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Recovery of iron hydroxides from electro-coagulated sludge for adsorption removals of dye wastewater: Adsorption capacity and adsorbent characteristics
Surfaces and Interfaces 18 (2020) 100439
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Recovery of iron hydroxides from electro-coagulated sludge for adsorption removals of dye wastewater: Adsorption capacity and adsorbent characteristics Tadele Assefa Aragaw
T
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Faculty of Chemical and Food Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar, Ethiopia
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro-coagulated Sludge Adsorbent DR28 Iron Hydroxide Iron Oxide Isotherm
The aims of this research work was to investigate the potentials of raw and calcined iron hydroxides/oxides sludge adsorption for DR28 dye removal. The adsorbent is prepared from electro-coagulated (EC) sludge in industrial wastewater treatment plant, as a Nobel adsorbent for decolorizing direct red 28 dye (DR28). The EC sludge adsorbent were prepared with soaking processes and calcination in a range of temperature. The surface properties of raw and calcined EC sludge adsorbent were examined using Zeta Potential, XRD and FTIR. Basic (effect of solution pH, temperature, initial dye concentration) operation parameters were examined for raw and calcined EC sludge adsorbent. The diffraction patter suggests that the crystalline hematite are produced during calcination and increases its intensity as temperature increases. Langmuir and Freundlich Equation were used to model the adsorption equilibrium; and pseudo first and second order Equation were used to model the adsorption kinetics. The results suggested that the adsorption of DR28 dye is chemisorption, the interaction between the adsorbent surfaces with dye is strong. The Langmuir adsorption capacity of DR28 dye with raw and calcined EC sludge adsorbent was calculated as 1.262 mg/g and 1.252 mg/g respectively with experimental conditions: mixing time=20 min, adsorbent dosage=0.5 grams, initial dye concentration= 20 mg/L and solution pH =2 at ambient temperature. The adsorption kinetics model data were consistent with the pseudosecond-order. The removal efficiency of 97% was recorded at pH 2, ambient temperature 20 mg/l concentration and 1 g/100 ml for 1 hour. High direct red 28 dye uptake capability and cost-effectiveness of sludge utilization from textile wastewater treatment plant make it potentially attractive for dye removal.
1. Introduction The textile factory is producing a huge amount of dye-containing wastewater which is non-biodegradable organic compounds. Recently, the environmental impact of dye wastewater from textile, paint, and leather industries are required special attention to the treatment of azo dye. To eradicate this problem and reuse the electro-coagulated (EC) sludge, adsorptions of this kind of wastewater are important into natural effluents. Different dye contaminant removal such as physical, chemical and biological methods have been developed for the last decays. So far, the treatment of textile wastewater in Ethiopia has been based mainly on aerobic biological processes followed by chemical coagulation. Chemical and biological methods are effective for removing dyes, but they require specialized equipment and are quite energy-intensive; also, a large amount of byproduct is often generated that creates a sludge
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disposal problem [1]. With the high cost and secondary sludge production problems involved in the above methods, further investigation of different adsorbents, non-conventional, for the adsorption process including waste sludge utilization is attracting scholar's attention. The author of this work previously has been done promising low-cost none conventional adsorbent from Ethiopian kaolin and also synthesized zeolite with hydrothermal methods for dye and hardness removal respectively [2,3]. Also finding another good low-cost and non-conventional adsorbents contribute to the sustainability of the environment [4] and also offer promising advantages with sludge waste management for commercial purposes in the future. Carbon based, biochar, materials as an adsorbent for the removal of organic pollutants especially azo dyes attracts attention now a days due to its diversified functionalities and have high potentials for contaminant removal in the wastewater steams. A developments of biochar-based composite with a foreign materials like magnetic-biochar,
Corresponding author. E-mail address: [email protected].
https://doi.org/10.1016/j.surfin.2020.100439 Received 17 December 2019; Received in revised form 9 January 2020; Accepted 11 January 2020 Available online 13 January 2020 2468-0230/ © 2020 Elsevier B.V. All rights reserved.
