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Background <p>The complex chemical structure and recalcitrant nature of sewage sludge biochar (SSBC) pose significant challenges for determining its true total elemental content. Direct wet digestion methods often fail to completely decompose the matrix, while spectroscopic techniques are hindered by strong interferences and limited sensitivity.</p> Results <p>This study explores a novel approach that combines dry-ashing (DA) with two distinct digestion methods: <i>Aqua regia</i> (AR) and hydrofluoric acid with nitric acid (HF). As a reference method, Instrumental Neutron Activation Analysis (INAA), which does not require sample dissolution, was employed. INAA technique enabled the determination of aluminium (Al), arsenic (As), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), potassium (K), manganese (Mn), nickel (Ni), and zinc (Zn). Dry-ashing followed by HF digestion (DA + HF) showed superior performance compared to dry-ashing with <i>Aqua regia</i> (DA + AR) or wet digestion methods, particularly for Al, Cr, and K, which were significantly underestimated using DA + AR. A regression model showed very strong correlations (R<sup>2</sup> ≥ 0.9) between DA + HF and INAA for all elements studied. The pyrolysis temperature (up to 820&#xa0;°C) did not significantly impact the effectiveness of decomposition by DA + HF. However, DA + HF still failed to completely dissolve corundum (Al<sub>2</sub>O<sub>3</sub>) and titanium dioxide (TiO₂), which are resistant to HF and were present in SSBC.</p> Conclusions <p>The findings support DA + HF as the most effective digestion method for SSBCs, though challenges remain in fully dissolving certain mineral residues. It also highlights the critical role of HF in accurate determination of total elemental contents in sewage sludge biochars. These insights are crucial for ensuring accurate assessment of potentially toxic elements (PTEs) and nutrients in SSBCs, facilitating their safe application in agriculture.</p> Graphical abstract <p></p>

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Optimization of digestion methods for total elemental analysis in sewage sludge biochars

  • 期刊论文

  • 出版模式：

    开放获取

  • 发表日期：

    2026年6月3日

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  • Leidy Marcela Ulloa-Murillo,
  • Jan Kučera,
  • Václav Tejnecký,
  • Altyn Taisheva,
  • Chandra Sekhar Paul,
  • Zdeněk Košnář,
  • Jiřina Száková,
  • Pavel Tlustoš,
  • Filip Mercl

摘要

Background

The complex chemical structure and recalcitrant nature of sewage sludge biochar (SSBC) pose significant challenges for determining its true total elemental content. Direct wet digestion methods often fail to completely decompose the matrix, while spectroscopic techniques are hindered by strong interferences and limited sensitivity.

Results

This study explores a novel approach that combines dry-ashing (DA) with two distinct digestion methods: Aqua regia (AR) and hydrofluoric acid with nitric acid (HF). As a reference method, Instrumental Neutron Activation Analysis (INAA), which does not require sample dissolution, was employed. INAA technique enabled the determination of aluminium (Al), arsenic (As), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), potassium (K), manganese (Mn), nickel (Ni), and zinc (Zn). Dry-ashing followed by HF digestion (DA + HF) showed superior performance compared to dry-ashing with Aqua regia (DA + AR) or wet digestion methods, particularly for Al, Cr, and K, which were significantly underestimated using DA + AR. A regression model showed very strong correlations (R2 ≥ 0.9) between DA + HF and INAA for all elements studied. The pyrolysis temperature (up to 820 °C) did not significantly impact the effectiveness of decomposition by DA + HF. However, DA + HF still failed to completely dissolve corundum (Al2O3) and titanium dioxide (TiO₂), which are resistant to HF and were present in SSBC.

Conclusions

The findings support DA + HF as the most effective digestion method for SSBCs, though challenges remain in fully dissolving certain mineral residues. It also highlights the critical role of HF in accurate determination of total elemental contents in sewage sludge biochars. These insights are crucial for ensuring accurate assessment of potentially toxic elements (PTEs) and nutrients in SSBCs, facilitating their safe application in agriculture.

Graphical abstract