Journal of Environmental Management 137 (2014) 157e162
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The application of electrocoagulation for the conversion of MSWI fly ash into nonhazardous materials Wing-Ping Liao a, Renbo Yang a, b, *, Wei-Ting Kuo a, Jui-Yuan Huang a a b
Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan Bio-way Environmental Science and Technology (BEST) Corp., Ltd., Taichung 404, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2012 Received in revised form 12 October 2013 Accepted 12 February 2014 Available online 13 March 2014
This research investigated the electrocoagulation of municipal solid waste incineration (MSWI) fly ash at a liquid-to-solid ratio (L/S) of 20:1. The leachate that was obtained from this treatment was recovered for reutilization. Two different anodic electrodes were investigated, and two unit runs were conducted. In Unit I, the optimum anode was chosen, and in Unit II, the optimum anode and the recovered leachate were used to replace deionized water for repeating the same electrocoagulation experiments. The results indicate that the aluminum (Al) anode performed better than the iridium oxide (IrO2) anode. The electrocoagulation technique includes washing with water, changing the composition of the fly ash, and stabilizing the heavy metals in the ash. Washing with water can remove the soluble salts from fly ash, and the fly ash can be converted into Friedel’s salt (3CaO$Al2O3$CaCl2$10H2O) under an uniform electric field and the sacrificial release of Alþ3 ions, which stabilizes the toxic heavy metals and brings the composition of the fly ash to within the regulatory limits of the toxicity characteristic leaching procedure (TCLP). Use of the Al anode to manage the MSWI fly ash and the leachate obtained from the electrocoagulation treatment is therefore feasible. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: MSWI fly ash Electrocoagulation Heavy metals Friedel’s salt Toxicity characteristic leaching procedure
1. Introduction Incineration has become the preferred option for the disposal of municipal solid waste (MSW) in Taiwan. The fly ash generated by MSW incineration (MSWI) is trapped in the air pollution control devices (APCDs). MSWI fly ash is classified as a hazardous waste and must be disposed of in landfills after the stabilization and solidification (S/S) of the heavy metals and dioxins. However, the increasingly limited landfill capacity and the restrictions of the new landfill policy developed by the Taiwanese Environmental Protection Administration (TEPA) in 2007 have caused the reuse of MSWI fly ash to become an essential issue in Taiwan. The four methods for processing MSWI fly ash that have been described in research papers include physical, biological, thermal and chemical treatment techniques. Physical treatment techniques often use washing with water as a pretreatment to promote the formation of hydrate phases that convert heavy metals to lessreactive forms and remove significant amounts of soluble salts
* Corresponding author. 5F, 66, Sec. 2, Taiyuan Rd., North Dist., Taichung 404, Taiwan. Tel.: þ886 4 22062926; fax: þ886 4 22062927. E-mail addresses: [email protected], [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jenvman.2014.02.012 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
from the fly ash (Abbas et al., 2003; Arvelakis and Frandsen, 2005; Colangelo et al., 2012; Kirby and Rimstidt, 1994; Mangialardi, 2003). Wu and Ting (2006) used a biological technique to treat fly ash and found that fungal bioleaching was more beneficial than chemical leaching. However, bioleaching requires more time for processing than does chemical leaching. Thermal treatment techniques that are used to handle fly ash such as sintering, melting, fusion, and vitrification are usually operated at high temperatures. The products of these thermally treated fly ashes are highly crystalline on the surface and are highly durable chemically, which causes them to be considered suitable for reuse (De Casa et al., 2007; Károly et al., 2007; Karamanov et al., 2003; Lam et al., 2011). Nevertheless, these thermal treatment processes are economically undesirable on the commercial scale because of their high energy consumption and the high costs of building and maintaining the facilities used to operate the thermal treatment processes (Sakai and Hiraoka, 2000). Chemical extraction or fixation techniques have been studied much more often than the other three techniques. The chemical agents that are typically used include HCl, HNO3, H3PO4, Ca(OH)2, NaOH, ethylenediaminetetraacetic acid (EDTA) disodium salt, Na2S, and thiourea (Astrup et al., 2006; Eighmy et al., 1997; Kida et al., 1996; Mizutani et al., 1996; Okada et al., 2007; Youcai et al., 2002). Many studies have
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addressed sequential extraction processes and confirmed the extractability of some heavy metals (Eighmy et al., 1995; Huang et al., 2007; Wan et al., 2006). These studies that have focused on the processing of MSWI fly ash appear to have resolved questions related to the toxicity of the fly ash; however, these methods are still defective in at least three aspects. The first defective area is the large amount of wastewater awaiting further disposal; the second defective area is the deficiency or absence of chlorine reduction in the fly ash; and the third defective area is the inability to keep up with the quantity of fly ash produced daily, which was the main reason that the chemical treatment methods of the fly ash were considered the most commercially optimal. With an increasing global focus on zero waste strategies and the continuous reuse of wastewater resources, this study investigated an innovative treatment of both MSWI fly ash and wastewater using direct electrolysis, wherein an electrocoagulation technique was used to resolve the above three problems, resulting in efficient handling of MSWI fly ash. The electrocoagulation technique involves the generation of coagulants in situ by dissolving aluminum (Al) ions electronically from Al electrode, which is also known as a sacrificial electrode. Metal ions are generated at the anode. Hydrogen gas that is released from the cathode helps to float flocculated particles out of the water (Jiang et al., 2002). The reactions occurring in electrocoagulation are shown in Eqs. (1)e(4). Anode: 2 H2O / 4 Hþ þ O2(g) þ 4 e
(1)
Al / Al3þ þ 3 e
(2)
Cathode: 2 H2O þ 2 e / 2 OH þ H2(g)
(3)
Al3þ þ 3 OH / Al(OH)3
(4)
The electrocoagulation technique is an effective method for removing toxic heavy metals from wastewater (Merzouk et al., 2009; Mouedhen et al., 2009; Zongo et al., 2009), and almost no research on electrocoagulation of MWSI fly ash is available in scientific literature. The main purposes of this research were to handle MSWI fly ash and retrieve the leachate (untreated wastewater) by means of the electrocoagulation technique while considering the following aspects: (a) the physicochemical features of the untreated fly ash, (b) a comparison of the untreated and the treated fly ash samples, as well as the wastewater that is generated within the regulatory limits after electrocoagulation, and (c) a mineralogical investigation to identify the composition of untreated and treated fly ash samples by scanning electron microscope/energy dispersive spectrometer (SEM-EDS) and X-ray diffraction (XRD). 2. Materials and methods 2.1. Sampling The MSWI fly ash used in this study consisted of approximately 20 kg of material that was obtained from the Taichung City refuse incineration plant. This plant is a large-scale incinerator located in central Taiwan. The material was collected from a side tube of the duct into the fly ash hopper. The fly ash hopper is used to provide fly ash to carry out S/S treatments. The proportion of general (nonhazardous) waste to general industrial waste in the incineration source material was 78:22. The plant uses a set of three stoker grate-type incinerators. Fly ash is produced from a semi-dry scrubber system of APCDs, consisting of two spray dryer injections, one injecting slurry lime for the neutralization of acidic gases and one injecting powdered activated carbon for the removal
