Improving On Electrokinetic Remediation In Spiked Mn Kaolinite

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Improving on electrokinetic remediation in spiked Mn kaolinite by addition of complexing agents M. Garc´ıa Nogueira, M. Pazos, M.A. Sanrom´an, C. Cameselle ∗ Department of Chemical Engineering, University of Vigo, 36320 Vigo, Spain Received 18 October 2005; received in revised form 2 February 2006; accepted 13 March 2006

Abstract During the electrokinetic treatment of kaolinite polluted with Mn, metal ions migrated to the cathode side, but the alkaline front generated by the reduction of water at the cathode provoked their precipitation as Mn hydroxide. It prevented their collection in the cathode chamber. Most Mn precipitated in a thin soil layer close to the cathode (68%), and only 26% of the initial Mn was found in the cathodic solution. Solubility of manganese can be enhanced with the addition of organic compounds (EDTA, oxalic acid and citric acid) that form complexes or chelates with Mn2+ ions. Besides, the complexing agents tested in this work are environmentally benign and do not provoked negative effects in soil. The addition of EDTA and oxalic acid to the spiked Mn kaolinite improved metal removal from kaolinite. Thus, 38% of Mn was colleted at the cathode chamber when oxalic acid was used as complexing agent. EDTA was more effective since 42% of Mn reached the cathode. The rest of the Mn remained into the kaolinite, mainly in the last section, close to the cathode. When citric acid was added to kaolinite, the behaviour of the electrokinetic treatment was totally different. Mn was completely removed and no accumulation was detected into the kaolinite. All contaminated metal was recovered in the cathodic solution. This good result was attributed to the complexing activity of citric acid and to the high electro-osmotic flow induced by the presence of citric acid into the kaolinite sample. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electrokinetic remediation; Mn; Citric acid; EDTA; Oxalic acid; Electro-osmotic flow

1. Introduction In electrokinetic remediation contaminants are removed from soil and groundwater by the action of an electric potential applied across electrodes embedded in the polluted medium. The electroremediation process is governed in part by the electrode reactions that are inherent to the process. The main reaction is the electrolysis of water. The H+ generated at the anode side move towards the cathode creating an acid front. It makes easier the desorption of the adsorbed cations on the soil surface, as well as forces the dissolution of precipitated contaminants (i.e. carbonates, hydroxides, etc.). On the other hand, the OH− ions generated at the cathode cause the precipitation of the metals, preventing their transport and reducing the treatment efficiency [1]. Furthermore, solubility of most heavy metals may be significantly reduced in the presence of certain anions such as carbonates, sulphides and sulphates. Since these substances

might be naturally present in the soil and because the pH can be increased by the electroremediation process itself, diverse approaches have been proposed for metals to be mobilised in soils: changing the acidity, changing the system ionic strength, changing the redox potential and forming complexes [2,3]. The addition of complexing agents can convert soil bound heavy metal ion into soluble metal complexes or it may modified ␨-potential that increases the electro-osmotic flow [4]. The aim of this work is to determine the influence of complexing agents in the removal of Mn from soil and sludge by electrokinetic remediation. EDTA, oxalic acid and citric acid were the complexing agents selected in this work. Their influence in the electrokinetic treatment was tested in model samples (Mn spiked kaolinite). The use of simple systems with a small number of chemical species permitted to evaluate the influence of each complexing agent more precisely. 2. Background

∗

Corresponding author. Fax: +34 986 812380. E-mail address: [email protected] (C. Cameselle).

Complexes consist of one or more central atoms or central ions, usually metals, with a number of ions or molecules, called

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ligands, surrounding them and attached to them. Some ligands can form several bonds to a single metal ion. The resulted complexes are called chelates, which are usually more stable than those complexes with a single bond metal–ligand [5]. It makes chelates more adequate in the electrokinetic treatment. In the selection of a chelating agent some considerations should be taken into account [6]: • Chelates should be stable over a wide pH range. • The ligand-to-metal molar ratio should not be higher than 1:1. • The chelating agents and their chelates should not be adsorbed onto the soil particles. • The chelating agent should be cost effective. Complexing agents can be added to the electrode rinse or purge streams and will be introduced into the soil by electroosmotic flow or, if charged, by ionic migration [7–9]. Kharkats [10] reported that an effective decontamination in the whole soil thickness is only obtained when complexant is homogeneously distributed in the soil. Another researchers [11–13] obtained good results introducing ammonia as complexing agent into the soil. They remove Cu, Cr and As from different soil samples. Under alkaline pH conditions, As is highly mobile and Cu and Cr can be mobilised forming stable complexes with ammonia.

