Environ. Sci. Technol. 1993, 27, 652-660
Mineralogy and Surface Properties of Municipal Solid Waste Ash Carl S. Kirby' and J. Donald Rlmstldt
Department of Geology, Virginia Polytechnlc Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 2406 1-0420 Bulk chemical analysis shows most elements enriched over average soil abundances except for Si, which is 60%of the concentration in soils. Eleven minerals were identified using powder X-ray diffraction (XRD). Standard additions using XRD gave the following weight percent of minerals (f2a) in combined bottom and fly ash: gypsum, 1.8 f 1.9; hematite, 3.7 f 1.7; quartz, 2.3 f 1.0; spinel, -3.5; halite, 0.5 f 0.4; calcite 3.5 & 1.9; rutile, 1.1f 1.3. Mullite, sylvite,anhydrite, and wiistite were also identified. The ash contains 18% minerals, 9% structural and adsorbed water, and 72 % glass. An estimated composite glass composition is reported. Chemical sequential extraction showed that most Cr is present in phases resistant to chemical weathering, while significant Cd and P b are sequestered in acid-soluble (carbonate) phases. Little of these toxic trace metals are water soluble or in exchangeable surface sites. The ash has a pH-dependent cation exchange capacity of 25-55 mequivl100 g, suggesting surface sites are available to sequester metals.
Introduction Worldwide there is an ever-increasing production of refuse, and increasingly, industrialized countries are turning to incineration as a way to manage municipal and hazardous wastes. The main advantage for incinerating these wastes is volume reduction. In the case of municipal refuse, there is a volume reduction from trash to ash of 90 % ;thus ash landfills fill one-tenth as quickly as refuse landfills. Incineration also transforms the chemical energy stored in refuse into a usable form, usually electricity or heat (steam). This transformation is commonly termed waste to energy (WTE). Municipal solid waste (MSW) ash may be a potential hazard due to the toxic elements and organic compounds that are enriched in it. Effective and safe handling of ash depends upon thorough knowledge of its chemical properties. Conversely, MSW ash may be a resource due to the enrichment of scarce elements over their abundances in average soils and rocks. Economical extraction of scarce elements requires knowledge of how the elements are bound in ash. Ash has also been used as a construction material on a limited basis. A thorough knowledge of its chemical and mineralogical properties will help engineers assess its chemical and physical durability. Lisk (I) and Ujihara and Gough (2) presented reviews of the current state of knowledge about MSW ash. This paper describes methods for characterizing the basic physicochemical properties of the inorganic portion of MSW ash. We examine the major element chemistry and demonstrate methods to quantify the mineralogy and distribution of elements among various chemical phases. We present a method for quantifying surface charge which relates to the ability of ash to adsorb trace metals on surface sites. We examine how trace metals (Pb, Cr, Cd) are sequestered in various phases using several techniques. The techniques presented here to study MSW ash are applicable to coal ash, hazardous waste ash, slags, refuse
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from gassification, mixtures of ashes, and incinerated contaminated soil as well as MSW ash. As new incineration technologies are developed, these methods of characterizing the solid residue will continue to be useful. For example, the first incineration of trash combined with hazardous waste is being proposed now in Virginia (3). The effects on ash properties of mixing waste streams can be evaluated using our techniques. Data derived from these methods will aid regulators and plant operators to develop methods to handle MSW ash in the short term (e.g., setting exposure limits for workers, suggesting ash handling techniques). In addition, these data can be used as a basis for geochemical modeling of long-term ash stabilization. Such models will aid engineers in developing disposal methods that are environmentally benign over the long term.
