Predicting soil-water partition coefficients for Hg(II) from soil properties.

The metal adsorption characteristics for fifteen Taiwan soils by Hg(II), were evaluated using pH as the major variable. The soil samples were thoroughly characterized for their physical chemical properties and composition, particularly organic matter and metal oxides. The adsorption of Hg(II) increased with increasing pH between pH 2.5 and 5.5, whereas the adsorption significantly decreased above around pH 5.5. Below pH 5.5, greater adsorption was found for soils with a higher organic matter content at constant pH and metal concentration. To better understand the mechanism of adsorption, the experimental results for Hg (II) were tested in a partition coefficient model to relate the adsorption of the Hg(II) by the different soils with soil components: organic matter, iron oxide, aluminium oxide and manganese oxide. This model was not successful when applied to measurements at the differing natural soil pHs because of the importance of pH. At pH greater than 5.5 the model fails because of the complexation of Hg by the dissolved organic matter. However, partition coefficients obtained from experimental data were highly correlated with those calculated for a partition coefficient between mercury and organic matter alone at lower pH. Normalization of the partition coefficients, Kd, for the organic matter content of the soils, Kom, greatly improved the correlation between the partition coefficient and pH under pH 5.5 (R2 increased from 0.484 to 0.716). This suggests that the surficial adsorption sites are principally due to organic matter for pH less than 5.5. For the 24-hour equilibration period employed, diffusion of Hg through this superficial organic matter coating to underlying sorptive materials, including metal oxides, is not important in the partitioning of Hg. At pH above 5, a decrease of mercury adsorption with increasing solution pH was also found. This result may be explained in part by the complexation of mercury by soil dissolved organic matter whose concentration increased with increasing pH.

Characterization of visibility and atmospheric aerosols in urban, suburban, and remote areas.

Visibility data from over the past four decades accumulated from urban areas of central Taiwan indicated that air pollutants have significantly degraded visibility in recent years. Currently, the annual average visibility in urban areas of the same region is approximately 8-10 km, while the visibility in remote areas is approximately 25-30 km. To understand how aerosols affect the visibility in this region, here we selected three sites in central Taiwan to measure the soluble ionic and carbonaceous species of PM(2.5) and PM(2.5-10) during 1997-1998. A MOUDI cascade impactor was used to measure the size distributions of atmospheric sulfate, nitrate, and carbonaceous particles. The aerosol data were then analyzed together with meteorological and air quality data. Comparing the results obtained from urban, coastal suburban and remote sites revealed that sulfate, carbonaceous species and local wind speed significantly affected the visibility in the urban area. However, sulfate concentration and humidity influenced visibility in the coastal area of central Taiwan. The particulate concentration at the remote station was roughly one-fifth of that in the city. Regression analysis results indicated that humidity is a dominant factor affecting remote visibility.