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2. Materials and methods
nanometallic oxides/hydroxides-biochar and layered nanomaterial coated-biochar; and also activation of biochar with different chemicals are well studied to produce a high removal capacities of the material with a range of organic contaminants [5]. Also a composites of graphite and layered hydroxides nanomaterial sorbents for water treatment have a higher capacities due to their distinctive surface and structural properties. This materials have special removal capacities for heavy metal ions, radionuclides and azo dyes due to their large surface area and ion exchangeability [6].Thus, the iron oxides/ hydroxides from the EC sludge, after treatment, can be used as hybrid with carbon based adsorbents or/and directly as metal oxides as an adsorbent for different contaminant removal. Electrocoagulation wastewater treatment systems have been broadly used for different technological applications [7]. Due to the non-biodegradable natures of the dye wastewater, the textile wastewater treatment plant uses the electrocoagulation unit process and has huge sludge production. The electrocoagulation treatment processes in dye-containing wastewaters from the textile industry is a promising technique for not easily biodegradable compounds [8] but sludge production is still a problem as solid waste management. Thus utilizing the electro-coagulated sludge for adsorptions of wastewater is promising research in sludge management as well as resource recycling point of view. The primary interphase by-products of wastewater treatment in electrochemically induced by iron/steel electrodes are mainly ferrous hydroxide Fe (OH)2 and ferric hydroxide Fe(OH) [9] and oxidative transformation with calcination produces iron oxides as shown in Fig. 1. Especially hematite at higher temperature which have a crystalline nature. In addition to the pure iron hydroxides, oxy-hydroxides (goethite, α-FeOOH, and lepidocrocite, γ-FeOOH,) can also be formed and removed as a sludge in the electrocoagulation processes [10,11]. Thus to utilize the EC sludge for dye wastewater removal and preparing an adsorbents is attractive and promising interest. Saying this, the present study forestalls to remove direct red 28 dye as a modal azo dye with recycled EC sludge adsorbent. The FTIR spectra, XRD diffraction and zeta potential properties of the adsorbent are well discussed. Effect of basic adsorption parameters (pH, temperature, contact time and initial dye concentration); and adsorption isotherms, kinetics, and thermodynamics were carried out. The main objectives of the research are to study the adsorptions capacity of EC sludge, raw and calcined, for direct red 28 dyes which considers as a modal azo dye in the textile industries and adsorbent characteristics.
2.1. Chemicals and equipment Hot air oven, jaw crusher, disk mill, sieves, electronic balance, muffle furnace, pH meter, hot plate with a magnetic stirrer, measuring cylinder, test tubes, pipette, what man filter paper and centrifuge were used during adsorbent preparation and batch adsorption experiment. Also powder of dyes, concentrated sulphuric acid, sodium hydroxide, and potassium bromide chemicals, without purification, were used. 2.2. Adsorbent and adsorbate preparation 2.2.1. Sample collection and physical treatment Sludge sample was collected from Bahir Dar Textile factory wastewater treatment plant, Bahir Dar, Ethiopia. It was dried with an oven at 70 °C, ground and milled to passes less than 200 µm sieve using jaw crusher and disc mill. The desired product was used for wet treatment and oversize products have been returned to the crusher. For the separation of solid sludge from grease, soluble salts, organics to meet the needs of dye removal adsorbent; the beneficiation process was used. Ground and milled sludge were dispersed in de-ionized water for 24 h and bunged to separate all floatable oils, specks of dust and dissolved solid particles. With decanting the supernatant, the slurry was washed until the unstable suspended particle removed. The suspension after beneficiation were dried at 70 °C to remove absorbed water. Forward after, the beneficiated and dried solids were ground and milled for thermal treatment. 2.2.2. Thermal treatment The high temperature oxidative transformation of iron oxides and hydroxides are followed a range of temperature dependent phase interchanges from magnetite to hematite or/and maghemite [12–16]. Temperature dependency oxidative transformation stages of iron oxides formations are as follows [9]: (1) Temperature range 200- 400°C; 4 Fe3O4 +O2 → 6 γ-Fe2O3 (2) Temperature range 375 -550°C; 6 γ-Fe2O3→α-Fe2O3 (3) 550°C and above 4 Fe3O4 +O2 → 6 α-Fe2O3 The wet treated and dried powder was used for thermal treatment at 100, 300, 500 and 800 °C for 3 h in a muffled furnace to remove moisture and organic from the sludge. Also to transform goethite (FeOOH), which is precipitated during electrochemical effluent treatment to hematite and maghemite, α-Fe2O3 and γ-Fe2O3 with thermal oxidation.
Fig. 1. Photographic representations of (a) raw (b) calcined at 500 °C electro-coagulated (EC) sludge. 2
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Fig. 2. Calibration curve for the standard solution of Direct Red 28 Dye.
Where Co and Ct (mg/L) is the initial and concentration at time t of the dye in the solution, respectively.
2.2.3. Adsorbate preparation and analytical methods Modal dye, direct red 28, which can be used from the dye industry were taken from the commercial market. Stock solutions were prepared with 0.5 grams of powdered dye into 1 liter of double distilled water making 500 mg/L solution. Working and standard solutions (10, 20, 40, 60, 80 and 100 mg/L) were prepared with analytical methods. To know the maximum wavelength, λmax, and absorbance of the dye was taken and scanned from 200 to 700 nm using UV/VIS spectrometer (PerkinElmer Lambda 35) and it was found that 498 nm. To calculate the final dye concentration from each batch adsorption experiment, a calibration curve was prepared. The linear calibration curve as shown in Fig. 2 was used as a basis for determining the dye concentration variation as a result of the dye adsorption process for each experiment.
qe =
V(Co − Ce) m
(2)
qt =
V(Co − Ct ) m
(3)
Where qe and qt are, the amounts adsorbed (mg/g) at the equilibrium and at any time t, respectively. Co,Ce and Ct are the concentration of the dye in the solution (mg/L) at the initial, equilibrium and at time t, respectively; V is the volume of the solution (L), and m is the mass of the adsorbent (g).
