The values of organic matter of 38 organic wastes obtained by the loss-on-ignition method at 430°C have been correlated with both total organic carbon ...
Bioresource Technology 44 (1993) 203-207
RELATIONSHIPS BETWEEN ORGANIC MATTER A N D CARBON CONTENTS OF ORGANIC WASTES A. E Navarro, J. Cegarra, A. Roig & D. Garcia Consejo Superior de Investigaciones Cientificas, CEBAS, PO Box 4195, Murcia, Spain (Received 16 July 1992; accepted 30 October 1992)
against this method: mass loss of C02 from soil carbonates and loss of hygroscopic and structural water from clay minerals. Depending on the ignition temperature, this can be the cause of substantial errors in the process. A survey of the available literature provides no consensus on the time and temperature to be used for ignition. Davies (1974) recommends calcination at 430°C for 24 h, Ben-Dor and Banin (1989) suggest calcination at 400°C for 8 h, while Donkin (1991) thinks that 450°C and 6 h are the most suitable conditions, all being of the opinion that such methods avoid most of the interferences associated with loss of clay structural water and the presence of carbonates. Other authors use higher temperatures: Storer (1984) uses 500°C and Gallardo and Saavedra (1987) 600°C, while Imasuen et al. (1989) recommend 1025°C. Another way of calculating the organic matter of soils consists in multiplying the organic-carbon content by the empirically obtained and widely used factor of 1.724 (Schulze, 1949), on the assumption that approximately 58% of soil organic matter is organic carbon. Numerous later publications have dealt with this matter, and other values have been proposed for the factor (Broadbent, 1953; De Leenheer et al., 1957; Howard, 1965). However, there is very little information on methods of determining organic matter in organic wastes or concerning the search for factors or correlations between the carbon and organic-matter contents in these materials. The only work found on this topic is by Giovannini et al. (1985), who proposed the value of 1.68 in order to transform the organiccarbon content obtained by the wet-oxidation-backtitration method with potassium dichromate into the organic-matter content in seven samples of aerobically digested sewage sludges. In this work, we used a much larger number and a much wider selection of organic wastes, some of which contain carbonates, and propose the gravimetric losson-ignition method at 430°C as a way of determining the organic-matter content. The soundness of the method was checked by an analysis of the residual organic carbon in the ashes obtained by calcination. In
Abstract
The values of organic matter of 38 organic wastes obtained by the loss-on-ignition method at 430°C have been correlated with both total organic carbon measured by elemental microanalysis and oxidable organic carbon determined by the traditional method of Walkley and Black, duly adapted to organic materials. Calcination at 430°C for 24 h is the most suitable method for determining the organic-matter content of organic wastes. Linear and quadratic equations for the correlations between organic matter and carbon were tested, and high values of r, significant at the 99"9% probability level, were found. The use of the linear equations is recommended to transform either carbon content into organic matter or vice- versa easily. Key words: Organic wastes, organic matter, oxidable carbon, total carbon.
INTRODUCTION
The organic-matter content of a soil is an important characteristic used for the assessment of many of its physical, chemical, and biological properties and for estimating its nutrient status. The increasing production of organic wastes and their recycling and use in agriculture, either as amendments or as soil fertilizers, has led to the need for finding analytical methods, similar to those employed for soil, for the determination of their organic-matter content. The gravimetric loss-on-ignition method may be considered as one of the most accurate methods for determining the organic-matter content of soils. It is simple, does not need highly skilled personnel or advanced apparatus, and can be conducted on a routine basis on relatively large numbers of samples. However, two main criticisms have been levelled
Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 203
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A. F. Navarro, J. Cegarra, A. Roig, D. Garcia
addition, an attempt was made to correlate the values for organic matter obtained by the above method with those of total organic carbon measured by elemental microanalysis and with those of oxidable organic carbon determined by the traditional method of Walkley and Black (1934), duly adapted to organic materials.
