Removal of co-occurring ions during arsenic treatment

Packed-bed adsorptive treatment systems for arsenic removal will be widely employed. We have been conducting rapid small scale column tests (RSSCTs) on many adsorptive media, and frequently track the removal of vanadium, phosphate, silica, and uranium. Phosphate is reported to compete for arsenic adsorption sites. Silica affects surface charge and arsenic removal. Vanadium is listed on the EPA Candidate Contaminant List (CCL2), and some states currently have vanadium regulations. The regulation for uranium has only recently been enforced. All of these elements co-occur with arsenic in groundwaters. This presentation will include batch adsorption data and RSSCT breakthrough curves for these co-occurring ions from natural groundwaters and experiments with model waters. A few brief examples of the findings from continuous flow RSSCTs are summarized as follows: • ArsenxNP (a iron-modified hybrid ion exchange resin) removed uranium and arsenic for ∼ 25,000 bed volumes. Iron (E33) and titanium (Adsorbsia GTO, MetsorbG) medias only removed uranium for < 1000 bed volumes although arsenic was removed for >20,000 bed volumes. Experiments were just initiated on a separate groundwater to evaluate uranium removal using GFH, E33, MetsorbG, and ArsenxNP. • GFH removes more phosphate and silica than E33 • GFH removes As(III) significantly better than E33 • GFH removes more vanadium than any of the other media Batch tests have recently been conducted to compare vanadium removal by different iron and titanium based media. Adsorption isotherms were developed at multiple vanadium concentrations and variable adsorbent dosages. As the drinking water community begins to treat groundwaters for arsenic, renewed attention to the behavior of other trace elements is emerging as an important issue for several reasons. First, these elements potentially affect/compete for arsenic adsorption sites. Second, these elements accumulate in the adsorbent media and may affect the ability to dispose of the media to landfills. Third, it may be necessary to design treatment systems capable of removing these elements as current regulations become enforced and as regulations for other elements (vanadium, molybdenum, selenium) are evaluated.