Adsorption of different substituted phenols onto S23, F22 and SA_DRAW

In the following part, the single and competitive adsorptions of four phenolic compounds (PCP; PNP; PHBA and P) onto the two commercials (S23 and F22) and SA-DRAW (the best SBAC) are investigated and compared. The experimental conditions and procedures have been explained in details in chapter II.

a- Single solute solution Isotherm

Figure III.7.a, b and c shows the single component adsorption isotherms for the four substituted phenols in aqueous solution onto SA_DRAW, S23 and F22, respectively. These results confirm that for all phenols adsorption capacity of SA_DRAW is much lower than for the two commercial activated carbons. As mentioned before, this lower performance could be due to much lower BET surface area (265 m² g^-1), the less developed microporous structure of SA_DRAW and its high ash content. It should also be noticed that despite S23 and F22 have nearly the same surface area, S23 exhibits the greatest adsorption capacities. This seems to be related to either the basic nature of this activated carbon and/or its lower ash content (Haydara et al., 2003).

Whatever the activated carbon used, the single component isotherms revealed the strongest adsorption capacity of PCP - followed by PNP, P and PHBA. Different behaviour was observed in the case of SA_DRAW at higher concentrations, the adsorption capacities being increased strongly for PNP and PHBA, according to type II adsorption isotherm, whereas type I is convenient for all the other isotherms (Sun and Meunier, 2003). Type II isotherm indicates a multilayer adsorption consecutive to additional interactions between the molecules. The possibility of the formation of these layers generally occurs with mesoporous materials as SA_DRAW (Vmicro = 0.11 cm³ g^-1 and Vmeso = 0.17 cm³ g^-1). Even if the isotherms of PNP and PHBA seem to increase rapidly, type S isotherm, with constant value at higher concentrations, could be expected.

108 Fig.III.7. Adsorption isotherms at 25°C of single component of phenol, PHBA, p-nitrophenol and p-chlorophenol onto activated carbons (a) SA_DRAW, (b) S23 and (c) F22.

0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 Ce(mol/L)

0 0.005 0.01 0.015 0.02 0.025 0.03

Ce (mol/L)

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 Ce (mol/L)

109 Independently of the nature of ACs, the adsorption capacities are known to depend on several characteristics of the phenols compounds in water: solubility, hydrophobicity, nature of the functional group on the aromatic molecules and ability to oligomerize (oxidative coupling). It can be noticed that comparing these characteristics leads to different trends thus it becomes complex to analyze the difference in affinities for the molecules studied as shown subsequently. These characteristics are collected in Table III.7 for the four molecules studied.

The favorable or unfavorable effects are specified for each parameter and then discussed.

Table III.7

Favorable or unfavorable effects of molecules characteristics in single solute adsorption

P PHBA PNP PCP

Solubility 4 1 2 3

Hydrophobicity 3 4 2 1

Functional group

unfavorable unfavorable favorable favorable

Oligomerisation Favorable Unfavorable Unfavorable favorable

Moreno-Castilla et al. (1995a) reported that the adsorption capacity increased with decreased water solubility of the phenolic compounds. In the present study, this parameter seems to be secondary since PHBA is the less adsorbed although it is the less soluble compound.

On the other hand, the activated carbons are mainly hydrophobic and display a strong affinity for organic molecules which have a limited solubility in water. Hydrophobic compounds tend to be pushed away towards the adsorbent surface being then more adsorbed than hydrophilic compounds (Lu and Sorial, 2007). The higher capacity of the PCP may be explained according to the hydrophobicity of the adsorbed molecules. Among these four phenols, PCP is more hydrophobic than PNP, P and PHBA, as expected from the magnitude of the logarithm of the octanol-water partition coefficient; log Kow,PCP < log Kow,PNP < log Kow,P < log Kow,PHBA.

Another parameter generally taken into account is the dispersive interactions between the activated carbon (electron donating) and the aromatic molecules (electron acceptor). The

110 nature of aromatic group could increase (electron releasing effect) or decrease (electron withdrawing effect) the electron density of the aromatic ring and influence the interactions between the surface groups and the aromatic ring. The enhanced interaction in the case of PNP and PCP is due to the electron-withdrawing effect of the - NO2 and the - Cl substituents, as they reduce the overall electron density of the aromatic ring, increasing its ability to accept the electron of AC graphite layers (Dabrowski et al., 2005).

When phenols are concerned a special adsorption mechanisms may involve oligomerization which is generally regarded as irreversible adsorption. Many researchers have been confirmed that some phenolic compounds undergo oligomerization on the surface of activated carbon when molecular oxygen is present in the test environment (Uranowski et al., 1998; Lu and Sorial, 2007). As seen in chapter I, the critical oxidation potential (COP) is the most important parameter to identify the molecules which can be oligomerized: The more the value of COP, the less the molecule is able to oligomerize (Lu and Sorial, 2004). In this study, oligomerization could only occur for phenol (COP = 1.089V) and PCP (COP = 1.094V) explaining the high adsorption of PCP and rather high of phenol despite its low solubility and hydrophobicity.

The consideration of all these characteristics seems quite favorable to the adsorption of PCP and PNP in agreement with the isotherms obtained. For these single solute systems, the solubility seems to have a limited impact.

b- Langmuir and Freundlich models

For a comparison of single solute isotherm results in a quantitative manner, the isotherms were represented by Langmuir and Freundlich models and the constants obtained according to these two models are listed in Table III.8. The correlation coefficients square (R²) for both models at 25 °C suggested that the empirical Langmuir equation was much more convenient than Freundlich equation in describing the adsorption of all the phenolic compounds onto the two commercial ACs (S23 and F22). However, for SA_DRAW no good fit could be obtained with the two models, Freundlich model being found to better represent the adsorption data for PNP and PHBA, while Langmuir model better fitted the data of PCP and P.

The Langmuir equation being convenient for the two commercial AC clearly indicates that S23 provides the highest maximum adsorption capacity (qmax) for the four phenols, and

111 the best adsorption equilibrium constant (KL) for P and PCP. Concerning the adsorbates, the highest value for PCP proves its high affinity with this carbon surface compared to the other phenols studied in this work. For F22, the values of KL arenearly the same for PCP, PNP and PHBA, and the higher value of qmax indicates here again more affinity of PCP with the carbon surface.

Table III.8

Parameters of Langmuir and Freundlich models for the adsorption of single solute of P, PCP, PNP and PHBA on three types of activated carbons at 25 °C

Langmuir model Freundlich model