This paper reports an investigation of the effects of a trihydroxamate siderophore, desferrioxamine B (DFO-B), and a common biological ligand, oxalate, on the steady-state dissolution of goethite at pH 5 and 25 jC. The main goal of our study was to quantify the adsorption of the ligands and the dissolution of goethite they promote in a two-ligand system. In systems with one ligand only, the adsorption of oxalate and DFO-B each followed an L-type isotherm. The surface excess of oxalate was approximately 40 mmol kg À 1 at solution concentrations above 80 AM, whereas the surface excess of DFO-B was only 1.2 mmol kg À 1 at 80 AM solution concentration. In the two-ligand systems, oxalate decreased DFO-B adsorption quite significantly, but not vice versa. For example, in solutions containing 40 AM DFO-B and 40 AM oxalate, 30% of the DFO-B adsorbed in the absence of oxalate was displaced. The mass-normalized dissolution rate of goethite in the presence of DFO-B alone increased as the surface excess of the ligand increased, suggesting a ligand-promoted dissolution mechanism. In systems containing oxalate only, mass-normalized goethite dissolution rates were very low at concentrations below 200 AM, despite maximal adsorption of the ligand. At higher oxalate concentrations (up to 8 mM), the steady-state dissolution rate continued to increase, even though the surface excess of adsorbed ligand was essentially constant. Chemical affinity calculations and dissolution experiments with variation of the reactor flow rate showed that far-from-equilibrium conditions did not obtain in systems containing oxalate at concentrations below 5 mM. The dissolution rate in the presence of DFO-B at solution concentrations between 1 and 80 AM was approximately doubled when oxalate was also present at 40 AM solution concentration. The dissolution rate in the presence of oxalate at solution concentrations between 0 and 200 AM was increased by more than an order of magnitude when DFO-B was also present at 40 AM solution concentration. Chemical affinity calculations showed that, in systems containing DFO-B, goethite dissolution was always under far-from-equilibrium conditions, irrespective of the presence of oxalate. These results were described quantitatively by a model rate law containing a term proportional to the surface excess of DFO-B and a term proportional to that of oxalate, with both surface excesses being determined in the twoligand system. The pseudo first-order rate coefficient in the DFO-B term has the same value as measured for goethite dissolution in the presence of DFO-B only, while the rate coefficient in the oxalate term must be measured in the two-ligand system, since it is only in this system that far-from-equilibrium conditions obtain. These latter conditions do not exist in the system containing oxalate only, but they do exist in the DFO-B/oxalate system because the siderophore is able to remove Fe(III) from all Fe -oxalate complexes rapidly, leaving the uncomplexed oxalate ligand in solution free to react again wit...

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Clay-supported TiO 2 photocatalysts can potentially improve the performance of air treatment technologies via enhanced adsorption and reactivity of target volatile organic compounds (VOCs). In this study, a bench-top photocatalytic flow reactor was used to evaluate the efficiency of hectorite-TiO 2 and kaolinite-TiO 2 , two novel composite materials synthesized in our laboratory. Toluene, a model hydrophobic VOC and a common indoor air pollutant, was introduced in the air stream at realistic concentrations, and reacted under UVA (λ max = 365 nm) or UVC (λ max = 254 nm) irradiation. The UVC lamp generated secondary emission at 185 nm, leading to the formation of ozone and other short-lived reactive species. Performance of clay-TiO 2 composites was compared with that of pure TiO 2 (Degussa P25), and with UV irradiation in the absence of photocatalyst under identical conditions. Films of clay-TiO 2 composites and of P25 were prepared by a dip-coating method on the surface of Raschig rings, which were placed inside the flow reactor. An upstream toluene concentration of ~170 ppbv was generated by diluting a constant flow of toluene vapor from a diffusion source with dry air, or with humid air at 10, 33 and 66 % relative humidity (RH). be partially attributed to the contribution of gas phase reactions by short-lived radical species. When the reaction rate was normalized to the light irradiance, T r /I λ , the UV/TiO 2 reaction under UVA irradiation was more efficient for samples with a higher content of TiO 2 (P25 and Hecto-TiO 2 ), but not for Kao-TiO 2 . In all cases, reaction rates

