Oxidation, and precipitation

Part of the continuously recirculated solution is bled off and sent to the iodine finishing process. Iodine finishing consists of contacting this bleed of concentrated acidic iodide solution with gaseous chlorine, through which iodine is formed by oxidation and precipitated. After iodine precipitation, the resulting acidic mother Hquor, saturated with free iodine, is pumped back to acidify the clarified brine and to recover the remaining iodine. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

As seen in the above equations, the aqueous oxidation processes convert sulfur in the feed to dissolved sulfate, while arsenic is oxidized and precipitated as ferric arsenate compounds. So, problems of the emission of sulfur and arsenic oxides caused by roasting are avoided in the aqueous oxidation processes. The two different industrial methods which achieve the oxidation reactions are pressure oxidation and biological oxidation. 

Good separation from iron is achieved by formation of solutions of stable Ni11 and Co11 ammine complexes, whilst any Fe11 leached is oxidized and precipitates as Fem oxyhy dr oxides. 

Applied to certain compounding ingredients (particularly zinc oxide and precipitated whiting) which have been surface-treated to improve their reinforcing power or their ease of incorporating into rubber. Also applied to a lining which has... 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

An autocatalytic reaction is one promoted by its own reaction products. A good example in geochemistry is the oxidation and precipitation of dissolved Mn11 by C>2(aq). The reaction is slow in solution, but is catalyzed by the precipitated surface and so proceeds increasingly rapidly as the oxidation product accumulates. Morgan (1967) studied in the laboratory the kinetics of this reaction at 25 °C and pH > 9. 

Figure 8. Regression of batch oxidation and precipitation experiment of Bullen et al. (2001), where Fe(II),q was oxidized, followed by precipitation of ferrihydrite, over a 24 h period. The reaction progress (F) is well-fit by a first-order rate law, where the rate constant is 0.0827 F/h, with an of 0.96. In the model illustrated by Equations (5)-(8) in the text, this rate constant would be set to k,.

Equation (16) notes that the difference in measured 5 Fe values for Fe(II)aq and ferrihydrite precipitate is equal to difference in the Fe(III)aq-ferrihydrite and Fe(III)aq-Fe(II)aq fractionation factors, assuming that the proportion of Fe(III)aq is very small (<5%). In cases where the proportion of Fe(III)aq ratio is significant (>5%), the isotopic effects of combined oxidation and precipitation may still be calculated using an incremental approach and Equation (12), along with the pertinent fractionations between components (Eqns. 14 and 17). 

Figure 10. Fe isotope compositions for total aqueous Fe (Fe,(,T) and ferrihydrite (FH) precipitate and aqueous Fe-ferrihydrite fractionations from the batch oxidation and precipitation experiment of Bullen et al. (2001). (A) Measured S Fe values from Bullen et al. (2001), compared to simple Rayleigh fractionation (short-dashed lines, noted with R ) using 10 1naFe.,-FH = 0.9%o, as well as the two-step re-equilibration model discussed in the text (i.e., Eqn. 12), shown in solid gray lines for the aqueous Fe and ferrihydrite components the predicted 5 Fe value for Fe(III), is shown in the heavy dashed line, which reflects continual isotopic equilibrium between Fe(II), and Fe(III),(. Note that in the experiment of Bullen et al. (2001), aqueous Fe existed almost entirely as Fe(II),(,. (B) Measured fractionation between total aqueous Fe and ferrihydrite precipitate, as measured, and as predicted from simple Rayleigh fractionation (black dashed line) and the two-step model where isotopic equilibrium is maintained between aqueous Fe(II),q and Fe(III),q (solid gray line).

Iron and manganese are initially supplied to the sediments as a component of the sinking flux of POM and particifiate oxyhydroxides. Remineralization of the POM releases iron and manganese to the pore waters. In the presence of O2, the solubilized metals are oxidized and precipitate as oxyhydroxides, thereby increasing the inorganic particifiate phase in the oxic layer. Continuing sedimentation eventually carries this particulate Mn and Fe below the oxic zone. 

