Article pubs.acs.org/EF
Seed-Assisted Hydrothermal Treatment with Composite Silicon− Aluminum Additive for Solidification of Heavy Metals in Municipal Solid Waste Incineration Fly Ash De-zhi Shi,* Chao Zhang, Jin-lu Zhang, Peng-fei Li, and Yun-mei Wei Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Faculty of Urban Construction and Environmental Engineering, National Centre for International Research of Low-Carbon and Green Buildings, Chongqing University, Chongqing 400045, People’s Republic of China ABSTRACT: Owing to the high Ca content but low concentration of Si and Al in fly ash (FA) generated from municipal solid waste incineration (MSWI), hydrothermal treatment (HT) of MSWI FA directly could not result in the synthesis of calciumcontaining aluminosilicate minerals zeolites such as tobermorite, an important material for the hydrothermal solidification of heavy metals. According to the research results from the effect of the extra silicon−aluminum source added, with the higher amount (30%) of the composite additive in which the ratio of quality for coal fly ash (CFA) and diatomite is 1:1, tobermorite could be successfully synthesized, which is mainly attributed to the proper element proportion of Ca to Si and Al and Al to Si and Al, adjusted by additives. Furthermore, the tobermorite seed added also could not help the formation of tobermorite, even at a higher reaction temperature of 200 °C, due to the improper element proportion with a 10% mass of the composite additive. With a 30% mass of the composite additive and seed at a higher reaction temperature of 200 °C, the tobermorite seed added could not only promote the formation of tobermorite as early as the first 3 h but also inhibit the hibschite generated from tobermorite. Due to the leaching toxicity of Pb decreasing to the lowest level (0.25 mg/L) being related to the tobermorite largely formed, it can be concluded that the addition of a 30% mass of composite additive (including CFA and diatomite) and a 3% mass of tobermorite seed at a higher reaction temperature of 200 °C would be the recommended technological parameter for seed-assisted hydrothermal solidification of MSWI fly ash. hydrothermal treatment can convert the MSWI fly ash into more-stable aluminosilicate mineral forms12 and further facilitate the immobilization of heavy metal ions.11 Because MSWI fly ash belongs largely to the CaO−CaCl2−CaSO4− Al2O3−SiO2 system, Yao et al.13 have reported the synthesis of tobermorite (Ca5Si6(OH)2O16·4H2O) from MSWI fly ash by hydrothermal treatment in the presence of NaOH solution at 180 °C. Tobermorite is very rare naturally but is the most important compound in various hydrous calcium silicates and can act as a cation exchanger in particular due to its high adsorption capacity of heavy metal ions. Chemical composition analysis sets forth SiO2, CaO, and Al2O3 as being the major waste-insoluble components of MSWI fly ash, which is hence a suitable starting material for tobermorite synthesis, given the highly specific surface area of fly ash particles. However, in comparison with coal fly ash and MSWI bottom ash, the contents of Si and Al in MSWI fly ash were found to be lower, while the content of Ca was high due to the large amount of Ca(OH)2 injected into the semidry scrubber for removing acid gases.14 Accordingly, quartz and high amounts of silicon−aluminum additives could be used to solidify heavy metals of MSWI fly ash based on tobermorite and zeolite formation,5 such as coal fly ash (CFA), diatomite, bentonite, and so on. The compositional similarity to some volcanic materials, the precursors of natural zeolites, prompted Höller
1. INTRODUCTION Fly ashes generated from municipal solid waste incineration (MSWI) contain highly toxic organic contaminants (PCDD/ Fs, PCBs, PAHs, etc.) and carcinogenic heavy metals,1 which are harmful to the environment and human beings. Most countries classified MSWI fly ash as a hazardous material that must be detoxified or stabilized before final disposal. According to Chinese standard GB18485-2014,2 incineration fly ash is classified as hazardous waste (code: HW18) and requires special treatment. According to Chinese standard GB168892008,3 when the treated MSWI fly ash meets some special requirements, it could go to sanitary landfills. Because of the shortage of landfill and tighter environmental regulations, new ways of treating and utilizing MSWI fly ash are urgently needed. Recently, hydrothermal solidification for processing of fly ash is regarded as a promising technology that offers considerable advantages in terms of economic, technical, and environmental performance.4 Under hydrothermal conditions, the ion product of water is thousands of times that at room temperature and pressure; thus the reaction rate will be increased substantially, so hydrothermal processes that take millions of years during the diagenesis of sedimentary rocks occur within a short period in the laboratory.5 Therefore, MSWI fly ash might be solidified into a very tough and durable product. The hydrothermal technique has been successfully employed to convert concrete waste into construction materials and to solidify blast furnace water-cooled slag,6 coal fly ash (CFA),7 and MSWI bottom ash8 as well as MSWI fly ash.5,9−11 Using hydrothermal crystallization under alkaline conditions (NaOH and KOH), the © XXXX American Chemical Society
