Chemical pretreatment oxidation-reduction

Chemical Oxidation. Chemical oxidation can be appHed ia iadustrial wastewater pretreatment for reduction of toxicity, to oxidize metal complexes to enhance heavy metals removal from wastewaters, or as a posttreatment for toxicity reduction or priority pollutant removal. 

Figure 31.14 shows the Rp values of [2A] with three different chemical pretreatments and with a TMS plasma polymer on each of the three pretreated surfaces, as well as on the control [2A]CC surfaces (chromate conversion-coated 2A). It can be seen that the Rp values of [2A] were decreased to some extent by pretreatment of alkaline cleaning and were drastically reduced by alkaline cleaning plus deoxidization. As observed in the XPS results, the accumulation of Cu elements and removal of oxide layer on [2A] surfaces were presumed responsible for the reduction in corrosion resistance of these chemically pretreated [2A] panels. 

MF may be used to remove these heavy metals provided pretreatment chemicals are added to precipitate the metals to particles of filterable size. The chemical pretreatment step is crucial since it will affect the performance of the membrane and the resultant sludge volume as well as the contaminant removal efficiency. Reduction/oxidation, absorption/oxidation, and/or catalytic reactions are utilized along with pH adjustment to provide the optimum precipitation. Although conventional methods of waste water treatment may use a similar pretreatment chemistry, the final solid/liquid separation by gravity settling is usually not as effective as membrane filtration. 

The methodologies used for chromium speciation in liquid samples are very diverse preconcentration on different types of columns, chemical reactions such as complexation or oxidation-reduction, and other separation procedures have been used and will be reviewed briefly below. These pretreatments with subsequent atomic spectrometric determination allow good limits of detection and recoveries for samples with low concentrations of chromium (Comelis 1996). Polarography is also suitable for the determination of chro-... 

In comparison, for elemental analysis, the integrity of the analyte is not an issue, so much harsher sample pretreatment can be performed using inorganic acids. This breaks down the sample and extracts the analyte completely into the liquid phase. However, it is still necessary to ensure that the spike and sample isotopes are in the same chemical form, so repeated oxidation/reduction cycles may be necessary. This is particularly true if thermal ionization mass spectrometry (TIMS) is used, because different inorganic oxidation states can have quite different thermal properties. 

Chemical oxidizers used to disinfect RO systems include hydrogen peroxide (peroxide), halogens, and ozone. Although halogens (and specifically chlorine) are the most popular oxidizers using in conjunction with RO pretreatment, they do not have the highest oxidization-reduction potential (ORP). Table 8.8 lists the ORP for several oxidizers. As the table shows, ozone and peroxide have nearly twice the ORP or oxidative power as chlorine. Despite the relatively low ORP, chlorine is the most commonly used disinfectant in brackish water RO pretreatment due to its ease of use and its ability to provide residual disinfection (for seawater desalination using RO, bromine (as HOBr)... 

The surface crystal structure and particle size can also influence photoelectro-chemical activity. The mode of pretreatment, for example, dictates whether titanium dioxide exists in the anatase phase (as is likely in samples which have been calcined at temperatures below 500 °C) or in the rutile phase (from calcination temperatures above 600 °C) or as a mixture of the two phases for pretreatments at intermediate temperature ranges. The effect of crystalline phase could be easily demonstrated in the photocatalytic oxidation of 2-propanol and reduction of silver sulfate, where anatase is active for both systems. But when the catalyst was partially covered with platinum black, alcohol oxidation was easy, but silver ion reduction was suppressed. On rutile, redox activity was observed for Ag+, alcohol oxidation was negligible [85]. 

At low temperatures where the surface ionic mobility is restricted the catalytic activity of a divided oxide for oxidation or reduction processes is determined primarily by the nature and the concentration of lattice defects in the surface layer and by the strength of the bond between oxygen and these defects. The nature and concentration of the defects depend upon the chemical nature of the catalyst, its previous history, and on the course of the catal diic reaction itself. In some instances, a small modification in the preparation procedure or in the pretreatment may result in an important change of cataljrtic activity. Such abrupt changes of activity may be caused by the occurrence of different reaction paths on apparently similar catalysts. Since the catalytic act is localized on particular surface structures, the energy spectrum of the active surface is of paramount importance and correlations between catal3rtic activities and collective or average properties of the catalyst are crudely approximate. 

Naturally, the intensity of surface PL is strongly influenced by the chemical composition of the electrolyte, in particular, by whether oxidants or reductants are included in it because electrons or holes trapped at surface states can easily react with oxidants or reductants in the electrolyte, resulting in luminescence quenching. The intensity of surface PL is also strongly dependent on surface pretreatments. However, it is to be noted that bulk PL is also influenced by these factors because photo-generated electrons and/or holes in the semiconductor bulk can diffuse to the surface and react with an oxidant or reductant in the electrolyte and also surface species (surface states) before they recombine. This leads to a decrease in the densities of electrons and holes in the semiconductor bulk and hence to a decrease in the intensity of bulk PL. 

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