Scholarly article on topic 'Initial Water Content and Temperature Effects on Electrokinetic Removal of Aluminium in Drinking Water Sludge'

Initial Water Content and Temperature Effects on Electrokinetic Removal of Aluminium in Drinking Water Sludge Academic research paper on "Chemical engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Physics Procedia
OECD Field of science
Keywords
{"Electrokinetic Remediation" / Sludge / Aluminium / "Water content" / Temperature}

Abstract of research paper on Chemical engineering, author of scientific article — M. Cherifi, S. Hazourli, M. Ziati

Abstract Electrokinetics is a developping technology that is intended to separate and extract heavy metals, radionuclides, and organic contaminants from saturated or unsaturated soils, sludges and sediments, and groundwater. The goal of electrokinetic remediation is to effect the migration of subsurface contaminants in an imposed electric field. This technique is known as electrokinetic remediation, electroreclamation, electrochemical decontamination, electrorestoration, electromigration or electrochemical soil processing. Electrokinetics involves the installation of electrodes into the subsurface surrounding the contaminated region. After the electrodes are in place, a low electrical potential is applied across the anode(s) (positively charged electrode) and the cathode(s) (negatively charged electrode). As a result of the electrical gradient, different physico-chemical reactions occur and contaminant transport occurs due to various mechanisms within the soil and groundwater. Generally, for the migration to be significant, the contaminants should be in a soluble form. If they are not soluble, they need to be desorbed, dissolved, and/or solubilized into the pore solution before they can be adequately transported from the soil to an electrode wells/reservoirs. Different types of contaminants have been investigated and research has been conducted to optimize the electrokinetic variables. The present study was undertaken to systematically investigate the effect of initial sludge water content, and heating on the electrokinetic remediation of alumium-contaminated sludge. A total of four laboratory experiments were conducted using drinking water sludge. The first two tests studied the effect of variation of initial sludge water content under an ambient temperature, and the last two tests studied the effect of heating on electrokinetic remediation under conditions of both constant saturation and applied voltage.

Academic research paper on topic "Initial Water Content and Temperature Effects on Electrokinetic Removal of Aluminium in Drinking Water Sludge"

Available online at www.sciencedirect.com

ScienceDirect

Physics Procedia 2d (20099) 1021-1030

www.elsevier.com/locate/procedia

Proceedings of the JMSM 2008 Conference

Initial Water Content and Temperature Effects on Electrokinetic Removal of Aluminium in Drinking Water Sludge

M. Cherifi*, S. Hazourli, M. Ziati

Laboratory of Water Treatment and Valorization of Industrials Wastes, Chemistry Department, Faculty of Sciences, Badji Mokhtar University, Bp 12 Annaba, 23000, Algeria.

Received 1 January 2009; received in revi sed form 331 July 2009; accepted 3 1 August 2009

Abstract

Electrokinetics is a developping technology that is intended to separate and extract heavy metals, radionuclides, and organic contaminants from saturated or unsaturated soils, sludges and sediments, and groundwater. The goal of electrokinetic remediation is to effect the migration of subsurface contaminants in an imposed electric field. This technique is known as electrokinetic remediation, electroreclamation, electrochemical decontamination, electrorestoration, electromigration or electrochemical soil processing. Electrokinetics involves the installation of electrodes into the subsurface surrounding the contaminated region. After the electrodes are in place, a low electrical potential is applied across the anode(s) (positively charged electrode) and the cathode(s) (negatively charged electrode). As a result of the electrical gradient, different physico-chemical reactions occur and contaminant transport occurs due to various mechanisms within the soil and groundwater. Generally, for the migration to be significant, the contaminants should be in a soluble form. If they are not soluble, they need to be desorbed, dissolved, and/or solubilized into the pore solution before they can be adequately transported from the soil to an electrode wells/reservoirs. Different types of contaminants have been investigated and research has been conducted to optimize the electrokinetic variables. The present study was undertaken to systematically investigate the effect of initial sludge water content, and heating on the electrokinetic remediation of alumium-contaminated sludge. A total of four laboratory experiments were conducted using drinking water sludge. The first two tests studied the effect of variation of initial sludge water content under an ambient temperature, and the last two tests studied the effect of heating on electrokinetic remediation under conditions of both constant saturation and applied voltage © 2009 Elsevier B.V. All rights reserved

