NEW APPROACH FOR ASSESSING DECONTAMINATION PROCESSES BY CHEMICAL SPECIATION COMBINED WITH BIOASSAYS

Heinz Stichnothe, Alexander Weissbach, Helga Neumann-Hensel, Wolfgang Calmano

Technical University Hamburg-Harburg, Department of Environmental Science and Technology, Eissendorferstrasse 40, 21071 Hamburg, Germany

contact e-mail: stichnothe@tu-harburg.de

ABSTRACT

The combination of chemical speciation by means of sequential extraction, XAFS (x-ray fine structure spectroscopy) thermodynamical calculations and bioassays seems to have a promising future. Due to thermodynamical calculations with PHREEQC the formation of mixed Pb-salts is predicted. At the moment the signification of the formation of these mixed salts for the XAFS reference substances database has still to be evaluated. Nevertheless the existence of these mixed salts could be verified due to x-ray diffractometer analysis. Thermodynamical modelling is a useful tool for determination of reference substances, process parameters and also for prediction of chemical species during leaching process or operational methods like sequential extraction.

A simple relation between chemical species and toxicity effects could not bee observed. Even natural matrix compounds like humic acid, Fe-complexes or citric acid bring out toxicity effects for both applied bioassays algae test and soil contact test. The mechanism of the toxic effect is still under investigation. With these results the application of bioassays due to their extremely high sensitivity has to be examined before they can be used as an assessment tools for decontamination processes.

INTRODUCTION

Heavy metals in soils are still a challenging problem all over the world. Various remediation technologies are applied, e.g. extraction procedures, electrokinetic and soil washing. Due to the constraint that heavy metals, except elemental mercury, have a very high boiling point and can not be bioconverted, usually leaching processes have to be applied to remove heavy metals from soil. Such a leaching process must combine high extraction efficiency and low waste water volume. These prerequisites can be realized by a circuit process where heavy metals are removed from the extraction solvent by means of electrolysis and then reused again. But how to assess the efficiency of this process? Considering just the heavy metal content will not answer the question of detoxification of soil due to the treatment. A new approach using a model soil has been undertaken to combine heavy metal speciation by XAFS (x-ray fine structure spectroscopy) with thermodynamical calculations and bioassays to improve the extraction process as well as refer toxicity data to chemical species.

METHODS

A model soil composed of quarz, goethite, humic acid and clay is contaminated with different lead species, like PbO₂, PbCl₂, PbCO₃ and PbSO₄. The compounds are mixed and stirred for 24 h afterwards the soil is dried at room temperature. This soil is used for the further investigations. The speciation and distribution of the lead species on different soil compartments after establishing of equilibrium and after an extraction procedure are performed by sequential extraction and XAFS (X-ray adsorption fine structure spectroscopy). XAFS is able to provide us with information about the real binding form of heavy metals in soils without interacting with the matrix. Furthermore the XAFS –spectra contain information about the binding form of several metals in different soil components like humic acid or clay. The evaluation is performed by comparison of the real spectra data with linear combinations of reference substances, e.g. Pb-salts as well as assoziates of lead with soil compounds like humic acid Mn- or Fe-oxides /Welter/.

The sequential extraction scheme of Zeien&Bruemmer and the BCR were conducted according to URE to try to evaluate the operationally defined fractions with the XAFS- data. Both methods are selected to answer the question whether an extended use of defined fractions improves the characterization of heavy metals in soils. The Zeine&Bruemmer scheme is subdivided into seven defined fractions and the BCR method just in three.

To investigate the detoxification potential, a test battery containing algae bioassay, microbiological contact assay is conducted. The toxicity is determined indirectly by measuring the effect of the elutriate with the algae growth inhibition test with Pseudokirchneriella subkapitata. The method is carried out in miniaturized version according to the german DIN 38414 part 33.

The microbiological contact assay is used to determine toxicity directly in soil. The test organism Arthrobacter globiformis is a typical aerobic chemoheterotrophic bacteria in soils. The method is based on the conversion of resazurine to resorufine by bacterial dehydrogenase activity, and is measured against an uncontaminated control sample. The decontamination process contains a countercurrent extraction reactor where the extraction solvent is pumped up-stream through the soil and subsequently treated in an electrolysis cell to recycle the solvent.

