Effect Mechanism Of Fulvic Acid On Pb Bioaccumulation By Chlorella Pyrenoidosa

The alga-water interface is an important site for heavy metal migration and transformation.Heavy metal can be transferred from aqueous phase to algae phase through adsorption/absorption process.Eventually,it will be a detriment to human health through the transfer of the food chain.Dissolved organic matter?DOM?also plays an important role in aquatic ecosystem,and it can influence the metallic formation,and further alter the biological accumulation of heavy metals by algae.Previous studies found that in the presence of FA,the adsorption of Pb on the algal surface would increase by more than 10times within an appropriate concentration range.It was beyond to the predictions of the biological ligand model,and the adsorption isotherm was no longer fit the Langmuir model.The adsorption mechanism was still unclear.Therefore,based on the knowledge of coordination chemistry and environment chemistry,exposure experiments together with model fitting,trace amountanalysis,and microscopic analysis method have been employed to study the the relationship among DOM,metal,and algae and the accumulation mechanism of the heavy metal.The contents are summarized as following:First,the appropriate conditions of exposure experiments were investigated.The exposure concentration ranges of algae,Pb,and FA were determined by an orthogonal experimental design.The results determined the algal density of 0.02 OD₆₆₀,equivalent to3.2×10⁵cell/m L or 4.06 mg/L biomass,and the concentration of Pb and FA ranged from 1to 40 mg/L.The exposure time was determined to be 3 h according to adsorption kinetics experiments.In addition,EDTA elution experiment was used to distinguish Pb adsorbed and absorbed from algae,and the optional elution conditions were 0.01 M EDTA,1 min duration of exposure.Fixing the concentration of FA,the adsorption isotherm of Pb formed an L-S type curve,presenting two platforms.For Langmuir model,when FA increased was 0,5,10,20mg/L,the corresponding values of R²decreased from 0.997,0.906,and 0.919 to 0.771.The fitting degrees of Langmuir model decreased with the increase of FA concentration.When two-step model demonstrated was adopted,the corresponding values of R²was 0.996,0.984,0.999,and 0.990,respectively,showing good degrees of fitting.The two-step model indicated there were two stages of adsorption process.The first stage was monolayer adsorption of Pb ions,and the second stage was multilayer adsorption.On the other hand,when Pb concentration was fixed,the adsorption capacity of Pb increased first and then dereased as FA concentration increased,formed a prominent peak.The result indicated that both Pb²⁺and FA-Pb complex panticapated the bioaccumulation of Pb in algae.Therefore,it is speculated that a ternary complex of algae-Pb-?FA-Pb?was formed.Pb ions adsorbed in the first stage acted as a bridge between algae and FA-Pb complex,while FA-Pb complex was adsorbed in the second stage,where multi-molecular layer adsorption occurred.when adsorption capacity of Pb greater in the presence of FA was greater than that in the absence of FA,it could be defined that a ternary complex was formed.According to this criterion,a three-phase diagram was plotted to describe the proportion ranges of ternary complexe formation.Whereinto,algae was 3%?15%,Pb was 42%?92%,and FA was 2%?55%.Furthter experiments showed that,the amount of Pb absorbed by algae decreased as ternary complex formed,indicating that due to the occurrence of multi-molecular layer adsorption,more Pb²⁺was intercepted on the surface of algae,which inhibited the absorption of Pb by algae.An experiment was conducted at 0.02 OD₆₆₀,30 mg/L Pb and 10 mg/L FA to study the thermodynamic and kinetic of multi-layer molecular adsorption.The adsorption capacity of Pb in the presence of FA increased as temperature increased,which indicated the multilayer adsorption process was an endothermic reaction.The?H for the algae-Pb binary system was 4.910 KJ/mol,and it was 20.519 KJ/mol for the ternary system,which indicated that the multi-layer adsorption required more energy.From the results of the adsorption kinetics,the equilibration time of binary system was 40 min for Pb,and it was prolonged to 50 min for the ternary system.This indicated that the process of multilayer adsorption was more complicated and consumed more time.In addition,in the first two minutes,the adsorption capacity of binary was 1.938 mmol/g,accounting for 80.38%of equilibrium adsorption capacity,which was close to the equilibrium.The adsorption capacity of the ternary system was 3.746 mmol/g at 2 min and was much greater than that in binary system,but it only accounting for 36.37%of equilibrium adsorption capacity,and was far from the equilibrium.It revealed that the monolayer adsorption?the first layer?was almostly completed at the first 2 min,and at this time the multilayer adsorption?the second layer?had only just begun.Both adsorption processes of binary and ternary systems followed the pseudo-second order model,namely chemical adsorption processes.The rate constant K₂was 0.599g/?mmol·min?for the binary,which was much larger than that of the ternary with the value of 0.019 g/?mmol·min?.Thus,the adsorption speed from the second layer was much slower than that of monolayer.The effect of p H on Pb and FA increment in ternary system was investigated.In the presence/absence of FA,the adsorption capacity of Pb by algae increased with the increase of p H,indicating that the adsorption capacity of Pb was in competition with H⁺in solution.However,the adsorption of FA by algae decreased with the increase of p H in the binary system,indicating that the undissociated FA was more easily adsorbed by algae.In the ternary system,when p H<7,the adsorption capacity of FA increases with the increase of p H.It can be inferred that FA-PB complexes were adsorbed to algae.When p H>7,the adsorption of FA decreased,but the adsorption amount of Pb continued to increase,which should be attributed to the fact that the dissociated FA provided more adsorption points.To understand the mechanism of the multilayer adsorption on algae,the microscopic morphology observation of algae was conducted.Algae with various treatments,including unexposed algae,algae exposed to FA,algae exposed to Pb,and algae exposed to both FA and Pb,were pbserved with PSD.The algal sizes were in the order of algae-Pb-FA>algae-Pb>algae-FA>algae,and this phenomenon conformed to the formation of multilayer adsorption,which increased the size of particles.FTIR and XPS data indicated Pb ion was bined with carboxyl unit on algae when adsorption occurred.The amounts of the combined oxygen atom with Pb in ternary system were greater than that in binary,and the area of C-O and C=O in C1s spectrum was also larger than that in binary.The results illustrated each Pb ion can combine with both carboxyl groups of algae and FA.SEM observation showed the adsorbed layer for the ternary were much thicker than that for the binary.TEM observation indicated the ternary complex prevent more Pb enter into algae,and thus relieving the toxicity of Pb on algae.Based on classical coordination chemistry theory,chapter 7 proposed three rules for the formation machenism of multi-molecular layer adsorption?the formation of ternary complex?.This study can help us to deeply understanding of the behaviors of the accumulation of heavy metal by algae combined with DOM,and further accurately evaluate on the risk of heavy metal migration and transformation in aquatic ecosystem.