- Email: [email protected]
Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode
Accepted Manuscript Title: Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode Author: S. Elabbas N. Ouazzani L. Mandi F. Berrekhis M. Perdicakis S. Pontvianne M-N. Pons F. Lapicque J-P Leclerc PII: DOI: Reference:
S0304-3894(15)30327-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.12.067 HAZMAT 17347
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
12-10-2015 21-12-2015 30-12-2015
Please cite this article as: S.Elabbas, N.Ouazzani, L.Mandi, F.Berrekhis, M.Perdicakis, S.Pontvianne, M-N.Pons, F.Lapicque, J-P Leclerc, Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.12.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode S. Elabbasa,b, N. Ouazzania,b, L. Mandia,b, F. Berrekhisc, M. Perdicakisd, S. Pontviannee, M-N. Ponse, F. Lapicquee, J-P Leclerce* [email protected] a
Laboratoire d’Hydrobiologie, Ecotoxicologie et Assainissement (LHEA, URAC 33), Faculté de Sciences Semlalia, BP2390, Université Cadi Ayyad, Marrakech, Maroc.
b
Centre National d’Etude et de Recherche sur l’Eau et l’Energie (CNEREE), Université Cadi Ayyad, BP511, Marrakech, Maroc.
c
Equipe de Physico-chimie des Matériaux, Ecole Normale Supérieure, Université Cadi Ayyad, BP2400, 40000 Marrakech, Maroc d Laboratoire de Chimie Physique et Microbiologie pour l'Environnement (LCPME) UMR 7564, Université de Lorraine – CNRS, 405 rue de Vandœuvre, F-54602 Villerslès Nancy Cedex, France e Laboratoire Réactions et Génie des Procédés (LRGP) UMR 7274, CNRS, Université de Lorraine, 1 rue Grandville, BP 20451, 54001 Nancy cedex, France. *
Corresponding author. Tel: +33 (0)3 83 17 52 77; Fax: +33(0)3 73 32 29 75.
1
Highlights COD and Cr (III) species can be simultaneously removed by electrocoagulation Cu-containing Al alloy is more efficient than pure Al as electrodes Dilution of too concentrated tannery wastewater is required for efficient treatment
2
Abstract This paper deals with the ability of electrocoagulation (EC) to remove simultaneously COD and chromium from a real chrome tanning wastewater in a batch stirred electrocoagulation cell provided with two aluminium-based electrodes (aluminium/copper/ magnesium alloy and pure aluminum). Effects of operating time, current density and initial concentration of Cr (III) and COD have been investigated. The concentrations of pollutants have been successfully reduced to environmentally acceptable levels even if the concentrated effluent requires a long time of treatment of around 6 hours with a 400 A/m2 current density. The aluminium alloy was found to be more efficient than pure aluminium for removal of COD and chromium. Dilution of the waste has been tested for treatment: high abatement levels could be obtained with shorter time of treatment and lower current densities. Energy consumption of the electrocoagulation process was also discussed. The dilution by half of the concentrated waste leads to a higher abatement performance of both COD and chromium with the best energy efficiency.
Keywords: Aluminum electrodes; chromium; Current density; Electrocoagulation; Tannery wastewater
3
1. Introduction Leather production, associated to cloth and craft industries, is a very important activity in many countries since antiquity. It represents a large economic input in many developing countries but is also unfortunately an important source of pollution. The tanning process is a transformation of raw animal hides into leather. It involves various chemical and mechanical operations to remove the residual meat, fat and hair from the skin, to deeply clean the skin and to improve its aspect and its functional properties. Despite the existence of less pollutant treatments, chromium tanning remains the most employed technique because of the higher quality of the leather obtained. A report issued in 2003 has estimated the number of tanneries in Morocco to more than 60 units, most of them using chromium [1]. Tannery wastewater rise important environmental problems because of their complex composition and high concentrations of various organic and inorganic chemicals reflected in particular by high values of Chemical Oxygen Demand (COD) [2, 3] and chromium concentration over 5 g/L [4, 5]. Although new tanneries use modern equipments with optimal adjustment of the chromium quantity, conventional chromium bath tanning processes still use chromium salts in excess. Fahim et al. [5] estimated that the leather takes up only 60 to 80% of the introduced chromium. Depending on the efficiency of the operation, wastewater contains chromium species, which must be removed before disposal. Removal of chromium has been also studied for drinking water but the concentrations are there far much lower than in tannery wastewater [6]. Several methods have been attempted and a recent review summarizes the chemical and biological treatment technologies for leather tannery wastewaters and their efficiencies [7]. In comparison between electrocoagulation and chemical precipitation to remove metals from acidic soil leachate, Meunier et al. [8] have reported chromium removal of
4
more than 99% with an initial concentration of 100 mg/L by chemical precipitation in the presence of NaOH and Ca(OH)2. Keerthi et al. [9] have tested a hybrid membrane bioreactor with a COD removal of 72.69 % from wastewater with an initial Cr concentration at 1600 mg/L. Using coagulation and flocculation, Song et al. [10] have obtained a COD removal of 30 to 37% with an initial concentration of 3300 mg/L. Adsorption has also been used for treatment of tannery effluents [4, 5, 11, 12]. Fabbricino et al. [4] have reported 90% of Cr (III) removal from an initial concentration of 6100 mg/L using adsorption in an environment friendly cycle with recovery of the metal. However this last process may be not be economically feasible in the treatment of tannery wastewater because of high costs and the expertise required to implement and sustain the operation of such processes. Fahim et al. [5] reported absorption of nearly 99% of the chromium from tannery waste water using activated carbon at an final concentration of 44 mg/L from the initial level at 5500 mg/L. The adsorbent can also be a natural soil as explored by Tiglyene et al. [12]. Use of a cation exchange resin was reported to remove 96% of Cr (III) from a waste solution issued by tannery [13] but the concentrated effluent of 5480 mg/L was previously diluted 10-fold. Recent studies have been presented to facilitate the use of low chromium solutions by possible effective chromium recycling through re-concentration using nanofiltration [2]. Ouaissaa et al. [14] have obtained a 75% COD abatement of a low polluted tannery wastewater using an intensified electrocoagulation process by addition of an adsorbent. Costa et al. [15] have reported a 40% TOC abatement using dimensionally stable electrodes (DSA®), however with the risk of oxidation of Cr (III) to Cr (VI). Sundarapandiyan et al. [16] have studied the specific case of a very salty tannery wastewater (around 30g/L of Cl-): the high salinity was shown not to affect the efficiency of the treatment. Chromium from highly concentrated tannery wastewater (around 2.5 g/L) could be recovered at
5
99% using Pb anodes and Cu cathodes after dilution of the effluent to less than 0.3 g/L [17]. Sirajuddin et al. [18] studied several electrochemical processes to treat around 10 g/L of COD from tannery wastewater effluent but only the results concerning TOC were presented. Finally, for treatment of a very concentrated effluent (COD 5470 mg/L and Cr 6900 mg/L) ozonation could be used with COD abatement up to 88% (Houshyara et al. [19]) but this treatment can oxidize Cr (III) into Cr (VI), which generates another pollution issue. Finally, phytoremediation has investigated too for treatment of such waste, as done by Mandi et al. [20].
Electrocoagulation has been used in several of the above mentioned results and it has been successfully applied to treat numerous varieties of wastewater, of either industrial or urban origin. The main advantage of electrocoagulation is its ability to treat simultaneously a wide range of type of pollutants including organic matter, heavy metals, dyes, and minerals. It was found to be effective for the removal of dye [21, 22], heavy metals from coal acid drainage wastewater [23], non-biodegradable organic pollution and arsenic from paper mill wastewater [22], and several others types of effluents often listed in the literature. Phosphates were also successfully removed from (surface or ground) water using this technique [24]. Moreover the experimental data gained in treatment tests could lead to physicochemical modelling of electrocoagulation process [25] or in scaling-up and economic considerations [26].
Electrocoagulation uses a direct current source between two metal electrodes immersed in polluted water. The electrical current causes the dissolution of anodes made of iron or aluminium into wastewater. The metal species released in solution, at an appropriate pH, can form wide ranges of coagulated species and metal hydroxides that destabilize
6
and aggregate the suspended particles or precipitate and adsorb dissolved contaminants. In the case of aluminium, Al3+ ions are generated at the anode and combine to OH− ions provided by the cathode to form various monomeric species, which finally transform to solid Al(OH)3(s). Freshly formed amorphous Al(OH)3(s) “sweep flocs” have large surface areas, which are beneficial for rapid adsorption of soluble organic compounds and trapping of colloidal particles. These flocs are removed easily from the aqueous medium by sedimentation or flotation. In addition to ref. [25] other groups attempted in better understanding EC process [27, 28] in view to its more efficient integration in overall water treatment operations. Numerous papers have compared the effect of the electrodes nature (namely iron and aluminium) whereas the purity of the anodes was rarely investigated. Vasudevan et al [29] pointed out the lower efficiency of pure aluminium electrodes compared to aluminium alloys in the remediation of phosphate-contaminated water. The lower efficiency of pure aluminium was explained by passive film formation, lower dissolution efficiency, non-uniform dissolution and higher operating voltage with time. The direct oxidation of dissolved substances is also affected by the nature of the electrode as pointed out by Kruthika et al [30]. The aim of the present study was to investigate the treatment of highly polluted traditional tannery wastewater by electrocoagulation with initial concentrations around 6 g/L COD and 7 g/L Cr (III). In comparison with pollutant levels usually encountered, these high concentrations of COD and Cr (III) are due to the nature of the process involved: natural skin (inducing high organic pollution), implication of various chemicals during the treatment and incomplete chromium absorption by the skin resulting in a high level of chromium in wastewater [31]. Another reason is that the liquid released during this specific step is rejected directly with other effluents of the
7
tanning process. Removal of COD and Cr (III) was followed in discontinuous tests carried out with recirculation of the electrolyte solution depending on the operating conditions, and in particular depending on the nature of the Al-based electrodes. First tests conducted with the raw wastewater revealed moderate treatment efficiency for both pollutants, presumably because of its high contents in both Cr species and COD. It was then decided to dilute the wastewater prior to its treatment, for the sake of more efficient operation. The optimal dilution factor was determined for the sake of high removal yield and acceptable energy consumption per m3 of the original wastewater.
