dynamics of the soil solution cationic composition in a limed soddy-podzolic soil contaminated with...
TRANSCRIPT
965
ISSN 1064-2293, Eurasian Soil Science, 2008, Vol. 41, No. 9, pp. 965–972. © Pleiades Publishing, Ltd., 2008.Original Russian Text © G.V. Lavrent’eva, S.V. Kruglov, V.S. Anisimov, 2008, published in Pochvovedenie, 2008, No. 9, pp. 1092–1100.
INTRODUCTION
The contamination of soils with heavy metals(HMs) has become a global environmental problembecause of the large number and diversity of techno-genic sources, the active involvement of metals intobiogeochemical migration, and their toxicity for livingorganisms. Even at relatively low levels of technogeniccontamination, the concentrations of HMs in the envi-ronment continuously increase, because they are notsubjected to any physicochemical or biological degra-dation, in distinction from other groups of contami-nants. When arriving into the natural environment,HMs are accumulated in the upper soil horizons,involved into biogeochemical processes, concentratedin separate links of trophic chains, and poorly removedfrom ecosystems [1, 4, 8].
If HMs are firmly bound to the solid phase compo-nents and poorly available for biological uptake, theirnegative effect is low. When HMs pass to the liquidphase (soil solution), the probability increases for theirentry into plant, animal, and human organisms. Thus,the interphase distribution is a key factor determiningthe mobility of HMs in soils and their biological avail-ability.
The chemical analysis of the liquid phase providesuseful information on the soil processes and plantgrowth conditions; its results can be used for predictingthe accumulation and toxic effects of HMs [12, 21, 23].At the same time, the study of the composition anddynamics of the soil solution is related to significant
theoretical and experimental problems [3]. The ionicand concentration compositions of the liquid phasevary rapidly and continuously due to the contact withthe solid phase having a high exchange capacity, themigration of ions and their uptake by plants, andchanges in soil conditions. The extraction of undilutedsoil solution requires suitable equipment, and the effectof external factors on its composition should be mini-mized for obtaining a representative sample. The avail-able methods for separating soil solutions have somedisadvantages [2, 22].
The aim of this work was to study the cationic com-position and the dynamics of soil solution extractedwith vacuum samplers from soddy-podzolic soil con-taminated with Co and Cd at the separate and combinedaddition of metals, as well as the effect of the pH on theinterphase distribution of HMs and the K, Na, Ca, andMg cations.
EXPERIMENTAL
Experiments were carried out with the plow horizonof limed sandy loamy soddy-podzolic soil from Kalugaoblast. The content of physical clay in the soil was
20.1
±
1
.5%; that of humus (determined by Tyurin’smethod) was
1.9
±
0
.3%; the pH (1 M KCl solution ata phase ratio of 1 : 2.5 and a contact time of 3 h) was
6.68
±
0.03
; K
2
O and P
2
O
5
(determined by Kirsanov’smethod) were
268
±
22
and
167
±
14
mg/kg, respec-tively; and the CEC (0.01 M CaCl
2
) was
10.4
±
0.2
cmol/kg.
SOIL CHEMISTRY
Dynamics of the Soil Solution Cationic Composition in a Limed Soddy-Podzolic Soil Contaminated
with Co and Cd at Variable pH
G. V. Lavrent’eva, S. V. Kruglov, and V. S. Anisimov
All-Russia Institute of Agricultural Radiology and Agroecology, 109 km Kievskoe sh., Obninsk, Kaluga oblast, 249032 RussiaE-mail: [email protected]
Received May 4, 2007
Abstract
—The dynamics of the cationic composition in the liquid phase of a soddy-podzolic soil contaminatedwith Co and Cd under different pH (changed by addition of H
+
ions) was studied in a long-term experiment (for470 days). Soil solutions were extracted with vacuum samplers installed in the soil for the time of the experi-ment. It was shown that the concentrations of Co and Cd, as well as those of K, Na, Ca, and Mg, in the solutionchanged during 3–4 weeks after moistening the air-dry soil to 70% of the maximum water capacity andincreased during 5–6 months after addition of H
+
. This indicated the low rate of the processes and reactionsoccurring in the soil. The addition of Co and Cd to the soil affected the interphase distributions of K, Ca, andMg; the effect of Co was higher than that of Cd by 3–4 times. When added together, the Co increased the con-centration of Cd in the soil solution by 4–6 times, and the effect of Cd on the content of Co was no higher than1.5 times.
DOI:
10.1134/S106422930809007X
966
EURASIAN SOIL SCIENCE
Vol. 41
No. 9
2008
LAVRENT’EVA et al.
