Mise au point de techniques analytiques pour la spéciation du sélénium dans les boues de stations d'épuration d'eaux résiduaires urbaines Development of new analytical techniques for determining selenium speciation in sewage sludge: differential pulse cathodic stripping voltammetry (DPCSV) and electrothermal atomic adsorption spectrometry (ETAAS)

Résumé

Les stations d'épuration d'eaux résiduaires sont une des étapes du cycle du sélénium dans l'environnement et contribuent à sa redistribution dans le milieu naturel. Très peu étudié jusqu'à présent dans ces milieux, le sélénium n'en est pas moins un élément très important du point de vue écotoxicologique, sa teneur dans les boues de stations d'épuration destinées à l'épandage agricole faisant par ailleurs l'objet d'une norme.

Nous avons mis au point des techniques permettant la détermination spécifique de l'élément total dans ce type d'échantillon, par minéralisation classique ou assistée par micro-ondes et dosage par Voltamétrie de Redissolution Cathodique Différentielle Pulsée (DPCSV) et Spectrométrie d'Absorption Atomique ElectroThermique (ETAAS). Le contrôle qualité a été effectué sur deux échantillons certifiés fournis par le Bureau Communautaire de Référence (BCR) : la boue CRM 145 R et la boue CRM 007.

Cependant, lorsqu'on parle de risque toxicologique, il est important de s'intéresser à la détermination des différentes formes sous lesquelles cet élément peut être présent. Nous avons pour cela réalisé des extractions parallèles (spéciation de phases) du sélénium contenu dans les boues afin de déterminer quel pourcentage du sélénium total est réellement et potentiellement disponible pour les végétaux lors d'un épandage sur sol agricole. La spéciation d'espèces a été brièvement abordée dans le but de déterminer les teneurs en Se(IV) et Se(VI), espèces les plus toxiques.

Mots-clés:

• Sélénium,
• boues de station d'épuration,
• spéciation,
• minéralisation,
• extraction

Abstract

The great effort undertaken for about twenty years to improve the quality of surface waters has led to the construction of numerous waste water treatment plants, generating an increasing amount of sludge. Waste water and sludge treatment processes represent an important point in the hydrological cycle at which the disposal of substantial quantities of trace elements to the environment may be regulated. From the law on waste recovery and disposal in 1975 to the European guideline about wastes in 1991, the priority has been given to waste recovery and recycling. With increasing pressure to ban all sludge dumping at sea, and considering the prohibitive costs of land-filling and incineration, there is a great tendency to dispose of sludge on land (40% in 1988 to 60% in 1992).

Although numerous studies have demonstrated the intrinsic value of sludge for soil amendment, given its nitrogen, phosphorus and homogeneous organic matter content, evidence has accumulated in recent years that numerous environmental problems can arise because of the presence in sludges of high amounts of certain trace elements (potentially toxic to plants and to human beings and liable to be concentrated along the food chain), among which selenium is particularly interesting.

Selenium presents a complex case, as it is also an essential element for living organisms (including humans). The amendment with sewage sludge is sometimes used to increase the selenium content in crops, and afterwards in cattle, when there is a proven lack of this element in a given place. Nevertheless the boundary between essentiality and toxicity is relatively narrow and is expressed at trace levels. It is thus particularly important to survey the selenium concentrations encountered in sewage sludge, especially as guidelines and regulations concerning these data will probably be strengthened.

Presently, in France, sludge must not contain more than 200 mg Se·kg^-1 dry weight and must not be used on soils containing more than 10 mg Se·kg^-1 dry weight (AFNOR U 44-041 norm). This norm concerns only the total amount of selenium contained in sludge and does not take into account the different species (organic and inorganic Se(-II), Se(0), Se(IV) and Se(VI)) that could be present.

First of all we had to develop methods for the classical and microwave-assisted wet digestion of sewage sludge, and the determination of their total selenium concentration by Differential Pulse Cathodic Stripping Voltammetry (DPCSV) and ElectroThermal Atomic Absorption Spectrometry (ETAAS). Quality assurance involved the analysis of two BCR (Community Bureau of Reference) certified sewage sludge reference materials (CRM 145 R and CRM 007) and the different techniques were then applied to natural samples from a representative French sewage treatment plant located in the city of Tarbes (South-West of France).

The mixture HNO₃-H₂O₂-H₂O led to the best results for the digestion and analysis of certified samples, caused few problems for the analysis by DPCSV and ETAAS, and was therefore retained. The decrease of the digestion duration obtained by the use of microwaves was particularly interesting (from one to three days on a hot plate to less than one hour by the Microdigest 301 (PROLABO, France)), and reproducibility was also acceptable (between 3 and 10%). Concentrations obtained for the sewage sludge from the Tarbes treatment plant were very much lower than those for NF U 44-041: 1.08±0.11 mg Se·kg^-1 dry weight.

However knowledge of speciation, that is to say the determination of the different physicochemical forms of selenium present in a given medium, is necessary when speaking of the toxicological risk represented by an element. The mobility of selenium and its toxicity to the biosphere are related to its association with various sludge or soil constituents as well as to its total concentration. "Soft" or partial extraction techniques are necessary when the aim of the study to determine trace element speciation. The extractants used must separate selenium from the matrix without inducing any loss or change in the partitioning of individual chemical species. In parallel extractions the mechanisms involved for each extractant must correspond to processes occurring in nature and are then associated with special fractions of selenium: soluble, exchangeable, "oxidizable", and "mineral" fractions .

Parallel extractions with three types of extractants were chosen for this study and applied first to CRM 007: warm water (soluble fraction), ammonium phosphate-citric acid (soluble + exchangeable fraction) and sodium hydroxide (soluble + exchangeable + "oxidizable" fraction). The soluble, exchangeable, "oxidizable" and "mineral" fractions represent respectively : 11%, 14%, 39% and 36%. The same procedure was then applied to natural samples from Tarbes giving the following results: 36% soluble, 22% exchangeable, 42% "oxidizable". The sodium hydroxide extraction procedure allowed us to extract the entire Se content of this sludge (1.07±0.03 mg Se·kg^-1 dry weight), showing that all the selenium present is potentially available after agricultural land application. It was then possible in this fraction to deal with the species speciation of selenium by the mean of a separation of inorganic and organic species on an Amberlite CG-400 resin and a specific analysis by DPCSV. Se(IV) and Se(VI) represent respectively between 30 and 40% and between 2 and 20% of total selenium in the sludges from Tarbes.

Keywords:

• Selenium,
• sewage sludge,
• speciation,
• digestion,
• extraction