Dear Varsha,

If you add an anionic flocculant to a sewage stream that is also anionic, there is no electrostatic interaction between the 'particles' and the flocculant. The flocculant is even repelled from the particle surfaces by electrostatic repulsion. Adding a non-absorbing polymer to a particulate dispersion can lead to so-called depletion flocculation. This is by the way common practice in the treatment of wastewater, please check: https://en.wikipedia.org/wiki/Depletion_force#Water_treatment and its linked sources for more information.

Kind regards, Leo

Oh, sorry I forgot to add this to answer your question. In depletion flocculation the particulate species are not changed so their zeta-potential remains the same. The added flocculant will not contribute to the zeta-potential as dissolved polymers scatter far less light than particles, so the effect of their addition you will not see when performing the traditional light scattering micro-electrophoresis (like with a Zetasizer).

If, you add a cationic flocculant it neutralizes the charges on the particles and there you could see a change in zeta-potential.

Kind regards, Leo

Thank you so much Leo for your answer this has added something new to my knowledge.

However, I would also like to mention that in other flocculant that i have assessed which is also negatively charged, change in zeta potential have been observed: the magnitude of negative value of sewage wastewater sample has been decreased and zeta potential value was increased to -10 mV. This could be probably due to the polymer bridging phenomenon.

So this kind of variation in the behavior of anionic flocculating agent is acceptable? kindly suggest

regards

varsha

You may have some other component that scatters light so that it is measured by the instrument (assuming you are using light scattering). For example, your flocculants themselves. You could check that by analyzing solutions of just each of the flocculants.

Another consideration is how the measurement deals with multiple species of particles. Your initial -17.1mV could be from two different materials or just one. With the Zetasizer Nano, zeta potential is determined two ways. One only gives a mean value. The other can give a distribution. If the latter method is possible (the instrument decides) then you should look at the zeta potential distribution pre- and post-treatment.

If you are using a Zetasizer Nano (or later) then I'd be happy to look at dts files for pre- and post-treatment. There's a lot of extra information contained in the files beyond just the single zeta potential value that can be used. (I invented the analysis method that Malvern calls monomodal analysis).

Thank you for your inputs Sir.

Yes I have checked the zeta potentials of the flocculant aqueous solution which is also negative and near to the value of the wastewater itself. The instrument being used is Malvern only and yes it does gives the zeta potential distribution data for the analysis.

And due to the values obtained from this study I was in a dilemma how to relate all the things together.

kind regards

Varsha

What is the flocculant and its concentration? How does that compare to the typical level of particles in the water? It could simply be that the scattering from the flocculant is dominating the signal.

It might happens. Alternatively, the estimation of wastewater conductivity could be a possible replacement.