A Rapid Microwave Digestion Method for Colorimetric

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A rapid microwave digestion method for colorimetric measurement of soil organic carbon a

K. R. Islam & R. R. Weil

a

a

Department of Natural Resource Sciences and Landscape Architecture , University of Maryland , College Park, MD, 20742 Published online: 11 Nov 2008.

To cite this article: K. R. Islam & R. R. Weil (1998) A rapid microwave digestion method for colorimetric measurement of soil organic carbon, Communications in Soil Science and Plant Analysis, 29:15-16, 2269-2284, DOI: 10.1080/00103629809370110 To link to this article: http://dx.doi.org/10.1080/00103629809370110

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COMMUN. SOIL SCI. PLANTANAL., 29(15&16), 2269-2284 (1998)

A Rapid Microwave Digestion Method for Colorimetric Measurement of Soil Organic Carbon1 K. R. Islam and R. R. Weil Department of Natural Resource Sciences and Landscape Architecture, University of Maryland, College Park, MD 20742

ABSTRACT Soil organic matter is an active component of agroecosystems. Rapid and precise measurement of organic carbon (Corg) is essential to monitor changes in organic matter and soil quality. A new semi-micro wet digestion method for the determination of Corg was developed using rapid microwave energy applied at 500 J mL-1 digestion mixture to enhance oxidation of Corg by K2Cr2O7 and conc. H2SO4. This proposed method and three existing methods of soil C determination were compared with the LECO dry combustion carbon analyzer. The r2 value for the proposed microwave method regressed against LECO C was 0.9913. The recovery of Corg by the rapid microwave digestion method for spectrophotometric measurement was 91.7±1.2% (conversion factor 1.09) of C measured by the LECO dry combustion method. Compared to the traditional Walkley-Black's method, the proposed spectrophotometric with microwave digestion method was rapid and more precise, used smaller reagent volumes, and produced less waste.

1

Mention of a particular brand is for identification purposes only and does not imply a preference over other brands with similar properties.

2269 Copyright © 1998 by Marcel Dekker, Inc.

www.dekker.com

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ISLAM AND WEIL


INTRODUCTION Soil organic matter greatly influences soil quality and productivity. The fact that organic matter affects a wide range of biological, chemical, and physical properties accounts for much of the interest in its measurement. Measurements of soil organic matter are usually based on determination of Corg. Several methods ranging from direct wet or dry combustion to indirect wet oxidation have been employed to measure Co . Although traditional combustion methods to measure Cor (which include ashes, charcoal, and carbonates) are accurate, they are slow and expensive to carry out (Nelson and Sommers, 1982). As a result, titrimetric methods have been developed that use wet oxidation reactions. Among the titrimetric variants, the Walkley-Black method has been employed widely because of its simplicity and rapidity. The method depends on heat generated by mixing the cone. H2SO4 with aqueous Cr(IV) to facilitate hydrolysis and oxidation of C or . The amount of Cor oxidized by Cr(VI) is measured after back-titration of excess of Cr(VI) with a reducing agent (generally ferrous sulfate or ferrous ammonium sulfate solution) in the presence of an indicator, or by colorimetric measurement of either the concentration of remaining Cr(VI) or the concentration of Cr(III) formed during the reaction (Walkley and Black, 1934; Schollenberger, 1945; Graham, 1948; Sims and Haby, 1971). Since oxidation of Corg is accompanied by reduction of Cr(VI) to Cr(III), accompanied by a distinct change in color, spectrophotometry may be used as a more rapid alternative to the titration method (Sims and Haby, 1971; Heanes, 1984). As both the Cr(VI) and Cr(III) species absorb visible light, the wavelength used in spectrophotometric measurement of Cor is an important consideration. Chromium (III) has two absorption peaks at 450 and 590 nm whereas Cr(VI) has an absorption peak only at 450 nm (Sims and Haby, 1971). However, at the maximum absorption peak of Cr(III) at 590 nm, there is no light absorption interference from Cr(IV) species. An advantage to measuring Cr(III) is that it exists as the stable hexaquo ion [Cr(H2O)6]+3 in an acid medium (Cotton and Wilkinson, 1988; Prokisch et al., 1995). A problem with Cr(IV) is that chloride interference consumes this species as CrO2Cl2, but does not similarly react with Cr(III) species (Metson et al., 1979). Spectrophotometric measurement of Cr(III) absorption of 590 nm light in chromate-acid soil mixtures is directly proportional to the amount of Corg oxidized (Sims and Haby, 1971). Measurements of Cor by wet oxidation methods have been criticized for their low and variable recovery and incomplete oxidation of Co using only the heat of reaction (Heanes, 1984). Aside from the effect of chemical composition or oxidation state of Co , the wet oxidation of Co depends primarily on the activity of an oxidant in the strong acidic medium and the final temperature obtained during digestion (Charles and Simmons, 1986; Orlov, 1992). Several workers have supplemented the heat of reaction with external heat to promote the oxidation of Co by Cr(VI) in presence of cone. H2SO4 (Schollenbeger, 1945; Allison, 1960;


