Ewubare et al. 2024 — Heavy metals in the environment: effects on soil, plants, human health, and possible solutions (AJEE narrative review)
This open-access narrative review in the American Journal of Environmental Economics surveys heavy-metal sources, soil and plant uptake, human-health effects, and phytoremediation/microbial remediation strategies. The seven Nigerian co-authors (Department-of-Chemistry-affiliated, four institutions in Edo and Imo States) frame the review around three coupled compartments — soil, plants, humans — and assemble regulatory limits from WHO, FAO/Codex, the EEA, and the European Union, plus two African case studies and one global vegetable-accumulation comparison. The paper is a teaching-grade synthesis rather than a primary-data report. It cites Hazrat et al. (2019), Pourret et al. (2021), and Das et al. (2023) as recent umbrella references; no original sampling or analytical work is reported. Evidence tier C: useful as a leads document and as a textbook-style overview of the soil–plant–human pathway, not citable as direct evidence for occurrence values. All quantitative values appearing on this page are reattributions of cited primary sources.
Key numbers
WHO drinking-water guideline values cited by the review (Section “Heavy Metals and Human Health”, attributed to WHO 2004)
The review reports these as drinking-water permissible limits in ppm:
- Hg — 0.001 ppm
- Cd — 0.005 ppm
- Pb — 0.05 ppm
- Cr — 0.05 ppm
- As — 0.05 ppm
These values must be re-verified against the current WHO Guidelines for Drinking-water Quality before any certification-document use. The current WHO guideline value for Pb is 0.01 mg/L (provisional) and for As is 0.01 mg/L (provisional); the values the review cites as “WHO 2004” predate the current edition.
FAO/WHO Codex maximum limits for heavy metals in vegetables cited by the review (attributed to Wu 2014, Kesson et al. 2015)
- Hg — 0.05 mg/kg for all vegetables
- Cd — 0.2 mg/kg for leafy vegetables; 0.3 mg/kg for root vegetables; 0.1 mg/kg for other vegetables
- Pb — 0.15 mg/kg for all vegetables
- Cr — 0.1 mg/kg for all vegetables
- As — 0.1 mg/kg for all vegetables
Cross-check against Codex STAN 193-1995 (current revision) before applying. The Cd entries match the Codex revisions for leafy vs. other vegetables; the Cr entry is not a Codex value and likely originates from a national or supplementary standard.
EEA soil pollutant limit values cited by the review (attributed to European Union 2002; Baritz et al. 2023)
In ppm:
- Hg — 0.20 ppm
- Cd — 0.44 ppm
- Pb — 0.48 ppm
- Cr — 0.20 ppm
- As — 0.11 ppm
The review presents these as European Environment Agency soil-pollutant limit values. Verify against the EEA 2023 Soil-monitoring report and current Member State sludge/soil directives; the Pb and Cr values in particular look low relative to typical EU soil-quality guidelines.
WHO 1996 permissible limit for copper in vegetables cited within the Okra case study
- Cu — 10 mg/kg in vegetables (attributed to WHO 1996, “Permissible limits of heavy metals in soil and plants”).
Omokaro et al. (2023) — Copper in Okra (Abelmoschus esculentus) cultivated on dumpsite soils in Benin City, Nigeria (cited values, not primary)
- Cu in Okra on residential dumpsite soil — 36.0 mg/kg (highest uptake).
- Cu in Okra on farmland — 23.3 mg/kg (lowest uptake).
Both values exceed the WHO 1996 vegetable limit of 10 mg/kg cited above. The review does not specify wet or dry weight; cross-check with Omokaro et al. (2023, American Journal of Food Science and Technology 2(2):65–73, DOI 10.54536/ajfst.v2i2.2233) before applying.
Alexander et al. (2006) — Cd and Pb accumulation by six vegetables grown on heavy-metal-contaminated soil (cited values, not primary)
Cadmium concentrations (mg/kg dry matter) by species, descending:
- Lettuce — 8.6
- Spinach — 5.8
- Onion — 3.6
- Carrot — 2.0
- Pea — 0.29
- French bean — 0.07
Lead concentrations (mg/kg dry matter) by species, descending (sequence reported as “lettuce ≈ 2× onions”):
- Lettuce — 14.6
- Onion — 7.5
- Carrot — 5.8
- Spinach — 1.8
- Pea — 0.78
- French bean — 0.34
Verify against Alexander et al. (2006, Environmental Pollution 144(3):736–745) before applying. The species-by-species ranking is the review’s main quantitative observation about between-vegetable variation.
