Guerra Sierra et al. 2021 — Phytoremediation of heavy metals in tropical soils

Guerra Sierra and colleagues review the characteristics of tropical soils, the principal anthropogenic sources of heavy metal contamination affecting them, and the plant species and mechanisms relevant to phytoremediation in tropical and subtropical settings. The article is a narrative review, not a primary measurement study; it pulls regulatory soil limits, plant-accumulator examples, and emerging-contaminant context (nanoagrochemicals, pesticide residues, biofertiliser-derived metals) from the cited literature. For Heavy Metal Index purposes the source is context-only: it does not measure metal occurrence in food, ingredients, or consumer products.

Key numbers

Regulatory soil limits cited by the review

Table 4 (page 5) summarises soil limit values (mg/kg) drawn from Adnan Tutic et al. and Tóth et al.:

The review also reports (page 4) country-specific permitted arsenic concentrations in agricultural soil: 20 mg/kg in Australia, Canada, and China; 1 mg/kg in Tanzania; 0.11 mg/kg in the EU. Permitted Hg is 200 mg/kg in New Zealand and 0.3–1.0 mg/kg in China, citing reference [61].

Tropical-soil composition reference values

Page 3 reports that suggested organic carbon thresholds in tropical soils are approximately 2% (below which deterioration may occur), and that the carbon content in soils of various uses in the Colombian Caribbean region is 0.5–1%. Iron concentrations in soils worldwide are reported as 20 to 550 g/kg, with Brazilian soils containing iron oxides “from a few grams to about 800 g/kg.” Cuban tropical soils are cited as 69.6% with low organic matter and 43.3% with heavy erosion (reference [27]).

Industrial source–metal pairings

Table 3 (page 5, adapted from Caviedes Rubio et al.) lists metal panels associated with eleven industrial activities. Selected entries:

The review states (page 4) that “most low-quality fertilizers with low Zn content have a relatively high Cd content,” and that biofertilisers derived from pig and poultry manures accumulate “As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, and Sb” in soils.

Emerging-contaminant occurrence values

Page 6 reports glyphosate residues “of up to 1.42 µg/L in groundwater and 0.47 µg/L in human urine in the Yucatan Peninsula, Mexico” (reference [73]). Page 7 reports that Fe₂O₃ nanoparticles at 3.2 mg/kg “significantly reduced the biomass of mycorrhizal clover [by] 34% by reducing glomalin content and nutrient acquisition from arbuscular mycorrhiza fungi roots” (reference [91]).

Hyperaccumulator definitions

Page 10 reports that, by accepted definition, a plant species is considered hyperaccumulating “As or Pb when it accumulates ≥1000 mg/kg in its aerial organs and Cd when it accumulates ≥10 mg/kg (on a dry weight basis)” (reference [129]). The authors cite specific examples (page 10): Thlaspi caerulescens as a hyperaccumulator of Zn, Cd, and Ni; Sedum alfredii of Zn, Pb, and Cd; Trifolium alexandrinum of Cd, Pb, Cu, and Zn.

Tropical-species examples (Appendix Table A1)

The appendix tabulates representative tropical and subtropical species evaluated for phytoremediation. Selected entries:

Body-text figures (page 7) report Cecropia peltata removing 15.7–33.7% of soil Hg.

Cocoa / Cd note

Page 12 states that compared with other cocoa-producing regions, “some countries in Latin America and the Caribbean (LAC) are particularly affected by worrying cadmium levels in cocoa beans that are a concern for manufacturers of products with a high cocoa content,” and that Heliconia psittacorum has been used to remediate Cd-contaminated cocoa soils. No primary cocoa-bean Cd values are reported in this review.

Methods (brief)

Narrative review with 204 references. No primary sampling, no analytical chemistry methods, no statistical analysis. Inclusion criteria, search strategy, and quality appraisal are not described; the review does not follow PRISMA or any other systematic-review protocol, and it cites secondary tables (regulatory limits, source–metal pairings, hyperaccumulator examples) drawn directly from other reviews. Regulatory and occurrence values reported here should be re-verified against their original references before being treated as primary evidence.

