Magnetic seeding coagulation: Effect of Al species and magnetic particles on coagulation efficiency, residual Al, and floc properties.

Magnetic seeding coagulation (MSC) process has been used to accelerate flocs sedimentation with an applied magnetic field, offering large handling capacity and low energy consumption. The interactions of three typical Al species, aluminum chloride (AlCl), AlO(OH) polymer (Al), and (AlO)Al(OH) polymer (Al), with magnetic particles (MPs) were examined to clarify the MSC process. In traditional coagulation (TC) process, the aggregation of primary Al-dissolved organic matter (DOM) complexes with in-situ-formed polynuclear species generated a large average floc size (226 μm), which was proved to be efficient for DOC removal (52.6%). The weak connections between dissolved Al-DOM complexes and MPs led to the negligible changes of dissolved Al after seeding with MPs in AlCl. A significant interaction between MPs and Al was observed, in which the MPs-Al-DOM complexes were proposed to be responsible for the significant improvement of DOC removal (from 47% to 52%) and residual total Al reduction (from 1.05 to 0.27 mg Al L) with MPs addition. Al produced a lower floc fractal dimension (D = 1.88) than AlCl (2.08) and Al (1.99) in the TC process, whereas its floc strength (70.9%) and floc recovery (38.5%) were higher than the others. Although more detached fragments were produced with MPs addition, the effective sedimentation of these fragments with the applied magnetic field led to the decrease of residual turbidity and colloidal Al in Al. The dependence of coagulation behavior to MPs and different Al species can be applied to guide the application of an effective MSC process.