RAPID MICROWAVE DIGESTION METHOD

2271

Heanes, 1984). Procedures using external heating to digest Corg increase recovery, but are generally slow and may cause thermal decomposition of Cr(VI) to Cr(III) if the digestion temperature exceeds 150°C (Heanes, 1984; Charles and Simmons, 1986; Vickery et al., 1995). Temperatures between 125 and 150°C are recommended to avoid both incomplete oxidation of Corg by Cr(VI) at low temperature and thermal decomposition of Cr(VI) to Cr(III) at high temperature (Charles and Simmons, 1986). We sought a more rapid and improved oxidation of Cor that eliminates troublesome titration, reduces digestion time and decreases thermal decomposition of Cr(VI). Since the heat of reaction raises the temperature of the dichromateacid mixtures sufficiently to induce a substantial oxidation of Cor within a minute or so (Walkley, 1947), the use of microwave (MW) energy seemed desirable. Microwaves are non-ionizing, electromagnetic energy that can raise temperature in any absorbing medium by dipole rotation and vibration of the molecules without changing the molecular structure (Neas and Collins, 1988). The MW energy is quickly absorbed by polar and ionic molecules, raising sample temperatures almost instantly. Rapid acid digestion of a variety of compounds has been achieved with MW energy in both open and closed vessels with results equivalent to or better than the results obtained by conventional heating methods (Nakashima et al., 1988; Neas and Collins, 1988). Thus, we hypothesized that (i) microwave energy-accelerated dichromate-acid digestion of soil will ensure a maximum oxidation of Corg in soil and ii) measurement of Cr(III) absorption peak at 590 nm would provide a reliable methodology to determine Cor in routine soil testing laboratories. MATERIALS AND METHODS Soils Used To test the proposed method for Corg determination, a wide range of soils were collected (0-15 cm depth) from sites under various management practices. Sites included cultivated and uncultivated adjacent paired fields in Bangladesh, Malawi, Sri Lanka, and Zimbabwe, and a replicated cropping system experiment in the United States in Accokeek, MD. For each site, about 10 soil cores were pooled together to obtain a composite sample which was then gently passed through a 2mm sieve to remove stones, roots, and plant residues. Subsamples of soil were air dried for at least 48 hours and ground with a mortar and pestle to pass a 0.2-mm sieve before C analysis. The soils ranged in pH from 4.6 to 6.5 (no free org carbonates) with Corg contents ranging from 0.3-4.2% (Table 1). Microwave Digestion Method for Soil Organic Carbon Measurement A General Electric 650-W commercial MW oven was used with energy at high power supplied by a magnetron operating at 2450 MHz in the continuous mode.

2272


TABLE 1. Description of the sites and selected characteristics of the soils.

pH

no.