of dioxins, followed by a fabric filter for the collection of particulate matter. The fly ash samples were dried at 105 5 C for 24 h and stored in PE bottles at 25 C. 2.2. Experimental apparatus A schematic diagram of the laboratory-scale setup for the electrocoagulation experiment comprising two similar reactors is shown in Fig. 1. The first reactor handles the fly ash mixed fluid consisting of untreated fly ash mixed with deionized water and conducts a solid/liquid separation of the treated fly ash mixed fluid. The second reactor handles the leachate from the separation. The two processes were operated under the same conditions: the electrical current was fixed at 10 A, and the pump flow rate was 600 mL/min. The main components of the reactor are a DC power supply device, an electrocoagulation reactor that consists of a reactor tank, Al or iridium oxide (IrO2) stick anodes, a stainless steel cathode, an upper tank channel, and a quantitative pump. The upper tank channel uses triangular spill weirs to provide a uniform distribution of the inlet fluid. Prior to each batch experiment, an Al or IrO2 anode was fixed in the middle of the reactor tank, and the tank was modified for electrical conduction while the stainless steel cathode board was enclosed inside the body of the reactor tank. The bottom three-way valve of the reactor tank was used for control purposes and was connected by a transparent PVC tube to a quantitative pump to control the mixed fluid reflux. The three-way valve was opened in the direction in which the pump was moving the fluid when the mixed fluid was being processed for electrocoagulation. The fluid temperature increased to approximately 50 C after approximately 1 h of treatment time. The power was subsequently turned off, and the separation of solid and liquid proceeded while the treated fly ash mixed fluid rested for approximately 4 h to yield the treated fly ash and leachate. The clear supernatant liquor was carefully collected for the leachate and prepared for the next electrocoagulation experiment at the second reactor. The volume of the reactor device was approximately 3.5 L, which was sufficient for conducting electrocoagulation experiments in 3.0-L batches. 2.3. Experimental methodology The MSWI fly ash experiments involved three aspects: a reduction in the toxicity of the solid fraction of the fly ash, the recovery and reuse of the leachate, and the analysis of the fly ash samples by SEM-EDS and XRD. This research was divided into two unit experiments to be conducted on hazardous MSWI fly ash to confirm the feasibility of reusing the treated leachate. In the processing of Unit I experiments, two electrocoagulation experiments were conducted on the different Al and IrO2 anodes. The L/S ratio was maintained at 20:1, which had been determined to be the optimal ratio for the hydrolysis of the MSWI fly ash in our previous study (Yang et al., 2012). The samples were untreated water (A and B) and treated ash (A and B). These untreated water samples were also treated in the electrocoagulation experiment to yield treated water (A and B). The untreated water A was also electrocoagulated with the IrO2 anode to develop a contrast. In Unit I experiments, the total three ash samples and five leachate samples were checked to ensure that they adhered to the limits established by the toxicity characteristic leaching procedure (TCLP) regulations and the effluent wastewater regulations, respectively (TEPA website, 2012). Unit II experiments were conducted on the untreated fly ash for the purpose of reducing the toxicity of the fly ash with the leachate that was recovered from Unit I. The acceptable treated leachate was mixed with untreated fly ash for the electrocoagulation experiments under the same set of operating conditions that were
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Fig. 1. The setup for electrocoagulation in this study.
specified for Unit I. After electrocoagulation with the Al anode, the samples were denoted as treated ash A(r) and untreated water A(r). Next, the untreated water A(r) was used to conduct further electrocoagulation experiments using the Al anode. 2.4. Analytical methods The physicochemical parameters of the untreated fly ash samples that were analyzed included the pH, the electrical conductivity (EC), the leachable amount of Cl (%), the water content (%), the cation exchange capacity (CEC), the total amount of four heavy metals (Pb, Cu, Cr, and Cd), and the TCLP tests. These analyses were conducted according to the National Institute of Environmental Analysis (NIEA) methods provided by the Environmental Analysis Laboratory of the TEPA (TEPA EAL TEPA website, 2012). The pH and EC were determined by the electrode method. The chloride (Cl) concentration in the leachates was determined by titration with Hg(NO3)2, which was subsequently changed to calculate the leachable amount of Cl (%) in the fly ash. The water content (%) in the samples was determined by weighing the samples before and after placing them in an oven that was kept at 105 5 C to determine the amount of water lost. The CEC was determined using the sodium acetate method. The total amount of four heavy metals in the fly ash was determined by aqua regia digestion. The TCLP test was established by the TEPA based on the US EPA method (SW-846 3rd Ed., Method 1311, 1992) and was conducted with an extraction solution containing acetic acid at a pH of 2.9 mixed with fly ash at a fixed L/S ratio of 20:1. A continuous extraction with the rotary agitators operating at 30 rpm was conducted for 18 h to assess the leaching features of the four heavy metals in the untreated fly ash and the treated ash (A, B, and A(r)). The leachates were filtered through Whatman 934-AH membranes prior to analysis by a Perkin Elmer AAnalyst 100 flame atomic absorption spectrometer (FAAS). The sequential extraction analyses used to fractionate the Pb were conducted according to the method reported by Tessier et al. (1979). These researchers developed sequential chemical extractions for the partitioning of heavy metals into five fractions: the exchangeable fraction, the fraction bound to carbonates, the
Table 1 The characteristics of untreated fly ash, the TCLP limits and total amounts of heavy metals that were detected. Item
Untreated fly ash
EC (L/S ¼ 20/1) Water content CEC Cl (L/S ¼ 20/1) Total amount analysis
31.0 0.64 15.5 19.1 3410 725 115 92.1
Pb Cu Cr Cd
0.32a 0.07 0.16 0.11 92.9 15.2 7.6 6.2
Unit mS/cm % meq/100 g % mg/kg mg/kg mg/kg mg/kg
a Data are the average standard deviation based on the analysis of 3 different samples.
fraction bound to reducible FeeMn oxides, the fraction bound to oxidizable organic matter or sulfides, and the residual fraction. The easiest fraction to remove was the exchangeable fraction followed by the fraction bound to carbonates, the fraction bound to reducible FeeMn oxides, the fraction bound to oxidizable organic matter or sulfides, and the residual fraction (Kim et al., 2002; Tessier et al., 1979). The exchangeable and carbonate fractions contained the compounds that were weakly bound, while the oxidizable and the residual fractions contained compounds that were tightly bound. After the samples were subjected to the proper extraction sequence, the leachates were filtered through Whatman 934-AH membranes. Microwave digestion (CEM) was then conducted on a MARS-Xpress instrument, and the digested samples were analyzed by FAAS to determine the fractions of Pb present in the fly ash. Samples from all four fly ash types were collected after the experiments and subjected to a mineralogical investigation, which included SEM-EDS and XRD analyses. SEM-EDS analysis can determine the elemental composition of each sample, and XRD can determine the crystallization patterns of each sample. SEM-EDS analysis was conducted by a JEOL JSM-6700F instrument, and XRD analysis was conducted using a Bruker D8 discover facility at a scanning rate of 1 /min from 10 to 100 with an X-ray tube source of 40 kV/40 mA.