Keywords: Electrokinetic Remediation; Sludge; Aluminium; Water content; Temperature

1. Introduction

Electrokinetics is a developing technology that is intended to separate and extract contaminants from soils [1, 2, 3], in the recent decade, the electrokinetic process has been developed and applied in the field tests. Electrokinetic treatments of sludge wastes were also used for decontamination or recycling of valuable metals therein sludges [4, 5,

* Corresponding author. Tel.: 11 213 38 87 65 67; fax: 11 213 38 87 65 67. E-mail address: cherifimoun@yahoo.fr.

doi:10.1016/j.phpro.2009.11.058

6], ground water [7]. The goal of electrokinetic remediation is to effect the migration of subsurface contaminants in an imposed electric field. A typical in-situ electrokinetic remediation system is shown schematically in figure (1).

Fig. 1. In-Situ electrokinetic remediation

The major advantages of this technique are: (a) a unique applicability to low permeability soils (clays, silts, till lenses and layers). Such soils have greater ability to adsorb pollutants, but are resistant to standard in situ remedial techniques (b) the capability of removing a wide range of contaminants; and (c) a low electric power consumption

Laboratory experiments have shown that is possible to achieve high removal efficiencies by this method for various type of pollutants: heavy metals and inorganic species such as As, Cr, Cd, Fe, Hg, Ni, Zn, NO3- [1, 2], some radionuclides as uranium, thorium, strontium [8]as well as some organic compounds like phenol, acetic acid, etc [9, 10]. Essentially, electrokinetics involves the installation of electrodes into the subsurface surrounding the contaminated region. After the electrodes are in place, a low electrical potential is applied across the anode(s) (positively charged electrode) and the cathode(s) (negatively charged electrode). As a result of the electrical gradient, different physico-chemical reactions occur and contaminant transport occurs due to various mechanisms within the soil and groundwater. The pore fluid is used as the conductive mass. The successful implementation of the electrokinetic technique requires a thorough understanding of the physico-chemical reactions and the contaminant transport processes under an induced electric potential. Initially, it should be recognized that under an electric potential, the electrolysis of water occurs at the electrodes according to the following reactions:

At the anode: 2H2O ^ O 2( g) + 4H+ + 4e

At the cathode: 4H2O + 4e~ ^ 2H2 (g) + 4OH

Thus, the electrolysis reactions cause an acidic solution to be generated at the anode and an alkaline solution at the cathode. This in turn, causes contaminants to be desorbed and/or dissociated, and results in an initiation of electromigration ; i.e ; the transport of ions and polar molecules under the influence of the applied electric field. The applied electrical potential gradient also leads to the process known as electroosmosis, i.e. ; the flow of an ionic liquid under the action of an applied electric field relative to a charged surface. The three major contaminant transport mechanisms in electrokinetic remediation are electromigration, electroosmosis, and electrophoresis . Electromigration is the transport of ions and ions complexes to the electrode of opposite charge. electrophoresis is the transport of charged particles or colloids under the influence of an electric field; contaminants bound to mobile particulate matter can be transported in this manner. For non ionic species there is no electromigration transport; these species are only moved by electroosmotic advection, the direction and the velocity of their migration correspond to those of the electroosmotic flow.

The overall efficiency of the transport processes depends on many factors, including the contaminant ion concentration, the hydration of ions, the mobility of the ions, the viscosity of the groundwater, the dielectric constant of the medium, and the temperature [12, 13] investigation, the effect of increasing the voltage gradient, and/or the use of various remediation times, initial soil moisture content, etc.

2. Scope of the present study

In the previous study that employed drinking water sludge, a specific amount of water was added to the weight sludge to facilitate sludge placement into the electrokinetic cell. Electroosmotic flow is promoted at higher water contents [14]. It was hypothesized that increasing the initial moisture content may assist contaminant migration and removal. Increasing the moisture content should theoretically increase the amount of solubilized contaminant as well as the amount of particle dispersion. This is because chemical species have a distinct solubility, and the addition of more solution permits further contaminant solubilization into the liquid phase. Similarly, by diluting the soil with water, a more disperse or open soil matrix should result that is beneficial for contaminant migration and electroosmotic advection.

