The Proton Migration And Coagulation Characteristics Between The Protonated Groups Of Humic Acid And Aluminum Ions

Humic acid?HA?is heterogeneous group of macromolecules with complex structure and give high THMFP?trihalomethane formation potential?,which may have negative effects on water quality.Using advanced instrumental and chemical analytical methods,this paper studied the proton migration of HA and selective binding behavior between humic acid and aluminium salts,revealed the mechanism of competition and synergism between the protonated groups of humic acid and aluminum ions,and investigated the characteristic of in situ aluminum hydrolysis and the coagulation performance of in situ and preformed Al species.This study provided a theoretical basis for the safeguard of drinking water quality.The protonation/deprotonation of humic acid was analyzed by ¹H NMR.Aromatic and olefins protons were more easily to occur proton migration,resulting in the degree of deprotonation increased as pH increased.Moreover,the degree of deprotonation of aliphatic group increased first and remained unchanged as pH increased,while the protons contained in carbohydrate-like and methoxy changed little in the whole pH range.The migration behavior of aluminum ions in water without HA was monitored using photometric dispersion analyser?PDA?and Al-Ferron method.At pH<4,monomeric aluminum species?Al_a?was the dominate species.When the pH increased above 4,the content of Al_a decreased and soluble aluminum species transformed into amorphous hydroxide solids Al?OH?_3?s?as the Al concentration increased to 0.25m M,which was more obvious as pH increased.Based on above analyses,the morin fluorescence technique was applied to characterise the Al-HA complex quantitatively.It revealed that the linear complexation reaction occurred between aluminum and humic acid at pH<7,and the reaction rate increased as the pH increased from 2 to 6.While at pH?7,most of the dosed aluminum existed in the form of free aluminum and remained unreacted in the presence of HA until the concentration reached to trigger Al?OH?_3?s?formation.These differences are related to the reactivity of the protonated group of humic acid with the aluminum ion.Two-dimensional correlation spectroscopy indicated the reactivity of the protonated groups of humic acid during coagulation with aluminum.It revealed the preferential removal of carboxylic acid and carboxyl groups,as well as the delayed response of NH deformation of amide II and aliphatic hydroxyl at pH 5.At pH 7,the carboxyl group provided the fastest responses followed by the aliphatic hydroxyl group.Differentiating the change of molecular structures of HA by ¹H NMR and X-ray photoelectron spectra analysis,it revealed that there was a selective complexation between HA and Al.At pH 5,carboxylic carbon,ether or alcohol carbon,aliphatic carbon and unsubstituted aromatic carbon were selectively removed by coagulation.While at pH7,almost all components were removed proportionally by sweep adsorption without selectivity.To further prove the selective behavior,a series of well-characterized molecules containing the functional groups of HA were chose to investigate the coagulation removal behavior during coagulation.Combined with three dimensional fluorescence and fourier transform infrared analysis,it elucidated that the removal efficiency may be related to complexation capacity of ligand with aluminum at pH5.While coagulation removal enhanced with increasing number of functional groups,and the position of the functional groups influenced coagulation more than the number of groups at pH7.In addition to the coagulation characteristics of competition between the protonated groups,similar structural characteristics exhibited the same coagulation behavior and synergistic effect.The important structural characteristics included adjacent phenolic groups,adjacent carboxylic and phenolic groups.The phenolic groups have a strong electron-donating resonance effect,which is pronounced at the ortho and para positions and outweighs the weaker electron-withdrawing inductive effect.Thus,the presence of phenolic groups in the ortho or para position to other group can increase deprontonation,thereby favoring the bidentate complex and enhancing coagulation efficiency.In order to increase the removal of humic acid,the hydrolyzed aluminum species were investigated by ²⁷Al NMR and electrospray ionization-mass spectra?ESI-MS?method.Furthermore,the characteristic of in situ aluminum hydrolysis and the coagulation behavior of in situ and preformed Al species with humic acid were clarified.With pH increased,aluminium chloride hydrolyzed and aluminum speciation contained Al₁ to Al₂₀cores with different amounts of water molecules.Because of diversity of in situ aluminum species,AlCl₃ can meet the demand for different binding sites and had better complexation capacity with HA.AlCl₃ gave rise to better HA removal efficiency than preformed Al₁₃ at pH 5⁶,where the mutual effects of in situ Al₁₃3 and other aluminum species played an important role.Compared with the AlCl₃ hydrolysis,the dominated species of preformed Al₁₃3 was Al₁₃3 at different pH values.Al₁₃ is a high-charged cluster with seven positive charges,which has the capability to bind to all binding sties.Preformed Al₁₃ firstly bound to strong binding sites,resulting in partial supersaturation of sites and charge reversal,and HA was removed inefficiently.Moreover,Al₁₃ could gradually be converted to octahedral Al during coagulation and had less complexation capacity with HA,affecting coagulation efficiency.This study has important directive to to realize the enhanced natural organic matter represented by humic acid removal.