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November 30, 2005

Arsenic Leachability in Water-Treatment Adsorbents

C. Jing¹, S. Liu¹, M. Patel^1,2, and X. Meng^1
1Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ; ²New Jersey Department of Environmental Protection, Trenton, NJ

At NSLS beamline X19A, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and leaching tests to investigate arsenic (As) leachability in spent water-treatment adsorbents. The EXAFS results indicate that arsenic forms inner-sphere bidentate binuclear surface complexes on the adsorbents, which include granular ferric hydroxide, granular ferric oxide, titanium dioxide, activated alumina, and modified activated alumina. This study improved the understanding of arsenic bonding structures on adsorptive media surfaces and As leaching behavior for different adsorbents.

Chuanyong Jing

Suqin Liu

The U.S. Environmental Protection Agency (EPA) has adopted an arsenic maximum contaminant level (MCL) of 10 µg/L, which will be enforceable as of January 23, 2006. This more stringent arsenic drinking water standard requires the installation of new water-treatment systems and the upgrading of existing ones. Filtering arsenic in water using adsorbents is especially suitable for small community systems and individual homes because the filters do not require much effort to operate. Recently, more effective adsorbent media have been developed and used for arsenic removal. As a result of the enhanced removal efficiency, large amounts of spent media with elevated arsenic content will be generated. Accurately determining the leachability of arsenic in this waste is crucial for both economic concerns and the evaluation of environmental impacts.

Figure 1. The observed (dotted line) and model calculated (solid line) As K-edge FT EXAFS spectra resulting in a radial distance structure for the spent media. The peak positions are uncorrected for phase shift. Arsenic formed binuclear bidentate surface complexes on the media surface.

In this study, EXAFS spectroscopy at NSLS beamline X19A was employed to determine the arsenic local coordination environment of spent adsorbent media. Spent adsorbents were collected from five pilot-scale filters that were tested for their ability to remove arsenic from groundwater in New Jersey. The spent media included granular ferric hydroxide (GFH), granular ferric oxide (GFO), titanium dioxide (TiO2), activated alumina (AA), and modified activated alumina (MAA). The As leachability, determined with the Toxicity Characteristic Leaching Procedure (TCLP), was below 180 µg/L for all spent media. The leachate As concentration in the California Waste Extraction Test (WET) was more than 10 times higher than that in the TCLP, and reached as high as 6650 µg/L in the spent GFH sample. The EXAFS analysis shows that the first and strongest peak in the Fourier transform (FT) curve was contributed by four oxygen atoms at an average As-O distance of 1.69 Å for all five media samples (Figure 1). The constant As-O interatomic distance was independent of the media to which the As(V) was adsorbed. The second shell atom (distance) was Fe (3.32 Å), Fe (3.28 Å), Ti (3.27 Å), Al (3.14 Å), and Al (3.13 Å) for GFO, GFH, TiO2, AA, and MAA, respectively. The EXAFS results show that arsenic formed bidentate binuclear inner-sphere complexes on the surfaces of these five spent adsorbents. This stable structure can be used to explain the high affinity of arsenic for the adsorbent surfaces.

BEAMLINE
X19A

FUNDING
New Jersey Department of Environmental Protection

PUBLICATION
C. Jing, S. Liu, M. Patel, and X. Meng, Arsenic Leachability in Water Treatment Adsorbents, Environmental Science and Technology, 39, 5481-5487 (2005).

FOR MORE INFORMATION
Chuanyong Jing
Center for Environmental Systems
Stevens Institute of Technology
Hoboken, NJ
Email: cjing@stevens.edu