Reprinted from I&EC RESEARCH, 1996, 35. 245

Use of Kraft Pine Lignin in Controlled-Release FertilizerFormulations

Ma C. García,* J. A. Díez,f A. Vallejo, L. García, and Ma C. Cartagena

Departamento de Química y Análisis Agrícola, E.T.S.I. Agrónomos, Universidad Politécnica de Madrid,Ciudad Universitaria, sin 28040 Madrid, Spain

Lignin was used as a majority component in a controlled-release fertilizar coating, since thiswaste product displays properties that make it an ideal substance for application in soil togetherwith conventional mineral fertilizers. Pine lignin marketed under the ñame of Indulin AT andobtained by the Kraft process was used. Insoluble in water for all pH valúes that may occur insoil, this product was employed for coating pelleted urea. Seven series of fertilizar were obtained,which also contained various types of rosins and in some cases linseed oil as additives. Thephysical—chemical evaluation of these fertilizers showed that the most efficient are those whosecoating embodies a mixture of dimerized, esterified, and natural rosins, as well as lignin. Itwas also demonstrated that the efficiency of the producís noticeably increases by adding linseedoil as a sealing agent.

Introduction

The efficiency of nitrogenous controlled-release fertil-izers (CRFs) is usually greater than that of conventionalfertilizers in media where major water losses occurthrough drainage (Vallejo et al., 1993). One way toprepare CRFs is to coat soluble fertilizers with aninsoluble material. The nutrient is released slowlythrough surface pores or cracks (Jiménez, 1992). Coat-ings used for this purpose must have two additionalproperties: they must be biodegradable in soil and notleave toxic substances behind which will affect crops(Hignett, 1985).

As a low-cost waste product in the paper pulpmanufacturing process, lignin is a material that maybe ideal for application to soil together with solublefertilizers (Truskin and Kadyrov, 1978) since, accordingto Martin and Haider (1992) and Stevenson (1982),when embodied into soil, it acts as a humic acidprecursor substance. Flaig (1984) proved that ligninreduces the nitrification rate of urea, enabling thiscompound to remain in the soil longer.

The purpose of this paper was to study the possibilityof using lignin as a coating for soluble fertilizers inpreparing more controlled-release fertilizers. Sevenseries of CRFs were prepared by coating pelleted ureawith Kraft pine lignin, several types of rosin, and, insome cases, linseed oil. An additional aim was toevalúate the influence of the type of coating and itspercentage on the product's solubilization rate and onphysical properties related to the handling, transport,and subsequent use of the fertilizer (More, 1978).

Materials and Methods

Lignin-coated controlled-release fertilizers were pre-pared in the laboratory per the process described inSpanish Patent No. 536567 (Jiménez et al., 1984). Thebase fertilizer used was pelleted urea with 46% nitrogenrichness. The lignin embodied into the coating ismarketed under the ñame of Indulin AT and is apurified Kraft pine lignin completely free of hemicellu-lose producís and water insoluble for all possible pH

f Instituto de Ciencias Medioambientales del C.S.I.C., Madrid,Spain.

valúes a soil may display. Natural, dimerized (ResidysPolymer) and esterified (Resiester T) rosins were usedas the lignin's adhesive. Linseed oil was used as anadhesive or sealant in preparing some products.

Components of the products obtained were separatedby selective solubilization processes. Urea was watersolubilized and its contení was determined by íheKjeldhal meíhod (AOAC, 1990). Benzene-dissolvedrosins were jointly evaluated by visible—ultravioletspecírophoíometry using a UV-160 Model Shimadzuspectrophotomeíer. The same meíhod was employed íoanalyze íhe lignin coníení by using dioxane as solvení.

The urea reléase curve was characíerized in each ofthe products obtained as a funcíion of time, ai a consíanttemperaíure of 20 °C (Jiménez eí al., 1993). To do so,0.5 g of each fertilizer was placed in íesí íubes and 10mL of disíilled waíer was added. When íhe time foreach íest had elapsed, the samples were filtered andíhe urea contení was analyzed in each filíraíe by visiblespectrophoíomeíry using p-(dimeíhylamino)benzalde-hyde in a hypochlorite médium (AOAC, 1990). All tesiswere performed in triplícate, and the average valué wastaken as íhe resulí.

