Effects Of UV Light Radiation On The Molecular Structure Of Humic Acid And Its Coagulation In Drinking Water Treatment Process

Humic acid commonly exists in natural waters. The presence of humic acid indrinking water treatment process has received much attention in recent years becauseof its adverse affections on the removal of other organic pollutants and the formationof disinfection by products in chlorination. However, its removal ratio was usually lowin the plants with traditional coagulation-sedimentation-filtration treatment systems.The effects of UV light radiation as a pretreatment method on humic acid coagulationwas investigated in this paper. In such case, the effect of UV light radiation on themolecular structure of HA was studied. We also investigated the influence of waterquality including pH, turbidity, alkalinity, molecular structure characteristics of humicacid, and the concentrations of Ca2+and Mg2+on the removal of humic acid. Besides,the mechanism of the promoting effects of UV light radiation on HA coagulation wasanalysed. Therefore, some of the preliminary conclusions were made as below:pH and TOC varied significantly with UV light radiation time. It was found thatthe pH would decrease notably after UV light radiation when the solution is alkaline,which might be related to the increasing of organic acids with small molecular weight.Under acidic conditions, however, the solution pH would increase gradually, whichmight be related to the reactions rates of acidic function formation and the humic acidmineralization. Approximately10%of the HAwas mineralized at pH=7after UV lightradiation. Besides, UV light radiation could have an effect on the molecular structureof HA. It was found that as the increasing of UV light radiation time, the E4/E6value both decreased gradually at pH8.0and10.0. The partial of humic acid with lowmolecular weight varied little. Compared with the three-dimensional fluorescencespectra and infrared spectra with and without UV, it was found that some functionalgroups of humic acid molecules changed after UV irradiation.Our results also suggested that water quality could effect the removal of humicacid with and without UV pretreatment. Before UV light radiation, the optimumefficiency of HA coagulation is at pH=5, while the best point is at pH=6with UVpretreatment; When the pH is above5.5, the UV pretreatment of HA can promote thecoagulation, the residual of PHA in effluent is7.592mg/L before the UV lightradiation, and it will decreased to1.935mg/L after UV pretreatment. Although theexistance of turbidity and cation can promote the coagulation of HA with and withoutUV pretreatment, they had little adverse effect on the pretreatment process. Alkalinityhave inbibitional effect on the removal of HA in coagulation process, when increasedalkalinity from5mg/L to100mg/L, the lower value of the removal rate of HA issubstantially the same (abount15%). At the same time, the greater molecular weightand less hydrophilic group contained can contribute to the coagulation of HA.It was found that UV light radiation could notably improve the removal efficiencyof humic acid in the normal drinking water pH range. HA residual in the coagulatedwater could be controlled in the range of1.5~3.0mg/L at pH6.0~8.0, much lower thanthat without pretreatment (5-8mg/L). The zeta potential of the humic acid with UVlight radiation was higher than that without pretreatment at pH6.0-9.0, and thus led toan average colloidal diameter decrease; while AFM observation showed that both itsapparent colloidal diameters and heights increased from less than130nm and0.5~3.5nm to100~240nm and1.0~4.5nm after2h UV light radiation, which was provedto be caused by the accumulation of humic acid as the lower Zeta potential.