Environ. Sci. Technol. 1997, 31, 3330-3338
Heavy Metal Stabilization in Municipal Solid Waste Combustion Dry Scrubber Residue Using Soluble Phosphate T . T A Y L O R E I G H M Y , * ,† BRADLEY S. CRANNELL,† LESLIE G. BUTLER,‡ FRANK K. CARTLEDGE,‡ EARL F. EMERY,‡ DANIEL OBLAS,§ JAMES E. KRZANOWSKI,| J. DYKSTRA EUSDEN, JR.,⊥ ELIZABETH L. SHAW,# AND CARL A. FRANCIS3 Environmental Research Group, A115 Kingsbury Hall, University of New Hampshire, Durham, New Hampshire 03824, Chemistry Department, Louisiana State University, Baton Rouge, Louisiana 70803, Center for Advanced Materials, University of Massachusetts at Lowell, 1 University Avenue, Lowell, Massachusetts 01854, Mechanical Engineering Department, 134 Kingsbury Hall, University of New Hampshire, Durham, New Hampshire 03824, Geology Department, Carnegie Hall, Bates College, Lewiston, Maine 04240, Analytical Shared Experimental Facility, Center for Material Science and Engineering, Room 13-4137, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, and Harvard University Mineralogical Museum, 24 Oxford Street, Cambridge, Massachusetts 02138
The mechanisms of heavy metal stabilization of calciumbased dry scrubber residue using soluble PO43- were investigated. This stabilization technology is presently used in the U.S. and Japan to reduce metals leaching from municipal solid waste combustion residues. At an experimental dose of 1.2 mol of H3PO4/kg of residue and using a relatively dry mixing system, the reduction in the operationally defined fraction available for leaching (using the Dutch Total Availability Test) is 38% for Cd, 58% for Cu, 99% for Pb, and 28% for Zn. pH-dependent leaching (pH 4, 6, 8) showed that the treatment was able to reduce equilibrium concentrations by 0.5-2 log units for many of these metals, particularly Pb and Zn. Depth profiling of particles using secondary ion mass spectroscopy suggests that stabilization is by precipitation of metal phosphate reaction products rather than by adsorption of metals to phosphate particle surfaces. Bulk and surface spectroscopies show that the insoluble reaction products are nanometer-sized, crystalline and amorphous calcium phosphates, tertiary metal phosphates, and apatite family minerals. The geochemical thermodynamic equilibrium model MINTEQA2 was modified to include both extensive phosphate minerals and simple ideal solid solutions for modeling pH-dependent solid phase control of leaching. Both apatite family and tertiary metal phosphate end members and ideal solid solutions act as controlling solids for Ca2+, Zn2+, Pb2+, Cu2+, and Cd2+. The prevalence of small, nanometer-sized reaction products suggest that Ostwald ripening and precipitate maturation has not completely occurred during initial mixing.
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Nevertheless, soluble phosphate is an effective stabilization agent for divalent heavy metals in waste materials such as scrubber residues.
