B. Ruiz-Labrador, A. Coz, E. Cifrián, B. Galán, A. Andrés * Department of Chemical Engineering, University of Cantabria, 39005, Santander, Spain

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1 Management of hazardous waste on sustainable landfills: Influence of the compaction of stabilised/solidified metallic waste on the leaching characteristics B. Ruiz-Labrador, A. Coz, E. Cifrián, B. Galán, A. Andrés * Department of Chemical Engineering, University of Cantabria, 395, Santander, Spain *Corresponding author: ana.andres@unican.es, Tel , Fax: Abstract In this work, the study of the compaction process in stabilised/solidified metallic waste has been carried out in order to improve the mechanisms controlling release on sustainable landfills for nonhazardous monolithic waste. The compaction process has been previously optimised based on the density results of the compacted products. Later, a NEN 7345 diffusion leaching test has been developed for three samples at 4, 12 and 2 bars compaction press, in order to study the leaching behaviour of the compacted samples. Results of ph, electric conductivity, redox potential and heavy metal concentrations, Sb, Cd, Mo and Pb, have been analysed in the leachates. In all cases, diffusion is the dominant mechanism controlling release and the best immobility results have been obtained in the samples with a pressure equal to 12 bars. Finally, the compaction process of this kind of waste material as treatment prior to land disposal in a sustainable landfill for monolithic waste base on the leaching behaviour in a long time is recommended. Introduction In present times the management options of industrial waste on sustainable landfills have increased considerably. Some of the aims of sustainable landfill are the optimisation of the degradation of organic pollutants, the immobilisation of hazardous waste and the solubility control and flushing of inorganic waste (Pivato and Gaspari, 26). The European regulatory framework for granular materials gives the criteria for the disposal in a non-hazardous waste landfill (OJEC, 23). However, it proved impossible to derive values for stabilised monolithic waste due to the lack of suitable information for relating release from stabilised waste in monofills. The European Commission decided that additional information was needed prior to setting criteria for this type of landfill, such as infiltration, evaporation and run-off, some physical and technological properties of the monoliths and the dominant mechanisms controlling release (e.g. diffusion, solubility limitations, and kinetics) (van der Sloot et al, 27). In this work, products obtained by stabilisation/solidification of electric arc furnace dust at industrial scale have been compacted in order to improve the inertisation process and the immobilisation mechanisms on sustainable landfills for monolithic waste. Initially, the compaction process has been optimised and later a diffusion leaching test has been carried out in order to study the leaching characteristics of the monolithic products in a sustainable landfill. Three samples at different compaction intensities have been studied and ph, Electric Conductivity (EC), redox potential and heavy metal mobility have been analysed in the leachates. The leaching behaviour of stabilised/solidified metallic waste has been strongly studied (van der Sloot et al., 1997; Laforest and Duchesne, 26; Salihoglu and Pinarli, 28a and b); however, the compaction processes for these waste materials have not been developed. Materials and methods Products obtained through stabilisation/solidification of electric arc furnace dusts at industrial scale from a factory of Cantabria have been compacted. The compaction process has been previously optimised in a laboratory press Mignon SS/EA Nannetti, obtaining cylindrical pieces of 4 cm diameter. Water content (from 2 and 8%) and pressure values (from 4 to 2 bars) have been studied as input variable and the mass density has been analysed in the monoliths as output variable. Figure 1 shows a photograph of the obtained compacted products.

2 Figure 1. Photograph of the obtained compacted S/S waste. For three samples with 3% water content at 4, 12 and 2 bars, NEN 7345 diffusion leaching test has been determined to study the leaching behaviour and to give the dominant mechanisms controlling release. NEN 7345 leaching test (NEN, 1995) over 64 days was proposed by Dutch Building Material Decree (BMD, 1999) in order to study the leaching of inorganic components from monolithic materials in a closed tank. The ph value, electric conductivity, redox potential and metal concentration of cadmium, lead, molybdenum and antimony have been analysed in the leachates. In previous studies (Andrés et al., 26; Coz et al., 27), these metals have been obtained as the main pollutants for this stabilised/solidified waste material. Diffusion characteristics, diffusion coefficients, immission values and accumulated leaching concentration have been obtained in all cases. Furthermore, the immission values of the pollutants over 1 years have been compared with the regulated limit, according to the Building Material Decree (BMD, 1999). Results and discussion Figure 2 shows the results of the compaction process at 4, 12 and 2 bars. Typical curves for waste compaction have been obtained in all cases (Reddy et al., 29). The optimised products have been obtained for 3-4 percentages of water content and a high pressure value, from 12 to 2 bars. Density (g/cm 3 ) Water content (%) Figure 2. Optimisation of the compaction process. The leaching behaviour of three compacted samples at different compaction intensities have been studied through NEN 7345 diffusion leaching test. Figure 3 shows the ph, EC and redox potential results. The results of electrical conductivity and redox potential do not depend on the compaction process, giving similar behaviour for 4, 12 and 2 bars. The electric conductivity diminishes drastically at the beginning of the leaching test from µs/cm to µs/cm and later it remains constant from 5 to 931 µs/cm. The results of the potential redox have a maximum peak, with values of mv, at 4 hours and the final results of the leaching tests gives values from 187 to 225 mv. Regarding the results of ph values, in all cases very similar results are obtained at the end of the leaching tests; however, in some cases, ph values close to 4 are obtained.

