Environ. Sci. Techno/. 1995, 29, 2901-2903

Absorption of Formaldehyde byOleander (Nerium indicum)
TAKAYUKI KONDO,KIYOSHI H A S E G A W A , * A N D
R Y U T A R O UCHIDA
Chemical and Biochemical Engineering, Faculty ofEngineering, Toyama University, 3190 Gofuku,Toyama 930, JapanM A S A N O R I ONISHI A N D
AKIHIRO M I Z U K A M I

KENJI OMASAFIGURE 1. Apparatus for measuringthe absorption of formaldehyde
by trees: A, NZgas; B,massflow controller;C, formaldehyde solution(37%); D, pump; E, activated charcoal; F, H20 bubbler. G, mixingchamber; H, buffer tank; 1, thermometerand hygrometer;J. exposurechamber; K, fan; 1. pump; M, dry gas meter; N, lamp; 0, water filter;
P, to bubblers for sampling of formaldehyde gas,

The National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba, Ibaraki 305, Japanand formaldehyde concentration on the rate of absorption
by oleander.

Toyama Prefectural Environmental Science Research Center,17-1 Nakataikouyama, Kosugi, Toyama 939-03, Japan

Experimental Section

Introduction
Vegetation is known to act as an important sink for gaseous
air pollutants. Since Hill (1) demonstrated that alfalfa
absorbed inorganic air pollutants such as SOz, NO2, 03,and Cl2, a number of investigators have repeatedly confirmed the absorption of SO2 ( 2 , 3 ) ,NO2 ( 4 , 5 ) , and 0 3 (6,7 ) by plants. On the other hand, few studies haveinvestigated the absorption of organic air pollutants byplants. Bacci etal. (81, and Simonich etal. (9,101 observed
the leaf-air partitioning of lipophilic organic chemicals
such as chlorinated hydrocarbons and polycyclic aromatichydrocarbons. Accordingly, the planting of trees is one
strategy for reducing the concentration of air pollutants inpolluted urban and industrial areas (11).
Atmospheric formaldehyde is given off as an exhaustgas from fuel combustion, solvent use, garbage incineration,and other activities (12). It is also produced by secondary
reactions of hydrocarbons with 0 3 . Recently, the atmospheric levels and environmental fate of formaldehyde haveattracted attention as a major volatile organic pollutant.
Gabele (13)reported that atmospheric formaldehyde may
increase as a result of switching from gasoline and diesel
fuels to methanol fuels. On the other hand, the indoor
concentration of formaldehyde has been found to be higher
than outdoors because of indoor sources such as wood
products and furniture (14).
Girard et al. (15) and Mutters et al. (16) observed theincorporation of atmospheric formaldehydeinto sugars andamino acids in leaves of plants, although the absorption
rate was not determined. To estimate the effect of treeplanting on atmospheric formaldehyde, it is necessary to
determine the absorption rate of formaldehyde by trees.
Therefore,the rate of formaldehyde absorption by oleander(Neriumindicum),which is popular as a street tree or garden
tree in Japan, was measured.
This paper describes the development of a reliable
method for measuring the absorption of formaldehyde bytrees and the effects of light intensity, transpiration rate,* F a : 764-41-8432.