The comparison of the ion activity product (lAP) of the dissolved constituent ions (e.g. for goethite, Fe " and OH ) with JQo of a Fe oxide provides an indication of whether the oxide will precipitate or dissolve in a particular solution. If the lAP exceeds Kso> the solution is supersaturated with respect to the oxide and precipitation takes place. If lAP = K o, the system is in equilibrium and if lAP < K o, the oxide will dissolve until equilibrium is reached. Interference with nudeation may retard or even inhibit predpitation in a supersaturated solution and prevent true equilibrium from being attained. 

Iron oxidation and precipitation of ferric hy-droxysulfates by resting Thiobadllus ferrooxidans cells. Appl. Environ. Microbiol. 43 ... 

All the early work on plutonium was done with unweighable amounts on a tracer scale. When it became apparent that large amounts would be needed for the atomic bomb, it was necessary to have a more detailed knowledge of the chemical properties of this element. Intensive bombardment of hundreds of pounds of uranium was therefore begun in the cyclotrons at Berkeley and at Washington University in St. Louis. Sepa-ration of plutonium from neptunium was based on the fact that neptunium is oxidized by bromate while plutonium is not, and that reduced fluorides of the two metals are carried down by precipitation of rare earth fluorides, while the fluorides of the oxidized states of the two elements are not. Therefore a separation results by repeated bromate oxidations and precipitations with rare earth fluorides. 

The nodules are formed by the oxidation and precipitation of iron and manganese. The oxidation of Mn24 is catalyzed by a reaction surface io a tetravalent state that absorbs additional Fe2+ or Mn2+ which, in turn, becomes oxidized. A surface is required and the initial deposition may be of iron oxide, possibly from volcanic or geothermal sources. Proper conditions of pH, redox potential, and metal ion concentration are found in deep ocean waters. The rate of accumulation appears to be very slow. The growth also may be discontinuous, and is estimated at a faster rater rate near the continental margins. 

A common form of pretreatment is to remove the risk of fouling and staining by iron and manganese using an aeration tower with a coke or media catalyst, onto which the aerated water falls, promoting the oxidation and precipitation of insoluble ferric hydroxide and manganese dioxide. 

Absorption of atmospheric gases followed by oxidation and precipitation of metals in water samples... 

The reaction proceeds in two stages. In the first stage the reaction components are heated by direct or indirect steam in wooden vats which are lined with lead and provided with stirrers. A small quantity of bleaching powder is added to oxidize and precipitate the iron. The insoluble residue separated by filtration of the solution is still containing a certain quantity of boric oxide it is, therefore, charged into an autoclave with a stirrer, where after the addition of a fresh... 

Although the concentration of heavy metals in brines is usually not high enough to cause alarm, iron can sometimes be quite high (10-100 mg L-1), but it quickly oxidizes and precipitates out because of the high pH of the brine. 

The initial products of chemical weathering are (i) simple soluble constituents with further dependence on the species and the existing physico-chemical conditions, (ii) insoluble minerals such as clay minerals, iron-and manganese oxides and precipitates of ionically dissolved elements and (iii) highly resistant residual minerals. 

Upon dissolution of oxides and (hydr)oxides of manganese and iron under reducing conditions at depth, divalent cations of these elements can diffuse upwards through the pore waters to be oxidized and precipitated in near-surface oxic layers, leading to the characteristic near-surface enrichment of manganese and iron in the sediments of many well oxygenated lakes (see Section 3.2.4.3). Phosphorus and... 

The leaching was carried out in thickeners that were covered with dome-shaped roof to prevent ammonia losses. Compressed air was introduced into the thickeners to dissolve the nickel and oxidize and precipitate the iron in solution. The ammonia saturated air from the thickeners was passed through absorption towers where it was scrubbed wdth water to recover the ammonia. 

Figure 4 Change in pH as a function of the amount of pyrite oxidized under four scenarios (i) pyrite oxidizes to an acid ferrous sulfate solution without any further oxidation (solid line) (ii) pyrite oxidizes and the resultant ferrous sulfate solution is allowed to oxidize, hut no precipitation is allowed (upper dashed line) (iii) pyrite oxidizes, the ferrous sulfate solution oxidizes and precipitates ferrihydrite (p-Ksp = 4.89, lower dashed line) and (iv) pyrite oxidizes, ferrous sulfate solution oxidizes, and goethite precipitates (dotted line). Computed with PHREEQCI at 25 °C and 1 har, thermodynamic data from Nordstrom et al. (1990).

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