A

Cl Anions

Mg Na Cations

SiO2

Cl Anions

Figure 9 Carbon aerogel performance using 1 L samples of raw water that consisted of 75% Colorado River water and 25% California state project water. Mean water quality data (n = 3) taken using 1.4 V, 100mL/min, at pH 8.3. Adapted from Ref. [69].

Because of the relatively small average pore size (4-9 nm) of the carbon aerogel material, only 14-42 m2/g - less than 10% - of the aerogel surface area was available for ion sorption. Farmer et al. [66], in the original engineering study for CDI with carbon aerogels, concluded that CDI was best suited for very dilute process streams, such as contaminated groundwater, ultrapure water for semiconductor processing, and perhaps brackish water.

Lastly, in a RO system, the concentration of total organic carbon (TOC) - a mixture of natural organic material and antiscalant components - is highest in the RO concentrate. Work conducted by Gabelich et al [69,71] showed that even at moderate TOC levels, relative to RO concentrate, significant fouling issues resulted, which limited the sorption capacity of carbon aerogel electrodes. Sorption mechanisms for NOM may include electrostatic attraction as well as physical enmeshment with the carbon aerogel structure, resulting in NOM not being removed during regeneration. Electrostatic ion pumping - a simple variant of CDI - is currently being evaluated using smooth electrodes to mitigate the problems with high surface area electrodes [73].

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