1 Introduction

Medical waste is considered as a major environmental concern in terms of pollution and public safety; it requires proper waste management in order to minimize its impact on public health and protect the environment (Tsakona et al. 2007, Hossain et al. 2011). The world health organization (WHO) defines medical waste as waste generated by health care activities that include a broad range of materials from used needles and syringes to solid dressing, body parts, diagnostic samples, blood, chemicals, pharmaceutical, medical devices, and radioactive materials. There are several methods available for treatment of medical waste such as incineration, autoclaving, gas sterilization, chemical disinfection, and microwave irradiation (Lee et al. 2004). Incineration is widely used, whereby hazardous medical waste is burned at high temperature. The organic and burnable materials are reduced to inorganic matters and this will result in the reduction of the volume of the solid waste (Shaaban 2007). At the same time, incineration has many disadvantages such as emission of toxic gases, dioxin, chlorinated hydrocarbons, polycyclic aromatic hydrocarbons, and concentrating heavy metals in the ash (Alvim-Ferraz and Afonso 2005). The leachability of heavy metals from medical waste ash must be low in order to safely dump medical waste ash in landfills (Sukandar et al. 2006).

Many studies have been done to investigate the leachability of medical waste ash and the factors affecting this process. Zhongxin and Gang (2010) studied the characteristics of fly ash derived from the incineration of medical waste in China; the characteristics of medical waste ash were found by using ash leaching experiments. The results indicated that the concentration of heavy metals increases with the increase of leaching time, also it is the highest in the particle size range 250–900 μm. At neutral pH, the leaching concentrations of heavy metals were the lowest. Sukandar et al. (2006) investigated the metals’ leachability of medical waste incinerator fly ash in Japan on the basis of particle size. The TCLP (Sequential extraction and Toxicity Characteristics Leaching Procedure) analysis was done to quantify leaching amount of metals in each particle size and the result indicated that the leachability of Cd, Cr, Cu, Hg, Ni, Sn, and Zn did not change with the change of particle size but arsenic As had the highest leachability in particle size fraction of < 38 μm. In this study, it was found that the physical and chemical properties of fly ash depend on many factors such as the feeding waste composition, chemical reactions occurring during the incineration process, and incinerator operation parameter. Timatheaton et al. (2012) studied the characteristics of bottom ash samples obtained from two hazard waste incineration centers in Greece. The result showed that the major components of medical waste bottom ash were SiO2, CaO, and Al2O3. It was also found that the concentration of heavy metals in the leachate decreased as liquid-solid ratio increases.

The Jordan Ministry of Health (MOH) introduced medical waste management regulations in October 2001 in order to regulate medical waste handling and disposal (AlMomani et al. 2013, Fraiwan et al. 2013). Despite all dangerous effects of the medical waste on human health and the environment, only small attention has been oriented to proper handling and disposal. Several previous studies have indicated that there is a shortcoming in medical waste management and disposal in Jordan (Oweis et al. 2005, Abu Qdais et al. 2007, Al-Jaradin and Persson 2012, Fraiwan et al. 2013). Some hospitals in Jordan have their own incinerator, while others are sending their solid medical waste to Jordan University of Science and Technology (JUST) incinerator. The JUST incinerator has two combustion chambers: the primary chamber works at 800–900 °C while the secondary at a temperature range of 900–1200 °C. These ranges of temperature are matching those required by the ministry of health regulation. The residence time in each of these chambers is about 10 s. This incinerator has the capacity of 4.0 t/day and is fed by medical wastes containing plastic which forms the largest percentage of waste with 35%, miscellaneous (23.9%), glass (16.7%), paper (14%), metals (6.7%), and textiles (3.7%) (Bdour et al. 2007). The medical waste ash is collected and sent to the Al Ekaider dump site together with municipal solid waste. This site is the only official dump site in northern Jordan with an area 806 × 103 m2, used in almost equal proportions for disposal of industrial wastewater and municipal solid waste (Al-Jaradin and Persson 2012). To the researchers’ best of knowledge, there were no previous studies that have been reported on investigating ash characteristics and the leachability of the ash generated from the JUST medical waste incinerator.

The aims of the study are to investigate the leachability of heavy metals from the medical waste ash and to investigate factors that affect the leachability, including particle size, pH, temperature, and solid-liquid ratio. In this work, the ash properties and leaching characteristics of medical waste ash were investigated in order to have a safe proper treatment and disposal of the incinerator ash. The possible release of Zn, Cu, Cd, Pb, Fe, Mo, Si, and Sr from the ash was determined by batch leaching experiments. The level of the concentrations of the released heavy metals was examined under different experimental parameters such as leaching time, temperature, pH, particle size, and solid/liquid ratio.

