Huang et al. 2025 - biochar and antimony in the rice rhizosphere

Huang and colleagues ran a rice pot experiment using Sb-contaminated paddy soil from Anhui Province, China, to test how 3% Paulownia-derived biochar affects antimony mobility, rhizosphere fractions, and transfer into rice roots, shoots, and grain. The source is primary soil-to-rice transfer and remediation-context evidence for Sb. It is not market rice occurrence evidence and should not be pooled with retail or field-survey rice-grain measurements.

Key numbers

Starting soil and biochar

The paddy soil was collected in August 2022 from the 0-20 cm surface layer of a field adjacent to an Sb-Au mining site in Dongzhi County, Chizhou, Anhui Province, China. The soil had been contaminated by a 2010 tailings-dam breach.

Initial soil and biochar properties:

The soil was silty loam, with 61.8% silt and 22.4% sand. The biochar had carboxyl groups at 0.46 mol/kg and phenolic hydroxyl groups at 0.15 mol/kg.

Rhizosphere conditions

In the unamended planted soil (S1), pH rose from 5.70 to 6.63 during rice growth, a 16.32% increase. Eh fell from 456 mV to 158 mV, a 63.35% reduction. SOM decreased from 2.79% to 2.56% between tillering and jointing, then rose to 3.41% at maturity. DOC increased from 2.91 g/kg to 4.68 g/kg between tillering and jointing, then declined to 4.04 g/kg by maturity.

Adding biochar changed the initial rhizosphere state: pH increased from 5.70 to 7.74, SOM from 2.79% to 7.57%, and DOC from 2.91 g/kg to 4.38 g/kg, while Eh decreased from 455.85 mV to 379.28 mV. Across the rice growth period in S1-BC, pH declined from 7.74 to 6.90, SOM declined from 7.57% to 5.56%, and DOC increased from 4.38 g/kg to 6.66 g/kg.

Bioavailable Sb and Sb fractions

Bioavailable Sb increased over rice growth in both treatments:

The authors report that biochar increased bioavailable Sb by 1.57-32.97% across rice growth stages.

Sequential extraction showed changing Sb fractions. In S1, exchangeable Sb rose from 1.71% to 4.29% across growth stages, a 150.96% increase. Carbonate-bound Sb rose from 5.04% to 14.51%, a 188.17% increase. Residual Sb declined from 80.14% to 67.19%, a 16.16% reduction.

In S1-BC, exchangeable Sb rose from 2.20% to 2.81%, a 28.10% increase. Carbonate-bound Sb rose from 6.63% to 10.66%, a 60.72% increase. Fe-Mn oxide-bound Sb fell from 9.85% to 7.15%, a 27.34% reduction, and organic-bound Sb fell from 8.11% to 6.85%, a 15.48% reduction. The residual fraction showed no apparent change across the rice growth cycle.

Sb in rice plant parts and grain

In unamended soil, Sb in rice shoots decreased from 0.74 mg/kg to 0.45 mg/kg from tillering to maturity. Sb in rice roots rose from 15.26 mg/kg to 26.64 mg/kg between tillering and grain filling, then rose to 69.79 mg/kg at maturity.

Adding biochar reduced Sb concentrations in roots and shoots while slightly increasing grain Sb:

The authors interpret this as biochar reducing soil-to-root and root-to-shoot movement while increasing shoot-to-grain movement, likely through altered pH, DOC, and rhizosphere binding conditions.

Transfer factors

The bioconcentration factor (BCF) and translocation factors changed with growth stage:

The paper concludes that biochar decreased Sb accumulation in roots and shoots but increased Sb in grains, so biochar use for Sb-contaminated paddy remediation needs food-chain risk evaluation before being treated as protective.

Methods (brief)

Surface soil was air-dried, sieved to 2 mm, mixed, and characterized for pH, SOM, particle size, and total Sb. Paulownia bio-waste biochar was produced by a water-fire coupled process, dried at 85 degrees C, ground, sieved to 0.25 mm, and characterized by pH, elemental carbon analysis, ash content, FTIR, acidic functional group titration, and SEM.

Plexiglass columns received a 2 cm stone layer and 30 cm of soil, either unamended or amended with 3% biochar by weight. After 7 d waterlogging, 25-30 Nanjing 46 rice seeds were planted. Rhizosphere soil and plant samples were collected at tillering, jointing, grain filling, and maturity. Plants were separated into roots and shoots before maturity and into roots, shoots, and grains at maturity.

Bioavailable Sb was extracted from 1 g rhizosphere soil with 0.1 mol/L CaCl2 and measured by ICP-MS. Sb fractions used a modified Tessier sequential extraction into exchangeable, carbonate-bound, Fe-Mn oxide-bound, organic-bound, and residual fractions. Plant roots, shoots, and grains were digested with HF, concentrated HNO3, and H2O2, then Sb was measured by ICP-MS. Total soil Sb was measured by ICP-OES.

Implications

Certification: Do not use this source as ordinary rice occurrence evidence. The grain values come from a pot experiment using deliberately contaminated paddy soil near a mining/tailings site, not from a market, field-survey, or retail rice sample frame.

App: Useful for supply-chain and remediation context. The study warns that a mitigation intervention can reduce root and shoot Sb while increasing grain Sb, so mitigation claims need edible-tissue verification rather than only rhizosphere or root endpoints.

Courses: Useful for teaching why “bioavailable soil fraction,” “root concentration,” “shoot concentration,” “grain concentration,” BCF, and TF are distinct evidence objects and cannot be collapsed into a single contamination claim.

Wiki pages this source may touch

• antimony
• rice
• soil-to-plant-transfer
• remediation-evidence
• icp-ms

Verification notes

This page was built from the full PDF, including the abstract, soil and biochar preparation, pot experiment design, rhizosphere sampling method, ICP-MS/ICP-OES methods, Figures 1-5, Results and Discussion sections 3.1-3.5, Conclusions, supplementary-material note, and data-availability statement. Product routes are intentionally empty because the study is not retail or market rice occurrence evidence. Antimony speciation is not separated into Sb(III) and Sb(V); the authors identify this as a limitation and report total Sb/fractions rather than species-specific edible-grain Sb.

Page history

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