ABSTRACT
Annually huge amounts of fly ash are produced, instead of land filling this waste can be reutilized for obtaining new materials such as zeolites which can be further used in adsorption processes. The fly ash, collected from Power Plant Craiova (Romania) was used for obtaining the zeolites materials with good adsorption capacity for heavy metals and dyes from wastewater.
Fly ashes hydrothermally treated were used as a low cost adsorbents for the removal of Cd2+, Cu2+ cations and Methylene blue (MB) from synthetic wastewaters containing one, two and three pollutants. The new zeolite materials (ZCRns and ZCRs20) obtained from fly ash Craiova (FACR) has an increased specific surface area which led to good efficiencies. During the adsorption process the parameters: contact time and optimum amount of substrate were optimized for obtaining a maximum efficiency. The adsorption isotherms and kinetics were studied. The obtained results were fitted using Langmuir and Freundlich isotherms. Pseudo-first order, pseudo-second order and interparticle diffusion were employed for analyzing the kinetic data. The new material was characterized in terms of crystallinity (XRD) and surface properties: morphology.
The composite substrate proved good efficiencies for removal of heavy metals cations and methylene blue from the synthetic pollutant systems in adsorption. As the results show, MB removal runs with the highest efficiency in adsorption (93.33%) while heavy metals removal has almost similar values in adsorption processes.
1. INTRODUCTION
The growing need for water in industries has led to many problems like water pollution caused by the discharging of effluents loaded with heavy metals, dyes, surfactants, wax, etc. which poses environmental and health problems. Heavy metals like cadmium, copper, from these effluents led to groundwater and soil contamination, they are not biodegradable and tend to accumulate in living organisms and their toxicity, even at low concentrations leads to several diseases and disorders [1]. The main resource of heavy metals release in the environment is sewage from different industries such as mining, electroplating, cadmium-nickel batteries, plastic manufacturing [2] melting and casting industries [3] pesticides, petroleum refining processes, photography [4] printed circuit board manufacturing [5]. Cadmium affects mainly the kidneys, the lungs and the liver and it can also lead to bones demineralization [6]. Copper affects in the beginning the liver by disrupting its ability to detoxify, elevated copper level in the body leads to other problems for the nervous, reproductive system. Some of the copper salts are very toxic, like blue vitriol and by taken them in large amounts they cause severe vomiting, pain in the abdomen, severe headaches, diarrhea and paralysis. Nickel mainly affects the skin causing dermatitis; it also causes asthma, conjunctivitis [7]. The maximum acceptable concentration in drinking water declared by World Health Organization (WHO) [8] for these heavy metals is: 0.003 mg/L for Cd2+, 2 mg/L for Cu2+.
Several methods used for the removal of heavy metals and dyes from wastewater are: chemical precipitation [8], coagulation– flocculation, photocatalysis, ion exchange [9,10], nanofiltration membrane technology [11] and adsorption [12]. From all of these, the adsorption is one of the most economical regarding the fact that it uses a variety of adsorbents obtained from the wastes has simple equipment and can be easily operated with it [13]. The most known adsorbent is activated carbon, but it has the production cost and regeneration, because of this it is necessary the development of inexpensive materials.
Zeolites have different applications such as adsorbents [14] catalysts, molecular sieve and can be also used for ion exchange [15]. The synthesis of zeolites from fly ash by some conventional methods include hydrothermal synthesis, microwave heating [16] dry or molten-salt conversion, fusion method [17] and quantum methods [18].
Many researchers have tested the removal efficiency of heavy metals like Cd2+, Pb2+ [19], Cu2+ [20] Co2+, Cr3+, Ni2+, Zn2+ [9] onto zeolites obtained from fly ash.
This paper presents the synthesis, characterization of two zeolites ZCRns and ZCRs20 obtained from fly ash by hydrothermal method, and the results in removal of Cd2+, Cu2+ cations and MB from mono-, bi-, and three-component systems by adsorption.
2. EXPERIMENTS
2.1. SUBSTRATE PREPARATION
The material used for zeolites synthesis in this case was fly ash collected from Craiova Central Heat Power Plant, Romania. The composition of ashes depend on the type of fuel, combustion technology applied (type of furnace, the temperature developed and of cooling depends).