2 Experimental Work

2.1 Materials

Medical waste bottom ash was collected from Jordan JUST medical waste incinerator. The collected ash was sieved into several sieve fractions (180–300, 300–500, 500–1000, and 1000–2100 μm). The fractions were dried in an oven at 105 °C for 3 h and then stored in a desiccator for further use. Ultrapure water (Milli Q Type I) was used for leaching reagent preparation, while all other chemicals were of reagent grade. NaOH and HCl were used to adjust the pH of the leaching solution.

2.2 Characterization

Representative samples from the bulk ash sample as well as from the sieve fractions were finely ground and masses of about 0.2 g were subjected to total microwave digestion using trace analysis grade HNO3 (69%), HCl (37%), and HF (48%) following the method reported by Al-Harahsheh et al. (2009). The obtained aqueous samples were then analyzed using a Thermo Scientific iCAP Q inductively coupled plasma mass spectrometer (ICP-MS).

Representative samples, from the as-prepared four sieve fractions and one obtained after leaching, were analyzed for their mineral composition using XRD analysis. The representative samples were ground further to suit XRD sample requirements. A computer-controlled Hiltonbrooks® generator with a Philips® PW 1050 diffractometer with an automatic divergence slit, and Cu-Kα anode producing X-rays of wavelength λ = 1.54056 A° was used. The diffractometer was operating at 40 kV and 20 mA, and automatic routines allowed scanning for values 2θ from 5° to 90° using a step size of 0.02° and scan speed of 2°/min. The diffraction data were analyzed by Diffraction Technology “X-Pert High Score Plus®” X-ray analytical software. Identification of the minerals contained in the sample was achieved by comparing the X-ray spectrum with a database (Joint Committee on Powder Diffraction Standards - International Centre for Diffraction Data (JCPDS-ICDD)).

The pH measurement of the solution before and after leaching was performed using a Jenway pH meter model 3510. Fourier transform infrared (FTIR) analysis was also carried out for a representative sample of the bottom fly ash using Bruker FTIR, model Tensor II with an attenuated total reflection (ATR) attachment.

2.3 Leachability Tests

Batch leaching tests were conducted in this work to study the leachability of the bottom ash of the incinerator. The effects of the contact time (0.33, 0.67, 1, 2, 24, and 48 h), particle size (180–300, 300–500, 500–1000, and 1000–2100 μm), initial pH (2, 5, 7, 9, and 11), temperature (25, 35, 45, and 55 °C), and solid-liquid ratio (3/250, 3/200, 3/150, 3/100, and 3/50 g/ml). In a typical leaching experiment, a mass of 3 g was added to 150 ml of leaching reagent with a specified pH and shacked in a temperature-controlled water bath at a specified temperature for 48 h. Aliquot samples of about 1 ml were withdrawn from the leaching mixture employing a syringe with a 0.22-μm PTFE syringe filter. About 0.1 ml of the withdrawn sample was diluted 50 times with 1% HNO3 (trace analysis grade) and then analyzed using ICP-MS employing kinetic energy discrimination mode (KED). To ensure the quality of the analysis, internal standards (Sc and Y and Bi) were used to correct for instrumental drift. Table 1 shows the conditions of the leaching tests.

Table 1 Summary of leaching experiments conditions
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3 Results and Discussions

3.1 Characterization

3.1.1 Particle Size Analysis

Figure 1 shows the frequency distribution of the particle size analysis of the medical waste ash. The mass distribution of the different size fractions is not even; the major mass fractions are − 1000 + 500 μm (32.8%) and − 2100 + 1000 μm (30%).

Fig. 1
figure 1

Particle size distribution of the bottom ash

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3.1.2 Chemical Composition of the Bottom Ash

The chemical composition of the bulk bottom ash sample and the four size fractions used in the current study is presented in Table 2. The major elements present in the ash are Ca, Si, Al, Cl, Na, Fe, Ti, S, Mg, Ba, and K. The ash also contains appreciable amounts of heavy metals including Zn, Pb, Cu, Sr, Mn, Cr, and Ni. The latter two elements may have come from stainless steel needles present in the medical waste, while Zn and Ni may be from batteries. The high chloride content, especially in the small size fractions, indicates that the medical waste contains appreciable amount of PVC which decomposes into HCl and combustible hydrocarbons. HCl reacts with metals present in the waste forming chlorides such as NaCl, KCl, CaCl2, PbCl2, CdCl2, and FeCl2 (Al-Harahsheh et al. 2014, Al-Harahsheh et al. 2015, Al-Harahsheh 2018). These chlorides are highly soluble in water. Of course, part of the chloride may have come from chloride salts (NaCl) which is used frequently in medical treatment (Zhao et al. 2010). As the particle size increased, the chloride content decreases which is consistent with the above explanation meaning that HCl from PVC will have higher reactivity with the small particles.

