Simultaneous removal of CUII, NIII and ZNII by a granular mixture of zero-valent iron and pumice in column systems

The granular mixtures between zero-valent iron (ZVI) and other materials (e.g. sand, pumice) have been recently proposed to overcome the problems (e.g. clogging) related to the use of ZVI alone in treatment systems such as permeable reactive barriers or individual potabilisation systems. This paper presents the results of the research activity, carried out by column tests, aimed at comparing the performance of a granular mixture between ZVI and pumice (weight ratio 30:70) and of pure ZVI for the individual and combined removal of CuII, NiII and ZnII. The specific objective was to verify the occurrence of phenomena of mutual interaction and/or competition among contaminants. In fact, although metal removal by ZVI has been extensively documented in the past, the great majority of studies examined either very simple systems (i.e. single metal solutions) or very complicated ones (e.g. real acid mine drainage). In both cases, and for different reasons, the occurrence of mutual interactions (positive or negative) among the chemical species present in water is impossible to detect. For these reasons, this paper compares column experiments carried out in similar conditions using both monocontaminant and pluricontaminant solutions. The concentration values used for the three contaminants were of 500 or 50 mg/l for CuII and 50 mg/l for NiII and ZnII. In monocontaminant systems, in both reactive media the removal sequence observed is Cu > Zn > Ni. In pluricontaminant solutions, CuII removal is unaffected by the presence of the other metals while the removal efficiencies of Ni and Zn decrease in respect to monocontaminant tests. In the mixture ZVI/pumice, the long-term removal efficiency reduction is higher for Zn (about 58% in respect to the experiment carried out with the solution containing Zn only) than for Ni (reduction of about 33%), leading to the removal sequence Cu > Ni > Zn for the test with the highest CuII concentration. This behaviour has been explained, hypothesising the possible spontaneous formation of a bimetallic system between ZVI and Cu capable of enhancing Ni removal especially in the short term.