kinetic studies of ethylene oxidation by potassium permanganate adsorbed on rice hull ash, lahar...

28 The Philippine Agricultural Scientist Vol. 90 No. 1 (March 2007)

Ethylene Oxidation by Potassium Permanganate A.B. Hernandez et al.THE PHILIPPINE AGRICULTURAL SCIENTIST ISSN 0031-7454Vol. 90 No. 1, 28-39March 2007

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

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,Philippines2Crop Science Cluster, College of Agriculture, University of the Philippines Los Baños, College, Laguna 4031, Philippines*Author for correspondence; e-mail: [email protected]

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 withrespect 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 andintrinsic (permanganate-independent) rate coefficients (k’ and k, respectively) were calculated basedon pseudo-first order kinetics. The optimum KMnO4 concentration for scrubbing ethylene was found tobe 0.04 M. Experimental values of the rate coefficient k’ (in min-1) were 0.0216 ± 0.0020, 0.0127 ± 0.0003and 0.0085 ± 0.0006 for rice hull ash, lahar ash and coconut coir dust, respectively. Values of theintrinsic rate coefficient k (in min-1 g carrier/g KMnO4) were 1.87, 4.78 and 0.02 for rice hull ash, laharash and coconut coir dust, respectively. At the same KMnO4 loading, lahar ash was the most efficientKMnO4 carrier followed by rice hull ash and coconut coir dust. However, the most efficient KMnO4carrier (per gram) as ethylene scrubber was rice hull ash followed by lahar ash and then coconut coirdust.

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 stableand showed negligible changes in rate coefficient k’ for 27 d; lahar ash was the least stable carrierfollowed 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 thataffects physiological processes in plants. It is also a natu-ral product of plant metabolism and is produced by alltissues of higher plants and by some microorganisms. Asa phytohormone, even in trace amounts (less than 0.1 ppm),it regulates many aspects of growth and development, andhas been shown to be an inductive factor in rapid physi-ological changes (e.g., ripening and senescence) inpostharvest fruits, especially climacterics (Buffler 1986),and vegetables (Abeles et al. 1971; Kazuhiro and Watada1991; Jayaraman and Raju 1992; El Blidi et al. 1993). More-over, ethylene reduces the storage life of many postharvestcommodities if it is used at a high concentration.

Ripening or senescence of perishable commodities isdelayed by maintaining ethylene at low levels inside pack-

ages and storage rooms; this consequently extends thestorage 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 KMnO4which is impregnated into an inert and porous matrix with alarge surface area (Jayaraman and Raju 1992). Several ma-trices or carriers have been used as ethylene scrubbers andthey are usually made of siliceous materials such as ver-miculite and celite (Abeles 1973), mixtures such as cementand expanded mica (Wills et al. 1981) and commercial prepa-rations such as Purafil which consists of alumina.

Potassium permanganate (KMnO4) is a dark purple orbronze-like, non-volatile, odorless crystal that is stable inair. 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 aparticular KMnO4 scrubber is indicated by the rate coeffi-

The Philippine Agricultural Scientist Vol. 90 No. 1 (March 2007) 29

Ethylene Oxidation by Potassium Permanganate A.B. Hernandez et al.

cient for the reaction between ethylene and KMnO4adsorbed on a particular carrier; it is dependent on thepermanganate concentration and is influenced by the car-rier used (Kavanagh and Wade 1987).

The search for readily available materials as KMnO4carriers without the use of binders (e.g., cement, clay alu-mina, etc.) resulted in the development of the KMnO4-ricehull ash scrubber (Lizada and Artes 1989). The advantagesof rice hull ash as KMnO4 carrier include its inertness toKMnO4, its high silica content, which contributes to itshigh 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 arehighly 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 wasalso conducted on chitin as potential KMnO

4

carrier. Thekinetic order for the oxidation of ethylene by the carrier-adsorbed KMnO was determined for rice hull ash, laharash and coconut coir dust, as well as the intrinsic andpermanganate-dependent rate coefficients for the pseudofirst-order reaction. The results were used to assess theefficiency and long-term stability of each of the carriers asa component of the ethylene scrubber.