2.3. Experimental design and description 2.4. Adsorbent characterization In a conical flask of 250 ml, working solutions with the desired concentration and 1 gram of EC sludge adsorbent were added and agitated with a magnetic stirrer on a digital hot plate at 200 rpm. All batch adsorption experiments were conducted using raw and calcined (500 °C) EC sludge adsorbent. Because calcination at 500 and 800 °C have a similar feature as confirmed from XRD diffraction patterns, I prefer only one calcined EC adsorbent throughout the experiment. The pH of the solution was adjusted with inorganic acidic/basic solution (1 molarity of HCl and NaOH) before adding the adsorbent. The batch adsorption experiments were performed as initial dye concentration (20, 40, 60, 80 mg/L), temperature (ambient, 25 ± 2, 40, 60 and 80 °C), solution pH (2, 4, 6, 8, 10) and mixing time (10, 20, 40, 60, 80 and 100 min) at 0.5 grams of adsorbent. At the end of each experiment, 15 ml of the solutions were withdrawn at a predetermined time and separate the clear supernatant using centrifuge (SIGMA 3-18 KS) at 4000 rpm for 10 min. The absorbance after adsorption was measured using UV/Vis spectrometer (PerkinElmer lambda 35) for each run. The final dye concentration was calculated from the calibration curve and the removal efficiency was calculated as Eq. (1) [17]. The equilibrium state concentration (loading) of adsorbate in the solid phase (qe, mg/g) and concentration (loading) of adsorbate in the solid phase at any time (qt, mg/g) were determined as Eqs. (2) and (3) respectively [18].
Removal Efficiency (%) =
(Co − Ct ) ×100 Co
FTIR Spectroscopy was used to determine the functional groups in the raw and calcined (500 °C) EC sludge adsorbent and also after DR28 dye adsorption. The analysis was done at a wave number range of 400–4000 cm−1 using JASCO 6600 typeA spectrophotometer. 1:100 gram sample to KBr ratio were grounded uniformly using mortar and pestle, and pelletized and absorption spectra was recorded. Zeta Potential Instrument (Malvern Zetasizer Nano ZS series) was used at different pH values to determine the surface charge nature of raw and calcined (500 °C) EC sludge adsorbent. The values were recorded at (acidic media) pH 2, pH 4, pH6 and (basic media) pH 8 and pH 10. The crystalline and phase change of the raw and calcined (100, 300, 500 and 800 °C) EC sludge were measured using X-ray diffraction (SHIMADZU, MAXima_X XRD-7000) with CuK radiation, voltage of 40.0 kV, with continuous scanning range of 10-80.
2.5. Isotherm, kinetics and thermodynamics study 2.5.1. Adsorption isotherm model Langmuir and Freundlich isotherm models were used to determine the relationship between dye ions from the solution adsorbed on EC adsorbents and remaining in the solution. The adsorption isotherm with Langmuir and Freundlich model was governed as the equation in Eqs. (4) and (5), respectively [19,20].