0013-936x195/0929-2901$09.00/0

D 1995 American Chemical Society

Apparatus. Figure 1 shows the apparatus used for measuring the rate of formaldehyde absorption by trees.
Measurements of formaldehyde absorption were conductedusing a chamber system placed in a temperature-controlledroom. The system consisted of two major components:
gas conditioning instrumentation (Figure 1, A-G) and acylindrical exposure chamber (H-0).
Humidity-controlled pure air and formaldehyde-containing NZ were introduced into a mixing chamber (G).
Formaldehyde concentration in the gas mixture wascontrolled by the flow rate of Nz, and the humidity of thegas was regulated by the temperature of a HzObubbler (F),The exposure chamber system consisted of a 120-L
cylindrical acrylic chamber U) equipped with 10-L buffertanks (HI at its inlet and outlet. The rate of gas flow throughthe chamber was about 20 L min-l. Mixing of the chambergas, facilitated by two internally mounted fans (K), wasadequate for creating a uniform distribution of the formaldehyde concentration.
Mean inlet temperature and relative humidity were 26& 0.6 “Cand 70 5%,respectively. The inlet concentrationsof formaldehyde were within the range of 65-75 ppb andthe photosynthetic photon flux density (PPFD) was 600
pmol of photons m-z s-l (Koito Co. Ltd., IKS-25; calibrated
by Li-Cor Inc., Li-185A) at the height of the plant inside thechamber, except for the experiments in which the inlet
concentrations of formaldehyde or PPFD were varied.
Measurement of Formaldehyde and Calculation ofAbsorption Rate. Gas samples in the inlet and outlet buffertanks were drawn at a flow rate of 1 L min-I for 4 h throughtwo bubblers in series, each containing 10 mL of 0.5%boric
acid solution ( I 7). The solutions were combined, and theinlet and outlet concentrations of formaldehyde were
measured using a AHMT (4-amino-3-hydrazino-5-mercapto- 1,2,4-triazole)procedure (18). The absorption rateof formaldehyde was corrected by a “blank” experiment
performed with an empty chamber. The corrected absorption rate was calculated using the procedure of Hanson
et al. (5).

*VOL. 29. NO. 11.1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY2901

TABLE 1

Absorption Rate of Formaldehyde by OleanderaPPFDa(Irrnol of photons rn-z s-l)absorption rateb(ngh-l ppb-')100
200
400600

9.5 1.8
28.7 f 8.0
92.6i 21.1
103 f 26.5

Photosynthetic photon flux density.
deviation of four measurements.

Mean value and standard

1 1500r

Reliability of the Chamber System. The reliability ofthis chamber system was tested using cylindrical filter
papers coated with trimethanolamine, which is an absorbent of formaldehyde. The absorption rates of formaldehyde using this chamber system were compared with thosemeasured from the actual amount of formaldehyde absorbed on the filter papers. Prior to the experiments, thegas mixture was passed through the chamber for more than
30 min under the experimental conditions to equilibrate
the system, then the filter papers were placed quickly inthe chamber, and the experiment was started. After 4 h,
the filter papers were extracted with distilled water bysonication for 15 min. A portion of the extracted solution
was then analyzed using the AHMT procedure.
Absorption of Formaldehydeby Oleander. The effects
of light intensity and formaldehyde concentration on therate of absorption by oleander were examined. Afterconditioning for more than 30 min, the rate of absorption
by oleander was measured while varying the light intensityfrom 100 to 600ymol of photons m-* s-l. The transpiration
rate was determined from the decrease in the weight ofoleander planted in a pot, which was covered with a plasticbag. In the experiments whose inlet concentrations offormaldehyde were changed from 60 to 470 ppb, the rateof absorption of formaldehyde by oleander was measured
for 8 h at a sampling flow rate of 0.5 L min-I.

Results
The variation in the inlet concentration of formaldehyde
was measured every hour for 8 h. The relative standard
deviation was less than 5.0%. The outlet/inlet ratio of
formaldehydeconcentration was 98.2 i+ 2.0%(n= 9) in the"blank" experiment. Figure 2 shows the results of absorption of formaldehyde with the filter papers. The relationshipbetween the absorption rate determined by the chamber
method and the actual absorption rate of formaldehyde
was linear (1.01 k 0.10, n = 20).
Table 1 shows the rate of formaldehyde absorption byoleander at various light intensities. The absorption rates
were normalized for leaf area (one side) and the outlet
concentration of formaldehyde. Since a uniform distribution of formaldehyde concentration in the chamber mustbe attained, the outlet concentration of formaldehydewas
considered to be equal to that in the chamber itself (4).The
absorption rates of formaldehyde by oleander were within
the range 9.5-103 ng dm-2 h-l ppb-l and increased as thelevel of PPFD increased.
Figure 3 shows the linear relationship between theabsorption rate of formaldehyde and the transpiration rate
2902

ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1 1 , 1 9 9 5UI050100I150Transpiration rate (ma rim" h' mmHg'')

FIGURE3. Relationship betweenthe rate of formaldehyde absorption
and the transpiration rate of oleander.

