ALBERT

Description: The noble gases, which are chemically inert under normal terrestrial conditions but vary systematically across a wide range of atomic mass and diffusivity, offer a multicomponent approach to investigating gas dynamics in unsaturated soil horizons, including transfer of gas between saturated zones, unsaturated zones, and the atmosphere. To evaluate the degree to which fractionation of noble gases in the presence of an advective–diffusive flux agrees with existing theory, a simple laboratory sand column experiment was conducted. Pure CO 2 was injected at the base of the column, providing a series of constant CO 2 fluxes through the column. At five fixed sampling depths within the system, samples were collected for CO 2 and noble gas analyses, and ambient pressures were measured. Both the advection–diffusion and dusty gas models were used to simulate the behavior of CO 2 and noble gases under the experimental conditions, and the simulations were compared with the measured depth-dependent concentration profiles of the gases. Given the relatively high permeability of the sand column (5 x 10 –11 m 2 ), Knudsen diffusion terms were small, and both the dusty gas model and the advection–diffusion model accurately predicted the concentration profiles of the CO 2 and atmospheric noble gases across a range of CO 2 flux from ~700 to 10,000 g m –2 d –1 . The agreement between predicted and measured gas concentrations demonstrated that, when applied to natural systems, the multi-component capability provided by the noble gases can be exploited to constrain component and total gas fluxes of non-conserved (CO 2 ) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.

Description: A series of Cu-substituted goethites, single and co-substituted with Cr, Zn, Cd and/or Pb was prepared, having molar ratios equal to 2.00, 3.33 and 5.00 mol%. All the samples contained only goethite, except Cu-, (Cu,Zn)- and (Cu,Pb)-samples synthesized at 5.00 mol% where hematite was also formed. The presence of Cr/Cd suppressed the hematite-forming effects of Cu. The general sequence of metal entry into the single-metal-substituted goethites was Zn = Cr 〉 Cd 〉 Cu 〉 Pb and in di- (5.00 mol%) and tri- (3.33 mol%) metal-substituted goethites was Cu 〉 Zn 〉 Cd 〉 Cr 〉〉 Pb. Cu incorporation increased all the unit-cell parameters in single-metal-substituted goethite, and these parameters increased in combination with other metals as follows: Cd 〉 Zn 〉 Cr 〉 Pb in the multimetal-substituted goethites. The Cu-substituted goethite dissolved faster than pure goethite. Co substitutions of Cr/Pb reduced the dissolution rate (kFe), while substitutions of Cd/Zn increased kFe.