Modeling praseodymium toxicity in solution to wheat root elongation using the biotic ligand model theory

Praseodymium (Pr[Ⅲ]) is a rare earth element (REE) with chronic toxicity. With the increasing use of REE in various fields, considerable amounts of praseodymium have been released into the environment. Consequently, understanding the toxic effects and ecological risks of Pr(III) on organisms is crucial. This study utilized a soil-simulated solution culture method to investigate the influence of Ca^{2 +}, K⁺, Na⁺, Mg²⁺, and pH on acute toxicity to wheat through a single-factor control experiment and established a Pr(III) toxicity prediction model based on the biotic ligand model (BLM). These findings demonstrated that increasing the activities of Mg²⁺, Ca²⁺ and H⁺ reduced the toxicity of Pr(III) on wheat root elongation. In contrast, increasing the activities of K⁺ and Na⁺ exhibited no significant effects. Additionally, pH influenced both the solubility and speciation of Pr(III). At pH < 6.5, Pr(III) predominately exists as Pr³⁺ and PrCl²⁺, whereas at pH 7.0, the proportion of PrOH²⁺ significantly increased. Based on DPS9.0 software fitting results, the stability constants were determined as follows: logK_PrBL = 2.54, logK_PrClBL = 3.26, logK_PrOHBL = 3.18, logK_CaBL = 2.50, logK_MgBL = 2.61, logK_HBL = 3.88, and f_PrBL^50% = 0.36. These results suggest that the BLM effectively predicts Pr(III) toxicity by accounting for toxic species such as Pr³⁺, PrCl²⁺, and PrOH²⁺, along with the competition for binding sites by Mg²⁺, Ca²⁺, and H⁺. The improved Pr(III)-BLM performance is believed to be applicable to a wide range of land plants.