envireport_bioavailability and toxicity of metals

8/6/2019 Envireport_bioavailability and Toxicity of Metals

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` Bioavailability of metals released from mineral deposits

is very complex and dependent on many interrelatedchemical, biological, and environmental processes. These

processes may vary over time and among micro-

organisms, plants, and animals.` In soil and surface water, the mining method, presence or

absence of sulfide minerals, quantity of water, acid-

buffering capacity, presence of organic matter and iron

and manganese oxide minerals, element speciation, and

concentrations of other constituents in water may impact

dissolved and bioavailable metal and metalloid contents.


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` The fate of various metals in the natural environment is of greatconcern (Adriano, 1986; 1992), particularly near former mine sites,dumps, tailing piles, and impoundments, but also in urban areas andindustrial centers.

THESE MATELS are:

AND METALLOIDS, including

Chr iu ickel

Copper

a ga ese ercury

Ca iu ea

Arse ic A ti ony Seleniu ,


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` Soil, sediment, water, and organic materials in these areas

may contain higher than average abundances of theseelements, in some cases due to past mining and (or)

industrial activity, which may cause the formation of the

more bioavailable forms of these elements. In order to

put elemental abundances in perspective, data from landsand watersheds adjacent to these sites must be obtained

and background values, often controlled by the bedrock

geology and (or) water-rock interaction, must be defined.


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` Metals can be dispersed in soil, water, and air.Geoscientists are mainly concerned with metalsdispersed in soil and sediment, dissolved in ground andsurface water, suspended as particles in surface water,

and in pore fluid in sediment (fig. 1). In addition,metals can be dispersed into the atmosphere, by naturalgeochemical cycling and by other anthropogenicprocesses (such as smelting and burning leaded gasolineand coal) and by microbial activities; these metal fluxes

must be considered in overall metal bioavailabilitystudies.


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` Bioaccumulation of metals by biota in surface water andby plants and animals in terrestrial environments canadversely affect humans. In surface and ground water,sediment and air, bioavailability is a complex function of

many factors including total concentration and speciation(physical-chemical forms) of metals, mineralogy, pH,redox potential, temperature, total organic content (bothparticulate and dissolved fractions), and suspended

particulate content, as well as volume of water, watervelocity, and duration of water availability, particularly inarid and semi-arid environments.


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` In addition, wind transport and removal from the

atmosphere by rainfall (frequency is more important thanamount) must be considered. Many of these factors varyseasonally and temporally, and most factors areinterrelated. Consequently, changing one factor mayaffect several others. In addition, generally poorly

understood biological factors seem to strongly influencebioaccumulation of metals and severely inhibit predictionof metal bioavailability (Luoma, 1989). Some of themajor controls on the bioavailability of metals in surfacewater and soil and data concerning potentially hazardousmetals are described below.


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` In order to understand bioavailability, plant materials and

selective chemical leaches of soil must be analyzed andthe results compared. Elemental suites for which analyses

are performed and the type of selective leaches utilized

must be tailored to bedrock and soil types, and to

suspected anthropogenic inputs. Soil pH, organic matter,and sulfur and carbonate contents should be determined

to enable accurate assessment of elemental reservoirs,

mobility, and bioavailability. Additional work on

mineralogical residences of metals is also importantbecause metals can be associated with several sites (fig. 2).


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Figure 1.0 interrelationship of man, metals, andFigure 1.0 interrelationship of man, metals, and

environmentenvironment


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` Plant species and relative abundance and availability of necessaryelements also control metal uptake rates.

` Abundant bioavailable amounts of essential nutrients, including

phosphorous and calcium, can decrease plant uptake of non-essential but chemically similar elements, including arsenic andcadmium, respectively.

` More complex interactions are also observed: bioavailability may berelated to multi-element amounts or ratios.For example, copper toxicity is related to low abundances of zinc, iron,

molybdenum and (or) sulfate (Chaney, 1988).

` In the scientific literature, many studies describe anthropogenic (industrial ormining) contributions to elemental abundances, and their bioavailabilitycontrols, in the environment.

