Environ. Sci. Technol. 2005, 39, 5736-5741
Simultaneous Application of Dissolution/Precipitation and Surface Complexation/Surface Precipitation Modeling to Contaminant Leaching D E F N E S . A P U L , * ,† KEVIN H. GARDNER, AND T. TAYLOR EIGHMY Environmental Research Group, University of New Hampshire, Durham, New Hampshire 03824 A N N - M A R I E F A¨ L L M A N ‡ Swedish Geotechnical Institute, Olaus Magnus va¨g 35 SE-581 93 Linko¨ping, Sweden ROB N. J. COMANS Energy Research Center of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands
This paper discusses the modeling of anion and cation leaching from complex matrixes such as weathered steel slag. The novelty of the method is its simultaneous application of the theoretical models for solubility, competitive sorption, and surface precipitation phenomena to a complex system. Selective chemical extractions, pH dependent leaching experiments, and geochemical modeling were used to investigate the thermodynamic equilibrium of 12 ions (As, Ca, Cr, Ba, SO4, Mg, Cd, Cu, Mo, Pb, V, and Zn) with aqueous complexes, soluble solids, and sorptive surfaces in the presence of 12 background analytes (Al, Cl, Co, Fe, K, Mn, Na, Ni, Hg, NO3, CO3, and Ba). Modeling results show that surface complexation and surface precipitation reactions limit the aqueous concentrations of Cd, Zn, and Pb in an environment where Ca, Mg, Si, and CO3 dissolve from soluble solids and compete for sorption sites. The leaching of SO4, Cr, As, Si, Ca, and Mg appears to be controlled by corresponding soluble solids.
Introduction Steel slag is a byproduct of steel making and has traditionally been used in road construction because of its good engineering properties. In the U.S., approximately 6.5 million tons of steel slag is produced, three-quarters of which is used in road construction as asphaltic concrete aggregate (21%), as fill (21%), and as road base (37%). Other uses of steel slag are in railroad ballasts, ice control, soil remineralization and conditioning, neutralization of industrial discharge and mine drainage, roofing granules, and landfill daily cover materials (1). Steel slag is formed at very high temperatures (12001700 °C). After it cools to atmospheric conditions, it chemically weathers mainly due to oxic conditions, lower tem* Corresponding author phone: (419)530-8132; fax: (419)530-8116; e-mail:
[email protected]. † Present address: Mail Stop 307, 2801 W. Bancroft St., Department of Civil Engineering, University of Toledo, Toledo, OH 43606. ‡ Present address: The Swedish Environmental Protection Agency, S-106 48 Stockholm, Sweden. 5736
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peratures, and the presence of water. The secondary minerals that form are oxides of aluminum and iron, which may coat the surface of the steel slag grains (2-4). Amorphous oxide minerals of aluminum and iron may have a significant impact on the mobility of trace elements because of their large surface areas, microporous structures, and abundance of binding sites (5). In this research, we investigated the leaching of 12 anions and cations from a five-year old weathered steel slag by pH stat experiments and geochemical modeling of the ions in aqueous, sorbed, and precipitated phases. Considering that aging reactions and formation of new minerals may affect the release of contaminants by surface complexation, surface precipitation, and dissolution/precipitation reactions, we considered all these chemical reactions in conjunction with aqueous complexation. This contrasts the majority of research reported in the literature, which has focused on modeling only dissolution/precipitation reactions of major elements in the residues (6). Two exceptions are the works of Meima and Comans (7) and Dijkstra et al. (8) who investigated the interactions of metals with hydrous oxides and successfully predicted the release of Cu, Mo, Pb, and Zn from MSWI ash. They used the surface complexation model (SCM) and the surface precipitation model (SPM), which have been extensively applied to various surfaces such as pure metal (Al, Si, Mn, and Fe) hydroxides (9-11), carbonates (12), sulfides (13), natural soils (14), sediments (15), and bacterial surfaces (16). While SCM/SPMs have been used for many surfaces, most of these systems have been simpler, dual, or tertiary systems with few components, unlike the conditions of leaching from residues where dozens of ions are present and may competitively affect leaching. The goal of this research was to simultaneously consider surface complexation, surface precipitation, and solubility controls for multiple components. We modeled leaching in the presence of multiple analytes as opposed to modeling each element in isolation from other species because the nature of metal speciation on solid surfaces is a function of not only pH but also of surface coverage, competitive adsorption, ionic strength, and aqueous phase complexation (17). The steel slag sorptive surfaces were assumed to be characteristic of hydrous ferric oxides, and the generalized double layer model and the SPM described in detail in Dzombak and Morel (18) and Zhu (12) were used. All reactions were modeled using Visual MINTEQ, which includes the HFO sorption constants from Dzombak and Morel (18). Inputs for the model such as background analytes, sorbate, and sorbent concentrations were obtained from pH stat leaching experiments, availability tests, and selective chemical extractions of the steel slag, respectively.
