Comment on “Critical Evaluation of Desorption Phenomena of Heavy Metals from Natural Sediments

Comment on “Critical Evaluation of Desorption Phenomena of Heavy Metals from Natural Sediments

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  Correspondence Comment on “ Critical Evaluation of DesorptionPhenomena of Heavy Metals from NaturalSediments” History shows that scientists often go to great lengths todemonstrate that anomalous observations can be accom-modated by an established theory or, conversely, that they are erroneous. This process can perdure until eventually a“paradigm” or perspective shift occurs, whereby a new conceptual approach that satisfactorily accounts for thecontentiousobservationsemergesandsupplantstheearlierone ( 1 ). Upon reading the recent article by Gao et al. ( 2  ), itishardnottowonderwhetherresearchonmetaldesorptionfrom soils and sediments is not in the pre-shift phase and whether a slight change of perspective would not provide amore satisfactory conceptual framework than the currentviewpoint, which is focused almost entirely on the abioticsorbent phase (i.e., clays, oxides, humified organic matter).In their interesting article, Gao et al. ( 2  ) show how lead(Pb) and cadmium (Cd) sorbed to Utica sediments from theHudson River can be increasingly desorbed or redistributedfromthesedimenttosolutionbyrepeatedwashingswithanelectrolyte solution, by sequestration with EDTA, or by lowering the pH with a mineral acid. The authors observedsignificant desorption hysteresis when the replaced-super-natantmethodwasused.Thehysteresisappearedtoincrease with aging time, but it was not manifested when desorption was initiated by lowering the solution pH. Their resultssuggest that desorption of metal ions from the solid phaseof natural sediments depends not only on the surfacechemistryofthesolidsandonthepropertiesofthedesorbing metals but also on the strength of “sinks” that are presentin the system.Before Gao et al. ( 2  ), other authors made similar observa-tions.Workingwithasimple,modelsystem,Strawnetal.( 3  )showed that 98% of Pb, held to the surface of   γ -Al 2 O 3  by aninner-sphere bidentate bonding mechanism, could be de-sorbedwithin3daysatpH6.5usingacation-exchangeresin.Jensen-Spaulding et al. ( 4  ) showed that within 350 hextracellularpolymersinduceda2 - 4-foldincreaseincopper(Cu) and Pb release from long-contaminated ( > 20 yr) soilseven though Pb, particularly “aged” Pb, is generally con-sideredimmobileinsoils.Furthermore,asJensen-Spaulding et al. ( 4  ) point out, at the microscale of a bacterial cell, localpolymerconcentrationswouldprobablybemuchhigherthanthoseusedintheirexperimentsandthedesorptionofmetalsinto biofilm coatings is likely to be significantly increased,even in cases where the contaminants may have been agedfor over a decade.Thetraditionalviewofmetalreleasefocusesondesorptionandconsidersthatthekeyactorsaretheabioticsolidsorbents + metalcouple(withothersorbentsandcomplexingagentsin solution playing only secondary support roles). Theexperimental results of Gao et al. ( 2  ) and others suggest thatone should instead view the release of metals from abioticsolidphasesaspartofabroader repartitioning  process(Figure1), which involves on equal footage the metal, the abioticsolid sorbents, and whatever sink is present in the system. Along with the characteristics of the metal and the sorbent,thestrengthofthesinkdeterminestheextentandrateoftherepartitioning.Natural systems include a wide range of sinks beyondthoseconsideredbyGaoetal.( 2  ),suchasdispersedcolloidalparticles that have dissociated from the matrix and move with the soil solution, dissolved organic matter, and labilecomplexes. They also include micro- and macro-flora andfauna that influence metal repartitioning either passively oractively.Forexample,the“agingeffect”inwhichthereleaseofmetalsappearstobeimperceptivelyslowisthoughttobedue in part to physical or diffusional limitations. However,plants and microbes have evolved mechanisms that cir-cumventtheselimitationsorenhanceabioticreactions,suchas the release of phytosiderophores by plant roots to obtainiron under Fe deficiency stress. In addition to increasing Feuptake, this behavior is also associated, in graminaceous FIGURE1. Metalreleaseisgenerallyviewedasatwo-compartmentprocessinwhichonlythesorbentandmetalinsolution(i.e.,free-ionortotaldissolved,whichincludescomplexedand/orcolloidalspecies)areconsidered(A).However,mountingevidencesuggeststhatatleastthreecomponentsshouldbeconsidered:sorbent,metal,andsink.Sinksmaybenumerousandvariedandincludecomplexingagents,microbes,andplantroots,amongothers(B). 