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Trace metals and organic pollutants in treated and untreated residues from urban solid waste incinerators
Microchemical Journal 79 (2005) 291 – 297 www.elsevier.com/locate/microc
Trace metals and organic pollutants in treated and untreated residues from urban solid waste incinerators F. Bagnoli, A. Bianchi, A. Ceccarini, R. Fuoco*, S. Giannarelli Department of Chemistry and Industrial Chemistry, Via Risorgimento 35, 56126 Pisa, Italy Received 20 September 2004 Available online 2 November 2004
Abstract Fly ash and sludge samples from two different incineration plants of urban solid waste (USW) were submitted to two different stabilization– solidification processes based both on the use of Portland cement. The efficiency of these processes to stabilize/solidify the residues and to reduce the release of pollutants was evaluated by performing mechanical and leaching tests according to the Water Research Institute (IRSACNR) standard method. The leaching test was based on the treatment of the sample with an acetic acid solution adjusted at pH=5.2 (about 0.5 M), for 24 h under magnetic stirring. Two analytical procedures for the determination of trace metals (Pb, Cd, Cu and Se) and organic pollutants [polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenils (PCBs) and organic chlorinated pesticides (OCPs)] in the various samples were optimized. Trace metals were determined by differential pulse anodic/cathodic stripping voltammetry (DPASV/DPCSV), and organic pollutants by gas chromatography coupled with mass spectrometry (GC-MS). The concentration of trace metals in raw residues and the corresponding leaching solutions were higher than the regulatory limits, whereas the stabilized–solidified residues showed a compressive strength for both mixtures higher than the suggested limit value and the concentration of all the pollutants lower than the regulatory limits, after 28 days curing time. Finally, a polyurethane resin was added as an additive to the Portland mixture, and the effect on the pollutant release was investigated. D 2004 Elsevier B.V. All rights reserved. Keywords: Urban solid waste; Incineration plant; Fly ash and sludge samples; Stabilization–solidification; Portland cement; Mechanical and leaching test; Metals; Organic pollutants
1. Introduction Incineration is a very attractive process for urban solid waste (USW) treatment since it ensures a substantial reduction of both volume and weight of USW coupled with a recovery of energy [1]. On the other hand, it produces residues, namely fly ash and sludge, which are hazardous waste, owing to their high content of both heavy metals, such as Cd, Pb, As, Se, Hg, and organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenils (PCBs) and organic chlorinated pesticides (OCPs). Therefore, for a correct environmental protection, it is mandatory to monitoring the presence of such toxic substances in the residues, and so evaluate the correspond* Corresponding author. Tel.: +39 50 2219254; fax: +39 50 2219260. E-mail address: [email protected] (R. Fuoco). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.10.001
ing environmental impact. Various chemical, physical and thermal stabilized–solidified processes have been developed [2,3] in order to obtain materials with a lower environmental impact which might be disposed in ordinary landfills, or used as building materials, i.e. road foundation or coastal breakwaters. In the latter case, the development of assessment methods for the long-term release of pollutants is necessary [4], and would also lead to a better understanding of the mechanism involved with the leaching process of contaminants [5,6]. Physical stabilization– solidification processes are based on the use of thermoplastic polymers which encapsulate the residues in a matrix that coats and disperses them [7–10]. In a few cases, treatment at high temperature (1300 8C) might also be used to convert the residues into glass materials, with a lower environmental impact [11,12]. Portland cement is the most commonly used reagent in chemical stabilization–
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solidification of hazardous waste [13–15]. Several additives, i.e., lime and clay [16], soluble silicate [17,18], coal fly ash [19], organic compounds [20,21], sand and gravel [22,23], are generally used to improve the effect on setting and curing the cement and to reduce the release of pollutants. In this respect, it should be highlighted that there is a basic need of reliable analytical procedures for the determination of these pollutants in order to correctly evaluate the effectiveness of different treatments. In this study, two different chemical processes both based on the use of Portland cement to stabilize–solidify fly ash and sludge samples obtained from two incineration plants of USW were compared. The evaluation was based on both mechanical and chemical–physical tests. Two analytical methodologies for the determination of both trace metals (Pb, Cd, Cu and Se) and organic pollutants (PAHs, PCBs and OCPs) in residues (fly ash and sludge) and leaching solutions were optimized. The accuracy and reproducibility of pollutant determination were established by analyzing marine sediment reference materials and spiked solutions, respectively. Finally, some preliminary results concerning the effect of polyurethane resin additive to the stabilization–solidification mixture were finally discussed.
(SRM 2261) standard solutions were NIST (USA) products. HS-6 and HS-2 marine sediment certified reference materials for PAHs and PCBs content, respectively, were obtained from the National Research Council of Canada and used for quality control of analytical data. A Star 3400 CX (Varian, USA) gas chromatography coupled with a ion trap mass spectrometer bSaturn 2000Q (GC-MS/MS) was used for determining PAHs, PCBs and OCPs in the final extract. The GC was equipped with a split/ splitless injector (SP 1078, Varian) whose temperature profile was as follows: initial temperature, 60 8C; a temperature increase of 200 8C/min up to 300 8C. Chromatographic separation was always performed on a fused silica capillary column MS-5 (Hewlett Packard Italiana, I) 95% dimethyl– 5% phenyl polysiloxane chemically bonded stationary phase, 0.25 mm I.D., 0.25 Am film thickness, 30 m length, connected to 2 m long deactivated fused silica capillary precolumn 0.32 mm I.D. The temperature profile of the chromatographic oven was the following: initial temperature, 50 8C isothermal for 2 min; 10 8C/min up to 120 8C and isothermal for 5 min, 4 8C/min up to 295 8C and isothermal for 60 min. Helium 99.995% purity was used as a carrier gas at 190 kPa. 2.3. Fly ash and sludge samples
2. Materials and methods 2.1. Trace metals KCl and 30% H2O2 Suprapur grade were Merck (D) products whereas 30% HCl and 25% NH3 were Baker InstraAnalyzed Reagent (NL). Na2B4O7d 10H2O RPE grade was a Carlo Erba (I) reagent. Cd(II), Pb(II) and Cu(II) 1000 ppm standard solutions were Sigma (USA) reagents. Se(IV) standard solution was Merck (D) reagent. Ultrapure water (specific resistivity N18 MVd cm) was obtained by an Elgastat UHQ (UK) system. Polarographic measures were performed by a PAR 174 polarographic analyzer (EG&G, USA), coupled with an 18-bit AutoIon board (Dionex, USA). Differential pulse anodic stripping voltammetry (DPASV) and differential pulse cathodic stripping voltammetry (DPCSV) were used for the determination of Cd, Pb, Cu and total Se, respectively. The electrochemical cell was equipped with a hanging drop mercury (HMDE), a platinum wire counter-electrode and a saturated calomel reference electrode (SCE). A microwave oven (MLS-1200 Mega, Milestone-FKV, USA), equipped with a UV generation system was used for the decomposition of the organic interferences in the sample. 2.2. Organic pollutants n-Hexane, dichloromethane and acetone pesticide grade were Carlo Erba (I) products. PAHs standard solutions were AccuStandard (USA) products; PCBs (SRM 2262) and OCPs
The residues were collected at two incinerator plants which used two different treatment processes of fumes. These plants are two of the most commonly used models for solid waste incineration. The plant A was based on a dry process that produced both fly ash (separated from the fumes with an electrostatic precipitator) and sludge (obtained after the injection of a sodium carbonate solution (30% in weight) in the scrubber tower). The sludge samples were collected in a filter press. In plant B, a calcium hydroxide powder was injected in the gaseous flow coming from the boiler. The residues produced (fly ash, calcium hydroxide in excess, calcium chloride and calcium sulphate) were separated from the fumes by a bag filter.
