Kinetic Studies of Ethylene Oxidation by Potassium Permanganate Adsorbed on Rice Hull Ash, Lahar Ash, or Coconut Coir Dust

28 The PhilippineAgricultural ScientistVol. 90 No. 1 (March 2007)
Ethylene Oxidation by Potassium Permanganate A.B. Hernandez et al.THE PHILIPPINEAGRICULTURAL SCIENTIST ISSN 0031-7454
Vol. 90 No. 1, 28-39
March 2007
Kinetic Studies of Ethylene Oxidation by Potassium Permanganate
Adsorbed on Rice HullAsh, LaharAsh orCoconut CoirDust
Alvin B. Hernandez1, Edralina P. Serrano2 and Ernesto J. del Rosario1*
1Institute of Chemistry, College of Arts and Sciences, University of the Philippines Los Baños, College, Laguna 4031,
Philippines
2Crop Science Cluster, College ofAgriculture, University of the Philippines Los Baños, College, Laguna 4031, Philippines
*Author for correspondence; e-mail: ejdros@yahoo.com
The ethylene scrubbing (oxidizing) efficiency and stability of KMnO4 adsorbed on rice hull ash, lahar
(volcanic ejecta) ash or coconut coir dust as carrier were evaluated. Values of the kinetic order with
respect to ethylene of the reaction between C2H4 and KMnO4 were 1.35 ± 0.39, 0.84 ± 0.18 and 1.46 ±
1.09 for rice hull ash, lahar ash and coconut coir dust, respectively. The permanganate-dependent and
intrinsic (permanganate-independent) rate coefficients (k’ and k, respectively) were calculated based
on pseudo-first order kinetics. The optimum KMnO4 concentration for scrubbing ethylene was found to
be 0.04 M. Experimental values of the rate coefficient k’ (in min-1) were 0.0216 ± 0.0020, 0.0127 ± 0.0003
and 0.0085 ± 0.0006 for rice hull ash, lahar ash and coconut coir dust, respectively. Values of the
intrinsic rate coefficient k (in min-1 g carrier/g KMnO4) were 1.87, 4.78 and 0.02 for rice hull ash, lahar
ash and coconut coir dust, respectively. At the same KMnO4 loading, lahar ash was the most efficient
KMnO4 carrier followed by rice hull ash and coconut coir dust. However, the most efficient KMnO4
carrier (per gram) as ethylene scrubber was rice hull ash followed by lahar ash and then coconut coir
dust.
Scrubber stability was determined by measuring how fast the rate coefficient k’ and chromacity
(intensity of KMnO4 color) changed with time. The rice-hull-ash-based scrubber was the most stable
and showed negligible changes in rate coefficient k’ for 27 d; lahar ash was the least stable carrier
followed by coconut coir dust.
Key Words: chitin, coconut coir dust, ethylene oxidation, lahar ash, permanganate adsorption, rice hull ash, scrubber
Abbreviations: GC-FID – gas chromatograph with flame ionization detector
INTRODUCTION
Ethylene (ethene) is the simplest organic compound that
affects physiological processes in plants. It is also a natu-
ral product of plant metabolism and is produced by all
tissues of higher plants and by some microorganisms. As
a phytohormone, even in trace amounts (less than 0.1 ppm),
it regulates many aspects of growth and development, and
has been shown to be an inductive factor in rapid physi-
ological changes (e.g., ripening and senescence) in
postharvest fruits, especially climacterics (Buffler 1986),
and vegetables (Abeles et al. 1971; Kazuhiro and Watada
1991; Jayaraman and Raju 1992; El Blidi et al. 1993). More-
over, ethylene reduces the storage life of many postharvest
commodities if it is used at a high concentration.
Ripening or senescence of perishable commodities is
delayed by maintaining ethylene at low levels inside pack-
ages and storage rooms; this consequently extends the
storage and transport life of the produce (Wills et al. 1981).
Ethylene can be removed from the storage atmosphere us-
ing an ethylene-scrubbing material containing KMnO4
which is impregnated into an inert and porous matrix with a
large surface area (Jayaraman and Raju 1992). Several ma-
trices or carriers have been used as ethylene scrubbers and
they are usually made of siliceous materials such as ver-
miculite and celite (Abeles 1973), mixtures such as cement
and expanded mica (Wills et al. 1981) and commercial prepa-
rations such as Purafil which consists of alumina.
Potassium permanganate (KMnO4) is a dark purple or
bronze-like, non-volatile, odorless crystal that is stable in
air. It can oxidize ethylene into ethylene glycol and eventu-
ally into carbon dioxide (Abeles 1973; Wills et al. 1981;
McMurry 2000) as shown in Fig. 1. The efficiency of a
particular KMnO4 scrubber is indicated by the rate coeffi-

The PhilippineAgricultural ScientistVol. 90 No. 1 (March 2007) 29
Ethylene Oxidation by Potassium Permanganate A.B. Hernandez et al.
cient for the reaction between ethylene and KMnO4
adsorbed on a particular carrier; it is dependent on the
permanganate concentration and is influenced by the car-
rier used (Kavanagh and Wade 1987).
The search for readily available materials as KMnO4
carriers without the use of binders (e.g., cement, clay alu-
mina, etc.) resulted in the development of the KMnO4-rice
hull ash scrubber (Lizada andArtes 1989). The advantages
of rice hull ash as KMnO4 carrier include its inertness to
KMnO4, its high silica content, which contributes to its
high surface area to weight ratio, and its high porosity.
Based on these properties, some inert indigenous materi-
als that have high silica content (e.g., lahar ash) and/or are
highly porous (e.g., coconut coir dust) are potential per-
manganate carriers in ethylene scrubbers.
Our study deals with the evaluation of rice hull ash,
lahar (volcanic ejecta) ash and coconut coir dust as KMnO4
4
carriers in ethylene scrubbers. Preliminary evaluation was
also conducted on chitin as potential KMnO
4
carrier. The
kinetic order for the oxidation of ethylene by the carrier-
adsorbed KMnO was determined for rice hull ash, lahar
ash and coconut coir dust, as well as the intrinsic and
permanganate-dependent rate coefficients for the pseudo
first-order reaction. The results were used to assess the
efficiency and long-term stability of each of the carriers as
a component of the ethylene scrubber.
