International Journal of Applied Agricultural Research ISSN 0973-2683 Volume 13, Number 1 (2018) pp. 9-19 © Research India Publications http://www.ripublication.com

Lead Sequestration in the Soil Environment with an Emphasis on the Chemical Thermodynamics Involving as a Soil Amendment: Review and Simulations

Michael Aide* and Indi Braden Department of Agriculture, Southeast Missouri State University, USA.

(*Corresponding author)

Abstract Soil (Pb2+) contamination is a global issue and in many instances the extent of the impact is intense. In Missouri (USA) the legacy of lead mining and processing has resulted in widespread soil impact. We present a review of lead research focusing on the use of phosphate soil amendments to reduce the biological availability of lead and to illustrate how simulations of soil solution chemistry may guide researchers in defining their experimental designs. We demonstrate (i) the simulation of lead hydrolysis arising from the dissolution of and , (ii) the simulation of lead hydrolysis and the thermodynamically favored precipitation of lead species across selected pH intervals and (iii) provide activity diagrams showing the stability fields for selected soil chemical environments. Thermodynamic simulations are addressed as an important preparatory process for experimental design development and protocol selection involving field-based research. Keywords: Lead, hydrolysis, -pair formation, pyromorphite, galena.

INTRODUCTION In the eastern Ozarks of Missouri (USA), Mississippi Valley Type lead ores occur in Paleozoic carbonate rock as galena (PbS), with sphalerite (ZnS) and pyrite (FeS2) as important auxiliary minerals [1]. On a world-wide basis, lead (Pb2+) abundances vary with the rock type, ranging from 10 to 40 mg Pb kg-1 in felsic and argillaceous sediments [2]. Ultramafic and carbonate sediments typically have 0.1 to 10 mg Pb kg- 1, whereas the earth’s average crustal Pb abundance is 15 mg Pb kg-1 [2]. Table 1 lists selected lead and other important minerals germane to this discussion. 10 Michael Aide and Indi Braden

Table 1. Important Pb-bearing and accessory minerals. Mineral Composition Galena PbS Sphalerite ZnS

Cerussite PbCO3

Hydrocerrusite Pb3(OH)2(CO3)2

Anglesite PbSO4

Smithsonite ZnCO3

Hydrozincite Zn5(CO3)2(OH)6

Plumbogummite PbAl3(PO4)2(OH)5●H2O

Crandallite CaAl3(PO4)2(OH)5●H2O

Goyazite SrAl3(PO4)2(OH)5●H2O

Philipsbornite PbAl3(AsO4)2(OH)5●H2O

Mimetite Pb5(AsO4)3Cl

Wulfenite PbMoO4

Pyromorphite Pb5(PO4)3Cl

Tsumebite Pb2CuPO4(OH)3●3H2O

Tetraplumbite orthophosphate Pb4O(PO4)2

Hydroxyapatite Ca5(PO4)3OH

Phosgenite Pb2Cl2CO3

Larnakite Pb2(OH)2SO4

Triple Superphosphate Ca(H2PO4)2●H2O (0-45-0)

No mineral name Pb3(PO4)2

No mineral name PbHPO4

No mineral name Ca3(PO4)2

No mineral name Ca4H(PO4)3●3H2O

No mineral name Pb4(OH)6SO4

Aide [3] observed three Pb-impacted soil profiles at St. Joe State Park (Missouri, USA) for Pb, Cd, Zn, Ag and In, noting that each element was positively partitioned in the near-surface A horizons. Lead ranged from 224 to 589 mg Pb/kg, whereas cadmium ranged from 0.3 to 3.1 mg Cd/kg. Extractions with pyrophosphate and EDTA demonstrated that a small to substantial portion of the to Pb and Cd pools were Lead Sequestration in the Soil Environment with an Emphasis on the Chemical… 11 recovered, inferring potential availability. Aide et al. [4] at St. Joe State Park (Missouri, USA) amended the soil horizons of Pb-impacted soils with triple superphosphate (0- 45-0) and after an incubation period soil was employed in a greenhouse assay using soybeans (Glycine max L.). Plant tissue uptake of Pb and Cd were not significantly different because of superphosphate amendment, suggesting the possibility that calcium inhibited Pb-Phosphate interaction. Galena (PbS) typically experiences congruent weathering at the mineral surface to form Pb hydroxide/oxide, with the exact reaction stoichiometry a function of pH. In alkaline environments reaction products are dominated by thiosulfate, whereas elemental sulfur and predominate in acidic to neutral soil environments. For protonation of Litharge the reaction may be expressed as: + + PbOc + H = Pb(OH) with a log Qs10 = 12.72 [5].

