The effect of different TiO2 nanoparticles on the release and transformation of mercury in sediment

Abstract

The environmental safety of both nano-TiO2 particles and mercury (Hg) pollution has been of widespread concern. Soils and sediments were the main receiving environments of nano-TiO2 and Hg. There was a strong association between nano-TiO2 with Hg compounds. Very limited research has been carried out on the impact of nano-TiO2 on the environmental behavior of Hg in sediments. The objective of this study was to investigate the effects of nano-TiO2 on the release and transformation of Hg in sediments. Two types of TiO2 nanoparticles were employed. Sediment samples were collected in the water level fluctuating zone of the Three Gorges Reservoir, China. Sediment samples spiked with HgCl2 solution were taken as Hg-contaminated sediments. The flooding experiments were conducted using capped borosilicate glass bottles containing 300 ml TiO2 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 TiO2 addition were also prepared. The results showed that nano-TiO2 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-TiO2 treatments significantly decreased by the 10th day. The effect of nano-TiO2 greatly depended on anatase and rutile particles. After that, methyl Hg concentrations had no significant difference between nano-TiO2 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-TiO2 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. Both anatase and rutile nano-TiO2 were found to affect the release of Hg in sediment. Nano-TiO2 promoted THg release in sediments and posed an increased risk of Hg contamination in waters, especially for anatase particles. Moreover, nano-TiO2 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.