The soil was sieved through a 2-mm sieve, and min-eral fertilizers (water solutions of NH
4
NO
3
, KCl, andKH
2
PO
4
) were added at rates of 0.2, 0.14, and 0.14 g/kgfor N, P
2
O
5
, and K
2
O, respectively. The soil sample wasdivided into several parts, to which Cd, Co(II), andCd + Co were added at 50 and 100 mg/kg, respectively,by the separate and simultaneous addition of a watersolution of metal nitrates. The contaminated sampleswere thoroughly mixed, put into polyethylene vegeta-tion pots 5 l in volume, and wetted with deionized waterto the maximum water capacity (MWC). After incuba-tion for 1 month, they were used in a pot experiment for3 months and then stored in the air-dry state for6 months.
Before the beginning of the desorption experiment,the soil samples were sieved through a 2-mm sieve andput into 5-l polyethylene pots. The weight of the air-drysoil was
3.9
±
0.1
kg/pot. Cylindrical Teflon samplerswith porous walls (pore diameter of 2
µ
m) wereinstalled at an angle of
45°
in the center of each pot at adepth of 5–10 cm and hermetically connected by tubeswith containers for the collection of the soil solution. Aconstant vacuum of 0.7 atm could be maintained in thesystem using an automatic vacuum pump (PrenartEquipment ApS, Denmark). The use of samplersinstalled in the soil for the time of the experimentallowed sampling the soil solution equilibrated with thesolid phase without disturbing the soil.
The soil samples were wetted with deionized waterto 70% of the MWC and maintained in this statethroughout the experiment. It was previously found thatthis level of soil water content allows the extraction ofthe necessary amount of soil solution for 3–4 h. In thefirst 30 days, the pots were weighed and water wasadded every day; later on, the water content was con-trolled once a week. The extraction of the liquid phasebegan 3 h after the next correction of the soil water con-tent. The soil solution was sampled (about 20 ml/pot)on 1st, 9th, 23rd, 30th, 35th, 112th, 220th, 352nd,422nd, 456th, and 470th days. Immediately after thesampling, the pH of the solutions was measured bypotentiometry using a Thermo Orion pH meter.
To simulate different acidity levels, 0.01 M HCl wastwice added to the soil (on the 30th and 350th days ofthe experiment) at a rate of 25 mmol
ç
+
/kg of air-drysoil.
The contents of exchangeable and acid-soluble Co,Cd, K, Na, Ca, and Mg were measured in the extractsobtained by successive extraction with 1 MCH
3
COONH
4
(pH 4.8) and HCl at a phase ratio of 1 :10 and a contact time of 24 h.
The concentrations of Co and Cd in the soil solu-tions and extracts were measured by atomic emissionspectroscopy (ICP AES Varian Liberty II), and those ofK, Na, Ca, and Mg were measured using the atomicabsorption method (Varian SpectrAA 250+) with addi-tion of Cs and Sr as depressants to the test solutions.
The experiment was performed in duplicate, and itsresults were processed using Microsoft Excel 2002software.
RESULTS AND DISCUSSION
T h e c a t i o n i c c o m p o s i t i o n o f t h es o i l s o l u t i o n a n d i t s d y n a m i c s at the sep-arate and simultaneous addition of Cd and Co to sandyloamy soddy-podzolic soil. When added to the soils assalt solutions, the metals are distributed between theliquid and solid phases; most ions interact with the min-eral and organic components of the solid phase, arereversibly or irreversibly sorbed, and form insolublecompounds [4, 8]. The physicochemical changesoccurring in the soil affect the distribution of other cat-ions in the soil systems, because the binding of ions bythe solid phase components is competitive in its nature[6, 15, 19].
It is supposed that a dynamic equilibrium is estab-lished after some time between the exchangeable andmore strongly bound forms of ions in the soil solution.The estimation of the restoration time of the sorptionand ionic equilibria disturbed because of changes in thechemical composition of soils or soil conditions is ofgreat importance in the study of the rate of processesand reactions occurring in the soil, the determination ofthe time of soil incubation, and the planning and imple-mentation of pot experiments.
At the beginning of the experiment, the residencetime of the added Co and Cd in the soddy-podzolic soilwas more than 10 months, which seems to be sufficientfor the restoration of the equilibrium disturbed by theaddition of metals in the amounts exceeding their totalcontent (
4.8
±
0.6
and
0.6
±
0.1
mg/kg) by 20 and 80times, respectively. On the other hand, the wetting ofthe air-dry soil for a long time to 70% of the MWC alsoresulted in the desorption of cations and the shift of theequilibrium. The concentration dynamics of the Co andCd and the K, Na, Ca, and Mg macrocations in the liq-uid phase of the uncontaminated soddy-podzolic soiland the soil contaminated by the separate and simulta-
15
10
5
0
2.5
2.0
1.5
1.0
0.5
01 9 23 30
Co,
µ
mol
/l
Control
CoCo + Cd
t, days
1 9 23 30
Control
CoCd + Co
Cd,
µ
mol
/l
Fig. 1.
Dynamics of Co and Cd concentrations in the liquidphase of the soddy-podzolic soil at the separate and com-bined addition of metals.
EURASIAN SOIL SCIENCE
Vol. 41
No. 9
2008
DYNAMICS OF THE SOIL SOLUTION CATIONIC COMPOSITION 967
neous addition of water solutions of HM nitrates isshown in Figs. 1 and 2.
