Sweep flocculation as a second form of charge neutralisation—a review

Abstract

The coagulation/flocculation (C/F) processes are mainly due to charge neutralisation (CN) and sweep flocculation (SF) mechanisms. However, the SF mechanism has also its CN property moreover than its well-known weighting characteristic. On this weighting characteristic, the literature has usually focused without taking in consideration the SF’s CN property. This review discusses the implicated mechanisms in destabilisation of colloids and aggregation of flocs. Colloids are very small particles that have extremely large surface area. The consequence of this smallness in size and mass and largeness in surface area is that in colloidal suspensions: gravitational effects are negligible and surface phenomena predominate. Hence, during C/F process, colloids are removed by CN and SF mechanisms which act on the anionic charge of the colloid by its neutralisation prior to its removal by sedimentation/filtration. The sweep flocs can be described as large aggregates of Al(OH)₃/Fe(OH)₃ that are formed when Al/Fe salt is added to water. Further, sweep flocs are positively charged and the colloids of clay minerals abundantly found in the water are negatively charged. As a result, the colloidal particles are electrostatically attached to the sweep flocs in the neutral pH water as to the hydrolysed metalliccations. Even if researches have greatly elucidated the growth, breaking and the re-growth of flocs, further research is required to understand CN and SF mechanisms and optimise the C/F process at the nano level.

Keywords:

• Coagulation/flocculation (C/F)
• Colloid
• Sweep flocculation (SF)
• Charge neutralisation (CN)
• Mechanism
• Aluminium
• Iron