Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 SYNTHESIS AND HEAVY METAL IMMOBILIZATION BEHAVIORS OF MEDICAL WASTE ASH BASED GEOPOLYMER K. TZANAKOS 1, A. MIMILIDOU 1, K. ANASTASIADOU 1 A. STRATAKIS 2 and E. GIDARAKOS 1 1 Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, P.C , Greece 2 Department of Mineral Resources Engineering EXTENDED ABSTRACT Among the principal methods available for the disposal of medical waste, incineration and the subsequent disposal of the resultant ash by landfilling is the priority method used. The main advantages of incineration are the destruction of pathogens and the reduction in the volume and weight of the waste. However, incineration produces residues that are enriched by toxic chemicals, such as heavy metals. Because of this, the need to manage the produced ash in an environmentally friendly way is great. In the last decade, geopolymer binders have emerged as one of the possible alternatives to conventional cement binders for concrete industry and to the management of the produced ash. For the production of the geopolymer binders have been used many kinds of raw materials, such as lignite bottom ash, blast furnace slag and metakaolin. Due to the interesting findings of these studies, we decided to study the possibility of using the residual ash of hospital waste in combination with metakaolin as a raw material for the production of geopolymers. In the present work, bottom and fly ash, generated from incinerated hospital waste, was used as a source material for making geopolymer. Hospital waste ash, sodium hydroxide, sodium silicate solution and metakaolin were mixed. Four series of experiments were carried out. In the first series, only bottom ash was used as a raw material. In the second series, a quantity of calcium carbonate was added in order to study its effect on the geopolymer paste. In the third and fourth, fly ash and bottom ash at proportions of 75:25 and 50:50 (FA: BA) were used respectively. In all series three different proportions of Medical Waste Ash (MWA): Metakaolin (MK) of 20:100, 30:100 and 50:100 were applied. Geopolymers were cured at 50 0 C for 24 h. After a certain aging time of 7 and 28 days, the strength of the geopolymer mortars, the leachability of heavy metals and the mineralogical phase of the produced geopolymers were studied. The study demonstrates that geopolymers can be produced with the use of only medical waste bottom ash as a raw material. Moreover the addition of fly ash and calcium carbonate caused a significant increase in the compressive strength of the geopolymer mortars without simultaneously affecting their crystalline structure. All the geopolymers produced show a compressive strength higher than the regulatory limit of MPa, which proves that they stabilized. Moreover, the percentage increase of the ash used for the production of the geopolymers resulted in the increase of their compressive strength. Furthermore, it is a fact that the geopolymer binders of this study gave compressive strength levels lower than those of previous studies, but it is certain that either with the use of the same materials but in different proportions or by removing some material such as metakaolin, it would be possible to produce geopolymers with significantly greater strength. Finally it is concluded that geopolymerisation is able to immobilize all the heavy metals found in medical waste fly and bottom ash, as the leachability of the produced binders reduced drastically through this process. Keywords: Hospital waste, fly ah, bottom ash, geopolymerization, metakaolin, compressive strength

2 1. INTRODUCTION Among the principal methods available for the disposal of medical waste, incineration and the subsequent disposal of the resultant ash by landfilling is the priority method used. The main advantages of incineration are the destruction of pathogens and the reduction in the volume and weight of the waste. However, incineration produces residues that are enriched by toxic chemicals, such as heavy metals (Jung et al., 2004). Because of this, the need to manage the produced ash in an environmentally friendly way is great. Anastasiadou et al., 2011, have studied the feasibility of using fly and bottom ash in cement matrices, which can then be disposed safely in non-hazardous landfills, or even reused in the construction industry. Moreover, in the last decade, geopolymer binders have emerged as one of the possible alternatives to conventional cement binders for concrete industry (Chindaprasirt et al.,2009) and to the management of the produced ash. For the production of the geopolymer binders have been used many kinds of raw materials, such as lignite bottom ash (Sata et al., 2012), blast furnace slag (Oh et al., 2010) and metakaolin (Rovnaník et al., 2010). Due to the interesting findings of these studies, we decided to study the possibility of using the residual ash of hospital waste in combination with metakaolin as a raw material for the production of geopolymers. 2. EXPERIMENTAL PROGRAM 2.1 Materials Bottom and fly ash were sampled from a Medical Waste Incineration Facility (MWIF). Bottom ash was first dried in the oven for 24 hours and then was grounded in order to reduce its particle size class below 100 μm. The particle size of the fly ash was already below 100μm so there was no need for such pretreatment. The chemical composition of the two kinds of ash and the concentration of the heavy metals that was measured with the TCLP method are displayed at table 1 and 2 respectively. Chemical compositio n Bottom ash Table 1. Chemical composition of medical waste bottom and fly ash. SiO 2 CaO Na 2 O Al 2O 3 Fe 2O 3 Mg O Ba O TiO 5 SO 3 K 2 O Othe r Fly ash Heavy metals Bottom ash Table 2.TCLP leaching values for medical waste bottom and fly ash samples. Cr (mg/l) Ni (mg/l) Zn (mg/l) Cd (mg/l) Ba (mg/l) Pb (mg/l) <DL <DL Fly ash <DL <DL TCLP limit According to table 1, the main elements of the bottom ash are SiO 2 (39.74%), CaO (27.77%) and Na 2O (9.13%), while the major element of the fly ash is CaO (89.20%). In all the metals measured the concentration of heavy metals in bottom ash was within permissible limits for the US EPA TCLP test. On the other hand, concentrations of Zn and

