Behavior of multi-layer permeable reactive barriers for groundwater remediation

This paper aims to evaluate the efficiency of a multilayer configuration of a permeable reactive barrier (PRB) made up of granular mixtures of zero valent iron (ZVI) and lapillus. The latter is a volcanic material used to disperse ZVI particles. A high dispersion of ZVI improves the long-term hydraulic conductivity but can significantly reduce reactivity due to the lower amount of ZVI. In this research, the performance of two different combinations of a two-layer configuration was studied by means of long-term column tests. The first layer, named “pre-treatment layer”, had a thickness of 4 cm and a volumetric ratio (ZVI/lapillus) of 10:90 or 05:95, while the second layer had a volumetric ratio (ZVI/lapillus) of 20:80. A single layer configuration made only of the 20:80 ZVI/lapillus was used as a benchmark. The three tests were performed using a multi-contaminated solution of copper, nickel and zinc. Test results showed an early loss of the hydraulic conductivity in the single layer configuration and an increase of PRB longevity by 68 % in the presence of the pre-treatment layer. The pre-treatment zone containing 10 % ZVI delayed the clogging phenomenon, while the zone with 5 % ZVI ensured both the correct long-term hydraulic behavior and a removal efficiency higher than 77.6 % for Nickel and 99 % for copper and zinc at 23 cm of thickness for at least two months.