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Influence of operating parameters on phosphate removal from water by electrocoagulation using aluminum electrodes
Separation and Purification Technology 123 (2014) 124–129
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Influence of operating parameters on phosphate removal from water by electrocoagulation using aluminum electrodes A. Attour a,b, M. Touati a, M. Tlili a, M. Ben Amor a, F. Lapicque c, J.-P. Leclerc c,⇑ a
Laboratoire de Traitement des Eaux Naturelles. Centre de Recherches et technologies des eaux, Technopole de Borj-Cédria, 8020 Soliman, Tunisia Institut Supérieur des Sciences et Technologies de l’Environnement, Technopole de Borj-Cédria, Tunisia c Laboratoire Réactions et Génie des Procédés, UMR 7274 CNRS – Université de Lorraine, 1 rue Grandville, B.P. 20451, Nancy, France b
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 18 December 2013 Accepted 21 December 2013 Available online 3 January 2014 Keywords: Electrocoagulation Phosphate Aluminum electrodes pH Energy consumption Temperature effect
a b s t r a c t Treatment of water containing phosphate by electrocoagulation has been studied in a laboratory batch reactor. The effect of operating parameters on both phosphate removal efficiency and pH evolution has been investigated. Influence of distance between electrodes, current density, initial pH, temperature and conductivity has been extensively studied in a wide range of values. The results show that the removal efficiency depends on the electrical charges; the same efficiency is obtained with low current density with long time of treatment, or higher current intensity with short treatment time. The time evolution of the pH during the treatment strongly depends on the operating conditions but the final pH is more or less the same due to the buffering effect of AlðOHÞ3 =AlðOHÞ 4 mixture. Effects of the temperature, often disregarded in the literature shows that treatment rate is strongly increased with temperature whereas conductivity near 1 mS/cm is enough to ensure reasonable treatment rate. The electrical energy consumption (around 4 Kw/m3) is acceptable to achieve 90% of conversion but lower current density is preferable because of the lower voltage drop. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Excess of phosphorus in water is responsible for several possible types of problem. In industries, excess of phosphate in process water is responsible for progressive fouling of the pipes. Phosphate is not supposed to be toxic for human being because of the reasonable low concentration in the drinking water and aliments and because its presence in most cells and organisms. The major impact is relative to environment since it is one of the major eutrophication [1] for higher phosphate concentration. This phenomenon is responsible for the dramatic growth of algae occurring in natural water reserves. The growth in the human population and the rise in water consumption have increased the efforts to safe and to protect all water resources. Phosphorus compounds came from various sources but agriculture and cattle are the main direct and indirect origins of its presence. In addition of the necessary reduction of discharge of chemicals wastes containing phosphorous, treatment of water containing phosphorous is necessary. The discharge limits are very strict in all countries: 0.5–1.0 mg/L in USA, 5 mg/L in India [2], 1–2 mg/L in France and 10 mg/L in Tunisia [3]. The three mains categories of treatments (physical, chemical and biological) can be considered to eliminate the phosphorous. ⇑ Corresponding author. E-mail address: [email protected] (J.-P. Leclerc). 1383-5866/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.12.030
A complete review of these treatments including the advantages and disadvantages of each method for the case of phosphate has been published by Yeoman et al. [4]. In recent years, investigations have been focused on the treatment of wastewaters using electrocoagulation owing to the increase in environmental restrictions on wastewater. Numerous papers dealing with electrochemical pollutant removal in industrial or synthetic effluents using iron or aluminum electrodes have been published so far. Electrocoagulation is a simple and efficient method for the treatment of many types of water and wastewater. It has not been widely accepted because of the use of electricity and the unjustified apparent complexity as compared to other treatment technologies. Electrocoagulation technique uses a direct current source between metal electrodes immersed in the water to be treated. The electrical current causes the dissolution of metal plates resulting in iron or aluminium cations into the wastewater. The metal ions, at an appropriate pH, can form wide ranges of coagulated species and metal hydroxides that destabilize and aggregate the suspended particles or precipitate and adsorb dissolved or suspended contaminants [5]. The most widely used electrode materials in EC process are aluminium and iron because of the trivalent form of the metal. In the case of aluminium, main reactions are as: 3þ
Anode reaction : AlðsÞ ! Al
þ 3e
ð1Þ
A. Attour et al. / Separation and Purification Technology 123 (2014) 124–129
Cathode reaction : H2 O þ e !