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3. Results and discussion
100
Pb
90
3.1. Fly ash characteristics Table 1 shows the basic characteristics of the MSWI untreated fly ash. The analytical results indicate that the fly ash sample has a high pH of 13.2, a high EC of 31.0 mS/cm at an L/S of 20:1, and a high Cl content of 17.6%. The high EC will encourage the electrochemical reaction, so the addition of any electrolyte to drive the reaction is not required. The mass of Pb, Cu, Cr, and Cd in the fly ash was found to be 3,410, 725, 115, and 92.1 mg/kg, respectively. The Pb content in the fly ash was found to be higher than the other three heavy metals (Cu, Cr, and Cd). 3.2. The TCLP analysis of the fly ash after electrocoagulation treatments Table 2 shows the results of the TCLP analysis conducted on the four heavy metals in the fly ash after treatment with electrocoagulation. The heavy metal Pb contents in untreated fly ash and treated ash B clearly exceed the regulatory limits, while in treated ashes A and A(r) are compliant with the limits. The removal efficiencies of treated ash B, A, and A(r) are 82.0, 97.9, and 98.5%, respectively. A and A(r) seem to have the electrocoagulation effects to stabilize the Pb content in the fly ash significantly. The electrocoagulation effect generated by an electric field environment caused the Al anode to release Al3þ ions, as shown in Eq. (2). The Al3þ reacted not only with the OH produced by the cathode in Eq. (3) to form Al(OH)3 as shown in Eq. (4), but the ions also reacted with other ions to form new species in the mixed fly ash fluid. Clearly, the recovered leachate was used as the electrocoagulation disposal liquid instead of deionized water is feasible. A comparison of the Pb fractions in the fly ash finds that the total proportion of the weakly bound Pb fractions in the untreated fly ash and the treated ash B, as shown in Fig. 2, are 70.0% and 39.9%, respectively. These proportions are much higher than the weakly bound fractions of 3.6% in the treated ash A and 6.8% in the treated ash A(r), which shows that the electrocoagulation effect using Al as the anode material is able to stabilize the heavy metals by transforming the weakly bound Pb fractions into strongly bound fractions. Therefore, this technique has demonstrated the ability to convert the fly ash into nonhazardous materials by reducing the contents of the treated fly ash to levels that are compliant with the TCLP regulatory limits. 3.3. The recovery and reuse of the leachate The changes in the heavy metal concentrations, the pH, and the EC before and after electrocoagulation treatment of the leachates
Table 2 A comparison of the fly ash with the TCLP regulatory limits before and after different electrocoagulation treatments. Heavy metal
Untreated fly Unit I Unit II ash Treated ash Treated ash Treated ash Ba Aa A(r)b
Pb (mg/L) 48.3 Cu (mg/L) 0.16 Cr (mg/L) 1.27 Cd (mg/L) 0.11
8.67 0.18 0.17 0.04
(82.0)c () (86.6) (63.6)
1.01 0.03 0.14 0.02
(97.9) (81.3) (89.0) (81.8)
0.72 0.03 0.27 0.02
(98.5) (81.3) (78.7) (81.8)
Regulatory limits
5.0 15.0 5.0 1.0
a The electrocoagulation experiments make use of an Al anode in Group A and an IrO2 anode in Group B. b The ‘r’ in the brackets refers to the wastewater recovered from the Unit I experiments. c Removal efficiencies (%).
Relative ab und anc e (%)
80 Residual
70 60
Organic matter/sulfides
50 Fe-Mn oxides
40 30
Carbonates
20 Exchangeable
10 0
ted fly
a Untre
A B ash A(r) d ash d ash d ash Treate Treate Treate
Fly ash Fig. 2. A comparison of the Pb fractions found in the various types of fly ash.
are shown in Table 3. The Unit I experiments have shown that the Al anode was able to reduce heavy metal concentrations significantly so that the standards for effluent discharge are met. In particular, this reduction is notable in the Pb concentration, which is reduced from 26.6 to 0.45 mg/L after treatment. The final concentration of 0.45 mg/L is 10 times lower than the Pb concentration of 4.55 mg/L that was obtained in the leachate after treatment with the IrO2 anode. There is almost no disposal capacity when the IrO2 anode is used because of the untreated water B has already been treated to be compliant with regulatory limits. The Pb concentration of 0.89 mg/L in the untreated water B is reasonable, as most of the Pb remained in the solid fraction of the treated ash B. The treated ash B is therefore unable to meet the TCLP regulatory limits (see Table 2). Furthermore, the contrasting experimental result obtained from treating water A with the IrO2 anode proves that the inert IrO2 electrode is inferior to the sacrificial Al electrode. The use of the Al anode as a sacrificial electrode for treating the fly ash reaction strengthens the electrocoagulation effect. The high valences of the ions in Al(OH)3 aggregate or adsorb toxic heavy metals in the leachate (Merzouk et al., 2009; Mouedhen et al., 2009), thereby reducing the Pb and the Cd concentration in the leachate to comply with the regulatory limits. The IrO2 anode lacks the electrocoagulation effect and only relies on a small amount of high valence heavy metals to decrease the high Pb concentration in the treated water. The pH is reduced from 13.1 to 12.9 when the IrO2 anode was used, which is smaller than the change in pH from 13.1 to 10.6 when the Al anode was used. The changes in the EC with similar observations. These conclusions are confirmed by the results obtained from the treatment of the untreated water B samples. The decreased pH and EC values are helpful when considering the use of the recovered leachate for mixing the fly ash and treating the mixture with the Al anode in the Unit II experiments. In the Unit II experiments, the decreases in the concentrations of the four heavy metals after the treatment of the water sample A(r) are the same as the decreases exhibited in the Unit I experiments when the Al anode was used for treatment. For example, the removal efficiencies of Pb in Units I and II are 98.3 and 96.3%, respectively. The only difference that was observed between the Unit I and the Unit II experiments is in the EC value. The EC value from the Unit II experiments is approximately twice the EC value of the Unit I experiments because the liquid used for treatment in Unit II is the leachate recovered from the Unit I experiments. The increased EC is a result of the accumulation of soluble chloride in the leachate. The accumulated chloride concentration has
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Table 3 The concentrations of the heavy metals and the pH of the wastewater in comparison with the effluent wastewater regulatory limits, as well as the EC change in the leachate before and after electrocoagulation. Item
Pb (mg/L) Cu (mg/L) Cr (mg/L) Cd (mg/L) pH EC (mS/ cm) a b
Unit I
Unit II
Untreated water A
Treated water A (Al)a
Treated water A (IrO2)a
Untreated water B
Treated water B
Untreated water A(r)
Treated water A(r)
26.6 0.07 0.23 0.04 13.1 27.0
0.45 0.02 0.01 0.01 10.6 23.0
4.55 0.05 0.08 0.01 12.9 26.1
0.89 0.08 0.18 0.05 13.3 29.2
0.83 0.04 0.02 0.03 12.8 29.4
23.1 0.05 0.18 0.05 12.8 44.8
0.86 0.02 0.01 0.02 10.4 39.6
Regulatory limits
1.0 3.0 0.5 0.03 6.0e9.0 eb
The material in the brackets refers to the anode material. “e” indicates that there are no regulatory limits for this parameter.
