Physico-chemical and biological evaluation of the efficacy of in situ metal inactivation in contaminated soils.

 

J. Vangronsveld*, N. Spelmans, H. Clijsters, E. Adriaensens, R. Carleer, L. Van Poucke (Limburgs Universitair Centrum, Universitaire Campus, B-3590 Diepenbeek, Belgium); D. van der Lelie, M. Mergeay, P. Corbisier, J. Bierkens, L. Diels (VITO, Flemish Institute for Technological Research, Boeretang 200, 2400 Mol, Belgium)

* corresponding author: e-mail: jaco.vangronsveld@luc.ac.be);

 

ABSTRACT

In situ inactivation (immobilization) of metal contaminated soils is a developing technique which seems to be a valuable alternative for more expensive and complicated civil-engineering techniques. The main objective of in situ immobilization is to decrease the risks of the contamination for the environment and human health through reductions of relative mobility and bioavailability of the metals in the soil. Specific criteria for evaluation and monitoring of in situ inactivation of metals are needed. In this paper the combined use of sequential extraction procedures, microbial heavy metal biosensors (BIOMET®), phytotoxicity tests and a zootoxicity test is presented as a test system for evaluation and monitoring of efficacy and durability of in situ inactivation of metal contaminated soils. A good conformity was found between the different evaluation criteria. The soil additive cyclonic ashes alone or in combination with steelshots or bentonite gave the best immobilization.

 

Introduction

Current remediation methods for metal contaminated soils are primarily based on civil-engineering techniques (f.i. excavation). They are expensive and environmentally invasive. Alternative remediation techniques that are low cost and environmentally sound, but equally protective of human health and the environment, should be developed. Application of soil amendments modifying the physicochemical properties of the contaminating metals combined with the development of biological communities for further metal immobilization was shown to be very promising (for a review, see Vangronsveld and Cunningham, 1998). These amendments make metals less soluble and available for biota.

Regulatory acceptance of in situ immobilization as a secure technique depends on the solution of the following problems: (1) how to measure metal bioavailability and (2) how to assure and monitor efficiency of metal immobilization on the long term. A thorough evaluation of the overall effect of additives and the sustainability of metal immobilization may combine physico-chemical and biological methods. Biological methods complement physico-chemical evaluation methods, which do not directly address biological availability or toxicity. Biological evaluation of soil contamination asks for the employment of various living organisms from different trophic levels. In this paper we present the combined use of sequential extraction procedures, microbial heavy metal biosensors (BIOMET®), phytotoxicity tests and a zootoxicity test as a test system for evaluation and monitoring of efficacy and durability of in situ inactivation of metal contaminated soils. Further attention was paid to eventual changes in the microbial community (proportion of metal tolerant bacteria).

 

METHODS

A heavily contaminated sandy soil from the site of an old zinc smelter (Lommel, Maatheide (further mentioned as ‘MH’) in the NE of Belgium; Vangronsveld et al. 1995, 1996) was ‘diluted’ with a comparable (pH, texture, …) uncontaminated soil resulting in a series of : 0 % MH, 20 % MH, 50 % MH and 100 % MH. These soil mixtures were treated with several soil additives in different proportions : steel shots 1 % (SG; Mench et al. 1994), steel powder (SC) 1 %, cyclonic ashes 5 % (formerly called beringite; Vangronsveld et al. 1990, 1991, 1995a, 1995b, 1996) (CA), bentonite 5 % (BE), and combinations (CA 5 % + SG 1 %; CA 5 % + BE 5 %). Choice of the additives used in this experiments was based on previous experience. The effect of the amendments on metal mobility and bioavailability was evaluated at different moments after their application. In this paper, only results for 12 months are summerized.

Sequential extraction is based on the procedure of Tessier et al. (1979).

Bacterial availability of the metals Zn and Cd was assessed using a bacterial biosensor strain Alcaligenes eutrophus (BIOMET; Corbisier et al. 1994; Corbisier et al. 1996).

The effects of the soil additives on the bacterial ecology in treated soils was evaluated by determining the total populations of aerobic bacteria and their heavy metal resistant subpopulations. The amount of metal resistant subpopulations is presented by the ratio of the c.f.u. in presence of Zn to the c.f.u. when no metals are added. With low ratios, less metal resistant bacteria were counted, which indicates low bioavailability of the specific metal in the soil. Higher ratios indicate higher bioavailability of the metal.

