Electrokinetic extraction of surfactants and heavy metals from sewage sludge

Electrokinetic extraction of surfactants and heavy metals from sewage sludge

Electrochimica Acta 54 (2009) 2108–2118 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 54 (2009) 2108–2118

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Electrokinetic extraction of surfactants and heavy metals from sewage sludge Violetta Ferri, Sergio Ferro, Carlos A. Martínez-Huitle ∗,1 , Achille De Battisti 1 University of Ferrara, Department of Biology & Evolution, Pharmaceutical and Agrotechnological Resources Section, Via L. Borsari 46, 44100 Ferrara, Italy

a r t i c l e

i n f o

Article history: Received 30 April 2008 Received in revised form 28 July 2008 Accepted 20 August 2008 Available online 3 September 2008 Keywords: Electrokinetic treatment Sludge reclamation Surfactants Heavy metals Recycling

a b s t r a c t Waste management represents a quite serious problem involving aspects of remediation technologies and potential re-utilization in different fields of human activities. Of course, wastes generated in industrial activities deserve more attention because of the nature and amount of xenobiotic components, often difficult to be eliminated. However, also ordinary wastes of urban origin are drawing more and more attention, depending on the concentration of noxious substances like surfactants and some heavy metal, which may eventually require expensive disposal. In the present paper, a research has been carried out on the application of electrokinetic treatments for the abatement of the above xenobiotic components from sewage sludge generated in urban wastewater treatment plants. Experiments were carried out on a laboratory scale, in a 250 mm × 50 mm × 100 mm cell, using 250–300 g of sludge for each test and current densities between 2.4 and 5.7 mA cm−2 . As a general result, quite significant abatements of heavy-metal ions and surfactants were achieved, with relatively low energy consumption. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction The disposal of sewage sludge from urban wastewater treatment plants is a growing problem worldwide. The European Community [1] has developed the draft of “Working document on sludge” with the aim of updating the regulatory system for the (re)use of sewage sludge. In Italy, sewage sludge could be disposed on agricultural lands but, in recent years, this use has been limited by legislation in order to avoid contaminations and the accumulation of both heavy metals and undesired organic compounds in the soils. Analogous regional regulations have decreased the maximum attainable limit for these pollutants. As a consequence, recycling of sludge in agricultural lands requires specific treatments and electroremediation can supply interesting solutions [2–9]. The electrokinetic remediation technique, also referred as “electroremediation”, “electro-reclamation” or “electrochemical soil remediation”, is one of the most promising remediation processes, and offers high efficiency and time-effectiveness in decontaminating low-permeability soils and sludge contaminated with heavy metals, radio nuclides, or organic compounds [7,8]. In addition, the low operational costs and potential applicability to a wide range of contaminants [9] have suggested a number of case studies of waste remediation [10–18]. The electrokinetic approach generally requires low-level DC current densities in the order of a few

∗ Corresponding author. Tel.: +39 0532291124; fax: +39 0532240709. E-mail address: [email protected] (C.A. Martínez-Huitle). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.048

mA/cm2 between suitably located electrodes, which anyway induce physicochemical changes in the applied media, leading to species transport by coupled mechanisms, such as electromigration, electroosmosis and electrophoresis, which take place together with the electrolysis of water at electrodes [7–9]. These phenomena can be briefly described as follows: (i) electromigration is the transport of ions and ion complexes toward the electrode of opposite charge; (ii) electroosmosis is the movement of soil moisture or groundwater, which generally takes place from the anode to the cathode and is due to the existence of a space-charge on the solution side of the particle/solution interface. In consideration of the fact that ionogenic groups, like silanols, are ionized above pH 3 affording a negative charge to the solid surface, the spacecharge on the solution side is positive. Its migration toward the cathode, causes solution displacement too, thus generating a cathode-directed electroosmotic flow; (iii) 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 as well; (iv) the electrolysis of water produces oxygen gas and H+ in the anode compartment, while hydrogen gas and hydroxyl anions are formed at the cathode; then, both H+ and OH− are able to move across the soil, causing an acidic and an alkaline front to migrate through the porous media. The former, in turn, may cause contaminant desorption and/or dissociation, and results in an initiation of electromigration, i.e., the transport of ions

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under the influence of the applied electric field. On the other hand, the alkaline front tends to precipitate the heavy metals, an effect that should be minimized by suitably adjusting the pH in the cathode compartment. Transport of heavy metals can be caused by the above phenomena, the most important being the electrically induced ion migration [10]; this is especially true when the initial ionic conductivity of the pore fluid is high, or when the initial soil pH is low [7]. When an electric field is applied across a soil, heavy metal cations dissolved in the pore solution drift toward the cathode; the force applied to the ions is F = zi e × ∇ V

(1)

where F = force (N); zi = charge of ionic species; e = is the elementary charge (1.6 × 10−19 C); V = voltage gradient (V/m). Concurrent electroosmotic flow can accelerate the transport of inorganic cations toward the cathode. The system is also affected by electrolysis of water and reduction reactions at the cathode, as electrolysis reactions dominate the chemistry at the electrode surfaces [7,8]. In un-enhanced electrokinetic remediation process, the application of a direct electric current via electrodes immersed in a moisture-saturated soil results in water oxidation at the anode, which generates an acid front that causes desorption of metals [7]: 2H2 O → O2 (g) + 4H+ + 4e−

(2)

while water reduction at the cathode produces an alkaline front, which will move toward the anode: 2H2 O + 2e− → 2OH− + H2 (g)

(3)

This alkaline front abate the rate of migration of metal cations by precipitation. Secondary reactions may exist at the cathode, depending upon the concentrations of available species, for example: Men+ + n e− → Me (Me = Metal)

(4)

