This paper focuses on the use of pillared clays as catalysts for the Fenton-like advanced oxidation, specifically wet hydrogen peroxide catalytic oxidation (WHPCO). This paper discusses the limitations on the application of a homogeneous Fenton system, development of solid catalysts for the oxidation of phenol, advances in the synthesis of pillared clays, and their potential application as catalysts for phenol oxidation. Finally, it analyzes the use of pillared clays as heterogeneous Fenton-like catalysts for a real wastewater treatment, emphasizing the oxidation of phenolic compounds present in coffee wastewater. Typically, the wet hydrogen peroxide catalytic oxidation in a real effluent system is used as pretreatment, prior to biological treatment. In the specific case of coffee wet processing wastewater, catalytic oxidation with pillared bentonite with Al-Fe is performed to supplement the biological treatment, that is, as a posttreatment system. According to the results of catalytic activity of pillared bentonite with Al-Fe for oxidation of coffee processing wastewater (56% phenolic compounds conversion, 40% selectivity towards CO2, and high stability of active phase), catalytic wet hydrogen peroxide oxidation emerges as a viable alternative for management of this type of effluent.
One of the most effective technologies to remove organic pollutants from aqueous solutions is the Fenton’s reagent treatment [
Catalytic oxidation reactions are almost exclusively limited to transition elements because these may exist in more than one state of oxidation, making the establishment of a repetitive oxidation-reduction cycle possible [
A variety of solid catalysts among which are metal oxide (CuO and Cu/
Phenol and its compounds are extremely toxic to the environment. These pollutants are released into the surface water bodies by oil refineries, pulp and paper, pharmaceutical and pesticide industries, and by several other chemical plants [
Phenol is also relevant in the field of environmental research because it has been chosen frequently as a model pollutant. Much data is available on its removal and destruction, in particular with respect to wastewater treatments [
Widely studied methods for the removal of phenol include biological treatment [
Different solids are used in the wet hydrogen peroxide catalytic oxidation of phenol solutions, among which are pillared clays with Al-Cu [
One of the most popular catalysts for the oxidation of phenol with
In the search for other iron containing heterogeneous Fenton-type catalysts that have a low leaching of the active phase at pH 3-4, where the phenol oxidation with
Barrer and MacLeod first introduced the concept of transforming a lamellar solid into a porous structure by inserting laterally spaced molecular props between the layers of a smectite clay mineral [
The classical pillaring method involves two steps: first, the addition of the precursor polymer solution (pillaring agent) into the diluted clay mineral dispersion (intercalation). A second step is the thermal treatment of the intercalated clay mineral [
Variants of the traditional method have been researched in order to decrease the water volume and timing of the synthesis. Thus, it has become possible to decrease the volume of water by the use of concentrated suspensions, both clay and the pillaring agent [
Although, in the past 30 years, several studies highlighting some applications of industrial pillared clays have been published, such materials have not been used as commercial catalysts primarily due to the difficulty of extending the laboratory synthesis to an industrial scale [
A diversity of raw clays has been used for the preparation of these pillared materials, around 80% corresponding to bentonite-montmorillonite [
Bentonite-based catalysts for oxidation of phenol using Fenton-like AOPs.
