Photocatalytic treatment of water

An ultraviolet driven photocatalytic post-treatment technique for the purification of waste water distillates, reverse osmosis permeates and spacecraft habitat atmospheric humidity condensates is described. Experimental results show that organic impurity carbon content of simulated reclamation waters at nominal 40 PPM level are reduced to, PPB using a recirculating batch reactor. The organic impurities common to reclaimed waste waters are completely oxidized employing minimum expendables (stoichiometric oxygen). This paper discusses test results and parameteric data obtained for design and fabrication of a bread-board system. The parametric testing includes UV light source evaluation, photolysis vs photocatalysis comparison, oxygen concentration dependence, temperature dependence, reactor mixing, disinfection features, photocatalyst loading, photocatalyst degradation studies and power consumption estimates. This novel post-treatment approach for waste water reclamation shows potential for integration with closed-loop life support systems.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to the photocatalytic treatment of water for 
removal of impurities therein by an oxidation-reduction reaction. More 
specifically, the invention relates to the reclamation of contaminated 
water by the use of a photocatalytic process. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an improved, 
efficient process for the removal of compounds present in highly dilute 
concentrations in water. 
More specifically, it is an object of the present invention to provide an 
improved process for the removal of contaminants from water for the 
purpose of producing potable water from waste water.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Upon irradiating semiconductor powders with band-gap or higher energy 
photons, electron(e.sup.-)/hole(h.sup.+) pairs are created within the 
crystalline powder particles by the same physical processes that they are 
generated in photovoltaic devices (8). In a photovoltaic device these 
charges are made to run through a wire to do electrical work; whereas in a 
photoelectrochemical device these carriers are injected into solution 
species (organic molecules, for example) resulting in the performance of 
redox chemical reactions. 
As depicted in FIG. 1, the photoelectrochemical oxidation of organics is 
exoergic and would tend to occur spontaneously; however, the kinetics, in 
the absence of a suitable catalyst, are extremely slow. 
An important aspect of the photocatalytic process is the adsorption of the 
organic molecules onto the extremely large surface area presented by the 
finely divided powders dispersed in the water. A frequent result of 
chemical adsorption is a diminishment in the activation energy of 
reaction. This catalytic effect, depicted in FIG. 1a, compares the 
energetics of oxidizing a dissolved organic molecule catalytically and 
non-catalytically. Note that the activation energy, Ea, of the 
non-catalytic reaction (FIG. 1a) is large and the reaction products are 
lower in energy than the starting materials. FIG. 1b depicts the 
energetics of an organic molecule adsorbed onto a catalytic surface. 
Although the starting material and final products are at the same energies 
as in FIG. 1a, Ea is lower. Operationally this results in faster reaction 
rates than would occur in the absence of the catalyst. Metal and mixed 
metal oxides are commonly employed as catalysts to facilitate many 
ordinary chemical reactions of organic molecules including oxidations. 
However, in photoelectrochemical applications advantage is taken of the 
fact that the solid phase (a metal oxide semiconductor) is also 
photo-active and the generated charge carriers are directly involved in 
the organic oxidation. In this case the process is photocatalytic and is 
depicted in FIG. 2. The adsorption of the band-gap photon by the 
semiconductor particle results in the formation of an electron 
(e.sup.-)/hole(h.sup.+) pair. The electrons generated in the conduction 
band react with solution oxygen forming the dioxygen anion (O.sub.2 -) 
species which subsequently undergoes further reactions resulting in the 
production of the powerfully oxidizing hydroxyl radical species, OH. These 
powerful oxidants are known to oxidize organic compounds by themselves. 
Additionally, the strongly oxidizing holes generated in the valence band 
have sufficient energy to oxidize all organic bonds. It is the provision 
of a catalytic surface and the dual attack by the powerful oxidizing 
radicals and holes that make photocatalysis such an effective method for 
oxidizing aqueous organics. 
FIG. 3(a) shows the thermodynamic conditions necessary for the performance 
of redox chemistry. In the conventional band energy scheme electrons tend 
to spontaneously move downhill (lose energy) and holes, because of their 
positive charge, move `uphill`. The hole acceptor, in this example an 
organic molecule, must lie above the valence band and the electron 
acceptor, O.sub.2, must lie below the conduction band. The charge transfer 
from both bands must occur essentially simultaneously in order to preserve 
charge conservation. FIGS. 3(b) and (c) are examples of thermodynamically 
impossible redox reactions because either a conduction band (b) or valence 
band (c) charge transfer is forbidden due to the mismatched energetics of 
one of the redox couples. 
