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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 $\pu{10 g/L}$ at a $\mathrm{pH}$ of 8. Since there is so much ferric chloride, it will neutralize all of the alkalinity in the leakage and then start decreasing the $\mathrm{pH}$. To maintain the current $\mathrm{pH}$ of 8 lime, $\ce{Ca(OH)2}$, is added so that the ferric chloride will neutralize that instead.

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

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 $\mathrm{pH}$?

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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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