Zhao et al. 2023 - Rice-straw biochar sorption of Cu(II) and Pb(II)

Zhao and colleagues compared original, bleached, and NaOH-activated rice-straw biochars as sorbents for dissolved Cu(II) and Pb(II). The source is primary remediation-method evidence for metal immobilization mechanisms, not food or ingredient occurrence evidence: the measured endpoints are sorption coefficients, model fits, biochar functional groups, and DFT binding energies rather than concentrations in edible rice, rice straw as feed, soil after application, or food products.

Key numbers

Biochar preparation and sorption test

Rice straw was pyrolyzed under nitrogen at 300, 500, or 700 C for 4 h to produce O300, O500, and O700. Bleached materials (BL, BL300, BL500, BL700) were produced by three 5 h NaClO2/acetic-acid treatments. Activated materials (AN, AN300, AN500, AN700) were produced by three NaOH treatments at 150 C using 1 g material with 5 g NaOH.

The batch sorption test used separate Cu(NO3)2 and Pb(NO3)2 solutions diluted to nine concentrations from 1 to 50 mg/L in 0.01 mol/L NaNO3. Solution pH was held at 6.0 +/- 0.2. Vials were shaken at 25 C for 5 days, centrifuged, filtered through 0.45 um membranes, and measured by flame atomic absorption spectroscopy. Samples were analyzed in duplicate.

Surface chemistry

Table 1 reports XPS C 1s and O 1s peak fractions. Selected sorption-relevant values:

The authors report that bleaching increased oxygen-containing groups, while activation increased aromatic carbon and specific surface area. They found no strong relationship between specific surface area and Kd, but did find a significant positive relationship between COOH/C=O content and Kd.

Cu(II) sorption model fits

Table 2 reports isotherm fits. Values below retain the source’s units: Langmuir Q is reported as mg/kg; Kd values are L/g at Ce = 5 mg/L; Sips KS is mg/g.

For Cu(II), the highest source-reported Sips Kd at Ce = 5 mg/L was BL500 at 3.207 L/g. The highest Langmuir Q was BL300 at 34.27 mg/kg.

Pb(II) sorption model fits

For Pb(II), the highest source-reported Sips Kd at Ce = 5 mg/L was BL500 at 22.67 L/g. The highest Langmuir Q was O300 at 70.66 mg/kg. Pb(II) sorption was generally higher than Cu(II) sorption on the same biochar, which the authors attribute to Pb(II)‘s larger electron cloud and smaller hydration radius.

Mechanism findings

The authors report that Langmuir fits generally outperformed Freundlich fits for the isotherms, while the Sips model provided the best overall description. DFT calculations estimated complexation energies of -12.4 eV for Cu(II) and -9.3 eV for Pb(II). For Cu(II), cation-pi interaction energy (-14.8 eV) was larger than complexation energy (-12.4 eV). For Pb(II), complexation was higher than the cation-pi interaction. The authors conclude that complexation and cation-pi interactions jointly dominate Cu(II) and Pb(II) sorption, with oxygen-containing groups especially important.

Methods (brief)

The study prepared rice-straw-derived biochars at three pyrolysis temperatures and applied two chemical modification pathways: oxidative bleaching and NaOH activation. Biochars were characterized by elemental analysis, N2 physical adsorption for specific surface area, XPS for surface chemistry, FTIR for functional groups, and solid-state 13C NMR for carbon structure. Batch sorption experiments exposed the biochars to Cu(II) or Pb(II) nitrate solutions across 1-50 mg/L at controlled pH, then quantified remaining dissolved metal by flame atomic absorption spectroscopy. Sorption isotherms were fit with Langmuir, Freundlich, and Sips models, and DFT calculations compared -COOH complexation with aromatic cation-pi interactions.

Implications

Certification: Do not use this source in food, infant-food, supplement, or ingredient heavy-metal occurrence pools. It supports remediation and input-risk context: modified rice-straw biochar may immobilize Cu and Pb in aqueous or soil-remediation settings, but the paper does not demonstrate edible-crop concentration changes or consumer exposure reduction.

Courses: Useful case study for distinguishing a remediation sorbent endpoint from a food-safety endpoint. A strong Kd or isotherm fit is not a product concentration and cannot be converted into a benchmark percentile.

App: Route as context for soil-remediation, biochar amendment evaluation, and metal immobilization mechanisms. Keep any rice or rice-straw supply-chain implications context-only unless paired with field evidence showing metal changes in soil, crops, or amendments.

Wiki pages this source may touch

• copper
• lead
• soil-to-plant-transfer

Verification notes

The PDF has author attribution and DOI 10.3390/agronomy13051282; no DOI conflict was observed. This is primary laboratory evidence, but not food-occurrence evidence. ingredients and products are intentionally empty because the paper does not measure edible rice, infant cereal, rice-based products, or any consumer product matrix. The source specifies Cu(II) and Pb(II) in aqueous nitrate solutions; frontmatter uses the repo’s broader Cu and Pb metal slugs while this page preserves oxidation-state specificity in prose and tables. The paper’s supplementary Table S1 specific-surface-area values are cited by the authors but not reproduced in the PDF text extracted here, so the source page records the qualitative SSA finding rather than inventing values.

Source-internal K_L unit inconsistency: Table 2 header reports K_L in L/kg, while the Methods equation glossary (Equation 1) labels K_L as mg/L. The wiki tables reproduce the K_L numeric values but do not assert a unit, since the source itself is inconsistent.

Audit subagent 2026-06-02 flagged Table 1 column header “C=O / -COO in C 1s (%)” as misleading — verified against PDF Table 1 (C 1s panel has columns C=C, C-C, C-O, -COO, π-π only; C=O appears only in O 1s deconvolution); corrected header to “-COO in C 1s (%)“. Audit also flagged [[supply-chain/soil-to-plant-transfer]] as a slug outside the taxonomy snapshot — verified against wiki/supply-chain/soil-to-plant-transfer.md; finding was a false positive because the snapshot enumerates only ingredients/products/metals/regulations and supply-chain/ is its own valid directory.

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.