(215s) Degradation of Artificial Humidity Condensate in the Confined Space Using Large-Area Boron Doped Diamond Electrode

Degradation of artificial
humidity condensate in the confined space using large-area boron doped diamond
electrode

Hao Li, Lecheng
Lei, and Bin Yang^*

(keyangb@zju.edu.cn)

Key Laboratory of Biomass
Chemical Engineering of Ministry of Education, Department of Chemical and
Biological Engineering, Zhejiang
University, Hangzhou 310027, China

Key
words: BDD electrode; Ta substrate; electro-catalytic oxidation; water reuse

The
confined space is a particular location that is cut off from the atmosphere or
without any gas exchange with the outside. It will be a large cost if the water
is provided only by means of external supplements when human being lives there
for a long time, therefore water reuse is the most realistic and economical way
to produce drinking water. Humidity condensate generated by the body mainly
from breathing, sweating, skin evaporation and so on is of a relatively good
quality which can be used as drinking water after treatment^[1]. It
has been reported that humidity condensate has a major composition of small
molecule alcohol, acid, and inorganic salt, and the total organic concentration
are relatively low^[2]. The
methods of activated carbon adsorption and the medium temperature catalytic
oxidation are employed to treat humidity condensate. However, a large amount of
adsorbents are consumed for the former method and while the energy consumption for
the later method is relatively high. Therefore, we consider of choosing the electro-catalytic oxidation
technology to remove organic pollutants in water efficiently, economically
and reliably. The key to this technology is to fabricate high quality of anode.
The boron doped diamond (BDD) electrode has been proved as a candidate owing to
a wide potential window, low background current, high electrochemical stability
and corrosion resistance^{[3, 4]}.

Due
to the limitation of space in the confined space, the treatment facilities
should be compact and easy to install and operate. Herein, we devoted to
fabricating large-area BDD electrode. BDD films are usually deposited on the
substrate of Si, Ti, Nb, Ta, W and so on. The substrate of Si is most widely
used at present, however it is brittle and its conductivity is poor^[5]; Ti
is a much cheaper material with good electrical conductivity, but the adhesion
of diamond film between Ti is poor due to the formation of TiC transition layer^[6]. In
particular, Ti substrate show serious deformation when fabricating large-area diamond
films (see Figure 1a and 1b). The substrate materials of Nb, Ta and
W can be utilized, because they have good mechanical strength, high electrical
conductivity and good chemical stability. In the present study, large-area BDD films
(the size of tantalum plate: ¦µ100°Á1mm) are grown
on Ta substrates (see Figure 1c and 1d) using hot filament
chemical vapor deposition. The surface of the Ta/BDD electrode is smooth and
uniform distribution, and there don't have any deformation on the electrode. On
the basis of some literatures^[7], the
main composition of artificial humidity condensate is shown in Table 1.

Figure 1. The BDD electrode
deposited on Ti (a: top view; b: front view) and Ta (c: top view; d: front view)
substrates

Table 1 The composition of artificial humidity condensate

Figure
2
shows the SEM picture of the surface morphology of BDD electrode. It is obvious
that the substrate is totally covered by the clear-cut facet diamond crystals
with hexangular, triangular and rectangular habits in the diameter range from 1
to 3µm.
The XRD profile (see Figure 3) reveals that a diamond (111) facet preferential
orientation. Moreover the diffraction peaks of TaC and Ta₂C are
observed which are interlayers formed during the deposition process. Raman
spectrum and XPS are further investigated to identify the structure of BDD
electrodes. It is also shown that diamonds have a compact structure,
homogeneous composition and high levels of content.

Figure 2. The SEM image
of the surface morphology of BDD film

Figure 3. The XRD diagram
of the surface of BDD film deposited on Ta substrate

200 mL
artificial humidity condensate was treated through circular manner with the velocity
of 50mL/min by using the BDD electrode as anode and the stainless steel plate
as cathode. The final TOC removal efficiencies against the operating time for
different current densities are shown in Figure 4. The removal
efficiency for each current reaches up to 95%. The residual treated solution
was further identified by HPLC. It reveals that the degradation products are
composed of oxalic acid, formic acid and acetic acid, and among them oxalic
acid is the dominant component. The operating time are approximately 120, 90,
70 min, respectively in order to obtain the TOC removal efficiency of 80% in the
current density of 5, 10, 15 mA/cm²
(the corresponding power is 3.8, 9.9, 17.8 W). Accordingly, the energy
consumptions are 7.6, 14.8, 20.8 J respectively. Although the energy
consumptions in these three conditions are all relatively low, the current
density of 10mA/cm² is optimal with the consideration of treated time
together.

Figure 4. Influence of the
current density on the TOC removal efficiency during degradation of 200mL artificial
humidity condensate

One
of the biggest problems of using electro-catalytic oxidation technology is the
generation of large amount of hydrogen gas. When the space is filled with
hydrogen, it will induce combustion, explosion or other safety hidden troubles,
so hydrogen must be cleaned clearly in time. The generation of hydrogen under operating
condition is listed in Table 2. 0.61 L hydrogen gas was generated after
2 hours' treatment in current density of 10mA/cm² where the TOC
removal efficiency reaches about 90%. In particular, no more than 1°ë of
the volume is occupied in the confined space of 20m³, where 18L
hydrogen will be produced in case that 6kg water will be treated every day. It
demonstrates that the volume of generated hydrogen is small in comparison to
the size of the confined space. It can be collected and removed easily by other
purification means in the space.

Table 2 Hydrogen and total gas generation under
different current density with treating 200mL artificial humidity condensate in
2 hours

The
accelerated life test was carried out in 1mol/L sulfuric acid solution. The
result shows that the working life of the BDD electrode can reach hundreds of
hours, which indicates that the BDD electrode is
stable and effective when long-term using. What's more, to ensure our
daily water demand, we can use multiple sets of electrodes in combination in
order to achieve the effective treatment in a short period of time. In
conclusion, using the electro-catalytic oxidation technology to degrade artificial
humidity condensate is an efficient and economic method.

References:

1.             Samsonov,
N.M., et al., Long-duration space mission regenerative life support. Acta
Astronautica, 2000. 47(2¨C9): p. 129-138.

2.             Bobe,
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experience gained on the space stations "Salut", "Mir", ISS and development
prospects. Acta Astronautica, 2007. 61(1-6): p. 8-15.

3.             Panizza,
M. and G. Cerisola, Application of diamond electrodes to electrochemical
processes. Electrochim. Acta, 2005. 51(2): p. 191-199.

4.             Peralta-Hern¨¢ndez,
J.M., et al., A Brief Review on Environmental Application of Boron Doped
Diamond Electrodes as a New Way for Electrochemical Incineration of Synthetic
Dyes. International Journal of Electrochemistry, 2012.
2012: p. 1-18.

5.             Sun,
J., et al., Electrochemical oxidation of aqueous phenol at low concentration
using Ti/BDD electrode. Sep. Purif. Technol., 2012. 88: p.
116-120.

6.             Guo,
L. and G. Chen, Long-Term Stable Ti/BDD Electrode Fabricated with HFCVD Method
Using Two-Stage Substrate Temperature. J. Electrochem. Soc.,
2007. 154(12): p. D657.

7.             Pierre,
L.M., et al., Chemical analysis of potable water and humidity condensate: phase
one final results and lessons learned. SAE Technical Paper,
1999. 1999-01-2028.