The major constituents of the fly ash are oxides: SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O presented in Table 1.
Table 1 Fly ash composition
The fly ash collected (raw FACR) from the electro-filters of the plant is of F type because the sum of the major compounds, oxides SiO2 + Al2O3 + Fe2O3 is over 75% and it doesn’t aggregate in water. From this raw fly ash were obtained two types of zeolite materials with washed fly ash and unwashed:
1) A part of the fly ash was washed with ultra pure water by mechanical stirring for 48 h at room temperature (20-22°C). After stirring the washed sample was filtrated, from the filtrated solution were obtained the values of pH (11), conductivity (390µS/cm) and TSD (802mg/L). After filtration the ash was dried in the oven at 105-115°C, the dried substrate was sieved and the 20 μm fraction was selected for further experiments (FAw). For obtaining zeolite materials the washed fly ash was mixed with NaOH solution in autoclave for 5h at 150°C. After running time the material was washed with ultra pure water till constant pH, filtrated and dried over night. This new material (ZCRs20) was used in adsorption experiments.
2) The same amount of raw FACR unmodified by washing was hydrothermally treated with NaOH in the same conditions. This zeolite material was notated ZCRns. The substrates (ZCRns and ZCRs 20) were further used in adsorption experiments for removing heavy metals from synthetic wastewater loaded with: a) Cd2+; b) Cd2++Cu2+; c) MB+Cd2++Cu2+.
2.2. CHARACTERIZATION OF THE SUBSTRATES
The ZCRns and ZCRs20 crystalline structures were evaluated by X-ray diffraction (XRD). By using AFM (Ntegra Spectra, NT-MDT model BL222RNTE) the morphology studies (roughness and macro-pore size distribution) were done. Scanning was conducted on three or more different places with a certain area of 5 μm x 5 μm for each position, chosen randomly at a scanning grate of 1Hz. By using scanning electron microscopy (S-3400N-Hitachi) further surface investigations were done at an accelerating voltage of 20 kV. Surface compositions of zeolites were measured before and after adsorption using energy dispersive X-ray spectroscopy (EDX Thermo Scientific Ultra Dry). By using porosity analysis and BET surface (Autosorb-IQ-MP, Quantachrome Instruments), the characterization of the surface was completed.
2.3. ADSORPTION EXPERIMENTS
The pollutant systems were synthetically prepared using bidistilled water and CdCl2∙2.5H2O (Scharlau Chemie S.A., c<98%), CuCl2∙2H2O (Scharlau Chemie S.A., c<98%) and Methylen Blue (MB) (C16H18N3S) (Fluka AG, reagent grade). The experiments were done using heavy metals solutions in concentration range of C Cd2+ = 0,.., 560 mg/L, C Cu2+ = 0,…,330 mg/L and CMB = 0,03125 mMol/L.
For deciding which of the two zeolites materials is more efficient in removing heavy metals and MB from wastewater they were tested in following adsorption experiments: Three series of experimental test of adsorption were done:
Adsorption on ZCRs20/ ZCRns substrate in solution containing one, two and three pollutants:
- Cd²⁺ /Cu²⁺ from mono-cationic system, under mechanical stirring;
- Cd²⁺ and Cu²⁺ from bi-cationic system, under mechanical stirring;
- MB+Cd²⁺+Cu²⁺ from system with three pollutants, under mechanical stirring.
During the batch experiments the kinetic and thermodynamic adsorption parameters of mono-cationic, di-cationic and dye system were evaluated in each experiment. For this 0.1 g of ZCRns substrate or ZCRs20 respectively were stirred (200 rpm) at room temperature (20 - 23°C) with 50 mL solution at initial concentrations (CiCd2+ = 560 mg/L, CiCu2+ = 330 mg/L and CiMB = 0.03125mMo;/L). For the kinetic studies aliquots were taken each at 10, 15, 30, 45, 60, 90, 120, 150, 180 min., and the substrate was removed by filtration. The supernatant was analyzed using AAS (Analytic Jena, Zeenit 700), at λCd = 228.8 nm, λCu = 324.75 nm, while the dye was analyzed by UV-VIS spectrometry (Perkin Elmer Lambda 25).
