Université Gustave Eiffel – Interview with Myriam Duc and François Lansac

Significance of the topic

The characterization of carbon and sulfur species in soils and geomaterials is critical for geotechnical design, environmental risk assessment and material reuse. Accurate, rapid determination of total carbon (TC), total organic carbon (TOC), mineral carbon (TIC), total sulfur and pyritic (sulfide) sulfur supports decisions about foundation performance, swelling/shrinkage potential of clays, sulfate-driven deterioration of concrete and acid-generation risk from disturbed pyritic materials. Developing robust, soil-specific analytical protocols reduces turnaround time and enhances laboratory-to-field decision making.

Objectives and overview of the study

• Introduce the research activities of the Soil, Rock and Geotechnical Structures laboratory at Universite9 Gustave Eiffel, with emphasis on physico-chemical characterisation of geomaterials.
• Explain how a LECO SC832 carbon/sulfur analyser is used in the laboratory workflow to obtain TC, TOC, TIC and sulfur fractions, including a tailored method to separate sulfate sulfur from pyritic (sulfide) sulfur in soils.
• Summarise throughput, method advantages and future analytical needs identified by the team.

Methodology and analytical approach

The laboratory combines several complementary analytical techniques to characterise geomaterials at multiple scales:

• Mineralogy and microstructure: X-ray diffraction (XRD) and scanning electron microscopy (SEM), supported by petrography.
• Porosity and pore-size distribution: mercury intrusion porosimetry.
• Chemical composition: inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence (XRF), and thermal analysis (TGA).
• Carbon and sulfur speciation: LECO SC832 high-temperature combustion analyser for TC and total sulfur; acid-based sample pretreatments to discriminate inorganic (mineral) carbon and sulfate sulfur from organic carbon and sulfide sulfur.

Key analytical workflows described:

1. TOC determination: measure TC by combustion, then remove inorganic carbon by acid treatment and re-measure to calculate TOC (TC TIC/TOC approach).
2. Sulfur speciation for soils: (a) measure total sulfur by high-temperature combustion (approx. 1450 C) with catalyst; (b) dissolve sulfate by boiling acid treatment and filter the sample; (c) analyse the retained solid (including the filter) to quantify sulfide (pyritic) sulfur. Sulfate sulfur is inferred from the difference between total sulfur and residual sulfide sulfur.

Used instrumentation

• LECO SC832 carbon and sulfur analyser: combustion-based determination of TC, TOC (via acid pretreatment), total sulfur and pyritic sulfur (with adapted pretreatment).
• Complementary devices: XRD, SEM, TGA, ICP-OES, XRF and mercury intrusion porosimetry used for broader material characterisation.
• Potential extension noted: LECO CNS series (e.g., CNS928) for simultaneous carbon-nitrogen-sulfur analysis, requested by the team to measure nitrogen/ammonium content.

Main results and discussion

The research team adapted and validated a rapid two-step protocol for soils that provides both carbon and sulfur speciation with much shorter turnaround than traditional wet-chemistry approaches. Main outcomes:

• Analytical performance and speed: the adapted sulfur separation method reduces analysis time from days or weeks to typically two days in the research lab, and can be reduced to one day in professional lab settings.
• Practical reliability: analyzing the filtered solid fraction (including the filter) minimizes loss of sulfide-bearing particles (e.g., pyrite) and improves quantification of pyritic sulfur in soils where sulfate species (e.g., gypsum) dissolve during acid extraction.
• Throughput: initial use of the instrument yielded approximately 300340 samples analysed in eight months; each sample is measured in duplicate to ensure reproducibility.
• Method transfer: the team reports successful transfer of the adapted protocol to professional laboratories, indicating robustness and suitability for routine geotechnical and environmental analyses.

Benefits and practical applications of the method

• Faster decision-making for geotechnical projects: rapid identification of pyritic sulfur and organic carbon supports risk assessments for excavation, reuse of soils, and concrete durability evaluations.
• Versatility: the approach handles heterogeneous geomaterials where conventional soil standards for sulfur are lacking, by combining combustion analysis with selective acid extractions adapted for soils.
• Cross-disciplinary value: results feed into mechanical and hydraulic analyses (e.g., foundation design, permeability assessment) and environmental evaluations (acid generation, sulfate leaching).
• Operational advantages: relatively straightforward sample handling, short analysis times, and compatibility with existing LECO workflows facilitate adoption in research and professional labs.

Future trends and potential uses

• Expanded elemental suites: demand for nitrogen and ammonium analysis was expressed; coupling carbon/sulfur workflows with CN(S) analysers would provide fuller C-N-S budgets for soils and materials.
• Method standardisation: developing and publishing soil-specific standards for sulfur speciation would increase comparability across labs and support regulatory needs.
• Automation and throughput: integrating automated sample preparation and filtration protocols could increase sample throughput and reduce manual handling variability.
• Data integration: linking chemical speciation results with mineralogical, microstructural and geotechnical datasets (e.g., XRD, SEM, porosimetry, mechanical tests) to build predictive models of material behaviour.
• Coupling with speciation analytics: complementary techniques (e.g., sequential extraction, X-ray absorption spectroscopy) could provide further insight into sulfur oxidation states and mineral hosts in complex soils.

Conclusion

The Universite9 Gustave Eiffel team demonstrates a pragmatic, rapid analytical strategy using a LECO SC832 analyser to obtain reliable carbon and sulfur speciation in soils and geomaterials. By combining high-temperature combustion with targeted acid extractions and careful recovery of solid residues, the laboratory achieves fast turnaround, robust pyritic sulfur quantification and practical workflows that have been transferred to professional environments. Expanding element detection (notably nitrogen) and pursuing standardisation and automation are logical next steps to broaden applicability and throughput.

References

• Interview with Myriam Duc and FranE7ois Lansac, Universite9 Gustave Eiffel; discussion of LECO SC832 application in soil and geomaterial analyses.
• Instrument references: LECO SC832 carbon/sulfur analyser; mention of LECO CNS928 series as a potential extension for C-N-S analysis.