2. Mechanism of the electrocoagulation using aluminium electrodes In electrocoagulation processes, the coagulating species are produced in situ and the overall process consists of three successive stages: (i) formation of coagulants by electrolytic oxidation of the sacrificial electrode (ii) destabilization of the contaminants, particulate suspension and breaking of emulsion and (iii) aggregation of the destabilized phases to form flocs [25, 27-28]. The main reactions occurring at the electrodes are: At aluminum anode: Al→Al3+ + 3e–
(1)
At aluminum cathode: 2H2O + 2e– →
H2 + 2OH−:
(2)
Moreover in the presence of chloride ions, aluminium from the cathode can dissolve and form hydrogen: Al + 3 H2O → Al3+ + 3 OH- + 3/2 H2 In the solution: 8
(3)
Al3+ + 2OH−
→ Al(OH)3
(4)
Because of the standard potential of the Al/Al3+ couple near -1.8 V/NHE, occurrence of side reactions e.g. oxygen evolution or Cr (III) oxidation to Cr (VI) with standard potentials at 1.23 and 1.33 V respectively, is very unlikely and have not been considered here. Two types of aluminium electrodes were employed, namely the aluminium alloy (Duralumin) and pure aluminium aiming to assess the effect of the chemical composition of the electrodes on the treatment effectiveness. For both electrodes, operating time, current density and dilution rate have been investigated to determine the optimal conditions for chromium and organic matter removal and electrical energy consumption but also to point out the effect of aluminium electrodes composition on efficiency and super-faradaic charge yields.
3. Materials and methods 3.1 Tannery effluent samples Leather processing involves a number of unit operations as shown in Fig 1. The pretanning operation includes successively the following steps: soaking, liming, removal of extraneous tissues (unhairing), deliming and bating. Furthermore chromium tanning treatment involves degreasing, pickling by acidification, pressing and drying. Finally the shaving, dyeing and other operations - depending on the final use of the leather - are conducted for obtain a finished leather. The samples were obtained from the tank used for chromium tanning step in a small traditional tannery in Marrakech city (Morocco) having a capacity of treatment of about 500 hides/day. This tank is a large wooden cylinder (3.5 m high and with a radius of 1.56 m) placed horizontally. The effective volume of the tank was 17.14 m3. The barrel
9
can rotate on two hollow metal shafts for possible introduction of raw hides, chemicals, dyestuff and water through the square-shaped hatch of the tank. A gear wheel fixed on the flange of the barrel allows transmission of rotation. Large grab samples (10 L) were collected in polyethylene containers and stored at 4°C in the dark until use. Their average characteristics are given in Table 1. Sub-samples of 2 L liquid were used for each electrocoagulation test. The effluent was characterized by very high chromium content near 7 g/L and about 6 g/L of COD. The composition of tannery wastewater is strongly depending on the level of optimization of the factory. If the modern industry releases less polluted wastewater, the majority of semi-traditional industries used very concentrated solutions of chromium to guaranty the quality of the treatment. Several publications have shown the efficiency of the treatment of tannery wastewater, however most of the papers have focused their interest on the organic pollution characterized by TOC or COD [9, 32] and the papers including the follow-up of chromium are based on low chromium concentrations, generally lower than 100 mg/L [33, 34].
3.2 Electrode materials Two types of aluminum electrode have been used in this present study. Aluminum alloy (duralumin) A-U4G (2017-Al) had the following metal contents: Cu (4%), Fe (0.7%), Mg (0.7%), Mn (0.7%), Si (0.5%), Zn (0.25%) and Cr (0.1%). The “pure” aluminium (1050) electrodes contained actually impurities but to a low level as listed below: Si (0.25%), Fe (0.4%), Cu (0.05%), Ti(0.05%), Mn(0.05%), Zn(0.07%).
3.3 Experimental set-up
10
Electrocoagulation treatment was carried out in a discontinuous system under solution recirculation functioning. The electrocoagulation cell used, consisted of two identical polymethylmethacrylate (Perspex) halves being 20 cm long, 10 cm wide and 5 cm thick. Metal plates with an area of (157 cm2) acting as facing electrodes were imbedded in the polymeric halves, as described in [35]. The active electrode surface was first mechanically polished under water with abrasive paper, rinsed with ultra-pure water and dried prior to immersion in the wastewater. The area of each electrode was 105 cm2. The gap between anode and cathode was maintained at 2 cm. The electrodes were connected to a DC power supply (AFX 2930 SB, China) providing current intensities up to 5 A. The current was ranging from 2.1 to 4.2 A (200 – 400 A m-2 respectively) in the present investigation. The current was kept constant during each run and the cell voltage was monitored. The schematic diagram of the complete electro coagulation system is shown in Figure 2. 3.4 Experiments The experiments were conducted at ambient temperature ( 20°C) with 2 L wastewater gently stirred at 200 rpm in the tank. The solution was continuously circulated in the flow circuit by means of a peristaltic pump at 50 mL/min. Ten cm3 fractions were sampled at regular time intervals (every 10 minutes during the first hour and every 30 minutes later), allowed to settle for 24 hours, then filtered (0.45 µm pore size) before any analysis to determine effluent COD, Cr (III) and Al (III) concentrations. Experiments with diluted wastewater (1/2, 1/3, 1/4 and 1/6) were also conducted in order to study the influence of the pollutant concentrations and because the treatment of highly concentrated effluents was sometimes very difficult and energy consuming. The pH was continuously measured with a multi-parameter instrument (Consort C931, Turnhout, Belgium). COD was measured by the dichromate method [36] and 11
subsequent measurement of the optical density using a HACH 2400 spectrophotometer (Loveland, Colorado). Chromium and aluminium concentration were analysed by atomic absorption spectrometry after appropriate dilution in nitric acid solution. The removal efficiency of each pollutant (R) was calculated using the following equation:
R(%)
C 0 Ct x100 C0
(5)
where C0 and Ct represent, respectively, the initial and the final concentrations of COD or chromium. Moreover current efficiency of Al dissolution has been defined as the ratio of the weight amount of Al present in the solution or in the flocs at time t, mAl, , over that produced by anodic dissolution according to Faraday’s law, with three electrons exchanged per Al ion formed:
Al
m Al 60 It M Al x 3F
(6)
where I is the applied current (A), time t is in minutes, F is the Faraday constant and MAl is the molar weight of aluminium. 4. Results and discussion 4.1 Effect of current density and electrolysis time on treatment Cell current and electrolysis time are two important parameters in electrochemical processes. The value of the current density determines the coagulant production rate, adjusts the rate and size of the bubble production, and is hence to affect the growth of flocs. These parameters were varied in the range 200-400 A/m2 and 5-360 min respectively. The cell voltage varied typically from 2 to 3 V depending on the operating conditions: the moderate cell voltage is primarily due to the high conductivity of the waste to be treated, in particular in the treatment of the original wastewater (Table 1). 12
Fig. 3 shows the effect of current density on Cr (III) removal versus time using both types of electrodes: aluminium alloy (Duralumin) (Fig. 3a) and pure aluminum (Fig. 3b) respectively. The chromium removal efficiency increased with the current density and electrolysis time but in different proportions depending on the nature of the electrodes. With duralumin electrodes chromium removal yields of 93%, 95.4 and 99.7% were achieved after 360 min at current densities of 200, 300, and 400 A/m2, respectively. When aluminium electrodes were used, the abatement was lower at 200 A/m2 (75%) but similar 93 and 99 % at 300 and 400 A/m2 respectively. Fig. 4 shows the effect of current density versus time on COD removal using both types of electrodes. An increase in current density from 300 to 400 A/m2 yields to improved efficiency of COD removal from 81 to 95 % for the duralumin electrode after 360 min of treatment (Fig. 4a). Likewise what has been observed for chromium, duralumin electrodes are also more efficient than pure aluminium electrodes to remove COD (Fig. 4b). From the data shown in Figures 3 and 4, it can be observed that the treatment consumes an electrical charge ranging from 2.27 104 to 4.54 104 A.s L-1, which is really large in comparison with usual charges in other electrocoagulation processes which often range between 0.2 104 and 104 A.s L-1: assuming a 100% current efficiency for Al dissolution is to result in Al (III) concentration up to 4.23 g L-1 at the end of the run. At this point it can be wondered whether such a large amount of electricity consumed is due to particular interactions between the organic matter and Cr (III), or simply to the high concentrations of the substances to be removed. With pure aluminium electrodes, Zongo et al [37] noticed that the presence of Cr (VI) at 200 ppm does not affect COD abatement, but there are relatively few papers dealing with the competition between different types of pollutant at high concentrations. 13
4.2 Effect of the electrode nature on the process Initial pH plays an important role in performance of the electrocoagulation process and its change during the treatment is linked to the efficiency [38]. The pH increase is mainly attributed to the production of hydroxide ions (OH−) that are continuously generated from water reduction at the cathode as given by equation (3). However, the various equilibria between the Al hydroxide forms depending on the pH medium provide the system buffer properties. The pH of the waste under treatment can then attain a steady level after sufficient Al dissolution. It is known that the formation of solid Al(OH)3 is optimal for pH ranging from 6 to 8. Fig. 5 shows the variations of the tannery wastewater pH using both types of electrodes: duralumin (Fig. 5a) and pure aluminium (Fig. 5b). In both cases, the pH increased continuously with time for the three current density values considered. With duralumin electrodes, the final pH was between 6.5 and 7.0 depending on the current density, whereas with pure aluminium electrodes, a lower pH increase was observed up to 5.3 at the end of the run. These pH variations appear greatly dependent on the nature of the electrode materials. Fig. 6 show the concentration of aluminium dissolved versus the charge passed per liquid volume unit for the two electrode materials, for all values of the current density. As expected, the amount of dissolved Al species increases along time; interpretation of the data show that Al dissolution proceeds with current efficiency ranging from 1.2 to 3, depending on the operating conditions. The high concentration of chloride ions allows significant chemical dissolution of aluminium after reaction (3). Moreover the amount of Al (III) species is globally slightly higher with duralumin electrodes than for pure Al, as shown by comparison of Fig. 6a and 6b. As a matter of fact Cu-containing Al 14
electrodes are known to be more prone to pitting corrosion than pure aluminium. Chromium removal using duralumin was enhanced by the higher formation rate of Al(OH)3 whose flocs behave as an adsorbent for chromium ions. When the pH ranges between 4 and 7 co-precipitation of chromium hydroxides such as Cr(OH)3 can occur. The COD removal is also slightly higher using duralumin due to the affinity of the organic material to Al (III) species.