Examples include: occurrence of heavy metals in soil near and far from urban

pollution (Pouyat and McDonnell, 1991); formation of acid mine drainage(Filipek and others, 1987); uptake of heavy metals by plants in lab experiments(Brown and others, 1995); and uptake of metals by vertebrates in the vicinityof zinc smelters (Storm and others, 1994).


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Figure 2.0 the chemical forms of metals in solidFigure 2.0 the chemical forms of metals in solid

phasephase


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` Plant uptake of trace elements is generally the first step of their

entry into the agricultural food chain.

` Plant uptake is dependent on

movement of elements from the soil to the plant root

elements crossing the membrane of epidermal cells of the root

transport of elements from the epidermal cells to the xylem, inwhich a solution of elements is transported from roots to shoots

possible mobilization, from leaves to storage tissues used asfood (seeds, tubers, and fruit), in the phloem transport system.


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` After plant uptake, metals are available to herbivores andhumans both directly and through the food chain. The

limiting step for elemental entry to the food chain isusually from the soil to the root (Chaney, 1988). This

critical step usually depends on element concentrations insoil pore solutions, which are controlled by local soil

physical and chemical conditions including water content,pH, Eh, and other factors.


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` Climate strongly influences soil types which largely controlelement (metals and metalloids) mobility and availability.

Arid climatesin the western United States often result in smallsoil organic matter abundances and large salt and carbonateabundances. These phases often contain the metals ofinterest.

Humid climates, in the eastern United States, prevail and large

amounts of organic matter require determination of organicmatter-associated metals and their residence times or turn-over rates, because after some time, much of the organicmatter is oxidized and associated metals may be released oravailable.

In tropical climate conditions, accumulation of oxide minerals ofiron, manganese, and aluminum in soil profiles may limit themobility and bioavailability of both metals and metalloids.


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After discharge to an aquatic environment but before uptake by organisms,

metals are partitioned between solid and liquid phases.


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Within each phase, further partitioning occurs among ligands as

determined by ligand concentrations and metal-ligand bondstrengths. In solid phases, soil, sediment, and surface waterparticulates, metals may be partitioned into six fractions:(a) dissolved,(b) exchangeable(c) carbonate

(d) iron-manganese oxide(e) organic(f) crystalline (Elder, 1989; Salomons, 1995).

Various metals partition differently among these fractions as shownby sequential partial extraction procedures.


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` Partitioning is affected strongly by variations in (Elder,1989; Salomons, 1995): pH

redox state

organic content

other environmental factors

The relative mobility and bioavailability of trace metalsassociated with different fractions are shown in Table 1.


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` Dissolved fraction consists of carbonate complexes, whoseabundance increases with pH, and metals in solution, includingmetal cation and anion complexes and hydrated ions whosesolubilities are affected strongly by pH and tend to increasewith decreasing pH (Elder, 1989).

` Exchangeable fractions consist of metals bound to colloidal orparticulate material.

` Metals associated with carbonate minerals in sedimentary rocks and

soil constitute the carbonate fraction, which can be newlyprecipitated in soil.

` The iron-manganese oxide fraction consists of metals adsorbed toiron-manganese oxide particles or coatings.

` The organic fractionconsists of metals bound to various forms of

organic matter.` The crystalline fraction consists of metals contained within the

crystal structure of minerals and normally not available tobiota.


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` Hydrogen ion activity (pH) is probably the most importantfactor governing metal speciation, solubility from mineralsurfaces, transport, and eventual bioavailability of metals inaqueous solutions. pH affects both solubility of metalhydroxide minerals and adsorption-desorption processes.Most metal hydroxide minerals have very low solubilitiesunder pH conditions in natural water. Because hydroxide ionactivity is directly related to pH, the solubility of metalhydroxide minerals increases with decreasing pH, and moredissolved metals become potentially available for

incorporation in biological processes as pH decreases. Ionicmetal species also are commonly the most toxic form toaquatic organisms (Salomons, 1995).


