Alleviating cobalt and lead toxicity in rice using zero valent iron (Fe) amendments

Overview

This greenhouse pot study investigated the simultaneous effects of zero valent iron (Fe0) amendments and rice cultivar selection on cobalt and lead uptake, translocation, and bioaccumulation in rice grains. Two Malawian rice cultivars (Faya and Kilombero) were grown under continuous flooding in sand-loam soils amended with Fe0 at 0, 6.20, and 12.40 g Fe0 per kg soil. The study reports grain-level lead and cobalt concentrations and translocation factors, making it relevant as context for understanding iron amendment effects on metal accumulation in rice grain.

This is a soil amendment/pot study measuring metal uptake into grain. It provides context for uptake pathways rather than finished-food concentration survey data.

Key numbers

All values are reported in the paper’s own units (μg/kg dry weight unless otherwise noted). Four replicates per treatment.

Grain cobalt (Faya cultivar):

• Control: 93 ± 26 μg/kg
• 6.20 g Fe0/kg soil: 62 ± 34 μg/kg (33% reduction)
• 12.40 g Fe0/kg soil: 48 ± 11 μg/kg (48% reduction)

Grain cobalt (Kilombero cultivar):

• Inconsistent and non-significant variations across Fe0 dosages.

Grain lead (Faya cultivar):

• Control: 128 ± 25 μg/kg (mean across treatments)
• Faya accumulated 70% less grain-Pb than Kilombero overall.

Grain lead (Kilombero cultivar):

• Control: 423 ± 22 μg/kg
• Both cultivars showed Fe0-driven reduction: Fe0 dosages reduced grain-Pb from 279 ± 12 μg/kg (control mean) to 152 ± 90 μg/kg (49% reduction at 6.20 g/kg) and 86 ± 65 μg/kg (71% reduction at 12.40 g/kg).

Regulatory comparison: Control grain-Pb concentrations for both cultivars exceeded the WHO/EC maximum permissible limit of 200 μg/kg for lead in rice. Fe0-amended rice fell below this limit.

Porewater cobalt reduction: Fe0 reduced available cobalt in porewater by 38% (6.20 g/kg) and 53% (12.40 g/kg) compared to control (15,900 μg/kg).

CRM recovery: Cobalt 84% to 106% (mean 95.0%); lead 76% to 100% (mean 87.5%) using NIST1568a rice flour and DC73319.

Evidence fitness

EF-2: The study provides grain-level metal concentration data for both cobalt and lead across two cultivars and three Fe0 dosages with replication (n = 4 per treatment). Analytical methods used ICP-MS/MS with CRM validation. The greenhouse pot design is controlled but does not replicate field conditions. Grain data are directly relevant to understanding how iron amendments modulate lead and cobalt accumulation in rice grain.

Methods (brief)

Greenhouse pot experiment in Aberdeenshire, Scotland. Sand-loam soils (0 to 12 cm depth) mixed 1:1 with sand (v/v) and spiked with Fe0 at 0, 6.20, or 12.40 g/kg. One seedling per pot (Faya 14M69 or Kilombero), continuously flooded with at least 3 cm water above soil surface. Porewater sampled at 25, 40, 90, 110, and 130 days after transplanting via Rhizon samplers. Shoots and grains ball-milled, digested in 70% nitric acid (Mars 5 system), analyzed by ICP-MS/MS (Agilent 8800) in inorganic mode. Two-way ANOVA for cultivar, Fe0 dosage, and their interaction; Pearson correlations computed.

Implications

This study is relevant as upstream mitigation context for rice metal contamination rather than as finished-food survey data. The key finding that Fe0 soil amendments at 6.20 g/kg or above reduced grain lead below the 200 μg/kg WHO/EC limit in both cultivars, and reduced grain cobalt by up to 48% in Faya, suggests that zero valent iron amendments are a potential agronomic tool for reducing lead and cobalt accumulation in rice. The cultivar-dependent response (Faya more responsive for cobalt, Kilombero more responsive for lead) underscores that amendment efficacy varies by genotype, consistent with findings for arsenic and cadmium in the same experimental system (Mlangeni et al. 2020, 2021).

Provenance notes

• Open access under CC BY-NC-ND 4.0.
• Received 28 March 2022, accepted 17 November 2022.
• Soils were not pre-contaminated with cobalt or lead; background levels apply.
• The study focused on cobalt and lead, which are less commonly studied than arsenic and cadmium in rice systems.
• Fe0 sourced from Beijing North Yongbang Science and Technology, China (average diameter 3.9 ± 1.01 mm).

Related evidence

• Rice
• Lead

Update history

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