Determining Carbon in Soil Waste, Soil, Sediment and Sludge

The total organic carbon (TOC) content is a vital indicator for numerous applications. In agricultural science, carbon is a crucial parameter to gain an insight about the element cycling of sediments and soils. Organic carbon enters through the decomposition of animal and plant residues and acts as the chief source of nutrients for plants and microorganisms. TOC analysis, thus, provides vital information about microbiological activity and organic matter to characterize and assess soil and sediment.

Measuring TOC directly is a non-trivial analysis. Regularly total carbon is measured, and then non-organic carbon sources are subtracted. Besides organic carbon, inorganic carbon also exists in soils and sediments, commonly in the form of carbonates. The two most typical sources of carbonate in soils and sediments are the minerals dolomite and calcite. The corresponding bulk parameter, total inorganic carbon (TIC), includes not only these minerals but also other carbonate derivatives, such as bicarbonate and carbonic acid. Inorganic carbon can be differentiated from organic carbon as it is not an accessible form of carbon for biological systems, i.e. it is not bio-available.

Determining Carbon in Soil

Often it is not sufficient to just distinguish between TOC and TIC. Elemental carbon (ROC) is an additional common source of carbon, which is also not bioavailable. Particularly in solid wastes, but in other applications as well, separately measuring this third type of carbon offers a much more accurate determination of bio-available, and thus environmentally applicable, carbon sources.

TOC in Solid Samples

  • soli TOC cube
  • rapid CS cube
  • vario MAX cube
  • vario EL cube
  • acquray

Determining Carbon

Method of Choice

For the analyzes of TOC different techniques can be used.

Method 1: Determination of TOC at Combustion Temperatures over 900 °C with Prior Acidification

The classic TOC determination in soil, sediment, sludge and solid waste uses high temperature combustion at temperatures over 900 °C. In the presence of oxygen, carbon is converted into CO2 and then measured by a thermal conductivity or infrared detector. To remove the TIC, the sample is acidified before the measurement. The carbonate in the sample is converted into CO2 in the presence of mineral acid, which is removed during a drying step over several hours at higher temperatures. The procedures and principles are described in various standards, including ISO 10694, DIN EN 13137 and DIN EN 15936.

Method 2: Determination of TOC at Lower Combustion Temperatures

To overcome the time consuming acidification step to eliminate the carbonates, lower combustion temperatures can be used. By choosing an ideal combustion temperature, it is possible to reduce inorganic carbon loss while maintaining total recovery of organic carbon. Combustion temperatures from 450 °C to 650 °C have been demonstrated to give satisfactory results (For example Pitt et al, 2003).

Method 3: Determination of Different Forms of Carbon Using Temperature Programming

When assessing solid waste for example, it might be essential to establish the ROC separately, as elemental carbon is not bio-available. Therefore, a temperature ramp technique is used: TOC is determined at 400 °C, ROC between 400 and 600 °C, and TIC between 600 and 900 °C. The sample is heated at a rate of 70 °C per minute to the designated temperature, and then maintained for a particular hold time. The CO2 generated at the different temperatures signifies the different carbon fractions (see Figure 1).

Three-step temperature graph of the soli TOC cube

Figure 1. Three-step temperature graph of the soli TOC cube using a Cambisol B Horizon soil sample.

The separation of TIC and ROC can be further improved by the use of an inert carrier gas. Following the 400 °C temperature step, the carrier gas is switched from oxygen to nitrogen and the sample is directly pyrolyzed at 900 °C. In these conditions, the ROC stays in the sample container while the TIC is converted to CO2. After the TIC has been established, the system is supplied oxygen again, to oxidize the ROC. In a majority of cases, this method results in an improved separation of ROC and TIC as shown in Figure 2. Both techniques conform to the new DIN 19539.

Two-step temperature graph with gas switching of the soli TOC cube

Figure 2. Two-step temperature graph with gas switching of the soli TOC cube using a Cambisol B Horizon soil sample.

