REMEDIATION OF HEAVY METALS AND RADIONUCLIDES BY  PHYTOIMMOBILIZATION: EVALUATION OF SEQUESTERING AGENTS

Knox, A.S.1, Kaplan, D. I.2, Hinton, T.G.1, Sharitz, R.R.1, Serkiz, S.2

1The University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, SC, 29802, USA, Phone: 803 725-2752, Fax: 803 725-3309, achlopecka@srel.edu

2Savannah River Technology Center, Westinghouse Savannah River Company, Aiken, SC 29808, USA

 

ABSTRACT

Phytoimmobilization offers the advantage of reduced cost and environmental impact by reducing the quantity of solid waste that ultimately needs to be treated.   This study evaluated several stable, environmentally friendly, geological materials (apatite, zeolite, metallic iron, Fe oxide, gypsum, pyrite, and combination of these minerals) for their ability to retain Ba, (as an analogue for Ac) Co, Cr, Hg, Pb, and U.  The laboratory geomat studies showed that metallic iron was effective at removing Cr, Hg, Pb, Co, Ba, Eu, and U from solution, whereas apatite was effective at removing Eu, Pb, and U from solution.

 

INTRODUCTION

A novel phytoremediation technology, phytoimmobilization, was evaluated.  In the first stage of this technology, phytoextraction, plants are used to remove contaminants from soil by incorporation into the aboveground herbaceous and woody biomass (Fig 1.A).

 

Fig. 1.  Two stages of phytoimmobilization: A)  Phytoextraction - plants are used to remove contaminants from soil; B) Sequestration - contaminants are immobilized by geomat

 

In the second stage, the woody and herbaceous plant materials are allowed to fall to the soil surface and decompose.  As the contaminants are released from the decomposing plant material, they are immobilized in either a mineral-amended surface soil or a geomat deployed at the ground surface (Fig. 1.B).  Once the sediment has been deemed sufficiently decontaminated, the contaminated geomat can be either left in place or recovered, depending on human and ecological risk.  In conventional phytoremediation, plants are grown and harvested in successive cycles to remove the contaminants; no geomat is included.  This process produces a large quantity of contaminated vegetation, which must be disposed of at an appropriate facility.

 

An invention disclosure describing the phytoimmobilization process has been filed and a “proof-of-concept’ study is currently underway in an ecologically sensitive wetland, located on the TNX Outfall Delta Site on the Savannah River Site (SRS), Aiken, South Carolina.   This site is contaminated with uranium, thorium, several of their daughter products, mercury, and chromium.  The investigation is being conducted in two phases, a vegetative uptake study and a treatability study.  These studies will look at the influences of plant species and geomat design  (e.g. sorption characteristics) on the removal and fixation of soil contamination at the TNX Outfall Delta.  This paper presents data from the laboratory geomat immobilization study, a part of the vegetative uptake study.

 

MATERIALS AND METHODS

Due to the different geochemistry of the studied elements, three suites of different geomat materials were evaluated  (Cr experiment, Hg experiment and Ba, Co, Pb, and U experiment).  All experiments were conducted in centrifuge tubes for a period of one week.  Each treatment had three replicates.  All three experiments were conducted with two background solutions; pure water (MQ) and rain water with dissolved organic carbon (decomposed leaf litter).  Rain was collected in Aiken, SC, and leaf litter was collected at non contaminated area of TNX on the SRS. 

 

Cr experiment

The following geomat materials were tested: metallic iron (Fe), North Carolina apatite (NCA), hydroxylapatite (HA), and zeolite (clinoptilolite- ZC).  The metallic iron was tested by itself and in the presence of NCA, HA, or ZC.  Additionally, Fe was tested with mixtures of NCA and ZC, and HA and ZC.  For one week, 0.3 g of each geomat material was shaken with 30 mL of solution containing 1mg Cr(VI) L-1.  Chromium concentration was determined by atomic absorption spectrometry (AAS).  Sorption values were measured in the presence and in the absence of high dissolved organic carbon concentrations.  The partitioning of the Cr to the various geomat materials was quantified by the distribution coefficient, Kd (Equation 1).

 

Vspikex(Aspike– Afinal)

Kd = ---------------------------------                                                          Equation 1

Afinal x Mgeomat

 

Where Aspike and Vspike are the metal concentration (mg L-1) and volume (mL), respectively, of the solution added to the geomat sample; Afinal is the metal concentration in the solution after one week contact with the geomat material; and Mgeomat is the geomat material mass (g).

 

Hg experiment

Gypsum, pyrite, and metallic iron were evaluated for their ability to remove Hg from the aqueous phase.  All materials were tested in triplicate.   0.3 g of each material was added to 30-mL of a 2 mg/L Hg(II) solution.  After one week, the samples were taken from a shaker, centrifuged, and then analyzed by Cold-Vapor Atomic Fluorescence Spectrometer. 