3 Pb were found to be high in fly ash, exceeding the limits of the TCLP method. These results are in agreement with previous studies by Anastasiadou et al., 2011 and Kougemitrou et al., Materials used in constant quantities were sodium hydroxide solution of 10 M and sodium silicate solution with a composition of 8.9 wt.% Na 2O, 28.7 wt.% SiO 2, 62.5 wt.% H 2O, and metakaolin, produced by calcination at C for 4 h of kaolinite Mix proportions Four series of experiments were carried out in this study. In the first series, only bottom ash was used as a raw material. In the second series, a quantity of calcium carbonate was added in order to study its effect on the geopolymer paste. In the third and fourth, fly ash and bottom ash at proportions of 75:25 and 50:50 (FA: BA) were used respectively. In all series three different proportions of Medical Waste Ash (MWA): Metakaolin (MK) of 20:100, 30:100 and 50:100 were applied. The production of the geopolymer binders comprised the following steps: a) preparation of the alkaline solution by dissolving the quantity of sodium silicate and sodium hydroxide in distilled water, b) addition of the ash in the solution and stirring for 5 minutes c) addition of the quantity of the MK and stirring until a homogeneous and fluid paste was formed. Then the mixture was cast into 50 mm cubic moulds and the samples were cured in the oven at 50 0 C for 24 h to complete the geopolymerization reaction. It is important to note that the values of the temperature (Chindaprasirt et al., 2007; Panias et al., 2007), of the curing time (Palomo and Fuente, 2003) and of the concentration of the sodium hydroxide solution (Rattanasak and Chindaprasirt, 2009; Chindaprasirt et al., 2009) were chosen based on the literature and after a large number of trials Methods of analysis X-ray diffraction (XRD) was utilized to determine the mineralogical properties of the fly and bottom ash samples and of the produced geopolymers. The XRD patterns were recorded on a BrukerAXS D8 Advance 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 fly ash and bottom ash samples. Diffraction patterns were manually analysed using the Diffrac Evaluation software with database of International Centre for Diffraction Data (ICDD). Compressive strengths of synthesized geopolymers of all mix designs were determined using a calibrated hand-operated hydraulic compression apparatus. Total maximum loads were recorded at the point of fracture, and the compressive strength was determined using the formula f m = P/A, where f m is the compressive strength (MPa), P is the total maximum load (N) and A is the area of loaded surface (mm 2 ). The TCLP analysis simulates landfill conditions. It determines which of the contaminants identified by the United States Environmental Protection Agency (EPA) are present in the leachate and their concentrations. 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 0 C) was performed by shaking the material for 18 h. Subsequently, the leachate samples were filtered through a 0.8μm 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. 3. RESULTS 3.1 X-ray diffraction analysis Figure 1 shows the XRD patterns of bottom and fly ash samples as well as selected geopolymer samples. The main crystalline phase of the bottom ash was gehlenite, which is a composite mineral, while other crystalline phases such as quartz, hematite, halite,

4 stilbite and calcite were also detected. Similar results have been presented by Xu et al., 2010, and Li et al., The major mineral in the fly ash was calcite; halite, quartz, anhydrite were present in considerable amounts, and zeolite and thermonatrite in trace amounts. The results of the XRD analysis of fly ash are in agreement with the results of Kougemitrou et al., 2011; Anastasiadou et al., 2011; Sata et al., 2012 and Guo et al., Figure 1. X-ray diffractograms of the fly and bottom ash and of the produced geopolymers binders. The resulting geopolymers of the first, second and third series had almost amorphous phases and were similar to each other. The main chrystaline phase of these binders was quartz. The similarity between the XRD patterns indicate that the addition of calcite carbonate in the second series and fly ash in the third series did not affect the chrystalline phase of the geopolymers. Yip et al., 2005; Yip et al., 2008; and Li & Liu, 2007agree with this conclusion. On the other hand in the fourth series, where the quantity of the fly ash at the binders increased considerably, the calcite became the main chrystaline phase. This was due to the fact that a percentage of the fly ash added didn t react during the geopolymer process Compressive strength Figure 2 shows the compressive strength of all the geopolymer mortars. It is evident that all the geopolymer binders gave a compressive strength of MPa, higher than the standard stipulated for solidified waste forms, which is MPa after 28 days of solidification (Morgan &Bostick, 1992). The compressive strength of the geopolymer mortars increased with the use of calcium carbonate at the second series of the experiment and with the addition of the fly ash at the third and the fourth series. This increase was due to the increase of calcium oxide and of the ratio of SiO 2/ Al 2O 3 in the geopolymer binders.