Introduction Chemical stabilization of waste materials offers the potential to reduce the leachability of heavy metals in the waste. The principal objective during stabilization is to form new mineral phases with reduced solubilities and increased geochemical stability in a leaching environment. One stabilization agent of recent interest, particularly for Pb2+, is PO43- (1-4). It is used commercially to stabilize a variety of hazardous and industrial wastes. Numerous technologies employ either particulate (e.g., crushed waste phosphate rock) or soluble PO43- (e.g., dissolved superphosphates, H3PO4, etc.) as the added source of PO43-. PO43- combines with over 30 elements to form about 300 naturally occurring minerals (5, 6). Metal phosphates are frequently found as secondary minerals in the oxidized zones of lead ore deposits and as assemblages around ore bodies (6). They also occur in soils, sediments, and phosphatic beds (6). As such, they are very stable with respect to pH, Eh, and mineral diagenesis. Isomorphic substitutions are very common for both divalent cations (e.g., Pb2+ for Ca2+) and oxyanions (e.g., AsO43- for PO43-) in these naturally occurring minerals (6). Past research efforts have shown that phosphate minerals are likely controlling solids for Ca2+, Cd2+, Cu2+, Pb2+, and Zn2+ in natural soil systems (7-9). The use of PO43- to immobilize metals has been advocated for industrial wastewaters (9, 10) and lead-contaminated soils (3, 11). The metal of concern must be dissolved and available for precipitation with the PO43- for successful immobilization. System liquid-to-solid ratio (L/S; w:v), pH, ionic strength, mixing and mass transfer, and reaction time all play a role in the progress of the reaction, the particle size of the precipitate, and the activity of the precipitate. Stability field analyses can be used to assess the theoretical impact of PO43dose on ideal phase predominance (1, 6). However, assuming that Ostwald ripening is occurring, it is likely that crystallization kinetics can initially promote the formation of relatively soluble nanometer-sized crystallites, then metastable intermediaries, and finally stable micrometer-sized crystalline phases. In complex and heterogeneous waste stabilization systems, these crystalline phases are likely to be solid solutions rather than pure end members. Both PO43-containing minerals and soluble PO43- have been considered as sources of PO43-. The former involves the dissolution of the less stable added mineral [e.g., Ca5(PO4)3OH] and the subsequent precipitation of the more stable metal phosphate precipitate [e.g., Pb5(PO4)3OH]. The later involves the direct precipitation of a stable metal phosphate precipitate [e.g., Pb5(PO4)3OH]. In both cases, system L/S and pH can both influence or be controlled to promote the dissolution of the original mineral phases containing the heavy metals, the dissolution of the particulate source of PO43- (if used), and formation of the metal phosphate precipitates. Chemical stabilization can involve a continuum of reaction mechanisms from surface sorption of metals to existing or * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: (603) 862-2364. † Environmental Research Group. ‡ Louisiana State University. § University of Massachusetts at Lowell. | University of New Hampshire. ⊥ Bates College. # Massachusetts Institute of Technology. 3 Harvard University Mineralogical Museum.
S0013-936X(97)00407-0 CCC: $14.00
1997 American Chemical Society
newly formed particulate surfaces in a waste material, through the formation of new surface metal precipitates, to the formation of discrete heterogeneous or homogeneous metal precipitates (12). The research by Fulghum et al. (13) has demonstrated the utility of secondary ion mass spectroscopy (SIMS) in elucidating sorption versus precipitation mechanisms. Pb2+ was either sorbed to micrometer-sized CaCO3 or coprecipitated with Ca2+ to form micrometer-sized solid solution (Pb,Ca)CO3 precipitates. Depth profiles of each sample type showed surface enrichment of the 206Pb+ versus the 40Ca+ in the sorbed scenario. In the coprecipitation scenario, 206Pb+ and 40Ca+ were equally abundant with depth. The stabilization of a Ca-dominated waste with PO43- can produce a complex precipitation sequence with reactions that are considered reversible but which strongly tend toward a very stable end product (14). When Ca2+ and PO43- are titrated in solution, the reaction sequence generally involves the formation of Ca9(PO4)6 (nonstoichiometric amorphous calcium phosphate), CaHPO4‚2H2O (brushite), CaHPO4 (monetite), Ca8H2(PO4)6‚5H2O (octacalcium phosphate), β-Ca3(PO4)2 (whitlockite), and ultimately Ca5(PO4)3OH (calcium hydroxyapatite), the most geochemically stable calcium phosphate (15). The sequence is influenced by ion activity products (IAPs), pH, ionic strength, reaction