Ensaios básicos com fosfato

Compreendendo a análise química básica do fosfato

O fósforo é um componente-chave da vida. É um ingrediente essencial do protoplasma celular, tecido nervoso e ossos. Tal como o fosfato, que faz parte dos componentes das estruturas genéticas e membranas celulares formando fosfolípidos. A conexão do fósforo com a vida vegetal e animal se faz importante, mas também apresenta desafios.

Baixos níveis (especialmente na forma de fosfatos) necessitam ser mantidos em sistemas aquáticos para limitar o crescimento de micro fotossintético aquático e macro organismos. Prevenir este crescimento (especificamente de algas) impede que esses organismos se tornem incômodos ou apresentem riscos à qualidade da água.

Medir esses níveis de fósforo é um subconjunto especializado de procedimentos laboratoriais de águas residuais que merece um olhar mais atento.

Confira abaixo a matéria completa publicada pelo Water Environment Federarion

Phosphate testing basics
Understanding the chemistry underlying phosphate analysis
Peter E. Petersen

Phosphorous is a key component of life. It is an essential ingredient of cell protoplasm, nervous tissue, and bones. As phosphate, it is part of the components for genetic structures and phospholipids form cell membranes. The connection of phosphorous with plant and animal life makes it important but also presents challenges.

Low levels — especially in the form of phosphates — need to be maintained in aquatic systems to limit the growth of photosynthetic aquatic micro- and macro-organisms. Preventing this growth — algae in particular — prevents these organisms from becoming nuisances or water quality hazards.

Measuring these phosphorus levels is a specialized subset of wastewater laboratory procedure that deserves a closer look.

Phosphorus forms

Waters, such as lakes, rivers, streams, and wastewater, contain phosphorous, primarily as phosphates. These compounds may further be classified as

• orthophosphates (the dissolved form),
• condensed phosphates, such as pyro-, meta-, and other polyphosphates, (the inorganic form), and
• organically bound phosphates.

Organic phosphates come from the biological processes and enter the cycle via wastewater through body wastes and food residues. They also can come from the orthophosphates being released in the biological treatment process or by receiving water biota.

The differing forms come from different sources. Organophosphorous compounds appear in detergents, pesticides, nerve agents, and matches. Also, fertilizers often contain orthophosphates.

Most notable in the past were phosphates in laundry detergent. Prior to phosphate bans and changes in manufacturers’ cleaning formulations, influent wastewater typically had about 10 mg/L of phosphorous. (Since the changes, the reduction of phosphorus has varied by region.)

Within wastewater treatment, the microorganisms in secondary biological treatment processes (enhanced biological phosphorus removal systems) can reduce phosphorous concentrations to between 2 and 3 mg/L. A coagulation process using such metal ions as alum or ferric chloride can reduce the phosphorous level further. Even pH adjustment with lime can help with the removal.

Orthophosphate test with ascorbic acid

But to know how much phosphorus has been removed requires an accurate measurement. To ensure an orthophosphate test yields accurate results, it is important to acid-wash all glassware with a hot solution of one-part hydrochloric acid and one-part water and rinse everything with distilled water. The acid-washed glassware should be filled with distilled water and treated with all the reagents to remove the last traces of phosphorus that might be absorbed on the glassware.

Preferably, this glassware should be used only for the determination of phosphorus, and after use it should be rinsed with distilled water and kept covered until needed again. If this is done, the full hydrochloric acid wash is required only occasionally. Never use commercial detergents.

Reactant considerations

Dissolved orthophosphate is measured colorimetrically using an ammonium molybdate and antimony potassium tartrate solution with ascorbic and sulfuric acids. The procedure for preparing the ammonium molybdate–antimony potassium tartrate solution is found in the 2003 book, Operation of Wastewater Treatment Plants, by Kenneth D. Kerri.

The mixture of sulfuric acid, ascorbic acid, ammonium molybdate, and antimony potassium tartrate is only stable for 4 hours. The different components of this mixture have longer shelf lives, but once they are combined, the mixture should be used quickly or discarded.

Along the same lines, the ascorbic acid solution is only stable for a week, so it is important to make small batches fresh.

Also, do not allow the ammonium molybdate solution (a saturated solution) to precipitate out. This will give low phosphate readings. If there are any signs of precipitates in the solution, discard it, and use a fresh solution.