Tillage and vegetation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

NT-Shorea robusta, L. NT-Acacia auriculiformis, L. NT-Acacia minijiri, L. NT-Tektona grandis, L. CT-Sugarcane CT-Jute/Rice/Mustard NT-Shorea robusta, L. CT-Rice/Jute CT-Cotton NT-Mullacana brucifera, L. CT-Rice/Jute/Maize CT-Napier grass NT-Indigenous grass CT-Rice/Jute NT-Mullacana brucifera, L. CT-Banana plantations CT-Near A. albida, L./Maize CT-Near A. albida, L./Maize CT-Under A. albida, L./Maize CT-Under A. albida, L./Maize CT-Under A. albida, L./Maize CT-Under A. albida, L./Maize CT-Near A. albida, L./Maize CT-Near A. albida, L./Maize

4.9 5.1 4.8 5.0 5.2 6.0 4.9 4.8 5.3 5.7 6.2 5.6 5.5 6.2 5.7 5.8 6.1 6.0 6.5 6.0 6.7 5.2 6.1 6.3

Soil

(1:2.5)

NT 1

(g kg ) 1.02 1.25 1.24 0.50 0.89 0.72 1.17 1.36 0.99 1.32 0.43 1.18 1.69 0.90 1.24 3.40 2.00 2.20 2.90 2.30 2.30 0.80 1.00 0.90

Clay (g kg"1)

Soil texture

150 250 250 350 200 210 210 300 300 290 250 260 250 260 230 677 242 345 429 359 277 361 232 167

SiL SiL SiL CL

L L SiL SiL CL CL L SiL SiL L L C SCL CL C CL SCL SCL SCL SCL

55 c S >, 3


25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

KT*-Brachestegia forest CT-Maize rotation ïn-Leucaena leucocephalia, L. CT-Cassava/Maize rotation CT-Cassava/Maize CT-Maize/Maize rotation CT-Maize/Maize rotation CT-Maize rotation CT-Under/4. albida, L./Maize CT-UnderA albida, L./Maize CT-Near/4. albida, L./Maize CT-Pigeon peas/Maize CT-Cassava/Maize/Peas NT-Wheat/soy/com RT-Wheat/soy/corn RT-Soy/wheat/clover/corn CT-Continuous corn NT-Continuous tall fescue CT-Maize and wingbean CT-Maize and wingbean NT*-Woodland/savannah NT*-Grassland/savannah CT-Continuous maize

6.3 4.6 5.9 5.6 5.2 6.0 5.4 5.2 5.7 5.0 5.2 5.1 5.0 6.3 6.2 6.5 6.3 6.1 5.0 4.6 4.9 5.1 5.6

2.70 0.70 1.80 0.40 0.24 0.50 0.70 0.20 1.90 1.40 1.00 1.20 0.80 0.73 0.62 0.87 0.81 1.70 0.60 0.70 0.10 1.30 0.40

481 704 319 169 228 445 383 196 276 263 260 289 223 100 102 103 101 100 112 276 38 89 171

SL C CL SL L SC SC SL

L SCL SCL SCL SCL SL SL SL SL SL SL SCL SL SL SL

CT=conventional tillage, NT=no-till, NT*=undisturbed natural system, RT=ridge tillage, C=clay, CL=clay loam, L=loam, SC=sandy clay, SL=sandy loam, SCL=sandy clay loam, SiL=silt loam and NT=total N. Soils: 1-15 from Bangladesh, 16-37 from Malawi, 38-42 from United States, 43-44 from Sri Lanka, and 45-47 from Zimbabwe.

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2274

ISLAM AND WEIL

The energy output into the MW oven cavity was determined by measuring the rise in temperature of 1,000 mL distilled water in a 2 L Pyrex glass beaker placed at the center of the cavity and heated continuously at full power for 2 min (Neas and Collins, 1988):