The concentration of Co in the liquid phase of thecontaminated soil was higher than in the control soil bytwo orders of magnitude, and the content of Co in thesoluble form was about 0.4% of the added elementamount. When the two metals were simultaneouslyadded to the soil, the Cd favored the increase in the Cocontent in the liquid phase by 40–50%. The restorationof the quasi-equilibrium state disturbed by the changesin the soil water content occurred during 3–4 weeks; adecrease in the Co concentration in the soil solutionwas observed during this period (Fig. 1).
The concentration of Cd in the liquid phase of thecontaminated soil was higher than in the control soil by2.5–3.5 times, and the dissolved Cd made up 0.03% ofthe added element amount. The effect of Co on the dis-tribution of Cd between the liquid and solid phases ofthe soil was more significant: at the simultaneous addi-tion of the two metals, the content of Cd in the solutionwas 4–6 times higher than that at the addition of Cdalone. In distinction from Co, the concentration of Cdin the soil solution increased during at least three weeksafter wetting the air-dry soil.
The addition of Co and Cd affected the distributionof other cations in the soil; the effect of Co ions havinghigher sorption selectivity was significantly higher thanthat of Cd ions (Fig. 2). The effect of Co on the inter-phase distribution of K, Ca, and Mg was manifested asan increase in their concentration by 1.5–2.5 times onall the sampling dates. The effect of Cd was significantonly for the Ca and Mg ions, the content of which in theliquid phase increased by 20–60%. However, the con-centration of K in the soil solution after the simulta-neous addition of Cd and Co to the soil was signifi-cantly (by 2 times) higher than after the addition of Coalone.
At the addition of Na, the variation of the measure-ment results within the experimental treatments fre-quently exceeded the differences between the separatetreatments, which complicated the reliable detection ofCo and Cd effects. The dynamics of the Na concentra-tion in the soil solution, on the contrary, was more man-ifested. On the whole, it can be concluded that the con-tent of monovalent cations in the solution is moredependent on the time elapsed since the wetting of theair-dry soil than the content of the bivalent cations.After 30 days, the concentration of K
+
and Na
+
cationsin the liquid phase decreased by 1.3–1.8 times, whilethe content of soluble Ca
2+
and Mg
2+
varied less duringthis period (Fig. 2).
The differences in the interphase distributions of Coand Cd and their effect on the distributions of other cat-ions could result from the different contents of metalsarriving to the soil from the outside: the molar ratio was4 : 1. However, it is more probable that the experimen-tal results reflected the difference in the capacity of theions to compete for more energetically profitable bind-
ing sites and the sorption of two metals on the solidphase under the corresponding soil conditions.
The Co and Cd ions are bound by the solid phase ofthe soil due to the adsorption on aluminosilicates; Fe,Mn, and Al oxides and hydroxides; and carbonates andother minerals with uncompensated for negativecharges and a developed sorption surface via the ionexchange mechanism with the participation of func-tional groups of mineral and humus macromolecules,as well as due to their precipitation as insoluble com-pounds (metal hydroxides). The higher affinity of Co tosolid-phase components results in the displacement ofthe main cations from their positions in soil systemsand their transfer to less selective sorption sites. Cd ionsare less competitive than Co ions. When large amountsof ions of two metals are simultaneously added to thesoil, the effects of their interaction can be manifested,including those related to the stoichiometry of theresulting sorption products.
Changes in the state of cations are later manifestedthrough the changes in their concentration in the soilsolution. Among the elements considered, the Na ionsare characterized by the lowest sorption selectivity;therefore, the addition of Co and Cd has no significanteffect on the state and interphase distribution of Na inthe soil.
E f f e c t o f t h e p H o n t h e c a t i o n i cc o m p o s i t i o n a n d t h e d y n a m i c s o f t h es o i l s o l u t i o n . Different levels of soil acidity weresimulated by addition of equal amounts of
ç
+
ions ontwo dates: on the 30th and 350th days of the experi-ment. The differences between the separate experimen-tal treatments were small; therefore, the changes in the
1.5
1.0
0.5
0
3
2
1
020
15
10
5
0
4
3
2
1
0
Control ëd Co Cd + Co
1 9 23 30
t, days
K, m
mol
/lC
a, m
mol
/l
Mg,
mm
ol/l
Na,
mm
ol/l
1 9 23 30
Fig. 2.
Effect of Co and Cd at the separate and combinedaddition on the concentrations of mono- and bivalent cat-ions in the soil solution.
968
EURASIAN SOIL SCIENCE
Vol. 41
No. 9
2008
LAVRENT’EVA et al.
pH of the soil solutions averaged for the treatmentswith the Co- and Cd-contaminated soils are shown inFig. 3.
