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Behavioural properties of trace metals in soils
Applied Geochemistry. Suppl. Issue No.2. pp. 3-9,1993
0883-2927/93 $6.00 + .00
© 1992 Pergamon Press Ud
Printed in Great Britain.
Behavioural properties of trace metals in soils ALINA KABATA-PENDIAS
Institute of Soil Science and Cultivation of Plants (I. U .N.G.), 24-100 Pulawy, Poland Abstract-The speciation and spatial distribution of trace metals in soils are closely related to their behaviour in multiphase and dynamic soil systems. The surface reactions at soil-solution-root interfaces are the key to the determination of the amounts of mobile and plant-available metals. The behaviour of trace metals in the soil environment depends to some extent on their origin. Lithogenic metals are associated mainly with primary minerals , and may be phytoavailable under specificconditions in the soilplant system. Anthropogenic metals show great diversity, but in most cases they are likely to be very mobile and easily available to plants. Forms of the metals of both origins may be transformed due to pedogenic processes and they become "pedogenic" metals, greatly controlled by soil properties. In order to assess the phytoavailability and to accept proper guidelines for permissible trace metal loading, their behaviour in the specificsoil environments has to be defined .
pedogenic ongms of metals are difficult to distinguish . However, the association of trace metals with soil minerals can reflect their origin , and is an important factor in their distribution and behaviour. The distribution of trace metals among primary minerals separated from the soil (cambisol) varies within the soil horizons (Fig. 1). Minerals from the surface horizon of the soil are richer in Cu and Cr than are the same minerals from the parent rock . In contrast, Co, Ni and V are more concentrated in minerals from the parent rock than from the top soil. This may reflect a higher mobility of Cr and Cu under weathering conditions. In the dynamic chemical system of the soil, mineral transformations are continuous . The degree of weathering of parent material is likely to influence the metal content of the soils. In most soils quartz is the predominant (up to 70%) constituent of the sand and silt fractions (20-200 f.A.). Feldspars comprise -20%, and the heavy minerals <5% . Thus , the behaviour of trace metals associated with quartz and feldspars is likely to have a significant influence on their mobility behaviour in most soils. It is still an open question as to how trace metals are likely to form and exist in mineral compounds (KABATA-PENDIAS and BRUEMMER, 1991). It seems more likely that these metals are bound in soil minerals by isomorphic substitution or by fixation on free structural sites. The adsorption capacity of some soil components for trace metals is high, and therefore a considerable amount of metals can be bound before the formation of definite metal compounds takes place, e.g. phosphates, carbonates, sulfites, sulfates (LINDSAY, 1979). Trace metals associated with primary minerals are normally available to plants , provided that plants get an adequate supply of major nutrients from other sources . Contents of trace metals in clover grown on the surface layer of the soil and on parent rock were fairly similar . This indicates that root exudates can mobilize micronutrients from relatively insoluble
INTRODUCTION
SOIL, as a part of the terrestrial ecosystem, plays a significant role in element cycling. It has important functions as a filter and a buffer, and also as a storage and transformation system, supporting a homeostatic interrelationship between the biotic and abiotic environments. Behaviour of trace metals in soils depends upon complex reactions between trace cations and different components of the various soil phases: solid , aqueous and gaseous . Soil, as a chemical system is in semi-equilibrated or short-term-equilibrium state (SPARKS, 1986). The main features of the soil biogeochemical system are: (1) heterogeneous distribution of compounds and components; (2) seasonal and spatial alteration of major soil variables; (3) transformation of element species; (4) transfer between phases; and (5) bioaccumulation. Present-day soils contain trace elements of various origins. Lithogenic elements are those which are directly inherited from the lithosphere (parent material). Anthropogenic elements in soils are all those deposited into soils as direct or indirect results of human activities . Pedogenic elements are of lithogenic and anthropogenic origins but their distribution in soil horizons and soil particles are changed due to mineral transformation and other pedogenic processes. Thus, the behaviour of trace elements is highly related to their speciation and is likely to be governed by their origin.
LITHOGENIC METALS
The trace metal composition of the soil is primarily inherited from the parent material. The mobility of these elements during weathering is determined first by the stability of the host minerals, and second by their electrochemical properties. Lithogenic and 3
4
A. Kabata-Pendias
Soil borizor« :
r:::::J
~C
A
200
Cr
100
V
01
~ 50
Co
Cu
01
Ni
E
"5'" ...
.~ 70 E .5: 5
'" "0
~
'"... "0
~ .c.
... '" 0 Q. '" :!:! ~
!:! Ii
5-
~
... 0'"
... Q. E :!:!'" 1;
~
e:
oS!
"
0-
~
.~ E .c.
rr ... 0'"
Q.
'" :!:! oS!
..
.!!! 0
.
.~
"
.c.
!:! 0
0-
E
r ...'"
0
Q.
:!:!'" oS!
!:!
s...
l;
.~
0-
.c.
"
E
...'"0
s
!:!
~
::l 0"
:!:!
l;
FIG. 1. Association of trace metals in primary minerals (heavy minerals, mainly Fe oxides and pyroxene; feldspars ; quartz) separated from grain-size fraction 20-200 p of two horizons of the cambisol derived from basalt (KABATA-PENDIAS, 1966).
compounds, e.g., from primary minerals of granite and basalt (KABATA-PENDIAS, 1971). All lithogenic trace metals form a pool of relatively immobile elements. However, they are likely to be transformed into mobile species under a change of soil conditions, and by the activity of root exudate.