RESULTS AND DISCUSSION

To calculate the Pb-species distribution at equilibrium the added Pb-species are used as input parameters. The results are presented in tab. 1.

Table 1: Pb-species distribution after achieving equilibrium calculated with PHREEQC version 1.3

mineral phases	formula	M_r [g/mol]	amount [mol]	mass_Pb [mg]
anglesite	PbSO₄	303.25	5.35 10^-6	1
cerrusite	PbCO₃	267.20	4.09 10^-3	847
cotunnite	PbCl₂	278.10	4.08 10^-3	845
galenite	PbS	239.25	1.39 10^-15	0
lanarkite	PbO:PbSO₄	526.44	1.19 10^-2	4947
laurionite	PbOHCl	259.65	5.23 10^-5	11
phosgenite	PbCl₂:PbCO₃	543.30	7.83 10^-3	3246

The input species cerrusite and colunnite are transformed partlyto phosgenite after reaching equilibrium while anglesite and massicote result exclusively in larnakite. The formation of mixed Pb-salts like phosgenite (PbCl₂:PbCO₃) and larnakite (PbSO₄:PbO) can be evaluated by x-ray diffractometer analysis.

Figure 1: X-ray diffractometric spectra of a solid obtained from a batch experiment with PbO and PbSO₄; as reference the spectra of massicote (PbO) is marked with black arrows

These mixed salts has not been part of the reference substance database. Therefore the XAFS data have not been analysed yet.

The thermodynamical calculations are also useful to determine conditions like pH, solvent and solvent concentration for the decontamination process. Due to these calculations, a citrate solution of 0.1 mol/l and pH 6 are selected to conduct the leaching experiment. The complexation properties of citrate solution ensure the dissolution of all Pb-species and avoid an excessive dissolution of goethite at a solid/liquid ratio of 1:10 (fig. 2).

Figure 2: Solubility of Pb-species of the model soil in dependence of addition of 0,2 mol/l ammoniacitrate at a solid/liquid ratio of 1:10

This model soil has been decontaminated with the described circuit process from 10000 up to 50 mg/kg which would allow an unlimited reuse of the soil according to german law. The remove of lead species include all operationally defined fractions of the Zeien&Bruemmer and also reduce the TOC-content in the soil.

The bioassays are applied to the model soil before and after the extraction procedure, at first to the extraction agent (here citric acid), and the compounds humic acid and goethite, respectively. Additionally these single compounds in combination with citric acid and a pure PbNO₃ solution have been investigated. Citric acid at concentrations above 100 mg/l causes a complete inhibition regarding the test organism. Goethite itself shows no toxicity until it is extracted with citric acid. The formation of Fe-citrate-complexes seems to effect the test organism. In contrast to the behavior of goethite, the toxicity of humic acid decreases after extraction with citric acid. These results indicate that already the interaction of soil matrix compounds and citric acid effects both test organisms even in absence of heavy metal species. But likewise the toxicity of the pure Pb-solution is reduced by addition of citric acid. Despite these results the extraction of the Pb spiked soil with citric acid reduces the toxicity concerning both bioassays, but it could not be evaluated whether the reduction of TOC or humic acid, respectively, in this particular case or the removal of lead is responsible for the decreasing toxicity. The results are summarised in table 2.

Table 2: Data of the model soil before and after extraction with 0.1 mol/l citrate solution

model soil	Pb [mg/kg]	TOC [%]	Algae toxicity*	contact test toxicity *
before	9900	0.69	yes	yes
after	50	0.10	no	no
*toxicity criteria > 20% inhibition of the test organism

Further investigations have to be undertaken to answer this question.

REFERENCES

Ure, A.M., Quevauviller, P., Muntau, H. & Griepink, B. (1993): Intern. J. Environ. Anal. Chem. 51: 135-151.

Welter, E., Calmano, W., Mangold, S. and Tröger, L. (1999): Fresenius J. Anal. Chem. 364: 238-244.

Zeien, H. & Brümmer, G.W.(1989): Chemische Extraktionen zur Bestimmung von schwermetallbindungsformen in Böden. Mitteilgn. Dtsch. Bodenkundl. Gesellsch. 59: 505-510.