UV absorbance of suspended sediments measured in a laboratory using a submersible spectrophotometer

Measurements were taken using a commercially available absorbance sensor, Trios Opus (TriOS GmbH, Germany) at 1 minute intervals. Sediments were sieved into size fractions and particle size was quantified using a Malvern Mastersizer 2000 (Malvern, UK) and organic content was assessed using the Loss on Ignition method.

The experimental setup consisted of a 5 L beaker filled with 4 L of deionized water, a large stirrer set to a constant rpm, and an opaque surround to minimize interference from ambient light on sensor measurements. The sensor was placed in the centre of the beaker, ~5 cm above the stirrer blade. To verify uniform mixing, subsamples were taken directly beside the optical window of the sensor and at two other locations within the beaker. These subsamples were analysed using a commercial handheld turbidimeter (Sper Scientific Portable Turbidity Meter 860040) at predefined calibration points to confirm consistent turbidity. Turbidity readings were stable and comparable across all sample positions at each calibration point.

Each measurement cycle consisted of 15 calibration points spanning SSC concentrations from 0 to 5000 mg/L. During calibration, sensors recorded data continuously for 20 minutes, and once measurements had stabilized, 10 readings were randomly selected for further analysis.

All absorbance data were recorded by the TriOS Opus sensor and exported in CSV format, LOI data (weight measurements) were initially recorded on paper, then transferred into an Excel spreadsheet and particle size data from the Mastersizer were exported as CSV files and imported into statistical software for analysis. Records were screened for any missing or null values and data points were checked to ensure they fell within expected ranges (e.g. in case of data entry errors).