Table 1 Composition of the stabilization–solidification mixtures per kg of residue Components
Portland Cement 42.5 Portland Cement+CaCO3 Ca(OH)2 MgO Na2SiO3 Additives Water Total mixture
Mixture MDB (plant bAQ)
Mixture Diwamil (MD) (plant bBQ)
kg/kg fly ash
kg/kg sludge
kg/kg fly ash
0.58
1.04 1.20
0.36 0.12 0.13
0.60 0.20 0.25
0.69 1.88
1.04 3.13
0.15 4.05 5.40
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2.4. Stabilized–solidified residues
3. Analytical procedures for raw materials
The stabilization–solidification experiments were carried out by mixing either fly ash or sludge samples with two different stabilization–solidification mixtures, namely MDB and MD. Table 1 shows the composition of the mixtures along with the amount of reagents used per kg of residues, whereas Table 2 shows the composition of mixed residues obtained from plant B. The stabilized–solidified residues were prepared as follows: residues (fly ash or sludge) and stabilization–solidification mixture were mixed and water was added progressively to the mixture. After mixing for about 15 min, the mixture was transferred into cylindrical moulds (about 75 cm3 in volume). Five replicates per each category of material were prepared and cured for 28 days at room temperature. Water-soluble Idroben 201 and Idroben 270 polyurethane resins were added to the stabilization–solidification mixture. After mixing the residues and MDB mixture with 8% in weight of a resin, the mixture was transferred into cylindrical moulds, following the above-described procedure.
3.1. Trace metals
2.5. Compressive strength The compressive strength of stabilized–solidified residues was determined, after 28 days curing time, applying a compressive force by a hydraulic press CBR T 104 (Control, I), on the base of the cylindrical sample, which was steadily increased until the fracture of the blocks occurred. The compressive strength, expressed as kg/cm2, was calculated by dividing the total load (in kg) to the basis area of the cylinder (in cm2). The samples that passed the test were submitted to the leaching test according to the official standard method adopted in Italy (IRSA-CNR standard method [24]). 2.6. Leaching test The test involved the immersion of the sample in a suitable container with an amount of bidistilled water equal to 16 times the sample weight. The solution was adjusted to pH=5.2 with 0.5 M acetic acid solution and stirred for 24 h at room temperature. Every 30 min, the pH of the solution was tested and eventually adjusted. Finally, the solution was filtered through a 0.45-Am membrane filter and diluted with bidistilled water to a total volume equal to 20 times the sample weight.
Table 2 Composition of mixed residue as obtained from plant bBQ per kg of fly ash Components
kg/kg Fly ash
Ca(OH)2 CaCl2 CaSO4
2.0 1.36 0.18
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The concentration of trace metals in the raw materials was measured after the mineralization of samples with nitric and perchloric acids. In particular, nitric acid was added to the sample at room temperature. After 2 h, the suspension was heated (T=100 8C) then, after 2 h, cooled at room temperature. Nitric and perchloric acids were then added to the suspension and heated (T=100 8C) until solution decolorizing (about 30 h). The solution was reduced to 2 ml, added with 20 ml of water, heated, hot-filtered and finally diluted to 100 ml. 3.2. Organic pollutants The concentration of organic pollutants was measured after the extraction of residue samples with two aliquots of a 3:1:1 hexane/dichloromethane/acetone mixture in an ultrasonic bath for 30 min. The organic extract was separated from the sample, treated with sodium sulphate for water elimination, reduced to 1 ml under a mild nitrogen flow, and finally analyzed.
4. Analytical procedures for leaching solution 4.1. Trace metals The concentration of Cd, Pb and Cu [25] was measured in 60 ml of acetic acid solution, used in the leaching test. The solution was added with 50 Al of 30% H2O2 and 150 Al of 30% HCl, and then submitted to UV irradiation for 1 h. Once the solution had been equilibrated at room temperature, it was transferred to the voltammetric cell and deaerated for 10 min with nitrogen flow for DPASV measurement. The preelectrolysis was performed at 800 mV (vs. SCE) for 3 min under magnetic stirring. After a 30-s rest period, the potential was scanned from 800 to 50 mV with a scan rate of 5 mV/s and a pulse amplitude of 25 mV. The concentration of total Se [Se( II)+Se(IV)+Se(VI)] was measured in another 60 ml aliquot of the acetic solution which was added with a 25% NH 3 solution (final pH about 10), 1.2 ml of Na2B4O7d 10H2O 0.1 M and 60 Al of 30% H2O2. The solution was then submitted to UV irradiation for 1 h. In these conditions, Se( II) and Se (VI) species, if present in the sample, are oxidized and reduced to Se(IV), respectively [26]. Once it had been equilibrated at room temperature, HCl was added up to a pH of about 2, then it was transferred to the voltammetric cell and added with 100 Al of 1.00 g/l Cu(II) solution. Finally, the solution was deaerated for 10 min with nitrogen flow, and a DPCSV measurement was performed. A pre-electrolysis potential of 400 mV was applied for 3 min while stirring the solution. After a 30-s rest period, a potential scan was performed from 400 to 900 mV with a scan rate of 5 mV/s and a pulse amplitude of 25 mV.
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Table 3 Characteristic m/z values of ion fragments used for the determination of PAHs, PCBs and OCPs by GC-MS/MS m/z PAHs NAP ACY ACE FLU PHE, ANT F, PYR BaA, CRY BbF, BkF, BaP IP, BP DBA
70, 102, 128 98, 122, 156 98, 122, 123 86, 110, 161 122, 150, 152 199, 200, 201 224, 225, 226 249, 250, 256 265, 274, 275 276, 277, 280
PCBs PCB29, PCB28 PCB50, PCB52, PCB47, PCB44 PCB70, PCB66 PCB101 PCB87 PCB77 PCB151, PCB138, PCB 128 PCB118, PCB105, PCB126 PCB153 PCB187 PCB201 PCB180, PBC170 PCB195, PCB194 PCB206 PCB209
186, 255, 220, 289, 258, 220, 323, 254, 290, 359, 360, 359, 393, 427, 428,
188, 257, 222, 291, 291, 222, 325, 256, 325, 361, 393, 361, 395, 429, 430,
256 290 294 324 324 290 327 324 364 398 426 392 426 431 496
OCPs Esachlorobenzene (HCB) g-Lindane (g-HCH) p,p-DDE p,p-DDD, o,p-DDT p,p-DDT Mirex
249, 145, 246, 199, 165, 237,
251, 146, 248, 200, 200, 239,
282 147 281 237 237 270
4.2. Organic pollutants The organic pollutants were extracted from the acetic leaching solution with a 4:1 hexane/dichloromethane mixture for 30 min at room temperature. The organic phase was then recovered, treated with sodium sulphate for water elimination, reduced to about 1 ml under a mild nitrogen flow, and finally analyzed [27,28]. Table 3 shows a list of the pollutants that have been considered in the present study, and the corresponding m/z values of the characteristic ion fragments used for their determination. The volume injected was 40 Al.
