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

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.