We measured the temperature and azimuthal dependence of the diffusion rates of hydrogen ( 1 H) and deuterium ( 2 H) on Ni(111) from 65 to 240 K where the diffusion rates vary by 8 orders of magnitude. The diffusion of hydrogen crosses over from an activated behavior above 110 K to another activated behavior below 110 K with a smaller activation energy. By ruling out the possibility of a step effect, we suggest that the crossover is from a classical overbarrier hopping to a quantum mechanical underbarrier tunneling. [S0031-9007(97)

The transport of Pb(II) or Cd(II) in subsurface has been studied in the literature; however; their co-transport in porous media in presence of colloids has not been clearly understood. In this work, a series of column experiments were conducted to study Pb(II) and Cd(II) co-transport in saturated porous media under various experimental conditions with different combination of colloidal solution (montmorillonite colloid, manual loessial soil colloid and humic acid), flow rate (0.1, 0.5 and 1.0 ml min −1 ) and sand grain size (0.4-0.8mm and 0.8-2.0mm). The results showed that increase of flow rate and grain size promoted the mobility of Pb(II) and Cd(II), furthermore, the presences of mobile colloids also enhanced the mobility of the two heavy metals, meanwhile, Cd(II) showed higher mobility than Pb(II) in the columns. Findings from this work enhanced current understanding of the competitive transport and colloidfacilitated transport of heavy metals in saturated porous media.

Ball-milled biochar (BMBC) is a typical engineering material that has promising application prospects in remediating contaminated soil and water. It is fundamental to rate the transport behaviors of BMBC in the underground environment before extensive use. In this study, the effects of the ubiquitous cations (Na+, Mg2+, and Al3+) and model organic matter (humic acid) on the transport of BMBC were investigated using laboratory column experiments. The results demonstrated the facilitated effect of HA on the transport of BMBC due to the negatively charged surface and steric effect under neutral conditions. HA and ionic strength manifested an antagonistic effect on the transport of BMBC, where the presence of one could weaken the effect from the other. We also found the charge reversal of the BMBC surface in the presence of Mg2+, thus enhancing the deposition of BMBC onto the medium surface. On the other hand, the charge reversal from Al3+-coupled acid conditions led to the restabilization and transport of BMBC in porous media. Therefore, the rational usage of BMBC is indispensable and more attention should be paid to the composition and change in underground water that might facilitate the transport of BMBC and thus lead to negative environmental implications.

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