The Chemistry of Lead and Cadmium in Soil: Solid Phase Formation1

JAVIER SANTILLAN-MEDRANO AND J. J. JuRiNAK2

ABSTRACTEquilibrium batch studies were conducted to obtain solu-

bility data of Pb and Cd in soils. The data were plotted onequilibrium solubility diagrams using pH as the master vari-able. In the construction of the diagram the hydroxide, car-bonate, and phosphate compounds of Pb and Cd were givenparticular attention. Both Pb and Cd solubility decreased inthe soils as pH increased. The lowest values were obtained inthe calcareous soil. Under a given set of conditions, however,Cd activity in solution was always notably greater than thatof Pb. In noncalcareous soils the solubility of Pb appearedto be regulated by Pb(OH)2, Pb.,(PO4)2, Pb4O(PO4)2,Pb5(PO4)3OH, depending on the pH. In calcareous soils,PbCO3 also assumed importance. At higher Cd concentrationsthe precipitation of Cd3(PO4)2 and/or CdCO3 regulated cad-mium solubility. At low Cd concentrations the equilibriumsolution was undersaturated with regards to both Cd3(PO4)2and CdCO3.

Additional Index Words: equilibrium solubility diagram.

CONCERN over environmental quality has generated inter-est in the chemistry of Pb and Cd in soils. Lagerwerff

(1972) has presented a pertinent review of the currentstatus of our knowledge concerning these heavy metal soilcontaminants. As with other heavy metals, the chemistryof Pb and Cd in soil can qualitatively be described asaffected by (i) the specific adsorption or exchange adsorp-tion at a given mineral interface, (ii) the precipitation ofsparingly soluble compounds of which they are constituent,and (iii) the formation of relatively stable complex ionsor chelates which result from the interaction with soilorganic matter. However, few data are available to delineate

the relative importance of these mechanisms in the retentionof Pb and Cd in soils.

Bittel and Miller (1974), using pure clay systems, re-ported the selectivity coefficients of Pb2 + , Cd2+ and Ca2+

ions on montmorillonite, illite, and kaolinite. Their datashow that, in the systems studied, Pb2+ was preferentiallyadsorbed over Ca2+ whereas the Cd2+-Ca2 + exchange hada selectivity coefficient near unity.

Lagerwerff and Brower (1972, 1973) studied the ex-change reactions of both Pb2+ and Cd2+ with aluminum,calcium, and sodium ions in three soils. Generally, aGapon-type equation was found to describe the reactions.In the Na-system, Pb2+ was found to precipitate. It wasspeculated that Pb(OH)2 was one compound formedthough other unidentified Pb compounds were also consid-ered co-precipitated. The solubility of the solid lead phaseformed was found to increase with a decrease in both pHand the concentration of salt (NaCl). Under similar condi-tions, no precipitation of Cd2 + was noted.

The importance of indigenous soil phosphate as a poten-tial buffer agent for lead in the soil solution has been sug-gested by the work of Nriagu (1972, 1973a, 1973b). Histhermodynamic data suggest that the pyromorphite form oflead, i.e., PbB(PO4)3X, where X = halide ion, OH~, etc.,should be considered when studying the fate of Pb. He con-cluded that in the presence of chloride ion, Pb5(PO4)3Cl(chloropyromorphite) was the stable modification of leadin natural systems. No similar study has been reported inthe literature for cadmium.

This report represents a continuing attempt to ascertainthe possible reactions in soils which are included in deter-mining the concentration of lead and cadmium ions in thesoil solution. Of particular interest was the development ofequilibrium solubility diagrams with reference to the forma-tion of solid phase hydroxide, carbonate and phosphatecompounds of lead and cadmium.

THEORYWhen Pb or Cd is added to a soil-water system, multiphase

equilibria are assumed to be established between the soil solu-

852 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

tion, the exchanger (adsorbed) phase, the inorganic solid phase,and the organic phase. For simplicity the viewpoint is taken thatthe precipitation or dissolution of an appropriate sparingly sol-uble compound is the primary mechanism that controls the activ-ity of Pb or Cd in the equilibrium solution. In the following, theionic equilibria of Pb will be developed noting that an identicalapproach was also used to describe the Cd equilibria.