Temperature also is an important parameter that can influence the electrokinetic remediation process. During in situ treatment, the soil can be subject to external temperature fluctuations. Moreover, inherent to the process is the Joule effect. It corresponds to a partial conversion of the electrical energy into Joule heating, which finally results in energy lost for the ions transport and induces the increase of the soil temperature. Whose magnitude is directly proportional to the electrical current (I) and the resistance of the medium (R). Joule heat P is defined by :

P= I2 R =ie kS (1) where: k is the conductivity of the solution, ie, electrical potential gradient and S is the total cross sectional area.

The largest increases of temperature during electrokinetic remediation processing have been reported for pilot or field scale experiments. The primary effect of temperature is trough the change of viscosity. A decrease in viscosity with rising temperature will produce an increase in electroosmotic flow; relative permittivity decrease with both increasing temperature and polarity [13] (Tableau 1), which will reduce the electroosmotic flow.

Table 1. Viscosity and electrical permittivity changes with temperature

Temperature (°C) 25 50 75

Deionised water:

Viscosity (x10-3 Ns/m2 ) 0,8903 0,5467 0,3788

Relative electrical 78,303 69,909 62,425

permittivity

0,1 molar Sodium sulphate solution:

Viscosity (x10-3 Ns/m2 ) 0,929 0,585 0,391

Relative electrical 76,103 67,709 60,225

permittivity

According to some experimental observation, it appears that a temperature rise should increase the ionic electromigration velocity and the electroosmotic flow. Then, the cationic species should be greatly enhanced as the two phenomena are additive. But the delaying or enhancing influence of temperature on anions transport will depend on the ratio of the variation of each of the two terms.

3. Experimental methodology

3.1. Preparation of sludge

The sludge used in this study was collected from water treatment plant drinking right just after the stage of raw water clarification with aluminum sulphate as a coagulant. After recovering sludge studied, it is dried in an oven at 1050C for 24 hours, then, it is ready for analysis

3.2. Chemical analyses

The moisture content of sludge was determined by the lost weight fraction at a temperature of 1050C for over night. The pH value of the sludge was measured in a suspension of 1g of a dewatered sludge in 10 ml of distilled water using a pH meter GRISON 2000. The hydraulic conductivity was also measured by the same way using a conductimeter METTELER TOLEDO MC 226. The content of organic matter was determined as a percentage of volatil solids in total solids by the weight lost after ignition of approximately 1g of dried sample at 5000C for 20 mn. The heavy metal content in sludge was determined by wet extraction method, sludge (2g) was digested with 20ml of HNO3/HCl solution (1:3, v/v) in a 250ml teflon beaker at 1000C for 2H. However, to determine the aluminium content, a digestion with HNO3/HCl 6N solution (1:3, v/v) was carried out. The liquid sample obtained after digestion was filtered using paper filter and diluted to 100ml. The content of heavy metals and aluminium was determined by atomic absorption spectrometer PERKMIN ELMER 3110.

Experiments were run on drinking water sludge. The experimental apparatus is represented in figure (2). It consists of an horizontal glass cell. The saturated sample is confined_in the central part, separated from the two end chambers by filter papers. A direct power supply provides a constant voltage between the two graphite electrodes immersed in each end chamber solution, then the cell is put into a bath marie to maintain the studied temperature.

3.3. Test variables

Table (2) shows the testing program and the variables used in the four different electrokinetic tests. As seen in this table, the first two tests were performed using a different initial sludge water contents, while Tests 3, and 4 were conducted using the same initial sludge water content but at different temperatures. All four experiments were performed using drinking water sludge and tap water as the anode and cathode electrolyte solutions. The properties

Fig. 2. Schematic representation of the experimental device

of this sludge are shown in Table 3. In addition, for all the four experiments, the voltage gradient applied across the electrodes was 10 V and the test duration was 10 days.

Table 2. Electrokinetic testing programme

Test Designation Saturation report Temperature

1 EK1 75g of weight sludge/75ml of tap water 20° C

2 EK2 75g of weight sludge/100ml of tap water 20° C

3 EK3 75g of weight sludge/100ml of tap water 10° C

4 EK4 75g of weight sludge/100ml of tap water 35° C

Table 3. Properties of drinking water sludge

Property value

Moisture content 1.52

pH 6.38

Hydraulic conductivity 1711 us/cm

Organic matter 32%.

