Effect Of Organic Acid On Biosorption And Bioavailability Of Pb By Chlorella Pyrenoidosa And Its Mechanism

For the intractable and persistent properties of heavy metal in freshwater ecosystems,even in trace amounts they can accumulate to a level where they become toxic then threaten the organism and human health.Dissolved organic matter?DOM?as another kind of important components in aquatic environment,contain multiple functional groups,can combine with various metal ions in freshwater.Therefore,in natural water,especially in eutrophic water,metal ions mostly exist in form of Metal-DOM,thus change even control the bioavailability and toxicity of metal.However,DOM are complicated in structure and wide varieties,their effects to the biogeochemical behavior of heavy metal are changing over individual characters.Hence,a detailed classification before assess their effect to the ecological risk of heavy metal has more scientific and guiding significance.The project aims at research the influence of low molecular weight DOM on bioavailability and bioaccumulation situation of Pb.We choose two kinds of low molecular weight organic acid,malic acid and citric acid as representation,use freshwater alga Chlorella pyrenoidsa as bio-adsorbent,carry out systemic research from three respects: adsorption kinetics,adsorption isotherm and influence factors.The main results are shown as following:In the first part,we investigated the adsorption kinetics of Pb by the freshwater algae Chlorella pyrenoidsa with the presence of organic acid.The results show:?1?In the ternary system of Pb-DOM-Chlorella,the equilibrium process of Pb is slower than that in the binary system of Pb-Chlorella,the reaction is more complicated as well;the kinetics process could be well-described by the pseudo-second-order rate equation,means the adsorption belongs to chemical adsorption.?2?Compared to malic acid at the same molarity,with the presence of citric acid,the adsorption rate is slower,the equilibrium adsorption amount is grater,moreover the equilibrium adsorption amount is positively related to carboxyl concentration.This result proves that the quantity of carboxyl on DOM can influence the adsorption rate and final equilibrium adsorption amount,the more the quantity of carboxyl,the slower the adsorption rate is,the greater the equilibrium adsorption amount is.In the second part,we designed six day long-term experiment to make a comparison between the two situation where there exist organic acid and not,draw adsorption isotherms of Pb on Chlorella,examined Chlorella's tolerance and it's enrichment.The results show:?1?When there was no organic acid exist,the increasement of Pb didn't inhibit the growth of Chlorella obviously,after added organic acid,it stared to appear varying degrees of inhibition,the inhibition at the concentration of 0.104mmol/L was stronger than 0.052mmol/L.?2?The process of Pb absorbed by Chlorella could be described by Langmuir isotherm model,within the range of organic acid's highest concentration is 0.104mmol/L and Pb's highest concentration is 0.0772mmol/L,each experiment system reached adsorption equilibrium.Organic acid could promote enrichment,and higher concentration promoted more,citric acid promoted more than malic acid of the same molarity.From the relationship curve between carboxyl concentration and the maximum adsorption amount shows they have fine linear correlation.So increase the concentration of organic acid or the amount of carboxyl in unit volume?replace malic acid with citric acid?both can promote the enrichment of Pb by Chlorella.In the third part,eluted Chlorella samples with 0.01mol/L EDTA solution to differentiate Pb absorbed from internalized.The results shows that the presence of organic acid increased the adsorption amount of Pb,and higher concentration increased more;citric acid had a more increasing than malic acid of the same molarity.Meanwhile the internalization and bioavailability of Pb both also fits the law above mentioned,increase the concentration of organic acid or the amount of carboxyl in unit volume both promote biological enrichment of Pb inside and outside the Chlorella cell,increase bioavailability.The experiment results match the ternary complex model,the complex of organic acid and Pb would interact with membrane of Chlorella,forming ternary complexes Pb-organic acid-Chlorella,then the organic acid molecules play a role of bridge between cell membrane and Pb,increase the number of adsorption sites as well as sending more Pb to cell.Our results found a correlation between the enrichment capacity of Pb and organic acid concentration or carboxyl amount,suggest a stoichiometric relationship,further confirm the presence of ternary complexes.Also,organic acid improves the percentage of internalized Pb.In the last part,in view of the previous studies,because K2HPO4,one of a important components can generate precipitations with Pb to interfere bioenrichment,we studied several concentrations of K2HPO4.The results shows:?1?K2HPO4 plays an important role in the growth of Chlorella,the concentration of it in a normal BG11 medium is 40mg/L,but it is proved in this experiment that when it's concentration fell to 20mg/L,Chlorella can reach the same growth level;when it's concentration is less than or equal to 4mg/L,it doesn't generate enough precipitation with Pb²⁺ to interfere bio-sorption.?2?Chlorella?living?own the features of adsorption precipitation,the effect of phosphonic acid is complicated.In concentrations range from 0 to 40 mg/L,if the concentration of HPO42-is higher than Pb²⁺ and generate obvious precipitation,the enrichment amount is increase,but the reaction consumes quite a part of free Pb²⁺,so the internalized absorption is reduced,the role of the former was stronger than the latter,total performance is biosorption amount increased.Organic acid promotes the enrichment of Pb by forming ternary complexes under certain concentrations,and this process is relevant to concentration of organic acid and carboxyl.Our experiment results have practical significance in perfecting the ecological risk assessment of heavy metals in water environment,also provide a potential biosorbant and it's scope of application.