The sparingly soluble lead compounds considered under theexperimental conditions and used to develop the equilibriumdiagrams were Pb(OH)2, PbCO3, Pb3(PO4)2, Pb4O(PO4)2, andPb5(PO4)3OH. The solubility isotherms of PbO, PbHPO4 andPb5(PO4)3Cl were also included in the equilibrium diagrams forreference.

The experimentally determined concentration of lead insolution was corrected for the hydroxide, carbonate, phosphate,and acetate ion pairs as well as the indifferent salt effect asmanifested by the appropriate activity coefficients. The acetate(OAC~) anion was introduced into the system with Pb2 + , Cd2+,and Ca2+ ions which were added at various concentrations toeffect solution equilibria.

The mass balance equations used to define the ionic equilibriaof lead in solution are as follows:

CPb = [Pb2-1-] + [PbOH+] + [PbOAC+] + [PbHCOt]d

+ [PbCO°] + [PbHPO°] [1]

COAC = [HOAC] + [OAC-] + [CaOAC+] + [PbOAC+] [2]

Cco2 = [H2CO3] + [HCO3 ] + [CO3 ] + [CaHCO3" ]

+ [PbHCO-] + [CaCO°] + [PBCO°] [3]

= [H3P04] + [H2P04-]-

+ [CaHP04°] + [PbHPO°]

,2-,

[4]

where Ci is the total analytical concentration of the ith ionicspecie in solution. At normal soil solution concentrations, it wasassumed only mono-ligand complexes were significant (Adams,1971). Thus in this study, multiligand complexes, e.g., Cd(OH)3~,Pb(OH)3~, etc. were not included.

From Eq. [1], [2], [3], and [4], the concentrations of any solu-tion specie, i.e., [PO4

3-], [CO32-], [CaOAC+], etc. were deter-

mined by using the appropriate thermodynamic relations andsubstituting into the mass balance equation. For example, todetermine the specie, [OAC~] the relationships used in Eq. [2]were

[HOAC] = [H+] [OAC-]/Kai

[CaOAC+] = [Ca2+] [OAC-]/KAC

[PbOAC+] = [Pb2+] [OAC-]/KPb [5]

where Kaj is the acetic acid dissociation constant, KAC andKPb are the ion pair dissociation constants (Kd) of [CaOAC+]and [PbOAC+] ion pairs, respectively (see Table 1). It is impor-tant to also consider additional ion-pair reactions of the P&+

and Ca2+ ions.The activity of Pb2+ in solution, APb, at any pH value can be

determined by the application of the ion activity product, KSD,principle to the system. The relation between APb and the solu-tion pH for the principle lead compounds considered areLead hydroxide, Pb(OH)2:

-log(APb) = 2pH-8.1 [6]

Lead carbonate, PbCO3:

-log (APb) = 2pH +log PCo2-5.33 [7]

Lead orthophosphate, Pb3(PO4)2:

f~^f~~\ f f V

-log (APb) = 14.87 + % log ——F°4YP°4 [8]y

Lead hydroxypyromorphite, Pb5(PO4)3OH:

C(—log (APb) = 12.56 + pH/5 + % log —

Tetraplumbite phosphate, Pb4O(PO4)2:

CC-log (APb) = 9.3 + pH/2 + 1/2 log —

o4 7po4 KI K2 K3

y[9]

o4 ypo4 K3

[10]

where y = [H+]3 + KJH+]2 + Kj K2[H+] + Kx K2 K3,KI, K2, and K3 are the 1st, 2nd, and 3rd dissociation constantsfor phosphoric acid, respectively; 7PO4 is the activity coefficientfor [PO4

3~]; Pco2 is the partial pressure of CO2 in atmospheres;and

CCpo4 = CPo4 - 2} MJ HPO°

i.e., the total phosphate concentration corrected for Ca and Pbphosphate ion pairs.

In the pH range of this study, the solubilities of Pb3(PO4)2,Pb5(PO4)3OH, and Pb4O(PO4)2 are similar. Thus, for ease ofpresentation, the Pb4O(PO4)2 isotherm was not plotted on thelead equilibrium diagrams. However, as a potential sink for leadit must be jointly considered with lead orthophosphate andhydroxypyromorphite.

