selection and test of effective chelators for removal of heavy metals from contaminated soils

Selection and test of effective chelators for removal of heavy metals from contaminated soils

Ting-Chien Chen, Edward Macauley, and Andrew Hong

Abstract: Heavy metal contamination of soil is a common problem at many hazardous waste sites. Chelating extraction of heavy metals has been proposed as a remediation technique for contaminated soils. A useful method was developed, which assessed 190 ligands for their ability in extraction and recovery of target metals, including cadmium, copper, lead, mercury, nickel, and zinc. Chelator performance was evaluated based on equilibrium calculations with an emphasis on the potential of recovering both the metals and chelating agents. Batch equilibration experiments over 24-h periods were performed to test three chelating agents, S-carboxymethyl-cysteine (SCMC), N-2-acetamidoiminodiacetic acid (ADA), and pyridine-2,6-dicarboxylic acid (PDA), which were deemed suitable for the extraction of cadmium, copper, lead, and zinc from soil. All three chelators demonstrated high extraction capability toward their respective target metals across a wide range of pH, metal, and ligand concentrations. In addition, all three chelators exhibited good recovery potential at moderately elevated pH values. The potential of many chelating agents and their effective pH ranges in the remediation of soils contaminated with heavy metals are reported.

Key words: heavy metal, soil, contamination, chelation, remediation.

RCsumC : La contamination du sol par des mCtaux lourds est un problbme que l'on observe rCgulikrement dans de nombreuses dCcharges de produits dangereux. L'extraction par chClation de ces mCtaux lourds a kt6 proposCe comme mCthode d'assainissement des sols contaminks. Une mCthode utile a CtC mise au point et a permis d'Cvaluer le potentiel de 190 ligands comme agents d'extraction et de rCcupCration de mttaux cibles, dont le cadmium, le cuivre, le plomb, le mercure, le nickel et le zinc. Le rendement des chtlateurs a CtC CvaluC par des calculs d'tquilibre mettant l'accent sur le potentiel de rCcuptration des mttaux et des agents chClateurs. Des essais d'kquilibration par lot sur des pCriodes de 24 heures ont CtC effectuts afin d'Cprouver trois agents chklateurs, i savoir la S-carboxymCthylcystCine (SCMC), l'acide N-2-acCtamidoiminodiacCtique (ADA) et l'acide pyridine-2,6-dicarboxylique (PDA), qui ont CtC jugts acceptables pour I'extraction du cadmium, du cuivre, du plomb et du zinc. Tous trois ont dCmontrC d'excellentes capacitCs d'extraction de leurs mCtaux cibles respectifs sur une vaste plage de pH et de concentrations de mCtaux et de ligands. En outre, tous trois ont prCsentC un bon potentiel de rCcupCration i des valeurs de pH modCrCment ClevCes. Le potentiel de nombreux agents chClateurs ainsi que leurs plages de pH efficaces pour l'assainissement des sols contamints par des mttaux lourds sont prCsentCs dans les pages suivantes.

Mots elks : mktal lourd, sol, contamination, chClation, assainissement. [Traduit par la rCdaction]

Introduction copper, zinc, nickel, and mercury are among the most fre-

Soil contamination by heavy metals is a encountered at many hazardous waste site

qukntly reported heavy metal contaminants in National Priority common problem List sites, where they are often found at extremely elevated " concentrations. Reported sites were found to contain con-

Received July 13, 1994. Revised manuscript accepted April 25, 1995.

T.-C. Chen, E. Macauley, and A. Hongl. Department of Civil and Environmental Engineering, University of Utah, Salt Lake City, UT 84112, U.S.A.

Written discussion of this paper is welcomed and will be received by the Editor until April 30, 1996 (address inside front cover).

' Author to whom correspondence should be addressed.

taminant levels as high as 80000 ppm zinc, 1500 ppm cadmium, 70 000 ppm chromium, and in excess of 200 000 ppm lead (Peters and Shem 1992a). High metal concentrations in soils pose a serious risk to groundwater supplies. Once released into the soil matrix, most heavy metals are strongly retained, ensuring prolonged adverse impacts on environmental quality and human health.

Various techniques have been used for the remediation of contaminated soils (U.S. EPA 1990; Sims 1990). One promis- ing method for the remediation of heavy metal contaminated soils is to use chelators, either as on-site soil washing agents

Can. J. Civ. Eng. 22: 1185- 1197 (1995). Printed in Canada I ImprimC au Canada

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Can. J. Civ. Eng. Vol. 22, 1995

or for in situ remediation. Only a few familiar achelators (e.g., EDTA and NTA) have been studied for heavy metals removal. EDTA has been most extensively studied (Allen and Chen 1993; Borggaard 1979; Brown and Elliott 1992; Elliott and Brown 1989; Elliott et al. 1989; Ellis et al. 1986; Peters and Shem 1992a, 1992b; Slavek and Pickering 1986). The adsorption of copper, zinc, and nickel onto mineral solids in the presence of EDTA was also studied (Bowers and Huang 1986; Elliott and Huang 1979; Huang et al. 1988). The potential of NTA in mobilizing metals in soil was inves- tigated (Elliott et al. 1989; Linn and Elliott 1988; Peters and Shem 1992b), as were the potentials of DTPA, EGTA (Sommers and Lindsay 1979), hydroxylamine (Ellis et al. 1986), and citrate (Francis et al. 1992). Although extensive studies have been conducted over the past several decades related to metals partitioning in the presence of natural and synthetic chelating agents, the use of these ligands in extracting heavy metals from contaminated soils has been relatively recent. There is a lack of a developed methodology for properly choosing ligands to be used in the treatment applica- tion. In addition, many metals of interest form very stable complexes with EDTA and NTA, rendering subsequent recovery of the metals and these ligands very difficult. The recovery issue has not been adequately addressed, and has only begun to be examined (Allen and Chen 1993; Chen and Hong 1994; Hong et al. 1993a, 1993b; Macauley and Hong 1995). In order to assess the full potential of this technology, it is vital to systematically examine a large number of ligands for their selective removal of heavy metals, as well as their recovery potential.

The objectives of this research were (i) to provide a methodology to assess a large number of chelating agents in terms of their extraction and recovery potential toward heavy metals, (ii) to evaluate the results of 190 preliminarily screened ligands for the extraction of six target metals using equilibrium chemical calculations, and (iii) to present experimental verification for the predicated extraction and recovery potential of three selected chelators used on four studied metals.

Screening of potential chelating agents

Screening criteria Prior to performing full equilibrium calculations, hundreds of ligands were screened for their potential use as metal- extracting agents. General screening criteria were the fol- lowing:

1. The ligands must have high metal complexing abilities as indicated by their equilibrium complexation constants, especially toward heavy and transition metals, as opposed to hard sphere cations such as Ca or Mg.

2. Metals of interest in soil remediation are transition metals (e.g., Cu" and Nin) or B-type (soft sphere) cations (e.g., Znn, CdI1, Pb", and HglI). These metals form stable complexes with ligands containing sulfur or nitrogen as donor atoms, whereas A-type (hard sphere) cations (e.g., Ca2+ and Mg2+) prefer chelators with oxygen as donor atoms. Thus, ligands containing sulfur and nitrogen as donor atoms are preferred.

3. Ligands with multiple coordinating sites (multidentate ligands, or chelators) typically form more stable complexes.

Therefore, chelators that are tri-, tetra-, or hexadentate ligands (i.e., possess three, four, or six ligand groups, respectively) are envisaged to perform better in a dilute metal solution than would a monodentate ligand, and are therefore preferred.

As a result of screening hundreds of organic compounds, 190 chelators were selected for further evaluation, as listed in Table 1. All equilibrium constants used in calculations were obtained from the literature (Martell and Smith 1974, 1977, 1982; Smith and Martell 1975, 1976, 1989).

