Abstract
Cadmium, Cu, Ni, Pb, and Zn removal via soil flushing with tannic acid (TA) as a plant biosurfactant was studied. The soil was treated for 30 h in a column reactor at a constant TA concentration and pH (3%, pH 4) and at variable TA flow rates (0.5 mL/min or 1 mL/min). In the soil leachates, pH, electrical conductivity (EC), total dissolved organic carbon, and metal concentrations were monitored. Before and after flushing, soil pH, EC, organic matter content, and cation exchange capacity (CEC) were determined. To analyze the organic matter composition, pyrolysis as well as thermally assisted hydrolysis and methylation coupled with gas chromatography-mass spectrometry were used. Metal fractionation in unflushed and flushed soil was analyzed using a modified sequential extraction method. The data on cumulative metal removal were analyzed using OriginPro 8.0 software (OriginLab Corporation, Northampton, MA, USA) and were fitted to 4-parameter logistic sigmoidal model. It was found that flushing time had a stronger influence on metal removal than flow rate. The overall efficiency of metal removal (expressed as the ratio between flushed metal concentration and total metal concentration in soil) at the higher flow rate decreased in this order: Cd (86%) > Ni (44%) > Cu (29%) ≈ Zn (26%) > Pb (15%). Metals were removed from the exchangeable fraction and redistributed into the reducible fraction. After flushing, the soil had a lower pH, EC, and CEC; a higher organic matter content; the composition of the organic matter had changed (incorporation of TA structures). Our results prove that soil flushing with TA is a promising approach to decrease metal concentration in soil and to facilitate carbon sequestration in soil.
Highlights
Two of the most important anthropogenic contaminants that are commonly found in soils are heavy metals/metalloids and hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs)
After the first hour of soil flushing at both flow rates, the pH in the leachate increased considerably, and it reached its maximum after 4 h of flushing at a flow rate of 0.5 mL/min and after 3 h at 1.0 mL/min This suggests that changes in the chemistry of the leachate were dynamic
Its conductivity quickly increased to a value higher than that in the original solution, but it decreased to values lower than the starting value (Figure 3b)
Summary
Two of the most important anthropogenic contaminants that are commonly found in soils are heavy metals/metalloids and hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Metals and metalloids have contaminated soil in over 5 million sites covering 20 million ha of land [2]. This widespread pollution is due to the fact that metals and metalloids are in soils in different concentrations and from various sources including mining, smelting, military training, electronics industries, fuels, waste disposal, and agriculture [3]. Ex situ soil washing and in situ soil flushing have been shown to have great potential for removing a wide range of contaminants from soils including various metals and metalloids [4]. Metals are usually removed more efficiently from acidic soils than from calcareous soil [5,6], both types of soil can
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