(1) 3
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Ce C 1 = e + qe qm qm KL
logq e = logkf +
thermal treatment, after adsorption with the calcined EC sludge is due to adsorbed carboxyl and amine function group in the dye molecule. The bands appearing at 450–650 cm−1 after calcination belong to the Fe-O stretching vibration from Fe2O3 [23], indicating that iron oxides (hematite or/and maghemite) are formed. The band corresponding to Fe-O stretching mode of Fe2O3 is shown at 576 cm−1 having a sharp (high) intensity in calcined than raw sludge suggesting that the FeOOH were oxidative transformed to iron oxide in a calcination treatment. The band appears at 1097 cm−1 is typically metal hydroxyl (M-OH) stretching vibration from raw EC sludge but disappears from calcined one due to oxidative transformation [26,27]. and Most Azo N=N stretching bands occur between 1580 and 1400 cm−1 which are shown in raw EC but disappeared from thermally treated. Also absorption bands at 3424 cm−1 which may be attributed to the -NH2 group, which overlaps with the stretching vibration of the OH groups. Also, the presence of the peak located around 2900 cm−1 corresponds to C-H vibrations in the raw EC sludge tells that there is adsorbed organic carbon from the sludge but disappear from calcined one [28,29]. Thus, thermal treatment of EC sludge is important for good adsorptions by removing the organic carbon which was embedded during electrocoagulation treatments of wastewater. Also, the peaks at 1430 cm−1 are disappeared and at1622 and 2340 cm−1 are insignificant in the calcined EC sludge. This is due to organic carbons in alkane; carboxyl and ester functional groups have decomposed during thermal treatment at 500 °C and the electrostatic interaction mechanisms with DR28 dye increases. This leads the adsorbent has good adsorption capacity as compared to raw EC sludge with having a large surface area and surface charge nature. As the pH of the reaction from the system decrease, EC sludge expected to have iron oxide and hydroxide appears positively charged which favors the adsorptions of direct red 28 and have higher zeta potentials which is anionic dye molecule. This is due to the electrostatic attraction between the positively charged adsorbent surfaces with the anionic dye molecule. As a result, at high pH, the uptake of direct red 28 dye, anionic dye, was low as shown in Fig. 4. Calcined EC sludge has high zeta potential value at pH 2 than raw EC sludge. This is because raw EC sludge has organic contaminant from the textile effluent as well as there is no transformed iron oxide which has high charge and magnetic nature. This can also be confirmed raw EC sludge is black but the calcined EC sludge is a red pigment powder as can be seen in Fig. 1. The red powder is an indication of Fe2O3 that has been produced by oxidative transformation of FeOOH during calcination. The x-ray diffraction patterns recorded for raw and calcined (100, 300, 500 and 800 °C) EC sludge from electrochemical unit processes of textile wastewater treatment plant are depicted in Fig. 5. The major diffraction intensity corresponds to α-FeOOH recorded at 2Ɵ = 21.3°, 30.2° and 35.5° in raw and calcined at 100 °C EC sludge. The XRD pattern of the this nanomaterial displays five major characteristic peaks situated at 2θ values of 30.1°, 33.3°, 35.7°, 40.7°, 43°, 49.6°, 54.1° and 62.5° that indicates the inverse spinel iron ferrite (Fe3O4) structure [30,31]. A sharp peak shown at 2Ɵ = 24.1° for calcined at 800 °C EC sludge tells that a crosstie pattern. Alternatively, the weak peaks at 2Ɵ = 32.2 and 33.3°, 40.7°, 49.6°, 54.1°, 57.7°, 62.5° and 64.2° are the characteristics to α-Fe2O3 structure and the intensity to this peaks becomes sharpen as the treatment temperature increases from 300 to 800 °C due to oxidative formations of α-Fe2O3. Dominantly, the hematite type of iron oxide peaks are high and sharp which are similar reports by [32]. The XRD diffractgram in Fig. 5 obtained after calcination with a range of temperatures illustrate that iron oxy-hydroxide, goethite (FeOOH), is disappeared after 300 °C and completely transformed to hematite up to 800 °C. But the presence of short peak at 2Ɵ = 57.7° is corresponds with maghemite and magnetite, suggests that it were not fully crystalline up to 800 °C. As the temperature increases, the hematite iron oxides have intense counts were recorded which thus confirmed the absence of any impurities. Also from the electro-coagulated metal hydroxide sludge in the
(4)
1 logCe n
(5)
Where qm is sorption capacity (mg/g), KL is sorption energy (L/g), Kf and n are the Freundlich model constants and indicating the relationship between sorption capacity and sorption intensity, respectively. If n = 1, n > 1, and n < 1, then the sorption process would be the linear, physical, or chemical in its nature, respectively. 2.5.2. Adsorption kinetic model The kinetic model was used to determine reaction kinetics from batch adsorption experiments of adsorption mechanism as a dependency of mixing time. Adsorption kinetics for pseudo-first and second-order models have been governed as a linear equation in Eqs. (6) and (7) respectively.