P = C p KATm/t

(V)

where P is the apparent power absorbed by the water sample (J s"1); Cp is the heat capacity of water (J ml/ 1 °K"'); K is a factor (4.184) to convert thermal chemical cal ml/ 1 °K-I to watts (J s 1 ); AT (°C) is the difference between final temperature and initial water temperature; m is the mass of the water (g); and t is duration (s) of MW energy application. Using this equation, the MW oven output at high power was calculated to be 640-W (J s-1) which is equivalent to an energy flux of 615 m J s"1 cnr2 of cavity. Several experiments were conducted to examine the effect of MW energy on oxidation and temperature rise in KjCr2O7 - cone. H2SO4 mixtures, alone or with glucose, sucrose, or soil. The MW energy was applied at 0, 100, 200, 400, 600, 800, and 1,000 J mL 1 of digestion mixture to determine the energy level that would achieve a temperature around 140°C. These energy levels were achieved by 0, 15, 30, 60, 90, 120, and 150 s of MW applied at 640 J s 1 . To construct standard curves, 0.50 mL of standard solution (containing either 0,0.25,0.50,1.0,2.0,5.0, or 10.0 mg of sucrose C) was added to 50 mL Erlenmeyer flasks, followed by 5.0 mL of 0.17M K2Cr2O7 and 5.0 mL of cone. H2SO4. Each flask was covered with a 25-mm diameter short stem glass funnel to promote refluxing and placed on a mechanical turntable to ensure a uniform rotation of the samples in the oven cavity. The MW energy was applied at 500 J mL 1 of digestion mixture which is equivalent to running the MW oven (640 W) for 75 s when using a total of 100 mL of digestion mixture. After digestion, the flasks were cooled and the contents were transferred to Folin-Wu tubes and brought to 3 0 mL with distilled deionized water. Absorbance of the digestate was measured at 590 nm by a spectrophotometer. For spectrophotometric analysis with microwave digestion of Cor (MW-Spec), exactly 0.20 g oven-dried equivalent of 0.2 mm sieved air-dried soil was added to 50 mL Erlenmeyer flasks, and the procedure followed as for the sucrose C standards. After digestion, the digests were allowed to cool, and diluted to 30 mL with distilled deionized water. The diluted contents were transferred to polycarbonate tubes and centrifuged at 5,000 rpm for 5 min. About 10 mL of supernatant was poured into a glass cuvette to measure the absorbance at 590 nm in a spectrophotometer. Standard Methods Used for Soil Carbon Measurements Several commonly used methods were compared to the proposed MW-Spec method of Corg analysis.


2275

LECO Dry Combustion Method As a reference method, Co was measured in all soils by the LECO dry combustion CHN analyzer (LECO, 1988). Samples of 0.1 to 0.2 g ODE of airdried soil in Zinc capsules were analyzed in the LECO CHN analyzer which uses infrared spectroscopy to measure CO2-C liberated by high temperature combustion.


Heat of Solution Method The Walkley-Black wet oxidation method (with only heat of solution) was used to measure Corg in air-dried soils by measuring the absorbance of Cr(III) at 590 nm, using sucrose C as standard (NH-Spec) and by using back titration with 0.5M ferrous ammonium sulfate in presence of ferroin indicator (NH-Tit) to measure unreacted Cr(IV). External Heating Method Heanes (1984) spectrophotometric with hot-plate digestion (HP-Spec) method was used to measure Corg. Exactly 0.20 g ODE of air-dried soil was placed into 50 mL Folin-Wu tubes along with 5 mL of 0.17M KjCr2O7 and 5 mL of cone. H2SO4. The mixture was digested under a fumehood using an aluminum block holder placed on a hot plate preheated to 135±3°C. A 25-mm short-stem glass funnel was placed on the mouth of each tube to promote refluxing of the dichromateacid-soil mixture. The digests were allowed to cool, transferred to polycarbonate tubes and centrifuged as for the MW-Spec method. Absorbance of the digests at 590 nm was determined by a spectrophotometer. Statistical Analysis For all Co analytical methods, soils collected from fields in Bangladesh, Malawi, Sri Lanka, and Zimbabwe were analyzed in duplicate except LECO dry combustion. The Co measured by titration and spectrophotometric methods were not corrected with recovery factors. The variances of Co analyses were calculated for each soil and for each method. Regression and correlation analyses were used to compare the performance of MW-Spec method with the LECO method to measure Co . Statistical analyses were done at/K0.05 level. RESULTS AND DISCUSSION At all MW energy levels, the digestion temperature was about 5°C lower with soil than in the reagent blank (Figure 1). With soil (and to a lesser degree with sucrose), KjCr2O7 is reduced upon oxidizing a relatively complex organic C and the presence of organic C may reduce the possibility of thermal decomposition of Cr(IV) to Cr(III) more than in a reagent blank. The KjCrjO^conc. HjSO4 mixtures, alone or with glucose (C6U12O^, sucrose (C^H^O,,) or soil, all reached the boiling