The lowest decrease of the pH of the solutions wasobserved already in the first 30 days of the experiment,which could be due to the slow desorption of
ç
+
afterthe change in the water content of the soil stored in theair-dry state for a long time and probably due to theadsorption of CO
2
from the air. The adsorption of HMcations by the solid phase also results in the release ofprotons [6, 25, 28], although other mechanisms (includ-ing, e.g., the hydrolysis of metals [10]) are also pro-posed to explain the decrease in the pH of the equilib-rium solutions. However, in our case, Co and Cd wereadded to the soil 10 months before the beginning of theexperiment. It should be assumed, in this case, that theresulting changes in the soil were preserved for a rela-tively long time.
After the first addition of
ç
+
ions, their concentra-tion in the liquid phase abruptly increased and the pHvalue decreased after 24 h from
8.3
±
0.3
to
7.7
±
0.2
.During the next week, this value increased again to
8.0
±
0.4
due to the solution contact with the solidphase having a higher buffer capacity. A gradualdecrease in the pH to 7.6 was observed in the followingmonths due to slow soil processes and reactions. Thesecond addition of
ç
+
ions to the soil decreased the pHof the solution by almost 1, but the variation of theparameter value was not recorded because of the largersampling intervals. On the whole, the addition of50 meq
ç
+
/kg of soil in two portions decreased the pHvalue from
8.3
±
0.3
to
6.8
±
0.1
.
At the addition of
ç
+
ions to the soil, their majorpart is used for the protonation of functional groupswith a corresponding change in the particle charge. Insoils with a low content of organic matter, the protona-
tion reaction predominantly proceeds with the partici-pation of hydroxyl groups occurring on the surface ofamorphous and crystalline Fe, Mn, and Al hydroxidesand on the basal and lateral faces of soil mineral crys-tallites. In soils enriched with humus substances, car-boxyl and amino groups and phenol hydroxyls oforganic compounds are primarily involved in the proto-nation and the reaction occurs almost instantaneously.The concentration of protons determines the degree ofdissociation of the functional groups; the averagecharge of the humic acids; and, hence, their capacity forbinding metal ions. The stability of the mineral andorganic metal complexes also depends on the pH. Allthese factors affect both the distribution of metalsbetween the solid and liquid phases and the ratiobetween the free ions and their hydrolyzed forms insolution [7, 26, 31].
The decrease in the pH more affected the interphasedistribution of Cd than that of Co; the soil processesand reactions occurred at low rates in both cases andwere of durable character (Fig. 4). The addition of
ç
+
ions, which decreased the solution pH from
8.3
±
0.3
to
7.6
±
0.2
, also favored a rapid increase of the Co con-centration in the liquid phase by about 1.5 times, but theamount of dissolved Co continued to increase until thenext portion of protons was added to the soil. The con-tent of Cd in the liquid phase increased by 2–4 timesimmediately after the acidification, but more significantchanges were observed on the following samplingdates. The new addition of
ç
+
ions to the soil, whichdecreased the pH of the soil solution from
7.6
±
0.1
to
6.8
±
0.1
, resulted in an increase in the concentration ofCo and Cd in the solution by 2.5–3.5 times during 2–3 months.
It is noteworthy that the shape of the curves in all thecases indicates the gradual release of Cd and Co ionsfrom the solid phase into the soil solution, which con-tinues for several months after the addition of
ç
+
to thesoil. The
ç
+
ions have a low displacing capacity withrespect to the adsorbed Co
2+
and Cd
2+
ions, but theincrease in the
ç
+
concentration in the soil favors the
pH9.0
8.5
8.0
7.5
7.0
6.5
6.00 100 200 300 400 500
t, days
Fig. 3.
Dynamics of the soil solution pH (arrows indicatethe addition of H
+
ions to the soil).
2.0
1.5
1.0
0.5
100200
300400
0
40
60
80
100
20
5000
10
6
4
2
100200
300400
0
20
30
40
50
10
5000
8
Co,
µ
mol
/l
t
, days
Controlëd
Co
Co + Cd
Control
Cd + Co
Co,
µ
mol
/lC
d, m
ol/l
Cd,
mol
/l
Fig. 4. Dynamics of Co and Cd concentrations in the soilsolution after addition of H+ (indicated by arrows) to thesoil. The results of the control treatment are given on theright ordinate.
EURASIAN SOIL SCIENCE Vol. 41 No. 9 2008
DYNAMICS OF THE SOIL SOLUTION CATIONIC COMPOSITION 969
dissolution of their hydroxides, carbonates, and othercompounds.
To compare the effects of H+ ions added to the soilon the interphase distributions of Co and Cd, the distri-bution coefficients Kd (calculated as the ratios betweenthe metal concentrations in the solid phase and the soilsolution) averaged for the corresponding pH ranges aregiven in Table 1.
