EVALUATION OF Pb REMEDIATED SOILS BY MEANS OF DIFFERENT AVAILABILITY INDICES

Geebelen, W.1*,2, Adriano, D.C.2, Vangronsveld, J.1 & Clijsters, H.1

1 Environmental Biology, Centre for Environmental Sciences, LUC, Universitaire Campus,

B-3590 Diepenbeek, Belgium, tel. ++32 11 26 83 77, fax ++32 11 26 83 01, wouter.geebelen@luc.ac.be

2 University of Georgia, Savannah River Ecology Laboratory, Aiken, South Carolina, 29802, USA

 

ABSTRACT

Lead immobilization was examined on a Pb contaminated soil (1500 mg Pb kg-1) using 7 different soil treatments: bentonite, zeolites, cyclonic ashes, compost, lime, steelshots and hydroxyapatite. The immobilizing effect of the soil additives was evaluated by 3 availability indices: the modified distribution coefficient (Kmd), bioavailability factor (BF) and the transfer factor (TF). The Kmd is defined as the ratio of Pb in soil to its concentration in the soil solution. The BF is defined as the ratio of Pb content in the exchangeable phase to the total Pb concentration in soil. The TF is defined as the ratio of the Pb content in plant tissue (lettuce) to the total concentration of Pb in soil. Kmd and BF correlated well with the capacity of some enzymes in test plants known to react under (metal induced) oxidative stress. Application of zeolite was problematic due to soil compaction while steelshots released toxic amounts of Fe and Mn which killed lettuce seedlings. Cyclonic ashes, compost, hydroxyapatite and lime substantially increased Kmd and decreased BF and TF.

 

INTRODUCTION

In many countries, remediation goals and guidelines are based on the total metal analysis of the soil. This old paradigm is changing in favor of a more realistic approach which differentiates the bioavailability of heavy metal contaminants. Bioavailability refers to the fraction of the total chemical that can interact with a biological target (Vangronsveld & Cunningham, 1998). This paper discusses 3 availability indices which can be used to evaluate the efficacy of soil amendments (Knox & Adriano, 1998; Knox et al., 2000) in decreasing Pb bioavailability in artificially contaminated soils. The Kmd is defined as the ratio of Pb in soil to its concentration in the soil solution, based on the exchangeable fraction. The BF is defined as the ratio of Pb content in the exchangeable phase to the total Pb concentration in soil. The TF is defined as the ratio of the Pb content in plant tissue (lettuce) to the total concentration of Pb in soil.

 
METHODS

1500 mg Pb kg-1 was added to an acid sandy soil (pH-H2O=4.27; electrical conductivity= 375 µS cm-1; CEC=5.35 cmol e-1; 4.05 % humus); 30 days after Pb addition, the following soil amendments were applied (w/w): 1 % bentonite, 0.5 % zeolite, 5 % cyclonic ashes, 5 % compost, 1 % lime, 1 % steelshots and 1 % hydroxyapatite. After application the soil mixtures were rehydrated up to field capacity and equilibrated (recovery of chemical and biological equilibrium) for a minimum of 100 days at room temperature before experiments were performed.

 

Kmd, BF

Soil samples were air dried (60 °C; 48 h) and sieved through a 2 mm sieve preceding analysis. Samples of 5 g soil were shaken in plastic flasks with 50 ml 0.1 M Ca(NO3)2 (Chen et al., 1997a) for 24 h. Extracts were filtered through ash-free paper; metal analysis was performed in triplicate and in acid-washed (10 % HCl) glass labware. Metal concentration was determined by atomic absorption spectrometry (AAS).

Kmd was calculated as the concentration of Pb in the soil after desorption to the concentration of Pb extracted by Ca(NO3)2 and is expressed in L kg-1.

BF was calculated as the concentration of Pb extracted by Ca(NO3)2 to the total Pb concentration in soil, expressed in %.