5. Quality control of analytical data The accuracy and reproducibility of pollutants determinations by the proposed procedures were established by analyzing five aliquots of the CRM-HS-6 marine sediment reference materials for PAHs and five aliquots of the
CRM-HS-2 marine sediment reference materials for PCBs, since certified reference materials for fly ash and sludge were not available. The error percent of both PAHs and PCBs was always negative and lower than 35% with a coefficient of variation (CV) lower than 20% (Table 4). Moreover, a blank acetic acid leaching solution was spiked with Cd, Pd and Cu at two concentration levels, and a few selected PAHs, PCBs and OCPs. The results show that the error percent for PAHs, PCBs and OCPs was lower than 30%, and the CV of the mean values better than 15%. The results obtained for trace metals (Table 5) exhibit an error percent of 13% at most, and a CV always better than 14%. As for the organic pollutants, although the reproducibility is quite satisfactory, the recovery indicates the presence of a systematic loss of analytes due to both extraction and cleanup steps.
6. Results and discussion 6.1. Analysis of raw residues In order to evaluate the variability of the pollutant content in the residues generated from both incineration plants, three samplings were performed in a period of time of 18 months. Table 6 shows the total concentration of trace metals (Cd, Pb, Cu and total Se) and organic pollutants, respectively, as obtained by analyzing all fly ash and sludge samples collected during the three samplings. From these results, it comes out that both fly ash and sludge samples showed a quite good reproducibility of the pollutant level. In particular, the highest CV was found for sludge samples from plant A whose value was typically 30–40% for both trace metals and organic pollutants. Fly ash samples from the same plant showed the highest reproducibility: the CV was lower than 10% for trace metals and the PAH total content, and lower than 25% for both PCB and OCP total content. Moreover, sludge samples from plant A showed the highest trace metal concentrations, being those of Cd and Pb higher than the corresponding limit value by a factor of 5 and 1.5, respectively. Fly ash samples from plant B showed the lowest concentrations of both organic pollutants and trace metals which never exceeded the regulatory limit, with the exception of Cd whose concentration was very close to the limit value. Fly ash samples from plant A showed a similar total PAH content of sludge samples but a much higher content of BaP, PCBs, OCPs and HCB. However, it should be highlighted that the concentration values of all samples were about five orders of magnitude lower than the proposed limit value. Comparing the results of fly ash samples from both plants A and B, it follows that the total content of trace metals and organic pollutants is higher in the former by a factor of about 2 and 5, respectively. Such a difference should be explained in term of the lower content of fly ash
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Table 4 Determination of selected PAHs, PCBs and OCPs in certified marine sediment samples and in a blank spiked acetic solution Certified marine sedimenta,b
Organic pollutants
PAHs PHE FLU CRY BbF BaP BPE PCBs PCB28 PCB52 PCB101 PCB118 PCB138 PCB153 PCB180 OCPs g-HCH p,pV-DDE p,pV-DDD p,pV-DDT
Blank spiked solutions (ng/l)
Certified
Found (CV*)
3.0 (F0.6) 3.5 (F0.6) 2.0 (F0.3)
2.1 (14%) 2.2 (10%) 1.6 (20%)
30 35 20
2.2 (F0.4) 1.8 (F0.7)
2.2 (10%) 1.2 (20%)
– 35
6.9 (F0.5) 6.1 (F0.7) 3.7 (F0.3)
Error (%)
6.1 (10%) 5.4 (15%) 3.0 (15%)
12 12 19
Spiked
Found (CV*)
Error (%)
0.10 0.10 0.10
0.08 (11%) 0.09 (15%) 0.08 (7%)
20 10 20
0.10 0.10 0.10 0.10 0.10 0.10 0.10
0.08 0.08 0.08 0.08 0.09 0.09 0.07
(13%) (10%) (15%) (11%) (13%) (8%) (15%)
20 20 20 20 10 10 30
0.10 0.10 0.10 0.10
0.08 0.08 0.08 0.09
(15%) (9%) (10%) (14%)
20 20 20 10
(*) On five replicate measurements. a PAHs in CRM-HS-6 (mg/kg). b PCBs in CRM-HS-2 (Ag/kg).
in the mixed residue of plant B (Table 2). The dilution factor is about 4, although the concentration ratio of trace metals does not agree exactly with it due to the trace metals content of Ca(OH)2 used for fume treatment. Whereas, organic pollutants show differences that can be attributed to the higher efficiency of the combustion process in plant B, rather than to the dilution effect. In fact, fly ash samples from plant A showed the presence of PAHs with five aromatic condensed rings, including BaP, and PCBs with more than five chlorine atoms, while samples from plant B showed PAHs with four aromatic condensed rings (BaP was not detected) and PCBs with no more than four chlorine atoms. Finally, comparing the content of organic pollutants in sludge and fly ash samples from plant A, it follows that PAH total concentrations were quite similar, but BaP content was 10 times higher in the former. Moreover, PCB and OCP concentrations of sludge were about 10
Table 5 Determination of Cd, Pb and Cu in blank spiked acetic solution samples Trace metals
Concentration (Ag/l) Spiked 1
Found (CV*)
Error (%)
Cd
180
+8
449
Pb
824
194 (14%) 799 (1%) 70.0 (3%)
3
2061
3
181
Cu
72.4
(*) On five replicate measurements.
Spiked 2
Found (CV*)
Error (%)
460 (12%) 1803 (12%) 196 (12%)
+2 13 +8
and 100 times lower than those of fly ash samples, respectively. The differences observed for PCBs and OCPs could be explained in terms of higher volatility. 6.2. Leaching test on fly ash and sludge samples All stabilized–solidified samples were submitted to the compressive strength before analyzing them. Table 7 shows the values of compressive strength obtained of samples cured for 28 days. For all samples, the experimental values were always higher than the suggested limit value. These results show that both stabilization–solidification proceTable 6 Mean concentration of trace metals and of organic pollutants in fly ash (FA) and sludge (S) samples from plants A and B (values higher than the regulatory limits are in bold) FA-A (CV*)
FA-B (CV*)
S-A (CV*)
Trace metals (mg/kg) Cd 206 (7%) Pb 3385 (4%) Cu 1150 (1%) Total Se 13 (10%)
98 1802 444 5
551 7583 3690 24
(32%) (32%) (31%) (19%)
100 5000 5000 100
Organic pollutants (lg/kg) P PAHs 7.5 (4%) BaP 0.03 (24%) P 6.3 (27%) P PCBs OCPs 43.2 (26%) HCB 42.9 (17)
1.4 – 0.1 0.7 0.6
15.7 0.3 0.7 0.5 0.3
(25%) (38%) (44%) (36%) (44%)
– 5.105 5.105 – –
(16%) (21%) (8%) (42%)
(21%) (26%) (20%) (18%)
Regulatory limit (**)
(*) On three replicate measurements; FA=fly ash; S=sludge; (**) Ref: Italian D. L.gs N. 152/99, Tab. 3.
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Table 7 Compressive strength (kg/cm2) of stabilized–solidified fly ash (FA) and sludge (S) samples from two incineration plants (A and B), after 28 days curing time
Table 10 Concentration (mg/l) of trace metals in the 0.5 M acetic acid leaching solution of stabilized–solidified sludge (S) samples from plant A without SA(MDB) and with 8% polyurethane resin S-A(MDB)/I270
Sample
Compressive strength (kg/cm2) (CV*)
Trace metals
S-A(MDB)
S-A(MDB)/I270
FA-A(MDB) FA-B(MD) S-A(MDB) Suggested limit value (**)
32 (5%) 56 (1%) 68 (6%) 30
Cd Pb Cu Total Se
0.007 0.03 0.04 0.0003
0.005 0.006 0.003 bd.l.