obviously not reduced the effectiveness of the electrochemical reaction. The utilization of the Al anode to handle the fly ash fluid and leachate could therefore reduce the concentrations of the four heavy metals in the leachate to meet the concentration standards set for effluent discharge, while the pH of the leachate is still approximately 10.0, which exceeds the regulatory limit of 6.0e9.0. Although the leachate can not be discharged directly, the leachate can still feasibly be recovered for reuse in the electrocoagulation disposal of untreated fly ash, achieving the goal of recycling the wastewater in the process. A determination of the number of cycles for which this wastewater can be reused for the treatment of the fly ash is a subject that warrants examination in the future. 3.4. The SEM-EDS analysis of fly ash samples The elemental composition of the fly ash samples by weight proportion is examined by SEM-EDS and is shown in Table 4. The main elements in the fly ash are found to be oxygen (O), sodium (Na), silicon (Si), chlorine (Cl), potassium (K), and calcium (Ca). Other than these elements, which comprises a large proportion of the materials in the untreated fly ash, such trace elements as Al and Zn are lower in concentration and are unable to be measured by SEM-EDS because the water washing conducted during the electrocoagulation treatment washed out most of the soluble ionic elements (Na, Cl, and K) that are easily hydrolyzed (Abbas et al., 2003; Arvelakis and Frandsen, 2005; Kirby and Rimstidt, 1994; Mangialardi, 2003), resulting in an increased relative content of these trace elements. In the untreated fly ash, O, Ca, and Cl make up the largest composition proportions of 37.8, 25.9, and 24.9%, respectively. In addition, Na, K, and Si are also present in certain proportions, accounting for 4.96, 4.84, and 1.68%, respectively. Therefore, NaCl, KCl, silicon chloride, and SiO2 compounds should be found, together with the calcium chloride and the calcium oxide in fly ash. The treated ash samples A and A(r) including the Al composition is reasonable because of the release of Al3þ ions from
the anode, which increases the concentration of Al in the reaction. The proportions of elements such as Ca, Cl, and Al are higher in treated ash A(r) than in A, clearly accumulating twice the amount of Cl due to the reusing of the recovered leachate from Unit I, which contributes to an increase in the Cl content. This result is proven for the EC data for untreated water A to A(r) in Table 3 (Yang et al., 2012). 3.5. XRD analysis of the fly ash samples The results of the XRD analysis are displayed in Fig. 3 to determine the compounds originally present in the fly ash. The untreated fly ash consists mainly of KCl, NaCl, SiCl4, SiO2, CaClOH, CaSO4, and Ca(OH)2, confirming the previous hypothesis that the fly ash contained KCl, NaCl, SiCl4, SiO2, and Ca(OH)2. The treated ash samples B, A, and A(r) do not show the presence of soluble salts such as KCl, NaCl, SiCl4, and CaClOH because these salts are easily washed out by water, while ash B retains Ca(OH)2, CaSO4, SiO2, and produces the new compound, CaCO3. The new compound (CaCO3) was formed under the same condition that occurred in ashes A and A(r), but the CaSO4 was changed to produce a new compound, 3CaO$Al2O3$CaCl2$10H2O (Friedel’s salt). The sacrificial Al anode releases the Al3þ ions when an electric field is applied, and these Al3þ ions react with fly ash, resulting in the precipitation of Friedel’s salt. Friedel’s salt is in a stable state, contains chlorine ions that experience great difficulty in dissolution, and can absorb heavy metals (Dai et al., 2009; Wu et al., 2010), causing the heavy metals
Table 4 The elemental analysis of the fly ash in terms of weight fractions. Element
Untreated fly ash
Treated ash A
Treated ash B
Treated ash A(r)
O Na Al Si Cl K Ca Zn Sum (%)
37.8 4.96 ea 1.68 24.9 4.84 25.9 e 100.08
69.1 0.26 2.87 0.07 4.19 2.45 20.9 e 99.84
59.4 0.14 e 1.61 4.41 0.84 29.2 2.90 98.50
53.7 e 3.47 0.71 8.36 2.83 29.3 e 98.37
a
“e” showed which element was not measured.
Fig. 3. XRD analyses of the four different types of fly ash after treatment by electrocoagulation.
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in the fly ash to be transformed into abundant associations with the strongly bound fractions rather than with the weakly bound fractions. From this research, the composition of the fly ash is found to have changed into harder material after electrocoagulation, showing that treated ashes A and A(r) are compliant with the regulatory limits and are nontoxic. 4. Conclusions This research used an innovative electrocoagulation technique to dispose of MSWI fly ash and succeeded in changing the composition of the fly ash to convert it into nonhazardous materials. The three main functions in the application of this technique are washing with water, changing the fly ash composition, and stabilizing the heavy metals in the ash. The soluble salts such as KCl, NaCl, SiCl4, and CaClOH can be washed out and cannot be determined by XRD analysis in the treated fly ash. In addition, the release of Al3þ ions that reacts with Ca- and Cl-containing compounds converts the fly ash into Friedel’s salt, which has changed the composition of the fly ash. Friedel’s salt contains insoluble chlorine atoms and can also absorb and stabilize heavy metals to allow the fly ash to comply with the TCLP regulatory limits. The use of the IrO2 anode lacks the electrocoagulation effect, and only the water washing is effective for fly ash treatment. Electrocoagulation has the potential to recover the leachate for use as the fluid to mix the fly ash with, and the converted fly ash has a potential recycling value. This work has resolved the problem of wastewater disposal, as well as the problem of reducing and stabilizing chlorine levels in the fly ash. The third problem associated with commercializing the process will be left to future work. References Abbas, Z., Moghaddam, A.P., Steenari, B.M., 2003. Release of salts from municipal solid waste combustion residues. Waste Manage. 23, 291e305. Arvelakis, S., Frandsen, F.J., 2005. Study on analysis and characterization methods for ash material from incineration plants. Fuel 84, 1725e1738. Astrup, T., Mosbæk, H., Christensen, T.H., 2006. Assessment of long-term leaching from waste incineration air-pollution-control residues. Waste Manage. 26, 803e814. Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2012. Soluble salt removal from MSWI fly ash and its stabilization for safer disposal and recovery as road basement material. Waste Manage. 32, 1179e1185. Dai, Y., Qian, G., Cao, Y., Chi, Y., Xu, Y., Zhou, J., Liu, Q., Xu, Z.P., Qiao, S., 2009. Effective removal and fixation of Cr(VI) from aqueous solution with Friedel’s salt. J. Hazard. Mater. 170, 1086e1092. De Casa, G., Mangialardi, T., Paolini, A.E., Piga, L., 2007. Physical-mechanical and environmental properties of sintered municipal incinerator fly ash. Waste Manage. 27, 238e247. Eighmy, T.T., Crannell, B.S., Butler, L.G., Cartledge, F.K., Emery, E.F., Oblas, D., Krzanowski, J.E., Eusden, J.D., Shaw, E.L., Francis, C.A., 1997. Heavy metal stabilization in municipal solid waste combustion dry scrubber residue using soluble phosphate. Environ. Sci. Technol. 31, 3330e3338.
Eighmy, T.T., Eusden, J.D., Krzanowski, J.E., Domingo, D.S., Staempfli, D., Martin, J.R., Erickson, P.M., 1995. Comprehensive approach toward understanding element speciation and leaching behavior in municipal solid waste incineration electrostatic precipitator ash. Environ. Sci. Technol. 29, 629e646. Huang, S.J., Chang, C.Y., Mui, D.T., Chang, F.C., Lee, M.Y., Wang, C.F., 2007. Sequential extraction for evaluating the leaching behavior of selected elements in municipal solid waste incineration fly ash. J. Hazard. Mater. 149, 180e188. Jiang, J.Q., Graham, N., André, C., Kelsall, G.H., Brandon, N., 2002. Laboratory study of electro-coagulation-flotation for water treatment. Water Res. 36, 4064e4078. Károly, Z., Mohai, I., Tóth, M., Wéber, F., Szépvölgyi, J., 2007. Production of glasse ceramics from fly ash using arc plasma. J. Eur. Ceram. Soc. 27, 1721e1725. Karamanov, A., Pelino, M., Hreglich, A., 2003. Sintered glasseceramics from Municipal Solid Waste-incinerator fly ashes-part I: the influence of the heating rate on the sinter-crystallisation. J. Eur. Ceram. Soc. 23, 827e832. Kida, A., Noma, Y., Imada, T., 1996. Chemical speciation and leaching properties of elements in municipal incinerator ashes. Waste Manage. 16, 527e536. Kim, S.O., Moon, S.H., Kim, K.W., Yun, S.T., 2002. Pilot scale study on the ex situ electrokinetic removal of heavy metals from municipal wastewater sludges. Water Res. 36, 4765e4774. Kirby, C.S., Rimstidt, J.D., 1994. Interaction of municipal solid waste ash with water. Environ. Sci. Technol. 28, 443e451. Lam, C.H.K., Barford, J.P., McKay, G., 2011. Utilization of municipal solid waste incineration ash in Portland cement clinker. Clean Technol. Environ. Policy 13, 607e615. Mangialardi, T., 2003. Disposal of MSWI fly ash through a combined washingimmobilisation process. J. Hazard. Mater. 