Degradation of Phenol Using the Mixed (Al-Fe) Pillared Bentonite as a Heterogeneous Photo-Fenton Catalyst∗

In this work the mixed Al-Fe pillared bentonite was developed and tested as a heterogeneous catalyst for the photo-Fenton oxidation of phenol under solar light irradiation. The structural characteristics of the catalyst were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Al-Fe pillared clay exhibits higher basal distance than original bentonite. The effect of pH to the degradation of phenol and the reusability of the catalyst were addressed.The results of photocatalytic experiments indicate that the heterogeneous photo-Fenton process employing the Al-Fe pillared clay as a photocatalyst possesses a high potential for phenol degradation. During the photocatalytic reaction, the amount of iron leaching out increases as a function of time. Significantly lower concentration of Fe and Fe in the solution after the treatment could give a great advantage to the mixed Al-Fe pillared bentonite/H2O2 system over the homogeneous Fenton system. [DOI: 10.1380/ejssnt.2011.490]

Fe(OH) 2+ + hv → Fe 2+ + HO • (4) However, homogeneous Fenton reaction requires high concentration of Fe (II), and iron ions must be treated after used.In addition, low pH condition of this reaction makes it impractical because of the high cost of acidifying before treatment and neutralizing after treatment.Therefore, immobilizing transition metal ions to improve efficient of Fenton process is the objective of this project.
In order to decrease the price and increase the availability of catalysts, clay is one of the most promising carriers to immobilize metal ions.Pillared interlayered process is employed to immobilize metal ions into interlayers of clay.Pillared interlayered clay is used as catalyst to degrade organic pollutants in Fenton process.Clay can be pillared with only iron cation [5,6] or with the mixture of iron and other cations, such as aluminum [7][8][9], zirconium [10].In pillaring process, clays are swelled in water, and

Pillared solution
Pillared solution was prepared at 338K by adding NaOH solution gradually into AlCl 3 and FeCl 3 solution while continuously stirring until the molar ratio OH/(Al + Fe) is 2/1.The molar ratio Al/Fe was 8.5/1.5.The pillared solution was aged overnight.

Al-Fe pillared clays
A 2 wt% clay suspension was adjusted to pH 9.1 and swelled for 2 days.Pillared solution was added into clay suspension until achieving 10 mmol Al-Fe/1g clay.The suspension was stirred in 4 hours and aged for one day at room temperature.After that, the suspension was filtered and washed with deionized water repeatedly until completely eliminate chloride ion, then dried at 313, 343 and 383 K in 12 hour.Finally the dried solid was calcined at 673 K in 3 hours.

C. Phenol photocatalytic oxidation
The reaction was carried out in a sequencing batch reactor.300 mL of approximate 100 mg/L phenol solution was added to the 500 mL beaker, then 0.5 g pillared clay was added, and the suspension was stirred continuously to make uniform suspension.H 2 O 2 was added to the suspension at the beginning of solar light irradiation.The pH of the solution was adjusted by HCl and NaOH solution.Phenol and iron ion concentration was determined.

D. Analytical method
Catalytic characterization was investigated by X-ray diffraction method using D8 ADVANCE instrument (Bruker-Germany), SEM (JEOS JSM -5410 LV, Japan).Concentration of phenol was determined by spectrophotometric method using UV-VIS Novaspec II instrument (Germany) with 4-amino antipyrine as a color agent at 510 nm.Concentration of iron was measured using 1,10phenanthroline as a color agent.Cation exchange capacity of clay was measured by methylene blue adsorption method according to American Petroleum Institute.

A. Material characterization
Chemical composition and physicochemical properties of original bentonite and Al-Fe pillared clay were represented in Tables I, II, III and IV.
XRD patterns (Fig. 1) showed that interlayer distance  of bentonite modified by Fe/Al mixture was higher than that of original bentonite.From SEM patterns (Fig. 2), there is difference between pillared clay and original Bentonite.It proves that pillared clay was successfully synthesized.