4.3 Effect of pollutant concentration on treatment efficiency and energy consumption The effluent has been diluted from 1 to 1/6, corresponding to conductivities of 60, 43, 31, 23 and 16 mS/cm respectively and treated by electrocoagulation at 200 A/m2 current density for 180 min. Figures 7 and 8 illustrate the amounts of chromium and COD removed versus aluminium produced for both electrode materials, for reaction times limited to 70 min. For the sake of clarity in the illustration, the results have been zoomed on the first part of the treatment, i.e. for amount of COD and Cr (III) removed below 3 g/L. In all cases, the amount of removed species increased obviously with the amount of Al (III) generated. For the highest dilution factors (3 or more), a plateau in the removed amounts can be observed, corresponding to nearly complete treatment. In spite of significant dispersion all data for low abatement degree below 50% converge in a regular variation with the amount of Al dissolved. Generation of Al (III) is little affected by the dilution in spite of lower concentration of sodium chloride, as exemplified for the case of pure aluminium in Figure 9, also in the first part of the runs: NaCl concentration remained actually at a sufficient level even in 1:6 dilution to promote Al dissolution, and the current efficiency in Al (III) varied from nearly 3 in the first minutes, down to 1.1 within 0.2 during the last hour. Comparable observation was done with duralumin (data not shown) 15
Table 2 shows the removal efficiency at the end of the treatment depending on the dilution factor. Depending on this factor, the initial concentrations of chromium and COD varied between 915 to 6672 mg/L, and 928 to 5864 mg O2/L respectively. The removal efficiency of COD is very high for the two Al-based materials when the dilution factor is higher than 1/2. On the contrary the abatement is lower for the concentrated effluents. For chromium, the abatement is lower but at acceptable level for the diluted effluents. However, as expressed above, it appears that Cr (III) and COD can be removed by adsorption of Al (III), therefore more diluted wastes can be efficiently treated with moderate amounts of coagulant, contrary to the original wastewater. The energy consumption was calculated in terms of kWh per m3 of wastewater to analyse the possible positive effect in this specific case of very concentrated effluents. The following relation has been used:
EEC
UIt103 60(C0 Ct )V
(7)
where U is the cell voltage measured during the electrolysis (V), I the applied electrical current (A), t the current time of treatment (min), and V the reactor volume (L). C0 and Ct represent the initial pollutant concentration and the concentration at time t (in mg/L). As exemplified in Fig. 10 for the example of Cr (III) removal using duralumin electrode, the specific energy increases along the run. Efficient treatment of the raw waste requires more than 20 kWh m-3: this value is high in comparison to usual energy demand in electrocoagulation processes but this is probably due to the high amounts of both COD and Cr (III). Dilution of the waste allows significant reduction of the energy demand per m3 of liquid treated in the cell: taking into account the dilution made, this would correspond to values ranging from 10 to 35 kWh m-3 raw waste depending on the dilution factor.
16
As a matter of fact the energy demand was calculated per kg of pollutant removed: Figs 11 and 12 show the energy consumption per kg of Cr(III) and COD removed, respectively, for both types of electrode (duralumin in Figs 11 a and 12 a, pure aluminium 11 b and 12 b). For both electrodes the energy consumption globally increased with an increase in Cr (III) and COD removal efficiency – with exception of a few data with Al electrodes for unknown reasons -: abatement of the last percents from waste is often more energy consuming. In spite of appreciable scattering of the data, energy consumption appears to be higher for the highly diluted solution (1:4 and 1:6). This is mainly due to the reduced conductivity, resulting in higher resistances of the cell [39] as confirmed by the cell voltage which attained 3.2 V for the 1:6 diluted waste. Comparing Table 2 and Fig 11 shows that a limited dilution allows considerable reduction in the chromium concentration in the effluent with very reasonable energy consumption: twofold dilution appears as the best compromise, allowing Cr (III) removal with an energy consumption near 1 kWh/kg, in comparison to nearly the double from the raw waste. The removal of COD in comparison with chromium was found to be very effective and less energy-demanding for both types of electrodes (Figure 12). The effect of the dilution factor is less visible than for Cr (III) removal because of the data scattering but diluting two times the waste issued from the tannery plant allowed energy consumption near 0.7 kWh/kg COD removed. Electrocoagulation process is very acceptable from the energy consumption point of view. For the sake of comparison, data reported in [40] revealed 3.8 kWh/kg COD removed with a 58% of abatement when treated tannery wastewater using an electroFenton process. Strong variations of the energy consumption can be expected: Daneshvar et al. [41] reported energy consumption between 4.7 and 7.57 kWh/kg COD
17
under optimized conditions to remove dyes by electrocoagulation but it rose to more than 15kWh for nearly complete abatement. A trade-off between abatement degree and energy consumption should be made.
5. Conclusions The treatment by electrocoagulation of tannery wastewater highly concentrated in chromium and COD was investigated using two different types of aluminium based electrodes. Duralumin aluminium alloy was found to be more efficient for COD and Cr removal compared to pure aluminium electrodes. The removal of chromium is mainly due to adsorption on aluminium hydroxide. Duralumin much more easily dissolved and the concentration of aluminium versus charge is higher, which favours the removal of chromium. The COD elimination is also due to the affinity of the organic material to Al (III) species in the form of Al(OH)3. Dilution of the very concentrated effluents can allow reduction in the energy consumption and improvement of the efficiency of the treatment: for the waste considered, twofold dilution appeared optimal. The noticeable differences we noticed between electrodes made of pure aluminium and the Al alloy show the necessity to deeply investigate the influence of the nature of the electrode materials for other types of effluents.
ACKNOWLEDGEMENTS: The authors would like to acknowledge the France-Morocco cooperation for the financial support of this work in the framework of the “Water and Environment Competency Pole” project. In addition, the authors greatly thank the manager for the leather industry Mr Slaissi Abd Aljalil for his help.
18
REFERENCES [1]
J. D. Sautet, O. Gand, P. Jardin, J.-M. Mornas, A. Daif, Rapport de synthèse – Etude d’analyse du potentiel du cuir (2003) http://www.abhatoo.net.ma/maalamatextuelle/developpement-economique-et-social/developpementeconomique/industrie/industrie-du-textile-et-du-cuir/rapport-de-synthese-etude-danalyse-du-potentiel-du-secteur-cuir [in French]
[2]
P. Religa, A. Kowalik, P. Gierycz,Application of nanofiltration for chromium concentration in the tannery wastewater, J. Hazard. Mater.186 (2011) 288–292.
[3]
A. Benhadji, M-T. Ahmed, R. Maachi, Electrocoagulation and effect of cathode materials on the removal of pollutants from tannery wastewater of Rouïba, Desalination 277 (2011) 128–134
[4]
M. Fabbricino, B. Naviglio, G. Tortora, L. d’Antonio, An environmental friendly cycle for Cr(III) removal and recovery from tannery wastewater, J. Environ. Manag. 117 (2013) 1-6
[5]
N. F. Fahim, B.N. Barsoum, A. E. Eid, M. S. Khalil, Removal of chromium (III) from tannery wastewater using activated carbon from sugar industrial waste, J. Hazard. Mater. B136 (2006) 303-309.
[6]
S. Vasudevan, J. Lakshmi, G. Sozhan, Studies on the Al–Zn–In-alloy as anode material for the removal of chromium from drinking water in electrocoagulation process. Desalination 275 (2011) 260-268.
[7]
G. Lofrano, S. Meriç, G. E. Zengin, D. Orhon, Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: a review, J. Sci. Total Environ. 461-462 (2013) 265-281.
[8]
N. Meunier, P. Drogui, C. Montane, R. Hausler, G. Mercier, J.-F. Blais, Comparison between electrocoagulation and chemical precipitation for metals removal from acidic soil leachate, J. Hazard. Mater. B137 (2006) 581–590.
[9]
V. Keerthi Suganthi, M. Mahalakshmi, N. Balasubramanian, Development of hybrid membrane bioreactor for tannery effluent treatment. Desalination 309 (2013) 231–236.
