Metal-hydroxide precipitates

A fourth mechanism is called sweep flocculation. It is used primarily in very low soflds systems such as raw water clarification. Addition of an inorganic salt produces a metal hydroxide precipitate which entrains fine particles of other suspended soflds as it settles. A variation of this mechanism is sometimes employed for suspensions that do not respond to polymeric flocculants. A soHd material such as clay is deUberately added to the suspension and then flocculated with a high molecular weight polymer. The original suspended matter is entrained in the clay floes formed by the bridging mechanism and is removed with the clay. 

OH- ions combine with ions of some metals to form insoluble metal hydroxides (precipitation). Precipitated metals settle out and thus are removed from the water adsorption, using activated carbon, improves this separation process. Iron is one of many metals which is commonly removed in this way. 

Clay - The use of elay based floeculating agent(s) in eonjunetion with a strong metal preeipitator has proven sueeessful in many wastewater treatment applieations where the objeetives are aimed at metals removal. Clay based floeeulants eleans the wastewater and in some eases replaees multistage conventional treatment system and saves the traditional operational difficulties of treatment with several chemicals such as metal hydroxide precipitation, coagulant, floeeulants and other methods. Commercial clay-based floeeulants usually consist of bentonite and other... 

The aqueous decomposition of thiourea to sulfide and cyanamide has been found to be catalyzed by metal hydroxide species and colloidal metal hydroxide precipitates. Kitaev suggested that Cd(OH)2 is actually required for CdS film formation to occur by adsorption of thiourea on the metal hydroxide particles, followed by decomposition of the Cd(OH)2-thiourea complex to CdS. Kaur et al. [241] found... 

The first step in the precipitator is the addition of polyelectrolyte solution in the flash mix tank [T-98], surge tank [T-99], and then into the slow mix unit [T-100] containing a variable speed mixing paddle. The purpose of this unit is to coagulate and flocculate53 the metal hydroxide precipitates. 

From the slow mix unit [T-100], the waste flows into the lamellar portion of the sedimentation clarifier [T-101].54 55 The lamella in the clarifier concentrates the metal hydroxide precipitates. Clarified effluent can be discharged to the sewer. 

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... 

TETRA HDS [High density solids] A process for aiding the removal of heavy metals from wastewaters. It is a physical process which controls the characteristics of heavy metal hydroxide precipitates so that they settle quicker. The precipitates have a hydrophobic surface, so they are easy to de-water. Developed and licensed by Tetra Technologies, Houston, TX. Widely used by the iron and steel industry in the United States. Not to be confused with hydrodesulfurization, often abbreviated to HDS. 

Hess GG, McKenzie DE, Hughes BM. 1986. Selective preconcentration of polynuclear aromatic hydrocarbons and polychlorinated biphenyls by in situ metal hydroxide precipitation. J Chromatogr 366.197-204. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Iron is a natural coagulant that agglomerates fine and colloidal metals in water. It is a natural absorbent of heavy metals present in water, and its solids are more dense than other metal hydroxide precipitates. 

A fourth mechanism is called sweep flocculation. It is used primarily in very low solids systems such as raw water clarification. Addition of an inorganic salt produces a metal hydroxide precipitate which entrains fine panicles of other suspended solids as it settles. 

Pretreatment involving filtration and clarification for removal of suspended solids and turbidity may improve treatment efficiency. To reduce the interference of inorganic and organic compounds, other treatment processes may have to be combined with UV/H202 systems for effective treatment. In some situations, pH control may be required to prevent precipitation of metal salts during the oxidation process and to avoid a loss in efficiency due to the precipitates. Generally, metal hydroxide precipitation can be avoided for pH less than 6. Alkaline pH can adversely affect the reaction rate, possibly due to the base-catalyzed decomposition of H202. 

