# Coagulation with Ferric Chloride

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 $$\mathrm{pH}$$?

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