This 2021 review published in Toxics synthesizes global literature on heavy metal accumulation in rice and six categories of edible aquatic plants (water spinach, lotus, watercress, water chestnut, taro, and wild rice), with a focus on bioaccumulation mechanisms, health effects, and mitigation strategies. The authors compile findings from sites across Asia (Bangladesh, Sri Lanka, Thailand, China), Africa (Nigeria), and Australia, showing that Cd, As, Cr, Pb, and Hg repeatedly appear in these food plants above WHO/FAO permissible limits. Rice is identified as the dominant exposure pathway for Hg in inland Asian populations due to paddy methylation dynamics, not fish consumption.
Key numbers
Permissible limits compiled from WHO/FAO (Table 2):
- As: 0.1–0.2 mg/kg (cereals/vegetables)
- Cd: 0.05–0.4 mg/kg
- Cr: 1.3 mg/kg
- Hg: 0.03 mg/kg
- Ni: 0.1 mg/kg
- Pb: 0.05–0.3 mg/kg
Observed concentrations compiled across studies (Table 2, mg/kg wet produce, asterisked values exceed permissible limits):
- Rice: As 0.09–0.13, Cd 0.003–0.06, Pb 0.12–0.37*, Cr 2.6–5.3*, Ni 0.01–0.53*
- Water spinach (I. aquatica, Bangkok sites): Hg up to 1.44 µg/kg dry weight; Pb 0.06–1.10*, Cd 1.44*, Zn 16–36
- Indian lotus (N. nucifera, Yichang China): Cd and As exceeding national food standards; edible tuber peel concentrations 1.3–9.0× higher than inner flesh
- Watercress: As 2.0*, Pb 0.86*, Cd 0.10, Cr 0.30*, Zn 0.34
- Rice–fish aquaponic system: Cd in rice grain reached 5.86 mg/kg when Cd added to water (above threshold)
Bioconcentration factors (BCF) in brown rice: As 0.001–0.224, Cd 0.001–2.434, Pb 0.001–0.048.
Average blood Pb in Nigerian children aged 1–6 years: 10.6 µg/dL (exceeding CDC maximum of 10 µg/dL), linked to Pb-contaminated rice varieties.
Methods (brief)
This is a narrative review; no primary analytical method is described. The authors synthesize reported values from ICP-MS, AAS, and ICP-OES studies across the corpus. The review covers rice and the aquatic plant species listed in Table 1. No LOD/LOQ are given for the review itself; these vary by original source. The main limitation is that compiled values aggregate wet-weight and dry-weight data without always distinguishing basis, and geographic diversity is wide.
Implications
Certification: Rice is confirmed as the highest-priority grain matrix for Cd and As; the review supports treating Pb in rice as a secondary concern in certain production regions (Nigeria, Southeast Asia). The peel-vs-flesh gradient in lotus root illustrates why whole-vegetable vs edible-part matrix definitions matter in certification scope.
Courses: Useful overview-level source for modules on aquatic food plant contamination, paddy methylation of Hg, and the distinction between essential micronutrients (Fe, Zn, Cu) and non-essential toxic metals. Covers the food-chain exposure narrative clearly.
App: Supports flagging water spinach, lotus root, and watercress as higher-risk aquatic vegetables. Reinforces rice as a high-priority ingredient for As and Cd profiling.