Evaluation of chelators for extraction and recovery potential

A complete summary of evaluation results for the screened chelators is given in Table 1. Three factors are relevant in assessing chelation potential for the removal of metals: (i) soil-metal interaction, (ii) aqueous equilibrium chemistry of metals, and (iii) coordination chemistry of metals with ligands. The first factor can impact partition of species between the soil and liquid phases, but its exact influence on metal extraction by chelation is difficult to assess without a full knowledge of the soil mineralogy and the equilibrium constants of various pertinent interfacial processes, such as complexation reactions among the metal and the mineral phases. The selection of suitable chelating agents as addressed in this paper is primarily based on consideration of the latter two factors.

Metal cations such as those divalent ions of interest here (Pb2+, Cd2+, Cu2+, Zn2+, Ni2+, and Hg2+) can be present in aqueous solution only at very low concentrations in the pH range (e.g., 6-9) of natural waters. When the concentra- tiois of these metals exceed the solubility of their corres- ponding hydroxide or carbonate phase at a given pH value, the metals will precipitate. Therefore, the metals, in particu- late forms, willintermingle with and become an integral part of the soil or sediment matrix.

Soils contain mineral and humic constituents which carry hydroxyl and carboxylic groups. The acid-base characteristics of these functional groups contribute to the formation, at the soil surface, of electrically charged groups important for metal retention. Thus, in addition to physical entrapment of metal hydroxide or carbonate solids, the soil can accommo- date metals through more specific interactions, including surface complexation and surface precipitation mechanisms (Dzombak and Morel 1990).

Equilibrium chemistry is important when considering the solubility of a particular metal. For example, lead in a carbonate-bearing natural water may be present as Pb2+, Pb(OH)+, Pb(OH),O, Pb(OH),-, Pb(OH),2-, Pb,(OH),+, P~,(oH),~ +, P~,(oH),~ +, P~~(oH): +, P ~ C O , ~ , P ~ ( c o ~ ) ~ ~ - , and PbHC03+ (Martell and Smith 1982; Smith and Martell 1976, 1989). The total aqueous lead concentration (Pb,') can be defined as

[I] Pb,' = [Pb2+] + [PbOHf] + [Pb(OH)?] + [Pb(OH),-]

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Table 1. Calculated effective pH ranges of various chelators toward enhancing metal solubility. Conditions: Me, = 1 mM, C, = 1 mM, L, = 10 mM. (Metal solubilities as S determined by stable phases of carbonates, hydroxides, or oxides.) CD 3

Cu2 + Pb2 + Cd2+ Zn2 + Ni2+ Hg2 +

2 Chelator

!E

1. Aminoacetic acid (glycine) CzHsOzN 3.1-11.1 (none) (none) 6.4-10.1 5.7-12.0 2.0- 10.3 2. L-2-Aminopropanoic acid (alanine) C3H70,N 3.3-11.0 (none) (none) (none) 6.1-11.3 (none) 3. DL-2-Aminobutanoic acid C4H902N 3.8-10.6 (none) * (none) 6.5 - 10.8 * 4. L-2-Amino-3-methylbutanoic acid (valine) CSHI ,O2N 3.6- 10.7 * (none) (none) 6.3-11.0 * 5. L-2-Amino-4-methylpentanoic acid (leucine) C6H~30zN 2.8-10.9 * (none) (none) 6.3 - 10.7 * 6. L-2-Amino-3-phenylpropanoic acid (phenylalanine) I O2N 3.2-11.0 (none) (none) (none) 5.8-11.0 * 7. L-Aminobutanedioic acid (aspartic acid) C4H704N 0.9-11.2 4.0-5.2 (none) 1.6-9.9 5.3- 12.1 * 8. L-2-Aminopentanedioic acid (glutamic acid) C5H904N 1.3-10.7 (none) (none) 4.1 -7.5 6.4- 10.8 * 9. L-2-Amino-3-(4-hydroxypheny1)propanoic acid (tyrosine) C 9 H ~ 1 ~ 3 ~ 3.4-11.4 (none) (none) 7.7- 10.1 6.1 - 12.2 *

10. L-2-Amino-3-(3,4-dihydroxypheny1)propanoic acid (L-DOPA) C 9 H ~ lo,N 3.2-14.0 (none) (none) 4.9- 14.0 5.9- 14.0 * 11. L-2-Amino-3-hydroxypropanoic acid (serine) C3H703N 2.5-14.0 (none) (none) 6.4-10.1 6.2- 11.4 * 12. L-2-Amino-3-hydroxybutanoic acid (threonine) C4H903N 1.7-14.0 (none) * 6.5-9.3 5.7-11.5 * 13. L-2-Aminobutanedioic acid 4-amide (asparagine) C4Ha03Nz 3.0-10.7 * * 5.6-10.3 5.4-12.1 * 14. L-2-Aminopentanedioic acid 5-amide (glutamine) C5H~003Nz 3.4- 10.4 (none) (none) 6.0- 13.3 5.9- 10.8 * 15. L-2-Amino-5-guanidopentanoic acid (arginine) C6H 1d0zN4 3.5- 10.2 * * 5.6- 12.5 2.9-12.0 (none) 16. L-2-Amino-3-mercaptopropanoic acid (cysteine) C3H702NS * 3.8- 14.0 4.2-14.0 5.2-13.6 4.0-14.0 (none) 17. D-2-Amino-3-mercapto-3-methylbutanoic acid (penicillamine) CSHIIOZNS * 4.5-14.0 4.2-14.0 4.5-14.0 5.2-14.0 2.9-8.7 18. DL-2-Mercaptohistidine C6H90zN3S * (none) 8.6-14.0 6.6-13.4 6.6- 13.8 (none) 19. DL-2-Amino-4-(methy1thio)butanoic acid (methionine) C 5 H ~ IOZNS 3.3- 10.6 (none) (none) 6.6-9.0 5.8- 10.9 (none) 20. 3,3'-Dithiobis(L-2-aminopropanoic acid) (cystine) C6HI2o4N2S2 0.6- 10.6 * (none) 5.7-10.2 3.3-11.7 * 21. L-2,5-Diaminopentanoic acid (ornithine) C~HIZOZNZ 3.1-11.4 * * (none) 1.3-12.2 (none) 22. L-2-Amino-3-(4-imidazolyl)propanoic acid (histidine) C6H90zN3 3.1 -13.7 (none) (none) 5.4-11.1 2.4-13.8 * 23. S-(1-Carboxy-3-phenylpropy1)-L-alanyl-L-proline (enalaprilat) CI8H2,OSN2 2.3-9.8 * * 1.8-13.0 * * 24. L-Py rolidine-2-carboxylic acid (proline) C5H90zN 3.8-11.5 * * (none) 6.7-11.8 * 25. N-(Phosphonomethy1)glycine (glyphosate) C3H805NP 2.3-11.4 * (none) 4.5- 11.4 5.0-12.0 * 26. DL-1,3-Thiazolidene-4-carboxylic acid (thiaproline) C4H702NS 2.2-9.0 * * 4.9-8.2 4.9-9.6 * 27. N-2-Aminoethylglycine (EDMA) C4H~oNz0z 2.3 - 13.9 (none) 8.9- 13.3 5.2-12 4.7- 14.0 * 28. Diethylenetriamine-N-acetic acid C~HISOZN~ 2.1 - 14.0 * * 3.8-13.9 4.3-14.0 * 29. Ethylenediiminodiacetic acid (EDDA) C6H 1zO4Nz 1.7-12.4 3.8- 12.8 (none) 3.6- 12.9 3.9- 13.8 * 30. Ethylenediiminodipropanedioic acid (EDDM) C~HIZOBNZ 1.9-12.3 3.4- 12.3 * 3.6-12.3 3.6-14.0 1.3- 11.1 3 1. Ethylenediiminodibutanedioic acid (EDDS) C~oH~60aNz 1.3-14.0 3.5-12.9 4.2-13.4 3.2-12.9 3.4-14.0 1.1-14.0 32. Ethylenediiminodi-2-pentanedioic acid (EDDG) C12H2008N2 2.8-11.8 3.8-7.6 (none) 4.2-12.0 4.1-13.6 0.7-14.0 33. DL-2,3-Diaminopropanoic-N,N '-dimalonic acid C~HIZOIONZ 1.1-13.6 4.5-12.7 3.5-13.7 3.4-12.5 4.2-13.4 1.6-14.0 34. Ethylenediiminobis[(2-hydroxy pheny1)acetic acid] (EHPG) 1 8 ~ 2 0 ~ 6 ~ 2 1.1-14.0 3.5-13.7 8.0-14.0 3.5-14.0 4.8-13.2 * 35. 6-Oxa-3,9-diazaundecanedioic acid CaH I 60sNz 3.0-11.6 * * 5.1-11.3 4.8-11.2 * 36. N,N-Dimethylglycine C4H90zN 4.7-9.8 * * * 7.2 - 10.2 * 37. N,N-Bis(phosphonomethyl)glycine C4Hl108NP2 3.3-10.3 * * * * * 38. N,N'-Bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid C20H2406N2 1.6-14.0 10.3-14.0 3.2-14.0 3.1-14.0 3.8-14.0 * 39. DL- 1-Methylethylenedinitrilotetraacetic acid N,N1-diamide CIIH,o06N4 0.0-14.0 0.0-14.0 1.5- 14.0 1.8-12.8 2.7-14.0 * 40. DL-1-Ethylethylenedinitrilotetraacetic acid N,N1-diamide CloH2204N4 0.1 -14.0 0.0-14.0 0.0-10.7 1.2-11.7 2.7- 14.0 * 41. N,Nf-Bis(2-aminoethyl)ethylenediiminodiacetic acid C~oHzz04N4 3.0- 14.0 * * 3.9-14.0 4.2-14.0 *

