CHEMICALLY MODIFIED CLAYS USED FOR ENVIRONMENTAL QUALITY

In this work, catalysts were synthesized based on chemically modified clays through pillaring process with Al (III) and Fe (III) using commercial bentonite and acidtreated montomorillonite like KSF montmorillonite and K10 montmorillonite from Aldrich, as raw materials. The raw materials and chemically modified clays were characterized by: BET (Brunauer-Emmet-Teller) method and XRD (X-ray diffraction). Afterwards these catalysts were tested during wet oxidation with peroxides using phenol as target compound. Experimental results show that chemically modified clays with Fe(III) presented a higher catalytic activity in Fenton like processes than Al-Fe clays.


INTRODUCTION
The urban distribution of qualitative water for human consumption, domestic activities and industrial use is a fundamental objective concerning the sustainability of life quality for the population.However, many industrial processes generate a series of pollutants, which have a negative impact on ecosystems and on humans (toxic, carcinogenic and mutagenic properties).Particularly, phenol is considered as one of the most toxic pollutants, causing an increase in the demand of oxygen in water sources and an unpleasant taste and smell in drinking water even when it is found in very small quantities [1,2].Phenols are present in wastewaters from various industries, such as refineries (6 -500 mg/L), coking operations (28 -3900 mg/L), coal processing (9 -6800 mg/L), and manufacture of petrochemicals (2.8 -1220 mg/L).Phenols are also the main organic constituents present in condensate streams in coal gasification and liquefaction processes.Other sources of waste steam containing phenols are pharmaceutical, plastics, wood products, paint, and pulp and paper industries (0.1 -1600 mg/L) [3].
As a particular case we can mention olive oil mill wastewaters, rich in phenol and polyphenol that give rise derivatives to relevant phytotoxicity.Phenol-containing wastewaters must not be released into open water sources without treatment because of the toxicity of phenol.Due to the toxic nature of some of these compounds the Environmental Protection Agency has set a water puri cation standard of less than 1 part per billion (ppb) of phenol in surface waters [3].
Catalytic wet peroxide oxidation (CWPO) is part of the advanced oxidation processes (AOP) and is an efficient process because H 2 O 2 is a strong oxidative agent.Pillared clays are successfully used as catalysts in these processes.Study on chemically modified clays by pillaring in Fenton and Fenton like processes were investigated due to its low cost, mild conditions (room temperature, atmospheric pressure) and also the lack of toxicity for the environment of clays, hydrogen peroxide and iron salts [1,3,4].Among the catalysts used in Fenton (Fenton like) reactions the pillaring clays presented a high interest in the late years due to their special texture and catalytic properties.This catalyst confers a high catalytic activity for phenol conversion and moreover, it can be reused for many times, without losing its catalytic activity [5].The pillared clays synthesis method at a laboratory level is essentially based on the mixture of a clay diluted suspension (2 %) with a pillaring solution, a process generally involving considerable time and high volumes of water [6].The current trend in the pillaring process focuses on the synthesis of concentrated suspension, both for clay and for the pillaring agent.Schoonheydt and Leeman synthesize Al -saponite through the addition of clay dust to the aluminum pillaring solution [7].Storaro have used 50% w/w chlorhydrol to obtain Al and Al-Fe-pillared clays from 50 % w/w aqueous or acetone suspensions of bentonite [8].Vicente and Lambert have described a new synthetic pillaring path, by using a dialysis membrane, clay dust and small volumes of a pillaring solution.On the other hand, clays modified with mixed pillars offer a great potential as catalysts in several reactions of environmental interest, due to the possibility of introducing metal cations with properties leading to the desired reaction [9].
Several studies have been reported in the literature using iron as the pillar agents' source.Fe -pillared intercalated layered clays (PILC S ) show magnetic and high catalytic activity.The disadvantage of Fe -PILC s is the small basal spacing.However, in some works, delaminated structured PILC S with basal spacing values higher than 1.4 nm were obtained at room temperature and upon calcinations a decrease in d-spacing occurred [10][11][12][13].
In this work, catalysts were synthesized based on chemically modified clays through pillaring process with Fe (III) and Al (III) -Fe (III).Afterwards these catalysts were tested by wet oxidation with peroxides using phenol as target compound.The catalytic activity was evaluated by rate constant, considering a first order reaction.Verifying the hypothesis that the reaction is of order 1, we observed that the value of the rate constant K increases with the decrease of the phenol concentration.

