Effects of UV radiation on humic acid coagulation characteristics in drinking water treatment processes

• UV pretreatment could improve the removal of HA in coagulation at pH above 6.0.
• At pH < 5.5, the removal of HA would be inhibited after pretreatment.
• The molecular diameters and heights of HA increased notably after pretreatment.
• The E4/E6 ratio of HA solution decreased gradually as the radiation time increase.
• The enhanced removal of HA was mainly caused by the increase of zeta potential.

Humic acid (HA) commonly exists in natural waters and drinking water treatment processes. The effects of UV light radiation on the coagulation of HA were investigated in this paper. It was determined that UV light radiation could effectively improve the removal of HA at pH above 6.0. The concentration of residual HA in the coagulated water was in the range of 1.5–3.0 mg/L, which was significantly lower compared to that without pretreatment. However, in waters tested with pH below 5.5, the UV light radiation showed an adverse effect on HA removal using coagulation, which might be connected to its high oxidation function under acidic conditions. The zeta potential of the HA solution with UV light radiation was approximately 10 mv higher compared to that without pretreatment at pH above 6.0; while its apparent diameters and heights increased from less than 130 nm and 0.5–3.5 nm to 100–240 nm and 1.0–4.5 nm respectively, after 2.0 h of UV light radiation as the aggregation of HA. The compressed double electric layer caused by the formation of positive charged sites after UV light radiation was considered to be the primary factor that led to the enhanced removal of HA in the following coagulation process.