 

TF

Lettuce seedlings (Lactuca sativa cv. Hilde) precultivated for 3 weeks on a non-polluted substrate, were transplanted to the Pb polluted soil mixtures (3 pots per treatment; 5 plants/pot) and grown in a greenhouse for 4 more weeks. The aboveground plant parts were rinsed with distilled water, dried (80°C; 48 h), and extracted with concentrated ultra pure HNO3 (Milestone MLS-1200 MEGA). Pb analysis was performed by AAS.

TF is calculated as the Pb concentration found in aboveground parts of lettuce to the total Pb concentration in soil.

 

Phytotoxicity

After 1 day vernalisation and overnight imbibition, test plants (Phaseolus vulgaris cv. Limburgse vroege) were sown in polyethylene pots of 400 ml (3 pots per treatment, 5 plants/pot). The plants were cultivated in a plant growth chamber under standardised environmental conditions (22 °C, 65 % RH, 12 h light, photosynthetic active radiation: 150 µmol m-2 s-1) and watered daily with deionized water. Fourteen days after sowing samples (1 g fresh weight) of primary leaves and roots were stored at –70°C for enzyme and protein analysis. The following enzymes were measured spectrophotometrically (Shimadzu UV-1602) as described by Van Assche et al. (1988): guaiacol peroxidase (GPOD; E.C.: 1.11.1.7), malic enzyme (ME; E.C.:1.1.1.40) and glutamate dehydrogenase (GDH; E.C.: 1.4.4.2) in roots and POD and ME in primary leaves. They are directly or indirectly involved in the defence against oxidative stress (Van Assche et al., 1988; Van Assche & Clijsters, 1990; Vangronsveld & Clijsters, 1994). Enzyme capacity was expressed in mU per gram fresh weight. Soluble protein content was measured in primary leaves and roots using the Biorad method (Bradford, 1976).

Pearson-correlations were calculated using Statistica and were considered significant when p < .05

 

RESULTS AND DISCUSSION

Kmd slightly increased when bentonite (46.1 L kg-1), steelshots (80 L kg-1) and zeolite (151,9 L kg-1) were applied, compared to the untreated (26.3 L kg-1) (fig. 1). Kmd however, was strongly enhanced when compost (369.7 L kg-1), cyclonic ashes (412.1 L kg-1), hydroxyapatite (689.1 L kg-1) and lime (1714.9 L kg-1) were used indicating strong Pb immobilization.

 

Comparable results were found when bioavailability index (BF) was used: Pb bioavailability was reduced when bentonite (17,8 %), steelshots (11,7 %) and zeolite (6,2 %) were applied (BF untreated= 27.6 %) (fig. 2). Strong reduction were found when compost (2,6 %), cyclonic ashes (2,5 %), hydroxyapatite (1,4 %) and lime (0.6 %) were used.

 

Enzyme capacity was found to be high for all enzymes measured when no soil additive was applied (table 1). Cyclonic ashes, compost, hydroxyapatite and lime substantially reduced these capacities reflecting lower Pb phytotoxicity. To a lesser degree bentonite and steelshots decreased induction of oxidative stress while application of zeolite resulted in higher enzyme capacities. This increased stress reaction however occurred independent of soil Pb content and was due to soil compaction caused by the additive (Geebelen et al., 2000). A same pattern was found for soluble protein content measured in roots and leaves (data not shown).

 

Table 1: Enzyme capacity of GPOD, ME and GDH in roots and GPOD and ME in primary leaves (in mU g-1 fresh weight) of 14 days old Phaseolus vulgaris seedlings grown on lead enriched soils (1500 mg Pb kg-1) treated with several soil additives.