Table 2 Chemical composition of the bulk ash and the sieve fraction of the bottom ash
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The iron content increases with the increase of particles size which is probably due to the fact that most iron is coming from needles which accumulate in the large fractions. Silicon also increases with the increase of particle size because the glass particles also accumulate in the large fractions. Cadmium (< 0.03 μg/g) and selenium (0.067 μg/g) were below the detection limit of the analysis method. The low concentration of these elements could be due to the high volatility of their compounds (Zhao et al. 2010), for example, the boiling point of H2Se is − 41 °C.

3.1.3 XRD Analysis

The X-ray diffraction pattern of the bottom ash before and after leaching is shown in Fig. 2. The main mineral phases present in the ash, before leaching, are calcite (CaCO3), halite (NaCl), sylvite (KCl), anhydrite (CaSO4), hematite (Fe2O3), hydrochlorborite (Ca2B4O4Cl(OH)7·7H2O), cristobalite (SiO2), melanterite FeSO4·7H2O), and chlormayenite (Ca12Al14O32). These results are consistent with the previously reported data on the mineral composition of medical waste bottom ash (Chang and Wey 2006, El Bakkali et al. 2013). After leaching, the following phases have disappeared: halite, sylvite, and anhydrite; the disappearance of the first two is explained by high solubility of these salts in water. However, the disappearance of anhydrite could be related to the formation of amorphous intermediate hydrated calcium sulfate (Jones 2012). Additionally, the cristobalite peaks have disappeared after leaching which suggests formation of some hydrated calcium silicates (Kim and Olek 2014) as cristobalite is considered a reactive silica.

Fig. 2
figure 2

XRD pattern of the two fractions of the bottom ash before and for one after leaching

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3.1.4 FTIR Analysis

Figure 3 shows the FTIR spectra of the as-received medical waste bottom ash. The broad peak centered around 3400 cm−1 is related to OH stretching of bonded and non-bonded OH group; the presence of the peak at 1635 cm−1 indicates bending of OH group in water, which means that the sample contains some unbound water. It is important to point out that the hot ash is quenched in water after combustion at JUST incinerator. The strong peak at 1415 cm−1 and the less strong peak at 1472 cm−1 are due to the asymmetric stretching of CO3. The presence of peaks 709 and 872 cm−1 in the finger print region confirms the presence of calcium carbonate in the sample (Reig et al. 2002). These results are consistent with both chemical and XRD analyses which suggest that the major constituent of the ash is Ca in the form of carbonate. The small peak at 563 cm−1 is assigned to Fe-O stretching (Al-Harahsheh et al. 2018), while the peak at 677 cm−1 and the shoulder at 1139 cm−1 may be assigned to S-O bending and stretching of sulfates, respectively. The peaks at 463 and 519 cm−1 could be assigned to mixed Si-O-Si and O-Si-O bending which are common for silicate minerals (Ojima 2003).

Fig. 3
figure 3

FTIR spectra of as-received and dried bottom ash sample (size fraction 180–300 μm)

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After drying the sample, almost all peaks related to moisture have disappeared (broad peak at 3400 cm−1 and the peak at 1635 cm−1).

3.2 Leachability of Bottom Ash

3.2.1 Effect of Contact Time

Initially, the effect of contact time was studied to determine the equilibrium time for the subsequent leaching tests. Figure 4 demonstrates the effect of time on the leaching of several elements from the medical waste solid ash within 48 h under agitated conditions.

