Ammonia Removal from Rodent Habitat Operations in Space Using Phosphoric Acid Treated Activated Carbon

doi:10.4236/ajac.2013.412095

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American Journal of Analytical Chemistry, 2013, 4, 776-780

Published Online December 2013 (http://www.scirp.org/journal/ajac)

http://dx.doi.org/10.4236/ajac.2013.412095

Open Access AJAC

Ammonia Removal from Rodent Habitat Operations in

Space Using Phosphoric Acid Treated Activated Carbon

Zhe Lu1*, Jacob A. Hines2*, Daniel J. Rozewicz1, Michael L. Hines3

1Lockheed Martin Space OPNS, Moffett Field, USA

2University of California, Santa Barbara, USA

3NASA Ames Research Center, Moffett Field, USA

Email: *[email protected], *[email protected]

Received October 30, 2013; revised November 29, 2013; accepted December 15, 2013

permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

To accommodate long duration biology research with rodent habitats on the International Space Station while providing

a healthy living and working environment for crewmembers, NASA Ames Research Center developed a new exhaust

filter for odor control for the Animal Enclosure Module (AEM), which houses mice and rats. The new exhaust filter

uses activated carbon pellets as adsorbents, with phosphoric acid (H3PO4) impregnated on the surface. The deodoriza-

tion performance of the new exhaust filters for AEM units housed with mice was evaluated. The ammonia breakthrough

time of the exhaust filters was also investigated. The results indicated that H3PO4 treated activated carbon exhibited a

high ammonia adsorption capacity of more than 90%. Furthermore, the new exhaust filter can effectively control the

odor from the AEM units for a 45-day (minimum) flight mission with a given animal biomass.

Keywords: Activated Carbon; Impregnation; Adsorption; Ammonia; International Space Station

1. Introduction

For centuries, humans have observed animals in order to

better understand aspects of human biology. In modern

times, animal research has become the gold standard for

basic biology and medicine. With animal subjects, it is

possible to control and reproduce environmental condi-

tions, experimental subjects, and protocol, a set of ad-

vantages that is often difficult if not impossible to achieve

with human subjects. The value of animal research ap-

plies equally well to research in space as it does on the

ground. Space animal research is essential for under-

standing the impacts of spaceflight on physiological sys-

tems and for the development of therapies that will miti-

gate detrimental responses to spaceflight. The rodent, for

example, is an ideal surrogate for establishing the tem-

poral baseline effects of long term exposure to space-

flight.

The National Aeronautics and Space Administration

(NASA) Animal Enclosure Module (AEM), designed to

provide a flight habitat environment for rodents, has been

routinely used for spaceflight to conduct rodent micro-

gravity research studies in space. The rodents in the

AEM are a source of numerous airborne contaminants.

Odor complaints that have been risen about the rodents

involve both the unpleasant character of the odor, as well

as the potential adverse health effects.

Although little attempt has been made to identify the

chemical composition of the exhaust airstream from the

AEM, over 100 chemical compounds have been identi-

fied in the airstream from various animal houses [1-4].

Ammonia is one of the major odor causing compounds in

animal houses and presents the greatest risk to the envi-

ronment. It is produced by the decomposition of nitro-

gen-containing compounds in the excreta, especially in

the urine. Levels of 50 - 100 ppm of ammonia can cause

irritation of eyes, throat, and nose, and has a detection

threshold between 5 and 18 ppm for humans [5]. Ammo-

ia levels for a well-ventilated animal house are in the

range of 5 to 10 ppm as set by the US Occupational

Safety and Health Administration (OSHA) [6]. The Space-

craft Maximum Allowance Concentration (SMAC) of am-

monia in the International Space Station (ISS), set by the

NASA/JSC Toxicology Group, is 30 ppm for less than 1

hour of exposure and 10 ppm for an exposure of more

than 7 days [7].

*Corresponding authors. NASA Ames Research Center (ARC) has undertaken

Z. LU ET AL. 777

the effort to build a new lot of exhaust filters for odor

control, especially for ammonia removal, for upcoming

long-term science payload missions with AEM payloads.

Many approaches have been tested to mitigate ammo-

nia, but most of them remain on laboratory scales. These

techniques include absorption by solutions, separation

using membranes, catalytic decomposition, and adsorp-

tion by porous solids [8,9]. However, the adsorption on

porous solids will be an excellent option to remove am-

monia in the flight habitat environment, as liquid solu-

tions are difficult to maintain in space. The ammonia

adsorbents are mainly zeolite, activated carbon, and acti-

vated alumina. The ammonia adsorption on porous solids

can take place through physisorption and chemisorption

processes [10]. The chemisorption process is usually

stronger than physisorption, because the chemisorption

process involves a chemical reaction between ammonia

and the adsorbents, while the physisorption process re-

lates to a pore’s filling being driven by weak van de

Waals forces.