3. RESULTS AND DISCUSSION
3.1. CHARACTERIZATION OF THE SUBSTRATES
The composition of silicon-aluminous of the fly ash is confirmed by XRD spectra, Fig.1. The major crystalline components of raw fly ash are: αSiO2 (quartz), hematite (Fe2O3), identified by the sharp peaks in the range of 2θ = 26°.
Fig. 1 XRD data of (1) ZCRs20 and (2) ZCRns
The diffractogram (Fig.1) data show that SiO2 phases of raw FACR and washed fly ash are absent in the new materials (ZCRns and ZCRs 20), while the new crystalline phases are present in zeolites materials: phillipsite (Na6Al6Si10O32∙12H2O), sodium aluminum silicate hydrate Na8(AlSiO4)6∙4H2O, Na8(AlSiO4)6∙(OH)2∙4H2O (sodium aluminum hydroxide hydrate) and other phases of the aluminosilicates, typical for zeolite materials. The quartz syn, hematite and maghenite are in small proportion in new zeolites while the area picks of new alumino-silicates are higher after treatment. The hydrothermal process of rawFACR or washed fly ash further promotes the surface interactions, including dissolution, re-crystallization processes.
The spherical particles were gradually disappeared and prismatic shape of zeolite occurred and crystalline phases increased in both substrates from 38.5% to 58.5% for ZCRs20 and 41.9% for ZCRns.
The values for both materials ZCRs20and ZCRns show high conversion of SiO2 at crystalline zeolite (NaP1) by hydrothermal treatment. The content of quartz and mullite were found to be mainly responsible for hinder formation of zeolite and their activity [21]. The composition of crystalline phases, crystalline degree, and morphological changes are presented in table 2.
Table 2 The characteristics of the raw fly ashes and of the zeolite material
Other information related to the morphology and characteristics of the surface were obtained from the AFM and SEM micrographs (Fig. 2 and Fig. 4).
By washing and hydrothermal treatment, the roughness shows a strong decrease of surface from 74.1 nm to 17.2 nm. The roughness can give information about knowing the level of investigation (the millimetric scale usually permits one to distinguish the main surface treatments) [22].
Fig.2 The AFM topography and phase distribution
The chemical report (Si/Al) and structural changes are mirrored in morphology modifications, Fig. 2 (a,b,c)resulting large differences in the substrates’ affinity for heavy metals. On phase’s distributions, it can be seen more agglomerates in the new material adsorbent, so more macro-pores and meso-pores are ready to lodge the cations of heavy metals.These AFM images were used to characterize the surface morphology: the uniformity, grain size and pore size distribution [23] of the samples Fig.3.
Fig. 3. The interparticle voids distribution
The raw FACR, ZCRs20 and ZCRns have a rough surface with larger pores/voids heterogeneously distributed, confirmed by the pores/voids distribution curves with two or three maxima.
Fig. 4. Scanning Electron Microscopic images (a) rawFACR; (b) ZCRs20; (c) ZCRns
The SEM images Fig. 4 show significant modifications during the fly ashes processing. Most raw FACR particles are spherical with particles size from 7.94μm to 44.1μm while in zeolite materials large agglomerates (18.8 μm) are formed from mostly spherical and minority prismatic particles (3.47μm average) Fig.4. Surface compositions (Si, Al, Fe, Mn, Ti) of raw fly ash and of ZCRs20, ZCRns zeolites were evaluated using energy dispersive X-ray spectroscopy (EDX), Table 3.
Table 3. Surface compositions of raw fly ash and ZCRs20, ZCRns zeolites
The polar and disperse contributions to the surface energy of ZCRs20 and ZCRns were calculated according to the model developed by Owens, Wendt, Robel and Kaelble and are presented in Table 4.