98, 225e240. Merzouk, B., Gourich, B., Sekki, A., Madani, K., Chibane, M., 2009. Removal turbidity and separation of heavy metals using electrocoagulationeelectroflotation technique: a case study. J. Hazard. Mater. 164, 215e222. Mizutani, S., Yoshida, T., Sakai, S.I., Takatsuki, H., 1996. Release of metals from MSW I fly ash and availability in alkali condition. Waste Manage. 16, 537e544. Mouedhen, G., Feki, M., De Petris-Wery, M., Ayedi, H.F., 2009. Electrochemical removal of Cr(VI) from aqueous media using iron and aluminum as electrode materials: towards a better understanding of the involved phenomena. J. Hazard. Mater. 168, 983e991. Okada, T., Tojo, Y., Tanaka, N., Matsuto, T., 2007. Recovery of zinc and lead from fly ash from ash-melting and gasification-melting processes of MSW e comparison and applicability of chemical leaching methods. Waste Manage. 27, 69e80. Sakai, S.i., Hiraoka, M., 2000. Municipal solid waste incinerator residue recycling by thermal processes. Waste Manage. 20, 249e258. TEPA EAL, 2012. Environmental Analysis Laboratory of Environmental Protection Administration in Taiwan. Website: http://www.niea.gov.tw/. TEPA website, 2012. Taiwanese Environmental Protection Administration. http:// www.epa.gov.tw/. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844e851. Wan, X., Wang, W., Ye, T., Guo, Y., Gao, X., 2006. A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure. J. Hazard. Mater. 134, 197e201. Wu, H.Y., Ting, Y.P., 2006. Metal extraction from municipal solid waste (MSW) incinerator fly ash-Chemical leaching and fungal bioleaching. Enzyme Microb. Technol. 38, 839e847. Wu, Y., Chi, Y., Bai, H., Qian, G., Cao, Y., Zhou, J., Xu, Y., Liu, Q., Xu, Z.P., Qiao, S., 2010. Effective removal of selenate from aqueous solutions by the Friedel phase. J. Hazard. Mater. 176, 193e198. Yang, R., Liao, W.P., Wu, P.H., 2012. Basic characteristics of leachate produced by various washing processes for MSWI ashes in Taiwan. J. Environ. Manage. 104, 67e76. Youcai, Z., Lijie, S., Guojian, L., 2002. Chemical stabilization of MSW incinerator fly ashes. J. Hazard. Mater. 95, 47e63. Zongo, I., Leclerc, J.P., Maïga, H.A., Wéthé, J., Lapicque, F., 2009. Removal of hexavalent chromium from industrial wastewater by electrocoagulation: a comprehensive comparison of aluminium and iron electrodes. Sep. Purif. Technol. 66, 159e166.
Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
The application of electrocoagulation for the conversion of MSWI fly ash into nonhazardous materials Wing-Ping Liao a, Renbo Yang a, b, *, Wei-Ting Kuo a, Jui-Yuan Huang a a b
Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan Bio-way Environmental Science and Technology (BEST) Corp., Ltd., Taichung 404, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2012 Received in revised form 12 October 2013 Accepted 12 February 2014 Available online 13 March 2014
This research investigated the electrocoagulation of municipal solid waste incineration (MSWI) fly ash at a liquid-to-solid ratio (L/S) of 20:1. The leachate that was obtained from this treatment was recovered for reutilization. Two different anodic electrodes were investigated, and two unit runs were conducted. In Unit I, the optimum anode was chosen, and in Unit II, the optimum anode and the recovered leachate were used to replace deionized water for repeating the same electrocoagulation experiments. The results indicate that the aluminum (Al) anode performed better than the iridium oxide (IrO2) anode. The electrocoagulation technique includes washing with water, changing the composition of the fly ash, and stabilizing the heavy metals in the ash. Washing with water can remove the soluble salts from fly ash, and the fly ash can be converted into Friedel’s salt (3CaO$Al2O3$CaCl2$10H2O) under an uniform electric field and the sacrificial release of Alþ3 ions, which stabilizes the toxic heavy metals and brings the composition of the fly ash to within the regulatory limits of the toxicity characteristic leaching procedure (TCLP). Use of the Al anode to manage the MSWI fly ash and the leachate obtained from the electrocoagulation treatment is therefore feasible. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: MSWI fly ash Electrocoagulation Heavy metals Friedel’s salt Toxicity characteristic leaching procedure
1. Introduction Incineration has become the preferred option for the disposal of municipal solid waste (MSW) in Taiwan. The fly ash generated by MSW incineration (MSWI) is trapped in the air pollution control devices (APCDs). MSWI fly ash is classified as a hazardous waste and must be disposed of in landfills after the stabilization and solidification (S/S) of the heavy metals and dioxins. However, the increasingly limited landfill capacity and the restrictions of the new landfill policy developed by the Taiwanese Environmental Protection Administration (TEPA) in 2007 have caused the reuse of MSWI fly ash to become an essential issue in Taiwan. The four methods for processing MSWI fly ash that have been described in research papers include physical, biological, thermal and chemical treatment techniques. Physical treatment techniques often use washing with water as a pretreatment to promote the formation of hydrate phases that convert heavy metals to lessreactive forms and remove significant amounts of soluble salts
* Corresponding author. 5F, 66, Sec. 2, Taiyuan Rd., North Dist., Taichung 404, Taiwan. Tel.: þ886 4 22062926; fax: þ886 4 22062927. E-mail addresses: [email protected], [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jenvman.2014.02.012 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
from the fly ash (Abbas et al., 2003; Arvelakis and Frandsen, 2005; Colangelo et al., 2012; Kirby and Rimstidt, 1994; Mangialardi, 2003). Wu and Ting (2006) used a biological technique to treat fly ash and found that fungal bioleaching was more beneficial than chemical leaching. However, bioleaching requires more time for processing than does chemical leaching. Thermal treatment techniques that are used to handle fly ash such as sintering, melting, fusion, and vitrification are usually operated at high temperatures. The products of these thermally treated fly ashes are highly crystalline on the surface and are highly durable chemically, which causes them to be considered suitable for reuse (De Casa et al., 2007; Károly et al., 2007; Karamanov et al., 2003; Lam et al., 2011). Nevertheless, these thermal treatment processes are economically undesirable on the commercial scale because of their high energy consumption and the high costs of building and maintaining the facilities used to operate the thermal treatment processes (Sakai and Hiraoka, 2000). Chemical extraction or fixation techniques have been studied much more often than the other three techniques. The chemical agents that are typically used include HCl, HNO3, H3PO4, Ca(OH)2, NaOH, ethylenediaminetetraacetic acid (EDTA) disodium salt, Na2S, and thiourea (Astrup et al., 2006; Eighmy et al., 1997; Kida et al., 1996; Mizutani et al., 1996; Okada et al., 2007; Youcai et al., 2002). Many studies have
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addressed sequential extraction processes and confirmed the extractability of some heavy metals (Eighmy et al., 1995; Huang et al., 2007; Wan et al., 2006). These studies that have focused on the processing of MSWI fly ash appear to have resolved questions related to the toxicity of the fly ash; however, these methods are still defective in at least three aspects. The first defective area is the large amount of wastewater awaiting further disposal; the second defective area is the deficiency or absence of chlorine reduction in the fly ash; and the third defective area is the inability to keep up with the quantity of fly ash produced daily, which was the main reason that the chemical treatment methods of the fly ash were considered the most commercially optimal. With an increasing global focus on zero waste strategies and the continuous reuse of wastewater resources, this study investigated an innovative treatment of both MSWI fly ash and wastewater using direct electrolysis, wherein an electrocoagulation technique was used to resolve the above three problems, resulting in efficient handling of MSWI fly ash. The electrocoagulation technique involves the generation of coagulants in situ by dissolving aluminum (Al) ions electronically from Al electrode, which is also known as a sacrificial electrode. Metal ions are generated at the anode. Hydrogen gas that is released from the cathode helps to float flocculated particles out of the water (Jiang et al., 2002). The reactions occurring in electrocoagulation are shown in Eqs. (1)e(4). Anode: 2 H2O / 4 Hþ þ O2(g) þ 4 e
(1)
Al / Al3þ þ 3 e
(2)
Cathode: 2 H2O þ 2 e / 2 OH þ H2(g)
(3)
Al3þ þ 3 OH / Al(OH)3
(4)
