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for Arsenic Drinking Water Sorbents
During EPA advisory meetings in 2003 and 2004, the majority of participants felt that the disposal of arsenic spent sorbent media would not be a problem.
Preliminary work at PACS laboratory has shown that caution needs to be taken in the disposal of these used media. The present disposal methods, sewer and non-hazardous landfill, may result in arsenic release into the environment. When brought into contact with a real-world municipal wastewater plant influent and humic acid, bound arsenic was released in both tests. These test methods indicate that new technology is needed for disposal of spent media to avoid release to the environment. Additionally, there are known microbes that have high risk to release arsenic and change the ferric iron to a more mobile ferrous form.
Sorbents are Ideal Solution
Sorbents are ideal for purifying water for many reasons. They are easy to operate and can be placed in a cartridge. All of the water goes through the media, so no water is wasted. Media usually can be purchased from several different vendors, and it is fully utilized in lead/lag designed adsorption systems (exhausted mg arsenic/gram sorbent) compared to a batch process. Spent media is contained in a closed container. The estimated service-life can be predicted before installation. Small mini-columns can be used to obtain estimated sorbent usage in just a few days. Much prior knowledge and professional experience is readily available to users, who range from homeowners to large and small municipal drinking water facilities. Often small water plants need a turn-key packaged system due to their small operational staff and limited budget. For these systems, sorbent adsorption offers the best economics and performance. Once the number of water connections becomes critically low, the point-of-use (POU) technology needs to be applied. In the POU-sized systems, arsenic adsorption media also have economic advantages.
Spent Media Arsenic Leaching
There are three important questions concerning the disposal of arsenic spent iron oxide and other media. Will the arsenic come off of the sorbed media? What happens to the basic iron and other media in the present and proposed disposal processes? Is there a way to recycle the media?
On all three of these basic concerns: arsenic release; fate of media; and recycling of media—the iron oxide media appear to have problems and opportunities for future improvements. Tables I and II summarize experimental data from the PACS labs, that shows concern with the present disposal methods. One gram of spent media was prepared and challenged with 500 ml of either fresh municipal sewer water from a local domestic plant or was challenged with a .2 molar humic acid solution prepared at the lab. In both challenges, arsenic was released from the media. Control experiments with distilled water did not reveal any arsenic release from spent media with either As(III) or As(V) loaded sorbent.
The practice of blending water can be risky. The GFO media can remove lead, chromate and many other toxics. Combining GFO treated water with untreated raw water will allow some toxics into the distribution system that GFO could have removed if given the opportunity. It is recommended to pass all raw water through the GFO media, as a safety precaution.
A recent article in Science points out that the microbiological world contains organisms that use arsenate As(V) compounds as energy sources. These organisms take in arsenate and emit As(III) arsenite. The iron-based media have relatively small surface areas and are soft. These physical values for GFO are about 20% of typical activated carbons with surface areas of 1,000 to 1,200 sq meters per gram and ball-pan hardness values of 95 to 99. Microorganisms (micron sizes) are expected to have access to arsenic compounds on the outer surface of the spent GFO media. Anaerobic microorganisms reduce metal ions efficiently, and ferric iron should be converted to ferrous iron in addition to As(V) to As(III) microbial reduction.
Water conditioning and purification professionals are well acquainted with iron staining of household fixtures and the taste of irony water. One example can be found at the web site: www.pacslabs.com. The site includes some pictures listed as “Painted Pots” in Kootenay Mountain Park in British Columbia, Canada. The brownish-red color is due to iron oxide in the spring water coming up under the pot—a hole in the earth that can hold the iron leached water from the rocks. The rim of the pot accumulates the iron until it develops the red color, and the overflow from the pot creates the reddish stream down the hillside. Eventually, the rim buildup at the top of the pot builds up until the increased water pressure ruptures the pot and the spring water leaves the pot high-and-dry. Landfill disposal of iron based materials and microbial actions in nature may be able to create the “Painted Pot” phenomena.
The iron in the present media is in valence III, which is relatively water insoluble. Microorganisms are known to use iron III as an energy source and emit iron II, the water soluble and colorless form. Simple air oxidation can convert the colorless iron II back to the reddish-brown water insoluble iron III, which is the problem in bathroom plumbing fixtures. Present disposal practices are promoting large plugs of iron spent media to be placed in landfills. It will be an interesting opportunity for microbiologists and chemists to revisit these man-made iron III rich deposits. A hypothesis is that these iron deposits will have some similarities to the “Painted Pots” images described above.
There are solutions to this apparent spent arsenic media disposal problem. The canisters and vessels containing the spent media can be sent back to the manufacturer or local water service professionals for regeneration, which is a technology based on competitive desorption. Hydroxide or other higher valence anions can competitively displace arsenite and arsenate from spent media for reprocessing.
We have long disposed of ion exchange regeneration liquid residuals to the domestic sewer, only to recently find that there are better brine disposal practices. Vendors can take the spent ion exchange media to central locations for regeneration. This creates a whole new business opportunity for water conditioning and purification professionals. Possibly, the marketplace will develop a new disposal practice for the arsenic spent media. There is no doubt that toxic arsenic needs removal from our drinking waters. Some scientists think 10 ppb is even too high for good health. The practice of blending water to stay just below 10 ppb may need to be reexamined in the future. wqp