Electrochemical reduction of Ni(taab)2+ [taab = (20), see p. 231] occurs in two one-electron steps, to complexes formulated as [Nim(taab)] + and [Niu(taab)]°. The relationship between the annulene taab and the two-electron reduction product, the porphyrin-like taab2-, is discussed.102 The preparation of macrocycles of type (87) by a template synthesis requires a minimum ring size of x = y = 3 and depends upon the strong complexing of the metal ion at the pH of the reaction, otherwise the metal hydroxide precipitates. The NiL(C104)2,nH20 (n = 0, 1, or 2) species have been prepared for x = 3, y = 4.479... 

The solubility of metal-hydroxide precipitates in water varies depending on ionic strength and number of pairs and/or complexes (Chapter 2). A practical approach to determining the pH of minimum metal-hydroxide solubility, in simple or complex solutions, is potentiometric titration, as demonstrated in Figure 12.3. The data show that potentiometric titration of a solution with a given heavy metal is represented by a sigmoidal plot. The long pH plateau represents pH values at which metals precipitate the equivalence point, or titration end point, indicates the pH at the lowest metal-... 

Precipitation and dissolution of metal hydroxides The solubility product principle can also be applied to the formation of metal hydroxide precipitates these are also made use of in qualitative inorganic analysis. Precipitates will be formed only if the concentrations of the metal and hydroxyl ions are momentarily higher than those permitted by the solubility product. As the metal-ion concentration in actual samples does not vary much (10—1 —10 3 mol -1 is the usual range), it is the hydroxyl-ion concentration which has the decisive role in the formation of such precipitates. Because of the fact that in aqueous solutions the product of hydrogen- and hydroxyl-ion concentrations is strictly constant (A = 10 14 at 25°C, cf. Section 1.18), the formation of a metal-hydroxide precipitate depends mainly on the pH of the solution. Using the solubility product principle, it is possible to calculate the (minimum) pH required for the precipitation of a metal hydroxide. 

Results of similar calculations, made on several metal-hydroxide precipitates, are summarized on the graphs of Fig. 1.13. (Erdey, 1963). The shaded area in each graph shows the pH region in which the precipitate is formed the areas left white correspond to circumstances under which the ions are in the dissolved phase. The upper ends of the oblique lines, drawn as boundaries, correspond to solutions containing 10 2 mol 1 metal ions, these are therefore the pH values at which precipitation begins. The lower ends, on the other hand, mark... 

McNeill, L. and M. Edwards (1997). Predicting as removal during metal hydroxide precipitation. J. Am. Water Works Assoc., 89, 1, 75-86. 

In less acidic (or more basic) solutions, hydroxide or oxide bridges between metal atoms form, the high positive charge promotes more hydrogen ion dissociation, and a large aggregate of hydrated metal hydroxide precipitates. A possible first step in this process is... 

The effects of pH on the binding of iron to various types of cellulose samples after 2k hours incubation at 30°C is shown in Table I. Carboxymethylcellulose bound as much as 70 of the ferrous iron at pH 7.0, compared to Whatman 3 filter paper which only bound 18 at pH = 7-0. Control samples of buffer and metal at each pH did not show any sign of metal hydroxide precipitation at the concentrations and temperature used in the study. The samples of pH 6.0 and 7.0 did however change to a faint yellow upon addition of iron. 

Metal hydroxide precipitate phases can also form in tire presence of non-Al-bearing minerals (Scheinost et al., 1999 Sparks, 2002, 2005). Using diffuse reflectance spectroscopy (DRS), which is quite sensitive for discriminating between Ni-0 bond distances, it was shown that a-Ni (OH)2 formed upon Ni2+ sorption to talc and silica (Figure 3.6). 

The formation of metal hydroxide surface precipitates and subsequent residence time effects on natural sorbents can greatly affect metal release and hysteresis. It has generally been thought that the kinetics of formation of surface precipitates was slow. However, recent studies have shown that metal hydroxide precipitates can form on time scales of minutes. In Figure 3.7 one can see that mixed Ni-Al hydroxide precipitates formed on pyrophyllite within 15 minutes, and they grew in intensity as time increased. Similar results have been observed with other soil components and with soils (Scheidegger et ak, 1998 Roberts et ak, 1999 Sparks, 2002, 2005). 

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