* * 2 42. 1,4,7-Triazacyclononane-1,4,7-triacetic acid CIZHZ,O~N~ 1.3-14.0 2.0-14.0 2.2-14.0 1.6-14.0 A

03 4

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Table 1 (continued). A

2

03

~i'+ 03 Chelator Cu2+ Pb2+ Cd2 + Zn2+ Hg2 +

43. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) 44. 1,4,7,10-Tetraazacyclotridecane-l,4,7,10-tetraacetic acid (TRITA) 45. Iminodiacetic acid (IDA) 46. 2,2'-Iminodipropanoic acid 47. N-Methyliminodiacetic acid (MIDA) 48. N-Propyliminodiacetic acid 49. N-Allyliminodiacetic acid 50. N-Benzyliminodiacetic acid 5 1. Nitrilotriacetic acid (NTA) 52. DL-2-Methylnitrilotriacetic acid 53. DL-2-Ethylnitrilotriacetic acid 54. DL-2-Hexylnitrilotriacetic acid 55. DL-2-(2-Methylthioethy1)nitrilotriacetic acid 56. N-(2-Hydroxybenzyl)iminodiacetic acid 57. 5'-Bis(carboxymethyl)an1inomethyl-3,3'-dimethyl4'-hydroxy-

fuchson-2"-sulfonic acid (Semi-Xylenol Orange) 58. DL-N-(2,3-Dihydroxypropy1)iminodiacetic acid 59. N-[1 , 1-Bis(hydroxymethyl)ethyl]iminodiacetic acid 60. N-(2-Carbamylethy1)iminodiacetic acid 61. Uramil-N,N-diacetic acid 62. N-(2-Hydroxyethy1)ethylenedinitrilotriacetic acid (HEDTA) 63. Ethylenedinitrilotetraacetic acid (EDTA) 64. DL-(Methylethy1ene)dinitrilotetraacetic acid (PDTA) 65. DL-(Hexylethy1ene)dinitrilotetraacetic acid 66. 1,2-Dimethylethylenedinitrilotetraacetic acid 67. trans-1,2-Cyclohexylenedinitrilotetraacetic acid (CDTA) 68. Trimethylenedinitrilotetraacetic acid (TMDTA) 69. DL-(1,3-Dimethyltrimethylene)dinitrilotetraacetic acid 70. DL-2,3-Diaminopropanoic-N,N,N',Nf-tetraacetic acid 71. 5,5'-Bis[bis(carboxymethyl)aminomethyl]-3,3'-dimethyl-

4'-hydroxyfuchson-2"-sulfonic acid (Xylenol Organge) 72. Glycyl-L-histidine 73. 1,2-Pheny lenedinitrilotetraacetic acid 74. Pyridine-2-carboxylic acid (picolinic acid) 75. Pyridine-2,6-dicarboxylic acid (dipicolinic acid, PDA) 76. Pyridine-2-acetic acid 77. N-(2-Pyridylmethy1)iminodiacetic acid 78. N-(6-Methyl-2-pyridy1methyl)iminodiacetic acid 79. L-2-Amino- 1-(3,4-dihydroxypheny1)ethanol (noradrenaline) 80. Ethylenediamine (en) 8 1. 4,7-Dithia- 1,lO-diazadecane 82. 2,2-Bis(aminomethy1)propylamine 83. Cis,cis-1,3,5-Triaminocyclohexane

1.6- 14.0 1.5 - 14.0 1.7- 12.0 1.8- 10.9 2.1 - 12.2 2.1 - 12.0 1.9-11.5 1.7-11.1 1.5 - 12.3 1.1-11.3 0.0- 11.2 1.4- 10.8 0.2- 10.0 1.1-12.4

3.3-8.5 1.3 - 10.0 1.8-11.6 1.5 - 10.6 0.5- 13.0 0.9- 13.0 1 .O - 14.0 1.2 - 14.0 1.2 - 14.0 1.3-14.0 1.1- 14.0 1.5-14.0 1.6- 14.0 1.5- 12.5

(none) 3.9- 14.0 0.7-11.9 0.8- 10.7 0.0- 11.5 2.7-8.3 0.9- 13.7 1.5 - 10.7 5.0- 14.0 4.2-13.1 4.6- 12.6 4.0- 12.6 5.8-11.1

* *

4.5-5.5 *

4.0-5.7 * *

3.6-6.6 1.8-12.3 1.4- 12.7 1.8- 12.5 1.9 - 12.0

4:

*

2.1-13.1 2.3- 11.5 1.7-13.8 2.6-8.8 1.2-13.5 1.6-13.9 1.2-13.8 1.3 - 14.0 1.3 - 14.0 1.9-14.0 2.5- 14.0 2.9-13.2 4.4-12.9 2.1 - 14.0

0.0- 13.2

* *

0.6-11.0 *

0.9- 13.7 1.5- 10.7 (none) (none) (none)

* *

A

4

(none)

9.3-13.3 9.9-13.4 7.6- 13.2 8.2-9.8 2.9- 14.0 2.1 - 13.4 2.5-13.2 2.6-13.1 (none)

*

11.7-13.2 3.5- 13.2 3.3-5.9 7.4-9.7 1.3-14.0 1.8-14.0 1.6- 14.0 1.6- 14.0 1.6- 14.0 1.7- 14.0 1.4- 14.0 3.1 - 14.0 3.8 - 14.0 2.4- 14.0

(none) (none)

* 1.4- 14.0 1.5 -9.4

* 2.0 - 14.0 2.7-13.8 (none) (none) (none) (none)

*

1.8- 14.0 2.0- 14.0 4.7-11.3

i;