(*) On three replicate measurements; (**) Ref: Quad. Ist. Ric. Acque, 64 n.3, 1984.
dures are able to give a final material with acceptable mechanical characteristics in about 3 weeks, and also that all residues behave quite similarly. Tables 8 and 9 show the concentration of trace elements and organic pollutants in the 0.5 M acetic solution used for the leaching test performed on fly ash and sludge samples (untreated and stabilized–solidified), respectively. Owing to the high content of Ca(OH)2, it was impossible to performed the leaching test on untreated fly ash samples from plant B. From these results, it comes out that fly ash samples from plant A (Table 8) showed higher concentration than the regulatory limits in the leaching acetic solution for all metals, with the exception of selenium. In particular, the values were 7.0, 12.8 and 2.8 mg/l for cadmium, lead and copper, respectively. Owing to that, the sample should be disposed in a special landfill. The situation changed quite a lot when the sample was submitted to the stabilization–solidification process using MDB mixture. The concentration values were, for all metals, below the regulatory limit, although Cd and Pb
should deserve special attention since their concentrations were quite close to the limit values. The results concerning fly ash samples from plant B (Table 8) indicate that the samples treated with MD release cadmium, lead, copper and total selenium in a very small amounts. Finally, the results of the leaching test of untreated sludge samples (Table 8), evidenced higher metal concentrations than the regulatory limits (3.4, 11.9 and 7.8 mg/l for cadmium, lead and copper, respectively). The MDB mixture applied to sludge samples evidenced a much higher efficacy at reducing the metal release. In fact, metal concentrations in the leaching solution was at least 4 times lower than the regulatory limit, with the exception of copper. Comparing the stabilization–solidification MDB and MD mixtures on the release of trace elements it follows that the concentration values for fly ash samples from plant B treated with MD were about 10 times lower than those from plant A treated with MDB. However, it is opportune remark that the amount of stabilization–solidification mixture added to 1 kg of fly ash was about 2 and 4 kg for MDB and MD, respectively. The results of leaching test indicate that fly ash
Table 8 Concentration (mg/l) of metals in the 0.5 M acetic acid leaching solution of fly ash (FA) and sludge (S) samples from plants A and B (values higher than the regulatory limits are in bold) Trace metals
Regulatory limit (*)
Fly ash (FA) samples Untreated plant A
Stabilized–solidified by MDB plant A
Stabilized–solidified by MD plant B
Sludge (S) samples Untreated
Stabilized–solidified by MDB plant A
Cd Pb Cu total Se
0.02 0.2 0.1 0.03
7.0 12.8 2.8 0.01
0.01 0.1 0.03 0.0003
0.008 0.01 0.003 0.00002
3.4 11.9 7.8 0.02
0.007 0.03 0.04 0.0003
(*) Ref: Italian D. L.gs N. 152/99, Tab. 3.
Table 9 Concentration (ng/l) of organic pollutants in the 0.5 M acetic acid leaching solution of fly ash (FA) and sludge (S) samples from plants A and B Organic pollutants
Regulatory limit (*)
Fly ash (FA) samples Stabilized–solidified by MDB plant A
Stabilized–solidified by MD plant B
Untreated
P P PAHs P PCBs OCPs
Untreated plant A
Stabilized–solidified by MDB
– – 5.104
44.1 1.8 24.1
15.6 – 0.6
21.0 0.3 4.6
256 0.3 9.8
26.5 – 0.8
(*) Ref: Italian D. L.gs N. 152/99, Tab. 3.
Sludge (S) samples
F. Bagnoli et al. / Microchemical Journal 79 (2005) 291–297 Table 11 Concentration (ng/l) of PAHs in the 0.5 M acetic acid leaching solution of stabilized–solidified fly ash (FA) and sludge (S) samples from plant A without FA-A(MDB) and S-A(MDB), and with 8% polyurethane resin FAA(MDB)/I270 and S-A(MDB)I270 Organic pollutants P PAHs BaP
FA-A (MDB)
FA-A (MDB)I270
S-A (MDB)
S-A MDB)I270
10.9 bd.l.
5.7 bd.l.
15.4 bd.l.
8.6 bd.l.
of plant A (Table 9) released 12%, 0.7% and 1% of PAHs, PCBs and OCPs, respectively. The percentages were calculated on the basis of the samples weight and the volume of the acid acetic leaching solution. The amount released further decreased for stabilized–solidified samples. In particular, PCBs were quantitatively retained in the stabilized–solidified residues. Similar conclusions were made for fly ash samples of plant B (Table 9), even if there are no concentration values for the untreated samples. PAH and OCP concentrations found in the leaching solution of sludge samples (Table 9) were 10 times higher than those of stabilized–solidified samples. In this case, PCBs were not quantitatively retained in the stabilized–solidified residues since a concentration of 0.3 ng/l was measured. Finally, comparing the stabilization–solidification MDB and MD mixtures on the release of organic pollutants, it follows that there are no substantial differences on the release of PAHs, while OCP concentrations of fly ash samples from plant B treated with MD mixture were about 8 times higher than those of plant A treated with MDB. 6.3. Effect of polyurethane resins Preliminary measurements were finally performed in order to evaluate the effect of I270 polyurethane resin added as an additive to the MDB mixture. Tables 10 and 11 show the results obtained using 8% in weight of the polyurethane resin in the stabilization–solidification of fly ash and sludge samples. As for the trace metals (Table 10), the release of Pb and Cu decreased by a factor of about 5 and 10, respectively. Fly ash samples showed a very similar release with and without resin. As for the organic pollutants (Table 11), the presence of I270 resin decreased the PAH release by a factor of about 2 both for fly ash and sludge samples. OCPs and PCBs showed no differences.
7. Conclusions MDB and MD Portland-based stabilization–solidification mixtures allow residues with a compressive strength higher than the suggested limit value to be obtained. Moreover, both the stabilization–solidification processes guarantee the respect of the current regulatory limits for all the pollutants
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considered. In particular, the stabilization–solidification mixture MD is more efficient than the MDB mixture for immobilizing lead, copper and selenium, but it is less efficient for OCPs. The use of more efficient stabilization– solidification processes based on polymeric additives looks promising for a further reduction of pollutant release and, consequently, for a better environmental protection.
