Toxicity, degradation and metabolic pathway of 4-chlorinephenol in Chlorella vulgaris

Environmental contamination by 4-chlorinephenol (4-CP) has received widespread attention due to its frequent detection and adverse environmental effects. The traditional water treatment approaches and equipment are relatively mature, making them easier to operate and maintain. However, 4-CP is particularly difficult to degrade using the technologies, often resulting in incomplete removal, secondary pollution, or high energy consumption. The present study explored the toxicity of 4-CP to Chlorella vulgaris and its removal mechanism in microalgae. The results revealed that the photosynthetic activity and the growth of C. vulgaris were inhibited at high-level 4-CP (≥ 60 mg/L) during the whole exposure period. The dry weight and Chlorophyll-a content of C. vulgaris in the 4-CP treatment at 90 mg/L were only 72.3 % and 72.1 % of the control group, respectively. The structure and function of C. vulgaris was dramatically impacted under 4-CP stress. Malondialdehyde content, superoxide dismutase and catalase activities were significantly increased under multiple 4-CP concentrations, rising by 217 %, 299 %, and 78 % at 90 mg/L 4-CP compared to the control. Additionally, the removal of 4-CP by C. vulgaris aligned more closely with the pseudo-first-order model than with the pseudo-second-order model. As 4-CP concentration rose from 10 to 90 mg/L, the 4-CP removal rate decreased from 86.0 % to 56.7 % and the half-life was increased from 5.419 to 7.673 d based on the pseudo-first-order kinetics of 4-CP removal by C. vulgaris. Biodegradation is the major pathway for 4-CP removal by C. vulgaris, accounting for 72.54 % of the total removal rate at 30 mg/L on the 6th day. A total of 5 metabolites were detected for 6-day cultivation, including 4-chlorocatechol, hydroquinone, pyrogallol, 1,4-benzoquinone and succinic acid. This work provided an insight into the removal mechanism and biodegradation pathway of 4-CP in C. vulgaris.