Role of amino acid functional group in alga-amino acid-Zn ternary complexes

The bioaccumulation of heavy metals (HMs) at the water-algal interface determines their threat degree to aquatic ecosystem. Amino acids (AAs), as predominant components of dissolved organic matter (DOM), play pivotal roles in shaping the speciation of HMs in water and their subsequent accumulation by algae. However, the diverse nature of amino acid poses a challenge in generalizing the underlying mechanisms. This study explores four amino acids (asparagine, aspartic acid, glycine, lysine), each possessing distinct amino and carboxyl groups, and investigates their impact on zinc (Zn) bioaccumulation through batch experiments. Upon the introduction of AAs, the adsorption process obeyed the pseudo-second-order kinetic adsorption model demonstrating chemisorption (R2 >0.99), and it affected Zn externalization (up to 3.994 mmol g-1). This phenomenon followed a two-step model, indicating multilayer adsorption. The study proposed that the protonated amino group of AA could serve as a link between Zn-AA complex and microalgae. Amino acids adsorbed onto the surface of microalgae via protonated amino groups did not occupy the Zn-binding sites, instead, they furnished additional functional groups. This differs from the previous results of organic acids containing only carboxyl functional groups as DOM. Multiple linear regression equations and correlation analyses were used to explain the effects of amino and carboxyl morphology on microalgae accumulation of Zn. The equation showed that the leading model terms explain 75.8% of the accumulation amount variance. This study enhances our understanding of the effects of functional group-different DOM on the accumulation of heavy metals in microalgae, offering insights for future research and prediction.