METHODS
Thirty-eight organic materials were chosen to give a wide selection of urban and agricultural wastes and industrial by-products. They were divided into four groups: city refuse (CR), sewage sludges (SS), manures (MA), and plant residues (PR). The CR group consisted of five materials of different degrees of maturity. The SS group was made up of five samples, one of which had undergone anaerobic digestion (SS1) and four of which had been treated aerobically. The MA group was formed of two rabbit (MA1, MA2), four sheep (MA3-MA6), and five poultry manures (MA7-MA11 ). The PR group was the most numerous and diverse and consisted of 17 materials, which included crop residues, forestal wastes, and by-products of the agro-food industry. The samples were air-dried, ground to < 0.02-ram sieve size, and kept until the carbon and organic-matter analyses were carried out. The total-organic-carbon (TOC) determinations were made by using an automatic microanalysis method (Navarro et al., 1990), which involved a previous acid treatment of the samples (4-6 mg) to avoid interference from the carbonates. The residual organic carbon (ROC) was determined from the ash after ignition of the organic wastes, although greater amounts of sample (25-30 mg) were taken, since the content of organic carbon in the ash was expected to be low. The content of inorganic carbon (IC), mainly from carbonates, was taken as the difference between the values of carbon determined by the microanalysis method in samples that had not undergone the acid treatment and the values of TOC. The oxidizable organic carbon (OXC) was determined according to the method of Walkley and Black (1934) as follows: A 50-rag sample was placed in a 500-ml Erlenmeyer flask, and M/6 K2Cr207 (10 ml) and concentrated H2SO 4 (20 ml) were added. The mixture was then shaken vigorously for 30 s, and, after half an hour, the dichromate excess was titrated after the addition of 10 ml of concentrated H3PO 4 with 0-5 M Fe(NH4)2(804) 2 .6H20. A blank was also run with 10 ml of Kzfr207 and 20 ml of concentrated HzSO 4. The OXC content, expressed as a percentage by weight of the sample used, can be calculated by the expression % OXC=0"15 x f x ( B - A )/W, where A and B are the volumes (ml) of the Fe(NH4)2(SO4)2.6H20 solution used to titrate the excess of K2Cr207 sample and blank analysis, respectively, f is a correction factor arising
from the titration of the flesh K2Cr207 solution with Fe(NH4)2(SO4) 2 . 6H20, and W is the weight (g) of the sample. The content of organic matter in wastes was taken as the gravimetric loss-on-ignition produced by ashing the samples (previously dried in an oven at 105°C until a constant weight was reached) in a Heraeus Thermicon T muffle furnace for 24 h at 430°C (OM-1). An ashing at 600°C for the same period of time was also carried out on a parallel set of samples, the gravimetric losson-ignition being denoted by OM-2 in this case. The crucibles containing the ashes were kept dry for further analysis of their residual organic carbon (ROC). The OM-1 values were corrected by using the corresponding ROC values, multiplying the OM-1/TOC value of each material by its ROC value, and adding the product obtained to OM-1. The values thus obtained were called OM-1C and, together with those of OM-1, were taken to establish the correlations between the organic-matter and carbon contents of