Reaction Fronts in Brick-Sand Layers: Column Experiments and Modeling

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Abstract
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.