Biogeochemical Cycles Of Toxic Metals In Farmland Soil-Plant System

With the development of urbanization and the interference of intensiveanthropogenic activities,a large amount of toxic metals from such contamination,asindustry,traffic and sewage,were emitted to the soil.The metals,persistent inenvironment,can not be degraded by the microorganism in soil.On the one hand,theaccumulated metals in agriculture soils can be transferred through food chains andconsequently cause adverse health effects on humans.On the other hand,toxic metalsin soil can be transformed to the more harmful organic metal compounds.In addition,under such dynamic conditions,as runoff,resuspension,leaching,and weathering,soils could contribute metals to the hydrosphere and the atmosphere.As a result,theecological environment is becoming deteriorated increasingly.It is noticeable that 2 ofthe 10 major environmental crises in the 20th century were associated to metalpollution.Therefore,toxic metal pollution in soil is one of the major globalenvironmental puzzles,accordingly,arouses a worldwide concern of the researchersand governments.With the support of the National Nature Science Foundation,the bidding projects ofShanghai Environmental Protection Bureau and the Key Program of Shanghai ScienceFoundation,the toxic metal (e.g.,Cd,Pb,As,Hg and Cr) pollution characteristics inShanghai soils were studied.The environmental biogeochemical cycles of metals infarmland were investigated in an all-round way,including atmospheric deposition,transfer in soil-plant system and surface runoff,as well as the leakage in soils.Additionally,the varied in-out put fluxes of metals in farmland were listed.Besides,the isothermal adsorption of Pb²⁺ and Hg²⁺ were explored deeply.The mainconclusions can be summed up as follows.(1) Metals in the agriculture soils were relatively low,up to the national soilstandards for the plants,whereas metals in surface dust have been accumulatedsignificantly.Metals in surface soils accumulated distinctly.Cd in surface soils wasmainly in weak acid-soluble and reducible fractions,while Pb and As were controlledby residual fraction,with the residual and reducible fractions for element of Hg.Thespeciation of metals in the soils of different profile was similar.Moveable coefficientsof Cd,Pb,As and Hg in surface soils were 36.43%,0.17%,1.08% and 2.76%,respectively.The order for bioavailable percentages of metals was as following:Cd＞Hg＞Pb＞As.Fertilization,industrial and domestic wastes were the dominant sources of metals in the farmland soils,while the metal accumulation in dust wasassociated with contaminants introduced by traffic and industry.(2) Metal contents in urban soils were much higher than those in farmland soils,and metal pollution was even severer in urban dust.In Shanghai urban area,thepollution hotspots tended to associate with city core area,major road junctions,andthe regions close to industrial zones.In attempt of identifying the source of metalsthrough geostatistical and multivariate statistical analyses,it was concluded as follows:Pb,Zn and Cu mainly originated from traffic contaminants;soil Ni was associatedwith natural concentration;Cd largely came from point-sourced industrial pollution;and Cr,Ni in dust were mainly related to atmospheric deposition.(3) Metal contents in wheat were varied during the different growing stages.As forthe wheat organs,wheat fruit accumulated less metal,whereas metal contents in rootwere elevated.Although both Setaria viridis and wheat belongs to Gramineae,theaccumulation of metals in Setaria viridis was different,with higher metal contents infruits and low values for roots.Vegetable species could influence metal concentrations.Generally,higher metal enrichment was present in vegetable roots.Among theinvestigated metals,the bioconcentration factor of Cd in plants was the highest,lowvalue in plants for As.Metal contents in plants were dominated by bioavailablefraction in surface farmland soils,and a cubic function was fitted well.The results ofhealth risk assessment indicated that carcinogenic metals (Cd and As) in wheat andBrassica campestris L.ssp.Chinnesis could cause high risks,and the risk ofnoncarcinogenic metals (Pb and Hg) in vegetables and wheat can be ignored.