Characteristics Of Distribution And Transport Of Heavy Metals In Multi-phase System Of Natural Waters

The solid phases, such as deposited sediments (DS), suspended particulatematters (SPM) and surface coatings (SC), and water phase constitute thecomplex multi-phase system controlling the behavior of heavy metals in naturalwaters. The great differences of three solid phases on chemical compositions,characteristics and functions were caused by their state of having existence andforming conditions. Moreover, due to the different research backgrounds andmethods on the adsorption capacity and contribution of chemical components ofSC, SPM and DS in controlling the behavior of heavy metals in natural waters,the results gained are difficult to be compared. Anoxic sediments could beoxidized by extraction reagent and by air in dryness pre-treatment procedure,which resulted in different impact on speciation patterns of heavy metals duringthe sequential extraction procedure. The heavy metals in sediments could bereleased, transported and combined again which changed their bioavailabilityand toxicity when the environmental conditions have been altered. Hence, theobjective of this study is to contrast the chemical components and mineralcompositions of SC, SPM and DS in the same water body, investigate thecontents and distributions of heavy metals in solid phases and in theircomponents in nature water by extracting the Cu, Pb and Zn in Fe, Mn oxidesand organic matters in three solid phases, determine the main adsorption sites ofheavy metals on the solid phases, realize the adsorption difference of heavymetals on the three solid phases and their chemical components. Furthermore,the spatial distribution of sediments in Yitong River was investigated, the impactof extraction reagent and dryness procedure on anoxic sediments during thesequential extraction procedure was discussed, and the releasing process and theimpact degree on the release of heavy metals in sediments under the aeration aswell as cooperation effect produced when several effect factors coexisted innatural waters were analyzed too. These results could help to further understandthe behavior, bioavailability of heavy metals and the self-purification mechanismof natural waters.Firstly, the main chemical components, mineral composition andconcentration of heavy metals in DS, SPM and SC of different natural waterswere studied. The results indicated that no significant difference about thecontents of Fe-oxides in DS, SPM and SC was found, but the order of contents isfollowed as SC > SPM >DS for the Mn-oxides and organic matters. Thesignificant crystal characters were observed among DS, SPM and SC, thecomplex of mineral and crystal degree decreased in the order DS > SC > SPM, inaddition, Quartz is the single most important determinant of mineralcompositions in the selected waters. The enrichment capacity increased in theorder of SPM > SC >DS for Pb, Cu and Zn, which suggesting that SPM and SCplay more important roles in controlling transport and fate of heavy metals thanDS disregarding the quantity of the solid phases in aquatic environment.Secondly, the results gained by different extraction methods to compare tothe adsorption capacity and contribution of components of DS, SPM and SC toheavy metals were difficult to ensure the components extracted to be comparable.Accordingly, the extraction conditions to remove the main components in SPMwere optimized on the basis of earlier researches, in which the methods of0.006mol/L NH2OH·HCl+0.01mol/LHNO3 shaking30 min for Mn oxide and0.8mol/L Na2S2O4(pH=6.0)shaking 40min for Fe, Mn oxides meet therequirement of less extraction to no-aim component and more aim components.And then, the methods of 0.008 mol/L NH2OH·HCl+ 0.01 mol/L HNO3oscillating 30 min to extract manganese oxides;0.8 mol/L Na2S2O4, pH 6.5,oscillating 40 min to remove amorphous iron oxides and part of manganeseoxides;0.1MHNO3,30%H2O2,85℃ in water bath for suspended particulatematers and sediments and 0.02M HNO3,30%H2O2 , 24h for surface coatings toremove organic matters can receive better effect. By comparing with Tessiersequential extraction scheme, the conclusion can be drawn that iron andmanganese oxides extracted by C6H806+ (NH4)2C2O4 can represent nearly all theiron and manganese oxides which play an important role in heavy metalsadsorption. Consequently the method could be employed to extract themanganese oxides, iron oxides and organic matters in DS, SPM and SC, and theextracted components were proved to be comparable.According to the methods described above, the adsorption sites of heavymetals in DS, SPM and SC of natural waters were determined. The analysissuggested organic matters was very significant sorbents for Cu adsorption, Pbwas controlled mainly by Fe oxides, but Mn oxides and organic matters playedan important role for Zn in three solid phases. In addition, the extraordinarypredominance of Mn oxides on adsorption capacity was found in DS, SPM andSC for Cu, Pb and Zn compared with Fe oxides and organic matters, and theconcentration difference of heavy metals in three solid phases mostly came fromthe contents difference of Fe, Mn oxides and organic matters.Furthermore, the study on the spatial distribution of sediments in YitongRiver indicated that the concentration varied dramatically from upstream todownstream for Zn and Pb, but others like Cu, Ni, Co, As kept constant, whichcontributed to the different sources of heavy metals. Heavy metals in ZiyouBridge and Weixing Bridge showed identical trends with sediment depth, but thehigher loadings of Cu, Pb, Zn in the layer 25cm and 50cm below the surfacesediment in Dong Bridge have been revealed, Cu, Pb, Zn and Co, As displayedthe similar depositing characteristics respectively. According to the conclusion ofspeciation patterns analysis, the major binging forms of Cu were bound toorganic/sulfides, and the remaining Cu was mostly present as Fe oxides fraction;Pb, Zn, Ni mainly existed in Fe oxides and organic/sulfides fraction, but thepercentage in Mn oxides and carbonates forms for Zn and Ni was greater than Pb.On the basis of PCA, the binging behavior of Cu in association with carbonateswas not similar to Pb, Zn, Ni, Fe, Mn, and the correlation between carbonatesand the six heavy metals was insignificant, but the binging behavior of Cu, Pb,Zn, Mn in association with Mn oxides was similar, and Fe, Mn, Pb, Zn insediments were dominated by Fe oxides. The entractants employed in sequentialextraction procedure and the pre-process of dryness could alter the distributionpatterns of heavy metals in anoxic sediments. Cu was mainly associated withorganic materials other than sulfides, indicating the less effect by the extractionprocedure, but Pb, Zn, Ni, Fe, Mn are predominantly associated with sulfidesmeaning an under-representation of organic/sulfide fraction in the metalsspeciation patterns of anoxic sediment samples, and the amount of Fe, Mn inorganic/sulfide fraction was significantly less than in SEM induced by higheroxidation rate of FeS and MnS.In the end, heavy metals would release to waters and cause pollution againwhen the environmental conditions has been changed. In this study, the influenceon the release of heavy metals in sediments by the factors such as aeration, pH,salinity and complexant was simulated in Lab. The results suggested that Znrelease was more slowly than Pb, and the concentration of Cu in solutionchanges slightly, the re-adsorption or Co-precipitation process of Pb and Znoccurred after a period of aeration, the concentration peaks of the heavy metalsin the solutions appeared more rapidly, which meant that the sediment had moreecological risks, the heavy metals in the sediment changed from more stableforms to those easily assimilated by microorganisms, which increased the heavymetals toxicity to them. When three factors, pH, salinity and complexant,coexisted in one water system, the proportion of release decreased in the order ofCu>Zn>Pb, which was mainly relative to the stability of EDTA complex ofmetals. The correlation between pH and the release amount of Cu and Zn wassignificant atρ<0.01, but it was significant atρ<0.05 for Pb;no obviouscorrelation has been found between salinity and heavy metals release;moreover,significant correlation (ρ<0.01) between complexant and release of Cu, Pb andZn was detected in this study. The cooperation effect of pH, salinity andcomplexant showed that the present of pH and complexant in waters promotedthe release of Cu, other than Pb and Zn.