Heavy Metals and Ash
Chapter on ash and trace metals in biochar: concentration during pyrolysis, feedstock drivers, EBC class limits, and agronomic salinity/micronutrient issues. Report context: 2026-09-16.
Related notes: Contaminants and Quality · Woody vs Manure Ag Feedstocks · Gasification Residues · Properties · PFAS · Testing EBC IBI S668 · Key Sources
1. Ash conservation and concentration
Biomass ash is largely conserved into the solid while C/H/O leave as volatiles, so metal and mineral concentrations in biochar exceed those in the parent biomass (EBC 10.5E §7.5; Woolf 2021 eq. 2). Woolf Table 1 mean ash: wood 2.2% vs manure 28.5% vs sewage sludge 39.4% (open PDF).
Exception: mercury (and some semi-volatiles) can leave in the gas phase at pyrolysis temperatures (EBC §7.5).
High ash → lower FC (Gasification Residues, Woody vs Manure Ag Feedstocks) and harder O/C_org analytics (H-Corg and O-Corg; S668 §14).
2. EBC heavy-metal limits (v10.5E Table 2)
Indicative dry-matter limits from EBC 10.5E (FeedPlus/AgroOrganic often on 88% DM basis for some elements):
| Metal | FeedPlus / AgroOrganic (stricter) | Agro / Urban / Materials | Basic |
|---|---|---|---|
| Pb | 5 g t⁻¹ (88% DM) | 45 / 120 g t⁻¹ DM | Declaration |
| Cd | 0.5 g t⁻¹ (88% DM) | 0.7 / 1.5 g t⁻¹ DM | Declaration |
| Cu | 70 g t⁻¹ DM | 70 / 100 g t⁻¹ DM | Declaration |
| Ni | 25 g t⁻¹ DM | 25 / 50 g t⁻¹ DM | Declaration |
| Hg | 0.1 g t⁻¹ (88% DM) | 0.4 / 1 g t⁻¹ DM | Declaration |
| Zn | 200 g t⁻¹ DM | 200 / 400 g t⁻¹ DM | Declaration |
| Cr | 70 g t⁻¹ DM | 70 / 90 g t⁻¹ DM | Declaration |
| As | 2 g t⁻¹ (88% DM) | 13 g t⁻¹ DM | Declaration |
Aligned with EU fertiliser regulation and national soil/chem ordinances as cited by EBC (§7.5). Cu/Zn exceedances may be marketed as micronutrient fertilizers if labeled and rate-guided—not as ordinary Agro biochar.
Biosolids chars often fail Agro/Urban metal limits; EBC channels them toward EBC-Basic unless a country annex allows nutrient-to-metal ratio approaches (§4.9).
3. Salts, EC, and agronomy
Leachate electrical conductivity estimates salt content—a feedstock contamination / manure signal (EBC §7.6). Ash-rich chars can lime and fertilize but also salinize sensitive soils. Pair with nutrient declaration (N, P, K, Mg, Ca, Fe mandatory in EBC — §7.4).
4. Testing
- EBC: endorsed labs, class-specific packages.
- U.S.: EPA trace-metal methods per standardizes biochar lab testing §12.2 · PDF.
- Always report on stated moisture basis (DM vs 88% DM for feed).
5. Practice takeaway
- Metals track feedstock cleanliness more than kiln brand.
- Concentration math: >50% mass loss roughly doubles non-volatile metal content—plan feedstock specs accordingly.
- Separate micronutrient labeling from general soil-amendment claims when Cu/Zn are high.
Primary links
- EBC 10.5E PDF
- Woolf et al. 2021 · models soil GHG from biochar
- ANSI S668 PDF · standardizes biochar lab testing
- EU 2019/1009