Implications

Certification: This source does not contribute to HMT&C threshold-setting evidence pools for food, supplements, or personal care. It supports supply-chain background and educator material on tropical-region contamination sources and remediation options. It is one of several places where soil-regulatory limit values from Tutic et al. and Tóth et al. and country-by-country agricultural-soil limits appear in summary form; route only as soil-context evidence.

Courses: Useful as a structured overview of tropical-soil characteristics (oxisols, ultisols, alfisols, inceptisols, entisols), the principal anthropogenic sources of heavy-metal contamination in tropical agriculture, and the plant families and species that have been evaluated for phytoremediation in tropical and subtropical settings. Particularly relevant for educator content addressing Latin American supply chains where soil contamination is a structural driver of ingredient-side risk (cacao, cassava, rice, and tropical fruits grown in former mining or industrial regions).

App: No contamination_profile blocks are touched by this source. No food, ingredient, or product values are reported.

Microbiome: The review touches arbuscular mycorrhizal fungi (AMF) and plant-growth-promoting rhizobacteria (PGPR) interactions with soil heavy metals (pages 7, 13), including the reduction of mycorrhizal colonisation in glyphosate-treated plants and the suppression of microbial biomass by ZnO, TiO₂, and Fe₂O₃ nanoparticles. Tag for WikiBiome federation review of soil-microbiome-metal interactions; this source is secondary and would only be used as a literature signpost, not as primary evidence.

Wiki pages this source may touch

• lead
• cadmium
• mercury-total
• arsenic-total
• nickel
• chromium
• copper
• zinc
• aluminum
• soil-to-plant-transfer

Verification notes

The PDF DOI 10.3390/su13052574 is open-access under CC BY 4.0 (MDPI Sustainability). All numeric values reported in this page are taken from the explicit figures in the review’s tables or body text and carry page references. Because this is a secondary review, regulatory limits and source attributions reproduced here (Table 3, Table 4, country-specific As/Hg permits) should be re-verified against their original references — Tutic et al. [66], Tóth et al. [67], Caviedes Rubio et al. [54], Aprile et al. [48] — before being treated as authoritative for HMT&C threshold work or course content.

Evidence-tier set to B rather than A because the document is a narrative review without inclusion criteria or systematic methodology, even though Sustainability is a peer-reviewed journal. No primary measurements are produced; the document is a structured synthesis of other reviews and primary studies.

Frontmatter metals: includes tAs (not iAs) and tHg (not MeHg) because the review does not distinguish arsenic species or mercury species when reporting regulatory limits or hyperaccumulator concentrations; Cr (not Cr-VI) for the same reason. Aluminum appears because the review discusses aluminum oxides in tropical soils, but Al is not a primary remediation target in the cited examples. Zinc and copper appear because they figure prominently in both source-metal pairings and hyperaccumulator examples even though they are not HMT&C-program analytes; they support educator and supply-chain content.

No brand names are quoted from the source (the review does not name product brands). Instrument vendors and reference materials are not described, since the document reports no analytical methods of its own.

2026-06-02 audit (fresh-context subagent, ingest-next-manual-fetch-pdf v2 skill, Phase 3 apply): verdict PROMOTE. All 5 checks clean; 3 advisory concerns reviewed. (a) Audit subagent flagged the Colombian Caribbean carbon figure “0.5–1%” as potentially mis-normalized from a “g/kg” source value — verified against PDF p. 3: source explicitly reports “between 0.5–1%”, not “g/kg”; finding was a false positive (likely a PDF-render symbol-confusion artifact). (b) Audit subagent suggested adding “(as reported in source)” hedge to the worldwide-soil Fe range “20 to 550 g/kg” because it appears anomalously high — verified against PDF p. 3: range is reproduced verbatim from the source as the source reports it (ref [36]); the figure is plausible for Fe-rich tropical oxisols (≈2–55% Fe by mass) and the wiki page already attributes it to the source’s page. No hedge added; the verbatim citation is the faithful representation. (c) Auditor questioned whether supply-chain/soil-to-plant-transfer is a real wiki page — verified: wiki/supply-chain/soil-to-plant-transfer.md exists in the corpus. No content changes from this audit pass; verification-note record only.

Page history

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