Removal of Hydrolyzable and Condensed Tannins from Aqueous Solutions by Electrocoagulation Process

Removal of Hydrolyzable and Condensed Tannins from Aqueous Solutions by Electrocoagulation Process

J. Hassoune

; S. Tahiri

; A. Aarfane

; M. El krati

; A. Salhi

; and M. Azzi

Abstract: Vegetable tanning effluents have very high and difficult-to-treat chemical oxygen demand (COD). The main environmental dam- age is due to the low biodegradability of tannins, that can create environmental problems. Tannins are recalcitrant molecules and resist microbial attack; they are toxic to a variety of microorganisms. In this work, electrocoagulation with aluminum electrodes in a batch reactor was applied to remove polyphenolic compounds of vegetable tannins from aqueous solutions. Two types of commercial tannin extracts were used: chestnut, as a representative of hydrolyzable tannins, and mimosa, as a typical condensed tannin. The effects of operating parameters on the efficiency of electrocoagulation — current density, pH, ionic strength, interelectrode distance (IEA), electrolysis time, concentration of tannin extracts, and the like — have been investigated. It has been shown that the removal efficiency of tannin polyphenols increased with the increasing applied current density and increasing ion strength. The optimum current density ( J ) was 47 . 6 A = m

for chestnut and 71 . 4 A = m

for mimosa at 1,000 mg = L. Within these values, 97.4 and 98.8% of tannin polyphenols can be removed, respectively. On the other hand, the most effective removal capacity can be achieved at normal pH (without correction) of each tannin solution in the optimum range. However, the removal efficiency of vegetable tannins from water decreases at more acidic and alkaline pH. The addition of NaCl to the solution helps increase removal efficiency and could save power consumption significantly. The optimal electrode distance is determined to be 2 cm for the electrolysis cell employed. The optimal time for achieving maximum removal of polyphenols increases with increasing tannin concentration.

Nevertheless, it can be reduced by increasing the applied current. DOI: 10.1061/(ASCE)EE.1943-7870.0001196. © 2017 American Society of Civil Engineers.

Author keywords: Removal; Vegetable tannins; Polyphenols; Electrocoagulation; Aluminum electrodes.

Introduction

Vegetable tannins are polyphenolic compounds that occur naturally in the bark and leaves of many plants. They have multiple adjacent polyhydroxyphenyl groups in their chemical structure, which have extremely high affinity for proteins, metal ions, and other macro- molecules like polysaccharides (Haslam 1989). According to their

chemical nature and structural characteristics they are subdivided into two groups: hydrolyzable tannins that undergo hydrolysis in the presence of acids or enzymes, and condensed tannins that instead tend to polymerize (Chabaane et al. 2011; Mabrour et al.

2004). Vegetable tannins are one of the oldest natural products used for tanning hides and skins because they are able to interact and transform animal proteins (collagen) into resistant insoluble prod- ucts (leather). They convert a putrescible material into a lasting material that is resistant to the attack of microorganisms. In veg- etable tanning, the collagen chains of the skins are reticulated through hydrogen bonds between phenolic groups of tannins and NHCO groups of the collagen (Bickley 1992).

Vegetable tannin extracts are used a great deal in tanning and retanning processes so as to ensure full penetration of tannins and complete reaction between tannins and skins or leathers.

Consequently, only a part of the tannins present in the initial tan- ning solution reacts with the skins. A part of tannins and most non- tannins inevitably remains in the tanning-exhausted bath, creating environmental problems owing to the high content of organic mat- ter in the wastewaters discharged (Cassano et al. 2003). In fact, vegetable tanning effluents have very high and difficult-to-treat chemical oxygen demand (COD). The main environmental damage is due to the low biodegradability of tannins. Thus it is necessary to substantially remove the residual tannins from the tanning effluent before evacuating it in the receiving environment. The maximum permissible limit for COD in industrial wastewater effluent is 500 mg O

= L according to Moroccan norms.

Treatment of water containing tannins and polyphenolic com- pounds has been the subject of many studies. For example, Marsal et al. (2009) studied the adsorption of polyphenols of vegetable extracts by organo-bentonites. Borchate et al. (2012) studied the applicability of coagulation-flocculation for raw-vegetable tannery

1

Ph.D. Student, Laboratory of Water and Environment, Dept. of Chemistry, Faculty of Sciences of El Jadida, Université Chouaïb Doukkali, P.O. Box 20, El Jadida 24000, Morocco. E-mail: hassounejamila@

yahoo.fr

2

Professor, Laboratory of Water and Environment, Dept. of Chemistry, Faculty of Sciences of El Jadida, Université Chouaïb Doukkali, P.O. Box 20, El Jadida 24000, Morocco (corresponding author). E-mail: tahiri.