Initial contents of metals in sludge

(mg/Kg of dry weight)

Cr 0.6mg/Kg,

Pb 2.6g/Kg,

Cu 2g/Kg,

Zn 4.6g/Kg,

Fe 5.5g/Kg

Al 16.4g/Kg

3.4. Testing procedure

About 75 g of the prepared sludge was saturated with tap water, then packed into the electrokinetic cell for each experiment, the anode and cathode electrolyte solutions were placed into the electrode compartment, the studied temperature was fixed with a bath marie. The level of the processing fluid in the anode and cathode chambers was kept the same to avoid sludge dewatering and electroosmotic cessation. During experiments, the overall courant drops across EK cell was measured, pH variation of both electrode solutions and sludge bed was also determined.

After EK treatment the sludge specimen was removed from the cell and sectioned equally into five segments. The concentration of aluminium in the sludge and both electrode solutions was determined.

4. Experimental results

The results of the electrokinetic experiments were analysed to investigate the effect of variation in initial sludge water content and temperature on aluminium removal from drinking water sludge

4.1. Electrical current

Figure (3) shows that the current values for all the tests exhibited a similar trend. Generally, the current increased rapidly during the first few hours, then gradually declined and several other researchers have also observed similar behaviour [11], when water is added to the soil, the salts that are associated with the dry soil particles dissolve into the water and produce a pore solution with a high ionic strength [15]. Furthermore, the dissolved ionic metals contaminants that are present will also contribute to increasing the ionic strength. Initially, when the voltage gradient is first applied, the current is low because it takes time for the solution to migrate into the soil from the electrode reservoirs and for the salts and/or contaminants to dissolve. Within a few hours, however, the initial current reaches

a peak value due to the strong ionic concentration. Then, the current gradually decreases because the cations and anions are electromigrating towards their respective electrode. In addition, the products of the electrolysis reactions or other chemical species may reduce the current by neutralizing the migrating ions. For instance, H+ ions migrating towards the cathode could be neutralized by OH~ ions migrating towards the anode, thereby forming water and diluting the number of ions in solution. Time-dependent pH changes due to electrolysis reactions could also affect the current by causing changes such as mineral dissolution, or chemical precipitation/dissolution

Figure (3a) shows that the initial sludge water content significantly affected the current; as the initial moisture content increased, the current increased. Increasing the moisture content might have been beneficial because the resulting sludge might have been more disperse, and this should make it easier for the ionic species to migrate through the pore network. The presence of additional water molecules may also facilitate a greater amount of charged species hydration and ionic dissolution. The dilution of the ionic strength at the higher moisture content could also lead to an increased diffuse double layer thickness. In the EKj, the current decreased at a slightly more gradual rate than in the EK2 with higher initial water content, and this indicates that using a lower initial water content may be advantageous for sustaining the current. The lower water content apparently hindered the electromigration of charged species, so it took additional time for the ions to reach the electrodes. Since the ions remained in the pore solution for a longer duration, the current was sustained.

Figure (3b), shows that current profiles under temperature influence have the same behaviour observed in Figure (3a). During the experiments, the current increased rapidly in the first few hours, then it decrease during the later stages of the treatment.

The increase of current was accompanied with the rise of temperature, the highest electrical current was achieved at 35°C; this was possibly due to the increased electroosmotic flow and ionic mobility.

Fig. 3. Evolution of the electrical current with time a) EKi and EK2; b) EK2, EK3 and EK4

4.2. Sludge pH

During the experiment, variations of pH in the anode and cathode electrolyte solution were monitored. As discussed earlier, the electrolysis of water results in the formation of H+ ions (low pH solution) at the anode and OH-ions (high pH solution) at the cathode; and on completion of the experiment the pH in the sludge bed was also measured.