MATERIALS AND METHODS

Scrubber PreparationPreparation of Carriers. Rice hull ash was obtained fromCandelaria, Quezon and coconut coir dust from Victoria,Laguna. Lahar ash was sampled from a lahar area inPampanga; chitin was purchased from Aldrich ChemicalCompany, USA. The materials (except chitin which wasused as received from Aldrich) were dried in a hot air cham-ber and sequentially passed through 10-mesh and 60-meshsieves. Only particles that passed through the 10-meshsieve 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 (ricehull ash, lahar ash, coconut coir dust or chitin) was mixed

with a specified volume of distilled water; the latter wasmeasured 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 butdid not produce any evident wetting of the surface of thecontainer (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 ofdefinite concentration (0.03 M, 0.04 M and 0.05 M) whichwas ten times the optimal volume capacity of the carrier asdetermined 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 liquidbut we ascertained that it had no KMnO4 as free-flowingliquid.

One gram (or 2 g for determination of scrubber stabil-ity) of each KMnO4-carrier mixture was measured using atop-loading balance and placed inside a 3 cm x 3.5 cm (5.5cm x 5 cm for determination of scrubber stability) cellulosicnon-woven fabric sachet.

Preparation of Ethylene Scrubber VesselThe ethylene scrubber vessel (Fig. 2) consisted of a 250-mL Erlenmeyer flask which had been cleaned, dried andflushed with ample amounts of air. The flask was coveredwith a rubber stopper with two holes through which wereinserted two short glass tubes that were connected by asoft rubber tube. Sampling of the gas in the flask, which isalso 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 wassealed in order to prevent the escape of gas from the ves-sel and the rubber tube for sampling was replaced regu-larly.

Preliminary ExperimentsDetermination of pH of Carriers and Scrubbers. Based onthe determined optimal volume capacity of each scrubber,the corresponding optimal volume of distilled water wasadded to 1 g of each carrier and transferred quantitativelyinto a 50-mL volumetric flask. The pH of the mixture wasthen measured using the JEMCO Analog pH meter. Thisprocedure was repeated but instead of distilled water,KMnO4 was added at concentrations of either 0.03 M, 0.04M or 0.05 M. The pH was measured in order to elucidatethe chemical changes that occurred in the KMnO4 solu-tion when added to the carrier. For example, the KMnO4adsorbed on coconut coir dust at acidic pH was convertedto Mn2+, based on color change from purple to colorless;this makes the scrubber inefficient. Experimental data onthe maximum liquid holding capacity of different carriersare presented in Table 1.

by the carrier was calculated as the difference between theinitial mass of KMnO4 4 and the mass of residual KMnO ofthe filtrate after 10 min. This was used as basis for calculat-ing the mass fraction of adsorbed KMnO4, which is themass of KMnO4 adsorbed by the carrier divided by themass 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) andtheir absorbance was measured at 535 nm. A standard curvewas 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 orions (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 (Laidlerand Meiser 1999; Atkins and de Paula 2002). Eq. (1a) maybe rewritten as a linear relationship between and 1/cwith 1/K as the slope:

(1b)

Determination of Kinetic Order with Respect to EthyleneEach packed freshly prepared ethylene scrubber (using0.04 M KMnO4 solution) was placed inside the ethylenescrubber vessel which had been thermally equilibrated at20 oC. The laboratory room where the experiments wereconducted was maintained at 20 oC. Then the flask wasclosed 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 werehomogenized by manual agitation and then two 1-mLsamples of the headspace gas were obtained at suitabletime intervals. Ethylene purity was close to 100% and sourcefor the experiments was a 1000 ppm stock solution. Theethylene stock solution was placed in the laboratory wherethe temperature was maintained at 20 oC. In the experiment,approximately 4 ppm ethylene was obtained from a 1000ppm stock solution and was prepared using the C1V1 =C2V2 formula. The volume of 1000 ppm stock that wouldgive approximately 4 ppm solution was calculated from theformula. However, the actual concentration was verifiedexperimentally using a gas chromatograph with flame ion-ization detector (GC-FID).

