Effect of Soil Washing with Ferric Chloride on Cadmium Removal and Soil Structure

Jing Dong,Xue Zhang,Yuan Zhou,Jin Yuan,Qi Sun,Longchao Xia

doi:10.3390/app112210956

Abstract

In China, arable soils contaminated with cadmium (Cd) threaten human health. Ferric chloride (FeCl3) is a highly efficient agent that can remove Cd from contaminated soils. However, it is unknown whether FeCl3 damages the soil structure and consequently affects crop growth. In this study, we investigated the impacts of Cd extraction by FeCl3 on the structure of a paddy soil on the basis of comparisons of control (without washing agents) and hydrochloric acid (HCl) treatments. According to our results, the removal efficiency increased with the decrease in soil initial pH, as adjusted by FeCl3. However, the low pH of 2.0 caused a partial loss of soil mineral components, with an Al release of 4.4% in the FeCl3-treated soil versus 1.3% in the HCl-treated soil. In contrast, the amount of released Al was less than 0.2% in the control and in the FeCl3 treatments with initial pH values of 3.0 and 4.0. The washing agents caused soil TOC loss of 27.1%, 17.5%, and 2.76% in the pH 2.0, 3.0, and 4.0 FeCl3 treatments, compared with 15.5% in the initial pH 2.0 HCl treatment. The use of FeCl3 represents an optimum tradeoff between removal efficiency and the loss of soil components to restore Cd-polluted soils by adjusting the initial pH to 3.0 with the addition of FeCl3. Under this condition, the amount of Al loss was less than 0.2%, and the extraction efficiency reached 40.3%, compared to an efficiency of 39.7% with HCl at an initial pH of 2.0. In conclusion, FeCl3 could effectively remove Cd from contaminated soil.

Highlights

Due to rapid industrialization and urbanization, arable soils in China are increasingly subjected to cadmium (Cd) pollution [1]
In a previous study, a significant portion of metal–Ethylenediaminetetraacetic acid (EDTA) complexes were retained in the soil after washing [10], most likely because metal–EDTA complexes, such as Ni–EDTA, Co(II)–EDTA, and Cd–EDTA, have a quinquedentate configuration; the carboxylate group not coordinated to the metal center can be adsorbed onto the soil mineral surface [11]
The removal efficiency increased with the decrease in soil initial pH, as adjusted by FeCl3

Summary

Introduction

Due to rapid industrialization and urbanization, arable soils in China are increasingly subjected to cadmium (Cd) pollution [1]. Ethylenediaminetetraacetic acid (EDTA) is an efficient chelating reagent for extracting Cd from soils due to its strong chelating ability and the high solubility of metal–EDTA complexes in solution [8,9]. In a previous study, a significant portion of metal–EDTA complexes were retained in the soil after washing [10], most likely because metal–EDTA complexes, such as Ni–EDTA, Co(II)–EDTA, and Cd–EDTA, have a quinquedentate configuration; the carboxylate group not coordinated to the metal center can be adsorbed onto the soil mineral surface [11]. In a previous study, the Cd from Cd–LMWOA complexes was adsorbed onto the soil particles with the biodegradation of the LMWOAs. Renewal of LMWOAs is necessary to maintain the steady release of Cd from the soils [12]

Objectives

Methods

Results

Conclusion