Upon their migration to the electrodes, extraction and removal of the organic and inorganic pollutants can be accomplished by electrodeposition, precipitation, or ion exchange, either at the electrodes or in an external extraction system placed in a unit cycling the processing fluid [7]. The present work concerns a laboratory investigation on the use of an electrokinetic (EK) method for the removal of heavy metals and anionic surfactants, like linear alkylbenzenesulphonates (LAS [19]), from urban wastewater sludge. 2. Experimental 2.1. Reagents Chemicals were of the highest quality commercially available, and were used without further purification: NaNO3 , KNO3 and Na2 SO4 were purchased from Fluka. Aqueous solutions were prepared using double-distilled deionized water. Ether, chloroform, ethanol, n-butanol and deuterated solvents were purchased by Fluka at high purity degree. 2.2. Sludge sampling Sludge samples were supplied by Gruppo HERA (“Holding Energia Risorse Ambiente” group) and withdrawn from urban wastewater treatment plants located in Emilia Romagna, Italy. Specimens were stored for a few days in 3 l plastic barrels and kept at 4 ◦ C before utilization and analysis. Two EK remediation studies

Fig. 1. Diagram of the cell for electrokinetic sludge remediation: (A) general dimensions, (B) vertical photo and (C) lateral photo. Parts of the electrokinetic cell: (1) anode electrode, (2) separation membranes, (3) cathode electrode, (4) anode compartment, (5) sludge sample compartment and (6) cathode compartment.

were performed, i.e., the removal of heavy metals and anionic surfactants (LAS), on three different sludge samples, supplied by HERA from plants in Ferrara (sludge 1 and 2) and Forlì-Cesena (sludge 3); the latter was considered in particular for its larger LAS content (due to important touristic activities). 2.3. Physicochemical characteristics All sludge samples were analyzed for particle-size distribution, sludge conductivity and pH (CRISON micropH 2001), basing on Italian standard method IRSA CNR (No. 64); dry weight and humidity were determined according to the UNI EN 12880:2002 method. Concerning the heavy metals content, the sludge was digested in HNO3 , following the UNI-EN 13346:2002 and EPA-SW 846-3051 official European methods. Heavy-metal concentrations (Cd, Co, Cr, Cu, Ni, Pb, V, Zn, Fe and Mn) of the digested sludge samples were determined by atomic absorption spectroscopy (AAS, PerkinElmer AAnalyst 800) in accordance with the IRSA CNR-No. 64 method. 2.4. Description of electrokinetic cell The cell used for the electrokinetic sludge remediation tests is presented in Fig. 1. The experimental apparatus consisted of a closed system, comprising the sludge bed, two electrode compartments and electrical contacts (see Fig. 1C); the cell dimensions were 250 mm × 50 mm × 100 mm. Either a Ti/Pt or a Ti/IrO2 anode, and a Ti cathode were used, with working areas of 45 cm2 ; filter-paper separations were placed between the sludge sample and electrode reservoirs. Gases produced by electrode reactions were extracted

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from the cell by appositely arranged needles, and vented from electrode reservoirs using a fuming hood. Supporting electrolyte was manually added in the electrodes compartments, when necessary, while stirring was assured by small magnetic bars. 2.5. Electrokinetic experiment conditions Firstly, optimal operating conditions were determined, and then different current intensities were investigated, for different times. To gain information about the process, pH, redox potential (vs. SCE, Pt wire), cell potential, variation in electrolyte volume (electroosmotic flow), conductivity and temperature were measured before and after each treatment, in both electrolyte compartments (anode and cathode). The EK cell was filled with homogenized sludge and all experiments were conducted using a sludge compartment having a thickness of 50 mm, equivalent to about 300 g of sludge. During EK remediation, both cell voltage and pH in the two reservoirs were monitored. A DC power source (AMEL model 553 potentiostat) was used to provide a constant potential values. After treatment, the sludge was extracted and properly analyzed. In the case of the reservoir solutions, total carbon (TC) and total organic carbon (TOC) measurements were made using a Dohrmann DC80 Organic Carbon Analyzer. Also, some samples of electrolyte were analyzed by ion chromatography for determining anions content (Dionex DX-120 Ion Chromatograph). In a first series of tests, the EK cell was used in horizontal position; subsequently, a few tests were performed investigating the effects of a vertical arrangement. In the case of experiments carried out in order to eliminate the LAS from the sludge, the organic removal was determined through extraction and analytical investigations (by nuclear magnetic resonance (Varian 400 MHz), high-performance liquid chromatography coupled with mass spectrometry (Agilent Technology, HP1200; mass spectrometer HP6410) or spectrophotometrically). 2.6. Organic extraction procedures for organic species identification and quantification

by method B described above, was dissolved in 0.6 ml of deuterated solvent and NMR spectra were obtained. After that, known amounts of two internal standards were added to the sample and the NMR analyses were repeated. 1 H NMR spectra provide specific information about the protons in the molecule(s), which is proportional to the concentration of the organic compound(s): using the area of the proton signals, obtained by 1 H NMR, for both the LAS aromatic ring and the added standards, the unknown concentration of LAS in the sample could be thus estimated. 2.7. Energy consumption and costs Key points for the development of clean methodologies are the energetic and economic aspects: to make predictions, even if only indicative, about these aspects, allows indications about the feasibility of the process. The energy consumption for the removal of one m3 of sludge was calculated by Eq. (5); subsequently, energy consumption used for the treatment of one ton was estimated (kWh/ton), considering that the average sludge density is about 1250 Kg/m3 (Eq. (6)). Finally, taking into consideration an electrical energy cost of about 0.08D per kWh (Italy), the process expenditure was estimated by Eq. (7). Energy consumption (kWh/m3 ) =

Test energy (Wh)/1000] Volume(m3 )

(5)

Energy consumption(kWh/ton) =

Energy (kWh/m3 ) 1.25 (ton/m3 )

(6)

Cost (D /ton) = Energy consumption (kWh/ton) ×0.08 (D /kWh)

(7)

3. Results and discussion 3.1. Physicochemical characterization of the sludge

Two extraction methods were employed, in order to identify and quantify the organic species contained in the untreated and EK-treated sludge samples: (A) a weighted sludge sample (electrokinetically treated or not), previously dried for 4 days, was subjected to an organic extraction in soxhlet (for 6 h), using 180 ml of organic solvent (99.8% of purity). Once cooled and made anhydrous with Na2 SO4 , the solution was filtered and concentrated under vacuum. Since under the above extraction conditions, both polar and nonpolar organic species were extracted, successive extractions were used in order to obtain a higher separation between the organic species; three different solvents were used: ether, chloroform and methanol, in this order, and the above described procedure was thus repeated three times. (B) a weighted sludge sample (electrokinetically treated or untreated) was placed in a vial and mixed with distilled water and n-butanol; the vial was sonicated for 2 h and subsequently centrifuged for 30 min at 9000 rpm. The obtained liquid extract was filtered and concentrated under vacuum conditions.