4.4- 12.1 4.5- 12.4 3.6- 12.1 4.0-11.4 2.4- 12.8 1.9-11.6 2.3-11.3

* 4.1-9.8 3.0- 14.0

2.3-13.7 2.8- 10.9 2.2- 12.0 2.6-11.1 1.5-13.2 1.9-13.2 2.1 - 14.0 1.7-13.9 1.6-14.0 2.0-13.8 1.6- 14.0 2.5- 13.4 2.6- 13.8 2.3- 13.4

0.0- 12.9 5.3- 14.0 1.4- 12.8 2.2- 10.5 1.6-11.0 5.2-7.7 1.8-11.4 2.3-11.0 6.6- 14.0 6.7-11.0 (none) 6.7- 10.5 7.6- 13.2

* 4:

1.7-5.6 *

(none) * * 'I;

1.0-7.6 * * * * *

(none) (none)

* *

0.4- 14.0 0.0- 14.0 0.0- 13.2 0.4- 13.2 0.6- 13.3 0.1 - 14.0 0.5-11.6 1.4-12.1 0.4- 14.0

'I;

* 0.8-10.5 0.3-8.6 0.0- 10.9 2.0-7.7 0.3- 12.7 0.6- 12.7

*

2.2-13.6 * %

*

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Table 1 (continued). 9 0

Pb2 + Cd2 + N,'+ 3

C helator Cu2+ Zn2 + HgZ+ m &

84. L-2-(Methylamino)-1-(3,4-dihydroxypheny1)ethanol (adrenaline) C9H1303N 5.1 - 14.0 (none) (none) 7.0- 14.0 7.4-10.7 * E.

85. N-Methylethylenediamine C3H~oN2 4.3 - 12.6 * 8.4- 14.0 (none) 6.0- 12.5 4.5- 12.0 86. 1,4,7,10-Tetraazadecane (trien, 2,2,2-tet) C6H~8N4 2.4-14.0 5.7-10.7 7.7-13.4 5.1-12.1 4.8-14.0 1.4-14.0 87. 1,4,8,11 -Tetraazaundecane (2,3,2-tet) C1H20N4 2.7- 14.0 (none) 7.8-13.5 5.4-12.5 4.8-14.0 3.0-12.8 88. 1,4,9,12-Tetraazadodecane (2,4,2-tet) CsH22N4 4.3-13.9 * (none) 6.7-11.0 5.3-13.1 1.7-12.9 89. 7-Thia- 1,4,10,13-tetraazatridecane CsHz2N4S 3.4-14.0 * * 5.6-11.7 4.8-14.0 * 90. Nitrilotris(2-ethylamine) (tren) C6H~8N4 3.3 - 14.0 * 7.5-13.7 4.9-14.0 5.0-14.0 1.5-13.2 91. 1,4,7,10-Tetraazacyclododecane ([12]aneN4) CsH20N4 0.4-14.0 3.2-13.9 4.0-14.0 3.0-13.6 4.2-14.0 0.0-14.0 92. 1,4,7,1O-Tetraazacyclotridecane ( [ 13]aneN4) C9H22N4 0.0-14.0 4.8-13.1 7.7-14.0 3.7-13.5 4.0-14.0 0.2-14.0 93. 1,4,8,11-Tetraazacyclotetradecane ([14]aneN4) C1oH24N4 0.3-14.0 (none) 9.7-13.8 4.3- 13.4 3.2- 14.0 1.0- 13.3 94. 1,4,8,11-Tetramethyl-1,4,8,ll-tetraazacyclotetradecane C~4H32N4 1.9-13.5 * 9.1-14.0 5.0-14.0 5.9-11.8 1.3-11.9 95. 1,4,8,12-Tetraazacyclopentadecane ([15]aneN4) I I H26N4 2.0- 14.0 * * 4.5- 13.3 4.2 - 14.0 2.1 - 13.6 96. 1,4,7,10,13,16-Hexaazacyclooctadecane ([18]aneN6) C~2H30N6 0.2-13.4 5.9-13.5 4.1-14.0 4.1-14.0 4.2-14.0 1.2-14.0 97. 1,4,8,11-Tetra(2-hydroxyethy1)-l,4,8,11 -tetraazacyclotetradecane C,8H4004N, 2.2-12.1 (none) 7.1-14.0 6.3-12.1 5.9-12.1 1.4-10.8 98. 2-(Methanesulfonamidomethy1)pyridine ClHlo02N2S 4.0- 12.1 (none) (none) (none) 6.5 - 11.8 (none) 99. 2,s-Bis[l-(2-aminoethylamino]ethyl]pyridine (epyden) C13H25N5 2.2- 14.0 * * 4.3- 13.4 4.3- 14.0 * 100. 2,2'-Bipyridyl C,oH8N2 0.6-10.9 3.6-5.4 2.2-7.1 1.6-10.8 2.0-14.0 0.0-11.8 101. 1,9-Di(2-pyridy 1)-2,8-diaza-5-oxanonane C~6H220N4 1.7-12.0 * * 4.1 - 10.9 3.8 - 13.7 * 102. 1,9-Di(2-pyridyl)-2,8-diaza-S-thianonane 1 6 ~ 2 2 ~ 4 ~ 0.8-13.3 * * 3.1-11.4 3.3-14.0 * 103. 1-w-Di(2-pyridy1)-2,5,8-triazanonane (pydien) C~6H23N5 0.9- 14.0 * * 2.7- 13.0 2.7-14.0 * 104. 2,6-Di(2-pyridy1)pyridine (2,2',2"-terpyridine) C I ~ H I L N ~ 0.0- 14.0 * 2.6-6.0 2.2-9.1 1.6- 14.0 * 105. 1,lO-Phenanthroline C ~ ~ H ~ N 2 0.4- 14.0 (none) 1.4-13.4 1.2-12.8 1.6-14.0 0.0-13.8 106. Iminobis(methylenephosphonic acid) C2H9O6NP2 4.2- 10.0 4.4-6.9 (none) 5.3 - 10.2 6.0- 10.5 * 107. Ethylenedinitrilotetrakis(methylenephosphonic acid) C6HloO12N2P4 1.2- 14.0 * * 2.6- 14.0 3.9- 14.0 * 108. 3-Hydroxypropanoic acid C3H603 (none) (none) (none) (none) (none) * 109. DL-(2-Mercaptopropiony1)glycine C5H,0,NS * 3.6-10.8 6.6-13.5 5.0-10.8 4.8-14.0 * 110. Ethanedioic acid (oxalic acid) C2H204 1.2-8.4 1.4-6.6 2.9-5.9 1.6-8.6 4.0-9.5 0.0-6.6 11 1. Propanedioic acid (malonic acid) C3H404 2.5-7.3 4.7-5.3 (none) 3.9-8.1 5.2-8.9 * 112. But-3-ene-1,l-dicarboxylic acid (allylmalonic acid) C6H~04 2.9-6.7 * (none) 5.0-6.9 5.5-8.2 * 1 13. L-Hydroxybutanedioic acid (malic acid) C4H605 2.8-7.8 3.2-5.3 (none) 4.0-7.5 5.0-8.5 * 114. Thiodiacetic acid C4H604S 2.6-7.1 1.8-5.8 3.6-5.2 3.5-8.1 4.3-9.4 0.0-8.8 115. (Ethy1enedithio)diacetic acid C6H~004S2 0.0-9.0 2.7-5.9 3.3-4.8 3.6-7.1 0.6-9.1 0.0-10.3 1 16. Ethylphosphinodiacetic acid C6H I lo,P * * * * 4.5- 10.4 0.0- 14.0 117. 3-Carboxymethylpentandioic acid C~H~006 (none) * * (none) (none) * 118. 2-Hydroxypropane-l,2,3-tricarboxylic acid (citric acid) CsH801 1.3-10.6 3.8-5.9 (none) 3.5- 12.3 4.7- 10.2 1.9-7.2 1 19. Ethylenediphosphinotetraacetic acid C~oH~608P2 * 1.2-7.7 1.3-6.8 3.8-8.0 1.3-14.0 0.0-14.0 120. Benzo- l,3-dioxole-2,2-diphosphonic acid CIH~O~P~ 3.0-8.7 * * 3.7- 10.6 5.4- 10.4 * 121. 7-Chloro-6-demethyltetracycline (demethylchlorcycline) C2,H2208N2C1 2.5-10.3 * (none) 2.0-9.9 4.7-11.5 * 122. 2-Mercaptoethanol C2H60S * 0.0-5.4 8.6-13.3 6.0-11.4 4.2-8.9 * 123. L-2-Amino-3-phenylpropanoic acid (phenylalanine) 'gH1 4.3-13.8 * (none) 6.0- 12.5 6.2- 13.3 * 124. L-2-Amino-3-(3-hydroxy-4-oxo-1,4-dihydro-l-pyridinyl)propanoic acid C8Hlo04N2 0.0-11.6 2.0-12.3 (none) 2.5-11.6 * * 125. DL-1 ,3-Diaminopropane-N,N1-dibutanedioic acid C,,H1808N2 2.4- 10.2 (none) * * * 0.0- 14.0

