  Electrodes composed of activated carbon (AC) particles can desalinate water
by ion electrosorption. To describe ion electrosorption mathematically,
accurate models are required for the structure of the electrical double layers
(EDLs) that form within electrically charged AC micropores. To account for salt
adsorption also in uncharged ACs, an "attraction term" was introduced in
modified Donnan models for the EDL structure in ACs. Here it will be shown how
instead of using an attraction term, chemical information of the surface
structure of the carbon-water interface in ACs can be used to construct an
alternative EDL model for ACs. This EDL model assumes that ACs contain both
acidic groups, for instance due to carboxylic functionalities, and basic
groups, due to the adsorption of protons to the carbon basal planes. As will be
shown, this "amphoteric Donnan" model accurately describes various data sets
for ion electrosorption in ACs, for solutions of NaCl, of CaCl2, and mixtures
thereof, as function of the external salt concentration and of the cell voltage
between two AC electrodes in a process called capacitive deionization (CDI).
The amphoteric Donnan model can be used to relate the conditions of activation
treatment and electrode preparation (which both influence the carboxylic acid
content) to the EDL-structure and desalination performance of ACs.