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` Adsorption, which occurs when dissolved metals areattached to surfaces of particulate matter (notably iron,

manganese, and aluminum oxide minerals, clay, and

organic matter), is also strongly dependent on pH and, ofcourse, the availability of particulate surfaces and totaldissolved metal content (Bourg, 1988; Elder, 1989).

Metals tend to be adsorbed at different pH values, and

sorption capacity of oxide surfaces generally varies from

near 0 percent to near 100 percent over a range of about2 pH units.


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` The adsorption edge, the pH range over which the rapid change insorption capacity occurs, varies among metals, which results inprecipitation of different metals over a large range of pH units.Consequently, mixing metal-rich, acidic water with higher pH,metal-poor water may result in dispersion and separation of metals

as different metals are adsorbed onto various media over a range ofpH values. Cadmium and zinc tend to have adsorption edges athigher pH than iron and copper, and consequently they are likely tobe more mobile and more widely dispersed. Adsorption edges also

vary with concentration of the complexing agent; thus, increasingconcentrations of complexing agent increases pH of the

adsorption edge (Bourg, 1988). Major cations such as Mg+2

andCa+2 also compete for adsorption sites with metals and can reducethe amount of metal adsorption (Salomons, 1995).

`


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` Particulate size and resulting total surface area available foradsorption are both important factors in adsorption processes andcan affect metal bioavailability (Luoma, 1989). Small particles withlarge surface-area-tomass ratios allow more adsorption than anequivalent mass of large particles with small surface-area-to-mass

ratios. Reduced adsorption can increase metal bioavailability byincreasing concentrations of dissolved metals in associated water.The size of particles released during mining depends on miningand beneficiation methods. Finely milled ore may release muchsmaller particles that can both be more widely dispersed by waterand wind, and which can also serve as sites of enhanced

adsorption. Consequently, mine tailings released into fine-grainedsediment such as silty clays found in many playas can have muchlower environmental impact than those released into sand orcoarse-grained sediment with lower surface area and adsorption.


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` Temperature exerts an important effect on metal speciation,because most chemical reaction rates are highly sensitive totemperature changes (Elder, 1989). An increase of 10 C candouble biochemical reaction rates, which tendency are often

the driving force in earth surface conditions for reactions thatare kinetically slow, and enhance the tendency of a system toreach equilibrium. Temperature may also affect quantities ofmetal uptake by an organism, because biological process rates(as noted above) typically double with every 10 C temperature

increment (Luoma, o 1983; Prosi, 1989). Because increasedtemperature may affect both influx and efflux rates of metals,net bioaccumulation may or may not increase (Luoma, 1983).


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` In recent organic carbon-rich sediments, trapped interstitial fluidscan commonly form a strongly reducing (anoxic) environment.Low redox potential in this environment can promote sulfatereduction and sulfide mineral deposition. During diagenesis, muchof the non-silicate-bound fraction of potentially toxic metals such

as arsenic, cadmium, copper, mercury, lead, and zinc, can be co-precipitated with pyrite, form insoluble sulfides, and becomeunavailable to biota (Morse, 1994). Seasonal variation in flow ratesor storms that induce an influx of oxygenated (sea)water can resultin rapid reaction of this anoxic sediment and thereby releasesignificant proportions of these metals. Pyritization and (or) de-

pyritization of trace metals probably can be an important processin controlling bioavailability of many trace metals, especially in themarine environment (Morse, 1994).

`


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Numerous geochemical environmental factors may affect metalavailability to aquatic organisms and plants (discussed in previous

sections).


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` metal concentrations of solutions

` solute metal speciation

` metal concentration in food

` metal partitioning among ligands within food` influence of other cations

` Temperature

` pH

` redox potential


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The extent of bioavailability is largely controlled by elementalspeciation or chemical siting in soil, which determine solubility.


