The effect of different TiO2 nanoparticles on the release and transformation of mercury in sediment
Purpose The environmental safety of both nano-TiO 2 particles and mercury (Hg) pollution has been of widespread concern. Soils and sediments were the main receiving environments of nano-TiO 2 and Hg. There was a strong association between nano-TiO 2 with Hg compounds. Very limited research has been carried out on the impact of nano-TiO 2 on the environmental behavior of Hg in sediments. The objective of this study was to investigate the effects of nano-TiO 2 on the release and transformation of Hg in sediments. Materials and methods Two types of TiO 2 nanoparticles were employed. Sediment samples were collected in the water level fluctuating zone of the Three Gorges Reservoir, China. Sediment samples spiked with HgCl 2 solution were taken as Hg-contaminated sediments. The flooding experiments were conducted using capped borosilicate glass bottles containing 300 ml TiO 2 suspensions (0.2 g L −1 and 0.4 g L −1 ) and 30 g original or Hg-contaminated sediment samples outdoors. All experiments were conducted in triplicate. Water samples were collected at determined interval times over a 60-day period. Sediments were collected on the 60th day. Total Hg (THg) and methylmercury (MHg) in both water and sediment samples and Hg speciation just in sediment samples were determined. A series of control samples without TiO 2 addition were also prepared. Results and discussion The results showed that nano-TiO 2 particles at high content (4 g kg −1 ) promoted the release of Hg in sediments, and the anatase particles showed a stronger effect. Methyl Hg concentrations in waters of nano-TiO 2 treatments significantly decreased by the 10th day. The effect of nano-TiO 2 greatly depended on anatase and rutile particles. After that, methyl Hg concentrations had no significant difference between nano-TiO 2 treatments and the control, except 2 g kg −1 rutile treatment by the 20th day. The changes of Hg speciation in sediments demonstrated that nano-TiO 2 inhibited the transformation of Hg-bound organic matter into the oxidated state, especially upon 2 g kg −1 rutile treatment. This was consistent with the lower MHg concentrations in the 2 g kg −1 rutile treatment. Conclusions Both anatase and rutile nano-TiO 2 were found to affect the release of Hg in sediment. Nano-TiO 2 promoted THg release in sediments and posed an increased risk of Hg contamination in waters, especially for anatase particles. Moreover, nano-TiO 2 inhibited the methylation process of Hg in sediments at the initial stage of flooding and could affect the transformation of Hg speciation in sediments, especially for low-content rutile particles.
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