Removal of nitrates from groundwater by electrocoagulation

Removal of nitrates from groundwater by electrocoagulation

Chemical Engineering Journal 171 (2011) 1012–1017 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsev...

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Chemical Engineering Journal 171 (2011) 1012–1017

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Removal of nitrates from groundwater by electrocoagulation ˜ Engracia Lacasa, Pablo Canizares, Cristina Sáez, Francisco J. Fernández ∗ , Manuel A. Rodrigo Department of Chemical Engineering, Faculty of Chemical Sciences, University of Castilla-la Mancha, Avda. Camilo José Cela, 12, 13071 Ciudad Real, Spain

a r t i c l e

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Article history: Received 2 December 2010 Received in revised form 21 March 2011 Accepted 23 April 2011 Keywords: Electrocoagulation Coagulation Nitrates Iron electrodes Aluminium electrodes

a b s t r a c t In this study, coagulation and electrocoagulation processes were compared with regard to their respective efficiencies as to the removal of nitrates from water. The results indicate that electrocoagulation is an effective technology for nitrate removal because nitrate anions preferentially adsorb onto the surfaces of growing metal-hydroxide precipitates. Other similar results were observed when using iron or aluminium electrodes whenever coagulation reagents, aluminium or iron, were plotted in molar units and the same adsorption isotherm was obtained. Since electrocoagulation merely acts as a dosing coagulant technology, current density does not influence the removal of nitrates from water. However, it strongly affects the feasibility of nitrate removal because current density increases the operational cell potential. In other words, current density influences the power consumption that is required to provide a specific dose of reagent. On the other hand, the coagulation results indicate that this technology is not suitable for removing nitrate from water. The huge increase in conductivity observed during coagulant dosing (in comparison to the conductivity that was obtained when using electrocoagulation) appears to promote competition among nitrates and coagulant counter ions. It also decreases the widths of the double layers that form around the precipitate particles. Both of these processes most likely explain why nitrates cannot be removed from water using a coagulation process. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The excessive application of fertilizers, the intense exploitation of farms and the ubiquity of other agriculture-related industries have increased the nitrogen load discharged into waterways. This growth in the nitrogen load has resulted in a decrease in water quality and has even let to health problems related to oxidized forms of nitrogen. The removal of nitrogen can be carried out by using either biological or physico-chemical methods. Nitrogen and phosphorus compounds are typically removed from wastewater via biological nutrient removal (BNR) processes because they are the most inexpensive of all the available processes. However, these processes are not suitable for all wastewater types because the feasibility of BNR depends on biodegradability as well as on the carbon to nitrogen ratio in the water [1]. Furthermore, these processes are very sensitive to the presence of toxic compounds and to other parameters, such as temperature, pH, and conductivity. In contrast, physico-chemical treatments are robuster and commonly used to improve the quality of drinking water and to remove large quantities of nutrients from industrial wastewaters.

∗ Corresponding author. Tel.: +34 902204100; fax: +34 926 29 53 18. E-mail address: [email protected] (F.J. Fernández). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.053

Among the various physico-chemical processes, the most commonly used techniques involve ion exchange, reverse osmosis, chemical precipitation, and electrocoagulation [2,3]. The main motivation for choosing among the different physico-chemical processes is economic, and available capital and operating costs typically determine the preferred method of choice. Therein, ion exchange and reverse osmosis processes are expensive and, therefore, unattractive. Among the cheapest physico-chemical processes, which include chemical precipitation and electrocoagulation, the last one has several advantages, involving the ability to deliver a precise coagulant dose via control of the amount of applied electrical current, easy automation, low energy requirements [4], and the ability to destabilize, aggregate, and separate the pollutants in a single stage [5,6]. Electrocoagulation includes the in situ generation of coagulants via the electro-dissolution of a sacrificial anode, which usually consists of iron or aluminium [6]. The interaction between the coagulant and the pollutant is the most complicated aspect in the electrocoagulation process. Once the coagulant metal is dissolved into water, it is hydrolyzed to form different hydroxo-metallic monomeric and polymeric species as well as metal hydroxide precipitates. The types and amounts of produced species (speciation) primarily depend on the metal concentration and pH [7]. Likewise, the coagulation/destabilization mechanisms and removal efficiencies closely relate to the coagulant species present in the system [8]. Thus, it has been reported

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in the literature that nitrates can be removed from wastewater via their adsorption onto the surfaces of hydroxide precipitates, which are generated from metals and released by the electrodes [9–11]. The goal of this work was to increase the knowledge on the removal of nitrates through electrocoagulation and to go into the mechanisms that this process involves. The performances of the coagulation and electrocoagulation processes were compared to one another in order to identify the highest nitrate removal efficiencies using the lowest necessary coagulant dosages.

