Previous studies show that sediments of the Cantabrian estuaries (Northern Spain) contain significant concentrations of heavy metals and organic pollutants derived from intensive industrial, agricultural and urban activities. In the most polluted sediments of two estuaries (the bay of Santander and estuary of Suances), physico-chemical and environmental characterisation has been carried out.

Water content, density, pH, specific surface, porosity and particle diameter distribution have been evaluated as physical parameters. Total metal contents have been analysed as inorganic parameters. Loss of ignition (LOI), phenol index and extractable organic halogens (EOX) have been determined as organic parameters. Two leachate assays have been carried out in order to evaluate the environmental behaviour of the sediments. Total availability has been analysed using the NEN 7341 test and the mobility of the pollutants has been determined using the EN 12457 leaching test at two liquid/solid ratios. The obtained results have been compared with European Union regulations.

The physical parameters and inorganic compounds are very similar in the studied sediments. However, the sediments from Suances have high concentrations of LOI and EOX. The obtained correlation of the physical parameters with the particle size distribution is useful in the study of the sediment addition to ceramic process as a sediment management option. A comparison of the results obtained from the total, availability and mobility analysis of the inorganic pollutant concentrations of As, Ba, Ni, Pb and Zn present in sediments has been made, giving knowledge for the development of regulations and tools that would contribute to the integral management of the estuarine areas.

Introduction

Coastal ecosystems are subject to the discharge of pollutants via urban, industrial and accidental effluents. The majority of the pollutants are adsorbed in the particulate matter and deposited in the sediments. Estuaries form sinks for sediments usually contain a complex mixture of compounds originating from human activities (Ridgway and Shimmield, 2002).

Sediments geochemistry, together with sediment deposition pattern and contaminant sources are controlling factors of the adsorption-desorption of contaminants onto sediments; these processes are responsible for availability, fate and behaviour of the contaminants in the environment. Organic matter, particle size, mineral fraction and microstructure of sediments have been related as the main factors depending on the behaviour of organic and inorganic contaminants in sediments (Karickhoff et al., 1979; Evans et al., 1990; Mitra et al., 1999; Cornelissen et al., 1999; Riba et al., 2002).

The site-specific determination of physico-chemical characteristics, total pollution contents, availability and mobility of contaminants could be used to determine sediment management options and source control (Andres et al., 2002; Förstner, 2002; Apitz and White, 2003; Del Valls et al., 2004). Sediments from Cantabrian estuaries (Northern Spain) have been previously characterised by different chemical and ecotoxicological parameters in order to study the pollution of these coastal areas (Viguri et al., 2002; Rodríguez et al., 2003; González-Antolin et al., 2004). In this work, the physico-chemical characterisation and the total content, availability and mobility of the As, Ba, Ni, Pb and Zn elements from the most polluted sediments have been carried out. These results could give useful information and criteria to evaluate alternatives to sediment management as disposal or introduction into ceramic matrices.

Materials and methods

Samples of 20 kg of 50 cm polluted depth sediment (Viguri et al., 2002) were obtained from the estuary of Suances (S1) and the bay of Santander (S2 and S3) using a grab sampler. All samples were homogenised and transferred to clean glass jars, capped with aluminium foil and kept in the dark at 4°C until analysis.

A set of physico-chemical parameters were determined using standardised methods: water content (UNE-EN 12880:2001); density, measured by standard test method for screening bulk density of waste; pH value by EPA9040A method for waste solids, sludge and sediments with water content higher than 20%. Microstructural parameters as specific surface and porosity were analysed through BET surface using a Micromeritics ASAP 2000 and a Micromeritics Poresizer 9310 equipment respectively. Particle size distribution was evaluated through UNE 77314:2001 method. Samples were powdered, digested by aqua regia and the metal content was determined by ICP-OES as inorganic parameters by an external laboratory (Activation Laboratories in Canada). Organic matter has been estimated by loss of ignition (LOI) according to the DIN 38414-3 method. Phenol index has been analysed by distillation and UV spectrophotometric determination by the DIN 38409-H16-3 method. Extractable organic halogens (EOX) have been determined according to the NEN 5735 method by microcoulometer analysis using a EUROGLAS ECS 2000 instrument.

Two leaching assays have been carried out in order to evaluate the environmental hazard of the sediments. Total availability has been analysed using the two step NEN 7341 leaching for solid building and waste materials (NEN, 1995); sample is milled to a small particle size (95% < 125 μ m) and mixed with fresh water at L/S = 100 l kg−1 at two extractions with pH values of 7 (applied to optimise the leachability of oxyanionic species) and 4 (applied as a lower limit of pH in natural environments), are used to calculate the availability of inorganic components.

The small particle size and the high L/S ratio minimises, diffusion barrier of solid particles and solubility limits of the constituents to be analysed, respectively. The European standard EN 12457 leaching test for granular materials aimed at compliance testing of waste destined for landfill (Council European Union, 2003), was used to study the mobility of the pollutants. For leaching studies, an amount corresponding to 100 g of dry weight was placed into 1000 ml flasks. Water was then added to obtain liquid to solid (L/S) ratios of 10:1 and 2:1 in order to assess leachability under mild extraction conditions for material reuse options. The mixture was shaken by end-over-end rotation at 10 rpm for 24 h. After this, the mixture was filtered at 0.45 μ m (Millipore®) and used for chemical analyses.