The activity of Pb2+ in solution was determined experiment-ally by analyzing the solution for total lead, i.e., CPb, then theconcentration of free lead ion, [Pb2-1-], and its activity coefficient,7Pb, was calculated as follows:

[Pb2+] = CPb/« [11]

Table 1—Equilibrium constants for Pb and Cd at 25C

Equilibrium Constants for Pb and Cd at 25 CSparingly soluble salts

Compound

PbOPbHPO,Pb(OH)2PbCOjPbj (PO, ),

Pbs(P04)ClPb4O(PO4 )2Cd(OH)jCdCO,CdjfPO,)!

"Ksp15. 32t11. 43«19. 9t12. 8t44.6'76.8-84. 4t65. 17*14. 3t.11. 2t,38. 15,

14.7'11.6!32. 6t

Ion PairsSpecie

PbOH+

PbHPO?PbCO?PbOAC+

CdOH+

CdCO?CdOAC+

CdHPO,

pk

7 R»3.7|,3. 4!

4.60*3. 1*2.2»3. 2!

d

7.3'3. it

2.52"

* Nriagu (1972).T Nriagu (1973a).' Determined in this study.™ Davis (1962).* Wagman et al. (1968).

SANTILLAN-MEDRANO & JURINAK: CHEMISTRY OF LEAD AND CADMIUM IN SOIL 853

where

COACK i\

Pb v1 + [H+]/Kai + [Ca2+]/KAC)

32 Kh Pco2 [OH~][H]2KCP KOP

and KPb, KCP, and KOP are the [PbOAC+L [PbCO3°], and[PbOH+] ion pair dissociation constants, respectively, Kci andKc2 are the 1st and 2nd dissociation constants of carbonic acid,and Kh is Henry's law constant. The contribution of [PbHCO3

+]and [PbHPO4°] ion pairs was assumed to be negligible. In cal-culations involving concentrations, e.g., Eq. [5] and [11], thethermodynamic equilibrium constants were corrected for theionic strength of the solution (Stumm and Morgan, 1971).

The activity of lead is given by

= [Pb2+]yPb [12]

where 7Pb was calculated from the extended law of Debye-Huckel [Freiser and Fernando: 1963] using a value of 4.0 and5.0 for the "a" factors of Pb and Cd, respectively.

In terms of the total analytical concentrations of Pb2+, theactivity of Pb2+ is now

where

APb = CPb/f

MATERIALS AND METHODS

[13]

[14]

The soils used in this study were Yolo loam, Handford sandyloam, and Nibley clay loam. Some of their important chemicalproperties are listed in Table 2. Prior to use, the soils werecalcium-saturated by washing several times with \M CaCl2. Theexcess salt was removed by leaching with distilled water. Thesoils were then dried at 70C and passed through a 60-meshscreen.

Equilibrium solution data were obtained by placing 0.1 g,0.5 g, or 1.0 g of Yolo, Nibley, or Hanford soil, respectively, in125-ml Erlenmeyer flasks then adding 50 ml of solution with agiven heavy metal/Ca2+ ratio. The initial concentration ofPb2+ or Cd2+ varied from 4.5 X 10-6 to 0.9 X 1Q-3 M whereasthe Ca2+ concentration varied from 0.0 to 0.5 X 10~3 M. Theacetate form of all cations in equilibrium with their carbonatesalt was used in an effort to reduce the acidity associated withthe more concentrated solutions of Pb and Cd. The suspensionswere agitated at a temperature of 25 ± 1C for a period of 100hours. The solutions were separated from the suspensions bymicropore filtration and analyzed by atomic absorption spectro-photometry. The equilibrium concentration data were processedto yield either the activity of lead, APb, or cadmium, A^ (seeEq. [13]), and the negative logarithm of the cation activity wasthen plotted against the pH of the equilibrium solution.

Table 2 — Properties of soils used in studying Pb and CdReactions

PHCEC, meq/lOOg% organic carbonWater-soluble P, moles*Total (resin) extractableP. K«/g*Total Pb, j,g/gtTotal Cd, (jg/gt% CaCOj

Yolo loam6.8

27.01. 12x i<r»

44.015.0<0. 05

--

Hanford si6.64.50.4

4. 1 x 10"'

38. 134.50. 18

--

Nibley cl8. .4

26.01.7

6. 4 x 10"'

49.327.20. 150.5

* Method of Watnabe and Olsen (1963).t Method of Amer et al. (1955).* Soil digested with mixture of 3AT HC10, and 6N HC1.