Solutions employed in the parametric tests were made by dissolving in 
deionized water (DI) organic concentrates containing organic compounds 
which occur in waste water RO permeates, distillates and spacecraft 
humidity condensates. The solution compositions and concentrations are 
shown in Table 1. In some experiments urea and dioctylphthalate were 
absent from the test cocktail because of their resistance to 
photocatalytic oxidation. This is discussed below. 
In most of the experiments to date the annular recirculating batch reactor 
having an ID of 3.8 cm, OD of 5.1 cm, and length of 56 cm shown in FIG. 4 
was employed. The total system volume was approximately 0.7 L. including 
the pump and connecting tubing. The light sources, either a 350 nm 
emitting UV fluorescent or 254 nm emitting low-pressure Hg lamp was at the 
center of the annulus. In most experiments the 254 nm light of the 
low-pressure Hg lamp was utilized because the photocatalytic process did 
not appear to differentiate between the two sources. In the temperature 
dependance experiments, some of the data were collected employing a 350 nm 
front illuminated recirculating batch reactor. 
TABLE 1 
______________________________________ 
THE ORGANIC COMPOUNDS AND THEIR 
CONCENTRATIONS COMPRISING THE NOMINAL 40 
PPM TEST ORGANIC COCKTAILS 
COCKTAIL 
DESIGNATION 
A B C 
(TOC) (TOC) (TOC) 
COMPOUND PPM PPM PPM 
______________________________________ 
ACETIC ACID 9 9 9 
BENZOIC ACID 3 3 3 
BENZYL ALCOHOL 3 3 3 
BENZALDEHYDE 0.5 0.5 0.5 
CAPROLACTAM 2 2 2 
ETHANOL 2 2 2 
2-BUTOXYETHANOL 
0.5 0.5 0.5 
N,N-DIMETHYL 0.5 0.5 0.5 
FORMAMIDE 
OCTANOIC ACID 3 3 3 
PHENOL 3 3 3 
CRESOL 0.5 0.5 0.5 
PROPIONIC ACID 9 9 9 
DIOCTYLPHTHALATE 
3 0 0 
UREA 1 0 0 
IONS 0(DI H.sub.2 O) 
0(DI H.sub.2 O) 
*PRE- 
SENT 
______________________________________ 
*ION CONC. IN PPM: Ca.sup.+2 (7), Na.sup.+ (37), K.sup.+ (15), 
SO.sub.4.sup.-2 (28), PO.sub.4.sup.-3 (3), CI.sup.- (41), F.sup.- (4) 
In a typical experiment, approximately 5 ml aliquots were removed from the 
reactor, through a stopcock, at intervals, for subsequent total organic 
carbon (TOC) analysis on a Dohrman DC-80 total organic carbon (TOC) 
analyzer. The analyzer was calibrated on a daily basis employing potassium 
biphthalate as a standard. The aliquots were acidified and gas purged to 
remove dissolved carbon dioxide prior to injection into the analyzer. The 
TOC value obtained prior to illumination was the time zero sample to which 
subsequent measurements with illumination are compared. 
During a run, the temperature was controlled to 35.degree. C. using a water 
jacketed reactor. Because oxygen was rapidly consumed in the initial phase 
of photocatalytic purification, the solution oxygen concentration, was 
monitored by an oxygen probe (YSI) and controlled, to approximately 20% of 
saturation, by the addition of pure oxygen via a fine catheter. 