Methods Sampling. The ash used in this study came from the University City Resource Recovery Facility (UCRRF) in Charlotte, NC. This incinerator facility handles -2 x lo5 kg/day of mainly residential refuse. A voluntary curbside recycling program removes some aluminum, newsprint, and glass prior to incineration. The plant generates electricity, and it sells waste steam for space heating in the cooler months. The twin furnaces operate at -1000 OC. Fly ash is collected by an electrostatic precipitator and mixed with bottom ash on site. The combined ash is then wetted with water for dust control. All ash used in this study was bottom and fly ash combined at the incinerator. No attempt was made in the sampling to obtain a truly representative sample of the ash stream of the UCRRF because our goal was to develop a general procedure to study MSW ash rather than to characterize the ash from this particular facility. Therefore, the ash examined in this study is not a representative average sample of UCRRF ash because spatial and temporal variations due to extreme heterogeneity of the refuse stream are not addressed by our grab samples. In addition, we presorted much of the metal from the ash. We chose to concentrate on the development of techniques which allow thorough inorganic characterization of ashes. The protocols developed here are designed to characterize ashes from a variety of sources (e.g., MSW and hazardous waste incinerators, domestic stoves and furnaces, coal-fired power plants). However, to characterize ash from a particular facility, care must be taken to ensure that sampling procedures account for the spatial and temporal variations in ashes (4). Approximately 15 kg of freshly wetted ash was collected. Except as noted, the ash used in this study was dried at 55 O C , run through a jaw crusher, hand sorted for metal particles, cone and quartered to l/ls splits, and sieved to less than 20 mesh. Then a riffle splitter was used to homogenize the ash into l/2048 splits. Bulk Chemical Analysis. Three bulk chemical analyses were performed at an outside laboratory (Chemex Labs; Sparks, NV) using wet chemical techniques. Assay 0013-936X/93/0927-0652$04.00/0
0 1993 American Chemical Society
Table I. Bulk Chemical Analysis of Ash and Estimated Weight Percent of Oxides in the Composite Glass Fraction*
species
%f2a
estimated wt % in av glass
Si02
33.4 f 0.8
42
A1203 LOP CaO Fez03 NazO SO4
13.4 f 6.6 9.7 f 4.6 9.1 f 1.0 8.7 f 3.6 4.2 f 0.2 3.5 f 0.8
13 10 9
K2O Ti02 Mi30 ZnO PZOS
2 0.7
c1
1.7 f 0.4 1.6 f 0.1 1.5 f 0.4 >1.25 f ? 1.1f 0.52 1.04 f 0.08
PbO MnOz cuzo BaO Bz03 Crz03 SrO CdO SnOz
0.41 f 0.12 0.24 f 0.08 0.22 f 0.06 0.12 f 0.06 (5.5 f 0.4) X le2 (5.0 f 0.4) X 1k2 (2.6 f 0.2) X (1.6 f 0.6) X (1.4 f 0.2) X e
0.4 0.2 0.2 0.1
NiO vzo5 AgzO coo SeOz Hg Au
(1.4 f 0.6) X (1.1f 0.4) X (3.0 zk 1.8) X 103 (2.3 f 1.2) x 10-3 2.0 x 10-3 d 5.0 X 10-4 5.0 X
1 x 10-2 1 x 10-2 3 X 1P3 2 x 10-3 2 x 10-3 5 x 106 5 x 105
sum
91.3 f 9.0
91
2 5
3
1 1 1 0
1 x 10-1 1x 10-1
3 x 10-2 2 x 10-2 1x 10-2
bulk anal. method carbonate fusion, gravimetric ICP gravimetric ICP ICP ICP induction furnace, Leco-IR detector ICP ICP ICP ICP ICP AgCl precipitation, titration AAS ICP ICP ICP neutron activation ICP ICP ICP NHJ sublimation, extraction, AAS ICP ICP AAS ICP AAS flameless AAS fire assay, AAS
a Values reported are averages for n = 3 analyses except where noted. Cu was above linear range for method used. LOI, loss on ignition. n = 1. n = 2.
type and digestion methods are listed in Table I. One aliquot of ash was analyzed without having passed through a sieve or riffle splitter. The second and third aliquots were passed thorough the riffle splitter. Organic content was determined by rinsing ash twice for 3 h with a liquid to solid ratio of 10 to remove soluble salts, drying, adding 30 5% Hz02 to the preweighed ash, evaporating to dryness, and weighing. Loss on ignition (LOI) involved drying ash overnight at 60 "C, recording initial mass, firing at 600 "C in a muffle furnace for 6 h, and then recording mass upon removal from oven. Physical and Chemical Separations. A divide and conquer approach is an essential tool for studying a heterogeneous material such as MSW ash. The physical and chemical separations necessary to concentrate similar phases were performed on the bulk ash for X-ray and electron microscope analysis as described below. Acetylene tetrabromide (sp gr 2.96) was employed as a heavy liquid to isolate the densest phases. An electromagnet was used to separate magnetic materials. Suspended particles from acetone suspensions were employed to isolate small particles on the basis of densityand particle shape. Soluble salts were isolated by dissolution in deionized water and subsequent reprecipitation at room temperature. Ultrasonic dispersion in acetone to reduce agglomeration of particles preceded magnetic, suspension, and heavy liquid separations; this step greatly aided separation of phases.