Physical and chemical characteristics (dry weight, humidity and pH) of the sludge samples were obtained. Sludge samples were dried at two temperatures, 60 and 105 ◦ C, for 12 h or more (until a constant weight was obtained: UNI EN 12880:2002 method). Two specimens (from sludge-samples 1 and 2) were initially considered: the corresponding dry weight values were 21.3 and 21.0% at 60 ◦ C, whereas 20.5 and 20.1% at 105 ◦ C. Accordingly, the sludge humidity was relatively constant, for the two sludge samples and at both temperatures, ranging from 79 to 80%. In the case of the sludge sample 3, dry weight and humidity values obtained at 105 ◦ C were about 22.7 and 77.3%, respectively. For pH measurement, sludge specimens (50 g) were centrifuged at 20 ◦ C and 9000 rpm for 30 min; by using the small amount of extracted liquid, a pH evaluation was performed: sludge samples 1 and 2 had an analogous pH, equal to 8.12, while a value of 7.90 was measured for sample 3. Other data for the used sludge samples were supplied by HERA; they have been collected in Table 1. 3.2. Metal concentrations in sludge samples

On all final extracts the following determinations were performed: IR spectra on the product as such extracted (data not showed) and 1 H NMR and 13 C NMR analyses (in deuterated chloroform or methanol). In addition, quantification of the LAS content in the EK treated or untreated sludge samples was estimated using the following procedure: a carefully weighted amount of extract (2–3 mg), obtained

As-received sludge samples were dried at 105 ◦ C and subsequently ground in an Agate mortar; a weighted amount of powder (1–2 g) was then mineralized according to UNI-EN 13346:2002 and EPA-SW 846-3051 methods. Residual matter was dissolved in 1 ml of HNO3 and 20 ml of distilled water and boiled. Finally, the solution was filtered, diluted to a known volume with water, and analyzed by

V. Ferri et al. / Electrochimica Acta 54 (2009) 2108–2118 Table 1 Characteristics of the different sludge samples



Dry weight (105 C) Organic carbon Total N (Kjeldhal) Total P (as P) Total K (as K) Salinity (mequiv./100 g) Fluorides (mg/kg DM) Chlorides Sulfates

Sample 1

Sample 2

Sample 3

20.5% 32.1% 4.72% 1.38% 0.11% 24.75 n.d. n.d. n.d.

20.1% 31.0% 4.22% 1.45% 0.21% 29.96 2 18.9 212

22.7% 28.8% 4.18% 1.97% 0.26% 42.91 n.d. n.d. n.d.

n.d.: not determined.

AAS for determining the heavy metals content (Cd, Co, Cr, Cu, Ni, Pb, V, Zn, Fe, Mn and Hg). Metal concentrations (mg per kg of dry matter) are reported in Table 2. High concentrations of Zn and Cr were found, which can be due to the use of anticorrosion agents containing those metals in the cooling systems of the wastewater plants. Also the iron content of the sludge was quite important, reaching 14 g/kg; concerning manganese, its content was nearly 400 mg/kg: in addition to its potential toxicity, Mn is important because it can affect the mobility of other metals such as Zn, Cu, and Ni [18]. 3.3. Preliminary electrokinetic experiments: current–potential behavior 300 g of sludge were homogeneously mixed with 10 ml of water and used for preliminary tests (PT1 and PT2, see Table 3). Characterization of the flow cell and sludge resistance were determined progressively increasing the cell potential and recording the respective current values, using NaNO3 as electrolyte at both electrode compartments. As expected, a linear dependence was obtained, which allowed to estimate a sludge resistance of about 65  (since the current was varied, no values are reported in Table 3 for that parameter). A similar behavior was found for sludge samples 1 and 2. Moreover, important variations in pH (acidic and alkaline values in the anode and cathode reservoirs, respectively) and conductivity values were observed, as a result of water electrolysis (see Table 3). 3.4. Supporting electrolyte experiments In order to enhance the effectiveness of the process, in terms of removal of heavy metals from sludge samples 1 and 2, initial experiments were carried out in order to optimize the composition of the anode and cathode solutions. In fact, the supporting electrolyte has

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a significant influence on the metals removal [20]. Different solutions (NaNO3 , Na2 SO4 and KNO3 ) were tested; 0.1 M NaNO3 was chosen at the beginning (tests PT1, PT2 and 1), but it was subsequently replaced (test 2) by Na2 SO4 at the anode and KNO3 at the cathode, with the purpose of supplying nutrients to the bacteria present in the sludge (through the electromigration of sodium and nitrates, respectively). A third test (no. 3) was carried out inverting the latter electrolytes (in order to supply the bacteria with sulfates and potassium): with this last configuration, the sludge assumed a characteristic smell, after the EK treatment, typical of bacterial activity. Accordingly, Na2 SO4 and KNO3 were chosen as supporting electrolyte for the cathode and anode reservoirs, respectively, for all successive tests (Tables 3 and 7). 3.5. Electrokinetic experiments for removing heavy metals: horizontal cell After the preliminary experiments, different current intensities were investigated, for different times (t) in order to ascertain optimal conditions for heavy metals removal, as reported in Table 3. Fig. 2A and B show the variations in heavy metals concentration in the sludge samples, after the electrokinetic treatment. Desorption of metal species from sludge particles occurred along with the migration of the acid front, during the treatment, and metal contaminants were gradually transported towards the cathode by electromigration and electroosmotic purging. The migration of different metal species showed similar behaviors and their overall concentration decreased with time, in agreement with result reported by other authors [9–18]. The percent removals of metal concentrations in the sludge for each experiment (tests 1–5) are collected in Table 4. In general, good efficiencies were achieved with increasing the current density and the treatment time. In the first case (test 1), the lowest efficiencies were obtained, which ranged from 1 to 13%; when the supporting electrolyte was changed, replacing the NaNO3 with Na2 SO4 and KNO3 , better results were obtained (tests 2 and 3). Under otherwise similar experimental (J, t) conditions, results showed some differences between tests 2 and 3 (inversion of electrolyte solutions): modest decreases in the removal efficiencies of some heavy metals (Al, Cd, Co, Cu, Ni, Pb and Zn) were observed, while the removal efficiency for Cr and V was increased. Test 4 showed that, increasing the treatment time, a more homogeneous metals removal could be achieved. However, discrepancies in the obtained results are possible, due to problems of homogeneity (sludge samples were withdrawn to measure the concentration of heavy metals). Noteworthy, two differently colored areas were created in the sludge during the electroki-

