Six primary chemicals have been used to treat AMD. Each chemical has
characteristics that make it more or less
appropriate for a specific condition. The best choice among alternatives
depends on both technical and economic factors.
The technical factors include acidity levels, flow, the types and concentrations
of metals in the water, the rate and degree of chemical
treatment needed, and the desired final water quality. The economic
factors include prices of reagents, labor, machinery and equipment,
the number of years that treatment will be needed, the interest rate,
and risk factors.
Metal Precipitation and pH
Enough alkalinity must be added to raise water pH and supply hydroxides
(OH-) so dissolved metals in the water will form insoluble metal hydroxides
and settle out of the water. The pH required to precipitate most metals
from water ranges from pH 6 to 9 (except ferric
iron which precipitates at about pH 3.5). The types and amounts of metals
in the water therefore heavily influence the selection of an AMD treatment
system. Ferrous iron converts to a solid bluish-green ferrous hydroxide
at pH >8.5. In the presence of oxygen, ferrous iron oxidizes to ferric
iron, and ferric hydroxide forms a yellowish-orange solid (commonly called
yellow boy), which precipitates at pH >3.5. In oxygen-poor AMD where
iron is primarily in the ferrous form, enough alkalinity must be added
to raise the solution pH to 8.5 before ferrous hydroxide precipitates.
A more efficient way of treating high ferrous AMD is to first aerate the
water (also outgassing CO2), causing the iron to convert from ferrous
to ferric, and then adding a neutralizing chemical to raise the pH to
6 or 7 to form ferric hydroxide. Aeration after chemical addition is also
beneficial. Aeration before and after treatment usually reduces the amount
of neutralizing reagent necessary to precipitate iron from AMD. Aluminum
(Al) hydroxide generally precipitates at pH > 5.0 but also enters solution
again at a pH of 9.0. Manganese precipitation is variable due to its many
oxidation states, but will generally precipitate at a pH of 9.0 to 9.5.
Sometimes, however, a pH of 10.5 is necessary for complete removal of
manganese. As this discussion demonstrates, the appropriate treatment
chemical can depend on both the oxidation state and concentrations of
metals in the AMD (U.S. Environmental Protection Agency 1983). Interactions
among metals also influence the rate and degree to which metals precipitate.
For example, iron precipitation will largely remove manganese from the
water at pH 8 due to co-precipitation, but only if the iron concentration
in the water is much greater than the manganese content (about 4 times
more or greater). If the iron concentration in the AMD is less than four
times the manganese content, manganese may not be removed by CO-precipitation
and a solution pH of >9 is necessary to remove the manganese. Because
AMD contains multiple combinations of acidity and metals, each AMD is
unique and its treatment by these chemicals varies widely from site to
site. For example, the AMD from one site may be completely neutralized
and contain no dissolved metals at a pH of 8.0, while another site may
still have metal concentrations that do not meet effluent limits even
after the pH has been raised to 10.
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