Optimized coagulation pathway of Al13: Effect of in-situ Aggregation of Al13

The coagulation mechanism for removing particles by Al-13 has been extensively investigated for water treatments. It was widely accepted that Al-13 played important roles in coagulation mainly by charge neutralization and electrostatic patch. However, the discovery of Al(13 )aggregates (Al(13)agg) in flocs indicated that the real coagulation process should be different from the previous understanding, including when (Al(1)(3)agg) were generated and how it interacted with negative particles. The aggregation process of Al-13 during coagulation and its micro-interfacial effect on particle coagulation remains to be explored. In this study, to investigate the aggregation of Al(13 )and its effect on coagulation performance, two parallel coagulation jar tests were conducted on silica suspensions by preformed Al(13)agg and Al-13, respectively. The results showed that optimized coagulation for particle removal by Al-13 occurred from pH 7 to pH 9, which was dominated by the in-situ aggregation of Al-13. The results confirmed that Al(13)aggwere both present in flocs generated in two tests, however, the morphology and distribution of surface Al of flocs were different for two tests. The in-situ formed Al(13)aggcovered all over the silica particles in flocs, resulting in compact structure with rough surfaces, while the preformed Al(13)agg mainly distributed on joint sites between particles, generating denser flocs with smooth surfaces. This difference verified that the in-situ aggregation of Al(13)was the key factor to optimized particle coagulation. The overall optimized particle coagulation by Al-13 should undergo the following pathway: charge neutralization - in-situ aggregation of Al-13- inter-particle bridging. (C) 2019 Elsevier Ltd. All rights reserved.