For the Co from natural sources (the backgroundcontent in the soddy-podzolic soil), the Kd values werehigher by 4–6 times than those for the Co added to thesoil in the soluble form. For Cd, on the contrary, theadded metal fraction was characterized by higher Kdvalues. At the simultaneous contamination of the soilwith both metals, the Co decreased the Kd(Cd) value byabout 4 times and the Cd decreased the Kd(Co) value byno more than 1.5 times. The changes in the pH of thesolutions from 8.3 to 6.8 decreased the Kd(Co) by 5times on average and the Kd(Cd) by 25–40 times andmore, which indicated the different stabilities of the Coand Cd sorption products in the pH range considered.
The changes in the concentrations of ä+ and Ca2+
cations in the liquid phase of soddy-podzolic soil afterits acidification are shown in Fig. 5. The shapes of theNa and Mg curves are basically similar to those for Kand Ca; therefore, they are not presented. As at theaddition of Co or Cd, the concentrations of the maincations in the solution continue to increase during sev-eral months after the addition of ç+ ions to the soil, butthe dynamics vary among the separate ions.
The differences revealed between the experimentaltreatments with the separate and simultaneous additionof Co and Cd were also manifested after the soil acidi-fication. The effect of Co on the concentration of Cd inthe soil solution was stronger by 2–3 times than theeffect of Cd on the concentration of dissolved Co. Theeffect of these metals on the content of other cations inthe soil solution was also noted, although it was lesspronounced. The least significant differences wereobserved for Ca, whose concentration in the soil solu-tion exceeded the concentrations of other cations byalmost two orders of magnitude. The observed effectswere hardly due to ion interactions alone at this exper-imental stage; they more probably reflected the featuresof processes and reactions proceeding at the sorption ofCo and Cd by the soil solid phase.
The considered mechanisms of cation sorption usu-ally include physical adsorption on the surface of soilminerals (secondary clay minerals; amorphous andcrystallized Fe, Mn, and Al hydroxides; and carbon-ates) and ion exchange involving the functional groupsof minerals and humus macromolecules. However,some studies showed that, along with the exchangephenomena and simple adsorption, other processes andreactions occurring on the solid–liquid interface (for-mation of surface complexes or surface settlings, diffu-sion of sorbate into the sorbent volume) should also beconsidered.
Studying the kinetics of the related sorption–des-orption processes reveals two groups of reactions pro-ceeding with different rates. It was noted that the sorp-tion of cations on the surface of oxides and hydroxidesis a two-stage process [14, 16, 18, 24, 27]. The rapidfirst stage includes primary adsorption on the surface;different mechanisms, namely, the diffusion of adsor-bate inside the sorbent particles [16] or the slow rear-rangement of adsorbate and time-dependent changes inthe stoichiometric configuration of surface complexes[17, 18], were proposed to explain the second slowstage. Correspondingly, the observed effects areexplained differently: by the slow rate of the diffusion-controlled processes or by the formation of very stablesurface complexes, which are more difficultly desorbed(e.g., bidentate complexes, when the simultaneous rup-ture of at least two bonds is necessary for desorption)[23].
Table 1. The values of Kd (cm3/g) for Co and Cd depending on the soil solution pH
Solution pHControl Co Co + Cd Cd
Co* Cd* Co Cd* Co Cd Co* Cd
8.3 1396 116 308 135 218 331 1855 1240
8.3 – 7.6 578 38 213 44 157 21 870 101
7.6 – 6.8 246 4 59 4 38 14 224 18
* Fraction of metal from natural sources.
100 200 300 400 5000
0.5
1.0
1.5
2.0
100 200 300 400 5000
100
200
300
400
ëdControl Cd + CoCo
K, m
mol
/l
t, days
Ca,
mm
ol/l
Fig. 5. Effect of Co, Cd, and H+ on the concentrations of Kand Ca in the soil solution.
970
EURASIAN SOIL SCIENCE Vol. 41 No. 9 2008
LAVRENT’EVA et al.
In studying the desorption kinetics of Co and Cdsorbed by the clay fraction isolated from two soils, itwas found that the increase in the duration of the sorp-tion period significantly reduces the desorbed portionof metals and decreases the rate of desorption reactions;i.e., the reactions do not attain equilibrium even if nofurther sorption of ions occurs [27]. This is explainedby the gradual displacement of sorbed Co and Cd ionsto the sites with the lower rate of desorption or, as analternative mechanism, the slow diffusion of ions insideof particles resulting in their translocation to less acces-sible sites.
Other authors [29, 30] consider the surface precipi-tation of metal ions with the formation of three-dimen-sional products rather than monolayers. It is noted thatthe sorption of Co, Cr, Mn, Ni, and Zn by soils and soilcomponents results in the formation of a new solidphase composed of the metal hydroxide layer on thesurface of soil minerals. The surface precipitation isobserved at ion concentrations and in pH ranges signif-icantly lower than those corresponding to the saturationof the solution with respect to the solid phase, when theprecipitation of metal hydroxide can be expected underordinary conditions. In other words, the transition fromthe adsorption to the surface-induced precipitation ofions occurs and the surface of the minerals acts as amatrix for the formation of surface precipitate. The pre-cipitate has a structure analogous to that of Me(OH)2,but it is less ordered and has high concentrations ofvacant sites for metals [30].