and forms moderately stable metal complexes [16]. Moreover, oxalic is one of the strongest organic acids and therefore, it is able to attack and dissolve hydrous oxides [17]. Cameselle et al. [18] satisfactory extracts iron present as oxides and hydroxides from kaolin using oxalic acid. Iron was extracted by the direct attack of H+ ions and was maintain in solution forming a complex whose ratio Fe3+ :oxalate was 3:1. Ribeiro et al. [19] used oxalic acid in the electrodialytic decontamination of a Cu, Cr, and As contaminated industrial waste. They removed 93% of Cu, 95% of Cr and 99% of As. 2.3. Citric acid Citric acid was used to improve the removal of metals from soil because of being readily available, relatively inexpensive, and environmentally benign [13]. Shiau et al. [20] showed that citric acid removed up to 80% of copper from a polluted wood waste. Yang et al. [21] proved that citric acid is effective in removing several metals as Pb, Zn, Cu and Mn from a contaminated soil. This complexing agent was also reported in electrokinetic decontamination of soils. Yang and Lin [3] used citric acid in the anode reservoir fluid and they recovery 53% of Cr while EDTA only reached 11%. Citric acid was also used to depolarise the cathode reaction [22] and to improve the electroosmotic flow towards the cathode [23].

2.1. EDTA 3. Materials and methods Ethylenediaminetetraacetic acid can attach to a metal ion up to six sites, since each of the acetate groups and two nitrogen atoms have free electron pairs necessary for coordinate bond formation. The feasibility of this compound as a solubilizing or complexing agent has been reported in several works, specially due to its strong chelating ability for a variety of heavy metals [14]. Laboratory studies have shown that EDTA is effective in removing Pb, Zn, Cu and Cd from contaminated soil, although extraction efficiency depends on many factors such as the liability of heavy metal in soil, the strength of EDTA, electrolytes, pH and soil matrix. In electrokinetic remediation the removal of Pb and Zn was enhanced adding EDTA in the cathode well. EDTA forms negatively charged complexes, which migrate towards the anode, at high pH values. The pH on the anode well was maintained above neutral to assure that EDTA solubilizes and complexes the precipitated metals [2]. Reed et al. [9] added EDTA to the cathode well in order to avoid the formation of high pH zones. However, the low pH measured in the soil prevents the formation of metal complexes, and metals were recovered at the cathode. The main disadvantage of using EDTA in the remediation of metal-contaminated soils is their high cost. However, EDTA offers good possibilities for recovery and recycling [15] and the reagents cost can be reduced with a simple treatment of the liquid stream.

3.1. Electrokinetic cell The experiments were conducted in the electrokinetic cell previously described by Ricart et al. [1]. A schematic diagram of the experimental set-up is shown in Fig. 1. Cylindrical samples of 100 mm length and 32 mm diameter are placed between two porous stones and then an electric field is applied across two inert electrodes of graphite located at both sides of the soil sample. Three auxiliary electrodes allow measuring the field distribution along the sample. The gas produced by the electrode reactions is purged from the electrode chambers with a manual valve.

2.2. Oxalic acid Oxalate was tested as a soil extractant metal because it is biodegradable, naturally occurring and relatively inexpensive,

Fig. 1. Schematic diagram of experimental set-up: (1) soil sample; (2), porous stones; (3) graphite electrodes; (4) electrode chambers; (5) auxiliary electrodes; (6) gas valves.

