Study On Surface Activity Of Soil Humic Acid And Its Sub-fractions

Humic substances(HS)is a mixture of various organic substances including origanic macromolecules derived from the remains of plants and animals through complicated physical,chemical and biological processes,and it widely occurring in water,soils and sediments.Humic acid(HA)is an important fraction of HS,as the main complexing agent or adsorbent of pollutants in the environment that have amphiphilic properties of HAs with hydrocarbon chains as hydrophobic fractions,ionic groups such as carboxylic and nonionic polar portions such as phenols,alcohol,aldehydes,ketones,amides and amines as hydrophilic fractions.As a kind of natural surfactant,HA can form micelle-like structure at high concentration,reduce the surface tension of aqueous solution.In this paper,HA and its sub-fractions were isolated from forest soil.The critical micelle concentration(CMC)was determined by measuring the surface tension.The similarity and difference of surface activity of HA and its sub-fractions were studied by dynamic light scattering(DLS)and laser Doppler electrophoresis technique(LDE)were used to study the particle size and distribution of HA aggregates in micellar-like structure.The follows results can be summarized:1.The reducing maximum tension and efficiency with CMC of HA and its sub-fractions on the surface tension of aqueous solution and the similarity and difference of CMC.HA and its sub-fractions(HA,HA1,HA2,HA3,HA4,HA5 and HA6)isolated from Jiufeng Forest Park in Beijing were used.The surface tension data were fitted by exponential model.The curve of surface tension of HA and its sub-fractions with concentration was fitted by exponential function model.The results showed that the reducing maximum tension of HA ranged from 22.8 to 35.6 m N/m,the reducing maximum tension was,in order to HA6>HA3>HA2>HA5>HA4>HA1,and the reducing efficiency was,in order to HA6>HA5>HA3>HA4>HA2>HA1.According to the linear equation,the CMC values of HA sub-fractions were determined,ranging from 216 to1024 mg/L,with an average of 723±292 mg/L,in the order to HA1≈HA2>HA3>HA4>HA5>HA6,indicating that the effects capacity of aqueous solutions surface tension with HA sub-fractions are different.The multiple linear regression method of CMC was fitted,CMC=18896-6.9×C-296×alkyl-C-331×aromatic-C-17019×H/C+4054×HB/HI(R~2=0.909,P<0.05,n=16),with error ranged from-199～236mg/L.The predicted results of CMC by this equation were better than those of the reported models.2、According to the characterization results of HA and its sub-fractions,The Zeta potential value was related to the carboxyl content in the structure at the same concentration(R~2=0.582,P<0.20,n=7).HA and its sub-fractions were filtered by 0.45μm membrane,and the particle size of HA and its sub-fractions ranged from 111 to 187 nm.However,HA and its sub-fractions process abiotic self-assemble into few supramicrometer-sized particle aggregates(>1000 nm)by particle size distributions(PSDs).The PSDs spectra showed that more than 97.6%of HA aggregates were in the range of 0～100 nm.The Gaussian model was used to analyze the number-base on PSDs spectra of HA sub-fractions.The peaks central location were 0.96～5.57 nm,1.86～10.60 nm,3.75～45.94 nm and 8.96～55.46nm,respectively.The results show that the PSDs of HA1,HA3,HA4 and HA6 aggregates are basically within 20 nm,which are supramolecular polymers formed by hydrogen bonding and hydrophobic effect,while the particle sizes of main aggregates of main aggregates of HA2 and HA5 are larger than 20 nm,which suggests that they may be formed by association of smaller aggregates.