Elsevier

Chemosphere

Volume 77, Issue 4, October 2009, Pages 510-517
Chemosphere

Characterization of a former dump site in the Lagoon of Venice contaminated by municipal solid waste incinerator bottom ash, and estimation of possible environmental risk

https://doi.org/10.1016/j.chemosphere.2009.07.046Get rights and content

Abstract

Bottom ash from a municipal solid waste incinerator on a former contaminated site, the island of Sacca San Biagio (Lagoon of Venice), was examined in order to evaluate levels of pollutants and their potential mobility and availability. Heavy metal concentrations were determined and the actual contamination of the site was compared with national legislation on polluted sites. The site was mainly contaminated by zinc, copper and lead. Physico-chemical characterization of bottom ash was carried out by SEM (Scanning Electron Microscopy) with micro-analysis by EDS (Energy Dispersive X-ray Spectroscopy) and XRD (X-ray Diffractometry), for information on newly formed minerals. SEM-EDS analysis revealed the presence of particles, compounds and clusters containing heavy metals and, in particular, the presence of barium sulfate, which was assumed to be a site-specific compound. Similarities between bottom ash and atmospheric PM10 collected on the adjacent island of Sacca Fisola were studied and a risk of aerodispersion of the fine fraction of ash was assumed. Lastly, in order to evaluate the potentially available fraction of metals (non-residual fraction) and the directly exchangeable fraction, two single extraction procedures with HCl and citric acid were carried out, respectively. Results showed a relatively low concentration of readily phyto-available metals, as well as the high concentrations found for some heavy metals (Cu, Pb, Zn) in the potentially mobilizable fraction.

Introduction

Former disposal sites of hazardous materials may represent a long-term source of environmental contamination, since pollutants can be directly propagated to surface and ground waters and air. In particular, bottom and fly ashes, which are the main residues of MSWI (Municipal Solid Waste Incinerator) activity, contain high concentrations of potentially hazardous elements (such as heavy metals) (Chandler et al., 1997, Zevenbergen et al., 1998, Huang et al., 2006). There is evidence that bottom ash is unstable in atmospheric conditions, since weathering changes the mineralogical characteristics of bottom ash, mainly by reducing its chemical reactivity by carbonation and trapping of heavy metals in newly formed minerals (Piantone et al., 2004).

Although several studies have investigated weathered and unweathered MSWI bottom ash, to our knowledge there are no published data about bottom ash more than 12 years old (Zevenbergen et al., 1998, Meima and Comans, 1999). It is therefore of interest to study the 35-year-old MSWI bottom ash of a former disposal site located in the Lagoon of Venice.

In this study we examine the environmental risk related to ash stored on the island of Sacca San Biagio (SB in Fig. 1), near the city centre of Venice, over a period of about 10 years, during incinerating activity which started in 1973.

This paper provides data on metal contamination and the main mineralogical characteristics of MSWI bottom ash stored on the site. Mineralogical neoformations due to weathering processes are discussed with reference to data on younger bottom ash (Zevenbergen et al., 1998, Zevenbergen et al., 1999, Meima and Comans, 1999, Piantone et al., 2004). Comparisons of airborne particle collected from the nearby island of Sacca Fisola (SF in Fig. 1) is also discussed. Lastly, two single extractions with cold HCl and citric acid were carried out, to evaluate the potentially mobilizable and the directly exchangeable metal fractions, respectively, and to gain information on the risk associated with the diffusion of contaminants through the trophic chain.

The island of the former incinerator, called Sacca San Biagio (SB), is located in the middle of the Lagoon of Venice, at the western end of the island of Giudecca. The site covers about 45,000 m2 (Fig. 1). It was originally a natural deposit of sediments emerging at low tide, filled with inert materials and rubble in the post-war period. In 1970, a municipal solid waste incinerator was built, and operated from 1973 to 1984. During this time, approximately 60,000 m3 of bottom ash were collected and dumped on the site.

In 1994, the island was secured against erosion with walling made of reinforced concrete sheet piles, but total impermeabilization was not guaranteed.

The incinerator was demolished in summer 2003. Currently, the island is used as a temporary waste dump and is almost completely covered with spontaneous mainly herbaceous, pioneer vegetation.

Section snippets

Sampling and pre-treatment of bottom ash

Bottom ash samples were collected from various sub-areas of SB. In particular, as shown in Fig. 1, the island was divided into four sites, each containing three sampling points. A total of 12 samples was collected, from reciprocally distant points on the island for optimal representativity of the site itself. Samples were collected after a period of 72 h without rain. Primary samples (1 kg per sample) were collected between depths of 0 and 30 cm and were stored in polyethylene bags for transport

Total metal concentrations in bottom ash

Table 1 shows total element concentrations determined in the four sampling sites, and limits imposed by Italian regulations for various soil uses (mg kg1 dry weight).

No significant statistical differences were noted between the average values of the four sites. This statement is supported by statistical analysis. The Wilks–Lambda and Fisher exact tests were combined to yield a significativity index (significativity level set at 0.05). Only As and Al turned out to discriminate between sites. As

Conclusions

Analyses carried out on bottom ash indicated the presence of secondary minerals. Carbonates (mainly calcite) and aluminosilicates, with heavy metal impurities, were extensively observed. Collected evidence supports other authors’ conclusions (Zevenbergen et al., 1999, Meima and Comans, 1999, Meima et al., 2002, Piantone et al., 2004) about adsorptive reacting bulks and the progressive decrease in pH values during weathering processes.

The study site is extensively contaminated by heavy metals.

Acknowledgements

The authors thank Lorena Gobbo for technical assistance and VESTA Spa for financial support.

References (28)

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