Geochemical Modeling of Leaching from MSWI Air-Pollution-Control

Apr 27, 2006 - leaching conditions after initial washout of highly soluble salts from residues. Leaching experiments were performed at a range of pH-v...
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Environ. Sci. Technol. 2006, 40, 3551-3557

Geochemical Modeling of Leaching from MSWI Air-Pollution-Control Residues T H O M A S A S T R U P , * ,† J O R I S J . D I J K S T R A , ‡ R O B N . J . C O M A N S , ‡,§ HANS A. VAN DER SLOOT,‡ AND THOMAS H. CHRISTENSEN† Institute of Environment & Resources, Technical University of Denmark, Building 115, DK-2800 Lyngby, Denmark, Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands, and Wageningen University, Department of Soil Quality, P.O. Box 8005, 6700 EC Wageningen, The Netherlands

This paper provides an improved understanding of the leaching behavior of waste incineration air-pollution-control (APC) residues in a long-term perspective. Leaching was investigated by a series of batch experiments reflecting leaching conditions after initial washout of highly soluble salts from residues. Leaching experiments were performed at a range of pH-values using carbonated and noncarbonated versions of two APC residues. The leaching data were evaluated by geochemical speciation modeling and discussed with respect to possible solubility control. The leaching of major elements as well as trace elements was generally found to be strongly dependent on pH. As leaching characterization was performed in the absence of high salt levels, the presented results represent long-term leaching after initial washout from a disposal site, that is, liquid-tosolid ratios above 1-2 L/kg. The leaching of Al, Ba, Ca, Cr, Pb, S, Si, V, and Zn was found influenced by solubility control from Al2O3, Al(OH)3, Ba(S,Cr)O4 solid solutions, BaSO4, Ca6Al2(SO4)3(OH)12‚26H2O, CaAl2Si4O12‚2H2O, Ca(OH)2, CaSiO3, CaSO4‚2H2O, CaZn2(OH)6‚2H2O, KAlSi2O6, PbCO3, PbCrO4, Pb2O3, Pb2V2O7, Pb3(VO4)2, ZnO, Zn2SiO4, and ZnSiO3. The presented dataset and modeling results form a thorough contribution to the assessment of long-term leaching behavior of APC residues under a wide range of conditions.

Introduction Residues from cleaning of flue gases at municipal solid waste incinerators (MSWI), that is, fly ashes and reaction products from gas cleaning processes, are alkaline and contain very high levels of salts and heavy metals due to volatilization and condensation processes in the boiler and flue gas cleaning systems (1). The main environmental concern with respect to these residues, usually termed flue gas ashes or airpollution-control (APC) residues, is the release of contaminants by leaching. Considerable amounts of metals can leach to the surrounding environment when the residues are utilized or landfilled, a release that may continue for several * Corresponding author phone: +45 4525 1600; fax: +45 4593 2850; e-mail: [email protected]. † Technical University of Denmark. ‡ Energy Research Centre of The Netherlands. § Wageningen University. 10.1021/es052250r CCC: $33.50 Published on Web 04/27/2006

