Reduction of hexavalent chromium with polyphenol-coated nano zero-valent iron: column studies

Nano zero-valent iron (nZVI) is considered as a highly efficient reductant with wide applicability in the remediation of contaminated sites, due to the small particle size, large specific surface area, and high reactivity. In this study, nZVI was synthesized by mixing Camellia Sinensis extract (green tea, GT) with a ferric iron solution. Polyphenols in the GT extract act as: (a) strong reductants, reducing thus Fe(III) to its elemental state and (b) capping agents that stabilize the Fe(0) nanoparticles and inhibit their potential rapid oxidation. The remediation potential of ZVI nanoparticles, produced with the Green Tea method (GT-nZVI), for hexavalent chromium (Cr(VI)) was evaluated by column tests. The tests were carried out using polyethylene columns, with 2.63 cm internal diameter and ~10 cm length. The columns were filled with 65 g of natural soil and pure silica sand at a ratio of 50% w/w. The reductive capacity of GT-nZVI was examined for high (5 mg L−1) concentration of hexavalent chromium in aqueous solution. The soil was initially treated in the first column by introducing 11 pore volumes of GT-nZVI suspension through the packed column while in the second column 24 pore volumes were introduced. It was found that about 210 and 420 mg of ZVI nanoparticles were retained by the soil during this stage in column I and in column II, respectively. After this step, the solution of hexavalent chromium was introduced in the columns at a flowrate of 1.2 mL min−1, in order to determine the breakthrough curve of Cr(VI) in the immobilized GT-nZVI on the soil. The total amount of reduced Cr(VI) was equal to 17 mg in column I and 33 mg in column II. Based on the available experimental data, it was estimated that a GT-nZVI-treated zone in the subsurface, with a length (L) of 0.75 m, could provide a sufficiently long contact time in order to reduce Cr(VI) levels from 5 mg L−1 to less than 5 μg L−1, under the condition that groundwater Darcy velocity is either equal to or less than 0.1 cm min−1.