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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals a
b
a
Fozia Anjum , Muhammad Shahid , ShaziaAnwer Bukhari & J. Herman Potgieter
c
a
Bio-analytical Lab, Department of Chemistry, Government College University, Faisalabad, Pakistan b
Bioassays Section, PMBL, Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad, Pakistan c
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, Wits 2050, South Africa Accepted author version posted online: 22 Jul 2013.Published online: 25 Aug 2013.
To cite this article: Fozia Anjum, Muhammad Shahid, ShaziaAnwer Bukhari & J. Herman Potgieter , Environmental Technology (2013): Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals, Environmental Technology, DOI: 10.1080/09593330.2013.824992 To link to this article: http://dx.doi.org/10.1080/09593330.2013.824992
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.824992
Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals Fozia Anjuma , Muhammad Shahidb , ShaziaAnwer Bukharia and J. Herman Potgieterc∗ a Bio-analytical
Lab, Department of Chemistry, Government College University, Faisalabad, Pakistan Section, PMBL, Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad, Pakistan c School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, Wits 2050, South Africa b Bioassays
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(Received 29 January 2013; accepted 20 June 2013 ) The mineralogy, as well as elemental composition, of the incinerated hospital waste (HW) ashes are not well known and need to be investigated for the safe handling and disposal of such ash. A study was conducted to investigate the chemical composition, mineralogy and bioleaching of selected metals from incinerated HW bottom ash using Aspergillus niger under the combined effect of ultrasonic radiation. Different techniques were utilized to determine the elemental composition (Electron Dispersive X-ray Spectroscopy [EDX], atomic absorption spectrophotometry, inductively coupled plasma-optical emission spectroscopy, ultraviolet-visible light spectrophotometer) and mineralogy (X-ray Diffraction) of the raw sample, as well as the bioleached samples. Chemical leaching tests were performed to determine the effect of different organic acids on metals dissolution. Microbes were tested for acid production and leaching capabilities of selected metals from medical waste (MW) bottom ash. Wet chemical and EDX analyses showed that the ash was enriched with metallic elements like Na, K, Ca, Fe and Al with a concentration range of 22–115 (g/kg). Furthermore, the ash contained heavy metals such as Cu, Cr, Ni, Sn and Ti in the range of 0.51–21.74 (mg/kg). Citric and oxalic acids generated by fungi could be important leaching agents acting to dissolve these metals. Under ultrasonic treatment, metals dissolution by the acidic metabolites was at its maximum after just 9 d of leaching. The results showed that the dissolution of metals was much higher in citric and oxalic acid than with other acids. Extraction of metals from incinerated MW ash indicated that this ash may be a potential source of metals in the future. Keywords: hospital wastes; incinerator bottom ash; ultrasonic treatment; bioleaching; Aspergillus niger; organic acids; metals dissolution
1. Introduction The term medical waste (MW) includes all infectious wastes disposed from medical institutions.[1] New criteria define infectious waste on the basis of the form of waste, place of waste generation, and kind of infectious diseases it might contain. Incineration has been proved to be the best treatment option for MW, as it could reduce the MW volume by up to 90%, while simultaneously degrading the infectious materials effectively.[2] Tanaka et al. [3] estimated that the quantity of incinerated infectious waste from medical establishments in Japan comprises 82.3% of the total incinerated volume. Usually identification of waste characteristics is conducted from the point of waste generation to intermediate treatment residues until it is finally disposed. Based on the characteristics of the waste, the appropriate treatment and disposal system is established to minimize the environmental impact. However, such stages cannot be applied to MW due to the strict regulations governing its disposal. For example, all infectious materials are sealed in hermetically tight containers, and then placed in a special storage area not to be opened until incinerated.[4] ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
At medical institutions, infectious waste is collected separately from non-infectious waste. The quantity of infectious waste is normally about 20–25% of the total waste generated from medical institutions. Incineration is the preferred option for the treatment of infectious wastes.[5] However, ash from the incinerator may cause secondary pollution to the environment if not properly treated.[4] Previous studies have shown that bottom ash from incineration of municipal solid waste (MSW) might be a valuable material, because it can be used as a secondary aggregate in roads and construction materials.[6] Due to the similarity in the chemical compositions of MW ash and MSW ash, there is a risk that MW ash might be reused in the same way.[6,7] However, MW bottom ash has some special characteristics that must be taken into consideration before it can be reused. MW contains large amounts of disposed metallic or plastic materials. Therefore, the bottom ash from MW incineration may contain a large proportion of toxic metallic elements that might prevent its reuse. Previous studies have indicated that MW bottom ash contains higher amounts of
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F. Anjum et al.
heavy metals such as Cd, Cr, Ni, Pb and Zn than does MSW bottom ash.[7,8] Currently, the major ways to treat the MW ash are chemical stabilization, cement solidification and chemical leaching that are used to detoxify MW incineration ash. Unfortunately, these techniques are highly energy intensive, and also involve hazardous chemical usage during the treatment.[9] Biohydrometallurgical approaches are generally considered a low-cost ‘green technology’ with low-energy requirements. Microbial bioleaching is based on the natural ability of microorganisms to transform solid compounds into a soluble and extractable form. This may involve enzymatic oxidation or reduction of the solid compound, or an attack on the solid compound by metabolic products of the microorganisms.[10] Three main groups of microorganisms have been used for the bioleaching process, namely autotrophic bacteria like Thiobacilli species, heterotrophic bacteria like Pseudomonas and Bacillus species and heterotrophic fungi like Aspergillus species and Penicillium species.[11] The extraction of metals from low-grade ores, mining wastes, sewage sludge, secondary raw materials and industrial intermediate products using these microorganisms, have been reported previously.[10–12] High population densities, limited landfill area and potential health hazards all contributed to the fact that incineration is the most popular method of intermediate MW treatment. However, incineration of waste produces residues such as ash that is enriched by several metals.[9] These heavy metals can be bioleached for use after microbial treatment of the ash. Despite slow recovery rates of the process, this technology is attractive as it is not energy intensive. Among various physical methods for speeding up the leaching process, the application of ultrasound is proving to be of considerable interest for intensifying the microbial growth, as well as performance of the process.[10,13] To shorten the leaching time without a decrease or possible enhancement in the final metals recovery values, a process intensification study has been carried out with the use of ultrasound. Ultrasound can rapidly extract metals from ash through a process of cavitation in the form of shock waves and micro-jet formation through the cavitating medium such as water or dilute acids.[14] Bioleaching has shown a steady increase in interest to recover valuable metals by the fact that they are relatively inexpensive, involving low energy consumption and being environmentally safe. Sonochemical extraction techniques together with classical methods gave a fast and selective extraction of metals.[13,14] A large collection of literature on the application of ultrasound to metal leaching operations has been reported in earlier work.[15,16] The use of ultrasonic waves for the extraction of copper, zinc and nickel from their ores has previously been reported by some of the current authors.[10] Treatment with ultrasonic waves not only increases the rate of dissolution of metals, but also increases the rate of diffusion of soluble species in the liquid phase.