10.1021/es0493594 CCC: $27.50  󰂩  2004 American Chemical Society VOL. 38, NO. 17, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY  9 4701 Published on Web 07/24/2004  species, with uptakes of Zn, Ni, and Cd up to 200% higherthan in controls ( 5  ).Similarly,bacteriainsoilsandsedimentscanthemselvesactively or passively influence metal partitioning. The cell walls of bacteria are (predominantly) electronegative andsituated in such a way as to allow intimate interaction withsoluble components, such as metals, in the bacterium’senvironment. These cells behave as surface nucleation sitesand can passively accumulate large quantities of metals ( 6  ).In actively respiring bacteria, the metabolically influencedmicroenvironment surrounding the cell can lead to thedevelopmentofmineralphases,whichwouldnothavebeenpredictedbasedonthebulkgeochemistry.Forexample,thesediments in Green Lake (Fayetteville, NY) consist of calciteeventhoughthenaturalgeochemistryofthebulklakewatershould favor the precipitation of gypsum ( 7  ).Soils and sediments are multicomponent systems withpropertiesthatarecontinuallybeinginfluencedbyphysical,chemical, hydrological, geological, and biological processes( 6  ). In these environments, biological sinks exhibit a widerange of responses; under given conditions, one organismmay act as a sink, whereas another may not be able to doso. For instance, earthworms ( Eisenia veneta  ) have beenobserved to accumulate Cd under conditions where leafy Swiss chard did not ( 8  ). The presence of multiple sinks insomecasesmaycausethebehaviorofonetoaltertheactivity of others, as observed by Sayer et al. ( 9  ), who found that ryegrass ( Lolium perenne  ) grown in a blend of sand andpyromorphite (one of the least soluble and most stable leadminerals) took up 30 times more Pb when the fungus  Aspergillus niger   was present. A key feature of many sinks involved in metal reparti-tioning in soils and sediments is that they are susceptible totransportwiththeliquidphase.Thistransportisoftenpassive(as in the case of colloids or dissolved organic matter,remnants of plant roots, fungal hyphae, and lysed bacterialcells that are carried away by the moving liquid phase), butit may be active as well (as in the case of motile bacteria ormacrofauna).Regardlessofitsnature,themovementofsinksthroughsoilsandsedimentsmayleadtosignificantfacilitatedtransport of the metals and ultimately to the contaminationof groundwater reservoirs or surface water bodies. Inparticular, the often overlooked role of biological sinks inthis context may be vital to characterizing otherwise un-explained losses of metals from soil and sediment profiles( 10  ,  11 ) .  Further research on this issue appears necessary tofully understand metal contamination in these ecosystemsand to manage it appropriately. Literature Cited (1) Kuhn, T. S.  The Structure of Scientific Revolutions  ; TheUniversity of Chicago Press: Chicago, IL, 1962.(2) Gao, Y.; Kan, A. T.; Tomson, M. B.  Environ. Sci. Technol. 2003 , 37  , 5566 - 5573.(3) Strawn, D. G.; Scheidegger, A. M.; Sparks, D. L.  Environ. Sci.Technol.  1998 ,  32  , 2596 - 2601.(4) Jensen-Spaulding,A.;Shuler,M.L.;Lion,L.W. WaterRes  . 2004 , 38  , 1121 - 1128.(5) Romheld, V.; Awad, F.  J. Plant Nutr  .  2000 ,  23  , 1857 - 1866.(6) Ledin, M.  Earth-Sci. Rev  .  2000 ,  51 , 1 - 31.(7) Schultze-Lam,S.;Fortin,D.;Davis,B.S.;Beveridge,T.J. Chem.Geol.  1996 ,  132  , 171 - 181.(8) Oste, L. A.; Dolfing, J.; Ma, W.-C.; Lexmond, T. M.  Environ.Toxicol. Chem  .  2001 ,  20  , 1339 - 1345.(9) Sayer, J. A.; Cotter-Howells, J. D.; Watson, C.; Hillier, S.; Gadd,G. M.  Curr. Biol  .  1999 ,  9  , 691 - 694.(10) Baveye,P.;McBride,M.B.;Bouldin,D.;Hinesly,T.D.;Dahdoh,M. S. A.; Abdel-Sabour, M. F.  Sci. Total Environ  .  1999 ,  227  ,13 - 28.(11) Qureshi, S.; Richards, B. K.; Hay, A. G.; Tsai, C. C.; McBride,M. B.; Baveye, P.; Steenhuis, T. S.  Environ. Sci. Technol  .  2003 , 37  , 3361 - 3366.  Astrid R. Jacobson* and Philippe Baveye Department of Crop and Soil ScienceCornell University 1002 Bradfield HallIthaca, New York 14853-1901ES0493594 4702  9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 17, 2004
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