B. Catalytic efficiency of Al-Fe pillared clay
Preliminary test proved that phenols can not be degraded by H 2 O 2 without Al-Fe pillared clay.Phenol removal caused by adsorption process on pillared clay was not significant in comparison with that caused by oxidation process.Experiments proved that unpillared bentonite can not degrade phenol.Experimental data of phenol conversion degree is presented in Table V and Fig. 3. Reaction condition: 129 mg/L phenol solution, room temperature, sunlight.
Figure 3 shows that Al-Fe/Bent material has high efficiency under sunlight and room temperature condition.This is a promising material to apply in practice to treat wastewater polluted by phenol in particular, and by organic pollutants, in general.

C. Effect of pH on heterogeneous Fenton reaction
Fenton reaction takes place rapidly at pH < 3 (if higher, Fe(II) is precipitated, so that the efficiency decreases significantly).To investigate the effect of pH on catalytic property of Al-Fe pillared clay, experiments were carried out at different pH.Reaction was carried out at room temperature with sunlight irradiation.Results are showed in Fig. 4. From obtained data it can be concluded that the catalytic efficiency of the material is significant when pH >3, while homogeneous Fenton process is not efficient.However, when increasing pH, the induction period increases, so that time for phenol to degrade completely is longer.When pH = 3, after only 90 mins, the phenol conversion is 93.80%, but when increasing pH to 3.5, 4 and 4.4, the conversion of phenol decreases to 88.73%, 86.53% and 6.42% respectively.Consequently, induction period is a function of pH.
Many scientists think that the induction periods occur in condensed phase, involving polymers [11], and it is not in solution state.Aqueous Fenton reactions catalyzed by pillared clay were observed several times [12], but their mechanism can not be clarified yet, because of lack of details and involvement of condensed phases and adsorption onto material surface.There can be two factors in response for induction periods: activation processes of surface iron by formation of complex with reactant surface before oxidation takes place; or the need of time to dissolve iron so that homogeneous Fenton reaction can occur.The second reason has less possibility because concentration of iron is small (<2.

D. Test on the iron leaching
During reaction, iron from pillared clay is dissolved into solution.Concentration of iron tends to decrease when increasing pH (Fig. 5).Iron keeps leaching even when phenol is completely degraded.At higher pH, amount of iron is smaller (at pH = 4.4, after reaction, concentration of iron is 0.22 mg/L).It shows that amount of iron leaching out and reusability not only depend on gradually oxidation between iron and phenol but also depend on pH.In this work, concentration of iron after 7 hours of reaction is smaller than 0.25 mg/L at pH 4.4.Consequently, the higher the pH, the smaller the leaching of iron.However, optimum pH is chosen as 4.0 because of the shortest conversion time (1.5 h) and the lowest concentration of iron (0.48 mg/L).

E. Reusability of Al-Fe -bent catalyst
The used material was collected, washed with large amount of deionized water and dried at room temperature.It was then reused as catalyst for Fenton reaction.Conditions were: 107.63 mg/L phenol solution, pH 4, room temperature, sunlight irradiation.Results are presented in Fig. 6.
It can be seen from Fig. 6 that Al-Fe-bent material still has catalytic activity, although it decreases.For new material, it takes 1.5 hours to completely degrade phenol, but for reused material, it takes 2.5 hours.

IV. CONCLUSION
Al-Fe pillared clay was successfully synthesized from Thanh Hoa bentonite and Al, Fe ions.Results proved that phenol treatment by adsorption is insignificant in comparison to that by catalysis process.The experimental data show that phenol degradation decreases while pH increases, and the optimum achieves at pH 4. Amount of iron leaching into wastewater strongly depends on pH, and it decreases sharply when pH increases.

TABLE I :
Mineral composition of enriched Thanh Hoa clay.

TABLE II :
Chemical composition of original Thanh Hoa clay.

TABLE III :
Chemical composition of enriched Thanh Hoa clay.

TABLE IV :
Interlayer distances of bentonite and modified bentonite.

TABLE V :
Interlayer distances of bentonite and modified bentonite.