Materials and Methods Steel Slag and Laboratory Measurements. The steel slag was obtained from a Swedish electric arc furnace steel plant designed to produce low alloy steel from scrap steel. The slag is alkaline and dominated by Ca (22%), Al (2%), Fe (24%), Mg (4%), Mn (4%), Si (6%), and Cr (0.8%). The slag was placed in lysimeters at the Swedish Geotechnical Institute in Linko¨ping, Sweden in December 1992 and excavated in October 1997. Further details of sample treatment and description of lysimeters and laboratory experiments are given in the Supporting Information. Excavated samples were stored and sieved (4 mm sieve) under N2/Ar to minimize the impact of O2 or CO2 on leachate characteristics. pH dependent leaching experiments were performed at room temperature, for 24 h, at a liquid to solid (L/S) ratio of 5 L/kg. Neutral to 10.1021/es0486521 CCC: $30.25
2005 American Chemical Society Published on Web 06/28/2005
alkaline pHs (6, 8, 10, and 12) were selected to bracket the pH between freshly produced slag (pH ∼12) and potential future pH values associated with carbonation (pH 7-8). The leachates were filtered through 0.2 µm filters and split into three samples. One split was preserved with HNO3 for analysis of Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, Pb, S, Si, V, and Zn by AAS, GFAAS, or ICP-MS. The second split was left unpreserved and analyzed by IC for Cl, Na, K, and SO4. The third split was analyzed for carbonates by high-temperature catalytic combustion and IR analysis of CO2. For those ions that are unsaturated with respect to their solids, the sum of the concentrations associated with aqueous and surface species (available sorbate concentrations) is necessary for the SCM/SPMs. These total availability values were estimated from maximum potential leaching of the metals in the raw steel slag. (See Supporting Information for more details on sample collection and experiments.) To determine mineral surface concentrations, X-ray photoelectron spectroscopy (XPS) and operationally selective chemical extractions were used. For particle surface analysis using XPS, steel slag samples were ground to FehCrO4-, >FehH2AsO4, >FehHAsO4-, >FehMoO4-, >FehO-, >FehOHAsO4-3, >FehOHCrO4-2, >FehOHMoO4-2, >FehOHSO4-2, >FehSO4-, and >HghOH2+) with concentrations less than 0.01% of the total high affinity surface site concentration (horizontal dashed line at 4.42 × 10-9 M) and surface precipitates are not shown in the figure. Surface complexed ions are denoted as >FehIon. >FehO- and >SOHh are unoccupied high affinity surface sites. The notation for Cd, Cu, Zn, and Pb excludes Fe to indicate that these metals have surface precipitates in addition to surface complexes. arc furnace steel slags using IR spectroscopy. Dolomite was also observed with XRD in ladle slag from an electric arc furnace steel production plant (24). Zevenbergen et al. (25) also reported dolomite as a secondary mineral resulting from weathering of MSWI bottom ash at high water infiltration rates. Total dissolved Ca and Mg concentrations predicted in the model are within 1 order of magnitude of the leachate concentrations measured in pH-dependent leaching experiments. Dolomite did not appear to be controlling the solubility of carbonate; therefore, carbonate concentrations (in modes 3 and 4) were fixed in the model to aqueous
FIGURE 3. Surface coverage distribution on low affinity sites at pH 6, 8, 10, and 12. Surface precipitates are not shown. Surface complexes (>CdOH2+, >FeH2AsO4, >FeHAsO4-, >FeMoO4-, >FeOBa+, >FeOHAsO4-3, >FeOHCrO4-2, >FeOHMoO4-2, >FeOHSO4-2, >FeSO4-, and >HgOH2+) with concentrations less than 0.01% of the total low affinity surface site concentration (horizontal dashed line at 1.77 × 10-3 M) and surface precipitates are not shown in the figure. Surface complexed ions are denoted as >FeIon. >FeO- and >SOH are unoccupied weak surface sites. The notation for Cd, Cu, Zn, and Pb excludes Fe to indicate these metals have surface precipitates in addition to surface complexes. concentration measured in pH-dependent leaching experiments. SiO2 was detected in the weathered steel slag sample using XPS. Quartz (SiO2) and amorphous silica in steel slags have been observed by Kartbaoui et al. as well (22). The equilibrium predicted concentrations of Si are higher than the concentrations measured in pH-dependent leaching experiments possibly because the duration of leaching experiments was too short to allow such high concentration dissolution and equilibration of Si. The rapid release of Si to solution through the dissolution of glasses, quartz, and other silicates might have been slower than the precipitation of SiO2 (am), as has also been hypothesized for MSWI ash leaching (26). VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Adsorption was not expected to control Cr concentrations because Cr(VI) adsorption to iron oxides is relatively weak and decreases in the presence of CO3 and SO4 (27, 28). Ba(S0.96Cr0.04)O4 was selected as the solubility controlling solid based on earlier evidence (29). Ba and SO4 and Ba, SO4, and Cr were found to be associated in naturally weathered archae-metallurgical slags (30) and coal fly ash samples, as well (31). In modes 3 and 4, Ba concentration in the model was fixed to the aqueous concentrations measured in pHdependent leaching experiments because this element could not be modeled accurately. Cd was undersaturated with respect to any of the solids in the Visual MINTEQ database. SCM (mode 3) overestimated and SPM (mode 