Electrokinetic Remediation Of Bottom Ash From Municipal Solid Waste Incinerator

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Electrokinetic remediation of bottom ash from municipal solid waste incinerator Giombattista Traina a,∗ , Luciano Morselli b , Giuseppe Persano Adorno a a

b

Istituto Giordano S.p.A., Rimini, Italy Department of Industrial Chemistry and Materials, University of Bologna, Italy

Received 27 September 2005; received in revised form 3 April 2006; accepted 23 May 2006

Abstract The electrokinetic remediation was studied to verify the possibility to reclaim the bottom ash from municipal solid waste incineration (MSWI). In Italy, a production of 1 million tons per year of this kind of residue has been estimated, 90% of which is still landfilled. This work shows the results of four electrokinetic remediation tests for the removal of Pb, Cu, Zn, Cd and chlorides, using an open cell with graphite electrodes and without enhancing agents. The four tests have, respectively, been performed at a constant current density of 0.89, 1.67, 2.04 and 2.48 mA cm−2 , with duration of 42, 68, 47 and 40 h. Heavy metals occur in ashes in various forms, such as exchangeable, adsorbed, precipitated, organically complexed and residual phases. In order to determine the nature of any given system, in terms of specific chemical species and pertaining mobilities, sequential extraction analyses have been performed. The release of pollutants was investigated for treated and untreated ash. After treatment, the concentration of pollutants in the leachate was reduced by 31–83%, better results being obtained for chlorides. Both the low amount of heavy metal extracted and the increase of ash pH during the electrokinetic tests, suggest to use enhancing agents or a cation exchange membrane at the cathode, to prevent the precipitation of metals as hydroxides. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electrokinetic remediation; Bottom ash; Electroreclamation; Municipal solid waste incineration; Heavy metals

1. Introduction Municipal solid waste incineration is one of the most popular means of dealing with non-recyclable solid waste. Although the volume of waste to be disposed in landfills could be reduced to 90% by incineration, considerable amounts of ash are discharged through this process. The solid phase generated from the MSWI can be divided into two distinct parts: the bottom ash, that is the inert and incombustible residue from the combustion process, and the fly ash, which derives from the cleaning process of the flue gas. Bottom ash is the most important by-product of the incineration process; therefore, the improvement of ash quality seems to be a very important R&D task for sustainable waste management. The use of incinerator bottom ash (IBA) for road construction is widespread in a large number of European countries with consequent environmental benefits in reduced

∗

Corresponding author. Tel.: +39 338 8006175; fax: +39 541 345540. E-mail address: ing [email protected] (G. Traina).

primary aggregate consumption [1]. About 1 million tons per year of IBA are currently produced in Italy, 90% of which is still landfilled. The obstacles of recycling IBA are due to the leaching of some pollutants, such as Cu, Cr, Pb, chlorides and sulphates, that often exceeds the limits established by Italian laws (according to the D.M. 5/2/98 and EN 12457-4), and therefore expensive treatments of the ash are required before bottom ash re-use. The most common treatment, used to improve the environmental characteristics of this non-hazardous waste in view of recycling, is encapsulation (stabilization/solidification). The encapsulation of ash from incineration is typically carried out by mixing the ash with cement or other pozzolanic materials to form a monolithic material that effectively excludes moisture (physical encapsulation). Sometimes, this process needs various additives, which can increase the cost of the treatment. Moreover, the matrix may not be effective at binding some cations and the leaching of chloride salts can lead to loss of physical strength and durability. In addition, the final product can fail the leaching test. This work is aimed at the evaluation of electrokinetic remediation (EKR) of the bottom ash from

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MSWI, for the removal of both heavy metals and salts (chlorides). The paper presents four lab tests carried out with a basic system, using two graphite electrodes in direct contact with the ash and without using enhancing agents or ion exchange membranes.

Since the technique has not been considered for the removal of heavy metals from MSWI bottom ash, yet, the present paper aims to report preliminary results on this possible application.