The electrocoagulation technique is an effective method for removing toxic heavy metals from wastewater (Merzouk et al., 2009; Mouedhen et al., 2009; Zongo et al., 2009), and almost no research on electrocoagulation of MWSI fly ash is available in scientific literature. The main purposes of this research were to handle MSWI fly ash and retrieve the leachate (untreated wastewater) by means of the electrocoagulation technique while considering the following aspects: (a) the physicochemical features of the untreated fly ash, (b) a comparison of the untreated and the treated fly ash samples, as well as the wastewater that is generated within the regulatory limits after electrocoagulation, and (c) a mineralogical investigation to identify the composition of untreated and treated fly ash samples by scanning electron microscope/energy dispersive spectrometer (SEM-EDS) and X-ray diffraction (XRD). 2. Materials and methods 2.1. Sampling The MSWI fly ash used in this study consisted of approximately 20 kg of material that was obtained from the Taichung City refuse incineration plant. This plant is a large-scale incinerator located in central Taiwan. The material was collected from a side tube of the duct into the fly ash hopper. The fly ash hopper is used to provide fly ash to carry out S/S treatments. The proportion of general (nonhazardous) waste to general industrial waste in the incineration source material was 78:22. The plant uses a set of three stoker grate-type incinerators. Fly ash is produced from a semi-dry scrubber system of APCDs, consisting of two spray dryer injections, one injecting slurry lime for the neutralization of acidic gases and one injecting powdered activated carbon for the removal
of dioxins, followed by a fabric filter for the collection of particulate matter. The fly ash samples were dried at 105 5 C for 24 h and stored in PE bottles at 25 C. 2.2. Experimental apparatus A schematic diagram of the laboratory-scale setup for the electrocoagulation experiment comprising two similar reactors is shown in Fig. 1. The first reactor handles the fly ash mixed fluid consisting of untreated fly ash mixed with deionized water and conducts a solid/liquid separation of the treated fly ash mixed fluid. The second reactor handles the leachate from the separation. The two processes were operated under the same conditions: the electrical current was fixed at 10 A, and the pump flow rate was 600 mL/min. The main components of the reactor are a DC power supply device, an electrocoagulation reactor that consists of a reactor tank, Al or iridium oxide (IrO2) stick anodes, a stainless steel cathode, an upper tank channel, and a quantitative pump. The upper tank channel uses triangular spill weirs to provide a uniform distribution of the inlet fluid. Prior to each batch experiment, an Al or IrO2 anode was fixed in the middle of the reactor tank, and the tank was modified for electrical conduction while the stainless steel cathode board was enclosed inside the body of the reactor tank. The bottom three-way valve of the reactor tank was used for control purposes and was connected by a transparent PVC tube to a quantitative pump to control the mixed fluid reflux. The three-way valve was opened in the direction in which the pump was moving the fluid when the mixed fluid was being processed for electrocoagulation. The fluid temperature increased to approximately 50 C after approximately 1 h of treatment time. The power was subsequently turned off, and the separation of solid and liquid proceeded while the treated fly ash mixed fluid rested for approximately 4 h to yield the treated fly ash and leachate. The clear supernatant liquor was carefully collected for the leachate and prepared for the next electrocoagulation experiment at the second reactor. The volume of the reactor device was approximately 3.5 L, which was sufficient for conducting electrocoagulation experiments in 3.0-L batches. 2.3. Experimental methodology The MSWI fly ash experiments involved three aspects: a reduction in the toxicity of the solid fraction of the fly ash, the recovery and reuse of the leachate, and the analysis of the fly ash samples by SEM-EDS and XRD. This research was divided into two unit experiments to be conducted on hazardous MSWI fly ash to confirm the feasibility of reusing the treated leachate. In the processing of Unit I experiments, two electrocoagulation experiments were conducted on the different Al and IrO2 anodes. The L/S ratio was maintained at 20:1, which had been determined to be the optimal ratio for the hydrolysis of the MSWI fly ash in our previous study (Yang et al., 2012). The samples were untreated water (A and B) and treated ash (A and B). These untreated water samples were also treated in the electrocoagulation experiment to yield treated water (A and B). The untreated water A was also electrocoagulated with the IrO2 anode to develop a contrast. In Unit I experiments, the total three ash samples and five leachate samples were checked to ensure that they adhered to the limits established by the toxicity characteristic leaching procedure (TCLP) regulations and the effluent wastewater regulations, respectively (TEPA website, 2012). Unit II experiments were conducted on the untreated fly ash for the purpose of reducing the toxicity of the fly ash with the leachate that was recovered from Unit I. The acceptable treated leachate was mixed with untreated fly ash for the electrocoagulation experiments under the same set of operating conditions that were
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159
Fig. 1. The setup for electrocoagulation in this study.
specified for Unit I. After electrocoagulation with the Al anode, the samples were denoted as treated ash A(r) and untreated water A(r). Next, the untreated water A(r) was used to conduct further electrocoagulation experiments using the Al anode. 2.4. Analytical methods The physicochemical parameters of the untreated fly ash samples that were analyzed included the pH, the electrical conductivity (EC), the leachable amount of Cl (%), the water content (%), the cation exchange capacity (CEC), the total amount of four heavy metals (Pb, Cu, Cr, and Cd), and the TCLP tests. These analyses were conducted according to the National Institute of Environmental Analysis (NIEA) methods provided by the Environmental Analysis Laboratory of the TEPA (TEPA EAL TEPA website, 2012). The pH and EC were determined by the electrode method. The chloride (Cl) concentration in the leachates was determined by titration with Hg(NO3)2, which was subsequently changed to calculate the leachable amount of Cl (%) in the fly ash. The water content (%) in the samples was determined by weighing the samples before and after placing them in an oven that was kept at 105 5 C to determine the amount of water lost. The CEC was determined using the sodium acetate method. The total amount of four heavy metals in the fly ash was determined by aqua regia digestion. The TCLP test was established by the TEPA based on the US EPA method (SW-846 3rd Ed., Method 1311, 1992) and was conducted with an extraction solution containing acetic acid at a pH of 2.9 mixed with fly ash at a fixed L/S ratio of 20:1. A continuous extraction with the rotary agitators operating at 30 rpm was conducted for 18 h to assess the leaching features of the four heavy metals in the untreated fly ash and the treated ash (A, B, and A(r)). The leachates were filtered through Whatman 934-AH membranes prior to analysis by a Perkin Elmer AAnalyst 100 flame atomic absorption spectrometer (FAAS). The sequential extraction analyses used to fractionate the Pb were conducted according to the method reported by Tessier et al. (1979). These researchers developed sequential chemical extractions for the partitioning of heavy metals into five fractions: the exchangeable fraction, the fraction bound to carbonates, the
Table 1 The characteristics of untreated fly ash, the TCLP limits and total amounts of heavy metals that were detected. Item
Untreated fly ash
EC (L/S ¼ 20/1) Water content CEC Cl (L/S ¼ 20/1) Total amount analysis
31.0 0.64 15.5 19.1 3410 725 115 92.1
Pb Cu Cr Cd
0.32a 0.07 0.16 0.11 92.9 15.2 7.6 6.2
Unit mS/cm % meq/100 g % mg/kg mg/kg mg/kg mg/kg
a Data are the average standard deviation based on the analysis of 3 different samples.
fraction bound to reducible FeeMn oxides, the fraction bound to oxidizable organic matter or sulfides, and the residual fraction. The easiest fraction to remove was the exchangeable fraction followed by the fraction bound to carbonates, the fraction bound to reducible FeeMn oxides, the fraction bound to oxidizable organic matter or sulfides, and the residual fraction (Kim et al., 2002; Tessier et al., 1979). The exchangeable and carbonate fractions contained the compounds that were weakly bound, while the oxidizable and the residual fractions contained compounds that were tightly bound. After the samples were subjected to the proper extraction sequence, the leachates were filtered through Whatman 934-AH membranes. Microwave digestion (CEM) was then conducted on a MARS-Xpress instrument, and the digested samples were analyzed by FAAS to determine the fractions of Pb present in the fly ash. Samples from all four fly ash types were collected after the experiments and subjected to a mineralogical investigation, which included SEM-EDS and XRD analyses. SEM-EDS analysis can determine the elemental composition of each sample, and XRD can determine the crystallization patterns of each sample. SEM-EDS analysis was conducted by a JEOL JSM-6700F instrument, and XRD analysis was conducted using a Bruker D8 discover facility at a scanning rate of 1 /min from 10 to 100 with an X-ray tube source of 40 kV/40 mA.
