Huang et al. 2025 - Biochar, antimony, rhizosphere-rice system

Huang and colleagues ran a five-month rice pot experiment in mining-contaminated soil from Dongzhi County, Anhui Province, China to test how a 3% addition of Paulownia bio-waste biochar reshapes antimony bioavailability, soil-fraction redistribution, and root-to-grain translocation across four rice growth stages. The source is primary evidence for antimony behaviour in a rhizosphere-rice system under a contaminated-field intervention, not a market-basket distribution. Total soil Sb was 94.41 mg/kg (contaminant factor 38.35, “high contamination” category) and grain Sb in the biochar-amended treatment rose slightly from 0.35 to 0.38 mg/kg, so the grain values should be routed as agronomic-intervention evidence rather than a normal commercial-rice benchmark pool.

Key numbers

Soil and biochar baseline

The paddy soil was silty loam (silt 61.8%, sand 22.4%) with pre-experiment pH 5.88 and soil organic carbon 1.01%. Total Sb was 94.41 mg/kg and bioavailable (CaCl2-extractable) Sb was 2.21 mg/kg. The contaminant factor of 38.35 placed the site in the high-contamination category against the cited reference threshold of 5. The Paulownia bio-waste biochar produced by a water-fire coupled process had pH 7.98, carbon content 69.58%, ash 20.85%, and BET-style specific surface area 85.70 m2/g. Acid-titration-determined carboxyl and phenolic hydroxyl group concentrations were 0.46 mol/kg and 0.15 mol/kg.

Environmental factors across rice growth stages

Figure 1 reports pH, Eh, SOM, and DOC at five sample points (unplanted CK plus tillering TIL, jointing JOI, grain filling GRF, and maturity MAT).

Values without explicit text quotes are read from Figure 1; only the values printed in the Results text (CK, MAT, percentage changes) are exact. Adding biochar shifted the CK baseline from pH 5.70 to 7.74, SOM 2.79% to 7.57%, DOC 2.91 g/kg to 4.38 g/kg, and lowered Eh from 455.85 mV to 379.28 mV.

Bioavailable Sb across rice growth stages

Figure 2 reports CaCl2-extractable Sb at the same five sample points.

Bioavailable Sb increased 1.57-32.97% with biochar addition across all growth stages.

Sb sequential-extraction fractions

Modified Tessier extraction produced five fractions: exchangeable (EXC), carbonate-bound (CARB), Fe-Mn oxide-bound (Fe-Mn), organic-bound (OM), and residual (RES). The text reports percentage changes from the first to last rice growth stage in each treatment.

Figure 3 shows that across both treatments RES dominates throughout the rice cycle (roughly 65-80% of total Sb) and that biochar amendment compressed the EXC and CARB shifts that occurred in the unamended soil.

Sb in rice tissues

Figure 4 reports Sb in shoots and roots at four growth stages.

The text reports that biochar reduced Sb in roots by 38.00-48.97% and in shoots by 30.57-47.42% relative to S1, and that grain Sb rose slightly from 0.35 mg/kg (S1) to 0.38 mg/kg (S1-BC).

Transfer factors

The text and Figure 5 report bioconcentration factor (BCF = C_root / C_rhizosphere-soil) and translocation factors (TF_R-S = C_shoot / C_root, TF_S-G = C_grain / C_shoot).

Biochar therefore decreased soil-to-root and root-to-shoot transfer but increased shoot-to-grain transfer, with a small net rise in grain Sb.

Methods (brief)

Soil was collected in August 2022 from the 0-20 cm surface layer of a paddy adjacent to an Sb-Au mining site in Dongzhi County, Chizhou, Anhui Province, China (30 deg 32 min 53 sec N, 116 deg 44 min 52 sec E) that had been contaminated by a tailings-dam breach in 2010. Soil pH was measured at a 1:5 g:mL soil:water ratio with a pH meter (FE22-Standard, Mettler Toledo). Soil organic matter used hydrated-heat K2Cr2O7 oxidation colorimetry. Soil particle size used a Mastersizer 2000 (Malvern). Total soil Sb was measured by inductively coupled plasma optical emission spectrometry (ICP-OES, Avio 200, PerkinElmer).

Biochar was produced from Paulownia (Paulownia fortunei) bio-waste using a water-fire coupled process, oven-dried at 85 deg C, ground, and sieved through a 0.25 mm mesh. Carbon was quantified with a Vario Micro cube elemental analyzer (Elementar, Langenselbold). Ash was determined by mass loss after 4 h in a muffle furnace (SXG18123, Tianjin) at 800 deg C. Functional groups were characterized by Fourier transform infrared spectroscopy (Nicolet iS50, Thermo Fisher). Acidic functional group titration followed the International Humic Substances Society protocol. Surface morphology used scanning electron microscopy (S-4800, Hitachi).

Pot units were 30 cm diameter plexiglass columns with a 2 cm gravel base and 30 cm of soil either unamended (S1) or amended with 3% biochar by weight (S1-BC). After 7 days of waterlogging, 25-30 Nanjing 46 rice seeds were sown per column. Soil water was kept between 25% and 60% by weight (TMS-4 logger, TOMST) and Hoagland solution supplied nutrients. Unplanted soil columns served as CK and CK-BC controls. All treatments were performed in triplicate. Rhizosphere soil and plant samples were collected at TIL, JOI, GRF, and MAT; the MAT stage was the only one that included grains.