PEDOGENIC METALS
Soils consist of an heterogeneous mixture of different organic and organo-mineral substances, clay minerals, (hydrous) oxides of AI, Fe and Mn, and other solid components, as well as of a variety of soluble substances. The binding mechanisms for trace metals are therefore complex and vary with the composition of the soil, soil acidity and redox conditions. The complexity of all possible reactions in natural heterogeneous soil systems reflects various soil processes which can be generalized as follows: (1) dissolution ; (2) sorption; (3) complexation; (4) migration ; (5) precipitation; (6) occlusion; (7) diffusion; (8) binding by organic substances ; (9) absorption and sorption by microbiota; and (10) volatilization. All these processes are governed by several soil properties of which pH and redox potential are known to be the most important parameters. Figure 2 illustrates the variation in metal sorption on humic acid as affected by solution pH of the media. The sorption of most metals greatly increases with increasing pH of the solution, while Hg and Mn are little affected. Due to a great chemical variability in natural soil organic compounds it is difficult to describe an equilibrium constant of trace metal cations between organic matter and external solutions (GAMBLE, 1986). The activity of trace metals in relation to the soil pH is
known to be modified by the amount and type of organic matter. Several metal-organic complexes are easily soluble at pH values above 6 and 7 (BRUEMMER et al., 1986; KABATA-PENDIAS and PENDIAS, 1992). Both the distribution and phytoavailability of pedogenic trace metals are influenced by the specific adsorption of metals on various soil constituents, in particular hydrous oxides of Fe and Mn. The pedogenic formation of these oxides seems to be a significant factor governing the distribution of metals in solid phase in the soil profile (Fig. 3). The concentration of Cu, Co, Zn, Ni, and Pb in Fe-Mn nodules and concretions was much higher than in organic matter and in the biomass of Bacillus megaterium grown in a soil extract. Metals fixed by Fe and Mn oxides are slightly mobile, and may be unavailable to plant roots. Such an immobilization of metals is observed at the root-soil interface zone where these oxides are precipitated under specific soil condition (HILLER et al., 1988). The concentration of trace metals in the Fe-plaque formed on roots may also be a source of the excess of metals as compared to the surrounding soil medium (Orrs et al. , 1987). Amounts of clay minerals in most soils range from 10 to 30% of mineral constituents. Their influence on trace metal behaviour is significant, especially due to the high sorption capacity for metals (ADRIANO, 1986; KABATA-PENDIAS and PENDIAS , 1992). The solubility of metals fixed by various minerals may considerably differ, but in most cases increases in a sequence from kaolinite and biotite , to montmorillonite and freshly precipitated Fe-AI oxides (Table 1). Different analytical procedures involving successive extractions have been developed to measure the speciation of metals. The distribution patterns of
Behavioural properties of trace metals in soils
5
100
Zn Ni
..•••Co .. o· .00
..
.....
00·
a Cr
0··
00 •• 0
.....
Mn 5.8
3.7
2.4
pH of sol ution
FIG. 2. Effects of pH on the sorption of metals on humic acid (GAMBLE, 1986). Sorption is given in percentage of initial concentrations of metals, 0.5 x 10- 4 mole of each metals in 100 ml,
metal species in soils vary widely, and are comparable only when the same method is applied. Figure 4 illustrates different measurements of Cd species in ground water of podzol, and in a solution of the soil amended with sewage sludge. In ground water, 39% of the Cd occurs as the cationic species, while in the solution of the sludge-amended soil 95% of the Cd is in the cationic form (ALLOWAY et al., 1984; Fie, 1987). Depending upon the variability in physicochemical characteristics of metals, their affinity to soil components governs their speciation (Fig. 5).
_
Fe
Easily mobile metals (Zn and Cd) exist mainly as organically bound, exchangeable, and water soluble species, while slightly mobile metals (Pb, Ni and Cr) are mainly bound in silicates (residual fraction); Cu and Mo predominate in organically bound and exchangeable species. Their stability is, however, strongly influenced by changing soil environmental conditions. Soluble, exchangeable and chelated metal species are the most mobile fraction of metals in soils (Table 2). Root exudates are known to be very active in the ~ Organic mottw ( HA &FA J
nodules a
Biomoss b
~ Mn nodules _
Fe - Mn conc rectians
!f 1000 <,
! VI
c ~
~
0
u
"5VI
100
!; ~
s.. ~
10
Cu
Co
Zn
Ni
Pb
FIG. 3. Distribution of trace metals among soil components (KABATA-PENDIAS and PENDlAS, 1992). (a) Data for Pb are given for Fe-rich soil spots. (b) Biomass of Bacillusmegateriumgrowing in soil extract.
6
A. Kabata-Pendias Table 1. Relative retention and solubility of Co and Cu from minerals" (KABATAPBNDIAS, 1973) Retention and solubility of metalst (%) Co Saturated Mineral Biotite Kaolinite Montmorillonite Fe-AI oxides
Cu Blank
Saturated
Blank
R
S
S
R
S
S
35 2 25 n.d.
8 14 44 47
1 5 1 16
58 3 29 n.d.
4 16 90 62
5 15 86 10
R = retention, percent of the added amount; S = soluble, percent of the total content (fixed and initial amounts). "Mineral saturated with metal chlorides at the concentration of 100 mgll, in solution 0.1 N CaCI2 , at sample to solution ratio 1:20. tMetals released from saturated minerals into the nutrient solution through the cellulose dialysis membrane of porous size 20-80 A.
absorption of trace metals existing in different (even slightly soluble) species in soils. The composition of root exudates varies with plant species and plant varieties, microorganism association, and the condition in which the plant grows. Roots can also develop mechanisms of metal avoidance due to different kinds of physiological barriers (NUORTEVA, 1990). Because the laboratory chemical simulation of root exudates is very difficult, the key for solving the chemical and biological factors controlling the bioavailability of metals has to be found in their mobile fraction (KABATA-PENDIAS and PENDIAS, 1992; VERLOO and EEcKHouT, 1990). Evaluation of the availabilityof metals should alwaysbe related to their
~11~~~~~~~~Cd-CI ~ Cd-SO,
Mo Zn
........
Cd Cu Pb Ni
f-:'-:'-';--;-~:-':-'::,-:",-:"'-;-'-;'-+-:--;-:'-':-':-'
Cr o 15 SpecieS 01 metals :
o
~
1\11
so
roo %
7S
residual
R
exchangeab le
bound OM
•
eaSily soluble
associated with OXides I Fe ,M n }
FIG. 5. Speciation of trace elements in soils (in percent of total content). Data for Mo are for the chernozem (CUMAKOV, 1988), and all other metals for the podzolic loamy sand (DuDKA et al., 1990).
content in both solid and aqueous phases, as well as to the rate of metal solubility, e.g. the transfer from the solid to the aqueous phase (BRUEMMER et al., 1986). 22% 0.5%
ANTHROPOGENIC METALS
B d- cal. 95 %
Cd-argo 1.7%
Cd-neulr. 2.5 %
FIG. 4. Relative distribution of Cd species in the soil aqueous phase as determined by different methods. (A) In ground water of the forest podzol, by GEOCHEM method (Fie, 1987). (B) In solution ofthe soil amended with sewage sludge, by ion exchange procedure (ALLOWAY et al., 1984).
