Sir:
Dr. R&C has made some comments [I] on the properties of chemically heterogeneous complexants, especially by referring to some of our papers [2-41, which suggest that he has missed the important points given in these papers. Consequently, we take this opportunity to clarify these points, particularly as it is not a simple field and our explanations may be of general interest to the readers of the journal. We will discuss mainly the concepts which require clarification, without going into all the details of RuiiC's paper.
1. Nature of physico-chemical heterogeneity
Contrary to what is suggested by the introduction of RuiiC's paper, we have repeatedly stated in our papers [2-81 that the nature of colloidal and macromolecular natural complexants (such as fulvic and humic acids, clays with their coating of organic matter or iron hydroxides, etc.) is heterogeneous both due to physical and chemical reasons, and that, in particular their heterogeneity results from the following three characteristic features (e.g., [2,6,8]): 1. polyelectrolyte properties, due to the large number of dissociable functional groups per molecule;