A new green approach to Fenton’s chemistry using tea dregs and coffee grounds as raw material

1 Introduction

Coffee and tea are the mostly consumed beverages in the world, and the coffee grounds and tea dregs which are waste materials thereof, are daily produced in large amounts throughout the world. The world production of coffee green beans and tea was estimated to be 7.42×10⁹ and 3.52×10⁹ kg, respectively [1]. During the entire coffee processing chain, several residues are obtained. These residues can be divided into two categories: those generated in the producing countries, representing >50% of the coffee fruit mass, and those produced in the consuming countries after beverage preparation. Finding a use for these residues should be desirable. More sustainable uses for coffee and tea residues are increasingly important. Over the past years, several strategies have been tentatively applied, particularly in the producing countries where the direct discarding of these residues has been the cause of numerous environmental problems over decades. In particular, efforts are being made to implement adequate disposal approaches and potential reuses, including horticultural, animal feed, biodiesel, fuel pellets, or activated carbons [2–5]. In an in vitro experiment, a ‘green’ one-step approach has been developed for the synthesis of nanoscale zero-valent iron particles at room temperature [6]. In some of our previous works [7, 8] we showed that coffee and tea residues enriched with ferrous sulfate and zinc sulfate are effective in increasing iron and zinc in edible parts of leaf radish and rice. However, actually, the spent coffee and tea materials are used only in composts, deodorants and the like and a new recycle method for these materials is necessary.

In this work, we tried to use ferric iron (Fe³⁺) and coffee grounds or tea dregs as raw materials to develop a new ‘green’ iron catalyst material and to test it as iron fertilizer and as a catalyst on the Fenton’s reaction for degradation of methylene blue (MB), and disinfection of Escherichia coli (E. coli).

2 Materials and methods

2.2 Methods

2.2.1 Synthesis of ‘green’ iron catalysts

Figure 1 shows the production processes of the catalysts. Used tea leaves or coffee grounds (88 g) were mixed with 12 g of ferric chloride anhydrous (FeIIICl₃) and 300 ml of water. The mixture was heated at 98°C for 24 h and dried at room temperature. Then, it was ground before use. The Fe catalysts developed from used tea leaves were named TEA-Fe and those developed from coffee grounds were named CAF-Fe.

Figure 1

Photographic image of the reagents: (A) ferric chloride anhydrous solution (FeIIICl₃) with (B) tea dregs or coffee grounds and (C) the product of the reaction. The detection of reduced Fe was made by adding 2,2′bipyridyl to the C solution. The red color indicates the presence of ferrous iron.

2.2.2 Chemical characterization of iron catalysts

The chemical composition of used tea leaves and coffee grounds are described by Morikawa and Saigusa [7]. The surface morphology of the Fe catalysts was characterized by a scanning electron microscope (Hitachi s4800, Tokyo, Japan) and the chemical state of iron on both materials was measured by X-ray photoelectron spectroscopy (Thermo Fisher Scientific K-Alpha, MA, USA). The amount of Fe²⁺ and Fe³⁺ in the Fe catalysts was measured by a colorimetric method using 2,2′-dipyridil [9] by a spectrophotometer at 525 nm. The total Fe in plant tissues and other samples was analyzed by inductively coupled plasma optical emission spectroscopy (Thermo Fisher Scientific iCAP, Tokyo, Japan). The total amount of hydroxyl radicals generated during 60 s of reaction was measured by chemiluminescence (ATTO Octa AB-2270, Tokyo, Japan) [10].

2.2.3 Treatment protocols

2.2.3.1 Test the ‘green’ iron catalysts as iron fertilizer

A pot experiment was conducted in a greenhouse to evaluate the ‘green’ iron catalysts as fertilizer using a horizon C of Andosol (ammonium bicarbonate – DTPA extractable Fe of 12.6 mg kg^-1) with soil pH adjusted to 6.5 and an alkaline soil (ammonium bicarbonate – DTPA extractable Fe of 12.6 mg kg^-1) with original pH of 9.2. Pack-choi (Brassica rapa var. chinensis) was chosen as the test plant due to the high ascorbic acid content that enhances the absorption of iron by humans. Increased amounts of the new ‘green’ iron catalysts TEA-Fe and CAF-Fe of 0 (control), 0.25 and 0.5 g kg^-1 air dried soil were applied. The treatments were named as follows: no application or control (CNT), 0.25 g kg^-1 dry soil of TEA-Fe (TEA-Fe 1) or CAF-Fe (CAF-Fe 1) and 0.5 g kg^-1 dry soil of TEA-Fe (TEA-Fe 2) or CAF-Fe (CAF-Fe 2). For all treatments, NPK was applied at the rate of 0.5, 0.5 and 0.5 g plant^-1. Plants were harvested after 60 days of growth.