Figure (4) shows the normalized distance from the anode versus the sludge pH for all the tests; it illustrates that the acidic solution generated at the anode typically migrates through the sludge towards the cathode, thereby lowing the pH along most of the sludge profile. Conversely, it is also evident from this figure that an alkaline solution,

generated by the electrolysis reaction at the cathode, migrated towards the anode and increased the pH in the sludge region nearest to the cathode. Compared to H+, OH- ions have larger ionic radii and a lower mobility and hence the H+ ions usually migrate faster through the soil. Krishna. R et al [11] reported that the test with more initial water content produced the highest cumulative flow, and this test had the lowest pH in the sludge adjacent to the cathode. If there is a high electroosmotic flow towards the cathode, the pH should be lower near the cathode, because the direction of electroosmotic flow was from the anode towards the cathode, thereby opposing the electromigration of OH ions towards the anode. Moreover, when the concentration of H+ ions increases near the cathode, more OH ion neutralization may occur. The tests with the lower water contents produced a similar amount of electroosmotic flow, and they possessed similar pH profiles. The lower flow in the test with less initial water content might have allowed greater OH penetration, because the second sludge section from the cathode possessed a lightly higher pH than in the other test. The greatest variation between the three tests occurred in the soil section adjacent to the cathode, and this mainly appeared to be a consequence of the variable electroosmotic flow rates of these experiments. These results indicate that the initial sludge water content does have an effect on the pH, but the effect was minor and was mostly confined to the region nearest to the cathode.

As seen in figure (4b), the pH profiles for the three tests using different temperatures were above of the initial sludge pH especially in the three first section areas nearest anode. However, in the other two sections, the test with the highest temperature has a higher alkalinity. As discussed earlier, this was possibly due to the variation in ionic mobility coefficient of hydrogen, hydroxyl and other ions and changing H+ adsorption with temperature. Adsorption can increase with temperature and so an increase hydrogen ion adsorption would retard the hydrogen ion front.

Fig. 4. pH profile in drinking water sludge sections after electrokinetic treatment. a): EK1 and EK2,b): EK2, EK3 and EK4 4.3. Aluminium migration

Based on their solubility when they are alone in water, J. Virkutyte et al [16] reported that the metal hydroxide precipitation was at a minimum level if the pH value is below 4.5. Based on its solubility when it is alone in a simple aluminium solution containing 10-5M of aluminium, Al (III) has a low solubility and starts precipitating as Al(OH)3 at pH value of approximately 4.5. Therefore, the Al (III) ion that was electromigrating towards the higher pH region close to the cathode, might precipitate in the order of Al(OH)3.

Figure (5) shows the normalized concentration of Al that was present in the sludge for the tests conducted with different initial water contents and at different temperatures.

The water content of the sludge affects the current, increasing the water content increases the flow and may have a beneficial effect, because the sludge becomes more dispersed and it will facilitate the migration of ions through the pore water. The presence of water molecules can further improve the hydration of many species and dissolution of ions, so it seems clear that inadequate water inhibits electromigration cash charge which requires more time for the ions to join their corresponding electrodes. As shown in figure (5a), it may be noted for the two tests accumulation of aluminum in the five sections of the sludge bed, but this accumulation was higher in the two last sections and so in the cathode chamber with a more important rate aluminium reduction in the three sludge sections nearest the anode chamber compared with test with less initial water content. Therefore, aluminium migrated towards the cathode and then precipitated in the sludge regions near the cathode. So These could be explained by the duration test, that would increase to achieve more product arriving at the cathode or the precipitation phenomenon because of the sludge pH in the five sections area that are noticed high at the end of the test which favour Al3+ precipitates.

Since aluminium is in a cationic form, under the electrokinetic processing conditions, it will migrate toward the cathode. As discussed earlier, rising temperature will produce an increase in electroosmotic flow [39, 40] and the ionic electromigration velocity. Elecromigration and electroosmosis were the predominant transport process. As illustrated in figure (5b), during the test which was monitored at lowest temperature 10°C, the migration of aluminium did not appeared and a small amount of aluminium did accumulate in the cathode chamber. Compared to the two other tests (20 and 35°C), the displacement of aluminium toward cathode region was remarkable. Although, little difference in the aluminium migration was observed in these two tests. Apparently, the high temperature affects more the contaminant removal. By observing figure (5), it can be seen also that accumulation rate of aluminium increased in the two last sludge sections and in the cathode chamber at the high temperature (35°C)

Fig. 5. Concentration profile of aluminium in sludge bed and both electrolyte compartment after electrokinetic treatment: a) EK1 and EK2, b)

EK2, EK3 and EK4

But, under the developed conditions, the temperature rise enhances the cationic transport. As a consequence, the time necessary for a species to move from the point to another would be reduced, then the treatment time and associated costs especially, power consumption P, as

P = juIdt (2)

where: U, voltage, could be minimized

Alternatively, an increase of the electrical current I (when working under constant voltage) results from the transport acceleration (temp effect) as I is related to the ionic fluxes, according to equation (3)

I = J (3)

The current density J is related to the ionic fluxes according to:

J = £ zJ (4)

where: Ji is the molar fluxe and zi is the charge of the species.

