ARSENIC AND OLD MINERALS: HOW EARTH'S OXIDATION LED TO ARSENIC'S TOXIC RELEASE TO THE CONTINENTAL HYDROSPHERE

In the reducing geochemical conditions of the early Earth, the toxic metalloid arsenic was sequestered by very stable and relatively insoluble silicate and sulfide minerals. Thus it posed no threat to the simple single-cell organisms eking out an anaerobic existence then. With the onset of the “great oxidation event” geochemical conditions changed dramatically, and now these stable silicate and sulfide minerals began to oxidize and dissolve in some environments. Those minerals that had been buried below the seawater-sediment interface remained largely unaffected. Where geologic and biologic conditions exposed these minerals to molecular oxygen, the iron and sulfur in them were oxidized, and in the process, arsenic was released to solution. Because energy (electrons) was involved in these new “redox” chemical reactions, micro-organisms evolved in a way to take advantage of these energetic reactions. Thus a complex “tug-of-war” began then (and continues to the present) between microbial processes, mineral stabilities, mineral(loid)-surface complexation reactions, the fate of decomposing organic matter, and the geochemistry of arsenic. The nature of the new minerals and their surfaces had some good and bad consequences for simple, and ultimately less-simple, organisms on the planet. Where geologic processes operated to bury sediments and organic matter under the oceans, arsenic behaved pretty much as it always had: sulfate-reducing bacteria made pyrite framboids in anoxic marine sediments that sequestered arsenic. Where geochemical conditions became very oxidizing, bright-colored orange and yellow hydrous ferric-oxide minerals formed and they were almost equally efficient at sorbing and sequestering arsenic as the sulfide minerals had been. However, certain micro-organisms eventually evolved that coupled their respiration to the destruction of these orange and yellow iron minerals, which again led to release of the sequestered arsenic to the hydrosphere. Late in the 20^th Century, humans in places like Bangladesh and India began pumping groundwater from aquifers in this “middle ground” of the redox geochemical spectrum where arsenic has been released by microbes from minerals, thus triggering the major environmental calamity we are dealing with today.