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Micropore size distribution and surface characteristics co-influence on 4-chlorophenol adsorption mechanism from organic solvents Ewa Lorenc-Grabowska, and Piotr Rutkowski Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01493 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018
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Micropore size distribution and surface characteristics co-influence on 4-chlorophenol adsorption mechanism from organic solvents Ewa Lorenc-Grabowska*, Piotr Rutkowski
E. Lorenc-Grabowska, Piotr Rutkowski Wrocław University of Science and Technology, Faculty of Chemistry, Department of Polymer and Carbonaceous Materials, Gdańska 7/9, 50-344 Wrocław, Poland e-mail:
[email protected]* corresponding authors
[email protected] ACS Paragon Plus Environment
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Abstract The effects of co-influence of a pore size distribution (PSD) and a surface chemistry of activated carbon (AC) on the p-chlorophenol (PCP) adsorption from water, heptane and cyclohexane have been studied. To modify the surface basicity, the commercial activated carbon and the ash free commercial activated carbon were subjected to a heat treatment in a hydrogen atmosphere. The ACs were also oxidized with a hydrogen peroxide to increase the acidity. All applied modifications caused negligible changes in the porous texture and a significant modification on the surface characteristics. The adsorption of PCP was carried out in static conditions at an ambient temperature. The time needed to obtain the adsorption equilibrium from organic solvent was shorter than from water. The boundary layer effect was found to increase in the direction of water CWZ H2O2 > CWZ > CWZ dem > CWZ H2 > CWZ demH2. Regarding water a slightly different trend is found. The qte is the highest for CWZ H2 and then decreases in the direction of CWZ H2 > CWZ H2O2 > CWZ dem > CWZ > CWZ demH2 > CWZ demH2O2. Therefore, in the case of the organic medium the tendency is similar to the tendency of the heteroatom (O+N) content, whereas for water medium no such relationship is found. The adsorption from water requires much longer equilibrium time compared to heptane and cyclohexane. To attain equilibrium in non-aqueous systems only 2.5-3.5 hours are needed whereas for water system 7 hours are required for hydrogen treated carbon, over 23 hours for CWZ demH2O2 and around 16 hours for other adsorbents. It is generally accepted that the molecules with a bigger molecular mass have a higher affinity toward the adsorbent surface.33 The molecular mass of heptane and cyclohexane is a few folds bigger (Table S2) than water molecules. Thus, the organic solvents’ molecules move faster toward the surface entailing the dissolved p-chlorophenol molecule. On the other hand, the bigger size of the organic solvents suppresses the uptake of PCP on active sites, which is reflected in a lower adsorption capacity determined at the equilibrium time. 9 ACS Paragon Plus Environment
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Due to the adsorption kinetics being controlled by a diffusion mechanism, the intraparticle diffusion model based on the theory proposed by Weber and Morris34 was applied to determine the rate-limiting step of the PCP adsorption. The equation describing the model is given in Table S3. The intraparticle diffusion model plots for the ACs studied are shown in Fig. 4 and the calculated values of equation parameters are given in Table 2. The multilinearity, indicating that the adsorption kinetics involves several steps, is observed exclusively for the systems were water is used as the solvent. The kinetics for heptane and cyclohexane are characterized by a single straight line indicating a similarity in the mechanism of the PCP adsorption from the organic medium. The Weber-Morris equation applied to all experimental data gives a strong correlation in the case of non-aqueous solvents only. The calculated correlation coefficient, R2 (Table 2), for organic solvents varies between 0.891 and 0.972 for heptane and between 0.911 and 0.968 for cyclohexane. If the equation is applied to a full range of points for the adsorption from water the obtained correlation coefficient is considerably lower and varies between 0.397 and 0.959. The division of the plot to two regions significantly improved the correlation coefficient. The different shapes of Weber-Morris plot indicate differences in the adsorption kinetics from the different media. However, a lack of boundary diffusion was observed regarding the adsorption from heptane. In this work, the occurrence of an external layer of the diffusion process for all solvents can be inferred from the fact that the plots do not pass through the point of origin. From the intercept of the plots, the boundary layer effect can be evaluated and is expressed as CI. It can be seen that the calculated CI value decreases in the direction: water>cyclohexane>heptane (except the PCP/water/CWZ demH2O2 system). In the same direction the hydrophobicity of the solvents (Table S2) decreases. On the other hand, the CI value decreases with an increase in the (O+N) content in ACs for the PCP/water system. A reverse effect is observed for PCP adsorption from a non-aqueous solution. The observed relationship is given in Fig.5. In our
