Agglomeration and sedimentation of titanium dioxide nanoparticles (n-TiO2) in synthetic and real waters

The recent detection of titanium dioxide nanoparticles (n-TiO2) in wastewaters raised concerns about its fate in the aquatic environment, which is related to its mobility through water bodies. Laboratory experiments of n-TiO2 (particle size distribution: 10-65 nm) dispersed into both synthetic and real aqueous solutions under environmentally realistic concentrations (0.01, 0.1, 1 and 10 mg/l) were conducted over a time of 50 h to mimic duration of ecotoxicological tests. Agglomeration and sedimentation behaviour were measured under controlled conditions of salinity (0-35 parts per thousand), ionic composition and strength, pH and dissolved organic carbon (DOC). Physico-chemical parameters and particle agglomeration in the dispersions were investigated by transmission electron microscopy, Brunauer, Emmett and Teller method and dynamic light scattering. A fluorescence spectrophotometer operating in the nephelometric mode was employed to obtain the sedimentation rates of n-TiO2. The overall results showed that agglomeration and sedimentation of n-TiO2 were affected mainly by the initial concentration. Sedimentation data fitted satisfactorily (R-2 in the range of 0.74-0.98; average R-2 : 0.90) with a first-order kinetic equation. The settling rate constant, k, increased by approx. one order of magnitude by moving from the lowest to the highest concentration, resulting very similar especially for all dispersions at 1(k = 8 x 10(-6) s(-1)) and 10 mg/l (k = 2 x 10(-5) s(-1)) n-TiO2, regardless the ionic strength and composition of dispersions. The implication of these results on toxicological testing is discussed.