Catalyst preparation
The clays used in this study (KSF and K10) were homoionizated with a NaCl (Lach-ner).For Na-bentonite clay, the stage of homoionization with sodium chloride solutions was eliminated, because this is natural sodium clay.The polyhydroxocationic solution that contained Fe (III), was prepared by drop-wise addition of a NaOH solution (Lach-ner) over a FeCl 3 solution, at room temperature under vigorous stirring.The preparation method for chemically modified clays by pillaring only with iron (4 mmoles Fe/g clay) was detailed by authors in another paper [14].
The polyhydroxocationic solution that contained Fe and Al, was prepared using a molar ratio 5:1 (AlCl 3 :FeCl 3 ) that according to literature report [15] was considered optimal.The intercalation was realized at 333 K.After ageing through microwave irradiation for 10 minutes, the intercalation of the pillars is improved by repeated washing with water until chloride ions were eliminated.In order to stabilize the internal structure of the intercalated products the clay was dried in steps at 393 K.The calcination was carried out after drying at 673 K for 2 h.

Characterization of the solids
The raw clays and pillared clays were characterized determining the specific surface area through BET method, using nitrogen adsorption at 77 K and a Quantachrome Autosorb-1-MP surface area and porosity analyzer and also through X -ray diffraction (XRD) study (Siemens D5000 diffractometer using CuK radiation -filtered with Ni, = 0.15401 nm).Table 1 shows the surface areas of the raw materials and the materials obtained after pillaring.The specific surfaces of the Fe -pillared clays, shown in Table 1, were characterized in another comparative study [5].

Catalytic evaluation
Solids modified with the Fe and Al-Fe polymeric precursor were evaluated in the phenol oxidation reaction in diluted aqueous medium according to the catalytic test procedure reported in prior works [5].Phenol concentrations were measured through 4 -aminoantipyrine spectrophotometer method using a Thermo Spectronic Genesys 20 spectrophotometer.The detection limit according to SR ISO 6439 is 0.005 mg/L [16,17].
The catalytic activity of the pillared clays in the phenol oxidation reaction in diluted aqueous medium is represented by Fenton like reaction (Fe 3+ /H 2 O 2 ) and is evaluated by catalytic coefficient considering the reaction of first order.The mechanism of the Fenton like process includes the reactions reported in other studies [5].

RESULTS AND DISCUSSION
According to the basal distance (d 001 ) values obtained by XRD analysis, that are presented in Figure 1, Figure 2 and Figure 3, the incorporation of Fe +3 and Al +3 cations in the structure of clays type K10, KSF and Be-Na did not positively influence because the values of the basal distance of new structure are smaller than the basal distance of the raw clay.The pillaring processes determine an increase of the basal distance and surface area in the clay's structure [1,6,9].The results obtained in this study showed a decrease in the values of the basal distance for the pillared clays.Also the surface area values for the chemically modified clays by pillaring (M 0 -10 Be-Na-Fe) increased more than three times comparatively to the raw material.The decrease of the basal distance values according to Figure 1 2 and 3 may be the result of a structural deformation, due to calcinations at a high temperature or to failure of the dialysis step during the chemical modification of clays, step which according to scientific results from specialized literature [18], is necessary because it favors classic cationic exchange between Na + and Keggin Al 13 7+ ions.Also the decrease of the basal distance may be caused by the failure in obtaining Fe 3+ pillars in the chemically modified clays by pillaring confirming thereby previous data found in specialized literature at a series of authors that expressed skepticism regarding the ability of Fe ions to produce pillared clays [19,20].

