Equilibrium modeling of lead adsorption onto a “red soil” as a function of the liquid-phase composition.

"Red soils" are silty-clay materials typical of Central Italy, with a high content of iron and aluminum (hydro)oxides, resulting from dissolution of carbonates and mixed with particles of volcanic origin. These soils are becoming interesting because of their large availability in Central and Southern Italy and large attenuation capacity with respect to heavy metals due to their geochemical characteristics (high surface area, cation-exchange capacity, and pH). Thus, their possible use as a daily landfill coverage or as a low-cost sorbent material is under evaluation. Lead adsorption, as representative of heavy metals in polluted streams, has been studied as a function of the composition of the liquid phase. Different background electrolytes (NaClO 4, NaNO 3, NaCl, and CH 3COONa) and different pHs (4-7) were used in order to obtain different lead speciation in the liquid phase. Lead adsorption occurred not only to the account of free lead, and positively charged complexes (PbNO 3 +, PbCl +, and CH 3COOPb +) strongly contributed to the total metal removal. An adsorption model was developed based on the surface complexation concept, which can be proposed as a general approach for characterizing metal adsorption on heterogeneous natural sorbents as a function of the composition and pH of the liquid phase. The model was characterized by the presence of two surface complexation and one ion-exchange sites onto which the different lead species can be sorbed. A maximum sorption capacity of 21.7 mg g -1 was calculated for lead and gives to the red soil a particular interest as a possible alternative low-cost sorbent.