OI
Formaldehyde concentntion (ppb)

FIGURE4. Relationship betweenthe rate of formaldehyde absorption
and formaldehyda concentration.

of oleander, obtained from the experiments in which thelight intensity was varied (T = 0.963, n = 16). The
transpiration rate was normalized for leaf area and the vapor
pressure of water differential between the actual value andsaturated value inside the chamber.
Figure 4 shows the relationship between the absorption
rate and the concentration of formaldehyde, Within theconcentration range of 35-245 ppb, the rate of formaldehyde absorption increased linearly with increasing
formaldehyde concentration.

Discussion
The results of the "blank" experiment indicated that theadsorption of formaldehyde to the walls of the chamber
and the formation or decomposition of formaldehyde in
the chamber were very small using this system. From theresults of formaldehyde absorption with the filter papers,
it was confirmed that this chamber system was applicableto the measurement of formaldehyde absorption by trees.
The rate of formaldehyde absorption by oleander was
103 ng dmW2
h-l ppb-l at 600 ymol of photons m-2 s-l.
Rogers et al. (4) reported that the rates of NO2 uptake byloblolly pine and white oak were 5.2 x lo-' and 1.1 x lo-'

pg dm-2 min-' p p h - ' at 492 pmol of photons m-2 s-l,respectively. In our study, the absorption rate determinedusing the same units as those of Rogers et al. (4)wasestimated to be 1.7 x low2pg dm+ min-l pphm-' at 600
pmol of photons m-2 s-l. Thus, it was confirmed that
atmospheric formaldehyde was absorbed by oleander at
an absorption rate similar to that of atmospheric NOZ.
The rate of formaldehyde absorption by oleander
increased as the level of PPFD increased. This relationshipagrees with the results obtained by Rogers e t a l . ( 4 )for NO2
uptake by corn and soybean. They explained that theincreased rate of NO2 uptake with increasing light intensity
was due to a decrease in total diffusive resistance. In ourresults, a linear relationship between the rate of formaldehyde absorption and transpiration rate was observed.
Since the wind speed inside the chamber was almost
constant, it is reasonable to consider that the resistance ofthe leaf boundary layer was constant and that the increasein the transpiration rate was equal to the increase instomatal conductance. Consequently, the absorption offormaldehyde increased as stomatal conductance increased. Thus, we can conclude that formaldehyde isabsorbed through the stomata. It was found by Hanson
et al. (5)and Johansson et al. (19) that NO2 absorption bytrees is strongly influenced by stomatal conductance.
Omasa et al. (20, 21) concluded that the absorption ratesof Son, NOZ, and 0 3 can be explained by factors such as
boundary layer and stomatal resistance, which are related
to gaseous diffusion. The mechanism of formaldehyde
absorption by trees is similar to that of inorganic air
pollutants such as SOn, NOz, and 0 3 because of its
hydrophilic property.
As shown in Figure 4, increasing the concentration offormaldehyde did not affect the absorption ability ofoleander, and no visible injury was observed during the8-h exposure period even at a high concentration of 245
ppb. Singh et al. (22)showed that formaldehyde concentrations in urban sites in the United States were within therange of 6.6-45.9 ppb. Zhang et al. (14) found that indoor
formaldehydeconcentrations in six residential houses werewithin the range of 26.9-101.7 ppb. Therefore, trees havesufficient ability to absorb atmospheric formaldehyde inurban areas or within the home.
Mutters etal. (16)observed that plant growthwas higher
when plants were exposed to high concentrations of gaseous
formaldehyde (400 ppb) and that higher concentrations of
sucrose and glycine in leaf tissue were associated withformaldehyde treatment. They concluded that atmosphericformaldehyde would probably have no harmful effect on

short-term growth of beans at 400 ppb. Krd et al. (23)reported that barley incorporatedformaldehyde into sugars,
amino acids, and choline in the leaves under light irradiation. Girard e t al. (15) found that formaldehyde wasincorporated predominantly into amino acids of younger
leaves. From our experimental results, we cannot discuss
the metabolism of formaldehyde in leaf tissue of oleander;
however, it seems that similar metabolic pathways to thosedescribed above (15, 16,231 play an important role in theabsorption of formaldehyde. It is concluded that trees ingeneral could act as an important sink for atmosphericformaldehyde.

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Received for review April 17, 1995. Revised manuscript received August 21, 1995. Accepted August 23, 1995.
ES950261M

VOL. 29, NO. 11, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY2903