 2006 American Chemical Society

thousands of years (2, 3). To assess the environmental impacts from residue disposal, it is necessary to quantify this leaching. Quantifying leaching in a long-term perspective, however, requires a detailed knowledge about the minerals controlling leaching and release of contaminants. When residues are placed at their final destination, infiltrating water will dissolve primary minerals and precipitate secondary minerals according to the leachate composition. Highly soluble salts such as NaCl are almost completely dissolved and washed out by the leachate. As infiltration continues, heavy metals are removed from the residues and pH gradually drops from the initial high value (typically around 12-13) toward the neutral/acidic range. During leaching, CO2 in the infiltrating water affects the mineralogy by converting hydroxides into carbonates (e.g., Ca(OH)2 into CaCO3). These carbonation reactions affect the acid neutralizing capacity of the residues and thereby pH, which in turn influences metal solubility and complexation. Initial leachate from APC residues contains extremely high levels of Cl, K, and Na; however, these elements easily leach from the residues and the concentrations significantly decrease within a liquid-to-solid (L/S) ratio of 1-2 L/kg. In column experiments on APC residues, often the specific conductivity decreases several orders of magnitude within L/S 1-2 L/kg (see Supporting Information). For typical landfills, L/S ratios of 1-2 L/kg correspond to periods of about 20-100 years (4). Leaching control in a long-term perspective beyond this period is not likely to involve easily leachable salts. Therefore, to enable an unbiased assessment of leaching processes in a long-term perspective, modeling should be based on leaching experiments with salt levels comparable to those expected to occur after L/S ratios of 1-2 L/kg. Because of the long periods involved, also effects on leaching induced by carbonation should be evaluated. Leaching from residues may be controlled by a number of processes, for example, dissolution/precipitation, redox, and sorption. Either of these processes may be dominating for specific elements in the leachate at specific pH levels. As pH is a key parameter affecting metal release, a useful method to investigate leaching control is to evaluate the pH dependency of leaching and then identify the processes controlling leaching at various pH-values. A first step is then to identify the potential solubility-controlling solids for the individual elements. Geochemical speciation modeling has proven to be a suitable tool for investigating leaching controlling processes (5, 6). In this context, leachate concentration versus pH curves are very useful. Element concentrations as well as modeled solubility control then represent a potential rather than a prediction at the individual pH-values. Within the past decade, a substantial amount of work has been done on characterization of MSWI bottom ashes (2, 5-12); however, significantly less effort has been devoted to determining the geochemical processes controlling leaching from APC residues (13-16), despite the higher pollution potential of these ashes. A main part of the work on APC residues has focused on evaluation of geochemical processes with respect to stabilization of APC residues, for example, with phosphate (14) or cement (15, 16). A few studies have focused on combined bottom and fly ashes (17, 18). A number of studies have addressed the APC residue mineralogy without including geochemical modeling (19-25). Although a few of the mentioned studies characterized fly ash mineralogy in pre-leached samples (13, 22), none of the studies has investigated leaching in a long-term perspective, nor included potential effects from carbonation on leaching control over a wide pH range. Consequently, a better understanding of VOL. 40, NO. 11, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the processes controlling leaching from APC residues at conditions relevant for long-term leaching is needed to further improve the basis for predicting leaching. This paper aims to provide an improved understanding of the leaching behavior of waste incineration APC residues with an emphasis on leaching in a long-term perspective. Leaching is evaluated on the basis of batch experiments providing conditions relevant for leaching after the initial washout of highly soluble salts from a disposal site. The leaching experiments are carried out on carbonated and noncarbonated versions of two municipal solid waste incineration APC residues. The leaching data are evaluated by geochemical speciation modeling and discussed with respect to possible solubility control.