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previous works23,27,28 we observed a relationship between the boundary layer thickness and the affinity of phenol for the surface with hydrophilic groups when adsorption was carried out from water. The greater the number of hydrophilic functional groups in the ACs the thinner the boundary layer. The presence of a hydrophilic group promotes the water molecules to move toward the surface. Consequently, the PCP molecules surrounded by water are attracted more by the surface of a higher hydrophilicity. The same mechanism gives a reverse relationship for organic solvents. The lower boundary layer thickness is observed for carbon with a lower hydrophilicity as the hydrophobic solvent has higher affinity toward a hydrophobic surface. However, for an organic solvent the effect is not so pronounced. The attraction leads to a faster molecules movement in the film of boundary layer. The diffusion in the boundary layer performs when the concentration of adsorbate in a solution is high and should not be mistaken for affinity of the PCP molecules toward active sites that is described by the equilibrium adsorption isotherms. The boundary layer is the first stage in the adsorption processes that might be rate limiting. The second stage is the movement of molecules to the active sites through transporting pores, shown in the first part of the plot (I). The final stage of the intraparticle diffusion is the diffusion in micropores, shown in the last part of the plot (II). In the case of intraparticle diffusion the slope of the plot indicates the rate of adsorption. Regarding the adsorption from an organic solvent the kp is similar and varies between 1.457-3.146 for cyclohexane, and 1.701-2.578 for heptane. As mentioned above, regarding the adsorption from water the plot can be divided in two parts. The value of the intercept for the first part of plot (kpI) (that corresponds to kp in the case of a non-aqueous solution) is a few times bigger and varies between 4.128 and 15.37. The higher the value of kpI the faster the molecules movement in porous structure. However, the adsorption from water is described by additional step (II part). This is almost parallel to the X axis and represents the final adsorption on the active sites. The 11 ACS Paragon Plus Environment
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presence of the slow penetration into micropore to the active sites causes the adsorption from water to have a much longer time requirement to attain the equilibrium time. Those findings are further supported by the equilibrium studies. Equilibrium adsorption of phenol The processes of the p-chlorophenol adsorption from water were carried out in unbuffered conditions. Taking into account the pH of the solution after the adsorption (6.7-8.5), the electrostatic forces do not influence the adsorption processes on CWZ carbon as the PCP molecule was in a non-ionic form. The adsorption isotherms for PCP on the ACs are shown in Fig. 6. The shape of the equilibrium isotherms of the PCP adsorption from the different solvents presents almost a whole range of the Giles classification isotherms.3. In the case of the adsorption from water the L2-type is observed for the CWZ and CWZdem. The S4-type for the hydrogen peroxide oxidized carbons and the H-type for the hydrogen treated carbons is found. When cyclohexane is used as a solvent, the L2-type isotherms are noticed for the CWZ, CWZdem, CWZH2 and CWZdemH2, whereas for the same set of carbons for heptane these are the S4-type isotherms. Finally, the C-type isotherms are recorded for the organic medium systems/the oxidized carbons. The shape of the adsorption isotherm is one of the most direct manifestations of the interaction between the components of the studied system. The different shapes reveal various regimes of the adsorption with solvents. In the wider literature, the interpretation of the shape of the phenolic compounds adsorption isotherm from the organic solvents is extremely rare. In the case of the adsorption on polymeric adsorbent the C-type isotherm was found for heptane20 whereas the L1-type was found for cyclohexane medium.36 The adsorption of phenol on a surface modified ACs from a different solvent was presented by Anhert et. al.14,18 The shape of isotherms for all solvents was changing from a S-type through a C-type to a L-type. The L2-class of isotherms is
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commonly reported for the adsorption of phenols from an aqueous solution.1,11,12 This occurs when there is not a strong competition between the adsorbate and the solvent for occupying the adsorption sites. The S-type was observed in systems where the adsorption was carried out on a strongly oxidized ACs. This type occurred when there was a strong competition with a solvent17,37,38. Moreover, the H-type was presented for the phenols adsorption on ACs with a high basicity.28 The three used solvents differ in their hydrophobicity in the direction of water
