Studies of metal species in water extracts from metallophytes employing solid phase extraction and size exclusion chromatography coupled to inductively coupled plasma mass spectrometry

Metallophytes have the ability to grow in metal-rich soils without showing signs of toxicity. It is believed that chelating of metals to organic ligands is a mechanism that metal-tolerant plants use for detoxification. To obtain information about metal species, water extracts of roots, stems and leaves from two metallophytes (Blepharis aspera, and Helicrysum candolleanum) were analysed using inductively coupled plasma mass spectrometry (ICP-MS) in combination with cation exchange solid-phase extraction (off-line) and on-line with size exclusion chromatography (SEC). The extracts were analysed for 10 metals that were present at higher than normal concentrations and for 8 elements that have physiological functions in plants. The elements studied were Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Ni, P, S, Si, Ti, V and Zn. The total concentration and the percentage of water-extractable metal were determined in all plant parts. For both plants, K had the highest extraction efficiency (>70%), followed by Mg, Mn, Ni, P and Zn. The main elements Ca, K and Mg were found to be present only in cationic form in the extracts of both plants, whereas Al, Co, Cr, Cu, Fe and Ni were present partly in non-cationic form, indicating complex formation. Size exclusion chromatography showed that all the plant parts had a similar size distribution of the dissolved organic matter, exhibiting mainly one broad peak. All the metals co-eluted with the first part of this peak, suggesting that the metals were associated with relatively large organic molecules, and more so for H. candolleanum than for B. aspera, and more pronounced for the stems than for the roots and leaves. Such data indicate that the two plants may have different mechanisms of dealing with high metal concentrations.