Ordering Information:
ORDER NUMBER:  91211
DATE AVAILABLE: Summer 2008

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PRINCIPAL INVESTIGATORS:
Timothy A. Kramer, Richard H. Loeppert, and Hunyoung Wee

OBJECTIVES:
The objectives of this research were to (1) conduct a general physical and chemical characterization of arsenic-contaminated residuals generated in the laboratory and at operating water utilities, (2) evaluate environmental and chemical factors impacting the release of arsenic from residuals, and (3) evaluate methods to stabilize arsenic against release from a variety of residual materials.

BACKGROUND:
The recent reduction in the maximum allowable contaminant level for arsenic in municipal drinking water has increased the need for effective strategies to remove arsenic from drinking water and has led to the increased generation of arsenic-contaminated residuals from water treatment systems. Arsenic is commonly removed by adsorption onto oxides or hydroxides of iron or aluminum. These materials must be disposed of in a manner that ensures that the arsenic will not be released after disposal.

HIGHLIGHTS:
The release of arsenic from water treatment residuals was lower from iron-based residuals compared to aluminum-based residuals or residuals produced by lime/soda ash treatment. Arsenic release was determined by residual composition and predominant arsenic species, but was also highly impacted by pH, counter-ion, and the presence of competing ligands. More favorable arsenic retention was generally achieved with iron-based residuals, with arsenate compared to arsenite, and at pH values approaching neutrality. Increasing desorption of arsenate was observed at pH values greater than 8; however, this pH effect could be largely eliminated with calcium instead of sodium as the counter-ion.

APPROACH:
The research team undertook the following steps during the course of the project:

1. Produced residuals in the laboratory and contaminated them with arsenate or arsenite
2. Measured the release of arsenic under conditions of the Toxicity Characteristics Leaching Procedure (TCLP) test and over a range of pH and concentrations of phosphate, sulfate, or chloride
3. Examined the ability of residuals to maintain pH in regions of low arsenic release
4. Measured the effect of calcium on arsenic release
5. Determined arsenic release from residuals from water treatment plants and pilot plants in TCLP tests and in the presence of phosphate
6. Treated two residuals by Portland cement, lime, fly ash, and ferrous sulfate and measured arsenic release from them

RESULTS/FINDINGS:

Sorption Desorption Studies
The extent of arsenic release decreased in the following order: iron-based was greater than aluminum-based, which was greater than calcium carbonate residuals. Release was affected by pH, with higher release at extreme pH values within the range of pH 4 to 10, probably due to dissolution of the solid phase. The presence of sulfate or chloride had little effect on arsenic release, but phosphate increased release.

Buffering Capacity of Residuals
The residuals can affect the pH of leaching solutions and thereby affect the amount of arsenic released. Models can be developed to predict this effect.

Inhibition of Desorption by Calcium
The presence of calcium reduced the release of arsenic whether the calcium was present during the formation of the residuals or whether it was added only during the desorption experiment. This effect could be due to formation of solid phases containing calcium and arsenic or by interactions between calcium and the surfaces of the residuals.

Analysis of Field Samples
Phosphate stimulated arsenic release, but sulfate and chloride had little effect. Arsenate release was greatest at pH 10, while arsenite release was greatest at pH 4.

Stabilization Techniques
The addition of lime resulted in reduced arsenic release, probably by forming calcium arsenate solid phases.

IMPACT:
Both laboratory generated and field residuals retained arsenic well under typical environmental conditions of pH and water quality. However, increased leaching at extreme pH indicates that such conditions should be avoided. Arsenic release was associated with dissolution of solid phases in the residual, so an intact residual is evidence that arsenic will be retained well. The detrimental effect of high pH can be mitigated by also providing sufficient calcium. Solidification/stabilization processes using Portland cement/lime can be effective. The presence of chloride and sulfate had little effect on arsenic release, so their presence in landfill leachates should not promote arsenic release.

RESEARCH PARTNER:
USEPA

PARTICIPANTS:

• El Paso Water Utilities
• Naval Air Station Fallon
• Sierra Pacific Power Company
• Public Utilities Department of the City of Billings
• Helena Water Treatment Utilities