s@ucd.ac.ma; t_soufiane@yahoo.fr

3

Research Associate, Laboratory of Water and Environment, Dept. of Chemistry, Faculty of Sciences of El Jadida, Université Chouaïb Doukkali, P.O. Box 20, El Jadida 24000, Morocco. E-mail: aaarfane@yahoo .com

4

Professor, Laboratory of Water and Environment, Dept. of Chemistry, Faculty of Sciences of El Jadida, Université Chouaïb Doukkali, P.O. Box 20, El Jadida 24000, Morocco. E-mail: elkrati1@yahoo.fr

5

Ph.D. Student, Laboratory of Water and Environment, Dept. of Chemistry, Faculty of Sciences of El Jadida, Université Chouaïb Doukkali, P.O. Box 20, El Jadida 24000, Morocco. E-mail: anassalhi@hotmail.com

6

Professor, Laboratory of Interface Materials Environment, Dept.

of Chemistry, Faculty of Sciences Aïn Chock, Univ. Hassan II, P.O. Box 5366, Casablanca 20000, Morocco. E-mail: azzimed57@

gmail.com

Note. This manuscript was submitted on July 5, 2016; approved on October 27, 2016; published online on February 21, 2017. Discussion per- iod open until July 21, 2017; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, © ASCE, ISSN 0733-9372.

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wastewater with polyaluminum chloride alone and used in combination with flocculent. A membrane treatment based on nanofiltration (NF) of exhausted vegetable tannin liquors from the leather industry was developed by Cassano et al. (2003). These authors showed that NF permits recovery of water and an increased tannin/nontannin (T/NT) ratio in the retentate solution. Advantages include reduction of environmental impact, simplification of waste- water cleaning, lower disposal costs, and savings in chemicals and water (Cassano et al. 2003, 2001). Membrane technology has also been applied by Scholz and Lucas (2003) to recover chemicals from unhairing, vegetable tanning, and chrome liquors, and to pol- ish saline part streams for reuse. Tahiri et al. (2013) proposed a chain of treatment processes to reach better management of mineral and vegetable tannery wastewaters. The supernatant recovered after precipitation of chromium was used to dilute vegetable tanning wastewaters characterized by a high concentration of polyphenols.

Diluted effluent was then treated by liming and tangential micro- filtration (MF).

The objective of the present study was to examine the feasibility of electrocoagulation in removing hydrolyzable and condensed vegetable tannins from their aqueous solutions. The electrochemi- cal process consists of in situ generation of coagulants by electrical dissolution of sacrificial aluminum electrode and production of insoluble flocs that are easily separated from clear water. In order to highlight the optimal conditions required for removing tannin polyphenols, the effects of several operating parameters on treat- ment efficiency, such as applied current density, initial pH, ionic strength, electrolysis time, concentration of tannins, and distance between the cathode and the anode, have been investigated.

Materials and Methods

Vegetable Tannins

Two types of commercial tannin extracts were used in this work:

chestnut as a representative of hydrolyzable tannins and mimosa as a typical condensed tannin. The hydrolyzable tannins are glucose esters of gallic and ellagic acids (Fig. 1) (Mabrour et al. 2004).

Condensed tannins have a flavanoid core as a basic structure.

Mimosa tannin has a polymeric structure containing, on average, four flavanoid units, typified by the formula given in Fig. 2 (Martinez 2003). The tannin content of the extracts is 76% gallo- tannin for chestnut tannin and 67% flavanoid for mimosa tannin, according to the data sheets of these tannic products (Chabaane et al. 2011). Samples of vegetable tannins used in the present study were collected from a tannery plant in Mohammedia, Morocco.

Electrocoagulation

Electrochemical treatment was performed on a laboratory scale with an electrolytic cell. Two flat, rectangular aluminum electrodes ( 18 × 3 × 2 mm) as anode and cathode were installed in parallel.

The active surface area was 21 cm

. The aluminum plates consisted of 99% Al. In order to give a regulated electricity current to the electrochemical cell, a DC power supply (Advance Electronics PP10A, 0 – 50, 0 – 1 A, U.K.) was used. Electrodes were connected to the positive and negative terminals of the DC power supply. The experimental setup used in the present study is shown in Fig. 3.

Electrocoagulation experiments were carried out at 25 2 °C with 400 mL of tannic solutions containing 1 g = L tannins and 1 g = L NaCl. A magnetic stirrer (200 rpm) was used to maintain homogeneous mixing of the solution in the reactor. At the end of the treatment, obtained suspensions were allowed to stand for approximately 30 min to reach spontaneous separation of formed flocs. The liquid phase was then centrifuged prior to analysis.