References [1] J. Carlin, Chemistry and Industry (1998, November) 922. [2] J.R. Conner, S.L. Hoeffner, Critical Reviews in Environmental Science and Technology 31 (4) (1998) 325. [3] J.R. Conner, S.L. Hoeffner, Critical Reviews in Environmental Science and Technology 31 (4) (1998) 397. [4] R. Barna, Z. Rethy, A. Imyin, Y. Perrodin, P. Moszkowicz, L. TirutaBarna, Waste Management 20 (8) (2000) 741. [5] P.G. Baker, P.L. Bishop, Journal of Hazardous Materials 52 (2–3) (1997) 311. [6] X.D. Li, C.S. Poon, H. Sun, I.M.C. Lo, D.W. Kirk, Journal of Hazardous Materials 82 (3) (2001) 215. [7] H.R. Lubowitz, C.C. Wiles, Toxic and Hazardous Waste Disposal 1 (1980); P.D. Kalb, P. Colombo, J.H. Heiser, Waste Form Development Program, Annual Report, BNL-51756, Upton, NY, 1983. [8] P.D. Kalb, P. Colombo, PE Solidification of low level Wastes, Topical Report, BNL-51867, Upton, NY1984. [9] S. Balzamo, G. De Angelis, The Solid Waste 6 (6) (1992) 401. [10] P.D. Kalb, P. Colombo, J.H. Heiser, Waste Form Development Program, Annual Report, BNL-51756, Upton, NY, 1983. [11] R.P. Pojasek, Toxic and Hazardous Waste Disposal, vol. 1, Arbor Science Publisher, Ann Arbor, MI, 1979. [12] P. Wunsch, C. Greilinger, D. Bienik, A. Kettruo, Fresenius Environmental Bulletin 6 (1997) 234. [13] F.K. Cartledge, L.G. Butler, D. Chalasani, H.G. Heaton, Environmental Science and Technology 24 (1990) 673. [14] R. Derie, Waste Management 16 (8) (1996) 711. [15] G. Dinelli, P. Caramuscio, G. De Angelis, S. Balzamo, Proceedings. Nat. Symp. on Waste Incineration, Bologna, March 1995. [16] C. Collivignarelli, G. Bertanza, G. Andreottola, The Solid Waste 6 (1) (1992) 27. [17] C. Collivignarelli, G. Bertanza, G. Andreottola, The Solid Waste 6 (1) (1992) 413. [18] D.M. Montgomery, C.J. Sollars, R. Perry, Waste Management and Research 6 (1988) 217. [19] Y. Zimmel, D. Scharz, Reviews in Chemical Engineering 9 (3–4) (1993) 385. [20] D.M. Montgomery, C.J. Sollars, R. Perry, S.J.T. Pollard, Journal of Hazardous Materials 28 (1991) 313. [21] S. Balzamo, G. De Angelis, The Solid Waste 6 (6) (1992) 401. [22] L.G. Carbone, W.H. Gutemann, D.J. Lisk, Chemosphere 19 (12) (1989) 1951. [23] S. Balzamo, G. De Angelis, F. De Poli, Proc. Int. Symp. on Cem. Ind. Solutions Waste Management, 1992, p. 189. [24] Quaderni dell’Istituto Ricerche 64 (3) (1984). [25] M. Betti, R. Fuoco, P. Papoff, Annali di Chimica 79 (1989) 689, (Rome). [26] P. Papoff, F. Bocci, F. Lanza, Microchemical Journal 59 (1998) 50. [27] R. Fuoco, M.P. Colombini, C. Abete, Annali di Chimica 81 (1991) 383, (Rome). [28] R. Fuoco, M.P. Colombini, C. Abete, International Journal of Environmental Analytical Chemistry 55 (1994) 15.
Trace metals and organic pollutants in treated and untreated residues from urban solid waste incinerators F. Bagnoli, A. Bianchi, A. Ceccarini, R. Fuoco*, S. Giannarelli Department of Chemistry and Industrial Chemistry, Via Risorgimento 35, 56126 Pisa, Italy Received 20 September 2004 Available online 2 November 2004
Abstract Fly ash and sludge samples from two different incineration plants of urban solid waste (USW) were submitted to two different stabilization– solidification processes based both on the use of Portland cement. The efficiency of these processes to stabilize/solidify the residues and to reduce the release of pollutants was evaluated by performing mechanical and leaching tests according to the Water Research Institute (IRSACNR) standard method. The leaching test was based on the treatment of the sample with an acetic acid solution adjusted at pH=5.2 (about 0.5 M), for 24 h under magnetic stirring. Two analytical procedures for the determination of trace metals (Pb, Cd, Cu and Se) and organic pollutants [polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenils (PCBs) and organic chlorinated pesticides (OCPs)] in the various samples were optimized. Trace metals were determined by differential pulse anodic/cathodic stripping voltammetry (DPASV/DPCSV), and organic pollutants by gas chromatography coupled with mass spectrometry (GC-MS). The concentration of trace metals in raw residues and the corresponding leaching solutions were higher than the regulatory limits, whereas the stabilized–solidified residues showed a compressive strength for both mixtures higher than the suggested limit value and the concentration of all the pollutants lower than the regulatory limits, after 28 days curing time. Finally, a polyurethane resin was added as an additive to the Portland mixture, and the effect on the pollutant release was investigated. D 2004 Elsevier B.V. All rights reserved. Keywords: Urban solid waste; Incineration plant; Fly ash and sludge samples; Stabilization–solidification; Portland cement; Mechanical and leaching test; Metals; Organic pollutants
1. Introduction Incineration is a very attractive process for urban solid waste (USW) treatment since it ensures a substantial reduction of both volume and weight of USW coupled with a recovery of energy [1]. On the other hand, it produces residues, namely fly ash and sludge, which are hazardous waste, owing to their high content of both heavy metals, such as Cd, Pb, As, Se, Hg, and organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenils (PCBs) and organic chlorinated pesticides (OCPs). Therefore, for a correct environmental protection, it is mandatory to monitoring the presence of such toxic substances in the residues, and so evaluate the correspond* Corresponding author. Tel.: +39 50 2219254; fax: +39 50 2219260. E-mail address: [email protected] (R. Fuoco). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.10.001
ing environmental impact. Various chemical, physical and thermal stabilized–solidified processes have been developed [2,3] in order to obtain materials with a lower environmental impact which might be disposed in ordinary landfills, or used as building materials, i.e. road foundation or coastal breakwaters. In the latter case, the development of assessment methods for the long-term release of pollutants is necessary [4], and would also lead to a better understanding of the mechanism involved with the leaching process of contaminants [5,6]. Physical stabilization– solidification processes are based on the use of thermoplastic polymers which encapsulate the residues in a matrix that coats and disperses them [7–10]. In a few cases, treatment at high temperature (1300 8C) might also be used to convert the residues into glass materials, with a lower environmental impact [11,12]. Portland cement is the most commonly used reagent in chemical stabilization–
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solidification of hazardous waste [13–15]. Several additives, i.e., lime and clay [16], soluble silicate [17,18], coal fly ash [19], organic compounds [20,21], sand and gravel [22,23], are generally used to improve the effect on setting and curing the cement and to reduce the release of pollutants. In this respect, it should be highlighted that there is a basic need of reliable analytical procedures for the determination of these pollutants in order to correctly evaluate the effectiveness of different treatments. In this study, two different chemical processes both based on the use of Portland cement to stabilize–solidify fly ash and sludge samples obtained from two incineration plants of USW were compared. The evaluation was based on both mechanical and chemical–physical tests. Two analytical methodologies for the determination of both trace metals (Pb, Cd, Cu and Se) and organic pollutants (PAHs, PCBs and OCPs) in residues (fly ash and sludge) and leaching solutions were optimized. The accuracy and reproducibility of pollutant determination were established by analyzing marine sediment reference materials and spiked solutions, respectively. Finally, some preliminary results concerning the effect of polyurethane resin additive to the stabilization–solidification mixture were finally discussed.
