Chemical speciation and mobilization of copper and zinc in naturally contaminated mine soils with citric and tartaric acids

Abstract

A one-step extraction procedure and a leaching column experiment were performed to assess the effects of citric and tartaric acids on Cu and Zn mobilization in naturally contaminated mine soils to facilitate assisted phytoextraction. A speciation modeling of the soil solution and the metal fractionation of soils were performed to elucidate the chemical processes that affected metal desorption by organic acids. Different extracting solutions were prepared, all of which contained 0.01 M KNO₃ and different concentrations of organic acids: control without organic acids, 0.5 mM citric, 0.5 mM tartaric, 10 mM citric, 10 mM tartaric, and 5 mM citric +5 mM tartaric. The results of the extraction procedure showed that higher concentrations of organic acids increased metal desorption, and citric acid was more effective at facilitating metal desorption than tartaric acid. Metal desorption was mainly influenced by the decreasing pH and the dissolution of Fe and Mn oxides, not by the formation of soluble metal–organic complexes as was predicted by the speciation modeling. The results of the column study reported that low concentrations of organic acids did not significantly increase metal mobilization and that higher doses were also not able to mobilize Zn. However, 5–10 mM citric acid significantly promoted Cu mobilization (from 1 mg kg⁻¹ in the control to 42 mg kg⁻¹ with 10 mM citric acid) and reduced the exchangeable (from 21 to 3 mg kg⁻¹) and the Fe and Mn oxides (from 443 to 277 mg kg⁻¹) fractions. Citric acid could efficiently facilitate assisted phytoextraction techniques.

Introduction

Mining and smelting activities contribute significantly to metal contamination due to the discharge and dispersion of mine wastes into nearby agriculture soils, food crops and water ecosystems (Navarro et al., 2008). The accumulation of heavy metals in soil can negatively impact the health of humans and animals, plant growth and soil microbial activity (Vamerali et al., 2010).

A promising strategy for remediating metal-contaminated sites involves phytoextraction, an in situ technique in which plants are used to remove pollutants from the environment (Vamerali et al., 2010). However, the slow desorption of metals in soils has been a major limitation for successful phytoextraction. The bioavailability of metals is affected by several soil factors, such as pH, cation exchange capacity, organic matter content, the speciation of the metal and the metal itself (Evangelou et al., 2007).

Phytoextraction can be improved by the addition of appropriate amendments or chelating agents to the soil to increase metal mobility and enhance its uptake by plants. Various synthetic chelating agents have been used in assisted phytoextraction studies. One of them is ethylenediaminetetraacetic acid (EDTA), which is probably the most efficient chelating agent for enhancing metal uptake by plants (Blaylock et al., 1997). However, the slow degradation rate and the long persistence of EDTA in the soil could lead to an increased risk of groundwater pollution due to the leaching of metals; moreover, this compound could be toxic for plants and microorganisms (Evangelou et al., 2007).

Several compounds have been proposed as alternatives to EDTA and other synthetic chelating agents. These proposed compounds should be a compromise between their fast degradation and their ability to efficiently enhance phytoextraction. Among them, low-molecular-weight organic acids (LMWOAs) are natural compounds that originate from root exudates, microbial metabolites and the decomposition of soil organic matter (Jones, 1998). These easily biodegradable carboxylic acids, such as citric and tartaric acids, reduce soil pH and are capable of forming soluble complexes, thereby playing an important role in metal mobility and in the subsequent accumulation of metals in plants (Evangelou et al., 2007). However, several studies have observed low extraction efficiencies when these compounds have been applied at low doses because of their rapid biodegradation and their sorption onto soil particles (Evangelou et al., 2006, Evangelou et al., 2008, Liu et al., 2008). Although the effects of LMWOAs on metal desorption from soils with induced contamination have been widely studied (Qin et al., 2004, Evangelou et al., 2006, Evangelou et al., 2008, Nascimento, 2006, Schwab et al., 2008), there is limited information on the desorption behavior of metals from naturally contaminated soils with long aging times.

The aims of this study were to assess and compare through a one-step extraction procedure and a soil column experiment the effects of two natural LMWOAs, citric and tartaric acids, which are commonly present in soils, on Cu and Zn mobilization in naturally contaminated mine soils to facilitate future assisted phytoextraction techniques. This study also attempted to elucidate the main chemical processes that affect metal desorption by LMWOAs in these soils by means of speciation modeling using Visual Minteq and metal fractionation.