Grantee Research Project Results
Final Report: Limestone-Based Material for Arsenic Removal From Drinking Water
EPA Contract Number: 68D03064Title: Limestone-Based Material for Arsenic Removal From Drinking Water
Investigators: Williamson, Terrence E.
Small Business: HydroTech Engineering
EPA Contact: Richards, April
Phase: II
Project Period: October 1, 2003 through December 31, 2004
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2003) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , Watersheds , SBIR - Water and Wastewater
Description:
Arsenic contamination of drinking water is a major problem in many areas of the United States and around the world. The problem has been highlighted by the U.S. Environmental Protection Agency’s (EPA) decision to mandate the reduction of arsenic’s maximum contaminant level from 50 ppb to 10 ppb by January 2006. Current remediation technologies are quite expensive. Thus, lowering the standard will put economic pressure on communities with high levels of arsenic in their drinking water. Urgent action is needed to address this problem.
A limestone-based technology developed by HydroTech Engineering, L.L.C., will 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. Work by HydroTech Engineering has demonstrated arsenic removal of greater than 95 percent by limestone. This research project included characterization of limestone sorbents and potential dopants, system dynamics, efficiency improvements, a field test, and commercialization activities. Limestone-based material was tested and evaluated after the addition of dopants. Direct comparisons of HydroTech Engineering’s material with granulated ferric hydroxide and other commercially available materials were made. Finally, as a precursor to commercialization and certification, a field-scale trial was conducted on a single well with 70 ppb arsenic in the community of Keystone, South Dakota.
Phase I of this project focused on understanding the physical and geochemical processes that govern arsenic retention and transport in groundwater, with emphasis on adsorption by limestone. Phase I clearly showed that HydroTech Engineering’s technology has the potential to reduce arsenic in drinking water at the source, and Toxicity Characteristic Leaching Procedure (TCLP) tests have indicated that the arsenic waste material is stable and can safely be disposed of in a landfill or incorporated into cement as aggregate. Phase II focused on increasing the efficiency of limestone material by evaluating a variety of doping mechanisms. Column studies were conducted to determine the total adsorptive capacity of limestone and the breakthrough of arsenic from column leaching.
During Phase II, HydroTech Engineering also focused on understanding the physical and geochemical processes that govern arsenic retention and transport in groundwater, with an emphasis on adsorption by limestone. This research has advanced understanding of the surface chemistry, the adsorptive capacity of limestone, waste-disposal options, and methods to improve the efficiency of the process. Specific tasks included BET measurements, particle-size analysis, x-ray diffraction analysis, scanning electron microscopy, batch and column experiments, interference ions, temperature dependence, pH dependence, arsenite removal, kinetics, isotherms, pelletization, cartridge design, dopants, and a field trial.
Summary/Accomplishments (Outputs/Outcomes):
Arsenic removal by limestone is a process that operates effectively over a wide range of pH values. The following results were not dependent on pH between the range of 4 and 10.
In laboratory experiments, oxidation of arsenite prior to the adsorption process was accomplished by the addition of an oxidant, such as sodium hypochlorite (NaOCl), to a solution containing As (III). Previous research has determined that the oxidation of As(III) by chlorine is very rapid and that complete oxidation can be obtained in less than 1 minute. Initially, a 2 ppm NaOCl solution was added to 200 ppb arsenite solution adjusted to pH 8. The solution was magnetically stirred for 10 minutes to ensure complete oxidation of the As(III) to As(V). Batch experiments then were performed using 100 mL of oxidized As(III) solution and varying mass amounts of Minnekahta limestone (< 0.5 mm sieve size). Each batch test was run for 48 hours. Results indicate that limestone effectively removes arsenite at 85 percent (average) after pretreatment while it effectively removes arsenate at 79 percent (average). Therefore, the high removal rate of arsenite confirmed that the arsenite was completely oxidized.
Batch experiments were performed to examine and compare the percent arsenic removal at various pH levels using very low quantities of activated alumina (AA) in limestone. These batch experiments were conducted using AA of 48/100 mesh size as a dopant, and Minnekahta limestone of less than 0.5 mm sieve size. The surface area of AA of 48/100 size is 358 m2/g. Each batch test was conducted for a duration of 48 hours and initially contained 100 mL of 100 ppb arsenic solution. The pH of the initial solution ranged from 4 to 10. Results indicate that 0.005 g of AA showed a maximum arsenic removal rate of 95 percent at pH 7, while 0.01 g of AA had a maximum of 98 percent removal rate at pH 8 (pH 8.2 is the typical zero point charge for AA). The removal efficiency of AA deteriorated at pH 10. At pH 8, 0.01 g of AA-doped limestone showed a maximum removal rate of 99 percent. About 98 percent removal rate, however, also was consistently achieved at pH 4 using various amounts of AA. Results indicate that increasing pH from 4 to 10 had almost no effect on sorption of arsenic by AA-doped limestone.
The TCLP testing performed during Phase I (Method SW 846-1311) showed that an arsenic-treated limestone waste product is nonhazardous and suitable for disposal in municipal landfills. Results from previous research also indicate that a limestone waste product could be used as an aggregate in making concrete. The potential for the solid arsenic-limestone waste product to be used as a raw material in cement kilns was evaluated in Phase II. Thermal analysis of arsenic desorption from the waste product was performed by the Materials Characterization Center at Western Kentucky University. All of the samples were analyzed on a TA 2960 SDT. The samples were heated from room temperature to 1,550ºC at a heating rate of 20ºC/min under a flowing atmosphere (100 mL/min).