MATERIALS AND METHODS
Scrubber Preparation
Preparation of Carriers. Rice hull ash was obtained from
Candelaria, Quezon and coconut coir dust from Victoria,
Laguna. Lahar ash was sampled from a lahar area in
Pampanga; chitin was purchased from Aldrich Chemical
Company, USA. The materials (except chitin which was
used as received fromAldrich) were dried in a hot air cham-
ber and sequentially passed through 10-mesh and 60-mesh
sieves. Only particles that passed through the 10-mesh
sieve but not through the 60-mesh sieve were used as per-
manganate carrier.
Determination of Maximum Liquid Holding Capac-
ity of Carrier. One gram of each permanganate carrier (rice
hull ash, lahar ash, coconut coir dust or chitin) was mixed
with a specified volume of distilled water; the latter was
measured using a 10-mL pipette and the mass of each car-
rier was determined using a top loading balance. The fol-
lowing volumes (in mL) were used in the experiment: 0.1,
0.2, 0.3, … up to 10.0 (in increments of 0.1 mL). The maxi-
mum volume of liquid used for each carrier was the maxi-
mum volume of H2O that was absorbed by the carrier but
did not produce any evident wetting of the surface of the
container (Petri dish).
Preparation and Packaging of Ethylene Scrubber.
Ten grams of each carrier were weighed using a top-load-
ing balance and added to a volume of KMnO4 solution of
definite concentration (0.03 M, 0.04 M and 0.05 M) which
was ten times the optimal volume capacity of the carrier as
determined in the previous section. Then the KMnO4-car-
rier mixture was mixed manually using a glass stirring rod.
The mixture was a moist solid that contained some liquid
but we ascertained that it had no KMnO4 as free-flowing
liquid.
One gram (or 2 g for determination of scrubber stabil-
ity) of each KMnO4-carrier mixture was measured using a
top-loading balance and placed inside a 3 cm x 3.5 cm (5.5
cm x 5 cm for determination of scrubber stability) cellulosic
non-woven fabric sachet.
Preparation of Ethylene ScrubberVessel
The ethylene scrubber vessel (Fig. 2) consisted of a 250-
mL Erlenmeyer flask which had been cleaned, dried and
flushed with ample amounts of air. The flask was covered
with a rubber stopper with two holes through which were
inserted two short glass tubes that were connected by a
soft rubber tube. Sampling of the gas in the flask, which is
also shown in Fig. 2, was done by piercing the rubber tube
C C
H
H
H
H
2CO2H2C CH2
OH OH
KMnO4
MnO2
Fig. 1.Fig. 1.Fig. 1.Fig. 1.Fig. 1. Oxidation of ethylene by potassium permanga-
nate.
Fig. 2.Fig. 2.Fig. 2.Fig. 2.Fig. 2. Ethylene scrubber vessel (Gas sampling indi-
cated).

with the hypodermic needle of a syringe and applying suc-
tion by means of the syringe. The scrubber vessel was
sealed in order to prevent the escape of gas from the ves-
sel and the rubber tube for sampling was replaced regu-
larly.
Preliminary Experiments
Determination of pH of Carriers and Scrubbers. Based on
the determined optimal volume capacity of each scrubber,
the corresponding optimal volume of distilled water was
added to 1 g of each carrier and transferred quantitatively
into a 50-mL volumetric flask. The pH of the mixture was
then measured using the JEMCO Analog pH meter. This
procedure was repeated but instead of distilled water,
KMnO4 was added at concentrations of either 0.03 M, 0.04
M or 0.05 M. The pH was measured in order to elucidate
the chemical changes that occurred in the KMnO4 solu-
tion when added to the carrier. For example, the KMnO4
adsorbed on coconut coir dust at acidic pH was converted
to Mn2+, based on color change from purple to colorless;
this makes the scrubber inefficient. Experimental data on
the maximum liquid holding capacity of different carriers
are presented in Table 1.
by the carrier was calculated as the difference between the
initial mass of KMnO4 4and the mass of residual KMnO of
the filtrate after 10 min. This was used as basis for calculat-
ing the mass fraction of adsorbed KMnO4, which is the
mass of KMnO4 adsorbed by the carrier divided by the
mass of KMnO4 in the filtrate.
Standard solutions of KMnO4 containing the follow-
ing KMnO4 concentrations (in mM) were prepared (0, 0.025,
0.050, 0.10, 0.20, 0.50, 1.0, 2.0, 5.0, 10, 20 30, 40 and 50) and
their absorbance was measured at 535 nm.Astandard curve
was prepared by plotting absorbance against KMnO4 con-
centration. The residual concentration of KMnO4 was de-
termined from the calibration curve.
The Langmuir equation may be written as
(1a)
where θ is the fraction of sites on the carrier (adsorbent)
surface which is occupied by the adsorbed molecules or
ions (adsorbate), which is KMnO4, c is the adsorbate con-
centration and K is the adsorption equilibrium constant,
which is equal to the ratio of the rate constants for adsorp-
tion and desorption of the adsorbate on the carrier (Laidler
and Meiser 1999;Atkins and de Paula 2002). Eq. (1a) may
be rewritten as a linear relationship between and 1/c
with 1/K as the slope:
(1b)
Determination of Kinetic Order with Respect to Ethylene
Each packed freshly prepared ethylene scrubber (using
0.04 M KMnO4 solution) was placed inside the ethylene
scrubber vessel which had been thermally equilibrated at
20 oC. The laboratory room where the experiments were
conducted was maintained at 20 oC. Then the flask was
closed using a rubber stopper with sampling tube. Ethyl-
ene gas was introduced into the flask to a final concentra-
tion of approximately 4 ppm by injecting it from a hypoder-
mic syringe with a metal needle. The vessel contents were
homogenized by manual agitation and then two 1-mL
samples of the headspace gas were obtained at suitable
time intervals. Ethylene purity was close to 100% and source
for the experiments was a 1000 ppm stock solution. The
ethylene stock solution was placed in the laboratory where
the temperature was maintained at 20 oC. In the experiment,
approximately 4 ppm ethylene was obtained from a 1000
ppm stock solution and was prepared using the C1V1 =
C2V2 formula. The volume of 1000 ppm stock that would
give approximately 4 ppm solution was calculated from the
formula. However, the actual concentration was verified
experimentally using a gas chromatograph with flame ion-
ization detector (GC-FID).