1 H2ðgÞ þ OH 2
ð2Þ
Al3+ and OH ions generated by electrode reactions (1) and (2) react to form various monomeric species, which finally transform into Al(OH)3(s) according to complex precipitation kinetics. 3þ
Al
þ 3H2 O ! AlðOHÞ3 þ 3Hþ
ð3Þ
In the bulk solution, there is also possibility of precipitation of little soluble aluminium phosphate as follows: 3þ
Al
þ PO3 4 ! AlPO4ðsÞ
ð4Þ
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. Finally, these flocs are separated from the liquid either by flotation upon the action of gas bubbles (hydrogen and oxygen), either by settling because of their higher density [6]. Electrocoagulation has been considered for a number of wastewaters in a very broad range of nature and composition: in particular oil suspensions [7], wastes from textile industry [8,9], tannery [10], food industries [11], arsenic [12] or heavy metal as chromium for example [13] – a list far from exhaustive. Examination of the available literature reveals that the capacity and efficiency of electrocoagulation depends on the nature and initial concentration of the pollutants [14], as observed by Khemis et al. [15] or Sanchez-Calvo et al. [16] and to some extents by the design of electrocoagulation device and the flow conditions. The papers relative to the treatment of effluents by electrocoagulation are mainly focused on the influence on pollutant removal efficiency of operating parameters including current density, pH, operating time, pollutant concentrations. Optimal parameters could be determined to obtain higher pollutant removal rate with reasonable energy consumption. Relatively few papers are devoted to phosphorous removal. Some of the major contributions are gathered in Table 1. The phosphate concentration was chosen all the time equal to 100 mg/L except the work presented by Bektas et al. [20] who investigated a concentration ranging from 10 to 200 mg/L. Distance between the two electrodes was also unchanged at 5 mm. One again only Bektas et al. [20] investigated value for this parameter. Vasudevan et al. [2] published a paper on this subject covering in details the effect of numerous operating parameters and effluents composition. In particular they analyzed the effect of co-existing anions on the final removal efficiency. Lacasa et al. [21] proposed some mechanistic models for the two types of electrodes (aluminum and iron). They concluded that adsorption of phosphate on the hydroxide is the major effect with iron whereas direct precipitation and adsorption coexisting with aluminum electrode. The electrical consumption is lower using aluminum electrode. Finally, the study of Mahvi et al. [22] has not been included in the table since it concerns the removal of phosphate in wastewater. However it presents interesting analysis of the process including the separation step between solid and liquid phase. The aim of this paper is to study thoroughly the elimination of phosphate by electrocoagulation using aluminum electrodes. A large type and range of parameters have been studied and optimal operating conditions are proposed. Special attention was has been paid on parameters that have been not or little studied up to now. In particular, the influence of temperature has been investigated since local temperature may strongly vary from one country to another. Moreover the effect of other minerals present in the water on removal kinetics has been observed. Surface aspect of the electrodes after treatment has been also qualitatively analyzed for several operating conditions. Evolution of pH has been carefully followed during all the experiments.
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2. Materials and methods 2.1. Experimental set-up Fig. 1 shows the experimental set-up. The electrocoagulation experiments have been carried out in a jacketed batch reactor 10 cm diameter and being 23 cm high. The liquid was stirred at 150 rpm with a magnetic stirrer (Fisher Bioblock). The stirring speed is keeping low enough to avoid shearing of the flocs. The temperature of the reactor was kept constant by the water circulating in the jacket and heating by a thermostatic bath. Its value was measured inside the reactor. The two electrodes of aluminum are vertically immersed in the liquid for an active surface of 50 cm2 (5 10 cm2). Distance between anode and cathode was taken at 5 mm, but some preliminary experiments have conducted with 10 and 20 mm gaps. After each experiment and prior to the next experiment, the electrodes were pumiced and degreased with 400 grit super fine sandpaper and rinsed with 0.1 M HCl and clean water to avoid any effect due to the history of the electrodes. The electrodes were connected to a direct current power supply (ALR 3002 M) delivering a current up to 2.5 A, with a voltage ranging from 0 to 30 V. Electrochemical coagulation experiments were carried out under galvanostatic conditions in the range 0.1–0.9 A. Time t = 0 of run corresponded to switching on the current supply. 2.2. Solutions and chemical analysis A phosphate synthetic solution of 100 mg P/L was prepared by dissolving 0.439 g of potassium dihydrogénophosphate KH2PO4 (Prolabo) per liter of distilled water. The initial pH was adjusting with 1 M NaOH (Chemi-pharma) or with HCl (Reagent grade ACS, ISO Chemicals Developing and Manufacturing (CDM)) solutions. To study the effect of the electrolyte support, sodium chloride (Chemi-pharma) was added at various amounts, so its concentration varied from 4.5 mM to 25.5 mM. A UV spectrophotometer (HACH DR/4000U) at 430 nm was used for phosphate analyses in accordance with the standard vandomolybdophosphoric acid calorimetric method [23] after separating the particles through the 0.45 lm mixed cellulose ester syringe filters. The conductivity was measured continuously using a consort D292 conductimeter. pH and temperature measurements were performed continuously using a pH meter (HANNA instruments, pH 213). 3. Results and discussions 3.1. Influence of the electrodes gap Fig. 2 shows the phosphate removal efficiency obtained for 3 different distances. The kinetics of abatement and the maximum phosphate removal percentage increase when the gap is reduced. The ohmic voltage drop is increased with the distance between the two electrodes. Voltage has been recorded continuously during the experiments. At steady state and for i = 10 mA/cm, it reaches 5.5, 7.9 and 13.3 for a gap of 0.5, 1 and 2 cm respectively. They are several recent papers that confirm the effect of the electrodes gap on the pollutant removal efficiency. During the treatment of textile wastewater, Merzouk et al. [25], were measured a decrease of the turbidity removal when electrode gap is larger. The optimal gap for the three distance tested (10, 20 and 30 mm) was found to be 10 mm as we observed in our experiments. Recently, Anand et al. [26] observed that COD removal efficiency decreased with an increase in electrode gap. Electrical conductivity is inversely proportional to the distance between
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Table 1 Review of the major contributions dealing with treatment of phosphate containing in water by electrocoagulation. References
Type of electrodes (anode/cathode)
Current density (mA/cm2)
Distance between electrodes (mm)
Anode area (cm2)
Operating time (min)
Initial concentration [PO3 4 ] (ppm)
Phosphorous removal efficiency (%)
20 36 60 30 15
100 100 100 100 10–50 100–200 27
52–90% 93% 68–98% 68–99% 90% i = 5 mA cm2 90% i = 7.5 mA cm2 99.6%
Irdemez et al. [17] Irdemez et al. [18] Vasudevan et al. [2] Vasudevan et al. [19] Bektas et al. [20]
6 Pairs of aluminum electrodes 6 Pairs of iron electrodes Steel/stainless steel Aluminum/stainless steel 8 Pairs of aluminum electrodes
0.25–1 0.5 0.1–0.5 0.2–2 2.5–10
5 5 5 5 3
1500 1500 200 200 1500
Lacasa et al. [21]
1 Pair of electrodes: aluminum or iron
0.1–5.0
9
100
100–500
Fig. 1. Experimental set-up. Fig. 3. Phosphate removal efficiency versus time for 5 different currents density between 2 and 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm).