Based on the generalization and analysis of thesestudies, Sparks [29] formulated a very important con-clusion. Although sorption and ion-exchange equilibriaplay an important role in soil systems, many processesand reactions on the solid–liquid interface proceed witha limited rate and can be controlled by diffusion, andreal soils are rarely in an equilibrium state. Therefore,the time scale of chemical reactions in soils can cover arange from milliseconds to several years.
F o r m s o f C o , C d , a n d m a c r o e l e -m e n t s i n t h e s o i l . The forms of occurrence(fractions of elements differing in the predominantgeochemical association, mechanism, and bindingenergy with soil components and extracted during thechemical fractionation of soils) are widely used for
characterization of the metal state in soils [9, 20]. Thefraction of HMs in the soil solution, where they occuras hydrated free ions or soluble complexes with mineraland organic ligands, is the most mobile form readilyavailable for plants. In the solid phase, HMs occur inthe exchangeable and nonexchangeable states. Theyenter into the composition of fine mineral particles andhumic substances; are adsorbed by amorphous andcrystallized Fe, Mn, and Al hydroxides; and form a con-stituent part of weakly soluble compounds [4, 5, 11].
Soil samples for determining the forms of Co, Cd,and macroelements (K, Na, Ca, and Mg) were takenbefore the beginning of the experiment and after itscompletion. The time of occurrence of the added Coand Cd in the soil was 10 months in the former case andmore than 2 years in the latter case. The results of themeasurements are given in Table 2.
The amounts of Co and Cd extracted during the suc-cessive treatment of the contaminated soil with 1 Msolutions of CH3COONH4 (pH 4.8) and HCl were 55–71 and 80–96% of their total amounts. The contributionof the fractions extractable by 1 M CH3COONH4 was30–40% for Co and 60–75% for Cd, which, however,could not be unambiguously interpreted as the predom-inant fixation of Cd by the solid phase through the ionexchange mechanism [13]. At the contact of the soilwith an ammonium acetate buffer solution of pH 4.8,the transition of the metals into the equilibrium solution
could be due not only to the displacing action of ions but also to the dissolution of compounds less stablein the acid solution. Along with carbonates, amorphousand poorly crystallized Fe and Mn hydroxides werepartially dissolved; metal fractions from organic col-loids could also be extracted [9, 11].
Two years later, the total content of exchangeableand acid-soluble Co was lower by 20–30% than10 months after its entry to the soil. For Cd, the differ-ences were less pronounced. The total amount of Cdextracted after 2 years in the treatments without themetal addition was lower by 10% and that in the treat-ments with the soil contamination was 15% higher thanafter 10 months of the Cd residence in the soil.
Unfortunately, the problems considered in the workdid not include the changes in the element forms with
NH4+,
Table 2. Contents of Co and Cd extracted during the successive treatment of loamy soddy-podzolic soil with reagent solutions, mg/kg
Treatment
1M CH3COONH4 (pH 4.8) 1M HCl
10 months 2 years 10 months 2 years
Co Cd Co Cd Co Cd Co Cd
Control 0.04 ± 0.02 0.39 ± 0.06 0.08 ± 0.02 0.46 ± 0.19 2.6 ± 0.2 0.30 ± 0.03 1.9 ± 0.1 0.28 ± 0.03
Co 21.6 ± 1.2 0.38 ± 0.02 16.7 ± 2.3 0.51 ± 0.17 50.7 ± 1.4 0.29 ± 0.03 41.0 ± 4.9 0.19 ± 0.10
Co + Cd 25.6 ± 3.2 35.9 ± 2.3 24.2 ± 9.9 28.4 ± 2.6 49.1 ± 1.3 10.8 ± 1.1 36.4 ± 9.6 12.2 ± 4.5
Cd 0.03 ± 0.01 36.0 ± 3.4 0.12 ± 0.01 28.2 ± 5.3 2.5 ± 0.1 12.5 ± 2.6 1.8 ± 0.3 14.0 ± 3.2
EURASIAN SOIL SCIENCE Vol. 41 No. 9 2008
DYNAMICS OF THE SOIL SOLUTION CATIONIC COMPOSITION 971
time under stable conditions and no corresponding con-trol treatments were used. Therefore, it could not beconcluded whether the revealed differences resultedfrom the Co fixation (effect of ageing), although ç+
ions were added to the soil during this period, or theywere due to other reasons.
Because of the high error of the determination of theelement forms at low concentrations, the total contentof Cd in two extracts exceeded its total content by10−15% during the successive extraction from theuncontaminated soil and in the treatments with additionof Co alone.
The variation of the measurement results did notallow revealing the mutual effects of Co and Cd at theirsimultaneous addition to the soil. No reliable correla-tion was found between the relative contents of theforms of other cations and the contamination of the soilwith Co and Cd; therefore, the results of the determina-tions are not presented.