Anthropogenic trace metals enter the soils by a variety of pathways: (1) aerial deposition; (2) pesticide and fertilizer application; (3) waste utilization; (4) dredged sediment disposal; and (5) river and irrigation waters. The speciation and distribution of anthropogenic metals in soils are related to their chemical forms at the time of impaction. Aerial particles transporting trace metals are most commonly in the form of oxides, silicates, carbonates, sulfates and sulfides (when originating from coal combustion a glassy structure of metal compounds predominates). Metals entering soils with plant residues are organically bound or chelated, and those entering with sewage sludges differ with the sources and treatments of the wastes. In dredged sediments,
Behavioural properties of trace metals in soils
7
Table 2. Distribution and phytoavailability of metals in soils Metal classification and association
Metal species
Soil phase
Phytoavailability
Bound inside mineral particles Bound inside organic substances Precipitated compounds
Solid-mineral Solid-organic
Slight when decomposed (after weathering) Slight when decomposed
Solid-mineral and organic
Slight when dissolved
Exchangeable and chelated
Solid-mineral and organic
Moderate
Simple cations, organic and inorganic complex cations
Aqueous
Good
L: Primary and secondary minerals P: Organic matter, especially humic acids P: Concretions of oxides and clay minerals P and A: Particle surface and low molecular weight organic compounds A and P: Soil solution
Trace metal classification: L
= lithogenic; P = pedogogenic; and
metals are likely to be fixed by organic substances, clay minerals, and Fe-Mn-Al oxides. Thus, anthropogenic metals may form different species in soils, depending upon reactant surface and external binding sites with different bonding energy. The speciation of trace metals changes with soil depth. In a temperate climate, especially, metals often migrate with the water percolating through the soil. However, the input-output budget of trace metals in the top horizons of most soils in Europe is positive, which clearly indicates a higher input than output for metals (KABATA-PENDIAS and PENDIAS,
A
= anthropogenic.
1992). Increased leaching over the deposition of metals has been reported for acid forest soils in the Scandinavian countries (BERGKVIST et al., 1989; TYLER et al., 1987) and in Germany (MAYER and SCHULTZ, 1987). The concentration of trace meals in soil solutions is a good index of the mobile pool of metals in soils. Any chemical stress in soils is reflected in variations in the trace metal content of the soil solutions (Fig. 6). The behaviour of anthropogenic trace metals, as reflected in their speciation and phytoavailability, depend upon their forms or the compounds added to
500000
....... ......
....
.............
........................····COs
;:100000
•
~50000
.V' 'Co .: rJOOO
i 5000 Q.
§ ~ 1000
!
5OO.1--,---------:~-------:-r:__---+-----.
e
s 5000 j
'""
50
------------------------_.,
Cl
.
o
s
1000
!
500
f
1(}()
50
... """ ........
--
..................~-----------. ............ .."""....",.".. --.
.........
-- -- -
p " .. , /B .~ FIls .. -, Zns ./ "'.Mns - - - - F ep
~._._._.--'"
"'-'''_
Mnp '''-'''-Znp
10"'--~-----_,__-----r---t_--~
o
50 Doses 01 cool osh
100
400
Ifl ho I
FIG. 6. Impact of coal ash added to podzolic loamy sand on the concentration of Ca, Mg and Na, and on some trace elements in the soil solution and ryegrass (Lotium perenne L) (KABATA-PENDIAS et al., 1987). p = plant, s = soil solution.
A. Kabata-Pendias
8
BARLEY
CU
20
Zn
700
a
300 200
_a ------
-- ----pP
fOO
SOO
1000
0
lSOO
OATS
,0
,"
lSOO
Ni
, ,,
30
20
300
150
30
o
- ------p
- - _ . - - --p
p or::::==:::;====!!.-fOO 300 SOO
20
lII.tals in soil {mgllrg 1
- - - - grain FIG .
-straw
7. Metal uptake by barley and oats as influenced by their origin (CHLOPECKA , 1991; GRUPE and KUNTZE, 1987 and 1988). a = anthropogenic metals, p = pedogenic metals.
the soil. There is always a significant correlation between the concentration of metals in plants and their mobile species in soils. Because anthropogenic trace metals are easily mobile under most soil conditions their phytoavailability is higher than that of lithogenic and pedogenic metals (Fig. 7). However, when the solubility of metals is inhibited due to pollution effects, as for example after the addition of coal ash, their uptake by plants also decreases (Fig. 6). The importance of an anthropogenic origin of trace metals in terms of their mobility and availability is particularly important for Cd, Cu, Zn and Ni. Other metals such as Cr, Mn and Pb seem to be influenced less by their origin but it varies with variable conditions of the environment (CHLOPECKA, 1991; KABATA-PENDIAS and PENDIAS, 1992). Detailed information about the specific behaviour of anthropogenic metals from different sources of soil-solution-root interfaces is needed for a precise evaluation of their phytoavailability.
MODELLING OF TRACE METAL REBAVIOUR IN SOILS
Chemical modelling of trace metal equilibria in soil systems has been proposed for predicting their behaviour . Modelling programs are based on calculations of chemical equilibrium (MAITIGOD et al., 1985; Srosrro et al., 1984). Thermodynamic data can predict the final state of a system and give valuable but limited information on the mechanisms of chemical reactions in the complex multiphase soil system
(SPARKS, 1986). However, there are a number of problems with present models of trace element mobility in soil. Solubility equilibria may change during short time periods and within a few centimeters in the soil. In addition, all reactions related to the surface chemistry of root cells and of microbiota (including the rhizosphere) for root-solution and root-solids interfaces are not included in the applied models. The ability of roots to take up trace metals is based on the interactions with soil components including processes of changing Eh and pH, cation exchange and organic complexing. Despite these problems, chemical equilibrium models may be useful in an attempt to predict the mobility and phytoavailability of trace metals in soil systems.
SUMMARY
Trace metal behaviour in soils can be viewed in terms of reactions within solid, aqueous and gaseous phases . Processes at soil-solution-roots interface are fundamental to the understanding of trace metal interactions in multicomponent dynamic soil systems. The behaviour of trace metals in soils is related to their origin and forms. Lithogenic trace metals should be considered as slightly mobile but potentially available to plants under specific conditions of the soil-plant system. Behaviour of pedogenic metals reflects specific soil conditions . Anthropogenic metals are generally more mobile than lithogenic and pedogenic metals. However, all soil processes control the speciation and spatial distribution of trace metals from all sources. Therefore, the mobility and
Behavioural properties of trace metals in soils
phytoavailability of these metals should always be related to a given soil unit with definite properties and characteristics. Editorial handling : Brian Hitchon.