Fig. 2. Influence of electrodes gap on the phosphate removal efficiency 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6,5 mM, pHi = 7, T = 30 °C, i = 10 mA/cm ).
the two electrodes. As the distance increases, the resistivity of the solution increases, therefore, the amount of metal getting dissolved into the solution also decreases. Hence, the removal efficiency decreases with an increase in electrode gap. Zhang et al. [27] have estimated the effect of several electrode gaps from 10 mm to 60 mm on phosphate removal from landscape water. The maximal phosphate removal efficiency decreased from 86.4% to 63% except for a gap of 25 mm for which the efficiency reached 90%. The reason of this exception is not clearly explained. 3.2. Influence of current density Current density is the most sensitive operating parameter of electrocoagulation process. Fig. 3 shows the phosphate removal
Fig. 4. Phosphate removal efficiency versus electric charge for 5 different currents density between 2 and 18 mA/cm2 ð½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm.
efficiency versus time for 5 values of the current density between 2 and 18 mA/cm2. The kinetics is very sensitive to this parameter and the treatment is faster when the current density is higher. Many papers focused on the optimal conditions for the treatment including time of treatment and operating current density. In fact these parameters are linked together. Fig. 4 represents the phosphate removal efficiency versus the electric charge for the different current densities. This representation shows that the kinetics is affected by these parameters and several couples of operating time and current density can be selected to obtain the same abatement when the charge remains the same. However, with higher current
A. Attour et al. / Separation and Purification Technology 123 (2014) 124–129
Fig. 5. pH versus electric charge for 5 different currents density between 2 and 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm).
density, the fast production of small bubble of hydrogen facilitates the flotation. This is why, it is interesting to study the coupling between electrocoagulation process and separation of solid and liquid as it has been proposed by Zodi et al. [6]. Fig. 5 represents the pH evolution during the treatment. The pH increased with time and with appreciable effect of the current density, because of the OH production flux rate at the cathode. When the pH attains approximately 10, a stabilization is usually observed because of the buffering effect of AlðOHÞ3 =AlðOHÞ 4 mixture [13]. As already pointed out, one of the problems induced by using aluminum as electrode metal is the formation of a passive oxide film [24]. Fig. 6 shows several photos of the electrode surfaces after treatment and under different current densities. For low current densities, i.e. from 2 to 6 mA/cm2, we observe a white layer on the surface of anodes (see photo a). With higher current density coagulation and settling processes are faster and the deposit layer is less significant for medium current density (see photo b for 10 mA/cm2), and there is no real layer below 14 mA/cm2 (see photo c). The higher current densities are also responsible for chemical corrosion of the cathode due to intensive production of hydroxide anions (see photo d). 3.3. Influence of initial pH It has often been reported that the initial pH is one of the most sensitive operating parameter during electrocoagulation treatment. However, if they are numerous experiments in the literature to analyze the effect of the initial pH on the final abatement of the pollution, very few papers present the time variation of the pH for a large range of initial pH. Fig. 7 shows the phosphate removal
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Fig. 7. Phosphate removal efficiency versus time for different initial pH 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
Fig. 8. pH versus time for different initial pH varied from 2 to 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
11
efficiency for different initial pH. Except for the very low value of pH = 2, an acid pH = 3 favors the kinetic of abatement because of the predominant form of AlPO4(s). These results are in agreement with the optimal initial pH value of 3 suggested by Irdemez et al. [18]. Conversely, when pH increases both phosphate precipitation and adsorption on aluminum hydroxides compete. The kinetics of adsorption is slower than the kinetics of precipitation. When the pH is increasing the formation of AlPO4(s), and Al(OH)3 decreased and the abatement is limited. The evolution of the pH along treatment is shown in Fig. 8. From pH = 3–9, production of OH at the cathode increases progressively the pH up to 10 which corresponds approximately to value of the buffering AlðOHÞ3 =AlðOHÞ 4 mixture. When the initial pH is high
Fig. 6. Views of electrode surface at different current densities: white layer on the anode at low current densities (a) 2 mA/cm2 (similar aspect for i = 6 mA/cm2), (b) 10 mA/ cm2, (c) the film is not visible on the anode at 18 mA/cm2 (similar aspect for 18 mA/cm2) and (d) oxidation of the cathode (brown color) at 14 mA/cm2 and 18 mA/cm2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. pH versus time for different temperature (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, [NaCl] = 6.5 mM, d = 0.5 cm, i = 10 mA/cm2). Fig. 9. Phosphate removal efficiency versus time for different conductivities 2 (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
Fig. 12. Phosphate removal efficiency versus electrical energy consumption (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C). Fig. 10. Phosphate removal efficiency versus time for different temperatures 2 (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, [NaCl] = 6.5 mM, d = 0.5 cm, i = 10 mA/cm ).
