Environ. Sci. Technol. 1999, 33, 1424-1429
Complexation of Cu with Dissolved Organic Carbon in Municipal Solid Waste Incinerator Bottom Ash Leachates JEANNET A. MEIMA,† A N D R EÄ V A N Z O M E R E N , A N D ROB N. J. COMANS* Netherlands Energy Research Foundation (ECN), P. O. Box 1, 1755 ZG Petten, The Netherlands
The complexation of Cu with dissolved organic carbon (DOC) in leachates from fresh and 1.5-year old municipal solid waste incinerator (MSWI) bottom ash was studied using a competitive ligand-exchange solvent extraction procedure. At least two different ligands appear to be involved in the complexation of copper with DOC. The dissolved Cu appears to be 95-100% organically bound in leachates from both the fresh and the weathered bottom ash, and geochemical modeling indicates that the leaching of Cu from these ashes is primarily controlled by the availability of the organic ligands in the bottom ash. The mechanism that binds Cu to the solid phase is likely to be tenorite in the fresh bottom ash, and sorption to amorphous Fe/Al-(hydr)oxides in the weathered bottom ash.
Introduction Municipal solid waste is being incinerated in increasing quantities, leaving behind large amounts of bottom ash enriched in potentially harmful elements relative to soils and sediments. For the environmentally safe usage or disposal of these residues, knowledge of the processes controlling the release of potentially harmful elements is required. This paper focuses on the relatively high amounts of copper that are often leached from municipal solid waste incinerator (MSWI) bottom ash (1, 2). It has been suggested that the leaching of Cu from MSWI bottom ash is significantly enhanced by the complexation of Cu with dissolved organic carbon (1-5). Dissolved organic carbon (DOC) has been shown to be a major component of MSWI bottom ash leachates (6, 7), and copper is known to have a very high affinity for organic ligands (e.g., 8, 9). Bottom ash leachates were calculated to be strongly oversaturated with respect to common secondary copper minerals such as tenorite (CuO(s)) and Cu(OH)2(s) in case only inorganic complexation reactions were taken into account (1, 2). However, when the residual organic material was removed by heating the bottom ash for 8 h at 550 °C, a strong reduction in the leaching of Cu was obtained (1). Typical values for LOI (loss on ignition) for MSWI bottom ash from mass-burn incinerators range from 2 to 6%, which represent mainly organic carbon, carbonates, and tightly bound water (5). Ferrari (10) investigated the nature of DOC in fresh MSWI * To whom correspondence should be addressed; telephone: +31 224 564218; fax: +31 224 563163; e-mail:
[email protected]. † Present address: Netherlands Institute of Applied Geoscience TNO, P. O. box 6012, 2600 JA Delft, The Netherlands 1424
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bottom ash leachates and found the DOC in alkaline extracts to consist mainly of low molecular weight (99% recovery of Cu from the toluene, compared to approximately 91% in the single extraction with 2 mL of 1 M HNO3. The total concentration of Cu in the combined HNO3 fractions was determined by ICP-AES. Titrations. To determine the concentrations and stability constants of Cu-DOC complexes at different ligand saturations, bottom ash leachates were titrated with acac and Cu prior to CLE-SE analysis. Cu was added to 50 mL subsamples of the leachates at concentrations in the range 2-20 µmol/L. The subsamples were agitated continuously for 24 h. After 2 and 22 h of equilibration the pH was readjusted to the value prior to Cu addition using analytical-grade 1 M HNO3 or 1 M NaOH. The solutions were filtered through prewashed 0.2 µm membrane filters and subjected to CLE-SE analysis using 10-5 M acac. Titrations with acac were performed in filtered bottom ash leachates with no extra Cu added. Used acac concentrations varied between 10-5 and 10-2 M. Speciation Calculations. Equation 1 gives the mole balance for Cu and also includes the species that result from the addition of acac and toluene.
[Cu]T ) [Cu*] + [Cu(acac)20] + [Cu2+] + [Cu(acac)+] + [CuXi] + [CuLi] (1) [Cu]T is the total analytical concentration of Cu, [Cu*] the Cu(acac)20 transferred to the toluene phase, [CuXi] the sum of the inorganic complexes, and [CuLi] the sum of the naturally occurring Cu-DOC complexes. [Cu]T and [Cu*] were measured by ICP-AES as described above. The concentrations of the remaining species were calculated from the equations given in Table 1, and from the distribution coefficient of Cu(acac)20 between toluene and water (the Kd(Cu), see below).
