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Arsenic in the Water: How Far Have We Come? Published: January 17, 2002 Background Mining in Montana has both enriched the state economically and impoverished the state environmentally. Western Montana has the dubious honor of having the largest Superfund site in the nation, along with several smaller hazardous waste sites. These waste sites are a legacy of the state’s hard rock mining history. Clean-up efforts involve representatives of national, state and local government agencies, the mining industry, various public groups, and researchers in the field of mine waste technology. One focus of the clean up involves removal or stabilization of arsenic in drinking water. Arsenic can be released into the environment through the mining process and it is one of the most dangerous contaminants known to exist. According to the Environmental Protection Agency (EPA) web site at (http://www.epa.gov/safewater/arsenic/basicinformation.html):
For years, the EPA had a limit of 50 parts per billion (ppb) of arsenic in drinking water. While this is a very tiny amount of arsenic, it can still have detrimental effects to humans and other animals over a long period of time. Evidence of arsenic causing cancer and other illnesses in India and Bangladesh point to the historic standard as not being restrictive enough to prevent arsenic’s harmful effects. On January 22, 2001, the EPA set a new limit of 10 parts per billion of arsenic in water. This new standard should go into effect January 22, 2006 to give time for municipal systems to come into compliance. Larry Twidwell: Arsenic Research at Montana Tech Professor Larry Twidwell, Department of Metallurgical & Materials Engineering, has been involved in arsenic removal, recovery, and stabilization for the past fifteen years. Dr. Twidwell received the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME) Mineral Industry Education award in February 2001 for his work as an educator and for his accomplishments in the development of arsenic waste treatment technologies. Dr. Twidwell has worked with Master’s Degree candidates in Metallurgical Engineering to develop processes to remove arsenic from solutions with an emphasis on recovering the arsenic, and to stabilize solids that contain arsenic. Dr. Twidwell asks, "when you recover the arsenic, can you make it into a form that can be stored?" If the arsenic can be stored safely, then it will no longer pose a threat to the environment. Arsenic Stabilization, Removal, and Storage Technologies Developed at Montana Tech Apatite: Arsenic StabilizationOver the years, Dr. Twidwell and his students have developed three technologies for arsenic removal and stabilization from water supplies. The first technology involves sequestering (tying up) arsenic in a solid compound called apatite, which occurs in nature as a Calcium Hydroxy Phosphate. Arsenic (in the form of arsenate) is about the same molecular size as phosphate. The researchers substitute the arsenate for the phosphate into the apatite structure creating Calcium Hydroxy Arsenate Phosphate. This apatite structure now contains arsenic (in the form of arsenate) and is stable for storage out-of-doors. Dr. Twidwell’s students have tested the compound by injecting air (sparging) into the compound for four years without freeing the arsenic into the environment. This technique is a huge improvement over the old method of adding lime to water and forming Calcium Arsenate to remove the arsenic. Unfortunately, Calcium Arsenate is not stable for long-term storage. Carbon Dioxide in the air converts the calcium to Calcium Carbonate and that allows the arsenic to go back into solution. Many gold operations used the Calcium Arsenate method for arsenic storage and the arsenic is beginning to re-dissolve into the water again. The apatite structure developed at Montana Tech is much more stable than Calcium Carbonate when exposed to Carbon Dioxide. Dr. Twidwell and his student researchers have collaborated with MSE Technology Applications, Incorporated, to create a demonstration process for the new apatite structure. MSE is a corporation that specializes in working with and developing commercial applications of new technologies. The process has been demonstrated at ASARCO, a lead processing plant in East Helena, and at a gold mine in Jardine, Montana with excellent results. Iron-Arsenic Precipitation: Arsenic Removal Another technique developed at Montana Tech removes arsenic from a solution through precipitation (the separation of a solid from a solution). Researchers add iron to the arsenic-bearing solution and precipitate out an iron-arsenic compound. The iron-arsenic compound is easily filtered out of the water. Montana Tech is overseeing a commercial application of this process at a site in California where 30 gallons of ground water are treated per minute. The technique has proven very successful in the removal of arsenic from ground water. The process is supposed to reduce the arsenic from 100 parts per million to less than 25 parts per billion, but is actually reducing the arsenic to much lower levels. Dr. Twidwell states, "Since being in operation, the plant has never exceeded 1-2 ppb…[this is a] very successful operation…Montana Tech will receive royalties from the marketing of this system." The iron-arsenic precipitation process may be appropriate for municipal water systems in Montana. According to Dr. Twidwell, "there are hundreds of little municipal systems in the Rocky Mountain Range…they run 20-50 ppb of arsenic in the water. They [local communities] will all have to treat the water when the new [EPA] standard is adopted." Montana Tech plans to market this system commercially and all royalties will go to the school. Iron filings treatment: Another technique for arsenic removal The third technology has application both for drinking waters in municipal systems and effluent waters that flow from streams, rivers, or industrial outflows. Dr. Twidwell explains, "[we] run water through iron filings and kick the arsenic out of the water." The arsenic in solution is exchanged for the iron, putting iron into the water and removing the arsenic. Repeated test results demonstrate the process is successful in reducing the arsenic down to <1 ppb (the analytical detection limit). Dr. Twidwell states, "[We] have to control parameters such as pH (acidity) and oxygen potential in a system, but [this is] a fairly simple process. For example, with drinking water where the pH is about neutral, you don’t put much iron in solution because this process takes out arsenic and exchanges it for iron, i.e., it puts a small amount of iron in solution. You don’t want high iron, because it tastes bad in drinking water. But at neutral pH you put very little iron in solution. This process should be really applicable to drinking water…but it probably won’t be applicable to mine drainage waters because they are too acidic." The Tech group continues to look at new lines of research to improve this process. They are studying the effects of different types of alloys to "further limit the amount of metal that gets put back in solution." Arsenic: The Research Continues Arsenic occurs naturally in water in many areas of the world including Montana. Hard rock mining in Montana has released more arsenic into our water systems than would naturally occur. Researchers in mine waste technology are working on methods to reduce the amount of arsenic in our waters. So far, Dr. Larry Twidwell and his graduate students have developed three viable techniques for the stabilization and removal of arsenic from water. These techniques are relatively simple to control and are effective in reducing arsenic below the new EPA standard of 10 ppb in water. The research into arsenic stabilization and removal continues and Dr. Twidwell is confident that more and/or better processes will be developed.
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