Kcθ = 1 + Kc

1 1 = 1 +

θ Kc

Table 1. Maximum liquid holding capacity of different car-riers.

Carrier Maximum Volume/ gCarrier, mL

Rice hull ash 1.5Coconut coir dust 6.5Lahar ash 0.2Chitin 3.8

Determination of KMnO4 Adsorption Equilibrium ConstantDetermination of Equilibration Time for KMnO4 Adsorp-tion. To one gram of each carrier, half of the previouslydetermined optimal volume of 0.01M KMnO4 was added ina 50-mL volumetric flask. The flask was then filled withdistilled 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, eachcontaining 1g of carrier, were added 10 mL solutions ofvarying concentrations of KMnO4 (0–50 mM). The mixturewas agitated vigorously for 1 min in a vortex mixer, andallowed to equilibrate for 10 min. Then the suspensionswere filtered and the filtrate was analyzed for residualKMnO4 based on absorbance measurements at 535 nmusing the SECOMAM UV-Vis Spectrophotometer. The con-centration of residual KMnO4 in the filtrate was taken asthe unadsorbed KMnO4, as well as the equilibrium con-centration after adsorption. The mass of KMnO4 adsorbed

1θ



After obtaining two 1-mL samples of headspace gas,air (2 mL) was injected into the flask in order to maintainconstant headspace gas volume; the air was again homog-enized. Each headspace gas was analyzed for ethyleneusing the Shimadzu Gas Chromatograph equipped with aflame ionization detector and alumina column.

The procedure was repeated for ethylene concentra-tions of approximately 8, 12, 16 and 20 ppm. Measurementof 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 thesame volume to mass ratio of liquid and carrier, respec-tively). Ethylene analysis was done using the operatingparameters of the gas chromatograph given below.

Gas Chromatographic Parameters for Ethylene Analysis

Parameters Value

Injector/Detector temperature 160 oCColumn temperature 110 oCFlow rate of carrier gas (N2) 1.25 kg/cm2

Flow rate of H220.6 kg/cm

Flow rate of air 0.6 kg/cm2

Retention time for ethylene 30 sec

Determination of Rate Coefficients and ScrubbingEfficiencyEach freshly prepared and packed ethylene scrubber wasplaced inside the ethylene scrubber vessel which had beenthermally equilibrated at 20 oC. After injecting ethylene to afinal 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 foreach ethylene scrubber. The procedure was repeated us-ing each ethylene scrubber which had been prepared with0.04 M and 0.05 M KMnO4 solutions. Each scrubber ex-posed to ethylene was set aside for measurement of theresidual permanganate concentration. The residual perman-ganate of each scrubber was eluted with distilled waterand transferred quantitatively into a 50-mL volumetric flask.The absorbance of the eluate solution was measured at535 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

where A = C2H4 and B = KMnO4.The initial velocity (vo) for each scrubber at a speci-

fied initial concentration of ethylene was determined bycalculating the initial slope of the plot of ethylene concen-

kA B C+ ⎯ ⎯→

tration against time. Based on the reaction above, the ratelaw 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 kineticorder with respect to ethylene, n.

Assuming that the reaction follows pseudo-first orderwith respect to ethylene and assuming that the KMnO4concentration 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 reactionfollows 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 mo o o

d[A] d[B]v k[A] [B]

dt dt= − = − =

no 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 'toA A e−=

4

k 'k

KMnO loading=



RESULTS AND DISCUSSION

Liquid-Holding Capacity of CarriersCoconut coir dust had the highest liquid-holding capacitywhile lahar ash had the lowest (Table 1). The results maybe explained by the presence of hydrophilic groups in co-conut coir dust. This material has been reported to contain42.3% lignin (Suzuki et al. 1998), as well as 24.1% celluloseand 27.3% pentosan (Gonzalez 1970), which contain phe-nolic groups (for lignin) and hydroxyl groups; these groupsare 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 ashornblende, feldspar, mica, quartz, pyroxene and magne-tite and amorphous materials such as amorphous SiO2 (Phil-ippine Bureau of Soils 1991 unpublished); most of thesecomponents have low hydrophilicity. Rice hull ash, whichcontains mainly amorphous silica (Kamath and Proctor 1998;Kalapathy et al. 2002), exhibited moderate liquid-holdingcapacity since the oxygen atoms in the silicate structurecan interact with water molecules through H-bonding.