REFERENCES ADRIANO D. C. (1986) Trace Elements in the Terrestrial Environment. Springer. ALLOWAY B. J. , TILLS A. R. and MORGAN H. (1984) The speciation and availability of cadmium and lead in polluted soils. In Trace Substances in Environmental Health (ed. D. D. HEMPHILL), Vol. 18, pp. 187-201 . University of Missouri. BI!RGKVIST B., FOLKESON L. and BERGGRI!N D. (1989)Fluxes of Cu, Zn, Pb, Cr, and Ni in temperate forest ecosystems. Water Air Soil Pollut. 49, 217-222. BRUEMMER G. W., GERTH J. and HERMS U. (1986) Heavy metal species, mobility and availabilityin soil Z. Pfianzenernahr. Bodenkunde 149, 382-398. CHLOPECKA A. (1991)Impact of different compounds of Cd, Cu, Pb, and Zn on their speciation in soil and their contents of cereal plants. Manuscript in Polish. CuMAKOV A. (1988)Trace Elements in Slovakian Soils and Plant Nutrition. Ph.D. thesis. Institute of Sugar Industry, Bratislava, Czechoslovakia. DUDKA S., PIOTRoWSKA M., CHLOPECKA A. and LOPATEK J. (1990) Speciation, Mobility and Phytoavailability of Trace Metals in a Sewage Sludge Amended Soil. Rept. 270, lUNG, Pulawy, Poland. FIC M. (1987)Adsorptions- und Desorptions-Verhalten von Cadmium, Chromium, Kupfer und Zind an ausgewahlten Boden und Sanden. Ph.D . thesis. Christian-AlbrechtsUniversitat, Kiel, Germany. GAMBLE D. S. (1986) Interaction between natural organic polymers and metals in soil and fresh water systems: equilibria. In The Importance of Chemical Speciation in Environmental Processes (eds M. BRINCKMAN and P. J. SADLER), pp. 217-236. Springer. GRUPE M. and KUNTZE H. (1987) Zur Ni-Mobilitat einer geogen belasten Braunerde . Mitt. Dtsch . Bodenkundl. Ges. 55, 333-338. GRUPE M. and KUNTZE H. (1988) Zur Ermittlung der Schwermetallverfiigbarkeit lithogen und anthropogen belasteter Standorte. 1. Cd und Cu. Z. Pfianzenernahr. Bodenkunde. 151,319-324. HILLER D. A., BRUEMMER G. W. and ACKERMAND D. (1988) Gehalte an Haupt-und Nebenelementen in Konkretionen aus Oberboden von Marsche-Untersuchungen mit einer Midrosonde. Z. Pfianzenemiihr, Bodenkunde 151, 47-54. KABATA-PENDIAS A. (1966) Geochemistry of soils derived
9
from granites and basalts of Lower Silesia, Poland. Roczn . Nauk Roln . 9O-A-l , 1-60 (in Polish). KABATA-PENDIAS A. (1971)Micronutrient uptake by clover from different soil horizons. Pam . Pul. 45, 127-145 (in Polish). KABATA-PENDIAS A. (1973)Uptake of cobalt and copper by clover from minerals impregnated by these cations. Roczn. Glebozn. 24,273-287. KABATA-PENDIAS A. and BRUEMMER G. H. (1991) Ecological consequences of As, Cd, Hg, and Pb enrichment in European soils. In Global Perspectiveson Lead , Mercury and Cadmium Cycling in the Environment (eds T. C. HUTCHINSON, C. A. GORDON and K. M. MEEMA), pp. 107-114.SCOPE, Wiley Eastern. KABATA-PENDIAS A. and PENDIAS H. (1992) Trace Elements in Soils and Plants (2nd Edition). CRC Press. KABATA-PENDIAS A. , PIOTROWSKA M. and WIACEK K. (1987) Impact of coal ashes on soils and plants. Arch. Ochrony Srod . 1-2,97-104 (in Polish). LINDSAY W. L. (1979) Chemical Equilibria in Soils . WileyInterscicnce. MATTIGOD S. V., SPOSITO G. and PAGE A. L. (1985) Factors affectingthe solubilitiesof trace metals in soils. In Chemistry in the Soil Environment (ed. M. STELLY) , Chap. 11, pp. 203-221. Soil Sci. Soc. Am. MAYER R. and SCHULTZ R. (1987) Effects of soil acidification on heavy metal cycling in forest ecosystems. In Heavy Metals in the Environment (eds S. E. LINDBERG and T. C. HUTCHINSON), Vol. 2, pp. 402-406. CEP Consultants, Edinburgh, U.K. NUORTEVA P. (1990) Metal Distribution Patterns and Forest Decline. Seeking Achilles Heels for Metals in Finnish Forest Biocoenoses. University of Helsinki. Orrs M. L. , Buns E. P., RIEMER L., ROZEMA J. and BROEKMAN R. A. (1987) The iron-plaque on the roots of
saltmarsh plants: a barrier to heavy metal uptake? In Heavy Metals in the Environment (eds S. E. LINDGERG and T. C. HUTCHINSON), Vol. 1, pp. 407-409. CEP Consultants, Edinburgh, U.K. SPARKS D. L. (1986) Kineticsof reactions in pure and mixed systems. In Soil Physical Chemistry (ed. D. L. SPARKS) , Chap. 3, pp. 83-145. CRC Press. SPOSITO G., LECLAIRE J. P., LEVESQUE S. and SENESI N. (1984) Methodologies to Predict the Mobility and Availability of Hazardous Metals in Sludge-Amended Soils.
University of California. TYLER G., GERGGREN D., BERGKVJST B., FALKENGRENGRERUP U., FOLKESON L. and RUHLlNG A. (1987) Soil acidification and metal solubility in forests of southern Sweden. In Effects of Atmospheric Pollutants on Forest, Wetlands and Agricultural Ecosystems (eds T. C. HUTCHINSON and K. C. MEEMA) , NATO AS Series, Vol. G16, pp. 347-359. Springer. VERLOO M. and EECKHOUT M. (1990) Metal species transformation in soils: an analytical approach. Int. J. Env iron. Anal. Chem. 39, 179-186.