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3.2. Methodology Samples of Mn spiked kaolinite were prepared by adding a solution of MnSO4 or MnCl2 to kaolinite clay. In the experiments with complexing agents, EDTA, citric acid or oxalic acid were added to the Mn solution (before mixing with kaolinite) in a molar ratio of 1:1 (1 mol of complexing agent:1 mol of Mn2+ ). The pH of the resulting solution was adjusted to the original value of the Mn solution with NaOH and/or H2 SO4 . Kaolinite clay and the Mn/complexants solution was mixed thoroughly to obtain a slurry of 5 g Mn/kg dry kaolinite. The mixture stayed for 24 h to let the sorption of soluble ions onto the kaolinite surface and the formation of complexes of Mn. The slurry was compacted to a constant density and pressure in a consolidometer unit [1] in order to create homogeneous, near-saturated soil matrixes. It resulted in a kaolinite sample of 65–70 g (dry) in the tube of the electrokinetic cell, with a moisture content of 33–35% and a Mn concentration of 2.6–3.3 g/kg dry soil. The pH of the spiked kaolinite was about 4. Kaolinite has a particle size average of 3 ␮m and a specific surface of 13.5 m2 /g. The mineralogy analysis by X-ray diffraction indicated the presence of kaolinite clay 85%, mica 14% and quartz 1%. A constant potential difference of 30 V DC, equivalent to a potential gradient of 3 V/cm, was applied for about 7 days, except in one experiment that lasted for 22.7 days. Readings of voltage between the anode, the auxiliary electrodes and the cathode, current intensity and the pH in the electrode chambers were taken regularly during the tests. Upon completion of all experiments, the soil sample was divided into five sections of equal size and they were analysed for water content, Mn and citrate concentration and pH. Liquid samples from the electrode chambers were collected and analysed for Mn and citrate concentration and pH. 3.3. Analytical methods The test protocols for chemical extraction and analysis procedures were performed in accordance with EPA Method 3010 (Acid Digestion of Aqueous Samples and Extracts for Total Metals for Analysis by FAA and ICP Spectroscopy) and Method 3050 (Acid Digestion of Sediments, Sludges, and Soils). Flame atomic absorption spectroscopy (FAA) was used to analyse Mn. The soil pH was measured by mixing distilled water or KCl 1 M and dry soil in a ratio 2.5 mL/g. After 1 h of contact time the pH was measured in the supernatant fluid. When KCl was used the soil pH was calculated as follows [24]: pH = pH(KCl) + 0.5 where pH is the soil pH and pH(KCl) is the pH of the supernatant fluid. Citric acid was measured in the solution of electrodes chambers and in the interstitial fluid of soil with the acetic anhydride–pyridine method [25].

Fig. 2. Mn and pH distribution after the electrokinetic treatment in spiked kaolinite.

4. Results and discussion Electrokinetic remediation was successfully used to remove heavy metals from different sorts of soils. In order to assess the feasibility of the electrokinetic remediation in the removal of Mn from porous materials, some experiments with and without complexing agents were carried out. This metal was selected in this study because of there are some sites in Galicia (NW Spain) contaminated with this metal. A preliminary experiment with a saturated clay sample (kaolinite) was carried out. A constant potential difference of 3 V/cm was applied to the sample with an initial Mn concentration of 2.6 g/kg dry weight and a moisture content of 35%. Fig. 2 shows the distribution of Mn in the kaolinite sample at the end of the process. It can be observed that the manganese effectively migrated through the kaolinite towards the cathode. The Mn removed from the first four sections of the kaolinite (the closest to the anode) was very high ranging from 87% to 99% with an average value of 94%. However, most of the Mn removed was accumulated at the cathode end (68.7%) where a brown layer (due to Mn precipitation) was observed. A fraction of the removed Mn (26.6%) reached the cathode solution and precipitated as manganese hydroxide. Since deionised water was used as processing fluid in the electrode chambers, the electrolytic decomposition of water took place, resulting in an increase of the pH in the cathodic solution (about 11) and a reduction in the anodic one (about 2). A constant value was achieved very quickly, approximately 1 day after start-up. The H+ and OH− ions generated on the electrodes migrated through the kaolinite sample towards the opposite electrode forming an acid and a basic front. When both fronts met, soil was divided in two zones: a high and a low pH zone with a sharp pH jump in between [26]. The pH jump was next to the cathode because of H+ ions have almost twice as high ionic mobility as OH− ions [27]. Mn ions migrated towards the cathode, but when they reach the pH jump, reacted with OH− ions forming a precipitate of manganese hydroxide, which formed a narrow brown layer within the kaolinite close to the cathode. The precipitation of Mn clogged the soil pores, hindering further transport of more Mn cations and other species. Furthermore, the meeting of acid

