Model Development on Using Clay Membranes for Arsenic Removal from Drinking Water
EMSL Project ID
5096
Abstract
The ability of fine-grained clay minerals to act as semi-permeable membranes and restrict the passage of dissolved solutes is well documented. This unique property of clay, often referred to more generally as “membrane” behavior, results in chemico-osmosis (the movement of pore fluid in response to a solute concentration gradient) as well as hyperfiltration (restricted diffusive solute flux). Clay membrane behavior is important for many geohydraulic, geochemical, and geotechnical processes in natural or engineered systems. Chemico-osmosis and hyperfiltration are believed to be fundamental processes in the migration of water and solutes in highly-consolidated materials of high exchange capacity in deep sedimentary basins. Most recently, membrane behavior has been shown to be an important mechanism in the retardation of dissolved contaminants through engineered pollutant barriers such as landfill liners, slurry cutoff walls, and geosynthetic clay liner (GCL) materials. Use of clay membranes for arsenic removal from drinking water has been explored at the University of Missouri-Columbia, supported by a CAREER grant from the National Science Foundation. Arsenic is a toxic and carcinogenic metalloid that presents a major human health threat. Many municipalities draw drinking water supplies from groundwater containing elevated levels of arsenic. In recognition of the high risk associated with arsenic in drinking water, US EPA has recently revised arsenic maximum contamination level (MCL) down from 50 mg/L to 10 mg/L. In our preliminary study, clay membranes made of smectite and kaolinite are shown to reject arsenate at over 90% under some conditions.
It is hypothesized that the ability of clay membranes to reject arsenate is due to the negative charge associated with the clay particles. The charged surface results in development of an electrical double layer that provides resistance to the passage of arsenate. The exact mechanism of the solute rejection by clay membranes, however, is unknown. There is no model available to describe the solute rejection processes, and there is no modeling tools available to assess how various parameters, such as pH and ionic strength, will affect the membrane efficiency. The objective of this EMSL User Facility Proposal with Dr. Chongxuan Liu is to develop a mathematical model by incorporating the hydraulic and electrical double layer properties of clay membranes, aiming to predict/simulate water permeability and solute rejection. It is expected that model be applied to explain the experimental observations on arsenate treatment. In addition, the model may be modified to model many other geohydraulic, geochemical, and geotechnical processes.
Project Details
Project type
Exploratory Research
Start Date
2003-10-15
End Date
2006-10-22
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Liu, C. (2007) “An Ion Diffusion Model in Semi-Permeable Clay Materials”, Environ. Sci. Technol. ASAP Article 10.1021/es0624117 S0013-936X(06)02411-4.