Aluminum Phosphate Precipitation

Aluminum ion interacts with water to form hydroxo-complexes and solid A1(OH)3(s), and with orthophosphate to form the solid aluminum phosphate, AlP04(s). The concentrations of P04 and OH, of course, are 

let us consider the interaction of aluminum ion with water, with the only important species in solution being AP+, A10H +, and Al(OH)4  

We will now proceed in an identical fashion for the second component ion of the A1P04j i precipitate, P04  

Using Eq. 6-37, we can determine Ct.po (= Ct,ai) cts a function of pH. This relationship is plotted in Fig. 6-13. It shows that AlP04(s) has a minimum solubility in the neighborhood of pH 5.5. This information is useful to us because it indicates that it would be inappropriate to attempt AlP04(s precipitation for phosphate removal from solutions containing in the range of 10 to 10 mole P04/liter (which is typical of wastewaters) at 

Results of A1P04( s) precipitation from wastewater agree in general with the above observations. An optimum pH of about 6 was observed and it was necessary to use twice as much aluminum salt as required for phosphate precipitation because Al(OH)3ts also precipitated. Also, the A1(OH)3(si was of importance because it aided the removal of the fine, rather difficult to settle, AlP04 s). 

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Tetanus and diphtheria toxoids combined aluminum phosphate precipitated [FDA former]... 

Sodium alumiaate is an effective precipitant for soluble phosphate ia sewage and is especially useful ia wastewater having low alkaliaity (20,21). Sodium alumiaate hydrolyzes ia water to Al(OH)2 and Al" which precipitate soluble phosphate as aluminum phosphate [7784-30-7], AlPO. Sodium alumiaate has also been described as an effective aid for the removal of fluorides from some iadustrial waste waters (22). Combiaations of sodium alumiaate and other chemicals are being used to improve the detackification of paint particles ia water from spray-painting operations (23). 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

In the manufacture of sodium phosphates, the removal of contaminants from the wet process acid takes place in a series of separate neutralization steps. The first step involves the removal of fluosilicates with recycled sodium phosphate liquor. The next step precipitates the minor quantities of arsenic present by adding sodium sulhde to the solution, while barium carbonate is added to remove the excess sulfate. The partially neutralized acid still contains iron and aluminum phosphates, and some residual fluorine. 

A number of aluminum compounds, such as alum and sodium aluminate, have also been used by Layer and Wang [20] as phosphate precipitants at an optimum pH range of 5.5-6.5, as... 

To control the size of the spherical aluminum phosphate particles, urea can be used as a homogeneous precipitation agent. The decomposition of urea at elevated temperatures plays an essential role in controlling the particles size. 

Figure 6.1.10 shows the mean diameter of the spherical aluminum phosphate particles prepared by aging a solution containing 3.98 X l ) 2 mol/dm3 A1(N02)2, 3.98 X I0-2 mol/dm3 Na2HP04, 0.1 mol/dm3 HNOj, and varied amounts of urea at 100°C for 19 h (12). The mean particle size steeply decreases from 560 to 100 nm at urea concentration below 0.2 mol/dm3 and then decreases monotonously to 36 nm as the urea concentration is increased to 1 mol/dm3. Morales et al. (13) proposed a mechanism of alumin um phosphate precipitation by the reaction of aluminum salts with H,P04, which follows steps (a) (d) ...

If catalysts are prepared by coprecipitation, the composition of the solutions determine the composition of the final product. Often the composition of the precipitate will reflect the solution concentrations, as was shown for CuO/ZnO catalysts for methanol synthesis [18], but this is not necessarily the case. For al-minum phosphates it was found that at low P A1 ratios the precipitate composition is identical to the solution composition, but if the P A1 ratio in the solution comes close to and exceeds unity, the precipitate composition asymptotically approaches a P A1 ratio of 1 [19]. Deviations from solution composition in coprecipitation processes will generally occur if solubilities of the different compounds differ strongly and precipitation is not complete or, if in addition to stoichiometric compounds, only one component forms an insoluble precipitate this the case for the aluminum phosphate. 