This treatment enables the lixiviant to penetrate the solid (ore) particles more easily.[17] Furthermore, the frequency of 40 kHz and at the intensity of 1.5 W could increase the growth of fungi.[18]
2. Materials and methods All chemicals and reagents used in the present research work were of analytical grade. Pure standards of organic acids citric, oxalic, maleic and tartaric acids were obtained from Sigma-Aldrich Co. (St Louis, MO, USA). All culture media were purchased from Oxoid, UK.
2.1. Medical waste incinerator bottom ash A representative sample (1.00 kg) of the MW incinerator bottom ash was collected from the waste management department of the hospital in the city of Lahore, Pakistan. The incinerator was of medium scale and produced bottom ash at a temperature of 400–500◦ C. The incinerator was fuelled by diesel oil and natural gas. The representative sample of bottom ash was dried at room temperature, before being passed through a strainer to remove the metal and glass pieces from the ash. About 200–300 g of sample was packed in a polyethylene bag. The bags were brought to the Bioassay Section of the Biochemistry Laboratory of UAF, Faisalabad, Pakistan for further research work.
2.2. Pretreatment of the sample The sample was oven-dried (109◦ C) and ground to generate particles of ≤200 mesh size (74 micron) when using an ASTM sieving machine. The powdered samples were shifted into plastic bottles which were labelled and properly catalogued. This was further used for chemical and mineralogical analysis and for shake flask bioleaching experiments.
2.3. Elemental and mineralogical analysis Elemental analysis was carried out by atomic absorption spectrophotometry (AAS) (Perkin Elmer, AAnalyst 300), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and UV/VIS spectrophotometry (Hitachi, UV200; Japan) after acid digestion. The digestion was done with 3:1 HNO3 :HClO4 at 200◦ C and followed by subsequent redissolution in HCl.[19] The mineralogical composition was determined by X-ray Diffraction (XRD), using a RigakuRint 300 Series diffractometer and the JCPDS diffraction software database.[20] To determine the elements present at the surface of the particular area of the ash sample, EDX (Electron Dispersive X-ray Spectroscopy) analysis was performed at the same time by following the modified method reported by Barzik et al. [20]
Environmental Technology 2.4.
Determination of aluminium in the raw sample and leached solutions Dissolved aluminium was determined by UV/VIS spectrophotometer using Eriochrome cyanine R as the chromogenic agent. The 1:1 violet red aluminium complex was measured by determining the absorbance at λmax 540 nm.[21]
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2.5.
Chemical extraction of metals with and without ultrasonic treatment Chemical/sonochemical extraction of metals was performed by using citric, malic, oxalic and tartaric acids, each in pure form (1.0%), MW ash (2.0%) and the volume was adjusted to 100 mL by distilled water in 250 mL flasks. The flasks were incubated in an orbital shaker (Gellen Kamp, England) at 28◦ C and 150 rpm for 5 and 10 days for chemical/sonochemical leaching of metals. The pH was recorded every day, while liquid samples were withdrawn, filtered and analysed for metals at the end of each incubation period.[10] 2.6.
Production of organic acid metabolites from microbial strain Aspergillus niger culture was used as the isolated and purified strain as reported previously by some of the same authors.[10] For growth in the liquid medium, the culture medium consisted of (g/L): KH2 PO4 , 5.0; NH4 NO3 , 2.0; (NH4 )2 SO4 , 4.0; MgSO4 · 7H2 O, 0.2; peptone, 2.0; trisodium citrate, 2.5; and yeast extract, 1.0. The final volume was made up to 1000 mL with distilled water.[10] For bioleaching experiments, two sets of 250-mL Erlenmeyer flasks, each containing 100 mL of culture medium, were prepared in triplicate. The medium in each flask was autoclaved and 5.0% (m/v) of the given substrate (molasses) was added in each flask and then inoculated with 1.0 mL of A. niger spore suspension as inoculum (2.4 × 108 spores mL−1 ) in both sets of flasks. All the flasks were sealed with removable cotton and incubated in an orbital shaker (Gellen Kamp, England) at 28◦ C at an operating speed of 150 rpm for six days. 2.7.
Leaching of the metals with organic acids produced by A. niger Supernatant liquors of the microbial cultures were collected after six days of cellular growth, sterilized, centrifuged (8000 × g for 10 min at 15◦ C) and then filtered to remove any solid biomass before high performance liquid chromatography (HPLC) analysis for the determination of organic acid metabolites. Supernatants containing organic acid metabolites were used to leach the metals from the MW bottom ash. An amount of 1.00% (w/v) of the MW bottom ash was added to each medium in the both sets of flasks and incubated on an orbital shaker to keep everything
3
in a homogeneous slurry form at 28◦ C and 150 rpm for 18 days. To enhance the extraction of metals, ultrasonic radiation treatment was applied for 10 min/day to one set of shaking flasks. Sampling was done at determined time intervals to register the changes in pH value, and dissolved metals concentration. Finally, the solid phase was separated, and the pH value of the liquor was measured by a Metrohm pH meter (model 744). The dissolved metals concentration was measured by AAS as well as the UV/VIS spectrophotometer.[10] 2.8. Ultrasonic radiation treatment Enhanced bio-extraction of metals was performed ultrasonically in one set of the shaking flasks by following the modified method described earlier.[10] Ultrasonic treatment was applied by placing the experimental flasks containing the metabolites and ash sample in an ultrasonic bath for 10 min at 40 kHz per day during the bio-extraction period. 2.9. Statistical analysis All experiments were performed in triplicate and the results were reported as the mean value ± SD (standard deviation).[22] 3.
Results and discussion
The physical appearance of the MW ash sample was black. It was found to be strongly alkaline (pH 10.1) and insoluble in water. This was probably due to the presence of alkaline metals (Na, K, Ca and their associated anions) in the ash, which is a well-known phenomenon in various ash types. The organic matter content estimated by the loss of weight on ignition at 550◦ C for 6 h was 26.25%. This indicated a high concentration of unburned organic matter, which was contributed to the low operating temperature of the incinerator during combustion.[23] An elemental analysis of the exposed surface of the bottom ash sample (MW) by EDX is shown in Figure 1(a). Some of the numerous elements present on the exposed surface of the representative sample were Na, K, O, C, Ca, Fe, Si, Al, Cu, Cr, Ni, Sn and Ti. After bioleaching, the apparent morphology was quite different, as shown in Figure 1(b) and 1(c). In the case of the bioleached sample under ultrasonic treatment, the elements Fe, Si, Ca and C were detected on the surface (Figure 1(b)), whereas the bioleached sample without ultrasonic treatment had Fe, Al, Si, Cr, O, Ca and C present on the surface (Figure 1(c)), indicating that most of the elements having affinity for the microbial metabolites were extracted. The XRD results in Table 1 reveals the predominance of kaolinite, illite, wollastonite, sphelarite, gehlenite, natrolite, halite, hematite, calcite, anorthite, quartz and Ni-sulphide as the mineral phases in the ash from the incinerated hospital wastes (HWs). Metal oxides such as Ca3 Al2 O6 , Ca5 Cr 3 O12
F. Anjum et al.
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Figure 1. (a) EDX analysis of the raw sample (HW) before leaching, (b) EDX analysis of the sample (HW) after leaching with ultrasonic treatment and (c) EDX analysis of the sample (HW) after leaching without ultrasonic treatment.