(i.e. 11), the formation of AlðOHÞ 4 fixed the OH produced and the pH of the solution decreased to the buffering value. At higher pH values, the formation of hydroxide aluminum is more important and can deposit on the anode surface. This leads to an increasing ohmic drop (because of the inert, little conducting layer formed) and an increase in the electrical consumption. Lower pH values are thus preferable both from efficiency and maintenance point of view. However, these results are very promising for the industrialization of the method since except for very high or very low value, it is not necessary to adjust the initial pH to obtain full abatement of the phosphate.
3.4. Influence of conductivity It is well known that pollutant removal efficiency is improved when conductivity is higher. However the impact has to be quantified since it is affected by the effluent nature. Fig. 9 shows the phosphate removal efficiency versus time for several conductivities upon NaCl addition. The impact on the kinetic is notable. In particular when the conductivity is lower than 1 mS/cm, even the final conversion is affected. At this level, it should be recalled that the experiments have been conducted with synthetic solution containing only phosphate. Real natural waters contain usually several types of anions and cations with a natural conductivity high enough to ensure high treatment rate. 3.5. Influence of temperature The effect of temperature on electrocoagulation process is often neglected in the literature. In industrial configuration, this process
Fig. 13. Voltage versus time for 5 different current intensities from 2 to 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C).
is often run in few heating industrial premises. Depending of the geographical location, temperature of functioning can strongly varied from 15 °C to 40 °C in extreme cases (North of Europe and SubSahara area). This effect has been little studied by Vasudevan et al. [19]. The authors observed a lower efficiency under 22 °C but they just presented the final conversion. El-Naas et al. [28] evaluated the effect of temperature during the treatment of from refinery wastewater by electrocoagulation at two different temperatures, 25 and 40 °C. The observed an higher abatement on the removal of sulfate and COD at 25 °C. They assume that the solubility of aluminum sulfates decrease with temperature and therefore, the precipitation of the aluminum sulfate is enhanced at lower temperatures. Song et al. [29] noted that the color removal increased slightly with the temperature in the range from 20 °C to
A. Attour et al. / Separation and Purification Technology 123 (2014) 124–129
60 °C. The reason could be that with the increase in temperature the mobility and collisions of the ions with the hydroxyl polymers increases. Fig. 10 shows the kinetics of phosphate removal efficiency for 4 different temperatures between 20 and 50 °C. We observe a strong effect of the temperature on the kinetics rate which shown the positive effect of the temperature. The evolution of the pH represented Fig. 11 which is lower at high temperature because the formation of Al(OH)3 is favored and the concentration of OH is thus lower. Even if that point has not been evaluated in the present study, the temperature changed will also result in a variation of surface tension phenomena, with probably different hydrodynamic behavior of the suspended matter. In the presence of hydrogen bubble, it will certainly affect the flotation efficiency. 3.6. Electrical energetic consumption The electrical energy consumption was calculated in terms of kwh per m3 of treated effluent using the equation given below:
E ðkwh=m3 Þ ¼
U I t EC V
where U is cell voltage (V), I is current (A), tEC is the time of electrocoagulation treatment (h) and Vi is the volume (L) of effluent to be treated. Fig. 12 shows the phosphate removal efficiency versus the electrical energetic consumption. The cost, to remove the 70 first percentage of phosphate, is acceptable with a maximum of 4 kwh/m3. In this regard low current density is to be preferred since 90 of percentage removal have been obtained with less than 4 kwh/m3 at 6 mA/cm2. The energy consumption is non-constant for a given electrical charge since the tension does not vary linearly with the current density as illustrated Fig. 13. Moreover, the cell voltage is time dependent because of the change in pH, even if in the present case the variations are relatively small. This result confirms the conclusion obtained from Lacasa et al. [21] and based on another analysis. 4. Conclusion The treatment of water containing phosphate by electrocoagulation using electrodes of aluminum has been studied in batch reactor. Phosphate removal efficiency and the pH evolution versus time have been investigated for a wide range of values of the operating parameters: distance between electrode, initial pH, current densities and several conductivities. The better operating conditions are an electrodes gap of 5 mm, an initial pH of 3 and a conductivity of 3.2 mS, in these conditions the phosphate is totally removed with a rapid kinetic rate. The results show that removal efficiency depending on the electrical charge (time of treatment and current densities are linked together). However, the electrical energy consumption is lower with smaller current density because of the tension drop at high current density. At lower values of pH, the phosphate is removed by precipitation of AlPO4(s), whereas the adsorption on Al(OH)3 is predominant when the pH increased. The evolution of the pH is due to the formation of OH at the cathode. The buffering effect of the AlðOHÞ3 =AlðOHÞ 4 mixture makes the final pH more or less around 10 whatever the initial conditions. The rate and the final abatement of phosphate is strongly improved with the temperature. References [1] A.E. Durrant, M.D. Scrimshaw, I. Stratful, J.N. Lester, Review of the feasibility of recovering phosphate from wastewater for use as a raw material by the phosphate industry, J. Environ. Technol. 20 (1999) 749–758.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Influence of operating parameters on phosphate removal from water by electrocoagulation using aluminum electrodes A. Attour a,b, M. Touati a, M. Tlili a, M. Ben Amor a, F. Lapicque c, J.-P. Leclerc c,⇑ a