Kc
θ =
1 + Kc
1 1
= 1 +
θ Kc
Table 1. Maximum liquid holding capacity of different car-
riers.
Carrier Maximum Volume/ g
Carrier, mL
Rice hull ash 1.5
Coconut coir dust 6.5
Lahar ash 0.2
Chitin 3.8
DeterminationofKMnO4AdsorptionEquilibriumConstant
Determination of Equilibration Time for KMnO4 Adsorp-
tion. To one gram of each carrier, half of the previously
determined optimal volume of 0.01M KMnO4 was added in
a 50-mL volumetric flask. The flask was then filled with
distilled water up to the mark. The concentration of per-
manganate in the suspension was determined at the fol-
lowing time intervals (in min): 1, 3, 5, 7, 10 and 15.
Adsorption of KMnO4 by Carriers. To test tubes, each
containing 1g of carrier, were added 10 mL solutions of
varying concentrations of KMnO4 (0–50 mM). The mixture
was agitated vigorously for 1 min in a vortex mixer, and
allowed to equilibrate for 10 min. Then the suspensions
were filtered and the filtrate was analyzed for residual
KMnO4 based on absorbance measurements at 535 nm
using the SECOMAM UV-Vis Spectrophotometer.The con-
centration of residual KMnO4 in the filtrate was taken as
the unadsorbed KMnO4, as well as the equilibrium con-
centration after adsorption. The mass of KMnO4 adsorbed
1
θ

After obtaining two 1-mL samples of headspace gas,
air (2 mL) was injected into the flask in order to maintain
constant headspace gas volume; the air was again homog-
enized. Each headspace gas was analyzed for ethylene
using the Shimadzu Gas Chromatograph equipped with a
flame ionization detector and alumina column.
The procedure was repeated for ethylene concentra-
tions of approximately 8, 12, 16 and 20 ppm. Measurement
of ethylene concentration at zero time involved the proce-
dure given above; however, instead of using a KMnO4-
carrier mixture, a water-carrier mixture was used (using the
same volume to mass ratio of liquid and carrier, respec-
tively). Ethylene analysis was done using the operating
parameters of the gas chromatograph given below.
Gas Chromatographic Parameters for Ethylene Analysis
Parameters Value
Injector/Detector temperature 160oC
Column temperature 110oC
Flow rate of carrier gas (N2) 1.25 kg/cm2
Flow rate of H2
20.6 kg/cm
Flow rate of air 0.6 kg/cm2
Retention time for ethylene 30 sec
Determination of Rate Coefficients and Scrubbing
Efficiency
Each freshly prepared and packed ethylene scrubber was
placed inside the ethylene scrubber vessel which had been
thermally equilibrated at 20 oC.After injecting ethylene to a
final concentration of approximately 4 ppm using a syringe,
the ethylene scrubbing capacity of each scrubber was de-
termined as earlier described. Three trials were done for
each ethylene scrubber. The procedure was repeated us-
ing each ethylene scrubber which had been prepared with
0.04 M and 0.05 M KMnO4 solutions. Each scrubber ex-
posed to ethylene was set aside for measurement of the
residual permanganate concentration. The residual perman-
ganate of each scrubber was eluted with distilled water
and transferred quantitatively into a 50-mL volumetric flask.
The absorbance of the eluate solution was measured at
535 nm using the SECOMAM UV-Vis Spectrophotometer.
Kinetic Analysis. Consider a bimolecular reaction be-
tween reactants A and B giving product C with the corre-
sponding rate coefficient k
whereA= C2H4 and B = KMnO4.
The initial velocity (vo) for each scrubber at a speci-
fied initial concentration of ethylene was determined by
calculating the initial slope of the plot of ethylene concen-
k
A B C+ ⎯ ⎯→
tration against time. Based on the reaction above, the rate
law is given by the equation:
(2a)
Assuming [B]o is in excess and approximately con-
stant
(2b)
where ok ' k[B]=
o oln v ln k ' n ln[A]= +
k ' k[B]=
taking the natural logarithms of both sides,
(3)
From the linear plot of ln vo against ln [A]o the correspond-
ing slope that would be obtained is equal to the kinetic
order with respect to ethylene, n.
Assuming that the reaction follows pseudo-first order
with respect to ethylene and assuming that the KMnO4
concentration is constant, equation (2b) becomes
(4)
where
Integration of equation (4) results in
(5a)
Taking the natural logarithm of both sides gives
oln[A] ln[A] k 't= − (5b)
A plot of ln [A] against t gives a slope equal to -k’.
If the Pearson rho value R is equal to 1, then the reaction
follows first order with respect to ethylene, and
k’ = - slope (6)
Given the calculated permanganate-dependent rate co-
efficient (k’), the intrinsic rate coefficient (k) can be calcu-
lated using the equation:
(7)
n m
o o o
d[A] d[B]
v k[A] [B]
dt dt
= − = − =
n
o o
d[A]
v k '[A]
dt
= − =
o
d[A]
v k '[A]
dt
= − =
o
A t
A 0
d[A]
k' dt
[A]
= −∫ ∫
k't
oA A e−
=
4
k '
k
KMnO loading
=

RESULTS AND DISCUSSION
Liquid-Holding Capacity of Carriers
Coconut coir dust had the highest liquid-holding capacity
while lahar ash had the lowest (Table 1). The results may
be explained by the presence of hydrophilic groups in co-
conut coir dust. This material has been reported to contain
42.3% lignin (Suzuki et al. 1998), as well as 24.1% cellulose
and 27.3% pentosan (Gonzalez 1970), which contain phe-
nolic groups (for lignin) and hydroxyl groups; these groups
are capable of favorably interacting with water molecules,
resulting in the high water-holding capacity of the mate-
rial. Lahar ash is composed of primary minerals such as
hornblende, feldspar, mica, quartz, pyroxene and magne-
tite and amorphous materials such as amorphous SiO2 (Phil-
ippine Bureau of Soils 1991 unpublished); most of these
components have low hydrophilicity. Rice hull ash, which
contains mainly amorphous silica (Kamath and Proctor 1998;
Kalapathy et al. 2002), exhibited moderate liquid-holding
capacity since the oxygen atoms in the silicate structure
can interact with water molecules through H-bonding.