REACTOR DESIGN 
Some parameters considered in designing the photocatalytic water 
purification reactors are the following: 
Mass Diffusion 
The turbulence generated in the photocatalytic reactors gives rise to eddys 
of macroscopic size. Since the submicron photocatalyst particles are much 
smaller than the eddy size, mass transfer rates to the particles are 
governed by the diffusion of reactants through a stagnant layer 
surrounding the particles. Consider, for example, a molecule such as 
trichloroethylene (TCE) that has very rapid photocatalytic decomposition 
kinetics. In decomposition experiments performed with 100 PPM TCE, which 
occurs with an 18% quantum efficiency (ratio of the rate of TCE molecules 
decomposed to the rate of incident photons), a 9.times.10.sup.-7 moles 
L.sup.-1 s.sup.-1 mass consumption rate is observed. Based on calculations 
for the diffusion of TCE and oxygen to the catalyst particles, about 
7.5.times.10.sup.-3 moles L.sup.-1 s.sup.-1 and 3.4.times.10.sup.-2 moles 
L.sup.-1 s.sup.-1 reaction rates, respectively, should be supportable at 
20.degree. C. Calculation of the rate of oxygen diffusion from a finely 
divided gas phase (100 um diameter bubbles) into the solution yields a 
1.1.times.10.sup.- 3 moles L.sup.-1 s.sup.-1 rate of mass arrival. Thus by 
considering the primary diffusive processes, the rate of arrival of 
reactants to the particles' surface is 4 to 5 orders of magnitude greater 
than the rate of reactant consumption. Therefore, the photocatalytic 
decomposition rates of TCE and, most probably, the organics employed in 
this study, are not diffusion controlled at the light intensities employed 
here. 
Chemical Reactions 
The photocatalytic oxidation of aqueous organics is dominated by their 
reaction with hydroxyl radicals. These types of homogeneous reactions have 
a typical rate constant on the order of 10.sup.9 to 10.sup.10 L 
moles.sup.-1 s.sup.-1. Under aerobic conditions the steady state 
concentration of photocatalytically generated hydrogen peroxide resulting 
from aqueous O2 reduction or water oxidation has been measured at about 
15.times.10.sup.-6 M. Assuming that the steady state hydroxyl radical 
concentration is of the same order as the hydrogen peroxide, then for 
7.5.times.10.sup.-4 M (100 PPM) TCE the theoretical rate of TCE reaction 
with hydroxyl radicals can be calcuiated by the following expression: 
##EQU1## 
This is substantially larger than our experimentally observed rate for the 
photocatalytic oxidation of 9.times.10.sup.-7 moles L.sup.-1 s.sup.-1. 
Therefore, the hydroxyl radical attack, intrinsic to aqueous 
photocatalytic oxidation, does not appear to be a rate limiting step. 
Bulk Mixing 
The following illustrates the most basic considerations of photocatalyst 
light adsorption and its relationship to convective mixing. For a 0.1 wt % 
photocatalyst loading, experiments have shown that 90% of the light is 
absorbed within 0.08 cm. This is primarily due to the large UV absorption 
coefficient of the photocatalyst and therefore, most of the 
photoelectrochemistry occurs within this illuminated region. The onset of 
turbulence occurs for a Reynolds number (Re) of approximately 2000. Then 
flow characterized by Re equal to 4000 should provide adequate bulk mixing 
within the reactor. Using Deissler's empirical formula and dimensionless 
values for velocity and distance, a viscous sublayer 0.03 cm thick is 
calculated to adhere to the reactor tube wall. By operating the reactor 
with an Re 4000, a significant portion of the photoactive region is 
ensured of being within the well mixed turbulent zone. 
Adsorption of Organics 
The photocatalytic process must be dependent, to a degree, on the 
adsorption of the organics onto the photocatalyst particles' surface. The 
adsorption isotherm of an organic is dependent on factors such as pH and 
the presence of other solutes. Presently, the dependence of photocatalytic 
reaction rates on this variable are not well understood and this is 
essentially an uncontrolled parameter in the water purification process. 
Measurements indicate that in an initially 100 PPM TOC cocktail made using 
concentrate "B", approximately 2 PPM of the available TOC is adsorbed onto 
the photocatalyst powder and reactor wall surface. 
Interfacial Charge Transfer 
The transfer of charge carriers (e.sup.- and h.sup.+) across the 
semiconductor/solution interface is considered to be rate limiting for the 
slow reaction of photoelectrochemical water splitting. However, in the 
photocatalytic degradation of aqueous organics, the reduction of dissolved 
molecular oxygen is fast. This is because oxygen is a reactant at 
relatively high concentration and is a one electron transfer reduction, in 
comparison to the low concentration multielectron transfers involving 
water splitting intermediates. Simultaneously, rapid hole injection is 
expected because of the surface concentration of the water and adsorbed 
organics, both of which are hole acceptors. Although there are techniques 
for increasing interfacial charge transfer rates, their success is 
dependent on the exact photoelectrochemical process and process 
conditions. It is unclear at present whether charge transfer is rate 
limiting for this system. 
