IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 40 Analysis and Characterization Monday, 24 August 2026 / Hall B+C SL14 Fast and Accurate Method to Estimate Biochar Pore Size Distribution, Water Retention Curves and Cation Sorption Capacity by Low-Field NMR Etelvino Henrique Novotny1, José María García de Castro Barragán2, Heike Knicker2 1 Embrapa Soils, Rua Jardim Botânico, 1024. Rio de Janeiro-Brazil. CEP 22460-000. etelvino. novotny@embrapa.br 2 Instituto de la Grasa (IG-CSIC), Campus de la Universidad Pablo de Olavide, E-41013 Sevilla, Spain Biochar has been widely used as a soil amendment, with the potential to improve physical, chemical, and biological soil properties, including increased cation exchange capacity (CEC), pH elevation, enhanced water retention, and stimulation of microbial activity. It has also been studied as a promising material for the remediation of contaminated soils and water [1]. However, these effects are highly variable and strongly dependent on the biochar‘s intrinsic properties, which are primarily governed by feedstock composition, pyrolysis conditions, and ageing in the soil. Among these properties, pore size distribution is one of the most important – and most challenging – to determine [2]. While ¹H Nuclear Magnetic Resonance (NMR) relaxometry has been used to characterise soil porosity, it often presents an „ill-posed problem.“ Traditional spin-lattice and spin-spin relaxometry results are confounded by two correlated unknowns: pore length scales and surface relaxivity. To overcome this, we propose a method that directly accesses NMR diffusion eigenmodes [3]. This approach is governed solely by pore geometry, making it entirely independent of unknown surface relaxivity. Our refined low-field NMR method is fast (results in under 90 minutes), precise (unprecedented resolution and accuracy), and comprehensive (simultaneously determining pore size distribution, the water retention curve, and hydraulic conductivity). As a further refinement of our previously published method, we introduce a T₁ filter that enables the detection of smaller pores, improving the sensitivity of the technique by more than one order of magnitude. Concomitantly with the determination of these key hydro-physical properties, the method also allows for the assessment of paramagnetic ion sorption capacity. Mn²⁺ was used as a probe and as a practical application example. Results obtained by low-field NMR were validated against a conventional reference method (water
RkJQdWJsaXNoZXIy NDA4Mjc=