* 2

126. Oxybis(ethyleneiminomalonic acid) C ,oH I 60gN2 1.9 - 14.0 4.4-13.5 4.5-11.7 4.7-12.9 0.1-14.0 A

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Table 1 (continued).

Chelator Cu2 + Pb2+ Cd2 + Zn2 + Ni2+

127. 3-(3-Hydroxy-4-oxo-1,4-dihydro-l-pyridinyl)propanoic acid CeH904N 2.3-11.8 2.7-12.5 (none) 4.0-11.3 * 128. 1 ,5-Diazacyclooctane-N,N1-diacetic acid CloH1804N2 1.0-13.6 * * 3.6-13.7 4.8-14.0 129. N-(2-Carboxyethyl)iminodiacetic acid C7H ,lo,N 1.8-10.6 * 3.6-5.6 2.8-11.1 3.8-12.6 130. Ethylenebis-N,Nf-(2,6-dicarb0xy)piperidine C16H2,08N2 1.6-13.4 2.1 -14.0 3.0-14.0 2.4-13.3 3.2-14.0 131. L-Histidylglycine CSHIZO~N~ 3.3- 14.0 * * 5.4-9.4 4.8-12.1 132. L-5-Glutamyl-L-cysteinylglycine (glutathione) C~oH~706N3S * 3.2-14.0 3.5-14.0 4.9-13.3 6.2-11.1 133. N-(2,5-Dicarboxyphenyl)iminodiacetic acid CIZHI 108N 0.3-9.7 * 0.0-7.9 2.1-9.9 3.5-11.8 134. 1,4,8-Triazaoctane (2,3-tri) C5H~sN3 3.8-14.0 * (none) 6.5-11.9 5.6-14.0 135. 3-Hydroxy-1-methyl-l,4-dihydropyridin-4-one C6H,0zN 2.4-11.5 2.8-12.5 (none) 4.1 - 11.5 * 136. 1,4,7,10,13-Pentaazacyclopentadecane C~oH25Ns 0.8-14.0 3.7-14.0 3.1-14.0 3.1-14.0 * 137. 2-(Methylaminomethy1)pyridine C7H ION, 2.2-11.6 * (none) 6.1-9.0 5.0- 12.4 138. 7-(4-Sulfo-l-naphthylazo)-8-hydroxyquinoline-5-sulfonic acid C19H1307N3S2 0.7-13.3 1.5- 12.8 * 2.1 -12.0 3.2-13.6 139. Ethylenebis(irninoethylenephosphonic acid) C6H1806N2P2 2.4- 13.4 * * * 4.6-13.4 140. Ethylenebis[irnino(dirnethyl)methylenephosphonic acid] C8H2206N2P2 2.3 - 14.0 * * * 5.1 - 12.4 141. l-Hydroxy-3-(dirnethylamino)propane-l,l-diphosphonic acid C5H1507NP2 2.4- 10.6 * * 4.4-11.2 5.3-11.1 142. Nitrilotris(rnethylenephosphonic acid) C3HI2O9NP3 1.2- 13.0 * * 0.8-13.7 4.3-12.3 143. N,N'-Dimethylethylenebis(nitrilomethylenephosphonic acid) C6HI8O6N2P2 1.8-14.0 * * * 4.3- 14.0 144. Oxydiacetic acid (diglycolic acid) C4H60s 2.9-6.4 2.5-6.1 3.5-5.0 2.9-7.9 4.6-8.8 145. 2,3-Dihydroxynaphthalene-6-sulfonic acid C~oH80sS 5.0-14.0 * 12.0-13.6 8.0-13.1 7.5- 13.5 146. L-2-Amino-3-(3-indolyl)propanoic acid (tryptophan) CllH1202N2 2.7- 11.6 (none) (none) 6.3- 10.9 5.7- 12.6 147. N,N'-Bis(2-pyridylmethyl)ethylenedinitrilo-N,N'-diacetic acid C18H22N404 0.7-14.0 * 1.8-14.0 1.8-13.4 2.8-14.0 148. N-(3,3-Dimethylbutyl)iminodiacetic acid C,oH~904N 1.9-12.3 4.2-5.9 8.8-13.6 4.4-12.2 4.7-14.0 149. N-(Cyanomethy1)irninodiacetic acid C6Ha04Nz 1.4-9.2 * 2.5-7.1 2.0-9.9 3.2- 11.5 150. cis(C0,H)-2,6-Dicarboxypiperidine-N-acetic acid C9H~306N 1.1-10.9 1.6-12.4 2.8-13.1 2.0-11.2 3.8-12.3 151. N-(2-Hydroxyethy1)irninodiacetic acid (HIDA) C6H~~0~N 0.7-14.0 0.9-13.6 3.5-13.3 2.5-13.2 4.1-13.1 152. N-(2-Methoxyethyl)irninodiacetic acid C7H~30~N 1.0-14.0 2.0-13.3 3.5-13.6 2.7-12.5 4.3-13.3 153. N-(2-0xopropyl)irninodiacetic acid (N-acetonyliminodiacetic acid) C7H~ 1 ~ 5 ~ 1.9-10.3 2.5-8.5 3.1-9.5 3.0-10.5 4.3-12.2 154. N-(Benzoylmethy1)irninodiacetic acid CIZHI~O~N 1.7- 10.4 * 2.6- 13.3 2.9-10.7 4.1 -12.8 155. N-(Carbarnylrnethy1)iminodiacetic acid