the wastes. All the determinations for organic matter and carbon were run in triplicate. RESULTS AND DISCUSSION Carbon content Table 1 shows the TOC and OXC values of the organic wastes, the former always being higher than the latter. The IC values, expressed as percentages of CaCO 3, show that of the 38 waste materials examined, thirteen contained carbonates, manure MA5 showing the highest value (20"8%). The ROC content of the ash after calcination at 430°C was generally low and was only higher than 0"5% of the dry weight of the organic sample in six wastes (SS1, SS3, MA2, MA3, MA4, and MA5). The highest value (1"16%) was found in the ash of sample MA5, and this also represents the maximum value (4-70%) if expressed as a percentage of TOC. An analysis of ROC in the ashes obtained by calcination at 600°C (data not shown) revealed only traces of organic carbon, never exceeding 0.1% of the weight of the organic wastes. Organic-matter content The OM-1 values were generally quite similar to those of OM-2 (Table 2). However, samples SS1, MA2, MA3, MA4 and MA5, which had the highest carbonate content (Table 1), also showed clear differences between the organic-matter content obtained at 430°C and that obtained at 600°C. These samples, furthermore, can be classified among those which preserved a high ROC after calcination at 430°C (Table 1). This, together with the probable partial decomposition of the carbonates at 600°C, may explain the differences observed. This suggests that calcination at 430°C for 24 h is the most suitable method for determining the organic-matter content of organic wastes, since ROC values detected after calcination in these conditions can generally be considered of little importance and
Organic matter and carbon contents of wastes
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Table 1. Values of total organic carbon (TOC), oxidizable carbon (OXC), residual organic carbon (ROC), and inorganic carbon (IC) in the organic wastes (%of dry matter) Material
TOC
OXC
ROC(1)*
ROC(2)*
IC(3)*
CR1 CR2 CR3 CR4 CR5
25"51 29"93 33.14 32.59 31.88
23.04 27-36 32-01 26.66 28.39
0"10 0.40 0.13 0.22 0.09
0.38 1.33 0.39 0.68 0.28
0-0 0-0 0-0 5-0 11.7
SS 1 SS2 SS3 SS4 SS5
20.87 25.86 28.99 29"72 29.05
17.17 21.24 23.14 24.77 21.67
0.82 0.19 0-80 0-13 0"22
3.94 0.75 2.77 0.43 0,75
15.0 0.0 0"0 0"0 4.2
MA1 MA2 MA3 MA4 MA5 MA6 MA7 MA8 MA9 MA10 MA11
39"98 28"14 34.06 33"16 24.62 40.56 41"79 40-55 38.10 39"09 31.68
35"41 24"96 30"48 28"63 20"11 37'01 37.50 34.13 32.50 33.67 25.65
0"08 0"95 0"62 0"88 1-16 0-07 0"14 0.12 0.21 0-25 0.40
0.21 3.38 1"82 2.65 4.70 0.17 0.32 0.30 0.54 0-64 1.25
7.5 15"0 17.5 13.3 20.8 4.2 4.2 0-0 0.0 7.5 0.0
PR1 PR2 PR3 PR4 PR5 PR6 PR7 PR8 PR9 PR10 PR11 PR12 PR13 PR 14 PR15 PR16 PR17
48.63 40.89 44.36 47.99 45.63 47.21 49.01 37"35 49.40 44.81 49"66 44"11 56-65 48-21 53"49 38-03 36.47
44.92 38.66 40.47 42.74 41 '33 41.70 42.79 34.54 46.82 39'88 48.63 41.12 50'06 43.54 47.80 33"92 34-10
0-03 0-01 0"01 0"01 0.01 0-04 0"02 0"01 0.04 0"09 0"01 0"14 0"08 0.04 0"00 0-03 0-04
0.06 0.02 0.02 0.02 0.02 0.09 0-04 0.03 0.08 0.21 0.01 0.32 0.14 0.08 0.00 0.07 0.11
9.2 0.0 0"0 0"0 0"0 0.0 0"0 0"0 0"0 0-0 0.0 0.0 0'0 0"0 0"0 0"0 0'0
*( 1 ) % in the organic wastes after calcination at 430°C. (2) % of TOC in the organic wastes. (3) as % of CaCO3 in the organic wastes.
decomposition of the carbonates minimal or even nil (Davies, 1974).