[10] Z. Song, C.J. Williams, R. G. J. Edyvean, Treatment of tannery wastewater by chemical coagulation. Desalination, 164 (3) (2004) 249–259. [11] S. Elabbas, L. Mandi, F. Berrekhis, M.N. Pons, J-P. Leclerc, N. Ouazzani, Removal of Cr(III) from chromium tanning wastewater by adsorption using two natural carbonaceous materials: Eggshell and powdered marble. Environ. Manag. J. in press (1-7) [12] S. Tiglyene, A. Jaouad, L. Mandi, Treatment if tannery wastewater by infiltration percolation :chromium removal and speciation un soil. Environ.Technol.29 (2008) 613624 19
[13] S.K. Sahu, P. Meshram, B.D. Pandey, V. Kumar, T. R. Mankhand, Removal of chromium (III) by cation exchange resin, Indion 790 for tannery waste treatment, Hydrometallurgy 99 (2009) 170-174. [14] Y.A. Ouaissaa, M. Chabania, Abdelatif, A. Bensmailia, Integration of electro coagulation and adsorption for the treatment of tannery wastewater – The case of an Algerian factory, Rouiba, Procedia Engineering 33 (2012) 98-101. [15] C. R. Costa, C. M. R. Botta,E. L. G. Espindola, P. Olivi, Electrochemical treatment of tannery wastewater using DSA® electrodes, J. Hazard. Mater.153 (2008) 616-627. [16] S. Sundarapandiyan, R. Chandrasekar, B. Ramanaiah, S. Krishnan, P. Saravanan, Electrochemical oxidation and reuse of tannery saline wastewater, J. Hazard. Mater. 180 (2010) 197-203. [17] L. Sirajuddin, G. Kakakhel, M.I. Lutfullah, A. Bhanger, A. Shah, A. Niaz, Electrolytic recovery of chromium salts from tannery wastewater, J. Hazard. Mater. 148 (2007) 560-565. [18] E. Isarain-Chávez, C. De la Rosa, L.A. Godínez, E. Brillas, J.M. PeraltaHernández, Comparative study of electrochemical water treatment processes for a tannery wastewater effluent, J. Electroanal. Chem. 713 (2014) 62-69. [19] Z. Houshyar, A. B. Khoshfetrat, E. Fatehifar, Influence of ozonation process on characteristics of pre-alkalized tannery effluents, Chem. Eng. J. 191 (2012) 59-65. [20]
L. Mandi, S. Tiglyene, A. Jaouad, 2009. Depuration of tannery effluent by phytoremediation and infiltration percolation under arid climate in: M. El Moeujabber, L., Mandi, G. Trisorio-Liuzzi, G., I. Martin, A. Rabi, R. Rodriguez, R. (Eds), Technological Perspectives for rational Use of Water Resources un the Mediterranean Region. Publisher: Bari CIHEAM (2009), pp. 199-205.
[21] S. Zodi, O. Potier, F. Lapicque, J.-P. Leclerc, Treatment of textile wastewaters by electrocoagulation: Effect of operating parameters on the sludge settling characteristics, Sep. Purif. Technol. 69 (2009) 29-36. [22] S. Zodi, B. Merzouk, O. Potier, F. Lapicque, J.-P. Leclerc, Direct red 81 dye removal by a continuous flow electrocoagulation /flotation reactor, Separation Sep. Purif. Technol. 108 (2013) 215-222. [23] M.S. Oncel, A. Muhcu, E. Demirbas, M. Kobya, A comparative study of chemical precipitation and electrocoagulation for treatment of coal acid drainage wastewater, J. Environ. Chem. Eng. 1 (2013) 989-995. [24] A. Attour, M. Touati, M. Tlili, M. Ben Amor, F. Lapicque, J.-P. Leclerc, Influence of operating parameters on phosphate removal from water by electrocoagulation using aluminum electrodes, Sep. Purif. Technol. 123 (2014) 124-129.
20
[25] P. Cañizares, F. Martinez, C. Jiménez, C. Sáez, M.A. Rodrigo, Coagulation and electrocoagulation of oil-in-water emulsions, J. Hazard. Mater. 151 (2008) 44-51 [26] D. Valero, J.M. Ortiz, V. Garcia, E. Espósito, V. Montiel, A. Aldaz, Electrocoagulation of wastewater from almond industry, Chemosphere 84 (2011)1290-1295 (27] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC) – science and applications, J. Hazard. Mater. B84 (2001) 29-41 [28] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, J. Hazard. Mater. B114 (2004) 199-210 [29] S. Vasudevan, J. Lakshmi, J. Jayaraj, G. Sozhan, Remediation of phosphate contaminated water by electrocoagulation with aluminum, aluminum alloy and mild steel anodes, J. Hazard. Mater. 164 (2009) 1480-1486. [30] N. L. Kruthika, S. Karthika, G. Bhaskar Raju, S. Prabhakar, Efficacy of electrocoagulation and electrooxidation for the purification of wastewater generated from gelatin production plant, J. Environ. Chem. Eng. 1 (2013) 183188. [31] S.Saravanbahavan, P. Thaikaivelan, J. Raghava Rao, N. B. Nair, T. Ramasami, Natural leathers from natural materials: progressing toward a new arena in leather processing, J. Environ. Sci. Technol. 38(3) (2004) 871–879. [32] J. Feng., Y. Sun., Z. Zheng., J. Zhang., S. Li., Y. Tian., Treatment of tannery wastewater by electrocoagulation. J. Environ. Sci. 19 (2007) 1409-1415 [33] C. Regina Costa, P. Olivi., Effect of chloride concentration on the electrochemical treatment of a synthetic tannery wastewater. Electrochim. Acta 54 (2009) 20462052. [34] F.R. Espinoza-Quiñones, M.M.T. Fornari, A.N. Módenes, S.M. Palácio, F.G. Silva Jr., N. Szymanski, A.D. Kroumov, D.E.G. Trigueros, Pollutant removal from tannery effluent by electrocoagulation, Chem. Eng. J. 151 (2009) 59–65. [35] S. Zodi, J.N. Louvet, C. Michon, O. Potier, M.N. Pons, F. Lapicque, J.P. Leclerc, Electrocoagulation as a tertiary treatment for paper mill wastewater: Removal of non biodegradableorganic pollution and arsenic. Sep. Purif. Technol. 81 (2011) 62-68 [36] S .Zodi, Étude de l'épuration d'effluents de composition complexe par électrocoagulation et des couplages intervenants entre le traitement électrochimique et l'étape de séparation : application à l'industrie textile et papetière. PhD Dissertation, Université de Lorraine, Nancy, France (2012). [in French] [37] I. Zongo, J.-P. Leclerc, H. G. Maiga, J. Wethee, F. Lapicque, Removal of hexavalent chromium from industrial wastewater by electrocoagulation: A 21
comprehensive comparison of aluminium and iron electrodes, Sep. Purif. Technol. 66 (2009) 159-166. [38] P. Canizares, C. Jiménez, F. Martínez, M.A. Rodrigo, C. Sáez, The pH as a key parameter in the choice between coagulation and electrocoagulation for the treatment of wastewaters, J. Hazard. Mater. 163 (2009) 158–164. [39] H.A. Moreno-Casillas, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson, Electrocoagulation mechanism for COD removal, Sep. Purif. Technol. 56 (2007) 204–211. [40] U. Kurt, O. Apaydin, M. Talha Gonullu, Reduction of COD in wastewater from an organized tannery industrial region by Electro-Fenton process, J. Hazard. Mater. 143 (2007) 33-40. [41] N. Daneshvar, A. Oladegaragoze, N. Djafarzadeh, Decolorization of basic dye solutions by electrocoagulation: An investigation of the effect of operational parameters, J. Hazard. Mater. B129 (2006) 116–122.
22
Figure Caption Fig. 1.Various unit processes and operations in leather processing Fig. 2. Experimental set-up for electrocoagulation tests Fig. 3: Effect of current density on the chromium concentration versus time (a) duralumin (b) pure aluminium. Fig. 4: Effect of current density on the COD concentration versus time (a) duralumin (b) pure aluminium. Fig. 5: pH versus time (a) duralumin, (b) pure aluminium. Fig. 6: Al (III) concentration versus the electrical charge: (a) duralumin, (b) pure aluminium electrode. Fig. 7: Chromium removed versus Al (III) produced at 200 A m-2 for several dilution factors: (a) duralumin, (b) pure aluminium. Fig. 8: COD removed versus Al (III) produced at 200 A m-2 for several dilution factors: (a) duralumin, (b) pure aluminium Fig. 9: Concentration of Al (III) produced versus the electrical charge with pure aluminium electrodes. Current density = 200 A m-2 Fig. 10: Energy consumed for the treatment of 1 m3 diluted waste along the run for Cr (III) removal. Duralumin electrodes were used at 200 A m-2 Fig. 11: Effect of effluent dilution on the energy consumption per kg of chromium removed versus chromium removal efficiency; current density = 200 A m-2; (a) duralumin, (b) pure aluminium Fig. 12: Effect of effluent dilution on the energy consumption per kg of COD removal versus COD removal efficiency; current density = 200 A m-2; (a) duralumin, (b) pure aluminium.
