Grantee Research Project Results
Final Report: Accelerated Acid Digestion
EPA Contract Number: 68D98132Title: Accelerated Acid Digestion
Investigators: Later, Douglas W.
Small Business: Mountain States Analytical Inc. (MSAI)
EPA Contact:
Phase: I
Project Period: September 1, 1998 through March 1, 1999
Project Amount: $69,957
RFA: Small Business Innovation Research (SBIR) - Phase I (1998) RFA Text | Recipients Lists
Research Category: SBIR - Pollution Prevention , Pollution Prevention/Sustainable Development , Small Business Innovation Research (SBIR)
Description:
Mountain States Analytical, Inc.'s InSciTe Research Division (MSAI-IRD) has innovated and tested the concepts for automating inorganic sample preparations using a new, automated acid digestion technique entitled Accelerated Acid Digestion (AAD). AAD is a new sample preparation method for digesting inorganic elements in solid matrices for analysis by instrumental techniques such as AA and ICP. The AAD system uses conventional strong acids such as nitric acid and hydrochloric acid (the same acids used in open-beaker, hot plate acid digestions for USEPA SW-846 Method 3050) at elevated pressures and temperatures to increase the efficiency and decrease the time of sample digestion.A prototype AAD system has been designed, constructed and tested for the digestion of metals in solid samples for the U.S. Environmental Protection Agency (USEPA) Phase I SBIR contract awarded to MSAI-IRD in August of 1998. By controlling acid strength and digestion temperature, under elevated pressure conditions, a simulation of various USEPA digestion methods, for example hot plate (SW-846 Method 3050) and microwave (SW-846 Method 3051), was achieved.
Five specific objectives, or purposes, for this research were set forth in the USEPA SBIR Phase I contract work plan. Each of these objectives is listed below. A brief summary follows regarding the results obtained from this research for each objective.
Objective #1: | Verify the mechanical design concepts and chemical performance of a multiple-cell, automated AAD prototype system. |
Objective #2: | Study the key variables that influence the dissolution reaction kinetics of Accelerated Acid Digestion. |
Objective #3: | Perform a comparison study of Method 3050A and AAD in preparation for development of an SW-846 AAD method. |
Objective #4: | Demonstrate the feasibility of AAD sample preparation technology for the analysis of Mercury using ICP. |
Objective #5: | Demonstrate the applicability of AAD for digestion of other sediment, sludge and soil sample matrices. |
A multiple-cell AAD bench scale prototype was designed and fabricated for the AAD SBIR Phase I project. The system was designed to process 12 samples in series. A crude electronic interface with computer control was also completed for unattended operation of the system. This prototype system was initially tested with a Standard Reference Material (SRM) used in previous work with the bench-scale, experimental, single-cell AAD device. Results and data from this earlier system were used as the basis for a patent application and the submission of an SBIR Phase I proposal. Data from the prototype AAD compared very favorably with that from the experimental system. An average recovery for 22 metals of 101.4% with an 8.5% relative standard deviation was obtained for a commercial SRM using the prototype AAD.
A theoretical discussion of the acid dissolution process is provided in this report. Experimentation to determine the effects of temperature, pressure, acid concentration, sample size and cycle time was conducted. It was determined that the most important controlling factor in an AAD digestion is the temperature. The next most important parameter is acid quantity and concentration. Pressure, cycle time and sample size are important factors, but not necessarily controlling factors. These studies led to the determination of parameter set points that simulated the USEPA SW-846 Method 3050 acid digestion. Once determined, a proof of concept comparison study was conducted.
Three different SRMs, as well as spiked standard sand and soil were evaluated in the comparison study. The standard sand and soil were obtained commercially and spiked in the laboratory at low (near the method detection limit) and high levels. The SRMs were selected based on their concentration ranges for the 10 target metals studied: arsenic, barium, cadmium, chromium, copper, lead, mercury, selenium, silver and zinc. Seven replicates of each of the SRMs and spiked standards were digested using both AAD and USEPA SW-846 Method 3050. The digestates were then analyzed by ICP and the concentration of each metal in each sample determined and compared. Over 100 digestions and 1000 individual metals analyses were completed for this proof of concept study as prescribed by the USEPA Office of Solid Waste, Methods Development Section. With AAD parameters set to simulate Method 3050, the data produced using the AAD technique compared well with the Method 3050 data. In general, biases (recoveries compared to certified values) were in the 100% range with precision generally less than 10%, except for the two low level spiked standards. The research reported here provides the base for a formal validation study that will eventually lead to an USEPA approved method.
A total of six SRMs were analyzed for mercury by AAD/ICP and compared with data for the same sample processed by SW-846 Method 7471A protocol. Again, the data from AAD was very comparable with the current USEPA method. When mercury concentrations were above 1 mg/kg in the SRM, then the bias values were generally 90-110% with precision less than 10%. Method detection levels as determined from this work were on the order of 0.025-0.25 mg/kg, depending on the sample matrix. The results of these studies indicate that mercury could be routinely digested and analyzed by AAD/ICP along with other metals of interest normally determined by this technique. Thus, the number of times a single sample must be analyzed can be reduced from two analyses to a single analysis. This represents a potential 50% savings in time and effort to obtain the required metals data for a sample.
A total of 15 reference materials were digested by AAD and analyzed during the Phase I research. These included an agricultural soil, a sewage sludge, a municipal incinerator ash, a soil/sediment sample, a sewage sludge amended soil, two commercially prepared spiked soils, and a laboratory spiked commercial blank soil and sand. In addition six standard reference ores were obtained from a local mining company for evaluation. Data for these sample matrices is presented in this USEPA SBIR Phase I contract Final Report. In all cases, Method 3050 comparable data was obtained using AAD as the digestion technology.
In conclusion, the research performed in this USEPA SBIR Phase I contract clearly demonstrates the potential of using AAD as a sample digestion procedure for not only environmental samples, but solid samples in general that require determination of metals content. Advantages of the AAD as compared to the current USEPA methods include smaller acid volumes, shorter digestion times and a flow through design for ease of automation. Estimates based on this research indicate that a potential reduction in acid consumption of up to 80% can be realized using AAD. In addition, AAD digestions on a per sample bases can be completed in 15 minutes as compared to 3-4 hours for open-beaker, hot-plate style digestions. This report presents detailed data and documentation that supports these conclusions.
Supplemental Keywords:
Scientific Discipline, Sustainable Industry/Business, cleaner production/pollution prevention, Chemistry, Technology for Sustainable Environment, Analytical Chemistry, New/Innovative technologies, waste reduction, Accelerated Acid Digestion, innovative technology, pollution prevention, innovative technologiesThe 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.