However, many phases are physically inseparable due to intergrowths and exsolution textures. A hydrofluoric acid leach based on Hulett et al. (5) was employed to preferentially attack glasses over silicate mineral phases. A 0.75-g sample of ash was washed successively for 15 min with 10 mL of deionized water, 5 mL of concentrated HN03,5 mL of concentrated HC1, 5 mL of 0.25 moVkg ethylenediaminetetraacetic acid (EDTA),and 5 mL of concentrated NHdOH, followed by 4-24 h with 10 mL of 1% HF, with 10-mL deionized water rinses between steps. X-ray Diffraction (XRD). A Scintag powder X-ray diffractometer automated by the Scintag Diffraction Management System was used for all XRD work. Acetone slurries of ash ground in acetone for at least 5 min were prepared on glass slides; samples for standard additions were ground 25 min. Cu Kar radiation was used with a Ni filter. The accelerating voltage was 45 kV, and the current was 35 mA. Data were recordeddigitally, and peak position and intensity were determined either on screen or using the peakfinder feature in the software. Automated searchmatch software was not an effective method of mineral identification due to false matches caused by the complexity of patterns. Therefore, all mineral identifications were based on a manual search of the JCPDS data file guided by operator knowledge of the bulk chemistry. After confirming the presence of 11minerals by use of the separations outlined above, we performed quantitative determinations of concentrations of the most abundant minerals using a standard additions technique (6). Background-subtracted area intensities must be used, and the peak used from the phase of interest must be free from overlap with other peaks in the bulk pattern. Because X-ray intensities were only on the order of 100-300 counts/ s, and because the variation in intensity upon turning the samples was up to 20%, we rotated the sample mounts 45" eight times for each addition. Three to four additions of different weight fractions thus resulted in 24-32 data points per mineral. The error for each mineral concentration is reported as U =
(jstd error)2 slope
+
(std error)n)W intercept
Surface Chemistry. Potentiometric titrations of samples not run through the riffle splitter were performed to determine point of zero net charge (PZNC; see ref 7 for discussion). Four grams of ash suspended in 20 mL of 0.001, 0.01, and 0.1 mol/kg NaCl solutions was titrated with concentrated HC1. Samples were briefly shaken upon addition of acid, and the pH of the suspension was measured after 1min of settling. Blanks containing NaCl but no ash were also titrated. Cation and anion exchange capacity (CEC/AEC) determinations were performed in three pH ranges on samples which were not run through the riffle splitter. Three 10-min washes of 0.25 mol/kg CaCl2 (with 0.1 mol/ kg HC1,0.005 mol/kg HC1, and no HC1, respectively, for low, intermediate, and high-pH-range experiments) were used to saturate 0.75 g of ash with Ca2+and C1-. Three 10-min washes of 0.05 mol/kg CaCl2 (hHC1 as above) followed. The mass of the final wash was recorded to permit calculation of occluded volume, and the final wash was saved for Ca2+and C1- analyses. Saturating ions Ca2+ and C1- were displaced with three 20-mL, 10-min washes of 0.25 mol/kg Mg(N0312 followed by a 38-mL rinse with Environ. Sci. Technoi., VoI. 27, No. 4. 1993
653
Table 11. Sequential Extraction Procedurea step
reagent
deionized water 0.5 M Ca(NO& (3) Aa exchangeable 0.5 M AeNOq (4) acjd soluble 0.5 M CH3COOH + 0.1 M Ca(N0.h ( 5 )organically bound 0.1 M Na4P207 (6) amorphous iron 0.175 M (NHa)2C204 + 0.1 M oxide occluded H2C~04,cover tube to keep out light (7) crystalline iron 0.1 M NazEDTA + 0.3 M oxide occluded NHZOH-HCl (8) residual 0.1 g of dried solid ash from previous step with 3 mL of conc "03 + 3.5 mL of conc HF
time, h
(1)water soluble (2) Ca exchangeable
~~
a
-1,1*
24 10 min
(microwave)
See text for step 8 details.