Environmental Technology Table 1.
Chemical composition of raw sample of bottom ash determined by XRD.
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Elements
Chemical formula
Kaolinite Pyrite Quartz Aluminium oxide Illite Muscovite Dolomite Ni-sulfide Calcite Hematite Anorthite Dichlorine oxide Silica Phosphorus pentoxide Anhydrite
Al2 SiO2 O5 (OH) FeS SiO2 Al2 O3 KAl2 (Si3 Al)O10 (OH2 ) (K, Na) Al2 (Si, Al)4 O10 (OH)2 CaMg(CO3 )2 (NiFe)x S, (NiFe)S2 CaCO3 Fe2 O3 CaAl2 Si2 O Cl2 O SiO2 P 2 O5 CaSO4
and CaO · Al2 O3 , and various sulphur, phosphorus and chlorine compounds were also detected, together with calcium sulphate anhydrite (CaSO4 ), sodium thiosulphate (Na2 S2 O3 ) and iron phosphate Fe3 (PO4 )2 . Similar findings have previously been reported by other researchers.[1,24, 25] Non-metallic elements like chlorine might be due to the presence of a considerable amount of Polyvinyl chloride polymer in the waste. Moreover, chloride salts are frequently used in medical supplements and can contribute to the high Cl contents in the ash. Zhao et al. [9] reported highly significant values of oxides of metals such as Ca, Si, Al and non-metals such as P, S and Cl in their investigation, while Ibanez et al. [7] reported a smaller amount of these elements than those found in this study. Table 2 lists the metal concentrations in the MW bottom ash obtained by AAS and ICP-OES. The data indicated the enrichment of ash with various metallic elements. The major metal elements in the ash were (g·kg−1 )Ca (11.5), Al (60.1), Mg (51.4), Fe (51.3), Na (36.0) and K (22.1). Other Table 2. Metals concentration of raw sample of bottom ash by ICP/OES. Metal ions Na K Ca Mg Al Fe Li Ti Zn Cu Ba Mn Pb Ag Cd
5
HW Conc. (mg/kg) 36,040 22,050 11,504 51,440 60,120 51,320 0.07 13.06 24.11 21.74 1.36 1.47 9.11 0.99 1.36
Metal ions Co Bi Rb Cr Ni Sb Sn Ga Ge Hg Sr As Mo Li
HW Conc. (mg/kg) 2.01 1.03 2.22 9.02 3.59 1.03 0.51 6.22 0.42 0.73 0.47 1.22 0.78 0.07
Elements Sphelarite Wollastonite Silicocarnotite Akermanite Chromium oxide Gehlenite Natrolite Halite Silicocarnotite Tricalcium aluminate Calcium aluminate Sodium oxide Sodiumthiosulphate Iron phosphate
Chemical formula (ZnFe)S CaSiO3 Ca5 Cr 2 SiO12 KAlSi3 O8 Ca2 MgSi2 O7 Cr 2 O3 Ca2 Al2 SiO7 Na2 Al2 Si3 O10 · 2H2 O NaCl Ca5 Cr 3 O12 Ca3 Al2 O6 CaOAl2 O3 Na2 O (Na2 S2 O3 ) Fe3 (PO4 )2
(heavy) metals were also detected in significant amounts, e.g. Zn (24.1), Ti (13.1), Cr (9.0), Ni (3.6), Rb (2.2–2), Co (2.0), Cu (1.7), Ba (1.4), Mn (1.5), Cd (1.4), Ga (1.2), As (1.2), Pb (1.1), Bi (1.0) and Sb (1.0) (mg·kg−1 ). Metals such as Ag, Mo, Hg, Sn, Sr, Ge and Li were present in smaller quantities in the concentration range of 0.07–0.99 (mg·kg−1 ). In contrast to these results, earlier work reported higher levels of Zn, Ni, Cd, Cr, Cu and Pb in the incinerator bottom ash of MW.[26–29] The infectious wastes of hospitals are packed in coloured plastic bags before being sent for incineration and these coloured plastic materials may be the potential source of some of the heavy metals encountered. The MW bottom ash also contains much higher amounts of Zn, Ti and Cr (9.02 mg kg−1 ) than previously reported.[30] Kuo et al. [31] and Ibanez et al. [7] have indicated that MW bottom ash contains high concentrations of heavy metals such as Cd, Cr, Ni, Pb and Zn. These elements are commonly used in medical facilities, e.g. metal alloys containing Zn and Ti are widely used in medical instruments, while Cr is widely present in used needles and syringes. Kuo et al. [31] reported Fe, Al, Cu, Zn, Cr and Pb in the bottom ash of MW, while Kougemitrou et al. [8] reported the presence of Fe, Cu, Cr, Zn, Pb, Co and Cs in bottom ash of a MW incinerator at Athens. If such heavily contaminated ash is dumped on land, it may cause a dust hazard, soil contamination and, if exposed to rainfall, the leaching potential for heavy metals is enhanced. Blowing wind can contaminate the air with fine dust particles and, therefore, create a potential route of heavy metals intake through inhalation. The fallout from these metals in the area surrounding an incinerator can be substantial. These heavy metals can, therefore, enter biological chains.
3.1.
Chemical/sonochemical extraction of metals from HW bottom ash
Chemical and sonochemical extraction of metals were performed in shaking flasks containing ash (2%) and organic
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Metals dissolution (mg/Kg)
(a)
70 60 50
Fe
40
Al
30
Zn
20 10 0
O.A
C.A T.A Organic acids
M.A
Metals dissolution (mg/Kg)
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(b)
35 30 25
Fe
20
Al
15
Zn
10 5 0
O.A
C.A T.A Organic acids
M.A
Figure 2. (a) Sonochemical leaching of metals by organic acids after five days of incubation. (b) Chemical leaching of metals by organic acids after 10 days of incubation. Note: OA, oxalic acid; CA, citric acid; MA, malic acid; TA, tartaric acid.