23
24
25
26
27
28
29
30
31
32
33
34
35
Tables Table 1: Main characteristics of the wastewater Parameter pH Conductivity (mS/cm) COD (g O2/L) Cl-(g/L) SO42- (g/L) Cr (III) (g/L)
Value 3.8 54.2 5.8 23.3 1.3 7.0
36
Table 2: Effect of effluent dilution on the maximum COD and chromium removal efficiency using duralumin and pure aluminum electrodes; operation at 200 A/m2 for 180 minutes. Dilution factor Removal efficiency (%) Duralumin COD
Cr (III)
1:1
1:2
1:3
1:4
1:6
58.1
97.6
100
100
100
Pure Al
43.7
100
100
100
100
Duralumin
73.3
91.7
94.5
96.2
98.1
Pure Al
57.2
91.2
92.9
97.9
98.1
37
S0304-3894(15)30327-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.12.067 HAZMAT 17347
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
12-10-2015 21-12-2015 30-12-2015
Please cite this article as: S.Elabbas, N.Ouazzani, L.Mandi, F.Berrekhis, M.Perdicakis, S.Pontvianne, M-N.Pons, F.Lapicque, J-P Leclerc, Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.12.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode S. Elabbasa,b, N. Ouazzania,b, L. Mandia,b, F. Berrekhisc, M. Perdicakisd, S. Pontviannee, M-N. Ponse, F. Lapicquee, J-P Leclerce* [email protected] a
Laboratoire d’Hydrobiologie, Ecotoxicologie et Assainissement (LHEA, URAC 33), Faculté de Sciences Semlalia, BP2390, Université Cadi Ayyad, Marrakech, Maroc.
b
Centre National d’Etude et de Recherche sur l’Eau et l’Energie (CNEREE), Université Cadi Ayyad, BP511, Marrakech, Maroc.
c
Equipe de Physico-chimie des Matériaux, Ecole Normale Supérieure, Université Cadi Ayyad, BP2400, 40000 Marrakech, Maroc d Laboratoire de Chimie Physique et Microbiologie pour l'Environnement (LCPME) UMR 7564, Université de Lorraine – CNRS, 405 rue de Vandœuvre, F-54602 Villerslès Nancy Cedex, France e Laboratoire Réactions et Génie des Procédés (LRGP) UMR 7274, CNRS, Université de Lorraine, 1 rue Grandville, BP 20451, 54001 Nancy cedex, France. *
Corresponding author. Tel: +33 (0)3 83 17 52 77; Fax: +33(0)3 73 32 29 75.
1
Highlights COD and Cr (III) species can be simultaneously removed by electrocoagulation Cu-containing Al alloy is more efficient than pure Al as electrodes Dilution of too concentrated tannery wastewater is required for efficient treatment
2
Abstract This paper deals with the ability of electrocoagulation (EC) to remove simultaneously COD and chromium from a real chrome tanning wastewater in a batch stirred electrocoagulation cell provided with two aluminium-based electrodes (aluminium/copper/ magnesium alloy and pure aluminum). Effects of operating time, current density and initial concentration of Cr (III) and COD have been investigated. The concentrations of pollutants have been successfully reduced to environmentally acceptable levels even if the concentrated effluent requires a long time of treatment of around 6 hours with a 400 A/m2 current density. The aluminium alloy was found to be more efficient than pure aluminium for removal of COD and chromium. Dilution of the waste has been tested for treatment: high abatement levels could be obtained with shorter time of treatment and lower current densities. Energy consumption of the electrocoagulation process was also discussed. The dilution by half of the concentrated waste leads to a higher abatement performance of both COD and chromium with the best energy efficiency.
Keywords: Aluminum electrodes; chromium; Current density; Electrocoagulation; Tannery wastewater
3
1. Introduction Leather production, associated to cloth and craft industries, is a very important activity in many countries since antiquity. It represents a large economic input in many developing countries but is also unfortunately an important source of pollution. The tanning process is a transformation of raw animal hides into leather. It involves various chemical and mechanical operations to remove the residual meat, fat and hair from the skin, to deeply clean the skin and to improve its aspect and its functional properties. Despite the existence of less pollutant treatments, chromium tanning remains the most employed technique because of the higher quality of the leather obtained. A report issued in 2003 has estimated the number of tanneries in Morocco to more than 60 units, most of them using chromium [1]. Tannery wastewater rise important environmental problems because of their complex composition and high concentrations of various organic and inorganic chemicals reflected in particular by high values of Chemical Oxygen Demand (COD) [2, 3] and chromium concentration over 5 g/L [4, 5]. Although new tanneries use modern equipments with optimal adjustment of the chromium quantity, conventional chromium bath tanning processes still use chromium salts in excess. Fahim et al. [5] estimated that the leather takes up only 60 to 80% of the introduced chromium. Depending on the efficiency of the operation, wastewater contains chromium species, which must be removed before disposal. Removal of chromium has been also studied for drinking water but the concentrations are there far much lower than in tannery wastewater [6]. Several methods have been attempted and a recent review summarizes the chemical and biological treatment technologies for leather tannery wastewaters and their efficiencies [7]. In comparison between electrocoagulation and chemical precipitation to remove metals from acidic soil leachate, Meunier et al. [8] have reported chromium removal of
4
more than 99% with an initial concentration of 100 mg/L by chemical precipitation in the presence of NaOH and Ca(OH)2. Keerthi et al. [9] have tested a hybrid membrane bioreactor with a COD removal of 72.69 % from wastewater with an initial Cr concentration at 1600 mg/L. Using coagulation and flocculation, Song et al. [10] have obtained a COD removal of 30 to 37% with an initial concentration of 3300 mg/L. Adsorption has also been used for treatment of tannery effluents [4, 5, 11, 12]. Fabbricino et al. [4] have reported 90% of Cr (III) removal from an initial concentration of 6100 mg/L using adsorption in an environment friendly cycle with recovery of the metal. However this last process may be not be economically feasible in the treatment of tannery wastewater because of high costs and the expertise required to implement and sustain the operation of such processes. Fahim et al. [5] reported absorption of nearly 99% of the chromium from tannery waste water using activated carbon at an final concentration of 44 mg/L from the initial level at 5500 mg/L. The adsorbent can also be a natural soil as explored by Tiglyene et al. [12]. Use of a cation exchange resin was reported to remove 96% of Cr (III) from a waste solution issued by tannery [13] but the concentrated effluent of 5480 mg/L was previously diluted 10-fold. Recent studies have been presented to facilitate the use of low chromium solutions by possible effective chromium recycling through re-concentration using nanofiltration [2]. Ouaissaa et al. [14] have obtained a 75% COD abatement of a low polluted tannery wastewater using an intensified electrocoagulation process by addition of an adsorbent. Costa et al. [15] have reported a 40% TOC abatement using dimensionally stable electrodes (DSA®), however with the risk of oxidation of Cr (III) to Cr (VI). Sundarapandiyan et al. [16] have studied the specific case of a very salty tannery wastewater (around 30g/L of Cl-): the high salinity was shown not to affect the efficiency of the treatment. Chromium from highly concentrated tannery wastewater (around 2.5 g/L) could be recovered at
5
99% using Pb anodes and Cu cathodes after dilution of the effluent to less than 0.3 g/L [17]. Sirajuddin et al. [18] studied several electrochemical processes to treat around 10 g/L of COD from tannery wastewater effluent but only the results concerning TOC were presented. Finally, for treatment of a very concentrated effluent (COD 5470 mg/L and Cr 6900 mg/L) ozonation could be used with COD abatement up to 88% (Houshyara et al. [19]) but this treatment can oxidize Cr (III) into Cr (VI), which generates another pollution issue. Finally, phytoremediation has investigated too for treatment of such waste, as done by Mandi et al. [20].
Electrocoagulation has been used in several of the above mentioned results and it has been successfully applied to treat numerous varieties of wastewater, of either industrial or urban origin. The main advantage of electrocoagulation is its ability to treat simultaneously a wide range of type of pollutants including organic matter, heavy metals, dyes, and minerals. It was found to be effective for the removal of dye [21, 22], heavy metals from coal acid drainage wastewater [23], non-biodegradable organic pollution and arsenic from paper mill wastewater [22], and several others types of effluents often listed in the literature. Phosphates were also successfully removed from (surface or ground) water using this technique [24]. Moreover the experimental data gained in treatment tests could lead to physicochemical modelling of electrocoagulation process [25] or in scaling-up and economic considerations [26].
Electrocoagulation uses a direct current source between two metal electrodes immersed in polluted water. The electrical current causes the dissolution of anodes made of iron or aluminium into wastewater. The metal species released in solution, at an appropriate pH, can form wide ranges of coagulated species and metal hydroxides that destabilize
6
and aggregate the suspended particles or precipitate and adsorb dissolved contaminants. In the case of aluminium, Al3+ ions are generated at the anode and combine to OH− ions provided by the cathode to form various monomeric species, which finally transform to solid Al(OH)3(s). Freshly formed amorphous Al(OH)3(s) “sweep flocs” have large surface areas, which are beneficial for rapid adsorption of soluble organic compounds and trapping of colloidal particles. These flocs are removed easily from the aqueous medium by sedimentation or flotation. In addition to ref. [25] other groups attempted in better understanding EC process [27, 28] in view to its more efficient integration in overall water treatment operations. Numerous papers have compared the effect of the electrodes nature (namely iron and aluminium) whereas the purity of the anodes was rarely investigated. Vasudevan et al [29] pointed out the lower efficiency of pure aluminium electrodes compared to aluminium alloys in the remediation of phosphate-contaminated water. The lower efficiency of pure aluminium was explained by passive film formation, lower dissolution efficiency, non-uniform dissolution and higher operating voltage with time. The direct oxidation of dissolved substances is also affected by the nature of the electrode as pointed out by Kruthika et al [30]. The aim of the present study was to investigate the treatment of highly polluted traditional tannery wastewater by electrocoagulation with initial concentrations around 6 g/L COD and 7 g/L Cr (III). In comparison with pollutant levels usually encountered, these high concentrations of COD and Cr (III) are due to the nature of the process involved: natural skin (inducing high organic pollution), implication of various chemicals during the treatment and incomplete chromium absorption by the skin resulting in a high level of chromium in wastewater [31]. Another reason is that the liquid released during this specific step is rejected directly with other effluents of the
7
tanning process. Removal of COD and Cr (III) was followed in discontinuous tests carried out with recirculation of the electrolyte solution depending on the operating conditions, and in particular depending on the nature of the Al-based electrodes. First tests conducted with the raw wastewater revealed moderate treatment efficiency for both pollutants, presumably because of its high contents in both Cr species and COD. It was then decided to dilute the wastewater prior to its treatment, for the sake of more efficient operation. The optimal dilution factor was determined for the sake of high removal yield and acceptable energy consumption per m3 of the original wastewater.