The potential soil phytotoxicity was evaluated using a bioassay including several morphological and biochemical parameters (Van Assche and Clijsters 1990; Vangronsveld and Clijsters 1992). Bean (Phaseolus vulgaris ) was used as test plant.

Zootoxicity was tested using the earthworm Eisenia fetida. Every toxicity test was conducted according to the OECD-guidelines (1984) with minor modifications.

 

RESULTS

The original MH soil (pH 5.9) contained 4800 mg Zn kg-1, 2360 mg Pb kg-1, 830 mg Cu kg-1 and 20 mg Cd kg-1. The total metal content showed the same gradient as the dilution factor of the polluted soil. pH was not affected by the dilution. After application of CA and CA+SG the pH increased to 7.5 – 8.0, while for BE and CA+BE an increase to 8.5 – 9.0 was observed. The additives SG and SC did not affect the pH in the 50 and 100 % MH soil mixtures, but an increase was observed in the unpolluted and in the 20% MH soil.

Sequential extractions demonstrated that 12 months after application the majority of the soil additives significantly affect the exchangeable metal fraction as compared to the untreated soils (not shown). The combinations of CA+BE and CA+SG, in particular, were shown to be particularly effective. Easily soluble Zn (exchangeable fraction) for instance was reduced with 98 % after CA+SG or CA+BE addition and the exchangeable fraction of Cd was reduced with 96 %. The residual fraction of both Cd and Zn was increased for all treatments.

The results obtained for bioluminescence induction 12 months after soil treatment (not shown) fitted very well with the chemical extraction data. A linear increase in light production was observed when increasing concentrations of untreated Maatheide soil were tested. This linear increase in bioluminescence was proportional with the water extractable fraction of Zn and Cd. SG and SC treatments only resulted in a slight effect but other additives caused up to 80-90% reduction of bioluminescence as compared to the untreated MH soil, indicating that they efficiently immobilized most of the metals present. Data were ‘translated’ to a bioavailability index (Table 1).

The results of the bacterial countings (not shown) partly confirmed these findings. In the untreated soil mixtures a strong increase in metal resistant subpopulations was observed with increasing contamination. Twelve months after the application of the soil additives, a decrease in all subpopulations of Zn-resistant bacteria was observed for the soils to which CA or the combination CA+SG were applied.

Table 1 shows the phytotoxicity indices (PI), based on morphological and biochemical parameters, for the untreated and treated MH soil mixtures (Vangronsveld & Clijsters, 1992). The untreated soils showed a gradual increase of toxicity from phytotoxicity index (PI) 1 (‘not toxic’, 0 % MH) over PI 3 (‘moderately toxic’, 20-50 % MH) to PI 4 (‘strongly toxic’, 100 % MH). 12 months after application of the soil additives phytotoxicity generally decreased on 50 and 100 % MH, except for SG and SC. On the soil mixtures without any or with slight contamination (0-20 % MH), however, overall phytotoxicity tended to increase with SG, SC and BE. The Mn-bearing properties of SG and SC (Mench et al. 1998) clearly induced Mn-toxicity in bean, which appears to be highly sensitive to increased available Mn. SG and SC also reduced the plant calcium (Ca) and manganese (Mg) uptake (not shown). The combined application of SG and CA resulted in the restoration of normal levels of these elements and decreased PI for 50 and 100 % MH. The relatively high PI for BE (3 : ‘moderately toxic’) may be explained by changed physical properties of the soil (compacting), waterlogging and reduced plant availability of Ca and K due to this additive.

Severe weight loss of earthworms was already observed with 20% MH in the control soil mixtures (Table 1). Higher concentrations of MH resulted in almost 100 % mortality. 12 months after treatment of the contaminated soil mixtures, BE and the combination of CA-BE strongly reduced soil zootoxicity. In 100 % MH soil treated with these additives weight loss was only 1.5 % and 5.5 %. For treatment with CA and the combination of CA-SG this weight loss was –12.0 % and -10.7%.

No significant improvement of zootoxicity was found for the additives SC and SG applied to 100 % MH.

In general a good correlation was found between the different evaluation criteria (Table 1). The soil additive cyclonic ashes alone or in combination with steelshots or bentonite gave the best and most sustainable immobilisation of the heavy metals, the lowest phyto- and zootoxicity indices as well as a strong reduction of heavy metal bioavailability for bacteria. These results were subsequently used for large-scale field demonstrations to treat heavy metal contaminated soils using in situ inactivation.

