Evaluation of the actinia-shaped composite coagulant for removal of algae in water: Role of charge density

A series of novel cationic modified actinia-shaped composite coagulant (AMS-C), with similar tentacle length and distribution but different charge density (CD), was successfully designed and fabricated by combination of a cationic graft starch and attapulgite (ATP). AMS-C shows a high efficiency in coagulative removal of Microcystis aeruginosa from water over a wide pH range. The algae-harvesting efficiency of optimized AMS-C can reach to 92.27 % only within 1.0 min after settlement and its maximal harvesting efficiency is as high as 99.25 % at the optimum dosage of 1.5 mg/L. This can be attributed to its special composited structure with abundant cationic long tentacle chains. CD of AMS-C is a key structural factor. AMS-C with a relatively high CD obviously enhanced the coagulation efficiency and settling performance through the improved charge neutralization, besides, the distinct long tentacle chains of AMS-C allowed its easy accessibility and tightly contacted with the algal cells, and thus facilitated the formation of large, dense and fast regrowing algal flocs by the enhanced bridging and sweeping effects. The aforementioned effects were together contributed to the effective removal of algae. The effective interactions between microalga cells and the composite coagulants were also verified using extended Deryaguin-Landau-Verwey-Overbeek theory. Moreover, AMS-C was able to remove Microcystins-LR without destroying the cells, and still maintained a high algae-harvesting efficiency in real water bodies. Therefore, AMS-C, with the advantages of high-performance, environmentally-friendliness and low-cost, has notably promising application prospects in effective treatment of harmful algal blooms.