Environ. Sci. Technol. 2002, 36, 2875-2883
Reaction Fronts in Brick-Sand Layers: Column Experiments and Modeling VOLKER KARIUS,* KAY HAMER, AND TANJA LAGER Universita¨t Bremen, Fachbereich Geowissenschaften, Postfach 330 440, D-28334 Bremen, Germany
Admixing waste materials with common raw materials in brick production is a promising treatment technology to overcome contamination problems, because organic pollutants are destroyed and inorganic contaminants are thought to be immobilized. During their use in constructions and after the use as part of the demolition masses bricks can be leached by runoff waters and seepage waters. A possible application of recycling crushed bricks consists of their use as a surface layer material on sports grounds or in road construction. To investigate the potential leaching during acidification of a brick-sand layer and the resultant leaching of heavy metals, crushed material from two bricks was examined in several column experiments. Deionized water at pH 4 percolated through the water-saturated columns at a Darcy velocity which was varied between 0.37 and 2.2 m/d. Another column was run under unsaturated conditions. A reaction front evolved in all experiments characterized by a pH increase from pH 4 to pH 8. The chemical composition of the percolating water changed at the reaction front. Several heavy metals (Cd, Co, Cu, Ni) and Al were immobilized at this front. Other parameters such as Ca, S as SO4, V, and Mo were depleted within several days. The reaction front moved forward depending on the Darcy velocity in the column and the buffer capacity of the brick sand. Thermodynamic calculations (PHREEQC 2.0) indicated that mobilization of As was influenced by Ba(AsO4)2. The solubility of Ba and Mn was controlled by barite and manganite, respectively. Reactive transport modeling was applied to describe the dissolution of the bricks with regard to their main components Ca, SO4, Al, and Si.
Introduction The objective of the present study is to examine the composition of a seepage water evolving from a layer of crushed bricks. Such layers are used at sports grounds. Bricks are produced in a long-lasting (several days) high-temperature process (T >1000 °C). One advantage of this process is that organic compounds are completely destroyed in the course of brick production, and most inorganic contaminants become immobilized in the silicate matrix of the bricks (1, 2). This gives the opportunity to admix waste materials containing various organic or inorganic contaminants into the common raw material to overcome environmental problems (3-5). * Corresponding author phone: +49-421-2188934; fax: +49-4212184321; e-mail:
[email protected]. 10.1021/es010158z CCC: $22.00 Published on Web 05/21/2002
2002 American Chemical Society
To maintain ship traffic, an amount of about 300 000 m3 of wet sediment (measured in situ) has to be annually dredged out of the ports of Bremen-City. According to the German Law of Recycling Economy (6) such material is classified as waste. If the current practice of depositing harbor sediments in landfills is maintained, landfill capacity will only last until 2016. A technically feasible alternative consists of producing bricks from harbor sediments (7). The environmental characteristics of such a brick need to be assessed during various stages of the bricks use. The life-cycle of bricks includes storage prior to use, their immediate use in masonry or being crushed to brick sand to be applied on sport grounds or in road construction, and finally disposal in the form of demolition waste after the stage of use. There are several batch leaching tests to characterize the leaching properties of such material (8). These tests are mostly adequate to compare the results with environmental specifications or with other types of material. A former study based on batch leaching tests was able to demonstrate that bricks manufactured from sediments from Bremen’s harbor contained no hazardous potential exceeding that of other bricks made of common raw materials (2). Besides the comparative assessment of various bricks by applying batch leaching tests, it is much more difficult to estimate actual concentrations and loads in runoff waters from facades or seepage waters from tennis courts or roads consisting of layers of crushed bricks. Runoff-water concentrations can be estimated by leaching tests on account of different liquid to surface area ratios (9). The present study focuses on the composition of water which percolates through a layer of crushed bricks. The recent data are closer to a natural exposure situation than batch leaching test data or pH-static data were until now (5, 7).
Materials and Methods Characterization of Materials. Two different kinds of bricks were used in three column experiments. First, a brick made of 50 wt % harbor sediment, 35 wt % natural clay, and 15 wt % brick chippings was crushed, and a fraction of 200-630 µm was separated by dry sieving. This material which was used in column experiment C1 will be referred to as sediment brick (SB) in the following. Second, a commercial brick (CB) made of common raw materials was treated identically and used in two column experiments C2 and C3. Total acid digestions (HNO3, HClO4, HF) were performed on both materials prior to the analysis of the total element content. All solutions and leachates were analyzed in a Finnigan Mat Sola ICP-MS (trace elements) and a Perkin-Elmer Optima 3300 RL ICP-AES (major elements). Additionally, pH-stat experiments were performed on both materials prior to the column experiments in order to determine the acid neutralizing capacity over 24 h at pH 4 (ANC24). Therefore 50 g of brick sand was leached for 24 h with 500 mL of deionized water. The pH was held constant at pH 4 by titrating with HNO3. The suspended brick-sand samples were agitated with a PTFE-propeller reaching into a glass-beaker from above. This method is described in more detail elsewhere (2, 10). To get information about the phase composition of the brick an electron microprobe investigation was conducted on SB. This investigation was performed with a JEOL JXA 8900 R equipped with wavelength dispersive spectrometers (WDS) and energy dispersive spectrometers (EDS). Preparation of Columns. The brick sand was filled into Perspex columns (5 cm I.D., 50 cm length) and saturated with pure water (
