Reduction-precipitation system

A modified reduction-flotation system (Figure 6.5) is very similar to the existing conventional reduction-precipitation system (Figure 6.4), except that a DAF clarifier [T-101F] is used for clarification15-57 instead of using a conventional sedimentation clarifier (Tank T-101, Figure 6.4). 

It should be noted that the chemical reactions of the conventional reduction-precipitation system (Figure 6.4) and the modified reduction-flotation system are identical. 

It is seen from Figure 6.7 that this system is much simpler, more cost-effective, and easier to operate in comparison with all other process systems discussed earlier. The treatment efficiency of the new flotation-filtration system is expected to be higher than that of the conventional reduction-precipitation system. The new flotation-filtration system also requires much less land space.15... 

Table 6.3 shows the characteristics of a typical effluent discharge from a conventional reduction-precipitation system. The effluent quality meets industrial pretreatment requirements. 

The treatment efficiencies of the two innovative flotation-filtration wastewater treatment systems (Figures 6.6 and 6.7) are expected to be higher than those of the conventional reduction-precipitation system. 

Figure 6.4 shows an example of an existing plating facility and its conventional reduction-precipitation wastewater treatment system in New Britain, TN.15... 

FIGURE 6.4 Conventional reduction-precipitation wastewater treatment system. 

The first system, shown in Figure 6.6, is identical to the conventional reduction-precipitation in chemistry (i.e., neutralization, chromium reduction, pH adjustment, metal hydroxide precipitation, and so on). However, a flotation-filtration clarifier (Tank T101SF, as shown in Figure 6.6) is used. The unit consists of rapid mixing, flocculation, high-rate DAF, and sand filtration.1557... 

The treatment efficiency of this system (Figure 6.6) is much higher than that of the conventional reduction-precipitation wastewater treatment system (Figure 6.4).15... 

Three methods for trace metal preconcentration were examined liquid-liquid extraction aided by a chelating agent, concentration on a synthetic chelating resin and reductive precipitation with NaBTLt. The latter method gave 1000-fold preconcentration factors with total recovery of Pb and other elements17. Preconcentration of nanogram amounts of lead can be carried out with a resin incorporating quinolin-8-ol (3)18. Enhancement factors of 50-100 can be achieved by such preconcentration procedures followed by determination in a FLA (flow injection analysis) system limits of detection are a few pg Pb/L19. 

The experimental equipment system for the preparation of nano copper powder by reduction-precipitation is shown in Fig. I4.l, where the submerged circulative impinging stream reactor (SCISR) has the same structure as was used in the investigations described in the previous chapters in Part II of this book, with the same effective volume of 3.6x10 3 m it is also operated without the top cover but is made of titanium for anti-corrosion of Cl-. 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

The chromium reduction process can be employed as batch treatment or continuous treatment. For small daily volumes of water or wastewater that are less than 150,000 L (40,000 gal), the most economical system is batch treatment in which two tanks are provided, each with a capacity of one day s flow. Reduction, precipitation, and sedimentation are carried out in one tank, while the other is used to collect the waste. In a typical batch system, the required dosage of acid and sodium metabisulfite is added to the tank and the contents are mixed for 15 min to ensure complete reduction of the chromium. 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

In the EK-PRB treatment system, the removal mechanism for Cr(VI) can be illustrated by Fig. 23.7. Since the removal is believed to involve the reduction of Cr(VI) to Cr(III) by the oxidation of Fe° to Fe(II)/Fe(III) and the accumulation of Cr(VI) in the reservoirs, two simple removal paths are sketched. The first path is reduction of Cr(VI) due to the corrosion of the acid front passing through the barrier and bringing the dissolved Fe°/Fe + ions into the EO flow. The second path is the migration of Cr(VI) ions directly into a PBR reacting with the ZVI. The reaction scenarios of this reductive precipitation mechanism are described as below. 

The strong catalytic activity of platinum group metals has a significant impact on the properties of the solid products obtained in above described precipitation systems after longer hydrothermal treatment (9-72 h) at I60°C. In the course of hydrothermal treatment, the products of TMAH decomposition (methanol and trimethylamine) affect the reduction of metal cations and the formation of platinum group metal nanoparticles. These nanoparticles act as the catalyst for the reduction of Fe(lll) to Fe(ll) by the products of TMAH decomposition and the formation of characteristic Fe304 octahedra, which is followed by Mossbauer spectroscopy and FE-SEM (Fig. 23.19). 

Applications. Both industrial emissions reduction and indoor air-poUution abatement uses will grow. For example, the development of adsorbents with higher capacity for removal of radon from humid air could allow the development of a one-bed, delay-for-decay system in which radon adsorbs, decays to lead, and is precipitated onto the adsorbent. 

In earlier procedures, the ReO anion was precipitated from water as the relatively insoluble potassium salt. Reduction of KReO with hydrogen gas gives rhenium metal, but the metal is contaminated with ca 0.4 wt % potassium that cannot be separated easily. Although suitable for some purposes, rhenium formed from KReO is found to be unsatisfactory in appHcations such as those for use in filaments in mass spectrometer systems. The route involving NH ReO avoids this problem. 

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