During electrocoagulation, the generation of aluminum ions takes place at the anode and hydrogen gas H

is released from the cathode. The gases produced at the cathode during the electrolysis of water and metal dissolution allow the resulting flocs to float (Daneshvar et al. 2006; Chavalparit and Ongwandee 2009).

The main reactions occurring at the electrodes are

O HO

OH OH

(a) (b)

HO

OH

OH

HO HO

O

O O

O

Fig. 1. Molecular structure of (a) gallic acid; (b) ellagic acid

OH OH

R

OH R ₁

HO O

Where R1 is H for resorcinol Where R1 is OH for phlorogucinol Where R2 is H for pyrocatechol Where R2 is OH for pyrogallol

Fig. 2. Basic repeating unit in condensed tannins

A

V 1

6

5 3 3

4 2

(1) DC power supply (2) Electro-coagulation cell (3) Aluminum electrodes (4) Solution to be treated (5) Magnetic stirring bar (6) Magnetic stirrer

Fig. 3. Experimental setup of electrocoagulation treatment

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• Anode (oxidation): Al → Al

þ 3 e

; and

• Cathode (reduction): 2 H

O þ 2 e

→ H

ð g Þ þ 2 OH

ð aq Þ . The cathode may also be chemically attacked by OH

ions, gen- erated during H

evolution at high pH values (Picard et al. 2000), according to the following reaction:

2 Al þ 6 H

O þ 2 OH

→ 2 Al ð OH Þ

þ 3 H

At relatively low pH, the electrolytic dissociation of the alumi- num anode produces cationic species such as Al

, Al ð OH Þ

, and so forth. When the pH increases, these species are transformed into Al ð OH Þ

. The flocs produced have large surface areas that are gen- erally beneficial for rapid adsorption of soluble organic compounds and trapping of colloidal particles.

After the electrocoagulation experiments, the specific energy consumption and specific Al metal consumption could be calcu- lated for specific electrical energy consumption (SEEC) (Tezcan Un et al. 2009):

SEEC ð kWh = g compound removed Þ ¼ U · I · t

ðC

− C

Þ · V ð1Þ and for specific metal consumption (SMC):

SMC ð g = g compound removed Þ ¼ I · t · M

Z · F · ðC

− C

Þ · V ð2Þ

where U = cell voltage; I = current; t = electrolysis time; C

and C

= initial compound concentration and the final compound concentration at time t , respectively; V = volume of the solution to be treated; F = Faraday ’ s constant; M molecular weight; and Z = number of electrons transferred.

Analysis

Infrared IR spectra of tannins were recorded using a Tensor 27 FTIR spectrometer (Bruker, Karlsruhe, Germany) equipped with a DLaTGS detector. Spectra were obtained by co-adding 50 scans at a resolution of 4 cm

and a scanner velocity of 10 kHz HeNe frequency, 4,000 − 550 cm

. For instrumental and measurement control, spectra treatment, and data manipulation, OPUS version 6.5 (Bruker, Karlsruhe, Germany) was employed. Spectra recorded were the average of 5 spectra of each sample with a smoothing of 9 points.

Folin-Ciocalteu reagent was used to determine total polyphenol concentration. It consists of a mixture of phosphomolybdic and phosphotungstic acids that are reduced by phenols to their respective oxides in alkaline medium, resulting in a blue color.

Calibration was carried out using gallic acid. In this work, the de- veloped coloration was measured at 760 nm using a spectrometer (Visible V-1,200, MAPADA, Shanghai, China). Sometimes appro- priate dilution was processed to ensure that the concentration of the solution was within the dynamic range of the calibration curve.

Turbidity was measured using a device (TN-100/T-100, Eutech Instruments, Netherlands). The pH and conductivity were measured directly by a pH meter (WTW series, inolabo pH 730, Germany) and a conductivity meter (Model EC 214, Hanna Instruments, Woonsocket, Rhode Island), respectively.

All chemical reagents used were of analytical reagent grade and all solutions were prepared in distilled water. All laboratory devi- ces/instruments were carefully washed before use with a neutral detergent solution and then distilled water.