deionized water, with all displacing washes saved. The C1- concentration in samples was analyzed using a conductometric titration with Ag+ ions with no pretreatment. Samples for Ca2+ determination were acidified with concentrated nitric acid and analyzed by inductively coupled argon plasma spectrometry (ICAP). Electron Microscope and Microprobe. The composition of selected particles of ash was determined using a Cameca SX-50 electron microprobe automated by a Sun workstation, operabd in both wavelength-dispersive (WDS) and energy-dispersive (EDS) modes with 15-20-kV accelerating voltage and 20 nA. Polished geologicalmaterials were used as standards for quantitative analysis. The morphology and composition of selected ash particles was observed using a Camscan Series 2 scanning electron microscope (SEM) operated with 20-kV accelerating voltage and equipped with an hnu Si(Li) X-ray EDS detector. The SEM-EDS semiquantitative analysis used the standardless analysis software packaged with the hnu system. Both polished sections of grains embedded in epoxy and slurries of acetone-grain suspensions mounted on polished carbon substrates were examined. Sequential Chemical Extraction. Tessier et al. (8) discussed rationale and methods of sequential extraction. Our sequential chemical extraction was performed on 0.5 g of ash ground in acetone for 5 min in an automatic mortar and pestle and then air-dried to determine Pb, Cr, and Cd speciation as operationally defined in Table 11. Residual determination is problematic due to the likelihood of forming precipitates and is further described below. Place in 30-mL PTFE screw-top digestion bomb. Place eight bombs in covered plastic container in a microwave oven for 10 min at 400 W. Remove from oven carefully (some vapor may escape bombs), place in fume hood to cool, and reduce internal pressure. Pour solution into polyethylene bottle, wash bombs with saturated boric acid solution, and pour into bottle. Dilute to -100 g of total mass solution with boric acid solution. The boric acid step is necessary to solubilize fluoride precipitates (including a lead fluoride) formed in HF digestion. The Pb, Cr, and Cd contents of each extraction solution was analyzed using ICAP.
Results and Discussion Bulk Chemistry. Our bulk chemical analysis covered all major elements and many minor elements. In Table 654
Envlron. Sci. Technol., Vol. 27, No. 4, 1093
I, we have cast our analysis in terms of weight percent of oxide (using the highest reasonable oxidation state for each element) because the ash in this study formed under oxidizing conditions. The conversion to an oxide basis allows us to calculate an analytical s u m using mass balance, thus providing a check on the completeness of analysis. Other analyses (9-13) lack some major element data, usually Si, and thus cannot be used for mass balance. Our analysis including loss on ignition sums to 91% rather than to 100% ,but the sum of the squares of the individual errors could account for this discrepancy (Table I). The difficulty of obtaining complete digestion of MSW ash has been noted by several workers (11, 12). Differing analytical schemestherefore produce differingresults when used on the same ash. The sequential extraction section below discusses some difficulties we encountered in obtaining complete dissolution. There is considerable heterogeneity among bulk chemical analyses of ashes from different facilities, and ash from a single facility varies with time. However, this variability is rarely greater than 1 order of magnitude as illustrated in Figure 1. In Figure la, the low C1 value (9) is an outlier; this sample is from a lagoon sludge from which soluble salts were probably partially rinsed. Though feedstocks vary widely, it is likely that time-averaged MSW ash bulk chemistry is constrained to fall within this approximate order of magnitude variation. Figure 2 compares the weight percent of elements in ash (data from Table I) vs average soil (14)and can help select elements which are the greatest potential problems or possible resources. Si is