Laboratoire de Traitement des Eaux Naturelles. Centre de Recherches et technologies des eaux, Technopole de Borj-Cédria, 8020 Soliman, Tunisia Institut Supérieur des Sciences et Technologies de l’Environnement, Technopole de Borj-Cédria, Tunisia c Laboratoire Réactions et Génie des Procédés, UMR 7274 CNRS – Université de Lorraine, 1 rue Grandville, B.P. 20451, Nancy, France b
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 18 December 2013 Accepted 21 December 2013 Available online 3 January 2014 Keywords: Electrocoagulation Phosphate Aluminum electrodes pH Energy consumption Temperature effect
a b s t r a c t Treatment of water containing phosphate by electrocoagulation has been studied in a laboratory batch reactor. The effect of operating parameters on both phosphate removal efficiency and pH evolution has been investigated. Influence of distance between electrodes, current density, initial pH, temperature and conductivity has been extensively studied in a wide range of values. The results show that the removal efficiency depends on the electrical charges; the same efficiency is obtained with low current density with long time of treatment, or higher current intensity with short treatment time. The time evolution of the pH during the treatment strongly depends on the operating conditions but the final pH is more or less the same due to the buffering effect of AlðOHÞ3 =AlðOHÞ 4 mixture. Effects of the temperature, often disregarded in the literature shows that treatment rate is strongly increased with temperature whereas conductivity near 1 mS/cm is enough to ensure reasonable treatment rate. The electrical energy consumption (around 4 Kw/m3) is acceptable to achieve 90% of conversion but lower current density is preferable because of the lower voltage drop. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Excess of phosphorus in water is responsible for several possible types of problem. In industries, excess of phosphate in process water is responsible for progressive fouling of the pipes. Phosphate is not supposed to be toxic for human being because of the reasonable low concentration in the drinking water and aliments and because its presence in most cells and organisms. The major impact is relative to environment since it is one of the major eutrophication [1] for higher phosphate concentration. This phenomenon is responsible for the dramatic growth of algae occurring in natural water reserves. The growth in the human population and the rise in water consumption have increased the efforts to safe and to protect all water resources. Phosphorus compounds came from various sources but agriculture and cattle are the main direct and indirect origins of its presence. In addition of the necessary reduction of discharge of chemicals wastes containing phosphorous, treatment of water containing phosphorous is necessary. The discharge limits are very strict in all countries: 0.5–1.0 mg/L in USA, 5 mg/L in India [2], 1–2 mg/L in France and 10 mg/L in Tunisia [3]. The three mains categories of treatments (physical, chemical and biological) can be considered to eliminate the phosphorous. ⇑ Corresponding author. E-mail address: [email protected] (J.-P. Leclerc). 1383-5866/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.12.030
A complete review of these treatments including the advantages and disadvantages of each method for the case of phosphate has been published by Yeoman et al. [4]. In recent years, investigations have been focused on the treatment of wastewaters using electrocoagulation owing to the increase in environmental restrictions on wastewater. Numerous papers dealing with electrochemical pollutant removal in industrial or synthetic effluents using iron or aluminum electrodes have been published so far. Electrocoagulation is a simple and efficient method for the treatment of many types of water and wastewater. It has not been widely accepted because of the use of electricity and the unjustified apparent complexity as compared to other treatment technologies. Electrocoagulation technique uses a direct current source between metal electrodes immersed in the water to be treated. The electrical current causes the dissolution of metal plates resulting in iron or aluminium cations into the wastewater. The metal ions, at an appropriate pH, can form wide ranges of coagulated species and metal hydroxides that destabilize and aggregate the suspended particles or precipitate and adsorb dissolved or suspended contaminants [5]. The most widely used electrode materials in EC process are aluminium and iron because of the trivalent form of the metal. In the case of aluminium, main reactions are as: 3þ
Anode reaction : AlðsÞ ! Al
þ 3e
ð1Þ
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Cathode reaction : H2 O þ e !
