Changes in organic matter composition caused by EDTA washing of two soils contaminated with toxic metals

Erika Jez,Ladislau Martin-Neto,Simon Gluhar,Maria De Nobili,Marco Contin,Carlo Bravo,Domen Lestan

doi:10.1007/s11356-021-15406-z

Abstract

Two soils contaminated with potentially toxic metals (PTMs) contrasting in pH and mineralogy were remediated with CaEDTA, and changes in soil organic matter (SOM) composition were investigated. Previous studies showed no significant loss of SOM from CaEDTA-treated soils, but the results of our study reflected significant decreases (from 46 to 49%) in the free fraction of humic acids (HAs). Remediation affected the composition of the free HA fraction via disturbance of intermolecular bonds — an increase in phenolic and aromatic groups with a simultaneous decrease in carbohydrates — which was confirmed by FTIR spectroscopy in both soils. Because non-radical molecules such as carbohydrates were selectively removed, the concentration of free radicals in the free HA fraction increased in acidic soil. The bound fraction of HAs and fulvic acids (FAs) in SOM, which are important due to their stability and the permanent effects they have on the soil’s physical properties, remained unchanged in both remediated soils. The effect of soil recultivation was observed only in the excitation emission matrix (EEM) fluorescence spectra of HAs. In terms of SOM, CaEDTA soil washing can be considered moderately conservative; however, the restoration of free humic fractions is likely to be a long-term process.

Highlights

Soils contaminated by potentially toxic metals (PTMs) and in particular by lead (Pb), zinc (Zn), and cadmium (Cd) are a widespread problem (Edogbo et al 2020)
In spite of the fact that a higher CaEDTA dose was used (60 versus 100 mmol kg−1), Pb extractability was hindered in the calcareous soil by the abundance of Ca ions, derived from the dissociation of carbonates, which stabilized the CaEDTA complex (Manouchehri et al 2006)
The total soil organic C (TOC) was not significantly changed by the soil washing process, we found a statistically non-significant Total organic carbon (TOC) decrease (−6.1%) in the acidic soil, which did not recover

Summary

Introduction

Soils contaminated by potentially toxic metals (PTMs) and in particular by lead (Pb), zinc (Zn), and cadmium (Cd) are a widespread problem (Edogbo et al 2020). If the concentration of PTMs in soils contaminated by anthropogenic activities is several times the threshold value for toxicity, soil washing can be an effective alternative to solidification/stabilization and landfilling (Liu et al 2021). Several inorganic acids (e.g., hydrochloric, sulfuric, and nitric acid), organic acids (e.g., oxalic, citric, gluconic, and ascorbic acid), synthetic or biodegradable chelating agents (e.g., EDDS, EDTA, GLDA, NTA), and surfactants (e.g., rhamnolipids and sophorolipids) have been tested to improve the efficiency of soil washing (Gluhar et al 2020; Peng et al 2018). EDTA soil washing is an efficient and permanent way of removing PTMs from contaminated soils, it may affect soil properties (Gluhar et al 2020). In addition to targeted PTMs, EDTA forms strong complexes with a variety of cations, including alkaline earth metals

Methods

Results

Conclusion