Passage VII

Some soil bacteria can remove sulfate (SO₄^2-) from acid mine drainage (AMD, polluted water drainage from mining areas). The bacteria use the soils organic carbon for energy and remove SO₄^2- by reducing it to sulfide (S^2-) Oxygen-poor conditions are required for the reduction process. Researchers studied other organic materials containing bacteria populations to see if those mixtures could also reduce SO₄^2- in AMD.

Study 1

Four organic materials that contained bacteria populations were selected for testing: straw, hay, leaf compost, and manure. The materials, either individually or in mixtures of 2 of the materials, were packed to a depth of 50 cm in 10 cm diameter glass columns (see Figure 1). Duplicates of each column were prepared. The moist and dry densities and the carbon content (in kg/m³) of each column's contents were measured. The results are shown in Table 1.

Study 2

An aqueous solution with a concentration of 100 ppm SO₄^2- (simulated AMD) and a pH of 6.5 was put through each column at a flow rate of 80 mL/day. On Day 50, the S^2- concentration in the water in each column was measured at various depths. The results are shown in Figure 2. Table 2 shows the average S^2- concentration in a column's water on Days 50, 100, and 150.