Pore waters acids

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

Pre-acidified pore water (100 pi, diluted with Millipore Q-water if necessary) was transferred, using an Eppendorf pipette, into a 10 ml volumetric Pyrex flask. To this flask nitric acid (50 pi) was added, and the solution was then brought to volume with Millipore Q-water. Standards were made up by adding various amounts to stock metal solutions (lmg/1), nitric acid (50 pi), and a seawater solution (100 pi) of approximately the same salinity as the samples to be analysed. This final addition ensures that the standards are of approximately the same ionic strength and contain the same salts as the samples. 

I consider a system in which organic matter is oxidized at a steady rate that is a specified function of depth in uniform calcium carbonate sediments. The oxidation of organic matter increases the total dissolved carbon in the pore water of the sediment. The resultant increase in acidity causes the dissolution of calcium carbonate and a consequent increase in alkalinity as well as another increase in total dissolved carbon. The total dissolved carbon and alkalinity are transported by diffusion between different depths in the sediment. 

Copper concentrations in sediment interstitial pore waters correlate positively with concentrations of dissolved copper in the overlying water column and are now used to predict the toxicity of test sediments to freshwater amphipods (Ankley et al. 1993). Sediment-bound copper is available to deposit-feeding clams, especially from relatively uncontaminated anoxic sediments of low pH (Bryan and Langston 1992). The bioavailability of copper from marine sediments, as judged by increased copper in sediment interstitial waters, is altered by increased acid volatile sulfide (AYS)... 

Shea and MacCrehan [10] determined hydrophillic thiols in sediment pore water using ion-pair chromatography coupled to an electrochemical detector. Down to 2p mole absolute of these compounds could be determined including cysteine, monothiogylcerol, glutathione, mercaptopyruvic acid, 3-mercaptopropionic acid and 2-mercaptopropionic acid. 

Recently, Burkhard (2000) reviewed contaminant sorption by dissolved organic matter. Using several hundreds of UCC-water partition coefficients (A doc) reported in these studies, he found that UCC-water partition coefficients for naturally occurring DOC (humic and fulvic acids, sediment pore water, soil pore water, groundwater, and surface water) was best described by... 

While the extracts of SPMDs are generally less difficult to purify than are extracts of tissue or sediment, certain interferences can be problematic for some types of analyses. The most important of these potential interferences are codialyzed polyethylene oligomers (i.e., the so-called polyethylene waxes), oleic acid, and methyl oleate. The latter two interferences are residual from the synthesis of the triolein. Also, oxidation products of triolein may be present in dialysates of SPMDs that have been exposed (especially in the presence of light) to air for periods exceeding 30 d. For a standard 1-mL triolein SPMD, the mass of all these interferences in dialysates is generally <30 mg or about 6 mg g of SPMD (Huckins et al., 1996). Another potential interference is elemental sulfur, which is often present in sediment pore water and is concentrated by SPMDs. However, both polyethylene waxes and elemental sulfur are readily removed using the previously described SEC procedure. 

Various membrane materials are to be compared for corrosion resistance in hydrochloric acid. Membrane samples are ultrasonically cleaned with Freon for 5 minutes and dried at 200°C for 2 hours followed by similar steps of ultrasonic cleaning with demineralized water and drying. The conditioned membrane samples are then immersed in 35% HG solution, making sure that no air bubbles are trapped in pores. The acid exposure at the test temperature (e.g. 25°C) continues for a given period (e.g. one week). The tested samples are ultrasonically washed with demineralized water for 5 minutes and dried at 200°C for 2 hours. The weights of the cleaned membrane samples before and after the acid exposure are compared to assess the relative corrosion resistance of various membrane materials. 

In very narrow pores, water may be held even after drying at 100-120°. Furthermore, many types of silica contain hydroxyl groups within the bulk of the structure. This is especially true of silica which has been prepared by condensation of low-molecular silicic acids. It can easily be imagined that a few silanol groups will remain in the network not having... 

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

Quartz is one of the most common minerals in the sedimentary rocks typically associated with coal. A relatively inert mineral, quartz does not significantly affect the acid-alkali balance of pore waters, and to that extent it is environmentally neutral . On the other hand, quartz is the principal agent of silicosis, which is one of the most severe respiratory health problems associated with coal mining. 

Exchangeable acidity was determined by the BaCl2-TEA (triethanolamine) method (20). The equilibrium calcium-ammonium exchange coefficient (Gapon coefficient, Kq) was determined on the Ca2+-saturated, NH4+-saturated portions and unaltered sediment by methods described by Baes and Bloom (21). This determination was accomplished by allowing exchange solutions with compositions similar to LRL pore waters (Ca2+ = 6.6-52.9 xequiv/L NH4+ = 8.7-44... 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

Figure 4. A generalized profile of Fe and S chemistry in sediment pore waters from an acidic lake. Numbered points are discussed in the text. (Reproduced with permission from reference 35. Copyright 1989 American Geochemical...

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

The much higher reactivity of polysulfide ions, compared with HS, towards organic Michael acceptors suggests that, in coastal marine sediments and salt marshes, polysulfide 10ns could be more important than HS for organosulfur formation. In fact, preliminary studies indicate that TBP treatment of anoxic sediment slurries or extracted pore water from Biscayne Bay (Florida) releases substantial amounts of 3-MPA (22), suggesting the presence of polysufido-propionic acids or the disulfide of 3-MPA This aspect is currently under study. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Early studies of reduced sulfur in the environment centered on the role of sulfate reducing bacteria in sediments and the measurement of hydrogen sulfide and other simple inorganic sulfur compounds in pore waters. Simple colorimetric assays are probably still the best for such studies. Typical methods include the methylene blue determination of sulfide (26.27) and the dithio-nitrobenzoic acid (DTNB) method for total thiols (28.29). 

Temperature and pressure variations in natural systems exert major influences on carbonate mineral solubility and the distribution of carbonic acid chemical species. For example, the solubility of calcite decreases with increasing temperature, as does the solubility of CO2 gas in water. These two effects on solubilities can lead to precipitation of calcite as a cement in a marine sediment-pore water system that undergoes moderate burial. 

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

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