2. Experimental 2.1. Experimental procedure The treatability and operational costs of the coagulation and electrocoagulation processes were evaluated using benchscale coagulation and electrocoagulation studies. Both coagulation experiments were carried out in a batch operation mode. The experimental setups and procedures used herein have been described in previous investigations [12,13]. As to the electrochemical experiments, the coagulant reagent was derived from the dissolution of iron or aluminium electrodes that had been placed in a single compartment electrochemical flow cell. Both electrodes (anode and cathode) were square in shape (100 cm2 ), and the electrode gap was 9 mm. An electrical current was applied via a Promax FA-376 DC power supply. The synthetic groundwater was stored in a glass tank (5000 cm3 ) and recirculated through the electrolytic cell thanks to a peristaltic pump. In order to develop the discontinuous chemical coagulation experiments, a standard jar test experimental setup was used. In these experiments, a fixed amount of coagulant (AlCl3 or FeCl3 ) was added to the solution and pH values were adjusted using 0.1 M H2 SO4 and/or 0.1 M NaOH whenever the experiments required this pH modification. The synthetic groundwater consisted of sodium nitrate (25 mg N dm−3 ) and a supporting electrolyte in order to increase its conductivity (3000 mg dm−3 Na2 SO4 ). This nitrate concentration is reported to be the average concentration of nitrates into groundwater in agriculture zones which have an excessive use of fertilizers [14,15].

2.2. Analysis procedure Nitrate ions were characterized using ion chromatography by means of a Shimadzu LC-20A system. A Shodex IC I-524A column was used for the anionic separation and it was packed with an ion exchange resin that was formulated by bonding a quaternary ammonium group to a hydrophilic gel. The mobile phase consisted of an aqueous solution of 2.5 mM phthalic acid with a pH of 4.0, and the flow rate of the mobile phase was 1 ml min−1 . The nitrate peak exhibited a retention time of 6.28 min. To measure the ammonium ions was used the same ion chromatography equipment (column, Shodex IC YK-421; mobile phase, 5.0 mM tartaric, 1.0 mM dipicolinic acid and 24 mM boric acid; flow rate, 1.0 ml min−1 ). The peaks were identified and quantified using LCsolution Chromatography Workstation, version 1.23. Total aluminium or iron concentrations were measured off-line using an inductively coupled plasma spectrometer (Liberty Sequential, Varian) according to a standard method [16] (plasma emission spectroscopy). In order to evaluate the total metal concentrations, samples were diluted to 50:50, v/v using 4 N HNO3 so as to ensure the total solubility of the metal.

Fig. 1. Variation of nitrogen concentration (a) and conductivity (b) with metal molar concentration during the electrocoagulation of nitrate solutions, using aluminium ( NO3 − –N,  NH4 + –N) and iron ( NO3 − –N,  NH4 + –N) electrodes. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , j: 1.0 mA cm−2 , pH is not modified).

3. Results and discussion 3.1. Electrocoagulation vs. coagulation Fig. 1 represents changes in nitrogen-species concentration and conductivity as a function of the amount of coagulant reagent that was added during the electrocoagulation of a solution that contained 25 mg N–NO3 − dm−3 with iron and aluminium electrodes. Therein, it was observed that the nitrate concentration decreased to nearly zero during the discontinuous process, and both coagulation reagents behaved similarly. This was an important observation because it showed that nitrate anions can be efficiently removed by electrocoagulation using iron and aluminium electrodes. Concerning the conductivity, Fig. 1 shows that it slightly increases during the electrolytic process as a consequence of the electrochemical dosing of coagulant reagents. In this point, an important observation is the occurrence of a small concentration of ammonium during the electrocoagulation with aluminium electrodes, which is not obtained in the electrocoagulation with iron electrodes. These ammonium ions seem to behave as reaction intermediates. Their production can be easily explained in terms of the chemical reaction between aluminium and nitrate, previously proposed in literature by Murphy [17], and their decay by their adsorption onto the growing flocs of aluminium hydroxide. During the electrocoagulation process, ammonium ions disappear completely from the treated solution, and its concentration does not exceed the 10% of the nitrogen mass balance. In literature [18,19], the nitrate reduction is also reported by iron, promoted at a pH range between 2.0 and 4.5. However, it has not been observed in our experiments, and this can be explained by the operation pH used. Fig. 2 shows pH and zeta potential changes with the metal molar concentration. Thus, pH increased during the initial stages up to a final value of around 8.4 for iron and 9.4 for aluminium. The primary