2. Mechanism of the electrocoagulation using aluminium electrodes In electrocoagulation processes, the coagulating species are produced in situ and the overall process consists of three successive stages: (i) formation of coagulants by electrolytic oxidation of the sacrificial electrode (ii) destabilization of the contaminants, particulate suspension and breaking of emulsion and (iii) aggregation of the destabilized phases to form flocs [25, 27-28]. The main reactions occurring at the electrodes are: At aluminum anode: Al→Al3+ + 3e–
(1)
At aluminum cathode: 2H2O + 2e– →
H2 + 2OH−:
(2)
Moreover in the presence of chloride ions, aluminium from the cathode can dissolve and form hydrogen: Al + 3 H2O → Al3+ + 3 OH- + 3/2 H2 In the solution: 8
(3)
Al3+ + 2OH−
→ Al(OH)3
(4)
Because of the standard potential of the Al/Al3+ couple near -1.8 V/NHE, occurrence of side reactions e.g. oxygen evolution or Cr (III) oxidation to Cr (VI) with standard potentials at 1.23 and 1.33 V respectively, is very unlikely and have not been considered here. Two types of aluminium electrodes were employed, namely the aluminium alloy (Duralumin) and pure aluminium aiming to assess the effect of the chemical composition of the electrodes on the treatment effectiveness. For both electrodes, operating time, current density and dilution rate have been investigated to determine the optimal conditions for chromium and organic matter removal and electrical energy consumption but also to point out the effect of aluminium electrodes composition on efficiency and super-faradaic charge yields.
3. Materials and methods 3.1 Tannery effluent samples Leather processing involves a number of unit operations as shown in Fig 1. The pretanning operation includes successively the following steps: soaking, liming, removal of extraneous tissues (unhairing), deliming and bating. Furthermore chromium tanning treatment involves degreasing, pickling by acidification, pressing and drying. Finally the shaving, dyeing and other operations - depending on the final use of the leather - are conducted for obtain a finished leather. The samples were obtained from the tank used for chromium tanning step in a small traditional tannery in Marrakech city (Morocco) having a capacity of treatment of about 500 hides/day. This tank is a large wooden cylinder (3.5 m high and with a radius of 1.56 m) placed horizontally. The effective volume of the tank was 17.14 m3. The barrel
9
can rotate on two hollow metal shafts for possible introduction of raw hides, chemicals, dyestuff and water through the square-shaped hatch of the tank. A gear wheel fixed on the flange of the barrel allows transmission of rotation. Large grab samples (10 L) were collected in polyethylene containers and stored at 4°C in the dark until use. Their average characteristics are given in Table 1. Sub-samples of 2 L liquid were used for each electrocoagulation test. The effluent was characterized by very high chromium content near 7 g/L and about 6 g/L of COD. The composition of tannery wastewater is strongly depending on the level of optimization of the factory. If the modern industry releases less polluted wastewater, the majority of semi-traditional industries used very concentrated solutions of chromium to guaranty the quality of the treatment. Several publications have shown the efficiency of the treatment of tannery wastewater, however most of the papers have focused their interest on the organic pollution characterized by TOC or COD [9, 32] and the papers including the follow-up of chromium are based on low chromium concentrations, generally lower than 100 mg/L [33, 34].
3.2 Electrode materials Two types of aluminum electrode have been used in this present study. Aluminum alloy (duralumin) A-U4G (2017-Al) had the following metal contents: Cu (4%), Fe (0.7%), Mg (0.7%), Mn (0.7%), Si (0.5%), Zn (0.25%) and Cr (0.1%). The “pure” aluminium (1050) electrodes contained actually impurities but to a low level as listed below: Si (0.25%), Fe (0.4%), Cu (0.05%), Ti(0.05%), Mn(0.05%), Zn(0.07%).
3.3 Experimental set-up
10
Electrocoagulation treatment was carried out in a discontinuous system under solution recirculation functioning. The electrocoagulation cell used, consisted of two identical polymethylmethacrylate (Perspex) halves being 20 cm long, 10 cm wide and 5 cm thick. Metal plates with an area of (157 cm2) acting as facing electrodes were imbedded in the polymeric halves, as described in [35]. The active electrode surface was first mechanically polished under water with abrasive paper, rinsed with ultra-pure water and dried prior to immersion in the wastewater. The area of each electrode was 105 cm2. The gap between anode and cathode was maintained at 2 cm. The electrodes were connected to a DC power supply (AFX 2930 SB, China) providing current intensities up to 5 A. The current was ranging from 2.1 to 4.2 A (200 – 400 A m-2 respectively) in the present investigation. The current was kept constant during each run and the cell voltage was monitored. The schematic diagram of the complete electro coagulation system is shown in Figure 2. 3.4 Experiments The experiments were conducted at ambient temperature ( 20°C) with 2 L wastewater gently stirred at 200 rpm in the tank. The solution was continuously circulated in the flow circuit by means of a peristaltic pump at 50 mL/min. Ten cm3 fractions were sampled at regular time intervals (every 10 minutes during the first hour and every 30 minutes later), allowed to settle for 24 hours, then filtered (0.45 µm pore size) before any analysis to determine effluent COD, Cr (III) and Al (III) concentrations. Experiments with diluted wastewater (1/2, 1/3, 1/4 and 1/6) were also conducted in order to study the influence of the pollutant concentrations and because the treatment of highly concentrated effluents was sometimes very difficult and energy consuming. The pH was continuously measured with a multi-parameter instrument (Consort C931, Turnhout, Belgium). COD was measured by the dichromate method [36] and 11
subsequent measurement of the optical density using a HACH 2400 spectrophotometer (Loveland, Colorado). Chromium and aluminium concentration were analysed by atomic absorption spectrometry after appropriate dilution in nitric acid solution. The removal efficiency of each pollutant (R) was calculated using the following equation:
R(%)
C 0 Ct x100 C0
(5)
where C0 and Ct represent, respectively, the initial and the final concentrations of COD or chromium. Moreover current efficiency of Al dissolution has been defined as the ratio of the weight amount of Al present in the solution or in the flocs at time t, mAl, , over that produced by anodic dissolution according to Faraday’s law, with three electrons exchanged per Al ion formed:
Al
m Al 60 It M Al x 3F
(6)
where I is the applied current (A), time t is in minutes, F is the Faraday constant and MAl is the molar weight of aluminium. 4. Results and discussion 4.1 Effect of current density and electrolysis time on treatment Cell current and electrolysis time are two important parameters in electrochemical processes. The value of the current density determines the coagulant production rate, adjusts the rate and size of the bubble production, and is hence to affect the growth of flocs. These parameters were varied in the range 200-400 A/m2 and 5-360 min respectively. The cell voltage varied typically from 2 to 3 V depending on the operating conditions: the moderate cell voltage is primarily due to the high conductivity of the waste to be treated, in particular in the treatment of the original wastewater (Table 1). 12
Fig. 3 shows the effect of current density on Cr (III) removal versus time using both types of electrodes: aluminium alloy (Duralumin) (Fig. 3a) and pure aluminum (Fig. 3b) respectively. The chromium removal efficiency increased with the current density and electrolysis time but in different proportions depending on the nature of the electrodes. With duralumin electrodes chromium removal yields of 93%, 95.4 and 99.7% were achieved after 360 min at current densities of 200, 300, and 400 A/m2, respectively. When aluminium electrodes were used, the abatement was lower at 200 A/m2 (75%) but similar 93 and 99 % at 300 and 400 A/m2 respectively. Fig. 4 shows the effect of current density versus time on COD removal using both types of electrodes. An increase in current density from 300 to 400 A/m2 yields to improved efficiency of COD removal from 81 to 95 % for the duralumin electrode after 360 min of treatment (Fig. 4a). Likewise what has been observed for chromium, duralumin electrodes are also more efficient than pure aluminium electrodes to remove COD (Fig. 4b). From the data shown in Figures 3 and 4, it can be observed that the treatment consumes an electrical charge ranging from 2.27 104 to 4.54 104 A.s L-1, which is really large in comparison with usual charges in other electrocoagulation processes which often range between 0.2 104 and 104 A.s L-1: assuming a 100% current efficiency for Al dissolution is to result in Al (III) concentration up to 4.23 g L-1 at the end of the run. At this point it can be wondered whether such a large amount of electricity consumed is due to particular interactions between the organic matter and Cr (III), or simply to the high concentrations of the substances to be removed. With pure aluminium electrodes, Zongo et al [37] noticed that the presence of Cr (VI) at 200 ppm does not affect COD abatement, but there are relatively few papers dealing with the competition between different types of pollutant at high concentrations. 13
4.2 Effect of the electrode nature on the process Initial pH plays an important role in performance of the electrocoagulation process and its change during the treatment is linked to the efficiency [38]. The pH increase is mainly attributed to the production of hydroxide ions (OH−) that are continuously generated from water reduction at the cathode as given by equation (3). However, the various equilibria between the Al hydroxide forms depending on the pH medium provide the system buffer properties. The pH of the waste under treatment can then attain a steady level after sufficient Al dissolution. It is known that the formation of solid Al(OH)3 is optimal for pH ranging from 6 to 8. Fig. 5 shows the variations of the tannery wastewater pH using both types of electrodes: duralumin (Fig. 5a) and pure aluminium (Fig. 5b). In both cases, the pH increased continuously with time for the three current density values considered. With duralumin electrodes, the final pH was between 6.5 and 7.0 depending on the current density, whereas with pure aluminium electrodes, a lower pH increase was observed up to 5.3 at the end of the run. These pH variations appear greatly dependent on the nature of the electrode materials. Fig. 6 show the concentration of aluminium dissolved versus the charge passed per liquid volume unit for the two electrode materials, for all values of the current density. As expected, the amount of dissolved Al species increases along time; interpretation of the data show that Al dissolution proceeds with current efficiency ranging from 1.2 to 3, depending on the operating conditions. The high concentration of chloride ions allows significant chemical dissolution of aluminium after reaction (3). Moreover the amount of Al (III) species is globally slightly higher with duralumin electrodes than for pure Al, as shown by comparison of Fig. 6a and 6b. As a matter of fact Cu-containing Al 14
electrodes are known to be more prone to pitting corrosion than pure aluminium. Chromium removal using duralumin was enhanced by the higher formation rate of Al(OH)3 whose flocs behave as an adsorbent for chromium ions. When the pH ranges between 4 and 7 co-precipitation of chromium hydroxides such as Cr(OH)3 can occur. The COD removal is also slightly higher using duralumin due to the affinity of the organic material to Al (III) species.