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3. Results and discussion
100
Pb
90
3.1. Fly ash characteristics Table 1 shows the basic characteristics of the MSWI untreated fly ash. The analytical results indicate that the fly ash sample has a high pH of 13.2, a high EC of 31.0 mS/cm at an L/S of 20:1, and a high Cl content of 17.6%. The high EC will encourage the electrochemical reaction, so the addition of any electrolyte to drive the reaction is not required. The mass of Pb, Cu, Cr, and Cd in the fly ash was found to be 3,410, 725, 115, and 92.1 mg/kg, respectively. The Pb content in the fly ash was found to be higher than the other three heavy metals (Cu, Cr, and Cd). 3.2. The TCLP analysis of the fly ash after electrocoagulation treatments Table 2 shows the results of the TCLP analysis conducted on the four heavy metals in the fly ash after treatment with electrocoagulation. The heavy metal Pb contents in untreated fly ash and treated ash B clearly exceed the regulatory limits, while in treated ashes A and A(r) are compliant with the limits. The removal efficiencies of treated ash B, A, and A(r) are 82.0, 97.9, and 98.5%, respectively. A and A(r) seem to have the electrocoagulation effects to stabilize the Pb content in the fly ash significantly. The electrocoagulation effect generated by an electric field environment caused the Al anode to release Al3þ ions, as shown in Eq. (2). The Al3þ reacted not only with the OH produced by the cathode in Eq. (3) to form Al(OH)3 as shown in Eq. (4), but the ions also reacted with other ions to form new species in the mixed fly ash fluid. Clearly, the recovered leachate was used as the electrocoagulation disposal liquid instead of deionized water is feasible. A comparison of the Pb fractions in the fly ash finds that the total proportion of the weakly bound Pb fractions in the untreated fly ash and the treated ash B, as shown in Fig. 2, are 70.0% and 39.9%, respectively. These proportions are much higher than the weakly bound fractions of 3.6% in the treated ash A and 6.8% in the treated ash A(r), which shows that the electrocoagulation effect using Al as the anode material is able to stabilize the heavy metals by transforming the weakly bound Pb fractions into strongly bound fractions. Therefore, this technique has demonstrated the ability to convert the fly ash into nonhazardous materials by reducing the contents of the treated fly ash to levels that are compliant with the TCLP regulatory limits. 3.3. The recovery and reuse of the leachate The changes in the heavy metal concentrations, the pH, and the EC before and after electrocoagulation treatment of the leachates
Table 2 A comparison of the fly ash with the TCLP regulatory limits before and after different electrocoagulation treatments. Heavy metal
Untreated fly Unit I Unit II ash Treated ash Treated ash Treated ash Ba Aa A(r)b
Pb (mg/L) 48.3 Cu (mg/L) 0.16 Cr (mg/L) 1.27 Cd (mg/L) 0.11
8.67 0.18 0.17 0.04
(82.0)c () (86.6) (63.6)
1.01 0.03 0.14 0.02
(97.9) (81.3) (89.0) (81.8)
0.72 0.03 0.27 0.02
(98.5) (81.3) (78.7) (81.8)
Regulatory limits
5.0 15.0 5.0 1.0
a The electrocoagulation experiments make use of an Al anode in Group A and an IrO2 anode in Group B. b The ‘r’ in the brackets refers to the wastewater recovered from the Unit I experiments. c Removal efficiencies (%).
Relative ab und anc e (%)
80 Residual
70 60
Organic matter/sulfides
50 Fe-Mn oxides
40 30
Carbonates
20 Exchangeable
10 0
ted fly
a Untre
A B ash A(r) d ash d ash d ash Treate Treate Treate
Fly ash Fig. 2. A comparison of the Pb fractions found in the various types of fly ash.
are shown in Table 3. The Unit I experiments have shown that the Al anode was able to reduce heavy metal concentrations significantly so that the standards for effluent discharge are met. In particular, this reduction is notable in the Pb concentration, which is reduced from 26.6 to 0.45 mg/L after treatment. The final concentration of 0.45 mg/L is 10 times lower than the Pb concentration of 4.55 mg/L that was obtained in the leachate after treatment with the IrO2 anode. There is almost no disposal capacity when the IrO2 anode is used because of the untreated water B has already been treated to be compliant with regulatory limits. The Pb concentration of 0.89 mg/L in the untreated water B is reasonable, as most of the Pb remained in the solid fraction of the treated ash B. The treated ash B is therefore unable to meet the TCLP regulatory limits (see Table 2). Furthermore, the contrasting experimental result obtained from treating water A with the IrO2 anode proves that the inert IrO2 electrode is inferior to the sacrificial Al electrode. The use of the Al anode as a sacrificial electrode for treating the fly ash reaction strengthens the electrocoagulation effect. The high valences of the ions in Al(OH)3 aggregate or adsorb toxic heavy metals in the leachate (Merzouk et al., 2009; Mouedhen et al., 2009), thereby reducing the Pb and the Cd concentration in the leachate to comply with the regulatory limits. The IrO2 anode lacks the electrocoagulation effect and only relies on a small amount of high valence heavy metals to decrease the high Pb concentration in the treated water. The pH is reduced from 13.1 to 12.9 when the IrO2 anode was used, which is smaller than the change in pH from 13.1 to 10.6 when the Al anode was used. The changes in the EC with similar observations. These conclusions are confirmed by the results obtained from the treatment of the untreated water B samples. The decreased pH and EC values are helpful when considering the use of the recovered leachate for mixing the fly ash and treating the mixture with the Al anode in the Unit II experiments. In the Unit II experiments, the decreases in the concentrations of the four heavy metals after the treatment of the water sample A(r) are the same as the decreases exhibited in the Unit I experiments when the Al anode was used for treatment. For example, the removal efficiencies of Pb in Units I and II are 98.3 and 96.3%, respectively. The only difference that was observed between the Unit I and the Unit II experiments is in the EC value. The EC value from the Unit II experiments is approximately twice the EC value of the Unit I experiments because the liquid used for treatment in Unit II is the leachate recovered from the Unit I experiments. The increased EC is a result of the accumulation of soluble chloride in the leachate. The accumulated chloride concentration has
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Table 3 The concentrations of the heavy metals and the pH of the wastewater in comparison with the effluent wastewater regulatory limits, as well as the EC change in the leachate before and after electrocoagulation. Item
Pb (mg/L) Cu (mg/L) Cr (mg/L) Cd (mg/L) pH EC (mS/ cm) a b
Unit I
Unit II
Untreated water A
Treated water A (Al)a
Treated water A (IrO2)a
Untreated water B
Treated water B
Untreated water A(r)
Treated water A(r)
26.6 0.07 0.23 0.04 13.1 27.0
0.45 0.02 0.01 0.01 10.6 23.0
4.55 0.05 0.08 0.01 12.9 26.1
0.89 0.08 0.18 0.05 13.3 29.2
0.83 0.04 0.02 0.03 12.8 29.4
23.1 0.05 0.18 0.05 12.8 44.8
0.86 0.02 0.01 0.02 10.4 39.6
Regulatory limits
1.0 3.0 0.5 0.03 6.0e9.0 eb
The material in the brackets refers to the anode material. “e” indicates that there are no regulatory limits for this parameter.
