Why TOC Analysis is Crucial for Effective Waste Classification

The composition of waste material can vary considerably depending on its origin. Various uses exist for waste, including dumping it in landfills, burning it for energy production, and recycling.

The range of uses for waste requires numerous analytical techniques to determine various parameters of interest.

Total organic carbon (TOC) content is a key measure for waste classification, whether depositing domestic or industrial waste, sewage sludge, road waste (containing bitumen and tar), contaminated excavated soil, or rubble from building sites.

According to the European Landfill Directive, the TOC concentration of waste determines assignment to a landfill class.

On dumping sites, organic waste components undergo microbial decomposition, leading to landfill gases that cause unwanted smells and contribute to the greenhouse effect.

Decomposition products such as organic acids significantly contribute to heavy metal mobilization by complexation reactions. Heavy metals can transfer into the lower soil layers and cause serious seeping and groundwater contamination. Waste materials with high TOC content must be treated differently to prevent this.

The DIN EN 15936 (also EN 13137) describes the basics for TOC determination in solids. Several techniques can be employed, all of which include an inorganic non-oxidizing acid treatment step.

Carbonates react with the acid to form CO₂, which can be determined quantitatively using the indirect method (TOC = TC - TIC). Alternatively, the CO₂ is eliminated from the sample, and the TOC is measured directly afterward.

Materials and Methods

For all waste samples, TOC determination was performed using the direct method according to EN 15936, resp. EN 13137.

To achieve this, samples were weighted into ceramic sample boats, and different sample amounts were selected depending on homogeneity, with a higher sample weight applied to more inhomogeneous samples.

An acidification step was then implemented to remove carbonates and hydrogen carbonates (TIC), and samples were subsequently dried.

The boats were then placed onto the sampler FPG 48 and automatically introduced into the analyzer’s furnace. Different method settings, including temperature, sample introduction speed, and hold positions, were applied for the various waste matrices.

Combustion occurred inside a robust, catalyst-free ceramic combustion tube at temperatures ≥ 1000 °C and in a pure oxygen atmosphere.

Quantitative oxidation of all present organic compounds after the high combustion temperature and an oxygen surplus ensured TIC elimination.

During combustion, the CO₂ that formed was transferred by the carrier gas into the Focus Radiation NDIR detector. Combustion gases were purified between the furnace and the detection system using particle removal, moisture, and halogen absorption stages.

Sample Preparation

The waste samples were already homogenized (crushed or ground) to a satisfactory 1 to 3 mm particle size. Some samples, particularly road and industrial waste, displayed an erratic, non-uniform structure with uneven particle size and coloring. Samples were weighted into ceramic sample boats without additional homogenization.

Acidification was performed using an acid dispenser, and 500 µL of 10 % HCl was added to each sample.

The completeness of TIC elimination was confirmed by adding a few drops of HCl 25 % until no further gas (CO₂) was produced. All sample boats were then placed onto a heating plate at 40 °C and dried overnight for approximately 14 hours.

Following the method described here, three boats for each sample and a certified reference material were prepared.

Calibration

The solid TOC analyzer was calibrated by analyzing a single standard substance (Coal CRM) with different sample weights. The subsequent calibration curve covers a broad concentration range.

The coal standard (TC = 62.53 %) was weighted directly into ceramic sample boats in different portions, beginning with about 20 mg. The FPG 48 autosampler introduced these boats to the furnace of the solid TOC analyzer and then combusted them. Figure 1 shows the calibration curve.

Table 1. Calibration. Source: Analytik Jena US

Figure 1. Calibration curve. Image Credit: Analytik Jena US

Instrument Settings

Measurements were performed with a multi N/C 2100S duo consisting of a multi N/C 2100S basic unit equipped with an AS 60 and HT 1300 solid furnace and the FPG 48 solid sampler.

The instrument configurations displayed in Table 2 can also be used for TOC determination in waste samples of different origins by direct or differential methods.

Table 2. Further Instrument configurations. Source: Analytik Jena US

All instruments discussed above are equipped with a robust ceramic combustion tube unaffected by high concentrations of acid vapors, alkali, or earth alkali metals.

Combustion temperatures of up to 1300 °C (multi N/C duo systems) and 1500 °C (multi EA 4000 configurations) guarantee a quantitative digestion of all carbon compounds.

Method Parameters

Waste samples of different origins display different combustion behaviors due to variations in composition.

In cases where there is a high organic matter content, such as biowaste, the expected vigorous combustion process must be controlled to avoid falsified results by slowing the introduction of the sample into the hot furnace and by extra covering of the sample with an inert retarding material, such as annealed quartz sand.

Table 3 summarizes the parameter settings for combustion and sample introduction of the various waste samples.

Table 3. Method settings multi N/C 2100S duo. Source: Analytik Jena US

Results and Discussion

Table 4 displays the analysis results for different waste samples.

Measurements were performed in triplicates, and the measured standard deviations (SD) were within the expected range for heterogeneous matrices like waste.

Improved sample homogenization can enhance reproducibility by milling with cooled mills for samples like road waste containing tar. Routine labs often omit labor-intensive homogenization procedures, accepting relative SD values up to 5 %. Figure 2 displays some typical measuring curves.

Table 4. Results. Source: Analytik Jena US

Figure 2. Typical measurement curves of a) biowaste, b) contaminated soil, c) road waste, and d) industrial waste. Image Credit: Analytik Jena US

Summary

Multi N/C duo systems are perfectly suitable for analyzing waste samples of various origins, providing a fast and reliable application of the direct method of TOC determination.

Sample preparation can be performed for samples inside sample boats, reducing the preparation time and effort required. The sample analysis is fully automated, and the sampler FPG 48 can run up to 48 samples with different method settings for each sample matrix.

Simple calibration routines and the included wide-range Focus Radiation NDIR detector facilitate a broad range of measurement of up to 500 mg C absolute for all multi N/C duo instruments, making repeated analysis of samples with unexpectedly high concentrations unnecessary.

Preparation and measuring procedures are fully compliant with EN 15936. The multi N/C duo systems are suitable for automated analysis of TOC (TIC, TC), NPOC, POC, and TN_b in water samples without labor-intensive hardware modifications.

With a few mouse clicks, the software can alter the configuration setup and load the desired method, converting the solid TOC analyzer into a fully automated liquid analyzer.

Figure 3. Multi N/C 2100S duo. Image Credit: Analytik Jena US

References and Further Reading

1. Scirp.org. (2014). DIN EN 13137 (2001) Characterization of Waste-Determination of Total Organic Carbon (TOC) in Waste, Sludges and Sediments. Beuth-Verlag, Berlin. - References - Scientific Research Publishing. [online] Available at: https://www.scirp.org/reference/referencespapers?referenceid=1311722 [Accessed 8 Nov. 2024].
2. iTeh Standards. (2024). iTeh Standards. [online] Available at: https://standards.iteh.ai/catalog/standards/cen/d102db97-be7e-4014-b5f7-b9d28c3fb68e/en-15936-2012?srsltid=AfmBOooIs4mmwyT6F1e0i3XxlEINxh2g6IOgjRllATnpoVextBHWd3yk [Accessed 8 Nov. 2024].

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.