Rate of As

The advantages of electroless nickel over hard chromium include safety of use, ease of waste treatment, plating rates of as much as 40 p.m/h, low porosity films, and the ability to uniformly coat any geometric shape without burning or using special anodes. Increased chemical safety is another... 

Consideration of Faraday s law shows the advantage of electrochemistry for analytical purposes. With currents as small as 10 A easily measurable, conversion rates of as little as eq/s are readily detectable. This means detection limits in the range... 

In addition, plant growth rate also affects plant uptake and the bioavailability of trace elements. It was reported that the rate of As uptake by rice increased as the rate of plant growth increased (Onken and Hossner, 1995). 

The rate of As contamination of low-As aquifers as a result of pumping for drinking and irrigation water supply increases with As concentration and the rate of groundwater withdrawal. 

The extent and rate of As(III) oxidation on birnessite surfaces are affected strongly by sorbed or competitive metal ligands in solution. Figure 16.6 shows As(III) oxidation when Zn is preadsorbed or applied in solution. The abbreviations shown in the figure denote specific reaction conditions used. For example. 

This leads to the expression of the rate of as the number of times the reaction proceeds per second, the turnover frequency. The rate is thus a rate for the reaction, not the rate... 

The oxidation rate of As(III) in the presence of manganese and water may be substantially enhanced by manganese-oxidizing bacteria, such as Leptothrix ochracea (Katsoyiannis, Zouboulis and Jekel, 2004). Katsoyiannis, Zouboulis and Jekel (2004) found that the bacteria are important in oxidizing Mn(II) to Mn(IV), Fe(II) to Fe(III), and As(III) to As(V). The oxidation of Mn(II) leads to the precipitation of Mn(IV) (oxy)(hydr)oxides, which then abiotically oxidize additional As(III) and significantly sorb the As(V) that results from both abiotic and biotic oxidation. 

The rates of (aS-(a/ ) interconversion measured in CDC13 and C6D6 for analogs of colchicine differently substituted at C-l were found to be of the order of 10-4 to 10-5 sec-1 at 22°C, corresponding to a free energy of activation of 22-24 kcal/mol. Equilibration studies by H NMR revealed that an equatorial orientation of the acetamido group in colchicine and... 

In another publication (13) it was demonstrated that with proper maintenance and operating care each of two HPLC solvent delivery systems was capable of delivering smooth, precise flow rates of as low as a few microliters per minute. While not all chromatographers will desire this high degree of precision, it is clear that if people observe good laboratory practice,... 

Some actual data may help at this point. The rates of as a typical primary halide. You should not take too much... 

FIGURE 4.19. Etch rate of as-deposited CVD film and thermal oxide in NH4F. After Proksche et at, (Reproduced by permission of The Electrochemical Society, Inc.)... 

Speciation and pH also affect As adsorption rates. Arsenate adsorption was generally faster than adsorption of As(IIl), especially at lower pH (Ghosh and Teoh, 1985 Pierce and Moore, 1982). At a higher pH of 9, the rate of As(V) adsorption was slower and similar to As(lll) (Ghosh and Teoh, 1985 Ghosh and Yuan, 1987 Raven et al, 1998). 

Rates of As desorption are affected by pH. Darland and Inskeep (1997a) conducted column experiments using sand that contained Fe oxide coatings. The pH during both the adsorption and desorption parts of these experiments was maintained at a constant value of 4.5, 6.5, or 8.5. One pore volume of... 

Figure 7. Rate ofAs(V) desorption from ferrihydrite upon pH increase to 9.0, after adsorption at pH 8.0 for 144 h. Ferrihydrite precipitated and aged at pH 8.0 for 24 h prior to As(V) addition. Rate of As(V) adsorption at pH 9.0 shown for comparison. Reprinted from Fuller et al. (1993). with permission from Elsevier Science.

Arsenic is generally present as As(V) in oxidizing environments and as As(in) in moderately reducing environments. However, the rate of As(III) oxidation can be relatively slow in the presence of oxygen and As (III) can coexist with As(V). The rate of As(III) oxidation is greatly increased by the presence of other oxidants such as manganese oxides. 

The sorption of silicic acid (added 60 hours before arsenic) also decreased the rate and the total amount of arsenic sorbed (Waltham and Eick, 2002 Table 5.4). The amount of As(lll) sorbed decreased as the surface concentration of silicic acid increased. Furthermore, the inhibition of arsenite sorbed ranged from about 4% at a pH of 6 and 0.1 mM silicic acid up to 40% at a pH of 8 and 1 mol silicic acid. In all experiments except at a pH value of 8 and 1 mM silicic acid, the molar ratio of As(lll) sorbed to sihcic acid desorbed was greater than 1 (Table 5.4), indicating a greater quantity of arsenite sorbed compared with silicic acid desorbed. In contrast, silicic acid reduced the rate of As(V) sorption, which decreased by increasing pH and silicic acid concentration, but the total quantity of As(V) sorbed remained nearly constant, indicating that arsenate was able to replace silicate. Swendlund and Webster (1999) observed a reduction in As(V) sorption onto ferrihydrite at pH > 6 and attributed it to the polymerization of silicic acid. 

Study on the soil-to-carrot uptake rate of As 34 (hioavailahility). Inorganic arsenic species were prevalent in soil. The ingestion of the potentially toxic inorganic arsenic via consumption of carrots grown in soil contaminated at 30pgAsg was conservatively estimated at 37 pg per week... 

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