INFLUENCE OF NaOH SOLUTION ON THE SYNTHESIS OF HOSPITAL WASTE FLY ASH GEOPOLYMER

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 INFLUENCE OF NaOH SOLUTION ON THE SYNTHESIS OF HOSPITAL WASTE FLY ASH GEOPOLYMER A. MIMILIDOU 1, K. TZANAKOS 1, K. ANASTASIADOU 1 and E. GIDARAKOS 1 1 Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, P.C , Greece EXTENDED ABSTRACT In pursuing their aims of reducing health problems and eliminating potential risks to people s health, health-care services inevitably create waste that may itself be hazardous. In fact, medical and health-care wastes have sharply increased in recent decades due to the increased population, number, and size of health care facilities, as well as the use of disposable medical products. In Greece more than 14,000 tonnes of infectious hospital waste are produced yearly; a significant part of it is still mismanaged. Several types of treatment and disposal processes have been applied to medical waste. However, incineration has been identified as the best option for the disposal of medical waste in many areas. One of the inconveniences of incineration, is that generates solid residues, such as bottom and as well as off-gas cleaning residues with high levels of heavy metals, inorganic salts and other organic compounds. According to bibliography, possesses a high content of heavy metals, dioxins and furans. Therefore, attempts to stabilize the into a stable product, which is environmentally acceptable, has received considerable attention. In the last decades, geopolymerization has emerged as a promising process for the immobilization of several kinds of wastes. The present work focuses on the evaluation of the possibility to use geopolymerization technology to stabilize and solidify generated from incinerated medical waste. Medical waste (MWFA), sodium hydroxide, sodium silicate solution and metakaolin, produced by calcination at C for 4 h of kaolinite, were mixed. Four series of experiments were carried out as are shown in Table 1. NaOH solutions with concentrations of 1, 2, 4 and 6 M were used as an alkali activator, different for each sere. In all series three different percentages of MWFA, 20, 30 and 50 wt%, were applied. After a certain aging time of 7 and 28 days, the compressive strength of the geopolymer mortars, the leachability of heavy metals, as well as the mineralogical properties of the produced geopolymers were studied. This study demonstrates that medical waste incineration containing heavy metals, such as Zn and Pb, can be effectively stabilized/solidified using -based geopolymerization technology. Compressive strength tests performed on the geopolymer specimens after 7 and 28 days of solidification, revealed values higher than the regulatory limit of MPa. Moreover, the synthesized geopolymers showed a significant increase in compressive strength, as the percentage of, with regard to metakaolinite, was increased from 20 to 50%. Also, by increasing the ratio Na 2O/SiO 2 in the geopolymer binders from 0.63 to 0.73, an important decrease in compressive strengths was observed. Geopolymers with strengths of 7 MPa were obtained when 2 M NaOH and 50% were used. With regard to the leachability of heavy metals, it can be concluded that geopolymerisation technology is able to immobilize all the heavy metals found in medical waste. The leachability of the produced binders reduced drastically through this process. Finally, a significant decrease in leachability of heavy metals was noted by increasing NaOH concentration from 1 M to 6 M. Keywords: Hospital waste, fly ah, geopolymer, metakaolin, NaOH concentration