Results and Discussion

Fourier Transform Infrared Analysis

The main absorption bands recorded are generally similar to those corresponding to other tannins presented in the literature (Bharudin et al. 2013; Puica et al. 2006). Fourier transform mid-infrared (FT-MIR) spectra in attenuated total reflection (ATR) mode of the chestnut tannin reveal the presence of ─ OH at 3,350, C ═ C at 1,600, C ═ O at 1,727, C ─ OH at 1,350, CH

at 2,940, C ─ O ─ C at 1,197 cm

, and ester bonds at 1,050 cm

. The com- parison of the IR of the mimosa and chestnut tannins evidenced the absence of two bands (located at 1,197 and 1,727 cm

) in the mimosa spectra (Fig. 4). These two bands are characteristic of the C ─ O ─ C and C ═ O groups, respectively, indicating the useful- ness of FT-MIR in easily obtaining information on the class and nature of tannins. FT-MIR spectra of the chestnut and mimosa tannins were compared with those of other tanning materials:

hydrolyzable tannins (Chestnut AES, Dulcotan SP, Saviotan AS, Dulcotan N2, Saviotan RS) and condensed tannins [Seta Sun, Mimosa (S2) and Quebracho]. Chestnut N and Mimosa (S1) are the vegetable tannins used in this work. As can be seen, the tannins of the same class have the same spectra. This technique allows the determination of tannin type (hydrolyzable or condensed) without reagents and requires less time than usual analytical methods.

Effect of Current Density

Current density should have a significant impact on tannin removal efficiencies. The current density effect was investigated by varying current intensity from 0.02 to 0.4 A. Fig. 5 shows the evolution of voltage U ð V Þ and current density J ð A = m

Þ as a function of ap- plied current I ð A Þ . The active surface of the sacrificial electrode was 21 cm

. Experiments were carried out during 30 min at room temperature and at normal pH for each solution of tannin extract (4.3 for chestnut and 5.4 for mimosa) with a concentration of 1,000 mg = L in the presence of 1 g of NaCl per liter.

From the results shown in Fig. 6, it is clear that increasing the current density enhances vegetable tannin removal. This result can be explained by Faraday ’ s law [Eq. (3)]: when increasing current density, the aluminum released from the anode increases and hence increases removal of polyphenolic compounds because of their adsorption on Al ð OH Þ

and its polymeric compounds

m ¼ I · t · M

F · N

ð3Þ

where m = mass of dissolved aluminum; I = current (A); t = reaction time (s); M = molar mass of the aluminum (g = mol);

F = Faraday constant ( 96,500 C = mol); and N

= number of elec- trons involved in the reaction.

On the other hand, increasing current density leads to a rise in the bubble generation rate and a decrease in bubble size. Both effects are beneficial for high pollutant removal by H

flotation (Kobya et al. 2006; Essadki et al. 2008).

Fig. 6 shows that an increase in current from 0.02 (9.5) to 0.1 (47.6) and to 0.15 A ( 71 . 4 A = m

) for chestnut and mimosa tannins, respectively, increases the percentage removal of polyphenols from 14.4 to 97.4% and from 15.0 to 98.8%, respectively. These current densities are recommended to reach maximum removal of tannin polyphenols from their aqueous solutions at the operating condi- tions adopted in this study. The figure also shows that when the optimal current density is achieved, the removal percentage of tan- nins remains constant with any further increase in this parameter because there is an almost total elimination of tannins. This result

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was confirmed by the determination of final turbidity. As can be seen from the results shown in Fig. 7, the maximum reduction in turbidity was obtained for current densities 47.6 and 71 . 4 A = m

in the case of chestnut and mimosa, respectively.

The effect of current density on pH and conductivity changes is shown in Fig. 8, where pH increases as current density increases.

This can be explained by the excess of hydroxyl ions at the cathode due to reduced water. Obtained results show also that electrical

conductivity is not significantly affected by current density.

Even if the current density increases from 9.5 to 190 . 5 A = m

, the conductivity remains in the range of 2.50 – 2.58 mS/cm and 2 . 30–2 . 36 mS = cm for chestnut and mimosa tannins, respectively.

On the other hand, the cost of the electrocoagulation process is determined by the consumption of the sacrificial electrode and the electrical energy that are the economic advantages of this method.

From obtained results, it is clear that energy and Al consumption Fig. 4. ATR-FTIR spectra obtained for industrial vegetable tannins

0 5 10 15 20 25

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

I (A)

U (V)

0 20 40 60 80 100 120 140 160 180 200

J ( A /m

) U (V) Chestnut

U (V) Mimosa J (A/m2)

Fig. 5. Evolution of voltage and current density as a function of applied current

0 10 20 30 40 50 60 70 80 90 100

0 20 40 60 80 100 120 140 160 180 200 J (A/m

)

Removal of Polyphenols (%)

Chestnut Mimosa Volume = 400 mL

Electrolysis time = 30 min [Tannin] = 1 g/L [NaCl] = 1 g/L pH (Chestnut) = 4.3 pH (Mimosa) = 5.4

Fig. 6. Removal of polyphenolic compounds as a function of current density

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per gram of tannin removal increases with increasing current den- sity (Fig. 9). Therefore, it is necessary to use the optimal current density for the recovery of tannins from their aqueous solutions because it ensures the quickest removal rate with the lowest cost.

