Grantee Research Project Results
Final Report: Limestone-Based Material for Arsenic Removal From Drinking Water
EPA Contract Number: 68D02093Title: Limestone-Based Material for Arsenic Removal From Drinking Water
Investigators: Williamson, Terrence E.
Small Business: HydroTech Engineering
EPA Contact: Richards, April
Phase: I
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text | Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)
Description:
This research project focused on the development of a remediation technology that has shown the ability to reduce arsenic in drinking water at the source, with the added benefit of low-cost disposal of a stable and benign waste product in ordinary landfills. Previous work by the research team has demonstrated arsenic removal of greater than 90 percent by limestone. HydroTech Engineering, L.L.C., has identified a remediation method that will significantly concentrate the arsenic onto limestone. Treatment of large quantities of water with arsenic above drinking water standards will produce a relatively small and compact amount of solid limestone with adsorbed arsenic. Arsenic retention by limestone is an effective process that offers great potential for source reduction. Because of the ready availability of limestone, its use for arsenic remediation will be relatively inexpensive. This technology can be readily adapted to small water supply systems as well as private, domestic, and stock wells, helping operators of these systems to meet anticipated new rules. Additional benefits include the potential for low-cost disposal of the waste product in a stable form.
During the past 10 months, research has focused on understanding the physical and geochemical processes that govern arsenic retention and transport in groundwater, with emphasis on adsorption by limestone. This research has advanced understanding of the surface chemistry, adsorptive capacity of limestone, waste-disposal options, and methods to improve the efficiency of the process.
This Phase I project used batch, column, and adsorption tests to evaluate the adsorptive capacity of limestone for arsenic uptake as a function of the level of dissolved arsenic to be remediated as well as the surface area and particle size of the crushed limestone. Conditions were optimized for arsenic adsorption and retention under a variety of water quality conditions. This research also examined the total adsorptive capacity of limestone by determining the breakthrough of arsenic from column leaching. The specific technical objectives of the Phase I work were to: (1) establish the surface chemistry of the interaction between limestone and arsenic, (2) improve the adsorption efficiency of the arsenic-removal process, (3) characterize the long-term stability of the waste product, and (4) establish the efficiency of the process under real-world conditions through investigation of the engineering constraints.
Summary/Accomplishments (Outputs/Outcomes):
HydroTech Engineering, L.L.C.'s proposed mechanism for the removal of arsenic by limestone is a precipitation/adsorption of calcium arsenate, Ca3(AsO4)2, onto the amorphous, rough surface of the limestone. The solubility product of Ca3(AsO4)2 is 6.8 x 10-19. The solubility product of magnesium arsenate, Mg3(AsO4)2, is 2.0 x 10-20. The removal of arsenic, and the subsequent stability of the waste product, is facilitated by the alkaline surface pH of the limestone (pH 9-10) and the net positive surface charge of limestone.
Scanning electron microscopy (SEM) of the clean limestone surface and that of the waste product has been performed. No visible precipitates were seen on the waste product. However, it confirmed the nature of the amorphous, rough surface. Given the low concentrations (ppb range), it was not surprising that the calcium arsenate did not precipitate in large enough aggregates to be detectable by SEM.
Calcium arsenate crystals and calcium-rich arsenic compounds have been observed with the SEM in this research when samples were prepared with approximately 8,000 ppm arsenic concentrations. This was confirmed by microprobe analysis as well as x-ray fluorescence. For materials on which arsenate crystals were detected, the coverage was approximately 0.12 mg As/g limestone, with more space clearly available. In previous work, the coverage was approximately 0.0011 mg As/g limestone. This represents an improvement by a factor of 100.
Work by this research group has shown that as the particle size is reduced, the efficacy and capacity of the arsenic take-up are improved dramatically. Estimations of the surface area of the smallest particles used in earlier phases of this research were on the order of 0.001 m2/g of material. Clearly, this surface area is indicative of a fairly nonporous material. It is estimated that by size reduction the surface area has been increased to approximately 1 m2/g of material, or an increase of a factor of 1,000. To achieve such size reduction, wet ball milling can be used. Reduction to submicron size should be possible on the laboratory scale by subjecting the material to attrition milling. Submicron particles should provide on the order of 10 m2/g of material.
The reduced particle size with increased surface area per gram of material significantly improved the capacity and efficiency arsenic adsorption. Unfortunately, one of the drawbacks of having such fine particles is an increased pressure loss through the packed column. To maintain high specific surface areas, while limiting the pressure loss, granules of the fine powder initially were created. Using techniques described in the ceramics industry, fine (micron to submicron) powder was used to create millimeter-sized particles, while not losing the high specific surface area. Pellets were created by tumbling the fine powder in the presence of a binder, polyethylene glycol (PEG). The particles were sintered at elevated temperatures to remove the organic binder and create some particle strength, while minimizing the loss of surface area. Typical temperatures in the ceramic industry are above 400oC. This work has shown that there is a workable balance between particle size and column operation, and that such columns are commercially feasible.
The pH values in the pellet experiments indicated that some pellets underwent partial decomposition to lime (CaO). PEG appeared to help resist the decomposition of limestone during heating. The addition of dopants was investigated for improved arsenic retention. Use of certain dopants appeared to be quite effective, further demonstrating the ability of this technology to proceed to a successful commercial introduction.