0883-2927/93 $6.00 + .00
© 1992 Pergamon Press Ud
Printed in Great Britain.
Behavioural properties of trace metals in soils ALINA KABATA-PENDIAS
Institute of Soil Science and Cultivation of Plants (I. U .N.G.), 24-100 Pulawy, Poland Abstract-The speciation and spatial distribution of trace metals in soils are closely related to their behaviour in multiphase and dynamic soil systems. The surface reactions at soil-solution-root interfaces are the key to the determination of the amounts of mobile and plant-available metals. The behaviour of trace metals in the soil environment depends to some extent on their origin. Lithogenic metals are associated mainly with primary minerals , and may be phytoavailable under specificconditions in the soilplant system. Anthropogenic metals show great diversity, but in most cases they are likely to be very mobile and easily available to plants. Forms of the metals of both origins may be transformed due to pedogenic processes and they become "pedogenic" metals, greatly controlled by soil properties. In order to assess the phytoavailability and to accept proper guidelines for permissible trace metal loading, their behaviour in the specificsoil environments has to be defined .
pedogenic ongms of metals are difficult to distinguish . However, the association of trace metals with soil minerals can reflect their origin , and is an important factor in their distribution and behaviour. The distribution of trace metals among primary minerals separated from the soil (cambisol) varies within the soil horizons (Fig. 1). Minerals from the surface horizon of the soil are richer in Cu and Cr than are the same minerals from the parent rock . In contrast, Co, Ni and V are more concentrated in minerals from the parent rock than from the top soil. This may reflect a higher mobility of Cr and Cu under weathering conditions. In the dynamic chemical system of the soil, mineral transformations are continuous . The degree of weathering of parent material is likely to influence the metal content of the soils. In most soils quartz is the predominant (up to 70%) constituent of the sand and silt fractions (20-200 f.A.). Feldspars comprise -20%, and the heavy minerals <5% . Thus , the behaviour of trace metals associated with quartz and feldspars is likely to have a significant influence on their mobility behaviour in most soils. It is still an open question as to how trace metals are likely to form and exist in mineral compounds (KABATA-PENDIAS and BRUEMMER, 1991). It seems more likely that these metals are bound in soil minerals by isomorphic substitution or by fixation on free structural sites. The adsorption capacity of some soil components for trace metals is high, and therefore a considerable amount of metals can be bound before the formation of definite metal compounds takes place, e.g. phosphates, carbonates, sulfites, sulfates (LINDSAY, 1979). Trace metals associated with primary minerals are normally available to plants , provided that plants get an adequate supply of major nutrients from other sources . Contents of trace metals in clover grown on the surface layer of the soil and on parent rock were fairly similar . This indicates that root exudates can mobilize micronutrients from relatively insoluble
INTRODUCTION
SOIL, as a part of the terrestrial ecosystem, plays a significant role in element cycling. It has important functions as a filter and a buffer, and also as a storage and transformation system, supporting a homeostatic interrelationship between the biotic and abiotic environments. Behaviour of trace metals in soils depends upon complex reactions between trace cations and different components of the various soil phases: solid , aqueous and gaseous . Soil, as a chemical system is in semi-equilibrated or short-term-equilibrium state (SPARKS, 1986). The main features of the soil biogeochemical system are: (1) heterogeneous distribution of compounds and components; (2) seasonal and spatial alteration of major soil variables; (3) transformation of element species; (4) transfer between phases; and (5) bioaccumulation. Present-day soils contain trace elements of various origins. Lithogenic elements are those which are directly inherited from the lithosphere (parent material). Anthropogenic elements in soils are all those deposited into soils as direct or indirect results of human activities . Pedogenic elements are of lithogenic and anthropogenic origins but their distribution in soil horizons and soil particles are changed due to mineral transformation and other pedogenic processes. Thus, the behaviour of trace elements is highly related to their speciation and is likely to be governed by their origin.
LITHOGENIC METALS
The trace metal composition of the soil is primarily inherited from the parent material. The mobility of these elements during weathering is determined first by the stability of the host minerals, and second by their electrochemical properties. Lithogenic and 3
4
A. Kabata-Pendias
Soil borizor« :
r:::::J
~C
A
200
Cr
100
V
01
~ 50
Co
Cu
01
Ni
E
"5'" ...
.~ 70 E .5: 5
'" "0
~
'"... "0
~ .c.
... '" 0 Q. '" :!:! ~
!:! Ii
5-
~
... 0'"
... Q. E :!:!'" 1;
~
e:
oS!
"
0-
~
.~ E .c.
rr ... 0'"
Q.
'" :!:! oS!
..
.!!! 0
.
.~
"
.c.
!:! 0
0-
E
r ...'"
0
Q.
:!:!'" oS!
!:!
s...
l;
.~
0-
.c.
"
E
...'"0
s
!:!
~
::l 0"
:!:!
l;
FIG. 1. Association of trace metals in primary minerals (heavy minerals, mainly Fe oxides and pyroxene; feldspars ; quartz) separated from grain-size fraction 20-200 p of two horizons of the cambisol derived from basalt (KABATA-PENDIAS, 1966).
compounds, e.g., from primary minerals of granite and basalt (KABATA-PENDIAS, 1971). All lithogenic trace metals form a pool of relatively immobile elements. However, they are likely to be transformed into mobile species under a change of soil conditions, and by the activity of root exudate.
