Environmental Engineering Reference
In-Depth Information
Nanoporous carbon
Positive electrode
Contaminated
water
Pure
water
Negative electrode
SCHEME 2.8
Description of a process for electrochemical deionization (EDI) using a nanoporous carbon electrode.
The electronic charge related with ion adsorption appears as the so-called double-layer
capacitance. The amount of material adsorbed on the surface and capacitance is directly
proportional to the active surface area.
= C
F
sp
Γ sp
VS
(2.7)
sp
where Γ sp is the speciic coverage (mol/g), C sp the speciic capacitance (F/g), F the Faraday
constant, Δ V the change of potential, and S sp the speciic surface (m 2 /g ).
The accepted value C sp for carbon is 20 μF/c m 2 [145]. For a porous carbon ( S sp = 500 m 2 /g )
and a voltage change of 1 V, the amount of ions calculated by Equation 2.7 is ~1 mmol/g.
Since the concentration of dyes in water is quite low (<0.1 mmol/l) [146], 1 g of carbon
could decontaminate 10 liters of water. An advantage of capacitive deionization over sim-
ple adsorption is the fact that removal of the electrical charge results in release of the ions.
Therefore, the carbon electrode could be used to decontaminate water; the pure water
lushed out and replaced by waste solution where the contaminants are released. In that
way, the carbon is reused without the need for regeneration.
Figure 2.19 shows the insertion (at 0.0 V vs. SCE) and release (at 0.5 V) of methylene blue
dye. The cation (see chemical formula inside the graph) is inserted at 0.0 V and released at
0.5 V. The concentration of the dye was measured in situ by spectrophotometry at 430 nm.
2.4.2 Electroanalytical Sensing of Arsenic Ions with Nanocomposites
Redox metal oxides are widely used in electrocatalysis [147,148]. Therefore, they have also
been used in electroanalysis of toxic ions [149]. Cobalt oxide nanoparticles could be easily
deposited electrochemically onto lat GC electrodes [150]. They have been used for deter-
mination of hydrogen peroxide [151] and arsenic ions [152]. An important natural water
contaminant present as different ionic species is arsenic [153,154]. HPC (1PSC), modiied
with in situ produced cobalt oxide nanoparticles, was used to detect arsenic ions in neu-
tral solution (Figure 2.20). The cyclic voltammogram (CV) measured in the presence of As
ions (full line) shows a clear oxidation peak that is not present in the CV measured in the
absence of As ions (dashed line). The peak is likely due to the oxidation of As(III) to As(V)
species, catalyzed by cobalt oxide:
2 Co(OH) 2 → 2 CoOOH + 2 e
(III)
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