Mechanism of lead bioaccumulation by freshwater algae in the presence of organic acids

The accumulation of heavy metals by freshwater algae becomes more complex in the presence of dissolved organic matter but it is unclear how and to what extent. In this study, we comparatively assessed the effects of malic acid and citric acid on the bioaccumulation of Pb by Chlorella pyrenoidosa. A 6 h exposure experiment showed that adding organic acid (OA) to an algae–Pb binary system prolonged the adsorption equilibrium time and that the extended time was related to the concentration and number of carboxyl groups of the OA. A pseudo-second-order model fit the kinetic data well in the presence of OAs. The normality of carboxyl groups was negatively correlated with the bioaccumulation rate, k2, but positively correlated with the maximum bioaccumulation capacity, qmax, of Pb following a linear equation, Y = 121.3x + 52.2, R2 = 0.9877. Neither the Freundlich model nor the Langmuir model could fit the isotherm data in the presence of OA, suggesting that a new mechanism might exist. The quantitative relationship between bioaccumulated Pb and carboxyl group of OA indicated a stoichiometric relationship, which seemed to support the theory of a ternary complex of Pb, OA, and algal surface. Our results have added clarity to the current understanding of the accumulation of heavy metals by algae in freshwater ecosystems. [Display omitted] •The bioaccumulation of Pb by algae in the presence of organic acid (OA) was studied.•The adsorption equilibrium time was prolonged, and the bioaccumulated Pb increased.•Freundlich mode and Langmuir mode were no longer suitable for isotherm data.•Pb bioaccumulation capacity was linearly related to the carboxyl group amount.•The linear equation is BCPb = 121.3Ccarboxyl + 52.2, R2 = 0.9877.