Fly Ash Substrates for Complex Wastewater Treatment

World of Coal Ash (WOCA) Conference - May 9-12, 2011, in Denver, CO, USA http://www.flyash.info/

Fly Ash Substrates for Complex Wastewater Treatment Maria Visa1, Mihaela Nacu1 Radu Adrian Carcel1 1

Transilvania University of Brasov, RTD Dept. Renewable Energy Systems and Recycling, Romania, Eroilor 29, 500036 Brasov KEYWORDS: fly ash, wastewater, cadmium, copper, dyes adsorption

ABSTRACT Dyeing and finishing processes applied in the textile industry are among the most common source of surface water pollution. The waste water resulted in the dye industry contains a mixture of pollutants with variable structure, including dyes, heavy metals, surfactants. To remove these toxic compounds, the design of a treatment plant must combine adsorption on mixed substrates or adsorption with photo-catalysis or other advance treatment process. For understanding the complex process involving many pollutants, preliminary tests on binary (dye – heavy metal) and ternary (dye and two heavy metals) were developed and are presented. Experiments were done on synthetic wastewaters containing methyl orange, cadmium and copper, using alkali modified fly ash. The adsorption mechanisms are corroborated with the ash structure (XRD) and surface morphology (AFM). The dye, cadmium and copper adsorption isotherms on modified fly ash can be described using the Freundlich isotherm, proving the heterogeneous distribution of the active sites with different affinity on the substrate. The adsorption efficiency was studied on systems with different pollutant concentrations and the optimized conditions (contact time, amount of substrate for 50mL of pollutant) were evaluated in batch experiments using fly ash dispersions. Higher heavy metal removal efficiencies were obtained in dye containing systems, comparing to the systems without dyes proving that the dye acts as complexion agent. 1. INTRODUCTION Heavy metals and dyes are common pollutants, resulting from specific industries (electroplating, fertilizers, pesticides, pigments manufacturing and textile dye finishing) in huge amounts of wastewaters, with compositions ranging from tenth up to thousands of mg/L. For avoiding flora, fauna and health problems, the discharge limits are strict and require advanced wastewater treatment processes. Depending on the initial composition, many alternatives are proposed: adsorption, chemical precipitation, coagulation and flocculation, ion-exchange, reverse osmosis, membrane nanofiltration4, photo-degradation using wide band gap semiconductors:

TiO22, WO326 choosing one or a complex of solutions is subject of efficiency and cost analysis. Among these, adsorption technologies have several advantages: easy operation and well known technology, inexpensive equipments, less sludge, adsorbents’ reuse after desorption. For fulfilling the industrial requirements, low-cost adsorbents are intensively studied, mainly based on natural compounds or on wastes. Heavy metals (cadmium, cooper, zinc, nickel, iron) and dyes (methylen blue and methyl orange)23,24,25 removal was reported on inorganic oxides (natural zeolites11,19 , red mud20 resulted from the aluminium processing)18,6, wheat straw, or barley husks5. Organic artificial or synthetic wastes are also reported for these applications, like, bone char7 scrap rubber8, humus21 or bituminous coal. Ash, resulting from coal burning coal is a mixture of oxides with unburned carbon, with a predominant negative surface charge, having thus a good affinity for cations or polar substances/pollutants. The ash resulted from biomass (wood) burning has a larger amount of carbon but preserves the negative surface charge due to the oxides. Part of the oxides in fly ash are water soluble, therefore, washing the substrate before use is a prerequisite. Even so, the result is a rather heterogeneous surface with variable composition from one batch of fly ash to another, depending on the coal properties and burning conditions. Therefore, a further conditioning process must be developed to obtain a substrate with a larger degree of reproducibility. Previous studies proved the efficiency of alkali modified fly ash and optimized conditions were identified for cadmium and copper removal25. Previous studies investigated FA as substrate for heavy metals (cadmium, cooper, zinc, nickel) and dyes (methylen blue and methyl orange) removal, from mono, - bi or threecomponent (Cu2+, Cd2+, Ni2+ and MB) wastewaters, resulted from the dye finishing industry. The results showed that raw fly ash has limited efficiency, thus the surface of fly ash was modified with 1N, 2N, 4N NaOH solution25 with NaOH and Methylen Blue24 and with Methyl Orange23. The CET Brasov is a CPH plant which works with two boilers (420 t/h capacity) and two turbo-aggregates which generate 50 MW/each. Every year 6*105 tons of coal is burned and the fly ash resulted is 2*105 tons/year. Part of the ash is used in cement manufacturing but large amounts are still free and represent a major pollutions source. One possible application is to develop alternative processes (like wastewater treatment), involving the use of this waste. This paper presents a comparative study developed on fly ash (collected from the CET Brasov, Romania CHP), investigating the efficiency and optimization in removing heavy metals cations (cadmium and/or cooper) and methyl orange (MO) from multi-component solution.