CONCLUSIONS
To study the dynamics of the cation composition inthe liquid phase of the soddy-podzolic soil after the sep-arate and simultaneous contamination with Co and Cdunder variable pH, Teflon samplers with porous wallswere installed in the soil for the time of the experiment,which allowed the soil solution equilibrated with thesolid phase to be extracted without the soil disturbance.
It was found that, when arriving in the soil as watersolutions of salts, Co and Cd are not only differentlydistributed between the solid and liquid phases andmutually affect the sorption of each other at the simul-taneous addition but they also affect the interphase dis-tribution of other cations.
When Co and Cd were added in amounts exceedingtheir total content in the soil by 20 and 80 times, respec-tively, the concentration of Co in the liquid phaseincreased by almost two orders of magnitude and theconcentration of Cd increased by 2.5–3.5 times. At thesimultaneous addition of two metals to the soil, the Coincreased the concentration of Cd in the soil solution by4–6 times: when Cd alone was added, this increase wasno more than 1.5 times.
The fixation of Co and Cd ions by the solid phaseresulted in the displacement of other cations from theirpositions and their translocation to other less selectivesorption sites. Changes in the sorption of cations by thesolid phase were manifested in a 1.5- to 2.5-foldincrease in the concentrations of K, Ca, and Mg (but notNa) in the soil solution, which was traced during sev-eral months of the experiment. For Co ions, which havea higher affinity for the solid phase components, theeffect was higher than for Cd ions by 4 times andincreased at the simultaneous addition of the two met-als to the soil.
When the soil stored after contamination with themetals in the air-dry state was moistened to 70% of the
maximum water capacity, the concentrations of Co andCd, as well as those of K, Na, Ca, and Mg, in the soilsolution changed during 3–4 weeks. After the additionof ç+ to the soil, the contents of Co, Cd, and other cat-ions in the liquid phase increased during severalmonths. The slow desorption of ions could be due to thelow solubility of HM sorption products (surface precip-itation of hydroxides) in the pH range studied. It wasalso taken into account that, in distinction from soil sus-pensions, where the weight of the solution with thesolid phase exceeded the solid phase weight by manytimes, many processes and reactions in the soils withthe real water content proceeded more slowly, becausethey could be controlled by diffusion ensuring thetransport of ions.
The distribution of ions between the solid and liquidphases is a more sensitive and informative parameterthan the forms of elements determined using the chem-ical fractionation of soils. The results of determiningthe content of chemical element forms in the soil reflectthe different capacities of the two metals to be sorbedand retained by the solid phase components, but theycannot reveal the effect of the Co and Cd interaction,assess their effect on other ions, and determine the dif-ferences in the physicochemical state of the cations dueto the soil acidification.
REFERENCES1. Agroecology, Ed. by V. A. Chernikov and A. I. Chekeres
(Kolos, Moscow, 2000) [in Russian].2. A. E. Vozbutskaya, Soil Chemistry (Vysshaya Shkola,
Moscow, 1968) [in Russian].3. A. Kabata-Pendias, “Problems in the Current Bio-
geochemistry of Microelements,” Ros. Khim. Zh. (Zh.Ros. Khim. O–va D. I. Mendeleeva) 49 (3), 15–19(2005).
4. A. Kabata-Pendias and H. Pendias, Trace Elements inSoils and Plants (CRC, Boca Raton, 1985).
5. D. V. Ladonin, “The Effect of Iron and Clay Minerals onthe Adsorption of Copper, Zinc, Lead, and Cadmium inthe Nodular Horizon of Podzolic Soil,” Pochvovedenie,No. 10, 1197–1206 (2003) [Eur. Soil Sci. 36 (10), 1065–1073 (2003)].
6. D. V. Ladonin, “Ion Competition in Soils Polluted byHeavy Metals,” Pochvovedenie, No. 10, 1285–1293(2000) [Eur. Soil Sci. 33 (10), 1129–1136 (2000)].
7. D. S. Orlov, Soil Chemistry (Mosk. Gos. Univ., Moscow,1992) [in Russian].
8. D. S. Orlov, M. S. Malinina, and G. V. Motuzova, Chem-ical Contamination of Soils and Their Conservation(Agropromizdat, Moscow, 1991) [in Russian].
9. L. V. Perelomov and D. L. Pinskii, “Mn, Pb, and ZnCompounds in Gray Forest Soils of the Central RussianUpland,” Pochvovedenie, No. 6, 682–691 (2003) [Eur.Soil Sci. 36 (6), 610–618 (2003)].
10. D. L. Pinskii, “The Problem of the Mechanisms of Ion-Exchange Adsorption of Heavy Metals in Soils,” Poch-vovedenie, No. 11, 1348–1355 (1998) [Eur. Soil Sci. 31(11), 1223–1230 (1998)].
972
EURASIAN SOIL SCIENCE Vol. 41 No. 9 2008
LAVRENT’EVA et al.