N-2-acetamidoiminodiacetic acid (ADA) C6H~005Nz 1.2-14.0 1.5-9.3 2.9-9.2 2.5-10.9 3.9-11.7 156. N-[2-(Ethoxycarbamyl)ethyl]iminodiacetic acid C9H~606N2 1.5-11.2 3.4-8.9 8.0-9.2 3.8-11.2 4.4-13.0 157. N-(2-Mercaptoethyl)iminodiacetic acid C6H~ lo,NS * 2.2-14.0 2.3-14.0 2.6-13.5 4.4-14.0 158. N-[2-(Methylthio)ethyl]iminodiacetic acid C7H~304NS 0.8-12.3 2.1-11.0 3.1-13.6 2.7-11.6 3.9-13.4 159. N-(2-Aminoethyl)iminodiacetic acid CsH1204Nz 1.6-14.0 3.1-13.0 4.0-14.0 3.3-13.4 3.9-14.0 160. 2-[Bis(carboxyrnethyl)arnino]ethyltrimethylarnrnonium (perchlorate) C9H~904N2+ 1.4-10.0 2.5-6.8 2.9-7.1 2.4-10.0 3.5-11.9 161. DL-(2-Methylpropylethylene)dinitrilotetraacetic acid C 1 4 ~ 2 4 ~ 8 ~ 2 1.2-14.0 1.3-14.0 1.4-14.0 1.6-14.0 * 162. DL-(1,2-Dirnethy1ethylene)dinitrilotetraacetic acid C12H2008N2 0.9-14.0 1.2-14.0 1.4-14.0 2.4-14.0 2.4-14.0 163. meso-(1,2-Dimethylethylene)dinitrilotetraacetic acid C12H2008N2 0.6-14.0 0.9-14.0 1.3-14.0 1.3-13.9 2.6- 14.0 164. DL-(Phenylethylene)dinitrilotetraacetic acid C16H2008N2 1.0-13.5 1.0-14.0 1.3-14.0 1.4-13.8 2.6-14.0 165. DL-Ethylenedinitrilo-N,N1-di(2-butanoic acid)-N,N '-diacetic acid C14H2408N2 1.3-13.6 1.8-14.0 2.1-14.0 1.9-13.6 * 166. trans-1,2-Cyclopentylenedinitrilotetraacetic acid (CPDTA) CI 3H2008Nz * 1.3-14.0 1.5-14.0 * * 167. Ethylenedinitrilo-N,N'-di(3-propanoic)-N,N'-diacetic acid C,2H2008N2 2.2-12.4 3.0-13.0 3.4-13.8 2.6-13.2 3.5-14.0 168. Tetramethylenedinitrilotetraacetic acid C,2H2008N2 1.7-12.9 1.8-6.8 7.0-13.8 3.2-13.3 3.8-14.0

(none) (none)

* (none) (none)

*

(none) (none) 2.6-11.9 (none) (none) E = (none)

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Table 1 (concluded). 9 m

Chelator Cu2 + Pb2+ Cd2+ Zn2 + Nil+ Hg2 + 3

m +

169. Pentamethylenedinitrilotetraacetic acid C I ~ H ~ ~ O ~ N , 1.9-12.3 3.6-6.7 8.0-13.7 3.7-12.7 4.4-13.8 * E 170. Hexamethylenedinitrilotetraacetic acid C14H2,0,N2 3.6-12.1 3.5-6.9 8.1- 13.8 1.2-12.5 1.9- 13.8 1.7- 12.4 171. Octamethylenedinitrilotetraacetic acid C~6H280nN2 1.9-12.1 3.7-6.5 8.1-13.8 0.8-12.4 1.8-13.8 1.5-12.6 172. (2-Hydroxytrirnethy1ene)dinitrilotetraacetic acid C I I H I ~ O ~ N ~ 1.1-13.0 0.0-11.3 2.6-13.9 2.0-13.0 3.2-14.0 0.0-11.0 173. Oxybis(ethylenenitrilo)tetraacetic acid (EEDTA) C I ~ H ~ o O ~ N ~ 1.2-13.3 2.3-13.6 2.0-14.0 2.6-13.4 4.0-14.0 0.0-13.2 174. Ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) CI4H2,OloN2 0.0-13.0 0.0-13.5 1.7-14.0 0.2- 12.7 1.1 -13.8 0.0-13.2 175. Thiobis(ethylenenitri1o)tetraacetic acid (TEDTA) C12H2008N2S 1.2- 12.5 2.3-13.2 2.5-14.0 2.6- 12.9 3.7-14.0 0.0-13.6 176. Diethylenetrinitrilopentaacetic acid (DTPA) C14H23010N3 0.0- 14.0 0.8- 14.0 1.2- 14.0 0.3- 14.0 0.7- 14.0 0.5 - 14.0 177. Triethylenetetranitrilohexaacetic acid (TTHA) CIBH30012N4 0.0- 14.0 0.0- 14.0 1.4- 14.0 0.0- 14.0 0.2- 14.0 0.0- 14.0 178. 6-Methylpy ridine-2-carboxylic acid C7H702N 1.5-9.6 4.0-5.7 3.7-6.2 3.4-9.3 4.3-11.1 0.0-8.9 179. 4-Aminopyridine-2,6-dicarboxylic acid C7H60,N2 1.4-13.5 c 3.3-13.7 2.9-13.1 4.3-13.8 0.0-13.0 180. 2-Aminoethanethiol C2H7NS * 5.0-11.6 6.5-14.0 5.6-13.8 5.5-14.0 * 181. L-Cysteine methyl ester C4H902NS -F 4.1 - 13.4 4.2- 14.0 * 2.4- 14.0 .k

182. DL- l -Methylethylenediarnine (pn) C3H~oN2 4.0- 13.0 (none) (none) 6.5- 10.5 5.6- 13.8 3.6- 12.5 183. 1,4,7-Triazaheptane (dien) C4H~3N3 2.6- 14.0 (none) 8.4-13.8 6.0-12.3 5.3-14.0 0.0-13.3 184. 1,4,7,10,13-Pentaazatridecane (tetren) CnH23N, 1.8-14.0 7.8-10.8 6.0-14.0 4.6-13.3 4.5-14.0 1.8-14.0 185. Ethylenedinitrilotetrakis(2-ethylamine) (penten) C~~H2nN6 2.3 - 14.0 c 6.7-14.0 5.0-13.6 4.9-14.0 0.0-14.0 186. Ethylenebis[imino(phenyl)methylenephosphonic acid] C16H2206N2P2 1.9-12.6 * 4.1 - 13.4 3.4- 12.6 4.6- 12.7 * 187. Oxybis[ethyleneimino(dirnethyl)methylenephosphonic acid] CloH260,N2P2 3.9-12.3 4.2-6.7 9.7-13.9 5.3-11.5 (none) +:

188. Thiobis[ethyleneimino(dirnethyl)methylenephosphonic acid] CloH2606N2P2S 3.9- 12.2 4.1 - 13.4 8.7- 14.0 5.1 - 12.6 6.4- 12.5 * 189. Iminobis[ethyleneimino(dimethyl)methylenephosphonic acid] CloH2706N3PZ 2.3-14.0 3.9-14.0 7.8-14.0 4.3-13.5 5.0- 14.0 * 190. L-2-Amino-3-(carboxya1kglthio)propanoic acid

(-carboxymethyl-cysteine) (SCMC) C,H,O,NS 2.8-10.8 (none) * 5.5-9.8 5.0-11.2 * Notes: Effective pH range evaluated between 0 and 14; * denotes equilibriu~n constants not available; (none), the ligand is not effective toward the metal at any pH. Controlling mineral phases: Cu: Cu2(OH),C03(s) for pH < 8.1; CuO(s) for pH > 8.1; Zn: ZnC03(s) for pH < 9.0; ZnO(s) for pH >9.0; Cd: CdC03(s) for pH < 12.7; Cd(OH)2(s) for pH > 12.7; Pb: PbC03(s) for pH < 10.7; Pb3(OH)2(C03)2(~) for pH between 10.7 and 12.5; PbO(s) for pH > 12.5; Hg: HgO(s) for all pH; Ni: NiC03(s) for pH < 7.3; Ni(OH),(s) > 7.3

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Can. J. Civ. Eng. Vol. 22, 1995

Fig. 1. Lead speciation and solubility with and without PDA chelator as a function of pH.