Experimental Section APC Residues. Fly ash (FA) from an incinerator with a wet scrubbing system and residue from an incinerator with semidry (SD) flue gas cleaning technology were investigated. The residue samples were investigated in carbonated as well as noncarbonated versions. See the Supporting Information for details. Experimental Procedure. To investigate leaching control covering long time horizons, the experiments need to accelerate the leaching sequence occurring in real-life situations. A common approach is to investigate leaching as a function of pH (i.e., pH-dependent leaching) by testing residue samples in multiple batches, each fixed at specific pH-values using acid additions (5, 13, 26, 27). Solution samples are then taken from these batches and analyzed after filtration. Consequently, all soluble salts released from the residues are present in the analyzed solution samples covering the investigated pH-range. As previously discussed, in real disposal situations these highly soluble salts will wash out from the residues and only be present at low L/S ratios corresponding to very high pH-values (see the Supporting Information for further discussion). Performing geochemical modeling on leaching data from solution samples including the salts at all pH-levels will result in increased ionic strengths and potentially increased complexation levels; this may result in false model predictions. Therefore, to evaluate leaching control in a long-term perspective, salt levels have to reflect the situation that may be expected after initial wash out from a disposal site. To better resemble leaching conditions in a long-term perspective, a stepwise approach was used: (i) first the residues were treated with acidic solutions to obtain specific equilibrium pH-values (this step was identical to the commonly used pH-dependence test, e.g., ref 27), (ii) then the residues were washed to remove excess salts and metals “activated” by the pH-adjustments, and finally (iii) the leaching was determined by batch tests. First, 100 mL of water containing small amounts of HCl was added to 10 g of residue obtaining a series of batches with pH-values in the range of pH 4-13. HCl additions varied according to the desired end-point-pH and the acid neutralizing capacity of the residues. Batches were equilibrated in an end-over-end mixer for 72 h; solutions were separated from solids by centrifugation. The remaining solids were washed in two steps at L/S 2-3 L/kg for about 5 min, and then at L/S 10 L/kg for 24 h. After each step, the solutions were separated from the solids by centrifugation. Two leaching steps were then performed at L/S 10 L/kg for another 24 h, pH was measured, and solution samples were taken after centrifugation. This procedure allowed us to determine the leaching as a function of pH at conditions more likely to represent real-life conditions in a long-term perspective. For details, refer to the Supporting Information. Geochemical Modeling. Mineral saturation indices, solution speciation, and solubilities of minerals were calculated 3552

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using a database-expert system developed at ECN (The Netherlands) with the name “LeachXS” (28). Leaching data were subjected to speciation calculations using an application of the modeling framework ORCHESTRA (29). The MINTEQA2 (30) thermodynamic database, version 3.11, was used for all calculations. For changes to the MINTEQA2 database, we refer to Dijkstra et al. (12). The modeled “solubility curves” represent predicted element concentrations in equilibrium with specific minerals, calculated for each data point separately. In this paper, the leaching behavior of the elements Al, Ba, Ca, Cr, Pb, S, Si, V, and Zn is discussed. The elements were chosen as examples of important major elements, such as Ca and S, governing the solution chemistry and pH, while heavy metals, such as Cr, Pb, V, and Zn, are typical problem elements in leaching from waste incineration APC residues. Al, As, Ba, CO3, Ca, Cd, Cl, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sr, V, and Zn concentrations were included in the modeling. Further details are given in the Supporting Information.

Results and Discussion Overview of Results. In Figures 1 and 2, measured solution concentrations are shown as a function of pH in individual batches. For each element, solubility curves are shown for the minerals that most likely controlled the release to solution. It is important to note that element concentrations represent the residues potential for leaching at a specific pH rather than a prediction. Solubility control from a specific solid phase requires that the necessary elements are present in the residue at a given pH. The modeled solubility curves consequently provide information about which mineral phases may potentially control solubility at specific pHvalues. Figure 1 includes data for the major elements Al, Ba, Ca, S, and Si for carbonated and noncarbonated samples of the semi-dry (SD) residue as well as the fly ash (FA). Figure 2 includes data for the heavy metals Cr, Pb, V, and Zn. These elements are discussed individually in the following. The Supporting Information summarizes the mineral phases most likely to exercise solubility control. Aluminum, Al. Measured solution concentrations (10-710-4 M) followed a u-shaped dependency on pH, which is often observed for incineration ashes (e.g., 5, 13). Al concentrations, however, significantly decreased with increasing pH above pH 9.5. Overall, no significant difference between the leaching behavior of carbonated and noncarbonated samples was observed, except for the lower natural pH of the carbonated samples. Good agreement between the Al leaching from the SD and FA residues was observed; only a few data points for the FA residue at pH 5-6.5 seemed to deviate. Comparing measured data with the solubilities of Al(OH)3, Al2O3, and ettringite (Ca6Al2(SO4)3OH12‚26H2O), it was found that ettringite likely controlled Al leaching at pH-values above 9.5-10 for both residues. Below this, amorphous Al(OH)3 adequately predicted the measured concentrations: for the SD residue down to pH 5-6, and for the FA residue down to pH 6. The few outliers mentioned above for the FA residue were described by the solubility of Al2O3. Al(OH)3, Al2O3, and ettringite have previously been identified in waste incineration APC residues (13, 22); however, only amorphous Al(OH)3 has been suggested to control leaching in nonstabilized APC residues (13, 18). It should be noted that ettringite has been identified in cementsolidified APC residues and found to control Al leaching in these matrixes (15, 16, 31). From our experiments, it appears that ettringite may also be controlling in nonsolidified APC residues at high pH. Calcium, Ca. Variations in leached Ca with respect to changes in pH were less profound than for Al; concentrations were in the order of 10-2 M in the investigated pH range. No