Table 2 Concentration of heavy metals (mg/kg dry weight) for sludge samples 1 and 2 Heavy metal

As Cd Cr Cu Ni Pb Zn Co V Fe Mn Hg a b

Sludge sample ground. Sludge sample crushed.

Sludge 1

Sludge 2

Value

Error

%

Valuea

Error (%)

Valueb

Error (%)

19 4.03 88 563 61 139 1147 12 70 14,000 387 –

±2 ±0.03 ±12 ±36 ±11 ±16 ±71 ±1 ±9 ±1347 ±52 –

10.5 1.2 13.6 6.4 18.0 11.5 6.2 8.3 12.9 9.6 13.4 –

24.2 22.3 67.4 431.1 46.2 1110.5 1042.2 7.1 46.3 14230.8 396.9 3.2

±2.0 ±1.3 ±3.2 ±2.6 ±8.5 ±2.0 ±1.2 ±2.0 ±2.1 ±3.8 ±2.3 ±5.0

26.4 25.5 59.4 333.8 32.9 857.7 846.2 4.0 26.9 13380.7 324.5 2.5

±2.0 ±1.3 ±3.2 ±2.6 ±8.5 ±2.0 ±1.2 ±2.0 ±2.1 ±3.8 ±2.3 ±5.0

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Table 3 Electrokinetic experimental conditions for the removal of heavy metals from sludge samples Test

PT1 PT2 1 2 3 4 5 6h 7h a b c d e f g h

Sludge

1 1 1 1 1 1 2 2 2

ta

Jb

– – 3 8 8 16 48 8 8

– – 4 2.4 2.4 2.4 5.7 5 6

Rc

65 65 70 86 85 70 75 100 50

Anode reservoir

EFg

Cathode reservoir

Anode

Electrolyte

pH

Ti/Pt Ti/Pt Ti/Pt Ti/Pt Ti/Pt Ti/Pt Ti/Pt Ti/Pt Ti/Pt

NaNO3 NaNO3 NaNO3 Na2 SO4 KNO3 KNO3 KNO3 KNO3 KNO3

1.89 1.69 1.62 1.59 1.38 1.71 1.14 2.04 1.21

d

e

E

507 430 572 1116 1119 1163 1104 564 821

f

C

T

Cathode

Electrolyte

pH

12.75 13.57 14.21 20.80 22.10 28.00 62.90 11.04 21.40

23.2 23.0 23.1 24.7 21.6 18.2 20.0 27.2 31.9

Ti Ti Ti Ti Ti Ti Ti Ti Ti

NaNO3 NaNO3 NaNO3 KNO3 Na2 SO4 Na2 SO4 Na2 SO4 Na2 SO4 Na2 SO4

11.97 12.13 12.54 12.70 12.15 12.04 11.54 10.44 12.19

E −100 −190 −220 −818 −980 −984 −1008 −303 −925

C

T

10.21 11.67 12.03 17.59 18.15 20.70 30.60 9.28 25.00

22.9 22.6 23.0 23.4 22.0 18.0 18.5 24.0 31.4

– – 7.5 3.3 3.0 2.2 4.2 n.a. n.a.

Treatment time (h). Current density (mA/cm2 ). Average cell resistance (). Potential values vs. SCE (mV). Conductivity (mS/cm). Final temperature (◦ C). Electro-osmotic flow (ml/h). Experiments with the cell in vertical position. Electrolyte concentration: 0.1 M.

netic treatment, due to the change in pH at both sludge-bed sides. Therefore, in the successive test, anode and cathode sludge samples were analyzed separately, after the electrochemical treatment, with the aim of determining the influence of this aspect in metals migration. Test 5 was carried out under extreme conditions, in order to maximize the role of the different variables studied in the preceding experiments. According to results reported in Table 4, an important variation of removal efficiencies was observed with respect to previous EK experiments. At the cathode side, Cd concentration decreased from 22.29 to 20.01 mg/kg (about 10%); V, Zn and Hg concentrations reduced from 46.34 to 34.56 mg/kg (−25.4%), from 1042.2 to 753.3 mg/kg (−27.7%) and from 3.24 to 2.32 mg/kg (−28.4%), respectively. In the case of Pb, Cu and Cr, removal efficiencies from 36 to 45% were achieved, and even more important reductions were obtained for As, Co and Ni: their concentrations decreased by 65% (from 24.19 to 8.5 mg/kg), 71.9% (from 7.09 to 1.99 mg/kg) and 56.7% (from 46.16 to 20.01 mg/kg), respectively; in contrast, a low concentration decrease was achieved in the case of Fe (from 14230 to 13508 mg/kg) at the cathode side, while a −44% was found at the anodic one. Co, Cr, Zn and Ni presented an over-

all similar removal distribution through the sludge (i.e., analogous removals were found for the anodic and cathodic sides of the sludge bed), which indicates that these metals were able to migrate in the sludge sample but not completely transported toward the cathode or anode reservoirs. Interestingly, Mn average concentration in the cathode side of the sludge bed increased during the treatment (by 15%), while a practically complete elimination was achieved from the anodic side (−99.4%). In synthesis, important metal removals from the cathode side were achieved for As and Pb, while Co, Fe and Mn were removed principally from the anodic region (see Table 4); as a matter of fact, the electrochemically induced accumulation or electroconcentration of metals in a specific electrode area may anyway represent a useful treatment approach. For the other metals, the differentiation between the two sides of the sludge bed was less significant. Basing on obtained results, the electrokinetic process can be successfully suggested for the removal of heavy metals from a sludge, especially in the case of specific metal contaminations (more difficult is the situation when treating a mixture of species); it is important to remark that the above experimental tests were car-

