| Humic acid is widespread in all types of surface water environment. Due to it is a macromolecular organic mixture, the structural components is extremely complex. It's difficult to be removed during the water treatment process. At the same time, it is a major precursor of DBPs. The removal of humic acid in the purification process of drinking water has received great attention in recent years. In this paper, It was determined that not only the source of UV light direct mineralized humic acid, but also played an important role in promoting the removal rate of humic acid in the subsequent coagulation process. To reveal the mechanism of UV light in promoting the removal of humic acid in coagulation, the variation of the physical and chemical properties of the humic acid solution before and after UV light radition were investigated, and the effects of the changes in water quality conditions on the removal performance of HA in coagulation were observed in this paper. Major conclusions obtained were listed as following:(1) After UV light irradiation, the physical and chemical properties, including pH, TOC, color, viscosity, and hydrophilic and hydrophobic, of the humic acid solution would vary notably. The values of most parameters, such as solution pH, color, and viscosity decreased gradually during UV radiation. The proportion of hydrophilic component in the humic acid solution increased, which might be related to the generation of small humic acid molecules after UV irradiation.(2) Solution pH showed a notable effect on the photochemical reaction process of humic acid. When the solution pH increased from 7.0 to 9.0, the residual concentration of humic acid would increased slightly from 17.46 to 18.54 mg/L after UV light radiation pre-treatment. It turn out that eutral and acidic environment conducive to mineralization of humic acid decomposition. UV damage its ability to molecular structure is relatively strong.In addition, reducing the UV radiation during the pH value also will enhance the removal efficiency of coagulation of humic acid.(3) The presence of Fe2+ concentration can significantly improve the coagulation effect of humic acid. When the solution does not exist in iron, the removal of humic acid by UV radiation and coagulation treatment is 40.6%. When the concentration of iron ions was 0.020 mmol/L, the TOC removal under the same conditions is 45.28%.The humic acid removal can be increased to 84.02% when the concentration is 0.1 mmol/L.(4) It is different to solution pH and other water quality conditions, the impact of changes in viscosity of the solution on humic acid coagulation effect is relatively small after ultraviolet irradiation.With the increase of irradiation time, a slight rise in the temperature of the solution, which may be the main reason for decline in the viscosity of the solution. Thus, with the temperature of the system gradually returned to normal, its effect on coagulation is weak in the subsequent coagulation reaction process.(5) The hydrophilic molecules of humic acids increased slightly after UV radiation. The proportion of hydrophilic component was 17.34% when it has no light, the proportion of hydrophilic component was 28.64% when light 0.5h, the proportion of hydrophilic component was 32.87% when light 2h, the proportion of hydrophilic component was 39.11% when light 2.5h.The proportion of hydrophobic component changes small. Further studies showed that hydrophilic components are difficult to remove by coagulation compared with the hydrophobic components.Based on the results obtained in our study, it could be concluded that UV light radiation showed a notable impact on the physicochemical properties of the humic acid solution. Despite the hydrophilic properties of humic acid molecular increased slight after pre-treatment, the changes in physical and chemical properties, including the solution pH, temperature, and viscosity, played an important role in promoting the removal of humic acid in coagulation. |
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