To compare the toxic action of fluoride (F) and chloride (Cl) on freshwater algae, effects of F and Cl on Chlorella pyrenoidosa growth were investigated by determination of the algal biomass and model analysis. Results showed that the exponential growth equation (D = D0 + aebT) may be used to fit the relationship between cell density (D) of C. pyrenoidosa and exposure time (T), but F and Cl have significantly different effects on parameters a (initial quantity of algae at the exponential growth stage) and b (growth constant). In the range of experimental concentrations, F inhibited C. pyrenoidosa growth, and percent inhibition increased with increasing exposure time and F concentration. By contrast, Cl either inhibited or enhanced C. pyrenoidosa growth depending on Cl concentration and exposure time, and percent inhibition increased with increasing Cl concentration but decreased with increasing exposure time. The dose–response equation: I = min + (max − min)/(1 + 10lgEC50−lgC) may be used to fit the relationship between percent inhibition (I) and F or Cl concentration (C); however, F was distinct from Cl in terms of effects on parameter ‘min’. The EC50 values of F and Cl ranged from 118 to 170 mg/L and 19.73 to 36.33 g/L, respectively; the latter is much higher than the former. Thus F has higher toxicity than Cl.

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