The discussion and calculations above indicate that for the annular reactor 
and operating conditions employed in this study, purification rates are 
not mixing, diffusion, or chemical reaction rate limited. Experiments 
employing the annular reactor, in which the volumetric flow rate was 
varied from 8 to 30 L min.sup.-1, showed only small differences in the 
purification rates of water over this flow rate range. For this reactor, 
the lower flow rate corresponds to a Reynolds number of about 2000, the 
onset of turbulent flow. This indicates that mass transfer limitation has 
indeed been avoided. The relatively high quantum efficiency of 
photocatalytic degradation of TCE (about 18%), together with the 
considerations above, strongly suggests that the photocatalytic 
purification rates are light limited. Control of charge transfer or 
adsorption to enhance purification efficiency is not within the scope of 
this development effort. Rate increase should be observed with increased 
light intensity, however operating the low-pressure mercury lamps this way 
would reduce their life. The above considerations reduce to the fact that 
the most efficient operation and optimized purification rate of the 
photocatalytic system is achievable by minimizing the viscous sublayer 
depth in order to provide a deep well-mixed zone within the photoactive 
region. 
PHOTOCATALYTIC OXIDATION OF AQUEOUS ORGANICS 
Consider the following: photocatalytic reactions are usually described by 
Langmuir-Hinshelwood kinetics, which in the case of dilute solutions, 
reduces to the familiar exponentially decaying first-order expression(1) 
where C.sub.o is 
EQU fraction remaining=C(t)/C.sub.o =exp(-kt) (1) 
the initial concentration, k is the first order rate constant, and t is the 
time. Application of linear regression analysis to ln(C(t)/C.sub.o) vs 
time data yields a straight line with slope -k (FIG. 5). It must be noted 
that one constant, k, is being used to described the decomposition of a 
multicomponent system. Although actual kinetics may deviate somewhat from 
the oversimplified model above, the employment of first-order k's, when 
appropriate, facilitates comparison and discussion of various reactions 
and parameters. 
FIGS. 5 and 6 show the natural log and fraction of TOC remaining vs time 
data, respectively, for the photocatalytic decomposition of cocktails A, B 
and C. The photocatalytic decomposition of the organics in the two 
cocktails B and C exhibit almost identical kinetics, approximated by eq(1) 
throughout the run for a k value of 0.0470 min.sup.-1. Cocktail B was made 
using Dl water and is therefore practically ion-free. The water of 
cocktail C, however, had Ca.sup.+2, Na.sup.+, K.sup.+, SO.sub.4.sup.-2, 
PO.sub.4.sup.-3, Cl.sup.-, F.sup.- ion concentrations shown at the bottom 
of Table 1, approximating RO permeate. This demonstrates that ion 
concentrations found in RO permeate do not affect the aqueous 
photocatalytic oxidation process. This is expected because these ions are 
generally found to be electrochemically inactive (i.e. they have redox 
potentials that exceed those for water splitting or sufficiently slow 
kinetics that preclude significant reaction). Second order effects due to 
ion adsorption on the surface of the photocatalyst particles can cause 
shifting of the flat-band potential. For example, a drastic anodic shift 
in flat-band potential could slow down or thermodynamically preclude the 
vital oxygen reduction reaction. However, if flat-band potential 
modification due to these ions was occurring, the manifestation of the 
effect was negligible. 
For cocktails A and B (FIG. 5), the initial kinetics, down to about 0.37 
fraction remaining are approximately equal. For B, the small upward 
deviation from approximate linearity after 90 minutes is probably an 
experimental artifact attributable to higher uncertainty in TOC analysis 
owing to low level organic carbon contamination. For cocktail B, 500 PPB 
TOC concentration (denoted by dashed line in FIG. 5) is reached 
experimentally in 110 minutes; or, as predicted by equation (1), in the 
absence of contamination, in 98 minutes. For cocktail A, below the 0.37 
fraction, the decomposition of organics slows down markedly and almost 
stops at about 3 PPM. This decrease in rate is attributed to the extremely 
slow oxidation kinetics of urea and/or dioctylphthalate, whose individual 
decompositions are shown in FIG. 7. For cocktail A, lowering the pH to 
about 3 by the addition of 2 drops of 85% phosphoric acid, permitted the 
degradation of the remaining dioctylphthalate. This degradation is 
explained by the hydrolysis of dioctylphthalate to phthalic acid and 
2-ethylhexanol (the isooctyl group) at the lowered pH. These hydrolysis 
products are rapidly photocatalytically oxidized. Urea decomposition was 
probably not significantly assisted by the lowered pH and simply occurs 
very slowly over the duration of the experiment. 