(4) The rainwater in Shanghai was the acid rain,with a relatively low pH.Redoxpotential of the rain arranged from 120mV to 200mV mainly,and electricalconductivity in rainfall showed elevated values in winter,low numbers for summer.The ratios of NO₃^-contents to SO₄^2- levels revealed that H₂SO₄ controlled the acidityin the rainwater.Spatial analysis indicated that contributing rate of H₂SO₄ in the acidprecipitation was high in the rural area,followed by suburb and the city.Totalnitrogen (TN) contents in rain were from 0.37 mg·L^-1 to 11.69 mg·L^-1,and the averagevalue was 19.48mg.L^-1 for total carbon (TC).Spatial distribution of TN and TC inrains was regular,and the higher contents were in urban precipitation,with the lowervalues in rural rains.Total organic carbon was the main component of TC in therainwater.Metal contents in rains varied in the order of Pb＞Cr⁶⁺＞As＞Cd＞Hg,with thepollution degree of Hg＞Pb＞Cd＞As≈Cr⁶⁺.As to the metal concentration varieties in rainfalls,higher metal contents were present in urban rain,while low values were insuburban and rural precipitation.Metal concentrations in rains were different in variedmonths,in general,Cd,Pb and As in December rainfall were elevated,with the lowvalues in summer.The value of K,solid-liquid distribution coefficients of metals inrainwater,was larger than 1,meaning that soluble fraction was dominant.There werebetter correlations between metal contents in rainwater and precipitation as well aswind directions.The stable isotope compositions of Pb in rains were 1.1620-1.1748for ²⁰⁶pb/²⁰⁷pb and 0.40878-0.41103 for ²⁰⁷pb/²⁰⁸Pb.Sources identification ofpollutants in rains demonstrated that N,Cd,Pb and As were from the trafficcontaminants,while Cr and TIC (total inorganic carbon) were mainly correlated withsurface dust and building site dust,TOC and Hg related to coal combustion.(5) Wet atmospheric deposition fluxes of metals in urban area were higher thanthose in suburban and rural areas.It was computed that bulk atmospheric depositionfluxes of Cd,Pb,As and Hg in Shanghai were 404,15500,2934 and 283μg·m^-2·yr^-1severally,and wet atmospheric deposition made a major contribution to the totaldeposition.It was concluded that various types and intensities of human activitieswere the main reasons for the spatial variation of metal atmospheric deposition fluxes.According to the atmospheric deposition and emissions of metals,it could be gottenthat there were more than 200t (Pb) and 7t (Hg) transported to other regions.(6) Dissolved metal concentrations (DMC) in farmland runoff were lower thanthose in the rainfall.For the DMC in bare farmland runoff,the values increasedsteadily with relatively small valuesat the beginning,subsequently,the numbersdecreased after the peak contents.Meanwhile,metal contents of particles (PMC) inthe runoff were higher than those in surface soils,of which the grain size wasrelatively larger.The change of PMC in the process of runoff events was similar to theDMC trend.As a whole,particle and total metals losses in bare farmland runoff couldbe characterized by the first flush effect.Compared the DMC in bare land runoff,there were no consistent trend of DMC in plant-covered farmland runoff processes,whereas the PMC increased with the reduction of turbidity of runoff.Event meanconcentrations of dissolved metals in the plant-covered land runoff were present in theorder of Cr⁶⁺＞As＞Pb＞Cd≈Hg,and the first-flush phenomenon of these elements werenot obvious.Metals of Cd,Pb and Hg in rainwater were absorbed significantly by thesoil particles in the runoff,and desorption of soils was dominant for As and Cr⁶⁺.Particle levels and dissolved cotents of Cd,Pb and Hg in bare land runoff were higher than those in plant-covered land runoff,but As and Cr⁶⁺ showed the oppositediscipline.It was the metal contents of rainwater what determines which process wasdominant,adsorption or desorption.The critical levels of Cd,Pb,As and Hg in rainswere 0.095,1.068,1.049 and 0.066μg·L^-1 respectively.Furthermore,the solid-liquiddistribution coefficient (lgK_d) was varied among metals (As≈Pb＞Hg≈Cd),on whichthe particle concentration effect was remarkable.The grain size and clay content ofthe particles in runoff determined the PMC in farmland runoff.(7) Metal levels in the in soil leaching solution were relatively low,and the value ofCr⁶⁺ was highest,followed by Pb＞As＞Hg＞Cd.As for the contents variation indifferent layers of soil leaching solution,Cd and Hg contents in the depth of 20cmwere higher than those in the depth of 40 cm,the lowest for 60 cm.However,Cr⁶⁺was present the opposite distribution.There were no consistency and regularity for theelement Pb and As contents.Broadly speaking,metal levels in the plant-coveredfarmland soil leaching solution were higher than those in the bare land solution,whichwas caused by the acidic component of root exudates.Cd,Pb,As and Hg wereadsorbed by soil particles significantly,leading to lower levels in soil leachingsolution,meantime Cr was desorbed from the soils and higher value was investigatedin the leaching solution.There was a better linear correlation between precipitationand leakage.The elements of C and N leakage fluxes in different depth of soil layerswere present in the order of 20cm＜40cm＜60cm,and metals were quite different,noconsistent tendency observed in varied layer soil solution.Among the 5 studied metals,Cr⁶⁺ leakage flux was the largest,followed by the descending order of Pb,As,Hg andCd.Comparison of metal levels in varied solutions showed that Cd,As and Cr⁶⁺contents in runoff were higher than those in soil leaching solution,the oppositediscipline observed for Pb and Hg.Adsorption intensity was the primary cause ofdifferent metal levels in the two types of solutions.