 

 

        untreated

5%              cyclonic ashes

0.5%     zeolite

1%  bentonite

5%        compost

1%   steelshots

1% hydroxyapatite

1%            lime

GPODroot

18470±1486

6160±2054

23762±6276

16428±383

10696±1055

15922±1514

12691±2198

4491±545

MEroot

825 ± 64

335 ± 50

974 ± 49

684 ± 49

470 ± 93

566 ± 61

479 ± 28

380 ± 46

GDHroot

360 ± 63

124 ± 9

164 ± 17

169 ± 20

84 ± 34

129 ± 34

275 ± 170

196 ± 14

GPODleaf

2110 ± 286

579 ± 55

9227 ± 5003

1279 ± 290

193 ± 26

5436 ± 1987

2514 ± 274

1000 ± 175

MEleaf

923 ± 60

414 ± 150

1069 ± 61

622 ± 31

294 ± 31

1005 ± 176

745 ± 48

316 ± 23

 

Pb content (in mg Pb kg-1 fresh weight) found in edible part of lettuce (Lactuca sativa) was used to calculate transfer factor. TF was reduced by 30 to 40 % when cyclonic ashes (TF=0.0070), compost (0.0068), zeolite (0.0064) and bentonite (0.0054) were applied (TF untreated= 0.0102) (fig. 3). No values were obtained for the steelshots treated soil probably due to Fe and Mn phytotoxicity. Strong decreases in TF were also found when hydroxyapatite (0.0025) and lime (0.0028) were used.

 

Kmd as well as BF were found to correlate significantly with protein content, ME and GPOD capacity measured in roots of Phaseolus vulgaris. The main impact of Pb normally is observed at the root level where Pb is present in higher concentrations than above-ground tissues (Chen et al., 1997b). In primary leaves BF was found to correlate significantly with protein content and ME capacity. The correlations found indicate that the behaviour of the anti-oxidative stress enzymes mentioned reflects the extent of Pb bioavailability as affected by the treatments as calculated by the proposed availability/stability indices, especially at the root level. No correlations were found when TF values were used; these were based on Pb content found in lettuce cultivated under different circumstances (greenhouse), favouring Pb uptake.

The data also show addition of cyclonic ashes, compost, lime, and hydroxyapatite reduces bioavailable Pb.

 

REFERENCES

Bradford MM (1976), Anal. Biochem. 72: 248-254.

Chen ZS, Lee GJ, Liu JC (1997a), In: Proc. Extended Abstracts 4th Int. Conf. Biogeochemistry of trace elements. (IK Iskandar, SE Hardy, AC Chang, GM Pierzynski, Editors), Berkeley, pp. 421-422.

Chen J, Huang JW, Caspar T, Cunningham SD (1997b), In: Phytoremediation of soil and water comtaminants (EL Kruger, TA Anderson, JR Coats, Editors), ACS Symposium series 664, Washington, pp. 264-273.

Geebelen W, Vangronsveld J, Adriano DC, Carleer R, Clijsters H (2000), Amendment-induced immobilization of soil lead (submitted).

Knox AS, Adriano DC (1998), In: Proc. 4th Int. Symp. Exhib. Environmental Contamination in Central and Eastern Europe. Institute for International Cooperative Environmental Research florida State University, Tallahassee, FL, pp. 355-362.

Knox AS, Seaman J, Pierzynski G, Adriano DC (2000), In: Bioremediation of Contaminated Soils. (DL Wise, DJ Trantolo, EJ Cichon, HI Inyang, U Stottmeister, Editors), New York, Marcel Dekker, Inc., pp. 811-836.

Van Assche F, Cardinaels C, Clijsters H (1988), Environmental Pollution 52:103-115.

Van Assche F, Clijsters H (1990), Plant, Cell and Environment 13:195-206.

Vangronsveld J, Cunningham SD (1998), In: Metal contaminated soils: in situ inactivation & phytorestoration. (J Vangronsveld, SD Cunningham, Editors), Springer, Berlin, pp. 1-15.

Vangronsveld J, Van Assche F, Clijsters H (1990), In: Environmental Contamination; Proc. 4th Int. Conf. Barcelona (J Barcelo, Editor), Edinburgh, pp. 283-285.

 

 

 

 

Fig. 1: Kmd (L kg-1) of lead enriched soils (1500 mg kg-1) treated with several soil additives.

Fig. 2: BF (%) of lead enrichd soils (1500 mg kg-1) treated with several soil additives.

 

Fig 3:TF for edible parts of lettuce cultivated on lead enriched soils (1500 mg kg-1) treated with several soil additives.