The impact of recycling alum-humic-floc (AHF) on the removal of natural organic materials (NOM): Behavior of coagulation and adsorption

• Recycling alum flocs may promote the removal of NOM during coagulation process.
• The BET and Freundlich models indicated the adsorption of HA onto AHF was a multilayer adsorption.
• The charge neutralization was not the dominant coagulation mechanism for HA removal by AHF.
• AHF+PACl demonstrated a lower release of organic matter after breakage.
• The hydrogen bonding of AHF is the main mechanism for removal of HA.

Recycling alum flocs may promote the removal of NOM during the coagulation process. The performance of alum-humic-floc (AHF), poly aluminum chloride (PACl) and AHF and PACl mixtures (AHF+PACl) in the removal of humic acid (HA) were comparatively evaluated in regards to their coagulation behaviors and floc structure characteristics. Jar tests were conducted at neutral pH and in a dosage range from 6.11 to 26.49 mg Al/L. The dual coagulant, AHF with additional PACl, could significantly improve the removal efficiency of organic matter and residual turbidity, which probably depended on the physical adsorption and the charge neutralization mechanism. The experimental data using AHF fit with BET and Freundlich models, which indicated that the adsorption of HA onto AHF was a multilayer adsorption. Floc characterization was also performed to investigate the relationship between floc structure and organic matter removal. The highest removal efficiency of organic matter and residual turbidity occurred when using the combination of AHF with additional PACl as the coagulant, which suggested that fractal dimension of the flocs was the most significant factor in determining the removal of pollutants from the water solution. HA was released again into solution during floc breakage. The combination of AHF and PACl demonstrated a lower release of organic matter after breakage and the stronger dissolved organic carbon (DOC) removal capacity during re-growth process. The functional group on the AHF was the main factor in site-specific hydrogen bonding, which is the main mechanism for removal of HA.

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