Disposal of arsenic-enriched material is critical for achieving commercial viability. Because the arsenic is so strongly bound by the alkaline surface pH of the limestone, it was not expected to significantly leach out under normal waste disposal conditions. This has been confirmed by the favorable results of the Toxicity Characteristic Leaching Protocol (TCLP) test, which has been performed internally as well as by Mid-Continent Testing Laboratories in Rapid City, SD. This method was used to estimate the mobility of the waste product and determine if it was suitable for disposal in municipal landfills. The final arsenic concentration in the extraction fluid was 24 ppb. An earlier TCLP test was performed internally with the more acidic extraction fluid 2 on a second waste sample. The final arsenic concentration in this experiment was 8 ppb. The results were well within the standards set for disposal in a landfill. Even under high acid conditions, the arsenic remained on the limestone surface to a great extent or calcium arsenate re-precipitated onto the remaining limestone after the pH stabilized at approximately 6. This was facilitated by the highly alkaline limestone surface. The potential for the waste product to be incorporated into cement directly or into concrete as an aggregate has been successfully demonstrated.
During part of this research, arsenate crystals were mapped on a chip of limestone by using scanning electron microscopy. CO2 then was desorbed, and the material was converted to lime at approximately 1100 K. The general shape and orientation of the limestone chip was maintained. All of the previously mapped arsenate crystals were identified without difficulty. The x-ray fluorescence spectrum and the ratios of the arsenate crystals were unchanged from before and after testing. Thus, tests have indicated complete stability of the product, including thermal stability, and it should be possible to incorporate the waste into cement.
Batch and column experiments were conducted with limestone from several sources. The limestone was crushed in the laboratory with a rock crusher and then sieved to 16-mesh size (0.5-1 mm). For some of the work, ball milling also was used to prepare < 0.5-mm fines. The < 0.5-mm fines are the predominant absorbent in the batch experiments at the present time. It has been determined in earlier experiments that the smaller particle size increases the effective surface area per gram of adsorbent. Surface area analysis was conducted to determine the total surface area. Brunauer Emmet Teller measurements of the surface area of the 0.5-1 mm particles are 1.3 m2/g. Arsenic removal by limestone is a process that operates effectively over a wide range of pH values. Ten grams of limestone (1-2 mm) were agitated with a 100 ppb arsenic solution at varying pH values. The results were not dependent on pH between the range of 4 and 10.
When in use in a household or rural drinking water system, arsenic-removal columns will function dynamically. To size the column it is imperative to know the dynamic breakthrough time, or time until there is a measurable or unacceptable concentration of arsenic in the effluent. Work during Phase I in the past year has determined that breakthrough time is a function of many factors, including water flow rate, arsenic concentration in the supply water, and the kinetics of adsorption. During the last 4 months of the Phase I research, breakthrough curves were measured with a laboratory column using a range of continuous water flow rates, typical of what would be used in a household setting, demonstrating the potential commercial feasibility. An adsorption isotherm then was determined from the data.
Commercialization potential for the remediation of arsenic from water is based on global demand markets. Domestic demand markets are impacted by the mandate for arsenic levels in drinking water to be lowered to 10 ppb or less by February 2006. The U.S. Environmental Protection Agency estimates that there are from 2,500 to 4,000 smaller (10,000 or less population) domestic water systems that will require remediation. This is the primary target market. Larger systems are a separate market with unique requirements, but also represent a potential market.
Projections by Foresight Science and Technology, Inc., define the "overall market from $198-$622 million per year." These market projections are based on sales of cartridges to individual household users. Treating the entire community water system for a similar cost is HydroTech Engineering, L.L.C.'s goal. In special cases, a cartridge system may be recommended. Presently, limestone-based media appears to have the most cost-effective matrix for domestic point-of-entry remediation. Applications of this technology in countries with relatively low gross domestic products may be their only viable option.
In summary, the potential for commercialization is very high in the domestic market. Commercialization on a global basis is more difficult to evaluate in that demand and ability to pay for remediation often are not linked. A low-cost, simple-to-operate system with no disposal issues will be a major market contender in both the domestic and global markets. HydroTech Engineering, L.L.C.’s solution appears to answer these demands.
Conclusions:
Treatment of large quantities of water with arsenic above drinking water standards produces a relatively small and compact amount of solid modified limestone with arsenic in the form of low-solubility crystals of calcium-rich arsenate compounds. The efficiency of removal can be improved dramatically by several methods, including smaller particle sizes and dopants. The waste material can safely be disposed of in a landfill or incorporated into cement or concrete as aggregate. Because of the ready availability of limestone and low cost, its use for arsenic remediation is inexpensive, even for large water systems. Additionally, this technology is readily adapted to the needs of small rural water supply systems as well as private, domestic, and stock wells because it does not require initial high capital investment or specialized training for staff. Benefits of this research include a low-cost treatment technology for source reduction that will reduce arsenic below maximum contaminant levels, helping operators of water-supply systems to meet anticipated new rules. Additional benefits include the potential for low-cost disposal of the waste product in a stable form. This process is patent pending.
Supplemental Keywords:
limestone, arsenic removal, remediation, drinking water system, landfill, drinking water standards, calcium arsenate, Ca3(AsO4)2, magnesium arsenate, Mg3(AsO4)2, scanning electron microscopy, SEM, particle size, polyethylene glycol, PEG, ceramic industry, cement, concrete, Toxicity Characteristic Leaching Protocol, TCLP, small business, SBIR., RFA, Scientific Discipline, Water, Geographic Area, Environmental Chemistry, Arsenic, State, Analytical Chemistry, Environmental Monitoring, Drinking Water, Environmental Engineering, monitoring, public water systems, Safe Drinking Water, South Dakota, risk management, arsenic removal, chemical contaminants, community water system, treatment, arsenic exposure, contaminant removal, drinking water contaminants, drinking water treatment, water treatment, limestone-based material, groundwater, drinking water systemSBIR Phase II:
Limestone-Based Material for Arsenic Removal From Drinking Water | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.