Thermogravimetric analysis (TGA) curves (weight percent and derivative weight curves) were developed for the arsenic-limestone waste sample analyzed in air. The results show that substantial weight loss occurred at about 920ºC, and thermal decomposition was completed at around 1,000ºC. The major weight loss is estimated to be from the release of water and CO2. Acid digestion of the sample before and after thermal analysis showed that no arsenic desorbed from arsenic-limestone waste (2.22 mg/kg). The TCLP concentrations before and after thermal analysis, however, were 0.007 mg/kg and less than 0.001 mg/kg, respectively. The decrease in arsenic leachate concentration after thermal analysis could be attributable to CO2 desorption resulting in increased stability of calcium arsenate compounds. The weight loss of limestone waste in air is 43.3 percent. Therefore, it can be concluded from the results that the limestone waste product is thermally stable and can be used as a raw material in cement kilns for manufacturing cement.
Thermal stability studies conducted on activated alumina-doped Minnekahta limestone (1-2 mm sieve size) showed that approximately 50 percent of the arsenic desorbed from AA-doped limestone after thermal analyses. The sample, however, passed the TCLP limit for arsenic and aluminum. TGA curves (weight percent and derivative weight curves) of arsenic AA-doped limestone waste sample analyzed in air were developed. Substantial weight loss of the sample occurred at about 910ºC and thermal decomposition was completed at around 1,000ºC. The weight loss of the sample was 36.1 percent because of the release of water and CO2.
Thermal stability studies were performed on concrete mortar containing arsenic-limestone waste that was cast during Phase I. TGA curves were developed for concrete mortar samples analyzed in either air or nitrogen atmospheres. A comparison was made for the weight loss of the sample in air and in nitrogen. The results showed that substantial weight loss occurred at about 785ºC, with thermal decomposition completed at around 1,000ºC. The major weight loss was estimated to be from the release of water. The weight loss of concrete mortar in nitrogen and air was 15.55 percent and 17.85 percent, respectively. Derivative weight curves were developed for comparing the difference in the weight that may be caused by the differing atmospheres. Acid digestions performed on a concrete mortar sample before and after thermal analysis indicated that approximately 25 percent of arsenic desorbed from concrete mortar. The sample passed the TCLP limit, however, for arsenic of 5 ppm.
On November 12, 2004, Keystone City Well #4 was pumped for 1 hour and 40 minutes. At the end of the pumping period, water was collected for chemical analysis (sample ID KEY-2) for use in batch and column experiments. Flow from the well was about 60 gallons per minute. The total amount of water pumped from the well during this time period was about 6,000 gallons. A water sample was collected and analyzed. Total arsenic of the KEY-2 sample was 53 ppb, while dissolved arsenic was 50 ppb.
Batch experiments using Keystone City Well #4 water have been completed. Batch tests using 0.1, 0.5, 1.0, and 2.0 grams of ball-milled Minnekahta Limestone in 100 mL of water were completed. Two grams of limestone removed about 33 percent of the arsenic in solution (about 52 ppb As). In comparison, 1.5 grams of ball-milled limestone removed about 86 percent of the arsenic from a 100 ppb-mixed arsenic solution. Additional batch tests with higher mass amounts of limestone will be completed using Keystone well water to determine the maximum amount of limestone required in batch tests to remove arsenic to below 10 ppb.
The commercialization model used for this project was developed based on the concept that HydroTech Engineering would concentrate exclusively on research. At the appropriate time, a partnership will be developed with a company(s) capable of successfully launching the technology into the marketplace. The search for an appropriate commercialization partner has begun, involving a due diligence review and ranking of potential partners.
The unique qualities of this limestone process allow for arsenic remediation at the point of entry, allowing for community-wide filtration. The problem of arsenic in drinking water is global in nature, and HydroTech Engineering’s technology and commercialization model anticipates the enormous potential for remediation and profit.
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. 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 and low cost of limestone, 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. Limestone-based material offers the following potential benefits to the drinking water community: (1) high removal efficiency, (2) broad geographic/water system applicability, (3) reasonable cost, (4) ability to be adapted to a variety of water quality conditions, (5) compatibility with other water treatment processes and ease of technical use, and (6) efficient treatment of water at rural public water systems or individual households.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
limestone, arsenic removal, drinking water, small drinking water systems, landfill, ferric hydroxide, groundwater, Toxicity Characteristic Leaching Procedure, TCLP, adsorption, oxidation, arsenite, activated alumina, concrete, thermogravimetric analysis, TGA, small business, SBIR,, RFA, Health, Scientific Discipline, PHYSICAL ASPECTS, INTERNATIONAL COOPERATION, Water, POLLUTANTS/TOXICS, Environmental Chemistry, Arsenic, Chemicals, Risk Assessments, Environmental Monitoring, Physical Processes, Water Pollutants, Drinking Water, hybrid sorbent, exposure, arsenic monitoring, arsenic removal, human exposure, point of use, other - risk management, contaminant removal, drinking water treatment, human health, water treatment, arsenic exposure, human health riskSBIR Phase I:
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.