kinetics, the presence of precursor substrates or “seed”, and the presence of inhibitors like Mg2+ (14, 15). It is simple to synthesize binary or even ternary solid solutions of calcium phosphates (particularly apatities) where divalent metal cations such as Cd2+, Cu2+, Pb2+, and Zn2+ isomorphically substitute for Ca2+ (16). The approach taken by our group to understand stabilization mechanisms and identify reaction products involves a suite of methods. Neutron activation analysis (NAA), X-ray fluorescence (XRF), and X-ray photoelectron spectroscopy (XPS) are used for quantifying elements. Field-emission scanning electron microscopy (FESEM) is used to examine particle size and morphology. Scanning-transmission electron microscopy and X-ray microanalysis (STEM-XRM) are used to interrogate discrete particles. SIMS is used to examine stabilization reaction mechanisms. X-ray powder diffraction (XRPD) is used to identify bulk crystalline phase minerals. Magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR) allows for determination of bulk 31P chemical states. XPS is principally used to identify crystalline and amorphous speciation and abundance at particle surfaces. Operationally defined leaching tests and geochemical modeling of leaching behavior help to understand the effectiveness of the stabilization and identify mineral phases controlling leaching. Each of these methods provide specific and complementary information (2). We are employing this approach on a number of wastes including various combustion ashes, vitrification dusts, smelter dusts, and mine tailings. This study was designed to determine the mechanisms and reaction products of chemical stabilization of dry scrubber residues treated with soluble PO43-; a practice that is presently used in both the U.S. and Japan. Dry scrubber residues are the products of the use of CaO or Ca(OH)2 in the acid gas scrubbing of flue gases from municipal solid waste combustion. They comprise about 15% (by weight) of the total ash residue streams from a combustor. They contain fly ash, char, unreacted lime, and scrubber products (17). The more volatile heavy metals collected in the residue (e.g., Cd2+, Cu2+, Pb2+, and Zn2+) are relatively leachable (17). Soluble phosphate was effective in stabilizing these metals.
Materials and Methods Combustor Description. A 1500 ton per day mass burn facility was sampled. It consists of two parallel units comprised of reciprocating grates, water wall boilers, scrubber venturis, and Ca(OH)2 scrubbers with fabric filters. Sampling occurred over the period from January 3rd to January 7th,
1995. Dry scrubber residue was collected by plant personnel. A grab sample (1 kg) was collected from one of the scrubber transfer conveyors every 10 min to make a 4 h daily composite. This 24 kg daily composite was blended and subsampled to make a 5 kg daily working composite. Each of the five daily working composites was blended into a weekly composite which was sampled to make a 10 kg weekly composite working sample for subsequent analysis. A clean, lab-scale cement mixer was used for blending. Processing. A weekly composite working sample (5 kg) was treated. A single phosphate dose of 1.2 moles of H3PO4 per kg of residue (as received) was selected as an experimental laboratory-scale treatment formulation to ensure detection of stabilization reaction mechanisms and reaction products. This dose is somewhat higher than those typically used in practice in proprietary ash treatment systems at municipal solid waste combustors in the U.S. and Japan. This form of soluble phosphate is inexpensive and predominantly used in practice. Process mixing water at a L/S of 0.4 was used. The relatively dry mixture was thoroughly blended for 10 min in a Hobart mixer using a tined paddle (108 rpm) with a planetary orbit (48 rpm). The mixing regime is similar to full-scale treatment systems. Primary Standards. A number of primary standards were used with XRPD, MAS-NMR, SIMS, or XPS for (i) assessing detection limits in complex powder mixtures (XRPD), (ii) increasing databases for phase identification (XPS, MASNMR), or (iii) providing a standard reference material (SIMS). Minerals were obtained from the Harvard University Mineralogical Museum (HUMM), Excalibur-Cureton Mineral Company (EC), Aldrich (A), Fluka (F), or the National Institute for Standards and Technology (NIST). Standards included calcium chloroapatite [Ca5(PO4)3Cl; HUMM 10735; SIMS, XRPD, MAS-NMR, XPS], calcium hydroxyapatite [Ca5(PO4)3OH; HUMM 103003, A, F; XRPD, MAS-NMR, XPS], tricalcium phosphate [β-Ca3(PO4)2; F; XRPD, MAS-NMR], calcium bis(dihydrogen phosphate)monohydrate [Ca(H2PO4)2‚H2O; F; XRPD, MAS-NMR], monetite [CaHPO4; F; XRPD, MAS-NMR], octacalcium phosphate [Ca8H2(PO4)6‚5H2O; NIST; XRPD, MAS-NMR], lead chloropyromorphite [Pb5(PO4)3Cl; HUMM 110842; XPS, XRPD], whitlockite with