Comparison of Microwave-Assisted and Conventional Leaching Using EPA Method 3050B

Abstract

A microwave-heated EPA method 3050B for the leaching of key elements (cadmium, chromium, copper, lead, nickel, zinc) of environmental importance was tested and compared to conventional hot plate-heated EPA method 3050B. All commercially available temperature and power-controlled atmospheric pressure microwave systems were used for the adaptation of EPA method 3050B. Three temperature feedback control systems were evaluated for regulating the temperature of the leachate including outside (IR sensors) and inside the sample flask (gas bulb thermometer). Results, which show the efficiency and effectiveness of the microwave sample preparation method, are discussed for the leaching of three NIST Standard Reference Materials: 2704, 2710, and 2711. The elements were determined either using ICPMS, ET-AAS, or F-AAS. This study demonstrates that microwave heating with enhanced reaction control leads to improved precision compared to conventional heating sources.Leaching is a term that has been applied to the extraction of metals from environmental samples and has become common terminology of the EPA and in the environmental analytical field. Leaching is not a total decomposition, and leachable recoveries of analytes are generally lower than total concentrations. Recoveries can only achieve total values if an element is completely soluble in the leaching solvent. Leaching studies are an assessment of worst case environmental scenarios where components of the sample become soluble and mobile. Temperature is a key parameter for all leaching sample preparation methods as well as for extractions and digestions. The control of temperature is paramount in achieving reproducible leaching of elements. Temperature is a primary parameter used to increase the rate of leaching and to bring these tests into appropriate duration for laboratory evaluation. Previously, most leach methods have been accomplished in beakers on a hot plate. These methods are traditional, time consuming, fairly inefficient, and in general imprecise. During the early 1980s, fundamental research established temperature control as the most significant contributor to leach test error. 1,2 These experiments focused on the analysis of simulated nuclear waste glass materials. Temperature was found to be the dominant parameter in leaching uncertainty and imprecision. The control of temperature to within (0.04%, instead of (1% over a 28-day leach period, changed the interelement leaching uncertainty from 50% to 3%. By substituting microwave

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