FIGURE 1. Measured solution concentrations of Al, Ba, Ca, S, and Si as a function of pH. Solid and dashed lines represent model predicted equilibrium concentrations for the noncarbonated and carbonated residues, respectively. Model predictions are shown only for saturation indices within (3. If the model lines coincided, only the solid lines for noncarbonated residues are shown. Abbreviations are: gyp, gypsum; bar, barite; Ba(S,Cr)O4(1), XCr ) 0.23; cal, calcite; ett, ettringite; leu, leucite; por, porlandite; qtz, quartz; wai, wairakite; wol, wollastonite. significant differences between the two residues, carbonated and noncarbonated, were observed (see below). Ca concentrations slightly decreased with increasing pH from pH 9.5-10; however, the concentrations in the SD experiments slightly increased again at pH 11-12.5. This was not the case for the FA residue as this residue had a lower natural pH of 11.6. For both residues, gypsum (CaSO4‚2H2O) generally predicted the solution concentrations adequately at pH-values below 9.5. One outlier at pH 5 for the FA residue was better described by a solid solution with Ba: (Ca,Ba)SO4. At pH above 9.5, Ca was more likely controlled by ettringite as the solubility of gypsum increased. Although gypsum solubility

matched the measured Ca concentrations at pH 12.5 for the SD residue, it appears more likely that portlandite (Ca(OH)2) was controlling at this high pH. Gypsum has been reported in several studies on APC residues (13, 16-18, 22, 24) and has been suggested for solubility control (13, 18). Portlandite has been identified in nonstabilized APC residues (21, 22) but not reported to control Ca concentrations in these systems, although solubility control from portlandite has been observed for bottom ashes (5). Carbonation of the residues had only marginal effects on the pH-dependent leaching. It was expected that carbonation would result in solubility control from calcite in the carbonated residues; however, apparently this was not the case as VOL. 40, NO. 11, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Measured solution concentrations of Cr, Pb, V, and Zn as a function of pH. Solid and dashed lines represent model predicted equilibrium concentrations for the noncarbonated and carbonated residues, respectively. Model predictions are shown only for saturation indices within (3. If the model lines coincided, only the solid lines for noncarbonated residues are shown. Abbreviations are: Ba(S,Cr)O4(1), XCr ) 0.23; (2), XCr ) 8 × 10-5; caz, calcium zincate; cer, cerrusite; cro, crocoite; ota, otavite; smi, smithsonite; wil, willemite; zin, zincite. calcite was oversaturated at pH-values above 8. Possibly, the carbonation reaction was not complete throughout the individual ash particles. It is commonly observed, however, that calcite is oversaturated in incineration residue systems with saturation indices of about 1-2 (9, 11, 18). The fact that calcite often appears oversaturated may indicate that the thermodynamic data do not reflect the characteristics of typical calcite phases in ash systems; that is, small-sized crystals are thermodynamically less stable than larger crystals, and poorly ordered mineral phases may be more soluble than well-ordered versions (32). The apparent lack of equilibrium with calcite could also be due to slow reaction kinetics. Sulfate, S. Measured concentrations of S, interpreted as sulfate concentrations, followed those of Ca. The pHdependence of S was qualitatively similar to that of Ca, and concentrations were observed in the same order of magnitude as Ca (10-2 M). Carbonation did not seem to have significant effects on sulfate leaching. Gypsum solubility was found to match the measured data at pH-values below 9-10. The fact that both Ca and S were 3554