2 1. INTRODUCTION Medical and health-care wastes have sharply increased in recent decades due to the increased population, number, and size of health care facilities, as well as the use of disposable medical products (Mohee, 2005). In Greece more than 14,000 tonnes of infectious medical waste are produced yearly; a significant part of it is still mismanaged (Karagianidis et al., 2010). Several types of treatment and disposal processes have been applied to medical waste. However, incineration has been identified as the best option for the disposal of medical waste in many areas (Idris and Saed, 2002). One of the inconveniences of incineration, is that generates solid residues, such as bottom and as well as off-gas cleaning residues with high levels of heavy metals, inorganic salts and other organic compounds (Anastasiadou et al., 2012). According to bibliography, possesses a high content of heavy metals, dioxins and furans (Alba et al., 1997). Therefore, attempts to stabilize the into a stable product, which is environmentally acceptable, has received considerable attention. In the last decades, geopolymerization has emerged as a promising process for the immobilization of several kinds of wastes. Geopolymers are obtained by the reaction between an alkaline solution, the chemical activator, and an aluminosilicate binder (Lancellotti et al., 2010). This paper focuses on the evaluation of the possibility to use geopolymerization technology to stabilize and solidify generated from incinerated medical waste. 2. EXPERIMENTAL STUDY 2.1 Materials Fly ash sample was collected from the APOTEFROTIRAS S.A. medical waste incinerator plant located in Ano Liossia, near the greater Athens urban waste disposal site. The fly ash particle size was below 120 μm, so there was no need for any pretreatment. In previous study (Anastasiadou et al., 2012), chemical composition of was determined by X-ray fluorescence analysis, the results of which are displayed at Table 1. It is obvious that the major element of the investigated was CaO (89.2%). Metakaolin produced by calcination at C for 4 h of kaolinite, was used as the principal source of aluminosilicate. Table 1. Chemical composition of medical waste bottom and. Oxides SiO 2 CaO Na 2O Fe 2O 3 MgO Other Fly ash Geopolymer mixture preparation The production of geopolymer mixtures was held according to the following steps: a) Preparation of sodium hydroxide solution by dissolving sodium hydroxide pellets in distilled water. On the grounds that the dissolution process is highly exothermic, the solution was allowed to cool down to room temperature before the following steps. b) Preparation of sodium silicate solution, by the addition of solid Na 2SiO 3 in distilled water and stirring at high speed for 40 minutes using a vibrating table. c) Mixing of the above solutions. d) Eventual addition of MWFA to the alkaline solution and stirring for 5 minutes. e) Addition of metakaolin and intensive stirring until a homogeneous and fluid paste was formed. At steps d) and e), water was, also, added into the mix to promote hydration.

3 With the paste obtained, 50 mm cubic moulds were filled and compacted. All The specimens were then put in the oven after a predetermined delay time of 24 h. After curing at 50 0 C for 24 h, the specimens were demoulded the next day and kept at room temperature until the testing age. 2.3 Test series Four series of experiments were carried out in this study as are shown in Table 2. NaOH solutions with concentrations of 1, 2, 4 and 6 M were used as an alkali activator, different for each sere. In all series three different percentages of MWFA, 20, 30 and 50 wt%, were applied. Compressive strength was determined on each specimen after 7, and 28 days of curing. Series FLY ASH concentration Table 2. Series of experiments ΝaOH concentration (M) Curing time (h) Curing temperature ( 0 C) Hardening time (d) Α1 20,30, A2 20,30, B1 20,30, B2 20,30, C1 20,30, C2 20,30, D1 20,30, D2 20,30, Methods X-ray diffraction (XRD) was utilized to determine the mineralogical properties of the fly ash sample and of the -based geopolymers, as well. The test samples were placed in a holder which were then placed in a Rigaku X-Ray Diffractometer (XRD) with a copper x-ray tube and lynxeye detector system. A diffraction angle (2θ) between 4 0 and 70 0 and a scanning rate of 4 0 /min was applied to analyse the crystal phases of the samples. Diffraction patterns were manually analysed using the Diffrac Evaluation software with database of International Centre for Diffraction Data (ICDD). The compressive strength tests were performed at the age of 7 and 28 days in accordance with ASTM Test Method for Compressive Strength of Hydraulic Cement Mortars. The reported strengths are the average of three tests. Total maximum loads were recorded at the point of fracture, and the compressive strength was determined using the formula fm = P/A, where fm is the compressive strength (MPa), P is the total maximum load (N) and A is the area of loaded surface (mm2). In the leaching test for heavy metals the Toxicity Characteristic Leaching Procedure (TCLP) was applied. Manually crushed material (<1 cm) was leached using an extraction buffer of acetic acid and sodium hydroxide (ph 4.93±0.05) at a liquid/solid ratio of 20:1. The extraction (at 25±2 C) was performed by shaking the material for 18 h. Subsequently, the leachate samples were filtered through a 0.8 μm borosilicate glass fibre filter, and the resultant TCLP extract (filtrate) was analysed for heavy metals using Inductively Coupled Plasma-Mass Spectrometer (ICPMS) Agilent Technologies, model 7500cx.