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and basic fronts and the precipitation of Mn eliminated chargecarrying ions (e.g. H+ , Mn2+ and OH− ) creating a region of low conductivity. This caused an increase in the electric field in this region, which in a feedback effect accelerated the deionization. Moreover, low conductivity meant that the overall movement of ions by electromigration was low, so the remediation process would take a great deal of time [28]. This underlined that soil pH and metal solubility have a crucial importance for the successful removal of Mn from soil. Some substances, which can form stable complexes with Mn, can be introduced into the soil in order to keep Mn in solution in a wide range of pH, so that, Mn could be removed by the electrokinetic process even in the high pH zone. In the following set of experiments, some organic acids with complexing properties were used to enhance the removal of Mn from a spiked kaolinitic soil. 4.1. Addition of complexing agents In a first run of experiments, the complexing agents used were EDTA, oxalic acid and citric acid. These compounds were added to Mn spiked kaolinite, in a molar ratio 1:1 (metal:ligand). The purpose of the introduction of complexing substances is to enhance the electrokinetic remediation process forming metal complexes and/or increasing the electro-osmotic flow. Mn forms neutral or anionic complexes with EDTA and oxalate, therefore they might move towards the anode to maintain electroneutrality. Experimental results (Table 1) showed that complexing agents increased the efficiency of the electrokinetic remediation process. Nevertheless, manganese migrated towards the cathode. This indicated that non-anionic complexes were formed under the pH conditions prevailing in the soil. EDTA is soluble at pH above 3.5 [15] but the acid front reduced the soil pH up to a value lower than 3 during most of the treatment time. Therefore, EDTA did not complex manganese, which remained as a cation and migrated towards the cathode being precipitated in the last section of soil. Controlling the pH in anode solution could avoid the reduction of soil pH, so that EDTA could effectively form anionic complexes with Mn, which would be collected at the anode side [2]. However, this possibility was not assayed because the aim of the work was to

Table 2 Stability constants for formation of metal complexes of Mn2+ and citrate and equilibrium constants for citric acid dissociation [30] Stability constants for formation of complexes Mn[citrate]− 105.5 MnH[citrate] 109.4 Acidity constants for citric acid First Second Third

7.10 × 10−14 1.68 × 10−15 6.40 × 10−6

Solubility product Mn(OH)2

10−12.1

find a compound that could effectively separate the metal in a process as simple and economical as possible. The presence of EDTA in the soil also induced a moderate electro-osmotic flow towards cathode chamber that cooperated to manganese migration [3]. Moreover, hydrogen ions released from this organic acid neutralised a fraction of the hydroxyl ions generated in the cathode delaying the formation of the high pH zone. It resulted that more Mn ions reached the cathodic solution reducing the amount of metal accumulated in the soil. However, the larger energy expenditure and the cost of chemicals made EDTA no suitable for this process. Similar results were found for oxalic acid as shown in Table 1. A 38% of Mn reached the cathode chamber but a large fraction of Mn remained in kaolinite. This low efficiency and the high electric power consumption made oxalate not appropriate for large-scale applications. However, citric acid showed a completely different behaviour. Complete removal of manganese was achieved when citric acid was added to the kaolinite (Table 1). Citric acid forms negatively charged and neutral complexes with Mn. Table 2 shows the formation constants for the Mn–citrate complexes. These values were used to calculate the speciation in a solution that contains Mn2+ (0.1 M) and citric acid (0.1 M) in a wide pH range (Fig. 3). It tries to represent the environment in the interstitial fluid of the kaolinite sample. However, the influence of the kaolinite and the presence of others minor chemical species were no considered, so, the speciation shown in Fig. 3 was only used as a guideline for the discussion of the results obtained in this test.

Table 1 Mn distribution, pH and electric power consumption in the electrokinetic tests with and without complexing agents Complexing agent

Treatment time (days)

Sections: anode–cathode S1

S2

S3

S4

S5

Mn collected in the cathode (%)

Power consumption (W h)

–

7.9

Mn in kaolinite (%) pH

0.1 2.9

1.1 3.4

2.8 3.1

0.6 3.3

68.7 5.8

26.6

9.93

EDTA

7.0

Mn in kaolinite (%) pH

0.1 2.4

0.1 2.3

0.3 2.3

0.7 2.3

56.8 4.6

41.6

43.31

Oxalic acid

7.0

Mn in kaolinite (%) pH

0.3 2.6

0.4 2.8

1.3 3.1

5.4 3.2

54.1 4.9

38.3

39.44

Citric acid

8.8

Mn in kaolinite (%) pH

<0.01 2.7

<0.01 2.8

<0.01 3.2

< 0.01 3.3

< 0.01 3.5

99.9

31.98

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Fig. 3. Speciation of citric acid and Mn as a function of pH (L = citrate).