Table 4 Concentration of heavy metals (mg/kg dry weight) in the sludge, after the electrokinetic treatment, under the different conditions reported in Table 3 Heavy metal

Al As Cd Co Cr Cu Ni Pb V Zn Fe Mn Hg a b c d e f

Sludge 1, content a

31,467 19 4.03 12 88 563 61 139 70 1147 14,000 387 3.24

Sludge 2, content a

Test 1b (%)

2c (%)

3d (%)

4d (%)

−10 −5 – −8 −5 – −13 12 −6 −1 −5 −8 –

−25 −21 −8 −23 −20 −10 −25 −12 −17 −3 −14 −16 –

−19 −5 −6 −20 −30 −7 −13 −5 −20 −2 −15 −18 –

−9 −21 −3 −8 −19 −18 −25 −16 −13 −14 −12 −14 –

mg/kg dry weight. 0.1 M NaNO3 as supporting electrolyte at both reservoirs. 0.1 M Na2 SO4 and 0.1 M KNO3 as supporting electrolyte in anode and cathode reservoir, respectively. 0.1 M KNO3 and 0.1 M Na2 SO4 as supporting electrolyte in anode and cathode reservoir, respectively. Sludge cathode side. Sludge anodic side.

– 24.19 22.29 7.09 67.42 431.07 46.16 1110.49 46.34 1042.21 14230.8 396.93 3.24

Test 5d , e (%)

5d , f (%)

7 (%)

– −64.9 −10.2 −71.9 −36.3 −40.4 −56.7 −44.5 −25.4 −27.7 −5.1 15.3 −28.4

– −47.6 16.7 −78.4 −30.2 −21.1 −54.1 −13.0 −34.9 −21.0 −44.1 −99.4 −15.1

– −93.8 6.5 −33.1 12.2 −2.6 −8.8 12.6 36.6 4.5 4.1 −2.7 6.3

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the conductivity at both electrode compartments, at the end of the experiment, increased with respect to initial values, reaching 21.40 mS/cm (from 9.95) and 25.0 (from 13.59), in the anode and cathode sides, respectively. Also the temperature and the pH showed important variations with respect to values obtained for tests with the horizontal configuration. Removal efficiencies (Table 4) were somewhat inexplicable, being very high for As and sufficiently good for Co; for the other metals, the final concentrations were higher than the initial one, indicating that metals were able to migrate within the sludge sample, but could not be extracted and eliminated. As a matter of fact, problems in the withdrawal procedure (homogeneity of the sample for AAS analysis) have to be considered in order to justify the above data. However, in consideration of the inconveniences connected with the vertical configuration, further experiments were carried out maintaining the cell horizontal (tests 8–23). 3.7. General characteristics and analysis of the reservoir solutions after EK treatments

Fig. 2. Concentration variations of the heavy metals in the sludge samples after electrokinetic treatment, according to experimental conditions collected in Table 3. (A) Electrokinetic experiments performed using sludge sample 1 and (B) using sludge sample 2.

ried out using the electrokinetic cell in the horizontal position, as indicated in Fig. 1. 3.6. Electrokinetic experiments for removing heavy metals: vertical cell Some electrokinetic experiments were carried out setting the cell in a vertical position, with the anode on top (tests 6 and 7, reported in Table 3); these experiments were performed taking into consideration that the gravity effect could give an additional help in facilitating the mobilization (by electromigration and electroosmosis) of heavy metals in the sludge. However, different problems were found during the first experiment (test 6); in particular, the evolution of gases from electrodes caused the interruption of the electrical contact between the sludge bed and the supporting electrolyte lying below. As a result, the cell resistance increased, as can be seen for test 6 in Table 3. At the end of the treatment, the conductivity of electrolyte solutions resulted to be practically unchanged with respect to the initial value, while changes in pH and redox potential were lower than those observed in previous tests (see Table 3). In addition, no changes in electrolyte volume could be observed, probably due to the gas accumulation, which led to a pressure in opposition to the electroosmotic flow. Therefore, in the successive test (test 7), the gas accumulating in the electrode reservoirs was removed using particular outlets. The problems due to the anomalous resistance were solved and

During some experiments, a considerable amount of foam formed at both electrode compartments. This foam was caused by gas evolution at the electrodes (oxygen and chlorine at the anode, hydrogen and probably also ammonia at the cathode), concomitant with the EK extraction of the surfactants present in the sludge. Both chlorine and ammonia were identified, basing on their characteristic smell. Additionally, a change in color was also observed in the case of the cathode solution (which became yellowish), together with the formation of a white precipitate that was qualitative and quantitative analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). SEM and EDX investigations determined that the precipitate was CaCO3 , and contained Mn, Al, K, Cu and Pb, as shown in Fig. 3 (the percentage contents of elements present in the precipitate are collected in Table 5). Concerning the pH, acidic (∼1.5) and alkaline (∼12) values were always found, at the anode and the cathode respectively, as expected on the basis of electrode reactions (Table 3). On the other hand, no significant temperature increase due to Joule effect was observed during all tests. This outcome is in agreement with the data reported by Reed et al. [21], who observed a Joule effect only for current densities higher than 5.5 mA/cm2 . With reference to the redox potential of solutions, a positive value (∼1100 mV vs. SCE) was measured at the anode compartment, plausibly due to chlorine evolution in an acidic medium: 2Cl− → Cl2 + 2e (chloride ions are extracted from the sludge), while a negative value (−900 mV vs. SCE) was found for the cathodic solu-

Table 5 Percentage content of elements in the white precipitate found in the cathode compartment reservoir (EDX analysis) Element

C O Ca K Mg Na Cu Pb Al Cl S Si

Spectrum 1

Spectrum 2

Weight (%)

