Adsorption of red azo dyes on multi-walled carbon nanotubes and activated carbon : a thermodynamic study.
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2017
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Abstract
Carbonaceous materials have been extensively studied as highly efficient adsorbents for the removal of dyes
from wastewater. However, investigations of thermodynamic aspects of the interactions between these materials
and dyes remain scarce. This paper describes the thermodynamics of the interactions between the Ponceau 4R
(PR), Congo Red (CR), and Allura Red (AR) dyes and multi-walled carbon nanotubes (MWCNTs). The interactions
between the dyes and activated carbon (AC) were also evaluated for comparison. The investigation used a
combination of adsorption isotherms and isothermal titration nanocalorimetry (ITC) measurements, and a
thermodynamic approach provided full characterization of the adsorption process. For both MWCNT and AC, the
amount of adsorbed dye (ΓD) increased in the order ΓD (PR) < ΓD (AR) < ΓD (CR), and the adsorption capacity
normalized by the adsorbent specific area was up to 5.6 times higher for MWCNT. The maximum amount
adsorbed (ΓD,max) reached values of up to 2.00 μmol m−2 for CR adsorption on MWCNT. For both adsorbents, the
process of dye adsorption was enthalpically driven and entropically unfavorable. All the thermodynamic
parameters depended on the surface coverage and the structures of both dye and adsorbent. The adsorption
enthalpy change (ΔadsH) and entropy change (TΔadsSref) values were higher than −137.0 kJ mol−1 and
−114.2 kJ mol−1, respectively. In addition to the structural differences between MWCNT and AC, the ITC data suggested that the adsorption sites are heterogeneous and that the dyes preferentially adsorb on the more hydrophilic
sites on the adsorbent surfaces.
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FERREIRA, G. M. D. et al. Adsorption of red azo dyes on multi-walled carbon nanotubes and activated carbon: a thermodynamic study. Colloids And Surfaces A-Physicochemical And Engineering Aspects, v. 529, p. 531-540, set. 2017. Disponível em: <http://www.sciencedirect.com/science/article/pii/S0927775717305848>. Acesso em: 15 set. 2017.