|
|
|
Ordering Information:
ORDER NO: 3076
DATE AVAILABLE: Spring 2008
Printed Report
|
PDF
|
| Subscribers |
N/A |
|
| Non-Subscriber |
N/A |
N/A |
PRINCIPAL INVESTIGATORS:
Dongye Zhao, Thomas Steinwinder, Byungryul An, Mark O. Barnett, and Timothy A. Kramer
OBJECTIVES:
The research objectives of this project were (1) to develop and characterize a new class of innovative ion exchange (IX) materials, referred to as polymeric ligand exchangers (PLEs), which can remove As(V) highly selectively even in the presence of high concentrations of competing ions and requires much less regenerant brine (<10 percent of the brine need of conventional IX resins); and (2) to explore an engineered approach for brine treatment, which allows for reuse of the spent brine and production of environmentally safe arsenic-laden solid waste.
BACKGROUND:
The new arsenic (As) Maximum Contaminant Level (MCL) of 10 μg/L will impact ~ 4,100 water utilities serving nearly 13 million people nationwide, with an estimated compliance cost of ~ $600 million per year using current treatment technologies.
Although the ion exchange (IX) technology is identified by USEPA as one of the best available technologies for arsenic removal, conventional IX processes suffer from two major constraints: (1) the sorption capacity is severely retarded by ubiquitous competitive anions such as sulfate; and (2) large volumes of hazardous residuals are produced due primarily to frequent regeneration. Consequently, innovative IX materials are urgently needed for developing cost-effective IX processes for arsenic removal. This exploratory research addresses the urgent needs for enhanced arsenic removal in small drinking water systems.
APPROACH:
Five representative PLEs were synthesized and studied in detail. For comparison, two commercial strong-base anion (SBA) exchange resins were also studied. Isotherm tests were carried out in a series of batch experiments and in the presence of high concentrations of sulfate as competing anions to determine the equilibrium arsenate sorption capacity for the resins. Isotherm tests also resulted in the binary arsenate-to sulfate separation factors for comparing the relative affinity of various sorbents.
A series of bench-scale fixed-bed column tests and a semi-pilot scale column run were carried out to study breakthrough behaviors of arsenate and various competing ions.
Regeneration of the exhausted PLE was also conducted in the same column configuration. To minimize the brine need and reduce the production of process waste, the treatability and reusability of spent regeneration brine were studied. Ferric chloride was used to precipitate/co-precipitate arsenic in a simulated spent brine solution containing arsenate (300 mg/L), sulfate (600 mg/L), bicarbonate (5 meq/L), and NaCl (4 percent w/w).
Arsenic removal at various Fe/As molar ratios, pH, and aging time was tested in multiple batch reactors. After optimal conditions were determined, the spent brine was treated and the supernatant separated from the precipitates. Upon pH adjustment to 9–10, the recovered supernatant was reused for regeneration. To reduce As leachability from the resultant process waste sludge, an engineered procedure was developed by testing and optimizing the factors (Fe/As molar ratio, Ca addition, reaction contact time or aging, and temperature) that affect the leachability of arsenic. Both the standard TCLP and the California Waste Extraction Test (WET) were employed to determine the stability of the residuals resulting from various treatments.
RESULTS/FINDINGS:
Based on the results presented the following conclusions have been made:
- Optimal pH for nearly completely removing arsenic from spent brine is 3–7.
- Highly efficient (>99 percent) As removal from spent brine can be achieved at Fe/As molar ratio ≥2 at pH 6–7.
- Brine can be reused for regeneration after arsenic removal, and essentially no liquid hazardous waste residuals leave the plant.
- Contact time (i.e., the slurry aging) has little measurable effect on As removal from brine.
- WET and TCLP show a decrease (~10 percent WET) in leached arsenic after aging for 20 days as opposed to 2 days at room temperature (25ºC).
- In general, the arsenic stability increases (leachability decreases) with both increased temperature and aging period.
- Based on the WET and TCLP results, 60 days of aging of the wet sludge and at a moderately elevated temperature 50ºC can greatly reduce the arsenic leachability.
- The WET is much more powerful than the TCLP with a concentration of As in extraction fluid 4 orders of magnitude greater than the latter.
- A second, kinetically limited process occurs, leading to the decrease in the arsenic leachability.
RESEARCH PARTNERS:
This study was jointly funded by the Awwa Research Foundation and the U.S. Department of Energy through the Arsenic Water Technology Partnership. The report was published by WERC (a Consortium for Environmental Education and Technology Development at New Mexico State University).