Atomic (%)

Weight (%)

Atomic (%)

15.37 59.80 23.06 1.10 0.19 0.19 0.11 0.12 0.05 – – –

22.68 66.26 10.20 0.50 0.14 0.15 0.03 0.01 0.03 – – –

10.88 48.87 37.96 1.49 0.50 0.06 – – – 0.15 0.04 0.05

18.20 61.39 19.03 0.77 0.42 0.05 – – – 0.08 0.03 0.04

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Fig. 3. Scanning electron microscopy (SEM) image and energy dispersive X-ray (EDX) spectra for the white-colored precipitate found in the cathode compartment.

tion. Conductivity values increased, depending on time treatment and applied current density, as described before (see Table 3). In order to verify the movement of charged species from sludge to reservoir solutions, some analysis were carried out by ion chromatography; species detected in the electrode compartments (Cl− , NO2 − , F− , PO4 3− and SO4 2− ) are listed in Table 6. The data show an increase in phosphate and fluoride concentrations at the anode (tests 3, 4 and 5), whose accumulation can be related to the processing time, which increased from 8 h of treatment per test 3 (J = 2.4 mA/cm2 ), to 16 h per test 4 (J = 2.4 mA/cm2 ), and finally to 48 h per test 5 (J = 5.7 mA/cm2 ). In addition, a decrease in chloride concentration was observed, as a possible consequence of the development of gaseous chlorine at the anode surface. pH and ion chromatography data, on electrolyte solutions, show an increase of the final concentration of ionic species such as H+ , OH− , NO3 − , SO4 2− , Na+ and K+ , extracted from the sludge compartment. Total carbon (TC) and total organic carbon (TOC) were also measured, and results confirmed the extraction of both organic and inorganic substances (CO3 2− ) from the sludge, during the electrokinetic process. In all cases, TC and TOC values of the reservoir solutions increased after the EK process, depending on time treatment and applied current density (Fig. 4). To have indications about the nature of organic substances that were extracted, specific analyses were taken into consideration.

(b) Fatty acids: NMR spectra of fatty acids with unsaturated chain present peaks for vinyl groups between 5 and 6 ppm (multiplet), as well as peaks in the regions around 2.3 and 4 ppm due to protons close to the steric group. These signals are present in all extracts (ether, chloroform and methanol). (c) Oils: NMR spectra of oils show saturated aliphatic peaks in the area around 0.5 and 1.5 ppm. The spectra obtained from the various extracts present high integrations in aliphatic part, indicating the presence of many protons (different to the number of protons present in a surfactant and fatty acid). Analogous results were obtained for the sludge samples after the electrokinetic treatment, but higher quantities of organic compounds were detected with respect to the untreated ones. To quantify the organic content of the sludge, samples were dried and then extracted with n-butanol upon sonication (2 h), according to the protocol described in the experimental section (method B); then, NMR was employed to quantify the organic species. NMR analysis evidenced once again the presence of surfactants, oils and fatty acids.

3.8. Organic species identification and quantification (preliminary tests) Firstly, 10 g of sludge (electrokinetically untreated) were dried and treated according to procedure A described in Section 2. After that, the obtained extract was cooled, filtered and concentrated. Subsequently, NMR analyses were achieved for identifying the organic species contained in the sludge. According to the 1 H NMR and 13 C NMR spectra, considerable quantities of different classes of compounds were observed: (a) LAS: as sodium salt in MeOD, the spectrum presents peaks at 7.4 and 7.8 ppm (symmetrical doublet) due to protons of the aromatic ring (1 and 4 substituted) and peaks at 0.9, 1.4 and 1.6 ppm (multiples) characteristic of protons of an aliphatic chain. Similar peaks were found in the spectra of all obtained extracts.

Fig. 4. Increase of the total carbon (TC) and total organic carbon (TOC) in the reservoir solutions, as a result of the extraction of organic and inorganic substances from the sludge by electrokinetic process, according to experimental conditions collected in Table 3.

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Table 6 Anionic species detected by ion chromatography in the reservoir solutions Test

F− (ppm)

J (mA/cm2 )

t (h)

Cl− (ppm)

NO2 − (ppm)

NO3 − (ppm)

215 23

– 167

34 5,108

– –

18,347 –

– –

9,583 –

80 –

1,291 11,456

PO4 2− (ppm)

SO4 2− (ppm)

2

8

2.4

Anode Cathode

9 –

3

8

2.4

Anode Cathode

13 5

4

16

2.4

Anode Cathode

– –

160 –

– –

9,302 –

133 –

1,455 15,179

5

48

5.7

Anode Cathode

30 –

38 –

– –

15,112 29

408 –

6,383 5,599

19.1 –

Comparing the NMR spectra obtained from untreated and treated sludge extracts, a percentage of surfactants removal could be estimated. The presence of surfactants in the electrolyte solution (already highlighted by TOC analyses) was confirmed by extraction with n-butanol and NMR analysis. The cathode extract provided a defined NMR spectrum of the LAS, while the NMR spectrum of the anode extract showed the presence of both surfactants and other organic compounds (minor quantities); Fig. 5 shows a typical NMR spectrum, obtained after n-butanol extraction. 3.9. Electrokinetic remediation of LAS in the sludge Once defined the method for organics characterization, in a second series of tests (8–23), several EK experiments under dif-

Fig. 5.

1

ferent conditions were carried out in order to eliminate the LAS from the sludge (sample 3). As reported in Table 7, the tests were performed at different treatment time and current density (3, 4, 5 and 6 mA/cm2 , for periods of 8, 16, 24 and 48 h), using two EK cells working independently. As a second anode, a Ti/IrO2 net was used (its stability and electrical characteristics are not dissimilar to Ti/Pt); moreover, the distance between the electrodes and the sludge sample was minimized, thus allowing a reduction for the cell potential (which, in turn, allows a decrease in energy consumption and costs). At the end of each test, several parameters were analyzed, for both electrode reservoirs, and sludge samples were processed for the determination of their residual surfactant contents (by extraction, method B, and NMR analysis). Fig. 6 shows the LAS removal efficiencies (%), calculated as an extract to stan-

H NMR spectrum obtained after n-butanol extraction procedure for sludge sample and addition of standards for quantification of LAS content.

