Chemicals in Wastewater – Why Oxygen is Crucial to Treatment

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In this article I take a close look at the specific reactions of commonly found industrial chemicals in wastewater, in the presence of oxygen, as well as in its absence, and the part they play in the chemistry of wastewater.

Dissolved Oxygen In Wastewater

If there are chemicals in wastewater like dissolved oxygen (O2) and Potassium Hydroxide (KOH), the Manganous Sulfate present in wastewater can be turned into Manganic Sulfate. During this reaction a brown colored precipitate is formed called the Manganic Hydroxide. This will dissolve in Sulphuric Acid (H2SO4) and form Manganic Sulfate.

But if there are other chemicals in wastewater like Potassium Iodide (KI), then this Sulfate can react with it to breakdown and liberate Iodine. In testing laboratories, when analyzing the chemistry of wastewater, this final product is titrated with Hypo. For this titration starch serves as the indicator.

The above reactions are represented below.

The following chemical reaction occurs in the absence of dissolved oxygen:
MnSO4 +2KOH -> Mn(OH)2 + K2SO4
Manganese Sulphate reacts with Potassium Hydroxide to give Manganese Hydroxide and Potassium Sulfate. This reaction gives a precipitate which is white in color.

The following reaction occurs in the presence of dissolved oxygen
2Mn(OH)2 + O2 -> 2MnO(OH)2
Manganese Hydroxide reacts with dissolved Oxygen giving Dihydroxidooxidomanganate. This reaction gives a precipitate which is brown in color.

The other chemical reactions are as follows:
MnO(OH)2 + 2H2SO4 -> Mn(SO4)2 + 3H2O
Dihydroxidooxidomanganate plus Sulphuric Acid gives Manganese Sulphate and water

Mn(SO4)2 + 2KI -> MnSO4 + K2SO4 + I2
The reaction of Manganese Sulphate with Potassium Iodide gives Manganese Sulphate, Potassium Sulfate and Iodine. The result is the separation of Iodine.

And here’s the final reaction:
2Na2S2O3 + I2 -> Na2S4O6 + 2NaI
The Sodium Oxide Sulfide formed in wastewater reacts with Iodine to give Sodium Tetrathionate and Sodium Iodide

As we have seen, wastewater that carries industrial effluents contains different chemicals in varying concentration. Hence it is important to know about their reaction both in the presence and absence of oxygen, as it plays a vital part in wastewater treatment.

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