Among the potential ammonia adsorbents, activated

carbon is commonly used. Activated carbon exhibits high

surface area and highly developed porous structures,

which facilitate physical adsorption. Most importantly,

strong interactions between the gas and the adsorbent can

be enabled by tailoring the porous structure and surface

chemistry of activated carbon [8,11,12]. The alterations

of the activated carbon can be accomplished by impreg-

nation of inorganic compounds, which are anticipated to

develop stronger interactions between ammonia and the

surface of the activated carbon [13-15]. These impreg-

nants react instantaneously with ammonia in air that has

been filtered by previous sorbent layers to remove mois-

ture to form stable chemical compounds that are ire-

versibly bound to the media as inorganic [16]. The prop-

erties of activated carbon will be optimized by the im-

pregnation of suitable chemicals on the internal surface

for the chemisorption of certain gases.

The objective of this paper is to present the compre-

hensive design and configuration of the long-term odif-

erous organic compound filter, which uses phosphoric

acid loaded activated carbon as its adsorbent to remove

odorants, especially ammonia. Efforts were made to meas-

ure the effectiveness of the long duration exhaust filter

designed for odor control. The deodorization perform-

ance of the exhaust filters is examined based on 45-day

(or longer) science missions in space with mice-loaded

AEM units. The lifetime of the exhaust filter will also be

explored by testing the time until the odor breakthrough

of the exhaust filter.

2. Experimental

2.1. Materials

New exhaust filters (Figure 1) for the AEM have been

assembled and built at NASA Ames Research Center.

The exhaust filter is 21.9 cm× 35.5 cm× 6.5 cm and the

flowrate of theairstream from the AEM unitto the ex-

haust filter was controlled at 0.1 L/sec.

The exhaust filter in the AEM is designed to prevent

the escape of particulate matter into the cabin atmosphere,

as well as contain animal odor and neutralize urine within

the AEM. The exhaust filter, rated for 45 days of odor

control, is made up of 9 layers including Bondina, fabric

sorbents, activated carbon, filtrate, and zeolite, as shown

in Figure 2. Layer 1 directly exposed to the rodent cham-

ber. This layer, as well as layer 4, consists of phosphoric

acid (H3PO4) treated Bondina with 20 g pre-loaded H3PO4

impregnated on the surface of Bondina for removal of

alkaline gas, such as NH3 and amines. Layers 2, 3, 5, and

7 consist of nonwoven fabric with evenly distributed

holes for airflow, provide excellent liquidretention and

Figure 1. Picture of the exhaust filter for the Animal En-

closure Module (AEM).

Airflow

AirGap AirGap

123456789

1) Bondina with acid; 2) Sorbent, yellow, punched, with acid; 3) Sorbent,

yellow, punched; 4) Bondina with acid; 5) Sorbent, yellow, punched; 6)

Zeolite; 7) Universal Sorbent, yellow, punched; 8) Activated Carbon Bed,

with acid; 9) G200 Filtrete.

Figure 2. Diagram of the exhaust filter layers for the Ani-

mal Enclosure Module (AEM).

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Z. LU ET AL.

778

high air flow rate, and are used to stop urine. Air gaps

between layers 2 and 3, and layers 7 and 8, were de-

signed to separate fabric filter layers and allow a fast

airflow through the layers. Layer 7 contains zeolite pel-

lets from Lewcott Corp, Millbury, MA, which attempt to

facilitate the adsorption of both urine and ammonia.

Layer 8 is the carbon bed, which is packed with com-

mercially available Ammosorb (NUCON International,

Columbus, OH), a pelleted, activated carbon with phos-

phoric acid loaded on the surface to remove alkali gases

such as ammonia and amine vapors. The last layer of the

exhaust filter is Trion G200 filtrete, which will help to

reduce dust, mold spore and odor in the air stream.

Three AEM units (AEM 101, 102, and 103) were

loaded with an exhaust filter, mouse food bars, and dis-

tilled water, and were used for mice habitat. One AEM

unit (AEM 104), had this same setup and was used as a

control chamber for the odor evaluation. Ten (10) mice,

with 5 mice on each side of the split chamber, were

loaded in the three AEM units (101, 102, and 103). All of

the four AEM units were instrumented with temperature

probes and a gas port for periodic sampling of ammonia

(NH3), and daily hardware and animal health checks were

performed.