Based on the maximum liquid-holding capacity, thecorresponding volume of 0.04 M KMnO4 solution wasadded to each carrier. After addition of the KMnO4 solu-tion to each carrier, only rice hull ash and lahar ash showed

Determination of Scrubber StabilityThirty-six pieces of 2-g ethylene scrubbers (KMnO4-ricehull ash mixture, KMnO4-lahar ash mixture and KMnO4-coconut coir dust mixture), which had been prepared at theoptimum KMnO4 concentration (determined in the previ-ous section), were packed. Then three packed freshly pre-pared ethylene scrubbers were placed in a polyethylenebag and kept in the dark using an air-tight container.

For day 0, one polyethylene bag containing threepacked scrubbers was randomly picked from the storagecontainer. The ethylene scrubbing capacity was determinedusing the procedure mentioned earlier for the measurementof rate coefficients and ethylene scrubbing efficiency. Theneach ethylene scrubber was removed from the flask and itscolor analyzed using the Minolta Chromameter. The re-sidual permanganate of each scrubber was eluted with dis-tilled water and transferred quantitatively into a 100-mLvolumetric flask, followed by absorbance measurement at535 nm.

The same procedure mentioned above was used afterthe following number of days: 1, 3, 5, 7, 9, 11, 14, 17, 21 and27. Standards for chromacity were measured. Thechromacity values of freshly prepared and aged scrubbers,which had been exposed to air for 3 wk, were determined inorder to establish the chromacity standards.

desired free space of scrubber vesselfrom 1000ppm stock

stock 1000ppm

(ethylene concentration )(Volume )Volume ethylene =

ethylene concentration

[ ]sample2 4sample (in ppm) 2 4std in ppm

std

(peak height )(attenuation)C H = × C H

(peak height )(attenuation)

4

24

4 g KMnO(in )g carrier

2KMnO packing material (in cm )4

3(in ppm)

3ethylene

KMnO loading

(MM )(Area )mol KMnO / cm

mol ethylene / cm ethylene concentration 1g 1LMM 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 amountof KMnO4 (mole KMnO4 adsorbed on a particular carrier per unit area) to the amount of ethylene(mole ethylene per unit volume)

1 L1000 cm3

1 g1000 mg

from 1000 ppm stickstock 1000 ppm



purple coloration while chitin had a reddish purple colorand coconut coir dust gave a dark brown color.

Profile of KMnO4 Adsorption by CarriersThe fraction of permanganate adsorbed on the differentcarriers is plotted in Fig. 3 against the equilibrium perman-ganate concentration in solution; the latter was calculatedby subtracting the residual permanganate concentrationfrom the initial value based on experimental data. The plotsfor rice hull ash and chitin exhibited type L (Langmuir-type) adsorption isotherm (Fig. 3) while coconut coir dustshowed high-affinity type isotherm (Osipow 1962). Laharash exhibited what looks like a highly steep L-type iso-therm. The hyperbolic Langmuir-type physical adsorptionmechanism 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 arectangular hyperbola where the fractional saturation θ isplotted against the adsorbate concentration. It is similar tothe Michaelis-Menten equation where the initial velocityis plotted against the concentration of the substrate (vanHolde et al. 1998). The hyperbolic nature of the Langmuirisotherm, which describes the formation of a monolayer ofadsorbate molecules on the adsorbent surface, is con-trasted with non-hyperbolic phenomena exhibited duringcooperative binding in biological systems where sigmoi-dal or S-shaped binding curves are observed (van Holde etal. 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 andMeiser 1999).