1.1. History and principles of electrokinetic remediation

2.1. Bottom ash: characteristics and sample preparation

Electrokinetic soil processing is also referred to as electrokinetic remediation, electroreclamation and electrochemical decontamination. The process is substantially based on a lowlevel direct current of the order of some mA cm−2 , which crosses the area comprised between electrodes to remove contaminants. The low-level direct current results in physicochemical and hydrological changes in the soil mass, and leads to species transport by coupled mechanisms. Electrolysis of water produces hydrogen ions in the anodic compartment, which causes an acidic front to migrate through the soil/ash cell. This, in turn, causes desorption of contaminants from the surface of soil/ash particles, and results in their electromigration (the transport of ions under the influence of the electric field). If the soil pore is idealized as a capillary, the mobile cations form a concentric shell within the capillary. Under an applied electrical potential, this space charge, generally of cationic nature, moves toward the cathode, dragging the pore fluid and resulting in electro-osmosis (the hydraulic flow induced by an electric field) [2]. Electromigration and electro-osmosis are the two principal mechanisms involved in the electrokinetic remediation technique. This technology has recently made significant advances and has been tested for commercial application in the United States and The Netherlands [3]. Geokinetics (The Netherlands) and Electrokinetics Inc. (Baton Rouge, LA) have completed several large-scale pilot studies for the removal of heavy metals from clay soils. In addition, new applications have been investigated in the last few years, and the electrodialytic remediation (EDR) method, developed at the Technical University of Denmark [4], surely deserves to be mentioned. This approach has already been applied with success to the treatment of wood waste, biomass ash, contaminated harbour sediments and fly ash from MSWI [5–9]. However, the applied technique is quite different from the typical electrokinetic remediation: in the EDR, electrodes are placed in separate compartments where electrolyte solutions are circulated and where the heavy metals are concentrated at the end of the remediation. The electrolytes solutions also ensure that a good contact between the electrode surface and the surroundings is maintained, and that gases, due to electrode reactions, are transported away from the electrodes. Ion exchange membranes separate the soil/ash from the electrolyte solutions. An anion exchange membrane (AEM) that allows only the passage of anions is placed between the soil/ash and the electrolyte solution at the anode compartment, while a cation exchange membrane (CEM) is placed between the soil/ash and the electrolyte solution at the cathode side. The CEM is used, specially in soil remediation, to prevent OH− ions, produced by the electrolysis of water at the cathode, enrich the soil.

The IBA studied in this work derives from a grate furnace incinerator in the north of Italy. This incinerator of municipal solid waste has a nominal capacity of 18 tons/h and includes a heat recovery system to produce electricity. For each ton of incinerated waste, 300 kg of bottom ash and 30 kg of fly ash are produced. According to the Italian regulations, the former is a non-hazardous waste whereas the latter is hazardous, and it is usually landfilled with or without inertization. After the combustion, the bottom ash is usually quenched in water and then conveyed to the recycling process. However, not many plants have a complete recycling treatment and only the ferrous and coarse fraction separation is generally carried out. The coarse fraction, specially the incomplete incineration materials, return back to the grate furnace, whereas the residual 80% of bottom ash, with a typical particle size of less than 35–45 mm, is still landfilled, because of its high content of heavy metals and chlorides. This ash fraction is suitable of reuse as a lightweight aggregate, particularly in road base construction, but improvements in both environmental and mechanical characteristics are required. For carrying out this research, samples of ash were collected weekly during October and November 2004, from a single incinerator plant. Only the raw material was treated, to avoid the aging that usually causes changes in pH and other physicochemical characteristics. Hence, the ash was sieved at 20 mm, and the residual ferrous fraction was extracted by a hand magnet. After that the ash was inserted into the cell and compacted to obtain the desired density. No water was added before or during the process, the water content of the ash depending thus only on the quenching process. The ash was analysed using methods established by Italian standards and regulations in force. The average physicochemical properties of considered samples of ash are collected in Table 1. The electrokinetic remediation technique is not able to remove the heavy metals firmly bound to the soil/ash matrix. Heavy metals occur in ashes in various forms, such as exchangeable, adsorbed, precipitated, organically complexed and residual phases. These determine their environmental mobility and bioavailability, and finally their potentiality to contaminate the environment [10]. Heavy metals existing as loosely bound fractions, such as exchangeable or adsorbed forms on the clay/sand surface, or associated with organic matter and oxides with weak bonding strength, tend to be easily moved and dispersed. In order to determine the nature of any given system, in terms of the chemical species present and their relative mobilities, sequential extraction analyses have been performed [11]. This approach consists of several steps, which allow the determination of the form in which the polluting metals are present, and results obtained are helpful for the assessment of the risk of long-term contamination.