11. A. A. Ponizovskii and E. V. Mironenko, “Mechanisms ofLead(II) Sorption in Soils,” Pochvovedenie, No. 4, 418–429 (2001) [Eur. Soil Sci. 34 (4), 371–381 (2001)].
12. V. V. Snakin, A. A. Prisyazhnaya, and O. V. Rukhovich,Composition of Soil Liquid Phase (REFIA, Moscow,1997) [in Russian].
13. L. G. Suslina, L. N. Anisimova, S. V. Kruglov, andV. S. Anisimov, “Accumulation of Cu, Zn, Cd, and Pb byBarley from Soddy-Podzolic and Peat Soils at the Appli-cation of Potassium and Different pH,” Agrokhimiya,No. 6, 69–79 (2006).
14. N. J. Barrow, G. Gerth, and G. W. Brummer, “ReactionKinetics of the Adsorption and Desorption of Nickel,Zinc, and Cadmium by Goethite: II. Modeling the Extentand Rate of Reaction,” J. Soil Sci. 40, 37–52 (1989).
15. M. F. Benedetti, C. J. Miln, D. G. Kinniburgh, et al.,“Metal Ion Binding to Humic Substances: Application ofthe Non-Ideal Competitive Adsorption Model,” Environ.Sci. Technol. 29 (2), 446–457 (1995).
16. G. W. Brummer, G. Gerth, and K. G. Tiller, “ReactionKinetics of the Adsorption and Desorption of Nickel,Zinc, and Cadmium by Goethite: I. Adsorption and Dif-fusion of Metals,” J. Soil Sci. 39, 437–450 (1988).
17. J. A. Davis, J. A. Coston, D. B. Kent, and C. C. Fuller,“Application of Surface Complexation Concept to Com-plex Mineral Assemblages,” Environ. Sci. Technol. 32,2820–2828 (1998).
18. D. A. Dzombak and F. M. M. Morel, Surface Complex-ation Modeling: Hydrous Ferric Oxides (Wiley, NewYork, 1990).
19. H. A. Elliot, M. R. Liberati, and C. P. Huang, “Competi-tive Adsorption of Heavy Metals by Soils,” J. Environ.Qual. 15, 214–219 (1986).
20. C. Gleizes, S. Tellier, and M. Astruc, “Fractional Studiesof Trace Elements in Contaminated Soil and Sediment: aReview of Sequential Extraction Procedures,” TrendsAnal. Chem. 21 (6–7), 451–467 (2002).
21. P. E. Holm, T. H. Christenseir, C. J. Tjell, andS. P. McGrath, “Speciation of Cd and Zn with Applicationto Soil Solutions,” J. Environ. Qual. 24, 183–190 (1995).
22. S. E. Lorenz, R. E. Hamon, and S. P. McGrath, “Differ-ences between Soil Solutions Obtained from Rhizo-sphere and Non-Rhizosphere Soils by Water Displace-ment and Soil Centrifugation,” Eur. J. Soil Sci. 45, 431–438 (1994).
23. S. E. Lorenz, R. E. Hamon, P. E. Holm, et al., “Cadmiumand Zinc in Plants and Soil Solutions from ContaminatedSoils,” Plant Soil 189, 21–31 (1997).
24. J. Lutzenkirchen, “Evaluation of Experimental Proce-dures and Discussion of Two Different ModelingApproaches with Respect to Long-Term Kinetic ofMetal Cation Sorption onto (Hydr)Oxide Surfaces,”Aquat. Geochem. 7, 217–235 (2001).
25. M. B. McBride, “Cu2+ Adsorption Characteristics ofAluminum Hydroxides and Oxyhydroxides,” Clays ClayMiner. 30, 21–28 (1982).
26. McBride M.B. Reactions controlling heavy metal solu-bility in soils / In: B.A. Stewart (ed.) Advances in soilscience. Springer-Verlag, New York, 1–56 (1989).
27. R. G. McLaren, C. A. Backes, A. W. Rate, andR. S. Swift, “Cadmium and Cobalt Desorption Kineticsfrom Soil Clays: Effect of Sorption Period,” Soil Sci.Soc. Am. J. 62, 332–337 (1998).
28. M. E. Mesquita, “Application of Langmuir and Freun-dlich Isotherms to Cu–Zn Competitive Adsorption inTwo Soils: Effect of pH,” Agrochimica 45 (1–2), 32–45(2001).
29. D. L. Sparks, “Elucidating the Fundamental Chemistryof Soils: Past and Recent Achievements and Future Fron-tiers,” Geoderma 100, 303–319 (2001).
30. S. N. Towle, J. R. Bargar, Jr. G. E. Brown, andG. A. Parks, “Surface Precipitation of Co(II) (aq) onAl2O3,” J. Colloid Interface Sci. 187, 62–82 (1997).
31. B. Ulrich, “An Ecosystem Approach to Soil Acidifica-tion,” in Soil Acidity, Ed. by B. Ulrich and M. E. Summer(Springer, Heidelberg, 1991).