Acid front

Toth A, Lagzi I and Florvath D 1996 Pattern formation in reaction-diffusion systems cellular acidity fronts J. Rhys. Chem. 100 14 837-9... 

Fig. 1 Absorption scan of a chromatogram with 10 pg ( ) per chromatogram zone of the carboxylic acids tartaric acid (1), malic acid (2), lactic acid (3), succinic acid (4), fumaric acid (5), stearic acid + front (6).

Observations of the arrival of the C02 acid front at different wells clearly demonstrates that there is a great deal of heterogeneity in the reservoir. Furthermore, the arrival of waters which are undersaturated with respect to calcite suggests that single producing wells are recovering waters with widely different C02 contents. These observations have important implications for the interpretation of produced water compositions in complex reservoirs it must be done in conjunction with detailed reservoir flow models. 

Both adsorption and reaction plays an important role. Since the adsorption isotherm is favorable for the adsorption of propionic acid, the ester is formed at the beginning of the reactor thus ester and unreacted alcohol move ahead of the propionic acid front. Water is retained on the catalyst and stays behind the front. Thus no further reaction occurs. When the propionic acid front reaches the reactor outlet, the reaction takes place over the entire reactor volume, thus assur-... 

Electrokinetic methods are often in situ and involve inserting the electrodes directly into contaminated soils and sediments. When activated, the electrodes dissociate water, which produces oxidizing conditions and an acid front (perhaps at pH < 2) at the positively charged anode (Acar and Alshawabkeh, 1993, 2638) ... 

Edecrin , 50 mg (ethacrynic acid). Front (top) and back views. 

Figure 3 is a plot of in situ pH profiles as a function of depth in the column and time. It agrees with the results of several previous laboratory studies which showed that an acid front generated by electrolysis of water at the anode progressively moves through the soil column towards the cathode (Acar and Ashawabkeh, 1993 Eykholt and Daniels, 1994 Hicks and Tondorf, 1994). 

In natural soils which commonly contain illite and smectite, there can be a significant charge imbalance between Si4+ and Al3+ in the structures of these clay minerals. This results in a net negative charge on the clay mineral surfaces, resulting in more adsorption of mobile cations. When an acid front encounters these adsorbed mobile cations, they are very easily displaced by the H ion, which by virtue of its small size is strongly adsorbed to the clay surface. As a result, the measured CEC of such clay-bearing soils is predicted to increase, as we have observed in our experiments. 

STRECKER Ammoactdsynthesis Synthesis o( a-amino acids front aldehydes or ketones via cyanohydnns... 

When plutonium in a nitrate form enters soil it precipitates very soon due to hydrolytic reactions. It becomes attached where-wherever a pH-gradient occurs on the surface of the wetted mineral particles and at the top of the advancing acid front. After such an Irreversible type of adsorption it is improbable that the amount of plutonium 1n the soil water depends on ion exchange. Therefore one would suspect a relatively higher contribution coming from the dissolution of precipitated Pu(0H>4. 

In formulating a theory for the production of formic acid front methane in the presence of metallic oxides such as copper subaxide at temperatures ranging from 200° to 500° C., Nielson27 assumed the intermediate formation of carbon monoxide and water which then reacted to formic... 

The acid-front of the soil facilitates desorption of heavy metals from the soil surface and dissolution of hydroxyl complexes of heavy metals. As a result, this increases the heavy metals fractions present in the liquid phase, their mobilities, and the removal efficiency of heavy metal. In contrast, the base-front in the cathode zone can immobilize heavy metals by forming their hydroxides and heavy metal precipitation occurs in the soils close to cathode, causing low removal efficiency. 

The movement of acid-front to the cathode is due to migration (electric potentials), diffusion (chemical potentials), and advection (hydraulic potential) and causes desorption of heavy metals from clay surfaces and transports them into the pore fluid. Electro-osmotic flow and its associated phenomena constitute the mechanisms for removing heavy metals from soils. 

Figure 6.6.2 Photographs of motion of acid front from the anode and base front from the cathode for removal by electromigration of zinc from a cylindrical clay sample 0.2 m long, initially saturated with a 7.7 mol m" zinc solution, across which 8 V is applied. The frame times from top to bottom are 6, 8, 10, and 11.3 h, respectively. [Courtesy of Dr. Sebastian Tondorf. From Probstein Hicks 1993. Removal of contaminants from soils by electric fields. Science 260, 498—503. Copyright 1993 by the AAAS. With permission.]...

Electrokinetic processing of soils, by application of a direct current through a wet soil mass, results in the development of electrical, hydraulic, and chemical gradients. The formation of an acidic front at the anode from water electrolysis and the induced electroosmotic flux of the pore fluid enable the removal of those contaminants that can be solubilized, desorbed from the soil, or simply carried by the pore fluid. The fundamental basis of each of these two main processes is described below. 

The protons (H+) and hydroxyl (OH ) ions generated by electrolysis reactions (Eqs. 1.1 and 1.2) migrate toward the oppositely charged electrode. Acar et al. (1995) determined that, generally, H+ is about twice as mobile as OH , so the protons dominate the system and an acid front moves across the soil until it meets the hydroxyl front in a zone near the cathode, where the ions may recombine to generate water. Thus, the soil is divided into two zones with a sharp pH jump in between a high-pH zone close to the cathode and a low-pH zone on the anode side. The actual soil pH values will depend on the extent of transport of H+ and OH ions and the geochemical characteristics of the soil. 

In general, the electroremediation of heavy metals and metalloids in Table 4.1 are dependent mainly on the development of an acidic front through the soil since the acidification aids mobilization. In most of the experiments, the acidification had not reached through the whole soil specimen during the remediation period, and the remediation percentages given in the table were obtained only in a short distance from the anode. 

Mercury (Hg) It was shown by Thoming, Kliem, and Ottosen (2000) that the acidic front resulted in an increased oxidation rate of elemental Hg, but the oxidation process was slow and no Hg was removed from the soil in total. Nickel (Ni) General low removal efficiency without any enhancement even for 2 months processing. Only Clarke, Lageman, and Smedley (1997) showed that a high removal efficiency could be achieved but remediation time and conditions were not mentioned, so it could not be evaluated if the remediation was enhanced or not. 

For some combinations of heavy metals, it is also necessary to use enhancement solutions to ensure the simultaneous removal of all pollutants (Ottosen et ai, 2003). Especially, the presence of As in the soil necessitate alternative solutions to the acidic front since As generally has low mobility under acidic conditions, whereas As is more mobile under alkaline conditions, where most heavy metals are not mobile (Le Hecho, TelUer, and Astruc, 1998 Ottosen et aL, 2000). Le Hecho, Tellier, and Astruc (1998) conducted laboratory experiments with spiked soils, where the pollutants were As and Cr. Successful remediation was obtained in the developing alkaline front combined with the injection of sodium hypochlorite. As was mobile in the alkaline environment, and Cr(III) was oxidized to Cr(VI) by hypochlorite and mobilized in the alkaline environment. In loamy sand polluted with Cu and As from wood preservation. As and Cu were mobile simultaneously after the addition of NH3 to the soil (Ottosen et a/., 2000). As was mobile due to the alkaline environment and Cu formed charged tetra-ammine complexes. For the simultaneous mobilization and electrochemical removal of Cu, Cr, and As, ammonium citrate has shown to be successful (Ottosen et al, 2003). 

Enhancement of the remediation is necessary in various situations where the acidic front is not sufficient to obtain the crucial desorption. These situations are... 

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