In an attempt to elucidate the extremely slow photocatalytic decomposition 
of dioctylphthalate (octyl groups on 1,2 positions), the decompositions of 
2-ethylhexanol, dioctylterephthalate (octyl groups on 1,4 positions), 
diethylphthalate, and potassium biphthalate were investigated; the results 
are shown in FIG. 8. Note that except for the dioctylphthalates, the 
phthalates are generally rapidly photocatalytically decomposed. The 
difficulty in oxidizing dioctylphthalates was reflected in the TOC 
analyzer by the very long analyses times. The analyzer employs 185 nm UV 
light and simultaneous persulfate chemical oxidation for achieving organic 
oxidation to CO.sub.2. Since the extremely slow decomposition of the 
dioctylphthalates cannot be attributed to steric effects or unique 
resonance stability and because the hydrolizates are individually rapidly 
decomposed, their recalcitrance is attributed to their high water 
insolubility. Phthalate compounds were found in Space Lab humidity 
condensate and a source is placticizers commonly found in Tygon tubing. 
##STR1## 
is a product of human metabolism. Amides are generally more difficult to 
oxidize than amines owing to the resonance interaction of the electron 
pair on nitrogen with the adjacent carbonyl group. Urea's particularly 
slow kinetics is attributed to the additional resonance stability imparted 
by the second amine group on the lone carbon atom in the molecule (see 
structure above). The stability of urea towards radical attack is 
exemplified by comparing the rate constants of OH and other radical 
reactions with urea in Table 2. The important feature of this data is that 
urea reacts several orders of magnitude more slowly than the average 
organic. For the case of OH, urea was by far the slowest reactant when 
compared to the other organic compounds in the cited reference. 
TABLE 2 
______________________________________ 
RATE CONSTANTS FOR THE REACTION 
OF VARIOUS RADICALS WITH AQUEOUS UREA. 
THE AVERAGE RANGE OF OTHER AQUEOUS 
ORGANICS TABULATED FOR COMISON. 
K (urea) K (average) 
RADICAL (L mole-1 s-1) 
(L mole-1 s-1) 
REFERENCE 
______________________________________ 
.OH 7.9 .times. 10.sup.5 
109 to 10.sup.10 
(10) 
e.sub.aq - 
3.0 .times. 10.sup.5 
108 to 10.sup.10 
(10) 
.H &lt;3 .times. 10.sup.4 
10.sup.8 (10) 
CO.sub.3. - 
&lt;1 .times. 10.sup.3 
10.sup.6 to 10.sup.8 
(15) 
O.sub.3 5 .times. 10.sup.-2 
10.sup.2 to 10.sup.5 
(15) 
______________________________________ 
PHOTOLYSIS VS PHOTOCATALYSIS 
The 254 nm emission lines of Hg can excite electronic transitions that may 
ultimately lead to the decomposition of aqueous organics. FIG. 9 compares 
direct photolysis with photocatalysis (fraction of TOC remaining vs time). 
The photolytic degradation rates of the organics are shown to be 
negligible in comparison to photocatalysis. 
Two principal factors contribute to the ability of either photolysis or 
photocatalysis to effect the destruction of organics: 1. the degree of 
light absorption and 2. the lifetime of the excited state (i.e. the 
systems ability to remain in the excited state until degradation reaction 
pathways can be followed). 
Employing a UV spectrophotometer, it was found that approximately 50% of 
the 254 nm radiation is absorbed within 1 cm for an approximately 50 PPM 
TOC organic cocktail in the absence of the photocatalyst. Because this is 
relatively high light attenuation, the reason for the low photolytic 
decomposition of the organics has to be due to rapid de-excitation of the 
molecules. 
The electronic excited state is usually short lived because of the three 
photophysical intramolecular deexcitation pathways (intersystem crossing, 
fluorescence, and nonradiative or thermal decay) and intermolecular 
relaxation. These photophysical processes effectively compete with the 
fragmentation of and the ultimate destruction of the molecule. Also, 
oxygen, which is normally present in water, effects intermolecular 
relaxation because it is known to be an efficient excited state quencher. 
Photolysis experiments performed with oxygen present or absent did not 
significantly affect degradation rates. Therefore it is concluded that the 
failure of photolysis to effect significant organic degradation is 
attributed to rapid de-excitation via intramolecular pathways. 