2. Materials and methods

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Table 1 Physicochemical characteristics of bottom ash from MSWI Value SiO2 (%) CaO (%) Fe2 O3 (%) Al2 O3 (%) Zn (mg kg−1 ) Pb (mg kg−1 ) Cu (mg kg−1 ) Cr (mg kg−1 ) Cd (mg kg−1 ) Chlorides (mg kg−1 ) Sulfates (mg kg−1 )

36.81 34.96 5.39 4.51 2790 2820 2810 350 38 13811 9887

Value ± ± ± ± ± ± ± ± ± ± ±

0.7 0.71 0.34 0.28 63 95 12 51 5 92 21

Table 2 Chemicals used in the sequential extraction Exchangeable Weak acid soluble fraction Reducible fraction Oxidizable fraction Residual fraction

1 M MgCl2 (pH 7) 1 M CH3 COONa + CH3 COOH (pH 5) 0.04 M NH2 OH·HCl in 25% (v/v) CH3 COOH at 100 ◦ C 0.02 M HNO3 + H2 O2 (30%) + 3.2 M NH2 OAc in 20% HNO3 (pH 2) HF–HClO4

The five extraction steps are described in Table 2. Leaching of metals from the ash was determined using the EN 12457-4 test, which provides information on leaching of granular waste under fixed conditions (deionized water as leachant, 10 l/kg as liquid to solid ratio and agitating for 24 h). The resulting eluate, after filtration, was analysed for metal and chloride content, by atomic absorption spectrophotometry (AAS) and ion chromatography, respectively. 2.2. Electrokinetic remediation experiments The experimental rectangular cells (6 cm × 14 cm × 16 cm) used in this study were made of glass; graphite electrodes were used with dimensions of 5.8 cm × 9 cm × 0.5 cm. Two sheets of filter paper were placed at the interfaces between the ash bed and the electrodes (Fig. 1); no enhancing agents were used during the experiments, and the electrolyte was constituted by only the ash water, arising from the quenching process. A dc

100 g−1 )

CEC (meq Electrical conductivity (mS cm−1 ) pH Hydraulic permeability (cm s−1 ) LOI 550◦ (%) LOI 970◦ (%) Gravel (%) Sand (%) Silt/clay (%) Density (g cm−3 ) Water content (%)

6.7 8.29 12.78 2.5 × 10−5 2.40 5.01 54.41 42.49 3 1.88 25

power supply (60 V and 3 A) and a PC, to control the current density and to measure the energy consumption, completed the electrokinetic apparatus (Fig. 1). Both anode and cathode compartments were open at the top to vent the electrolysis gases. Four experiments were carried out, using four different current densities but maintaining the same distance between electrodes; experimental details are collected in Table 3. After each remediation test, the ash bed was divided into five slices, which have been oven-dried at 105 ◦ C for 24 h to evaluate the moisture content. 2.3. Analytical method Heavy metal concentrations were measured by carefully mixing 50 mg of ash with 300 mg of anhydrous lithium metaborate into a clean dry platinum crucible, fusing the mixture for 30 min at 800–900 ◦ C, hence suddenly cooling the mixture and then dissolving it in a beaker with 25 ml of hydrochloric acid (25%, w/w). The obtained solution was analysed by AAS. Ash pH was measured by mixing 10 g of dry ash and 25 ml of distilled water. After 1 h of contact time, the solution pH was measured using a radiometer pH electrode. Chloride and sulfate concentrations were measured by mixing 10 g of dry ash and 250 ml of distilled water, and heating at 120 ◦ C for 2 h; the liquid phase was vacuum-filtered with a 0.45 ␮m filter, to separate the solid particles, diluted to 250 ml and analysed by ion chromatography.