The fly ash was conditioned by washing and by optimized alkali treatment and the efficiency of these two substrates is kinetically and thermodynamically discussed. 2. ADSORPTION EXPERIMENTS 2.1. THE SUBSTRATES We collected only fly ash, from the electro-filters (FA-ELF). Raw fly ash was analysed by S.C. Prospecting S.A. Bucharest, Roman using AAS (Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, MnO), gravimety (SiO2, SO3, PC) and colorimetry (TiO2 and P2O5) methods. The major oxides from fly ash with role in heavy metals and dyes removal are presented in Table 1. Using emission spectrometry other metals were also identified in small amounts (Ba, Cu, Sn, Pb, Cr, Ni, V, Zn, Ti). According to the ASTM standards, the raw fly ash (FA-ELF) pertains of class F because the sum of the SiO2, Al2O3 and Fe2O3 is above 70%14. This has as direct consequence the lack of agglomeration in water (and in any water environment with reduced CaOderivates content), with positive influence on the use of this powder as adsorbent. Table 1. Raw fly ash composition FA Composition [%] SiO2 Al2O3 Fe2O3 53.32 22.05 8.97 *LOI: loss of ignition

CaO 5.24

MgO 2.44

K2O 2.66

Na2O 0.63

TiO2 1.07

MnO 0.08

LOI* 1.58

The fly ash was washed in ultra pure water, by stirring 200 g fly ash with 2000 mL ultra pure water, at room temperature (20-250C), for 48 h, till the filtrate solution had constant values of pH (10.2), and conductivity (1.710 mS/cm) with a TDS value of 850 mg/L. The washed fly ash was dried at 105-1200C, till constant mass and this substrate is further denominated as FA-W. Part of FA-W was further conditioned by stirring 48 h in with NaOH solution 2N (mFA-W : VNaOH2N = 1:10 g/mL), followed by filtration washing and drying and this substrate was notated (FA-NaOH). Both FA-W and FA-NAOH were separately sieved and the 40-100µm fractions were selected as substrate for experiments. The FA crystalline structure was evaluated by XRD (Bruker D8 Discover Diffractometer) and for morphology studies the images AFM were take using (Ntegra Spectra, NT-MDT model BL222RNTE). The surface area of the fly ash was measured by using Analyser “Tri Star II 3020” – Micromeritics, Debaser “flow Prep 060”- micromerics.

2.2 ADSORPTION EXPERIMENTS Two series of tests were done on multicomponent solutions of Cd2+, 0…700 mg/L, (CdCl2, Scharlau Chemie), Cu2+, 0…320 mg/L, (CuCl2, Scharlau Chemie), Methyl orange (MO) C14H14N3SO3Na, Merck), 0 - 1.999 mmol/L. The first set of experiments used FA-W as substrate and the second set used FA-NaOH. In both applications, the mixtures were stirred up to 240 min at room temperature, then the substrate was removed by vacuum filtration and the supernatant was analyzed. Heavy metals concentration was measured using AAS (Analytic Jena, ZEEnit 700), at λCd = 228.8nm, λCu = 324.75, while MO quantitative analysis was done by uv-vis spectroscopy at λmax = 464 nm (Perkin Elmer Lambda 25). 3. RESULTS AND DISCUSSION 3.1. THE SUBSTRATES The silicon-alumininous composition in the ash was confirmed by XRD spectra, Fig. 1. The major components of both ashes are: carbon (graphite), SiO2 (quartz) combined with Al2O3, (as rhombo H, mullite (3Al2O3.2SiO5), cristobalite and γ-Al2O3), hematite (Fe2O3) and MnO2 (ramsdellite), along with bixbyite (Mn2O3), TiO2 brookite. The traces of Cd2+, (0.00166 mg/l), and Cu2+ (0.00033 mg/L) were founded in washed water of fly ash. The unburned carbon graphite, and carbon hexagonal (chaoite or white), along with compounds as micro-sized crystallites represents a significant part of the FA and can explain the versatility of this material in adsorption processes of heavy metals, dyes and surfactants10.

αSiO2quartz Cgraphit

9000 8000

FA-NaOH FA-W

Intensity [a.u.]