2.3.3.2 Degradation of MB

Ferrous chloride tetrahydrate (FeII), Ferric chloride anhydrous (FeIII), FeEDTA, ‘green’ iron catalyst developed using tea dregs (TEA-Fe) and ‘green’ iron catalyst developed using coffee grounds (CAF-Fe) were tested for the Fenton’s reaction. The pH of the MB:H₂O₂ solution was 4.5. All of the experiments were carried out at 25°C. The treatments described in Table 1 were performed in triplicate.

Table 1

Volume of each reagent used in the degradation treatments.

Treatments1 name	0.017 mm Methylene blue (MB)(A)	100 mm Hydrogen peroxyde (HP)(B)	Deionized water(C)	1 mm Fe solution					Total volume
				Ferrous chloride tetrahydrate(D)	Ferric chloride anhydrous(E)	FeEDTA(F)|	CAF-Fe|(G)	TEA-Fe(H)
	ml
HP	80	10	10	–	–	–	–	–	100
FeII+HP	8O	10	–	10	–	–	–	–	100
FeIII+HP	80	10	–	–	10	–	–	–	100
Fe-EDTA+HP	80	10	–	–	–	10	–	–	100
CAF-Fe3+HP	80	10	–	–	–	–	10	–	100
TEA-Fe4+HP	80	10	–	–	–	–	–	10	100
CAF-Fe	80	–2	10	–	–	–	10	–	100
TEA-Fe	80	–	10	–	–	–	–	10	100

¹The order of addition of the reagents was of A+B+C+D+E+F+G+H to the microplate wells.

²The symbol ‘–’ means no addition of the respective reagent.

³CAF-Fe: ‘green’ Fe catalysts developed using coffee grounds.

⁴TEA-Fe: ‘green’ Fe catalysts developed using tea dregs.

Stock solutions of TEA-Fe and CAF-Fe were prepared dissolving 2.8 g of each catalyst in 1 l of deionized water and filtering in a 5A filter. The obtained solution contained 1 mmol l^-1 Fe as TEA-Fe and CAF-Fe catalysts. Stock solutions containing 1 mmol l^-1 Fe as ferrous chloride tetrahydrate, ferric chloride anhydrous and FeEDTA were prepared for the experiment. Eighty microliters of a 0.017 mmol l^-1 MB solution were added to a microplate of 96 wells. Then, 10 μl of 0.1 mmol l^-1 H₂O₂ and 10 μl of each Fe catalyst material were sequentially added to the solution, giving a final concentration of 10 mmol l^-1 H₂O₂, 0.1 mmol/l Fe and 0.014 mmol l^-1 MB. The absorbance of MB was determined spectrophotometrically (Thermo Fisher Scientific, Multiskan FC, Tokyo, Japan) at 650 nm. The measurements were made every 2 min of reaction, during a total of 30 min. The samples were mixed 5 s prior to the absorbance measurements.

2.4.3.3 Disinfection of E. coli

Pure E. coli ATCC11229 isolates were incubated in LB Broth Miller medium for 48 h at 37°C in a shaker table. A mixed cell culture was collected in order to determine the initial cell concentration and dilutions of 10^-4 to 10^-6 were spread in LB agar Broth medium-containing Petri dishes. Then, all plates were incubated for 24 h at 37°C and the colony forming units (CFU) per milliliter were counted. The LB Broth Miller medium was prepared adding the following to 800 ml deionized water: 10 g Bacto-tryptone, 5 g Bacto-yeast extract and 10 g NaCl. Then, the pH was adjusted to 7.5 with NaOH, the volume adjusted to 1 l with deionized water and sterilization by autoclaving at 120°C for 20 min was performed. The LB agar Broth medium was prepared adding agar to the LB Broth Miller medium before the autoclaving step. About 10 ml of LB agar was spread on the Petri dishes for bacteria incubation.