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Table 7 Experimental conditions of the EK experiments for removing LAS from sludge samples and results obtained after the EK process (LAS removal, energy consumption, cost and hydrogen production) Test

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

ta

24 24 24 24 16 16 16 16 8 8 8 8 48 48 48 48

Jb

3 4 5 6 3 4 5 6 3 4 5 6 3 4 5 6

Rc

52 40 39 43 51 58 42 44 58 66 39 37 77 60 33 47

Anode reservoir Anode

pH

Ti/Pt Ti/IrO2 Ti/IrO2 Ti/Pt Ti/IrO2 Ti/Pt Ti/IrO2 Ti/Pt Ti/IrO2 Ti/Pt Ti/IrO2 Ti/Pt Ti/Pt Ti/IrO2 Ti/IrO2 Ti/Pt

1.37 1.31 1.07 1.17 1.32 1.28 1.19 1.16 1.59 1.44 1.60 1.27 1.03 0.94 1.11 1.13

Cathode reservoir Ed 780 1095 1063 1098 1030 938 1042 998 820 803 997 1020 880 877 822 850

Ce

Tf

Cathode

pH

Ed

Ce

Tf

28.6 37.9 40.0 39.8 30.7 36.9 37.5 36.9 19.7 20.3 24.6 32.3 59.1 58.2 40.5 41.4

24.3 24.8 27.1 27.4 23.9 26.0 29.8 31.1 23.2 23.5 24.3 28.2 27.1 26.0 23.9 25.0

Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti Ti

12.72 12.80 12.92 12.86 12.84 12.84 12.83 12.84 12.59 12.74 12.37 12.41 12.82 12.81 12.81 12.90

−983 −1024 −1006 −1020 −1011 −1020 −1001 −1013 −981 −971 −994 −1000 −1009 −1004 −1001 −1002

39.3 41.3 47.2 47.2 39.7 45.8 40.7 43.7 24.9 34.7 30.7 39.5 54.8 38.5 41.0 45.2

24.7 25.3 27.4 28.8 24.5 26.7 30.3 34.3 27.1 25.5 25.9 28.2 42.2 35.0 29.0 34.3

In all experiments, the anode and cathode supporting electrolytes were 0.1 M KNO3 and 0.1 M Na2 SO4 , respectively. a Treatment time (h). b Current density (mA/cm2 ). c Average cell resistance (). d Redox-potential values, mV vs. SCE. e Conductivity (mS/cm). f Final temperature (◦ C). g Electroosmotic flow (ml/h). h EC = energy consumption (kWh/ton). i Hydrogen production during the treatment (dm3 ). j Estimation of annual hydrogen production (dm3 ).

EFg

LAS removal (%)

ECh

Cost (D /ton)

Vi

Vj

5.6 6.4 8.1 9.0 5.4 6.7 7.9 8.4 5.7 6.5 7.9 8.7 5.5 6.8 8.0 8.9

−17.3 −18.0 −19.7 −43.2 −27.3 −28.8 −28.3 −22.2 −22.4 −41.7 −31.9 −24.7 −29.1 −22.6 −18.7 −31.2

143 190 238 285 95 127 158 190 48 63 79 95 285 380 475 570

9.1 12.2 15.2 18.3 6.1 8.1 10.1 12.2 3.0 4.1 5.1 6.1 18.3 24.3 30.4 36.5

1.35 1.81 2.26 2.71 0.90 1.20 1.50 1.81 0.45 0.60 0.75 0.90 2.71 3.61 4.51 5.42

173 231 288 346 173 231 288 346 173 231 288 346 173 231 288 346

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8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Sludge

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under galvanostatic mode, only slight potential increases being observed. The measured resistance values were lower (49 ), if compared to values obtained for the previous EK experiments (∼74 ), but this behavior can be better justified by the decreased distance between electrodes (and thus to the minimization of ohmic drop in the electrolytes), rather than due to specific characteristics of the tested sludge sample. Again, and during all experiments, the production of a large amount of foam was observed. 3.10. Energy consumption and costs

Fig. 6. LAS removal efficiencies (%), calculated as an extract-to-standard signal ratio from NMR spectra. Test conditions are reported in Table 7.

dard (STD) signal ratio from NMR spectra, as discussed in Section 2. Also, in order to verify the LAS concentration estimated by NMR, data values were compared with those determined by HPLC–MS and methyl blue active substance (MBAS) methods. In this frame, NMR and HPLC–MS data exhibited similar results (e.g. NMR and HPLC–MS results for an untreated sample: 2131 and 2468 mg of LAS/kg, respectively), showing that the evaluation of LAS concentration estimated by NRM analysis is quite reliable. On the other hand, LAS removal efficiencies, obtained by NMR-based calculation and MBAS method, were consistent too. MBAS official method (CNR IRSA 5150 Q 100 1994) consists in the spectrophotometric determination of the surfactant concentration through the formation of blue salts, which form when methyl-blue reacts with the surfactants present in the water. After a chloroform extraction at pH 10, the absorbance of the extract is measured at 650 nm, and the concentration of the surfactant in the solution is thus deduced. The method is a very cheap but non-specific, and suffers from a number of interferences, which led to a systematic underestimation of LAS concentration; on these basis, the relative removal % was the only important parameter in our case. From the obtained results, LAS removals are shown to depend on experimental conditions but, in general, good efficiencies could be achieved for every experiment. On the other hand, the increase in LAS concentration in electrode reservoirs indicated that these organic species were able to migrate in, but not completely eliminated from, the sludge bed. As a final point, gravimetric analysis on the electrokinetic treated sludge revealed a reduction of LAS ranging from 17 to 43% (w/w), with respect to the untreated specimens. After the EK treatment, all electrolyte solutions had variations in pH, conductivity and potential, related with electrolysis of water (reactions (2) and (3)) (see Table 7). In fact, low (acidic) values of pH were achieved in the anode compartment, while high (alkaline) values were measured in the cathode reservoir. The conductivity of solutions increased as a result of the augmented ionic species concentration in solution (Table 7). All tests were carried out under open-reactor-conditions, allowing the escape of gases produced at anode (oxygen and chlorine) and cathode (hydrogen and ammonia). Redox potential values of 850 ÷ 1000 mV and −1000 mV vs. SCE were found for the anode and cathode solutions, respectively, as previously commented. No significant differences were observed in the initial and final temperatures, testifying that, under the chosen operating conditions, the amount of heat evolved for Joule effect was quite small. The cell potential remained rather constant around 12.5 V, operating

