Mesoporous Carbons of Well-Organized Structure in the Removal of Dyes from Aqueous Solutions

Mesoporous carbons with differentiated properties were synthesized by using the method of impregnation of mesoporous well-organized silicas. The obtained carbonaceous materials and microporous activated carbon were investigated by applying different methods in order to determine their structural, surface and adsorption properties towards selected dyes from aqueous solutions. In order to verify applicability of adsorbents for removing dyes the equilibrium and kinetic experimental data were measured and analyzed by applying various equations and models. The structural and acid-base properties of the investigated carbons were evaluated by Small-Angle X-ray Scattering (SAXS) technique, adsorption/desorption of nitrogen, potentiometric titration, and Transmission Electron Microscopy (TEM). The results of these techniques are complementary, indicating the type of porosity and structural ordering, e.g., the pore sizes determined from the SAXS data are in good agreement with those obtained from nitrogen sorption data. The SAXS and TEM data confirm the regularity of mesoporous carbon structure. The adsorption experiment, especially kinetic measurements, reveals the utility of mesoporous carbons in dye removing, taking into account not only the adsorption uptake but also the adsorption rate.

Adsorption kinetics Figure 1. Exemplary absorption spectra in the Vis range measured during the adsorption process for the MB (W3) system. In order to study thermal degradation of the dye/activated carbon system the measurements applying thermal analysis were made. In Figure S3(a-c) the TG, DTG and DSC curves measured for methylene blue, the pure carbon W3 and W3 loaded with dye are presented.
Analysing the process of thermal degradation of methylene blue ( Figure S3a) one can identify many stages on the DTG curve. This is a result of its complicated molecular structure -the presence of two benzene rings joined by sulphur and nitrogen atoms, and side dimethylamino groups. In the temperature range of 25-160 °C the endothermic peak corresponding to the removal of the physically bound, hygroscopic water is observed. The hygroscopicity of methylene blue is related to the hydrophilic character of molecule (Log P= -0.9) responsible for hydrogen bonds between water and sulphur and/or nitrogen atoms. The subsequent exothermic events up to 600 °C, with the predominant peak with the maximum rate at 444 °C, indicate oxidation and pyrolysis processes of the organic dye [76].
The DTG curve for the mesoporous carbon W3 ( Figure S3b) shows a well defined exothermic peak with the total weight loss of 91 wt.% of sample mass. In the temperature range smaller than 450 C successive desorption of thermally unstable oxygen complexes (carboxylic groups, lactones or lactols) bound to the carbonaceous surface occurs. It is well visible as an asymmetric peak on the DSC curve. In the temperature range 450-650 C the exothermic processes of oxidation of the more stable oxygen species (with a complex structure) and pyrolysis of solid bulk take place. This stage is visible as the single peak with the minimum at 490 C. Possible oxygen complexes which undergo degradation are as follows: phenolic, hydroquinonic, carbonylic, quinonic groups or ethers.
In the case of carbonaceous material loaded with dye ( Figure S3c), two peaks on the DTG curve are observed. The first one with the minimum at 60 C corresponds to removal of the weakly bound water whereas the subsequent one, with a bimodal character and the minimum at 421 and 474 °C can be attributed to the thermal decomposition of adsorbate and adsorbent. In comparison to the native material the thermal profile shows a shift towards lower temperature values (from 490 to 474 C). Therefore, one can state that the dye adsorption on W3 results in change of the degradation temperature range for the carbonaceous material. Volatile products released during the thermal degradation of methylene blue, carbonaceous material W3 and W3 loaded with dye were analyzed using the FTIR technique as shown in Figure S4. During the pyrolysis of all samples the most characteristic infrared bands centered at 2360, 2310 cm −1 and 669 cm −1 may be attributed to stretching and deformation vibrations of CO2, respectively. CO2 was generated in the thermal destruction of multiple benzene rings of dye, surface oxygen species and solid bulk of W3. At the temperatures above 243 °C some of these processes proceed with simultaneous release of CO with characteristic bands in the range of 2000−2500 cm −1 . Moreover, the wide absorption bands from 3500 cm −1 to 4000 cm −1 and additionally at 1756 cm −1 correspond to the O−H stretching vibrations and symmetric bending of water molecules in the vapor phase. The peaks at 1537 cm −1 and 1542 cm −1 indicate the stretching vibrations of C=C bonds in the aromatic dye ring or the carbon cyclic structure.
In the case of the spectra for pure dye and dye/W3 samples one can observe the signal recorded for SO2 as a band at 1370 cm −1 (asymmetric stretching vibrations) which come from sulphur atoms in the dye molecules. There is also a peak at 714 cm −1 on the spectra for both samples indicating the presence of HCN in the gaseous phase. In turn, the absorption bands at 2896 cm −1 and 2972 cm −1 corresponding to the CH stretching vibrations of methyl groups CH3 are well defined only for the dye sample. However, this signal is only recorded at 172 and 243 °C, and for these temperatures the spectra for dye/W34 are not shown. Figure 3. FTIR spectra of gas products of pyrolysis of methylene blue, W3 and methylene blue /W3 system at the temperatures corresponding to the process rate maxima. Table 3. Physicochemical properties of the amphiphilic triblock copolymers used in the silica synthesis.

Adsorption Experiment
Adsorption Equilibrium Table 5. Generalized Langmuir isotherm equation (GL) and its simplified forms.

Isotherm (code) Equation
Isotherm ( where: θ is the global (overall) adsorption isotherm (overall coverage), m and n are the heterogeneity parameters characterizing a shape (width and asymmetry) of the adsorption energy distribution function; K describes the adsorption equilibrium constant characterizing a position of the distribution function on an energy axis.
Adsorption Kinetics Table 6. Various kinetic equations applied in the study. where: aeq and ceq are the equilibrium adsorption and concentration, co is the initial concentration, c and a are the temporary concentration and adsorption, V is the solution volume, m is the adsorbent mass, k is the kinetic rate coefficient and t is time, F is the adsorption progress, f2<1 is normalized share of second-order term in the overall rate dependence.