Effect of pH

Usually pH is an effective factor that can affect considerably the efficiency of electrocoagulation. In this work, the effect of pH was studied for values ranging from 2 to 11, with the pH adjusted to the desired value by adding HCl or NaOH solutions. Obtained results shown in Fig. 10 reveal that the removal of tannin polyphe- nols depends strongly on the initial solution pH. During electrocoa- gulation, the dissolution of aluminum takes place and is influenced by pH. As can be seen, the removal efficiency of vegetable tannins

from water decreases at a more acidic and alkaline pH. This can be attributed to the behavior of Al ð OH Þ

, which leads to soluble Al

cations (at a more acidic pH) and to Al ð OH Þ

anions (at an alkaline pH). In fact, these soluble species are not useful for water treatment.

Generally, when the pH is between 4 and 9, the Al

and OH

generated in electrocoagulation cells react to form different oligo- meric species such as Al

ð OH Þ

, Al

ð OH Þ

, and Al

ð OH Þ

, to finally be converted into an insoluble amorphous compound in water, Al ð OH Þ

, via a complex kinetics of polymerization/precipi- tation (Bayramoglu et al. 2004).

On the other hand, pH strongly affects the size of the hydrogen bubbles. The typical size of bubbles produced during electrocoa- gulation by aluminum electrodes varies between 20 and 70 μ m (Adhoum et al. 2004). It is therefore essential to determine the pH values to be adopted before electrocoagulation treatment for 0

50 100 150 200 250 300 350

0 20 40 60 80 100 120 140 160 180 200 J (A/m

)

Turbidity (NTU)

Chestnut Mimosa Volume = 400 mL

Electrolysis time = 30 min [Tannin] = 1 g/L [NaCl] = 1 g/L pH (Chestnut) = 4.3 pH (Mimosa) = 5.4

Fig. 7. Evolution of residual turbidity as a function of current density

0 1 2 3 4 5 6 7 8

0 20 40 60 80 100 120 140 160 180 200 J (A/m

)

pH

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

EC (mS/cm)

pH final Chestnut pH final Mimosa

Conductivity (mS/cm) Chestnut Conductivity (mS/cm) Mimosa Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ; [NaCl] = 1 g/L

pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4

Fig. 8. Evolution of final pH and final electrical conductivity as a function of current density

0 2 4 6 8 10 12 14 16

0 25 50 75 100 125 150 175 200

J (A/m

) Energy consumption (kWh/g Tannin removed)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Al consumption (g Al/g Tannin removed)

Mimosa (Energy consumption) Chestnut (Al consumption) Mimosa (Al consumption)

Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ; [NaCl] = 1 g/L

pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4

Fig. 9. Specific energy consumption and specific Al consumption as a function of applied current density

0 10 20 30 40 50 60 70 80 90 100

0 1 2 3 4 5 6 7 8 9 10 11 12

pH

Removal of Polyphenols (%) ^Chestnut

Mimosa

Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ; [NaCl] = 1 g/L

J (Chestnut) = 47.6 A/m

; J (Mimosa) = 71.4 A/m

Fig. 10. Effect of initial pH on the removal of polyphenols by electrocoagulation

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overall control and for achieving small bubbles for efficient flotation. The minimum average diameter of the bubbles is gener- ally observed at neutral pH, but the value of the optimum pH may vary depending on the electrode material and the structure of the pollutant.

The final pH value, recorded after electrocoagulation, increases generally for an acidic initial pH less than 6.5. This is due to the evolution of hydrogen and the generation of OH

ions at the cath- odes. For initial basic media, the final pH does not change signifi- cantly because the OH

ions generated at the cathode are consumed by the metal ions produced at the anode forming the flocs (Fig. 11).

Effect of Ionic Strength

It is well understood that ionic strength is an important factors in the electrochemical process. The solution must have some mini- mum conductivity for the flow of the electric current (Khandegar and Saroha 2013). The initial conductivity of chestnut and mimosa solutions at 1 g = L is 136 and 231 μ S = cm, respectively. These val- ues are not sufficient to conduct electrocoagulation experiments.

To study the effect of ionic strength on the removal of polyphenolic compounds and the performance of the electrocoagulation process, NaCl was used to adjust the salt content of the tannin solutions between 0.05 and 2 g = L. The results shown in Fig. 12 indicate an obvious effect of salt content on removal efficiency. The removal of polyphenols increases by increasing ion strength because the sol- ution becomes more conductive as a result of the increased passage of electrons per unit time. Obtained results show that NaCl concen- trations of approximately 1 and 0 . 25 g = L are needed to remove, respectively, 97.4 and 97.7% of the initial amount of polyphenols of each vegetable extract from 400 mL of their solutions at 1,000 mg = L.