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found to match gypsum solubility is a strong indication that gypsum was actually a controlling mineral. Above pH 9-10, sulfate concentrations decreased and ettringite was a more likely candidate, although ettringite was somewhat oversaturated in the SD residue. A few data points were better described by (Ba,Ca)SO4 solid solutions (see below). Solid solutions of BaSO4 and CaSO4 have not been identified in APC residues, although BaSO4 (as Barite) (22) and CaSO4 (as anhydrite and/or gypsum) (13, 16-24) have been reported. The fact that Al, Ca, and S were all close to saturation with ettringite strongly supports ettringite as a controlling mineral at high pH. Barium, Ba. Leached concentrations of Ba were almost independent of pH and observed in the range of 10-5-10-7 M. One FA data point at pH about 11.6 was observed having a significantly higher concentration than the other data. No clear effect from carbonation could be observed. Figure 1 shows model curves for barite as well as a (Ba,Ca)SO4 solid solution. An indicative solubility constant of the latter phase was calculated assuming ideal solid solution behavior from the end members barite (BaSO4) and

gypsum (CaSO4). We did not check corresponding activity ratios in solution. The actual level of Ca substitution may well be different from the solids shown in Figure 1. However, the solubilities for Barite and Ba0.5Ca0.5SO4 are rather similar and may here illustrate the appropriate level. The solubility of BaCrO4 (not shown) was generally more than 3 orders of magnitude higher than the measured Ba concentrations, indicating that BaCrO4 was very undersaturated and unlikely for solubility control. Predicted concentrations for Ba(S,Cr)O4 solid solutions (33) were generally within 1 order of magnitude from the measured Ba concentrations, which indicates that such a solid solution may control Ba solubility. Similar conclusions have previously been made for bottom ash (34) and coal fly ash (35). It should be noted that also solid solutions of Sr in Barite have been suggested (35). In these experiments, Ba concentrations predicted by (Ba,Sr)SO4 were about 1 order of magnitude higher than the concentrations predicted by Ba(S,Cr)O4. This suggests that control from a solid solution with Sr was less likely than Ba(S,Cr)O4. Chromium, Cr. The Cr concentrations showed more variations in the investigated pH range than observed for the other elements. These variations, up to about 3 orders of magnitude, could not be explained solely by a dependency on pH. Carbonation did not seem to cause these variations, although all carbonated SD samples below pH 6.5 had low Cr concentrations. A number of SD and FA batches had significantly lower concentrations (in the order of 10-8 M) than the remaining batches (10-6-10-5 M). With respect to the modeling, Cr concentrations were treated as Cr(VI) species due to the oxidized nature of the residues. Except for the batches with low concentrations, it appeared that crocoite (PbCrO4) adequately described the Cr concentrations at pH-values below pH 8-9. Above this pH, the solubility increased dramatically, suggesting that crocoite was not controlling Cr leaching. BaCrO4 was highly undersaturated in most samples, typically with saturation indices in the range of -3 to -7. However, as observed for Ba, solid solutions of Cr in BaSO4 may be important for controlling Cr solubility. BaS0.77Cr0.23O4 (33) was undersaturated, typically 1-2 orders of magnitude at pH above 8; however, the shape of the model curves followed the measured data reasonably well. Cr solid phases have not been identified in APC residues; however, if Cr exists as Ba(S,Cr)O4 solid solutions (either in the original sample or formed as secondary phases), it is likely that the relative fractions of Cr and S vary among residues and possibly also at the scale of individual particles. Kersten et al. (34) investigated Ba(S,Cr)O4 solubility in bottom ash leachate and found that a mole fraction of XBaCrO4 ) 8 × 10-5 explained their data. Using a similar approach applying the same mole fraction (see Supporting Information), we found Cr concentrations in the order of 10-5 M (cf., Figure 2). Although the resulting model curves did not match the measured Cr concentrations exactly, the overall level was the same. This suggests that Ba(S,Cr)O4 phases may be suitable candidates for Cr solubility control and is further supported by the fact that Ba concentrations were close to saturation with Ba(S,Cr)O4. Decreases in Cr concentrations at high pH could potentially be caused by incorporation of Cr(VI) into low solubility minerals. Ettringite may be a candidate for incorporation of Cr(VI), as ettringite was close to equilibrium with Al, Ca, and S in these experiments. The Cr(VI) analogue of ettringite (36, 37) was not found to be close to saturation, but the potential role of ettringite phases with partial Cr(VI) substitution cannot be excluded on the basis of this calculation. Model curves presented in Figure 2 suggest that Ba(S,Cr)O4 phases may also have caused a similar decrease in solubility at high pH. The remaining batches having low concentrations (pH 4.58.5) are not easily explained by solubility control; however,