Based on the maximum liquid-holding capacity, the
corresponding volume of 0.04 M KMnO4 solution was
added to each carrier. After addition of the KMnO4 solu-
tion to each carrier, only rice hull ash and lahar ash showed
Determination of ScrubberStability
Thirty-six pieces of 2-g ethylene scrubbers (KMnO4-rice
hull ash mixture, KMnO4-lahar ash mixture and KMnO4-
coconut coir dust mixture), which had been prepared at the
optimum KMnO4 concentration (determined in the previ-
ous section), were packed. Then three packed freshly pre-
pared ethylene scrubbers were placed in a polyethylene
bag and kept in the dark using an air-tight container.
For day 0, one polyethylene bag containing three
packed scrubbers was randomly picked from the storage
container. The ethylene scrubbing capacity was determined
using the procedure mentioned earlier for the measurement
of rate coefficients and ethylene scrubbing efficiency. Then
each ethylene scrubber was removed from the flask and its
color analyzed using the Minolta Chromameter. The re-
sidual permanganate of each scrubber was eluted with dis-
tilled water and transferred quantitatively into a 100-mL
volumetric flask, followed by absorbance measurement at
535nm.
The same procedure mentioned above was used after
the following number of days: 1, 3, 5, 7, 9, 11, 14, 17, 21 and
27. Standards for chromacity were measured. The
chromacity values of freshly prepared and aged scrubbers,
which had been exposed to air for 3 wk, were determined in
order to establish the chromacity standards.
desired free space of scrubber vessel
from 1000ppm stock
stock 1000ppm
(ethylene concentration )(Volume )
Volume ethylene =
ethylene concentration
[ ]sample
2 4sample (in ppm) 2 4std in ppm
std
(peak height )(attenuation)
C H = × C H
(peak height )(attenuation)
4
2
4
4 g KMnO
(in )
g carrier
2
KMnO packing material (in cm )4
3
(in ppm)
3
ethylene
KMnO loading
(MM )(Area )mol KMnO / cm
mol ethylene / cm ethylene concentration 1g 1L
MM 1000mg 1000cm
=
⎛ ⎞⎛ ⎞⎛ ⎞
⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠⎝ ⎠
Formulas Used for Calculations
(a) Determination of the volume of desired initial ethylene concentration:
(b) Determination of ethylene concentration from peak height:
(c) Determination of reactant present in excess:
The reactant that is present in excess can be determined by calculating the ratio of the amount
of KMnO4 (mole KMnO4 adsorbed on a particular carrier per unit area) to the amount of ethylene
(mole ethylene per unit volume)
1L
1000cm3
1 g
1000mg
from 1000 ppm stick
stock 1000 ppm

purple coloration while chitin had a reddish purple color
and coconut coir dust gave a dark brown color.
Profile of KMnO4 Adsorption by Carriers
The fraction of permanganate adsorbed on the different
carriers is plotted in Fig. 3 against the equilibrium perman-
ganate concentration in solution; the latter was calculated
by subtracting the residual permanganate concentration
from the initial value based on experimental data. The plots
for rice hull ash and chitin exhibited type L (Langmuir-
type) adsorption isotherm (Fig. 3) while coconut coir dust
showed high-affinity type isotherm (Osipow 1962). Lahar
ash exhibited what looks like a highly steep L-type iso-
therm. The hyperbolic Langmuir-type physical adsorption
mechanism involves relatively weak van der Waals inter-
actions between permanganate and surface atoms or mol-
ecules of the adsorbent material.
The Langmuir adsorption isotherm is an equation of a
rectangular hyperbola where the fractional saturation θ is
plotted against the adsorbate concentration. It is similar to
the Michaelis-Menten equation where the initial velocity
is plotted against the concentration of the substrate (van
Holde et al. 1998). The hyperbolic nature of the Langmuir
isotherm, which describes the formation of a monolayer of
adsorbate molecules on the adsorbent surface, is con-
trasted with non-hyperbolic phenomena exhibited during
cooperative binding in biological systems where sigmoi-
dal or S-shaped binding curves are observed (van Holde et
al. 1998; Neet 1996) and with multilayer adsorption in non-
biological systems as described by the Brunauer-Emmett-
Teller isotherm (Atkins and de Paula 2002; Laidler and
Meiser 1999).
The results of the equilibration experiment for adsorp-
tion of permanganate by the different carriers are shown in
Fig. 4. Permanganate adsorption by the carriers (except
chitin) was completed after about 2 min; therefore the 10-
min equilibration time used in the experiments was suffi-
cient.
Data on KMnO4 loading of each carrier after equilibra-
tioninasolutioncontaining0.03M,0.04Mor0.05MKMnO4
are presented in Table 2a. The KMnO4 loading (expressed
in grams KMnO4 adsorbed per gram carrier) was calculated
by dividing the mass of KMnO4 contained in a particular
carrier by the mass of the carrier. The mass of KMnO4, in
turn, was determined by multiplying the molarity of the
KMnO4 solution by its optimal volume (required for each
carrier) and the molar mass of KMnO4.
The fractional saturation of adsorbent θ was calcu-
lated from the KMnO4 loading using the formula:
= KMnO4 loading/KMnO4 max
where KMnO4 max is the KMnO4 loading which gave the
maximalvalueofθ(AtkinsanddePaula2002;Osipow1962).
The KMnO4 max was determined from the maximal or limit-
ing y (ordinate) values in Fig. 3 at high equilibrium concen-
trations of KMnO4. The highest and lowest KMnO4 load-
ing values were observed in coconut coir dust and lahar
ash, respectively; chitin had slightly greater loading val-
ues compared to rice hull ash (Table 2a). Values of KMnO4
loading for rice hull ash were, on the average, 7.5 times
those for lahar ash but only one-fourth those for coconut
coir dust. Results of pH measurements of the KMnO4-car-
rier and water-carrier mixtures are given in Table 2b. The
change in pH was used as basis for evaluating acid-base
behavior of the carrier after KMnO4 adsorption and for
assessing the efficiency of the ethylene scrubbers.
