Spectroscopic and modeling approaches to understanding formation of microalgae-Pb-NOM ternary complex

Microalgae exhibit remarkable capacity for heavy metal (HM) accumulation, which can be enhanced by natural organic matter (NOM) in aquatic systems. This synergy often leads to deviations in HM bioaccumulation assessments. This study investigated the effects of NOM on lead (Pb) bioaccumulation by Chlorella sp., focusing on Pb concentration, functional group concentration, and stability constants of polymeric complexes. Results showed a remarkable Pb adsorption capacity of 2.438 mmol g^-1 (513 mg g^-1) at pH 6.5, primarily attributed to the formation of ternary complexes {alga-Pb-(NOM-Pb)}. A surface multilayer adsorption mechanism was identified, driven by the deprotonation of functional groups, with carboxyl groups preferentially adsorbing over amino groups. Notably, the ratio of algae-Pb to Pb-NOM stability constants is critical in ternary complex formation, surpassing the traditional emphasis on NOM functional groups. Advanced modeling approaches, including response surface methodology and random forest analysis, confirmed the paramount importance of stability constant ratios in predicting complex formation. These findings provide crucial implications for assessing and controlling ecological risks associated with ternary complexes in algal bloom waters, offering new perspectives on the biotransformation process of HMs. This study contributes to a more comprehensive understanding of HM-microalgae interactions and their environmental impacts.