Flooding by sea and brackish waters enhances mobility of Cd, Zn and Pb from airborne dusts in coastal soils.

Sea level rise and extreme weather conditions caused by climatic changes enhance the frequency and length of submersion events in coastal soils, causing deposited airborne dusts to get in contact with marine salts. The behaviour of Cd, Zn and Pb from pedogenetic minerals and from dusts from mining and smelting activities, added to two soils under different agricultural management (arable and grassland) was examined after soil flooding for 1, 7 and 30 days with waters of increasing salinities (0, 4.37, 8.75, 17.25 and 34.5 g L-1). A rain water event following 1 d flooding released an extra amount of metals. Concentration of potentially toxic elements (PTE), pH, dissolved inorganic and organic C were measured in solutions collected by gravity from soil columns. Speciation distribution of leached metals and oversaturation parameters were calculated by Visual Minteq 3.0 and showed that complexation by chloride ions for Cd and fulvic acids for Pb were the drivers of solubilisation, while Zn interacted with both. Results showed that marine salts enhance up to 300 times leaching of Cd, and several times that of Zn and Pb from contaminated soils and that airborne toxic elements are much more mobilized than pedogenic ones. Smelter exhaust metals, particularly Pb, were made more mobile than those in mine tailings (up to 55 against 0.7 ng µg-1 Pb). Soil management strongly also influence mobilization by saline water: much lower amounts were leached from the grassland soil. Soil organic matter quality (DOC and humification) affects the extent of mobilization. The length of the flooding period did not result in coherent time trend patterns for the three metals, probably because of the multiple changes in solution parameters, but leached metals were always highly linearly correlated negatively with pH and positively with DOC.

Kinetics of electron transfer reactions by humic substances: Implications for their biogeochemical roles and determination of their electron donating capacity.

Humic substances (HS) possess redox active groups covering a wide range of potentials and are used by facultative anaerobic microorganisms as electron acceptors. To serve as suitable electron shuttles for anaerobic respiration, HS should be able to re-oxidize relatively quickly to prevent polarization of the surrounding medium. Mediated electrochemical oxidation and decolorization assays, based on the reduction of the radical ion of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS•-) allow to determine the electron donating capacity (EDC) of HS, but uncertainties remain about the reaction time that should be allowed to obtain environmentally meaningful EDC values. In this work, we performed a kinetic analysis of the time trend of the reduction of ABTS•- by HS by Vis and Electron Paramagnetic Resonance (EPR) spectroscopies and by cyclic voltammetry. We found evidences of two concomitant separate mechanisms of electron exchange: a fast and a slow transfer processes which may have different environmental roles. These results can set a base to identify the appropriate conditions for the spectrophotometric determination of the fast and slow components of the EDC of HS.

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.