The results of the equilibration experiment for adsorp-tion of permanganate by the different carriers are shown inFig. 4. Permanganate adsorption by the carriers (exceptchitin) 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-tion in a solution containing 0.03M, 0.04M or 0.05 M KMnO4are presented in Table 2a. The KMnO4 loading (expressedin grams KMnO4 adsorbed per gram carrier) was calculatedby dividing the mass of KMnO4 contained in a particularcarrier by the mass of the carrier. The mass of KMnO4, inturn, was determined by multiplying the molarity of theKMnO4 solution by its optimal volume (required for eachcarrier) 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 themaximal value of θ (Atkins and de Paula 2002; Osipow 1962).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 laharash, respectively; chitin had slightly greater loading val-ues compared to rice hull ash (Table 2a). Values of KMnO4loading for rice hull ash were, on the average, 7.5 timesthose for lahar ash but only one-fourth those for coconutcoir dust. Results of pH measurements of the KMnO4-car-rier and water-carrier mixtures are given in Table 2b. Thechange in pH was used as basis for evaluating acid-basebehavior of the carrier after KMnO4 adsorption and forassessing 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

Fra

ctio

n p

erm

ang

anat

e ad

sorb

ed

rice hull ashcoconut coir dustlahar ashchitin

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 CarrierKMnO4Concen- Rice Hull Coconut Lahar Chitintration, 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-carrierand water-carrier mixtures.

pH of KMnO4- pH ofCarrier Mixture H2O-

Carrier Carrier0.03 M 0.04 M 0.05 M Mixture

Rice hull ash 8.9 9.1 9.3 9.3Coconut coir dust 5.5 5.7 5.9 5.3Lahar ash 4.5 4.8 4.9 4.4Chitin 8.2 8.2 8.3 8.4



The calculated value of KMnO4 loading is the totalamount of KMnO4 contained in the carrier, i.e., the amountof KMnO4 strongly bound to the carrier, plus the perman-ganate in the solution that wets the carrier particles. On theother hand, the amount of adsorbed KMnO4 is the amountthat could not be removed after washing with distilled wa-ter, i.e., this was either physically or chemically adsorbedon the carrier. Based on the methodology used in the ex-periments, it can not be assumed that the KMnO4 loadingand the adsorbed KMnO4 are identical. This point will betaken up again in relation to the discussion of the Langmuiradsorption 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 thereare strong molecular interactions, including chemical bondformation, between adsorbed and adsorbent molecules(Atkins and de Paula 2002). As mentioned earlier rice hullash and lahar ash contain inorganic combustion productswhich are not expected to chemically react with permanga-nate; similarly, chitin consists of poly-N-acetylglucosaminewhich appears stable in the presence of permanganate. Onthe other hand, coconut coir dust is composed of lignin,cellulose, pentosan and other organic compounds some ofwhich 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 excellentlinearity; an excellent linear fit would have indicatedLangmuir-type monolayer adsorption of permanganate oncoconut coir dust. The fact that a closer fitting line for theexperimental data in Fig. 5 with regression coefficient of0.98 (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 coirdust did not exactly follow the Langmuir mechanism. Asimilar plot for the other carriers gave negative values ofthe 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 thelinearized form of the Brunauer-Emmett-Teller equation(Osipow 1962; Atkins and de Paula 2002), especially at highpermanganate 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 isthat the carriers exhibited more complicated adsorptionmechanisms than those described by the Langmuir andBrunauer-Emmett-Teller equations. Needless to say, thisrequires further elucidation and research.