This paper reports steady-state dissolution rates of synthetic low-substitution Al-goethites (mol % Al < 10) at pH 5 in the presence of the trihydroxamate siderophore, desferrioxamine B (DFO-B), and the common biological ligand, oxalate. The siderophore-promoted Fe release rate increased both with the level of Al substitution and with DFO-B concentration up to about 100 µM, after which a plateau occurred, suggesting a saturation effect from DFO-B adsorption as a precursor to dissolution. At concentrations above 200 µM, oxalate also enhanced the Fe release rate, which however was not influenced by Al substitution. For Al-goethites with mol % Al < 4, the Fe release rate in the presence of 40 µM DFO-B together with varying concentrations of oxalate was typically greater than the corresponding sum of dissolution rates in the presence of the two ligands alone. This synergism may be the combined result of the ability of oxalate to adsorb strongly at the goethite surface, thus promoting Fe release, and of the high selectivity of DFO for Fe(III). Ferric oxalate complexes formed during dissolution will likely lose Fe 3+ by ligand substitution with DFO-B, leading to the production of Fe(HDFO-B) + and uncomplexed oxalate, the latter of which, in turn, could adsorb to the goethite surface again. For Al-goethites with mol % Al > 4, synergism was not apparent, which may signal the effect of a decreased surface density of Fe-OH sites associated with Al for Fe substitution. The oxalate-promoted release rates of Al did not scale with those of Fe, indicating incongruent dissolution. However, Al release rates in the presence of DFO-B did scale approximately with those of Fe but were not affected by the concentration of siderophore. These results are consistent with the presence of Al(OH) 3 inclusions in Al-goethite.

A series of chlorinated aliphatic compounds (RCI, including carbon tetrachloride (PCM), 1,1,1-trichloroethane (TCA), 1,1,2,2-tetrachloroethane (TeCA), pentachloroethane (PCA), hexachloroethane (HCA), trichloroethene (TCE), tetrachloroethene (PCE), trichloronitromethane (chloropicrin, CP), and trichloroacetonitrile (TCAN)) was reacted with ferruginuous smectite (sample SWa-1 from The Source Clays Repository), SWa, in aqueous suspension under anoxic conditions. Compounds highly polarizable or sharing substituents that facilitate charge delocalization adsorbed faster by reduced (SWa-R) than by unaltered (SWa-U) clay, indicating stronger dipole--dipole interactions between the substituents and the clay surface and/or hydrating water molecules. The reduction of the clay accelerated RCI adsorption up to 100-fold. Incubations with SWa-R promoted RCI reduction (CP, TCAN) or dehydrochlorination (TeCA and PCA). The reduction of structural Fe catalyzes the transformation of RCI via Brønsted and Lewis-basic promoted pathways. This study indicates that oxidation state of the structural Fe in SWa greatly alters surface chemistry and has a large impact on clay-organic interactions.

Abstract--The oxidation state of structural Fe greatly alters surface chemistry, which may have a large influence on clay-organic interactions. The effect of structural-iron oxidation state on chlorinated hydrocarbons at the clay-water interface was examined. Pentachloroethane (5CA) was reacted with oxidized, reduced, and reoxidized forms of three different smectites: montmorillonite, ferruginous smectite, and nontronite in aqueous suspension under controlled-atmosphere conditions. Pentachloroethane was found to adsorb at different rates for the three smectites. A series of 5CA-adsorption rate constants in the presence of these clays showed a strong correlation with the Fe(II) content of the clay (r 2 = 0.98). The clay surface behaves as a Br0nsted base and promotes 5CA dehydrochlorination. The adsorption kinetics at the clay-water interface were described by the formation of a precursor complex prior to 5CA dehydrochlorination.

Chloropicrin (trichloronitromethane, CP) reacted with reduced ferruginous smectite (SWa-1) in aqueous suspension. SWa-1 promoted CP-dechlorination to produce dichloro-(DCNM) and chloronitromethane (CNM) with yields of up to ∼80% within 30 min. The simultaneous formation of DCNM and CNM suggests that CP and SWa-1 surface participate in two electron-transfer pathways. A series of CPadsorption and dechlorination rate constants in the presence of SWa-1 showed a strong correlation with the Fe(II) content of SWa-1 (r 2 ) 0.96 and 0.98, respectively), {Fe(II)}. This study indicates that the oxidation state of structural Fe greatly alters surface chemistry and has a large impact on electron transfer during clay-organic interactions.

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