3 ph EC (μs/cm) Redox potential (mv) Figure 3. Results of ph, EC and redox potential in the NEN 7345 leaching test. The results of the accumulated mobility of the pollutants in mg/m 2 are shown in figure 4. The mobility of the pollutants in mg/m 2 is obtained as C*V/A, where C is the concentration of the leaching test in mg per litre, V is the volume of the leaching test in litres and A is the superficial area of the monolithic material in m 2. Only antimony, cadmium, molybdenum and lead have been studied due to in previous studies (Andrés et al., 26; Coz et al., 27) they have been obtained as the main pollutants for this stabilised/solidified waste material En* Sb (mg/m 2 ) 2 1 En* Cd (mg/m 2 ) En* Mo (mg/m 2 ) En* Pb (mg/m 2 ) 4 2 Figure 4. Results of heavy metal accumulated mobility in the NEN 7345 leaching test.

4 As it can be observed in figure 4, and in spite of the values of ph, EC and redox potential are very similar, the best immobilisation results are obtained when a 12 bar has been used as compaction press whereas the results at 4 and 2 bars are very similar in all cases. The results of lead are more similar to the compaction process; however the molybdenum, antimony and cadmium results at 12 bars differ %, % and %, respectively, in relation to the 4 and 2 bars. Table 1 shows the main parameter for the metal leaching behaviour. In all cases, diffusion is the dominant mechanism controlling release and the best immobility results have been obtained in the samples with a pressure equal to 12 bars. Therefore, a compaction process is recommended as treatment prior to land disposal for this kind of waste materials base on the leaching behaviour in a long time. Table 1. Results of the main parameters on the diffusion leaching test. Sb Cd Mo Pb 4 bars De (m 2 /s) 5.73E E E E-13 Mobility Low High High Medium ε 64 (mg/m 2 ) Regulated limit over 1 years bars De (m 2 /s) 1.2E E E E-13 Mobility Low Medium High Low ε 64 (mg/m 2 ) Regulated limit over 1 years bars De (m 2 /s) 1.75E E E E-13 Mobility Low Medium High Low ε 64 (mg/m 2 ) Regulated limit over 1 years De: effective diffusion coefficient. ε 64: immission value of the diffusion test in mg per m 2. The immission values of the pollutants over 1 years have been compared with the regulated limit, according to the Building Material Decree (BMD, 1999). The results of antimony and lead fulfil the regulated limits for building materials, however molybdenum exceeds the regulated value 3.5 times and cadmium exceeds 1 times. Conclusions Products obtained by stabilisation/solidification of electric arc furnace dusts at industrial scale have been compacted. The compaction process has been previously optimised and the best results have been obtained for 3-4 percentages of water content and a high pressure value, 12-2 bars. For three samples with 3% water content at 4, 12 and 2 bars, NEN 7345 diffusion leaching test has been determined to study the leaching behaviour and to give the dominant mechanisms controlling release. In all cases, diffusion is the dominant mechanism controlling release. The mobility results of the pollutants have been compared with the regulated limit for construction materials. The waste material can not be used as building material due to the molybdenum and cadmium concentrations. However, antimony and lead fulfil all the regulated limits.

5 On the other hand, in spite of the values of ph, EC and redox potential are very similar, the best immobilisation results are obtained when a 12 bar has been used as compaction press. So, a compaction process is recommended as treatment prior to land disposal for this kind of waste materials base on the leaching behaviour in a long time. Acknowledgements The Spanish Ministry of Education and Science have financially supported this work Project CTM References Andrés, A., Ruiz-Labrador, B., Coz, A., Viguri, J.R., 26. Testing of solidified-stabilized products at industrial-scale prior to disposal using new compliance leaching tests. Proceedings of the WASCON 26, Belgrade. BMD, 1999, Building Materials Decree. Sdu Uitgevers The Hague. docs/internationaal/bmdexplan.pdf. Coz, A., Ruiz-Labrador, B., Viguri, J., Andrés, A., 27. Factorial Experimental Design of Batch Leaching Tests in Stabilised/Solidified Metallic Waste. Proceedings of the ISWA World Congress 27, Amsterdam. Laforest, G., Duchesne, J., 26. Characterization and leachability of electric arc furnace dust made from remelting of stainless steel. Journal of Hazardous Materials, B135, NEN, Leaching characteristics of solid and stony building and waste materials. Leaching test. Determination of the leaching of inorganic components from building and monolithic waste materials with the diffusion test. NEN 7345:1995. OJEC, 23. The Official Journal of the European Community. Council Decision 23/33/EC of 19 December 22, Pivato, A., Gaspari, L., 26. Acute toxicity test of leachates from traditional and sustainable landfills using luminescent bacteria. Waste Management, 26, Reddy, K.R., Hettiarachchi, H., Parakalla, N.S., Gangathulasi, J., Bogner, J.E., 29. Geotechnical properties of fresh municipal solid waste at Orchard Hills Landfill, USA. Waste Management, 29, Salihoglu, G., Pinarli, V., 28a. Stabilization of Electric Arc Furnace Dusts from Steel Foundries. Environmental Progress, 27 (3), Salihoglu, G., Pinarli, V., 28b. Steel foundry electric arc furnace dust management: Stabilization by using lime and Portland cement. Journal of Hazardous Materials, 153, van der Sloot, H.A., Heasman, L., Quevauvillier, Ph., Harmonization of leaching/extraction test. Studies in Environmental Science, 7, Ed. Elsevier. van der Sloot, H.A., Zomeren, A., Meeussen, J.C.L., Seignette, P., Bleijerveld, R., 27. Test method selection, validation against field data, and predictive modelling for impact evaluation of stabilised waste disposal. Journal of Hazardous Materials, 141,