Pillared bentonite | Year | Synthesis condition | Development | Reference |
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B-Fe |
2003 | Pillaring solution: dilute |
This is the first work using natural bentonite from Valle del Cauca. |
[ |
B-Fe |
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Al-Fe |
2005 |
Pillaring solution: dilute |
Addition of Ce during the synthesis of the catalysts show favorable results in |
[ |
Al-Ce-Fe |
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Al-Ce-Fe |
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Al-Ce-Fe |
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B-AlCe |
2008 | Pillaring solution: dilute |
The clay pillared with the Al-Ce-Fe mixed solution is a very active and selective catalyst in the phenol oxidation. | [ |
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AlCe24US | Pillaring solution: dilute |
Ultrasound considerably reduces the modification time of the pillared clays. The catalysts show physical-chemical and catalytic properties similar to those when synthesizing by the conventional method. | ||
AlCeUS24 | ||||
AlCeFe24US | 2008 | [ | ||
AlCeFeUS24 | ||||
AlCeFe2424 | ||||
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B1-AlFe | Pillaring agent: solid |
Using powdered clay and a solid pillaring agent, the water volume needed in the synthesis of pillared clays was reduced. | ||
B2-AlFe | 2008 | [ | ||
B3-AlFe | ||||
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AlCe |
Pillaring agent: solid |
The use of microwaves and ultrasound significantly reduces the synthesis time of pillared clays. | ||
MAl-Fe | ||||
UAl-Fe | 2009 | [ | ||
AlCeFe |
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MAl-Fe-Ce | ||||
UAl-Fe-Ce | ||||
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MAlFe |
Pillaring solution: dilute |
The use of microwaves and ultrasound significantly reduces the synthesis time of pillared clays. | ||
MAlFe |
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UAlFe |
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UAlFe |
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MAlCeFe |
2009 | [ | ||
MAlCeFe |
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MAlCeFe |
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UAlCeFe |
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UAlCeFe |
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UAlCeFe |
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B2R, BT21 | Pillaring solution: dilute |
The use of microwaves significantly reduces the synthesis time. Additionally, the use of a clay suspension (30%) and dry clay reduces the amount of water used in the synthesis. | ||
B21, B22 | 2009 | [ | ||
B301, B302 | ||||
BD1, BD2 | ||||
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B-Al US | Pillaring agent: Solid |
With the methodology here used, the volume of water needed and the synthesis time of pillared clays were reduced. | ||
B-AlFe |
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B-AlFe |
2009 | [ | ||
B-AlFe |
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B-AlFe | ||||
B-AlCeFe |
The first pillared bentonites with Al-Fe and Al-Ce-Fe were synthesized using the conventional method of diluted pillaring solution and diluted suspension of clay (2 wt%), which involves the use of large volumes of water and long synthesis times. The pillared clays were very efficient catalysts in the reaction of a phenol oxidation in diluted aqueous media under mild experimental conditions (25°C and atmospheric pressure), as well as in the elimination of different intermediary compounds of the reaction, reaching high-mineralization levels. The catalysts showed high stability in the reaction media due to the strong interaction between the iron species and the catalyst support. The incorporation of cerium showed a favorable effect in pillaring of the materials, allowing the increase of the basal spacing and enhancing the catalytic activity of the catalysts [
To expand the pillaring process to an industrial scale, it was necessary to simplify procedures and optimization of the unit operations involved, particularly to decrease the volume of water and synthesis times. In this regard, Pérez et al. used ultrasound for aging and intercalating of pillared bentonite with Al-Ce and Al-Ce-Fe. The use of ultrasound showed a clear effect in the synthesis of this type of solids allowing the synthesis in a shorter time and preserving the physical-chemical characteristics as well as catalytic activity in the oxidation reactions [
With a procedure similar to that developed by Aouad et al. [
The synthesis of pillared clays by mixed systems Al-Fe and Al-Ce-Fe in a concentrated medium allows a 90%–95% decrease in the volume of water and a reduction in the intercalation times between 70% and 93% with respect to the conventional synthesis. The pillared clays using this new methodology show a catalytic activity and selectivity comparable to those of solids synthesized by the conventional method in a dilute medium [
The commercial use of PILCs based catalysts not only depends on the optimization of the synthesis, but also on the ability to shape the powder material into pellets, agglomerates, and so forth. These materials should keep their chemical properties, reactivity, and stability in the reaction medium. The manufacture of pellets, Raschig rings and monoliths, that involve pillared montmorillonites has basically been accomplished by the use of extrusion techniques [
With the developments in the synthesis of pillared clays in a concentrated medium, the manufacture of extruded materials with Al-Fe Al-Ce-Fe pillared bentonites was achieved. We found that the most adequate composition of the mixture of poly(hydroxo metal) bentonite/binder/water for the manufacture of extrudates with B-AlFe and B-AlCeFe was 42/28/30. The use of poly(hydroxo metal) bentonite (dried at 60°C) and not the pillared bentonite (after calcination at 400°C) considerably improved the mechanical stability of the extrudates. The mechanical resistance of B-AlFe and B-AlCeFe based extrudates depended on the calcination temperature. At 500°C, good mechanical strength and sufficient stability to immersion in water were achieved. The extrudates largely preserved the structural and textural properties of Al-Fe and Al-Ce-Fe-PILC and retained the catalytic properties of powdered pillared clays [
Advanced oxidation processes (AOPs) are based on physical and chemical processes capable of fundamental changes in the chemical structure of contaminants, as they involve the generation and use of transitory species with a high oxidation power, mainly the hydroxyl radical
