/الكلور/الزيوت والشحوم/المعادن/السيانيدCHLORINE/OIL&GREASE/METALS/CYANIDE

TECHNOLAB EL-BAHAA GROUP

GENERAL.DR

BAHAA BADR

Chlorine:

The pure element exists as the molecule, Cl2, which is a gas or a liquid at normal temperatures, depending on the pressure.

When dissolved in water, most of it reacts to form hypochlorous acid (HOCl) and hydrochloric acid (HCl) which make the water more acidic.

The HOCl dissociates, to some extent, to form H+ and OCl-, called hypochlorite ion.

(The HCl dissociates completely.)

If there is enough alkalinity to react with the hydrogen ions produced and keep the pH around neutral, most of the chlorine will be in the form of hypochlorous acid and hypochlorite ion.

Disinfection can be done using solutions of sodium hypochlorite, which produce the same substances in solution.

Hypochlorite ion is not considered as strong a disinfectant as HOCl, so the pH can affect the disinfectant efficiency.

Dissolved chlorine, hypochlorous acid, and hypochlorite ion, taken together, are all known as "free chlorine".

Free chlorine can react with ammonia in solution to form compounds called chloramines, which are weaker disinfectants than free chlorine, but have the advantage of not being used up by side reactions to the extent that free chlorine is.

Free chlorine (and chloramines) also react with organic nitrogen compounds to form organic chloramines, which are even weaker disinfectants.

The chloramines are termed "combined chlorine," and the sum of the free and combined forms are called "total chlorine."

(Note that a large enough amount of chlorine can oxidize ammonia to nitrogen gas; this can be used as a chemical means of destroying ammonia.)

Significance:

Chlorine is the most commonly used disinfecting agent for drinking water and wastewater.

It is coming into some disfavor because of toxic and carcinogenic byproducts, such as chloroform, which are formed when it reacts with organic matter present in the water.

Unless reduced to chloride, chlorine itself is toxic to aquatic life in receiving waters.

Pure chlorine liquid or gas is also a storage and transportation hazard because of the possibility of accidental releases to the atmosphere.

Some treatment plants are switching to hypochlorite solution because it is safer to handle.

Others are eliminating it entirely and using UV light or ozone for disinfection.

Measurement:

There are several choices for chlorine measurement, some of which can distinguish between free chlorine and the various chloramines.

There are titrations involving visual, color-indicator endpoints, as well as electrochemically measured endpoints. Some of them can be used to differentiate among the various forms of chlorine depending on whether iodide ion is added to the testing mixture.

The indicator known as DPD (full name, N,N-diethylparaphenylenediamine) can be used to measure free or total chlorine both colorimetrically or as a titration indicator.

"Amperometric titration" is a sensitive electrochemical method.

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Oil and Grease

is the name given to a class of materials which can be extracted from water using certain organic solvents.

They can be of biological origin (animal fat, vegetable oil);

they can be "mineral" (petroleum hydrocarbons);

or they can be synthetic organic compounds.

Fats and greases from restaurants and food processing industries can clog sewers, causing blockages and backups.

Petroleum products can be toxic and flammable, and can coat surfaces and interfere with biodegradation by microorganisms in wastewater treatment plants.

They are mostly biodegradable, especially biological oils and greases, but are a problem due to forming a separate phase from the water.

Measurement:

The major method of analysis is liquid-liquid extraction.

Currently, the chlorofluorocarbon known as CFC-113 is used, but is due to be phased out in favor of the hydrocarbon, hexane, because of the damage done by CFC's to the stratospheric ozone layer.

In the procedure, the sample is acidified, and then shaken several times with the solvent.

The solvent portions are combined and evaporated, and the residue is measured by weight.

In a CFC solution, the concentration of the oil/grease can also be measured by infrared spectrophotometry without having to evaporate the solvent.

To determine petroleum hydrocarbons alone, the extract solution can be treated with the material, silica gel, which absorbs the more polar biological compounds.

A newer method, solid phase extraction, passes the water sample through a small column or filter containing solid sorbent material which absorbs the oil and grease.

It is then desorbed from the sorbent using a solvent and analyzed as above.

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Metals:

Chemically, metals are classified as elements which tend to lose electrons in a chemical reaction.

As solids, they have easily movable electrons, which makes them good conductors of electricity and reflectors of light.

In compounds, they tend to be positively charged, because they have lost electrons (which carry a negative charge), and they tend to bind with non-metals.

This tendency makes some of them, such as iron and magnesium, biologically useful as part of biochemically active compounds like enzymes.

Others, such as lead, cadmium, and mercury are highly toxic because they interfere with the normal operation of these biological compounds.

The US EPA lists nine metals used in industry (arsenic, cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc) as toxic "priority pollutant" metals.

Measurement:

There are numerous colorimetric methods for metals.

Most of them are more useful in a purer medium, such as drinking water, than they are in wastewater, because of the presence of interfering substances.

The most popular methods in use today involve one form or another of atomic spectroscopy, as described previously.

Another technique, X-ray spectroscopy, is useful primarily for solid samples.

There are also electrochemical methods, like polarography and "anodic stripping voltametry" (ASV) which are quite sensitive;

but due to their complexity, they are usually thought of as being confined mostly to research purposes.
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Cyanide:

Cyanide is the name of an ion composed of carbon and nitrogen, CN-.

It is used in the mining and metal finishing and plating industries-- usually as the sodium or potassium salts, NaCN or KCN-- because of its ability to bind very strongly to metals to form water-soluble complex ions.

This same property makes it highly toxic to living things because it prevents the normal activity of biologically important, metal-containing molecules.

It is, however, biodegradable by some bacteria in low concentrations;

and they can become acclimated to higher concentrations if given enough time.

For unacclimated microorganisms in a wastewater treatment plant, however, a cyanide "dump" by an industry can lead to inhibition or even death, which can cause a severe "plant upset."

Measurement:

Cyanides are usually measured by a sensitive colorimetric/ spectrophotometric procedure which can detect levels down to about 5 parts per billion in water.

Since much of the cyanide in a sample is likely to be bound to metal ions, a digestion/distillation procedure is necessary to measure "total" cyanide.

Cyanide can also be measured by ion chromatography or an electrode method, though the latter is not considered too accurate.