Sequestrants, examples

Phosphonic acid is an intermediate in the production of alkylphosphonates that are used as herbicides and as water treatment chemicals for sequestration, scale inhibition, deflocculation, and ion-control agents in oil weUs, cooling tower waters, and boiler feed waters. For example, aqueous phosphonic acid reacts with formaldehyde and ammonium chloride in the presence of hydrochloric acid to yield aminotri(methylenephosphonic acid) [6419-19-8]. 

Deprotonation of enols of P-diketones, not considered unusual at moderate pH because of their acidity, is faciUtated at lower pH by chelate formation. Chelation can lead to the dissociation of a proton from as weak an acid as an aUphatic amino alcohol in aqueous alkaU. Coordination of the O atom of triethanolamine to Fe(III) is an example of this effect and results in the sequestration of iron in 1 to 18% sodium hydroxide solution (Fig. 7). Even more striking is the loss of a proton from the amino group of a gold chelate of ethylenediamine in aqueous solution (17). 

Aminotri(methylenephosphonic acid) [ATMP or AMP] is the least expensive phosphonate. It is a good, general-purpose, cost-effective scale inhibitor an effective chelant and the most thermally stable of all the common phosphonates. It is satisfactory up to at least 700 psia. However, if fed as a concentrate AIMP may easily form insoluble calcium phosphonate and it may also affect copper. ATMP has a sequestration value of 870 mg CaC03/g product at a pH level of 11 and for iron, a sequestration value of 150 mg Fe/g product at a pH level of 10. The pentasodium salt has a MW of 409. Examples include Dequest 2000/2006, Mayoquest 1230, Phos -2, Briquest 301-50A, Unihib 305, and Codex 8503. 

In addition, the sodium salts are metastable at best, producing white precipitates unpredictably and thus making formulatory work difficult. (It is also more expensive than ATMP.) HEDP has a sequestration value of 843 mg CaC03/g product at a pH level of 11. The tetrasodium salt has a MW of 294. Examples include Dequest 2010/2016, Mayoquestl500, Phos -6, Briquest ADPA 60A, Unihib 106, and Sequacel HD. 

Phosphonobutane-l,2,4-tricarboxylic acid (PBTC) is the most expensive of the commonly used phosphonates. However it is excellent at providing calcium carbonate control under highly stressed operating conditions. It is most resistant to the problem of calcium phosphonate precipitation and, from an environmental position, has the lowest phosphorus content of the common phosphonates. The acid material has a MW of 270. PBTC has a sequestration value of 280 mg CaC03/g product at a pH level of 11. It is very stable and can operate under very high pH conditions. However, it may also attack copper. Examples include Bayhibit AM, Mayoquest 2100, Phos -9, and Codex 551. 

Diethylenetriaminepenta (methylene phosphonic acid) [DETPMP] is the most effective sequestrant for barium sulfate. DETPMP has a sequestration value of 820 mg CaC03/g product at pH 11. The hexa-sodium salt has MW of 705. Examples include Dequest 2066 and Mayoquest 1860. 

Cliolestyramine (Questran) and colestipol (Colestid) are examples of bile acid sequestrants. Bile, which is manufactured and secreted by the liver and stored in the gallbladder, emulsifies fat and lipids as these products pass through the intestine Once emulsified, fats and lipids are readily absorbed in the intestine These drug bind to bile acids to form an insoluble substance that cannot be absorbed by the intestine, so it is secreted in the feces. With increased loss of bile acids, the liver uses cholesterol to manufacture more bile This is followed by a decrease in cholesterol levels. 

While metabolic engineers traditionally sought the rate-limiting enzyme to unlock flow through a pathway, now they understand that there may be many points of control and feedback with the metabolic network, and seek to empirically determine the dynamics of the interactions between rate controllers and other factors. For example, the sizes of metabolic precursor pools and the catabolism or sequestration of products affect accumulation as well as flux through the pathway. 

The disadvantage of the polyphosphates is that at the temperatures (100 °C or higher) used in many textile processes they can be hydrolysed into simpler phosphates that cannot retain the metal atom in the sequestered form. For example, dicalcium disodium hexametaphosphate hydrolyses on prolonged boiling to yield the insoluble calcium orthophosphate. This is one of the main reasons why polyphosphate sequestrants are used much less extensively than the more versatile and stable aminopolycarboxylates. 

Once the C02 is captured and compressed, it needs to be transported to the sequestration or utilization locations, unless the capture and sequestration processes are located at the same site. A C02 transportation infrastructure could be done with a rather conventional approach. On land, pipelines for long-distance C02 transport already exist. For example, a pipeline system more than 500 mi. long connects C02 fields in Southern Colorado to oil fields in West Texas. The C02 is purchased at about 15/ton for tertiary oil recovery. The cost of C02 transportation is a function of distance, whereas the costs of pipeline construction vary significantly by region (Doctor et al., 1997). The construction and operation of pipelines for ocean would be quite different from land-based pipelines. Generally, C02 is transported at supercritical pressures (-2000 psi). If C02 is sequestered at geological formations, the transferred C02 may require additional compression at the injection site depending on the specifics of the reservoir (Doctor et al., 1997). 

Cryptands of the type (217)-(220) tend to form stable complexes with a number of heavy metal ions. Of particular interest is the selectivity of (219) for Cd(n) the complex of this metal is approximately 106-107 times more stable than its complexes with either Zn(n) or Ca(n). This reagent may prove useful for removing toxic Cd(n) from biological systems as well as for other applications involving sequestration of this ion (for example, in antipollution systems). The selectivity observed in the above case appears to arise because (i) the nitrogen sites favour coordination to Zn(n) and Cd(n) relative to Ca(n) and (ii) the cavity size favours coordination of Cd(n) relative to Zn(n). 

---