Remediation techniques and heavy metal uptake by different rice varieties in metal-contaminated soils of Taiwan: New aspects for food safety regulation and sustainable agriculture

Abstract

Rice is one of the most important staple foods worldwide. Soil contamination with heavy metals and food safety problems occur in many countries as a result of numerous human activities, particularly wastewater and solid waste disposal. This review paper provides a schematic summary of heavy metals in identification processes, transport in soil to different rice varieties, and soil remediation strategies and techniques surrounding the agroenvironmental impact in paddy soils based on a description of Taiwan's experiences and database. In terms of the soil control standard, heavy metals including As, Cd, Cr, Cu, Hg, Ni, Pb and Zn are regulated by the Soil and Ground Water Pollution Remediation Act of Taiwan. Owing to the heavy metal source from wastewater along irrigation systems, heavy metals not only accumulate in the surface soil (0-30 cm), but are also highly distributed at the main entrance of irrigation water into individual paddy fields. Moreover, sediments in the irrigation canal have to be dredged and the irrigation system needs to be isolated from the discharge system of wastewater to maintain soil quality. Cadmium in rice grains accumulates more significantly in Indica varieties than in Japonica varieties, and this accumulation exceeds the food quality standard. The best well-performing metal uptake models have been developed to predict Cd levels in rice grains for Indica and Japonica varieties using soil bioavailable Cd and Zn concentrations extracted by 0.01 mol L )1 CaCl 2 . Soil remediation techniques, including turnover and dilution, in situ stabilization by chemical amendments and phytoremediation, have been tested and recommended in Taiwan. Although the high background levels of As, Cr and Ni, which were higher the soil control standard in some paddy soils, are derived from andesite and serpentinites in Taiwan, rice quality and yield were not adversely affected by these metals when labile concentrations were very low. Overall, it is necessary to identify the bioavailability of heavy metals in different soil types from specific case studies to provide reliable parameters for health-based risk assessments and to further achieve the goal of food safety and sustainable agriculture.

Subcellular Distribution of Cadmium in Two Paddy Rice Varieties with Different Cooking Methods

“…This was not in agreement with the observations of many researchers that have shown a 1.5 to 3.0 times higher Cd concentration accumulating in the grains of the Indica variety compared with the Japonica variety [32] [33], which is related to the oxidizing and translocation capacity of the roots [12] [34]. Although more Cd accumulated in the roots and shoots of the Japonica variety, the accumulated Cd would not be easily transferred to the grains.…”

“…He et al [17], in studying fish, reported that cooking treatments affected the subcellular distribution of HMs, and thus, the trophic availability of copper and zinc. Approximately half of the arable land in Taiwan is currently used to plant paddy rice, and Japonica is the major variety (approximately 90%) because of taste [12]. This desirability makes this variety more common than the Indica variety.…”

“…The ability to take up, transport, and accumulate Cd differs amongst rice cultivars [11]. Field studies in Taiwan have shown that more Cd accumulates in the grains of Indica varieties than in Japonica [12]. Furthermore, even when the total Cd concentration in the soil was lower than the control standard for croplands (5 mg/kg) enacted by Taiwan's Environmental Protection Agency (EPA), more than 40% of the Indica variety accumulated more than 0.4 mg/kg (food quality standard; FQS) of Cd in the grains [12].…”

“…This desirability makes this variety more common than the Indica variety. Although various paddy rice varieties have been planted in different Cd-contaminated sites to test the differences in accumulation capacity between varieties [12], knowledge of the factors governing Cd uptake and sequestration is insufficient. No studies on the effect of cooking treatments on the subcellular distribution and trophic availability of Cd in paddy rice grains have yet been published.…”

Global Experiences on Wastewater Irrigation: Challenges and Prospects

“…Loutfy et al ( 2010 ) showed all the studied contaminants in Egypt were much higher in henna and rosemary plants irrigated with wastewater, while in the case of Moghat (dried powder of peeled roots of Glossostemon bruguieri plants) samples, no difference was observed between freshwater irrigated samples and wastewater irrigated samples. Despite enriched As, Cr and Ni in some paddy soils from andesite and serpentinites in Taiwan, Hseu et al ( 2010 ) found these metals were not clearly accumulated in rice. The irrigation of olive trees and vegetable crops with treated wastewater in Jordan did not show any adverse effect on the chemical properties of fruits and leaves (Al-Hamaiedeh and Bino 2010 ).…”