Purification Of High Concentration Of Algae By Combining Nano-Fe₃O₄ With Polyaluminum Chloride

Eutrophication has been a serious problem in recent years and high concentration of algae is one of the problems caused by eutrophication. High concentration of algae is a threat of to large numbers of natural water bodies and some of the drinking water sources.Microcystis aeruginosa(M. aeruginosa) of Cyanophyta is one of the most predominant species during the water blooms in China. It can lead to poor water quality, and release microcystins which is a deadly threat to human body. The most widely used way to remove M. aeruginosa is coagulation with polyaluminium chloride(PACl) as the coagulant.Coagulation has good effect of algal removal as a traditional water treatment process. But the raising of algal concentration could result in the higher water pH, and high pH can increase the risks of Al dissolution from PACl. The high algal concentration also means a higher coagulant dosage and the more difficulties of settling coagulation floc. Therefore, this study uses nano ferroferric oxide(Fe3O4) as a coagulant aid to purify the algae containing water, in order to achieve a high removal efficiency on algae and other pollutants in water simutaneously, enhance the floc sedimentation efficiency, and reduce the PACl dosage.The research module includes:(1) the removal of high concentration of algae using nano-Fe3O4 and PACl and looking into its mechanism;(2) the removal of turbidity in high concentration of algal water using nano-Fe3O4 and PACl;(3) the removal of microsystins in high concentration of algal water using nano-Fe3O4 and PACl. The results and conclusions are shown as follows.(1) The addition of nano-Fe3O4 can increase the algae removal efficiency, decrease the settling time and reduce PACl dosage. The size and density of the floc using nano-Fe3O4 and PACl with the proportion of mPACl: mFe3O4=4:1 is much larger than the floc using only PACl with the same caugulant dosage. Adding nano-Fe3O4 can raise the algal removal rate from60% to 90% and above. In the mean time, the settling time for using only PACl is 60 min in order to reach the algal removal rate more than 60%, it only takes only 20 min to use nano-Fe3O4 and PACl, and the settling time could be reduced to less than 10 min with the aid of a magnetic field. Nano-Fe3O4 can also enhance the pH adaptability of PACl, the algal removal rate could be raised from below 15% to around 70% at acidic conditions.(2) Using nano-Fe3O4 and PACl have a good turbidity removal efficiency when the water containning algae and 20 mg/L kaolin. Dosing nano-Fe3O4 can promote the removal ofturbidity and the residual turbidity can reduce to less than 3 NTU. The results also show that the extracellular macromolecular organic matter and mineral salt in algal solution can promote the coagulation of turbidity. And the removal of turbidity prepared by DI water needs a further study.(3) The experiments of removing microcystins in high concentration of algal water show that coagulation with PACl could damage the M. aeruginosa cells and release microcystins to water. It is diffcult to remove the released microcystins especially at the low PACl dosage.Nano-Fe3O4 could promote the PACl hydrolysis to produce Alc, which benefits to coagulation and nano-Fe3O4 itself also have certain adsorption effect to microcystins. Dosing nano-Fe3O4 can enhance the pH adaptability of PACl, increase the removal effect at different pHs. At the condition of pH 5, the residual microcystins after dosing nano-Fe3O4 could be reduced to 7%of dosing only PACl.As a coagulant aid, the nano-Fe3O4 has been shown to improve the removal efficiency of M. aeruginosa under wider pH conditions, reduce the amount of PACl, and hasten sedimentation. In the season of algae overgrowth, it is a potential approach to control high concentrations of algae with low PACl when pH is alkaline. However, there are other engineering parameters that need to be further investigated for practical application, such as speed and duration of fast mixing and slow mixing, as well as PACl and nano-Fe3O4 concentrations for large scale water bodies.