acids (1.0%) after 5 and 10 d periods of incubation in chemical and sonochemical leaching (Figure 2(a) and 2(b)). During this period, the pH was recorded periodically every day. It was found that pH increased in both samples with and without ultrasonic treatment as the time progressed. Compared with conventional chemical leaching, ultrasonic treatment enhanced the rate of metal ions solubilization from the ash and improved the final yield in a decreased time. This may be attributable to breakdown of large ash particles into smaller and porous particles by the periodic sonication with a resultant increase in exposed surface area, which led to enhanced mineral dissolution by the acids.[10] Various synthetic organic acids exhibit different abilities to dissolve metal ions. This is analogous to the leaching ability of organic acid metabolites of fungal strains.[32,33] The solubilization of metals by organic acids is discussed below. 3.1.1. Chemical/sonochemical extraction of aluminium from bottom ash Different organic acids showed a different potential for aluminium extraction. Citric acid was the best organic acid for aluminium solubilization. A shaken flask containing citric acid solution used in conjunction with ultrasonic
vibration, achieved a maximum yield of aluminium of 86.6%, compared to the leaching of 43.1% in a similarly shaken flask without ultrasonic treatment, as illustrated in Figure 2(a) and 2(b). It was noted that ultrasonic treatment not only increases the rate of particle destruction by producing cracks and pits, but also improve the rate of metals solubilization by the acids from the raw sample. Thus, the final yield increased compared with conventional chemical leaching.[14] Huang et al. [34] used an inorganic acid for metal extraction to recover aluminium from fly ash of a MSW incinerator, and achieved a yield of 80%. While other acids also solubilize aluminium to a significant extent, the solubility was always comparatively higher in those samples that were treated ultrasonically. With the ultrasonic treatment, the maximum solubilization of aluminium was 52.0%, 48.6% and 25.3% in oxalic, malic and tartaric acid solutions, respectively, compared to 30.2%, 45.2% and 20.4%, respectively, without ultrasonic treatment. From all these values, it is clear that citric and oxalic acids are more effective leaching agents for aluminium than other acids under controlled conditions. Similar trends have been reported by other research workers.[35,36] 3.1.2. Chemical/sonochemical extraction of iron from bottom ash The results of iron extraction are shown in Figure 2(a) and 2(b). Under ultrasonic treatment conditions, the iron extraction was 87.3% in oxalic acid compared with 46.4% without ultrasonic treatment. Citric, malic and tartaric acid were also found to have great potential for iron extraction from the ash. Amounts of 63.3%, 56.8% and 35.1% were dissolved, respectively, when treated ultrasonically just after 5 days of incubation. In contrast, lower amounts of 37.2%, 48.7% and 32.6% of iron were recovered after 10 days of incubation in the shaking flasks that were not treated ultrasonically. Similar findings have been reported previously.[10] Huang et al. [33] recovered 82% iron from the MSW incinerator fly ash using an inorganic acid. 3.1.3. Chemical/sonochemical extraction of zinc from bottom ash The results of zinc solubilization in shaking flasks with and without ultrasonic treatment are schematically represented in Figure 2(a) and 2(b). Oxalic acid has the maximum potential for zinc extraction from bottom ash when compared to the other acids used. The maximum extraction of zinc (88.0%) occurred with oxalic acid at the end of the sonochemical leaching period. In the case of shaking flasks that were not treated ultrasonically, the maximum solubilization of zinc after 10 days of leaching was only 45.3%. Oxalic acid is a stronger acid than citric acid and has three times more affinity for zinc than citric acid. Citric acid and zinc oxide react to produce insoluble zinc citrate (Zn3 Cit2 ).[37]
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Environmental Technology In the interaction of zinc with acid, H+ ions govern the dissolution process and the maximum dissolved amount of zinc corresponds to the maximum amount of H+ ions available. The availability of these hydrogen ions has increased significantly with ultrasonic treatment.[38] Acids, like citric (77.6% and 37.5%), malic (48.3% and 39.6%) and tartaric acid (36.2% and 33.2%) also showed substantial dissolution of zinc with and without ultrasonic treatment, respectively. In the case of ultrasonically treated samples, high extraction of zinc after just 5 days of incubation might be due to an acceleration of the leaching process resulting from the ultrasonic radiation. Ultrasonic radiation increases the stirring effect and increased the exposed surface area of the ash particle in the slurry during leaching, thus resulting in an increased extraction of metal ions in shorter time.[14]
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metals. However, in the flask that was not treated ultrasonically, the pH increased smoothly to its maximum value of 7.6 at the end of the bioleaching period. It had been reported that solution pH was a key parameter that affected the bioleaching of heavy metal ions. This is borne out by the observed increase in bioleaching of Al, Fe and Zn with the increase of the initial pH. 3.4.
Variation in the total contents of metals of bottom ash
The leaching efficacy of metals was closely dependent on the solubilization of metals in the metabolites. The changes in Al, Fe and Zn contents of the MW ash are shown in Figure 3(b) and 3(c), respectively. During the bioleaching period, in the metabolites of A. niger treated ultrasonically, Al contents climbed sharply to a maximum value of about 85.7% on the 12th day indicating that ultrasonic waves not only increase the dissolution of metals, but also increase the efficiency of the process. After the 12th day, a decreasing trend in Al contents was again observed, that might be due to the desorption phenomenon by excessive ultrasonic treatment. However, in metabolites without ultrasonic treatment, the value of Al presented a gradually increasing trend. The maximum value (80.3%) was obtained on the 18th day. Xu and Ting [39] reported 12.3 mg/kg of Al leached from MSW incinerator fly ash. Once the bioleaching was ongoing under ultrasonic treatment, the contents of Fe in the liquid phase began to increase and reached up to 88.5% on the 9th day, then a decline occurs and the value falls to the level of 74.1% on the 12th day. It approached 87.7 on day 15 and then again decreased slightly to the level of 76.1% at the end of the bioleaching period. This irregular trend in iron dissolution after the 9th day of bioleaching might be due to the negative effect of ultrasonic treatment which leads to desorption of metals. Simultaneously, in the flask without ultrasonic treatment, the content of Fe rose to 72.8% on the 15th day and then began to decrease slightly to 68.9% on the 18th day. It was possibly because the shaking effect increased the solubility of the metal salts to some extent before the reverse phenomenon occurs. In contrast to our results, Wang et al.
3.2. HPLC analysis of metabolites Microbial metabolites at low pH contain a number of organic and inorganic acids. Concentration of organic acids has been detected by HPLC (Table 3). The major acids detected were citric, malic and oxalic acid, with tartaric acid in a small amount. These acids are responsible for the leaching of metals from the HW ash. 3.3.
pH changes of culture supernatant during bio-extraction/bioleaching period The pH changes that occurred in both set of samples during the 18 days of leaching are shown graphically in Figure 3(a). The pH was at its minimum value at the start of the bioleaching period. This corresponded to the maximum concentration/amount of acids present. As the bioleaching process commenced, an increase in pH was observed to occur. This phenomenon lasted till the end of the leaching period, and is due to the consumption of protons in the metals dissolution process. A distinct pH change occurred in the flask that was treated ultrasonically. On the 3rd day, the pH increased sharply from 3.2 to 4.4. The pH value approached its maximum (7.9) on the 12th day of bioleaching. After day12, an irregular trend in pH was observed. This might have been due to another phenomenon, i.e. desorption of
Table 3. Concentration of organic acid in fermented media after 6 days of growth of A. niger (Before leaching) and after leaching (after 18 days of sonobioleaching and bioleaching). Organic acids ((%) w/v) Medium 1 Before leaching After leaching 2 Before leaching After leaching
pH of fermented media
Citric
Malic
Oxalic
Tartaric
3.2 7.9
4.23 ± 0.72 0.09 ± 0.00
1.66 ± 0.07 0.02 ± 0.001
0.85 ± 0.05
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals a
b
a
Fozia Anjum , Muhammad Shahid , ShaziaAnwer Bukhari & J. Herman Potgieter
c
a
Bio-analytical Lab, Department of Chemistry, Government College University, Faisalabad, Pakistan b
Bioassays Section, PMBL, Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad, Pakistan c
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, Wits 2050, South Africa Accepted author version posted online: 22 Jul 2013.Published online: 25 Aug 2013.
