A Turbidity Compensation Method for Seawater COD Measurement Using UV-Visible Spectroscopy

Ultraviolet-Visible (UV-Vis) spectroscopy is widely used in in-situ seawater chemical oxygen demand (COD) measurement owing to its simplicity, rapidity, and pollution-free advantages. However, it is susceptible to interference from complex water components in coastal turbid water, which can result in a significant decrease in COD measurement accuracy. To improve the accuracy of COD measurement using UV-Vis spectroscopy in turbid water, an improved compensation method for COD in-situ measurement was proposed. The influence of turbidity on COD measurement was firstly analyzed using lab-prepared solutions with varying concentrations of marine sediments, it is found that a dynamic compensation method can be built based on the relationship between slopes of COD concentrations versus absorption at 254 nm and turbidity which can be well modelled by a least-squares fitting, and an accurate in-situ COD measurement in turbid coastal water can be achieved. Using mixed water samples with turbidity (0-200 NTU) and COD concentrations (1.00-75.00 mg/L) prepared in laboratory, it is demonstrated that the results derived by the compensation method have a root mean square error (RMSE) of 0.82 mg/L, and the determination coefficients (R²) are 0.9964 and 0.9978 for low and high COD concentrations, respectively. By collecting field samples with turbidity (6.91-107.05 NTU) and COD concentrations (0.96-9.15 mg/L) in a costal aquaculture farm, the COD measurements by this study has a relative error range of -4.27% to 6.64%, the RMSE of 0.25 mg/L, and the R² of 0.9939. Compared to uncompensated, direct subtraction compensation, and existing similar methods, the proposed method in this study significantly reduces measurement error and effectively improves the accuracy of in-situ COD measurement in turbid water, and has the potential to be applied in the development of in-situ instruments for COD measurements.