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1. Some of the chemical extractions are:2. (1) water or MgCl at neutral or ambient soil pH for easily soluble metals, (2) solubility in

weak base (pH 9) for 2 humic materials3. (3) weak acid or dilute acid in buffer solution (pH 2 to 5) to release metals associated

with carbonate phases4. a chelating (or complexing) agent such as EDTA (ethylenediaminetetraacetic acid)

(Borggaard, 1976) or DPTA (diethyenetriaminepentaacetic acid) buffered to a pH of 7(Crock and Severson, 1980)

` Other possible extractants include1. hydroxylamine hydrochloride for the "reducible" fraction associated with iron and

manganese oxides/hydroxides2. strong acid (HCl, pH1) to identify maximum mobility of most metals (Leventhal and

Taylor, 1990)3. oxidation by hydrogen peroxide to release metals associated with organic matter and (or)

sulfide minerals4. a strong oxidizing acid (HNO ) to execute steps (6) and (7) simultaneously5. 3 a mixture of strong acid and HF to dissolve residual silicate minerals. The choice of

extractants and the order in which they are used depends on the sediment/soil type,environmental conditions, and metals of interest.


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Brief summaries of some factors controlling bioavailability ofseveral metals and two metalloids are given below. Additional data areavailable in the references listed for each metal.


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` Arsenic mobility, bioavailability, and toxicity are dependent on speciation:arsenite (AsO3

+3 ) forms are much more toxic to biological species andare more mobile than arsenate (AsO4

+3) forms (Kersten, 1988). Arsenic ischemically similar to phosphorous. Arsenate interferes with phosphatemetabolism that is widespread in the biosphere. Metallo-organic forms ofarsenic also may be much more bioavailable than inorganic forms;

however, organic-bound arsenic is excreted by most species and does notappear to be highly toxic (Luoma, 1983). Adsorption-desorption on ironand aluminum oxide minerals is the main factor controlling arsenicbehavior in soil and sediment. Maximal adsorption occurs at different pHfor As{III} (pH 9.2) and As{V} (pH 5.5) as a function of the adsorbingmineral; As+3 mobility is enhanced under oxisizing conditions. Arsenic is

apparently highly mobile in anoxic sediment-water systems. Developmentof acidic and oxidizing conditions tends to release large amounts ofarsenic into solution due to decreased sorption capacity of both forms ofarsenic (see Lonard, 1991).


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` The redox potential of sediment-water systems exerts controlling regulation on

the chemical association of particulate cadmium, whereas pH and salinity affectthe stability of its various forms (Kersten, 1988). In anoxic environments, nearlyall particulate cadmium is complexed by insoluble organic matter or bound tosulfide minerals. Greenockite (CdS) has extremely low solubility under reducingconditions thereby decreasing cadmium bioavailability. Oxidation of reducedsediment or exposure to an acidic environment results in transformation ofinsoluble sulfide-bound cadmium into more mobile and potentially bioavailable

hydroxide, carbonate, and exchangeable forms (Kersten, 1988). Studies of lakeand fluvial sediment indicate that most cadmium is bound to exchangeable site,carbonate fraction, and iron-manganese oxide minerals, which can be exposed tochemical changes at the sediment-water interface, and are susceptible toremobilization in water (Schintu and others, 1991). In oxidized, near neutral water,CdCO3 limits the solubility of Cd

+2 (Kersten, 1988). In a river polluted by base-metal mining, cadmium was the most mobile and potentially bioavailable metaland was primarily scavenged by non-detrital carbonate minerals, organic matter,and iron-manganese oxide minerals (Prusty and others, 1994). Elevated chloridecontents tend to enhance chloride complex formation, which decreases theadsorption of cadmium on sediment, thereby increasing cadmium mobility(Bourg, 1988) and decreasing the concentration of dissolved Cd+2andbioavailability (Luoma, 1983). Also see, Stoeppler (1991) for additional data.


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` In a river polluted by base-metal mining, copper is mostefficiently scavenged by carbonate minerals and iron-manganese oxide minerals and coatings and is less mobilethan cadmium, lead, and zinc (Prusty and others, 1994); inmost other situations lead is less mobile than copper. Elevated

chloride contents decrease adsorption of copper on sediment,due to chloride complexation, which results in greatersolubility and mobility (Bourg, 1988; Gambrell and others,1991). In systems with high total copper contents,precipitation of malachite controls dissolved copper contents

at low pH ( B

ourg, 1988; Salomons, 1995). Sometimes,elemental substitution is more complex; for example, coppertoxicity is related to low abundances of zinc, iron,molybdenum, and (or) sulfate (Chaney, 1988).