Using x-ray adsorption fine structure spectroscopy, Scheckel and Ryan [6] demonstrated that soil lead was precipitated as pyromorphite (Pb5(PO4)3(Cl,OH,F). Given near-neutral soil pH conditions, phosphate amendments were shown to promote Cerusite-like mineral (PbCO3 or Pb3(OH)2(CO3)2) formation [7]. Using selective dissolution protocols, Kabala and Sing [8] and Mbila and Thompson [9] showed that Pb was primarily associated with residual and Fe-oxyhydroxide fractions. 2- Triple superphosphate (TSP as Ca(H2PO4)2H2O) reacts in soil to yield HPO4 or - H2PO4 , depending on the soil pH. In the presence of , TSP should act as a weak acid and form hydroxyapatite (Ca10(PO4)6(OH)2). In the presence of calcite, the aqueous 2- HPO4 activity should be sufficiently low that hydroxyapatite may be a substantial phosphate sink [10]. Phosphate-based amendments (hydroxyapatite, rock phosphate, H3PO4, and P-bearing fertilizers) have been used effectively to synthesize Pb in soil as either hydroxypyromorphite (Pb5(PO4)3OH), (PbAl3(PO4)2(OH)5(H2O)) or other Pb-bearing minerals [11 to 20]. Using soils in Florida, Ma et al. [12] documented that dissolution resulted in the synthesis of fluoropyromorphite minerals (Pb10(PO4)6F2), whereas rock phosphate dissolution resulted in a carbonate-bearing fluoropyromorphite (Pb10(PO4)3(CO3)3FOH). Using thermodynamic simulations involving hydrolysis and precipitation reactions, Porter et al. [10] showed that sufficient phosphate must be supplied to convert Ca to apatite-like minerals and then have sufficient phosphate to interact with Pb to form lead- phosphate minerals. Specifically, Porter and his colleagues theoretically demonstrated that variscite (AlPO4●2H2O) and vivianite (Fe3(PO4)2●8H2O) were not sufficiently effective in lowering the phosphate activity, allowing the conversion of galena (PbS) to pyromorphite (Pb5(PO4)3(Cl,OH,F). Alternatively, hydroxyapatite (Ca10(PO4)6(OH)2) formation from calcite (CaCO3) will suppress the phosphate activity sufficiently to inhibit pyromorphite formation [10]. Thus, P-induced Pb reductions in the presence of CaCO3 requires sufficient quantities of P to precipitate both Ca as hydroxyapatite and Pb as pyromorphite. 12 Michael Aide and Indi Braden

