Effect Mechanism Of Particulate Matter On Coagulation Process Of Coagulants With Different Aluminum Forms

Particulates in the water environment cause the water body to face the problem of turbidity pollution.The"Sanitary Standard for Drinking Water"（GB5749-2022）implemented in 2023 puts forward higher requirements for the turbidity of drinking water.As the basic link of water treatment,coagulation process is widely used in the removal of inorganic particles in water supply plants.Previous researchers have conducted in-depth research on the coagulation effect of inorganic particles in water,and the coagulation mechanism is relatively complete.However,in the coupling process of coagulation and ultrafiltration,which is likely to become popular in the future,the impact of inorganic particles with different particle sizes on the coagulation process of different aluminum form coagulants and the mechanism of flocs affecting membrane fouling are still unclear.Therefore,it is necessary to conduct relevant research to provide theoretical guidance for the further removal of inorganic particles in water and the alleviation of ultrafiltration membrane pollution.In addition,micro/nano plastic particles,a special organic particulate matter,have attracted great attention from the country and have been classified as a new pollutant category.Due to the large specific surface area and strong hydrophobicity,micro/nano plastic particles can absorb different kinds of pollutants and pathogens,and have comprehensive toxicity to organisms,threatening the safety of drinking water.However,the mechanism of these special pollutants on the coagulation process of coagulants with different aluminum forms is still unclear,and the effect in the complex system where multiple organic compounds coexist remains to be studied.Therefore,it is necessary to analyze the effect of micro-nano plastic particles on the removal of natural organic matter by aluminum-based coagulation and its mechanism.Taking nm-SiO₂ andμm-SiO₂ systems as the research object,coagulation-ultrafiltration experiments were carried out with different Al forms coagulants(Al Cl₃,Al₁₃,and Al₃₀)to investigate the effects of SiO₂ removal efficiency,residual Al in effluent and coagulation pretreatment on membrane flux at different p H values.Malvern laser particle size analyzer,SEM,BET and AFM were used to characterize the physical and chemical properties of floc and its distribution and force on the ultrafiltration membrane surface.The results showed that the removal efficiency of SiO₂ in the nm-SiO₂ system was lower than that in theμm-SiO₂system,and the membrane fluxes increased from 0.68 to 0.96（Al Cl₃）,0.86(Al₁₃)and 0.87(Al₃₀),respectively,after adding coagulants in the nano-particle system.The membrane flux of micron particle system increased from 0.79 to 0.80～0.84 after adding three coagulants.Micron-sized particles are the collisions between particles,while nano-sized particles mainly collide in the form of agglomeration.The particle sizes of oligomerized aluminum（Al_a）and particulate matter are more than 150μm,and the zeta potential of the system is negative and the repulsive force is generated on the membrane surface.Under neutral conditions,the strength factor of flocs formed by Al₁₃ and particles was much higher than that of Al Cl₃ and Al₃₀,and the medium polymeric aluminum（Al_b）transferred the irreversible membrane pollution caused by the blockage of small particulate matter inside the membrane pore to the reversible membrane pollution on the surface of the membrane pore.Highly polymerized aluminum（Al_c）has strong adsorption capacity of bridging and net sweeping.Amorphous and irregular agglomerated small particles form larger floc in this process to alleviate membrane pollution.Further,the effects of polystyrene nano plastic particles（PSNPs）and polystyrene micron plastic particles（PSMPs）on the removal of natural organic matter（NOM）by Al₁₃/Al Cl₃coagulation were analyzed.The variation of NOM during coagulation was analyzed by3DEEM,FTICR-MS and other organic characterization techniques.The interaction mechanism between polystyrene plastic particles,coagulants and NOM was explained by FT-IR,2D-COS,floc fractal dimension and other analysis methods.The results indicated that the presence of microplastics in water has an impact on the coagulation removal process of organic pollutants such as aromatic rings and lipids.Due to the size effect of particulate matter,PSNPs with small particle sizes and agglomerated characteristics are more likely to interact with coagulants,resulting in an increase in DBPs content due to residual organic pollutants that have not been removed,which affected the safety of drinking water.The interaction between aluminum ions and the largeπbond of polystyrene styrene ring generated a complex and formed a new Al-O bond(836 cm^-1),and micro/nano plastics participated in the coagulation process.NOM tends to be adsorbed on the micro/nano plastic interface,which is due to the hydrophobic interaction and high conformation entropy caused by the molecular structure reorganization of organic compounds.The presence of microplastics affected the reaction activity order of functional groups in Al₁₃ systems（aromatic C-H>C-OH>C-O>NH₂>aromatic C=C>aliphatic C-H）and Al Cl₃ systems（C-O>H-C=O>NH₂>C-OH>aliphatic C-H）,interfering with the removal of organic pollutants.The flocs formed by the interaction between coagulants and polystyrene plastic particles would absorb and release organic matter through hydrophobic forces,van der Waals forces,hydration,and polymer bridging during the coagulation process.