θ=
Kc
1 + Kc
0.00
0.20
0.40
0.60
0.80
1.00
0.000000 0.000200 0.000400 0.000600 0.000800 0.001000
Fractionpermanganateadsorbed
rice hull ash
coconut coir dust
lahar ash
chitin
Fig. 3.Fig. 3.Fig. 3.Fig. 3.Fig. 3. Plot of fraction permanganate adsorbed vs.
equilibrium permanganate concentration in so-
lution for various carriers.
equilibrium conc, M
Table 2a. Calculated KMnO4 loading for various carriers.
Initial KMnO4 Loading, g KMnO4 / g Carrier
KMnO4
Concen- Rice Hull Coconut Lahar Chitin
tration, M Ash Coir Dust Ash
0.03 7.11 x 10-3 3.08 x 10-2 9.48 x 10-4 1.80 x 10-2
0.04 9.48 x 10-3 4.11 x 10-2 1.26 x 10-3 2.40 x 10-2
0.05 1.19 x10-2 5.14 x 10-2 1.58 x 10-3 3.00 x 10-2
Table 2b. Results of pH measurements of KMnO4-carrier
and water-carrier mixtures.
pH of KMnO4- pH of
Carrier Mixture H2O-
Carrier Carrier
0.03 M 0.04 M 0.05 M Mixture
Rice hull ash 8.9 9.1 9.3 9.3
Coconut coir dust 5.5 5.7 5.9 5.3
Lahar ash 4.5 4.8 4.9 4.4
Chitin 8.2 8.2 8.3 8.4

The calculated value of KMnO4 loading is the total
amount of KMnO4 contained in the carrier, i.e., the amount
of KMnO4 strongly bound to the carrier, plus the perman-
ganate in the solution that wets the carrier particles. On the
other hand, the amount of adsorbed KMnO4 is the amount
that could not be removed after washing with distilled wa-
ter, i.e., this was either physically or chemically adsorbed
on the carrier. Based on the methodology used in the ex-
periments, it can not be assumed that the KMnO4 loading
and the adsorbed KMnO4 are identical. This point will be
taken up again in relation to the discussion of the Langmuir
adsorption isotherm for the different carriers.
Coconut coir dust exhibited a non-hyperbolic and gen-
erally flat plot (within experimental uncertainty) (Fig. 3);
this indicates a chemical adsorption mechanism where there
are strong molecular interactions, including chemical bond
formation, between adsorbed and adsorbent molecules
(Atkins and de Paula 2002).As mentioned earlier rice hull
ash and lahar ash contain inorganic combustion products
which are not expected to chemically react with permanga-
nate; similarly, chitin consists of poly-N-acetylglucosamine
which appears stable in the presence of permanganate. On
the other hand, coconut coir dust is composed of lignin,
cellulose, pentosan and other organic compounds some of
which can be oxidized by permanganate.
The double-reciprocal plot (solid line in Fig. 5) for co-
conut coir dust with y-intercept equal to 1.0 gave a regres-
sion coefficient of 0.87 which showed fair but not excellent
linearity; an excellent linear fit would have indicated
Langmuir-type monolayer adsorption of permanganate on
coconut coir dust. The fact that a closer fitting line for the
experimental data in Fig. 5 with regression coefficient of
0.98 (dashed line) did not pass through the predicted y-
intercept of 1.0, which is predicted by the Langmuir equa-
tion, shows that permanganate adsorption by coconut coir
dust did not exactly follow the Langmuir mechanism. A
similar plot for the other carriers gave negative values of
the y-intercept and did not fit the Langmuir equation. Al-
though the adsorption curves in Fig. 3 for rice hull ash,
lahar ash and chitin qualitatively indicate multilayer ad-
sorption, the plotted points did not give a good fit with the
linearized form of the Brunauer-Emmett-Teller equation
(Osipow 1962;Atkins and de Paula 2002), especially at high
permanganate concentrations. There are two possible ex-
planations for these experimental results; one is the differ-
ence between permanganate loading and the extent of ad-
sorption by the carrier. The other possible explanation is
that the carriers exhibited more complicated adsorption
mechanisms than those described by the Langmuir and
Brunauer-Emmett-Teller equations. Needless to say, this
requires further elucidation and research.
Figure 6 shows the plots of residual vs. initial KMnO4
4concentrations; zero residual KMnO
4
was observed for
coconut coir dust while values for RHA were 3.5–7 times
those for lahar ash. Although coconut coir dust had the
highest KMnO loading, it gave a light yellow solution
after elution which indicated negligible amounts of residual
KMnO4. This means that the adsorbed permanganate was
chemically reduced by coconut coir dust. This can be ex-
plained by the presence in coconut coir dust of lignin,
cellulose and pentosan (as earlier mentioned) which can
be oxidized by MnO4
-. This oxidation causes a color change
from purple MnO4
- to the colorless Mn2+
4
at low pH: the
observed pH of the coconut coir dust leachate was 5.5–
5.9. The much higher values of KMnO loading and re-
sidual concentration of rice hull ash compared to lahar ash
is explainable in terms of more extensive van der Waals
interaction, or physical adsorption mechanism, for rice hull
Fig. 4.Fig. 4.Fig. 4.Fig. 4.Fig. 4. Permanganate concentration of suspension of
various carriers at different equilibration times.
y = -2E-06x + 0.0002
y = 4E-07x + 0.0002
y = -8E-08x + 9E-05
y = 1E-07x + 6E-05
0.00000
0.00005
0.00010
0.00015
0.00020
0.00025
0 5 10 15
time, min
permanganateconc,M
rha
ccd
la
chi
Fig. 5.Fig. 5.Fig. 5.Fig. 5.Fig. 5. Plot of reciprocal of fraction of KMnO4 binding
sites occupied vs. reciprocal of equilibrium
KMnO4 concentration for coconut coir dust as
carrier.
solid line: y = 0.0003x + 1
R2
= 0.8711
dashed line: y = 0.0004x - 0.8473
R2
= 0.9804
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
0 5000 10000 15000 20000 25000 30000 35000 40000
/
1/1/1/1/1/θθθθθ
1/ec

ash due to more numerous hydrophilic groups present in
this material.
