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

The coagulation mechanism for removing particles by Al₁₃ has been extensively investigated for water treatments. It was widely accepted that Al₁₃ played important roles in coagulation mainly by charge neutralization and electrostatic patch. However, the discovery of Al₁₃ aggregates (Al₁₃agg) in flocs indicated that the real coagulation process should be different from the previous understanding, including when Al₁₃agg were generated and how it interacted with negative particles. The aggregation process of Al₁₃ during coagulation and its micro-interfacial effect on particle coagulation remains to be explored. In this study, to investigate the aggregation of Al₁₃ and its effect on coagulation performance, two parallel coagulation jar tests were conducted on silica suspensions by preformed Al₁₃agg and Al₁₃, respectively. The results showed that optimized coagulation for particle removal by Al₁₃ occurred from pH 7 to pH 9, which was dominated by the in-situ aggregation of Al₁₃. The results confirmed that Al₁₃agg were 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 Al₁₃agg covered all over the silica particles in flocs, resulting in compact structure with rough surfaces, while the preformed Al₁₃agg mainly distributed on joint sites between particles, generating denser flocs with smooth surfaces. This difference verified that the in-situ aggregation of Al₁₃ was the key factor to optimized particle coagulation. The overall optimized particle coagulation by Al₁₃ should undergo the following pathway: charge neutralization - in-situ aggregation of Al₁₃ - inter-particle bridging.