To cite this article: Fozia Anjum, Muhammad Shahid, ShaziaAnwer Bukhari & J. Herman Potgieter , Environmental Technology (2013): Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals, Environmental Technology, DOI: 10.1080/09593330.2013.824992 To link to this article: http://dx.doi.org/10.1080/09593330.2013.824992
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.824992
Combined ultrasonic and bioleaching treatment of hospital waste incinerator bottom ash with simultaneous extraction of selected metals Fozia Anjuma , Muhammad Shahidb , ShaziaAnwer Bukharia and J. Herman Potgieterc∗ a Bio-analytical
Lab, Department of Chemistry, Government College University, Faisalabad, Pakistan Section, PMBL, Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad, Pakistan c School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, Wits 2050, South Africa b Bioassays
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(Received 29 January 2013; accepted 20 June 2013 ) The mineralogy, as well as elemental composition, of the incinerated hospital waste (HW) ashes are not well known and need to be investigated for the safe handling and disposal of such ash. A study was conducted to investigate the chemical composition, mineralogy and bioleaching of selected metals from incinerated HW bottom ash using Aspergillus niger under the combined effect of ultrasonic radiation. Different techniques were utilized to determine the elemental composition (Electron Dispersive X-ray Spectroscopy [EDX], atomic absorption spectrophotometry, inductively coupled plasma-optical emission spectroscopy, ultraviolet-visible light spectrophotometer) and mineralogy (X-ray Diffraction) of the raw sample, as well as the bioleached samples. Chemical leaching tests were performed to determine the effect of different organic acids on metals dissolution. Microbes were tested for acid production and leaching capabilities of selected metals from medical waste (MW) bottom ash. Wet chemical and EDX analyses showed that the ash was enriched with metallic elements like Na, K, Ca, Fe and Al with a concentration range of 22–115 (g/kg). Furthermore, the ash contained heavy metals such as Cu, Cr, Ni, Sn and Ti in the range of 0.51–21.74 (mg/kg). Citric and oxalic acids generated by fungi could be important leaching agents acting to dissolve these metals. Under ultrasonic treatment, metals dissolution by the acidic metabolites was at its maximum after just 9 d of leaching. The results showed that the dissolution of metals was much higher in citric and oxalic acid than with other acids. Extraction of metals from incinerated MW ash indicated that this ash may be a potential source of metals in the future. Keywords: hospital wastes; incinerator bottom ash; ultrasonic treatment; bioleaching; Aspergillus niger; organic acids; metals dissolution
1. Introduction The term medical waste (MW) includes all infectious wastes disposed from medical institutions.[1] New criteria define infectious waste on the basis of the form of waste, place of waste generation, and kind of infectious diseases it might contain. Incineration has been proved to be the best treatment option for MW, as it could reduce the MW volume by up to 90%, while simultaneously degrading the infectious materials effectively.[2] Tanaka et al. [3] estimated that the quantity of incinerated infectious waste from medical establishments in Japan comprises 82.3% of the total incinerated volume. Usually identification of waste characteristics is conducted from the point of waste generation to intermediate treatment residues until it is finally disposed. Based on the characteristics of the waste, the appropriate treatment and disposal system is established to minimize the environmental impact. However, such stages cannot be applied to MW due to the strict regulations governing its disposal. For example, all infectious materials are sealed in hermetically tight containers, and then placed in a special storage area not to be opened until incinerated.[4] ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
At medical institutions, infectious waste is collected separately from non-infectious waste. The quantity of infectious waste is normally about 20–25% of the total waste generated from medical institutions. Incineration is the preferred option for the treatment of infectious wastes.[5] However, ash from the incinerator may cause secondary pollution to the environment if not properly treated.[4] Previous studies have shown that bottom ash from incineration of municipal solid waste (MSW) might be a valuable material, because it can be used as a secondary aggregate in roads and construction materials.[6] Due to the similarity in the chemical compositions of MW ash and MSW ash, there is a risk that MW ash might be reused in the same way.[6,7] However, MW bottom ash has some special characteristics that must be taken into consideration before it can be reused. MW contains large amounts of disposed metallic or plastic materials. Therefore, the bottom ash from MW incineration may contain a large proportion of toxic metallic elements that might prevent its reuse. Previous studies have indicated that MW bottom ash contains higher amounts of
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heavy metals such as Cd, Cr, Ni, Pb and Zn than does MSW bottom ash.[7,8] Currently, the major ways to treat the MW ash are chemical stabilization, cement solidification and chemical leaching that are used to detoxify MW incineration ash. Unfortunately, these techniques are highly energy intensive, and also involve hazardous chemical usage during the treatment.[9] Biohydrometallurgical approaches are generally considered a low-cost ‘green technology’ with low-energy requirements. Microbial bioleaching is based on the natural ability of microorganisms to transform solid compounds into a soluble and extractable form. This may involve enzymatic oxidation or reduction of the solid compound, or an attack on the solid compound by metabolic products of the microorganisms.[10] Three main groups of microorganisms have been used for the bioleaching process, namely autotrophic bacteria like Thiobacilli species, heterotrophic bacteria like Pseudomonas and Bacillus species and heterotrophic fungi like Aspergillus species and Penicillium species.[11] The extraction of metals from low-grade ores, mining wastes, sewage sludge, secondary raw materials and industrial intermediate products using these microorganisms, have been reported previously.[10–12] High population densities, limited landfill area and potential health hazards all contributed to the fact that incineration is the most popular method of intermediate MW treatment. However, incineration of waste produces residues such as ash that is enriched by several metals.[9] These heavy metals can be bioleached for use after microbial treatment of the ash. Despite slow recovery rates of the process, this technology is attractive as it is not energy intensive. Among various physical methods for speeding up the leaching process, the application of ultrasound is proving to be of considerable interest for intensifying the microbial growth, as well as performance of the process.[10,13] To shorten the leaching time without a decrease or possible enhancement in the final metals recovery values, a process intensification study has been carried out with the use of ultrasound. Ultrasound can rapidly extract metals from ash through a process of cavitation in the form of shock waves and micro-jet formation through the cavitating medium such as water or dilute acids.[14] Bioleaching has shown a steady increase in interest to recover valuable metals by the fact that they are relatively inexpensive, involving low energy consumption and being environmentally safe. Sonochemical extraction techniques together with classical methods gave a fast and selective extraction of metals.[13,14] A large collection of literature on the application of ultrasound to metal leaching operations has been reported in earlier work.[15,16] The use of ultrasonic waves for the extraction of copper, zinc and nickel from their ores has previously been reported by some of the current authors.[10] Treatment with ultrasonic waves not only increases the rate of dissolution of metals, but also increases the rate of diffusion of soluble species in the liquid phase.