The íype of reléase kineíics and the constaní ofreléase rate for the urea reléase process were deter-mined in each case by mathematically processing íheexperimeníally obtained data. It is thus possible íocharacíerize and compare the different fertilizers wiíheach oíher.

Physical properties were síudied by performing íesíson some of íhe fertilizers prepared: grain size (Kelly,1974), crushing sírengíh (TVA, 1970), apparení densiíy(USDA, 1977), critica! relaíive humidiíy (Hoffmeisíer,1979), humidiíy absorpíion raíe, and microphoíographicsíudy of íhe coaíing, enabling its thickness ío bemeasured and iís surface ío be examined by meíalcoaíing íhe fertilizer pellets and using an ISI Sx-25electrón scanner microscope.

Results and Discussion1. Composition of the Fertilizers. Table 1 shows

the composition of all fertilizers prepared, expressed asa percentage of their componenís. A figure has beeninserted in íhe nomenclaíure of each producí referringío iís coaíing perceníage.

0888-5885/96/2635-0245$12.00/0 © 1996 American Chemical Socieíy

246 Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996

Table 1. Description of the Composition of the Base Urea Fertilizer Products Expressed as % of Their Componentsseries

ULI

ULII

ULIII

ULIIA

ULIIIA

ULA

ULCA

fertilizer

ULI-20ULI-25ULI-32ULI-40ULII-20ULII-25ULII-30ULIII-16ULIII-22ULIII-29ULIIA-31ULIIA-33ULIIIA-18ULIIIA-34ULA- 12ULA-16ULCA-13ULCA-19ULCA-23

coating

20253240202530162229313318341216131923

nitrogen

36.233.930.727.036.233.931.638.035.332.031.230.237.129.839.838.039.536.635.0

lignin

12.315.222.324.111.313.717.59.6

13.919.217.518.110.219.86.69.27.1

11.113.4

total rosin

7.79.89.7

15.98.7

11.311.56.48.1

10.111.511.56.4

10.1

3.03.94.8

Residís polymer"

7.79.89.7

15.9435:65.72.12.73.35.75.72.13.3

1.01.31.6

Resiester T°

4.35.65.72.12.73.35.75.72.13.3

1.01.31.6

natural rosin

2.12.73.3

2.13.3

1.01.31.6

oil

2.03.42.05.05.47.02.94.04.8

0 Commercial producís from Unión Resinera Española, S.A.

2. Urea Reléase Rates in Water at a ConstantTemperature of 20 °C. 2.1. Series ULI, ULU, andULIII. Figure 1 shows urea reléase from the seriesULI, ULII, and ULIII, respectively, as a function oftime. The rate of urea reléase diminishes in all casesas the percentage of each series' coating ulereases. Asan exception, the ULI series is seen to behave in a fairlysimilar way to the two thickest coated products: ULI-32 and ULI-40. This may be due to the fact that, whenthe coating percentage reaches this order of magnitude,further increase barely alters the nutrient's rate ofreléase.

The reléase pattern for all of the fertilizers conformsto a first-order law. The reléase rate in this case isproportional to the mass of active agent containedwithin the device. The reléase rate is then given as

dM/dí = K(M0 - Mt) (1)

where MO is the mass of agent in the device at t = 0.Table 2 shows the valúes obtained for the first-order

reléase constant (K\) obtained for all fertilizers in theseries ULI, ULII, and ULIII, as well as their coefBcientsof correlation (r). As was to be expected, the valué ofK\s when the percentage of fertilizer coatingin each series increases. Series ULIII fertilizers showconsiderably lower KI valúes than fertilizers in otherseries with similar coating percentages.