 

References

Corbisier P, Thiry E, Diels L. (1996) Environ Toxicol and Water Quality 11:171-177.

Corbisier P, Thiry E, Masolijn A et al. (1994)In: Campbell A K, Cricka L J, Stanley P E, eds. Bioluminescence and Chemoluminescence: Fundamentals and Applied Aspects. Chichester, New York, Brisbane, Toronto, Singapore: John Wiley and Sons, pp. 150-155.

Mench M, Vangronsveld J, Didier V, et al. (1994) Environ Pollut 1994; 86:279-286.

Tessier A, Campbell PGC, Bisson M. (1979) Analytical Chemistry 51, 844-850.

Van Assche, F., Clijsters, H. (1990) Environ Pollut 66, 157-172.

Vangronsveld J., Clijsters H. (1992) In: Merian E, Haerdi W, eds. Metal compounds in environment and life, 4 (Interrelation between chemistry and biology). Wilmington: Science Reviews Inc., 117-125.

Vangronsveld J., Colpaert J., Van Tichelen K. (1996) Environ Pollut 94: 131-140.

Vangronsveld, J., Cunningham, S.D. (1998) Metal-Contaminated Soils: In-situ Inactivation and Phytorestoration, Springer Verlag, Berlin Heidelberg, ISBN 1-57059-531-3

Vangronsveld J., Sterckx J., Van Assche F., Clijsters H. (1995a) J Geochem Expl 52: 221-229.

Vangronsveld J., Van Assche F., Clijsters H. (1990) In: Barcelo J, ed. Proc Int Conf Environmental Contamination. Edinburgh: CEP Consultants, 283-285.

Vangronsveld J., Van Assche F., Clijsters H. (1991) In: Farmer JG, ed. Proc Int Conf Heavy Metals in the Environment. Edinburgh: CEP Consultants, 58-61.

Vangronsveld J., Van Assche F., Clijsters H. (1995b) Environ Pollut 87:51-59.

 

Table 1 : Phytotoxicity indices (PI), zootoxicity indices (ZI) and bio-availability index for Zn (BIOI) for the untreated and treated Maatheide soils 12 months after application of the different amendments) (CA : cyclonic ashes; SG : steel shots; SC : steel powder; BE : bentonite and combinations (CA+SG; CA+BE)).

Maatheide

Treatment

PI

ZI

BIOI

0 %

without

CA 5 %

SG 1 %

SC 1 %

CA 5 % + SG 1 %

BE 5 %

CA 5 % + BE 5 %

1

2

3

3

2

3

2

1

1

1

1

1

1

1

1

1

1

1

1

1

1

20 %

without

CA 5 %

SG 1 %

SC 1 %

CA 5 % + SG 1 %

BE 5 %

CA 5 % + BE 5 %

3

1

3

3

3

3

3

4

1

1

1

1

1

1

2

1

1

1

1

1

1

50 %

without

CA 5 %

SG 1 %

SC 1 %

CA 5 % + SG 1 %

BE 5 %

CA 5 % + BE 5 %

3

2

3

3

2

3

2

3

1

1

3

1

1

1

3

1

2

2

1

1

1

100 %

without

CA 5 %

SG 1 %

SC 1 %

CA 5 % + SG 1 %

BE 5 %

CA 5 % + BE 5 %

4

2

3

4

2

3

2

4

2

4

4

2

1

1

3

1

3

3

1

1

1

 

PI (phytotoxicity index for Phaseolus vulgaris)    

1 : “not toxic”

                                                               2 : “slightly toxic”

3 : “moderately toxic”

                                                               4 : “strongly toxic”

ZI (zootoxicity index for Eisenia fetida)

1 : “weight loss 0-10 %”

                                                               2 : “weight loss 11-20 %”

                                                               3 : “weight loss 21-50 %”

                                                               4 : “letale or weight loss 51-100 %”

BIOI  (bio-availability index for Zn)     

1 : “not bioavailable or < 60 mg Zn eq/ kg”

                                                               2 : “slightly bioavailable or 61-129 mg Zn eq/ kg”                                                                                          

               3 : “moderately bioavailable or 130-289 mg Zn eq/ kg”                                                                   

               4 : “strongly bioavailable or > 290 mg Zn eq/ kg soil”