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` The main sources of lead in the aquatic environment are leaded gasoline and mining (Prosi, 1989).

Leaded gasoline results in introduction of organometallic lead compounds, which eventually reachsurface water, into the atmosphere. Mining releases inorganic lead compounds. Both organic andinorganic forms of lead pose serious health risks to all forms of life (Ewers and Schlipkter, 1990).Inorganic lead compounds (sulfide, carbonate, and sulfate minerals) are commonly abundant in sedimentbut have low solubilities in natural water. Naturally-occurring lead in mineral deposits is not very mobileunder normal environmental conditions, but becomes slightly more soluble under moderately acidicconditions. Soluble lead is little affected by redox potential (Gambrell and others, 1991).

` Lead is tightly bound under strongly reducing conditions by sulfide mineral precipitation and complexionwith insoluble organic matter, and is very effectively immobilized by precipitated iron oxide minerals

under well-oxidized conditions (Gambrell and others, 1991). In the aquatic environment, total dissolvedlead abundances in water and pore water control primary uptake by organisms.

` Lead bioaccumulation is primarily dependent on the amount of active lead compounds (predominantlyaqueous species) in the environment and the capacity of animal species to store lead (Prosi, 1989).Particulate lead may contribute to bioaccumulation in organisms. For humans, particles that are inhaledbut not exhaled are especially important. Variations in physiological and ecological characteristics ofindividual species lead to different enrichment factors and tolerances for each organism. Studies ofbottom dwelling organisms suggests that iron-rich sediment inhibits lead bioavailability (Luoma, 1989).

` In a study of lake and fluvial sediment, most lead was bound to a carbonate fraction or to iron-

manganese oxide minerals, both of which respond to chemical changes at the sediment-water interface,and are susceptible to remobilization in water (Schintu and others, 1991).

` In a polluted river environment, lead is most efficiently scavenged by non-detrital carbonate and iron-manganese oxide minerals and is less mobile than cadmium (Prusty and others, 1994).


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` Mercury has three valence states in natural sediment-water systems:elemental mercury (Hg0), Hg+1, and

Hg

+2

(Kersten, 1988). Hg

0

is considerably more bioavailable for certain organisms than Hg

+2

becauseof the solubility Hg in lipid-rich tissue (Louma, 1983). However, Hg+2 is readily available to plants, butunder reducing conditions and in presence of free sulfide ligands, mercury is stabilized in the Hg+2 stateas extremely insoluble sulfide mineral precipitates or is bound as surface complexes with organic mattercontaining sulfur. Methylation of Hg+2 in natural environments leads to formation of volatile organiccomplexes that are several times more bioavailable than inorganic forms of mercury. Methylatedmercury species are also one of the most toxic pollutants in the biosphere. Natural production ofmethyl mercury occurs under anoxic conditions and is probably mediated by microbes, but methylationof Hg+2 is inhibited by elevated sulfide contents (Kersten, 1988). Increased pH appears to increaseavailability of mercury to marsh plants possibly by causing conversion of Hg+2 to Hg0. Sediment canserve as a sink for mercury discharged to the environment; partition coefficients, between suspendedmatter and water, are usually in the range of 10 (Kersten, 1988). Sorption is the main process forenrichment of mercury in sediment. Sorption can be influenced by chloride ligand concentration due toformation of chloro-mercurial complexes; in seawater, only organic matter retains its sorption capacityfor mercury. Most particulate mercury in natural aquatic environments is associated with humic andother organic materials as well as oxide and sulfide minerals. Several studies have shown that mercury isless bioavailable in sediment that is rich in organic matter (Luoma, 1989). Most mercury released frompoint sources during mining is bound to sulfide compounds and is relatively non-bioavailable. However,a large percentage of mercury becomes associated with organic complexes downstream from point

sources, possibly due to mobilization of mercury from pore fluids by humic acids. In natural water,suspended matter is the main transporting medium for mercury. In oxic sediment, most mercury isbound in unknown (complexed?) chemical forms that are readily susceptible to transformation, therebyaffecting its mobilization and bioavailability.


