Evaluation of soil loaded with green iron nanoparticles for hexavalent chromium reduction

Chromate is a pollutant often found in groundwater and considered a serious threat for humans and ecosystem. Iron nanoparticles produced by green tea polyphenols (GT-nZVI) is a powerful reductant, appropriate for the reduction of Cr(VI) to Cr(III). GT-nZVI suspension was initially conceived ideal for direct injection in the contaminated aquifers. However, many studies demonstrated that this suspension presents very low mobility in calcareous aquifers, which are typical in Mediterrannean countries. An alternative mode of GT-nZVI application is the incorporation of nanoiron in a permeable reactive barrier. This option was evaluated experimentally, assuming that the permeable barrier is filled with a calcareous soil previously loaded with nZVI. Namely an amount of soil was loaded with 0.40 mmol of nZVI per gram of soil (S-nZVI) and its efficiency for Cr(VI) removal was evaluated by conducting batch and column tests. Batch tests were carried out by mixing S-nZVI with contaminated groundwater (GW) containing 1300 ppb Cr(VI), at five doses from 5 to 50 grams of S-nZVI per liter of GW. Chromate concentration dropped below detection limit within 1 day at the highest dose. Reduction kinetics was slower with the other doses but after 20 days Cr(VI) dropped below the limit of 50 ppb at all doses except the lowest. Column tests confirmed the effectiveness of S-nZVI. Chromate concentration in column’s effluents remained below detection limit after the introduction of 390 pore volumes of GW.