The AEM exchanges chamber air with the exhaust fil-

ter, a process wherein the air from the rodent cage, pre-

viously drawn in through the inlet filter, flows through

the exhaust filter prior to exiting the AEM. High effi-

ciency air filters prevent the escape of particulate matter

into the cabin atmosphere, and treated activated carbon-

inside the filters helps contain animal odor and neutralize

urine within the AEM. After exiting the habitat through

the exhaust filter, the filtered air is drawn through the

exhaust fans into the cabin.

Gas samples were collected from the gas port that di-

rectly connected to the AEM main chamber. Concentra-

tions of NH3 were measured from test day 21 onwards

using two Dräger test pumps with a detection range of

0.20 - 5.0 ppm and 5 - 70 ppm, respectively.

2.2. Odor Measuring Method

The odor expelled from the four investigated AEM units

was evaluated using panelists to rank samples. A group

of volunteers were trained to conduct the daily odor

evaluations. An arbitrary scale was used to describe the

intensity of the odor from the AEM units. Each AEM

unit was covered with a blue shroud to prevent ambient

light from entering cage environment and prevented odor

panelists from viewing the cage interior.

During the evaluations, sniffers were asked to stand a

distance of 6 - 8 in from the AEM exhaust outlet and

scored the odor using a 5-point scoring system: 0 = un-

detectable, 1 = barely detectable, 2 = easily detectable, 3

= objectionable (disagreeable), and 4 = revolting (ex-

tremely offensive). Group training, instruction, and cali-

bration were provided to evaluators prior to the tests.

Score selections were based on odor interpretation with

respect to predetermined and formerly presented (sniffed)

standards for each level of the scoring system. Average

odor scores for each test day were then calculated.

The animal tests will be stopped when the average

odor scores are over 3.0 for two consecutive days, the

mouse food bars need to be replaced, or when the mice

are observed to be in bad health, whichever appears first.

The breakthrough time of the new exhaust filter will be

identified for AEM units 102, 103, and 104 when the

average odor scores go over 3.0 for two consecutive

days.

3. Results and Discussions

Figure 3 shows the odor evaluation results from the

AEM units on test days T + 0 through T + 50. The total

number of people in the panel was no less than 12 for

any test day. At the beginning of the test (T + 0), there

was an odor reading of 1.4 for the control AEM unit 104,

and 1.05, 1.4, and 1.6 for AEM unit 101, 102, and 103,

respectively. This indicated that some individuals identi-

fied a background odor, which waslikelythe odor of the

mouse food bars, and that the exhaust filters with H3PO4

treated activated carbon did not effectively remove the

odorants in the mouse food bars. We could not comment

on the reactions of odorants from mouse food bars against

the sorbent materials in the exhaust filter in the present

work, as the odor causing compounds in the mouse food

bars have not been identified yet.

From test day 7 to 50, the odor scores of the control

AEM unit decreased relative to the first test day and

varied in the range of 0.29 - 0.88, likely as a result of the

dehydration of the mouse food bars with operation time.

As indicated Figure 3, the average odor scores of animal-

loaded AEM units slightly decreased on day 7 for AEM

161116 21 26 31 36 41 46

Averag e Pa n eli st Od or S co re (0 -4 )

Test Day

Average Panelist Odor Score vs Time

AEM 101 Odor ScoreAEM 102 Odor Score

AEM 103 Odor ScoreAEM 104

(

)

Odor Score

Odor

Breakthrough

Figure 3. Odor evaluation results of the AEM units.

Open Access AJAC

Z. LU ET AL.

Open Access AJAC

779

units 102, 103, and 104, and then increased gradually

from day 14 to day 50. This can be attributed to the fact

that the odorants from the airstream of the AEM units

were removed by the exhaust filter, and that the odor

containment performance of the exhaust filter decreased

with longer operation duration. The decreased odor con-

trol performance of the exhaust filter could be either due

to the decrease of active sites, or the increased penetra-

tion of other odorants that were not absorbed by the filter

with extended operation time.

Despite the elevated odor scores from the AEMs, as

shown in Table 1, the odor score for each individual

AEM unit and test day was never over 3.0 for the test

duration. Furthermore, as given in Table 2, the ammonia

concentration in the airstream outlet from the AEM units

was maintained below 1 ppm. This indicated that acti-

vated carbon exhibited the capability to effectively ad-

sorb NH3 on its surface. The ammonia concentration in

the AEM internal airstreamvaried in the range of 2.61 -

6.30 ppm, as detected by the Dräger sensor inserted into

the gas sampling port. Thus, the exhaust filter exhibited

an ammonia containment efficiency of more than 90%.

