Graduate School of Engineering, Hokkaido University, N13W8, Sapporo, 060-8628, Japan.
Faculty of Engineering, Hokkaido University, N13W8, Sapporo, 060-8628, Japan.
Water Res. 2020 Sep 15;183:116093. doi: 10.1016/j.watres.2020.116093. Epub 2020 Jun 23.
Many PACl (poly-aluminum chloride) coagulants with different characteristics have been trial-produced in laboratories and commercially produced, but the selection of a proper PACl still requires empirical information and field testing. Even PACls with the same property sometimes show different coagulation performances. In this study, we compared PACls produced by AlCl-titration and Al(OH)-dissolution on their performance during coagulation-flocculation, sedimentation, and sand filtration (CSF) processes. The removal targets were particles of superfine powdered activated carbon (SPAC), which are used for efficient adsorptive removal of micropollutants, but strict removal of SPAC is required because of the high risk of their leakage after CSF. PACls of high-basicity produced by AlCl-titration and Al(OH)-dissolution were the same in terms of the ferron assay and colloid charge, but their performance in CSF were completely different. High-basicity Al(OH)-dissolution PACls formed large floc particles and yielded very few remaining SPAC particles in the filtrate, whereas high-basicity AlCl-titration PACls did not form large floc particles. High-basicity PACls produced by Al(OH)-dissolution were superior to low-basicity PACl in lowering remaining SPAC particles by the same method because of their high charge neutralization capacity, although their floc formation ability was similar or slightly inferior. However, high-basicity Al(OH)-dissolution PACl was inferior when the sulfate ion concentration in the raw water was low. Sulfate ions were required in the raw water for high-basicity PACls to be effective in floc formation. In particular, very high sulfate concentrations were required for high-basicity AlCl-titration PACls. The rate of hydrolysis, which is related to the polymerization of aluminum species, is a key property, besides charge neutralization capacity, for proper coagulation, including formation of large floc particles. The aluminum species in the high-basicity PACls, in particular that produced by AlCl-titration, was resistant to hydrolysis, but sulfate ions in raw water accelerated the rate of hydrolysis and thereby facilitated floc formation. Normal-basicity Al(OH)-dissolution PACl was hydrolysis-prone, even without sulfate ions. Aluminum species in the high-basicity AlCl-titration PACl were mostly those with a molecular weight (MW) of 1-10 kDa, whereas those of high-basicity Al(OH)-dissolution PACls were mostly characterized by a MW > 10 kDa. Normal-basicity Al(OH)-dissolution PACl was the least polymerized and contained monomeric species.
许多具有不同特性的 PACl（聚合氯化铝）混凝剂已在实验室和商业上进行了试制，但选择合适的 PACl 仍然需要经验信息和现场测试。即使具有相同特性的 PACl 有时也表现出不同的混凝性能。在这项研究中，我们比较了通过 AlCl 滴定和 Al(OH) 溶解生产的 PACl 在混凝-絮凝、沉淀和砂滤（CSF）过程中的性能。去除目标是超细粉末活性炭（SPAC）颗粒，SPAC 用于高效吸附去除微量污染物，但由于 CSF 后其泄漏的风险很高，因此需要严格去除 SPAC。AlCl 滴定和 Al(OH) 溶解产生的高碱度 PACl 在 Ferron 分析和胶体电荷方面是相同的，但它们在 CSF 中的性能却完全不同。高碱度 Al(OH) 溶解 PACl 形成大絮体颗粒，并在滤液中产生很少的剩余 SPAC 颗粒，而高碱度 AlCl 滴定 PACl 则不能形成大絮体颗粒。高碱度 Al(OH) 溶解 PACl 的电荷中和能力较高，因此在相同方法下降低剩余 SPAC 颗粒的效果优于低碱度 PACl，尽管其絮体形成能力相似或略低。然而，当原水中的硫酸根离子浓度较低时，高碱度 Al(OH) 溶解 PACl 效果较差。原水中需要硫酸根离子才能使高碱度 PACl 有效地形成絮体。特别是，高碱度 AlCl 滴定 PACl 需要非常高的硫酸盐浓度。水解速率是影响铝物种聚合的关键性质，除了电荷中和能力外，对适当的混凝，包括形成大絮体颗粒也很重要。特别是，高碱度 PACl 中的铝物种，尤其是通过 AlCl 滴定产生的铝物种，对水解具有抵抗力，但原水中的硫酸根离子会加速水解速率，从而促进絮体的形成。即使没有硫酸根离子，正常碱度的 Al(OH) 溶解 PACl 也容易水解。高碱度 AlCl 滴定 PACl 中的铝物种主要是分子量（MW）为 1-10 kDa 的物质，而高碱度 Al(OH) 溶解 PACl 中的铝物种主要是 MW>10 kDa 的物质。正常碱度 Al(OH) 溶解 PACl 的聚合度最低，含有单体物质。