Pb2+ substituted for Ca2+ [β-(Ca,Pb)3(PO4)2; HUMM 95.1.9; XPS], plumbogummite [PbAl3(PO4)2(OH)5‚H2O; EC; XPS], hinsdalite [PbAl3PO4SO4(OH)6; EC; XPS], cerrusite [PbCO3; HUMM 124936; XRPD, XPS], and gypsum [CaSO4‚2H2O; HUMM 12374; XRPD, XPS]. Analytical Scheme. Residues were subjected to a variety of analyses in either their untreated state or after treatment. Further, the residues were then subjected to rigorous leaching in the Dutch Total Availability leaching procedure (see below), producing four solid fractions for characterization: (i) untreated and unleached, (ii) untreated and leached, (iii) treated and unleached, and (iv) treated and leached. Presumably, solid phase reaction products would remain in the treated and leached residue after leaching. The untreated and treated weekly composite working samples were subsampled using cone and quarter techniques to produce the various fractions. About 500 g was produced for the unleached fractions. Because of sample loss during leaching, about 2 kg was needed for the leaching procedure to produce 500 g for the leached fractions. These samples were then cone and quartered to produce samples which could provide material for the various analyses described below. Total Composition. The dry scrubber residues were quantified for over 47 elements using NAA, XRF, and XPS. A single 3 g untreated and unleached sample was analyzed. Procedures are provided elsewhere (2, 17, 18). FESEM. FESEM was used to examine structure and morphology of discrete particles in all four residue fractions. An AMRAY 3300FE field emission scanning electron microscope equipped with a 305 Schottky field emission source was used. Samples (about 20 mg) were mounted on
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aluminum stubs using carbon paint. Specimens were tilted 10° and vacuums were around 10-7 Torr. Various magnifications (100-100000×) and accelerating voltages (3-10 kV) were used. STEM-XRM. STEM-XRM was used to examine discrete particle morphology and determine elemental composition and possible mineral formula in discrete particles of the two treated fractions. STEM examinations were conducted on a Hitachi H-600 TEM. The XRM work was done in the STEM mode using a Link AN-10000 analyzer. Samples were prepared by electrostatically adhering 100-5000 nm-sized particles to formvar-coated copper grids. Ten discrete particle assemblages were analyzed for each fraction. Spectra were collected for 60-120 s. Quantification of the spectra was done using the RTS-FLS program. Program sensitivity factors were modified by analyzing a variety of primary standards of known composition. SIMS. SIMS was used to elucidate the stabilization mechanism. Depth profiles for selected atomic masses (40Ca+, 208Pb+, 31P+, 35Cl+, and 28Si+) were collected for all four fractions so as to examine normalized concentrations (relative to 40Ca+) as a function of particle depth (particle exterior to interior). This would provide evidence of possible surface adsorption or evidence of new discrete phase precipitation (13). A VG SIMSLAB I (upgraded) quadrapole filter type mass analyzer was used to conduct positive ion (+m/z) depth profiling using an O2+ ion beam. Profiles were conducted at high vacuum (10-9 Torr). Preanalyzed indium foil was used for sample mounting to minimize charging. Generally, the ion source was operated at 10 keV and 20 nA. Target biases were usually 5-15 V. The profiles were done at 200× with the extractor operated at 1500 V. Estimated sputtering rates were roughly 0.1-1 nm/s. Profiles for 208Pb+, 31P+, 40Ca+, 28Si+, and 113In+ were typically conducted for 45 min to 1 h. Samples (about 20 mg) were run in triplicate for each of the fractions or standards that were analyzed. Typical results from one of the runs are presented. XRPD. XRPD was used to identify crystalline mineral phases in the four fractions. A Rigaku-Geigerflex goniometer was used (copper X-ray source, 45 kV, 35 mA, 1575 W). Powdered (e300 µm) fractions (about 5 mg) were fixed to a glass slide with double-sided tape, and all excess powder was removed. Scans were conducted from 6.00 to 90.00° at a rate of 0.5° of 2θ/min. Output from the goniometer was directed through an A/D board to a PC with programs used to translate, background adjust, smooth, and select peak locations. Samples were run in triplicate and a fourth run was conducted after tungsten was added to one of the replicates as an external standard. The drift in 2θ for the tungsten peaks was used to correct the drift for peaks always present in the samples. Drift corrections were always less than 0.20° of 2θ. The data were evaluated for possible crystalline phases using the PC-based search and match program MICRO-ID (Materials Data Corp., Livermore, California). Phases were based on PDF files 1-41 and determined using a 0.5˚ of 2θ error window, a 3 peak