Oxidation processes using hydrogen peroxide as oxidant have turned out to be a viable alternative for the wastewater treatments of medium-high total organic carbon concentrations. Furthermore, iron is an abundant and nontoxic element; hydrogen peroxide does not give origin to any harmful by-products and it is a nontoxic and ecological reactant [
Among the different materials used as support for oxidation reaction in a liquid phase, pillared clays represent around 7% in the existing literature. However, the interest for pillared clays has increased substantially in the last decade, given their use in different oxidation processes such as WHPCO and photocatalytic oxidation [
Pillared clays with Al-Fe and Al-Cu are promissory catalysts for wet hydrogen peroxide oxidation of phenol, because they combine a porous support and active sites for the adsorption of organic compounds in the activation of
In general, pillared clays with Al-Fe are efficient in phenol elimination under mild experimental conditions (atmospheric pressure and room temperature) without considerable leaching of metal ions [
Some examples of pillared clays-based catalysts for wet hydrogen peroxide oxidation of phenol.
Clay catalyst | Operation conditions | Best performances reached | Reference |
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Fe-Al pillared clays |
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[ |
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(Al-Cu)-pillared clays |
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[ |
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B-AlFe |
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[ |
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B-AlCeFe | As above |
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[ |
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AlCe24US |
As above |
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[ |
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AlCeFe24US |
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AlCeFeUS24 | As above | [ | |
AlCeFe2424 | |||
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Fe/Al-PILC |
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[ |
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B-Fe |
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[ |
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B-Cu |
As above |
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[ |
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(Al-Fe)-pillared clays |
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[ |
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B-AlFe extruded |
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[ |
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B-AlCeFe extruded | As above |
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[ |
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FAZA (Al-Fe) |
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[ |
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Al-Fe |
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[ |
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Al-Ce-Fe |
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Al-Ce-Fe |
As above | [ | |
Al-Ce-Fe |
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FAZA-1 |
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[ |
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Al/Fe-PILC |
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[ |
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MAlFe |
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MAlFe |
[ | ||
UAlFe |
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UAlFe |
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MAlCeFe |
As above |
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MAlCeFe |
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MAlCeFe |
[ | ||
UAlCeFe |
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UAlCeFe |
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UAlCeFe |
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B1-AlFe | As Above |
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B2-AlFe | [ | ||
B3-AlFe | |||
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Fe0.8Al12.2-PILC |
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[ |
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Al-Fe-PILC |
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[ |
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Extrudates Al-Fe pillared clay |
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FAZA powder |
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[ |
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FAZA |
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[ |
Cerium (Ce) was introduced into pillared clays with Al and Al-Fe, improving the metallic dispersion properties, increasing the pillars resistance [
Most research on the use of pillared clays for the oxidation of phenol was conducted with the powdered catalyst and a few with pellets of Al-Fe pillared clay [
Extrudates of an Al-Fe pillared clay catalyst suitable for packed-bed operations are evaluated for oxidation of phenol using hydrogen peroxide as the oxidant. The reaction was processed in a semibatch basket reactor under rather mild conditions. Operational parameters were studied under the following conditions: temperature from 25 to 90°C, atmospheric pressure, initial phenol concentration from 100 to 2000 ppm of the liquid phase, catalyst loading from 0 to 10 g/L, and input
Extrudates manufactured with pillared bentonites were also employed in phenol oxidation at 25°C and atmospheric pressure. Extrudates with B-AlFe and B-AlCeFe reached 100% conversion of phenol and TOC conversion between 30% and 62%, after 9 h of reaction. Once the reaction was completed (9 h), the catalyst was removed and the leachate was recovered and analyzed, showing Fe values in the range of 0.11–0.14 ppm. The outstanding differences in time for the phenol conversion and TOC of extruded materials compared to the powdered materials were a consequence of the agglomeration process and the inherent diffusional limitations [
The interest for pillared clays has dramatically increased in the last years, in great part due to their potential applications as catalysts. Metals incorporated in the pillared clay structure are crucial and make them suitable for a number of different applications, most of them belonging to the so-called “green chemistry” [
Table
Some examples of WPHCO of water pollutants (model molecules) or real effluents, using a Fenton reaction catalyzed by pillared clays.