such low concentrations may be caused by reduction of Cr(VI) by Al(0) present in the APC ash (38, 39). Cr(VI) is present as an anion and sorption at low pH-values may be relevant (40), although competing anions such as SO42- may dramatically reduce sorption (41). Lead, Pb. Generally, Pb concentrations followed a ushaped dependency on pH as typically observed for bottom ashes (6, 12) with concentrations in the order of 10-8-10-4 M. Two data points at pH 8.5 and 10.5 for the SD residue deviated from this pattern by having concentrations about 1-2 orders of magnitude higher than the others at similar pH-values. Again, no significant effect was observed from carbonation. Pb leaching appeared to be controlled by several minerals. At pH-values below 8-9, Pb concentrations seemed to be predicted relatively well by Pb2V2O7. Above pH 9, the solubility of Pb2V2O7 increased and concentrations were better described by the solubility of Pb2O3. It should be noted that Pb2O3 requires rather oxidizing conditions; however, Pb2O3 has previously been identified in incineration ashes (42, 43). A few data points with significantly higher concentrations could be explained by cerrusite (PbCO3). Except Pb2O3, we have found no reports of these minerals being identified in incineration residues; however, cerrusite has previously been suggested as a potential solubility-controlling mineral (13, 18). It should be noted that Pb leaching is probably influenced by sorption processes in bottom ashes (e.g., 6, 8, 9, 12) and good descriptions have been made using surface complexation models. Vanadium, V. Generally, V concentrations varied 1-2 orders of magnitude (10-9-10-7 M) in the pH range investigated, with no apparent correlation to pH (Figure 2). No effect of carbonation could be identified. Pb2V2O7 and Pb3(VO4)2 could to some extent predict the measured concentrations at pH less than 10; however, a few data for the SD residue around pH 8 and above pH 11 were not accounted for by these minerals. Although the V data were rather scattered, the observation that a Pb-V mineral could be important for both Pb and V indicates that such a mineral may potentially be controlling solubility. We have found no reports of these phases being identified or suggested for solubility control in incineration residues. Silica, Si. Overall, Si leaching increased as pH decreased: about 2 orders of magnitude in the range 10-5-10-3 M. Similar behavior has been reported for fly ashes (13). A few batches had concentrations about 1 order of magnitude higher than the others around pH 10-11. No significant effect of carbonation was observed. Quartz, although identified in APC residues (13, 17, 2124), was not found to control Si concentrations as the leaching behavior of quartz was qualitatively different from the measured data. However, Kirby and Rimstidt (18) suggested that quartz undersaturation at high pH in combined bottom and fly ashes may be caused by kinetic effects. In these experiments, it appeared more likely that leucite (KAlSi2O6) or wairakite (CaAl2Si4O12‚2H2O) controlled Si concentrations at pH-values below pH 10-11. At higher pH-values, concentrations seemed to be lower than predicted by either leucite or wairakite. Possibly, wollastonite (CaSiO3) controlled solubility at high pH, although this mineral was oversaturated by about 1-2 orders of magnitude. Wollastonite has been identified in APC residues (13, 22). We have found no reports of identification of either leucite or wairakite in APC residues. Zinc, Zn. The Zn concentrations were found in the order of 10-8-10-4 M with a u-shaped dependency on pH that is often observed for incineration residues (6, 7). Generally, willemite (Zn2SiO4) was close to saturation above pH 7. Willemite has been identified in fly ashes (44). Some batches had concentrations (about 1 order of magnitude) higher than expected for willemite in the pH range of 8-10; however, VOL. 40, NO. 11, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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these could generally be predicted by zincite (ZnO) solubility. At high pH (10.5-12), a few apparent outliers were adequately described by calcium zincate (CaZn2(OH)6‚2H2O), which is a solid solution of Zn in portlandite often