A microcosm study of permeable reactive barriers filled with granite powder and compost for the treatment of water contaminated with Cr (VI)

Abstract

The permeable reactive barrier (PRB) is a technology developed for the removal of contaminants in groundwater. It consists of a screen perpendicular to the flow of contaminated groundwater filled with a material capable of adsorbing, precipitating or degrading pollutants. Several materials have been tested for their use as reactive substrates for the construction of PRBs. Waste materials are of particular interest for this purpose due to the possibility of their reuse and their generally lower cost. With this aim, the Cr (VI) retention capacity of filler material consisting either of pine bark compost (PB) or a 50% mixture of compost and granite powder (PB50) was evaluated using an experimental device specifically designed for this study, which reproduces a permeable reactive barrier at the laboratory scale. Percolation experiments were carried out with a solution of 100 mg L<sup>-1</sup> Cr (VI) in 0.01M KNO<sub>3</sub>, followed by a leaching step with the saline background. The results show that compost is a highly efficient filler for permeable reactive barriers with almost 100% retention of Cr, whereas the retention efficiency of the mixture of PB50 oscillated between 18 and 46% during the experiment. The Cr retained by the filling material is strongly fixed, since no desorption was detected by leaching with the saline background, and concentrations in the standard Toxic Characteristic Leaching Procedure (TCLP) extracts were lower than 1 mg L<sup>-1</sup>. This behaviour minimizes the risk of release of the Cr retained by the material of the barrier in the event of it being traversed by water not contaminated with Cr. Modelling with Visual Minteq indicates that in the experiments with PB, the reduction of Cr (VI) to Cr (III) occurs and that Cr (III) is associated with dissolved organic matter, which is a form of lower toxicity than the initial Cr (VI) species. In turn, in the experiments with PB50, Cr (III) and Cr (VI) coexist and the oxidised form is not associated with dissolved organic matter, which suggests greater toxicity. The results indicate that pine bark compost is a potential candidate for use as filler material permeable reactive barriers.