Urea reléase in series ULIII fertilizers also conformsto a zero-order law. The reléase rate remains constantuntil the device is exhausted of active agent. Math-ematically the reléase rate, dMt/dt, from this device isgiven as

dM/dí = K (2)

where K is a constant, time is t, and the mass of activeagent released is Mt (Baker, 1987).

Zero-order reléase rate constants (feo) obtained for theULIII-16, ULIII-22, and ULIII-29 products are 3.73,2.69, and 2.07 days"1, respectively. Their coefficientsof correlation are significant at 0.001%, and feo reduceswith the percentage of coating, corroborating the factthat coatings improve when this percentage is increasedand reléase the nutrient more slowly.

100

80

80

40

8 8 1 0 1 2 * 4tima (days)

(b)

100

•o

80

40

20

ULI-20

%UREA *ol

ULJ-26 UU-32 ULMO

O 2 4 6 8 10 12time (days)

- ULII-20 -»- ULII-26 - ULII-30

(C)

e e 10time (days)

-*- UUII-22 -•

M «

- ULIII-29

Figure 1. Urea reléase in water, as a function of time, at aconstant temperature of 20 °C for (a) series ULI fertilizers withdifferent coating percentages; (b) series ULII fertilizers withdifferent coating percentages; (c) series ULIII fertilizers withdifferent coating percentages.

Table 2. Valúes of First-Order Reléase Constants andTheir Correlation Coefficients (r) Obtained by SimpleLinear Regression for the Series ULI, ULH, ULHI, ULCA,and ULA Fertilizers (n = 15)

series

ULI

ULE

ULIII

ULCA

ULA

fertilizer

ULI-20ULI-25ULI-32ULI-40ULII-20ULII-25ULII-30ULIII-16ULIII-22ULIII-29ULCA-13ULCA-19ULCA-23ULA-12ULA-16

ffi (days-1)

0.0990.0670.0590.0510.3160.1410.0890.0590.0350.0250.060.050.0430.0480.035

r*

-0.91***-0.87***-0.93***-0.94***-0.92***-0.90***-0.88***-0.98***-0.97***-0.99***-0.90***-0.94***-0.94***-0.98***-0.97***

*, significant at <0.001% level.

%UREA ioL

(a)

20

- ULII-30

e s 10time (days)

- ULIIA-31 -» ULIIA-33

(b)

«UREA sol.

e e 10time (days)

- uuihíe ULIII-2A -»- ULIIIA-18 -«- ULIIIA-34

Figure 2. Urea reléase in water, as a function of time, at aconstant temperature of 20 °C for (a) the ULII-30 fertilizer andthe series ULIIA fertilizers; (b) series ULIII and U1IIIA fertilizers.

The fact that the reléase kinetics of series ULIIIcomponente may be either a first- or zero-order patternis explained by taking into account the low magnitudeof the first-order reléase constants, involving smallvariations in the active agent's reléase rate.

It may be concluded that the most efficient coatingfrom a kinetic point of view is that of the series ULIIIand that the fertilizers releasing urea at the fastestrates are those of series ULII.

2.2. Series ULIIA and ULIIIA. Series ULIIA andULIIIA fertilizers were obtained by adding an externallignin and linseed oil coating on series ULII and ULIIIproducís. In fact, ULII-30 acted as a base for preparingtwo series ULIIA fertilizers. ULIIIA-18 and ULIIIA-34 were prepared from ULIII-16 and ULIII-29, respec-tively. Figure 2 shows that series ULIIA and ULULAfertilizers have slower solubilization process than the

Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 247

Table 3. Valúes of the Reléase Rate Constant and theCoefficient of Correlation Obtained by Simple LinearRegression for Series ULIIA and ULIIIA Fertilizers (n =15)

first order zero order

series

ULIIA

ULIIIA

fertilizer

ULIIA-31ULIIA-33ULIIIA-18ULIIIA-34

Ki(days-1)

0.0720.0610.0460.015

r"

-0.94***-0.96***-0.99***-0.98***

Ko(days-1)

3.583.463.251.32

r"

-0.89***-0.96***-0.98***-0.97***

" ***, significan! at <0.001% level.