The odor removal by the exhaust filter assembled on

AEM units suggests combined mechanics of physisorp-

tion and chemisorption of NH3 for H3PO4 loaded acti-

vated carbon. Ammonia can be absorbed on activated

carbon with pore size similar to its diameter (<4 Å) by

van de Waals forces. Only a small fraction of the acti-

vated carbon surface was utilized during the physisorp-

tion of ammonia due to the fact that activated carbon

hasa larger average pore size (usually 10 - 20 Å). The

impregnation of H3PO4 on the surface of activated carbon

blocks the pores of activated carbon as a result of pore

filling, and thus, creates more micropores that will ex-

hibit higher ammonia removal performance. Another con-

cern about the ammonia adsorption on activated carbon

is that the adsorbed ammonia easily desorbs from the

surface when being purged by air,as a result of the weak

nature of van de Waals forces.

Additionally, the surface acidity of activated carbon is

not ideal for ammonia adsorption. This limited the appli-

cation of activated carbon in ammonia removal as the

key factor that dominates the ammonia adsorption capac-

ity of the activated carbon is the surface chemistry, espe-

cially the acidic groups [8]. The chemical properties of

the carbon surface, especially acidic functional groups,

have a strong influence on NH3 adsorption [17]. The

impregnation of H3PO4 on the activated carbon induces

acidic groups at the basal planets of the activated carbon,

and thus, creates more active sites on the activated car-

bon surface. Ammonia is a basic gas and the introduction

of surface acidity by the impregnation of H3PO4 pro-

motes the NH3 adsorption capacity on activated carbon

via a Brønsted acid-base process [18]. It was reported

that the activated carbon impregnated with H3PO4 has a

highly oxidized surface, contributing to the high surface

acidity (pH < 3) [19]. It was reported that the ammonia

was captured on the surface of the H3PO4-loaded acti-

vated carbon through the following chemical reaction to

form NH4H2PO4 [14]:

H3PO4 + NH3 → NH4H2PO4

Therefore, the creation of acidic functional groups on

the surface of activated carboncaused an increase in the

amount of chemisorbed ammonia as well as an improve-

ment of physisorption properties at low relative pressures.

The deodorization lifetime of the exhaust filter, as in-

dicated by the breakthrough time, is partially dependent

on the amount of H3PO4 impregnated in the adsorbents

[14]. The breakthrough time of the exhaust filter was not

obtained in the current study, but the test results showed

that the new exhaust filter, which uses phosphoric acid

Table 1. Odor evaluation results from the four AEM units with non-ph osphoric acid impregnation exhaust filter.

Odor test scores

Days T + 1 T + 7 T + 14 T + 20 T + 24 T + 30 T + 36 T + 41 T + 45 T + 50

AEM 101 1.05 0 0.06 1.19 1.67 1.21 0.40 0.24 0.88 1.43

AEM 102 1.40 1.24 1.50 1.63 1.44 1.43 1.27 1.18 2.19 2.36

AEM 103 1.60 1.24 0.94 1.69 1.67 1.86 1.80 1.18 1.75 1.93

AEM 104 (c) 1.40 0.47 0.88 0.50 0.67 0.57 1.87 1.82 0.44 0.29

Average 1.35 0.82 0.83 1.12 1.64 1.50 1.64 1.59 1.60 1.90

S.D. 0.28 0.72 0.72 0.28 0.21 0.33 0.33 0.35 0.67 0.46

No. of evaluators 20 16 16 16 15 14 15 17 16 14

Table 2. Ammonia concentrations in the outlet airstreams of the AEM units.

Ammonia concentration in AEM Outlet, ppm

Days T + 21 T + 24 T + 31 T + 36 T + 41 T + 45 T + 50

AEM 101 0.41 0.72 0.75 0.75 0.45 0.45 0.38

AEM 102 <0.20 <0.20 0.26 0.27 <0.20 <0.20 <0.20

AEM 103 <0.20 <0.20 <0.20 <0.20 <0.20 <0.20 <0.20

Z. LU ET AL.

780

impregnated activated carbon as an adsorbent, could ef-

fectively remove ammonia from the exhaust airstream of

mouse habitats for a flight mission of at least 50 days.

4. Conclusion

A high performance, long duration exhaust filter was

developed by NASA Ames Research Center to remove

ammonia from the rodent housing Animal Enclosure

Module, and H3PO4 impregnated activated carbon was

used as its adsorbent. The odor evaluation results sug-

gested that the exhaust filter can effectively control the

odor from the mouse habitats during 45-day (minimum)

operation durations, and maintain the odor from AEM

units within acceptable levels. The AEM exhaust filter

exhibited more than 90% of overall ammonia contain-

ment efficiency. A better understanding of the reaction of

exhaust filter against the odorants in the AEM units

could be achieved through further identification of spe-

cific odor causing components in the exhaust airstream

of the AEM units.

5. Acknowledgements

The current work was supported by the NASA Ames

Research Center Rodent Habitat project.

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