Determination of Kinetic Order and Rate Coefficients
Determination of Kinetic Order. Scrubbers were prepared
by adding the optimal volume of KMnO4 at a particular
concentration to the carriers (except chitin). In determining
kinetic order with respect to ethylene, it was necessary to
ensure that KMnO4 was present in excess and that its con-
centration remained almost constant during the reaction;
0.04 M KMnO4 was sufficient for this requirement. Each
scrubber (carrier with KMnO4) was made to react with eth-
ylene at different initial concentrations of ethylene and the
latter were measured at specified times. The ethylene-con-
centration-versus-time data were used to determine the ki-
netic order of the reaction with respect to ethylene. This
was done by plotting the natural logarithm of the initial
velocity vo (initial slope of the plot of C2H4 concentration
against time) against the natural logarithm of the initial
C2H4 concentration [A]o. Values of the kinetic order for the
C2H4-KMnO4 reaction calculated from Fig. 7 were 1.35 ±
0.39, 1.46 ± 1.09 and 0.84 ± 0.18 for rice hull ash, coconut
coir dust and lahar ash, respectively. Based on these data
it can be concluded that the reaction of ethylene with
KMnO4 for rice hull ash and lahar ash as carriers most
probably followed first-order kinetics with respect to eth-
ylene; for coconut coir dust the kinetic order is less sure
because of the large experimental uncertainty. Gas adsorp-
tion involves collision of gas molecules with discrete ad-
sorption sites and is considered an elementary step; thus,
the kinetics with respect to the gaseous reactant is usually
first order. This means that the kinetic order is equal to the
molecularity of the process, namely one, with respect to
the gaseous reactant (Atkins and de Paula 2002; Laidler
1987).
The KMnO4 adsorbed on the carrier was in condensed
(solid or liquid) phase while ethylene was in gaseous phase;
with these conditions the reaction was modeled (Hernandez
2005) in terms of the molar ratio of KMnO4 adsorbed on a
particular carrier and C2H4 that participated in the hetero-
geneous chemical reaction. Values of this ratio varied from
5.35 for lahar ash to 173.89 for coconut coir dust at 0.05 M
KMnO4 and approximately 4 ppm C2H4. In general, this
ratio was large for all the carriers, except lahar ash which
had a ~1:1 ratio at a high C2H4 concentration (12–20 ppm)
and relatively low KMnO4 concentration (0.03–0.04 M).
Therefore, the assumption that the KMnO4 concentration
was in excess relative to that of C2H4 is valid except for
lahar ash at the lowest C2H4 concentrations and highest
KMnO4 concentrations.
Determination of Rate Coefficients. The R (Pearson
rho coefficient) values for the logarithmic plot of C2H4 con-
centration vs. time for the three scrubbers are equal to one
within experimental uncertainty (Fig. 8a-c); thus, it is valid
to assume that the reaction of C2H4 with KMnO4 was first
order with respect to C2H4. Based on the assumption of
pseudo-first order kinetics, the slope of the plots in Fig.
8a–8c gives the permanganate-dependent rate coefficient
(k’) of the reaction. This rate coefficient is the basis for
evaluating scrubber efficiency.
Plots of k’ vs. initial KMnO4 concentration are pre-
sented in Fig. 9; it can be seen that k’ values for the scrub-
bers treated with 0.03 M KMnO4 had the lowest values of
k’ while the k’ values for each carrier at 0.04 M KMnO4 and
0.05 M KMnO4 were not significantly different. Based on
this observation, the minimum effective concentration of
KMnO4 that should be equilibrated with the carriers was
found to be 0.04 M.
The k of a particular scrubber does not have signifi-
cant effect with respect to the varying initial KMnO4 con-
Fig. 6.Fig. 6.Fig. 6.Fig. 6.Fig. 6. Plot of residual permanganate concentration
vs. initial permanganate concentration for vari-
ous scrubbers.
-5.00E-05
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
0.03 0.03 0.04 0.04 0.05 0.05 0.06
Initial permanganate conc, M
Residualpermanganateconc,M
Rice hull ash
Coconut coir dust
Lahar ash
Fig. 7.Fig. 7.Fig. 7.Fig. 7.Fig. 7. Plot of ln initial velocity vs. ln initial C2H4
concentration for various scrubbers.
y = 1.3512x - 3.5272
R2
= 0.921
y = 1.4551x - 4.3371
R2
= 0.936
y = 0.8372x - 2.7998
R2
= 0.6241
-3.00000
-2.50000
-2.00000
-1.50000
-1.00000
-0.50000
0.00000
0.50000
1.00000
1.200 1.700 2.200 2.700 3.200
ln Co
lnVo
Rice hull Ash
Coconut coir dust
Lahar ash

centration added (Fig. 10). This observation on the k val-
ues is consistent with the rate equation
vo = k [A]n [B]m
which states that the intrinsic rate constant k is indepen-
dent of the amount or concentration of the reactants present
in the reaction (Atkins and de Paula 2002).
Figure 10 also shows that the intrinsic rate constant k
for lahar-ash-based scrubber, rice-hull-ash-based scrub-
ber and coconut-coir-dust-based scrubber are ~10, ~2 and
~0.20 g carrier – min-1/g KMnO4, respectively. The results
indicate that among the scrubbers, the KMnO4 adsorbed
in lahar ash is less tightly bound or more available for
oxidizing ethylene compared to the other carriers. This could
be due to the fact that the primary mineral components of
lahar ash such as hornblende, feldspar, mica, quartz, py-
roxene and magnetite are not involved in strong interac-
tions with KMnO4.
Plots of k’ vs. KMnO4 loading are presented in Fig. 11;
it can be seen that k’ values were highest for rice hull ash
followed by lahar ash and coconut coir dust. The data
Fig. 9.Fig. 9.Fig. 9.Fig. 9.Fig. 9. Plot of permanganate-dependent rate coeffi-
cient (k’) vs. initial permanganate concentra-
tion for various scrubbers.