Clay catalyst | Organic pollutant | Performances reached | Reference |
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Al/Fe-PILC | Treatment of municipal leachate from landfill by catalytic wet peroxide oxidation using an Al/Fe-pillared montmorillonite |
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[ |
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Cu-PILC |
Phenolic compounds (p-coumaric and p-hydroxybenzoic acids) and real olive oil milling wastewater (OMW) |
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[ |
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Cu/Al-PILC | Tyrosol, major compound of the polyphenolic fraction present in olive oil mill wastewaters |
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[ |
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(Al-Fe)PILC | Polyphenols in olive mill wastewater (OMW) |
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[ |
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Cu-PILC | Real wastewaters from agro-food production (Citrus juice production and olive oil milling wastewater) | Citrus juice production wastewater |
[ |
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Cu-AZA | Wet hydrogen peroxide catalytic oxidation of olive oil mill wastewaters using a Cu-pillared clay catalyst |
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[ |
Most treatment studies of OMW are focused on aerobic biodegradation or anaerobic digestion of that material [
Tyrosol, p-coumaric, and p-hydroxybenzoic acids have also deserved the attention of some researchers because they are representative compounds of the polyphenolic fraction typically found in olive processing wastewater [
Coffee is one of the largest agricultural-based products sold in international markets and has become the second best-marketed product worldwide, surpassed only by oil [
The coffee fruit consists of a coloured exocarp (skin), a fleshy, yellowish-white mesocarp (pulp), mucilage layers (covering the two beans) and two coats (the first, a thin, fibrous parchment and the second, a fine membrane, silver skin) [
The mucilage, a by-product of coffee wet processing, is primarily composed by pectin, sugars, water [
To determine the biological impact generated by coffee wet processing wastewater, the National Coffee Research Center of Colombia—Cenicafé conducted bioassays using
To complement the biological treatment, an integrated biosystem that uses macrophytes for posttreatment for coffee wet processing wastewater has been proposed, in order to generate the least negative impact on the coffee aquatic ecosystem [
Considering that coffee wastewater is highly biodegradable, biological treatments have been widely used [
The effluent from the anaerobic biological treatment of coffee wet processing wastewater (CWPW) developed in the National Coffee Research Center-Cenicafé still contains a nonbiodegradable compound that must be treated before it is discharged into a water source. It was found that chlorogenic acid, caffeic acid and tannins are toxic compounds because they inhibit the process of methanogenesis and limit the biodegradability of water during anaerobic digestion [
The physical chemical characterisation of the CWPW is detailed in Table
Physicochemical parameters of coffee wet processing wastewater before and after biological treatment in SMTA.
