¿Por qué no es eficaz añadir solamente sulfato de aluminio para remover el fósforo y el arsénico.

Why is not effective to add only Aluminum Sulfate, for Phosphorus and Arsenic removal in the water treatment directed to purification or depuration?

Lic. Sergio Enrique Molina Mejía. Chemist – MAIES – MBA – MSc

Specialty Gases Manager & Gas Technology Applications Consultant

Productos del Aire de Guatemala, S. A.

41 Calle 6-27 zona 8. Tel 2421 0400 ext 314

smolina@fabrigas.com 

It is common to find in the literature, that the process of coagulation with aluminum sulfate not only contributes to the removal of suspended solids in water treatment for either purification or depuration, but also enables the elimination of eutrophication-generating phosphates, and chronic-poisoning-generating arsenates, these features lacking in organic polymeric flocculants. However, experience in the most of coagulation processes indicates that the efficiency for phosphate ion does not exceed 3% and not more than 1% in terms of total phosphorus, being effective only for removal of the 87% solids suspended.

What important factor has been forgotten or pending of application, to convert this technical statement into a practical reality?

The coagulation process in the removal of suspended solids.

The coagulation process for the treatment of drinking water was introduced in this country in the early 1950s, by the Municipality of the City of Guatemala, using Aluminum Sulfate for the sole purpose of removing turbidity, because eutrophic ability of phosphate in natural waters had not been evidenced and there was no certainty of the presence of arsenate in the drinking water.

The use of aluminum sulfate or ferric sulfate for water clarification is based on the ability of hydrolysis that the cations of these inorganic salts have in neutral water, according to the following chemical reactions:

Al+3 + 3H2O → Al(OH)3 + 3H+

Fe+3 + 3H2O → Fe(OH)3 + 3H+

The aluminum or iron hydroxides generated in the chemical reaction, form a hydrated insoluble and very lumpy gel that traps solid particles suspended in water, enclosing within its globular shape or adsorbing on the surface, removing them when these clots settle by gravity, leaving a clarified water (transparent) that can undergo the following processes for disinfecting in case of drinking water, or oxidation process in the case of wastewater. During the development of this technique, it was found that in some cases the dosage with iron sulphate produced yellowing of water, so that these salts were replaced by aluminum ones that did not provide color to clarified. Back in the early 1960s, it was established that these coagulants also allowed occlusion inside their clots, of a certain amount of bacteria and yeasts that could be “mechanically” removed from the water. For these reasons, the coagulation with aluminum sulfate became an indispensable process in the purification of water, but not in the depuration of waste waters.

The important of these hydrolysis reactions is that at the end, obtaining of the hydroxyl ions extracted from the water molecules, releases hydrogen ions that acidify the aqueous medium, bringing the pH to values around 4.5 and 5.0 (or more acid according to the amount of added aluminum sulfate), allowing some inhibition to microbial growth, and thereby facilitating subsequent disinfection.

However, acidification of the aqueous medium resulting from the hydrolysis reaction of aluminum sulfate is the factor that prevents the proper removal of phosphorus and arsenic in its natural form of phosphates and arsenates. The free form of phosphate and arsenate may exist dissolved in water, in different species, which differ by the protonation of the anion, as describes below.

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The phosphate removal reaction, as shown below, needs to ensure the presence of sufficient free phosphate ion (PO4−3) to react with the aluminum ion (Al+3) and exceed the solubility product constant of aluminum phosphate (AlPO4) Ksp = 9.84 x 10−21 to form the insoluble salt that binds to the suspended solids and be jointly removed from the water, which happen at high alkalinity and a pH of 10, optimally between 12 and 14.

Al+3 + PO4−3 → AlPO4↓

Therefore, at the level of pH (acid) remaining the hydrolysis of aluminum sulfate when added to the coagulation process, the phosphorus present in the water would be in the form of dihydrogen phosphate (H2PO4−) that is unable to form insoluble salts with aluminum ion.

Similarly, the removal of arsenate, as shown below, needs to ensure the presence of sufficient concentration of arsenate ion (AsO4−3) allowing react with aluminum ion (Al+3) and overcome the solubility product constant of aluminum arsenate Ksp = 6.25 x 10−23, what happens at a pH greater than 10, optimally between 11 and 13.

Al+3 + AsO4−3 → AlAsO4↓

The phosphate occurs naturally in the water as a consequence of runoff resulting from passing through the soil, and collect phosphorus metabolized by bacteria in the degradation of plant and animal materials, and in waste water (in large quantities) from physiological waste discharged of the people. Arsenic occurs naturally in water as a result of geothermal and tectonic activity that emerges in volcanic eruptions, being oxidized in the atmosphere to arsenate and being fixed on the ground through its reaction with ferric iron (Fe+3) to form the insoluble ferric arsenate (like phosphorus to form insoluble ferric phosphate). If storm water runoff through a deforested area, both ferric arsenate and ferric phosphate will bind to suspended solids that can be dragged into the water bodies where they can be released into the different forms of arsenate or phosphate, as a result of anoxic, acidic and reducing conditions of the hypolimnion of eutrophic lakes, which transforms the ferric iron (Fe+3) in ferrous iron (Fe+2), enabling resolubilization of phosphorus and arsenic sequestered at the bottom of the lakes.

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