The bubble–particle attachment measurements were conducted with the Induction timer from the University of Alberta,Canada (Gu et al., 2003) as shown in Fig. 1. The sample and a supernatant
solution were transferred to a small cell under the bubble holder. The solution pH was 8.5–8.8. A small bubble (1.5 mm in diameter) was generated with a microsyringe, and the distance
between a bubble and an upper surface of particle bed was adjusted to be constant in all experiments. The bubble was kept in contact with the upper surface of particle bed for the controlled contact time (10–3000 ms). Then the CCD camera was used to visually observe whether particles were attached to the bubble.For each controlled contact time, ten measurements were repeated to determine attachment efficiency (expressed as a number of successful attachments in 10 observations). The contact time at which attachment efficiency is 100% represents attachment time(Albijanic et al., 2012).