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Fig. 2. Variation of pH values (a) and zeta potential (b) with metal molar concentration during the electrocoagulation of nitrate solutions, using aluminium () and iron () electrodes. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , j: 1.0 mA cm−2 , pH is not modified).

Fig. 3. Variation of nitrate–nitrogen concentration (a) and conductivity (b) with metal molar concentration during the coagulation of nitrate solutions, using aluminium () and iron () chloride. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , pH is not modified).

differences appear in the plot of z-potential vs. coagulant dosage. In both cases, there was an initial decrease and a later increase; however, these changes were very rapid for iron and markedly slower for aluminium. The initial decrease in the z-potential towards a more negative value indicates that a negative charge was formed on the surface of the solids; it may also refer to the adsorption of negatively-charged species, such as nitrate anions on the surface of the growing metal hydroxide precipitates. The later increase in the z-potential can be explained in terms of the aggregation of these initial flocs into greater flocs, in stages when nitrates are at smaller concentrations in the solution (need to bear in mind that the process is operated in a discontinuous mode). The increase could also have occurred due to the formation of new layers on the growing precipitates that covered surfaces where nitrates were present. In the case of iron, at the working pH, the primary species consisted of solid iron hydroxide, whereas, in the case of aluminium, both aluminium hydroxide precipitates and Al(OH)4− coexisted. The nitrate anion can be adsorbed onto the aluminium hydroxide surface, and it may explain the negative superficial charge that was observed over a long period of time. In this point, the neutralization noticed at long times can also be explained by the contribution of the positive charges of ammonium ions to the flocs net-charge. Figs. 3 and 4 represent the results that were obtained during the coagulation of the same nitrate solutions using iron and aluminium solutions. Similarly to the previous case, the pH was not modified during the coagulation; however, the pH was monitored and a fluctuation was observed according to the chemistry of the system. In these tests many significant observations were made. Firstly, nitrates were not removed using either of the two reagents regardless of reagent concentration. Additionally, conductivity was hugely increased especially in comparison with the increase that was obtained in the electrocoagulation process (y-axis scale is the same). In both cases, the pH decreased to a constant value which was around 1.2 for iron and 3.9 for aluminium. This observation

Fig. 4. Variation of pH values (a) and zeta potential (b) with metal molar concentration during the coagulation of nitrate solutions, using aluminium () and iron () chloride. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , pH is not modified).

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Fig. 5. Variation of nitrate–nitrogen concentration (a) and conductivity (b) with metal molar concentration during the coagulation of nitrate solutions, using aluminium () and iron () chloride. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , initial pH modified).

can be explained based on the acidic properties of iron chloride and aluminium chloride (Lewis acids), which promote the hydroxylation of M3+ cations with hydroxyl anions, thereby reducing the pH. This difference in pH could explain the observed differences in z-potential, which increased to positive values (reversing the surface charges of the precipitates) for iron and was maintained at approximately zero for aluminium. Again, the chemistry of the soluble hydroxoions of aluminium and iron explains the observed changes. In this pH range, the concentrations of iron hydroxocations [Fe(OH)2+ , Fe(OH)2+ ] and protons were significant. These species could have been adsorbed onto the surface of the iron hydroxide, explaining the resulting positive charge. The significance of this adsorption was smaller for aluminium, and this trend can be explained by the lower frequency of aluminium adsorption (the pH was significantly higher, which in this region, led to the presence of amorphous metal hydroxides instead of soluble cations). A comparison of Figs. 1 and 2 (electrocoagulation) to Figs. 3 and 4 (coagulation) demonstrates that the iron and aluminium doses had markedly different impacts on both systems. Initially, the most different changes in the pH seemed to be the cause of this difference in behaviour. In order to confirm this hypothesis, a new set of coagulation experiments was proposed wherein the pH was modified in order to try to match the pH values that were obtained in the electrocoagulation experiments. At this point, it was considered that the aqueous chemistry of iron and aluminium was not easy, and that it would be difficult to achieve the same pH due to the different buffering effects of the species in the solution. The results of these experiments are showed in Figs. 5 and 6. Again, no reduction in nitrate concentration was observed. The resulting conductivity was even greater (due to neutralization using sodium hydroxide), and the pHs were closer to those that were obtained by electrocoagulation; however, they were ultimately not identical (small additions of sodium hydroxide resulted in large changes). It is also inter-