PEDOGENIC METALS
Soils consist of an heterogeneous mixture of different organic and organo-mineral substances, clay minerals, (hydrous) oxides of AI, Fe and Mn, and other solid components, as well as of a variety of soluble substances. The binding mechanisms for trace metals are therefore complex and vary with the composition of the soil, soil acidity and redox conditions. The complexity of all possible reactions in natural heterogeneous soil systems reflects various soil processes which can be generalized as follows: (1) dissolution ; (2) sorption; (3) complexation; (4) migration ; (5) precipitation; (6) occlusion; (7) diffusion; (8) binding by organic substances ; (9) absorption and sorption by microbiota; and (10) volatilization. All these processes are governed by several soil properties of which pH and redox potential are known to be the most important parameters. Figure 2 illustrates the variation in metal sorption on humic acid as affected by solution pH of the media. The sorption of most metals greatly increases with increasing pH of the solution, while Hg and Mn are little affected. Due to a great chemical variability in natural soil organic compounds it is difficult to describe an equilibrium constant of trace metal cations between organic matter and external solutions (GAMBLE, 1986). The activity of trace metals in relation to the soil pH is
known to be modified by the amount and type of organic matter. Several metal-organic complexes are easily soluble at pH values above 6 and 7 (BRUEMMER et al., 1986; KABATA-PENDIAS and PENDIAS, 1992). Both the distribution and phytoavailability of pedogenic trace metals are influenced by the specific adsorption of metals on various soil constituents, in particular hydrous oxides of Fe and Mn. The pedogenic formation of these oxides seems to be a significant factor governing the distribution of metals in solid phase in the soil profile (Fig. 3). The concentration of Cu, Co, Zn, Ni, and Pb in Fe-Mn nodules and concretions was much higher than in organic matter and in the biomass of Bacillus megaterium grown in a soil extract. Metals fixed by Fe and Mn oxides are slightly mobile, and may be unavailable to plant roots. Such an immobilization of metals is observed at the root-soil interface zone where these oxides are precipitated under specific soil condition (HILLER et al., 1988). The concentration of trace metals in the Fe-plaque formed on roots may also be a source of the excess of metals as compared to the surrounding soil medium (Orrs et al. , 1987). Amounts of clay minerals in most soils range from 10 to 30% of mineral constituents. Their influence on trace metal behaviour is significant, especially due to the high sorption capacity for metals (ADRIANO, 1986; KABATA-PENDIAS and PENDIAS , 1992). The solubility of metals fixed by various minerals may considerably differ, but in most cases increases in a sequence from kaolinite and biotite , to montmorillonite and freshly precipitated Fe-AI oxides (Table 1). Different analytical procedures involving successive extractions have been developed to measure the speciation of metals. The distribution patterns of
Behavioural properties of trace metals in soils
5
100
Zn Ni
..•••Co .. o· .00
..
.....
00·
a Cr
0··
00 •• 0
.....
Mn 5.8
3.7
2.4
pH of sol ution
FIG. 2. Effects of pH on the sorption of metals on humic acid (GAMBLE, 1986). Sorption is given in percentage of initial concentrations of metals, 0.5 x 10- 4 mole of each metals in 100 ml,
metal species in soils vary widely, and are comparable only when the same method is applied. Figure 4 illustrates different measurements of Cd species in ground water of podzol, and in a solution of the soil amended with sewage sludge. In ground water, 39% of the Cd occurs as the cationic species, while in the solution of the sludge-amended soil 95% of the Cd is in the cationic form (ALLOWAY et al., 1984; Fie, 1987). Depending upon the variability in physicochemical characteristics of metals, their affinity to soil components governs their speciation (Fig. 5).
_
Fe
Easily mobile metals (Zn and Cd) exist mainly as organically bound, exchangeable, and water soluble species, while slightly mobile metals (Pb, Ni and Cr) are mainly bound in silicates (residual fraction); Cu and Mo predominate in organically bound and exchangeable species. Their stability is, however, strongly influenced by changing soil environmental conditions. Soluble, exchangeable and chelated metal species are the most mobile fraction of metals in soils (Table 2). Root exudates are known to be very active in the ~ Organic mottw ( HA &FA J
nodules a
Biomoss b
~ Mn nodules _
Fe - Mn conc rectians
!f 1000 <,
! VI
c ~
~
0
u
"5VI
100
!; ~
s.. ~
10
Cu
Co
Zn
Ni
Pb
FIG. 3. Distribution of trace metals among soil components (KABATA-PENDIAS and PENDlAS, 1992). (a) Data for Pb are given for Fe-rich soil spots. (b) Biomass of Bacillusmegateriumgrowing in soil extract.
6
A. Kabata-Pendias Table 1. Relative retention and solubility of Co and Cu from minerals" (KABATAPBNDIAS, 1973) Retention and solubility of metalst (%) Co Saturated Mineral Biotite Kaolinite Montmorillonite Fe-AI oxides
Cu Blank
Saturated
Blank
R
S
S
R
S
S
35 2 25 n.d.
8 14 44 47
1 5 1 16
58 3 29 n.d.
4 16 90 62
5 15 86 10
R = retention, percent of the added amount; S = soluble, percent of the total content (fixed and initial amounts). "Mineral saturated with metal chlorides at the concentration of 100 mgll, in solution 0.1 N CaCI2 , at sample to solution ratio 1:20. tMetals released from saturated minerals into the nutrient solution through the cellulose dialysis membrane of porous size 20-80 A.
absorption of trace metals existing in different (even slightly soluble) species in soils. The composition of root exudates varies with plant species and plant varieties, microorganism association, and the condition in which the plant grows. Roots can also develop mechanisms of metal avoidance due to different kinds of physiological barriers (NUORTEVA, 1990). Because the laboratory chemical simulation of root exudates is very difficult, the key for solving the chemical and biological factors controlling the bioavailability of metals has to be found in their mobile fraction (KABATA-PENDIAS and PENDIAS, 1992; VERLOO and EEcKHouT, 1990). Evaluation of the availabilityof metals should alwaysbe related to their
~11~~~~~~~~Cd-CI ~ Cd-SO,
Mo Zn
........
Cd Cu Pb Ni
f-:'-:'-';--;-~:-':-'::,-:",-:"'-;-'-;'-+-:--;-:'-':-':-'
Cr o 15 SpecieS 01 metals :
o
~
1\11
so
roo %
7S
residual
R
exchangeab le
bound OM
•
eaSily soluble
associated with OXides I Fe ,M n }
FIG. 5. Speciation of trace elements in soils (in percent of total content). Data for Mo are for the chernozem (CUMAKOV, 1988), and all other metals for the podzolic loamy sand (DuDKA et al., 1990).
content in both solid and aqueous phases, as well as to the rate of metal solubility, e.g. the transfer from the solid to the aqueous phase (BRUEMMER et al., 1986). 22% 0.5%
ANTHROPOGENIC METALS
B d- cal. 95 %
Cd-argo 1.7%
Cd-neulr. 2.5 %
FIG. 4. Relative distribution of Cd species in the soil aqueous phase as determined by different methods. (A) In ground water of the forest podzol, by GEOCHEM method (Fie, 1987). (B) In solution ofthe soil amended with sewage sludge, by ion exchange procedure (ALLOWAY et al., 1984).