4.3 Effect of pollutant concentration on treatment efficiency and energy consumption The effluent has been diluted from 1 to 1/6, corresponding to conductivities of 60, 43, 31, 23 and 16 mS/cm respectively and treated by electrocoagulation at 200 A/m2 current density for 180 min. Figures 7 and 8 illustrate the amounts of chromium and COD removed versus aluminium produced for both electrode materials, for reaction times limited to 70 min. For the sake of clarity in the illustration, the results have been zoomed on the first part of the treatment, i.e. for amount of COD and Cr (III) removed below 3 g/L. In all cases, the amount of removed species increased obviously with the amount of Al (III) generated. For the highest dilution factors (3 or more), a plateau in the removed amounts can be observed, corresponding to nearly complete treatment. In spite of significant dispersion all data for low abatement degree below 50% converge in a regular variation with the amount of Al dissolved. Generation of Al (III) is little affected by the dilution in spite of lower concentration of sodium chloride, as exemplified for the case of pure aluminium in Figure 9, also in the first part of the runs: NaCl concentration remained actually at a sufficient level even in 1:6 dilution to promote Al dissolution, and the current efficiency in Al (III) varied from nearly 3 in the first minutes, down to 1.1 within 0.2 during the last hour. Comparable observation was done with duralumin (data not shown) 15
Table 2 shows the removal efficiency at the end of the treatment depending on the dilution factor. Depending on this factor, the initial concentrations of chromium and COD varied between 915 to 6672 mg/L, and 928 to 5864 mg O2/L respectively. The removal efficiency of COD is very high for the two Al-based materials when the dilution factor is higher than 1/2. On the contrary the abatement is lower for the concentrated effluents. For chromium, the abatement is lower but at acceptable level for the diluted effluents. However, as expressed above, it appears that Cr (III) and COD can be removed by adsorption of Al (III), therefore more diluted wastes can be efficiently treated with moderate amounts of coagulant, contrary to the original wastewater. The energy consumption was calculated in terms of kWh per m3 of wastewater to analyse the possible positive effect in this specific case of very concentrated effluents. The following relation has been used:
EEC
UIt103 60(C0 Ct )V
(7)
where U is the cell voltage measured during the electrolysis (V), I the applied electrical current (A), t the current time of treatment (min), and V the reactor volume (L). C0 and Ct represent the initial pollutant concentration and the concentration at time t (in mg/L). As exemplified in Fig. 10 for the example of Cr (III) removal using duralumin electrode, the specific energy increases along the run. Efficient treatment of the raw waste requires more than 20 kWh m-3: this value is high in comparison to usual energy demand in electrocoagulation processes but this is probably due to the high amounts of both COD and Cr (III). Dilution of the waste allows significant reduction of the energy demand per m3 of liquid treated in the cell: taking into account the dilution made, this would correspond to values ranging from 10 to 35 kWh m-3 raw waste depending on the dilution factor.
16
As a matter of fact the energy demand was calculated per kg of pollutant removed: Figs 11 and 12 show the energy consumption per kg of Cr(III) and COD removed, respectively, for both types of electrode (duralumin in Figs 11 a and 12 a, pure aluminium 11 b and 12 b). For both electrodes the energy consumption globally increased with an increase in Cr (III) and COD removal efficiency – with exception of a few data with Al electrodes for unknown reasons -: abatement of the last percents from waste is often more energy consuming. In spite of appreciable scattering of the data, energy consumption appears to be higher for the highly diluted solution (1:4 and 1:6). This is mainly due to the reduced conductivity, resulting in higher resistances of the cell [39] as confirmed by the cell voltage which attained 3.2 V for the 1:6 diluted waste. Comparing Table 2 and Fig 11 shows that a limited dilution allows considerable reduction in the chromium concentration in the effluent with very reasonable energy consumption: twofold dilution appears as the best compromise, allowing Cr (III) removal with an energy consumption near 1 kWh/kg, in comparison to nearly the double from the raw waste. The removal of COD in comparison with chromium was found to be very effective and less energy-demanding for both types of electrodes (Figure 12). The effect of the dilution factor is less visible than for Cr (III) removal because of the data scattering but diluting two times the waste issued from the tannery plant allowed energy consumption near 0.7 kWh/kg COD removed. Electrocoagulation process is very acceptable from the energy consumption point of view. For the sake of comparison, data reported in [40] revealed 3.8 kWh/kg COD removed with a 58% of abatement when treated tannery wastewater using an electroFenton process. Strong variations of the energy consumption can be expected: Daneshvar et al. [41] reported energy consumption between 4.7 and 7.57 kWh/kg COD
17
under optimized conditions to remove dyes by electrocoagulation but it rose to more than 15kWh for nearly complete abatement. A trade-off between abatement degree and energy consumption should be made.
5. Conclusions The treatment by electrocoagulation of tannery wastewater highly concentrated in chromium and COD was investigated using two different types of aluminium based electrodes. Duralumin aluminium alloy was found to be more efficient for COD and Cr removal compared to pure aluminium electrodes. The removal of chromium is mainly due to adsorption on aluminium hydroxide. Duralumin much more easily dissolved and the concentration of aluminium versus charge is higher, which favours the removal of chromium. The COD elimination is also due to the affinity of the organic material to Al (III) species in the form of Al(OH)3. Dilution of the very concentrated effluents can allow reduction in the energy consumption and improvement of the efficiency of the treatment: for the waste considered, twofold dilution appeared optimal. The noticeable differences we noticed between electrodes made of pure aluminium and the Al alloy show the necessity to deeply investigate the influence of the nature of the electrode materials for other types of effluents.
ACKNOWLEDGEMENTS: The authors would like to acknowledge the France-Morocco cooperation for the financial support of this work in the framework of the “Water and Environment Competency Pole” project. In addition, the authors greatly thank the manager for the leather industry Mr Slaissi Abd Aljalil for his help.
18
REFERENCES [1]
J. D. Sautet, O. Gand, P. Jardin, J.-M. Mornas, A. Daif, Rapport de synthèse – Etude d’analyse du potentiel du cuir (2003) http://www.abhatoo.net.ma/maalamatextuelle/developpement-economique-et-social/developpementeconomique/industrie/industrie-du-textile-et-du-cuir/rapport-de-synthese-etude-danalyse-du-potentiel-du-secteur-cuir [in French]
[2]
P. Religa, A. Kowalik, P. Gierycz,Application of nanofiltration for chromium concentration in the tannery wastewater, J. Hazard. Mater.186 (2011) 288–292.
[3]
A. Benhadji, M-T. Ahmed, R. Maachi, Electrocoagulation and effect of cathode materials on the removal of pollutants from tannery wastewater of Rouïba, Desalination 277 (2011) 128–134
[4]
M. Fabbricino, B. Naviglio, G. Tortora, L. d’Antonio, An environmental friendly cycle for Cr(III) removal and recovery from tannery wastewater, J. Environ. Manag. 117 (2013) 1-6
[5]
N. F. Fahim, B.N. Barsoum, A. E. Eid, M. S. Khalil, Removal of chromium (III) from tannery wastewater using activated carbon from sugar industrial waste, J. Hazard. Mater. B136 (2006) 303-309.
[6]
S. Vasudevan, J. Lakshmi, G. Sozhan, Studies on the Al–Zn–In-alloy as anode material for the removal of chromium from drinking water in electrocoagulation process. Desalination 275 (2011) 260-268.
[7]
G. Lofrano, S. Meriç, G. E. Zengin, D. Orhon, Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: a review, J. Sci. Total Environ. 461-462 (2013) 265-281.
[8]
N. Meunier, P. Drogui, C. Montane, R. Hausler, G. Mercier, J.-F. Blais, Comparison between electrocoagulation and chemical precipitation for metals removal from acidic soil leachate, J. Hazard. Mater. B137 (2006) 581–590.
[9]
V. Keerthi Suganthi, M. Mahalakshmi, N. Balasubramanian, Development of hybrid membrane bioreactor for tannery effluent treatment. Desalination 309 (2013) 231–236.