At high pH values, the prevailing specie is the negatively charged Mn[citrate]− but nor citrate nor Mn were detected on the anode solution because of the pH throughout kaolinite remained acid (pH ≤ 3). Under this pH conditions, citric acid formed a neutral compound (MnH[citrate]) although a significant amount of the Mn remained as a free ion. Both species moved to the cathode side, the former by electroosmosis and the latter by electromigration and electroosmosis. Mn and citrate were recovered in the cathodic solution. Citric acid showed a triple action: forms metal complexes, provides an acid medium to delay the formation of high pH zones and induces an especially high electro-osmotic flow [23]. The electro-osmotic flow was so high that removed all the residual Mn and even dissolved precipitates formed in the soil in the first days of the experiment. As it was reported before [3], the electro-osmotic flow in this experiment is widely considered to be responsible for the removal of pollutants. 4.2. Electric power consumption The addition of these organic acids to the soil clearly changes the current intensity evolution and the electric power consumption. As an example, Fig. 4 shows the evolution of current intensity in two experiments with and without citric acid. The addition of organic acids resulted in higher intensity values because of the pore fluid contains a higher amount of dissolved ions that can carry the electric current. It also provoked a larger extension of the electrolytic reactions upon the electrodes including the OH− in the cathode. In fact, a brown layer (Mn precipitate) was observed in 15 h, whereas this layer was observed on the fourth day in the preliminary experiment. This increase of the intensity provokes high electric power consumption. On the other hand, the citric acid addition resulted in an especially higher electro-osmotic flow that corresponded with the two peaks of current intensity. In the first one, an average electro-osmotic flow of 10 mL/day was measured. In the second one, a flow as high as 30 mL/day was measured. It must be noticed that the total volume of the soil sample was 80 mL. The relation between current intensity and electro-osmotic flow was

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Fig. 4. Evolution of the electric current intensity for the experiments with and without citric acid.

previously reported by Hamed and Bhadra [29]. The electroosmotic flow had a low pH and was able to redissolved the precipitated Mn, so that Mn could be transported to the cathode chamber. The electric power consumption clearly increased when those organic acids were added to the soil sample. However, these values are acceptable for the treatment with citric acid due to the good results obtained (Table 1). Further studies will be conducted to optimise the citric concentration in order to reduce the electric power consumption and the cost of chemicals. 4.3. Optimisation of citric acid addition Two new experiments with citric acid were carried out (Table 3). In these experiments the metal-to-ligand molar ratio used was 2:1 to reduce the cost in chemicals. The system evolution for a molar ratio 2:1 (Mn:citric acid) is slower than the previous experiment with citric acid. The current intensity values were lower and the second peak of intensity was not detected. In fact, for a similar treatment time the amount of Mn collected at the cathode solution was only 52%, remaining the rest of Mn precipitated in the closest soil section to the cathode. Fortunately, the electric power consumption was also reduced by half. For longer treatment time (22.7 days), the second peak of current intensity was observed and a complete removal of Mn was achieved, although the electric power consumption was a bit higher. It means that the treatment time can only be diminished with an increase of the citric concentration. The optimum value should be determined for each specific application. Table 3 Electrokinetic treatment of Mn spiked kaolinite with citric acid Molar ratio Mn:citric

Treatment time (days)

Mn collected in the cathode (%)

Power consumption (W h)

1:1 2:1 2:1

8.8 7 22.7

99.9 52.0 99.9

31.98 15.86 45.86

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5. Conclusions In the experiment without additives, most of Mn was accumulated in a thin soil layer close to the cathode due to the penetration of OH− ions in the soil, and only 26.6% was found in the cathodic solution. The addition of EDTA or oxalic acid slightly increased the recovery of Mn in the cathode well and, therefore, reduced the accumulation in the soil sample but the high electric power consumption and the cost of chemicals make them inadequate for a large-scale application. The best results were obtained with citric acid. No accumulation of Mn was detected in the soil and it was completely collected in the cathodic solution. The most important effect of the addition of citric acid was the increasing in the electroosmotic flow that lets the transportation of all the contaminated metal to the cathode. The amount of citric acid added to a soil should be optimised in each case keeping in mind that the optimum solution will have to consider three factors: cost of chemicals, electric power consumption and treatment time. Acknowledgement This work was supported by Ministerio de Medio Ambiente, Spain (Project no.: 2.1-267/2005/2-B). References [1] M.T. Ricart, C. Cameselle, T. Lucas, J.M. Lema, Sep. Sci. Technol. 34 (16) (1999) 3227. [2] J.S.H. Wong, R.E. Hicks, R.F. Probstein, J. Hazard. Mater. 55 (1–3) (1997) 61. [3] G.C.C. Yang, S.L. Lin, J. Hazard. Mater. 58 (1–3) (1998) 285. [4] K. Popov, V. Yachmenev, A. Kolosov, N. Shabanova, Colloid. Surf. A 160 (2) (1999) 135. [5] J.C. Chao, A. Hong, R.W. Okey, R.W. Peters, Proceedings of the Conference on Hazardous Wastes Research, Snowbird, UT, USA, 1998, p. 142. [6] R.W. Peters, L. Shem, ACS Symp. Ser. 509 (1992) 70.

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