Immobilization of cadmium in contaminated soils is one of the decontamination techniques to improve soil quality. TiO2 nanoparticles are useful additive to bind cadmium. This study examines the impact of ionic strength and index cations on Cd-DTPA in a calcareous soil. The availability of Cd in soil and nanoTiO2 suspensions was tested as a function of ionic strength (10, 100 and 200 mM NaCl or CaCl2) in two soil to solution ratios (1:5 and 1:20) and different amounts of nanoparticles (0, 1 and 5% w/w). From Cd-DTPA measurements, we found that nano-TiO2 increased Cd-DTPA in NaCl treatments, whereas Cd-DTPA was not influenced by nano-TiO2 in CaCl2 treatments. For example, in soil to solution ratio of 1:20 and in 200 mM NaCl solution, the extent of Cd-DTPA increased from 3.54 to 5.56 mg kg-1 as the nanoparticle dosage was increased from 0% to 5%. In contrast, in CaCl2 suspension adjusted to an ionic strength of 200 mM, Cd-DTPA in 5% nano-TiO2 concentration and in blank treatment (without TiO2 nanoparticle) was 2.77 and 2.81 mg kg-1, respectively. Moreover, in all ionic strength treatments, after two weeks incubation time, soluble Cd decreased and pH increased in supernatant liquid in the presence of nano-TiO2. However, in the presence of CaCl2 solution, adsorption of cadmium to nanoparticles was less than of NaCl suspensions. For example, in soil to solution ratio of 1:20, reduction of soluble cadmium in 5% nano-TiO2 amount in comparison to blank treatment (without nano-TiO2) in suspensions adjusted to ionic strength of 200 mM of NaCl and CaCl2, was 47% and 8%, respectively Increase of Cd-DTPA in NaCl treatments was attributed to greater stability of nanoparticles as well as greater Cd adsorption to nano-TiO2 surfaces in Na+ in comparison to Ca2+ electrolytes. Cd-DTPA was unaffected by increasing the salt concentrations from 10 to 200 mM NaCl or CaCl2, whereas enhance of the salt concentrations increased soluble Cd and decreased pH of supernatant solutions. This indicates that under the range of ionic strength used in this study, stability of nanoparticles as well as Cd adsorption to nano-TiO2 was not influenced. Our observations showed that nano-TiO2 can remove soluble cadmium from soil suspension in the presence of both index cations. Furthermore, as the range of ionic strength applied in this study coincides with and also covers the range of most normal soils, it may be concluded that influence of nano-TiO2 on Cd-DTPA is mainly depended on type of index cation rather than electrolyte concentration.

Keywords