In addition, the speciation code MINTEQA2, version 3.11 (14), was used to calculate the aqueous speciation of acetylacetone and the complexation of Cu with inorganic ligands. Input in MINTEQA2 were the total concentrations of inorganic components in the leachates measured by chemical analysis and the concentration of acetylacetone added to the sample. The Davies equation was used for ionic strength corrections and the pH was fixed at the measured value. The conditional stability constant, Kcond(i) for the reaction Cu2+ + Li S CuLi as well as the total concentration of ligand Li, were determined by fitting the experimental data to a Langmuir isotherm with one or more ligands (eq 2), the number of ligands depending on the heterogeneity of the DOC. Although empirical in nature, this approach is consistent with present knowledge of the nature of the DOC in bottom ash leachates: mainly low molecular weight ( 7.5. Since tenorite has also been shown to precipitate from alkaline SO4-rich aqueous solutions at atmospheric pressure and temperature (25), tenorite may control the leaching of Cu from fresh MSWI bottom ash. In weathered/carbonated bottom ash, equilibrium with malachite and sorption to amorphous Fe/Al(hydr)oxides have been suggested to be potentially important controlling mechanisms for Cu, although the role of tenorite could not be excluded (2, 11). If inorganic complexation reactions are taken into account only (Figure 3a,b), measured [Cu]T are up to 3 orders of magnitude oversaturated with respect to tenorite, up to 1.5 orders of magnitude oversaturated with respect to malachite, and within 1-2 orders of magnitude from the predicted Cu leaching on the basis of sorption to amorphous Fe/Al-(hydr)oxides. However, if inorganic and organic complexation reactions are taken into account (Figure 3c,d), measured [Cu]T is within 1/2 an order of magnitude from the predicted [Cu]T on the basis of equilibrium with tenorite and malachite, and in the case of 1.5-year old bottom ash, also on the basis of sorption to amorphous Fe/Al-(hydr)oxides. Apparently, the differences in Cu leaching predicted on the basis of solubility control and sorption are largely masked by the large proportion of Cu bound to DOC. On the basis of these results we may conclude that the leaching of Cu from fresh and 1.5-year old MSWI bottom ash is primarily controlled by the availability of (strong) organic ligands. The mechanism that binds Cu to the solid phase is likely to be the thermodynamically most stable mineral or sorption process. For the fresh bottom ash, this appears to be equilibrium with tenorite. Above pH 7.5 Cu concentrations in equilibrium with tenorite describe almost exactly the measured [Cu]T in leachates from both fresh and weathered bottom ash. Below pH 7.5, however, equilibrium with tenorite (or any other secondary Cu mineral listed in the MINTEQA2 version 3.11 databases or in ref 26) would result in a strong increase in Cu leaching, which is not observed in the data (Figure 3c,d). Because the sorption model does predict relatively low Cu leaching down to a pH of 6.5 in the 1.5-year old bottom ash (Figure 3d), Cu leaching from weathered bottom ash may be controlled by sorption to amorphous Fe/Al-(hydr)oxides. In fresh MSWI bottom ash, however, the amount of sorption sites seems too low for sorption to be significant, as shown in Figure 3c. In conclusion, the leaching of Cu from fresh and 1.5-year old MSWI bottom ash is primarily controlled by the availability of strong organic ligands. In the absence of DOC the predicted Cu leaching would be 2-3 orders of magnitude lower. Future research, therefore, should focus on the identification of the nature of the DOC and its binding sites and the role of organic complexation in the long term.
Acknowledgments We thank Ronny Blust and Luc van Ginneken for calling our attention to the CLE-SE technique. Furthermore, we thank Nick Hoekzema for advice on statistical issues, Arjan de Koning for assistance with nonlinear regression analyses,
and Luc van Ginneken, Klaas Timmermans, Loes Gerringa, and Hans van der Sloot for stimulating discussions.
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Received for review December 29, 1997. Revised manuscript received December 28, 1998. Accepted February 2, 1999. ES971113U
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