7000

maghemite γ Fe2O3

6000

Hematite

Al2O3γ,ε

SiO2cristobalite

5000 4000

(2)

Cchaoitehexagonal Cchaoitehexagonal

3000

Al2O3γ,ε

2000 (1)

1000 20

30

40

50 2 theta [degree]

60

70

80

Fig. 1 XRD of raw FA-W and modified with NaOH 2N (FA-NaOH) The FA-NaOH specific surface is 10.95 m2/g, and has an average pore diameter of 25nm. During long time washing under stirring in ultra pure water, the soluble alkaline oxides, (K2O, Na2O, MgO, CaO) from raw fly ash were removed from the substrate in the solution, thus the pH raised up to 10.2, conductivity up to 1.710 and TDS up to 850, and a new surface appears with various roughness values, Fig. 2, good for adsorption heavy metals and dyes. This is confirmed also by the roughness decrease, ranging from 72.7 nm (FA-W), down to 30.5 nm (FA-NaOH). The changes in morphology are evident and the surface “leveling” is obvious the result of preferential dissolution of the high energy sites. Based on the results presented by Otero et al17. AFM data were used to get supplementary information about the FA pore association, density and size distribution, Fig. 3. The surface data show large mesopores, which are filled during the dye adsorption, Fig. 3. Still, the roughness after adsorption is increased, supporting the idea of preferential adsorption on corners and edges. One may conclude that there are adsorption sites with different affinity for the pollutants, both on FA-W and on Fa-NaOH. This effect is more pronounced on FA-W, with very large pores and, consequently, with lower specific surface.

(a) FA -W Average Roughness: 72.7nm

(b) FA - NaOH Average Roughness: 30.5 nm

(c) FA - NaOH after adsorption Average Roughness: 98.9 nm

Fig. 2 The AFM topography and average roughness

FA-W FA-N aO H after adsorption

Counts [a.u.]

Counts [a.u.]

FA C ET N aOH FA C ET N aOH M O

0

50 100 150 200 250 M e s o p o re s d ia m e te r [n m ]

300

a) FA-NaOH before and after adsorption

0

250

500

750

100 0

M eso p ores d iam eter [n m ]

b) FA-W before and after adsorption

Fig. 3 The pore size distribution 3.2. UPTAKE KINETICS OF THE HEAVY METALS AND METHYL ORANGE For metal removal kinetics studies, the metal uptake qe (mg/g) was evaluated by using i t the initial and current heavy metal concentrations ( ccation and ccation ), in a given solution volume, V, for a given amount ms of FA (eq. 1):

qe =

i t ( c cation - c cation )×V ms

(1)

Kinetics of heavy metals adsorption was modelled by the follows equations: - pseudo first-order Lagergren equation13, KL t log (q0 – qt) = log (q0) 2,303 −

(2)

where KL is the Lagergreen constant, and qt the metal uptake at moment, t. - pseudo-second order reaction model, developed by Ho and McKay9 :

t 1 t = + q t k 2 q e2 q e

(3)

where k2 is the equilibrium rate constant for the pseudo second-order adsorption (g mg-1min-1) and can be evaluated from the slope of the plot. While the first model corresponds to a substrate with very high affinity for the adsorbed species, the second one describes a process where the concentration of both participants (adsorbent – adsorbed specie) is of the same order of magnitude and

supports the idea of binary interactions. The model is mentioned in literature for heavy metals24 including Cu2+. A third adsorption mechanism is likely on porous substrates, the interparticle diffusion model, described by1.

q = k id t 1/2 + C

(4)

