The effectiveness of a treatment depends on a number of factors, including the oxidation state of the arsenic present (arsenate or arsenite), pH, and competing ions. Arsenate, due to its higher charge of +5, is more easily removed from water than arsenite. The oxidation state of arsenite can be altered from +3 to that of arsenate +5 using ozone or chlorination thereby making it easier to remove from the water. The EPA has approved a number of different methods to lower the concentration of arsenic in water. These include coagulation/filtration, adsorptive media, reverse osmosis, ion exchange, and iron oxide filtration. (U.S. Environmental Protection Agency 2007) (U.S. Environmental Protection Agency 2006) (Drinking Water Program, Maine Division of Environmental Health 2005)
This method involves adding a coagulant, such as ferric sulfate or ferric chloride, to the contaminated water. The coagulant alters the properties of the suspended or dissolved contaminants causing them to settle so that they can then be filtered out of the water. (U.S. Environmental Protection Agency 2006)
This involves passing the contaminated water through a media that adsorbs the contaminants. For arsenic, the most common media used is activated alumina (aluminum oxide, Al2O3). This process is more effective if the pH of the water is 6. Effectiveness decreases as the pH increases. (U.S. Environmental Protection Agency 2006)
This is a process in which the water is passed through a membrane with very tiny pores. Small water molecules pass through but larger elements like lead, iron, chromium, and arsenic are trapped. This works for arsenate (arsenic +5) but not for arsenite (arsenic +3). If the contamination is from arsenite, the water must pass through a pre-oxidizing system first to change it to arsenate. Reverse osmosis systems can be small and attached to a single faucet. (U.S. Environmental Protection Agency 2006) (Drinking Water Program, Maine Division of Environmental Health 2005)
This system works by passing the contaminated water through a resin bed of chloride ions. The arsenic ions knock the chloride ions out of the resin bed and take their place. When the resin bed is full of arsenic ions it is back washed with water saturated with salt. The trillions of chloride ions overwhelm the arsenic molecules sending them into the back wash water, which is then taken out of the system as waste water. The chloride ions also recharge the resin and the process can begin again. (U.S. Environmental Protection Agency 2006)
Iron Oxide Filtration
This works similarly to an ion exchange system with the added benefit of removing not just arsenate but also arsenite. Unlike the ion exchange system, the filters are not recharged but are disposed. It can be used in conjunction with a reverse osmosis system that is not effectively removing arsenate. (Drinking Water Program 2005)
Tests of these systems have shown that they reduce the concentration of arsenic to 5 ppb or less. (U.S. Environmental Protection Agency 2006) The technology chosen is dependent on various factors including volume of water and other contaminants to be removed.
One relatively new method of arsenic removal is filtering the water using granular ferric hydroxide (GFH) an adsorbent. Studies have shown that it is effective on both arsenite and arsenate with 5 to 10 times higher efficiency than activated alumina. Tests show that the adsorbent can treat 40,000 to 60,000 bed volumes of water before exceeding the arsenic limit of 10 µg/L. It leaves a residual waste of 5 to 25g/m3 of water treated. Under normal conditions the arsenic does not leach from the waste. Therefore it can be disposed of as non-hazardous. The system is cost effective, easy to install and does not require constant monitoring. (Pal 2001) In the US, GFH systems have been in use for some water systems since 2003. (Siemens AG 2009)
Arsenic has been and will always be with us. It is found almost everywhere on Earth - in the soil, in the rocks, and in the water. As the world increases its awareness of the danger and as technology makes it easier to control, we will be able to live with this abundant natural poison.
List of Visuals
- Reverse Osmosis
Courtesy US EPA
- Ion Exchange
Courtesy US EPA
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