Assessment of long-term performance and chromate reduction mechanisms in a field scale permeable reactive barrier

An innovative full-scale implementation of a permeable reactive barrier, consisting of a double-row of cylinders filled with zerovalent iron shavings, for chromate remediation was monitored over four years. Solid samples were analyzed to elucidate (i) the relevant corrosion mechanisms and products, (ii) the pathways of chromate reduction and immobilization, and (iii) the long-term performance of the barrier situated in a hydrological and geochemical complex groundwater regime. Sampling and analysis of groundwater and reactive material revealed an oxidative iron corrosion zone evolving in the inflow and a zone of anaerobic iron corrosion in the center and outflow of the barrier. Chromate reduction was mainly confined to the inflow region. The formation and thickness of corrosion rinds depended on sampling time, position, and depth, as well as on the size, shape, and graphite content In the inflow, the corrosion rinds mostly consisted of goethite and ferrihydrite. X-ray absorption fine structure spectroscopy revealed two distinct Cr(III) species, most likely resulting from homogeneous and heterogeneous redox reaction pathways, respectively. The longevity and long-term effectiveness of the PRB appears to be primarily limited by reduced corrosion rates of the ZVI-shavings because of the thick layers of Fe-hydroxides.