Good results were previously reported by22, in pollutants removal from binary component solutions, when the most likely model proved to be the pseudo-second order kinetics: (Cd2++MO /FA-NaOH 2N), with qe = 16.921 [mg/g] and k2 = 0.0972 [g/mg min] and (Cu2+ MO /FA-NaOH 2N) with: qe = 14.880 [mg/g] and k2 = 0.1046 [g/mg min]. These data show that the systems containing MO have almost constant kinetic parameters for heavy metal removal, as consequence of the uniform surface resulted from the dye adsorption on FA. The adsorption processes of pollutants from more component systems (three in the experiments here presented) can be well fitted using the pseudo second rate, for all the investigated systems. The results are presented in Table 2. Table 2 Kinetic parameters of the heavy metals and MO adsorption a Pseudo first-order Pseudo-second Interparticle diffusion reaction rate rate model FA-W KL k2 q 2 2 R2 Parameter R [min R [g/mg . e Kid C [mg/g] 1 ] min] Cd/(Cd + Cu+ MO) 0.347 0.924 3.285 5.624 0.835 Cu/(Cd + Cu +MO) 0.914 0.017 0.992 0.936 8.264 0.909 0.191 5.545 MO/(Cd +Cu+MO) 0.663 0.999 7.622 0.196 0.534 FA-NaOH Cd/(Cd + Cu+ MO) 0.971 0.011 0.175 0.979 2.194 8.140 Cu/(Cd +Cu + MO) 0.969 0.031 0.995 0.354 23.585 0.930 1.227 9.860 MO/(Cd + Cu+MO) 0.408 0.998 266.7 0.008 0.141 The data show that the adsorption capacity of FA-W and FA-NaOH for heavy metals from the multi-component systems is decreasing, comparative to the correspondent data for systems involving two pollutants, as result of competitive adsorption. Parallel mechanisms, of first order and interparticle diffusion are likely for copper adsorption. These processes are independently running, with lower rate and could be

the result of the higher copper mobility, due to its lower hydration number (4), comparative to cadmium (6) 12. An increase in the substrate capacity for copper adsorption was registered on Fa-NaOH, supporting the assumption of dye’s (stringer) bonding on the FA surface16, and suggesting that the ratio of ionization potential to ionic radius (strongly influencing the hydration number) can be suitable criteria for evaluating the relative sorption capacity. The kinetic data were obtained using a substrate: pollutant solution ratio of 1:10 g/mL. 3.2. OPTIMIZING THE ADSORPTION EFFICIENCY ON FA The Cd2+, Cu2+ and MO adsorption efficiency, η, on the two substrates was evaluated to optimize the contact time and the amount of dispersed FA in a given volume of wastewater, and was calculated with equation (8):

η=

i e (ccation − ccation ) × 100 i ccation

(8)

e

i

where ccation and c cation are the initial and equilibrated cation concentrations (mg/L), respectively mMol/L for MO.

100

100

80

80 2+

Cu (M)/ FA-NaOH 2+ Cd (M)/ FA-NaOH MO (M)/ FA-NaOH

60

40

Efficiency [%]

Efficiency [%]

The dynamic adsorption results are presented in Fig. 4a and 4b for the cadmium, cooper and MO, adsorption on FA-NaOH, compared with the results obtained using FAW as adsorbents.

2+

Cu (M) /FA-W 2+ Cd (M) /FA-W 2+ MO (M) /FA-W

60

40

20

20

0

0 0

20

40

60

80

100

120

140

160

180

Time [min]

Fig.4 a Efficiency immobilization vs. contact time

0

20

40

60

80

100

120

140

160

180

Time [min]

Fig. 4 b Efficiency immobilization vs. contact time

100

90

90

80

80

70

70 Efficiency [%]

Efficiency [%]

100

60 50 40 30 2+

Cu (M) /FA-NaOH 2+ Cd (M) /FA-NaOH MO (M) /FA-NaOH

20 10 0 0.0

0.5

1.0

1.5 massa [g]

2.0

2.5

60 50 40 30

10 0 0.0

3.0

Fig. 5a Efficiency immobilization, vs. mass of FA

2+

Cu (M) /FA-W 2+ Cd (M) /FA-W MO (M) /FA-W

20

0.5

1.0

1.5

2.0

2.5

3.0

massa [g]

Fig. 5b Efficiency immobilization, vs. mass of FA-W

The low adsorption efficiency of Cd2+ and Cu2+ cations on FA-W, comparative with the efficiencies on FA-NaOH, show that during the first hour of contact with substrate a share of pores are occupied with methyl orange molecules, as also supported by the AFM pictures and the average roughness values. The higher adsorption efficiency of copper vs. cadmium confirms the idea of the diffusion control of the process, which favors the cation with lower volume and higher mobility. Further, it may be concluded that in such complex systems, containing (two) cations and a dye, there are expected only average adsorption efficiencies of the heavy metals but very good results can be expected for dyes removal. The rate of adsorption is also depending on the amount of fly ash. While the dyes requires rather low amounts of ash, for efficient heavy metal removal (η> 90%) the amount of FA should rise up to 3g/50mL solution. The results are presented in Fig.5a and 5b. By increasing the amount of adsorbent, the initial pH increases up to 5.8 (bellow the precipitation value) while the adsorption efficiency of the heavy metals increases due to a larger amount of active centres in the system. The recommended parameters for simultaneous removal of the heavy metals and dyes are: (a) the FA-NaOH substrate: mFA-NaOH = 1.25 g for 50 mL solution; contact time: 60 min (b) the FA-W substrate:. mFA-W = 1.5 g for 50 mL solution; contact time: 90 min 3.3. ADOSRPTION THERMODYNAMIC STUDY The adsorption isotherm data were experimentally obtained after optimising the adsorption processes and the absorption parameters were calculated considering the Langmuir and Freundlich equations, respectively, as given below27.