1 H2ðgÞ þ OH 2
ð2Þ
Al3+ and OH ions generated by electrode reactions (1) and (2) react to form various monomeric species, which finally transform into Al(OH)3(s) according to complex precipitation kinetics. 3þ
Al
þ 3H2 O ! AlðOHÞ3 þ 3Hþ
ð3Þ
In the bulk solution, there is also possibility of precipitation of little soluble aluminium phosphate as follows: 3þ
Al
þ PO3 4 ! AlPO4ðsÞ
ð4Þ
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. Finally, these flocs are separated from the liquid either by flotation upon the action of gas bubbles (hydrogen and oxygen), either by settling because of their higher density [6]. Electrocoagulation has been considered for a number of wastewaters in a very broad range of nature and composition: in particular oil suspensions [7], wastes from textile industry [8,9], tannery [10], food industries [11], arsenic [12] or heavy metal as chromium for example [13] – a list far from exhaustive. Examination of the available literature reveals that the capacity and efficiency of electrocoagulation depends on the nature and initial concentration of the pollutants [14], as observed by Khemis et al. [15] or Sanchez-Calvo et al. [16] and to some extents by the design of electrocoagulation device and the flow conditions. The papers relative to the treatment of effluents by electrocoagulation are mainly focused on the influence on pollutant removal efficiency of operating parameters including current density, pH, operating time, pollutant concentrations. Optimal parameters could be determined to obtain higher pollutant removal rate with reasonable energy consumption. Relatively few papers are devoted to phosphorous removal. Some of the major contributions are gathered in Table 1. The phosphate concentration was chosen all the time equal to 100 mg/L except the work presented by Bektas et al. [20] who investigated a concentration ranging from 10 to 200 mg/L. Distance between the two electrodes was also unchanged at 5 mm. One again only Bektas et al. [20] investigated value for this parameter. Vasudevan et al. [2] published a paper on this subject covering in details the effect of numerous operating parameters and effluents composition. In particular they analyzed the effect of co-existing anions on the final removal efficiency. Lacasa et al. [21] proposed some mechanistic models for the two types of electrodes (aluminum and iron). They concluded that adsorption of phosphate on the hydroxide is the major effect with iron whereas direct precipitation and adsorption coexisting with aluminum electrode. The electrical consumption is lower using aluminum electrode. Finally, the study of Mahvi et al. [22] has not been included in the table since it concerns the removal of phosphate in wastewater. However it presents interesting analysis of the process including the separation step between solid and liquid phase. The aim of this paper is to study thoroughly the elimination of phosphate by electrocoagulation using aluminum electrodes. A large type and range of parameters have been studied and optimal operating conditions are proposed. Special attention was has been paid on parameters that have been not or little studied up to now. In particular, the influence of temperature has been investigated since local temperature may strongly vary from one country to another. Moreover the effect of other minerals present in the water on removal kinetics has been observed. Surface aspect of the electrodes after treatment has been also qualitatively analyzed for several operating conditions. Evolution of pH has been carefully followed during all the experiments.
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2. Materials and methods 2.1. Experimental set-up Fig. 1 shows the experimental set-up. The electrocoagulation experiments have been carried out in a jacketed batch reactor 10 cm diameter and being 23 cm high. The liquid was stirred at 150 rpm with a magnetic stirrer (Fisher Bioblock). The stirring speed is keeping low enough to avoid shearing of the flocs. The temperature of the reactor was kept constant by the water circulating in the jacket and heating by a thermostatic bath. Its value was measured inside the reactor. The two electrodes of aluminum are vertically immersed in the liquid for an active surface of 50 cm2 (5 10 cm2). Distance between anode and cathode was taken at 5 mm, but some preliminary experiments have conducted with 10 and 20 mm gaps. After each experiment and prior to the next experiment, the electrodes were pumiced and degreased with 400 grit super fine sandpaper and rinsed with 0.1 M HCl and clean water to avoid any effect due to the history of the electrodes. The electrodes were connected to a direct current power supply (ALR 3002 M) delivering a current up to 2.5 A, with a voltage ranging from 0 to 30 V. Electrochemical coagulation experiments were carried out under galvanostatic conditions in the range 0.1–0.9 A. Time t = 0 of run corresponded to switching on the current supply. 2.2. Solutions and chemical analysis A phosphate synthetic solution of 100 mg P/L was prepared by dissolving 0.439 g of potassium dihydrogénophosphate KH2PO4 (Prolabo) per liter of distilled water. The initial pH was adjusting with 1 M NaOH (Chemi-pharma) or with HCl (Reagent grade ACS, ISO Chemicals Developing and Manufacturing (CDM)) solutions. To study the effect of the electrolyte support, sodium chloride (Chemi-pharma) was added at various amounts, so its concentration varied from 4.5 mM to 25.5 mM. A UV spectrophotometer (HACH DR/4000U) at 430 nm was used for phosphate analyses in accordance with the standard vandomolybdophosphoric acid calorimetric method [23] after separating the particles through the 0.45 lm mixed cellulose ester syringe filters. The conductivity was measured continuously using a consort D292 conductimeter. pH and temperature measurements were performed continuously using a pH meter (HANNA instruments, pH 213). 3. Results and discussions 3.1. Influence of the electrodes gap Fig. 2 shows the phosphate removal efficiency obtained for 3 different distances. The kinetics of abatement and the maximum phosphate removal percentage increase when the gap is reduced. The ohmic voltage drop is increased with the distance between the two electrodes. Voltage has been recorded continuously during the experiments. At steady state and for i = 10 mA/cm, it reaches 5.5, 7.9 and 13.3 for a gap of 0.5, 1 and 2 cm respectively. They are several recent papers that confirm the effect of the electrodes gap on the pollutant removal efficiency. During the treatment of textile wastewater, Merzouk et al. [25], were measured a decrease of the turbidity removal when electrode gap is larger. The optimal gap for the three distance tested (10, 20 and 30 mm) was found to be 10 mm as we observed in our experiments. Recently, Anand et al. [26] observed that COD removal efficiency decreased with an increase in electrode gap. Electrical conductivity is inversely proportional to the distance between
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Table 1 Review of the major contributions dealing with treatment of phosphate containing in water by electrocoagulation. References
Type of electrodes (anode/cathode)
Current density (mA/cm2)
Distance between electrodes (mm)
Anode area (cm2)
Operating time (min)
Initial concentration [PO3 4 ] (ppm)
Phosphorous removal efficiency (%)
20 36 60 30 15
100 100 100 100 10–50 100–200 27
52–90% 93% 68–98% 68–99% 90% i = 5 mA cm2 90% i = 7.5 mA cm2 99.6%
Irdemez et al. [17] Irdemez et al. [18] Vasudevan et al. [2] Vasudevan et al. [19] Bektas et al. [20]
6 Pairs of aluminum electrodes 6 Pairs of iron electrodes Steel/stainless steel Aluminum/stainless steel 8 Pairs of aluminum electrodes
0.25–1 0.5 0.1–0.5 0.2–2 2.5–10
5 5 5 5 3
1500 1500 200 200 1500
Lacasa et al. [21]
1 Pair of electrodes: aluminum or iron
0.1–5.0
9
100
100–500
Fig. 1. Experimental set-up. Fig. 3. Phosphate removal efficiency versus time for 5 different currents density between 2 and 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm).