The disinfectant experiment was performed in 1.5 ml Eppendorf tubes, using the ‘green’ iron catalysts developed from tea dregs and coffee grounds. The E. coli ATCC11229 disinfection capacity of HP with TEA-Fe or CAF-Fe was compared to that of HP alone. Table 2 shows the treatments. After 10 min of contact time, 100 μl of 10^-1, 10^-2 and 10^-3 dilutions were inoculated into E. coli 3 M Petrifilms, incubated at 42°C for 48 h and then counted.

Table 2

Volume of each reagent used in the disinfection treatments.

Treatments1	108E. coli solution(A)	100 mm Hydrogen peroxide (HP)(B)	Deionized water(C)	10 mm Fe solution		Total volume
				CAF-Fe(D)	TEA-Fe(E)
	μl
Control	10	–2	990	–	–	1000
HP	10	100	890	–	–	1000
CAF-Fe3+HP	10	100	790	100	–	1000
TEA-Fe4+HP	10	100	790	–	100	1000

¹The order of addition of the reagents was of A+B+C+D+E.

²The symbol ‘–’ means no addition of the respective reagent.

³CAF-Fe: ‘green’ Fe catalysts developed using coffee grounds.

⁴TEA-Fe: ‘green’ Fe catalysts developed using tea dregs.

3 Results

3.1 Chemical characterization of iron catalysts

Figure 2 shows the scanning electron microscopy (SEM) images of (a) TEA-Fe and (b) CAF-Fe catalysts, and Figures 3 and 4 show the X-ray photoelectron spectroscopy (XPS) survey on the Fe2p3/2 and O1s regions. SEM images of the surface of both catalysts showed that they are composed of particles of variable sizes and shapes. For the Fe2p3/2 spectrum, the binding energy of the main peak was located at aproximately 711 eV and 725 eV, which are attributable to the ferric (Fe³⁺) and ferrous (Fe²⁺) iron. The absence of peak at approximately 707 eV suggests that both materials do not contain zero-valent iron. The photoelectron peak of O1s in Figure 4 can be decomposed into three separate peaks at 533 eV, 532 eV and 530 eV, representing the binding energies of oxygen in O-C, C=O and metal oxides chemically or physically adsorbed.

Figure 2

Scanning electron microscopy (SEM) images of ‘green’ (A) TEA-Fe and (B) CAF-Fe catalysts.

Figure 3

X-ray photoelectron spectroscopy (XPS) survey on Fe 2p3/2 of ‘green’ (A) TEA-Fe and (B) CAF-Fe catalysts.

Figure 4

X-ray photoelectron spectroscopy (XPS) survey on O1s regions of ‘green’ (A) TEA-Fe and (B) CAF-Fe catalysts.

The major components on the surface of both materials were C and O. Some minor Fe, Cl, N and P were also detected. There was some Si signal on the CAF-Fe catalyst. There was some Mg signal on the TEA-Fe catalyst. The chemical state of iron in both ‘green’ iron catalysts includes ferrous (Fe²⁺) and ferric (Fe³⁺) only, and Fe0 (zero-valent iron) was not found. From these results, we can conclude that the obtained results were not due to the presence of Fe0 in the materials.

Figure 5 shows the total amount of radicals generated during 60 s of reaction. The formation of hydroxyl radicals by Fe²⁺ and H₂O₂ is responsible for the luminol chemiluminescence [12]. Thus, the present study showed that the generation of hydroxyl radicals by both ‘green’ iron catalysts was higher than for the tested Fe salts (ferrous chloride tetrahydrate and ferric chloride anhydrous). The total amount of hydroxyl radicals {‧OH} generated was as follows: TEA-Fe>CAF-Fe>FeII>FeIII.

Figure 5

Total amount of hydroxyl radicals generated (as light) during 60 s of reaction of 1 mm of hydrogen peroxide with 0.1 mm of Fe as ferrous chloride tetrahydrate, ferric chloride anhydrous, CAF-Fe and TEA-Fe catalysts measured by a Luminescencer Octa AB-2270 using luminol.