Conducting the reaction

In the laboratory, the procedure generally has two steps. First, the condensed and organically bound phosphates are converted into dissolved orthophosphates. Second, the dissolved orthophosphates are measured colorimetrically.

In the first step, depending upon the amount of suspended phosphorus in the wastewater, filtration through 0.45-µm-pore membrane may be necessary to remove particulates. The filtrate, which contains dissolved reactive phosphorus, is then analyzed accordingly.

Total reactive phosphorus is defined as phosphorus that does not require preliminary hydrolysis or oxidative digestion and responds to direct colorimetric analysis and occurs in both dissolved and suspended forms. It is composed mostly of orthophosphate with small amounts of condensed phosphates, which are acid-hydrolyzed, that convert both the dissolved and particulate condensed phosphates into dissolved orthophosphates.

Then there are organically bound phosphates (dissolved and suspended) that must be converted into dissolved orthophosphates by an oxidation destruction method because of the organic matter being bound in the phosphates.

In the second step, orthophosphate is measured colorimetrically. Orthophosphate ions react with ammonium molybdate in an acidic solution to form phosphomolybdic acid, which, upon reduction with ascorbic acid, produces an intensely blue complex, which is called molybdenum blue.

Here’s how it works. When ammonium molybdate alone is added to the water or wastewater containing phosphorus, the reaction forms (very slowly) a yellow precipitate of ammonium phosphomolybdate. Adding sulfuric acid prevents the yellow precipitate and turns the solution blue. (The antimony potassium tartrate is added to increase the rate of reaction.)

The intensity of the blue color is measured colorimetrically at 650 nm with a spectrophotmeter. It is important to remember that the chemical reaction is time-sensitive. The reaction generally takes about 20 minutes to reach completion. After that, the intensity of the blue color begins to fade and gives inaccurate readings.

Orthophosphate test with stannous chloride

Using the same concepts as above, stannous chloride can be used rather than ascorbic acid as a reducing agent.

Condensed phosphates (pyro-, meta-, and other polyphosphates) and organically bound phosphates will not respond to this test. Also note that sulfide, thiosulfate, and thiocyanate will interfere with a true reading and cause lower-than-true test results.

This type of test is used in the field and at other times when a spectrophotometer is unavailable.

After following the instructions to obtain the blue color for the amount of phosphorus in the sample, the intensity of that blue color from the sample is compared with shades of blue from known concentrations of phosphorus.

The typical concentration of orthophosphate in water resource recovery facility (WRRF) influent ranges from 2 to 8 mg/L. Effluent values range from 1 to 6 mg/L.

Total phosphorus test

The total phosphorous test captures both the orthophosphate species described above as well as the phosphorus bound to particulates. The test is the same, but the sample preparation includes an ammonium persulfate and sulfuric acid digestion step to free the bound phosphorus from particulates. The sample and the digestion chemicals are autoclaved together to oxidize all forms of phosphorus into orthophosphates.

It is important to make sure that the digestion fully oxidizes the organic materials to release the phosphorous as orthophosphate; otherwise the results are meaningless.

Typical total phosphorus values entering a WRRF are 4 to 12 mg/L. Typical concentrations leaving a WRRF are 2 to 10 mg/L.

Too much phosphorus

Too much phosphorus can lead to water quality problems. Phosphorus follows a natural cycle through the water, soil, and sediments before it is released into the streams, taken up by plants, and then moved through the animal food chain to be returned. In this system, phosphorus is a limiting supply for plant life.

However, when phosphate (the biologically available form) becomes abundant as a result of, for example, phosphorus fertilizers, it can cause algae blooms. As these algae continue to grow, they can release toxins (depending on the type of algae), block out sunlight to aquatic plants below (leading to plant death), and will themselves die (causing a buildup of organic material in the water). Bacteria thrive on these dead plants and algae; the bacteria consume the dissolved oxygen in the water as they eat. This oxygen depletion can cause fish to die and add even more organic material. This leads to eutrophication — the excess buildup of nutrients in the water — as well as hypoxia — a lack of dissolved oxygen.

In short, according to the phosphorus cycle, it is important to recognize that even though orthophosphate is an important nutrient to both plant and animal life, phosphorus concentrations must be controlled to ensure water quality. Effectively controlling phosphorus means first properly measuring it.

Peter E. Petersen is a water chemist II at the Milwaukee Water Works.

Fonte: Water Environment Federarion