Table 4. Crushing Strength of Prepared Fertilizers

crushing strength standardfertilizer (kg/grain) deviation

ureaULI-20ULI-25ULI-32ULI-40ULII-20ULII-25ULII-30ULIIA-31ULIIA-33ULIII-16ULIII-22ULIII-29ULIIIA-18ULIIIA-34ULA-12ULA-16ULCA-13ULCA-19ULCA-23

1.101.171.411.711.781.131.181.401.401.422.352.562.732.402.751.101.111.101.121.14

0.420.350.370.380.380.400.390.400.380.380.300.310.310.300.290.360.380.350.330.36

ULII-30, ULIII-16, and ULIII-29 from which theyoriginate.

Table 3 demonstrates that the reléase rate constantsof oil-sealed fertilizers also diminish when the coatingpercentage increases and that the coefficients of cor-relation of zero-order constants obtained from ULIIAand ULIIIA fertilizers are comparable to those of thefirst-order constants.

2.3. Series ULCA and ULA. The series of producíscalled ULCA was prepared by using a mixture of theethanol solution of three rosins used in the series ULIIIand linseed oil, in equal parís, for adhering lignin íourea. The adhesive is made of linseed oil only in íheseries ULA fertilizer.

The linear regression analysis of íhese íwo series(Table 2) provides a beííer correlation by processing thedata according to a firsí-order paííern. The valúes ofthe reléase raíe consíanís thus obtained are between0.06 and 0.035 days"1. They are íherefore of the sameorder of magnitude as those obtained for series ULIIIproducís. Nevertheless, íhe physical properties of íhesefertilizers relaíed ío íheir coating strengíh did noí provesaíisfacíory.

3. Physical Tests. Once the producís prepared wereevaluaíed by deíerminaíion of íheir reléase rate in waterat a consíant temperature, it was necessary ío alsoconsider íheir physical properties since handling andíransport operations depend on such. The propertiessíudied were as follows.

3.1. Grain Size. Grain size íesís showed íhat thefertilizer grain size in all cases is between 1 and 4 mm,in accordance with current Spanish legislation, whichrequires 85% of a fertilizer ío be beíween íhese limiís.

3.2. Crushing Strength. In Table 4, uncoaled ureagives a compressive strength of 1.10 kg/pellet, which is

248 Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996

Table 5. Coating Thickness of Some Series ULIII andULJIIA Fertilizers

fertilizer

ULIII-16ULIII-29ULIIIA-18ULIIIA-34

mean thickness (um)

125196139238

extreme valúes

104-139160-210125-149205-245

Figure 3. Surface and cross section of a ULIII-29 pellet (29%coating) magnified 1000 times (left) and 200 times (right).

less than the mínimum valué of 2.3 kg/pellet recom-mended by the TVA. The latter figure is reached onlyin the case of ULIII and ULIIIA producís, in which theincrease in coating means an increase in this valué. Inthe case of ULIIIA-34, the 2.75 valué obtained is thehighest of all and represents an increase of 59% overthat of the uncoated base substance.

In the light of such results, it may be concluded that,except for these fertilizers, the remaining producísprepared by coating urea pellets do not meet thephysical hardness properties required for handling andtransport, despite some of them having raised goodexpectations from the kinetic standpoint.

3.3. Apparent Density. Coated producís have ahigher apparent density than uncoated producís. Thisdensity increases with the coating percentage, andproducís with a similar perceníage have the sameapparent density.

3.4. Critical Relative Humidity and HumidityAbsorption Rate. In the case of uncoated urea,humidity absorpíion commences ai a criíical relaíivehumidity of 80.5%. This figure agrees with referenceresults (Hoffmeister, 1979). The fertilizer's coaíingincreased the valué of the critical relative humidity to88% in all cases.