Table 4. Surface energy data for the substrates
The data show a large polar component, recommending the zeolites materials as a good adsorption substrate. The hydrothermal processes developed in autoclave with NaOH solution include changes in various chemical and physical properties such as surface structure (roughness), crystal structure and cation exchange capacity.
The mechanism of zeolite crystallization and the role of alkali solution on the synthesis reaction were considered. Three steps exist in alkali hydrothermal reaction of zeolite NaP1 synthesis: (1) the dissolution of Si4+ and Al3+ in coal fly ash, (2) the condensation of silicate and aluminate ions in alkali solution to make aluminosilicate gel, (3) the crystallization of aluminosilicate gel to make zeolite crystal. The OH− in alkali solution remarkably contributes to the dissolution step of Si4+ and Al3+ in coal fly ash, while Na+ in alkali solution makes a contribution to the crystallization step of zeolite.
3.2. ADSORPTION OF HEAVY METALS ON ZCRs20 and ZCRns SUBSTRATES
The zeolite materials obtained from fly ash were tested for heavy metals removal. The experiments were done on synthetic solutions containing one cation Cd²⁺ or Cu²⁺, two cations Cd²⁺ + Cu²⁺ and two cations with Methylene Blue (MB + Cd²⁺ + Cu²⁺). The parameters of the process such as contact time, substrate’s dosage and initial cation’s concentration were optimized considering the maximum efficiency of heavy metals cations, Ƞ, on ZCRns and ZCRs 20, which was calculated using the following eq. (2):
Fig.6. Adsorption efficiency vs. contact time in solutions containing one heavy metal
b) The system containing two heavy metals cadmium and cooper was chosen because the most toxic heavy metal is cadmium and it accumulates in the human body.
Fig.7. Adsorption efficiency vs. contact time in solutions with two heavy metals
The same numbers of active sites are occupied simultaneously by different cations so there is a competition between Cu²⁺ and Cd²⁺ cations hydrated with 4…6 water molecules. In aqueous solutions hydrated heavy metal ions can be subject of hydrolysis, according to the reaction (1), written for divalent metals:
Fig. 8. Adsorption efficiency vs. contact time in three -pollutants system
The planar structure of Methyllen blue with two aromatic rings can act as an electron donor in the interaction with Cd2+ cation, specific. The alkali form of MB may act as a supplementary complexion agent, which adsorbed heavy metals by terminal group - SO₃⁻ . By increasing the substrate mass the adsorption efficiency is better because on the substrate exist more active sites available for adsorption. A sharp increase in the percentage uptake of cooper cations can be seen until a maximum value (97.79 - 99.51%). In adsorption process in all cases the cations prefer substrate ZCRs20 with the largest BET surface. The affinity of new zeolites obtained from this fly ash for heavy metals depends on many factors for example:
- the properties of substrate (composition in alumino-silicates (Si/Al), surface area, the electrical charge of surface, types of exchangeable ions) [24];
- the properties of cations (hydrated ionic radius, hydrolysis constant, electronegativity value of the metal;
- the properties of wastewater (solutions) (pH, pollutant concentration and presence of other compounds (dyes, surfactants etc.)
Adsorption is a surface process, as consequence, not only the surface morphology and crystallinity, but also the surface charge is important. The surface charge depends on the pH solution and it must be discussed based on the value(s) of the point of zero charge (PZC), Fig.9.
Fig.9. The PZC determination is represented
According to the Pourbaix diagram, the copper ions can exists in aqueous solutions as (CuOH)+ at pH > 6.8 and as neutral hydroxide as precipitate, at pH > 7.5 [34]. Thus, to avoid copper precipitation the experiments should run in weak acidic media or close to the neutral pH. the channels of the lattice and they can replace the exchangeable Na⁺ , K⁺ , Ca²⁺ cations. As evidenced in the previous step, on the surface of zeolite materials new types of active centres were developed (≡ SiO⁻), (≡ SiONa) and (≡ AlO⁻) complex structures were formed with metal cations according to reactions (2, 3). During the adsorption process the heavy metal cations have to move through the pores.