match, and a minimum relative intensity of 1. Figure of merit (FOM) calculations were based 90% on peak location and 10% on peak intensity. Element exclusion was based on total composition data; elements present less than 100 mg/kg were excluded. A ranking algorithm was used to rank order phases within the three replicate analyses of a given fraction. For quality assurance purposes, samples consisting of the primary standards (i) Ca5(PO4)3Cl, (ii) PbCO3, (iii) CaSO4‚ 2H2O, (iv) a mixture of the three, and (v) a mixture of the three along with various loadings of amorphous glass (10, 30, 50, 75, 90, 95, and 99%) were run. The peak identification and search match software were able to identify all three standards in all of the mixtures with generally low FOMs and high rank order. Additionally, all other primary standards
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were individually analyzed and successfully identified with very low FOM values and the highest rank order. MAS-NMR. MAS-NMR was used to monitor the 31P isotropic chemical shifts and chemical shift anisotropy tensors of the component species in the two treated fractions. Calcium phosphate primary standards were also run. About 1-2 g of sample was analyzed. The spectra were taken on a 400 MHz Chemagnetics Infinity NMR spectrometer with a 9.4 Tesla magnet corresponding to a 31P NMR frequency of 161.9 MHz. The samples were spun in 7.5 mm (OD) zirconia rotors in a double resonance probe at spin rates between 1 and 7 kHz at a temperature of about 23 ( 2 ˚C. All spectra were recorded with proton decoupling while using 10 µs 90˚ 31 P pulses (except where noted); pulse lengths were calibrated by observing the 31P resonance of NH4H2PO4 (ammonium dihydrogen phosphate). The recycle delay was set to 10 s based on approximate 31P T1 measurements of some of the samples. The number of scans ranged from 500 to 4000. Chemical shifts were referenced to an external standard sample of NH4H2PO4 [δ (31P) ) 0 ppm with respect to 85% H3PO4 (19)]. The analysis of 31P NMR spectra of inorganic phosphates is generally done by a consideration of both the isotropic chemical shift and the individual chemical shift tensor elements (principal axis system). These values are obtained from the 31P NMR spectrum using the graphical method of Herzfeld and Berger (20) or by a Simplex and gradient search implementation of the Herzfeld-Berger method (21). The latter was used here as it is optimized for multiple component systems. The use of primary standards as reference materials was vital to the use of this technique because of the limited database available for chemical shift and tensor element values. XPS. A Perkin-Elmer Physical Electronics Division 5100 hybrid XPS was used to identify and quantify possible chemical phases as well as to quantify elements in the samples. Metal phosphate primary standards were also run. Samples (about 10 mg) were adhered to the sticky side of copper tape to facilitate analysis. For energy referencing, the entire system was calibrated to the gold 84.0 4f7/2 binding energy (BE). Correction for peak shift due to static charge buildup on the sample system was achieved through the adventitious carbon reference method using a C 1s BE of 284.8 eV as a conducting reference. The use of primary standards as reference materials was crucial because of the limited databases available for BEs of metal phosphates of interest. Methods related to assignment of full-width, half-maximum values, peak fitting, satellite assignments, and identification of species using the NIST and primary standards databases are given elsewhere (2, 17, 18). Total Availability Leaching Procedure. The procedure is based on the Dutch Total Availability Leaching test NEN 7341. It is used to operationally quantify the elemental mass fraction available for leaching. It was also used to produce untreated and treated leached fractions. The procedure involves two sequential extractions; the first was conducted at a pH of 7.0 and a L/S ratio of 100. For examination of mass fractions available for leaching, 8 g of residue was added to 800 mL of distilled, deionized water and stirred in the capped Teflon vessel. Concentrated HNO3 was added to bring the solution to pH 7.0, and then 3 N HNO3 was used to maintain pH throughout the leaching period. After 3 h, the leachate was filtered through a Costar (Costar Corp., Cambridge, MA) 0.45 µm polycarbonate filter. The second extraction entailed leaching the residue and filters from the first extraction at a pH of 4.0 for 4 h at a L/S ratio of 100. Concentrated HNO3 was added to bring the solution to pH 4.0, and then 3 N HNO3 was used to control pH. The pH stat leaching apparatus and analytical methods are described elsewhere (2, 17, 18). For production of the leached fractions, larger quantities of material were leached in larger vessels.