5. Conclusion

This study, performed to investigate the effects of initial sludge water content and temperature on aluminium migration in a drinking water sludge. With the experimental results, the important conclusions have been summarized as follows:

As the acid front generated by the electrolysis of water at the anode compartment migrated towards cathode, the complexed and/or adsorbed aluminium was dissolved and desorbed along with the migration of the acid front.

Application of 10V for 10 days with the same initial water content (75g of weight sludge/100mll of tap water) at temperature 20°or at more high temperature on drinking water sludge induces mobility of aluminium species. The removal of Al(III) is due to the flushing of the acid generated at the anode across the specimen resulting in desorption of Al(III) together with its migration and advection by electroosmotic flow.

Initial sludge water content influence on the electrokinetic sludge processing revealed that the electrical current increased considerably when the sludge contained high water content, but initial sludge water content affect on aluminium migration and removal appeared to be minimal. Furthermore, these experiments indicated that the moisture content remains nearly the same during the electrokinetic process although slight changes were evident. These slight changes were attributed to minor variations in the electroosmotic flow, which were, in turn, the result of the physico-chemical reactions and electrokinetic transport mechanisms that were occurring.

To study the temperature influence on an electrokinetic sludge processing, experiments are carry out at three temperatures with the same applied voltage gradient and initial water content. Under the developed conditions, no significant temperature influence is observed for pH. The temperature rise accelerates the aluminium movement. The electrokinetic velocities increase with temperature, this could then constitute an advantage for the polluant removal, which could reduce the necessary treatment time and corresponding costs. But a higher temperature also results in a higher current density.

Overall, it was concluded that the initial sludge water content affects the electrokinetic process but it does not significantly influence the migration and removal of total aluminium from the sludge, and the results indicate that the temperature rise could be beneficial for electrokinetic sludge processing if the processing time reduction at least compensated the energy lost generated.

RERERENCES

[1] S. O. Kim, K.W. Kim and al, Journal of environmental engineering. 8 (2002) 705-715.

[2] R. K. Srivastava, R.P. Tiwari, P. Bala Ramudu, J. Environ. Health, Sci. Eng. 4 (2007) 207-214.

[3] Y.B. Acar, N. Akram Alshawabkeh, J. Environment science health 27 (1992) 1835-186.

[4] J.-Y. Wang, D.-S. Zhang, O. Stabnikova, J.-H. Tay, J. Hazard. Mater. 124 (2005)139-146.

[5] C. Yuan, C. H. Weng, Chemosphere 65 (2006) 88-96.

[6] S. Glendinning, J. Lamont-Black, Colin J.F.P. Jones, J. Hazard. Mater. 139 (2007) 491-499.

[7] C .N. Mulligan; R. N. Yong and al, Engineering geology 60 (2001) 193-72.

[8] S. Pamukcu, J. K. Wittle. in 14 Annuel US Department of Energy Low-level radioactive Waste Management Conference Proceedings (1993)

256-278.

[9] Y. Acar, L. Heyi, and al, Journal of geotechnical engineering 118 (1992) 1837-1851

[10] J.-Y. Wang, X.-J. Huang, J. C.M. Kao, O. Stabnikova, Journal of Hazardous Materials 144 (2007) 292-299

[11] R. Krishna, R. E. Saichek and al, Indian geotechnical journal 32 (2002) 258-288.

[12] F. Baraud, S. Tellier, M. Astruc, J. Hazard. Mater. 64 (1999) 263-281.

[13] M.Penn and C.Savvidou,. Engineering geology and environment 2 (1997) 2081-2086

[14] D. H. Gray, Géotechnique 20 (1970) 81-93

[15] E. R. Saichek, R, Crishna, Chemosphere 51 (2003) 273-287.

[16] J. Virkutyte, M. Sillanpää, P. Latostenmaa, Sci. Total Environ. 289 (2002) 97-121