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` Molybdenum is an essential element for many animals and

plants as it is required in their enzyme system. Molybdenum

can be present in molybdate anions, MoO4-2, in soil where it

can be mobile and bioavailable, because it is geochemically

similar to sulfate. Molybdate ion is often associated with iron

oxyhydroxide minerals, where it competes with phosphate and

organic matter. Molybdenosis in animals is associated with soil

that contains large amounts of available molybdenum,especially in forage plants with low sulfur and copper contents

(Neuman and others, 1987).


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` In the arid western United States, a number of environmentalproblems are related to high selenium abundances. These problemsare mainly due to irrigation practices that allow selenium toaccumulate in drains, reservoirs, and wetlands. Under theseconditions, selenium can be bioavailable to plants and birds and

accumulate to toxic levels (Severson and Gough, 1992). Extensivework has been done by the U.S. Geological Survey to determine thegeochemistry of selenium, most of which is associated with(adsorbed on) oxide minerals, such as goethite (Balistrieri and Chao,1987), amorphous iron oxyhydroxide and manganese oxideminerals (Balistrieri and Chao, 1990), and is relatively immobile. A

chemical leach using 0.1 M KH2P

O4 can be used to determine thesoluble-available selenium (mainly as SeO4-2 ) content of soil with

low oxide mineral contents and high pH (Chao 4 and Sanzolone,1989).


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` In slightly basic, anoxic marsh sediment environments, zinc iseffectively immobilized and not bioavailable (Gambrell and others,1991). Substantial amounts of zinc are released to solution if thissediment is oxidized or exposed to an acidic environment. Veryhigh abundances of soluble zinc are present under well oxidizedconditions and at pH 5 to 6.5, whereas low abundances of solublezinc are present at pH 8 under all redox conditions and at pH 5 to6.5 under moderately and strongly reducing conditions (Gambrelland others, 1991). In polluted river environments, most zinc isscavenged by non-detrital carbonate minerals, organic matter, and

oxide minerals and is less mobile than cadmium (and perhaps lessmobile than lead) (Prusty and others, 1994). Elevated chloridecontents decrease adsorption of zinc on sediment (Bourg, 1988).


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Once metals are introduced and contaminate the environment, they will remain.

Metals do not degrade like carbon-based (organic) molecules. The only exceptions are

mercury and selenium, which can be transformed and volatilized by microorganisms.

However, in general it is very difficult to eliminate metals from the environment.


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` Traditional treatments for metal contamination in soilsare expensive and cost prohibitive when large areas of

soil are contaminated. Treatments can be done in situ(on-site), or ex situ(removed and treated off-site). Bothare extremely expensive. Some treatments that areavailable include:

1. High temperature treatments (produce a vitrified,granular, non-leachable material).

2. Solidifying agents (produce cement-like material).3. Washing process (leaches out contaminants).


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` Soil and crop management methods can help preventuptake of pollutants by plants, leaving them in the soil. The soil becomes the sink, breaking the soil-

plantanimal or human cycle through which the toxinexerts its toxic effects (Brady and Weil, 1999). Thefollowing management practices will not remove theheavy metal contaminants, but will help to immobilize

them in the soil and reduce the potential for adverseeffects from the metals Note that the kind of metal(cation or anion) must be considered:


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` Cationic metals are more soluble at lower pH levels, so

increasing the pH makes them less available to plants andtherefore less likely to be incorporated in their tissues and

ingested by humans. Raising pH has the opposite effect

on anionic elements


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` Drainage improves soil aeration and will allow metals to

oxidize, making them less soluble. Therefore when

aerated, these metals are less available. The opposite istrue for chromium, which is more available in oxidized

forms. Active organic matter is effective in reducing the

availability of chromium.


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` Heavy phosphate applications reduce the availability ofcationic metals, but have the opposite effect on anionic

compounds like arsenic. Care should be taken with

phosphorus applications because high levels of

phosphorus in the soil can result in water pollution.