(SRM 2261) standard solutions were NIST (USA) products. HS-6 and HS-2 marine sediment certified reference materials for PAHs and PCBs content, respectively, were obtained from the National Research Council of Canada and used for quality control of analytical data. A Star 3400 CX (Varian, USA) gas chromatography coupled with a ion trap mass spectrometer bSaturn 2000Q (GC-MS/MS) was used for determining PAHs, PCBs and OCPs in the final extract. The GC was equipped with a split/ splitless injector (SP 1078, Varian) whose temperature profile was as follows: initial temperature, 60 8C; a temperature increase of 200 8C/min up to 300 8C. Chromatographic separation was always performed on a fused silica capillary column MS-5 (Hewlett Packard Italiana, I) 95% dimethyl– 5% phenyl polysiloxane chemically bonded stationary phase, 0.25 mm I.D., 0.25 Am film thickness, 30 m length, connected to 2 m long deactivated fused silica capillary precolumn 0.32 mm I.D. The temperature profile of the chromatographic oven was the following: initial temperature, 50 8C isothermal for 2 min; 10 8C/min up to 120 8C and isothermal for 5 min, 4 8C/min up to 295 8C and isothermal for 60 min. Helium 99.995% purity was used as a carrier gas at 190 kPa. 2.3. Fly ash and sludge samples
2. Materials and methods 2.1. Trace metals KCl and 30% H2O2 Suprapur grade were Merck (D) products whereas 30% HCl and 25% NH3 were Baker InstraAnalyzed Reagent (NL). Na2B4O7d 10H2O RPE grade was a Carlo Erba (I) reagent. Cd(II), Pb(II) and Cu(II) 1000 ppm standard solutions were Sigma (USA) reagents. Se(IV) standard solution was Merck (D) reagent. Ultrapure water (specific resistivity N18 MVd cm) was obtained by an Elgastat UHQ (UK) system. Polarographic measures were performed by a PAR 174 polarographic analyzer (EG&G, USA), coupled with an 18-bit AutoIon board (Dionex, USA). Differential pulse anodic stripping voltammetry (DPASV) and differential pulse cathodic stripping voltammetry (DPCSV) were used for the determination of Cd, Pb, Cu and total Se, respectively. The electrochemical cell was equipped with a hanging drop mercury (HMDE), a platinum wire counter-electrode and a saturated calomel reference electrode (SCE). A microwave oven (MLS-1200 Mega, Milestone-FKV, USA), equipped with a UV generation system was used for the decomposition of the organic interferences in the sample. 2.2. Organic pollutants n-Hexane, dichloromethane and acetone pesticide grade were Carlo Erba (I) products. PAHs standard solutions were AccuStandard (USA) products; PCBs (SRM 2262) and OCPs
The residues were collected at two incinerator plants which used two different treatment processes of fumes. These plants are two of the most commonly used models for solid waste incineration. The plant A was based on a dry process that produced both fly ash (separated from the fumes with an electrostatic precipitator) and sludge (obtained after the injection of a sodium carbonate solution (30% in weight) in the scrubber tower). The sludge samples were collected in a filter press. In plant B, a calcium hydroxide powder was injected in the gaseous flow coming from the boiler. The residues produced (fly ash, calcium hydroxide in excess, calcium chloride and calcium sulphate) were separated from the fumes by a bag filter.
Table 1 Composition of the stabilization–solidification mixtures per kg of residue Components
Portland Cement 42.5 Portland Cement+CaCO3 Ca(OH)2 MgO Na2SiO3 Additives Water Total mixture
Mixture MDB (plant bAQ)
Mixture Diwamil (MD) (plant bBQ)
kg/kg fly ash
kg/kg sludge
kg/kg fly ash
0.58
1.04 1.20
0.36 0.12 0.13
0.60 0.20 0.25
0.69 1.88
1.04 3.13
0.15 4.05 5.40
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2.4. Stabilized–solidified residues
3. Analytical procedures for raw materials
The stabilization–solidification experiments were carried out by mixing either fly ash or sludge samples with two different stabilization–solidification mixtures, namely MDB and MD. Table 1 shows the composition of the mixtures along with the amount of reagents used per kg of residues, whereas Table 2 shows the composition of mixed residues obtained from plant B. The stabilized–solidified residues were prepared as follows: residues (fly ash or sludge) and stabilization–solidification mixture were mixed and water was added progressively to the mixture. After mixing for about 15 min, the mixture was transferred into cylindrical moulds (about 75 cm3 in volume). Five replicates per each category of material were prepared and cured for 28 days at room temperature. Water-soluble Idroben 201 and Idroben 270 polyurethane resins were added to the stabilization–solidification mixture. After mixing the residues and MDB mixture with 8% in weight of a resin, the mixture was transferred into cylindrical moulds, following the above-described procedure.
3.1. Trace metals
2.5. Compressive strength The compressive strength of stabilized–solidified residues was determined, after 28 days curing time, applying a compressive force by a hydraulic press CBR T 104 (Control, I), on the base of the cylindrical sample, which was steadily increased until the fracture of the blocks occurred. The compressive strength, expressed as kg/cm2, was calculated by dividing the total load (in kg) to the basis area of the cylinder (in cm2). The samples that passed the test were submitted to the leaching test according to the official standard method adopted in Italy (IRSA-CNR standard method [24]). 2.6. Leaching test The test involved the immersion of the sample in a suitable container with an amount of bidistilled water equal to 16 times the sample weight. The solution was adjusted to pH=5.2 with 0.5 M acetic acid solution and stirred for 24 h at room temperature. Every 30 min, the pH of the solution was tested and eventually adjusted. Finally, the solution was filtered through a 0.45-Am membrane filter and diluted with bidistilled water to a total volume equal to 20 times the sample weight.
Table 2 Composition of mixed residue as obtained from plant bBQ per kg of fly ash Components
kg/kg Fly ash
Ca(OH)2 CaCl2 CaSO4
2.0 1.36 0.18
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The concentration of trace metals in the raw materials was measured after the mineralization of samples with nitric and perchloric acids. In particular, nitric acid was added to the sample at room temperature. After 2 h, the suspension was heated (T=100 8C) then, after 2 h, cooled at room temperature. Nitric and perchloric acids were then added to the suspension and heated (T=100 8C) until solution decolorizing (about 30 h). The solution was reduced to 2 ml, added with 20 ml of water, heated, hot-filtered and finally diluted to 100 ml. 3.2. Organic pollutants The concentration of organic pollutants was measured after the extraction of residue samples with two aliquots of a 3:1:1 hexane/dichloromethane/acetone mixture in an ultrasonic bath for 30 min. The organic extract was separated from the sample, treated with sodium sulphate for water elimination, reduced to 1 ml under a mild nitrogen flow, and finally analyzed.
4. Analytical procedures for leaching solution 4.1. Trace metals The concentration of Cd, Pb and Cu [25] was measured in 60 ml of acetic acid solution, used in the leaching test. The solution was added with 50 Al of 30% H2O2 and 150 Al of 30% HCl, and then submitted to UV irradiation for 1 h. Once the solution had been equilibrated at room temperature, it was transferred to the voltammetric cell and deaerated for 10 min with nitrogen flow for DPASV measurement. The preelectrolysis was performed at 800 mV (vs. SCE) for 3 min under magnetic stirring. After a 30-s rest period, the potential was scanned from 800 to 50 mV with a scan rate of 5 mV/s and a pulse amplitude of 25 mV. The concentration of total Se [Se( II)+Se(IV)+Se(VI)] was measured in another 60 ml aliquot of the acetic solution which was added with a 25% NH 3 solution (final pH about 10), 1.2 ml of Na2B4O7d 10H2O 0.1 M and 60 Al of 30% H2O2. The solution was then submitted to UV irradiation for 1 h. In these conditions, Se( II) and Se (VI) species, if present in the sample, are oxidized and reduced to Se(IV), respectively [26]. Once it had been equilibrated at room temperature, HCl was added up to a pH of about 2, then it was transferred to the voltammetric cell and added with 100 Al of 1.00 g/l Cu(II) solution. Finally, the solution was deaerated for 10 min with nitrogen flow, and a DPCSV measurement was performed. A pre-electrolysis potential of 400 mV was applied for 3 min while stirring the solution. After a 30-s rest period, a potential scan was performed from 400 to 900 mV with a scan rate of 5 mV/s and a pulse amplitude of 25 mV.