The energy consumption for the removal of LAS from one ton of sludge was estimated, according to Eqs. (5)–(7). Table 7 reports the cost pertaining to each test, estimated taking into account the specific values of operating parameters (time and current) and considering an average cell potential value of 11 V; as expected, direct costs were found to be dependent on specific operating conditions, and estimated between 3 and 37 D /ton. While cost figures are not directly related with removal efficiencies, the obtained results suggest that the electrokinetic remediation method can be a feasible alternative for the treatment of sludge containing hazardous materials, such as surfactants. It is important to remark that, during the EK process, hydrogen gas is produced at the cathode and, if appropriately collected and stored, it could be used as an energy resource. In consideration of the cell dimensions, the amount of produced hydrogen was rather modest; however, considering a plant operating for 1 year, a significant volume would be produced. Both the hydrogen production during treatment and volume estimation for a year, assuming a semi-continue process (70% of the available time), are presented in Table 7, for a sludge volume of 250 g. 4. Conclusions From the results of the laboratory-scale study on the electrokinetic remediation of municipal sludge, the following conclusions can be drawn: (i) The concentrations of heavy metals and surfactants in the sludge provided by HERA were below the limits set by the Italian legislation. (ii) Values collected in Table 4 show that the removal efficiency increases both by prolonging the treatment time and raising the current density; further investigation is needed to understand the dependence of removal efficiencies for heavy metals on their speciation in the sludge matrices. (iii) The analytical method developed in this study (consisting of nbutanol/ultrasounds extraction) has been successfully applied to the determination of LAS by NMR. (iv) LAS removal efficiencies, ranging between 17 and 43%, were obtained. No substantial gain in LAS removal efficiency could be obtained by prolonging the application time to more than 16 h. The lack of a clear trend for data in Fig. 6 could be due to problems in sample homogeneity and analytical determinations. (v) Cost analysis was also attempted for the investigated electrokinetic systems for LAS removal: direct (energetic) costs were estimated between 3 and 37 D /ton (allowing removals between 17 and 43%), basing on a suggested expenditure of 0.08 D /kWh. The obtained results suggest the possibility to apply an electrokinetic method for the elimination of heavy metals and surfactants from sludge, obtaining significant removal levels and

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efficiency percentages. Finally, future work will be focused on a scale-up of the electrokinetic process, aiming to the optimization of parameters for increasing the LAS removal efficiency, as well as to the optimization of the LAS extraction procedure and NMR quantification. Acknowledgement The authors are grateful to HERA S.p.A.-R&D network division (Forlì, Italy) for the financial support to this work. References [1] Working document on sludge, 3rd draft (April 2000), http://ec.europa.eu/ environment/waste/sludge/pdf/sludge en.pdf. [2] M.T. Ricart, H.K. Hansen, C. Cameselle, J.M. Lema, Sep. Sci. Technol. 39 (2005) 3679. [3] M. Elektorowicz, S. Habibi, Can. J. Civil Eng. 32 (2005) 164. [4] S.O. Kim, S.H. Moon, K.W. Kim, S.T. Yun, Water Res. 36 (2002) 4765. [5] M.R. Jakobsen, J. Fritt-Rasmussen, S. Nielsen, L.M. Ottosen, J. Hazard. Mater. 106 (2004) 127. [6] J. Virkutyte, E. van Hullebusch, M. Sillanpa, P. Lens, Environ. Pollut. 138 (2005) 517.

[7] Y.B. Acar, A.N. Alshawabkeh, Environ. Sci. Technol. 27 (1993) 2638. [8] Y.B. Acar, R.J. Gale, A.N. Alshawabkeh, J. Hazard. Mater. 40 (1995)117. [9] S. Pamukcu, J.K. Wittle, in: Trantolo Wise (Ed.), Remediation of Hazardous Waste Contaminated Soils, Marcel Dekker, Inc., New York, 1994, p. 245 (Chapter 13). [10] R.L. Clarke, S. Kimmel, R. Lageman, S. Smedley, Proceedings of the 58th Annual Meeting American Power Conference, Illinois Institute of Technology, Chicago, 1996, p. 347. [11] C.N. Hsu, PhD dissertation, Texas A&M University, College Station, TX, 1997. [12] S.O. Kim, S.H. Moon, K.W. Kim, Environ. Technol. 21 (2000) 417. [13] S.O. Kim, K.W. Kim, J. Hazard. Mater. 85 (2001) 195. [14] S.O. Kim, S.H. Moon, K.W. Kim, Water Air Soil Pollut. 125 (2001) 259. [15] R. Lageman, Environ. Sci. Technol. 27 (1993) 2648. [16] J.K. Mitchell, T.C. Yeung, Electrokinetic Flow Barriers in Compacted Clay. Transportation Research Records No. 1289, National Research Council, Washington, DC, 1991. [17] A.P. Shapiro, PhD dissertation, Massachusetts Institute of Technology, Cambridge, 1990. [18] G.J. Zagury, Y. Dartiguenave, J.C. Setier, J. Environ. Eng. 125 (1999) 972. [19] Human and Environmental Risk Assessment on ingredients of European household cleaning products: LAS (linear alkylbenzene sulphonate)-Version 3.0, October 2007, http://www.heraproject.com/files/4F-HERA LASFinalReport2007revision10 07.pdf. [20] M.R. Jakobsen, J. Fritt-Rasmussen, S. Nielsen, L.M. Ottosen, J. Hazard. Mater. 106B (2004) 127. [21] B.E. Reed, M.T. Berg, J.C. Thompson, J.H. Hatfield, J. Environ. Eng. ASCE 121 (1995) 805.