Impact of pH on interaction between the polymeric flocculant and ultrafine coal with atomic force microscopy (AFM)

Solution pH plays a vital role in the interactions between polymer flocculants and solid colloid particles which determines the flocculation performance. The influence of solution pH on the flocculation of Magnafloc5250 on ultrafine coal suspension and the interactions between the polymers and solid surfaces were investigated by using atomic force microscopy (AFM), hydrodynamic radius measurements, particle floc size analyses, and flocculation dewatering tests. Our results revealed that the settling velocity and supernatant transmittance of ultrafine coal suspension without flocculant were low in alkaline conditions due to the strong repulsion, which agreed with the extended DLVO theory and direct force measurements by AFM. In the presence of Magnafloc5250, the highest transmittance was observed at pH 4 due to the compressed electric double layer among the coal particles, while the settling velocity reached a maximum value at pH 10, which was consistent with the largest average size of flocs (41.80 mu m) and more extended conformation of flocculant. The adhesion force measured by AFM was divided into two parts: flocculation force and bare force. And the flocculation force reached a maximum value with the broadest range of action at pH 10, which supported the settling tests and floc size measurements well. The strengthened bridging interaction (e.g., via hydrogen bonding) with increasing solution pH is attributed to the increase of effective binding sites between more extended polymer chains and coal particles, thus contributing to an enhanced flocculation behavior.

Automated summary

The pH of the solution has a significant impact on the interaction between polymer flocculants and solid colloid particles, influencing the flocculation performance. In ultrafine coal suspension, without the flocculant, low settling velocity and transmittance were observed under alkaline conditions, while the presence of the Magnafloc5250 showed improved performance at pH 4 with the highest transmittance and at pH 10 with the maximum settling velocity and floc size.