Wu et al. [21] investigated Zn and Pb impacted soils adjacent to tailing piles in China. They added K2HPO4, Ca3(PO4)2, Ca5(PO4)3OH, and Ca(H2PO4)2●H2O as soil amendments and observed that each treatment reduced Pb solubility and exchangeable Pb. The species Ca5(PO4)3OH was the most effective amendment for reducing Pb availability. All the phosphate-bearing amendments did promote an increase in solubility. Fayiga and Ma [22] demonstrated that phosphate reduced Pb availability; however, the arsenic availability increased. Cao et al. [23] demonstrated that phosphate effectively converted litharge, cerrusite and anglesite to pyromorphite in impacted soils. At pH 3 and pH 5 the Pb solubility followed the sequence litharge > cerrusite > anglesite, whereas at pH 7 the solubility sequence was anglesite > litharge > cerrusite. Chloropyromorphite was the dominant synthesized Pb -bearing product. Basta et al. [24] employed plant uptake and selected-sequential extractions to demonstrate that rock phosphate was effective in reducing Pb dissolution in simulations of gastrointestinal fluids, thus inferring reduced blood lead levels in children. In the UK, Caille et al. [25] demonstrated that arsenic solubility was increased in phosphate remediation of Pb-bearing soils. Debela et al. [26] demonstrated organic acids induced the release of Pb from pyromorphite. Debela et al. [27] observed that oxalic acid inhibited pyromorphite formation in Pb-impacted soils when amended with phosphate materials. They also demonstrated a similar response when citric acid was available, suggesting that organic acids may reduce the effectiveness of phosphate in remediating Pb-impacted soils. Topolska et al. [28] showed that Pseudomonas putida was able to mobilize Pb from pyromorphite and raising issues about the long-term stability of Pb- Phosphate sequestration. Zhang and Ryan [29] investigated chloropyromorphite formation kinetics from galena and hydroxyapatite in batch suspensions at pH levels ranging from pH 2 to pH 6 in unit increments. Lead solubility was greatest after 50 minutes of equilibration at pH 6 and the Pb decreased proportionally with increasingly acidic reactions. Soluble S concentrations were similar to the Pb concentrations, inferring the galena dissolution was stoichiometric. The galena dissolution was proposed as: + + 2+ PbS(s) + 2H = PbSH2 (s) = Pb + H2S In the suspensions the galena and apatite reaction to pyromorphite requires oxidation of sulfide to either elemental sulfur or sulfate to be thermodynamically favored: - + 2- 2+ Ca5(PO4)3(OH) + 5PBS + Cl +H + 10O2 = Pb5(PO4)3Cl + 5SO4 + Ca + 6H2O -1 ΔGf = -2,426KJ mol In a subsequent manuscript Zhang and Ryan [30] investigated chloropyromorphite formation kinetics from cerrusite and hydroxyapatite in batch suspensions at pH levels ranging from pH 2 to pH 6 in unit increments. The dissolution rates of cerrusite and apatite were rapid and chloropyromorphite formation was complete within 60 minutes when the amount of apatite was able to supply sufficient phosphate. The soluble Pb concentrations were related to the cerrusite dissolution. Lead Sequestration in the Soil Environment with an Emphasis on the Chemical… 13

Simulations employing MinteqA2 Using MinteqA2 software [31] Pb speciation may be estimated from thermochemical data for pH intervals from pH 4 to pH 8. Establishing a reasonably constant ionic strength using the back-ground solution chemistry [NO3, Cl, NH4, Ca, K, Mg, Na, SO4, PO4] of tile drainage effluent from the David M. Barton Agriculture Research Center [Missouri, USA], activity coefficients were calculated using the Debye-Huckel equation at 25ºC.

Simulation involving Galena and Pyromorphite Dissolution The initial lead solution concentrations [mol/L] were zero for the galena and pyromorphite dissolution simulation. Important input parameters for the dissolution of -4 -4 - galena and pyromorphite included: [NO3]=1.3 x 10 , [Cl]=8.5 x 10 , [NH4]=7.4 x 10 5 -4 -4 -4 -4 , [Ca] = 8.4 x 10 , [K] = 4.0 x 10 , [Mg] = 3.0 x 10 , [Na] = 1.4 x 10 , [SO4] = 2.7 x -4 -6 10 , [PO4] = 9.3 x 10 , pH = 7, PCO2 = 0.03 atm, 25ºC and Pe = 5 (296 mv or suboxic). Activity coefficients were estimated using the Debye-Huckel equation. Important Gibbs Free Energy and hydrolysis constants are in Tables 2 and 3. Clever and Johnson [32] provided Pb ion-pair thermodynamic data for may important Pb species.

Table 2. Important Gibbs Free Energy of Formation for Lead and Phosphorus Species

-1 Species ΔGf (kcal mole ) Pb2+ -5.8 Ca2+ -132.2 Cl- -31.37 OH- -37.59 - H2PO4 -270.2 2- HPO4 -260.3

H2O -56.69

Pb(H2PO4)2 -559.6 + PbHPO4 -281.8

Pb3(PO4)2 -565.0

Pb4O(PO4)2 [tetraplumbite orthophosphate] -617.3

Pb5(PO4)3OH [hydroxypyromorphite] -902.0

Pb5(PO4)3Cl [chloropyromorphite] -906.2

Ca5(PO4)3OH [hydroxyapatite] -1,511.6

PbAl3(PO4)2(OH)5●H2O [plumbogummite] -1,225.7

Pb2CuPO4(OH)3●3H2O (] -593 Source: [33]. 14 Michael Aide and Indi Braden

Table 3. Lead (Pb) hydrolysis constants. Species Log Kxy PbOH+ -7.71

Pb(OH)2 -17.12 - Pb(OH)3 -28.06 3+ Pb2(OH) -6.36 2+ Pb3(OH)4 -23.88 4+ Pb4(OH)4 -20.88 4+ Pb6(OH)8 -43.61 Sources: [5]