TABLE 1. Some Divalent Metal Phosphate Minerals and Their Solubility Products mineral
-log Ksp ∆Gf° (kJ/mol) footnote
dissolution reaction
apatites hydroxyapatite Ca5(PO4)3OH + H+ S 5Ca2+ + 3PO43- + H2O chloroapatite Ca5(PO4)3Cl S 5Ca2+ + 3PO43- + Clhydroxypyromorphite Pb5(PO4)3OH + H+ S 5Pb2+ + 3PO43- + H2O chloropyromorphite Pb5(PO4)3Cl S 5Pb2+ + 3PO43- + ClCd5(PO4)3OH Cd5(PO4)3OH + H+ S 5Cd2+ + 3PO43- + H2O Cd5(PO4)3Cl Cd5(PO4)3Cl S 5Cd2+ + 3PO43- + ClZn5(PO4)3OH Zn5(PO4)3OH + H+ S 5Zn2+ + 3PO43- + H2O Zn5(PO4)3Cl Zn5(PO4)3Cl S 5Zn2+ + 3PO43- + ClCu5(PO4)3OH Cu5(PO4)3OH + H+ S 5Cu2+ + 3PO43- + H2O Cu5(PO4)3Cl Cu5(PO4)3Cl S 5Cu2+ + 3PO43- + Cltertiary metal phosphates low whitlockite β-Ca3(PO4)2 S 3Ca2+ + 2PO43Pb3(PO4)2 Pb3(PO4)2 S 3Pb2+ + 2PO43Zn3(PO4)2 Zn3(PO4)2 S 3Zn2+ + 2PO43Cu3(PO4)2 Cu3(PO4)2 S 3Cu2+ + 2PO43Cd3(PO4)2 Cd3(PO4)2 S 3Cd2+ + 2PO43Mg3(PO4)2 Mg3(PO4)2 S 3Mg2+ + 2PO43tetra metal phosphates hilgenstockite Ca4O(PO4)2 + 2H+ S 4Ca2+ + 2PO43- + H2O Pb4O(PO4)2 Pb4O(PO4)2 + 2H+ S 4Pb2+ + 2PO43- + H2O other phosphate minerals AlPO4 S Al3+ + PO43AlPO4 monetite CaHPO4 S Ca2+ + PO43- + H+ brushite CaHPO4‚2H2O S Ca2+ + PO43- + 2H2O + H+ cornetite Cu3PO4(OH)3 + 3H+ S 3Cu2+ + PO43- + 3H2O libenthenite Cu2PO4OH + H+ S 2Cu2+ + PO43- + H2O pseudomalachite Cu5(PO4)2(OH)4 + 4H+ S 5Cu2+ + 2PO43- + 4H2O corkite PbFe3(PO4)(OH)6SO4 + 6H+ S Pb2+ + 3Fe3+ + PO43- + SO42- + 6H2O spencerite Zn4(PO4)2(OH)2AlPO4‚3H2O + 2H+ S 4Zn2+ + 2PO43- + 5H2O Zn rockbridgite ZnFe4(PO4)3(OH)5 + 5H+ S Zn2+ + 4Fe3+ + 3PO43- + 5H2O scholzite CaZn2(PO4)2‚2H2O S Ca2+ + 2Zn2+ + 2PO43- + 2H2O tarbuttite Zn2(PO4)OH + H+ S 2Zn2+ + PO43- + H2O faustite ZnAl6(PO4)4(OH)8‚4H2O + 8H+ S Zn2+ + 6Al3+ + 4PO43- + 12H2O plumbogummite PbAl3(PO4)2(OH)5‚H2O + 5H+ S Pb2+ + 3Al3+ + 2PO43- + 6H2O hinsdalite PbAl3(PO4)(SO4)(OH)6 + 6H+ S Pb2+ + 3Al3+ + PO43- + SO42- + 6H2O tsumebite CuPb2(PO4)(OH)3‚3H2O + 3H+ S Cu2+ + 3Pb2+ + PO43- + 6H2O a Estimated by the method of Nriagu (22). et al. (25).
