Profiles of and variations in aluminum species in PAC-MC used for the removal of blooming microalgae

Insights into the differential removal of various red tide organisms using modified clay: Influence of biocellular properties and mechanical interactions

In recent years, red tides have increased worldwide in frequency, intensity, involving a higher number of causative species during the events. As the most commonly used method for control of red tides, modified clay (MC) was found to have differential ability to remove various red tide species. However, the underlying mechanisms have not yet been completely elucidated. In this study, the use of MC to remove three typical disaster-causing species, Aureococcus anophagefferens, Prorocentrum donghaiense and Heterosigma akashiwo, was investigated, and differential removal of these species was probed with insights into their biocellular properties and mechanical interactions. The results showed that removal efficiencies of the three species by MC decreased in the order P. donghaiense > A. anophagefferens > H. akashiwo, while the sedimentation rates decreased in the order H. akashiwo > P. donghaiense > A. anophagefferens. Analyses of the cell surface properties and redundancy analysis (RDA) revealed that the highest surface zeta potential of -5.32±0.39 mV made P. donghaiense the most easily removed species; the smallest cell size of 3.30±0.03 μm facilitated the removal of A. anophagefferens; and the highest hydrophobicity with a H2O surface contact angle of 98.50±4.31° made the removal of H. akashiwo difficult. X-ray photoelectron spectroscopy (XPS) data indicated that the electronegativity of P. donghaiense was caused by carboxyl groups and phosphodiester groups, and the hydrophobicity of H. akashiwo was associated with a high C-(C, H) content on the cell surface. According to the extended Derjaguin, Landau, Verwey, and Overbeek (ex-DLVO) theory calculation, differences in the interaction energies between MC and the three red tide species effectively explained their different sedimentation rates. In addition, the degree of oxidative damage caused by MC to the three red tide species differed, which also affected the removal of red tide organisms.

Improvement of humic acid (HA) removal using a new inorganic-organic composite coagulant: α-costic acid as a modifier of polyaluminum chloride properties

In this work, α-costic acid (αCA), a plant sesquiterpenoid from Dittrichia viscosa, was grafted into polyaluminum chloride (PAC), forming a new eco-sustainable composite coagulant PAC-αCA with improved functionality. The α-costic acid fraction grafted into the PAC and the distribution of aluminum forms in the composite coagulant were evaluated for their effectiveness in removing bentonite and humic acid from synthetic water. The interaction mechanism between PAC and α-CA was examined by the Al-Ferron time spectrophotometric method, density functional theory (DFT), and FTIR analysis. By monitoring the aluminum speciation in the composite coagulant PAC-αCA, it was discovered that the introduction of α-CA impacted the distribution of various aluminum forms, including mononuclear Ala, highly polymeric Alb, colloidal, and medium polymeric Alc. The theoretical analysis identified the Alb species as particularly sensitive to reacting with α-CA. Furthermore, coagulation performance tests demonstrated that increasing the percentage of α-CA and promoting the prevalence of Alb and Alc species over Ala species in PAC-αCA led to improved removal of turbidity and UV254. This study provides an attractive and practical option for water treatment plants to remove colloidal suspensions in raw water effectively.

Mechanism by Which MC Controls Harmful Algal Blooms Revealed by Cell Morphology of Aureococcus anophagefferens

On the basis of field experience, a bloom does not continue after treatment with modified clay (MC), even though the residual harmful algal bloom (HAB) biomass accounts for 20–30% of the initial cells. This interesting phenomenon indicates that, in addition to causing flocculation, MC can inhibit the growth of residual cells. Here, from a cell morphology perspective, Aureococcus anophagefferens was used as a model organism to explore this scientific issue and clarify the mechanism by which MC mitigates harmful algal blooms (HABs). The results showed that, at an ~70% removal efficiency, neutral clay (NC) could not effectively inhibit the growth of residual cells, although it caused various forms of damage to residual cells, such as cell deformation, cell breakage, decreased extracellular polysaccharides (EPS), increased cell membrane permeability, and increased cytoplasmic granularity, due to physical collisions. After modification, some physical and chemical properties of the clay particle surface were changed; for example, the surface electrical properties changed from negative to positive, lamellar spacing increased, hardness decreased, adhesion chains increased, adhesion improved, and the number of absorption sites increased, enhancing the occurrence of chemical and electrochemical effects and physical collisions with residual cells, leading to severe cell deformation and chemical cell breakage. Thus, MC effectively inhibited the growth of residual cells and controlled HABs.