Heavy metals adsorption onto zeolites is an ion-exchange. Different stages are observed in the ion –exchange adsorption of the heavy metals: fast adsorption on the zeolite microcrystal surfaces during the first 30 min; then the inversion stage has a short-time prevalence of the desorption process connected with the diffusion flow from the zeolite microcrystal’s interior and is the moderate adsorption in the microcrystal’s interior [26].
3.3. THE ADSORPTION KINETIC
The metal uptake qe (mg/g) was evaluated for the kinetic studies by using the initial and current, t, heavy metal concentrations (ci cation and ct cation) in a given solution volume (V = 50 mL) for a given amount of ZCRns, respectively ZCRs 20 substrate (ms = 0.1 g) as given in eq. (3):
1. The pseudo first-order equation Lagergren 1989 [27]:
2. The pseudo-second order kinetic equation Ho Y.S, McKay 1999 [28]:
The interparticle diffusion is another possible kinetic model that can be applied in adsorption processes eq. (6) [29].
Table 6 Kinetic parameters of heavy metals adsorption
The pseudo-second order kinetics fitted the adsorption kinetics much better than the pseudo-first-order model for both adsorbents and supports the chemisorption process on these zeolite materials. Like in most other investigations, the adsorption capacity was the highest for Cu²⁺ for both zeolites. There are some cases where correlation coefficient R2 is not close to 1, which indicates departure from the pseudo first-order kinetics or intraparticle diffusion model. They are mainly in the case of cadmium, because the affinities of the adsorbents for nickel and cadmium are the smallest. On both zeolite materials the order is: Cu²⁺ > Cd²⁺ > MB which follows the order size of the hydrated ions [nm]: 0.29, 0.43 and 0.25.
3.4. ADSORPTION ISOTHERM
The adsorption isotherm data were experimentally obtained based on the optimized contact duration (90 min for ZCRns and 60 min for ZCRs20) and substrate amount (0.2 g ZCRns/ZCRs20 in 50 mL solution). The adsorption parameters were calculated considering the Langmuir and Freundlich equations (7) and (8) [32]:
1. the Langmuir isotherm – linearization:
2. the Freundlich isotherm – linearization:
Table 7 Adsorption isotherm parameters
The Langmuir model fits the equilibrium data much better than the Freundlich model for both adsorbents table 7. The Freundlich model can be fitted for cooper and cadmium on these substrates. Accordingly with data table the adsorption was rather homogeneous than heterogeneous and rather monolayer than multilayer.
The SEM images after adsorption of cations from bi-component solution present the competition between copper and cadmium cations. The percentages of cadmium and copper on surface scanned are: Cu (atom %) = 93.55 and Cu (atom %) =6.45.
Conclusions
Adsorption efficiency of cadmium, cooper cations is reduced compared to the adsorption efficiencies of mono-cationic systems, indicating the competition on similar active centers of powder zeolites (homogeneity substrates).
The competition between the different metal ions for surface sites of powder zeolites occurs and is dependent on the characteristics of the ions, substrate (crystalline phases, and morphological, surface area, changes), respectively.
The adsorption isotherms and kinetic data fitted well the Langmuir and pseudo-second order kinetic models, respectively. The order of affinities of ZCRs20 >ZCRns for the divalent metal ions reported is Cu2+> Cd2+ at optimized contact time 60 min. for ZCRs20 and substrate amount 4g/L multicomponent solution.
Three stages exist in alkali hydrothermal reaction of zeolite NaP1 synthesis: (1) the dissolution of Si4+ and Al3+ in coal fly ash; (2) the formation sodium aluminosilicate gel as zeolite precursor; (3) the crystallization of aluminosilicate gel to make zeolite crystal.
The zeolitic material ZCRs20 with high degree of crystallinity and specific surface area greater than ZCRns can be recommended for technologic adsorption process.
Two environmental problems are solved using zeolite materials obtained from fly ash: removing pollutants from wastewater and reducing huge amounts of fly ash.
Source: Maria Visa1, Popa Nicoleta2, Andreea Chelaru1
1 Transilvania University of Brasov, Center Renewable Energy Systems and Recycling
2 Department of Forest District Teliu, National Admin.State Forests Romsilva
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