Ferrous sulfate has a strong redox property, but it needs to be in alkaline environment to make it fully play the role of oxidation and coagulation. Fe2 + decomposed from ferrous sulfate in wastewater is a mononuclear complex, and its coagulation effect requires that when the pH value is more than 7, the Fe2 + of ferrous salt is oxidized to trivalent iron by dissolved oxygen in water, and then coagulation reaction occurs

If ferrous sulfate is used for decolorization, the optimal pH value is above 9. When the pH value is above 8, the polynuclear complex formed by hydrolysis and oxidation of ferrous sulfate makes the colored suspended dyes in the wastewater form flocs by net coagulation, and the decolorization rate can reach 92%. Of course, some pH values need to be adjusted by alkali agent (such as lime). Lime can not only adjust pH value, but also play the role of coagulation and phosphorus removal.

When ferrous sulfate is used as COD removal agent, the best pH is 7-8. Ferrous sulfate can neutralize and destabilize the suspended colloidal particles in water, and the ferric hydroxide colloid formed by ferrous sulfate and water is a kind of strongly adsorbed colloid. When the COD in water is insoluble, it will be neutralized by electricity and agglomerated. Along with it, it will be adsorbed by ferric hydroxide net and precipitated to form sludge. The removal rate can reach 88%.

It should be noted that: in the process of ferrous sulfate coagulation, with the continuous increase of ferrous sulfate dosage, the alkalinity of wastewater will continue to decrease, and the lower the alkalinity, the worse the coagulation effect. Therefore, we often add alkaline water treatment agent, such as dyeing wastewater decolorization treatment. We usually use ferrous sulfate + lime for dyeing wastewater decolorization treatment.