Short-Term Soil Flushing with Tannic Acid and Its Effect on Metal Mobilization and Selected Properties of Calcareous Soil

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.

Keywords: heavy metals; soil column; tannic acid sorption.

Figure 1

Tannic acid (TA) used as a soil flushing agent: (a) TA powder; (b) TA solution.

Figure 2

The setup for soil flushing: container for the flushing solution (1); column reactor (2); peristaltic pump (3); automatic sample collector (4).

Figure 3

Effect of soil flushing time (SFT) on (a) pH, (b) EC and (c) dissolved TOC in TA leachate from the column reactor.

Figure 4

Metal removal, cumulative metal removal (in mg/kg), and metal removal efficiency (in %) from soil as a function of soil flushing time (SFT) and TA flow rate: (a) Cd, (b) Cu, (c) Ni, (d) Pb, and (e) Zn. For metal removal efficiency, different letters indicate significant differences in metal removal between the two flow rates (ANOVA followed by Tukey’s honest significant difference test, p < 0.05).

Figure 4

Metal removal, cumulative metal removal (in mg/kg), and metal removal efficiency (in %) from soil as a function of soil flushing time (SFT) and TA flow rate: (a) Cd, (b) Cu, (c) Ni, (d) Pb, and (e) Zn. For metal removal efficiency, different letters indicate significant differences in metal removal between the two flow rates (ANOVA followed by Tukey’s honest significant difference test, p < 0.05).

Figure 5

Pyrolysis-GC-MS (above) and THM-GC-MS (below) total ion current chromatograms of soil samples after flushing with TA. The large peaks for catechol and pyrogallol (Py-GC-MS) and 3,4,5-trimethoxybenzoic acid methyl ester (S6; THM-GC-MS) indicate the presence of TA after flushing. Asterisks mark contamination (from sample handling and the analytical system). Other abbreviations for THM-GC-MS: S1 = 1,2,3-trimethoxybenzene; C1P6 = 4-methoxy-x-methyl-benzoic acid methyl ester); C5MS and C6MS indicate methylated forms of metasaccharinic acids (from carbohydrates); G18 = methylated ferulic/caffeic acid methyl ester; F16 and F18 are C16 and C₁₈ fatty acid methyl esters; C18:1 = monounsaturated C₁₈ fatty acid (oleic acid methyl ester). The compounds marked “un” are identified (numbers between parentheses indicate characteristic m/z fragments).