Chromium contamination is a problem at a number of hazardous-waste sites. At one site local concentrations of chromium reached 60,000 mg chromium/kg soil and 19,000 mg/L chromium in the water.
(a) Assuming that the aqueous chromium reached equilibrium with the adsorbed chromium, determine a sorption coefficient (in L/kg) for the soil at the site.
(b) At a site 200 ft downgradient, the aqueous chromium concentration was measured to be 1000 mg/L. Compute the sorbed concentration in equilibrium with the aqueous concentration using the equilibrium constant determined in part (a).
(c) In order to treat the contaminated groundwater at this site, a three step process is used. First, the water is pumped out of the ground and the pH is then reduced from 3.5 to 2.3. Next the chromium is chemically reduced from hexavalent Cr(VI) to trivalent Cr(III). Finally, the chromium is removed from the water by raising the pH, causing Cr(OH)3(s) to precipitate. The equilibrium expression for this final reaction is
Ksp = 10-30
If the pH of the water is raised to 8.5, what is the equilibrium concentration of the chromium (III) in moles/L? (d) What weight (in pounds) of precipitate is formed if groundwater containing 19,000 mg/L chromium is treated as described in part (c) and the volume of the precipitation tank is 6,000 gal?