3.2 Degradation of MB

From Figure 6, it is clear that the complete degradation of MB using TEA-Fe and CAF-Fe catalysts in the presence of H₂O₂ occurred within 20 min, while using FeII, FeIII and FeEDTA were around 50%, 25% and 10%, respectively. Lower degradation of MB was observed using HP, TEA-Fe and CAF-Fe alone. In general, the degradation of MB occurred gradually, indicating that the hydroxyl radicals {‧OH}, which are responsible for the degradation, were generated continuously as shown in Figure 5.

Figure 6

Effect of ‘green’ Fe catalysts and 10 mm hydrogen peroxide (HP) on degradation rate of 0.017 mm methylene blue (MB). Degradation rate versus time of MB with 10 mm HP, MB with 0.1 mm Fe as CAF-Fe and 10 mm HP (CAF-Fe+HP), MB with 0.1 mm Fe as TEA-Fe and 10 mMHP (TEA-Fe+HP), MB with 0.1 mm Fe as ferrous chloride tetrahydrate (FeII+HP), MB with 0.1 mm Fe as ferric chloride anhydrous (FeIII+HP), MB with 0.1 mm Fe as Fe-EDTA (Fe-EDTA+HP), MB with 0.1 mm Fe as CAF-Fe only (CAF-Fe) and MB with 0.1 mm Fe as TEA-Fe (TEA-Fe). The picture shows (A) HP and (B) TEA-Fe+HP treatments, respectively.

3.3 Disinfection of E. coli

Bacterial infections are a major cause of disease and even human death. Disinfectants are widely used as effective agents to kill or eliminate bacteria in various ways. They can be mainly divided into five agents: alkylating, sulfhydryl combining, oxidizing, dehydrating and permeable. In our experiment, the oxidizing capacity of Fenton’s reaction using the new ‘green’ iron catalysts developed was evaluated. As shown in Figure 7, the best disinfection effects were found using TEA-Fe and CAF-Fe catalysts in the presence of 10 mm H₂O₂. E. coli ATCC11229 was completely disinfected in those treatments, while it survived in the 10 mm H₂O₂ alone treatment. No sterilization effects were found using only TEA-Fe or CAF-Fe without H₂O₂.

Figure 7

Effect of 10 mm hydrogen peroxide (HP) and TEA-Fe (1 mm Fe) and CAF-Fe (1 mm Fe) with 10 mm HP on disinfection of Escherichia coli ATCC11229. The picture shows 3 M Petrifilms of (A) 10 mm HP and (B) 10 mm of HP with the ‘green’ TEA-Fe catalyst treatments, respectively. E.coli coliforms are blue colonies. nd means not determined.

4 Discussion

The problems related to the application of the use of Fe²⁺ and the standard Fenton’s reaction for in situ chemical oxidation can be alleviated through the use of chelates capable of binding ferrous (Fe²⁺) and ferric (Fe³⁺) iron. Nontoxic chelates such as citrate or gluconic acid have been used successfully in in-situ chemical oxidation process. Gluconic acid and HP can be generated on site, eliminating the need for transportation of highly concentrated HP [13]. The use of a chelate for remediation in aerobic environments minimizes Fe²⁺ oxidation by O₂. Dissolved oxygen has no effect on the degradation of trichlorophenol by the chelate-modified Fenton’s reaction using polyacrylic acid as the chelating agent [14]. Most of the tested chelates for Fenton’s reaction in the literature are of a high cost, making their practical use limited. We believe that the polyphenols of tea dregs and coffee grounds reduced and chelated the iron creating a good condition for degradation of MB by the Fenton’s reaction. However, more detailed studies concerning the mechanism of degradation by the developed materials are necessary.

The poor sterilization effect of 10 mm H₂O₂ solution could be explained by the possible production of catalase by the E. coli that can decompose H₂O₂, greatly hindering the inactivation effect, while in the TEA-Fe and CAF-Fe treatments, the decomposition of H₂O₂ into hydroxyl radicals {‧OH} occurred quickly, killing the bacteria before H₂O₂ decomposition by catalase.

