Trace Metal Thermodynamic Speciation With Humic Substances: The Nica-Donnan Model. This work presents a critical overview of the NICA-Donnan model for trace metal thermodynamic speciation with humic substances obtained from natural organic matter in soils, natural waters and anthropogenic sources. A first part covers the theoretical aspects of the model, how to obtain good experimental data and perform the modelling. A second part deals with the developments in the last decade namely the use of the generic parameters, criticism and alternatives to the electrostatic modeling, the understanding of the NICA equation and an analysis of the published data, ending with the needs and opportunities for model development.