Relationship between Cadmium Fractions Obtained by Sequential Extraction of Soil and the Soil Properties in Contaminated and Uncontaminated Paddy Soils

Toshimitsu Honma,Hirotomo Ohba,Tomoyuki Makino,Takuji Ohyama

doi:10.1155/2015/714680

Abstract

The method for the sequential extraction of cadmium from soil was adapted to investigate the relationship between different chemical forms of cadmium in soils and the soil properties of Cd-contaminated and uncontaminated paddy soils. Air-dried soil samples from each field site were sequentially fractionated into five forms: exchangeable Cd, inorganically bound Cd, organically bound Cd, oxide-occluded fraction, and residual Cd. The average and range of soil properties such as pH, total C, total N, CEC, exchangeable Ca, Mg, K, base saturation, available phosphate, particle size distribution, free iron oxide, oxalate extractable Al, and Fe were somewhat similar between uncontaminated and contaminated soils. The average total Cd in uncontaminated and contaminated soils was 0.26 and 0.65 mg kg−1, respectively. The proportions of soil Cd fractions did not differ between the uncontaminated and contaminated soils, although the Cd concentration of several fractions in contaminated soils was statistically higher than those in uncontaminated soils except for residual fraction. The proportion of exchangeable Cd was correlated with the CEC and phosphate absorption coefficient in contaminated soil but not in uncontaminated soil. Thus, soil properties appear to affect the proportions of soil Cd fractions in contaminated soil and should be considered when evaluating soil Cd mobility.

Highlights

Under Japan’s Food Sanitation Act, the Cd concentration of unpolished rice must not be higher than 0.4 mg kg−1 on or after February 28, 2011 [1]
The official method for extracting soil Cd to estimate the extent of soil contamination generally uses a 0.1 mol L−1 HCl solution, but the concentration of Cd estimated by this method tends to be unrelated to the Cd concentration of unpolished rice grains [2, 3]
It is probable that fields in this area produce rice grains with Cd concentrations above 0.4 mg Cd kg−1, the new international threshold established by the Codex Alimentarius Commission [16] for Cd concentrations in brown rice

Summary

Introduction

Under Japan’s Food Sanitation Act, the Cd concentration of unpolished rice must not be higher than 0.4 mg kg−1 on or after February 28, 2011 [1]. Various extraction methods have been tested to determine the plant-available Cd concentration in contaminated soil, and the Cd concentrations in some upland plants have shown high correlations with plant-available soil Cd concentrations [4, 5]. Despite this progress, the association of soil properties with the Cd concentration of paddy rice—especially the rice grain—has not been evaluated [2] because the rapid shifts between submerged and drained conditions affect the solubility of soil Cd due to the change in oxidation-reduction potential. An assessment of potential risk apart from the influence of the water condition of soils is useful for zoning high-risk fields and deciding on countermeasures or remediation

Objectives

Methods

Results

Discussion

Conclusion