Experimental analysis of magnesium hydroxide-reactive orange floc formation time and rate in coagulation process

• Application of magnesium hydroxide as a coagulant for treating high pH reactive orange wastewater was studied.
• Floc formation time and rate are determined using laser method.
• An empirical expression and a pseudo first-order model were used to estimate the floc formation rate order and coefficient.
• Floc morphology was measured.

Magnesium hydroxide as coagulant for treating high pH reactive orange wastewater was studied. The floc formation kinetics was investigated with a laser particle analyzing system under different experimental conditions. Floc formation time tended to decreases with the increase of coagulant dose, reactive orange concentration, temperature and pH value. An empirical expression and a pseudo first-order model were used to estimate the floc formation rate order and coefficient. The experimental data agreed well with the pseudo first-order equation. Floc formation rate order is 0.52 and 2.23 for pH value 11.5 and 12.0, respectively. It was also found that when pH value reached 11.5, floc formation time is not significantly influenced by initial reactive orange concentration. Floc image shows that magnesium hydroxide-reactive orange floc size is relative small and some aggregate together to form more compact ones under higher pH value.