Heavy metals have been measured on the obtained leachates by ICP-OES in a PERKIN ELMER Plasma 400 equipment, under the guidelines suggested by the manufacturer and the standard methods ISO 3120B (APHA, 1992) and SW6010B (EPA, 1996). The linear range for the method was 0.1–10 mg l− 1. Samples outside the linear range were diluted with deionized water and nitric acid such that all final solutions were in a 1% HNO3 matrix. The obtained results have been compared with EU regulation for inert waste disposal (Council European Union, 2003).

Results and discussion

Figure 1 shows the results of physical and organic parameters of the sediments. The results of phenol index in all samples have been smaller than the detection limit of the norm (1 mg kg− 1). As can be observed in figure 1, all sediments have a similar value of pH (7.17–7.76) and water content between 38–48%. The results of density and porosity are very similar in all cases; however, the specific surface in the third sediment is higher (21.71 m2g− 1) than the rest of sediments (8.91–9.43 m2 g− 1). The results of the particle diameter distribution show more sand contribution in the sediment S1 from Suances (73.63%) in relation to the sediments S2 and S3 from Santander (48–56%). The results of organic parameters in the sediment S1 from Suances are higher than the rest of the sediments with a very high EOX concentration, probably due to the intensive mining and industrial activity in this area where there are a pulp and paper mill, chlor-alkali and soda ash chemical plants (González-Antolin et al., 2004).

Figure 2 shows the evolution of the physical parameters with the percentage of particle diameters smaller than 63 μ m. The study of this behaviour is important due to the implications in the management of these sediments by introduction into ceramic materials. In the furnace of the ceramic process, if the particle diameter is high, the sediment will be only encapsulated in the matrix; however, if the particle diameter is low, the sediment will be part of the ceramic material (Elias, 2000). The water content and the density can influence the plasticity of the ceramic materials and the rest of the physical parameters are very important in the final product characteristics as mechanical, thermal and chemical resistance properties (Tourneret et al., 1999; Besegio et al., 2001). The results indicate that the density and specific surface are higher in the sediments with a high percentage of particle diameters smaller than 63 μ m. The water content decreases with this parameter and the LOI and porosity are very similar in all the intervals of particle distribution of the studied sediments. Based on these characteristics and taking into account previous results (Andres et al., 2004), all sediments are suitable to be introduced in ceramic materials. Two sediments stand out among them, the sediment S1 from Suances due to their high percentage of particle size smaller than 63 μ m and the sediment S3 from Santander because of their high specific surface. Furthermore, the utilisation of the sediment in ceramic matrices can be useful for the treatment of the organic pollutants.

Figure 3 shows the results of the total, the availability and the mobility concentrations of inorganic parameters As, Ba, Ni, Pb and Zn whose concentrations are higher than the detection limit of analytical equipment. Only the results of lead do not fulfil the limits established by the EU regulation for inert waste disposal in both Liquid/Solid ratios. The results of As, Ba and Ni total concentrations are very similar in all samples; the lead (797 mg kg− 1) and zinc (5,540 mg kg− 1) concentrations in the S1 sample are one order of magnitude higher than concentrations in S2 and S3 sediments. From the mobility results, it can be concluded that all sediments can be disposed of in a landfill for non-hazardous waste. However, all sediments should be pre-treated if their management option is the disposal in a landfill for inert waste, due to the values of lead mobility.

The availability results for all samples suggest that only a small part (between 0.5% and 20%) of the total concentrations are bioavailable, except for zinc, which has very high availability (between 23% and 100%; Figure 4). The percentages for mobility in relation to the total concentration ranges from 0.005% to 1.2% depending on the studied parameter, sediment and L/S ratio; lead shows the highest mobility values in S3 sediment with percentages of the total concentration of 1.2% at L/S = 10 and 0.92% at L/S = 2. From the point of view of management, lead (total contents and mobility) and zinc (availability) are the main inorganic parameters to take into account in the studied polluted sediments. The obtained mobility and availability concentrations for these metals (Figures 3D and 3E) are dependent of the final pH in the leachate; in this way, at the neutral pH values of the mobility assays, the lead concentration is higher than the zinc concentration, and higher than regulated limits for inert waste material landfilling, independently of the total and availability concentrations.

Although the metal concentration differences between the mobility test (mild conditions) and the availability test (worst-case environmental conditions) is several order of magnitude, systematically the metal concentrations in the leachates of the availability test overcome the regulated limits for landfill of inert waste (excepting As and Ba); therefore it is very important to remark that Ni, Pb, and Zn could be involved in the leaching process in the long-term. Consequently, a sustainable management strategy of sediments should take into account the inclusion of treatment technologies previous to landfill.

Conclusions

The present study demonstrates the main implications for sustainable management of the most contaminated sediments from the Cantabrian estuaries; both by introduction into ceramic materials and by landfill. The evolution of the physical parameters, density, porosity, specific surface and water content with particle size, show that studied sediments can be used as clay replacement in ceramic processes. Availability and mobility concentrations of inorganic parameters As, Ba, Ni, Pb and Zn, obtained by leaching tests, suggest that only a small part of the total concentrations are bioavailable for all samples except for zinc, which has very high availability. From the mobility results, it can be concluded that all sediments can be disposed of in a landfill for non-hazardous waste, but should be pre-treated if their disposal is to be in a landfill for inert waste, due to their high lead mobility.

Acknowledgements

This research project was supported by the financial help of the Spanish Education and Science Ministry Projects REN 2002-04202-C02-02 and PTR1995-0797-OP.

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