log(QE− qt) = logq e − t = qt
1 + k2q e2
k1 t 2.303
(6)
1 t qe
(7)
2.5.3. Thermodynamic behavior The thermodynamic property can be used for exothermic and endothermic reactions in the adsorption processes in terms of Gibb's free energy, enthalpy, and entropy. The value of standard change Gibbs free energy, enthalpy, and entropy can be determined with Eqs. (8) and (9) [19]. An exothermic reaction in thermodynamics of the adsorption process articulates as an increase of the atomic or molecular species release from adsorbent solid surface and re-enters to the liquid phase of adsorbate [21,22]. (8)
ΔG0=ΔH0 −TΔS0 lnK c =
−ΔG0 = ΔS0/R − ΔH0/RT RT
(9)
K c = q e/Ce
(10) −1
Where ΔG =standard change free Gibbs energy (kJ mol ), ΔH0= standard change enthalpy(Jmol-1), ΔS0= standard change entropy (J mol-1K-1) and R = universal gas constant (8.314 J mol−1K−1). Kc, the equilibrium constant, represents the ability of the adsorbent to retain the adsorbate and extent of movement of the adsorbate within the solution. Kc is the ratio of the equilibrium concentration of the dye (qe) adsorbed to adsorbent compared to the van't Hoff equation as equilibrium dye concentration in solution (Ce) [23], and can be deduced from Eq. (10). 0
3. Result and discussion 3.1. Surface and crystalline properties of the adsorbent FTIR spectra of raw and calcined (500 °C) EC sludge and after adsorption of DR28 dye with spectral scanning range 4000-400 cm−1 are depicted in Fig. 3 (a) and (b) respectively. Bands appearing at 3424, 2900, 1622, 1430, 1097 and 872 cm−1 were the stretching vibrations of eOH, which is assigned to surface OH groups of Fe3O4, C-H stretching, the asymmetric and symmetric bending vibration in C=O stretching in the carboxyl functional group [24], N=N stretching bands, C-O stretching and N-H deformation in the amines respectively from the raw Electro-coagulated (EC) sludge. The adsorption peaks at 3439 cm and 1632 cm−1 after heat treatment is also attributed to the water of hydration; the bending frequency of surface hydroxyl group was observed at 1392 cm−1 which are similar with the reports by [25]. A small peak appeared at 881 cm−1 and 1050 cm−1, which where disappear in 4
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Fig. 3. FTIR spectral analysis of (a) raw and calcined at 500 °C EC adsorbent (b) before and after direct red 28 dye adsorption.
calcined EC sludge at 500 °C were used due to calcination at 500 °C and 800 °C have similar surface and crystalline properties. A preliminary experiments on the effect of adsorbent was carried out at 0.1, 0.5, 1.0, 1.5, and 2.0 grams at solution pH of 2, ambient temperature, mixing time of 20 min and initial dye concentration of 20 mg/L and the optimum values were recorded at 0.5 g/ 100 L having 97.67% and 92.1% for calcind and raw EC adsorbent respectively. Thus 0.5 g/100 L adsorbent dosage were used as a constant throughout the experiments This is because examining the adsorbent dosage that can be enough in the available sorption surface for the desired adsorbate concentration is very important [34,35]. As can be seen in Fig. 6 (a) and (b) as initial dye concentrations are increased from 20 to 80 mg/L, the percentage of dye removed is being decreased with mixing time from 10 to 100 min at a solution pH of 2, ambient temperature (25 ± 2 °C) and adsorbent dosage of 0.5 g/100 ml of calcined and raw EC adsorbent respectively. This is due to the saturation of the available active sites of the adsorbents. At a low concentration, there will be unoccupied active sites on the surface of the adsorbent, and when the initial dye concentration increases, the active sites required adsorption of the dye molecules will be occupied [36]. Comparatively, the calcined EC sludge adsorbent have more efficiency (97%) than raw EC sludge adsorbent (92%) due to their surface and crystalline properties as described from Section 3.1. The effect of solution pH on calcined and raw EC sludge adsorbent was investigated at pH 2, 4, 6, 8 and 10 at ambient temperature (25 ± 2 °C), 20 mg/L dye concentration and 0.5 g/100 ml with different mixing time as shown in Fig. 7 (a) and (b) respectively. It has been observed that the removal efficiency is higher at pH 2 (97.6%) for Calcined and (92%) for raw EC sludge adsorbent at 20 min mixing time. Almost a similar percentage removal efficiency were recorded in solution pH from 4 to 8 but dramatically decreases at pH value of 10 for both raw and calcined EC sludge adsorbent. The maximum efficiency of the adsorbent to remove direct red 28 dye at pH 2 is due to released hydrogen ion in the solid surface to attract anionic natures of direct red 28 dyes solution together with positive charge nature of the adsorbents makes it a high electrostatic interactions in the solid-liquid phase system [37]. But, in the basic media, released hydroxyl ion has an electrostatic repulsion with the anionic direct red 28 dye makes it the solid-liquid phase in the adsorption process disturbs. Effect of temperature (ambient, 40, 60 and 80 °C in the adsorption
Fig. 4. surface charge and Zeta potentials of raw and calcined (500 °C) EC sludge adsorbent as a functions of pH.