Parameters | Water characteristics | |
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Beforea | Afterb | |
pH | 4.2 | 6.5 |
BOD5 (mg O2/L) | 2,990 | 284 |
COD (mg O2/L) | 6,580 | 551 |
COD/BOD | 2.2 | 1.9 |
Total suspended solids (mg/L) | 435 | 904 |
Acidity (mg/L) | 413 | 72 |
Total phenols (mg/L) | ND | 165 |
aAnalysis conducted at Cenicafé.
bAnalysis after biological treatment in hydrolytic acidogenic and methanogenic reactors (SMTA).
ND: Not determined because the sample contained a large amount of reducing sugars that generated interference and overestimated the total phenols content.
A preliminary analysis of the phenolic compounds in CWPW was performed by HPLC-ESI-MSn (high-performance liquid chromatography/electrospray ionization multiple mass spectrometry) by using a Shimadzu Liquid Chromatograph-Ion Trap-Time of Flight Mass Spectrometer (LCMS-IT-TOF, Kyoto, Japan). To conduct the analysis, 100 mL of WPCW were filtered through Millipore filters (0.45
Considering the results of the analysis of phenolic compounds in CWPW, the catalytic activity of the pillared bentonite with Al-Fe was evaluated in the oxidation reaction of three phenolic acids which were used as model molecules, using a similar procedure as that reported in previous studies for phenol [
Structure of phenolic acids.
Traditionally, the tests with phenol as a model system are performed with 40 mg/L but, for 3-CQA, a higher concentration was used, given that 3-CQA is one of the most abundant compounds in the CWPW [
Wet hydrogen peroxide catalytic oxidation tests of Al-Fe-PILC with chlorogenic (3-CQA), caffeic, and ferulic acids showed a beneficial effect when incorporating Fe in the bentonite, both with respect to the phenolic acids conversion (Figure
Phenolic acids conversion with AlFe-PILC.
Total organic carbon (TOC) conversion with AlFe-PILC.
The high conversion of chlorogenic acid can be explained by the presence of two electron-donating groups (as caffeic acid) and by three additional hydroxyl groups in the ester moiety. An example of the relationship between the chemical structure and its reactivity with hydroxyl radical would be the wet hydrogen peroxide photo-oxidation of p-coumaric, vanillic, ferulic, and caffeic acids when catalysed by (Al-Fe)-PILC, in which the degradation of phenolic compounds was 28%, 50%, 58%, and 86%, respectively [
The reaction scheme for evaluating the catalytic activity of AlFe-PILC in the oxidation of the CWPW was described in Figure
Diagram of the reaction system.
The AlFe-PILC achieved a high conversion rate of total phenolic compounds (56%) and mineralization towards
Catalytic oxidation of coffee wastewater with AlFe-PILC (continuous line) and Bent (dotted line). Total phenols conversion (star) and TOC conversion (diamond).
The combination of the two treatment methods, biological (developed by Cenicafé) and catalytic oxidation with AlFe-PILC, achieved a 96.7% reduction of COD in CWPW. These results are higher than those obtained when coffee processing wastewater was treated by coagulation-flocculation in combination with advanced oxidation processes (UV/
The pillared clays have received considerable attention in the last decade as catalysts for wet hydrogen peroxide oxidation of phenolic compounds because of their high activity (conversion of phenolic compound and degree of mineralization) and environmental compatibility (low cost, easy recovery of the catalyst, are reusable catalysts, oxidation that can be carried out at room temperature and atmospheric pressure, and that they show high stability of the active phase in the reaction medium). With the developments in the synthesis of pillared clays, it has been possible to reduce the water volume and synthesis times, and technical requirements for these materials can be useful at industrial level.
Given the excellent properties of pillared clays in the WHPCO for phenol and other model molecules, as well as real wastewater treatment, this advanced oxidation process can be integrated with biological process, as a pre- or posttreatment, depending on the physicochemical characteristics of the wastewater.
The high conversion of phenolic compounds, the selectivity to
The authors are grateful for the financial support provided by CSIC-COLCIENCIAS and DIB of Universidad Nacional de Colombia, Bogotá, for the development of several research projects conducted over the past 10 years. The authors also appreciate the cooperation received from the National Coffee Research Center—Cenicafé.