Comparison of Chelating Agents as Extractants for Metals in Diverse Soil Materials

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Abstract
Five chelating agents—DTPA, EDTA, EGTA, HEDTA, and NTA—were compared at pH 5.3 as extractants for Al, Fe, Mn, Zn, Cu, Cd, and Ni in 25 diverse soils. Extracting solutions contained 0.005M chelating agent, 0.01M CaCl2, and 0.1M acetic acid‐ammonium acetate buffer at pH 5.3. These extractants were compared to: a similar solution without added chelating agent, 0.1M HCl, and the DTPA soil testing solution (0.005M DTPA, 0.01M CaCl2, buffered at pH 7.3 by 0.1M triethanolamine). All extractants yielded clear filtrates that aspirated and burned cleanly during analysis by flame atomic absorption spectrometry. At the 5:1 extractant/soil ratio used, all buffered extractants maintained their pH satisfactorily. DTPA and HEDTA were among the most effective ligands for extracting all seven metals. EDTA extracted somewhat lower concentrations but was also a good extractant for all metals. NTA was among the best extractants for Al and Ni but among the least effective extractants for Fe, Mn, and Cu. EGTA was the least effective chelator of Fe, Zn, Ni, and especially Al. Despite its alkaline pH of 7.3, the DTPA soil‐testing solution was a moderately effective extractant for metals except Mn and ferrous Fe. Acid soils, reduced soils and sediments, and metal‐contaminated soils may release enough strongly complexed metal to reduce the effectiveness and the capacity of chelating extractants. Recently dredged lake sediment released particularly large concentrations of metals, primarily Mn2+ and Fe2+. Extracting metals effectively from metal‐rich soils may require increased concentration of chelating agents, higher extractant/soil ratios, or more than one extraction per sample.