Anthropogenic trace metals enter the soils by a variety of pathways: (1) aerial deposition; (2) pesticide and fertilizer application; (3) waste utilization; (4) dredged sediment disposal; and (5) river and irrigation waters. The speciation and distribution of anthropogenic metals in soils are related to their chemical forms at the time of impaction. Aerial particles transporting trace metals are most commonly in the form of oxides, silicates, carbonates, sulfates and sulfides (when originating from coal combustion a glassy structure of metal compounds predominates). Metals entering soils with plant residues are organically bound or chelated, and those entering with sewage sludges differ with the sources and treatments of the wastes. In dredged sediments,
Behavioural properties of trace metals in soils
7
Table 2. Distribution and phytoavailability of metals in soils Metal classification and association
Metal species
Soil phase
Phytoavailability
Bound inside mineral particles Bound inside organic substances Precipitated compounds
Solid-mineral Solid-organic
Slight when decomposed (after weathering) Slight when decomposed
Solid-mineral and organic
Slight when dissolved
Exchangeable and chelated
Solid-mineral and organic
Moderate
Simple cations, organic and inorganic complex cations
Aqueous
Good
L: Primary and secondary minerals P: Organic matter, especially humic acids P: Concretions of oxides and clay minerals P and A: Particle surface and low molecular weight organic compounds A and P: Soil solution
Trace metal classification: L
= lithogenic; P = pedogogenic; and
metals are likely to be fixed by organic substances, clay minerals, and Fe-Mn-Al oxides. Thus, anthropogenic metals may form different species in soils, depending upon reactant surface and external binding sites with different bonding energy. The speciation of trace metals changes with soil depth. In a temperate climate, especially, metals often migrate with the water percolating through the soil. However, the input-output budget of trace metals in the top horizons of most soils in Europe is positive, which clearly indicates a higher input than output for metals (KABATA-PENDIAS and PENDIAS,
A
= anthropogenic.
1992). Increased leaching over the deposition of metals has been reported for acid forest soils in the Scandinavian countries (BERGKVIST et al., 1989; TYLER et al., 1987) and in Germany (MAYER and SCHULTZ, 1987). The concentration of trace meals in soil solutions is a good index of the mobile pool of metals in soils. Any chemical stress in soils is reflected in variations in the trace metal content of the soil solutions (Fig. 6). The behaviour of anthropogenic trace metals, as reflected in their speciation and phytoavailability, depend upon their forms or the compounds added to
500000
....... ......
....
.............
........................····COs
;:100000
•
~50000
.V' 'Co .: rJOOO
i 5000 Q.
§ ~ 1000
!
5OO.1--,---------:~-------:-r:__---+-----.
e
s 5000 j
'""
50
------------------------_.,
Cl
.
o
s
1000
!
500
f
1(}()
50
... """ ........
--
..................~-----------. ............ .."""....",.".. --.
.........
-- -- -
p " .. , /B .~ FIls .. -, Zns ./ "'.Mns - - - - F ep
~._._._.--'"
"'-'''_
Mnp '''-'''-Znp
10"'--~-----_,__-----r---t_--~
o
50 Doses 01 cool osh
100
400
Ifl ho I
FIG. 6. Impact of coal ash added to podzolic loamy sand on the concentration of Ca, Mg and Na, and on some trace elements in the soil solution and ryegrass (Lotium perenne L) (KABATA-PENDIAS et al., 1987). p = plant, s = soil solution.
A. Kabata-Pendias
8
BARLEY
CU
20
Zn
700
a
300 200
_a ------
-- ----pP
fOO
SOO
1000
0
lSOO
OATS
,0
,"
lSOO
Ni
, ,,
30
20
300
150
30
o
- ------p
- - _ . - - --p
p or::::==:::;====!!.-fOO 300 SOO
20
lII.tals in soil {mgllrg 1
- - - - grain FIG .
-straw
7. Metal uptake by barley and oats as influenced by their origin (CHLOPECKA , 1991; GRUPE and KUNTZE, 1987 and 1988). a = anthropogenic metals, p = pedogenic metals.
the soil. There is always a significant correlation between the concentration of metals in plants and their mobile species in soils. Because anthropogenic trace metals are easily mobile under most soil conditions their phytoavailability is higher than that of lithogenic and pedogenic metals (Fig. 7). However, when the solubility of metals is inhibited due to pollution effects, as for example after the addition of coal ash, their uptake by plants also decreases (Fig. 6). The importance of an anthropogenic origin of trace metals in terms of their mobility and availability is particularly important for Cd, Cu, Zn and Ni. Other metals such as Cr, Mn and Pb seem to be influenced less by their origin but it varies with variable conditions of the environment (CHLOPECKA, 1991; KABATA-PENDIAS and PENDIAS, 1992). Detailed information about the specific behaviour of anthropogenic metals from different sources of soil-solution-root interfaces is needed for a precise evaluation of their phytoavailability.
MODELLING OF TRACE METAL REBAVIOUR IN SOILS
Chemical modelling of trace metal equilibria in soil systems has been proposed for predicting their behaviour . Modelling programs are based on calculations of chemical equilibrium (MAITIGOD et al., 1985; Srosrro et al., 1984). Thermodynamic data can predict the final state of a system and give valuable but limited information on the mechanisms of chemical reactions in the complex multiphase soil system
(SPARKS, 1986). However, there are a number of problems with present models of trace element mobility in soil. Solubility equilibria may change during short time periods and within a few centimeters in the soil. In addition, all reactions related to the surface chemistry of root cells and of microbiota (including the rhizosphere) for root-solution and root-solids interfaces are not included in the applied models. The ability of roots to take up trace metals is based on the interactions with soil components including processes of changing Eh and pH, cation exchange and organic complexing. Despite these problems, chemical equilibrium models may be useful in an attempt to predict the mobility and phytoavailability of trace metals in soil systems.
SUMMARY
Trace metal behaviour in soils can be viewed in terms of reactions within solid, aqueous and gaseous phases . Processes at soil-solution-roots interface are fundamental to the understanding of trace metal interactions in multicomponent dynamic soil systems. The behaviour of trace metals in soils is related to their origin and forms. Lithogenic trace metals should be considered as slightly mobile but potentially available to plants under specific conditions of the soil-plant system. Behaviour of pedogenic metals reflects specific soil conditions . Anthropogenic metals are generally more mobile than lithogenic and pedogenic metals. However, all soil processes control the speciation and spatial distribution of trace metals from all sources. Therefore, the mobility and
Behavioural properties of trace metals in soils
phytoavailability of these metals should always be related to a given soil unit with definite properties and characteristics. Editorial handling : Brian Hitchon.
