Environ. Sci. Technol. 2007, 41, 4286-4291
Copper(II) Binding to Dissolved Organic Matter Fractions in Municipal Solid Waste Incinerator Bottom Ash Leachate S U S A N N A O L S S O N , * ,† JORIS W. J. VAN SCHAIK,‡ JON PETTER GUSTAFSSON,† DAN BERGGREN KLEJA,‡ AND PATRICK A. W. VAN HEES§ Department of Land and Water Resources Engineering, KTH (Royal Institute of Technology), 100 44 Stockholm, Sweden, Department of Soil Sciences, Swedish University of Agricultural Sciences, Box 7014, 750 07 Uppsala, Sweden, and Man-Technology Environment Research Center, Department of Natural Science, O ¨ rebro University, 701 82 O ¨ rebro, Sweden
Information on Cu speciation in municipal solid waste incineration (MSWI) bottom ash leachate is needed for Cu leaching predictions and toxicity estimates. The complexation of Cu with dissolved organic matter (DOM) in leachates from a stored MSWI bottom ash was studied potentiometrically using a Cu-ion selective electrode. More than 95% of the copper was bound to DOM in the hydrophilic fraction of the leachate, indicating that the hydrophilic acids contribute to Cu complex formation. The hydrophilic acids constituted 58% of the dissolved organic carbon in the ash leachate. Comparisons between experimental results and speciation calculations with the NICA-Donnan model and the Stockholm humic model indicated differences between the ash DOM and the natural DOM for which the models have been calibrated. The ratio of carboxylic binding sites to phenolic binding sites was 2 times larger in ash DOM, and the Cu-binding affinity of the former was stronger than accounted for by the generic Cubinding parameters. The Cu-binding affinity of the phenolic sites, on the other hand, was weaker. When these parameters were adjusted, a good description of the experimental data was obtained.
Introduction The beneficial use of municipal solid waste incineration (MSWI) bottom ash as a construction material is restricted in many countries due to concerns about heavy metal leaching. Whereas reuse of the residues saves resources in the form of natural aggregates and decreases the burden of landfills, contaminant leaching is the most important environmental aspect that inhibits the potential use of the residue as an unbound sub-base layer material in road constructions (1). The leaching of copper (Cu) is of particular interest. It has a relatively high potential toxicity (2), and it may be leached * Corresponding author fax: +46 8 4110 775; e-mail: susannao@ kth.se. † KTH (Royal Institute of Technology). ‡ Swedish University of Agricultural Sciences. § O ¨ rebro University. 4286
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 12, 2007
in considerably amounts from the MSWI bottom ash compared to natural aggregates (3, 4). The leaching of Cu from MSWI bottom ash is facilitated by dissolved organic matter (DOM) (3, 5). Between 95 and 100% of the total amount of Cu in the leachate appears to be organically bound (6). It is well known that metal-organic complexation reduces the toxicity of Cu, and methods to account for this effect in exposure assessments have recently been developed (7). In order to make correct risk assessments for the management of MSWI bottom ash, it is therefore essential to understand the speciation of Cu in the leachate and the contribution from DOM. To describe the complexation of Cu by DOM, different geochemical speciation models can be used, such as NICADonnan (8-10), Model VI (11, 12), and Stockholm humic model (SHM) (13). Generic parameter values in these models are based on large datasets containing data mainly from natural waters and soils, in which fulvic acids, generally defined as the fraction retained in the Amberlite XAD-8 column and recovered with 0.01 M NaOH (14), constitute the dominant fraction (15-17). When modeling Cu complex formation in natural waters, humic and fulvic acids are commonly the DOM fractions considered. The differences in gel volume and intrinsic binding properties for DOM in natural waters have been found to be relatively small between different humic and fulvic acids, which suggest that generalization within these groups is possible (18). Whereas DOM from natural water and its capacity to complex copper has been thoroughly investigated (8, 19, 20), there are few publications on the intrinsic properties of the MSWI bottom ash DOM for Cu complex formation and the possibilities for predicting this by speciation modeling. Such knowledge would be crucial for modeling leaching behavior of the material or assessing leachate toxicity. Van Zomeren and Comans (21) used the NICA-Donnan model (9) to describe Cu binding to humic and fulvic acids in a MSWI bottom ash leachate, based on results from competitive ligand exchange-solvent extraction analysis. However, further verification is needed if models calibrated for DOM in natural water are to be used for describing Cu speciation in MSWI bottom ash leachate on a regular basis, such as in risk assessments. Furthermore, the possible contribution of hydrophilic acids, i.e., the acid fraction of DOM not retained in the Amberlite XAD-8 column, to Cu complex formation in MSWI bottom ash leachate is not yet enough understood, although the hydrophilic components may constitute a major part of the dissolved organic matter in the MSWI bottom ash leachate (21). In the present work, we study the Cu-binding properties of MSWI bottom ash DOM potentiometrically using a Cuion selective electrode (Cu-ISE). The importance of hydrophilic compounds are demonstrated, and new parameter values for the metal-ion and proton binding in the SHM and NICA-Donnan models, optimized for the MSWI bottom ash DOM, are suggested.