TEMPERATURE EFFECT 
The temperature dependence of first order rates is given to good 
approximation by the Arrhenius equation (2): 
EQU k(T)=A*exp(-Ea/RT), (2) 
where Ea is the activation energy, R is the gas constant, and A is 
sometimes called the frequency factor. FIG. 10 is a plot of In(k) vs 1/T 
for the photocatalytic decomposition of cocktail B performed in the 
annular reactor and cocktail A performed in a different, 
front-illuminated, recirculating reactor. The identical slopes of the 
plots reflect the fact that Ea is a thermodynamic quantity independent of 
reactor geometry, but the upwardly displaced slope of data obtained in the 
annular reactor reflects faster rates owing to more efficient reactor 
design and/or greater photon flux per volume of water. A value of Ea=3.5 
Kcal/mole was found for the two cocktails. Approximately the same 
activation energy values are reported in the literature for the 
photocatalytic oxidation of oxalic and formic acid. Since the organic 
cocktails are composed primarily of carboxylic acids, this value is 
concordant with the literature. It appears to be a general property of the 
photocatalytic oxidation of aqueous organics that anomalously low 
activation energy barriers are observed. This probably reflects the fact 
that the activation energy of the rate determining step is not provided by 
thermal processes. The reaction rate obtained by increasing the 
temperature from 35.degree. to 70.degree. C. is shown in FIG. 11 is 
increased by a factor of 1.8. For comparison, a classical reaction having 
Ea=15 kcal/mole would exhibit a factor of 12 increase in rate. 
OXYGEN DEPENDANCE 
The dependence of the photocatalytic rate constant on the oxygen 
concentration is shown in FIG. 12. It should be mentioned that up to 
approximately 0.5 fraction TOC remaining, the reaction appears to be 
insensitive to the oxygen concentration. After this point the dependence 
on oxygen concentration is slightly more pronounced. The k values in FIG. 
12 were obtained by linear regression analysis on the first three data 
points of a run. FIG. 12 shows that an oxygen concentration above 20% 
saturation (the equivalent oxygen derived from an air saturated solution) 
does not significantly increase reaction kinetics. However, below this 
value a noticeable overall decrease in rate is observed. 
The stoichiometric oxygen requirement for photocatalytically purifying 1 L 
of water containing 40 PPM TOC was determined from the quantity of O.sub.2 
required to oxidize all the organics in cocktail A (Table 1). This 
correlates to approximately 1.5 moles for every mole of organic carbon or 
0.112 L of oxygen per liter of 40 PPM TOC cocktail solution. This is the 
total amount of O.sub.2 consumed during the photocatalytic oxidation 
cycle. In a breadboard system the O.sub.2 will be metered into the 
photocatalytic reactor through a fine catheter to provide only the amount 
of O.sub.2 required for oxidation of the TOC. This control should prevent 
gas/liquid interfaces from forming as a result of exceeding O.sub.2 
saturation (43 mg L.sup.-1). The CO.sub.2 produced is more soluble in 
water (1688 mg L.sup.-1) than O.sub.2. Therefore, much higher amounts of 
TOC can be oxidized before gas/liquid interfaces would develop by 
exceeding CO.sub.2 solubility. 
In order to compare the quantity of oxygen consumed by photocatalytic water 
purification with that of respiration consider that one human needs 
approximately 3 L of water per day. Multiplication of 0.112 L O.sub.2 /L 
of H.sub.2 O by 3 yields 0.336 L/day/person of oxygen for water 
purification; an average 68 Kg human requires 3.5 mL/Kg-min (18) or 343 
L/day/person for respiration at rest. Clearly the amount of oxygen 
required to provide potable water is insignificant in comparison to that 
needed for respiration. 
PHOTOCATALYST LOADING 
Preliminary work has shown that photocatalyst loading of 0.01% and 0.2 wt % 
yielded rate constants of 0.0152 and 0.0295 min.sup.-1. Thus, for a factor 
of 20 increase in photocatalyst loading only an approximate doubling of 
the rate is observed. For photocatalyst loading higher than 0.2 wt %, no 
substantial rate increase was achieved. 
PHOTOCATALYST DEGRADATION 
In order to measure the useful life of the 0.2 wt % photocatalyst, 
accelerated degradation tests were performed using nominal 1000 PPM TOC 
solutions. Thus the decomposition of 1000 PPM TOC was equated to using the 
photocatalyst 25 times for the purification of water contaminated by 
compounds found in cocktail B (nominally 40 PPM TOC). The curves in FIG. 