Fig. 4
figure 4

Effect of time on the leachability of Sr, Cr, Al, and Pb (a); Rb, Cu, and Mg (b); Se, Ni, and Zn (c) and As, V, Co, and Cd (d) from bottom ash (size fraction − 180 + 300 μm) at a temperature of 25 °C, S/L ratio of 1:50, and initial pH of 7

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It can be observed that the concentration of Cr, Sr, Cu, Mg, Ca, K, and Ni increased with time. However, the concentrations of Zn and Al increased at the initial leaching stage (first minutes) followed by a sharp decrease in the follwing few hours of leaching then remain almost constant after that. The drop in concentration of Al and Zn could be due to the formation of certain solid precipitates. The concentration of Rb and Se reached equilibrium after 24 h and remained constant in the leachate. The leachability of other elements such as K and Ca was high, reaching values of 72 and 730 mg/l. The high concentration of Ca in the leaching solution has resulted in a fast increase of the initial pH to about 11 (see section 2.2.2). The maximum concentration of other elements such as Cd, As, Co, and V was below 0.3 μg/l as shown in Fig. 4d.

3.2.2 Effect of pH

Figure 5 shows the effect of the initial pH of the leaching solution on the leachability of several metal ions. Results indicate that leaching was almost constant for most of the elements; it can be observed that the initial pH has a slight effect on the level of leaching. When measuring the final pH of the solution after 48 h, it was between 10 and 11 irrespective of the initial pH, which explains the small variation in the leachability of the elements at different initial pH values. An additional test was devoted to monitor the change of the actual pH of the slurry with initial pH values of 2 and 11, as demonstrated in Fig. 6. From this figure, one can see that there is a sharp increase in the pH during the first few minutes of the leaching experiment when the initial pH was 2. It has then reached a pH value of 10.5 after 20 min then remained constant. When the initial pH was 11, a sharp drop was observed whereby the pH dropped to 8 within the first 3 min, then increased sharply to 11 after 7 min and then remained constant at this value.

Fig. 5
figure 5

Effect of initial pH on the leachability of Cr, Mg, and Al (a); Cu, Rb, Mo, and Fe (b); Pb, Se, Zn, and Ni (c); and Cd, As, Co, and V (d) from bottom ash (size fraction − 180 + 300 μm) at a temperature of 25 °C, S/L ratio of 1:50, and leaching time of 48 h

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Fig. 6
figure 6

Variation of the actual pH of leaching solution with time (S/L ratio 20 mg/ml, T = 25 °C, initial pH of 2 and 11)

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The behavior of pH profile after a short time being highly basic suggests that irrespective of the initial pH, the initial pH has a slight impact on the degree of leaching. Leaching level remains steady for various initial pH values. An exception can be observed for Ni, Zn, and Pb where the highest level of leaching was at low initial pH, then it remains steady.

3.2.3 Effect of Particle Size

Figure 7 shows the effect of particle size on the leaching level of several metal ions. It can be observed from the figure that the concentration of Fe, Mo, Rb, Pb, Se, and Zn decreases as particle size increases. For instance, the concentration of Mo drops from 110.5 μg/l for particle size range of + 180–300 μm to 65.2 μg/l at a size range of + 1000–2100 μm. While for Rb, it drops from 132 μg/l for particle size range of + 180–300 μm to 71 μg/l at a size range of + 1000–2100 μm. These data support the fact that as the particle size increases, the surface area available for mass transfer from the solid phase to the liquid decreases. This indicates that particle size has a significant impact on the leaching process. The behavior of other metal ions was different, for instance, the concentration of Al, Cr, and Cu increased initially then decreased after reaching a maximum value. This indicates that during sieving, some compounds are more concentrated at certain particle size.

Fig. 7
figure 7

Effect of particle size on the leachability of Sr, Cr, Mg, and Al (a); Cu, Rb, Mo, and Fe (b); Pb, Se, Zn, and Ni (c); and Cd, As, Co, and V (d) from bottom ash at a pH of 7, a temperature of 25 °C, S/L ratio of 1:50, and leaching time of 48 h

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As for the Ca concentration in the leaching solution, its change was found to be consistent with the chemical composition of the different size fractions and the overall dissolution % was between 11 and 12% of the initial Ca content. The high recovery of Ca could be related to the formation of CaCl2 during incineration of the ash due to the presence of PVC plastic materials. Calcium oxide chlorination by HCl is known to be one of the most thermodynamically preferable reactions (Al-Harahsheh 2018). Lead oxide is more thermodynamically preferable in terms of chlorination. However, less than 0.05% of the initial content is dissolved which is related to the formation of a solid precipitate of Pb(OH)Cl at pH above 7 (Chen et al. 2007, Al-Harahsheh et al. 2017).