4 3. RESULTS 3.1 X-ray diffraction analysis Results of XRD patterns of and selected geopolymer samples with different NaOH concentrations are shown in Figure 1. The main crystalline compounds of the investigated were calcite (CaCO 3), halite (NaCl), quartz (SiO 2) and anhydrite (CaSO 4). This result is in total agreement with the literature (Anastasiadou et al., 2012; Kougemitrou et al., 2011; Shi and Kan, 2009; Chang and Wey, 2006; Genazzini et al., 2003). In addition, the presence of more complicated crystal structures, such as defernite (Ca 6Si O26C 1.74O 5.8(OH) 8) and thermonatrite (Na 2CO 3(H 2O) were also verified. By comparing the XRD patterns of the geopolymer products with this of original, it is evident that no substantial new crystalline phases formed in the geopolymerization process, apart from the removal of defernite and the formation of faujasite. Previous studies have, also, demonstrated that geopolymerization did not significantly alter the degree of amorphous and crystallization of (Rattanasak et al., 2009; Li et al., 2012). At higher NaOH concentration of 4M and 6M, new crystalline silicate and aluminosilicate compounds were detected in addition to the amorphous gel. These compounds suggest a relatively high level of geopolymerization (Somna et al., 2011). However, there is a certain limit to the fraction of crystalline phases that can be tolerated by the matrix, after which increased crystallinity serve to weaken it (Van Jaarsveld et al., 1998). Figure 1. XRD patterns of and selected geopolymer samples with different NaOH concentrations 3.2. Compressive strength The compressive strengths of the geopolymer specimens synthesized at four different NaOH concentrations are shown in Table 3. The obtained compressive strengths were in the range of MPa, higher than the standard stipulated for solidified waste forms, which is MPa after 28 days of solidification (Morgan & Bostick, 1992). Generally, it can be observed that the synthesized geopolymers showed an important increase in compressive strength, as the percentage of, with regard to metakaolinite, was

5 increased from 20 to 50%. This increase was due to the increase of calcium oxide in the geopolymer binders, on the grounds that the investigated contains 89.2% CaO. Previous studies (Phair et al., 2001; Lee et al., 2002; Yip et al., 2005) have found that calcium has a positive impact on the compressive strength of geopolymeric binders. The current findings are in agreement with these studies. In addition, it is evident that the use of 1 M and 2 M NaOH led to higher compressive strengths. By increasing the ratio Na 2O/SiO 2 in the geopolymer binders from 0.63 to 0.73, a significant deterioration in mechanical properties was observed. High Na 2O content was found to promote an amorphous crystalline transformation in the system. The dense amorphous matrix exhibited the higher compressive strength (De Silva and Sagoe- Crenstil, 2008). The results conformed to previous researches (Songpiriyakij et al., 2010; De Silva and Sagoe-Crenstil, 2008; Chindaprasirt et al., 2006). However, many authors reported that the compressive strength of geopolymer pastes increase along with the increase of NaOH concentration (Somna et al., 2011; Hardjito et al., 2004). The NaOH concentration in the aqueous phase of the geopolymeric system acts on the dissolution process, as well as on the bonding of solid particles in the final structure. When comes into contact with NaOH, leaching of Si, Al and others minor ions begins (Somna et al., 2011). According to Panias et al. (2007), the increased NaOH concentration in the aqueous phase of geopolymeric systems causes positive, as well as negative effects on the mechanical properties of the geopolymeric materials. Thus, a compromise of the NaOH concentration in the aqueous phase is claimed. Table 3. Compressive strength values for all the synthesized geopolymers. Series FLY ASH concentration (%) Compressive Strengths (MPa) Hardening time: 7 d Hardening time: 28 d A B C D