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Table 3 Characteristic m/z values of ion fragments used for the determination of PAHs, PCBs and OCPs by GC-MS/MS m/z PAHs NAP ACY ACE FLU PHE, ANT F, PYR BaA, CRY BbF, BkF, BaP IP, BP DBA
70, 102, 128 98, 122, 156 98, 122, 123 86, 110, 161 122, 150, 152 199, 200, 201 224, 225, 226 249, 250, 256 265, 274, 275 276, 277, 280
PCBs PCB29, PCB28 PCB50, PCB52, PCB47, PCB44 PCB70, PCB66 PCB101 PCB87 PCB77 PCB151, PCB138, PCB 128 PCB118, PCB105, PCB126 PCB153 PCB187 PCB201 PCB180, PBC170 PCB195, PCB194 PCB206 PCB209
186, 255, 220, 289, 258, 220, 323, 254, 290, 359, 360, 359, 393, 427, 428,
188, 257, 222, 291, 291, 222, 325, 256, 325, 361, 393, 361, 395, 429, 430,
256 290 294 324 324 290 327 324 364 398 426 392 426 431 496
OCPs Esachlorobenzene (HCB) g-Lindane (g-HCH) p,p-DDE p,p-DDD, o,p-DDT p,p-DDT Mirex
249, 145, 246, 199, 165, 237,
251, 146, 248, 200, 200, 239,
282 147 281 237 237 270
4.2. Organic pollutants The organic pollutants were extracted from the acetic leaching solution with a 4:1 hexane/dichloromethane mixture for 30 min at room temperature. The organic phase was then recovered, treated with sodium sulphate for water elimination, reduced to about 1 ml under a mild nitrogen flow, and finally analyzed [27,28]. Table 3 shows a list of the pollutants that have been considered in the present study, and the corresponding m/z values of the characteristic ion fragments used for their determination. The volume injected was 40 Al.
5. Quality control of analytical data The accuracy and reproducibility of pollutants determinations by the proposed procedures were established by analyzing five aliquots of the CRM-HS-6 marine sediment reference materials for PAHs and five aliquots of the
CRM-HS-2 marine sediment reference materials for PCBs, since certified reference materials for fly ash and sludge were not available. The error percent of both PAHs and PCBs was always negative and lower than 35% with a coefficient of variation (CV) lower than 20% (Table 4). Moreover, a blank acetic acid leaching solution was spiked with Cd, Pd and Cu at two concentration levels, and a few selected PAHs, PCBs and OCPs. The results show that the error percent for PAHs, PCBs and OCPs was lower than 30%, and the CV of the mean values better than 15%. The results obtained for trace metals (Table 5) exhibit an error percent of 13% at most, and a CV always better than 14%. As for the organic pollutants, although the reproducibility is quite satisfactory, the recovery indicates the presence of a systematic loss of analytes due to both extraction and cleanup steps.
6. Results and discussion 6.1. Analysis of raw residues In order to evaluate the variability of the pollutant content in the residues generated from both incineration plants, three samplings were performed in a period of time of 18 months. Table 6 shows the total concentration of trace metals (Cd, Pb, Cu and total Se) and organic pollutants, respectively, as obtained by analyzing all fly ash and sludge samples collected during the three samplings. From these results, it comes out that both fly ash and sludge samples showed a quite good reproducibility of the pollutant level. In particular, the highest CV was found for sludge samples from plant A whose value was typically 30–40% for both trace metals and organic pollutants. Fly ash samples from the same plant showed the highest reproducibility: the CV was lower than 10% for trace metals and the PAH total content, and lower than 25% for both PCB and OCP total content. Moreover, sludge samples from plant A showed the highest trace metal concentrations, being those of Cd and Pb higher than the corresponding limit value by a factor of 5 and 1.5, respectively. Fly ash samples from plant B showed the lowest concentrations of both organic pollutants and trace metals which never exceeded the regulatory limit, with the exception of Cd whose concentration was very close to the limit value. Fly ash samples from plant A showed a similar total PAH content of sludge samples but a much higher content of BaP, PCBs, OCPs and HCB. However, it should be highlighted that the concentration values of all samples were about five orders of magnitude lower than the proposed limit value. Comparing the results of fly ash samples from both plants A and B, it follows that the total content of trace metals and organic pollutants is higher in the former by a factor of about 2 and 5, respectively. Such a difference should be explained in term of the lower content of fly ash
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Table 4 Determination of selected PAHs, PCBs and OCPs in certified marine sediment samples and in a blank spiked acetic solution Certified marine sedimenta,b
Organic pollutants
PAHs PHE FLU CRY BbF BaP BPE PCBs PCB28 PCB52 PCB101 PCB118 PCB138 PCB153 PCB180 OCPs g-HCH p,pV-DDE p,pV-DDD p,pV-DDT
Blank spiked solutions (ng/l)
Certified
Found (CV*)
3.0 (F0.6) 3.5 (F0.6) 2.0 (F0.3)
2.1 (14%) 2.2 (10%) 1.6 (20%)
30 35 20
2.2 (F0.4) 1.8 (F0.7)
2.2 (10%) 1.2 (20%)
– 35
6.9 (F0.5) 6.1 (F0.7) 3.7 (F0.3)
Error (%)
6.1 (10%) 5.4 (15%) 3.0 (15%)
12 12 19
Spiked
Found (CV*)
Error (%)
0.10 0.10 0.10
0.08 (11%) 0.09 (15%) 0.08 (7%)
20 10 20
0.10 0.10 0.10 0.10 0.10 0.10 0.10
0.08 0.08 0.08 0.08 0.09 0.09 0.07
(13%) (10%) (15%) (11%) (13%) (8%) (15%)
20 20 20 20 10 10 30
0.10 0.10 0.10 0.10
0.08 0.08 0.08 0.09
(15%) (9%) (10%) (14%)
20 20 20 10
(*) On five replicate measurements. a PAHs in CRM-HS-6 (mg/kg). b PCBs in CRM-HS-2 (Ag/kg).
in the mixed residue of plant B (Table 2). The dilution factor is about 4, although the concentration ratio of trace metals does not agree exactly with it due to the trace metals content of Ca(OH)2 used for fume treatment. Whereas, organic pollutants show differences that can be attributed to the higher efficiency of the combustion process in plant B, rather than to the dilution effect. In fact, fly ash samples from plant A showed the presence of PAHs with five aromatic condensed rings, including BaP, and PCBs with more than five chlorine atoms, while samples from plant B showed PAHs with four aromatic condensed rings (BaP was not detected) and PCBs with no more than four chlorine atoms. Finally, comparing the content of organic pollutants in sludge and fly ash samples from plant A, it follows that PAH total concentrations were quite similar, but BaP content was 10 times higher in the former. Moreover, PCB and OCP concentrations of sludge were about 10
Table 5 Determination of Cd, Pb and Cu in blank spiked acetic solution samples Trace metals
Concentration (Ag/l) Spiked 1
Found (CV*)
Error (%)
Cd
180
+8
449
Pb
824
194 (14%) 799 (1%) 70.0 (3%)
3
2061
3
181
Cu
72.4
(*) On five replicate measurements.
Spiked 2
Found (CV*)
Error (%)
460 (12%) 1803 (12%) 196 (12%)
+2 13 +8
and 100 times lower than those of fly ash samples, respectively. The differences observed for PCBs and OCPs could be explained in terms of higher volatility. 6.2. Leaching test on fly ash and sludge samples All stabilized–solidified samples were submitted to the compressive strength before analyzing them. Table 7 shows the values of compressive strength obtained of samples cured for 28 days. For all samples, the experimental values were always higher than the suggested limit value. These results show that both stabilization–solidification proceTable 6 Mean concentration of trace metals and of organic pollutants in fly ash (FA) and sludge (S) samples from plants A and B (values higher than the regulatory limits are in bold) FA-A (CV*)
FA-B (CV*)
S-A (CV*)
Trace metals (mg/kg) Cd 206 (7%) Pb 3385 (4%) Cu 1150 (1%) Total Se 13 (10%)
98 1802 444 5
551 7583 3690 24
(32%) (32%) (31%) (19%)
100 5000 5000 100
Organic pollutants (lg/kg) P PAHs 7.5 (4%) BaP 0.03 (24%) P 6.3 (27%) P PCBs OCPs 43.2 (26%) HCB 42.9 (17)
1.4 – 0.1 0.7 0.6
15.7 0.3 0.7 0.5 0.3
(25%) (38%) (44%) (36%) (44%)
– 5.105 5.105 – –
(16%) (21%) (8%) (42%)
(21%) (26%) (20%) (18%)
Regulatory limit (**)
(*) On three replicate measurements; FA=fly ash; S=sludge; (**) Ref: Italian D. L.gs N. 152/99, Tab. 3.
