Pressure-assisted chelating extraction: A clean approach for metal digestion


Traditionally, the analyses of trace metals that are present in solid matrices usually require oxidative decomposition or wet digestion techniques. This wet digestion utilizes a mixture of strong acids at elevated temperatures. For wet digestion procedures, target analytes are usually isolated prior to determination by a suitable detection technique. However these procedures take several hours to perform and consume large amounts of hazardous reagents such as strong acids and hydrogen peroxides. They are also prone to atmospheric contamination due to the use of open systems, which result in relatively high blanks from acid impurities. We have recently reported a modified use of ASE™ as a novel technique for digesting and removing heavy metals from solid matrices. The new technique, known as pressure-assisted chelating extraction (PACE), was demonstrated for the extraction of toxic metals, including lead, manganese, nickel, gold, iron, copper and cadmium. In PACE, solid samples are placed in stainless steel extraction cells and are equilibrated with appropriate chelating agents under static conditions. Using a programmed sequence of temperature, static times and pressure at several cycles, the dissolution of metal is achieved from the solid matrices under moderate temperature (up to 200°C) and pressure (up to 3000 psi). PACE achieves metal recovery that is equivalent to an ion-exchange technique while exceeding the efficiency of wet acid digestion. In addition, PACE provides for metal digestion by substituting the strong acids used during wet digestion with environmentally-friendly chelating agents. It uses less solvents (∼13.0 mL per 5g) and significantly less time (minutes vs. hours) for metal dissolution and removal. The percentage metal recoveries obtained ranged from 78 to 101% with an RSD of 5%. PACE has been validated using certified standard reference materials including industrial sludge, buffalo river sediments, coal fly ash and spent copper plating baths. The total time required to remove these metals was -20 minutes. Since PACE uses minimal, non-hazardous reagents, it has the potential to completely eliminate the use of concentrated acids in wet digestion. The automated digestion may further encourage wider acceptance for metal digestion. In this presentation, the concept, principles and wider applications of PACE using various chelating agents and analytical conditions will be discussed.

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Conference Proceeding

Publication Date


Journal Title

18th Annual Waste Testing and Quality Assurance Symposium, WTQA 2002 - Proceedings