Figure 6 shows the plots of residual vs. initial KMnO4

4concentrations; zero residual KMnO

4

was observed forcoconut coir dust while values for RHA were 3.5–7 timesthose for lahar ash. Although coconut coir dust had thehighest KMnO loading, it gave a light yellow solutionafter elution which indicated negligible amounts of residualKMnO4. This means that the adsorbed permanganate waschemically 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 canbe oxidized by MnO4

-. This oxidation causes a color changefrom purple MnO4

- to the colorless Mn2+

4

at low pH: theobserved 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 ashis explainable in terms of more extensive van der Waalsinteraction, or physical adsorption mechanism, for rice hull

Fig. 4.Fig. 4.Fig. 4.Fig. 4.Fig. 4. Permanganate concentration of suspension ofvarious 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 15time, min

per

man

gan

ate

con

c, M

rhaccdlachi

Fig. 5.Fig. 5.Fig. 5.Fig. 5.Fig. 5. Plot of reciprocal of fraction of KMnO4 bindingsites occupied vs. reciprocal of equilibriumKMnO4 concentration for coconut coir dust ascarrier.

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 inthis material.

Determination of Kinetic Order and Rate CoefficientsDetermination of Kinetic Order. Scrubbers were preparedby adding the optimal volume of KMnO4 at a particularconcentration to the carriers (except chitin). In determiningkinetic order with respect to ethylene, it was necessary toensure 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. Eachscrubber (carrier with KMnO4) was made to react with eth-ylene at different initial concentrations of ethylene and thelatter 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. Thiswas done by plotting the natural logarithm of the initialvelocity vo (initial slope of the plot of C2H4 concentrationagainst time) against the natural logarithm of the initialC2H4 concentration [A]o. Values of the kinetic order for theC2H4-KMnO4 reaction calculated from Fig. 7 were 1.35 ±0.39, 1.46 ± 1.09 and 0.84 ± 0.18 for rice hull ash, coconutcoir dust and lahar ash, respectively. Based on these datait can be concluded that the reaction of ethylene withKMnO4 for rice hull ash and lahar ash as carriers mostprobably followed first-order kinetics with respect to eth-ylene; for coconut coir dust the kinetic order is less surebecause 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 usuallyfirst order. This means that the kinetic order is equal to themolecularity of the process, namely one, with respect tothe gaseous reactant (Atkins and de Paula 2002; Laidler1987).

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 (Hernandez2005) in terms of the molar ratio of KMnO4 adsorbed on aparticular carrier and C2H4 that participated in the hetero-geneous chemical reaction. Values of this ratio varied from5.35 for lahar ash to 173.89 for coconut coir dust at 0.05 MKMnO4 and approximately 4 ppm C2H4. In general, thisratio was large for all the carriers, except lahar ash whichhad 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 concentrationwas in excess relative to that of C2H4 is valid except forlahar ash at the lowest C2H4 concentrations and highestKMnO4 concentrations.

Determination of Rate Coefficients. The R (Pearsonrho coefficient) values for the logarithmic plot of C2H4 con-centration vs. time for the three scrubbers are equal to onewithin experimental uncertainty (Fig. 8a-c); thus, it is validto assume that the reaction of C2H4 with KMnO4 was firstorder with respect to C2H4. Based on the assumption ofpseudo-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 forevaluating 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 ofk’ while the k’ values for each carrier at 0.04 M KMnO4 and0.05 M KMnO4 were not significantly different. Based onthis observation, the minimum effective concentration ofKMnO4 that should be equilibrated with the carriers wasfound 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 concentrationvs. 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

Res

idu

al p

erm

ang

anat

e co

nc,

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 C2H4concentration for various scrubbers.

y = 1.3512x - 3.5272

R2 = 0.921

y = 1.4551x - 4.3371

R2 = 0.936y = 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

ln V

o

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 presentin the reaction (Atkins and de Paula 2002).