HP possesses bactericidal and inhibitory activity due to its properties as an oxidant, and due to its capacity to generate other cytotoxic oxidizing species, such as hydroxyl radicals. The bactericidal activity of H₂O₂ coupled with rapid breakdown makes it a desirable sterilizing agent for use on some food contact surfaces and packaging materials in aseptic filling operations. Residual H₂O₂ is dependent on the presence or absence of peroxidase in the produce item [15].

The use of H₂O₂ in food industry for disinfection of whole and fresh-cut produce has been investigated in recent years. Microbial populations on whole cantaloupes, grapes, prunes, raisins, walnuts, and pistachios were significantly reduced upon treatment with H₂O₂ vapor [16]. Treatment by dipping in H₂O₂ solution reduced microbial populations on fresh-cut bell peppers, cucumber, zucchini, cantaloupe and honeydew melon, but did not alter sensory characteristics. Treatment of other produce was not as successful. H₂O₂ vapor concentrations necessary to control Pseudomonas tolaasii caused mushrooms to turn brown, while anthocyanin-bleaching occurred in strawberries and raspberries. Shredded lettuce was severely browned upon dipping in a solution of H₂O₂. Combinations of 5% H₂O₂ with acidic surfactants at 50°C produced a three to four log reduction of nonpathogenic E. coli inoculated onto the surfaces of unwaxed Golden Delicious apples [17]. It is important to note that most of the works on disinfection using H₂O₂ have been made with concentrations of H₂O₂ higher than those used in this work.

Figure 8 shows the estimated mechanism involved in the absorption of iron from the ‘green’ iron catalysts by pak choi plants and the estimated mechanism related to the Fenton reaction using the ‘green’ iron catalysts. Fenton’s chemistry is a very well-known process and it is one of the high potential oxidation technologies, because it produces a highly reactive species {‧OH} [18, 19]. In most of the studies, Fe²⁺ is used as a catalyst instead of Fe³⁺, as indicated in Reaction (I):

Figure 8

Estimated mechanism for disinfection and degradation processes using tea and coffee polyphenols and iron.

(I)Fe2++H2O2→Fe3++OH-+ · OH k=76 (I)

(II)Fe3++H2O2→Fe2++H++OH−+OH2− k=0.02 (II)

From the above equation, it is clear that the rate constant of Reaction (I) is more than that of Reaction (II). So the rate of oxidation of pollutant is also more in the case of Reaction (I) than Reaction (II). From Figure 5, it is clear that degradation of MB was increased more by the use of TEA-Fe and CAF-Fe than the FeII and FeIII treatments. It is also observed that when using TEA-Fe and CAF-Fe catalysts, no precipitation of iron hydroxide was found at the end of the reaction, while in the case of FeII, precipitation of ferric hydroxide was obtained. The Fenton applications are limited, due to the generation of excessive amounts of ferric hydroxide sludge that requires additional separation processes and disposal [20]. We supposed that the use of the developed ‘green’ iron catalysts could solve the problem.

No significant degradation of MB was obtained in the FeIII treatment. The MB was completely degraded within 20 min by TEA-Fe and CAF-Fe catalysts in the presence of H₂O₂. The higher generation of hydroxyl radicals {‧OH} (Figure 4) and the high content of active Fe²⁺ of ‘green’ iron catalysts was probably responsible for the results. The reduction and chelation functions of the polyphenols of the coffee grounds and tea dregs were estimated as a mechanism to give a stable Fenton reaction in our experiment. However, more detailed research is necessary to elucidate the real mechanism.

Tea and coffee contains substances such as ascorbic acid and polyphenols (catechins, chlorogenic acid, caffeic acid, caffeine, tannic acid and others) that are known to be natural antioxidants. These substances can reduce Fe³⁺ to Fe²⁺ and form complexes with both ions. The process of iron reduction is often attributed to both antioxidant and prooxidant activity of these compounds [21, 22]. Fukumoto and Mazza [23] noted dual antioxidant and prooxidant activities for a variety of plant-derived polyphenols, including gallic acid, protocatechuic acid, syringic acid, vanillic acid, ellagic acid, caffeic acid, coumaric acid, chlorogenic acid, ferulic acid, myricetin, quercetin, rutin, kaempferol, (+)-catechin, (−)-epicatechin, delphinidin and malvidin. The (ÿ)-epigallocatechin gallate (EGCG), a major catechin in green tea EGCG caused apoptotic cell death in osteoclastic cells, due mainly to promotion of the reduction of Fe³⁺to Fe²⁺ to trigger Fenton’s reaction, which affords a hydroxyl radical from HP [24]. Probably, the reduction of Fe³⁺ that generates Fe²⁺ which can catalyze the Fenton reaction seems to be the cause for the hydroxyl radical generation in our experiments.

