Investigation of FeCl3 induced coagulation processes using electrophoretic measurement, nanoparticle tracking analysis and dynamic light scattering: Importance of pH and colloid surface charge

• Zeta potential of latex particles is a function of FeCl3 coagulant concentration.
• Measurement of zeta potential allows controlling the latex coagulation and FeCl3 dosage.
• Charge neutralization mechanism is responsible for particle aggregation at optimal dosage.
• At high FeCl3 dosage, Fe(OH)3 nanoparticles are concomitantly formed.
• FeCl3 optimal dosage for coagulation is dependent on the initial pH suspension.

In water treatment processes, the optimal dosage of coagulant is highly dependent on suspended particle surface charge, size and concentration, pH and composition of water. One way to control the coagulation process can be based on the measurement of the electrophoretic mobility and determination of zeta potential.In this study we investigated the interaction between negatively charged polystyrene latex particles and iron(III) chloride as coagulant. We combined three methods, i.e. dynamic light scattering, nanoparticle tracking analysis, and modeling to thoroughly characterize our system.We have shown that stabilization of zeta potential occurred after 60–80 min after addition of coagulant. We demonstrated different behaviors of latex particles with FeCl3 depending on the dosage of iron ions. The optimal dosage of FeCl3 is equal to 1–2 mg/L for the rapid aggregation of 10 mg/L latex suspension. We found a good agreement between the aggregation rate and surface charge of the latex particles and that charge neutralization mechanism is responsible for particle aggregation. High dosage of coagulant was also found to result in formation of iron(III) hydroxide particles which diameter was about 200 nm. The initial pH is also important for latex particle coagulation. The lower initial pH of suspension is, the more rapidly the isoelectric point is achieved.

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