Relationships between organic matter and carbon Table 2 shows the values of the O M - 1 C / T O C and O M - 1 C / O X C ratios for all the organic wastes. T h e first ratio ranged between 1-71 and 2.19, and the second one fluctuated between 1.93 and 2"52, the average values of the two ratios being 1"94 and 2.22, respectively. T h e average O M - 1 C / O X C ratio is clearly different from that reported by Giovannini et al. ( 1985) for aerobically digested sludges (1.68). In order to obtain m o r e accurate relationships between organic matter and carbon in the wastes, linear and curvilinear relations were established between both parameters and the equations shown in Table 3 obtained. T h e linear equation y = a + bx and
the quadratic equation y = a + b x +cx 2, were tested and high values of r, significant at the 99.9% probability level, found. T h e s e values were practically the same for the linear equation when OM-1 or O M - 1 C was correlated with the T O C and O X C contents, respectively. Similar results were found with the quadratic equation, confirming the accuracy of the method here r e c o m m e n d e d for determining the organic matter. No substantial differences were noted in the values of the standard error of estimate, although this statistical parameter was slightly smaller in the quadratic equations than in the linear regressions. As shown in Table 3, a, b, and c were significant at different probability levels in the quadratic equations, but the values of the intercept (a) were not significant in three of the linear equations. This means that any of the reported quadratic equations can be considered suit-
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A . F . Navarro, J. Cegarra, A . Roig, D. Garcia
Table 2. Loss-on-ignition obtained by ashing at 430"C and 600°C and organic matter/carbon ratios in the organic wastes (% of dry matter) Material
OM- 1*
OM- 1C*
OM-2*
OM- 1C/TOC*
OM- 1C/OXC*
CR1 CR2 CR3 CR4 CR5
51.52 60-00 65"03 56.27 59.46
51.71 60"80 65"29 56"65 59"63
53"35 62-11 67"96 58.44 61-56
2.03 2.03 1.97 1.74 1.87
2.24 2.22 2.04 2.13 2.10
SS 1 SS2 SS3 SS4 SS5
34.71 43.79 51.34 52.12 50'81
36.02 44-12 52.76 52.35 51.19
38"72 46.51 52.55 54.39 53.13
1'73 1.71 1'82 1.76 1'76
2.12 2'08 2.28 2.11 2.36
MA1 MA2 MA3 MA4 MA5 MA6 MA7 MA8 MA9 MA10 MA11
83"70 56.14 62.34 58-77 45.96 82.82 85"93 84.56 80"04 84.27 58"68
83"88 58.04 63.48 60-33 48.12 82.96 86.21 84.81 80'47 84.81 59.42
84-14 59.37 66"76 63"09 51.11 83"51 86"60 85"76 81.33 86.03 59"76
2.10 2.06 1.86 1.82 1.95 2.04 2.06 2.09 2.11 2'17 1.87
2.37 2.32 2.08 2.10 2.39 2.24 2.30 2.48 2.48 2.52 2'32
PR1 PR2 PR3 PR4 PR5 PR6 PR7 PR8 PR9 PR10 PR11 PR12 PR 13 PR14 PR 15 PR16 PR17 Average
86"56 85"08 90.37 94.44 94-05 91"64 89.21 76.98 96.87 80'19 98"33 96"51 97-15 96"83 95"86 79.48 75"38
86.61 85'09 90"38 94.45 94.07 91"71 89.25 74.01 96.94 80'36 98.34 96'75 97"28 96"91 95"86 79.53 75.47
86"98 85"10 90.49 94.63 94.18 91"76 89'73 74.56 97.01 90'59 98.45 96'80 97.36 97'15 96'31 80'06 76.54
1'78 2.08 2'04 1.97 2.06 1.94 1.82 1.98 1.96 1"79 1.98 2.19 1.71 2.01 1.79 2.09 2-07 1.94
1.93 2.20 2.23 2.21 2.28 2'20 2.09 2.14 2.43 2.18 2.03 2"35 1-94 2.23 2.01 2.34 2.21 2.22
*OM-1: organic matter obtained by ashing at 430°C. OM-1 C: corrected OM-1, taking into account the residual organic carbon. OM-2: organic matter obtained by ashing at 600°C. TOC: total organic carbon. OXC: oxidizable carbon.