This treatment enables the lixiviant to penetrate the solid (ore) particles more easily.[17] Furthermore, the frequency of 40 kHz and at the intensity of 1.5 W could increase the growth of fungi.[18]
2. Materials and methods All chemicals and reagents used in the present research work were of analytical grade. Pure standards of organic acids citric, oxalic, maleic and tartaric acids were obtained from Sigma-Aldrich Co. (St Louis, MO, USA). All culture media were purchased from Oxoid, UK.
2.1. Medical waste incinerator bottom ash A representative sample (1.00 kg) of the MW incinerator bottom ash was collected from the waste management department of the hospital in the city of Lahore, Pakistan. The incinerator was of medium scale and produced bottom ash at a temperature of 400–500◦ C. The incinerator was fuelled by diesel oil and natural gas. The representative sample of bottom ash was dried at room temperature, before being passed through a strainer to remove the metal and glass pieces from the ash. About 200–300 g of sample was packed in a polyethylene bag. The bags were brought to the Bioassay Section of the Biochemistry Laboratory of UAF, Faisalabad, Pakistan for further research work.
2.2. Pretreatment of the sample The sample was oven-dried (109◦ C) and ground to generate particles of ≤200 mesh size (74 micron) when using an ASTM sieving machine. The powdered samples were shifted into plastic bottles which were labelled and properly catalogued. This was further used for chemical and mineralogical analysis and for shake flask bioleaching experiments.
2.3. Elemental and mineralogical analysis Elemental analysis was carried out by atomic absorption spectrophotometry (AAS) (Perkin Elmer, AAnalyst 300), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and UV/VIS spectrophotometry (Hitachi, UV200; Japan) after acid digestion. The digestion was done with 3:1 HNO3 :HClO4 at 200◦ C and followed by subsequent redissolution in HCl.[19] The mineralogical composition was determined by X-ray Diffraction (XRD), using a RigakuRint 300 Series diffractometer and the JCPDS diffraction software database.[20] To determine the elements present at the surface of the particular area of the ash sample, EDX (Electron Dispersive X-ray Spectroscopy) analysis was performed at the same time by following the modified method reported by Barzik et al. [20]
Environmental Technology 2.4.
Determination of aluminium in the raw sample and leached solutions Dissolved aluminium was determined by UV/VIS spectrophotometer using Eriochrome cyanine R as the chromogenic agent. The 1:1 violet red aluminium complex was measured by determining the absorbance at λmax 540 nm.[21]
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2.5.
Chemical extraction of metals with and without ultrasonic treatment Chemical/sonochemical extraction of metals was performed by using citric, malic, oxalic and tartaric acids, each in pure form (1.0%), MW ash (2.0%) and the volume was adjusted to 100 mL by distilled water in 250 mL flasks. The flasks were incubated in an orbital shaker (Gellen Kamp, England) at 28◦ C and 150 rpm for 5 and 10 days for chemical/sonochemical leaching of metals. The pH was recorded every day, while liquid samples were withdrawn, filtered and analysed for metals at the end of each incubation period.[10] 2.6.
Production of organic acid metabolites from microbial strain Aspergillus niger culture was used as the isolated and purified strain as reported previously by some of the same authors.[10] For growth in the liquid medium, the culture medium consisted of (g/L): KH2 PO4 , 5.0; NH4 NO3 , 2.0; (NH4 )2 SO4 , 4.0; MgSO4 · 7H2 O, 0.2; peptone, 2.0; trisodium citrate, 2.5; and yeast extract, 1.0. The final volume was made up to 1000 mL with distilled water.[10] For bioleaching experiments, two sets of 250-mL Erlenmeyer flasks, each containing 100 mL of culture medium, were prepared in triplicate. The medium in each flask was autoclaved and 5.0% (m/v) of the given substrate (molasses) was added in each flask and then inoculated with 1.0 mL of A. niger spore suspension as inoculum (2.4 × 108 spores mL−1 ) in both sets of flasks. All the flasks were sealed with removable cotton and incubated in an orbital shaker (Gellen Kamp, England) at 28◦ C at an operating speed of 150 rpm for six days. 2.7.
Leaching of the metals with organic acids produced by A. niger Supernatant liquors of the microbial cultures were collected after six days of cellular growth, sterilized, centrifuged (8000 × g for 10 min at 15◦ C) and then filtered to remove any solid biomass before high performance liquid chromatography (HPLC) analysis for the determination of organic acid metabolites. Supernatants containing organic acid metabolites were used to leach the metals from the MW bottom ash. An amount of 1.00% (w/v) of the MW bottom ash was added to each medium in the both sets of flasks and incubated on an orbital shaker to keep everything
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in a homogeneous slurry form at 28◦ C and 150 rpm for 18 days. To enhance the extraction of metals, ultrasonic radiation treatment was applied for 10 min/day to one set of shaking flasks. Sampling was done at determined time intervals to register the changes in pH value, and dissolved metals concentration. Finally, the solid phase was separated, and the pH value of the liquor was measured by a Metrohm pH meter (model 744). The dissolved metals concentration was measured by AAS as well as the UV/VIS spectrophotometer.[10] 2.8. Ultrasonic radiation treatment Enhanced bio-extraction of metals was performed ultrasonically in one set of the shaking flasks by following the modified method described earlier.[10] Ultrasonic treatment was applied by placing the experimental flasks containing the metabolites and ash sample in an ultrasonic bath for 10 min at 40 kHz per day during the bio-extraction period. 2.9. Statistical analysis All experiments were performed in triplicate and the results were reported as the mean value ± SD (standard deviation).[22] 3.
Results and discussion
The physical appearance of the MW ash sample was black. It was found to be strongly alkaline (pH 10.1) and insoluble in water. This was probably due to the presence of alkaline metals (Na, K, Ca and their associated anions) in the ash, which is a well-known phenomenon in various ash types. The organic matter content estimated by the loss of weight on ignition at 550◦ C for 6 h was 26.25%. This indicated a high concentration of unburned organic matter, which was contributed to the low operating temperature of the incinerator during combustion.[23] An elemental analysis of the exposed surface of the bottom ash sample (MW) by EDX is shown in Figure 1(a). Some of the numerous elements present on the exposed surface of the representative sample were Na, K, O, C, Ca, Fe, Si, Al, Cu, Cr, Ni, Sn and Ti. After bioleaching, the apparent morphology was quite different, as shown in Figure 1(b) and 1(c). In the case of the bioleached sample under ultrasonic treatment, the elements Fe, Si, Ca and C were detected on the surface (Figure 1(b)), whereas the bioleached sample without ultrasonic treatment had Fe, Al, Si, Cr, O, Ca and C present on the surface (Figure 1(c)), indicating that most of the elements having affinity for the microbial metabolites were extracted. The XRD results in Table 1 reveals the predominance of kaolinite, illite, wollastonite, sphelarite, gehlenite, natrolite, halite, hematite, calcite, anorthite, quartz and Ni-sulphide as the mineral phases in the ash from the incinerated hospital wastes (HWs). Metal oxides such as Ca3 Al2 O6 , Ca5 Cr 3 O12
F. Anjum et al.
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Figure 1. (a) EDX analysis of the raw sample (HW) before leaching, (b) EDX analysis of the sample (HW) after leaching with ultrasonic treatment and (c) EDX analysis of the sample (HW) after leaching without ultrasonic treatment.