electrocoagulation processes, different elements will be detected and dominantly were iron element about 83% as reported by [33]. 3.2. Effects of operation parameters The preliminary adsorption efficiency of the adsorbent with a different type of dyes, from cationic dye; Basic yellow 28, from anionic dye; direct red 28 and from reactive dyes; reactive red were tested without pH adjustment. The studied adsorbent was investigated as more effective for direct red 28 dyes removal. All the experiments were studied using direct red 28 dyes to examine the effectiveness of the raw and calcined electro-coagulated sludge as a potential adsorbent. For all effects of operation parameters, the calcined as compared with raw EC sludge has recorded better removal efficiency. This is due to the raw EC sludge surface occupied with different contaminants contained in the effluent and the calcined adsorbent is transformed in to iron oxides (hematite and maghemite) which are more stable than iron hydroxides or/ and oxy-hydroxides. For all batch adsorption experiments, the 5
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Fig. 5. X-ray diffraction pattern at different calcination temperatures and raw electro-coagulated (EC) sludge adsorbent.
from 10 to 100 min. As can be seen in Figs. 6-8 both in calcuned and raw EC sludge adsorbent, the percentage removal of direct red 28 dye is dramatically increased with increasing mixing time from 10 to 20 min. The further increase from 20 to 60 min, the percentage of dye removal is attained saturation of their performance. But further increase of mixing time after 60 min, the percentage removal decreases. The maximum removal efficiency was recorded as 97.5%, and 92% for calcined and raw EC sludge adsorbents respectively. It can be conclude that 10 minute is not enough for adsorbents to uptake the dye molecule and 20 to 60 min can be an equilibrium mixing time for the studied experimental conditions. Similar studies were reported for adsorptions of anionic dye, methyl orange, in the acidic media using mesoporous carbon materials as an adsorbent [28].
process was investigated at solution pH of 2, initial dye concentration of 20 mg/L and adsorbent dosage of 0.5 g/100 ml with different mixing time for both calcined and raw EC sludge adsorbent. As can be seen in Fig. 8 (a) and (b), with increasing reaction temperature, the percentage removal of dye shows that a slight decreases both in calcined and raw EC sludge adsorbents respectively. Even if the temperature effect is slight, recorded decreasing is due to the hole (active site) of the adsorbent at the surface become relaxed with high temperature in such a way that the adsorbed dye molecules can be released from the surface of the adsorbent and dissolved back to the liquid phase [21,22]. In the adsorption-desorption process, examining the exact required time is very important for the determinations of adsorbent performance. Thus six mixing time points were used for all batch experiments
Fig. 6. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent of with different initial concentration at 0.5 g adsorbent dosage, solution pH value 2 and at a temperature of 25 ± 2 °C. 6
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Fig. 7. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent with different solution pH value at 0.5 g dosage, 20 mg/L initial concentration and at a temperature of 25 ± 2 °C.
Fig. 8. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent with different adsorption temperature at 0.5 g adsorbent dosage, 20 mg/L initial concentration and solution pH 2.
3.3. Isotherm, kinetics and thermodynamics study
UV Vis spectra analysis before and after adsorptions at different pH values of direct red 28 dye were studied as shown in Fig. 9. The absorption peaks at 350 nm and 498 nm were disappeared at pH of 2, 4, 6 and 8 after adsorption of dye solutions which indicates that the color responsible chromosphere and azo functional groups are adsorbed on EC sludge adsorbent. But, at the far basic media at pH 10, the peak is similar to before adsorption of direct red 28 dye solutions even if the absorbance value has reduced as compared to before adsorption concentration. This is also confirmed from effect of initial dye concentration examination which has percentage removals below 25% at pH 10.
3.3.1. Thermodynamic study Effects of adsorption temperature on the uptake of dye solutions were studied. The thermodynamic parameters elucidate the feasibility, spontaneity and the nature of adsorbate-adsorbent were calculated with as the mathematical Eqs. (8) and (9). The values of ΔH° (J mol−1) and ΔS° (J mol−1K−1) were calculated from the slope and intercept of a linear plot lnKc versus 1/T as shown in Fig. 10 (a) and (b) for raw and calcined EC sludge adsorbent 7
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Fig. 9. UV-Vis spectra of before and after (pH from 2 to 10) direct red 28 dye adsorption on to calcined EC sludge adsorbent.