13 show the repeated cycling of the same photocatalyst four times. The 
purification rate was observed to slow down by the third and fourth 
cycles. At this time it was observed that significant amounts of 
photocatalyst was adhering to the outer reactor wall. Therefore, the rate 
decreases are presently attributed to the removal of large amounts of the 
photocatalyst from suspension. It is speculated that either the high 
concentrations of organics or the formation of intermediate oligomers 
caused adhesion of the photocatalyst to the outer wall. Also, the organic 
adsorbed onto the adhered photocatalyst was effectively shielded from the 
radiation by the remaining suspended photocatalyst and therefore, from 
further degradation. It is realized that loss of catalyst had some effect 
on loss of oxidation activity. The amount of loss has not yet been 
determined. Currently, degradation tests are being performed with 100 PPM 
TOC solutions in an attempt to avoid catalyst losses described above. 
Assuming that 0.2 wt % photocatalyst loading can be recycled at least 100 
times without significant activity loss are correct, then a 3L/person/day 
water requirement translates into a 0.6 g/person/day photocatalyst 
requirement. For a crew of six on a year mission this scales up to a total 
of 1314 g (2.9 lb) of photocatalyst. 
PHOTOCATALYTIC DISINFECTION 
Preliminary results on experiments to measure the disinfecting ability of 
photocatalytic water purification are reported in a Phase I study. The 
organism P. cepacia used in these studies had been shown to be extremely 
resistant to the standard disinfection concentrate, 10% iodine-providone. 
Experiments were carried out which measured the survival rate of P. 
cepacia cells that were maintained in the reactor for various periods of 
time. The initial concentration was 10.sup.7 cfu/ml. The solution oxygen 
concentration was approximately that of air saturation. After one hour of 
incubation in the illuminated reactor more than 99.99% of the cells had 
been destroyed, based on the fact that no colonies were present on agar 
plates which had been inoculated with 0.1 ml of a 100-fold dilution of the 
cell suspension. Cell death was not simply due to suspension in dilution 
medium, since cell viability was not affected after three hours of 
incubation in dilution medium in the absence of the photocatalyst and 
light. Furthermore, cell viability was not affected by incubation with the 
photocatalyst, with the recirculation pump on, in the dark for one hour. 
Therefore, cell destruction was not caused by the shear forces within the 
pump. Photocatalytic water purification appears to provide disinfection, 
but more rigorous controlled tests are required to fully asses its 
potency. 
CATALYST TICLE SEATION 
Preliminary work on the removal of the photocatalyst from the purified 
water have been successfully demonstrated by the utilization of a 
cross-flow filtration technique. In this procedure an Enka Microdyne 
Module (Model MD 020 CP 2N) constructed from 0.1 m2 of polypropylene 
capillary membrane having a pore size of 0.2 um and an ID of 1.8 mm was 
used. By employing periodic back-pulsing through the module, a small 
residual amount of photocatalyst was found adhering to the filter 
membranes. Further longevity testing of the process is in progress. 
SYSTEM DESIGN 
In the proposed prototype, the reactor geometry will be constrained by the 
low pressure Hg light source's cylindrical geometry, D=2.5 cm and active 
length =81 cm. For a one lamp reactor, annular geometry (FIG. 1) permits 
surrounding the light source with fluid. To meet NASA requirements, a 
multi-lamp reactor system is needed. Two options are: stringing together 
in series individual annular units or combining many lamps in one reactor 
vessel. At present, elementary fluid mechanical calculations indicate that 
the combination of several lamps in one reactor vessel may result in 
significant volumes of poor mixing or else require an average volumetric 
flow rate substantially greater than necessary with the annular reactor. 
The practical result of this may be inefficient use of light and increased 
electrical energy demands of the system. However, because the latter 
reactor design may offer weight savings, the final preprototype design is 
still under consideration. 