Environmental factors in the rhizosphere were measured as pH (FE22-Standard, Mettler Toledo), SOM (K2Cr2O7 oxidation colorimetry), Eh (HED-QX6550, Horde Electric), and DOC (TOC-L, Shimadzu). Bioavailable Sb was extracted with 0.1 mol/L CaCl2 (1:10 g:mL, 24 h shake at 25 deg C, 120 rpm) and quantified by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7800). Sb fractions used the modified Tessier sequential extraction with aqua regia (HNO3:HCl 1:3) substituted for the residue step. Plant Sb used 0.1 g dried tissue digested in 0.5 mL HF, 2 mL HNO3, and 1 mL H2O2 at 120 deg C for 2 h with the digestion repeated to completion, brought to a 25 mL final volume in ultrapure water, filtered through a 0.22 microm membrane, and analyzed by ICP-MS (Agilent 7800). The study measured total Sb only; the authors note in the Conclusions that Sb(III)-Sb(V) speciation and rhizosphere-microbe contributions were not addressed and are flagged as study limitations. Statistical processing used Microsoft 2010 and Origin 2024 (OriginLab, Northampton, MA).

Implications

Certification: This source should not be pooled as a representative China-market or any-market rice Sb occurrence distribution. It is a mining-contaminated-site agronomic intervention study with a single soil and a single rice cultivar in a pot system. It is useful for rice Sb mitigation evidence and for evidence that biochar redirects Sb migration (reducing soil-to-root and root-to-shoot transfer while increasing shoot-to-grain transfer, with grain Sb rising modestly from 0.35 to 0.38 mg/kg under biochar). The grain values cannot be compared to market baselines without acknowledging the 94.41 mg/kg total-Sb starting soil and the pot setting.

Courses: Useful case study for the rhizosphere fraction-redistribution framework and for the cautionary point that an amendment that lowers root and shoot uptake can still elevate grain accumulation through downstream translocation effects.

App: Route as rice Sb agronomic-intervention context for antimony. Flag contaminated-site basis, pot-experiment basis, China jurisdiction, total-Sb-only measurement, and biochar intervention before making any product-risk inference from the grain numbers.

Wiki pages this source may touch

• rice
• rice-bulk-grain
• antimony
• soil-to-plant-transfer
• agronomic

Verification notes

DOI 10.3390/toxics13050389 matches the published PDF; Toxics 2025, 13, 389 with received 6 April 2025, accepted 12 May 2025, published 13 May 2025. License is CC BY 4.0 per the publisher copyright statement on page 1. The paper measures total antimony only and explicitly flags Sb(III)/Sb(V) speciation as a study limitation in the Conclusions, so metals: [Sb] (not iSb or any speciated form) is correct. No brand-name products were sampled; all instrument and reference-material vendor names retained in the Methods section fall under the Part 12 scientific-method exception (Mettler Toledo pH meters, PerkinElmer ICP-OES, Agilent 7800 ICP-MS, Vario Micro cube Elementar, Nicolet iS50 Thermo Fisher, Hitachi S-4800, HED-QX6550 Horde Electric, TOC-L Shimadzu, TMS-4 TOMST, Origin 2024 OriginLab, SXG18123 muffle furnace). Exact tissue Sb concentrations from Figure 4 are reported only where the Results text gives them numerically (TIL/MAT shoot endpoints and TIL/GRF/MAT root endpoints); other Figure 4 points are quoted as approximate reads. The same convention applies to Figure 1 (pH/Eh/SOM/DOC at intermediate growth stages) and Figure 2 (bioavailable Sb at intermediate stages). Percentage changes and absolute endpoints come directly from the Results text.

Fresh-context audit subagent (2026-06-02) returned REVISE verdict; Checks 1 (numerical fidelity), 3 (speciation/methods), 4 (Part 12 brand firewall), and 5 (Part 2 wiki/HMTc firewall) clean or near-clean. Findings applied: (a) Eh S1-BC Span-column cell mislabel — verified against PDF p.6 (S1-BC Eh “decreased from 379.28 mV to 114.85 mV” CK-to-MAT is -69.72%; the “2.73-27.46%” range is the per-stage reduction of S1-BC vs matched S1 stage); cell relabeled to disambiguate both quantities. (b) Methods detail — added the 0.22 microm filtration step for the plant Sb digest and corrected “Microsoft Excel 2010” to “Microsoft 2010” per the exact PDF p.5 wording. Findings rejected as false positives: (c) Check 2 ❌ on [[supply-chain/soil-to-plant-transfer]] and [[mitigation/agronomic]] — both wikilinks resolve to live wiki pages (wiki/supply-chain/soil-to-plant-transfer.md and wiki/mitigation/agronomic.md) and are precedented across many other source pages (sun2023, lu2025, wang2020, rusanescu2023, etc.); the taxonomy snapshot is intentionally scoped to Ingredients / Products / Metals / Regulations only and does not enumerate the supply-chain or mitigation namespaces. (d) Check 2 ⚠️ matrices vocabulary concern — the soil and plant-organ matrices entries (rhizosphere-soil, contaminated-paddy-soil, biochar-amended-paddy-soil, rice-plant-organs, rice-grain) are precedented in the comparable rice-and-biochar source sun2023-biochar-rice-cadmium-field; the matrices vocabulary covers study-design matrix descriptors, not just consumer-food matrices.

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.