Environmental Technology Table 1.
Chemical composition of raw sample of bottom ash determined by XRD.
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Elements
Chemical formula
Kaolinite Pyrite Quartz Aluminium oxide Illite Muscovite Dolomite Ni-sulfide Calcite Hematite Anorthite Dichlorine oxide Silica Phosphorus pentoxide Anhydrite
Al2 SiO2 O5 (OH) FeS SiO2 Al2 O3 KAl2 (Si3 Al)O10 (OH2 ) (K, Na) Al2 (Si, Al)4 O10 (OH)2 CaMg(CO3 )2 (NiFe)x S, (NiFe)S2 CaCO3 Fe2 O3 CaAl2 Si2 O Cl2 O SiO2 P 2 O5 CaSO4
and CaO · Al2 O3 , and various sulphur, phosphorus and chlorine compounds were also detected, together with calcium sulphate anhydrite (CaSO4 ), sodium thiosulphate (Na2 S2 O3 ) and iron phosphate Fe3 (PO4 )2 . Similar findings have previously been reported by other researchers.[1,24, 25] Non-metallic elements like chlorine might be due to the presence of a considerable amount of Polyvinyl chloride polymer in the waste. Moreover, chloride salts are frequently used in medical supplements and can contribute to the high Cl contents in the ash. Zhao et al. [9] reported highly significant values of oxides of metals such as Ca, Si, Al and non-metals such as P, S and Cl in their investigation, while Ibanez et al. [7] reported a smaller amount of these elements than those found in this study. Table 2 lists the metal concentrations in the MW bottom ash obtained by AAS and ICP-OES. The data indicated the enrichment of ash with various metallic elements. The major metal elements in the ash were (g·kg−1 )Ca (11.5), Al (60.1), Mg (51.4), Fe (51.3), Na (36.0) and K (22.1). Other Table 2. Metals concentration of raw sample of bottom ash by ICP/OES. Metal ions Na K Ca Mg Al Fe Li Ti Zn Cu Ba Mn Pb Ag Cd
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HW Conc. (mg/kg) 36,040 22,050 11,504 51,440 60,120 51,320 0.07 13.06 24.11 21.74 1.36 1.47 9.11 0.99 1.36
Metal ions Co Bi Rb Cr Ni Sb Sn Ga Ge Hg Sr As Mo Li
HW Conc. (mg/kg) 2.01 1.03 2.22 9.02 3.59 1.03 0.51 6.22 0.42 0.73 0.47 1.22 0.78 0.07
Elements Sphelarite Wollastonite Silicocarnotite Akermanite Chromium oxide Gehlenite Natrolite Halite Silicocarnotite Tricalcium aluminate Calcium aluminate Sodium oxide Sodiumthiosulphate Iron phosphate
Chemical formula (ZnFe)S CaSiO3 Ca5 Cr 2 SiO12 KAlSi3 O8 Ca2 MgSi2 O7 Cr 2 O3 Ca2 Al2 SiO7 Na2 Al2 Si3 O10 · 2H2 O NaCl Ca5 Cr 3 O12 Ca3 Al2 O6 CaOAl2 O3 Na2 O (Na2 S2 O3 ) Fe3 (PO4 )2
(heavy) metals were also detected in significant amounts, e.g. Zn (24.1), Ti (13.1), Cr (9.0), Ni (3.6), Rb (2.2–2), Co (2.0), Cu (1.7), Ba (1.4), Mn (1.5), Cd (1.4), Ga (1.2), As (1.2), Pb (1.1), Bi (1.0) and Sb (1.0) (mg·kg−1 ). Metals such as Ag, Mo, Hg, Sn, Sr, Ge and Li were present in smaller quantities in the concentration range of 0.07–0.99 (mg·kg−1 ). In contrast to these results, earlier work reported higher levels of Zn, Ni, Cd, Cr, Cu and Pb in the incinerator bottom ash of MW.[26–29] The infectious wastes of hospitals are packed in coloured plastic bags before being sent for incineration and these coloured plastic materials may be the potential source of some of the heavy metals encountered. The MW bottom ash also contains much higher amounts of Zn, Ti and Cr (9.02 mg kg−1 ) than previously reported.[30] Kuo et al. [31] and Ibanez et al. [7] have indicated that MW bottom ash contains high concentrations of heavy metals such as Cd, Cr, Ni, Pb and Zn. These elements are commonly used in medical facilities, e.g. metal alloys containing Zn and Ti are widely used in medical instruments, while Cr is widely present in used needles and syringes. Kuo et al. [31] reported Fe, Al, Cu, Zn, Cr and Pb in the bottom ash of MW, while Kougemitrou et al. [8] reported the presence of Fe, Cu, Cr, Zn, Pb, Co and Cs in bottom ash of a MW incinerator at Athens. If such heavily contaminated ash is dumped on land, it may cause a dust hazard, soil contamination and, if exposed to rainfall, the leaching potential for heavy metals is enhanced. Blowing wind can contaminate the air with fine dust particles and, therefore, create a potential route of heavy metals intake through inhalation. The fallout from these metals in the area surrounding an incinerator can be substantial. These heavy metals can, therefore, enter biological chains.
3.1.
Chemical/sonochemical extraction of metals from HW bottom ash
Chemical and sonochemical extraction of metals were performed in shaking flasks containing ash (2%) and organic
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Metals dissolution (mg/Kg)
(a)
70 60 50
Fe
40
Al
30
Zn
20 10 0
O.A
C.A T.A Organic acids
M.A
Metals dissolution (mg/Kg)
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(b)
35 30 25
Fe
20
Al
15
Zn
10 5 0
O.A
C.A T.A Organic acids
M.A
Figure 2. (a) Sonochemical leaching of metals by organic acids after five days of incubation. (b) Chemical leaching of metals by organic acids after 10 days of incubation. Note: OA, oxalic acid; CA, citric acid; MA, malic acid; TA, tartaric acid.