Figure 10 also shows that the intrinsic rate constant kfor 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 resultsindicate that among the scrubbers, the KMnO4 adsorbedin lahar ash is less tightly bound or more available foroxidizing ethylene compared to the other carriers. This couldbe due to the fact that the primary mineral components oflahar 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 ashfollowed 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', m

in-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. initialpermanganate 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, g

car

rier

- m

in-1

/g K

Mn

O4

rice hull ash

coconut coir dust

lahar ash

y = -0.013x + 1.2557

R2 = 0.9604y = -0.0216x + 1.2469

R2 = 0.9697y = -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

ln e

thyl

ene

con

c

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 forrice-hull-ash-based scrubber at different KMnO4concentrations.

y = –0.013x + 1.2557R2 = 0.9604

y = –0.0216x + 1.2469R2 = 0.9697

y = –0.022x + 1.2925R2 = 0.9681

Fig. 8b.Fig. 8b.Fig. 8b.Fig. 8b.Fig. 8b. Plot of ln ethylene concentration vs. time forcoconut-coir-dust-based scrubber at differentKMnO4 concentrations.

y = -0.008x + 1.197R2 = 0.818

y = -0.0085x + 1.2807R2 = 0.9611

y = -0.0085x + 1.2961R2 = 0.8827

1.100

1.150

1.200

1.250

1.300

1.350

0 2 4 6 8 10 12

time, min

ln e

thyl

ene

con

c

0.03 M

0.04 M

0.05 M

y = –0.008x + 1.197R2 = 0.818

y = –0.0085x + 1.2807R2 = 0.9611

y = –0.0085x + 1.2961R2 = 0.8827

Fig. 8c.Fig. 8c.Fig. 8c.Fig. 8c.Fig. 8c. Plot of ln ethylene concentration vs. time forlahar-ash-based scrubber at different KMnO4concentrations.

y = -0.0098x + 1.3277

R2 = 0.9415y = -0.0127x + 1.296

R2 = 0.9452y = -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

ln e

thyl

ene

con

c

0.03 M

0.04 M

0.05 M

y = –0.0098x + 1.3277R2 = 0.9415

y = –0.0127x + 1.296R2 = 0.9452

y = –0.0128x + 1.3142R2 = 0.89



Fig. 11.Fig. 11.Fig. 11.Fig. 11.Fig. 11. Plot of permanganate-dependent rate coeffi-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', m

in-1

RHA

CCD

LA

points in Fig. 11 for each scrubber fit a straight line withpositive slope, which is equal to the intrinsic (permangan-ate-independent) rate coefficient k, in accordance with theequation

k’ = k (KMnO4 loading)

Based on the observed k’ values, it can be concludedthat rice hull ash was the best carrier in terms of ethylenescrubbing 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. Althoughlahar ash gave the highest k on a per gram scrubber basis,it was found inferior to rice hull ash which gave the highestk’ value of 0.0216 min-1 (at 0.04 M KMnO4).

Values of the permanganate-dependent rate coefficientk’ 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 kwere calculated using their data on the ratio of the volumeof 0.05 M KMnO4 to the mass of rice hull ash, as well astheir reported k’ values. The difference between the ratecoefficients reported by these researchers and those ob-tained in the present study can be explained by differencesin the chemical composition of the rice hull ash used in thetwo studies.

Determination of Scrubber StabilityVariation of Rate Coefficient with Time. As shown in Fig.12, the rate coefficient k’ for rice hull ash as carrier did notsignificantly vary with time, while those for both lahar ashand coconut coir dust showed significant reduction withtime. 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 otherhand, rice hull ash exhibited a slope of almost 0. The calcu-lated slope indicates scrubber stability; the less negativethe slope, the more stable the scrubber. Therefore, the moststable KMnO4 carrier was rice hull ash followed by coco-nut coir dust and the least stable carrier was lahar ash.

Determination of Residual Permanganate Adsorbedon the Scrubber. As shown in Fig. 13, the residual KMnO4for each scrubber decreased through time, except for coco-nut coir dust which had a negligible value. In general, thedecrease in the residual KMnO4 for a particular scrubberindicates 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 ofscrubber deterioration due to permanganate reduction, asshown by similar slopes in Fig. 13.