At constant current densities ( 47 . 6 A = m

for the chestnut sol- ution and 71 . 4 A = m

for the mimosa solution), increasing NaCl concentration causes an increase in electrical conductivity that leads to a decrease in electrical resistance and consequently in potential. Fig. 13 shows that when the concentration of NaCl in- creases from 0.05 to 2 g = L, the voltage decreases from 28.1 to 2.8 V for chestnut and from 35.6 to 3.6 V for mimosa. Therefore, more aluminum ions could be produced at the same power input.

This result is in accordance with that of other work. Generally, there 0

1 2 3 4 5 6 7 8 9 10 11

0 1 2 3 4 5 6 7 8 9 10 11

pH

pH

X=Y Chestnut Mimosa

Fig. 11. Final pH values of treated solutions versus their initial pH values

0 10 20 30 40 50 60 70 80 90 100

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 NaCl (g/L)

Removal of Polyphenols (%)

Chestnut Mimosa

Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ;

pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4 J (Chestnut) = 47.6 A/m

; J (Mimosa) = 71.4 A/m

Fig. 12. Removal of polyphenolic compounds as a function of NaCl concentration

0 5 10 15 20 25 30 35 40

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 NaCl (g/L)

U (V)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

EC (mS/cm)

U (Chestnut) U (Mimosa) EC (Chestnut) EC (Mimosa)

Fig. 13. Evolution of voltage and EC as a function of NaCl concentration

0 5 10 15 20 25

0 0.5 1 1.5 2 2.5

NaCl (g/L) Energy consumption (kWh/g Tannin removed)

0 0.05 0.1 0.15 0.2 0.25 0.3

Al consumption (g Al /g Tannin removed)

Mimosa (Energy consumption) Chestnut (Al consumption) Mimosa (Al consumption) Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ;

pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4 J (Chestnut) = 47.6 A/m² ; J (Mimosa) = 71.4 A/m²

Fig. 14. Energy and Al consumption as a function of NaCl concentration

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is an increase in current density with an increase in conductivity of the solution at a constant cell voltage or reduction in the cell voltage at constant current density (Bayramoglu et al. 2004;

Merzouk et al. 2010).

However, energy consumption per gram of tannin removed de- creases from 10.4 to 0.6 kWh and from 25 to 1 kWh for chestnut and mimosa, respectively, for the 30 min of electrolysis carried out.

At the same time, Al consumption (g Al = g tannin removed) decreases. However, further a increase in the value of NaCl con- centration beyond 1 g = L for chestnut and 0 . 25 g = L for mimosa has no effect on Al consumption (Fig. 14).

Effect of Interelectrode Distance

The distance between electrodes is one of the most important parameters affecting electrocoagulation efficiency. Maximum pol- lutant removal efficiency is obtained by maintaining an optimum distance between the electrodes (Khandegar and Saroha 2013).

In this work, the impact of the interelectrode distance (IED) on the removal of chestnut and mimosa tannins was investigated.

The variation in removal efficiency during 30 min with four distances ranging from 0.5 to 3 cm was studied. As can be seen, at constant current density, the highest removal efficiency was achieved at a distance of 2 cm (Fig. 15) for the electrochemical reactor previously described. However, minimum IED led to lower tannin removal efficiency because the generated metal hydroxides were degraded by collision with each other due to high electrostatic attraction (Daneshvar et al. 2004). Increasing the spatial separation of aluminum electrodes up to 3 cm reduces tannin polyphenols re- moval because of a decrease in current flow and coagulant gener- ation. Increasing the electrode distance more than the optimum decreases tannin removal because of increased travel time of the ions. As a result, the formation of flocs needed to coagulate the matter to be removed decreases (Daneshvar et al. 2004).

A 2-cm interelectrode spacing was hence chosen as optimal for subsequent tannin removal experiments. This IED reduces the electrostatic effects, resulting in slower movement of the generated ions. There is therefore sufficient time for the generated metal hydroxide to agglomerate and to form the flocs that remove the polyphenolic compounds from their aqueous solutions.

Effect of Electrolysis Time and Concentration of Tannin

The effects of operating time and tannin concentration on the re- moval of polyphenolic compounds are shown in Fig. 16. The elec- trocoagulation experiments were performed at different electrolysis times with optimum current densities of 47.6 and 71 . 4 A = cm

for chestnut and mimosa solutions, respectively. Obtained results show that the optimum electrolysis time depends strongly on both tannin concentration and applied current. In the case of solutions at 1 -g tannin = L, the concentration of total polyphenols drops, with the electrolysis time reaching removal rates over 97 – 98% (aver- age). The highest value of removal efficiency was researched at 30 and 40 min of electrolysis for chestnut and mimosa solutions at 1,000 mg = L, respectively. Longer reaction times do not enhance removal efficiencies because sufficient flocs are available for tannin removal. However, for the same current density applied in the case of each tannin compound, the optimal time necessary to achieve maximum removal of polyphenols increases with increasing tannin concentration. On the other hand, the electrolysis time required to remove chestnut or mimosa tannins from their aqueous solution at 2,000 mg = L can be reduced by increasing the applied current.

