The Effect Of Different Molecular Weight Dissolved Organic Matter(DOM) On Bioavailability Of Cu And Pb By Chlorella Pyrenoidosa In Fresh Water

The living algae is widespread and constitutes an important organic fraction in natural freshwater environments. Accordingly, their effect of bioaccumulating heavy metals may influence the environmental processes and behavior of the metals. Dissolved organic matter(DOM),as an important natural organic ligand in aquatic ecosystem, is an important factor in controlling metal speciation and bioavailability. In this study, we choosed Cu and Pb with different degrees of toxity to investigate their adsorption behaviour by the freshwater alga Chlorella pyrenoidsa in the presence of different molocullar weight DOMs(citric acid, fulvic acid and humic acid). The main results are shown as the following two parts.In the first part, we investigated the adsorption kinetics of Cu and Pbby the freshwater algae Chlorella pyrenoidsa and the influence of environmental factors. The results show that the process equilibrium of Cu and Pb reached quickly, the adsorption rate of Cu is greater than Pb,both of them could be well-described by the pseudo-second-order rate model. The adsorption capacities of Cu and Pb increased as the pH increased in the range that heavy metals did not precipitate(within pH=10). As the temperature rised, the adsorption capacities of Pb decreased, while the adsorption capacities of Cu increased, which indicated that the adsorption of Cu on Chlorella was endothermic and for Pb was exothermic.In the second part,we investigated the effect of three representative DOMs including citric acid, fulvic acid and humic acid on bioaccumulation(adsorption and absorption) of Cu and Pb by Chlorella.The results showed that: low molecular weight DOM like citric acid could improve Chlorella’s tolerance of heavy metals and promote the enrichment of heavy metals on Chlorella, because citric acid competed heavy metal ions with Chlorella and the complex of citric acid and heavy metal could through algae membrane by passive transport.But for the high molecular weight DOM, like fulvic acid and humic acid, the situation is more complicated, related to the specificity of heavy metals:(1) As for Cu, because of the fulvic acid and humic acid’s completion of free Cu with Chlorella, the toxicity of Cu was weakened.And these DOMs inhibited the enrichment of Cu. Besides, the inhibitory effect of larger molecular weight of humic acid was stronger.(2)As for Pb,when the Pb concentration was low, because the complex of fulvic acid or humic acid and Pb would interact with membrane of Chlorella, forming ternary complexes, the enrichment capacity of Pb would increased rapidly.And the promotion effect of fulvic acid which has more acidic functional groups was more significant. When the concentration of Pb reaches a certain value, the acidic functional groups of DOM were occupied all by Pb, so that there was no redundant functional group complexing with Chlorella and stop promoting the Pb bound to algae. The most important difference is that whether the binary complexes could interact with cell membrane to form ternary complexes.Experimental results show that low molecular weight DOM could promote the biological effectiveness of heavy metal, it has certain application value in dealing with heavy metal polluted water by Chlorella(living). High molecular weight DOM restricted by metal type,may promote or inhibit the bioaccumulation of different heavy metals. It has great significance to evaluate ecological risk of heavy metals in water environment.