Fig. 2. Influence of electrodes gap on the phosphate removal efficiency 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6,5 mM, pHi = 7, T = 30 °C, i = 10 mA/cm ).
the two electrodes. As the distance increases, the resistivity of the solution increases, therefore, the amount of metal getting dissolved into the solution also decreases. Hence, the removal efficiency decreases with an increase in electrode gap. Zhang et al. [27] have estimated the effect of several electrode gaps from 10 mm to 60 mm on phosphate removal from landscape water. The maximal phosphate removal efficiency decreased from 86.4% to 63% except for a gap of 25 mm for which the efficiency reached 90%. The reason of this exception is not clearly explained. 3.2. Influence of current density Current density is the most sensitive operating parameter of electrocoagulation process. Fig. 3 shows the phosphate removal
Fig. 4. Phosphate removal efficiency versus electric charge for 5 different currents density between 2 and 18 mA/cm2 ð½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm.
efficiency versus time for 5 values of the current density between 2 and 18 mA/cm2. The kinetics is very sensitive to this parameter and the treatment is faster when the current density is higher. Many papers focused on the optimal conditions for the treatment including time of treatment and operating current density. In fact these parameters are linked together. Fig. 4 represents the phosphate removal efficiency versus the electric charge for the different current densities. This representation shows that the kinetics is affected by these parameters and several couples of operating time and current density can be selected to obtain the same abatement when the charge remains the same. However, with higher current
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Fig. 5. pH versus electric charge for 5 different currents density between 2 and 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C, d = 0.5 cm).
density, the fast production of small bubble of hydrogen facilitates the flotation. This is why, it is interesting to study the coupling between electrocoagulation process and separation of solid and liquid as it has been proposed by Zodi et al. [6]. Fig. 5 represents the pH evolution during the treatment. The pH increased with time and with appreciable effect of the current density, because of the OH production flux rate at the cathode. When the pH attains approximately 10, a stabilization is usually observed because of the buffering effect of AlðOHÞ3 =AlðOHÞ 4 mixture [13]. As already pointed out, one of the problems induced by using aluminum as electrode metal is the formation of a passive oxide film [24]. Fig. 6 shows several photos of the electrode surfaces after treatment and under different current densities. For low current densities, i.e. from 2 to 6 mA/cm2, we observe a white layer on the surface of anodes (see photo a). With higher current density coagulation and settling processes are faster and the deposit layer is less significant for medium current density (see photo b for 10 mA/cm2), and there is no real layer below 14 mA/cm2 (see photo c). The higher current densities are also responsible for chemical corrosion of the cathode due to intensive production of hydroxide anions (see photo d). 3.3. Influence of initial pH It has often been reported that the initial pH is one of the most sensitive operating parameter during electrocoagulation treatment. However, if they are numerous experiments in the literature to analyze the effect of the initial pH on the final abatement of the pollution, very few papers present the time variation of the pH for a large range of initial pH. Fig. 7 shows the phosphate removal
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Fig. 7. Phosphate removal efficiency versus time for different initial pH 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
Fig. 8. pH versus time for different initial pH varied from 2 to 2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
11
efficiency for different initial pH. Except for the very low value of pH = 2, an acid pH = 3 favors the kinetic of abatement because of the predominant form of AlPO4(s). These results are in agreement with the optimal initial pH value of 3 suggested by Irdemez et al. [18]. Conversely, when pH increases both phosphate precipitation and adsorption on aluminum hydroxides compete. The kinetics of adsorption is slower than the kinetics of precipitation. When the pH is increasing the formation of AlPO4(s), and Al(OH)3 decreased and the abatement is limited. The evolution of the pH along treatment is shown in Fig. 8. From pH = 3–9, production of OH at the cathode increases progressively the pH up to 10 which corresponds approximately to value of the buffering AlðOHÞ3 =AlðOHÞ 4 mixture. When the initial pH is high
Fig. 6. Views of electrode surface at different current densities: white layer on the anode at low current densities (a) 2 mA/cm2 (similar aspect for i = 6 mA/cm2), (b) 10 mA/ cm2, (c) the film is not visible on the anode at 18 mA/cm2 (similar aspect for 18 mA/cm2) and (d) oxidation of the cathode (brown color) at 14 mA/cm2 and 18 mA/cm2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. pH versus time for different temperature (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, [NaCl] = 6.5 mM, d = 0.5 cm, i = 10 mA/cm2). Fig. 9. Phosphate removal efficiency versus time for different conductivities 2 (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, T = 30 °C, d = 0.5 cm, i = 10 mA/cm ).
