Is there future of sequential chemical extraction for speciation analysis of metal(loid)s in plants?

The properties of metal(loid)s in the environment are mainly determined by their chemical forms. The chemical forms of metal(loid)s affect their possible chemical and biochemical reactions, toxicity, mobility, and bioavailability in the environment. Sequential chemical extraction (SCE) has been successfully used to clarify the chemical forms of soil metal(loid)s. However, quite a few SCE procedures have been applied for speciation analysis of plant samples; there is no systematic discussion in such a field. The current review deals with the SCE of plant metal(loid)s, and compares the extraction procedures and the extractants in different SCE methods. It has been found that some chemical forms are unreliable for plant SCE analyses. The forms can be the phosphates or oxalates of some specific metals in plants, such as gadolinium or chromium, which cannot be fully extracted by the designated extractants, 2% acetic acid, or 0.6 M/L HCl. Therefore, SCE methods for the non-bivalent metal(loid)s have been emphasized in this work. Moreover, the application status, development trends, limitations, and future directions for SCE methods of plant metal(loid)s have been discussed.