Received: August 11, 2016 Revised: October 28, 2016 Published: October 31, 2016 A
DOI: 10.1021/acs.energyfuels.6b02019 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
FA was taken from the fly ash silo generated from the bag filter. All samples were sieved by a 35 mesh sieve, and the coarse fraction (with a pore diameter of 500 μm) was discarded. The fly ash was oven-dried at 105 °C ± 0.5 °C for 24 h and then ground and finally sieved. The fraction, which passed through a 75 mesh sieve (with a pore diameter of 200 μm) was analyzed to determine its principal properties.11 A pair of kinds of silicon−aluminum additives were selected. Coal fly ash (CFA) was collected from the Zhutuo Coal-Fired Power Plant, located in south of Chongqing. The diatomite was of chemical purity grade. 2.2. Seed Crystalline Preparation. Amorphous silica (20.84 g; 75−100 μm particle size), amorphous aluminum oxide (3.0 g; prepared by gibbsite calcination at 500 °C for 5 h) and calcium oxide (18.8 g; prepared by calcite calcination at 1100 °C for 6 h) solids were suspended in a 0.56 mol/L NaOH at solution-to-solid ratio of 5:128 to give stoichiometric ratios of Ca/(Al+Si)= 0.83 and Al/(Al+Si) = 0.15. The suspensions were mixed and transferred to a 300 mL Teflon vessel and heated to T = 150 °C (±1 °C). After being heated for a specific amount time (1−12 days), reaction mixtures were quickly quenched within 30 min. The solid was crushed, filtered, and washed with distilled water to remove residual ions and dried at 50 °C for 3−5 days. The crystallized seed was sieved by a 100 mesh sieve and stored in desiccators. The seeds from different crystallization time were analyzed, respectively, and the seed from the best parameters was used. 2.3. Hydrothermal Process. Coal fly ash (CFA) and diatomite used as silicon−aluminum additives were solely or combined added into MSWI raw fly ash for HT. For each kind of additive, the weightto-weight (w/w) ratio of additive and fly ash were 1:9 and 3:7, respectively (Table 1). To investigate the effect of seed, 3% masses of seed were added in the seed-induced HT process. Next, the samples were mixed adequately and stored in airtight polypropylene containers. Raw MSWI fly ash without additives and seed adjunction was used for a comparative study.
and Wrishing15 to synthesize zeolites from CFA. As industrial waste fly ash, owing to its high content of aluminosilicate glass, CFA has been widely used as materials for hydrothermal synthesis of zeolite, and different types of zeolites, such as NaP1 zeolite,16 Na-A zeolite,17 Na-Y zeolite,18 Na-X zeolite,19 etc., were successfully produced. Diatomite is the fossilized remnants of diatoms, tiny planktonic algae residing in all of the earth’s waters.20 The cell wall of diatoms is formed by the organism through the filtration of silica from water and is often referred to as the skeleton of the organism; it is made almost entirely of silica (SiO2).21 The death of large numbers of diatoms in an area leads to sedimentation of the minerals present in the cell walls leading to large deposits suitable for mining. As a result, diatomite is both nontoxic and odorless and present naturally in large quantities and at high purities; subsequently, it is available at low cost.22 CFA and diatomite could be reinforcing silica and alumina source addition for MSWI fly ash prior to hydrothermal synthesis. However, little research on aluminosilicate minerals formation with silica and alumina source addition during hydrothermal process of MSWI fly ash has been investigated. Among the hydrothermal synthesis techniques, the seedassisted or seed-induced hydrothermal method has been mainly used in synthesis of zeolites from chemical materials because it is widely realized that the usage of seeds can promote zeolite synthesis.23 Zeolite seeds was define as third type of structure directing agents in the synthesis of zeolites.24 The added seeds can accelerate crystallization so as to shorten the crystallization time as well as increase the product yield to complete conversion. The usage of seeds also forces the initial synthesis gel to produce the preferentially anticipated phase rather than other phases,25 thus expanding the range of gel composition,26 increasing the crystallization rate, or controlling the purity and properties (e.g., the crystal size) of the products.27 To the best of our knowledge, however, research on seed-assisted hydrothermal solidification of MSWI fly ash, one such kind of complex waste, has not been reported. Therefore, the objectives of this work were to investigate how to solidify heavy metals in MSWI fly ash by alkaline hydrothermal treatment (HT) and the effects of silicon− aluminum additive addition, seed addition, and reaction temperature on formation of aluminosilicate minerals zeolites in the solidified MSWI fly ash. For the further study on the mechanism of hydrothermal solidification of MSWI fly ash affected by various factors, HT products from different curing time were, respectively, investigated during the entire hydrothermal process. The relationship between tobermorite formation and heavy-metal stabilization was discussed. The optimal technological parameters of hydrothermal solidifying MSWI fly ash was summarized and recommended. The study is expected to provide fundamental information for seed-assisted hydrothermal treatment of MSWI fly ash.
