Chiutula et al. 2025 — Heavy metal accumulation in wastewater-irrigated vegetables, Blantyre, Malawi
Urban and peri-urban farmers in Blantyre, Malawi irrigate vegetable plots with treated effluent from the Soche Wastewater Treatment Plant. This study measures cadmium (Cd), chromium (Cr), lead (Pb), zinc (Zn), and copper (Cu) in wastewater, soils, and six vegetable species — three exotic (Brassica rapa subsp. chinensis, Brassica napus, Brassica rapa) and three indigenous (Cucurbita moschata leaves, Ipomoea batatas leaves, Amaranthus retroflexus leaves) — using flame AAS. Several measured concentrations exceed FAO/WHO permissible limits, and health risk index (HRI) calculations flag Brassica species as presenting potential chronic risk.
Key numbers
All concentrations are in mg/kg dry weight (AAS, flame atomization, acid digestion per APHA standard methods). Instrument detection limits: Cd 0.001, Cr 0.003, Pb 0.005, Zn 0.002, Cu 0.002 mg/L.
Soil (mean ± SD, n=3):
- Cd: 0.24 ± 0.01 mg/kg (below WHO limit of 0.8 mg/kg)
- Cr: 38.14 ± 1.35 mg/kg (below WHO limit of 100 mg/kg)
- Pb: 11.57 ± 0.06 mg/kg (below WHO limit of 85 mg/kg)
- Zn: 56.40 ± 0.54 mg/kg (exceeds WHO limit of 36 mg/kg)
- Cu: 23.62 ± 0.55 mg/kg (below WHO limit of 50 mg/kg)
Wastewater: Cd, Cr, and Pb all below detection limits; Zn 0.01 ± 0.001 mg/L; Cu 0.02 ± 0.018 mg/L.
Vegetables — key exceedances vs. FAO/WHO limits (Cd 0.2 mg/kg; Cr 2.3 mg/kg; Pb 0.3 mg/kg):
- Cr: 64% of vegetable samples exceeded 2.3 mg/kg. Highest in Cucurbita moschata stems (4.65 mg/kg) and Brassica napus stems (4.20 mg/kg).
- Cd: Several samples exceeded 0.2 mg/kg. Highest in Amaranthus retroflexus leaves (0.31 mg/kg).
- Pb: All vegetable species exceeded 0.3 mg/kg. Highest in Brassica rapa stems (4.09 ± 0.085 mg/kg).
- HRI (health risk index) values: Brassica rapa stems HRI = 92.3; Brassica rapa subsp. chinensis leaves HRI = 82.2 — both far exceeding the safe threshold of HRI > 1.
Note: The study measures TOTAL chromium (Cr); speciation into Cr(III) vs. Cr(VI) was not performed. The Cr data should not be treated as Cr-VI.
Methods (brief)
Agilent 240FS Atomic Absorption Spectrophotometer (AAS), flame atomization, air-acetylene flame. Vegetable samples oven-dried 60°C/12h, acid-digested (HNO3/HCl, 105°C/6h). One-way ANOVA and Duncan post hoc tests. PCA. Single time-point sampling (November, late dry/early rainy season). Authors note seasonal limitation: the study captures a snapshot and does not address temporal variability.
Implications
Certification: Supports vigilance for Pb, Cd, and total Cr in leafy vegetables and brassica species grown under wastewater irrigation or near industrial sites in sub-Saharan Africa. HRI values are extremely high for Brassica stems, raising consumer protection concerns. Courses: Case study for supply-chain contamination pathways (wastewater irrigation → soil accumulation → plant uptake). App: Contributes concentration ranges for leafy greens under wastewater-irrigation contamination conditions. Context is geographically specific (Malawi urban peri-agriculture) and may not generalize to commercial supply chains in certified markets. Microbiome: Not applicable.
Note on speciation: Study reports total Cr and does not speciate Cr(VI). Total Cr values may not be used as proxies for Cr(VI) exposure.