the only major element which is significantly depleted in ash; other major element abundances (upper right) in ash are similar to those of average soils or are enriched. Ca and Na are usually leached from soils by weathering, whereas Si is more resistant to weathering. In ash, these trends are reversed, though much Ca and Na would be leached from soluble salts in MSW ash quickly during landfill disposal if water reaches the ash. Among the minor elements, P b and Zn are both relatively abundant and quite enriched. Other elements (Au, Se, Ag, Cd) are considerably enriched, but their overall abundances are low. Au is lo7times enriched over average soils, however, Au is only enriched lo2times over average crustal rocks. Au occurs in our samples at -0.5 ppm, which is at the very low end of the range of mineable deposits. For MSW ash to be considered as an ore deposit, other elements, perhaps Cu, Ni, and Zn would have to be recovered, and large volumes of ash would have to be processed. X-ray Analyses. Comparison of Figures 3 and 4 illustrates the difficulty of using XRD to qualitatively identify mineral phases in MSW ash. Figure 3 is a diffractogram from a bulk ash sample; Figure 4 is from a magnetic fraction. Based on the bulk sample alone, identification of magnetite and hematite would be tenuous at best due to numerous possible peak overlaps; wustite could never be identified from a bulk sample. However, all three phases can be identified with confidence in the magnetic fraction (although only magnetite is magnetic, SEM and reflected light optical observations of polished sections show considerable exsolution of magnetite in glasses which also contain hematite and other phases). Separations of this kind are often used in XRD as part of a "divide and conquer" approach (15). After attempting to use computerized search-match software, we found that
1
100
X
m
1:
4 I
f
highest literature value lowest literature value
*
I
rn O
i
l
'
average literature value highest literature value lowest literature value
0
0
Au
Hg
.000001
Co
Ni
Cu
Zn
As
Se
Ag
Cd
Sn
V
Ba
Pb
ELEMENT Flgure 1. Bulk chemical analyses from this study, with error bars representing one standard deviation, and average and ranges from published literature values (9-13).
.12
-10
8
-6
4
-2
0
2
log(weight percent soil) Figure 2. Double-log plot comparing weight percent of elements in an average soil (74) to weight percent in ash from this study. The heavy ilne represents equal concentrations in both materials: upper lines represent order of magnitude enrichments. The more abundant the element, the closer the points are to the upper right corner.
the software often missed phases we were certain were present (e.g., quartz) and "identified" numerous mineral
phases which we judged were not present, based on examination of diffractograms, on chemical data, and on microscope observation. It is unlikely that one could successfully identify more than about five phases from a single diffractogram. As a test, we prepared a synthetic "ash" with the composition shown in Figure 3. Though the search was limited to elements present in the minerals composing the synthetic ash, our search-match software identified 4 out of 7 phases correctly and suggested 46 other possible phases. Therefore, we based all of our mineral identifications on separated fractions and a manual search-match procedure. We deem separations to be essential to accurate identification, and computerized search match software should be employed only with caution and healthy skepticism. Table I11lists the minerals identified and the separation fractions in which identifications were made. A spinel phase is listed in Table I11rather than magnetite because SEM-EDS observations demonstrate that the spinel structure phase contains cations (Ti, Al, Ca) in addition to Fe in solid solution. Once a mineral was identified using several peaks in a fraction, it could more confidently Environ. Scl. Technol., Vol. 27, No. 4, I993
06s
Is
qr P I
10
20
I
i
E
P
2oooo
Table IV. Inorganic Compounds Identified in MSW Ash in Five Studiesa phase