6 3.3. Leaching test for heavy metals In Table 4 leaching test results for the untreated, as well as for all the geopolymer binders cured at 28 days, are presented. It was observed that both Zn and Pb in the leachate of the untreated exceeded the USEPA regulatory limits by 2.36 and 1.2 times, respectively. This is in agreement with what has been reported by Anastasiadou et al., (2012). Also, lesser amounts of Ba, Cr, Cd and Ni, within permissible limits, were found. As far as the leaching test results for the solidified geopolymers are concerned, it is evident that the values for all the metals measured were within the permitted level for the TCLP test. These results indicate that geopolymerisation is able to immobilize the heavy metals found in. Moreover, as was expected, increase of the percentage of in the geopolymer matrices led to smaller stabilization rates of heavy metals. Finally, by comparing the leaching results of the four series of the experiment, it can be observed that as NaOH concentration increased from 1 M to 6 M, a significant decrease in leachability of heavy metals was noted. According to Zheng et al. (2010), a higher alkaline dosage can promote the formation of a new geopolymer gel considered to be responsible for the heavy metal fixation. Table 4. TCLP leaching values after 28 days for solidified geopolymer mortars. Heavy Metals Cr (mg/l) Ni (mg/l) Zn (mg/l) Cd (mg/l) Ba (mg/l) Pb (mg/l) TCLP limit untreated <DL* <DL ΝaOH concentration: 1 M 20% <DL <DL % <DL <DL % <DL <DL ΝaOH concentration: 2 M 20% <DL <DL % <DL <DL % <DL <DL ΝaOH concentration: 4 M 20% <DL <DL <DL % <DL <DL <DL % <DL <DL <DL <DL ΝaOH concentration: 6 M 20% <DL <DL <DL <DL % <DL <DL <DL <DL % <DL <DL <DL <DL * DL: Detection Limit 4. CONCLUSIONS This study demonstrates that medical waste incineration containing heavy metals, such as Zn and Pb, can be effectively stabilized/solidified using -based geopolymerization technology. Compressive strength tests performed on the geopolymer specimens after 7 and 28 days of solidification, revealed values higher than the regulatory limit of MPa. Moreover, the synthesized geopolymers showed a

7 significant increase in compressive strength, as the percentage of, with regard to metakaolinite, was increased from 20 to 50%. Also, by increasing the ratio Na 2O/SiO 2 in the geopolymer binders from 0.63 to 0.73, an important decrease in compressive strengths was observed. Geopolymers with strengths of 7 MPa were obtained when 2 M NaOH and 50% were used. With regard to the leachability of heavy metals, it can be concluded that geopolymerisation technology is able to immobilize all the heavy metals found in medical waste. The leachability of the produced binders reduced drastically through this process. Finally, a significant decrease in leachability of heavy metals was noted by increasing NaOH concentration from 1 M to 6 M. REFERENCES 1. Alba N., Gasso S., Lacorte T. and Baldasano J.M. (1997) Characterization of municipal solid waste incineration residues from facilities with different air pollution control systems, Journal of Air & Waste Management Association 47, Anastasiadou K., Christopoulos K., Mousios E. and Gidarakos E. (2012) Solidification/stabilization of fly and bottom ash from medical waste incineration facility. Journal of Hazardous Materials, In press. 3. Chang F.Y and Wey M.Y. (2006) Comparison of the characteristics of bottom and es generated from various incineration processes, Journal of Hazardous Materials B138, Chindaprasirt P, Chareerat T. and Sirivivatnanon V. (2006) Workability and strength of coarse high calcium geopolymer, Journal of Cement and Concrete Composites 29, De Silva P. and Sagoe-Crenstil K. (2008) Medium-term phase stability of Na2O Al2O3 SiO2 H2O geopolymer system, Journal of Cement and Concrete Research 38, Genazzini C., Zerbino R., Ronco A., Batic O. and Giaccio G. (2003) Hospital waste ashes in Portland cement mortars, Journal of Cement and Concrete Research 33, Hardjito D, Wallah SE, Sumajouw DMJ and Rangan BV (2004), In: 18th Australasian conference on the mechanics of structures and materials (ACMSM)1-3. Perth Australia. 8. Idris A. & Saed K. (2002) Characteristics of slag produced from incinerated hospital waste, Journal of Hazardous Materials B93, Karagiannidis A., Papageorgiou A., Perkoulidis G., Sanida G. and Samaras P. (2010) A multi-criteria assessment of scenarios on thermal processing of infectious hospital wastes: A case study for Central Macedonia, Journal of Waste Management 30, Lancellotti I., Kamseu E., Michelazzi M., Barbieri L., Corradi A. and Leonelli C. (2010) Chemical stability of geopolymers containing municipal solid waste incinerator, Journal of Waste Management 30, Lee W.K.W. and Van Deventer J.S.J. (2002) The effects of inorganic salt contamination on the strength and durability of geopolymers, Journal of Collides and Surfaces A 211, Li Q., Xu H., Li F., Li P., Shen L. and Zhai J. (2012) Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes, Journal of Fuel, In press. 13. Mohee R. (2005), Medical wastes characterization in healthcare institutions in Mauritius, Journal of Waste Management 25, Morgan I.L. and W.D. Bostick W.D. (1992), Solidification/Stabilization of Hazardous, Radioactive and Mixed Wastes,ASTMSTP 1123,American Society for Testing and Materials, Philadelphia, pp. 133.

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