 

Ba, Co, Eu, Pb, and U experiment

Immobilization of Ba, Co, Eu, Pb, and U was examined using six geomat materials: hydroxylapatite (HA), North Carolina Apatite (NCA), two natural zeolites – clinoptilolite (ZC)  and phillipsite (ZP), Fe oxide (Fe richTM waste byproduct from an industrial process that generates TiO2 pigment; E.I. Du Pont de Nemours, Wilmington, DE) ) and metallic Fe.  The concentration of each element in the spike solution was 50 mg L-1.   Suspensions composed of 0.3 g solid and 30mL of spiked solution were shaken for one week, phase separated by centrifugation, and then the aqueous phase was analyzed for metal content by ICP-MS.    

 

RESULTS AND DISCUSSION

Only metallic iron was effective at removing Cr(VI) from solution (Fig. 2).    Metallic iron removes soluble Cr(VI) from solution by converting the soluble Cr(VI) species to the sparingly soluble Cr(III) species (Cantrel et al., 1995).  Thus, the removal of Cr from solution is not via adsorption, as the Kd construct implies; instead, it is by reductive precipitation (Equation 2).

 

CrO42- + 3/2Feo + 5H+ = Cr(OH)3 + H2O + 3/2Fe 2+                         Equation 2

 

Addition of other materials such as apatite or zeolite decreased the effectiveness of the metallic  iron (Fig. 2).

 


Fig. 2. Cr(VI)-Kd values for various materials.

 


In the Hg experiment, both pyrite and metallic iron treatments resulted in a significant reduction in soluble Hg.  The reduction in soluble Hg concentrations in pyrite (FeS2) treated solutions is not surprising since Hg(II) can displace Fe 2+ to form an insoluble mercury-sulfide species (Bodek et. al., 1988).  Equation 3 illustrates the mechanism of soluble Hg removed from the pyrite treated system.

 

2 Hg 2+  + FeS2(s)       =      2 HgS(s) + Fe 2+                              Equation 3

 

In the metallic iron system, the pH increased to 10.2, the reduction potential (Eh) decreased to 110 mV, and the dissolved Hg concentrations decreased to 0.0294 mg L-1 (Table 1).  Faust and Osman (1981) reported similar reductions in soluble Hg concentrations in experimental systems when pH values  increased and Eh values decreased. 

 

Table 1.  Effect of geomat material on Hg concentration

Treatment

No  presence of dissolved  organic carbon

Presence of  dissolved organic carbon

 

Avg. pH

Avg. Eh

(mV)

Avg. Hg conc. (mg L-1)

Avg. pH

Avg. Eh

(mV)

Avg. Hg conc. (mg L-1)

Control

5.84

409

0.0000

4.97

290

0.0008

Hg Spike

3.10

498

1.8650

4.70

311

0.6304

Gypsum

3.30

510

1.5820

4.50

330

0.3442

Pyrite

3.01

345

0.0018

4.30

361

0.0090

Metallic iron

10.22

100

0.0294

5.85

110

0.1265

 

In the third experiment, five elements were added to the solution together at concentration approximately 50-mg L-1.  After one week concentrations of each tested element were significantly reduced by almost all the amendments (Table 2).

 

Table 2.  Kd values of Co, Ba, Eu, Pb, and U for various potential geomat materials.

Treatment

Co

Ba

Eu

Pb

U

Hydroxylapatite

7683

421

725426

138607

282448

North Carolina Apatite

470

222

313157

214916

264829

Zeolite (Clinoptilolite)

4114

6217

14473

1805864

7

Zeolite (Phillipsite)

223

6222

819

24828

9

Fe oxide (waste byproduct)

23037

3688

2561951

363449

8479

Metallic Fe

259629

26076

1525058

18108

452210

 

Hydroxylapatite and North Carolina Apatite were the most effective at reducing Eu, Pb, and U concentrations in the solutions and Kd values in these treatments were very high (Table 2).  Other researchers also reported high effectiveness of apatite in remediation of Pb or U contaminated soils through precipitation mechanism (Knox et al., 2000).  Both zeolites were effective for most elements, however, they did not remove U from solution (Kd values were very low; Table 2).  In the third experiment, metallic iron and Fe oxide (Fe-rich TM, waste byproduct) removed the most elements from solution.  Presented data showed that metallic iron could be attractive as a geomat material because it was effective at immobilizing  Cr, Hg, Co, Ba, Eu, Pb and U.  

 

REFERENCES

Bodek I, Lymn WJ, Reehel WF, Rosenblott DH, Walton BI, Conway RA (1988), Environmental Inorganic Chemistry: Properties, Processes, and Estimation Methods, New York, Pergamon Press.

Cantrell KJ, Kaplan DI, Wietsma TW (1995), J. Hazardous Materials. 42: 201-212.

Faust SD, Osman MA (1981), In Chemistry of Natural Waters, Ann Arbor Science Publishers, Inc.

Knox AS, Seaman JC, Mench MJ, Vangronsveld J, (2000), In: Environmental Restoration of Metals Contaminated Soils (IK Iskandar, Editor), CRC Press, Boca Raton, FL.