Exposure-pathway ratio cited by the review (attributed to Khan et al. 2015)
- ≈90 % of human exposure to heavy metals via consumption of edible crops.
- ≈10 % via inhalation of polluted air particles.
Other figures the review reports as standalone statements
- WHO statement, attributed via Abudawood et al. (2021) and Bhardwaj et al. (2021): higher exposure to heavy metals puts ~10 % of women at risk of infertility.
- Sparks et al. (2003): heavy metals have specific gravity ≥ 5 g/cm³ — the operating definition the review adopts.
- Cd specific gravity — 8.65× water (from review’s Cd toxicology section, attributed to Sobha et al. 2007).
Methods (brief)
Narrative review of secondary literature. No PRISMA, no inclusion/exclusion criteria, no quality assessment, no quantitative synthesis. Databases searched (per the Methodology section): Google Scholar, Frontier in Microbiology, African Journals Online (AJOL), Scopus, Web of Science, ScienceDirect, Directory of Open Access Journals (DOAJ). Search keywords: “heavy metals in the environment,” “heavy metal and human health,” “heavy metal and soil,” “heavy metal and plants.” Selection criterion: relevance, recency, and credibility of the publishing venue (the review does not define these terms operationally). No data extraction protocol described. Two author-created figures (Venn diagram of soil/plants/humans; bioremediation schematic) and two figures redrawn from Das et al. (2023). Published August 10, 2024 (received July 3, 2024; accepted August 6, 2024). DOI 10.54536/ajee.v3i1.3261, open access via e-palli (Delaware, USA). The 36-day review-to-acceptance window suggests light peer review.
Phytoremediation processes the review names and briefly describes:
- Phytoextraction (also phytoaccumulation) — root uptake, translocation to aerial parts, deposition in vacuoles; named hyperaccumulator candidates include Brassica juncea, Cassia alata, Celosia argentea, Kummerowia striata, Helianthus annuus, Momordica charantia, Nicotiana tabacum, Salix mucronata, Salix viminalis, Solanum lycopersicum, Solanum melongena, Swietenia macrophylla, Pteris vittata, Vigna unguiculata.
- Rhizofiltration — root assimilation from wastewater and groundwater; sunflower, tobacco, spinach for Pb.
- Phytostabilization — root-surface capture; black nightshade, sunflower, cowpea; Miscanthus giganteus, Avena sativa, Sinapis alba for soil stabilization.
- Phytovolatilization — uptake and atmospheric release of less-toxic volatile forms via leaves and stems.
- Phytodegradation (phytotransformation) — in-plant metabolism and degradation of organic compounds. The review groups inorganic heavy metals under this label without explaining why organometallic conversion would apply; phytodegradation is more properly an organic-pollutant pathway. Cite cautiously.
Microbial-remediation processes the review names:
- Bioaugmentation — adding specific microorganisms to a contaminated site.
- Biostimulation — modifying site conditions to promote native microbial activity.
- Mycoremediation — fungal degradation via extracellular ligninolytic enzymes (peroxidase, laccases); fungi named include Aspergillus sp., Bjerkandera adusta, Coriolus versicolor, Cryptococcus sp., Hirschioporus laricinus, Inonotus hispidus, Mucor sp., Penicillium sp., Phanerochaete chrysosporium, Phlebia tremellosa, Pleurotus sp., Trametes versicolor.
- Microalgal remediation — for nitrogen, phosphorus, carbon, BOD, and heavy-metal removal in wastewater treatment.
Implications
Certification: Limited direct value. The compiled WHO, FAO/Codex, and EEA limits are useful as cross-references for the auditor-guidance regulatory-context section but must be re-verified against the underlying primary documents before being inserted into HMTc certification text. The phytoremediation and microbial-remediation taxonomies are too high-level for supply-chain mitigation guidance.
Courses: Suitable as an introductory reading for a soil–plant–human pathway module. The species lists of phytoremediation candidates are useful starting points for the “plant-based contamination control” module. The review’s misclassification of inorganic heavy metals under phytodegradation should be flagged in any teaching adaptation.
App: Not a source of concentration values. The cited Alexander 2006 between-vegetable Cd/Pb ranking informs qualitative copy about leafy-vegetable risk but cannot drive app threshold logic without going back to the primary source.
Wiki pages this source may touch
- lead
- cadmium
- mercury-total
- mercury-methyl
- arsenic-total
- chromium
- chromium-hexavalent
- nickel
- copper
- zinc
- manganese
- cobalt
- antimony
- thallium
- molybdenum
- vegetables
- leafy-vegetables
- lettuce
- spinach
- carrot
- onions
- beans
Page history
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