Passage V

The processing of sulfur-rich ore (rock containing valuable metals) from mines produces liquid waste, which is often stored in a pond. Some of the liquid in the pond seeps down to the groundwater table, where it contaminates the groundwater beneath with sulfate (SO₄^2-). A barrier called a reactive wall can be installed in the ground near the pond to lessen SO₄^2- contamination. As polluted water flows through the wall, SO₄^2- is removed from the water (see Figure 1). The material that forms the wall—the reactive material (RM)—is composed of one or more organic substances.

A study evaluated the effectiveness of several reactive materials.

Study

Six reactive materials (RMs 1-6) were prepared from samples of 4 organic substances——sewage sludge, leaf mulch, sheep manure, and sawdust——that had been com­pletely dried. Table 1 show the composition of each RM.

The following procedures were performed for each RM: First, a 500 mL volume of the RM was placed in a 1 L flask. Next, 500 mL of a simulated liquid mine waste with an SO₄^2- concentration of 3,600 mg/L was added to the flask. The flask opening was then covered with an airtight cap. Every 5 days, a 10 mL sample of the liquid in the flask was collected through a seal in the cap. Each sample was analyzed for SO₄^2-. Once a sample was found to have an SO₄^2- concentration of zero, no additional samples were collected from the flask.

Figure 2 shows the results for each RM.

Table and figures adapted from K. R. Waybrant, D. W. Blowes, and C. J. Ptacek, "Selection of Reactive Mixtures for Use in Permeable Reactive Walls for Treatment of Mine Drainage.''©1998 by the American Chemical Society.