Behavior of complexing ligands during coagulation–flocculation by ferric chloride: A comparative study between sewage water and an engineered colloidal model.

In this work, a comparative study is conducted on sewage as an example of a natural water system and on an engineered colloidal model. The two water systems were treated by coagulation–flocculation with ferric chloride. Affinity of phosphate and sulfate used as complexing ligands to coagulant species was studied within the two systems. Aggregation dynamics were followed with jar tests and laser diffraction. Sulfate and phosphate were determined by using ion chromatography. Atomic absorption spectroscopy (AAS) and transmission electron microscopy coupled with energy dispersive X-ray spectrometry were used for elemental and ionic analysis of P, Fe, S, SO4, and PO4 in supernatants and sediments. Investigation of ligands revealed that phosphate and iron(III) species make strong complexes that enhance aggregation velocity, whereas sulfate is weakly complexed with iron(III) species. It can be released in suspensions without any consequences on aggregation dynamics. [ABSTRACT FROM AUTHOR]