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
0.025 0.030 0.035 0.040 0.045 0.050 0.055
initial permanganate concentration (M)
k',min
-1
Rice hull ash
Coconut coir dust
Lahar ash
Fig. 10.Fig. 10.Fig. 10.Fig. 10.Fig. 10. Plot of intrinsic rate coefficient (k) vs. initial
permanganate concentration for various scrub-
bers.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0.03 0.04 0.04 0.05 0.05 0.06
initial permanganate concentration (M)
k,gcarrier-min
-1
/gKMnO4
rice hull ash
coconut coir dust
lahar ash
y = -0.013x + 1.2557
R2
= 0.9604
y = -0.0216x + 1.2469
R2
= 0.9697
y = -0.022x + 1.2925
R2
= 0.9681
1.000
1.050
1.100
1.150
1.200
1.250
1.300
1.350
0 2 4 6 8 10 12
time, min
lnethyleneconc
0.03 M
0.04 M
0.05 M
Fig. 8a.Fig. 8a.Fig. 8a.Fig. 8a.Fig. 8a. Plot of ln ethylene concentration vs. time for
rice-hull-ash-based scrubber at different KMnO4
concentrations.
y = –0.013x + 1.2557
R2 = 0.9604
y = –0.0216x + 1.2469
R2 = 0.9697
y = –0.022x + 1.2925
R2 = 0.9681
Fig. 8b.Fig. 8b.Fig. 8b.Fig. 8b.Fig. 8b. Plot of ln ethylene concentration vs. time for
coconut-coir-dust-based scrubber at different
KMnO4 concentrations.
y = -0.008x + 1.197
R2 = 0.818
y = -0.0085x + 1.2807
R2 = 0.9611
y = -0.0085x + 1.2961
R2 = 0.8827
1.100
1.150
1.200
1.250
1.300
1.350
0 2 4 6 8 10 12
time, min
lnethyleneconc
0.03 M
0.04 M
0.05 M
y = –0.008x + 1.197
R2 = 0.818
y = –0.0085x + 1.2807
R2 = 0.9611
y = –0.0085x + 1.2961
R2 = 0.8827
Fig. 8c.Fig. 8c.Fig. 8c.Fig. 8c.Fig. 8c. Plot of ln ethylene concentration vs. time for
lahar-ash-based scrubber at different KMnO4
concentrations.
y = -0.0098x + 1.3277
R2
= 0.9415
y = -0.0127x + 1.296
R2
= 0.9452
y = -0.0128x + 1.3142
R2
= 0.89
1.100
1.150
1.200
1.250
1.300
1.350
1.400
0 2 4 6 8 10 12
time, min
lnethyleneconc
0.03 M
0.04 M
0.05 M
y = –0.0098x + 1.3277
R2 = 0.9415
y = –0.0127x + 1.296
R2 = 0.9452
y = –0.0128x + 1.3142
R2 = 0.89

cient (k’) vs. KMnO4 loading for various scrub-
bers.
y = 1.8729x + 0.0011
R2
= 0.7783
y = 0.021x + 0.0075
R2
= 0.75
y = 4.7804x + 0.0058
R2
= 0.7686
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.00E+00 2.00E-02 4.00E-02 6.00E-02
KMnO4 loading, g KMnO4/g carrier
k',min
-1
RHA
CCD
LA
points in Fig. 11 for each scrubber fit a straight line with
positive slope, which is equal to the intrinsic (permangan-
ate-independent) rate coefficient k, in accordance with the
equation
k’ = k (KMnO4 loading)
Based on the observed k’ values, it can be concluded
that rice hull ash was the best carrier in terms of ethylene
scrubbing efficiency followed by lahar ash and then coco-
nut coir dust.
The calculated slopes (k values) in Fig. 11 were 1.87,
4.78 and 0.02 g carrier – min-1/g KMnO4 for rice hull ash,
lahar ash and coconut coir dust, respectively. Although
lahar ash gave the highest k on a per gram scrubber basis,
it was found inferior to rice hull ash which gave the highest
k’ value of 0.0216 min-1 (at 0.04 M KMnO4).
Values of the permanganate-dependent rate coefficient
k’ reported by Lizada and Artes (1989) using a rice-hull-
ash-based scrubber are approximately ten times those ob-
tained in our study. Values of the intrinsic rate coefficient k
were calculated using their data on the ratio of the volume
of 0.05 M KMnO4 to the mass of rice hull ash, as well as
their reported k’ values. The difference between the rate
coefficients reported by these researchers and those ob-
tained in the present study can be explained by differences
in the chemical composition of the rice hull ash used in the
two studies.
Determination of ScrubberStability
Variation of Rate Coefficient with Time. As shown in Fig.
12, the rate coefficient k’ for rice hull ash as carrier did not
significantly vary with time, while those for both lahar ash
and coconut coir dust showed significant reduction with
time. Lahar ash showed the fastest decrease in k’ with time
(in days) with a slope of -5.93 x 10-4 and for coconut coir
dust, the corresponding slope was -2.57 x 10-4. On the other
hand, rice hull ash exhibited a slope of almost 0. The calcu-
lated slope indicates scrubber stability; the less negative
the slope, the more stable the scrubber. Therefore, the most
stable KMnO4 carrier was rice hull ash followed by coco-
nut coir dust and the least stable carrier was lahar ash.
Determination of Residual Permanganate Adsorbed
on the Scrubber. As shown in Fig. 13, the residual KMnO4
for each scrubber decreased through time, except for coco-
nut coir dust which had a negligible value. In general, the
decrease in the residual KMnO4 for a particular scrubber
indicates its deterioration and can be attributed to the pres-
ence of oxidizable components or contaminants in the car-
rier. Lahar ash and rice hull ash showed similar rates of
scrubber deterioration due to permanganate reduction, as
shown by similar slopes in Fig. 13.
Chromametric Analysis of Scrubber. Scrubber stabil-
ity could also be correlated with changes in chromacity of
the reflected light for each scrubber at different time inter-
vals. Chromacity indicates the color intensity of reflected
cient (k’) vs. time for various scrubbers.
y = 2.57E-05x + 2.60E-02
y = -2.67E-04x + 1.03E-02
y = -5.93E-04x + 1.79E-02
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
0.0350
0 5 10 15 20 25 30
time, days
k’,min
-1
Rice hull ash
Coconut coir dust
Lahar ash
y = 2.57E-05x + 2.60E-02
y = –2.67E-04x + 1.03E-02
y = –5.93E-04x + 1.79E-02
Fig. 13.Fig. 13.Fig. 13.Fig. 13.Fig. 13. Plot of the residual permanganate concen-
tration vs. time for various scrubbers.
y = -3E-06x + 0.0003
y = -3E-07x + 5E-06
y = -4E-06x + 0.0001
-5.00E-05
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
3.50E-04
0 5 10 15 20 25 30
time, day
Residualpermanganateconc,M
Rice hull ash
Coconut coir dust
Lahar ash
y= –3E-06x + 0.0003
y= –3E-07x + 5E-06
y= –4E-06x + 0.0001

light and can be measured in terms of the parameters L*
(lightness), a* and b* (chromacity coordinates, namely hue
and chroma). Values of the three chromacity parameters
L*, a* and b* for each scrubber at a particular time (Table
3a) were compared with those of both freshly prepared and
aged scrubbers (Table 3b). The data in Tables 3a and 3b
show that in terms of the three chromacity parameters, la-
har-ash-based scrubber exhibited the fastest color deterio-
ration which indicates chemical reduction of KMnO4.