1 I=O.l M

-CT= 1 mM PbCOs(s) Pb(OH)z(s)

P b ~ = 1 . 1 2 mM 1 PDA-enhanced Pb solubility

P D A T = ~ mM

-

PbHC03 + /

-

-

Figure 1 plots the total solubility of lead and solubilities of contributing lead species over a broad pH range. Equilibrium reactions and constants used to construct this figure are listed in Table 2. When the solubility of lead is exceeded, it can exist in solid forms as oxide, hydroxide, and (or) carbonate forms (e.g., PbO(s), Pb(OH),(s), PbCO,(s), and Pb,(OH),(CO,),(s)). The logarithmic function of the activity ratio, i.e., {mineral solid)/ {Pb2+), for Pb3(OH)2(C03)2(~) can be defined as (Stumm and Morgan 1981)

A determination of the relative activity ratios for PbCO,(s), Pb,(OH),(CO,),(s), PbO(s), Pb(OH),(s), and various other solid phases (Table 2) throughout the pH range 0- 14 sug- gests that the various predominant solids controlling lead solubility are PbCO,(s) for pH < 10.7, Pb,(OH),(CO,),(s) for pH between 10.7 and 12.5, and PbO(s) for pH > 12.5. It should be noted that PbCO,(s) and Pb(OH),(s) were used as controlling solids in solubility calculations for Fig. 1, because they are more commonly found than Pb,(OH),(CO,),(s) and especially when little aging has occurred.

In order to be effective at extracting metals from a soil and concomitantly forming soluble complexes, a ligand must overcome competing processes, including ( i ) metal precipitation as hydrous oxide and (or) carbonate precipitates, and (ii) surface complexation and precipitation on soil particles. Relevant equilibrium reactions and associated constants are needed to assess the complexation ability of a particular chelator toward a particular metal. The presence of metal- binding ligands will increase the total metal solubility. For example, in the presence of PDA, the total lead solubility (Pb,) can be written as

[3] Pb, = Pb,' + Pb(PDA)O + Pb(PDA),,-

Lead solubility in the presence of PDA ligand is also shown in Fig. 1.

For a complex-forming ligand, a degree of complexation (defined as PbT/PbTr) can be computed for that ligand over a wide pH range. Figure 2 shows the degrees of complexation of PDA toward Pb and toward Cd over a wide pH range; also plotted in this figure are the degrees of complexation of SCMC and ADA toward Cu and Zn, respectively. Accord- ing to Fig. 2, the effective pH range of PDA toward lead solubilization is determined to be below pH 8.9. The highest effective pH is taken at the intersection point between the degree of complexation curve with the chelator and the one without the chelator. In the interest of recovering the metal and the chelator, a suitable chelating agent for a specified metal should have an intersection at a moderate pH. An overly strong chelator that renders metal recovery difficult can be readily identified by a high Me-L complexation curve without the intersection. For example, EDTA is highly effective at enhancing metal solubility; however, the resulting metal complexes of EDTA are so stable that the metal cannot be reversibly released by the ligand even at high pH (> 11). This behavior has been confirmed by the absence of any intersection point between various Me-EDTA complexation curves and those respective Me precipitation curves without EDTA. Computation results of Table 1 suggest that EDTA is capable of solubilizing metals well beyond pH 13, thus EDTA is incapable of reversibly releasing an extracted metal.

The degrees of complexation of 190 ligands toward the six target metals have been evaluated using procedures similar to that shown for PDA toward Pb. The results are summarized in Table 1. Table 1 is intended as a guide to selecting suitable chelating agents for the extraction and recovery of the listed metals. As a preliminary guide, the results of Table 1 were calculated using equilibrium constants available for the lowest

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Chen et al. 1193

ionic strength (0, 0.1 M, or higher) without nonideality cor- rection. The total metal, carbonate, and ligand concentrations were set to be 1, 1, and 10 mM, respectively. However, nonideality due to high ionic strength (0.1 M) was considered in obtaining the calculated results plotted in Figs. 1, 2, and 4 (the remaining figures 3 and 5 plotted experimental results only).

In this study, three chelators (SCMC, ADA, and PDA) showing good extraction and recovery potential according to calculations have been selected for experimental verification, as the balance of this paper reports.

Chelating extraction experiments and results

Materials and methods Deionized water (18 MO . cm) obtained from a Milli-Q system (Millipore) was used in all procedures. The three chelators (SCMC, ADA, and PDA) were used as received (Fluka). Soils were taken from a Salt Lake City site, air- dried for one month, and then passed through a 2 mm sieve. The soil was characterized to show pH 7.8 and 7.5 in water and in CaC12, respectively; pH, , 8.1; specific gravity, 2.65; cation exchange capacity, 13.$Cmeq/100 g; respective size fractions of 90.4%, 7.5 %, and 2.1 % as sand, silt, and clay; 89.7 % and 10.3 % ash and organic contents, respectively, in the dry sieved fraction ( ~ 0 . 1 4 7 mm) (Chen and Hong 1994; USBR 1989); and background metal contents of 2, 5, 100, 6, and 290 ppm (i.e., mg/kg soil) for Cd, Cu, Pb, Ni, and Zn, respectively, as determined by batch extraction performed at pH 2 over 8 h. Experiments were conducted in 125 mL polyethylene and glass Erlenmeyer flasks using a batch solution volume, V, of 100 mL. No difference in results was seen between experiments carried out in glass flasks and in polyethylene flasks. Ionic strength, I, was held at 0.1 M using NaC104 (99 %) to eliminate variations among individual runs. Total carbonate concentration, CT, was added at 1 mM using NaHC03. All pH adjustments were performed manu- ally with either a 5 M HN03 or a 5 M NaOH solution, and pH measurements were made with an Orion model SA 720 pH meter. Stock solutions (1000 mg/L) of Cu, Zn, Ni, and Pb were prepared according to ASTM methods D1688, D1691, D1886, and D3559, respectively. A commercially available cadmium stock solution (1000 pg/mL) was used (J.T. Baker). A gyratory shaker table (New Brunswick Scien- tific Co., Model G-2) was used to provide agitation during adsorption and extraction procedures. Soils were kept in suspension by shaking at 260 rpm. All experiments were conducted at room temperature (22°C + 1 "C). In measuring the total aqueous metal concentrations (MeT), aliquots were withdrawn from the reaction mixtures, filtered through a 0.45 pm filter (Gelman Sciences sterile aerodisc), and acidified with nitric acid. Metal contents of samples were analyzed by atomic absorption (AA) spectrometry (Perkin Elmer Model 280) using ASTM methods D3557 for Cd, Dl688 for Cu, Dl886 for Ni, D3559 for Pb, and Dl691 for Zn. Standard procedures were followed when available (APHA et al. 1985; ASTM Annual Standards).

Typical concentration conditions in adsorption and extraction experiments were 50 mg/L Me, 2 g soil, I = 0.1 M, CT = 1 mM, and V = 100 mL. If necessary, an initial adjustment of pH was made. Mixtures were agitated continu-

Table 2. Equilibrium reactions of lead in carbonate-bearing water (Martell and Smith 1982; Smith and Martell 1976, 1989).

log K Equilibrium reaction (25"C, I = 0)

PbO(s, red) + H20 = Pb2+ + 20H- Pb(OH),(s) + 2H20 = Pb2+ + 20H- 0.5Pb20(OH)2(s) + 0.5H20 = Pb2+ + 20H- PbO(s, yellow) + H20 = Pb2+ + 20H- PbC03(s) = Pb2+ + C03'- Pb3(OH)2(C03)2(s) = 3Pb2+ + 20H- + 2C02- Pb,o(C03)6(0H)60(s) + 8H+ = 10Pb2+ +6C032-

+ 7H20 Pb2+ + OH- = PbOH+ Pb2+ + 20H- = Pb(OH),o

Pb2+ + 30H- = Pb(OH)3- Pb2+ + 40H- = Pb(0H);- 2Pb2+ + OH- = Pb20H3+ 3Pb2+ + 40H- = Pb3(0H)42+ 4Pb2+ + 40H- = Pb4(0H)44+ 6Pb2+ + 80H- = Pb6(OH)84+ Pb2+ + C03?- = PbCO? Pb2+ + 2C032- = Pb(CO3)?- Pb2+ + HC03- = PbHC03+ H+ + OH- = H20 H+ + C03'- = HC03- H+ + HC03- H2C030 H+ + P D A ~ - = HPDA- H+ + HPDA- = H~PDAO pb2+ + PDA?- = P ~ P D A O