respectively. Calculated parameters for the adsorption of direct red 28 dye on to EC sludge adsorbent are shown in Table 1. Change in Enthalpy (ΔH°) for the adsorption process is positive tells that the process is recorded as endothermic. Also changes in entropy (ΔS°) is positive, confirms that there is an increase in the disordered nature at the adsorbent-adsorbate interaction during the process. The randomness at the solid-liquid interface could result from the higher translational entropy acquired by the displaced water molecules as compared to that loss as a result of dye uptake [38]. ΔH° values obtained for the adsorptions of dye molecules onto EC sludge adsorbent recorded to be less than 40 kJ/mole which is 19.7 and 19.8 for calcined and raw respectively. This indicates that the physical sorption process takes place in the system. ΔG° values calculated were negative as shown in Table 1 suggests that the adsorption process is spontaneous and higher negative values, -7.572 and -7.326, for calcined and raw EC sludge adsorbent respectively at the higher
Table 1 Thermodynamic parameters for the adsorption of direct red 28 dye on to raw and calcined EC sludge adsorbent. Adsorbents
ΔG0 [KJ/KmoL]
ΔH0 [KJ/ KmoL]
ΔS0 [KJ/ Kmol
19.712 19.862
0.077 0.076
Temperature [K] 298 Calcined Raw
−4.482 −4.246
313
333
353
−6.027 −5.786
−7.572 −7.326
Fig. 10. Thermodynamic study for direct red 28 dye adsorption on to (a) Raw and (b) Calcined EC sludge adsorbents. 8
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studied adsorbents were 1.262 mg/g and 1.252 for raw and calcined EC sludge adsorbent respectively. The adsorption capacity of different hybrid and pure iron oxides/hydroxides for direct red 28 dye removal have been reported with different scholars. Such as using coated FeOOH porous thin films adsorption capacity is 14.4 mg/g [41], using magnetic nanopoweder is 54.46 mg/g [42], using Zn0.3Fe0.45O3 nanoparticle is 333.33 mg/g [43]. Also using non hybrid Fe3O4 spheres with loose structure is 22.64-28.46 mg/g [43], using pretreated Fe3O4 is 21.28 mg/g [44] and using Calcined magnesite is 0.2271 mg/g [45]. In this study, DR28 dye removal efficiency is more than 97% though the adsorption capacity is low as compared with hybrid and nanocomposite iron oxide adsorbents. This is due to the initial concentration and solid/liquid ratio in the adsorption process; the number of adsorption sites and available specific surface area; tendency of adsorbate to adsorb compared to stay in solution. Also the process depends on the adsorbate ionic nature, electronegativity, free energy of hydration and affinity of adsorbate-adsorbent. Volume of dye solution and mass of adsorbent plays an important role. In addition, increasing adsorption density of a direct red 28 dye on EC sludge adsorbent can have a capability to reduce the further tendency of adsorbent to adsorb owing to increasing competition among solute ions and a decrease in the number of vacant sites available for further adsorption. For the removal of dyes on to EC sludge adsorbent, it is also important knowing with what tenacity the solute is binding with the adsorbent. That is whether the retention is through physical or chemical sorption take place in the adsorption processes. Thus, it can be conclude that higher adsorption efficiency will have low adsorption capacity due to experimental condition, nature of the adsorbent and the adsorbent-adsorbate interaction in the adsorption phenomenon.
Table 2 List of calculated parameters from the Langmuir and Freundlich adsorption isotherm models for raw and calcined EC sludge adsorbents. Parameters
N Kf (mg/g) qm (mg/g) R2 KL (L/mg) RL
Langmuir Isotherm
Freundlich Isotherm
Raw EC Adsorbent
Calcined EC Adsorbent
Raw EC Adsorbent
Calcined EC Adsorbent
——— ——– 1.262 0.9504 14.40 1.05*10–2
——– ——– 1.252 0.9186 17.75 1.01*10–2
1.275 1.114 ———– 0.9844 ————– ————–
5.847 0.905 —————0.7156 ————– ———–
temperature (353.15 K) suggesting that the process at higher temperature was more spontaneous. 3.3.2. Adsorption isotherm model To this research adsorption isotherms was studied with Langmuir and Freundlich isotherm models. Thus, these models were examined by calculating the relationship between the amount of solute (dye) adsorbed onto the solid phase and the equilibrium concentration of the solute in solution at ambient temperature. Langmuir [39] and Freundlich [40] isotherm models were studied to understand the degree and extent of favorability of dye adsorption onto EC sludge adsorbents, respectively. These models, Langmuir and Freundlich, are given by the Eqs. (4) and (5), respectively. The main feature of the Langmuir equation is the separation factor (RL) that is defined as the Eq. (11). RL indicates of isotherm shape, to favorable adsorption 0< RL<1, for unfavorable adsorption RL>1, for linear adsorption RL=1, and irreversible adsorption RL=0 [40].
