Soil contamination by heavy metals: laboratory tests and numerical modelling of Al and Zn transport in unsaturated porous media

Soil pollution is of great environmental concern. Particular attention must be focused on urban areas, where heavy metals represent the key contaminants as a result of mainly anthropic activities. Under the evidence of their negative impacts on human health and ecosystems, environmental research is now working on reconstructing the fate of such elements, studying their behaviour during transport across soil layers. In the present thesis work, a small-scale approach has been adopted, with column and batch tests that investigated two target contaminants, zinc and aluminium. These two elements have been studied in the form of Zn(II) and Al(III) chloride solutions. The two selected elements have been analysed with UV-vis spectrophotometry technology. Moreover, to gain more consistent quantitative results, analytical detection methods were developed through colorimetric reagents. The developed method, different for zinc and aluminium, is based on the complexation reaction between the metal and a selective dye analyte. The formed complex takes on a characteristic colour that can be effectively detected in the visible spectrum. Zincon and pyrocatechol violet are the two ligand agents used for zinc and aluminium detection, respectively. The absorbance measurements have been done at 620 and 573.5 nm, where the linear correlation between absorbance and concentration was best expressed. Fate in saturated and unsaturated conditions for each contaminant has been studied on the same column, in succession. The results, analysed in terms of breakthrough curves (BTCs), were subsequently modelled with the HYDRUS-1D software. Sand-packed column saturated tests showed no retardation of both zinc and aluminium paths. Slight retention (10 – 20 %) on the contrary is observed. These results were confirmed by experiments in unsaturated conditions, apart from the aluminium that shows an unexpected behaviour. A similar result in terms of retention is obtained with column tests on standard soil, which however show a marked delay in contaminants’ BTCs. Batch adsorption tests for both contaminants showed that the amount of element adsorbed onto soil increases by increasing the initial concentration with a linear and a IV-type isotherm for aluminium and zinc, respectively. To simulate a real contamination, a final experiment has been conducted with a pulse injection of zinc-enriched material on a sand-packed column. The study indicates the general tendency of contaminants to spread deep. This could mean groundwater pollution. Note, however, that in sand material they seem to behave almost like non-reactive solute, while sorption mechanisms in soils can slow down the spread. New studies can be done on the topic, involving other contaminants’ concentrations and larger analytical scales.