Liu et al. 2023 - Heavy metals in Xingren paddy soil and rice

Liu and colleagues measured Pb, Cd, Cr, Hg, As, Cu, and Zn in 12 paired paddy-soil and rice-plant samples collected around an abandoned high-arsenic coal mine in Xingren County, Guizhou, China. The source is routeable rice occurrence evidence for dehulled rice grain, while its soil speciation, transfer-factor, and health-risk calculations are supporting context for contaminated paddy systems.

Key numbers

Dehulled rice grain concentrations

The rice-grain husk was removed before analysis. Section 3.4 reports the following rice-grain concentrations in mg/kg for Pb, Cd, Cr, Hg, As, Cu, and Zn, respectively:

For rice ingestion, Table 2 reports hazard-index values of 4.15 +/- 0.71 for adults and 21.70 +/- 3.76 for children. Rice-ingestion HQ contributions for adults were As 2.14, Pb 0.553, Cd 0.502, Cr 0.464, Cu 0.253, Hg 0.179, and Zn 0.0581. For children, the corresponding HQ values were As 7.22, Cu 8.53, Pb 1.86, Cd 1.69, Cr 1.56, Hg 0.603, and Zn 0.196.

Paddy-soil concentrations and speciation

Figure 2 reports paddy-soil summary concentrations in mg/kg:

Tessier extraction showed Pb mainly in residual, Fe-Mn-oxide-bound, and organic-bound forms. Cd had a higher exchangeable fraction than the other metals, which the authors interpret as a reason Cd was more available for rice uptake even though total soil As was the strongest exceedance signal.

Mean RSP values were Cd 5.73 +/- 1.00, Zn 4.33 +/- 0.93, Hg 3.11 +/- 2.49, Cu 1.39 +/- 0.20, Pb 1.22 +/- 0.26, As 0.04 +/- 0.00, and Cr 0.02 +/- 0.01. Mean RAC values were Cd 16.12 +/- 14.86%, Zn 13.63 +/- 13.87%, Cu 9.47 +/- 11.56%, Pb 8.63 +/- 12.78%, Hg 7.46 +/- 5.74%, Cr 3.85 +/- 5.46%, and As 0.23 +/- 0.28%.

Rice accumulation and transfer

The paper reports bioaccumulation/accumulation ranges from soil to rice for Cd 0.74-1.37, Zn 0.44-0.70, Cu 0.49-0.86, Pb 0.03-0.06, Hg 0.81-2.88, Cr 0.07-0.13, and As 0.15-0.41. Mean accumulation factors were ordered Hg 1.66 +/- 0.63, Cd 0.99 +/- 0.17, Cu 0.59 +/- 0.09, Zn 0.52 +/- 0.07, As 0.22 +/- 0.06, Cr 0.09 +/- 0.02, and Pb 0.04 +/- 0.01.

Root-to-grain transfer factors ranged from 0.48-1.56 for Cd, 0.17-0.56 for Zn, 0.07-0.35 for Cu, 0.07-0.18 for Pb, 0.07-0.22 for Hg, 0.07-0.16 for Cr, and 0.01-0.04 for As. Mean root-to-grain transfer factors were ordered Cd 0.91 +/- 0.28, Zn 0.28 +/- 0.11, Cu 0.18 +/- 0.08, Hg 0.15 +/- 0.05, Pb 0.13 +/- 0.03, Cr 0.10 +/- 0.02, and As 0.02 +/- 0.01.

Methods (brief)

In mid-August 2022, the authors collected 12 surface-soil samples and corresponding rice roots, stems, leaves, and grains from rice planting areas around the Xingren Coal Mine. Soil samples were composite samples from three random sub-locations per sampling point, dried, ground, and sieved through 200 mesh. Plant samples were dried and screened; rice grain husks were removed before grain analysis.

Soil Pb, Cr, Cu, Zn, K, and Cd were digested with HF:HClO4:HNO3, while soil Hg and As used aqua regia digestion. Cd was measured by graphite-furnace atomic absorption spectrometry, Hg and As by atomic fluorescence spectrometry, and other heavy metals by ICP-AES. Soil metal fractions were extracted by Tessier’s five-step sequential extraction method and measured by ICP-MS. The paper states that each sample was analyzed with three parallel samples, a national soil standard reference material (GSS-24) was used for quality control, error was kept within 5%, and recovery was 81-109%.

Health-risk calculations used adult and child exposure parameters, including rice ingestion rates of 420 g/day for adults and 150 g/day for children. The paper treats Pb, Cd, Cr, Hg, As, Cu, and Zn as non-carcinogenic elements and As, Cd, and Cr as carcinogenic elements. It explicitly notes that Cr concentrations are total Cr and that Cr(VI) risk factors were used for a maximum-risk assessment approach.

Implications

Certification: The dehulled-grain values are routeable occurrence evidence for products/rice-bulk-grain and ingredients/rice, but total arsenic must not be substituted for inorganic arsenic and total mercury must not be substituted for methylmercury. The study is China-market/local-field evidence from a high-arsenic coal-mining area, so it should not be silently pooled into a U.S.-market rice benchmark without jurisdiction and source-frame review.

App: Useful for explaining that a soil’s highest total contaminant is not always the element with the strongest plant-transfer signal. In this study, total soil As was the main exceedance, but Cd showed higher exchangeable fraction and root-to-grain mobility.

Courses: Useful for teaching rice-specific soil-plant transfer, Tessier fraction interpretation, total-versus-speciated analyte rules, and how health-risk outputs depend on exposure assumptions.

Wiki pages this source may touch

• rice-bulk-grain
• rice
• lead
• cadmium
• chromium
• mercury-total
• arsenic-total
• copper
• zinc
• agronomic
• index

Verification notes

This page was built from the full PDF, including the sampling section, chemical analysis methods, exposure-parameter Table 1, Figure 2 soil summary values, Tessier extraction/speciation discussion, RSP/RAC values, accumulation and transfer-factor results, Table 2 non-carcinogenic risk values, Table 3 carcinogenic-risk values, discussion, conclusion, and data-availability statement. The PDF extraction duplicates and overprints some lines, so table values were cross-checked against repeated captions and raw-text passages.

Speciation guardrails: the source reports total As and total Hg, not inorganic arsenic or methylmercury. The source explicitly says its Cr concentration is total Cr even though it uses Cr(VI) toxicity factors for risk assessment; this page routes chromium as total/broad Cr and does not treat the study as Cr(VI) occurrence evidence.

Paper-internal caution: Figure 2/table text shows Pb soil SD as 38.20 mg/kg in the repeated caption, while one prose extraction line renders 36.57 mg/kg. The repeated Figure 2 caption value is used here and the discrepancy is flagged rather than silently harmonized. The abstract/conclusion also refer to risk caused by ingesting “rice husk,” although the methods state that husks were removed before grain analysis; this page treats the concentration values as dehulled rice-grain data because that is the stated analytical matrix.

Page history

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