REFERENCES ADRIANO D. C. (1986) Trace Elements in the Terrestrial Environment. Springer. ALLOWAY B. J. , TILLS A. R. and MORGAN H. (1984) The speciation and availability of cadmium and lead in polluted soils. In Trace Substances in Environmental Health (ed. D. D. HEMPHILL), Vol. 18, pp. 187-201 . University of Missouri. BI!RGKVIST B., FOLKESON L. and BERGGRI!N D. (1989)Fluxes of Cu, Zn, Pb, Cr, and Ni in temperate forest ecosystems. Water Air Soil Pollut. 49, 217-222. BRUEMMER G. W., GERTH J. and HERMS U. (1986) Heavy metal species, mobility and availabilityin soil Z. Pfianzenernahr. Bodenkunde 149, 382-398. CHLOPECKA A. (1991)Impact of different compounds of Cd, Cu, Pb, and Zn on their speciation in soil and their contents of cereal plants. Manuscript in Polish. CuMAKOV A. (1988)Trace Elements in Slovakian Soils and Plant Nutrition. Ph.D. thesis. Institute of Sugar Industry, Bratislava, Czechoslovakia. DUDKA S., PIOTRoWSKA M., CHLOPECKA A. and LOPATEK J. (1990) Speciation, Mobility and Phytoavailability of Trace Metals in a Sewage Sludge Amended Soil. Rept. 270, lUNG, Pulawy, Poland. FIC M. (1987)Adsorptions- und Desorptions-Verhalten von Cadmium, Chromium, Kupfer und Zind an ausgewahlten Boden und Sanden. Ph.D . thesis. Christian-AlbrechtsUniversitat, Kiel, Germany. GAMBLE D. S. (1986) Interaction between natural organic polymers and metals in soil and fresh water systems: equilibria. In The Importance of Chemical Speciation in Environmental Processes (eds M. BRINCKMAN and P. J. SADLER), pp. 217-236. Springer. GRUPE M. and KUNTZE H. (1987) Zur Ni-Mobilitat einer geogen belasten Braunerde . Mitt. Dtsch . Bodenkundl. Ges. 55, 333-338. GRUPE M. and KUNTZE H. (1988) Zur Ermittlung der Schwermetallverfiigbarkeit lithogen und anthropogen belasteter Standorte. 1. Cd und Cu. Z. Pfianzenernahr. Bodenkunde. 151,319-324. HILLER D. A., BRUEMMER G. W. and ACKERMAND D. (1988) Gehalte an Haupt-und Nebenelementen in Konkretionen aus Oberboden von Marsche-Untersuchungen mit einer Midrosonde. Z. Pfianzenemiihr, Bodenkunde 151, 47-54. KABATA-PENDIAS A. (1966) Geochemistry of soils derived
9
from granites and basalts of Lower Silesia, Poland. Roczn . Nauk Roln . 9O-A-l , 1-60 (in Polish). KABATA-PENDIAS A. (1971)Micronutrient uptake by clover from different soil horizons. Pam . Pul. 45, 127-145 (in Polish). KABATA-PENDIAS A. (1973)Uptake of cobalt and copper by clover from minerals impregnated by these cations. Roczn. Glebozn. 24,273-287. KABATA-PENDIAS A. and BRUEMMER G. H. (1991) Ecological consequences of As, Cd, Hg, and Pb enrichment in European soils. In Global Perspectiveson Lead , Mercury and Cadmium Cycling in the Environment (eds T. C. HUTCHINSON, C. A. GORDON and K. M. MEEMA), pp. 107-114.SCOPE, Wiley Eastern. KABATA-PENDIAS A. and PENDIAS H. (1992) Trace Elements in Soils and Plants (2nd Edition). CRC Press. KABATA-PENDIAS A. , PIOTROWSKA M. and WIACEK K. (1987) Impact of coal ashes on soils and plants. Arch. Ochrony Srod . 1-2,97-104 (in Polish). LINDSAY W. L. (1979) Chemical Equilibria in Soils . WileyInterscicnce. MATTIGOD S. V., SPOSITO G. and PAGE A. L. (1985) Factors affectingthe solubilitiesof trace metals in soils. In Chemistry in the Soil Environment (ed. M. STELLY) , Chap. 11, pp. 203-221. Soil Sci. Soc. Am. MAYER R. and SCHULTZ R. (1987) Effects of soil acidification on heavy metal cycling in forest ecosystems. In Heavy Metals in the Environment (eds S. E. LINDBERG and T. C. HUTCHINSON), Vol. 2, pp. 402-406. CEP Consultants, Edinburgh, U.K. NUORTEVA P. (1990) Metal Distribution Patterns and Forest Decline. Seeking Achilles Heels for Metals in Finnish Forest Biocoenoses. University of Helsinki. Orrs M. L. , Buns E. P., RIEMER L., ROZEMA J. and BROEKMAN R. A. (1987) The iron-plaque on the roots of
saltmarsh plants: a barrier to heavy metal uptake? In Heavy Metals in the Environment (eds S. E. LINDGERG and T. C. HUTCHINSON), Vol. 1, pp. 407-409. CEP Consultants, Edinburgh, U.K. SPARKS D. L. (1986) Kineticsof reactions in pure and mixed systems. In Soil Physical Chemistry (ed. D. L. SPARKS) , Chap. 3, pp. 83-145. CRC Press. SPOSITO G., LECLAIRE J. P., LEVESQUE S. and SENESI N. (1984) Methodologies to Predict the Mobility and Availability of Hazardous Metals in Sludge-Amended Soils.
University of California. TYLER G., GERGGREN D., BERGKVJST B., FALKENGRENGRERUP U., FOLKESON L. and RUHLlNG A. (1987) Soil acidification and metal solubility in forests of southern Sweden. In Effects of Atmospheric Pollutants on Forest, Wetlands and Agricultural Ecosystems (eds T. C. HUTCHINSON and K. C. MEEMA) , NATO AS Series, Vol. G16, pp. 347-359. Springer. VERLOO M. and EECKHOUT M. (1990) Metal species transformation in soils: an analytical approach. Int. J. Env iron. Anal. Chem. 39, 179-186.