Subject anion adsorption

Selenium has a complex chemistry in the environment because of its multiple oxidation states and variable surface adsorption properties. Qualitatively it is analogous to sulfur occurring in the oxidation states +6 (selenate, Se04 ), +4 (selenite, SeOs "), 0 (elemental selenium) and —2 (Se, selenide) The Se anion closely resembles S (radii 0.20 and 0.185nm, respectively) and is often associated with sulfide minerals. Also, like S, Se is subject to volatilization through biological methylation. 

According to Bancroft,3 the charge on the diaphragm, which controls the direction of the osmotic flow, depends upon the relative adsorption of anions and cathions, being positive if cathions are adsorbed to a greater extent than anions. The same view is held by T. B. Briggs,4 who has recently contributed an important paper on this subject. 

The supernatant was subjected to fractional precipitation with ammonium sulfate (step 5) and then with acetone (step 6). PTTH was recovered in the precipitates with 35-55% acetone, while bombyxins were recovered with 55-75% acetone. The 35-55% acetone precipitates were subsequently purified through five steps of conventional chromatography gel filtration on Sephadex G-50 with 0.5M Tris-HCl (pH 8.5) (step 7), anion exchange on DEAE-Sepharose C1-6B with 0.2M sodium acetate (pH 5.2)(step 8), cation exchange on CM-Sepharose C1-6B 0.1-0.5M NaCl in 0.05M sodium acetate (pH 5.2) (step 9), Hydrophobic adsorption on Octyl-Sepharose C1-4B with 4M ammonium acetate, 0.2M ammonium acetate and 40% acetonitrile in 0.2M ammonium acetate (step 10) and gel filtration on Sephadex G-75 with O.OIM phosphate buffer containing 0.2M NaCl and 2% butanol (step 11). 

Salts are known to influence several properties of aqueous solutions in a systematic way (122,123). The effect of different aiuons and cations seems to be ordered in a sequence this theory was already proposed by Hofmeister in 1888 (124) from a series of experiments on the salts ability to precipitate hen-egg white protein. Numerous other properties of aqueous salt solutions are also found to be systematically salt dependent, such as the surface tension or the surface potential (122). However, the exact reason for the observed specific cation and anion sequences is still not fully understood (125). Model calculations (126), as well as nuclear magnetic relaxation experiments (127), propose a delicate balance between ion adsorption and exclusion at the solute interface. This balance is tuned by the solvent (water) stmcture modification according to the ion hydration (128, 129) and hence is possibly subject to molecular details. 

A very useful review of the subject of metal adsorption has recently appeared, which gives tables of the relative strengths of anionic and cationic adsorption for each of the metals in the periodic table, with separate tables for each of the different anion environments, i.e., F , Q", NO3, CbT, and sol". Table 2 shows some of the results from this reference that pertain to the catalytically important Group VIII and IB metals. There would seem to be ample scope for more detailed work along these lines for systems of more particular catalytic interest, e.g., those containing ammonia as a complexing ligand. Some more qualitative work has appeared in several papers, which will be discussed later. 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

As mentioned in the foregoing text, localized corrosion of metals starts with a local breakdown of passive films and localized metal dissolution then occurs at the breakdown site. The local film breakdown is caused by the adsorption of aggressive anions such as chloride ions, CP, which are hard bases. No reliable information is available about the acid-base characteristic of the passive film surface. It is however expected that the breakdown of passive films will be prevented, if some anions or molecules are firmly adsorbed expelling chloride ions from the film surface. In literature, though, we have seen almost no reliable studies on this subject. 

In the ion-exchange method, the cations C+ or the anions A" tend to saturate the first adsorption sites so that most of the compound adsorbs near the pore mouth, and a large concentration gradient develops within the pellet pore. There are several ways to obtain a more uniform distribution (1) a large supply of compound is used so as to saturate every adsorption site, (2) the support may be left for a long time in contact with the solution, and (3) competing ions are added to the solution, which will adsorb on the same sites as the ions of the metal precursor. This is the subject of the method discussed next. 

The adsorption of anions such as halides, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry [1 - 3]. It is well known that various electrochemical surface processes such as underpotential deposition of hydrogen and metal ions are strongly affected by co-adsorbed anions. Particularly, structures of the iodine adlayers on Pt, Rh, Pd, Au, and Ag surfaces have... 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

The latest developments in the issue are indicating that the view based on the H adsorption model is subject of some revision. In References 23 and 24 the voltammetric contribution of some specifically adsorbed anions (acetate, oxalate, chloride and bromide) was studied in the case of Pt(lll) electrodes by means of experiments involving the displacement of the adsorbed species by CO in acidic medium. The conclusion of this study was that the usual states correspond to the reversible adsorption/desorption of hydrogen, whereas the so-called unusual states would correspond to the adsorption/ desorption of anions. 

When a relatively small amount of surfactant is added to water, part of it is dissolved the dissolved molecules move freely in the aqueous phase. The other part is reversibly adsorbed on the interfaces present. The free and adsorbed surfactant molecules are subject to an adsorption/desorption equilibrium. The greater their affinity to the substrate, the stronger their adsorption. The well-known Langmuir equation is often used to quantitatively describe the adsorption of surfactants, particularly in the case of anionic surfactants. For other types of surfactants or mixtures of them, other equations may apply for details in this regard, the reader is referred to specialized textbooks [25, 26] and articles [27-29]. 

---