Table 3 Experimental details for the four remediation tests Test

Duration (h) Current density (mA cm−2 ) Initial voltage (V cm−1 ) Final voltage (V cm−1 ) Area of ash bed (cm2 ) Volume of ash bed (cm3 ) Length of ash bed (cm) Density (g cm−1 ) Initial water content (%)

I

II

III

IV

42 0.90 0.27 0.43 44 464 10.5 1.88 23

68 1.67 0.46 1.06 30 306 10.2 1.82 26

47 2.04 0.56 1.25 29 299 10.2 1.76 27.5

40 2.48 0.63 2.12 28 282 10 1.7 28

Fig. 1. Schematic diagram of experimental apparatus.

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Fig. 2. Heavy metal concentration in the five fractions investigated by the sequential extraction procedure.

3. Results and discussion 3.1. Heavy metals speciation The composition of used IBA, as obtained by particle size analysis, was as follows: 54.41% gravel, 42.49% sand and 3% silt/clay. The initial concentrations of metal contaminants in bottom ash are also collected in Table 1, whereas their chemical forms, determined by the sequential extraction method, are shown in Fig. 2. The sequential extraction procedure gave low concentrations of exchangeable metals (or carbonate-bound metals), Cd and Pb being the only exceptions. Cu is predominantly bound to organic matter, while Zn is adsorbed onto the Mn and Fe oxides; Cu leaching is predominantly due to the formation of highly soluble organo-copper complexes. Typically, bottom ash has a TOC of 1–4%. Although the exact speciation of organic material and complexes has not been identified in bottom ash from MSWI, still the high organic carbon content is correlated with high Cu leaching [12]. 3.2. Electrokinetic remediation results The mass balance for heavy metals, performed after each experiment and based on initial and final amounts, gave no realistic removal values (i.e., −2 to 30% for Zn, −50 to 15% for Cr, 13 to 49% for Cu), since it was influenced by the high nonhomogeneity of the ash; in contrast, the removal efficiency for chlorides (60%) seems to be a correct estimation (Fig. 5a). As a matter of fact, the behavior of pollutant concentrations (Fig. 3) in the ash bed after remediation could be a more representative result, as well as the comparison of pollutants release before and after the remediation treatment. In order to evaluate how the electroreclamation influences the leaching of pollutants, the release of contaminants from the ash treated in the second test was compared to the release of an untreated specimen, according to the EN 12457-4 test method. Results, collected in Table 4, show significant improvements for the environmental characteristics of the ash, as due to the electroreclamation process, specially for copper and chlorides, for which the leached concentrations were found to be approximately four (Cu) and six times lower

Fig. 3. Heavy metal concentration in the five slices, after each remediation test (1–5, from anode to cathode). The increase in Pb, Zn and Cu concentrations may be related to the presence of a non-ferrous screw, hidden in the ash (in slice 3), during test II.

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Table 4 Leaching results of pollutants for an untreated sample and for a sample treated by electroreclamation (according to EN 12457-4) Pollutants

UBAa

TBAb

% Reduction

Pb (ppm) Zn (ppm) Cu (ppm) Cr (ppm) Cd (ppm) Cl− (ppm)

0.16 0.689 1.204 0.19 0.181 348

0.11 0.324 0.277 0.095 0.101 60

31.25 52.98 76.99 50.00 44.20 82.76

a b

Untreated bottom ash. Treated bottom ash.

(Cl− ), from the treated specimen. A low Pb-extraction was recognized in all four experiments, probably due to its precipitation as hydroxide. In the second experiment, which was carried out using a current density of 1.667 mA cm−2 , a coarse non-ferrous screw, not extracted by the hand magnet, was found in the middle of the ash cell, after the electrokinetic treatment. In this part of the ash (slice III), a strange intensification of the Pb, Cu and Zn concentrations was found, as shown in Fig. 3, which might be due to the presence of the non-ferrous object. Screws or other extraneous metal components were found only once, during the whole experimental investigation, so no conclusions can be drawn about the role of the screw (brass) or how cation migration was influenced by it in run II. However, metal components might act like a false earth, and extra precipitation of heavy metals around the screw could have occurred in test II. When an electric field passes through a conductor, electrode reactions can take place: the object can be reduced or oxidised [9,13], and the heavy metal ions can be electrodeposited on it. In Fig. 4, electric field lines for situations

Fig. 4. Electric field lines in the case of: (a) a soil without coarse objects or conductors and (b) with a conductor (e.g., a screw) [13].