acids (1.0%) after 5 and 10 d periods of incubation in chemical and sonochemical leaching (Figure 2(a) and 2(b)). During this period, the pH was recorded periodically every day. It was found that pH increased in both samples with and without ultrasonic treatment as the time progressed. Compared with conventional chemical leaching, ultrasonic treatment enhanced the rate of metal ions solubilization from the ash and improved the final yield in a decreased time. This may be attributable to breakdown of large ash particles into smaller and porous particles by the periodic sonication with a resultant increase in exposed surface area, which led to enhanced mineral dissolution by the acids.[10] Various synthetic organic acids exhibit different abilities to dissolve metal ions. This is analogous to the leaching ability of organic acid metabolites of fungal strains.[32,33] The solubilization of metals by organic acids is discussed below. 3.1.1. Chemical/sonochemical extraction of aluminium from bottom ash Different organic acids showed a different potential for aluminium extraction. Citric acid was the best organic acid for aluminium solubilization. A shaken flask containing citric acid solution used in conjunction with ultrasonic
vibration, achieved a maximum yield of aluminium of 86.6%, compared to the leaching of 43.1% in a similarly shaken flask without ultrasonic treatment, as illustrated in Figure 2(a) and 2(b). It was noted that ultrasonic treatment not only increases the rate of particle destruction by producing cracks and pits, but also improve the rate of metals solubilization by the acids from the raw sample. Thus, the final yield increased compared with conventional chemical leaching.[14] Huang et al. [34] used an inorganic acid for metal extraction to recover aluminium from fly ash of a MSW incinerator, and achieved a yield of 80%. While other acids also solubilize aluminium to a significant extent, the solubility was always comparatively higher in those samples that were treated ultrasonically. With the ultrasonic treatment, the maximum solubilization of aluminium was 52.0%, 48.6% and 25.3% in oxalic, malic and tartaric acid solutions, respectively, compared to 30.2%, 45.2% and 20.4%, respectively, without ultrasonic treatment. From all these values, it is clear that citric and oxalic acids are more effective leaching agents for aluminium than other acids under controlled conditions. Similar trends have been reported by other research workers.[35,36] 3.1.2. Chemical/sonochemical extraction of iron from bottom ash The results of iron extraction are shown in Figure 2(a) and 2(b). Under ultrasonic treatment conditions, the iron extraction was 87.3% in oxalic acid compared with 46.4% without ultrasonic treatment. Citric, malic and tartaric acid were also found to have great potential for iron extraction from the ash. Amounts of 63.3%, 56.8% and 35.1% were dissolved, respectively, when treated ultrasonically just after 5 days of incubation. In contrast, lower amounts of 37.2%, 48.7% and 32.6% of iron were recovered after 10 days of incubation in the shaking flasks that were not treated ultrasonically. Similar findings have been reported previously.[10] Huang et al. [33] recovered 82% iron from the MSW incinerator fly ash using an inorganic acid. 3.1.3. Chemical/sonochemical extraction of zinc from bottom ash The results of zinc solubilization in shaking flasks with and without ultrasonic treatment are schematically represented in Figure 2(a) and 2(b). Oxalic acid has the maximum potential for zinc extraction from bottom ash when compared to the other acids used. The maximum extraction of zinc (88.0%) occurred with oxalic acid at the end of the sonochemical leaching period. In the case of shaking flasks that were not treated ultrasonically, the maximum solubilization of zinc after 10 days of leaching was only 45.3%. Oxalic acid is a stronger acid than citric acid and has three times more affinity for zinc than citric acid. Citric acid and zinc oxide react to produce insoluble zinc citrate (Zn3 Cit2 ).[37]
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Environmental Technology In the interaction of zinc with acid, H+ ions govern the dissolution process and the maximum dissolved amount of zinc corresponds to the maximum amount of H+ ions available. The availability of these hydrogen ions has increased significantly with ultrasonic treatment.[38] Acids, like citric (77.6% and 37.5%), malic (48.3% and 39.6%) and tartaric acid (36.2% and 33.2%) also showed substantial dissolution of zinc with and without ultrasonic treatment, respectively. In the case of ultrasonically treated samples, high extraction of zinc after just 5 days of incubation might be due to an acceleration of the leaching process resulting from the ultrasonic radiation. Ultrasonic radiation increases the stirring effect and increased the exposed surface area of the ash particle in the slurry during leaching, thus resulting in an increased extraction of metal ions in shorter time.[14]
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metals. However, in the flask that was not treated ultrasonically, the pH increased smoothly to its maximum value of 7.6 at the end of the bioleaching period. It had been reported that solution pH was a key parameter that affected the bioleaching of heavy metal ions. This is borne out by the observed increase in bioleaching of Al, Fe and Zn with the increase of the initial pH. 3.4.
Variation in the total contents of metals of bottom ash
The leaching efficacy of metals was closely dependent on the solubilization of metals in the metabolites. The changes in Al, Fe and Zn contents of the MW ash are shown in Figure 3(b) and 3(c), respectively. During the bioleaching period, in the metabolites of A. niger treated ultrasonically, Al contents climbed sharply to a maximum value of about 85.7% on the 12th day indicating that ultrasonic waves not only increase the dissolution of metals, but also increase the efficiency of the process. After the 12th day, a decreasing trend in Al contents was again observed, that might be due to the desorption phenomenon by excessive ultrasonic treatment. However, in metabolites without ultrasonic treatment, the value of Al presented a gradually increasing trend. The maximum value (80.3%) was obtained on the 18th day. Xu and Ting [39] reported 12.3 mg/kg of Al leached from MSW incinerator fly ash. Once the bioleaching was ongoing under ultrasonic treatment, the contents of Fe in the liquid phase began to increase and reached up to 88.5% on the 9th day, then a decline occurs and the value falls to the level of 74.1% on the 12th day. It approached 87.7 on day 15 and then again decreased slightly to the level of 76.1% at the end of the bioleaching period. This irregular trend in iron dissolution after the 9th day of bioleaching might be due to the negative effect of ultrasonic treatment which leads to desorption of metals. Simultaneously, in the flask without ultrasonic treatment, the content of Fe rose to 72.8% on the 15th day and then began to decrease slightly to 68.9% on the 18th day. It was possibly because the shaking effect increased the solubility of the metal salts to some extent before the reverse phenomenon occurs. In contrast to our results, Wang et al.
3.2. HPLC analysis of metabolites Microbial metabolites at low pH contain a number of organic and inorganic acids. Concentration of organic acids has been detected by HPLC (Table 3). The major acids detected were citric, malic and oxalic acid, with tartaric acid in a small amount. These acids are responsible for the leaching of metals from the HW ash. 3.3.
pH changes of culture supernatant during bio-extraction/bioleaching period The pH changes that occurred in both set of samples during the 18 days of leaching are shown graphically in Figure 3(a). The pH was at its minimum value at the start of the bioleaching period. This corresponded to the maximum concentration/amount of acids present. As the bioleaching process commenced, an increase in pH was observed to occur. This phenomenon lasted till the end of the leaching period, and is due to the consumption of protons in the metals dissolution process. A distinct pH change occurred in the flask that was treated ultrasonically. On the 3rd day, the pH increased sharply from 3.2 to 4.4. The pH value approached its maximum (7.9) on the 12th day of bioleaching. After day12, an irregular trend in pH was observed. This might have been due to another phenomenon, i.e. desorption of
Table 3. Concentration of organic acid in fermented media after 6 days of growth of A. niger (Before leaching) and after leaching (after 18 days of sonobioleaching and bioleaching). Organic acids ((%) w/v) Medium 1 Before leaching After leaching 2 Before leaching After leaching
pH of fermented media
Citric
Malic
Oxalic
Tartaric
3.2 7.9
4.23 ± 0.72 0.09 ± 0.00
1.66 ± 0.07 0.02 ± 0.001
0.85 ± 0.05
