Binding Characteristics Of Heavy Metals To Natural Fulvic Acid Fractions Sequentially Extracted From The Lake Sediments

Humic substance(HS)is a kind of complex natural macromolecular organic matter,which according to the difference of solubility,it can be divided into Humic Acid(HA),Fulvic Acid(FA)and Humin(HM).FA includes tons of acidic functional group,which has a strong adsorption and complexation to heavy metals,as well as a strong influence on landmark geochemical behaviors.There are significant differences composition,structure characteristics among FA fractions,which determined a significant difference of mechanism of mutual effect between FA fractions and heavy metals must be defined.The work was using sequentially alkali extractions of FA from sediment in lake Wuliangsuhai as absorbent,in order to study the adsorbent characteristics of FA fractions under various conditions include pH,ionic strength,different types of cations,temperature and illumination.It has combined with spectroscopy,topography,bi-Langmuir and ligand binding models to reveal the mechanism of FA fractions absorbed heavy metals.It is important in providing theoretical evidence for monitoring and controlling the heavy metal pollution such as Pb,Cu and Cd.The main findings are listed below:1.The element composition,functional group and appearance were similar among FA fractions(FAs),however,along the extracted times,the FAs were showed a series characteristics include decrease in organic matter content,acidic functional group content,aromaticity,sorption,molecular polarity,and increase in aliphaticity,humification degree.2.The result of isothermal adsorption experiment indicated that the adsorption of FAs to Cu2+,Pb2+,Cd2+ were saturated about 10 minutes,and the rate of adsorption were Cu2+>Cd2+>Pb2+.The adsorption rate constants of heavy metals adsorbed by different FA fractions were different,the rate constant decreased with the fractions increased which indicated that the adsorption rate of latter extracted FA fractions adsorbed heavy metal ions is slower.The results of isothermal adsorption experiments indicated along with the extracted times,the latter extracted FA fractions adsorbed heavy metals were reduction.3.The change of environment conditions would affect the FA adsorption of Cue2,Pb2+ and Cd2+.The FAs adsorption of heavy metals were improved with increasing of pH below the critical pH.With the increase of ionic strength,the all FAs adsorption capacity of Cu2+,Pb2+ and Cd2+ were decreased.Different ion types had different effects on FA adsorption of heavy metal.Adsorption reactions were spontaneous exothermic reactions,which makes FAs adsorption capacity decreased with the increase of temperature.Illumination can degrade FA and decrease the amount of adsorption.4.Ligand binding model and bi-Langmuir model had been well simulated in the FAs adsorption of heavy metals.Ligand binding indicated that there were strong and weak sites of adsorption,and the strong sites may conclude nitrogen and sulfur containing functional groups,the weak sites may contain carboxyl and hydroxyl groups.Although the adsorption capacity of strong sites stronger than weak sites,the number of weak site were more than those of strong site which played a leading role in adsorption.Bi-Langmuir model showed that the amount of adsorption declined along the extracted times,which had a positive correlation to acidic functional groups and a negative correlation to aliphaticity,molecular weight.5.After FAs adsorbed heavy metals,the composition and structure were changed.UV-visible analysis indicated that the degree of conjugation,aliphaticity,humification and polarity increased.FTIR analysis indicated that hydroxyl,carboxyl,nitrogen containing functional groups took part in the reaction.The fluorescence spectra showed that the fluorescence peak B(visible fulvic-like)appeared blue shift,and both peak A(humic-like)and peak B reacted quenched reaction,but the fluorescence intensity of peak C(UV fulvic-like)increased which indicated the carboxyl groups of FA participated in adsorption reaction,and the reaction process occured ?-?*transition.SEM showed that FAs gathered into clusters after sorption heavy metals.