Experimental Results

The TOC content has been established in six different solid samples according to the techniques explained before. It can be said that the TOC contents according to method 1 (acidification and combustion at 900 °C) and method 2 (combustion at 650 °C) are in the same range for all analyzed materials. The TOC contents detected by method 3 (combustion at 400 °C: TOC400) are the lowest among all the used methods.

As sludge and fluvisol reveal the lowest relative ROC contents, TOC400 is similarly compared to the TOC content established by using the other techniques. In contrast at higher ROC contents, results for TOC400 are clearly different from the results of the other techniques. However, the sum of TOC400 and ROC is always equivalent with the other TOC results.

Remarkably, the sum of TOC400 and ROC of slag and recycled building material reveal the largest difference between the sum of TOC400 + ROC and the TOC contents established by the other two approaches among all samples.

In summary, the comparability of the TOC results of the different approaches vastly depends on the carbon fractions that remain in the sample. It is important to consider this when choosing the correct technique to determine the TOC content.

Table 1. Determination of TOC contents in six different samples according to three different methods. The analyzes according to method 1 and 2 are performed using the vario MAX cube with a combustion temperature of 900 °C and 650 °C, respectively. The soli TOC cube has been used for the analyzes of TOC400, ROC and TIC900 by the temperature ramp method 3.

METHOD 1 METHOD 2 METHOD 3
SAMPLE TOC [%] TOC [%] TOC400 [%] ROC [%] TIC900 [%] TC [%] TOC400+ROC [%]
Fluvisol 4.02 4.12 3.64 0.25 0.04 3.93 3.89
Excavated soil 0.40 0.57 0.23 0.15 0.42 0.80 0.38
Recycled building material 0.95 1.18 0.34 0.42 0.90 1.66 0.76
Slag 0.17 0.28 0.03 0.09 0.16 0.28 0.12
Waste incinerator ash 0.80 1.25 0.57 0.56 0.18 1.31 1.13
Sewage sludge 2.24 2.29 2.06 0.12 0.03 2.21 2.18

Determining Carbon in Soil

Instrument Solutions

Elementar offers the most dynamic variety of products to serve various customer demands based on their application tasks. With the vario MAX cube, soli TOC cube and the acquray series, larger samples, on the order of grams, can be consistently analyzed. This makes them ideal instruments for the accurate analysis of inhomogeneous samples. The measurements of the TOC according to method 2 and method 3 are incredibly simple. For the analysis, the samples are weighed into crucibles and delivered directly to the instrument. In case of method 1, the sample is acidified directly in the crucible and after drying delivered to the instrument.

When using the rapid CS cube and the vario EL cube, the sample is weighed into silver foil cups, acidified and dried before the measurement. Both are restricted to method 1 but they are also able to determine extra elements. The general sample weight used for these instruments is 100-200 mg.

To directly establish the TIC of solid samples, the soliTIC module has been created to provide accurate and precise results. It is an extra device that can be simply connected to all cube models that can measure carbon, for example vario MAX cube, vario EL cube and rapid CS cube.

Table 2. Instrument solutions for the determination of carbon in soil, sludge, sediment and solid waste.

INSTRUMENT METHOD 1 METHOD 2 METHOD 3 N S H AUTOMATED SAMPLE FEEDING AUTOMATED ASH REMOVAL
vario MAX cube ✔*
soli TOC cube ✔**
acquray TOC + acquray solid
vario EL cube
rapid CS cube

*in CNS mode only, **with optional EC detector.

References

DIN EN 15936 - Sludge, treated biowaste, soil and waste – Determination of total organic carbon (TOC) by dry combustion.

DIN 19539 - Investigation of solids — Temperature-dependent differentiation of total carbon.

DIN EN 13137 - Characterization of waste - Determination of total organic carbon (TOC) in waste, sludges and sediments.

ISO 10694 - Soil quality - Determination of organic and total carbon after dry combustion (elementary analysis).

Pitt, J.L., T.L. Provin, F.M. Hons, F. Dou, and J.S. Waskom. Use of a total carbon/nitrogen analyzer for the determination of organic and inorganic carbon in soils, manure, and composts. Abstracts, 2003 Meeting of ASA, Denver, CO.

Elementar Analysensysteme GmbH

This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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