Fig. 12. Phosphate removal efficiency versus electrical energy consumption (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C). Fig. 10. Phosphate removal efficiency versus time for different temperatures 2 (½PO3 4 0 ¼ 100 mg P/L, pHi = 7, [NaCl] = 6.5 mM, d = 0.5 cm, i = 10 mA/cm ).
(i.e. 11), the formation of AlðOHÞ 4 fixed the OH produced and the pH of the solution decreased to the buffering value. At higher pH values, the formation of hydroxide aluminum is more important and can deposit on the anode surface. This leads to an increasing ohmic drop (because of the inert, little conducting layer formed) and an increase in the electrical consumption. Lower pH values are thus preferable both from efficiency and maintenance point of view. However, these results are very promising for the industrialization of the method since except for very high or very low value, it is not necessary to adjust the initial pH to obtain full abatement of the phosphate.
3.4. Influence of conductivity It is well known that pollutant removal efficiency is improved when conductivity is higher. However the impact has to be quantified since it is affected by the effluent nature. Fig. 9 shows the phosphate removal efficiency versus time for several conductivities upon NaCl addition. The impact on the kinetic is notable. In particular when the conductivity is lower than 1 mS/cm, even the final conversion is affected. At this level, it should be recalled that the experiments have been conducted with synthetic solution containing only phosphate. Real natural waters contain usually several types of anions and cations with a natural conductivity high enough to ensure high treatment rate. 3.5. Influence of temperature The effect of temperature on electrocoagulation process is often neglected in the literature. In industrial configuration, this process
Fig. 13. Voltage versus time for 5 different current intensities from 2 to 18 mA/cm2 (½PO3 4 0 ¼ 100 mg P/L, [NaCl] = 6.5 mM, pHi = 7, T = 30 °C).
is often run in few heating industrial premises. Depending of the geographical location, temperature of functioning can strongly varied from 15 °C to 40 °C in extreme cases (North of Europe and SubSahara area). This effect has been little studied by Vasudevan et al. [19]. The authors observed a lower efficiency under 22 °C but they just presented the final conversion. El-Naas et al. [28] evaluated the effect of temperature during the treatment of from refinery wastewater by electrocoagulation at two different temperatures, 25 and 40 °C. The observed an higher abatement on the removal of sulfate and COD at 25 °C. They assume that the solubility of aluminum sulfates decrease with temperature and therefore, the precipitation of the aluminum sulfate is enhanced at lower temperatures. Song et al. [29] noted that the color removal increased slightly with the temperature in the range from 20 °C to
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60 °C. The reason could be that with the increase in temperature the mobility and collisions of the ions with the hydroxyl polymers increases. Fig. 10 shows the kinetics of phosphate removal efficiency for 4 different temperatures between 20 and 50 °C. We observe a strong effect of the temperature on the kinetics rate which shown the positive effect of the temperature. The evolution of the pH represented Fig. 11 which is lower at high temperature because the formation of Al(OH)3 is favored and the concentration of OH is thus lower. Even if that point has not been evaluated in the present study, the temperature changed will also result in a variation of surface tension phenomena, with probably different hydrodynamic behavior of the suspended matter. In the presence of hydrogen bubble, it will certainly affect the flotation efficiency. 3.6. Electrical energetic consumption The electrical energy consumption was calculated in terms of kwh per m3 of treated effluent using the equation given below:
E ðkwh=m3 Þ ¼
U I t EC V
where U is cell voltage (V), I is current (A), tEC is the time of electrocoagulation treatment (h) and Vi is the volume (L) of effluent to be treated. Fig. 12 shows the phosphate removal efficiency versus the electrical energetic consumption. The cost, to remove the 70 first percentage of phosphate, is acceptable with a maximum of 4 kwh/m3. In this regard low current density is to be preferred since 90 of percentage removal have been obtained with less than 4 kwh/m3 at 6 mA/cm2. The energy consumption is non-constant for a given electrical charge since the tension does not vary linearly with the current density as illustrated Fig. 13. Moreover, the cell voltage is time dependent because of the change in pH, even if in the present case the variations are relatively small. This result confirms the conclusion obtained from Lacasa et al. [21] and based on another analysis. 4. Conclusion The treatment of water containing phosphate by electrocoagulation using electrodes of aluminum has been studied in batch reactor. Phosphate removal efficiency and the pH evolution versus time have been investigated for a wide range of values of the operating parameters: distance between electrode, initial pH, current densities and several conductivities. The better operating conditions are an electrodes gap of 5 mm, an initial pH of 3 and a conductivity of 3.2 mS, in these conditions the phosphate is totally removed with a rapid kinetic rate. The results show that removal efficiency depending on the electrical charge (time of treatment and current densities are linked together). However, the electrical energy consumption is lower with smaller current density because of the tension drop at high current density. At lower values of pH, the phosphate is removed by precipitation of AlPO4(s), whereas the adsorption on Al(OH)3 is predominant when the pH increased. The evolution of the pH is due to the formation of OH at the cathode. The buffering effect of the AlðOHÞ3 =AlðOHÞ 4 mixture makes the final pH more or less around 10 whatever the initial conditions. The rate and the final abatement of phosphate is strongly improved with the temperature. References [1] A.E. Durrant, M.D. Scrimshaw, I. Stratful, J.N. Lester, Review of the feasibility of recovering phosphate from wastewater for use as a raw material by the phosphate industry, J. Environ. Technol. 20 (1999) 749–758.
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