Materials and Methods Sampling. A 4 months old MSWI bottom ash sample from Uppsala municipality, central Sweden, was collected from outdoor piles in March 2004 and was further stored 4 months in room temperature (around 20 °C) with free access to air. The pH value was then approximately 8.5 (L/S 5, 24 h of equilibration), which indicates that the major part of the carbonation had occurred (3). To produce MSWI bottom ash leachate, samples of 40 g of material, from which metal parts, unburned material, and particles >10 mm had been 10.1021/es062954g CCC: $37.00
2007 American Chemical Society Published on Web 05/11/2007
removed, were leached with 200 mL of deionized H2O in polyethylene flasks for 24 h in an end-over-end rotator (L/S 5). Particles were removed by centrifugation and filtration (0.2 µm). To minimize the time for leachate storage, new leachates for the experiments (fractionation, acid-base titration, pH-stat titration, and alkalimetric titration) were produced prior to each experiment. All these leachates are here referred to as BA-0, since their chemical compositions were almost identical except some small variations (e.g., 2831 mg L-1 dissolved organic carbon and 4.51-4.53 µM Cu) which were accounted for in the modeling. No effects could be found on the relation between hydrophobic and hydrophilic components by further storage of the MSWI bottom ash. Before the titrations, the leachates were stored at +2 °C for a maximum period of 2 months. During this time, the composition of the leachate was continuously controlled, and no significant changes were observed. Characterization. The pH was determined with a Radiometer glass electrode (GK2401C). Cations and anions were analyzed by ICP-AES (Jobin-Yvon JY24 ICP) and ion chromatography (Dionex DX-120), respectively. Dissolved organic carbon was determined using a fully automated Shimadzu TOC-5000A analyzer. Low-molecular-weight organic acids (LMWOA) were determined by capillary electrophoresis (22). To determine oxalate and citrate, EDTA (final concentration 250 µM at pH 9) was added in a separate run to eliminate interference from Al and Fe. The LMWOAs were identified through spiking of samples with standards and comparison of migration times. Fractionation was made according to Leenheer (23). The hydrophobic fraction is the fraction of DOM in an acidified sample (pH 2) that is retained upon passage through a column containing Amberlite XAD-8 at a certain column capacity factor, k′, defined according to k′ ) VE/(2V0) - 1, where V0 ) void volume ) porosity‚bed volume and VE ) the volume of sample passing the column. A k′ value of between 40 and 50 was used (23). The fraction of DOM that passes the XAD-8 column under these conditions is denoted as the hydrophilic fraction. Titrations. Prior to titration, subsamples were exposed to different treatments. The untreated leachate (BA-0) was passed through an H+-saturated Bio-Rad AG-MP-50 cationexchange resin column to obtain a leachate in which both trace metals and major cations were removed (BA-CE). Part of the eluate was diluted with deionized water, pH-adjusted to 2, and consequently passed through a column containing Amberlite XAD-8 to obtain the hydrophilic fraction (BA-HPI). Concentrations of major solutes in the samples used are shown in the Supporting Information (Table S1). Acid-base titrations were performed on BA-0 using a TIM900 Titration Manager coupled with a ABU900 Autoburette (both Radiometer Copenhagen), using a Radiometer glass electrode (Radiometer Copenhagen PHM 93) for pH measurements. A custom program was defined and tested versus the results of some manual titrations for validation of the procedure and equipment used. The settings used for running the titration program were the following: burette speed, 0.5 mL min-1; maximum dose per addition, 0.250 mL; stability defined as ∆pH < 0.1 mpH s-1; and maximum stability time, 300 s. Sample volumes used were 30.00 mL; titrant solution was a 0.002 M NaOH solution. pH-stat titrations were performed at pH 6.0 and pH 9.0. A total of 30 mL of the sample (BA-0, BA-CE, and BA-HPI) was first adjusted to the desired pH with HNO3 or NaOH, and then it was subjected to titration with 0.1, 1, and 10 mM Cu(NO3)2 (ACS reagent, Sigma). The total Cu concentration ranged from 5 × 10-7 M to 5 × 10-4 M, and the ionic strength was 0.05-0.06 M. The pH was kept constant (within 0.05 units) by use of dilute NaOH. The pH values recorded were used for the modeling.
Alkalimetric copper titrations were performed on 40 mL sample solutions (BA-0, BA-CE, and BA-HPI) with 0.1 M or 0.001 M NaOH, depending on the buffering capacity of the sample. The NaOH solution was added in increments until a pH of about 10 was obtained. The ionic strength conditions were similar to those in the pH-stat titrations (0.05-0.06 M). Before the alkalimetric titrations, copper nitrate (Cu(NO3)2) was added in order to obtain a similar Cu to DOC (dissolved organic carbon) ratio in the subsamples (0.1 to 0.2 µM mg-1) (Supporting Information, Table S1). However, the original leachate (BA-0) contained sufficient native Cu (4.5 µM) and hence no copper nitrate was added to this sample. The pH values were adjusted to between 2 and 3 by adding HNO3 (HCl in the case of BA-0) or NaOH. Solutions were continuously stirred and purged with N2(g) before and during the titration period to eliminate the interference of carbonate, and the temperature was kept constant at 22 °C. After each copper or base addition in the pH-stat and the alkalimetric titrations, the Cu2+ activity in solution was measured by Cu-ISE (Cu-ISE, Orion 96-29, Thermo Electron Corporation, Beverly, MA), and the pH was measured using a PHM 93 reference pH meter (Radiometer Copenhagen). Values were recorded when the average change in potential was