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` Plants translocate larger quantities of metals to their

leaves than to their fruits or seeds. The greatest risk of

food chain contamination is in leafy vegetables likelettuce or spinach. Another hazard is forage eaten by

livestock.


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` Plants have been used to stabilize or remove metals from

soil and water. The three mechanisms used arephytoextraction, rhizofiltration, and phytostabilization. Thistechnical note will define rhizofiltration and

phytostabilization but will focus on phytoextraction.


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` Rhizofiltration is the adsorption onto plant roots or absorptioninto plant roots of contaminants that are in solutionsurrounding the root zone (rhizosphere). Rhizofiltration isused to decontaminate groundwater. Plants are grown ingreenhouses in water instead of soil. Contaminated water

from the site is used to acclimate the plants to theenvironment. The plants are then planted on the site ofcontaminated ground water where the roots take up the waterand contaminants. Once the roots are saturated with thecontaminant, the plants are harvested including the roots. In

Chernobyl, Ukraine, sunflowers were used in this way toremove radioactive contaminants from groundwater (EPA,1998).


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` Phytostabilization is the use of perennial, non-harvestedplants to stabilize or immobilize contaminants in the soil

and groundwater. Metals are absorbed and accumulated

by roots, adsorbed onto roots, or precipitated within the

rhizosphere. Metal-tolerant plants can be used to restore vegetation where natural vegetation is lacking, thus

reducing the risk of water and wind erosion and leaching.


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` Phytostabilizationreduces the mobility of the contaminantand prevents further movement of the contaminant into

groundwater or the air and reduces the bioavailability forentry into the food chain.


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Phytoextraction is the process of growing plants in metal contaminated soil.Plant roots then translocate the metals into aboveground portions of the plant. Afterplants have grown for some time, they are harvested and incinerated or composted torecycle the metals.


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` Phytoextraction is done with plants calledhyperaccumulators, which absorb unusually largeamounts of metals in comparison to other plants.Hyperaccumulators contain more than 1,000 milligrams

per kilogram of cobalt, copper, chromium, lead, ornickel; or 10,000 milligrams per kilogram (1 %) ofmanganese or zinc in dry matter (Baker and Brooks,1989). One or more of these plant types are planted at aparticular site based on the kinds of metals present andsite conditions. Tables 2 and 3 demonstrate theimportance of using hyperaccumulators.


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` Phytoextraction is easiest with metals such as nickel, zinc, and copperbecause these metals are preferred by a majority of the 400hyperaccumlator plants. Several plants in the genus Thlaspi (pennycress)have been known to take up more than 30,000 ppm (3%)of zinc in theirtissues. These plants can be used as ore because of the high metalconcentration (Brady and Weil, 1999). Of all the metals, lead is the most

common soil contaminant (EPA, 1993). Unfortunately, plants do notaccumulate lead under natural conditions. A chelator such as EDTA(ethylenediaminetetraacetic acid) has to be added to the soil as anamendment. The EDTA makes the lead available to the plant. The mostcommon plant used for lead extraction is Indian mustard (Brassisa juncea).Phytotech (a private research company) has reported that they havecleaned up leadcontaminated sites in New Jersey to below the industrialstandards in 1 to 2 summers using Indian mustard (Wantanabe, 1997).


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` Plants are available to remove zinc, cadmium, lead, selenium,and nickel from soils at rates that are medium to long-term,but rapid enough to be useful. Many of the plants that hyperaccumulate metals produce low biomass, and need to be bredfor much higher biomass production. Current genetic

engineering efforts at USDA in Beltsville, MD, are aimedtoward developing pennycress (Thlaspi) that is extremely zinctolerant. These taller-thannormal plants would have morebiomass, thereby taking up larger quantities of contaminatingmetals (Watanabe, 1997). Traditional cleanup in situmay cost

between $10.00 and $100.00 per cubic meter (m3), whereasremoval of contaminated material (ex situ) may cost as high$30.00 to $300/ m3. In comparison, phytoremediation mayonly cost $0.05/ m3 (Watanabe, 1997).

envireport_bioavailability and toxicity of metals

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