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Fig. 6. Variation of pH values (a) and zeta potential (b) with metal molar concentration during the coagulation of nitrate solutions, using aluminium () and iron () chloride. (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , initial pH modified).

esting to notice that, changes in z-potential were similar to those observed for the electrocoagulation process (an initial decrease followed by a later increase, which did not reverse the superficial charge), although there was a clear difference in the magnitude of the changes. The magnitude of the change was much smaller for the coagulation process, wherein the minimum z-potentials for aluminium were less than half of that obtained during electrocoagulation (this result suggests that a significantly lower quantity of nitrate was adsorbed during the first stages). Additionally, there was a much slower change in the z-potential for iron, which indicated that coagulation was a less efficient process. The primary difference between the electrocoagulation and coagulation processes was among the conductivities of the treated solutions. The obtained conductivities by means of coagulation were more than three times higher than those obtained using electrocoagulation. Although these conductivities did not appear to affect significantly the superficial charges of the metal hydroxide precipitates, these conductivities were likely to influence the widths of the double layers around the particles. This influence could have affected the nitrate removal efficiency. Another possible explanation for this observation is the competition among anions that were to be adsorbed onto the growing metal hydroxide precipitates, which is an effect that has been previously studied in several investigations [10,11,20,21]. During this process, the counter anion of the metal reagent that is added during the coagulation process competes with nitrates for adsorption onto the growing precipitates, thereby decreasing the removal of nitrates. 3.2. The impact of the current density on electrocoagulation results From the discussion presented in the previous section, it appears that electrocoagulation promotes the removal of nitrates from water, which contrasts the results that were obtained for coagulation. The most significant operational parameter in the elec-

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Fig. 7. Effect of the electrochemical dose of metal on the removal of nitrate-nitrogen by electrocoagulation with iron (open symbols) and aluminium (solid symbols) electrodes at several current densities: (, ) 0.1 mA cm−2 , (, ) 1.0 mA cm−2 , (, ♦) 3.0 mA cm−2 , (䊉, ) 5.0 mA cm−2 . (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , initial pH is not modified).

trocoagulation process was current density, which strongly relates to cell potential. In this way, it was important to examine the effects of current density and to confirm the range of current densities in which nitrates can be removed. Fig. 7 shows the effect of the electrochemical metal dose on the removal of nitrates using iron and aluminium electrodes at different current densities (ranging from 0.1 to 5.0 mA cm−2 ), which are typically used values for this technology. There was not a significant influence from using iron or aluminium on the nitrate removal and no significant differences were observed in the results, for any of the current densities researched (for a particular current charge applied or metal reagent dosed), since all data lie over the same line. In the onset of the Figure, nitrate removal data are plotted in the form of an adsorption isotherm. Although there is a great dispersion for higher nitrate concentrations, the data fit well to a Freundlich isotherm (q = 0.165(NO3 − –N)0.692 ), which clearly indicates that adsorption was the primary mechanism for nitrate removal. Fig. 8 shows the changes in the concentration of ammonium ions during the electrocoagulation processes. As it can be observed, the production of ammonium ions is nil for iron electrocoagulation, and it is also nil in the case of aluminium electrocoagulation when no current is applied (no actual electrocoagulation). In the remaining cases, within a first stage, it may be verified that the production rate of ammonium ions is the same for every current density applied. As stated before, this confirms that this production of ammonium ions is due to chemical reaction between aluminium

Fig. 9. Effect of the current density on the final pH ( iron,  aluminium) and on the cell potential ( iron,  aluminium) (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , initial pH is not modified).

