Pre-aggregation of Al13 in optimizing coagulation for removal of humic acid

The effective removal of humic acid (HA) by coagulation has been extensively investigated for water treatments. However, the limitations of pH variation and excessive residual aluminum issues were still factors needed to be considered. In this study, to investigate the coagulation mechanism for removing HA by Al-13 and optimize Al-13 operation for removing HA, Al-13 and preformed Al-13 aggregates (Al(13)agg) were applied to remove HA at different pH conditions. The results showed that preformed Al(13)agg exhibited superior HA removal performance than Al-13 due to its wide pH range and low residual Al level. During coagulation, Al-13 and Al(13)agg interacted with HA in their original status, but the DSlope(325-375) difference implied that the complexation capacity between HA and Al(13)agg was stronger than Al-13. The new peaks of HPSEC representing larger molecular weight substances were formed under acidic and neutral conditions, which indicated that HA firstly aggregated into larger complexed molecules by interacting with Al-13 or its hydrolysates and was subsequently removed by forming large flocs which was completely different from Al(13)agg situation. Therefore, the different coagulation mechanisms played the roles in HA removal for Al-13 and Al(13)agg which were studied in this paper. It was believed that the complexation and charge neutralization effects dominated coagulation process for Al-13 while sweep flocculation and adsorption coagulation were main driving force for Al(13)agg in HA removing. This work provides significant understanding of HA removal by Al-13 and Al(13)agg coagulation, which can help to design and optimize the high efficiency coagulant based on Al polycations. (C) 2021 Elsevier Ltd. All rights reserved.