The reductive dissolution of MnO2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO4(2-)) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe2O4), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.

A series of 23 unsaturated bench scale column experiments was carried out to study grain scale processes controlling the efficiency of soil vapor extraction for removal of volatile organic compounds (VOCs) from a desert soil sample (mass fraction of organic carbon, approximately 0.001). Experiments consisted of passing VOC-containing, humidified (>95% relative humidity) air through an unsaturated soil column until breakthrough occurred and then passing VOC-free air through the column until the VOC removal was complete. Effluent VOC concentrations were measured at frequent intervals. Experimental variables included VOC (benzene and p-xylene), soil moisture content (0 to 18% by volume), concentration of VOC in the inlet air stream, and interstitial velocity (0.2 to 0.6 cm s-i). Experimental breakthrough curves were modeled with an advectiondispersion model coupled with a first-order rate equation to describe mass transfer between phases. At 18% moisture, slow water-to-air mass transfer of benzene as the ratelimiting process explains the initial removal of benzene (C/Co > 0.1), but the desorption profiles strongly suggest that a slower process (intraparticle pore diffusion) becomes the dominant process at longer times. The breakthrough curves for p-xylene at 10 and 18% moisture also suggest that two sequential processes control removal ofp-xylene. The time constant for the faster process is comparable to that determined for the initial ratelimiting process for benzene at 18% moisture. The rate for the second process is the same order of magnitude for p-xylene at 10 and 18% and benzene at 18%. Calculations of the mass distributions of the VOCs among air, water, and soil strongly suggest that the more hydrophobic p-xylene tends to accumulate at the air-water interface (up to 60% of the total mass) and that benzene primarily accumulates in the aqueous phase.Paper number 95WR00004. 0043-1397/95/95WR-00004 $05.00 rate-limiting step. Grathwohl and Reinhard [1993] and Farrell and Reinhard [1994b] have suggested that intraparticle diffusion through mesopores and micropores is the rate-controlling process for long timescale removal of TCE from soil systems. Although the mechanism for a specific soil cannot be assigned a priori, the rate-controlling processes are on the particle scale.An important difference between saturated and unsaturated systems is the existence of the air-water interface. VOCs can accumulate at this interface [Karger et al., 1971; Pennell et al., 1992; Hoff et al., 1993a, b]. The amount of accumulation is controlled by the hydrophobicity of the compound [Hoff et al., 1993b]. One question investigated in this study is whether mass transfer across this interface could be a rate-limiting process in the sorption and desorption of different VOCs from unsaturated soil.The purpose of this research was to determine which ratelimiting processes control removal of VOCs in soil vapor extraction. Our hypothesis was that both mass transfer and intraparticle diffusion limited VOC removal from unsaturated soils. To te...

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Finely laminated sediments within Bainbridge Crater Lake, Galápagos, provide a record of El Niño–Southern Oscillation (ENSO) events over the Holocene. Despite the importance of this sediment record, hypotheses for how climate variability is preserved in the lake sediments have not been tested. Here we present results of long‐term monitoring of the local climate and limnology and a revised interpretation of the sediment record. Brown‐green, organic‐rich, siliciclastic laminae reflect warm, wet conditions typical of El Niño events, whereas carbonate and gypsum precipitate during cool, dry La Niña events and persistent dry periods, respectively. Applying this new interpretation, we find that ENSO events of both phases were generally less frequent during the mid‐Holocene (~6100–4000 calendar years B.P.) relative to the last ~1500 calendar years. Abundant carbonate laminations between 3500 and 3000 calendar years B.P. imply that conditions in the Galápagos region were cool and dry during this period when the tropical Pacific E‐W sea surface temperature (SST) gradient likely strengthened. The frequency of El Niño and La Niña events then intensified dramatically around 1750–2000 calendar years B.P., consistent with a weaker SST gradient and an increased frequency of ENSO events in other regional records. This strong interannual variability persisted until ~700 calendar years B.P., when ENSO‐related variability at the lake decreased as the SST gradient strengthened. Persistent, dry conditions then dominated between 300 and 50 calendar years B.P. (A.D. 1650–1900, ± ~100 years), whereas wetter conditions and frequent El Niño events dominated in the most recent century.