[10] Z. Song, C.J. Williams, R. G. J. Edyvean, Treatment of tannery wastewater by chemical coagulation. Desalination, 164 (3) (2004) 249–259. [11] S. Elabbas, L. Mandi, F. Berrekhis, M.N. Pons, J-P. Leclerc, N. Ouazzani, Removal of Cr(III) from chromium tanning wastewater by adsorption using two natural carbonaceous materials: Eggshell and powdered marble. Environ. Manag. J. in press (1-7) [12] S. Tiglyene, A. Jaouad, L. Mandi, Treatment if tannery wastewater by infiltration percolation :chromium removal and speciation un soil. Environ.Technol.29 (2008) 613624 19
[13] S.K. Sahu, P. Meshram, B.D. Pandey, V. Kumar, T. R. Mankhand, Removal of chromium (III) by cation exchange resin, Indion 790 for tannery waste treatment, Hydrometallurgy 99 (2009) 170-174. [14] Y.A. Ouaissaa, M. Chabania, Abdelatif, A. Bensmailia, Integration of electro coagulation and adsorption for the treatment of tannery wastewater – The case of an Algerian factory, Rouiba, Procedia Engineering 33 (2012) 98-101. [15] C. R. Costa, C. M. R. Botta,E. L. G. Espindola, P. Olivi, Electrochemical treatment of tannery wastewater using DSA® electrodes, J. Hazard. Mater.153 (2008) 616-627. [16] S. Sundarapandiyan, R. Chandrasekar, B. Ramanaiah, S. Krishnan, P. Saravanan, Electrochemical oxidation and reuse of tannery saline wastewater, J. Hazard. Mater. 180 (2010) 197-203. [17] L. Sirajuddin, G. Kakakhel, M.I. Lutfullah, A. Bhanger, A. Shah, A. Niaz, Electrolytic recovery of chromium salts from tannery wastewater, J. Hazard. Mater. 148 (2007) 560-565. [18] E. Isarain-Chávez, C. De la Rosa, L.A. Godínez, E. Brillas, J.M. PeraltaHernández, Comparative study of electrochemical water treatment processes for a tannery wastewater effluent, J. Electroanal. Chem. 713 (2014) 62-69. [19] Z. Houshyar, A. B. Khoshfetrat, E. Fatehifar, Influence of ozonation process on characteristics of pre-alkalized tannery effluents, Chem. Eng. J. 191 (2012) 59-65. [20]
L. Mandi, S. Tiglyene, A. Jaouad, 2009. Depuration of tannery effluent by phytoremediation and infiltration percolation under arid climate in: M. El Moeujabber, L., Mandi, G. Trisorio-Liuzzi, G., I. Martin, A. Rabi, R. Rodriguez, R. (Eds), Technological Perspectives for rational Use of Water Resources un the Mediterranean Region. Publisher: Bari CIHEAM (2009), pp. 199-205.
[21] S. Zodi, O. Potier, F. Lapicque, J.-P. Leclerc, Treatment of textile wastewaters by electrocoagulation: Effect of operating parameters on the sludge settling characteristics, Sep. Purif. Technol. 69 (2009) 29-36. [22] S. Zodi, B. Merzouk, O. Potier, F. Lapicque, J.-P. Leclerc, Direct red 81 dye removal by a continuous flow electrocoagulation /flotation reactor, Separation Sep. Purif. Technol. 108 (2013) 215-222. [23] M.S. Oncel, A. Muhcu, E. Demirbas, M. Kobya, A comparative study of chemical precipitation and electrocoagulation for treatment of coal acid drainage wastewater, J. Environ. Chem. Eng. 1 (2013) 989-995. [24] A. Attour, M. Touati, M. Tlili, M. Ben Amor, F. Lapicque, J.-P. Leclerc, Influence of operating parameters on phosphate removal from water by electrocoagulation using aluminum electrodes, Sep. Purif. Technol. 123 (2014) 124-129.
20
[25] P. Cañizares, F. Martinez, C. Jiménez, C. Sáez, M.A. Rodrigo, Coagulation and electrocoagulation of oil-in-water emulsions, J. Hazard. Mater. 151 (2008) 44-51 [26] D. Valero, J.M. Ortiz, V. Garcia, E. Espósito, V. Montiel, A. Aldaz, Electrocoagulation of wastewater from almond industry, Chemosphere 84 (2011)1290-1295 (27] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC) – science and applications, J. Hazard. Mater. B84 (2001) 29-41 [28] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, J. Hazard. Mater. B114 (2004) 199-210 [29] S. Vasudevan, J. Lakshmi, J. Jayaraj, G. Sozhan, Remediation of phosphate contaminated water by electrocoagulation with aluminum, aluminum alloy and mild steel anodes, J. Hazard. Mater. 164 (2009) 1480-1486. [30] N. L. Kruthika, S. Karthika, G. Bhaskar Raju, S. Prabhakar, Efficacy of electrocoagulation and electrooxidation for the purification of wastewater generated from gelatin production plant, J. Environ. Chem. Eng. 1 (2013) 183188. [31] S.Saravanbahavan, P. Thaikaivelan, J. Raghava Rao, N. B. Nair, T. Ramasami, Natural leathers from natural materials: progressing toward a new arena in leather processing, J. Environ. Sci. Technol. 38(3) (2004) 871–879. [32] J. Feng., Y. Sun., Z. Zheng., J. Zhang., S. Li., Y. Tian., Treatment of tannery wastewater by electrocoagulation. J. Environ. Sci. 19 (2007) 1409-1415 [33] C. Regina Costa, P. Olivi., Effect of chloride concentration on the electrochemical treatment of a synthetic tannery wastewater. Electrochim. Acta 54 (2009) 20462052. [34] F.R. Espinoza-Quiñones, M.M.T. Fornari, A.N. Módenes, S.M. Palácio, F.G. Silva Jr., N. Szymanski, A.D. Kroumov, D.E.G. Trigueros, Pollutant removal from tannery effluent by electrocoagulation, Chem. Eng. J. 151 (2009) 59–65. [35] S. Zodi, J.N. Louvet, C. Michon, O. Potier, M.N. Pons, F. Lapicque, J.P. Leclerc, Electrocoagulation as a tertiary treatment for paper mill wastewater: Removal of non biodegradableorganic pollution and arsenic. Sep. Purif. Technol. 81 (2011) 62-68 [36] S .Zodi, Étude de l'épuration d'effluents de composition complexe par électrocoagulation et des couplages intervenants entre le traitement électrochimique et l'étape de séparation : application à l'industrie textile et papetière. PhD Dissertation, Université de Lorraine, Nancy, France (2012). [in French] [37] I. Zongo, J.-P. Leclerc, H. G. Maiga, J. Wethee, F. Lapicque, Removal of hexavalent chromium from industrial wastewater by electrocoagulation: A 21
comprehensive comparison of aluminium and iron electrodes, Sep. Purif. Technol. 66 (2009) 159-166. [38] P. Canizares, C. Jiménez, F. Martínez, M.A. Rodrigo, C. Sáez, The pH as a key parameter in the choice between coagulation and electrocoagulation for the treatment of wastewaters, J. Hazard. Mater. 163 (2009) 158–164. [39] H.A. Moreno-Casillas, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson, Electrocoagulation mechanism for COD removal, Sep. Purif. Technol. 56 (2007) 204–211. [40] U. Kurt, O. Apaydin, M. Talha Gonullu, Reduction of COD in wastewater from an organized tannery industrial region by Electro-Fenton process, J. Hazard. Mater. 143 (2007) 33-40. [41] N. Daneshvar, A. Oladegaragoze, N. Djafarzadeh, Decolorization of basic dye solutions by electrocoagulation: An investigation of the effect of operational parameters, J. Hazard. Mater. B129 (2006) 116–122.
22
Figure Caption Fig. 1.Various unit processes and operations in leather processing Fig. 2. Experimental set-up for electrocoagulation tests Fig. 3: Effect of current density on the chromium concentration versus time (a) duralumin (b) pure aluminium. Fig. 4: Effect of current density on the COD concentration versus time (a) duralumin (b) pure aluminium. Fig. 5: pH versus time (a) duralumin, (b) pure aluminium. Fig. 6: Al (III) concentration versus the electrical charge: (a) duralumin, (b) pure aluminium electrode. Fig. 7: Chromium removed versus Al (III) produced at 200 A m-2 for several dilution factors: (a) duralumin, (b) pure aluminium. Fig. 8: COD removed versus Al (III) produced at 200 A m-2 for several dilution factors: (a) duralumin, (b) pure aluminium Fig. 9: Concentration of Al (III) produced versus the electrical charge with pure aluminium electrodes. Current density = 200 A m-2 Fig. 10: Energy consumed for the treatment of 1 m3 diluted waste along the run for Cr (III) removal. Duralumin electrodes were used at 200 A m-2 Fig. 11: Effect of effluent dilution on the energy consumption per kg of chromium removed versus chromium removal efficiency; current density = 200 A m-2; (a) duralumin, (b) pure aluminium Fig. 12: Effect of effluent dilution on the energy consumption per kg of COD removal versus COD removal efficiency; current density = 200 A m-2; (a) duralumin, (b) pure aluminium.
23
24
25
26
27
28
29
30
31
32
33
34
35
Tables Table 1: Main characteristics of the wastewater Parameter pH Conductivity (mS/cm) COD (g O2/L) Cl-(g/L) SO42- (g/L) Cr (III) (g/L)
Value 3.8 54.2 5.8 23.3 1.3 7.0
36
Table 2: Effect of effluent dilution on the maximum COD and chromium removal efficiency using duralumin and pure aluminum electrodes; operation at 200 A/m2 for 180 minutes. Dilution factor Removal efficiency (%) Duralumin COD
Cr (III)
1:1
1:2
1:3
1:4
1:6
58.1
97.6
100
100
100
Pure Al
43.7
100
100
100
100
Duralumin
73.3
91.7
94.5
96.2
98.1
Pure Al
57.2
91.2
92.9
97.9
98.1
37