Soil is a key element in human survival and can be described by various definitions
according to its main utility. Nowadays, it is accepted that soil is a very complex
heterogeneous medium which consists of solid phases containing minerals and organic
matter and soil solution, defined as the fluid phase where soil reactions, transport and
adsorption occurs (Alloway, 1995).
Unfortunately, in the last decades, soils have been used as a deposit of residues.
Therefore, the contamination and pollution of soils, sediments, surface and
groundwater are of main concern.
There are several ions and compounds of organic or inorganic nature that can
contaminate soil and water. The inorganic contaminants include heavy metals like
cadmium, chromium, cooper, lead and zinc, which are focused in this study. These
metals are commonly present in industrial wastes (electroplating, textile and dyes,
etc.), in fertilizers or in sewage sludge, but they can also contaminate soils through
atmospheric deposition or runoff water (Serrano et al., 2005).
Since adsorption is well recognized as the main process affecting the mobility of heavy
metals through soils (Bradl, 2004), the aim of this work is the study the non competitive
adsorption of these five metals, as well as the competitive adsorption in two different
typical soils of the North of Portugal
The soil samples were collected in Póvoa de Varzim, from O-horizon and A-horizon (0
cm -30 cm), one from a culture land and another from a beach land.
To evaluate the adsorption equilibrium, batch tests were performed by adding to 2 g of
air dried soil, 20 mL of 0.01 M CaCl2 solution, containing different initial concentrations
of Pb [Pb(NO3)2], Cd [CdN2O6·4H2O], Cu [Cu(NO3)2·3H2O], Zn [Zn(NO3)2·6H2O] and Cr
[K2Cr2O7], isolated (non-competitive system) and coupled (competitive system).To
eliminate the mass effect, the multi-metal solutions were prepared in terms of molar
concentrations (Echeverría et al., 1998). To avoid precipitation the pH of initial solution
was adjusted to 2. Isotherm models like Freundlich and Langmuir equations were
adjusted to experimental data.
Batch tests are useful for the understanding of the adsorption process. However,
advection and dispersion may occur in dynamic systems justifying the need of flow
experiments (Miretzky et al., 2006). For continuous experiments, soil samples were
packed in a column (25 cm × 3.2 cm) and a single – or multi – element solution of 50
mg/L was pass upwards during seven days. Then, using the CXTFIT code, the
convection-dispersion equation was adjusted to the breakthrough curves in order to
determine the retardation factor (R) and to compare the behavior of the different ions and soils
