Environmental Engineering Reference
In-Depth Information
revealed a charge transfer eficiency of almost 100% due to negligible effects of electrolysis
(due to the low current generated) and insigniicant back diffusion of ions from the elec-
trode chambers to the desalination chamber [108]. Further work with microbial desalina-
tion cells suggested them as a pretreatment to reduce salinity before RO treatment. To this
end, a microbial desalination cell was used to demonstrate salinity reduction between 43%
and 63% for 5-20 g/l NaCl using an anolyte with 2 g/l acetate while generating between
295 and 424 mW/m
2
[109]. In another developed cell more optimized for power generation,
called recirculation microbial desalination cells, reduction salinity of 20 g/l NaCl by 34%
with recirculation produced a power density of 931 mW/m
2
, while without recirculation,
salinity was reduced 39% with 698 mW/m
2
produced power density [110].
27.3.6 Asymmetry-Driven Ion Pumps
As with nanotechnology, biology has also provided clues to developing advanced water
desalination systems by following the ideas of ion transport in biological ion channels
and ion pumps [111]. While existential proof for the working of these systems has been
around in living organisms for a long time, implementing technologies for practical sys-
tems have continued to be a challenge due to several gaps in engineering these systems,
arising from a lack of mechanistic understanding of how to manipulate ion transport in
artiicial systems. However, some recent studies have shown promise in advancing the
U
I
(a)
(b)
Gate
metal
Source
reservoir
Drain
reservoir
KCl +
HCOOH
NaOH
SiO
2
insulation layer
Si
Source
electrode
Drain
electrode
z
(
T
= 115) (µm)
∆<
v
>
(nm/τ)
Gates
~35 nm
10
z
0
= 12.015 µm
200
Nanochannels
Microchannels
6
z
0
= -15 nm
100
2
r
0
/nm
100
200
FIGURE 27.16
(See color insert.)
(a) Schematic and SEM image shows the development of a device being used as a luidic tran-
sistor allowing electrostatic control of ions. (From Karnik, R., R. Fan, M. Yue, D. Li, P. Yang, and A. Majumdar,
Nano Letters
, 5, 943, 2005.) (b) Schematic depicts formation of a conical nanopore by electrochemical etching at
potential
U
with current
I
leading to data that show pumping of the potassium ions against a concentration gra-
dient as a function of pore diameter on the narrow side of the conical nanopore. (From Siwy, Z. and A. Fulinski,
Physical Review Letters
, 89, 19803-1, 2002.)