The simulation of the dissolution of galena and pyromorphite was performed. Given that the oxidation-reduction potential was established for suboxic conditions and the pH was standardized for pH 7, the following determinations were observed: (1) Pb2+ + and Pb(OH) were the dominant solution Pb-species, followed by PbSO4, PbCO3- + PbHCO3 , (2) galena dissolution permitted (supported) precipitation of hydroxylapatite and elemental sulfur, whereas the dissolution of pyromorphite supported only hydroxylapatite precipitation, (3) interestingly pyromorphite supported a slightly greater soluble Pb activity than galena (Table 4). Table 4. Lead hydrolysis, ion-pair formation and mineral dissolution in a defined chemical environment. ------log [activity] ------Species Galena Pyromorphite Pb2+ 10.5 (73.4%) 9.7 (72.4%) PbOH+ 11.1 (15.5%) 10.3 (15.1%) Pb(OH)2 13.6 12.8 - Pb(OH)3 17.6 16.8 3+ Pb2(OH) 20.3 18.8 2+ Pb3(OH)4 27.3 25.0 2- Pb(OH)4 22.2 21.4 4+ Pb4(OH)4 33.8 30.8 PbCl+ ----- 11.2 (1.7%) PbSO4 11.5 (5.3%) 10.7 (5.1%) + PbNO3 13.2 12.4 PbCO3 11.7 (3.6%) 10.9 (3.5%) + PbHCO3 11.9 (2.0%) 11.2 (2.0%) Galena: hydroxylapatite and Sº precipitate, Pyromorphite: hydroxylapatite precipitate 2+ -4 - Simulation initial parameters: [ ] = mol/L, [Pb ] = 0, [NO3]=1.3 x 10 , [Cl]=8.5 x 10 4 -5 -4 -4 -4 , [NH4]=7.4 x 10 , [Ca] = 8.4 x 10 , [K] = 4.0 x 10 , [Mg] = 3.0 x 10 , [Na] = 1.4 x -4 -4 -6 10 , [SO4] = 2.7 x 10 , [PO4] = 9.3 x 10 , pH = 7, PCO2 = 0.03atm, 25ºC and Pe = 5 (296 mv). Activity coefficients were estimated using the Debye-Huckel equation. Lead Sequestration in the Soil Environment with an Emphasis on the Chemical… 15

Simulation of Lead Hydrolysis and Precipitation Equilibria The initial lead concentrations were 10-3 mol Pb/L for Pb hydrolysis coupled with Pb and PO4 precipitation simulations. Important input parameters included the same solution chemistry as observed for the galena and pyromorphite dissolution. The pH was discretely varied in integral increments from pH 4 to pH 8. The total Pb solubility declined from pH 4 to pH 8, which roughly corresponds with the Pb solubility as proposed by Baes and Mesmer for PbO [5]. The dominant Pb species 2+ + + was Pb , followed by PbCl (pH 4 and 5), PbSO4 (pH 4 and pH 5), and Pb(OH) . For each of the pH intervals, the formation of pyromorphite was thermodynamically favored (Table 5). Anglesite was thermodynamically favored to precipitate at pH 5 and pH 6, whereas Larnakite was thermodynamically favored to form at pH 6 and Pb(OH)2 was thermodynamically favored to precipitate from pH 6 to pH 8.