0 20 40 60 80 100 120

0.5 1 2 3

Distance between electrodes (cm)

Residual polyphenols (mg/L)

Chestnut Mimosa

Volume = 400 mL ; Electrolysis time = 30 min [Tannin] = 1 g/L ; [NaCl] = 1 g/L

pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4 J (Chestnut) = 47.6 A/m

; J (Mimosa) = 71.4 A/m

Fig. 15. Residual concentration of polyphenols as a function of distance between electrodes

0 10 20 30 40 50 60 70 80 90 100

0 15 30 45 60 75 90 105 120 135 150 165 180 Time (min)

Removal of Polyphenols (%)

[Chestnut]= 1 g/L ; 0.1 A [Chestnut]= 2 g/L ; 0.1 A [Chestnut]= 2 g/L ; 0.2 A

0 10 20 30 40 50 60 70 80 90 100

0 15 30 45 60 75 90 105 120 135

Time (min)

Removal of Polyphenols (%)

[Mimosa]= 1 g/L ; 0.15 A [Mimosa]= 2 g/L ; 0.15 A [Mimosa]= 2 g/L ; 0.3 A (a)

(b)

Volume = 400 mL ; [NaCl] = 1 g/L pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4

Volume = 400 mL ; [NaCl] = 1 g/L pH (Chestnut) = 4.3 ; pH (Mimosa) = 5.4

Fig. 16. Removal of polyphenolic compounds versus time from (a) chestnut and (b) mimosa solutions at 1 g = L and 2 g = L under different current intensities

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Therefore, not only electrolysis time limits the elimination kinetics;

applied current density and the nature and concentration of tannin also play a role. During electrolysis, an increase of pH with time was observed (results not shown). This is mainly due to the greater formation of hydroxide ions at the cathode.

Conclusions

The results of this study have shown the applicability of electro- coagulation in the removal of vegetable tannins from their aqueous solutions. Based on the results of experiments, the following con- clusions may be obtained:

• When increasing current density, the aluminum released from the anode increases and hence increases tannins removal; in- creasing current density leads to an increased bubble generation rate and to decreased bubble size; both effects are beneficial for high polyphenols removal by H

flotation;

• The removal efficiency of vegetable tannins from water decreases at more acidic and alkaline pH; the final pH value increases for acidic initial pH less than 6.5 because of the evo- lution of hydrogen and generation of OH

ions at the cathode;

for initially basic media, the final pH does not change signifi- cantly because the OH

ions generated at the cathode are consumed by the metal ions produced at the anode forming the flocs;

• Increasing ion strength enhances polyphenol removal because the solution becomes more conductive as a result of increased passage of electrons per unit time (1 and 0.25 g of NaCl = L are needed to remove, respectively, 97.4 and 97.7% of chestnut and mimosa tannins from 400 mL of their solutions at 1,000 mg = L);

• Maximum tannin removal efficiency is obtained by maintaining 2 cm as the optimum distance between the electrodes of the electrolysis cell employed; removal efficiency was researched at 30 and 40 min of electrolysis for chestnut and mimosa tannins at 1,000 mg = L, respectively;

• For the same current density applied, the optimal time necessary to achieve maximum removal of polyphenols increases with in- creasing tannin concentration; the electrolysis time required to remove chestnut or mimosa tannins from their aqueous solution at 2,000 mg = L can be reduced by increasing the applied current.

On the other hand, results revealed that energy and Al consump- tion per gram of tannin removed increase with increasing current density and decrease when NaCl concentration increases.

References

Adhoum, N., Monser, L., Bellakhal, N., and Belgaied, J. E. (2004).

“ Treatment of electroplating wastewater containing Cu

and Cr(VI) by electrocoagulation. ” J. Hazard. Mater., 112(3), 207 – 213.

Bayramoglu, M., Kobya, M., Can, O. T., and Sozbir, M. (2004). “ Operating cost analysis of electrocoagulation of textile dye wastewater. ” Sep.

Purif. Technol., 37(2), 117 – 125.

Bharudin, M. A., Zakaria, S., and Chia, C. H. (2013). “ Condensed tannins from Acacia Mangium Bark: Characterization by spot tests and FTIR. ” AIP Conf. Proc., Vol. 1571, 153 – 157.