Table 1. Addition Ratio of Silicon−Aluminum Source Additives and the Sample Numbers addition ratio of additives (%)
samples FA-150 FA-200 C-10150 C-10200 CD-10150 CD-10200 C-30150 C-30200 CD-30150 CD-30200 CD-103-150 CD-103-200 CD-303-150 CD-303-200
2. MATERIALS AND METHODS 2.1. Materials and Pretreatment. The raw fly ash (FA) used in this study was collected from a full-scale operating stoker grate incineration plant (Tongxing MSW Incineration Plant) located in Chongqing, China. The plant was consisted of two parallel Martin SITY2000 reverse reciprocating grate style incinerators. Each stoker incinerator with a capacity of 600 tons per day had its own heat recovery system and air pollution control device (APCD). The semidry APCD was composed of a semidry scrubber, activated-carbon injector, and a bag filter.
a
B
diatomite
addition ratio of seed (%)
reaction temperature (°C)
− − −
− − −
150 200 150
10
−
−
200
90
5
5
−
150
90
5
5
−
200
70
30
−
−
150
70
30
−
−
200
70
15
15
−
150
70
15
15
−
200
87
5
5
3
150
87
5
5
3
200
67
15
15
3
150
67
15
15
3
200
addition ratio of FA (%)
CFA
100 100 90
−a − 10
90
−: not added. DOI: 10.1021/acs.energyfuels.6b02019 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels For the hydrothermal experiment, batches of 20 g of the pretreated samples were collected and then mixed with an alkali solution (0.5 mol/L NaOH) in a 300 mL Teflon vessel, of which the liquid-to-solid (L/S) ratio was adjusted to be 10 mL/g.9 The mixture was further stirred with a magnetic stirrer at 500 rpm for 12 h. The hydrothermal experiment was conducted without stirring in a Teflon line autoclave under an autogenous pressure.9 The reaction temperature (150 and 200 °C) was selected as a control parameter. For further study on the effect of seed-induced HT on the formation of minerals, different reaction times were elected, including 3, 6, 9,12, 24, and 48 h, respectively. After the hydrothermal treatment (HT), the autoclave was cooled to atmospheric conditions. Then, the suspension was immediately separated into the liquid phase and the solid phase via vacuum filtrating.11 The solid-ash-cake products (referred to as hydrothermally treated fly ash, “HT product”) was dried at 105 °C for 24 h before testing.10 The dried cake products were then ground and stored in airtight polypropylene containers until analyzed. The solution was also sampled by a polypropylene syringe, which was filtered through a membrane filter (0.45 μm opening). 2.4. Analytical Methods. The chemical compositions of raw FA and two kinds of additives were measured by X-ray fluorescence spectrometry (XRF, Alpha4000, Innov-X Systems Corporation). The crystalline properties of raw fly ash samples, additives, hydrothermally treated fly ashes, and crystallized seeds were examined by a polycrystalline X-ray powder diffractometer (XRD, PANalytical X’pert) with Cu−Kα radiation and scanning ranging from 5° to 60° with an increment of 0.02° at a scan rate of 10°/min. The working voltage was 40 kV, and the current was 250 mA. Field emission gun environmental scanning electron microscope (FEGE SEM, TESCANMIRA3) equipped with an energy-dispersive spectrometer (EDX, Oxford Inc.) were used to study the morphology and surface compositions of the ash particles. Before and after HT was performed, leaching toxicity of heavy metals including Pb, Zn, Cu, Cr, and Cd from fly ash were assessed in accordance with the Chinese regulations of HJ/T 299-2007.29 At the end of the leaching process, the suspension was filtered with a 0.45 μ polypropylene filter (0.45 μm) and then acidified with concentrated nitric acid to control the pH of obtained water sample within the range between 1 and 2.11 Next, the acidified water sample was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 2100DV).
Table 2. Chemical Composition of FA, CFA, and Diatomite Compared with Fly Ashes for HT from Literature (wt %) this study a
FA from literature
elements
FA
CFA
diatomite
Germanyb
Japanc
Chinad
Ca Cl Na K S Si Mg Fe Al
38.24 20.05 5.52 4.79 2.61 1.83 0.67 3.89 0.47
1.03
