How are metals removed from water?


The trace quantity for any metals is an important constituent of water. Some metals are classified as priority pollutants but some metals are necessary for the growth of algae, if the quantity of metal is not sufficient then that can be an obstacle for the growth of algae, on the other hand, if the quantity of the metals are higher than the recommended standards than that would cut the beneficial use of water by increasing the toxicity of the water.

Sources and Classification of Metals-

The discharges from commercial, residential, or industrial dwellings are the major source of trace metals, along with groundwater infiltration make the water beneath the surface earth impure.

Following are some classes of metals-

  • Dissolved metals:

Metals present in un acidified samples that can pass through 0.45 µm membrane filter.

  • Suspended metals:

Metals present in un acidified samples that can be retained on a 0.45 µm membrane filter.

  • Total metals:

The total of dissolved or suspended solids or the concentration of metals is determined on an unfiltered sample.

  • Acid extractable metals:

Metals in solution after the treatment of an unfiltered sample treated with hot dilute mineral acid.

Metals with their health concerns and removal methods-

Metal – Arsenic (As)

Use – Used in alloying as an additive for metals

Concern – Carcinogen and Mutagen, some long-term effects can incite fatigue or loss of energy.

Method of Removal –

Silver Diethyldithiocarbamate Method: 

The inorganic arsenic is reduced to AsH3 with the help of zinc in an acid solution in a Gutzeit generator, this AsHis led through glass wool impregnated with Pb(Ch3COO)2 solution in a scrubber into an absorber tube containing Ag- diethyldithiocarbamate (AgSCN(C2H5)2) dissolved in pyridine this is where arsenic reacts with this Ag-based salt eventually forming a red soluble complex.

Atomic Absorption Spectrometric Method:

Arsenic is converted to AsH3 by reducing to +3 state, the converted AsH3 is aspirated into Ar-H2 flame, reducing the amount of arsenic down to 2.5 ppb.

Standard Discharge Limit (µg/L): 20 (daily)

Metal – Selenium (Se)

Use – Majorly used in electronics, photocells, xerographic plates, solar batteries, a trace element in animal feed, ceramics, magnetic computer cores.

Concern – Depression, weakness, irritation of nose and mouth, red staining of teeth, hair, and finger are some of the long-term effects.

Method of Removal –

Diaminobenzidine Method:

This method involves the oxidation and reduction of Selenium, first Se is oxidized to Seo4 2-  and then reduced to SeO32- in a solution of warm 4N HCL and allowed to form a chelate with diaminobenzidine at an acidic pH of 1.5, then the latter is extracted into toluene.

Atomic Absorption Spectrometric Method:

Same as Arsenic, here Selenium is converted into SeH2 and then aspirated into Ar-H2 flame.

Standard Discharge Limit (µg/L): 5 or 10 ppb (parts per billion).

Metal – Cadmium (Cd)

Use – Used in fire protection systems, nickel-cadmium storage batteries power batteries, photography and lithography, brazing alloys, fungicide, basis of pigments used in ceramic glazes, photoelectric cells.

Concern – Highly toxic, leads to food poisoning in several cases, small quantities o Cadmium can damage the arteries of human kidneys, some long-term effects include hypertension suspended effect, thyroid.

Method of Removal –

Spectrophotometric Method (Dithizone Method):

Under optimum conditions Cadmium (Cd2+)  and forms a pink to red color that can be extracted with CHCl3.

Atomic Absorption Spectrometric Method:

Almost the same as Arsenic but here the extract is aspirated into an air-C2H2 flame.

Standard Discharge Limit (µg/L): 1.1 or 0.4 to 60 ppb (potable water).

Metal – Chromium (Cr)

Use – Used in industrial processes, nuclear and high-temperature research, protective coating for automobile and equipment accessories.

Concern – Carcinogenic and corrosive on tissues, skin, and kidney damage.

Method of Removal –

Diphenylcarbazide Method:

Chromium or CrVI in an acid medium reacts with diphenylcarbazide to form a red-violet color, some elements like Mo, V, Fe, and Cu are present in small amounts that can be removed by preliminary extraction with cupferron.

Atomic Absorption Spectrometric Method:

Same procedure as Cadmium.

Standard Discharge Limit (µg/L): 5.6 or 3 to 40 ppb (potable water).

Metal – Lead (Pb)

Use – Very commonly found in storage batteries, a gasoline additive, solder, and fusible alloys, cable covering, piping, tank linings.

Concern – Long-term effects include birth defects, brain, and kidney damage.

Method of Removal –

Dithizone Method:

In this method lead forms a pink complex, lead dithionite with dithizone CCl4, some interfering metals are removed by extraction at a pH of 2-3, once they are removed tartarate is added to prevent the formation of hydroxide, and the solution is turned to basic (pH 8-9) with ammoniacal sulfite and cyanide and then extracted with dithizone in CCl4.

Atomic Absorption Spectrometric Method:

Same procedure as Cadmium, but aspiration into an oxy-acetylene flame.

Standard Discharge Limit (µg/L): 5.6 or 200 ppb (potable water).

Metal – Mercury (Hg)

Use – Used in mercury vapor lamps, arc lamps, boilers, amalgams, catalyst electrical apparatus, mirror coating.

Concern – Toxic to the central nervous system and cause birth defects in the longer run.

Method of Removal –

Dithizone Method:

Mercury in its Hg2+ state reacts with dithizone in 1N H2SO4  medium to form an orange chelate which is then extracted into CCl4.

Flameless Atomic Absorption Method:

This method is also known as Mercury Analyzer, this method involves the reduction of Hg into the elemental Hg0 state by SnCl2 followed by the sweeping of Hg0 by air into absorption cell.

Standard Discharge Limit (µg/L): 2.1 or 2 ppb (drinking water).

All the living organisms need some amounts of metallic elements to facilitate their growth, but too of metal or too little of metal any these condition can cause a serious threat, hence balance seems to be the key and to maintain that science, technology, scientists, doctors, engineers, researchers work round the clock to save your lives by filtering what you put inside your body so the next time if you feel like wasting water just think about the resources and skills used to provide humanity with a single drop of water.

Source and References:

  • Anil K De, Environmental Chemistry.
  • Metcalf and Eddy, Wastewater Engineering Treatment and Reuse

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