The preliminary system design concept is depicted in FIG. 14. This 
preprototype will be operated in the batch mode with discrete unit 
operations for the photocatalytic oxidation of organics and separation of 
the photocatalyst from the purified water. Design considerations for the 
problems of fluid handling in microgravity include the use of hydraulic 
accumulators and bladder lined tanks for the holding and transfer of the 
liquids to prevent the problems of gas/liquid phase mixing. The 
waste-water distillate/RO permeate will be transferred under positive 
pressure from a hydraulic accumulator to the reactor loop where the 
photocatalyst and oxygen will be added. Stoichiometric oxygen, plus an 
excess equivalent to the air saturated value, will be added to the system 
over time by an oxygen injector. The oxygen injector will be a fine 
stainless steel catheter backed by a metering unit. The total amount of 
oxygen added to the system will be enough to ensure complete and rapid 
oxidation of organics and also avoid a gas/liquid phase mixture at the end 
of a run. A dissolved oxygen sensor will be employed for monitoring. An 
expansion tank, utilizing a diaphragm or floating piston, will be employed 
to accommodate the small temporary pressure/volume changes caused by the 
addition and consumption of oxygen and temperature changes. The reaction 
mixture will be recirculated by a centrifugal pump through the 
photocatalytic reactor loop to effect purification. Once purified, the 
photocatalyst/water slurry will be transferred under positive pressure to 
another hydraulic accumulator that acts as an interface between the 
reactor and filtration unit. A second centrifugal pump then circulates the 
slurry through the cross-flow filter to effect separation. The permeate 
(pure water) will be collected in a bladder lined reservoir for storage 
while the slurry retentate is concentrated in the hydraulic accumulator. 
The slurry is then transferred from the hydraulic accumulator under 
pressure to the reactor, completing the cycle. The backpulse hydraulic 
accumulator stores a small volume of purified water to periodically 
back-wash the filter to prevent excessive photocatalyst caking and 
plugging of the filter. The backpulsing is effected by pneumatic 
pressurization through a solenoid valve to the hydraulic accumulator 
without interruption of the slurry circulation through the filtration 
unit. 
ESTIMATED POWER REQUIREMENTS 
The principal electrical energy demands of the 4.5 L photocatalytic water 
purification prototype system are UV light generation (five 30 W 
low-pressure Hg lamps) and recirculating pump power (1/25 HP @ 
approximately 130 W). The data suggests that the 500 PPB TOC level of TOC 
will be reached in less than 90 minutes. After water purification is 
achieved, additional pumping power is required for fluid transfer and to 
effect particle separation. The latter pumping needs will be met by 
employing a motor having similar power demands as the recirculating pump. 
However, in the latter case, it estimated that this process will take only 
6 minutes. Therefore, a coarse estimate of the systems power demands are 
the following: 
__________________________________________________________________________ 
LIGHT GENERATION: 5 lamps .times. 30 W/lamp .times. 1.5 hr 
225 W-hr. 
PUMPING: 130 W .times. 1.5 hr = 
195 W-hr. 
FLUID TRANSFER AND FILTERING: 
130 W .times. 0.1 hr = 
13 W-hr. 
TOTAL: 433 W-hr. for 4.5 L. 
__________________________________________________________________________ 
The photocatalytic water purification process described here is effective 
for oxidizing organic impurities common to reclaimed waste waters and 
humidity condensates to carbon dioxide at ambient temperatures. TOC 
concentrations below 500 PPB are readily achieved. The temperature 
dependance of the process is well described by the Arrhenius equation and 
an activation energy barrier of 3.5 Kcal/mole. Urea and some insoluble 
phthalate esters are only slowly oxidized. Preliminary work, in a previous 
Phase I study, indicated that sterilization features are exhibited by the 
destruction of greater than 99.99% of initially 10.sup.7 colony forming 
units of P. cepacia. The only expendable required by the photocatalytic 
system is oxygen at an excess slightly greater than stoichiometric. For a 
40 PPM TOC contamination, the stoichiometric oxygen requirement is 
approximately 0.11 L O.sub.2 /L H.sub.2 O. The estimated energy 
requirements of the system are 100 W-hr/L H.sub.2 O. Preliminary 
photocatalyst degradation studies indicate that at least 5 L of initially 
40 PPM water can be purified per gram of photocatalyst. Particle 
separation subsequent to water purification appears to be readily 
accomplished with minimal energy consumption by the utilization of 
cross-flow filter technology. Separation of the catalyst particles from 
the purified water under microgravity will be facilitated by the avoidance 
of gas/liquid phase mixing. Other microgravity considerations implemented 
in the breadboard system will be the utilization of hydraulic accumulators 
and bladder tanks. The product of this development will be a breadboard 
photocatalytic purification system which employs multiple batch cycle 
operations for post-treatment of reclaimed waters for application in 
closed-loop life support systems in space based environments.