Chromametric Analysis of Scrubber. Scrubber stabil-ity could also be correlated with changes in chromacity ofthe reflected light for each scrubber at different time inter-vals. Chromacity indicates the color intensity of reflected

Fig. 12.Fig. 12.Fig. 12.Fig. 12.Fig. 12. Plot of permanganate-dependent rate coeffi-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’, m

in-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

Res

idu

al p

erm

ang

anat

e co

nc,

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 hueand chroma). Values of the three chromacity parametersL*, a* and b* for each scrubber at a particular time (Table3a) were compared with those of both freshly prepared andaged scrubbers (Table 3b). The data in Tables 3a and 3bshow 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 ratecoefficients of rice-hull-ash-based scrubber did not varysignificantly 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 isattributed to the MnO2 precipitate formed when KMnO4was made to react with an oxidizable substance. This MnO2precipitate is not a good oxidizing agent for ethylene.

Tables 3a and 3b also show that the chromacity valuesfor rice-hull-ash-based scrubber approached the corre-sponding chromacity values for the aged scrubber withthe 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 threeKMnO4-carrier mixtures (rice hull ash, lahar ash and coco-nut coir dust) were studied. The kinetic order with respectto ethylene of the reaction between C2H4 and KMnO4 wasequal to 1.35 ± 0.39, 1.46 ± 1.09 and 0.84 ± 0.18 for rice hullash, coconut coir dust and lahar ash, respectively. TheKMnO4-dependent and intrinsic rate coefficients (k’ and k,

Table 3b. Values of chromacity parameters for freshlyprepared and aged scrubbers.

Rice Hull Coconut Coir LaharAsh 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 18.211 35.2 22.3 13.3 14.4 3.6 10.9 35.6 3.1 16.914 34.8 22.4 12.7 14.8 3.2 8.6 35.6 3.0 18.017 35.5 21.5 13.4 14.1 3.8 6.9 36.9 2.0 20.821 36.6 22.1 16.2 14.5 4.5 6.9 36.1 1.5 18.827 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 orderkinetics; these were used to evaluate the ethylene scrub-bing efficiency of the carriers. For the carriers which hadbeen treated with 0.04 M KMnO4 (optimum KMnO4 con-centration), the k’ values (in min-1) were 0.0216 ± 0.0020min-1, 0.0127 0.0003 min-1 and 0.0085 ± 0.0006 min-1 for ricehull ash, lahar ash and coconut coir dust, respectively.These k’ values indicate that per gram of scrubber rice hullash was the most efficient oxidizer of ethylene, followed bylahar ash and then coconut coir dust. The observed valuesof k’ could be related to the residual KMnO4 in the scrub-ber. Rice hull ash was found to contain the highest residualpermanganate followed by lahar ash and coconut coir dust.

The intrinsic rate coefficient (k) was calculated as theratio of k’ to the permanganate loading. The calculatedvalues of k (in g carrier-min-1/g KMnO4) were 1.87, 4.78 and0.02 for rice hull ash, lahar ash and coconut coir dust, re-spectively. These k values indicate that at the same KMnO4loading, 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 thanthe intrinsic rate coefficient k (rate coefficient per g KMnO4),Therefore, the most efficient scrubber is that based on ricehull ash.

The scrubber stability was determined by measuringhow much the rate coefficient k’ changed with time. Thelahar-ash-based and rice-hull-ash-based scrubbers showedsimilar rates of decrease in k’ with time. Therefore, rice hullash scrubber was the most stable. This was confirmed bymeasuring the residual KMnO4 in the scrubber as a func-tion of time in terms of the chromacity of the reflected lightfrom the scrubber. The chromacity values for rice-hull-ash-based scrubber exhibited the slowest change followed bythe coconut-coir-dust-based and lahar-ash-based scrub-bers.

ACKNOWLEDGMENTSThe authors are grateful to Dr. E. B. Rodriguez for provid-ing technical data, Dr. M. Belarmino and Mr. R. Artificio foradvice on mathematical treatment of data and W. Absulio,L. Artes, N. Garcia and E. Esguerra for valuable technicalassistance.

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kinetic studies of ethylene oxidation by potassium permanganate adsorbed on rice hull ash, lahar...

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