2276

ISLAM AND WEIL

200

400

600

800

1000

MW energy applied G"/mL) FIGURE 1. Microwave energy applied and temperature rise in dichromate-sulfuric acid mixtures alone or with carbon from glucose, sucrose, or soil.

point with MW energy applied at 500 J mL"1 of digestion mixture. When the MW energy was applied at more than 500 J mL 1 , digestion temperatures increased above 140°C. Heanes (1984) reported that thermal decomposition of KjCr2O7 occurred rapidly above 150°C. Therefore, MW energy applied at 500 J mL"1 of digestion mixture was selected for Cor oxidation. Sucrose was selected as a standard organic C source to use in the measurement of C or , because sucrose C represents a carbohydrate analogous in composition to Corg (Metson et al., 1979; Stevenson, 1994). Compared to sucrose, glucose C standard solutions gave relatively higher Cr(III) absorbance values at high concentrations (Figure 2). Light absorption by the Cr(III) generated when sucrose C standards are oxidized by dichromate consumption methods, with or without external heating, are presented in Figure 3. The data suggest an improved oxidation of sucrose C by Cr(VI) in the presence of cone. H2SO4, and more nearly stoichiometric formation of Cr(III) by the HP-Spec and MW-Spec external heating methods than by the NH-Spec method. The mean values of the Co measured by five methods are presented in Table 2. Averaged across the soils, the LECO C was 14.94 g kg 1 , NH-Tit C was 8.37 g kg 1 , NH-Spec C was 11.45 g kg', HP-Spec C was 14.39 g kg 1 and MW-Spec C


2277

2.0

1.6 c o

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• C 6 H 12 0 6 =0.164«X r 2 =0.9998

O C 12 H 22 0ii=0.148*X r 2 =0.9999

1.2


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0

2 4 6 8 Organic C standards (mg)

10

FIGURE 2. Relationship between glucose or sucrose C absorption of 590 nm light by Cr(III).

2.0 • HP-Spec=0.159*X

1.6

r2=0.9999

c o

O MW-Spec=0.148»X r 2 =0.9999 A NH-Spec=0.119*X r 2 =0.9998

-p Q. l_

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0.8

0.4

0.0

0

2 4 6 8 Sucrose C standards (mg)

10

FIGURE 3. Relationship between glucose or sucrose C absorption of 590 nm light by Cr(III) with HP-Spec, MW-Spec, and NH-Spec methods.

2278


TABLIï 2. Soil organic C (g kg 1 ) as determined by• five methods. Soil

no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

LECOdry combustion 9.7 7.3 18.1 3.8 7.4 7.4 12.5 17.6 8.3 11.3 4.8 9.1 13.0 12.0 15.9 40.3 32.1 17.5 41.6 34.6 30.8 14.3 13.5 9.1

Titration 8.3 6.1 16.5 3.2 6.3 6.5 9.0 15.6 7.4 10.3 4.0 8.0 10.0 10.2 12.9 xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx

NH 8.1 6.3 16.0 3.0 6.1 6.4 9.1 15.0 7.3 9.2 3.8 7.9 10.2 10.8 13.6 28.3 24.8 8.0 29.0 25.4 23.8 7.3 8.5 7.5

Spectrophotometric MW 9.1 6.9 17.2 3.4 7.3 7.0 11.3 17.3 8.0 10.5 4.1 9.1 12.5 11.7 15.9 36.3 30.2 15.1 36.1 31.3 26.6 11.1 13.6 9.1

