The Combined Toxic Effect Of Nanoscale Titanium Dioxide（nTiO₂） And Environmental Pollutants On Scenedesmus Obliquus

In order to well understand the combined toxic effects of nanoscale Titanium dioxide（nTiO₂） and the existing environmental pollutants（BPA and Cd） on green algae Scenedesmus obliquus, the following studies were conducted: 1) The physical and chemical properties of nTiO₂ in S.obliquus culture medium. 2) The separate toxic effect of nTiO₂, BPA and Cd on S.obliquus. 3) The combined toxic effects of nTiO₂ and BPA/Cd at different toxic ratios according to three classical joint toxicity evaluation methods（toxic unit, additive index and mixture toxic index）. 4) The combined effect mechanism taking nTiO₂ +Cd group as example. Results are as follows:1) nTiO₂ in the S.obliquus culture medium could form greater aggregates with an average hydrodynamic diameter of 3⁵ micrometers. However, these aggregates were loose and there were still plenty of nano-particles around their irregular edges. Their Zeta-potential values were measured to be-19.6 m V^-23.6 m V which are independent to the exposure time and concentration. The maximum absorbance wavelength of these aggregates was measured to be 334 nm.2) The separate 72 h EC50 of nTiO₂, BPA and Cd on S.oblignus was 28.7 mg·L-1,1.81 mg·L-1 and 0.011 mg·L-1 respectively.3) When nTiO₂ and BPA did coexist, the 72 h EC50 values（showed by BPA） at different toxic ratios（4:1, 3:1, 2:1, 1:1, 1:2 and 1:3） were 2.198, 1.58, 1.153, 0.428, 0.306 and 0.189 mg·L-1, respectively. The 72 h EC50 values（showed by nTiO₂） at different toxic ratios（4:1, 3:1, 2:1, 1:1, 1:2 and 1:3） were 8.71, 8.35, 9.14, 6.79, 9.70 and 8.99 mg·L-1 respectively. The combined toxic effects shifted from antagonism（when the toxic ratio of BPA to nTiO₂ was 4:1） to additive effect（when the toxic ratio was 3:1） and then to synergism（when the toxic ratio were 2:1, 1:1, 1:2 and 1:3） as the toxic ratios changed. When nTiO₂ and Cd did coexist, the 72 h EC50 values（showed by Cd） at different toxic ratios（4:1, 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4） were 0.021, 0.016, 0.017, 0.019, 0.0048, 0.0013 and 0.00069 mg·L-1, respectively. The 72 h EC50 values（showed by nTiO₂） at different toxic ratios（4:1, 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4） were13.698, 13.915, 22.177, 49.573, 25.047, 10.175 and 7.201 mg·L-1 respectively. The combined toxic effects again turned from antagonism（when the toxic ratios of Cd to nTiO₂ were 4:1, 3:1, 2:1 and 1:1） to additive effect（when the toxic ratio was 1:2） and then to synergism（when the toxic ratios were 1:3 and 1:4） as the toxic ratios changed.4) The adsorption of cadmium by nTiO₂ lived up to the Freundlich model. The free Cadmium ions in culture medium decreased with the decreasing of toxic ratios of Cd to nTiO₂（Content of nTiO₂ increased）.When the concentration of nTiO₂ was relatively low, there was no Ti or Cd entering in the algae cells. However, when the concentration of nTiO₂ was high, quantities of nTiO₂ particles gathering around the algae cells were found. Besides, the nanoscale particles around the aggregates were observed to disrupt the cell membrane and some contents of Ti and Cd were ditected entering in the algae cells. For quantitative analysis, the accumulation of Cadmium by S.obliquus showed a relationship as 1:4>single Cd>1:1>1:4.