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Table 7 Compressive strength (kg/cm2) of stabilized–solidified fly ash (FA) and sludge (S) samples from two incineration plants (A and B), after 28 days curing time
Table 10 Concentration (mg/l) of trace metals in the 0.5 M acetic acid leaching solution of stabilized–solidified sludge (S) samples from plant A without SA(MDB) and with 8% polyurethane resin S-A(MDB)/I270
Sample
Compressive strength (kg/cm2) (CV*)
Trace metals
S-A(MDB)
S-A(MDB)/I270
FA-A(MDB) FA-B(MD) S-A(MDB) Suggested limit value (**)
32 (5%) 56 (1%) 68 (6%) 30
Cd Pb Cu Total Se
0.007 0.03 0.04 0.0003
0.005 0.006 0.003 bd.l.
(*) On three replicate measurements; (**) Ref: Quad. Ist. Ric. Acque, 64 n.3, 1984.
dures are able to give a final material with acceptable mechanical characteristics in about 3 weeks, and also that all residues behave quite similarly. Tables 8 and 9 show the concentration of trace elements and organic pollutants in the 0.5 M acetic solution used for the leaching test performed on fly ash and sludge samples (untreated and stabilized–solidified), respectively. Owing to the high content of Ca(OH)2, it was impossible to performed the leaching test on untreated fly ash samples from plant B. From these results, it comes out that fly ash samples from plant A (Table 8) showed higher concentration than the regulatory limits in the leaching acetic solution for all metals, with the exception of selenium. In particular, the values were 7.0, 12.8 and 2.8 mg/l for cadmium, lead and copper, respectively. Owing to that, the sample should be disposed in a special landfill. The situation changed quite a lot when the sample was submitted to the stabilization–solidification process using MDB mixture. The concentration values were, for all metals, below the regulatory limit, although Cd and Pb
should deserve special attention since their concentrations were quite close to the limit values. The results concerning fly ash samples from plant B (Table 8) indicate that the samples treated with MD release cadmium, lead, copper and total selenium in a very small amounts. Finally, the results of the leaching test of untreated sludge samples (Table 8), evidenced higher metal concentrations than the regulatory limits (3.4, 11.9 and 7.8 mg/l for cadmium, lead and copper, respectively). The MDB mixture applied to sludge samples evidenced a much higher efficacy at reducing the metal release. In fact, metal concentrations in the leaching solution was at least 4 times lower than the regulatory limit, with the exception of copper. Comparing the stabilization–solidification MDB and MD mixtures on the release of trace elements it follows that the concentration values for fly ash samples from plant B treated with MD were about 10 times lower than those from plant A treated with MDB. However, it is opportune remark that the amount of stabilization–solidification mixture added to 1 kg of fly ash was about 2 and 4 kg for MDB and MD, respectively. The results of leaching test indicate that fly ash
Table 8 Concentration (mg/l) of metals in the 0.5 M acetic acid leaching solution of fly ash (FA) and sludge (S) samples from plants A and B (values higher than the regulatory limits are in bold) Trace metals
Regulatory limit (*)
Fly ash (FA) samples Untreated plant A
Stabilized–solidified by MDB plant A
Stabilized–solidified by MD plant B
Sludge (S) samples Untreated
Stabilized–solidified by MDB plant A
Cd Pb Cu total Se
0.02 0.2 0.1 0.03
7.0 12.8 2.8 0.01
0.01 0.1 0.03 0.0003
0.008 0.01 0.003 0.00002
3.4 11.9 7.8 0.02
0.007 0.03 0.04 0.0003
(*) Ref: Italian D. L.gs N. 152/99, Tab. 3.
Table 9 Concentration (ng/l) of organic pollutants in the 0.5 M acetic acid leaching solution of fly ash (FA) and sludge (S) samples from plants A and B Organic pollutants
Regulatory limit (*)
Fly ash (FA) samples Stabilized–solidified by MDB plant A
Stabilized–solidified by MD plant B
Untreated
P P PAHs P PCBs OCPs
Untreated plant A
Stabilized–solidified by MDB
– – 5.104
44.1 1.8 24.1
15.6 – 0.6
21.0 0.3 4.6
256 0.3 9.8
26.5 – 0.8
(*) Ref: Italian D. L.gs N. 152/99, Tab. 3.
Sludge (S) samples
F. Bagnoli et al. / Microchemical Journal 79 (2005) 291–297 Table 11 Concentration (ng/l) of PAHs in the 0.5 M acetic acid leaching solution of stabilized–solidified fly ash (FA) and sludge (S) samples from plant A without FA-A(MDB) and S-A(MDB), and with 8% polyurethane resin FAA(MDB)/I270 and S-A(MDB)I270 Organic pollutants P PAHs BaP
FA-A (MDB)
FA-A (MDB)I270
S-A (MDB)
S-A MDB)I270
10.9 bd.l.
5.7 bd.l.
15.4 bd.l.
8.6 bd.l.
of plant A (Table 9) released 12%, 0.7% and 1% of PAHs, PCBs and OCPs, respectively. The percentages were calculated on the basis of the samples weight and the volume of the acid acetic leaching solution. The amount released further decreased for stabilized–solidified samples. In particular, PCBs were quantitatively retained in the stabilized–solidified residues. Similar conclusions were made for fly ash samples of plant B (Table 9), even if there are no concentration values for the untreated samples. PAH and OCP concentrations found in the leaching solution of sludge samples (Table 9) were 10 times higher than those of stabilized–solidified samples. In this case, PCBs were not quantitatively retained in the stabilized–solidified residues since a concentration of 0.3 ng/l was measured. Finally, comparing the stabilization–solidification MDB and MD mixtures on the release of organic pollutants, it follows that there are no substantial differences on the release of PAHs, while OCP concentrations of fly ash samples from plant B treated with MD mixture were about 8 times higher than those of plant A treated with MDB. 6.3. Effect of polyurethane resins Preliminary measurements were finally performed in order to evaluate the effect of I270 polyurethane resin added as an additive to the MDB mixture. Tables 10 and 11 show the results obtained using 8% in weight of the polyurethane resin in the stabilization–solidification of fly ash and sludge samples. As for the trace metals (Table 10), the release of Pb and Cu decreased by a factor of about 5 and 10, respectively. Fly ash samples showed a very similar release with and without resin. As for the organic pollutants (Table 11), the presence of I270 resin decreased the PAH release by a factor of about 2 both for fly ash and sludge samples. OCPs and PCBs showed no differences.
7. Conclusions MDB and MD Portland-based stabilization–solidification mixtures allow residues with a compressive strength higher than the suggested limit value to be obtained. Moreover, both the stabilization–solidification processes guarantee the respect of the current regulatory limits for all the pollutants
297
considered. In particular, the stabilization–solidification mixture MD is more efficient than the MDB mixture for immobilizing lead, copper and selenium, but it is less efficient for OCPs. The use of more efficient stabilization– solidification processes based on polymeric additives looks promising for a further reduction of pollutant release and, consequently, for a better environmental protection.
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