Thomas J. Sorg

Arsenic species in drinking water wells in the USA with high arsenic concentrations

Arsenic exists in ground water as oxyanions having two oxidation states, As(III) and As(V), and its concentrations vary widely and regionally across the United States (USA). Because of the difference in toxicity and removability of As(III) and As(V), arsenic speciation is important in the selection and design of an arsenic treatment systems. Identifying the arsenic species is also helpful in explaining and understanding the behavior and characteristics of arsenic in the environment. Although laboratory methods exist for speciating arsenic in water samples, the lack of a universal preservation method has led to the predominant use of field separation methods that are somewhat complex and costly. Thus, very few studies have incorporated arsenic speciation. A U.S. Environmental protection Agency (EPA) arsenic treatment research program provided a unique opportunity to speciate the naturally occurring arsenic in 65 well waters scattered across the USA with many of them being speciated monthly for up to three years. Speciation test data showed that 31 wells had predominantly As(V), 29 had predominantly As(III) and five had a mixture of both. A general pattern was found where As(III) was the dominant species in midwest ground waters where anoxic conditions and elevated iron concentrations prevailed and the well waters in the east, west and farwest had either As(III) or As(V) as the dominant species. The monthly (12-36) speciation tests results at many of these sites also found no major changes in arsenic species over time.

Regeneration of iron-based adsorptive media used for removing arsenic from groundwater

Adsorptive media technology is regarded as a simple, low cost method of removing arsenic from drinking water particularly for small systems. Currently, when the effluent of a treatment system reaches the USEPA maximum contaminant level (MCL) of 10 ug/L, the exhausted media is removed and replaced by new virgin media. Although the commonly used iron-based media products are reasonable in price, the replacement cost accounts for around 80% of the systems total operational costs. One option to media replacement is on-site regeneration and reuse of the exhausted media. To determine whether an iron based media can be successfully regenerated and reused, laboratory batch and column regeneration tests were conducted on six exhausted iron-based media products obtained from six full scale arsenic removal treatment systems. Batch tests conducted on three of the media products to evaluate the effectiveness of 1-6% caustic regenerant solutions found that arsenic desorption increased until around 4%. Using 4% caustic solutions, the columns tests on the six exhausted media products showed arsenic removals ranged from 25 to 90% with the best results obtained with the Severn Trent E33 media. Exposing the media to caustic (pH ≥ 13) and acid (pH ≤ 2) solutions found minimal media loss with the caustic solution, but significant media dissolution with a pH 2 acid solution. A six column pilot plant test at an Ohio test site with the lab regenerated media products found that the regenerated media could achieve arsenic removals somewhat similar to virgin media.

Regenerating an Arsenic Removal Iron-Based Adsorptive Media System, Part 1: The Regeneration Process

Adsorptive media technology is frequently used by small water systems to remove arsenic because of its simplicity and efficiency. Current practice is to replace the media when it no longer reduces arsenic below the maximum contaminant level of 10 μg/L that the US Environmental Protection Agency has set for drinking water. Media replacement typically accounts for approximately 80% of the total operational and maintenance costs. One potential option to reduce the cost is onsite regeneration and reuse of the media. To evaluate the regeneration option, three consecutive regeneration studies were conducted on a full-scale adsorptive media system. This article, the first of a two- part series, describes the regeneration process and its efficacy in stripping arsenic and other contaminants from exhausted media. Study results found that a three- step regeneration process of media backwash, caustic regeneration, and acid neutralization conditioning proved effective for stripping arsenic and other contaminants from the exhausted media.

Investigation of sequential and enzymatic extraction of arsenic from drinking water distribution solids using ICP-MS

A sequential extraction approach was utilized to estimate the distribution of arsenite [As(iii)] and arsenate [As(v)] on iron oxide/hydroxide solids obtained from drinking water distribution systems. The arsenic (As) associated with these solids can be segregated into three operationally defined categories (exchangeable, amorphous and crystalline) according to the sequential extraction literature. The exchangeable As, for the six drinking water solids evaluated, was estimated using 10 mM MgCl(2) and 10 mM NaH(2)PO(4) and represented between 5-34% of the total As available from the solid. The amorphously bound As was estimated using 10 mM (NH(4))(2)C(2)O(4) and represented between 57-124% of the As available from the respective solid. Finally, the crystalline bound As was estimated using titanium citrate and this represented less than 1.5% of the As associated with the solids. A synthetic stomach/intestine extraction approach was also applied to the distribution solids. The stomach fluid was found to extract between 0.5-33.3 microg g(-1) As and 120-2,360 microg g(-1) iron (Fe). The As concentrations in the intestine fluid were between 0.02-0.04 microg g(-1) while the Fe concentration ranged from 0.06-0.7 microg g(-1) for the first six drinking water distribution solids. The elevated Fe levels associated with the stomach fluid were found to produce Fe based precipitates when the intestinal treatment was applied. Preliminary observations indicate that most of the aqueous Fe in the stomach fluid is ferric ion and the observed precipitate produced in the intestine fluid is consistent with the decreased solubility of ferric ion at the pH associated with the intestine.

Regenerating an Arsenic Removal Iron-Based Adsorptive Media System, Part 2: Performance and Cost

Replacement of exhausted, adsorptive media used to remove arsenic from drinking water accounts for approximately 80% of total operational and maintenance costs of this commonly used small system technology. Results of three full-scale system studies of an onsite media regeneration process (discussed in the first article of this two-part series) showed it to be effective in stripping arsenic and other contaminants from a granular ferric oxide (GFO) exhausted adsorptive media. This second article details the performance of the regenerated media to remove arsenic through multiple regeneration cycles and the approximate cost savings of regeneration over media replacement. Results indicated that media regeneration did not appear to have a major detrimental effect on the performance of the GFO media, and the regeneration cost was potentially less than the media replacement cost. Therefore, onsite regeneration offers small systems a possible alternative to media replacement when removing arsenic from drinking water using iron-based adsorptive media technology.