Hexavalent chromium (Cr6+, as chromate CrO4 2-) is a mobile, toxic groundwater contaminant. A remediation strategy involves injecting a reducing agent to convert Cr6+ to Cr3+. Why does this transformation reduce both mobility and toxicity?
ACr3+ is radioactive and decays to a non-toxic element
BCr3+ is highly insoluble at near-neutral pH (forming Cr(OH)3 precipitates), immobilizing it in the aquifer matrix, and it is far less toxic than Cr6+ because it does not cross cell membranes as readily as the chromate oxyanion
CThe reducing agent destroys the chromium atoms
DCr3+ is volatile and escapes to the atmosphere
Cr6+ exists as the chromate oxyanion, which is soluble and mobile in groundwater across a wide pH range, and is a potent carcinogen because it enters cells through sulfate transport channels. Cr3+ is a cation that forms insoluble hydroxide precipitates at pH > 5, effectively immobilizing it. Cr3+ is also far less toxic, being an essential trace nutrient at low concentrations. This redox-based remediation exploits the dramatic difference in geochemical behavior between the two oxidation states.
Question 2 True / False
Dilution of contaminated groundwater by clean recharge water is sufficient to remediate most groundwater contamination plumes.
TTrue
FFalse
Answer: False
Dilution reduces concentrations but does not remove the contaminant mass. Sorbed contaminants desorb slowly, dissolved-phase NAPL (non-aqueous phase liquid) dissolves over decades, and mineral-hosted contaminants (e.g., arsenic on iron oxides) can be released by changing redox or pH conditions. Effective remediation must address the source, not just the dissolved plume. Natural attenuation (biodegradation, sorption, precipitation) can work for some contaminants but requires demonstration that it is occurring at sufficient rates. Most contamination requires active intervention at the source zone.
Question 3 Short Answer
Explain why acid mine drainage (AMD) can have pH values below 2 and extremely high dissolved metal concentrations.
Think about your answer, then reveal below.
Model answer: When sulfide minerals (primarily pyrite, FeS2) are exposed to oxygen and water during mining, they oxidize: FeS2 + 3.5O2 + H2O -> Fe2+ + 2SO4 2- + 2H+. The sulfuric acid produced drops pH dramatically. At low pH, Fe2+ is oxidized to Fe3+ (catalyzed by iron-oxidizing bacteria like Acidithiobacillus), and Fe3+ acts as an additional oxidant for pyrite (Fe3+ + FeS2 -> Fe2+ + S/SO4 + H+), creating a self-sustaining acid-generation cycle. The extremely low pH dissolves other metal-bearing minerals (Cu, Zn, Cd, Pb, As sulfides and secondary minerals), producing metal concentrations orders of magnitude above natural levels. AMD can persist for decades to centuries after mining ceases if sulfide source material remains exposed.
The autocatalytic nature of AMD -- where the product (Fe3+) accelerates the reaction -- explains why it is so persistent and severe. Bacterial catalysis increases the Fe2+ to Fe3+ oxidation rate by factors of 10^5-10^6.