Liu et al. 2022 — Heavy metals in rice, maize, and soil, Chengdu Plain, China
This study quantified concentrations of As, Cd, Hg, Pb, Cu, and Zn in rice grain (n = 40), maize grain (n = 10), and their respective soils (n = 50 total) from the Shifang area of the Chengdu Plain, Sichuan — a region where heavy industrial activities (metallurgy, chemicals, cement, phosphorus mining) coexist with intensive rice and maize cultivation. Analysis was by AAS (graphite furnace for Pb, Cd, Cu, Zn) and AFS (atomic fluorescence spectrometry) for As and Hg. A certified rice flour reference material (GBW10044) was used for quality control with recoveries of 91–105% across all metals.
The dominant finding is severe cadmium contamination: mean Cd in rice was 0.46 mg/kg and in maize 0.26 mg/kg (dry weight). 78% of all grain samples failed China’s national maximum limit for Cd (GB 2762-2017: 0.2 mg/kg rice; 0.1 mg/kg other grains). Mean Pb in rice (0.30 mg/kg) and maize (0.29 mg/kg) also exceeded the national limit of 0.2 mg/kg for both. Soil Cd mean (0.84 mg/kg) was approximately six times the local background value (0.14 mg/kg) and 98% of paddy soils exceeded China’s risk-control screening value (0.30 mg/kg). Estimated daily intakes (EDI) for As, Cd, Hg, Cu, and Zn exceeded oral reference doses for adults consuming local staple quantities; Pb EDI was below its reference dose.
Key numbers
- n = 40 rice, 10 maize grain samples; n = 50 soil samples.
- Analytical method: AAS (Analyst 800 P.E. with THGA-800 graphite furnace) for Pb, Cd, Cu, Zn; AFS-830a for As and Hg.
- Rice Cd: range 0.09–1.78 mg/kg dw; mean 0.46 ± 0.06 mg/kg dw; MPL 0.2 mg/kg.
- Maize Cd: range 0.05–0.77 mg/kg dw; mean 0.26 ± 0.08 mg/kg dw; MPL 0.1 mg/kg.
- 78% of all grain samples exceeded national Cd MPL.
- Rice Pb: range 0.06–0.58 mg/kg dw; mean 0.30 ± 0.02 mg/kg dw; MPL 0.2 mg/kg.
- Maize Pb: range 0.02–0.50 mg/kg dw; mean 0.29 ± 0.05 mg/kg dw; MPL 0.2 mg/kg.
- Rice As: range 0.04–0.17 mg/kg dw; mean 0.07 ± 0.01 mg/kg dw; MPL 0.15 mg/kg for rice, below limit on average.
- Rice Hg: range 0.002–0.21 mg/kg dw; mean 0.01 ± 0.01 mg/kg dw; MPL 0.02 mg/kg; maximum value (0.21) exceeds MPL.
- Soil Cd mean: 0.84 mg/kg (range 0.51–1.90); background 0.14 mg/kg; RSV 0.30 mg/kg at pH 6.5–7.5.
- Soil As mean: 8.01 mg/kg; RSV 30 mg/kg (within limit).
- Basis: dry weight throughout.
- Note: As reported as total arsenic (tAs); no speciation into iAs vs organic As.
Methods (brief)
Systematic random sampling on a 3 km × 3 km grid; 5–7 subsamples composited per site. Soil digested with HNO3/HClO4/HF; crop grain digested with HNO3/HClO4 (5:1). CRM validation: rice flour GBW10044 (91–105% recoveries) and Sichuan basin soil GBW07428. EDI and target hazard quotient (THQ) calculated using USEPA methodology. Limitation: As reported as total, not inorganic; Hg reported as total, not methylmercury; dry-weight basis throughout.
Implications
Certification: This dataset shows extreme Cd pollution in a specific Chinese industrial-agricultural interface zone. Mean rice Cd of 0.46 mg/kg dw is approximately 2.3× the Chinese limit and would fail HMT&C certification at any threshold level. The 1.78 mg/kg maximum represents a 9× exceedance. Highly relevant for supply-chain risk profiling of Chinese-origin rice. Courses: Illustrates the industrial-agricultural coexistence risk model; quantifies soil-to-grain transfer in high-contamination contexts; provides full health risk assessment (EDI, THQ) worked example. App: Rice Cd contamination profile — Chinese industrial zone data contributes to geographic_breakdown for elevated Cd in specific Chinese origin regions.