obviously not reduced the effectiveness of the electrochemical reaction. The utilization of the Al anode to handle the fly ash fluid and leachate could therefore reduce the concentrations of the four heavy metals in the leachate to meet the concentration standards set for effluent discharge, while the pH of the leachate is still approximately 10.0, which exceeds the regulatory limit of 6.0e9.0. Although the leachate can not be discharged directly, the leachate can still feasibly be recovered for reuse in the electrocoagulation disposal of untreated fly ash, achieving the goal of recycling the wastewater in the process. A determination of the number of cycles for which this wastewater can be reused for the treatment of the fly ash is a subject that warrants examination in the future. 3.4. The SEM-EDS analysis of fly ash samples The elemental composition of the fly ash samples by weight proportion is examined by SEM-EDS and is shown in Table 4. The main elements in the fly ash are found to be oxygen (O), sodium (Na), silicon (Si), chlorine (Cl), potassium (K), and calcium (Ca). Other than these elements, which comprises a large proportion of the materials in the untreated fly ash, such trace elements as Al and Zn are lower in concentration and are unable to be measured by SEM-EDS because the water washing conducted during the electrocoagulation treatment washed out most of the soluble ionic elements (Na, Cl, and K) that are easily hydrolyzed (Abbas et al., 2003; Arvelakis and Frandsen, 2005; Kirby and Rimstidt, 1994; Mangialardi, 2003), resulting in an increased relative content of these trace elements. In the untreated fly ash, O, Ca, and Cl make up the largest composition proportions of 37.8, 25.9, and 24.9%, respectively. In addition, Na, K, and Si are also present in certain proportions, accounting for 4.96, 4.84, and 1.68%, respectively. Therefore, NaCl, KCl, silicon chloride, and SiO2 compounds should be found, together with the calcium chloride and the calcium oxide in fly ash. The treated ash samples A and A(r) including the Al composition is reasonable because of the release of Al3þ ions from
the anode, which increases the concentration of Al in the reaction. The proportions of elements such as Ca, Cl, and Al are higher in treated ash A(r) than in A, clearly accumulating twice the amount of Cl due to the reusing of the recovered leachate from Unit I, which contributes to an increase in the Cl content. This result is proven for the EC data for untreated water A to A(r) in Table 3 (Yang et al., 2012). 3.5. XRD analysis of the fly ash samples The results of the XRD analysis are displayed in Fig. 3 to determine the compounds originally present in the fly ash. The untreated fly ash consists mainly of KCl, NaCl, SiCl4, SiO2, CaClOH, CaSO4, and Ca(OH)2, confirming the previous hypothesis that the fly ash contained KCl, NaCl, SiCl4, SiO2, and Ca(OH)2. The treated ash samples B, A, and A(r) do not show the presence of soluble salts such as KCl, NaCl, SiCl4, and CaClOH because these salts are easily washed out by water, while ash B retains Ca(OH)2, CaSO4, SiO2, and produces the new compound, CaCO3. The new compound (CaCO3) was formed under the same condition that occurred in ashes A and A(r), but the CaSO4 was changed to produce a new compound, 3CaO$Al2O3$CaCl2$10H2O (Friedel’s salt). The sacrificial Al anode releases the Al3þ ions when an electric field is applied, and these Al3þ ions react with fly ash, resulting in the precipitation of Friedel’s salt. Friedel’s salt is in a stable state, contains chlorine ions that experience great difficulty in dissolution, and can absorb heavy metals (Dai et al., 2009; Wu et al., 2010), causing the heavy metals
Table 4 The elemental analysis of the fly ash in terms of weight fractions. Element
Untreated fly ash
Treated ash A
Treated ash B
Treated ash A(r)
O Na Al Si Cl K Ca Zn Sum (%)
37.8 4.96 ea 1.68 24.9 4.84 25.9 e 100.08
69.1 0.26 2.87 0.07 4.19 2.45 20.9 e 99.84
59.4 0.14 e 1.61 4.41 0.84 29.2 2.90 98.50
53.7 e 3.47 0.71 8.36 2.83 29.3 e 98.37
a
“e” showed which element was not measured.
Fig. 3. XRD analyses of the four different types of fly ash after treatment by electrocoagulation.
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in the fly ash to be transformed into abundant associations with the strongly bound fractions rather than with the weakly bound fractions. From this research, the composition of the fly ash is found to have changed into harder material after electrocoagulation, showing that treated ashes A and A(r) are compliant with the regulatory limits and are nontoxic. 4. Conclusions This research used an innovative electrocoagulation technique to dispose of MSWI fly ash and succeeded in changing the composition of the fly ash to convert it into nonhazardous materials. The three main functions in the application of this technique are washing with water, changing the fly ash composition, and stabilizing the heavy metals in the ash. The soluble salts such as KCl, NaCl, SiCl4, and CaClOH can be washed out and cannot be determined by XRD analysis in the treated fly ash. In addition, the release of Al3þ ions that reacts with Ca- and Cl-containing compounds converts the fly ash into Friedel’s salt, which has changed the composition of the fly ash. Friedel’s salt contains insoluble chlorine atoms and can also absorb and stabilize heavy metals to allow the fly ash to comply with the TCLP regulatory limits. The use of the IrO2 anode lacks the electrocoagulation effect, and only the water washing is effective for fly ash treatment. Electrocoagulation has the potential to recover the leachate for use as the fluid to mix the fly ash with, and the converted fly ash has a potential recycling value. This work has resolved the problem of wastewater disposal, as well as the problem of reducing and stabilizing chlorine levels in the fly ash. The third problem associated with commercializing the process will be left to future work. References Abbas, Z., Moghaddam, A.P., Steenari, B.M., 2003. Release of salts from municipal solid waste combustion residues. Waste Manage. 23, 291e305. Arvelakis, S., Frandsen, F.J., 2005. Study on analysis and characterization methods for ash material from incineration plants. Fuel 84, 1725e1738. Astrup, T., Mosbæk, H., Christensen, T.H., 2006. Assessment of long-term leaching from waste incineration air-pollution-control residues. Waste Manage. 26, 803e814. Colangelo, F., Cioffi, R., Montagnaro, F., Santoro, L., 2012. Soluble salt removal from MSWI fly ash and its stabilization for safer disposal and recovery as road basement material. Waste Manage. 32, 1179e1185. Dai, Y., Qian, G., Cao, Y., Chi, Y., Xu, Y., Zhou, J., Liu, Q., Xu, Z.P., Qiao, S., 2009. Effective removal and fixation of Cr(VI) from aqueous solution with Friedel’s salt. J. Hazard. Mater. 170, 1086e1092. De Casa, G., Mangialardi, T., Paolini, A.E., Piga, L., 2007. Physical-mechanical and environmental properties of sintered municipal incinerator fly ash. Waste Manage. 27, 238e247. Eighmy, T.T., Crannell, B.S., Butler, L.G., Cartledge, F.K., Emery, E.F., Oblas, D., Krzanowski, J.E., Eusden, J.D., Shaw, E.L., Francis, C.A., 1997. Heavy metal stabilization in municipal solid waste combustion dry scrubber residue using soluble phosphate. Environ. Sci. Technol. 31, 3330e3338.
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