Passage VII

Some soil bacteria can remove sulfate (SO42-) from acid mine drainage (AMD, polluted water drainage from mining areas). The bacteria use the soils organic carbon for energy and remove SO42- by reducing it to sulfide (S2-) 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 SO42- 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 SO42- (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 S2- concentration in the water in each column was measured at various depths. The results are shown in Figure 2. Table 2 shows the average S2- concentration in a column's water on Days 50, 100, and 150.

39. Suppose oxygen had been continuously pumped through the columns containing straw over the first 100 days of Study 2. Based on the information provided, the resulting S2- concentration measured at Day 100 for the straw columns would most likely have been:

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

Look at the fifth line of the first paragraph of the article Oxygen-poor conditions are required for the reduction process, so the increase in oxygen is not conducive to SO42- becomes S2-. So look at table 2, on Day 100, S2- concentration will be less than 60