Environ. Sci. Technol. 1997, 31, 735-744
Performance and Design of the Availability Test for Measurement of Potentially Leachable Amounts from Waste Materials A N N - M A R I E F A¨ L L M A N Swedish Geotechnical Institute, S-581 93 Linko¨ping, Sweden
The availability test is used for measuring the potential leachability of components from solid, mainly inorganic residues and for distinguishing between leachable substances and those likely to be retained in the matrix. The objective of the presented study was to obtain results for discussion of the principles on which the availability test is based. Test conditions influencing the leaching process were identified. Experiments were based on the Dutch two-step pH static availability test (NEN 7341) and were performed in a two-level fractional factorial design on a municipal waste incineration bottom ash. The particle size, leachate to solid ratios (L/S), and time in the second leaching step (pH ) 4) showed the largest impact on the leached amounts. A modification in comparison to existing and proposed protocols were recommended as follows: particle size of 95% less than 125 µm, L/S ratio of 100 in each step, and leaching time of 18 h in the second step. A simplified model of the leaching by diffusion for each of the particle size distributions tested showed that more than 80% leaching of the available fraction by an efficient diffusion coefficient of pDe less or equal to 15 is possible to be obtained in the test.
Introduction The determination of leaching of substances from solid waste needs one step where the potential leachability is measured. A test for this purpose shall distinguish between the part of the composition that is likely to be active in a leaching process and the part that will be retained in the matrix for a long period of time. Solid residues from combustion processes consist of a mix of melt products and original material (1). In the melt products, both amorphous and crystalline phases may be present, and the systems may show different degrees of thermodynamic stability (2). This also applies to metallurgical slags (3). Elements identified in the residue may thus be part of a stable mineral with a varying degree of solubility, or they may be part of a thermodynamically unstable phase. The measurement of potential leachability is also needed in studies of the kinetics in the leaching process (4). Leaching by diffusion is related to the driving force of the concentration differences between the material and the leachate. The potentially leachable amount is aimed at giving an upper limit of the amounts that can be active in a leaching process. This value is more appropriate than the total content in the material for use as the concentration creating the driving force. The definition of the extent of leaching included in the term potential leachability has not yet been clearly stated. A * E-mail:
[email protected]; telephone: +46 13 201800; fax: +46 13 201914.
S0013-936X(96)00385-9 CCC: $14.00
1997 American Chemical Society
test for this purpose therefore needs to be operationally defined. A comparison can be made with other operational leaching/extraction tests such as the sequential extraction procedure (5), which uses different extraction media for identifying the partitioning of trace metals in the chosen fractions. The operational conditions under which the test is carried out need to be clearly defined and carefully designed to fulfill the end point criteria in the test. The Dutch availability test (NEN 7341) is a test for potential leachability that aims to “indicate what quantity of a particular component may leach out from a material under extreme circumstances (such as e.g. in the very long term, after disintegration of the material, full oxidation and loss of acid neutralisation capacity)” (6). It has also been described as measuring the part of elements not bound in silicate minerals or other poorly soluble mineral phases (7). The definition of this test is as yet not clear enough and needs to be improved. The basic principles used in the availability test are the maximization of the water solubility of the constituents and the diffusion from the particles to the bulk solution. This is obtained by (1) increase of solubility of the substances in relation to pH, (2) maintenance of a steep concentration gradient between the pore and bulk solutions, and (3) large specific surface area created by the small maximum particle size, which facilitates a high leaching rate. Reactions slower than approximately 1 day cannot be taken into account in the test. The test is conducted in two steps using a high solubility pH for oxyanions in the first step (neutral) and for cations in the second step (acidic). In the Dutch availability test, NEN 7341 (6), 16 g of material milled to 95% less than 125 µm is leached in demineralized water in two serial steps, each of a leachate to solid ratio (L/S ratio) of 50. pH in the first step is held constant at pH 7, and in the second step is held constant pH 4. Nitric acid is used for acidification. The duration of each step is 3 h. The resulting two leachates are combined before analysis. The leached amounts are reported as the potentially leachable amounts or fractions (if related to total composition). The test is conducted open to the atmosphere. This test is only applicable for inorganic species. No test of this kind exists for organic substances. Three modifications of this test have been proposed. In the pending Nordtest recommendation on the availability test (8), the L/S ratio of 100 is used in accordance with NVN 2508 (9), which was the predecessor of NEN 7341. van der Sloot et al. (10) propose an increase in grain size limit for the test sample to 300 µm. Fully oxidized conditions were not obtained in availability tests of some ashes and metallurgical slags (11, 12). As the test in its present performance cannot guarantee fully oxidized conditions, a modification of the test for oxidised conditions has been made (12). The availability test is becoming more frequently used in environmental assessment studies of solid wastes and secondary raw materials. It is therefore important to create a better scientific ground for the use as well as the improvements of the test before it will be incorporated in different jurisdictions. The objective of this study was to obtain results for discussion of the principles on which the test is based. The parameters giving main impact on the leaching results for a broad spectrum of solid residues needed to be identified. For this purpose, the impact of different test conditions on the leached amounts and measured redox potentials was studied. All results in this study are related to municipal solid waste incineration bottom ash, which is a widely produced residue. This ash has a complex composition that includes substances of both low and high solubility and of crystalline to amorphous nature (2, 4).
VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
735
TABLE 1. Total Composition of MSWI Bottom Ash (11)a
b
element
MSWI BA
element
MSWI BA
Si Fe Ca Al Na K Mg S Ti P Cub Znb Ba Mn Pbb
208 800 108 000 87 900 56 900 26 400 14 300 11 300 8 560 6 200 4 500 3 400 3 080 1 660 1 360 737
Sr Cr Zr Nib Sn V W Cob Asb Mo Nb Cdb Be Hgb LOI, 550 °C
285 274 200 138 130 58.7 33.9 19.1 16.0 16.0 13.1 5.8 1.78
