Comparison of tungstate and tetrathiotungstate adsorption onto pyrite

- This study demonstrates that pyrite plays an important role in determining the fate and transport of W in natural waters.
- WO42 − has a greater adsorption kinetics and higher level of adsorption capacity onto pyrite than WS42 −.
- The different specific adsorption of W may be attributed to the different inner-sphere complexation on the pyrite surface.

Adsorption of tungstate (WO42 −) and tetrathiotungstate (WS42 −) onto pyrite was investigated as a function of W species concentrations, pH, and ionic strength. Additional experiments examined the adsorption of these W species onto goethite. The adsorption experiments indicate that pyrite is a strong scavenger of W in aquatic environments. Specifically, results showed that WO42 − and WS42 − adsorption onto pyrite increased with decreasing pH. The greatest amounts of WO42 − and WS42 − adsorbed onto pyrite occurred at pH 4.95 and 5.2, respectively. Kinetic experiments indicate that WO42 − adsorption onto pyrite occurs more rapidly than WS42 −. The kinetic behavior of the adsorption of both W species onto pyrite is well described with a pseudo-second-order model. More specifically, rapid external adsorption followed by intraparticle diffusion were the rate-controlling steps during both WO42 − and WS42 − adsorption onto pyrite, and intraparticle diffusion of both W species onto pyrite took up > 99% of time, suggesting that it was the major rate-limiting step. The mono-surface Langmuir model provided the best fit to the adsorption data indicating that WO42 − and WS42 − were mainly adsorbed onto one specific site on the pyrite surface. Our model calculations suggest that the specific adsorption of WO42 − onto pyrite was greater than the corresponding adsorption of WS42 − onto pyrite in all cases. The difference of specific adsorption between WO42 − and WS42 − may be attributed to their different inner-sphere complexation on the pyrite surface. The adsorption experiments also showed that W species were less adsorbed onto pyrite than goethite. The results of the adsorption experiments demonstrate that pyrite plays an important role in determining the fate and transport of WO42 − and WS42 − in natural waters. Furthermore, comparison of our results with previous studies of Mo indicates that WS42 − is less particle reactive with respect to pyrite than MoS42 −, which we propose can explain, in part, the apparent stability of W in sulfidic waters compared to Mo as well as the elevated Mo/W ratios in Black Sea sapropels.