5 Compressive strength (MPa) Compressive strength (MPa) Compressive strength values after 7 days MWA:MK 100% bottom ash 100% bottom ash (addition of CaCO3) 75-25% bottom-fly ash 50-50% bottom-fly ash Compressive strength values after 28 days MWA:MK 100% bottom ash 100% bottom ash (addition of CaCO3) 75-25% bottom-fly ash 50-50% bottom-fly ash Figure 2. Summary of compressive strength values for all the produced geopolymers. Many researchers have studied the effect of increasing calcium oxide, either with the addition of calcium hydroxide and calcium carbonate (Alonso &Palomo 2001; Temuujin et al., 2009), or with the addition of blast furnace slag rich in CaO (Yip et al., 2008; Li et al., 2007), on the strength of the produced mortars. Calcination activates material by changing their crystalline structure into amorphous structure to store extra energy and increase their activity, and increasing compressive strength (Khale & Chaudhary, 2007). The results of the second series of the experiment are in agreement with the results of Pinto et al., 2004, and Buckwald et al., 2005, who had concluded that the compressive strength of the binders increases when the percentage of added source of calcium is up to 20% and 30 % respectively. Moreover, many researchers have studied the effect of the ratio of SiO 2/ Al 2O 3 on the compressive strength of the mortars (Silva et al., 2007; Fletcher et al., 2005; Duxson et al., 2005). A relatively small variation in initial molar concentrations of Na 2O, SiO 2 or Al 2O 3 in a geopolymer system can dramatically change its longterm properties (Silva &Sagoe- Crenstil, 2008). According to Songpiriyakij et al., 2010, the increase in compressive strength is due to the fact that as the mixture is enriched by greater amounts of silicon, the bonds between the Si-O-Si become stronger. Also, by increasing the SiO 2/Na 2Oratio, complex polymeric structures were formed, which led to the increase of the mechanical strength of the binders. The results of the third and fourth series are in agreement with the results of Yip et al., 2005 and Yip et al, 2008, who established the favorable effect of

6 the addition of slag high in calcium oxide (43% CaO), at rates between 20% and 40% of the amount of metakaolin, on the compressive strength of the binders Leaching test for heavy metals In table 3 the TCLP leaching values after 28 days of solidification are displayed for all the geopolymer binders. In all the geopolymer mortars all the measured heavy metals were found at concentrations lower than the permitted limits for the US EPA TCLP test. These results indicate that geopolymerisation is able to immobilize the heavy metals found in fly and bottom ash. Table 3. Summary of TCLP leaching values after 28 days for solidified geopolymer mortars. Concentration of heavy metals (mg/l) Heavy metals Cr Concentration Ni of heavy Zn metals in leachate Cd Ba Pb TCLP limit (mg/l) % bottom ash 20% <DL <DL % <DL <DL % <DL <DL % bottom ash (addition of CaCΟ 3) 20% <DL 0,0162 <DL , % <DL 0,0194 <DL , % <DL 0,0349 <DL % bottom-fly ash 20% <DL <DL % <DL <DL % <DL <DL % bottom-fly ash 20% <DL <DL % <DL <DL % <DL <DL It is also important to note that as the percentage of medical waste ash that was used for the production of the geopolymers was increased the incremental increase in leach rates became smaller than expected. Finally, comparing the results of the four series of the experiment, it was seen that the addition of calcium carbonate to the produced geopolymers (second series of experiment) did not affect the leachability of the heavy metals, while the addition of fly ash (third and fourth series) caused the expected, based on the concentration of these metals in both kinds of ash, change in the concentration of the final stabilized specimens. 4. CONCLUSIONS The study demonstrates that geopolymers can be produced with the use of only medical waste bottom ash as a raw material. Moreover the addition of fly ash and calcium carbonate caused a significant increase in the compressive strength of the geopolymer mortars without simultaneously affecting their chrystalline structure. All the geopolymers produced show a compressive strength higher than the regulatory limit of MPa, which proves that they stabilized. Moreover, the percentage increase of the ash used for the production of the geopolymers resulted in the increase of their compressive strength. Furthermore, it is a fact that the geopolymer binders of this study gave compressive strength levels lower than those of previousstudies (Sathonsaowaphak et al., 2009; Chindaprasirt et al., 2007), but it is certain that either with the use of the same materials but in different proportions or by removing some material such as metakaolin, it would be

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