4) underestimated the measured concentrations. While SPM provides a better estimate of the measured Cd concentration than SCM, the criteria set by Dzombak and Morel (18) with regard to the likeliness of surface precipitation were not met. Dzombak and Morel (18) suggested that as a rule of thumb, SPM is likely to become significant when (i) the dissolved sorbate concentration exceeds one-tenth of its solubility or (ii) the dissolved sorbate concentration exceeds one-half of the total surface site concentration. The modeling results suggest that leaching of Pb and V is controlled by Pb3(VO4)2 solubility at pH 6 and 8. At higher pH values, the solid is completely dissolved, and sorptive surfaces dictate the extent of leaching of Pb and V by surface precipitation and surface complexation reactions, respectively. The importance of surface reactions in the release of Pb was anticipated as Pb is well-known to have a high affinity for iron oxides (32). Carlier et al. (30) also observed Pb in hydrous iron oxides in the archeo-metallurgical slags. The need for SPM for both Pb and Zn was also expected since the dissolved Zn and Pb concentrations were close to one-tenth of their solubility values (18). Surface complexation provided good estimates of Zn except at high pHs where consideration of surface precipitation reactions was needed. While Zn and Pb formed aqueous complexes with carbonate, nitrate, hydroxide, sulfate, and chloride, these aqueous complexes were less than 1 % of total Zn or Pb in the system except at pH 6 when 4% of Pb and 96% of Zn was in aqueous phases. The percentage of Pb and Zn that precipitated on the surface varied from less than 1 % to more than 96% for both of the ions as the pH increased from 6 to 12. Consideration of surface reactions for Pb and Zn is essential because the modeling results show that (i) both Pb and Zn mainly speciate onto the surfaces and (ii) the surfaces are mainly occupied by Pb and Zn (Figures 2 and 3) where they compete for available sites on the surface. Our modeling observation of surface precipitation at high pHs is consistent with literature data. Shuman (33) proposed that Zn may form Zn(OH)2(s) upon sorption to hydrous Al oxide at pH values above 8. In a Zn-amorphous silica system, Roberts at al. (34) showed that the formation of an amorphous Zn(OH)2 precipitate with tetrahedral coordination between Zn and O occurred only at the highest pH that they studied, which was 7.5. Van der Hoek and Comans (35) have shown that As leaching in coal fly ash was controlled by sorption onto HFO. However, they observed more than 2 orders of magnitude difference in the amounts leached between pHs 6 and 12; in this paper, the amounts leached were almost the same across this pH range. These observations suggest that sorption may not be the major mechanism controlling the release of As and points to the presence of a solubility controlling solid in the system. Evidence from modeling for the presence of BaHAsO4/H2O as a secondary mineral in the weathered steel slag is in agreement with its occurrence in groundwater and drinking water systems (36-38). 5740
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On the basis of saturation indices, Cu(OH)2 was determined to be the only solid species in the database that could have controlled the release of Cu. Cu was significantly undersaturated with respect to Cu(OH)2 at low pHs, suggesting that interactions of Cu with sorptive surfaces and other aqueous complexes were the major processes controlling the release of Cu. SCM and SPM underestimated the measured concentration at neutral pHs. We would expect Cu concentrations to be higher if we had considered Cu complexation with dissolved organic carbon. However, the measured dissolved organic carbon was low at pHs 8 and 10 (2 and 3 mg/L) (where an increase in modeled Cu concentration would have fit the measured data better) and high at the end pHs (10 and 11 mg/L), suggesting that the release of Cu is possibly more complicated than what we considered in the model. The major species of Cu in the model were sorbed (all modeled pHs) and coprecipitated Cu (at pH ) 12 and 10) followed by Cu complexation with CO3 (at pH ) 6), NO3 (at pH ) 6) (experimental artifact due to added HNO3), and hydroxide (at pH ) 12). Modeling results for Mo are inconclusive. None of the Mo containing solids had low saturation index values, and modeling of Mo speciation onto sorptive surfaces did not appear to explain the leaching behavior of Mo even when sorption constants for Mo in Visual MINTEQ database estimated by linear free energy relationships (18) were replaced by Gustafsson’s (39) measured sorption constants for Mo.
Acknowledgments We greatly appreciate Mindy Weimer’s contribution to the early stages of this work. We thank Jon Petter Gustafsson for his helpful comments on the use of Visual MINTEQ software. This work was funded through a cooperative agreement (DTFH61-98-X-00095) between FHWA and the University of New Hampshire. The experimental part was financially supported by Swedish Waste Research Council under Grants AFN Dnr 261/97 and the Swedish Geotechnical Institute, which is greatly acknowledged.
Supporting Information Available Text describing the details of sample collection and treatment, and tables providing more information on model runs. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review August 28, 2004. Revised manuscript received May 19, 2005. Accepted May 20, 2005. ES0486521
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