Table 5. Lead hydrolysis, ion-pair formation and mineral formation in selected chemical environments. All values are -log [activity}. ------All values are -log [activity]------Species pH 4 pH 5 pH 6 pH 7 pH 8 Pb2+ 3.26 3.26 3.85 5.84 7.85 PbOH+ 6.86 5.86 5.44 6.45 7.44 Pb(OH)2 12.35 10.36 8.94 8.94 8.94 - Pb(OH)3 19.35 16.35 13.94 12.9 11.94 3- Pb2(OH) 8.92 7.91 8.09 11.09 14.09 2+ Pb3(OH)4 17.67 13.67 11.43 13.43 15.44 2- Pb(OH)4 26.96 22.96 19.54 17.55 15.55 4+ Pb4(OH)4 17.03 13.03 11.39 15.38 19.39 PbCl+ 4.83 4.83 5.4 7.4 9.4 PbSO4 5.1 5.1 5.89 6.89 8.89 + PbNO3 6.01 6.01 6.59 8.59 10.6 PbCO3 10.4 8.45 7.04 7.04 7.04 + PbHCO3 7.73 6.73 6.32 7.32 8.32 Note: Precipitated species ------All values are mol / liter------Pyromorphite 3.1 x 10-6 3.1 x 10-6 3.1 x 10-6 3.1 x 10-6 3.1 x 10-6 Anglesite 2.2 x 10-4 2.2 x 10-4 Larnakite 2.5 x 10-4 -4 -4 Pb(OH)2 3.0 x 10 9.8 x 10 9.8 x 10-4 Initial Pb concentration is 10-3 mol/L. Simulation initial parameters: [ ] = mol/L, -4 -4 -5 -4 -4 [NO3]=1.3 x 10 , [Cl]=8.5 x 10 , [NH4]=7.4 x 10 , [Ca] = 8.4 x 10 , [K] = 4.0 x 10 , -4 -4 -4 -6 [Mg] = 3.0 x 10 , [Na] = 1.4 x 10 , [SO4] = 2.7 x 10 , [PO4] = 9.3 x 10 , pH = 7, PCO2 16 Michael Aide and Indi Braden

= 0.03 atm, 25ºC and Pe = 5 (296 mv). Activity coefficients were estimated using the Debye-Huckel equation. The simulation was repeated for pH 6 with the inclusion of citrate at 10-3 mol/L. The Pb2+ - log[activity} was 4.37 (5.7%) and the Pb-Citrate -log[activity] was 3.06 (93.7%). Pyromorphite was predicted to form a finite solid phase; however, anglesite, larnakite and Pb(OH)2 were not favored to form finite solid phases.

Activity Diagram for Alamosite, Hydroxypyromorphite, Hydroxyapatite and Brushite Hydroxypyromorphite dissolution may be represented as: + - -4.14 Pb5(PO4)3OH + 7H = 5 pH + 3H2PO4 + H2O, where the Ksp is 10 [18] Similarly, Alamosite dissolution may be represented as: + 2+ 5.94 PbSiO3 + 2H + H2O = Pb + H4SiO4, where the Ksp is 10 [18] Obtaining the logarithms of these equations, coupled with their equivalents for hydroxyapatite and Brushite (CaHPO4●2H2O) the representations is plotted as an 2+ - activity diagram with log(Pb ) on the ordinate and log(H2PO4 ) on the abscissa. We do assume that unit activities are valid for the solid species and liquid water. We also -2 specify pH =7, temperature is 25ºC, the partial pressure of CO2 is 10 atm, the calcium activity is controlled by calcite and the silicic acid (H4SiO4) activity is controlled by noncrystalline silica (Figure 1). Thus, the activity diagram shows that solution equilibria for hydroxypromorphite is below the hydroxypyromorphite-alamosite line and between the stability lines for hydroxyapatite-hydroxypyromorphite and brushite- hydroxypyromorphite.

-4

-5 Alamosite

-6 Log(Pb2+) -7 Hydroxypyromorphite -8 Hydroxyapatite

-9 Brushite -10

-10 -9 -8 -7 -6 -5 -4 -3 log(H2PO4-)

Figure 1. Activity diagram for Alamosite, Hydroxypyromorphite, Hydroxyapatite and Brushite. Lead Sequestration in the Soil Environment with an Emphasis on the Chemical… 17

Using Thermodynamic Simulation to Refine Experimental Designs Given preliminary and relevant data collection of the proposed research study area, we use thermodynamic simulations to assist in experimental design and protocol advancement. Advantages of employing thermodynamic simulation prior to installing the research design include: (i) elucidating likely chemical speciation attributed to hydrolysis and complexation, (ii) assessing the role of temperature, (iii) potential mineral dissolution or precipitation, (iv) likelihood of oxidation – reduction reactions, and (v) the importance and influence of adsorption and cation exchange. The investigator has the potential to assess whether the thermodynamic system is open or closed and then assess and specify the role and intensity of CO2 and O2. Caution assessments include: (i) consideration that equilibrium is never completely attained, (ii) soils/sediments are much more complicated and ever-changing systems than simulations may accurately portray, (iii) kinetics may limit selected reactions even if thermodynamically favored, and (iv) the selection of data and the amount of data from the study area to provide realistic assessment.

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