Degradation efficiency of phenol
The conversion degree of phenol depending on the reaction time was achieved at constant values of the following factors: temperature (T = 298 K), initial concentration of phenolic solution (C 0 phenol = 250 mg/L), ratio solid/liquid (R 1 = 5 g/L), ratio H 2 O 2 /phenol (R 2 = 14 mol/mol), pH of the solution (pH = 3.5).The catalytic activity of the catalyst obtained from sodium bentonite showed favorable results in the degradation process of phenol.The catalysts obtained from sodium bentonite (M 0-10 Be-Na-Fe), showed a high catalytic activity.The total degradation of phenol took place after only 50 minutes.The values for phenol concentrations in time are presented in Figure 4.The values of samples tested only with iron shown in Figure 4 were previously mentioned by authors in another comparative study [5].
The high specific surface area of the acid -treated montmorillonite clays (K10, KSF) didn't lead to higher catalytic activity in Fenton like processes.That is probably why the chemically modified clays by pillaring (catalysts) obtained from these materials hindered the total degradation of phenol.Figure 4 shows that the only catalyst that determined a total conversion of phenol in just 50 minutes is the one obtained from sodium bentonite with Fe (4 mmoles Fe/g clay).Clays pillared with Fe -Al (l.67 mmoles Fe and 8.33 mmoles Al) did not lead to a total conversion of phenol after 50 minutes.By testing the catalysts obtained from Be-Na (M 0-10 Be-Na-Fe-Al), KSF (M 0 -10 KSF-Na-Fe-Al) and K10 (M 0-10 K10-Na-Fe-Al) by pillaring with Fe-Al, a 99 % conversion degree of phenol was achieved after 60 minutes.

Effect of catalyst concentration on phenol degradation
The effect of catalyst concentration on phenol degradation probably depends on the quantity of iron incorporated in clay interlayers.The incorporation of an important quantity of stabilized iron was possible probably only in the catalyst M 0-10 Be-Na-Fe (obtained from sodium bentonite and chemically modified only with iron).That is probably why using this catalyst the degradation of phenol was achieved in only 50 minutes (Figure 4).The success of the pillaring process with iron is sustained by the results obtained through BET method (Table 1).
A total conversion of phenol was achieved at concentrations of 5 g/L, 10 g/L, 15 g/L and 20 g/L M 0-10 Be-Na-Fe catalyst.The experimental data reveals that a M 0-10 Be-Na-Fe catalyst concentration of 5 g/L is enough for the total degradation of phenol from a synthetic solution containing 250 mg/L phenol, in maximum 50 minutes reaction [5].

Kinetics of phenol degradation. Rate constant
The catalytic activity was evaluated by the rate constant, considering a first order reaction.To check this hypothesis, the rate constant was determined for each material synthesized and mean square deviation for which a significant decrease of phenol concentration was achieved in 60 minutes was calculated.The mean square deviation values (R 2 ) that represent a key indicator for the estimation of errors are in all cases > 0.96 which confirms that the reaction is of first order (Figure 5, Figure 6).Verifying the first order kinetics hypothesis that the reaction of order 1 it can be observed that the value of the rate constant k increases with the decreasing of the phenol concentration.Thus the catalysts obtained from sodium bentonite present higher values for k then the catalysts obtained from montmorillonite KSF.

CONCLUSIONS
Using the method proposed in the present study in which clays were modified by pillaring, using as raw material sodium bentonite the incorporation of an important amount of stabilized iron is possible.A small dose of catalyst (5 g/L) obtained by pillaring Na -bentonite with Fe 3+ , determined a total conversion of phenol after only 50 minutes.For the clays pillared only with iron the ratio Fe/clay (mmoles/g) was 4. The polyhydroxocationic solution used for clay intercalation with Al and Fe, was prepared using a molar ratio of 5:1 (AlCl 3 : FeCl 3 ) with 8.33 mmoles Al and 1.67 mmoles Fe.Higher iron content in the pillaring solutions makes a high surface area while higher aluminum content causes a small surface area.The catalytic activity in a Fenton like reaction of the pillared clay depends on the amount of accessible iron species incorporated.

Table 1 .
The surface areas of raw materials and materials obtained after pillaring.