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Effects of grain size and structural heterogeneity on the transport and retention of nano-TiO2 in saturated porous media

Lv, Xueyan, Gao, Bin, Sun, Yuanyuan, Dong, Shunan, Wu, Jichun, Jiang, Beilei, Shi, Xiaoqing
The Science of the total environment 2016 v.563-564 pp. 987-995
environmental impact, ionic strength, models, porous media, prediction, preferential flow, sand, titanium dioxide
Accurately predicting the fate and transport of nano-TiO2 in porous media is critical to assess its environmental impact. This study was designed to understand the effects of gain size and structural heterogeneity under different ionic strength (IS) on the fate and transport of nano-TiO2 in saturated porous media. In the columns packed homogenously with sand of different grain sizes (920, 550, 390, and 275μm), the transport of nano-TiO2 decreased when the IS increased from 0.1 to 1 or 10mM. For all the three IS conditions, the retention of the nano-TiO2 particles in the columns increased when the gain size decreased, and the mobility of the nano-TiO2 was the lowest in the sand at size of 275μm with recovery rates of 0.30% to 1.72%. The mass recovery rates of TiO2 in other homogeneous columns were higher and ranged from 0.37% to 59.9%. Structural heterogeneity created two flow domains for the retention and transport of nano-TiO2 particles in the saturated porous media. The fast-flow domain dominated the flow and transport processes of the nano-TiO2 in the heterogeneous columns under the tested conditions. As a result, the transport of nano-TiO2 in the heterogeneous porous media was faster and higher than that in the homogeneous columns under similar experimental conditions. Because of the dominance of the fast-flow domain, the recovery rates of the nano-TiO2 in the heterogeneous columns were similar and ranged from 59.8% to 66.9%. These results reflected the importance of preferential flow to the fate and transport of nano-TiO2 particle in porous media. Simulations from a two-domain model matched the experimental breakthrough curves very well.