and nitrate proposed in literature by Murphy [17] and not to electrochemical processes. It seems also clear from data that a certain current helps to promote the nitrate reduction although this does not support an electrochemical reduction of nitrate but a chemical process. This may be explained taking into account that electrodissolution of the aluminium helps to remove protective oxides layer, formed on the surface of the aluminium electrodes. The removal of ammonium is carried out by adsorption onto the growing flocs of aluminium hydroxide. Consequently, the higher the current density is the faster the removal of ammonium ions and the smaller the concentration are observed, because for the same electrolysis-time, the current charge applied is higher. In every case the maximum generation of ammonium ion was always less than 3 mg N dm−3 and this cation concentration was completely removed in a quicker way than the nitrate one. Fig. 9 shows the impact of current density on the pH and cell potential. In the first case, the higher the current density, the higher the resulting pH, and the range of final pH values covered the pH value range that was used in the coagulation experiments depicted in Figs. 4 and 6. This result confirms that pH was not a primary parameter and that the negligible removal of nitrates during pHcontrolled coagulation cannot be described by the small differences in pH. Instead, the nitrate removal can be explained by the competing effects of counter ions that were added during the coagulant dosing. On the other hand, as it was expected, the cell potential increased with current density. This observation argues against the use of larger values because of an unnecessary increase in operational costs. The savings are very significant for very small current densities. Since the anodic surface is not a limiting step in this process and since it behaves as a sacrificial electrode, the electrode can work using very low current densities in this type of process. 4. Conclusions From this work, the following conclusions can be drawn:

Fig. 8. Variation of ammonium–nitrogen concentration with time during the electrocoagulation with iron (open symbols) and aluminium (solid symbols) electrodes at several current densities: (*, ×) 0.0 mA cm−2 , (, ) 0.1 mA cm−2 , (, ) 1.0 mA cm−2 , (, ♦) 3.0 mA cm−2 , (䊉, ) 5.0 mA cm−2 . (150 mg NaNO3 dm−3 + 3000 mg Na2 SO4 dm−3 , pH is not modified).

1 Electrocoagulation is an effective technology for the removal of nitrates from wastewaters. Nitrate adsorption onto growing metal hydroxide precipitates appears to be the primary mechanism behind the observed large nitrate removal efficiency. 2 For the same reagent dose (in molar units), the same amount of nitrate removal was obtained using both iron and aluminium anodes. Both adsorption isotherms supported the prediction that nitrate ion adsorption was the primary coagulation mechanism; however, the z-potential indicated a different process in terms of superficial charge using iron and aluminium, which can be explained by the different speciation of both coagulants.

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3 Small amounts of ammonium ions are produced during electrocoagulation with aluminium electrodes. These cations are adsorbed onto the growing aluminium hydroxide flocs and they are completely removed from the treated water before than nitrates. 4 Current density did not affect nitrate removal efficiency when the same dose of reagent was added to the treated water; however, current density could significantly affect operational costs due to increases in the resulting cell potential. 5 Coagulation cannot compete with electrocoagulation for this particular application. For the same reagent doses that achieve significant nitrate removal using electrocoagulation, coagulation exhibits no nitrate removal. The significant increase in conductivity seems to be the cause of this trend, which could be explained by an increase in the competition between anions for adsorption onto growing metal hydroxide precipitates and by a decrease in the widths of the double layers of precipitate particles. Acknowledgements This work was supported by the MCT (Ministerio de Ciencia y Tecnología, Spain) and by the EU (European Union) through projects CTM2010-18833/TECNO and through the CONSOLIDERINGENIO 2010 (CSD2006-044) project. References ˜ [1] A. De Lucas, L. Rodríguez, J. Villasenor, F.J. Fernández, Influence of industrial discharges on the performance and population of a biological nutrient removal process, Biochem. Eng. J. 34 (2007) 51–61. [2] A.S. Koparal, U.B. Ogutveren, Removal of nitrate from water by electroreduction and electrocoagulation, J. Hazard. Mater. B89 (2002) 83–94. [3] S. Vasudevan, F. Epron, J. Lakshmi, S. Ravichandran, S. Mohan, G. Sozhan, Removal of NO3 − from drinking water by electrocoagulation—an alternate approach, Clean – Soil, Air, Water 38 (2010) 225–229. [4] G.H. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41.

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