Pb2+ + 2PDA2- = Pb(PDA)22- H+ + EDTA4- = HEDTA3- H+ + HEDTA3- = H2EDTA2- H+ + H,EDTA~- = H~EDTA- H+ + H3EDTA- = H4EDTA0 H+ + H~EDTAO = H,EDTA+ H+ + H,EDTA+ = H~EDTA'+ Pb2+ + EDTA4- = PbEDTA2- P ~ E D T A ~ - + H+ = P~HEDTA- PbHEDTA- + H+ = PbH2EDTA0 PbH2EDTAo + H+ = PbH3EDTA+

"25°C and I = 0.3. 90°C and I = 0.1. '25°C and I = 0.1. "25°C and I = 1.0.

ously, and maintained in suspension by a shaker table. Initial pH values (pHo) between 4.2 and 5.9 were used to allow complete dissolution of 50 mg/L of the tested metal (MeT,-, = 50 mg/L), ensuring that the soil was introduced to a homo- geneous metal solution (the metal solubilities are > 1000, 900, 790, and > 1000 mg/L for Cd, Cu, Pb, and Zn, respectively, based on various metal hydroxides at respeEtive employed pH values of 4.2, 5.3, 5.3, and 5.9). Adsorption and extraction tests were performed consecutively such that completion of an adsorption run was immediately followed by an extraction run, initiated by adding the chelator. A time period of 24 h was allowed for equilibration during adsorption and extraction runs.

Procedures involving complex separation and metal segre-

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Fig. 2. Degrees of complexation of various chelating agents toward metals as a function of pH.

10 I I I I

with ligand Cd~=28.4 mg/L; PDA= 1 mM - - - - - - - - - - - - without ligand CUT=^^ mg/L; SCMC=2.4 mM Controlling solids Me(OH)z(s) P b ~ = 2 3 2 mM; PDA=2 mM -

- I=O.l M CT= 1 mM Z n ~ = 5 0 mg/L; ADA=2.3 mM

- 25 "C CU-SCMC

gation (as precipitate) from single and multimetal complex solutions were accomplished by incremental increases of pH to promote precipitation. Typically, a 12-h period was allowed for equilibration after each incremental increase in pH.

Results and discussion

Metal extraction from soil The extraction potential of SCMC, ADA, and PDA with regard to the Cu, Zn, Cd, and Pb metals was examined as a function of the ligand-to-metal mole ratio (&/Me,). The chelating agents SCMC and ADA were used for Cu and Zn, respectively, while PDA was used for both Pb and Cd. According to equilibrium calculations, this should show both effective chelation and recovery at moderate pH. For different batches containing different metal solutions (Me,,, =

50 mg/L), Fig. 3 first plots the equilibrium concentrations of metals remaining in the aqueous phase after the addition of 2 g/100 mL soil (solid symbols), then it plots equilibrium concentrations of metals after the addition of various amounts of chelators to these batches (open symbols). The results show that 80- 100% of various metals were extracted from soil and resolubilized, as the &IM* ratios were increased sufficiently.

Recovery of metal and chelator Figure 4 shows results of experiments verifying that various complexed metals can be released from properly selected chelating agents and be separated as solid precipitates. This figure indicates an increasing precipitation of complexed metals (measured by MeT/Meadded) as a function of increasing pH. Experimental results (solid symbols and curves) showed that precipitates of Pb, Cu, Zn, and Cd were signifi- cantly recovered from respective metal - ligand complexes (Pb-PDA, Cu-SCMC, Zn-ADA, and Cd-PDA) at approximate pH values of 8, 11, 11.5, and 12.5, respec-

tively. The recovery of extracted metals by precipitation followed by separation processes is a major objective of this work. However, discrepancies exist as to the pH values of metal -chelator separation; preliminary calculations of Table 1 determine the highest effective pH values for the complexation of these metal-ligand pairs to be 11, 10.8, 10.9, and 9.4, respectively. Upon further refined calculations, i.e., including the effects of I and Me,, these values are adjusted to 9.5, 12, 12, and 11, respectively, as shown in Fig. 4 (open symbols and broken curves for the calculated). Thus, the experimental and calculated curves of Fig. 4 are shifted by approximately 0.5-1.5 pH units, and the results of preliminary calculations and that of refined calculations differ by about l .5 pH units. Figure 4 indicates that the measured pH values of separation are lower than the calculated, except for the special case of Zn in which calcium was added to aid the separation.

Experiments were conducted to test the sequential release of multiple extracted metals by a single chelator SCMC as the pH was increased gradually. Figure 5 shows that up to pH 8, all five metals Cd, Cu, Ni, Pb, and Zn in a single batch, and each initially at 10 mg/L, were mostly soluble in the presence of 4.8 times excess (mole basis) of SCMC (L, = 3 mM). SCMC solubilized high concentrations of metals by its high metal-complexing ability; without the presence of SCMC, these metals would not have remained as soluble at pH 8. When the pH of the solution containing the mixed-metal complexes was further elevated, precipitates appeared as various Me, in solution reduced. At pH 13, all metals precipitated extensively. The order of metal precipitation from first to last was Pb, Zn, Cd, Cu, and Ni approximately at pH 11, 11, 11.9, 12.5, and 13, respectively. This is consistent with the order of precipitation listed in Table 1 for Zn, Cu, and Ni at pH 9.8, 10.8, and 11.2, respectively. The sequential precipitation of various metals at different pH values may provide a means of segregating metals

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Fig. 3. Adsorption (solid symbols, error bars shown) of metals to soil and desorption (open symbols) upon addition of chelating agents. Solid symbols indicate metal concentrations remaining in solution after adsorption equilibrium with added soil; open symbols indicate metal concentrations in solution after extraction equilibrium with added chelators.

50

LT/Meadded (mole ratio)

Fig. 4. Separation of metals from chelators as solid precipates as pH is increased. Solid symbols and curves show experimentally measured values; open symbols and curves show calculated values.

experimental . - . - . - . - calculated

- C T = ~ mM; I = O . l M

during the recovery process and it merits further study and model must be addressed. The degree of complexation calcu- optimization. lation enables one to determine at equilibrium the solubiliza-

tion and subsequent recovery of a metal as a function of pH, Limitation of the model and calculated results given a set of conditions such as the metal, chelator, and car- The methodology of chelator selection introduced is an bonate concentrations. However, the model cannot predict important part of this work; the potential limitation of the the degree of extraction from soil without further informa-

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Fig. 5. Sequential release of multiple metals from SCMC chelator as pH is increased gradually.

tion specific to the soil such as mineral types, contents, and thermodynamic constants governing the mineral-metal interactions (e.g., complexation constants). The interfacial interaction of metals with soils was not considered in this work. The results of Table 1 are presented as a screening aid for selecting potentially recoverable chelating agents that are useful for extraction of target heavy metals from contaminated soils.

Summary

A method based on equilibrium calculations has been developed for assessing the potential of chelating agents toward extractive remediation of heavy metal contaminated soils, with an emphasis on the recoverability of the metals and the chelators. A total of 190 prescreened chelating agents were assessed for their extraction and recovery potential towards six target metals and the results are reported. The developed approach is supported by experimental results using test chelators SCMC, ADA, and PDA for the extraction and recovery of copper, zinc, cadmium, and lead from soil. The assessment methodology and results reported here are intended as a guide to selecting appropriate chelating agents for the removal and recovery of specific metal contaminants from soils.

Acknowledgements

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