the semiempirical quantum chemical method PM6. The percentage of dye removed (Crem) by the ACs was close to 100% at an equilibrium concentration (Ceq) of less than 0.1 mM but decreased with increasing dye concentration. This decrease was stronger at Ceq of less than 1 mM, and Crem was less than 50% at a Ceq of 10–20 mM. For PVA and the PVA/AC composite containing C-7, the Crem values were minimal (<75%). The free energy distribution functions (f(ΔG)) for dye adsorption include one to three peaks in the −ΔG range of 1–60 kJ/mol, depending on the dye concentration range used and the spatial, charge symmetry of the hydrated dye ions and the structural characteristics of the adsorbents. The f(ΔG) shape is most complex for MO with the most asymmetrical geometry and charge distribution and adsorbed at concentrations over a large Ceq range. For symmetrical CR ions, adsorbed over a narrow Ceq range, the f(ΔG) plot includes mainly one narrow peak. MB has a minimal molecular size at a planar geometry (especially important for effective adsorption in slit-shaped pores) which explains its greater adsorptive capacity over that of MO or CR. Dye adsorption was greatest for ACs with the largest surface area but as molecular size increases adsorption depends to a greater extent on the pore size distribution in addition to total and nanopore surface areas and pore volume.

Symmetric geometry and charge structure of Congo red (CR) result in a narrow and symmetrical free energy peak of dye adsorption onto ACs in contrast to less symmetrical methylene blue (MB) or asymmetrical methyl orange (MO).Figure optionsDownload high-quality image (74 K)Download as PowerPoint slideHighlights
► Activated carbons with higher burn-off adsorb larger amounts of dyes.
► Blocking of AC pores by polymer in PVA/AC composites decreases dye adsorption.
► Free energy of adsorption of dyes is lower for AC than PVA/AC composites.