The chromacity data of the scrubbers could be corre-
lated with values of the rate coefficients k and k’. The rate
coefficients of rice-hull-ash-based scrubber did not vary
significantly with time, while for the coconut-coir-dust-
and lahar-ash-based scrubbers, the rate coefficients sig-
nificantly decreased with time (except for coconut coir dust,
k = 0). The aged scrubbers had a brownish color which is
attributed to the MnO2 precipitate formed when KMnO4
was made to react with an oxidizable substance. This MnO2
precipitate is not a good oxidizing agent for ethylene.
Tables 3a and 3b also show that the chromacity values
for rice-hull-ash-based scrubber approached the corre-
sponding chromacity values for the aged scrubber with
the lowest rate followed by coconut-coir-dust- and lahar-
ash-based scrubbers. This means that the rice-hull-ash-
based scrubber was the most stable while the lahar-ash-
based scrubber was the least stable.
CONCLUSION
The ethylene scrubbing efficiency and stability of three
KMnO4-carrier mixtures (rice hull ash, lahar ash and coco-
nut coir dust) were studied. The kinetic order with respect
to ethylene of the reaction between C2H4 and KMnO4 was
equal to 1.35 ± 0.39, 1.46 ± 1.09 and 0.84 ± 0.18 for rice hull
ash, coconut coir dust and lahar ash, respectively. The
KMnO4-dependent and intrinsic rate coefficients (k’ and k,
Table 3b. Values of chromacity parameters for freshly
prepared and aged scrubbers.
Rice Hull Coconut Coir Lahar
Ash Dust Ash
Freshly Prepared
L* 33.5 14.2 32.3
a* 29.2 2.5 11.4
b* -5.9 7.9 4.1
Aged
L* 61.5 23.3 51.0
a* 4.0 4.4 0.3
b* 25.8 15.7 20.0
L* (lightness), a* (hue), b* (chroma)
Table 3a. Values of the chromacity parameters for scrubbers at different days.
Time Rice Hull Ash Coconut Coir Dust Lahar Ash
(Days)
L* a* b* L* a* b* L* a* b*
0 33.3 27.8 -3.5 16.4 2.5 6.5 30.9 11.7 5.9
1 33.7 25.6 2.9 15.3 2.4 6.6 33.3 8.2 9.0
3 34.0 24.5 5.6 15.5 2.7 6.3 34.8 5.9 13.5
5 34.3 23.2 7.8 15.2 3.0 6.8 35.7 4.5 12.7
7 34.8 22.7 8.3 14.6 3.7 7.5 35.7 4.6 15.8
9 35.6 23.2 11.4 14.4 3.1 10.0 35.8 3.4 18.2
11 35.2 22.3 13.3 14.4 3.6 10.9 35.6 3.1 16.9
14 34.8 22.4 12.7 14.8 3.2 8.6 35.6 3.0 18.0
17 35.5 21.5 13.4 14.1 3.8 6.9 36.9 2.0 20.8
21 36.6 22.1 16.2 14.5 4.5 6.9 36.1 1.5 18.8
27 37.0 20.4 17.0 14.8 3.7 11.5 36.8 1.0 20.4
L* (lightness), a* (hue), b* (chroma)
respectively) were calculated based on pseudo-first order
kinetics; these were used to evaluate the ethylene scrub-
bing efficiency of the carriers. For the carriers which had
been treated with 0.04 M KMnO4 (optimum KMnO4 con-
centration), the k’ values (in min-1) were 0.0216 ± 0.0020
min-1,0.0127 0.0003min-1 and0.0085±0.0006min-1 forrice
hull ash, lahar ash and coconut coir dust, respectively.
These k’ values indicate that per gram of scrubber rice hull
ash was the most efficient oxidizer of ethylene, followed by
lahar ash and then coconut coir dust. The observed values
of k’ could be related to the residual KMnO4 in the scrub-
ber. Rice hull ash was found to contain the highest residual
permanganate followed by lahar ash and coconut coir dust.
The intrinsic rate coefficient (k) was calculated as the
ratio of k’ to the permanganate loading. The calculated
values of k (in g carrier-min-1/g KMnO4) were 1.87, 4.78 and
0.02 for rice hull ash, lahar ash and coconut coir dust, re-
spectively. These k values indicate that at the same KMnO4
loading, the lahar ash scrubs ethylene more efficiently, fol-
lowed by rice hull ash and then coconut coir dust. How-

ever, in practical terms k’(rate coefficient per gram of scrub-
ber) is a better gauge of ethylene scrubbing efficiency than
the intrinsic rate coefficient k (rate coefficient per g KMnO4),
Therefore, the most efficient scrubber is that based on rice
hull ash.
The scrubber stability was determined by measuring
how much the rate coefficient k’ changed with time. The
lahar-ash-based and rice-hull-ash-based scrubbers showed
similar rates of decrease in k’ with time. Therefore, rice hull
ash scrubber was the most stable. This was confirmed by
measuring the residual KMnO4 in the scrubber as a func-
tion of time in terms of the chromacity of the reflected light
from the scrubber. The chromacity values for rice-hull-ash-
based scrubber exhibited the slowest change followed by
the coconut-coir-dust-based and lahar-ash-based scrub-
bers.
ACKNOWLEDGMENTS
The authors are grateful to Dr. E. B. Rodriguez for provid-
ing technical data, Dr. M. Belarmino and Mr. R.Artificio for
advice on mathematical treatment of data and W. Absulio,
L. Artes, N. Garcia and E. Esguerra for valuable technical
assistance.
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