Fig. 5. (a) Chloride concentration in the five slices (1–5, from anode to cathode) after each remediation test and (b) moisture content in the ash after remediation.

without and with coarse objects or conductors buried inside the ash bed are represented [9]. At the end of each test, and particularly at the end of the second one, blue salts (probably copper hydroxyl carbonate or other copper minerals) were found on both electrode surfaces. For that reason, electrodes were scratched and analysed to evaluate which metal deposited on anode and cathode, respectively. This analysis showed Cu-migration toward both electrodes, but predominantly toward the anode; the same pattern was verified for Zn, while Cd and Pb moved toward the cathode. However, during all experiments, only small amounts of Pb were deposited on the electrodes. Conversely, high chloride mobility was found, even if chlorides, moving toward the anode (and thus against the electro-osmotic flow), need at least 60 h to get out of all ash bed. In the third experiment, carried out for 68 h, 60% of chlorides were removed, and increasing the current density, chlorides migration has further been enhanced (Fig. 5). As above anticipated, the electro-osmotic flow moved toward the cathode and its rate increased by increasing the current density. The pattern of moisture content in the ash (Fig. 5), after tests I and IV, clearly shows this experimental evidence. 3.2.1. Variation of pH and ohmic drop in the ash bed during the electrokinetic removal experiments After the electrokinetic removal experiments, the pH of the ash was measured for all slices (Fig. 6). The electrolysis of water in the anode and cathode compartments generated H+ and OH− ions, which moved toward the electrode of opposite charge. While, in soil remediation, the hydrogen action usually leads to a decrease of soil pH, in these experiments the overall pH of the ash increased. An explanation can be found in the high pH

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metallic object. Experiments showed an increase of ash pH, also nearby the anode, which depended on applied current density. At 2.5–3 mA cm−2 , the precipitation of heavy metal as hydroxides, especially nearby the cathode, rapidly occurred, leading to an increase of bed resistivity and thus in a cell voltage. Precipitation of metals nearby the cathode seems to be the most significant limit of the electrokinetic remediation of bottom ash, which suffers from the high pH and buffer capacity of that material. Further investigations are planned to verify the efficiency of electroreclamation of bottom ash from MSWI, acidifying the ash or using enhancing agents and a cation exchange membrane to prevent OH− to enrich the ash. Acknowledgement This research was jointly supported by Consortium Spinner, Emilia Romagna Region, Ministry of Labour and Social Policies and European Social Fund. References

Fig. 6. (a) pH variation of the ash, after each experiment and (b) cell voltage during the ash electroreclamation tests.

and high buffering capability of the ash, which decreases the dissolution and desorption rates of adsorbed and/or complexed species [14]. Starting from an initial value of 12.78, an increase of pH was found also in the first slice, where the role of protons should have been more evident. Fig. 6 also shows the overall cell voltage trends; the initially linear pattern changed to an exponential development, possibly related to cation precipitation as hydroxides. This in fact resulted in an increasing resistance and voltage drop within the ash bed. The change in pattern happens sooner by increasing the current density. This detail reveals that if no enhancing agents are used, the process is rapidly arrested by heavy metal precipitation (in form of hydroxides, when cations meet the OH− front that moves from the cathode), a phenomenon that takes place very quickly while using current densities as high as 2.5–3 mA cm−2 . 4. Conclusions In this study, four electrokinetic remediation tests were carried out, without using any enhancing agent, on bottom ash from MSWI. After test II, a metallic, non-ferrous screw was found in the middle of the ash bed, and an abrupt increase of Pb, Cu and Zn concentrations was found in the ash surrounding the

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