The Langmuir isotherm - linearization: C eq q eq

=

1 qmax ⋅ a

+

C eq

(9)

qmax

Where qmax (mg/g) represents the monolayer adsorption capacity, is a constant related to the free energy of adsorption, qeq is the amount of metal adsorbed from a solution with the equilibrium concentration, Ceq. The Freundlich isotherm - linearisation:

1 ln q eq = ln k F + ln C eq n

(10)

Where kF is Freundlich adsorption constants, being an indicator of the adsorption capacity and 1/n dimensionless parameter is a measure of the adsorption density. Table 3 presents the adsorption parameters for the heavy metal ions (Cd2+, Cu2+ and MO) calculated from the slope of the linearization plots of the two isotherm equations. Table 3 Adsorption isotherm parameters Langmuir Isotherm Parameters FA -W Cd/(Cd + Cu+ MO) Cu/(Cd + Cu +MO) MO/(Cd +Cu+MO) FA_NaOH Cd/(Cd + Cu+ MO) Cu/(Cd + Cu +MO) MO/(Cd +Cu+MO)

qmax

a

[mg/g]

[L/mg]

7.502 7.236 8.968-

Freundlich Isotherm R2

n

KF

R2

0.017 0.015 -

0.999 0.917 0.196

2.060 5. 201 1.242

0.00006 0.002 0.001

0.972 0.903 0.903

0.002 -

1 0.737 0.196

1.928 2.705 0.926

0.037 0.167 1.878

0.969 0.960 0.957

The data show that the Freundlich model could better describe the adsorption on FANaOH and FA-W as result of heterogeneous substrates resulted after washing and after treating with the NaOH 2N modifier. The higher surface energy on FA-W, confirmed by the higher surface roughness allows strong interactions with the heavy metals, thus the Langmuir model can also apply. Methyl orange, a significantly less ionic molecule, although adsorbs well, does not exhibit chemi-sorption and the Langmuir model is not suited for describing this process. The Langmuir model could well describe the adsorption of cadmium and cooper ions

attendance MO with bill of the homogenous of the substrate. The cadmium and copper behavior is quite different, again supporting the diffusive control that allows a larger amount of copper to be linked on the substrate. CONCLUSIONS Fly ash was investigated as substrate for complex adsorption processes, involving tricomponent pollutant system: two heavy metals (copper and cadmium) and a dye (methyl orange). The washed fly ash has a rough surface with a large amount of high energy sites. By further alkali treatment the surface becomes smoother with a larger amount of small meso-pores. These changes result in a controlled adsorption affinity. The substrates proved to be highly efficient in the dyes adsorption: methyl orange adsorbs fast and forms a rather homogeneous substrate. Therefore, the adsorption conditions were optimised considering the heavy metals uptake and are presented in the paper. The adsorption process of heavy metals in the experimental conditions is significantly controlled by diffusion and the low volume – high mobile copper cation has a faster adsorption rate, comparing to cadmium. The data show that, if optimized, fly ash can be a suitable substrate for efficient adsorption of a complex pollutants load from wastewaters. Considering its low cost, upscaling will be further studies. AKNOWLEDGEMENTS This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Found and by the Romanian Government under the contract number POSDRU ID 59323. REFERENCES [1] Allen S.J., McKay, G., Khader, K.Z.H, Interparticle Diffusion of a basic dye during adsorption onto sphangum peat, Environment Pollution, 56, 1999, pp.39-50. [2] Andronic, L.,Duta, A., The influence of TiO2 powder and film on the photodegradation of methyl orange, Mat. Chem. and Phys., 112, 2008, pp. 1078-1082. [3] Banerjee S.S., Joshi M.V., and Jayaram R.V., Removal of Cr(VI) and Hg (II) from aqueous solutions using fly ash and impregnated fly ash, Sep. Sci. and Techn, 39, (7), 2004, p. 1611. [4] Bowen, R.W., Welfoot, J.S., Modeling of membrane nanofiltration – pore size distribution effects, Chemical Engineering Science, 57, 2002, pp. 1393 – 1407.

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