Table 3. Organic matter versus carbon in the organic wastes OM- 1 OM-1 OM-1C OM-1C
TOC TOC TOC TOC
- 0"95 Ns - 53.42*** 1"06 Ns -49"21"**
1.95"** 4.82*** 1'91"** 4.66***
OM-1 OM-1 OM-1C OM-1C
OXC OXC OXC OXC
7-05 Ns -29"54* 8"18" -28-74*
1'96"** 4"27*** 1'95"** 4'29***
- 3.73 x 10 -2*** - 3 . 5 7 x 10 -z*** - 3 " 4 1 x 10 -2** - 3.49 x 10 -2.*
y = a + bx y =a "l- b x -[- c x 2 y=a +bx y=a+bx+cx 2
0.954**** 0"969**** 0.954**** 0.968****
5.51 4.62 5.43 4.60
y = a + bx y=a+bx+cx y=a+bx y=a+bx+cx
0.956**** 0-967**** 0.950**** 0.962****
5.51 4.73 5"63 5.02
2 2
*, **, ***, ****, N S = levels of significance greater than 0"1, 0'05, 0"01, 0"001, and not significant, respectively.
Organic matter and carbon contents of wastes
able for transforming the T O C and O X C values of the wastes into organic-matter content. Nevertheless, the use of the linear equations can be r e c o m m e n d e d for convenience, and even the intercept value can be ignored when T O C values are being transformed into organic-matter content. In this case, the use of the regression coefficient (1"91) obtained from the correlation between O M - 1 C and T O C can be sufficient if the values of the organic-matter content do not need to be very exact. To transform O X C content into organic matter, the equation ( O M - 1 C ) = 8-18 + 1"95OXC can be recommended. T h e linear equations found for the correlations OM-1 versus T O C and OM-1 versus O X C can also be employed for the rapid calculation of the values of both T O C and O X C parameters in the wastes by using their respective inverse equations: T O C = 0-51 ( O M - 1 ) + 0.48 O X C = 0-51 (OM-1) - 3.59 Thus the very simple routine analytical determination of the organic matter in wastes by loss-on-ignition at 430°C can easily be transformed into the corresponding carbon values.
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ignition' method. Commun. Soil Sci. Plant Anal,, 20, 1675-95. Broadbent, F. E. (1953). The soil organic fraction. Adv. Agron., 5, 153-83. Davies, B. E. (1974). Loss-on-ignition as an estimate of soil organic matter. Soil Sci. Soc. Amer. Proc., 38, 150-1. De Leenheer, L., Van Hove, J. & Rurjmbeke, M. (1957). Determination quantitative de la matirre organique du sol. Pedologie, 7, 324-47. Donkin, M. J. ( 1991 ). Loss-on-ignition as an estimator of soil organic carbon in A-horizon forestry soils. Commun. Soil Sci. Plant Anal., 22,233-41. Gallardo, J. F., Saavedra, J., Martin-Patino, T. & Millan, A. (1987). Commun. Soil Sci. Plant Anal., 18,699-707. Giovannini, G., Riffaldi, R. & Levi-Minzi, R. (1985). Determination of organic matter in sewage sludges. Commun. Soil Sci. Plant AnaL, 16,775-85. Howard, P. J. A. (1965). The carbon-organic matter factor in various soil types. Okios, 15,229-36. Imasuen, O. I., Fyfe, W. S., Olorunfemi, B. N. & Asuen, G. O. (1989). Zonal mineralogical/geochemical characteristics of soils of Midwestern Nigeria. J. African Earth Sci., 8, 41-4. Navarro, A. F., Cegarra, J., Roig, A. & Bernal, M. P. (1991). An automatic microanalysis method for the determination of organic carbon in wastes. Commun. Soil Sci. Plant AnaL, 22, 2137-44. Schulze, F. (1949). Anleitung zur Untersuchung der Ackererden auf ihre wichtingsten physikalisches Eigenschaften und Bestandteille. J. Prakt. Chem., 46, 241-8. Storer, D. A. (1984). A simple high sample volume ashing procedure for determination of soil organic matter. Commun. Soil Sci. PlantAnal., 15, 759-72. Walkley, A. & Black, I. A. (1934). An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci., 37, 29-38.