b
Veillard and Tardy (23). c Nriagu (6), some of which are estimates.
pH-Dependent Leaching. The pH-dependent leaching procedure is a means of determining the equilibrium leaching behavior over a range of pH values relevant to regulatory leaching and landfill disposal. Each extraction was done at a L/S ratio of 10.0 so as to ensure solid phase control. The leaching tests were conducted in a pH stat (see refs 2, 17, 18) for 24 h, a duration typically used for achieving pseudoequilibrium (2, 17, 18). The sample (80 g) was placed into the Teflon vessel to which 800 mL of distilled, deionized water was added. HNO3 (3 N) was used to control the pH at various set points (4, 6, 8). The leachates were filtered and analyzed as described previously (2, 17, 18). Additionally, Pb2+ was quantified using isotope dilution procedures and thermal ionization mass spectrometry after ion exchange concentration (2). PO43- was also determined by using a Lachat lowlevel colorimetric assay (2). Leaching Modeling. The geochemical equilibrium model MINTEQA2 (2, 17, 18) was used to determine which solid phase controls leachate composition as a function of pH. MINTEQA2 was modified to include all the 33 phosphate mineral phases shown in Table 1. The free energy data are from either estimates made using standard estimation techniques (see ref 22), data compilations (6, 23-25), or estimates provided by Nriagu (6). The likelihood of solid solution formation during dissolution and reprecipitation required further modification to MINTEQA2 to use idealized solid solutions as possible controlling solids. A simplistic zero heat of mixing and ideal site substitution model was assumed (26). Standard free energies of formation for the solid solutions (∆G°f,ij) between end members i and j were used to determine theoretical Ksp values for the solid solutions using
d
38.15 46.89 62.80 84.43 42.49 49.66 49.10 37.53 51.62 53.96
-6279 -6223 -3774 -3791 -3924 -3859 -4309 -4137 -6279 -3168
a b b b a b c a c a
32.69 44.36 27.11 36.85 32.60 24.38
-3884 -2364 -2633 -2051 -2456 -3538
b d b b b b
17.36 36.86
-4588 -2582
b b
17.00 19.09 18.93 5.94 14.00 19.83 28.66 24.77 68.55 34.10 12.55 65.70 29.36 15.10 9.36
-1601 -1681 -2154 -1567 -1204 -2771 -3388 -3953 -4799 -3553 -1621 -10 355 -5108 -4753 -2478
b e b c c c c c c c c c c d d
Rickard and Nriagu (24). e Wagman
∆G°f,ij ) x∆G°f,i + (1 - x)∆G°f,j + nRT[x ln x + (1 - x)ln(1 - x)] (1) where x is the mole fraction of end member i, ∆G°f,i and ∆G°f,j are the free energies of formation of end members i and j, respectively, n is the number of sites in the mineral undergoing substitution (e.g., 1.0), R is the universal gas constant, and T is temperature in Kelvin. Solid solutions (43) were added to the database. No attempts were made to evaluate the likelihood of these solid solutions with respect to theoretical (e.g. Ksp,i/Ksp,j) or experimental distribution coefficients.
Results and Discussion Total Composition. The major constituents (>10000 mg/ kg) in the scrubber residue were (in decreasing abundance) O, Ca, Cl, C, Si, Al, S, Na, and Zn. Minor constituents (100010000 mg/kg) included K, Ti, Fe, Mg, Pb, and Br. Trace constituents (
