Remediation of cadmium contamination in paddy soils by washing with chemicals: Selection of washing chemicals

doi:10.1016/j.envpol.2006.01.017

Environmental Pollution

Volume 144, Issue 1, November 2006, Pages 2-10

https://doi.org/10.1016/j.envpol.2006.01.017 Get rights and content

Abstract

The efficiencies of neutral salts, strong acids, and chelates were tested for extracting cadmium (Cd) from three paddy soils. The higher the selectivity of the cations of the added neutral salts toward soil adsorption sites, the lower the pH in the extracts and the more soil Cd could be extracted. In addition, soil carbon and nitrogen contents and mineral composition were closely associated with the amount of Cd extracted. Calcium chloride and iron(III) chloride were selected as wash chemicals to restore Cd-contaminated paddy soils in situ. Washing with calcium chloride led to the formation of Cd chloride complexes, enhancing Cd extraction from the soils. The washing also substantially decreased soil levels of exchangeable and acid-soluble Cd, which are the major forms of bioavailable Cd for rice (Oryza sativa L.). The optimum conditions for in situ soil washing were also determined for calcium chloride.

Introduction

Japanese paddy fields have suffered from serious soil pollution by cadmium (Cd) and various heavy metals caused by the rapid increase in industrial activities during the 1960s (Kitagishi and Yamane, 1981). This contamination led to the establishment of Japan's Agricultural Land-Soil Pollution Prevention Law in 1970. This law designated certain paddy fields, in which rice (Oryza sativa L.) crops contained more than 1 mg Cd kg⁻¹, in its unpolished grains, as “polluted paddy fields”. In these fields, permanent countermeasures should be taken to decrease the Cd content in rice grains to levels below 0.4 mg Cd kg⁻¹. The number of polluted paddy fields in Japan had expanded to 66 sites, with a total area of some 62.6 km², as of 1996. The use of unpolluted soil as a top dressing has been the primary permanent countermeasure used to decrease the Cd content in rice grains.

Recently, the CODEX Alimentarius Commission of the United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO) proposed a new international standard for Cd concentrations in a variety of staple foodstuffs; for polished rice, this level equals 0.4 mg Cd kg⁻¹(CODEX, 2005). The Japanese government consulted with the Committee for Foodstuff Safety to review the Cd concentrations in rice grains.

Since about half of the Cd intake of the Japanese people comes from rice (Toyoda et al., 1998), a staple food, it is urgent that Japan decreases the Cd content in rice to values as low as possible. Thus, it has become a priority to develop cost-effective and environmentally sound technologies for the restoration of Cd-contaminated paddy fields.

A variety of in situ (on-site) and ex situ remedial methods have been used for the restoration of soils contaminated with heavy metals. These include excavation and landfilling, solidification, stabilization, thermal treatment, electroreclamation, phytoextraction, phytovolatilization, and soil washing (Vangronsveld and Cunningham, 1998, Calmano et al., 2001). Although soil-washing techniques offer the great advantage of a high Cd-removal efficiency in contaminated soils, they are generally not environmentally sound technologies, and are thus not directly applicable to paddy fields. In addition, Cd-contaminated paddy fields are distributed across the nation and Cd concentrations are skewed toward low values (less than 1 mg Cd kg⁻¹, oven-dry basis). This wide distribution of low levels of Cd contamination makes it extremely difficult to transport contaminated soils to an ex situ treatment plant. However, there are many intractable problems posed by in situ detoxification of contaminated soils, such as difficulty of selection of environmentally friendly wash chemicals, cost-effectiveness, and the use of environmentally sound technologies for treatment of wastewater, among others.

In view of these problems, we established four objectives to guide the development of potential on-site detoxification technologies for Cd-contaminated paddy soils:

1.
The identification of wash chemicals with a minimal environmental impact on the paddy field and its surrounding environment, but with a high Cd-removal efficiency.
2.
The potential for a cost-effective and environmentally sound soil-washing and on-site wastewater-treatment system.
3.
Preservation of favorable soil fertility and plant growth after the washing treatment.
4.
The stability of the washing effect.

The objectives of this first paper are to report the results of efforts to select promising washing chemicals and to determine the optimal washing conditions for an on-site washing trial.

Section snippets

Soils

Soil samples were collected from the plow layers of three paddy fields in Nagano, Toyama, and Hyogo prefectures. Soils were sampled from five points in each parcel of fields and were mixed. All samples were air-dried at 25 °C and a relative humidity of approximately 60%, and were then passed through a 2 mm mesh sieve before use.

Analysis of soil chemical properties

Soil pH was determined by means of the glass-electrode method (Horiba, PH81, Japan) with a ratio of soil to either water or 1 mol L⁻¹ KCl of 1:2.5. The total carbon (TC)

Soil properties

The relevant chemical properties of the soils used in this study are summarized in Table 2. X-ray diffraction analysis identified kaolin minerals, mica, and chlorite in the Nagano and Hyogo soils, and kaolin minerals, chlorite, smectite, and chlorite-smectite intergrade minerals in the Toyama soil.

The total Cd contents of the soils increased in the following order (Table 2): Nagano < Toyama ≪ Hyogo. These Cd concentrations were substantially higher than the mean values in uncontaminated soils,

Conclusions

Calcium chloride and FeCl₃ were selected as soil-washing chemicals for use in Cd-contaminated paddy soils on the basis of their Cd-extraction efficiency, cost-effectiveness, and relatively low environmental impacts. The high efficiency of CaCl₂ in the extraction of soil Cd was mainly attributable to the high selectivity of Ca for soil adsorption sites compared with Cd, the concurrent lowering of the solution pH, and the formation of Cd–Cl complexes. The successive formation of Cd–Cl complexes

Acknowledgements

The authors gratefully acknowledge the useful suggestions during this work by Dr H. Imai, Dr M. Saito, Dr T. Otani and Dr S. Ono, National Institute for Agro-Environmental Sciences. Thanks are also due to Dr Y. Maejima for his comments in the classification of soils, and to Ms J. Hino, G. Bao, F. Ochida, M. Yasuda, T. Ishijima, M. Asakawa, and M. Miyata for their assistances in experiments. This work was supported in part by a Grant-in-aid (Hazardous Chemicals) from the Ministry of Agriculture,

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Remediation of cadmium contamination in paddy soils by washing with chemicals: Selection of washing chemicals

Abstract

Introduction

Section snippets

Soils

Analysis of soil chemical properties

Soil properties

Conclusions

Acknowledgements

Journal of Hazardous Materials

Chemosphere

Adsorption of lead, copper, zinc, cobalt, and cadmium by soils that differ in cation-exchange materials

Journal of Soil Science

Nickel, cadmium, and lead

Contamination in soil-plant system by cadmium, zinc, lead and copper

Models predicting the Cd concentrations in the winter wheat grain using topsoil analytical data

Clean-up and assessment of metal contaminated soils

Removal of heavy metals from contaminated soil and sediments using the biosurfactant surfactin

Journal of Soil Contamination

Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride

Soil Science Society of America Proceeding

Chloride as a factor in mobilities of Ni(II), Cu(II), and Cd(II) in soil

Soil Science Society of America Journal

Oxalate extraction of Pb and Zn from polluted soils: solubility limitations

Journal of Soil Contamination

Particle size analysis

Visual MINTEQ, Version 2.30

Cadmium, lead, zinc, copper and nickel in agricultural soils of the United States of America

Journal of Environmental Quality