Increasing the amount of bioavailable Fe and other micronutrients in plant foods for human consumption is a challenge which is particularly important for developing countries. Despite its abundance in soil, iron is still one of the most common nutrients limiting plant growth and development, because it exists mostly in a low-soluble form which is hardly available for plants. Therefore, plants have evolved two distinct uptake strategies: the reduction (Strategy I) mechanism of dicotyledonous plants, including pak choi and non-gramineous plants and the chelation (Strategy II) mechanism of graminaceous plants. In strategy II, or the chelation mechanism, which is exhibited by graminaceous plants, there is a release of Fe-chelating substances like mugineic acid (MA) in response to Fe-deficiency stress. The MA released in the plant rhizosphere solubilizes inorganic Fe³⁺-compounds by chelation, and the Fe³⁺-MAs complexes are taken up through a specific transport system in the root plasma membrane. In strategy I, or the reduction mechanism, plant iron mobilization is attained by releasing substances that increase the solubility of oxidized iron in the soil and reduction to Fe²⁺ by a ferric chelate reductase induced by iron deficiency [25, 26]. The ‘green’ iron catalysts were capable of increasing the content of Fe in the edible parts of pak choi when applied to the soil. Probably, chelated iron is protected from oxidation, precipitation and immobilization by the polyphenols of tea dregs and coffee grounds. In certain conditions, the organic molecules (the ligand polyphenols) can combine and form a ring encircling the iron. The pincer-like manner in which the Fe is bonded to the ligand changes the iron’s surface property and favors the uptake efficiency by plants. On calcareous soils, soil application of Fe fertilizers based on organic Fe salts, Fe complexes of lignosulfonates, citrates, gluconates and synthetic Fe chelates of limited stability [e.g., FeEDTA, iron(III) diethylenetriaminepentaacetic acid (FeDTPA) and iron(III) hydroxyethylenediaminetriacetic acid (FeHEDTA)] has limited or no results, because these fertilizers are not able to maintain Fe in soil solution. Only Fe chelates of higher stability [iron(III) ethylene diamine-N,N′-bis(hydroxy phenyl acetic acid) (FeEDDHA) and derivatives, with phenolic functional groups] are effective and provide the most efficient treatment to control Fe deficiency [27]. The developed catalysts were effective on supply iron to pak choi plants, especially in alkaline and calcareous soils.

All of the results using the new ‘green’ iron catalysts were obtained in controlled laboratory conditions. The effectiveness of these new materials on catalyzing the Fenton reaction could be influenced by many factors, such as the presence of scavengers and pH of the solution in natural conditions. Thus, more detailed studies concerning the effectiveness of the developed iron materials to catalyze the Fenton reaction in natural conditions are necessary.

In conclusion, the developed ‘green’ iron catalysts can be used as iron fertilizer in agriculture and in the Fenton process, to degrade contaminants or disinfect pathogens like E. coli. The advantage of the Fenton process is the complete destruction of contaminants to harmless compounds, for instance, carbon dioxide and water. However, its application has been limited, due to the low stability of Fe²⁺ and the generation of an excess amount of ferric hydroxide sludge that requires additional separation processes and disposal. Therefore, the use of developed ‘green’ iron catalysts could minimize these disadvantages. In addition, based on the strong oxidation effect, the new Fenton reaction is expected to be applied in various fields such as food, medicine, public health, agriculture, environmental cleanup, etc. The developed ‘green’ approach Fenton method is highly safe for a human body and the environment. Therefore, the developed method is expected to contribute to the effective use of tea and coffee wastes from the beverage industry, by creating a novel application method for the food waste materials.