Aggregation Properties Of Aqueous Humic Acid Based On Asymmetric Flow Field-flow Fractionation

Humic acid is one of major components of water organic pollutant.It mainly comes from the decomposition of animal and plant residues,and exists in colloidal form in natural waters.The content of it is about dozens of milligrams per liter.With huge surface area and strong surface energy,it can transform stable complexes or chelates with many kinds of metal ions.Humic acid is also an important precursor of disinfection byproducts,and it has important implications on the migration and transformation of other toxic pollutants in water.Carrying more reactive functional groups,its morphological characteristic is likely to be affected by the water environment conditions(pH,concentration,anion and cation concentration,solid particles,etc.).The inherent characteristics of humic acid are the fundamental factor affecting the complexation with other substances.In order to understand the relationship between the morphological changes of humic acid and chemical conditions of the water,and to improve the removal conditions of humic acid,charge properties and aggregation characteristics of humic acid under different pH,ionic strength and humic acid concentrations were studied.The study of Zeta potential by Malvern Nano Particle Sizer instrument showed that,in general,humic acid in aqueous solution is negatively charged.Along with the increasing of humic acid concentration and the ionic strength,the electronegativity of humic acid weakened;while with the increasing of pH,the electronegativity of humic acid enhanced.For complexations of humic acid with calcium or magnesium ions,,its negative charges decrease in terms of increasing the concentration of calcium or magnesium ions,and increases with the increasing of pH and ionic strength.The molecular weight distribution of humic acid and its complexes under different conditions were studied by asymmetric field flow fractionation technique.Combined with the theory of field flow fractionation,Stokes law and the principles of Einstein equations,the size distribution of humic acid and its complexes under various conditions were obtained.The results are as follows:(1)With the increase of humic acid concentration,the molecular weight increased,the size reduced and the crimp properties enhanced.Due to the impact of hydrogen bonds,molecular weight and size of humic acid reduced with the increase of pH.The increase of ionic strength inhibited the ionization of acid groups,leading to the increase in molecular weight and size of humic acids.(2)Calcium or magnesium ions,dissolved in water,can play a bridging role in promoting the aggregation of humic acid.The complexes of humic acid with calcium ions are loose,and the complexes of humic acid with magnesium ions are much tight.pH also mainly affect the formation of hydrogen bonds among humic acid complexes.The polymerization degree of the complexes decreases with the rise of p H.Sodium ions will compete with with calcium or magnesium for binding potential,impacting the combination of humic acid with calcium or magnesium.