Kinetic data for Pb and Cd reaction with soil was obtained byplacing 0.5 g of Nibley soil into 125-ml Erlenmeyer flasks. Thesoil was allowed to react with 50 ml of solution for < 0.1 min-ute to 200 hours. The reacting solution initially had a concen-tration of 7.0 X 10~5M of either Pb or Cd acetate. In addition,both solutions contained 1.0 X 10~3M calcium acetate. Thesamples and solution data were handled in a manner identicalto that used in the equilibrium studies.

The lack of published values for many of the ion pair disso-ciation constants required made it necessary to experimentallyestimate their values. Briefly, the method consisted of equili-brating either CdCO3 or PbCO3, at 25 ± 1C, with variousconcentrations of the complexing ligand, Ln~, and solving thefollowing equation for Kd.

= Ksp (1.0KdOH Krt

y)

where CHM is the total heavy metal concentration, KdoH is thehydroxyl complex (MOH+) dissociation constant, Ksp is thesolubility product of the carbonate, 7 is the activity coefficientof the given metal cation, and CO3

2~ and OH~ are the car-bonate and hydroxyl ion activity, respectively. The ionic strengthof the solutions used to calculate the values of Kd varied from0.001 to 0.0001 molar. The values of Kd found in this study aregiven in Table 1. Several of the Kd values found in literatureare also reported to give some indication of the accuracy of themethod used.

The KSD value of Cd3(PO4)2 used in this study was determinedby potentiometric titration. This method was essentially thatdescribed by Jurinak and Inouye (1962). The pKSD of Cd3(PO4)2was determined to be 38.1. This value was used in the construc-tion of the phase diagrams. The KSD of Pb3(PO4)2 was alsodetermined using potentiometric titration. The pKSD value ob-tained for Pb3(PO4)2 was 42.4 which compares favorably withthe pKSD value of 44.6 reported by Nriagu (1972).

RESULTS AND DISCUSSION

Figures 1 through 6 show the equilibrium solubility dia-grams for Pb and Cd in the three systems studied. Thephase diagrams were constructed assuming the followingvalues: CP04 = 10~6-3M, CCP04 = 10-«-6M, PC02 = 1Q-3

atm, Ca2+ = 10~3M, CL~ = 10~aM and ionic strength(I) = 10~Z-2M. The concentration of phosphorus is the aver-age value for the water soluble phosphorus of the threesoils studied.

Figures 1 and 2 show the activity of Pb plotted vs. solu-tion pH for the Yolo and Hanford soils, respectively. Thesolubility isotherm for Pb5(PO4)3 Cl is included in thediagrams for reference only since the Cl~ concentration inthe systems was negligible. The dependence of Pb solubilityon pH is noted. The inference from these data are that,under the experimental conditions Pb(OH)2 appears toregulate Pb2+ activity when the solution pH is less thanabout 6.6. As the pH increases the formation of lead ortho-phosphate, lead hydroxypyromorphite, and also tetra-plumbite phosphate become a distinct possibility. Data scat-ter and similarity in solubilities prevent a more definitivestatement as to the dominant solid phase present. It wasobserved that data scatter was most prevalent in the pHregion where several solubility isotherms intersected. Thissuggests that the precipitate formed may be the productof the co-precipitation of several compounds each of whichcould in principle exist alone. The resulting phase thus isa solid solution of mixed composition and variable solubil-

854 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

0

Fig. I—The solubility diagram for Pb in Yolo loam soil.

ity as compared to the pure compound isotherms shownin the diagrams.

Figure 3 shows the activity of Pb2+ in the slightly cal-careous Nibley soil. The buffer capacity of the soil limitedthe pH range of the data. However, a general decrease insolubility with increasing pH is evident. The data in the pHrange of 7.5-8.0 shows the complexity of determining thepredominate solid phase regulating Pb solubility in calcare-ous soils unless highly accurate data are available. In thispH range the possibility exists for the formation and/ormixture of PbCO3, Pb(OH)2> Pb3(PO4)2> Pb5(PO4)3OH, orPb40(P04)2.

These data tend to support the contention of Nriagu(1972, 1973a) that Pb phosphate formation can serve as asink for Pb in ecosystems. This situation would be particu-larly important where high levels of indigenous phosphateexists or in systems to which phosphate has been added.