(8) Equilibrium time of adsorption increased with the increase of sediment mass insolution,while both adsorbed Pb²⁺ on per unit of sediment and Pb²⁺ concentration inthe solution after equilibrium decreased.More than 95% of Pb²⁺ in solution wasremoved when sediment contents larger than 0.6g·L^-1.The experimental data closelyfollowed by both Langmuir and Redlich-Peterson isotherms,and sediment had aconsiderably high Langmuir monolayer capacity of Pb²⁺.Both pseudo-first-order andpseudo-second-order kinetics were tested and it was found that the latter gave a betterexplanation of the adsorption process.The equilibrium adsorption capacities calculated from the pseudo-second-order model could represent the true value.Therewas no significant correlation between initial adsorption rate of Pb²⁺ and the amountof sediment in solution.However,the pseudo-second-order rate constant increased inthe solution with more adsorbent,namely chemical adsorption controlled the process.Elovich equation could explain the mechanism of sorption in the solution with highercontents of sediment;nevertheless,the process of low concentration of adsorbentadsorbing Pb²⁺ disagreed well with Elovich equation.In terms of adsorption rate inthe sorption,intra-particle diffusion dominated in the more sediment solution.On theother hand,multi-linearity was presented for the adsorption rate in less adsorbentsolution.The first,sharper portion represented adsorption on the external surface.Thesecond portion indicated Pb²⁺ diffused gradually into the interior of particles andintra-particle diffusion controlled.pH of the solution,co-existence of EDTA,sedimentgrain size and humic acid level could influence the adsorption of Pb²⁺ onto sedimentgreatly.Adsorption rate of Hg²⁺ on sediment was greater than 97% after 0.3 min withthe initial content of 10μg·L^-1,and physical adsorption controlled in the process.Theimpacting factors of Hg²⁺ adsorbed on sediment mainly included adsorbent amount,pH of solution,co-existence of EDTA and sediment humic acid content.It wasobserved that the co-existence of Cd²⁺ could not impact adsorption of Hg²⁺ due to thetrace initial level of adsorbate.(9) Adsorption rate of Pb²⁺ onto sediment core statically was relatively smaller.Lower initial level of Pb²⁺ resulted in the lower equilibrium content and smallerdiffusion rate in interface between water and sediment,and initial contents of Pb²⁺were correlated well with diffusion rates and diffusion fluxes.Stem powder ofEichhornia crassipes (SPE) could truly be termed as a high-effective and low-costadsorbent,and the adsorption rate of Pb²⁺ with varied initial levels of 0.1-40 mg·L^-1was above 90%.Both elevated initial Pb²⁺ and less SPE could conduce to greaterequilibrium uptake and activation energy variation.It was revealedpseudo-second-order kinetics provided a better explanation of the adsorption of Pb²⁺on SPE.In terms of adsorption rate in the sorption,chemical adsorption controlled theadsorption rate.Furthermore,two-step linearity was presented for the adsorption rate.(10) Suppose the farmland was a black box,inputs of toxic metals in the farmlandwere dominated by atmospheric deposition,with the contribution rates of more than90%.The input fluxes ofCd,Pb,As and Hg were 0.4020,15.711,3.139 and 0.250mg·m^-2·yr^-1 respectively,which were smaller than the outputs values of 0.3247, 10.5033,1.4944 and 0.0960 mg·m^-2·yr^-1.As far as the environmental fates of themetals introduced to farmland were concerned,more than 80% of Cd wasaccumulated in plants;about 50% of Pb was transported to surface water by runoff;larger than 80% of As was accumulated in soils and lost with runoff;residue of Hg insoils accounted for nearly 60% the input.Innovations of this PhD dissertation are mainly:(1) Based on the intersecting of several subjects and the combination ofmacroscopic phenomena and microcosmic mechanisms,the environmental behaviorsof toxic metals in soil-plant system were investigated according to the input-output ofmetals in farmland.(2) Paying more attention to the natural conditions and field observations,the studyon biogeochemical cycles was improved greatly;and the traditional researches toomuch relied on the simulation experiment.(3) Some new equipment for sampling and observation were developed in thisstudy;and methodology for sample preparation was conducted with tha aid of SINEOMDS micro-wave digestion system.(4) Wet and dry deposition of toxic metals in Shanghai was studied completely,andit was found that waste gas discharge of Shanghai was a main source of metals in theglobal atmosphere.(5) Major sources of toxic metals in soils and atmosphere were identified by use ofisotope tracer technique.(6) Kinetics and isotherms adsorption of metal ions onto small rever sedimentswere investigated thoroughly;and a new type of biological material adsorbing metalion was explored.(7) The input-output fluxes of toxic metals in farmand were estimated systemically,and the accumulation rates of metals in soils were computed.