found in strongly alkaline/cementitious systems (8). The fact that portlandite was found in equilibrium with Ca at high pH supports the likelihood of calcium zincate controlling Zn concentrations at high pH (8). Below pH 7, Zn concentrations appeared to be undersaturated with respect to willemite; this may, however, be attributed to the solubility of ZnSiO3. Generally, smithsonite (ZnCO3) was undersaturated by more than 1 order of magnitude; Zn concentrations were closest to smithsonite solubility at pH 7-9. As the shape of the smithsonite solubility curves did not match the observed data, we conclude that this mineral was not controlling Zn leaching. It should be noted that smithsonite has previously been suggested to control Zn leaching in fly ashes (13) while smithsonite does not appear to be important in bottom ashes (6-8, 11). Zincite, but not smithsonite, has been identified in APC residues (13, 22, 24). In bottom ash systems where solubility control did not adequately describe experimental data, Zn leaching behavior has been explained by also considering sorption processes (9, 12). Although most Zn data in these experiments appeared to be relatively well matched by solubility curves, sorption may potentially be important for Zn in systems with APC residues. Implications for Leaching Prediction. Geochemical modeling of the two APC residues investigated in this paper revealed that the same mineral phases were likely to control leaching from both residues, carbonated and noncarbonated versions. For several elements, the same mineral phase was found to control the leaching; for example, ettringite controlled the leaching of Al, Ca, and S at high pH, while Ca and S leaching was controlled by gypsum at lower pH. Overall, the geochemical modeling predicted the concentrations of most elements (Al, Ba, Ca, Pb, S, Si, and Zn) within 1 order of magnitude in the range of pH 4.5-12.5, based on solubility control only. A few data points for Pb and Zn, and several data points for Cr and V, could not be adequately explained by solubility control from the mineral phases in the thermodynamic database. Because the observed concentrations in some cases were below concentrations predicted by mineral solubility, other mechanisms such as sorption, redox processes, or incorporation into other mineral phases may be important for these elements at certain pH-values. Additional modeling efforts on APC residues are needed to draw conclusions in this regard. To perform a complete assessment of impacts related to APC residue disposal, it is necessary to combine the modeling results presented in this paper with the results reported in the literature focusing at conditions relevant for “short-term” leaching including high salt levels. Knowledge about potential solid phases important for solubility control may provide a basis for leaching prediction using geochemical models. When interpreting results from such modeling efforts, researchers should realize that other aspects such as preferential flow, permeability changes, biological activity, and extremely slow weathering reactions may potentially play a role in a real disposal situation. Within this framework, we find that the leaching tests evaluated in this paper provide us with an improved basis for determining the solubilitycontrolling phases in a long-term perspective and that the modeling results provide a useful foundation for leaching prediction using geochemical models.

Acknowledgments This research was financed jointly by the Center for Waste Research (DHI - Water and Environment, DK), the Danish 3556

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Research Agency, and partially by the Dutch Ministry of Spatial Planning, Housing and Environment as part of the environmental research program of ECN.

Supporting Information Available Further information about residue samples (chemical composition, handling), experimental procedure (methods and analytical techniques used), and geochemical modeling (Cr modeling and tables summarizing modeling results). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review November 9, 2005. Revised manuscript received February 20, 2006. Accepted March 13, 2006. ES052250R

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