Study On The Morphology Al-humic Flocs Under Different Molecular Weight Distribution In Shear Force Field

This paper selected lake bottom sediment as raw material, the use of IHSSrecommended method for purification of the sediment to obtain natural humicacid,humic acid purified by means of elemental analysis（EA） is mainly composed of theelements of C,O,N,H,S,about98%of the total mass of humic acid. By ultrafiltration andorganic carbon mass balance calculations to separate the humic acid of differentmolecular weight, the results show that the humic substances which molecular weightgreater than30000up to about41%of the total molecular weight,and the substances ofmolecular weight less than1000at least,accounting for6.7%of the total.Substances offour molecular weight level of10000³0000、5000¹0000、3000⁵000、1000³000account for11.5%,13.3%,11.8%,15.7%of the total respectively.Determination of aluminum flocs’ size,settling velocity and other parameters tocalculate the fractal dimension and strength through microscopic photography, imageanalysis and particle image velocimetry, and use the fractal theory to discuss thestructural characteristics of humic flocs which formation in the cogulation withaluminum sulfate as coagulant and humic acid of different molecular weight level underthe shear force field.The results show that the fractal dimension of flocs decrease withthe molecular weight decreases.Under the conditions of weakly acidic（pH=5.0）,therange of flocs’ fractal dimension of three level which molecular weight greater than30kDa,10kDa～30kDa,5kDa～10kDa was1.83¹.94,1.8¹.87,1.74¹.81respectively,but the value was ranging from1.77to1.78,1.77to1.83,1.71to1.78respectively atneutral （pH=7.0）.Under weakly acidic conditions,the change of HA flocs’ fractal dimension with theincrease of aluminum dosage showing the trend of first increses and then decreases, then increases and finally stable, and the maximum value approximately1.93ataluminum dosage was0.2mg-Al/mg-TOC; The fractal dimension of humic acid flocsincreases with increasing aluminum dosage gradually and eventually and reach a steadyvalue at pH=7.0,the maximum at1.5mg-Al/mg-TOC of aluminum dosage is about1.87.Overall,the fractal dimension of floc at weakly acidic is greater than the value atneutral. At pH=5.0,the flocs’ fractal dimension, strength and settling velocity arepositively correlated and inversely related the average particle size of flocs,but there is acertain positive relationship between flocs’ fractal dimension and average particle sizeat pH=7.0.HA flocs’ fractal dimension of different molecular weight showed similar variationwith the change of shear force,the flocs’ fractal dimension slowly increase with theincrease of the shear force,and the maximum at the critical broken point,and then thevalue decreased,and eventually stabilized. The best shear force of coagulation withhumic acid which molecular weight greater than30kDa are generally G=120s^-1andG=150s^-1;At the humic acid molecular weight was ranging from10kDa to30kDa, theflocs began broken at G=90s^-1;For the humic acid molecular weight level of5kDa～10kDa,the phenomenon of flocs broken increased at G=90s^-1,this shows that the shearforce changes more significant impact on floc which formed in coagulation with lowermolecular weight hunic acid.