The development of a kinetic biotic ligand model to predict acute toxicity of Gadolinium for Daphnia magna

The metal Gadolinium (Gd) is a member of the lanthanide (Ln) group and is recognized as an emerging pollutant due to its widespread application in modern technology. Its acute toxicity depends on its free ion concentrations (Gd³⁺), which is directly related to chemical speciation. The Biotic Ligand Model (BLM) is a valuable tool for risk assessment which predicts the metal bioavailability and toxicity to specific organisms. However, studies developing BLM for any Ln species are rare. Consequently, this study aimed to develop a kinetic BLM to predict the acute toxicity of Gd to the freshwater crustacean Daphnia magna. A series of 48-hour toxicity tests were conducted using different major cation concentrations, in order to estimate their affinity constants for the biotic ligand (BL). The model was then validated, first in the presence of dissolved organic matter (DOM), and then with water samples collected from lakes, rivers, and estuaries in France and Germany. The outcome revealed that three major cations (potassium, magnesium, and calcium) act as strong competitors. The model was successfully validated in the presence of organic matter and in the majority of surface freshwater samples (9 out of 13 samples). In this case, the predicted survival had a strong fit with the observed data. However, this was reduced when applying the model to samples of elevated electroconductivity and a pH below 6.8, when survival was consistently overestimated, potentially a multi-stressor effect. The kinetic BLM predicted 48 h measured EC₅₀ ranging from 4 to 30 mg L^-1 which agreed with the data from the literature. The model could also predict chronic effect of Gd by estimating the no-effect concentration (NEC) under prolonged exposure time ranging from 0.1 to 1 mg L^-1.