Bickley, J. C. (1992). “ Vegetable tannins and tanning. ” J. Soc. Leather Technol. Chem., 76, 1 – 5.

Borchate, S. S., Kulkarni, G. S., Kore, S. V., and Kore, V. S. (2012).

“ Application of coagulation-flocculation for vegetable tannery waste- water. ” Int. J. Eng. Sci. Technol., 4(5), 1944 – 1948.

Cassano, A., Adzet, J., Molinari, R., Buonomenna, M. G., Roig, J., and Drioli, E. (2003). “ Membrane treatment by nanofiltration of exhausted vegetable tannin liquors from the leather industry. ” Water Res., 37(10), 2426 – 2434.

Cassano, A., Molinari, R., Romano, M., and Drioli, E. (2001). “ Treatment of aqueous effluents of the leather industry by membrane processes — A review. ” J. Membrane Sci., 181(1), 111 – 126.

Chabaane, L., Tahiri, S., Albizane, A., El Krati, M., Cervera, M. L., and de la Guardia, M. (2011). “ Immobilization of vegetable tannins on tannery chrome shavings and their use for the removal of hexavalent chromium from contaminated water. ” Chem. Eng. J., 174(1), 310 – 317.

Chavalparit, O., and Ongwandee, M. (2009). “ Optimizing electrocoagula- tion process for the treatment of biodiesel wastewater using response surface methodology. ” J. Environ. Sci., 21(11), 1491 – 1496.

Daneshvar, N., Oladegaragoze, A., and Djafarzadeh, N. (2006). “ Decolori- zation of basic dye solutions by electrocoagulation: An investigation of the effect of operational parameters. ” J. Hazard. Mater., 129(1 – 3), 116 – 122.

Daneshvar, N., Sorkhabi, H. A., and Kasiri, M. B. (2004). “ Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections. ” J. Hazard. Mater., B112(1 – 2), 55 – 62.

Essadki, A. H., Bennajah, M., Gourich, B., Vial, C. H., Azzi, M., and Delmas, H. (2008). “ Electrocoagulation/electroflotation in an external-loop airlift reactor — Application to the decolorization of textile dye wastewater: A case study. ” Chem. Eng. Process., 47(8), 1211 – 1223.

Haslam, E. (1989). Plant polyphenols — Vegetable tannins revisited, Cambridge University Press, Cambridge, U.K.

Khandegar, V., and Saroha, A. K. (2013). “ Electrocoagulation for the treat- ment of textile industry effluent — A review. ” J. Environ. Manage., 128, 949 – 963.

Kobya, M., Demirbas, E., Can, O. T., and Bayramoglu, M. (2006).

“ Treatment of levafix orange textile dye solution by electrocoagula- tion. ” J. Hazard. Mater., 132(2 – 3), 183 – 188.

Mabrour, J., et al. (2004). “ Effect of vegetal tannin on anodic copper dissolution in chloride solution. ” Corros. Sci., 46(8), 1833 – 1847.

Marsal, A., Bautista, E., Ribosa, I., Pons, R., and Garcia, M. T. (2009).

“ Adsorption of polyphenols in wastewater by organo-bentonites. ” Appl.

Clay Sci., 44(1 – 2), 151 – 155.

Martinez, S. (2003). “ Inhibitory mechanism of mimosa tannin using molecular modelling and substitutional adsorption isotherms. ” Mater.

Chem. Phys., 77(1), 97 – 102.

Merzouk, B., Madani, K., and Sekki, A. (2010). “ Using electrocoagulation- electroflotation technology to treat synthetic solution and textile wastewater, two case studies. ” Desalination, 250(2), 573 – 577.

Picard, T., Cathalifaud-Feuillade, G., Mazet, M., and Vandensteendam, C.

(2000). “ Cathodic dissolution in the electrocoagulation process using aluminum electrodes. ” J. Environ. Monitor., 2(1), 77 – 80.

Puica, M. N., Pui, A., and Florescu, M. (2006). “ FTIR spectroscopy for the analysis of vegetable tanned ancient leather. ” Eur. J. Sci. Theol., 2(4), 49 – 53.

Scholz, W., and Lucas, M. (2003). “ Techno-economic evaluation of mem- brane filtration for the recovery and re-use of tanning chemicals. ” Water Res., 37(8), 1859 – 1867.

Tahiri, S., et al. (2013). “ Management of tannery wastewaters: Treatment of spent chrome tanning bath and vegetable tanning effluents. ” Desalin.

Water Treat., 51(22 – 24), 4467 – 4477.

Tezcan Un, U., Koparal, A. S., and Ogutveren, U. B. (2009). “ Electrocoa- gulation of vegetable oil refinery wastewater using aluminum electrodes. ” J. Environ. Manage., 90(1), 428 – 433.

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