For the fluorometric method, uranium is concentrated by co-precipitation with aluminum phosphate, dissolved in diluted nitric acid containing magnesium nitrate as a salting agent, and the co-precipitated uranium is extracted into ethyl acetate and dried. The uranium is dissolved in nitric acid, sodium fluoride flux is added, and the samples fused over a heat source (EPA 1980). 

To precipitate the phosphate ion as aluminum phosphate, alum is normally used. The chemical reaction is shown next ... 

Now, compare akoh), = 10 and A"jp aipo., = 10" for the case of using alum. This situation is similar to the case of the ferric salts. The concentrations of the aluminum ion to precipitate either the aluminum hydroxide or the aluminum phosphate are comparable. This means that the hydroxyl ion is also a competitor in the phosphate precipitation. The process operates poorly at high pH, although it has a better pH range of equal to or less than 5 compared to that of the ferric salts. 

However, the use of buffers in parenterals is not always benign, and numerous instances have been summarized where buffers or other excipients have caused stability problems.For instance, the com-plexation of Ca(II) and Al(III) with phosphate buffer solutions has been studied at great length, as well as the kinetic characteristics of the subsequent precipitation of calcium and aluminum phosphate salts. The use of metal complexing excipients, such as citric... 

Aluminum phosphate adjuvant is not a stoichiometric compound. Rather, the degree of phosphate group substitution for hydroxyl groups depends on the precipitation recipe and conditions. 

Monobasic sodium phosphate should not he administered concomitantly with aluminum, calcium, or magnesium salts since they bind phosphate and could impair its ahsorption from the gastrointestinal tract. Interaction between calcium and phosphate, leading to the formation of insoluble calcium phosphate precipitates, is possible in parenteral admix-tures.< >... 

R. Holcomb and R. R. McKibbin report that after long standing a precipitate of iron and/or aluminum phosphate frequently settles out of solutions of potassium dihydrogen phosphate. These impurities are not removed easily by recrystallization from water because they remain suspended as colloids in the concentrated salt solution. For this reason Holcomb and McKibbin recommend that a 1/5 molar solution of the salt be stored for 24 hours at 85° and then filtered through a thick filter. The filtrate is evaporated and the salt recrystallized in the usual manner. 

Phosphate ions find their way into our water system from the fertilizers dissolved in the runoff from agricultural fields and from detergents that we send down our drains. Some of these phosphate ions can be removed by adding aluminum sulfate to the water and precipitating the phosphate ions as aluminum phosphate. Write the net ionic equation for the reaction that forms the aluminum phosphate. 

Analysis of Sample. Fifty ml. of the sample aliquot was centrifuged with an International Centrifuge Size 2 at 2000 r.p.m. (approximately 1000 G) for 20 minutes. The centrifugates were analyzed for their pH, concentrations of aluminum and phosphate, and the rate of aluminum-aluminon color development. The precipitates were washed with 30 ml. of water four times, dissolved in 10 ml. of N HCl, and analyzed for their concentrations of aluminum, phosphate, and sodium. 

Prior to the isoelectric point, stable turbid suspensions of positively charged colloidal particles were obtained, but in all instances only a very small portion of aluminum and of phosphates was found in the precipitate (Table I). These suspensions were stable for at least six months, during which time there was no noticeable increase in the amount of aluminum or phosphate precipitated. 

Variation in the solution pH values was only slight (Table I) the values did not appear to be affected significantly by the precipitation reaction. The precipitates were amorphous and dissolved in IN HCl almost instantaneously no gibbsite or any crystalline phase of aluminum phosphate was observed with x-ray diffraction or electron microscope. No change in composition, crystallinity, and morphology was noticed during aging up to at least six months. 

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