Hem (1973) concluded that in many natural surfacewaters the Pb activity was regulated by either PbCO3 orPb3(OH)2(CO3)2. Calculations, based on the conditions ofthis study, show that the solubility isotherm of Pb(OH)2-(CO3)2 is similar to that of Pb(OH)2.

Figures 4, 5, and 6 show how the activity of Cd2+ varieswith pH in the three soils. The treatment of experimentaldata was identical to that used for Pb. The obvious differ-ence between Pb and Cd systems is the paucity of thermo-dynamic data available to define Cd interaction. This factis reflected in the limited number of Cd solubility isothermsplotted in the diagrams.

Figures 4 and 5 show the activity of Cd2+ in Yolo andHanford soil, respectively. In both soils the general patternof Cd solubility is similar, i.e., a decreasing solubility withincreasing pH. In the Yolo soil, at pH < 7.5, the Cd2 +

activity is approximated by the orthophosphate isotherm.Above pH 7.5, the solution becomes undersaturated withrespect to Cd3(PO4)3. In the Hanford soil, Cd2+ activityapproaches the Cd3(PO4)2 isotherm when the pH valuesare < 6.8. As the pH increases the activity of Cd2+ againdrops sharply.

Wagman et al (1968) reported a pKsp value of 32.6 forCd3(PO4)2. Data from our studies inferred that this valuegreatly overestimated the amount of cadmium in solution.

n.OQ.

105.0 5.5 6.0 6.5

PH

7.0 7.5 8.0

Fig. 2—The solubility diagram for Pb in Hanford sandy loamsoil.

Fig. 3—The solubility diagram for Pb in Nibley clay loam soil.

The pKsp value of 38.1 for Cd3(PO4)2 used in the con-struction of the diagrams was calculated from experimentaldata. Attempts to detect other stable Cd-phosphate com-pounds that might assist in explaining the solubility datawere not successful. However, the possibility of the forma-tion of other solid phase cadmium compounds cannot beprecluded.

Cadium equilibrium in Nibley soil is shown in Fig. 6.Although data scatter is more pronounced than in the pre-vious systems, a decrease in Cd2+ activity is again notedas the pH increases. The formation of both CdCO3 andCd3(PO4)2, or a mixed compound, is considered possiblein this system. It is noted that for any given soil at any givenpH the activity of Cd2+ is considerably greater than that ofPb2 + . These data take on significance in view of the pol-lution hazard presented by Cd in the environment (Lager-werff, 1972; Friberg et al.; 1971). These data suggestgreater mobility of Cd2+ in the soil relative to Pb2 + andalso potentially greater availability of Cd for plant uptakewhen compared to Pb. The data further suggest that cal-careous soils are a more efficient sink for both Cd and Pbwhen compared to noncalcareous soils.

Equilibrium data obtained in this study were based on100 hours reaction time. Initial kinetic studies showed that

uOa

SANTILLAN-MEDRANO & JURINAK: CHEMISTRY OF LEAD AND CADMIUM IN SOIL

0

855

1°60 6.5 7.0 7.5 8.0PH

8.5 9.0

Fig. 4—The solubility diagram for Cd in Yolo loam soil.

•ouOa

106.0 6.5 7.0 7.5

PH8.0 8.5 9.0

Fig. 5—The solubility diagram for CD in Hanford sandy soil.

a reaction time of 200 hours did not materially effect theconclusions. Figures 7 and 8 show representative data ofhow the activity of Cd2 + and Pb2+ and pH of the systemsvaried with time in a Nibley soil suspension. The resultsshow that the reaction time used provided a reasonableapproximation of a system at equilibrium or at least arelatively stable system.

13Uaa

8

1°60 6.5 7.0 7.5 8.0 8.5 9.0P H

Fig. 6—The solubility diagram for Cd in Nibley clay loam soil.

timea- 0bO.63minc-2.Omind- 1 hr«-50 hrf-200hr

8 ~-^-°?r-=2-3i

1 0 . , __r—— . . . .5.0 5.5 6.0 65 7.0 7.5 8.0 B.5 90

pHFig. 7—The reaction kinetics of Fb in Nibley clay loam soil.

856 SOIL SCI. SOC. AMER. PROC,, VOL. 39, 1975