HP 8.7 7.3 18.0 4.0 7.2 7.0 12.0 17.4 8.0 11.0 5.0 8.9 12.6 11.5 15.3 39.6 30.6 16.3 38.6 32.4 28.8 12.0 12.9 9.0

e


25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 CV(%)

31.3 19.4 23.7 6.5 4.1 8.6 7.5 3.3 37.9 26.0 20.8 23.2 12.1 8.9 8.8 8.3 7.4 13.0 4.9 4.7 3.9 20.8 5.1 —

xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx

6.4 5.6 4.8 4.6 11.6 xxxx xxxx xxxx xxxx xxxx

8.6

26.2 14.9 19.9 5.5 3.6 7.3 6.6 2.5 26.5 22.1 19.8 17.2 7.4 6.4 5.7 4.9 4.7 11.6 3.8 4.3 3.1 14.4 4.8 6.7

30.1 16.4 21.9 6.5 3.6 8.0 7.4 2.9 36.2 23.4 19.6 21.9 11.2 8.4 8.2 8.3 7.3 12.9 5.3 4.8 3.9 16.9 5.5 3.8

29.9 17.4 23.6 6.8 4.2 8.0 6.9 3.4 38.4 22.8 20.9 22.8 11.8 8.6 8.5 8.3 7.4 12.9 5.2 4.4 3.4 21.4 5.3 3.1

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NH-Spec=no heat-spectrophotometric, MW-Spec=microwave energy digestion-spectrophotometric, HPSpec=hot plate digestion-spectrophotometric, x=not analyzed. to VO

2280

ISLAM AND WEIL


TABLE 3. Relationship among different methods of soil carbon analysis. Methods used NH-Tit NH-Spec HP-Spec MW-Spec MW-Spec

Regression equation (y=bx) Co™ = 0.830 »LECOC O r , = 0.755 »LECOC O r , = 0.960 »LECOC O r , = 0.917 •LECOC Cc I = 0.954 * HP-Spec CQ, C C C

r2 0.9321 0.9562 0.9948 0.9913 0.9922

No. of samples 20 47 47 47 47

NH-Tit=titration, NH-Spec=no heat-spectrophotometric, HPSpec=hot plate digestion-spectrophotometric, MW-Spec=microwave energy digestion-spectrophotometric.

was 13.85 g kg"1. The LECO dry combustion method always measured higher C values than the other methods. The biggest differences among methods were for African soils (Malawi and Zimbabwe) subjected to annual fire, suggesting that the LECO method may have included some charcoal C not included by the other methods. The relationships between Cor measured by the LECO dry combustion and that measured by the NH-Tit, NH-Spec, HP-Spec and the proposed MW-Spec method are presented in Table 3. As the intercepts of the relationships were not significantly different from zero, regression equations were used that forced the line through the origin. The Cor measured by the MW-Spec correlated better (Figure 4) with the LECO C (r 2 =0.9913) than did the NH-Tit (r 2 =0.9342) or the NH-Spec (r2=0.9562) methods (Table 3). There was a close relationship (1^=0.9922) between the MW-Spec method and the HP-Spec method. Of the dichromate consumption methods, the NH-Tit and NH-Spec, which oxidized Cor with only heat of dilution, gave by far the lowest recoveries of Corg (Table 4). Based on the recovery percentage given in Table 4, the C as measured by the four methods tested should be multiplied by a correction factor as follows: NH-Tit x 1.214, NH-Spec x 1.324, HP-Spec x 1.042 and MW-Spec x 1.09. Therefore, it is recommended that a factor of 1.09 be used to convert MW-Spec measured chemically oxidizable organic C to Corg. The convenience of the methods tested in this study is described in Table 4. The rapid MW-Spec, HP-Spec and LECO dry combustion methods to measure Cor gave results that were not significantly different from each other. The dichromate consumption with MW-Spec method produces much less waste (3 L 100 1 samples) than the Walkley and Black's heat of reaction methods (14-17 L).


2281

50

cn 40 O

Y=0.917*LEC0 C o r g r 2 =0.9913 (N 47)


30 0) 0 O O