Coagulation of humic acid by PACl with high content of Al13: The role of aluminum speciation

The role of aluminum speciation in coagulation–flocculation of humic acid (HA) by polyaluminum chloride (PACl) with high content of Al13 was investigated using 27Al NMR spectroscopy, solid-state 27Al NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and Al-Ferron method. Experimental results show that, in the coagulation process, the Al13 polymers are the dominant species in acidic pH and some of them change to Al precipitates at higher pH. The mechanism of HA removal is directly related to the form of aluminum in solution. The maximum removal of the HA is found to occur in the pH range from 5.0 to 6.0, which means that charge neutralization by Al13 polymer is the main mechanism of coagulation at slightly acidic pH. The results of zeta potential and solid-state 27Al NMR spectroscopy of the flocs also confirm that positive polymers are very effective in charge neutralizing with HA molecules. Adsorption and sweep coagulation of HA by amorphous Al(OH)3(s) play an important role at pH > 6.0 or over dosing.