The percentage of water absorbed by the differentproducts íesíed is differentiated according to the typeof coaíing and decreases wiíh increasing coaíing per-centage. The highest valúes are íhose of series ULIIproducís by far, according ío íhe kineíic results obtainedin water solubilization íesís. ULIII and ULIIIA showíhe smallesí water absorption percentages. Waíerabsorpíion as a funcíion of time is noticeably higher inuncoaíed urea and in series ULII fertilizers.

3.5. Microphotographic Coating Study. Thissíudy enables íhe coating's íhickness and homogeneiíyto be deíermined and was carried ouí wiíh ULIII andULIIIA fertilizers, which preved to be the mosí interesí-ing in íheir overall physical-chemical properties. Theextreme valúes and average thicknesses of the micro-phoíographically studied fertilizer coatings are given inTable 5. lí can be deduced from these data íhaí íhe

Figure 4. Surface of a ULIIA-18 pellet (18% coating) cut throughthe middle and magnified 250 times.

íhickness increases as íhe coating proportion increases.For íhis effect, the products shown in íhis table arecomparable with each other, since in all cases íhecoating is formed by íhe same basic components: ligninas íhe coaíing maíerial and a mixíure of íhree rosins(naíural, dimerized, and esíerified) as íhe adhesive.These valúes have been worked out from the micropho-íographs íaken, some of which are shown in Figures 3and 4.

NomenclaturaULI = series of fertilizers containing urea, lignin, and

polymerized rosinULII = series of fertilizers containing urea, lignin, and

polymerized and esterified rosinsULIII = series of fertilizers containing urea, lignin, and

polymerized, esterified, and natural rosinsULIIA = series of fertilizers containing urea, lignin,

polymerized and esterified rosins, and linseed oilULIIIA = series of fertilizers containing urea, lignin,

polymerized, esterifíed, and natural rosins and linseedoü

ULA = series of fertilizers containing urea, lignin, andlinseed oil

ULCA = series of fertilizers containing urea, lignin, po-lymerized, esíerified, and natural rosins, and linseed oil

Literatura CitedAOAC. Offícial Methods of Analysis, 15th ed.; Association of

Official Analytical Chemistry, Inc.: Virginia, 1990.Baker, R. W. Controlled Reléase Biologically Active Agents;

Wiley-Interscience: New York, 1987.Flaig, W. Soil organic matter as a source of nutrients. Int. Rice

Res. Inst. 1984, 73-91.Hignett, T. P. Fertilizer manual; Martinus Nijhoff and Dr. Junk

Publishers: Dordrecht, The Netherlands, 1985.Hoffmeister, G. Physical Properties of Fertilizers and Methods for

Measuring Them, TVA Bulletin Y-147, 1979.Jiménez, S. (Ed.) Fertilizantes de liberación lenta; Mundi Prensa:

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Jiménez, S.; Cartagena, M. C.; Vallejo, A.; Ramos, G. Kineticproperties of urea coated with rosin and tricalcic phosphate.Agrie. Med. 1993, 123, 47-54.

Kelly, W. J. Solids handling and metering in an NPK prilling plant.Proc. Fert. Soc. London 1974, 141.

Martin, J. P.; Haider, K. Effect of concentration on decompositionof some 14C-labeled phenolic compounds, benzoic acid, glucose,cellulose, wheat straw and Chlorella protein in soil. Soil Sci.Soc. Am. J. 1979, 43, 917-920.

More, A. I. Products and techniques for plant nutrient efficiencyproduction and use of slow reléase fertilizers, storage, handlingand transport of fertilizers. Proceedings of the Britísh SulphurCorporations. Second International Conference on Fertilizers;British Sulphur Corporation: 1978.

Stevenson, F. J. Humus Chemistry; Wiley-Interscience: New York,1982.

Trushkin, A. V.; Kadyrov, S. K. Effect of lignin stimulatingfertilizers on agrochemical properties of soils and on thenutrient regime of cotton plants. Gidroliz. Lesokhim. Promst.1978, 3, 6-7.

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Received for review January 19, 1995Accepted July 31, 1995®

IE950056F

* Abstract published in Advance ACS Abstraéis, November15, 1995.