3.2.4 Effect of Solid-Liquid Ratio

Figure 8 shows the effect of S/L ratio (mg solid ash/ml solution) on the leachability of several metal ions. Generally, it can be seen that the concentration decreased as the S/L ratio is decreased (Fig. 8a, b). This indicates that water is capable of dissolving appreciable amount of these heavy metals. For elements such as Pb and Zn, the concentration increases with the increase of S/L ratio up to a certain level then decreases (Fig. 8b). For most of the leached metals, the highest leaching level was at a S/L ratio of 60 mg/ml. Figure 8c shows that the release of Ni, Se, Zn, and Pb was below 35 μg/l irrespective of the S/L ratio with no trend relation except for Se; the release of which increases with the increase of S/L ratio. The release of Co, V, As, and Cd was below 0.3 μg/l irrespective of the S/L ratio, except that of Co at the smallest S/L ratio (12 mg/ml) as can be seen in Fig. 8d. The results obtained here are consistent with those obtained by (Zhongxin and Gang 2010).

Fig. 8
figure 8

Effect of S/L ratio on the leachability of Al, Cr, Sr, and Mg (a); Cu, Mo, Fe, and Rb (b); Pb, Se, Zn, and Ni (c); and Cd, As, Co, and V (d) from bottom ash at a pH of 7, a temperature of 25 °C, size range − 300 + 180 μm, and leaching time of 48 h

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3.2.5 Effect of Leaching Temperature

Figure 9 shows the effect of temperature on the leachability of several metal ions. Data indicate that the concentration of Cu, Pb, Al, Ni, and Co increased with the increase of temperature, while the concentration of Cd, As, and Zn has a trend of decrease as temperature increases. Elements including Se and Rb remain almost steady over the temperature range studied. Similar trends of element concentrations change in the leachate as a function of temperature was observed by Baba et al. (2008).

Fig. 9
figure 9

Effect of temperature on the leachability of Sr, Cr, Al, Mg (a); Pb, Cu, Rb, Mo, and Fe (b); Se, Zn, and Ni (c); and Cd, As, Co, and V (d) from bottom ash at a pH of 7, size range − 300 + 180 μm, S/L ratio of 1:50 and leaching time of 48 h

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3.3 Hazardous of Medical Waste Ash

Table 3 shows the absolute highest leached amounts of heavy metals obtained in the current study in comparison with the corresponding EPA limits. The extracted quantities of all the heavy metals were less than the limits set by EPA, even at the harshest conditions. Despite such result, it is recommended to use such ash for production of valuable construction and adsorbent materials such as geopolymer (Tzanakos et al. 2014). The presence of an appreciable concentration of Ca, Si, Al, and Na is encouraging to utilize such ash for the production of geopolymeric materials. Geopolymerization of such ash would also enhance its capacity for stabilization against possible leaching of heavy metals. Furthermore, to further reduce the leachability of heavy metals from such ash, minerals such as clinoptilolite can be added to the ash which will reduce the release of metals such as Cu and Zn as reported by Çoruh (2008), Demır et al. (2008), and Mesci et al. (2009). Stabilization/solidification can be also another alternative to deal with waste such materials (Suzuki and Ono 2008, Mesci et al. 2009, Çoruh 2012).

Table 3 Amount of leached heavy metals vs EPA limits (Sukandar et al. 2006)
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4 Conclusions

The following conclusions can be drawn from this work:

  1. 1.

    The major elements found in the ash were Ca, Si, Al, Cl, Na, Fe, Ti, S, Mg, Ba, and K, while the main mineral phases found in the ash were calcite, halite, sylvite, anhydrite, hematite, hydrochlorborite, cristobalite, melanterite, and chlormayenite.

  2. 2.

    The leachability of most of the metal ions increased with the increase of leaching time.

  3. 3.

    The ash was found to be highly basic and the initial pH was found to have a slight impact on the degree of leachability. Acid was consumed in the first few minutes of all leaching runs and reached final values between 10.5 and 11.

  4. 4.

    There was a variation on the effect of particle size on leachability, and the concentration of Fe, Mo, Rb, Pb, Se, and Zn increased as particle size decreased. The effect of particle size on the leachability of the other metals was different; during sieving, some compounds are more concentrated at certain particle size.

  5. 5.

    The concentration of the leached metal ions decreased as the S/L ratio was decreased, and for most of the leached metal ions, the highest leaching level was at a S/L ratio of 60 mg/ml.

  6. 6.

    Temperature does not have a specific trend effect on leaching, and the concentration of some compounds increased with increased on temperature, while others increased initially then decreased.

  7. 7.

    The extracted quantities of all the heavy metals at the harshest conditions were less than the limits set by EPA.