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I'm having a hard time understanding the use of coagulation wastewater treatment. I have read in literature that ferric chloride is used as a coagulant to remove COD, TSS and Ammonia from landfill leakage. Their dose was $10 \ \mathrm{g/L}$ at a pH of 8. Since there is so much ferric chloride, it will neutralise all of the alkalinity in the leakage and then start decreasing the pH. To maintain the current pH of 8 lime, $\ce{Ca(OH)2}$, is added so that the ferric chloride will neutralise that instead.

$$\ce{2FeCl3 + 3Ca(OH)2 -> 2Fe(OH)3 + 3CaCl2}$$

What I don't understand is what exactly is going on in this reaction? What do the products do in the leakage? What will happen to the reaction at higher or lower pH's?

If anyone can really explain this in-depth, that would be great!

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According to this Wikipedia article:

Iron(III) chloride is used in sewage treatment and drinking water production.$\mathrm{^{[1]}}$ In this application, $\ce{FeCl3}$ in slightly basic water reacts with the hydroxide ion to form a floc of iron(III) hydroxide, or more precisely formulated as $\ce{FeO(OH)−}$, that can remove suspended materials:

$$\ce{[Fe(H2O)6]^3+ + 4 HO− → [Fe(H2O)2(HO)4]− + 4 H2O → [Fe(H2O)O(HO)2]− + 6 H2O}$$

1) Water Treatment Chemicals, Akzo Nobel Base Chemicals. 2007.

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