Cationic contaminants

Rytwo et al. (2002) show that diquat and paraquat adsorb on neutral sites of sepiolite the authors speculate that this might be a general pattern for organic cation contaminants interacting with sepiolite. 

The Ba[Pt(CN)4] 4H20 used as the starting material for this preparation is synthesized and purified as described by Maffly et al.6 The Cs2S04 used should be 99.9% pure to minimize any possible cation contamination, especially by potassium. All other chemicals are reagent grade. Distilled water is used throughout the procedure. 

The relatively strong interaction of cationic contaminants with negatively charged soil constituents, for example, is expected to decrease bioavailability. This has been shown to be the case for diquat, in which intercalation into internal clay surfaces eliminates microbial degradation of the compound (Weber and Coble, 1968). Decreased bioavailabilities forbenzylamine in association with montmorillonite (Miller Alexander, 1991), quinoline bound to hectorite or montmorillonite (Smith etal., 1992), and cationic surfactants with humic materials or montmorillonite (Knaebel et al., 1994) have also been reported. 

Tn previous reports (1,2,3) we described the surface potential (AV) response of films of isoelectric phospholipids in the presence of anionic and cationic contaminants, either when the lipids were spread on electrolyte solution or when the latter was injected under the lipid film that had been spread on water. In the presence of 10 wt % acidic contaminant (dipalmityl phosphate, dicetyl phosphate, and dicapryl phthalate (DCP)... 

A subsequent series of glucose tests (10% concentration) was performed with added acids and bases to model expected anion and cation contaminants from manure hydrolysates (see Fig. 4). The contaminants were added at 100 ppm. The ammonium (added as carbonate) showed a decided inhibition of the catalysis. Calcium (added as carbonate) had a mild effect and the nitric acid contaminant even less, nearer the range of experimental variation. The effect of potassium (added as carbonate) was negligible (within experimental variation), as was the effect of the other acids. 

For TiC>2, this difference is more than the unit of pK. That is because in the case of metal oxides, that contains some anionic or cationic contaminants, as was previously mentioned, beside adsorption reactions, that form the electric charge at the interface, some ion or isotope exchange processes take place. The contribution of these processes is visible in Fig. 9 that presents the adsorption as a function of pH dependence, as an increase of cation adsorption below pHpzc and/or anion above pHpzc. This method may be applied to the determination of complexion constants only of the very pure metal oxides. 

FIGURE 4.10. Idealized phase diagram of the M-W-O system [4.45] (A) monotungstates and polytungstates (B) oxide and bronzes of ideal composition (C) oxygen-deficient oxides (D) reduced polytungstates (E) bronzes with cation excess (F) bronzes with cation deficiency (G) oxygen-deficient oxides with cation contamination. 

Cationic contaminants include numerous heavy metals and transition metals and several alkaline earth and alkali metals. The adsorption reactions of Pb, Cd2+, Co- . 1 Ig- C Cu- -. Zii- C i-". L Oz- - Sr- C Cs+, and NpO+ onto different oxide, hydroxide, and aluminosilicate minerals have all been investigated using XAS. The nature of the surface complexes formed has been found to be a function of crystal structure, sorbing cation, ligands present in solution, and surface coverage. 

Type two substitution is important because through it is possible to obtain active catalysts and to eliminate certain cations contaminating water or solutions. 

The most surprising finding in this series of experiments was that similar amounts of cationic contaminants have been found in the electrolyte membrane in stacks using metallic and graphitic bipolar plates. The cations found not only represented the composition of stainless steel (Fe, Ni, Cr) but also typical irais found in the tap water (ca, Mg, Cu, Zn) from which the deminerahzed cooling water... 

The results demonstrating reversibility of binding of TPP and divalent metal ions to apo-TK are summarized in Table II. Resolved apo-TK showed no activity when no Mg and no TPP was added. Addition of TPP, however, was effective in relatively high concentrations in the absence of divalent cations indicating trace amounts of divalent cations contaminating the assay system. In order to test the stability of co-factor binding, the resolved enzyme was incubated for 60 min at 25° in tris buffer... 

Chapter 8 provides a comprehensive review of membrane cationic contamination with macroscopic physics-based mathematical models. The introduction of cationic species into lire electrol5de membrane can displace the proton associated with a sulfonate group. In general, the presence of cationic contaminants will not significantly affect performance for many hours of operation. However, cation contaminants tend to be stable in the membrane and build up over time. 

Three main effects of cationic contamination include a loss in fuel cell performance an increase in high frequency cell resisfance, which does not wholly account for the performance losses and a decrease in limiting current. In addition to the effect on the membrane conductivity, the presence... 

The general behavior of the membrane with cation contamination, with and without the water effects, is presented and discussed based on predictions from the model. The model is extended to include the potential from the metal on each side into and through the ionomer in the electrodes. Because the work is focused on the membrane, this aspect is restricted to only include a hydrogen pump cell with hydrogen electrodes on both the anode and cathode sides. 

In future work, more contamination modeling is needed, especially to account for the effect of different types of contaminants, such as anion and cation contaminants, on cell performance. In reality, of course, multiple contaminants are often the case, so a multicontaminant model needs to be developed that will take into accoimt both anode and cathode contaminants, as well as membrane contamination. 

Cationic contaminants may emanate from many sources. Metals, such as iron and copper, in system components may ionize due to corrosion exchange with protons in the membrane. Metallic salts, such as sodium and calcium, may enter the fuel cell from coastal water or from deicing agents. The most likely source of cationic contaminants is from the fuel line. Hydrogen from reformed hydrocarbons usually contain parts per million (ppm) of ammonia. This ammonia can be oxidized to ammonium ions and enter the polymer electrolyte. 

Cationic contaminants tend to build up in the polymer electrolyte. This is because the sulfonate sites have a higher affinity for most other cations than protons and because most other cations do not partake in a suitable reaction to exit the polymer electrolyte phase [2,3]. In the case of ammonia, there is a suitable reaction at the cathode to remove ammonium ions from the system, but this reaction is likely slower than proton reduction. Some other metal ions, such as copper and cobalt, are electrochemically active in the fuel cell potential window and tend to "plate out" of the system. In general, once a cationic contaminant is in the polymer electrolyte phase it tends to stay there until the membrane has an acid treatment. 

Although research has been limited, several studies conducted show the effects of cationic contamination on PEMFC performance. These studies, in general, have been of two types. In the first, researchers introduced cationic contaminant to the feed stream and measured fuel cell performance over time. In the second, they precontaminated the fuel cell to known levels to understand how contamination level directly relates to the percentage of protons displaced. A brief summary of the experiments and results from these studies follow below. 

Hydrogen pump cells, where hydrogen oxidizes at the anode and reduces back to hydrogen at the cathode, were also utilized to better imderstand the nature of performance loss due to cationic contamination. The results of the hydrogen pump also showed a limiting current under contaminated conditions, a phenomenon not known for fully protonated systems [11]. This indicates that a mechanism of proton limitation exists in contaminated cases. 

We infer three main effects of cationic contamination on PEMFC systems from these experimental results. First, the system performance seems to drop all across the polarization spectrum. Second, the high frequency resistance of the fuel cell increases as contamination increases, but this increase does not fully account for the decrease in system performance. Third, the limiting current of PEMFC systems decreases as contamination increases. We can explain these effects through physical modeling of the fuel cell system. 

Before describing fhe modeling of cation contamination in the membrane, we should describe how we intend to treat the membrane itself as... 

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