10000
6
ref 16“ 17d 18“ I&‘ 1 9
Fez03 (hematite) 3.7 f 1.7 X CaC03 (calcite) 3.5 f 1.9 x NaCl (halite) 0.5 f 0.4 X X X Si02 (quartz) 2.3 f 1.0 x x x x AB204 spinel (“magnetic”) -3.5 f 2 x x x Ti02 (rutile) 1.1 f 1.3 X CaS04.2H20 (gypsum) 1.8 1.9 x Cas04 (anhydrite) X x x x -3(A1203).2(SiOZ) (mullite X FeO (wiistite) X KCl (sylvite) X X X amorphous and BDL less LO1 74 f 2 LO1 less gypsum waters 9.3 f 4.6 CaSOq0.5HzO (bassinite) X CaO (lime) X Caz(Mg,Al)(Al,Si)z07 (gehlenite) X CaTiOa (perovskite) X Na2HP03 X A1 (aluminum metal) X (Na,Ca)(Al,Si)dOa (plagioclase) X complex silicate(s)? x ZnSO4 X NaZS04 X &So4 X KAI(S04)z x x A1203 X K2ZnC14 X PbS04 x x (Fe,Mn,Ca)3(P04)z x a All determinations are by powder X-ray diffraction except ref 18. Some authors reported chemical formulas rather than mineral names. BDL refers t o minerals below detection limit using standard additions on bulk ash; LO1 is loss on ignition (organics, structural and absorbed water). Phases “identified” by Dipietro (20): Bottom ash (16 phases) SiOz, CaC03, Ca4O(PO4)2, Li(Mn,Fe)POd, Ti407, PbS04,PbC03,KZFeO4,Pb5(VO4)3Cl,Ca(Mg,Fe)SizOd,(Fe,Mg)SiOa, Zn3(PO4)2, MgSOq3H20, CdSiO3, KSCN, KAlSi308. Fly ash (7 phases) CaS04, (Fe,Mg)SiOa,CdS04, (K,Ba,Na)(Si,AlhOa,BaAlzSizOa, CuFezSa, CaMg(Si03)~.* This study; weight percent (f2u) in combined ash. ‘ Reference 16; fly ash. Reference 17; combined ash. e Reference 18; fly ash, fourier transform infrared. f Reference 18; fly ash. g Reference 19: flv ash.
*
40
10
60
50
70
20,” Flgure 3. X-ray dlffractograms for a bulk ash sample and a synthetic “ash”. Abbrevlations: cc, calcite (CaC03);g, gypsum (CaS04.2H20); qtz, quartz h, halite (NaCI);he, hematite (Fe203);mt, magnetite(Fe304); (SOp);r, ruth (Ti02). Note slmllar scales and intensities; synash is shifted up 5000 unlts for clarity. Synash contains the following weight percents: glass, 85.71; qtz, 3.85; cc, 1.10; mt, 1.10; he, 2.75; h, 1.10; g, 4.40. 50000
1
111t
1
E l U
60000
t
I
/I
20
10
30
50
40
28,
70
60
”
Figure 4. X-ray diffractogram for a magnetic separate: he, hematite (Fep03);mt, magnetite(Fe304);wu, wustite (FeO). Note clarity of peaks and glass “hump” shift compared to Figure 3. A glass hump occurs in glasses In lieuof a single peak becauseglass is much more disordered than minerals; therefore, the distribution of X-ray-reflecting surfaces has a wlde variance.
Table 111. List Showing in What Fraction Minerals Were Identified by XRDa mineral
A
B
quartz gypsum mullite rutile spinel hematite wiistite calcite anhydrite sylvite halite
$
0
0
o 0 0 0 0
C
fraction D E
F
*
*
G
H
0
0 0
*
O 0
0
*
$
**
$
* *
* *
*
0
0
o
0 0 0
0 Fraction A, bulk ash; B, residue from HF leach; C, magnetic; D, reprecipitated soluble salts; E, C325-mesh sieve fraction; F, heavy magnetic; G, heavy nonmagnetic; H, light nonmagnetic. *, definitively identified in this fraction; 0,identified in this fraction after positive identification in another fraction.
be identified using fewer peaks in other fractions or a bulk sample. Workers wishing to use separations for other ashes should be able to employ the principles involved in our separations but should be flexible in approach. For example, a search strategy we employed for Pb-rich phases 656
Environ. Sci. Technol., Vol. 27, No. 4, 1993
was to examine the heavy nonmagnetic fraction. While we found no Pb-rich phase using XRD (we did find several Pb-rich glasses and minerals using SEM-EDS), workers dealing with very fine fly ash or ash from emissions escaping the stack might find lead sulfates or carbonates in a heavy nonmagnetic fraction using XRD. Table IV shows the phases we identified with XRD and also lists phases identified by other workers. Qur separations are detailed above. Giordano et al. (16)examined bulk, water-soluble, and bulk minus water-soluble fractions. Qntiveros (17, performed some size fractionation and some separation by particle color. Henry et al. (18) examined bulk, water-soluble, and water-insoluble fly ash trapped after passing an electrostatic precipitator, Le., very small particles. Liu et al. (19)examined only particles