RL =
1 1 + KL Co
3.4.3. Adsorption kinetics Reaction kinetic studying is important to know the information about reaction mechanisms in the adsorption process. Pseudo-firstorder [46] and pseudo-second-order [47] were selectively used to analyze kinetic modeling of the processes. These two models are governed with mathematical Equations given by (13) and (14) respectively:
(11)
Where KL is the Langmuir constant related to the energy of adsorption (L/mg) and Co is the highest initial dye concentration (mg/L). As can be seen in Table 2, Freundlich isotherm is best described the uptake of direct red 28 dye onto raw EC sludge adsorbent (R2=0.9844) which indicates that dominantly multilayer adsorption (physical adsorption) take place. But Langmuir isotherm fits for calcined EC sludge adsorbent (R2 = 0.9186) which is dominated by monolayer adsorption (chemisorption) role in the dye solution uptake which is also confirmed with pseudo-second-order kinetic model. The calculated values shows that RL value is 1.01*10–2 and 1.05*10–2 for calcined and raw EC sludge adsorbent respectively which exists between 0 and 1 while the value of n was greater than 1. This indicates that the adsorption process was favorable. Graphically, the Langmuir isotherm model is presented in Fig. 11 (a) and (b) for raw and calcined EC sludge adsorbents respectively. The applicability of the Freundlich adsorption isotherm was analyzed with the same set of experimental data with that of the Langmuir model which shows in Fig. 12 (a) (b) for raw and calcined EC sludge adsorbent respectively. The linearized Equation of the Freundlich model can be mentioned as Eq. (12).
log q e = log kf +
1 log Ce n
dqt = k1(qe− qt) dt
(13)
dqt = k2 (q e − qt) dt
(14)
Where, qe and qt are the amounts of adsorbate adsorbed (mg.g−1) at equilibrium and at any time t, and k1 (min−1), k2 (g.mg−1min−1) are the pseudo-first and second-order rate constants, respectively. The pseudo-first-order rate has been expressed in terms of Eq. (13) and applying boundary conditions, t = 0 to te and qt = 0 to qt; with Eq. (6). The values k1 and qe were obtained from the slope and intercepts of the plot of log (qe – qt) versus contact time respectively. Results of Pseudo first order for adsorption has been shown in Fig. 13 (a) and (b) for the raw and calcined EC sludge adsorbents respectively. Also, the plot of the pseudo-second-order model is shown in Fig. 14 (a) and (b) for raw and calcined EC sludge adsorbent respectively which expresses a positive linear equation. The correlation coefficient as shown in Table 3 of the pseudosecond-order model (R2=0.99996) for both raw and calcined EC sludge adsorbents suggesting that the adsorption process is dominated with chemisorption mechanisms of adsorption, where the removal of dye from a solution is due to physicochemical interactions between solidliquid phases [48,49]. and
(12)
Freundlich isotherm model plotted as log (qe) versus log (Ce) and the parameters' values are mention in Table 2. The favorability of isotherm models characterized in terms of the magnitude exponent (n). If the value of sorption intensity (n) is in the range of 2 to 10 represent well, 1 to 2 moderately difficult, and less than 1 poor adsorption characteristics [3]. Thus, the values of n were 5.847 and 1.275 for calcined and raw EC sludge adsorbents respectively which indicate the favorability of adsorption is well fitted in calcined EC adsorbent and moderately fitted in raw one. The adsorption capacity (qe) of the
Conclusion Utilization of EC sludge for the preparations of adsorbent in development of dye removal technology is low-cost and bifunctional as environmental management. Adsorbents were successfully prepared 9
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Fig. 11. Langmuir isotherm for direct red 28 dye removal onto (a) raw (b) Calcined EC sludge adsorbents.
Fig. 12. Freundlich isotherm for direct red 28 dye removal onto (a) raw (b) Calcined EC sludge adsorbents.
diffraction patterns tells that the crystalline and pure hematite type of iron oxide are produced at a temperature of 500 °C and above. From 20 to 60 min adsorption, more than 97% and 92% of direct red 28 dyes are removed at the initial dye concentration of 20 mg/L, pH 2, adsorbent dosage 0.5 g/100 ml at ambient temperature for calcined and raw EC sludge adsorbents. The absorption kinetics study for direct red 28 dye was fitted with a pseudo-second-order model suggesting that the adsorption process is chemisorption. The Langmuir adsorption capacity of DR28 dye with raw and calcined EC sludge adsorbent was calculated as 1.262 mg/g and 1.252 mg/g respectively with experimental conditions: mixing time=20 min, adsorbent dosage=0.5 grams, initial dye concentration= 20 mg/L and solution pH =2 at ambient temperature. It can be deduce that EC adsorbent can be recycled from wastewater sludge having electrochemical processes in their effluent treatment
from EC sludge and effectively utilized for direct red 28 dye removal in aqueous solution. Surface and crystalline characteristics of prepared adsorbent suggesting that dominantly iron oxides (hematite and maghemite) is contained and it has a good adsorptive and magnetic nature as observed during batch experiment. The recycled EC sludge as an adsorbents performance for effective direct red 28 dye removal have been successfully tested. Regarding the preparation parameters of adsorbent, removal efficiencies of direct red 28 dye both in raw and calcined EC sludge are mainly affected by solution pH in the adsorption process. FTIR functional groups identified iron oxide vibration sharp peaks after heat treatment, and after adsorption of direct [28] dye is favored. Also iron hydroxide functional group peaks disappeared after heat treatment which tells that transformation of iron hydroxide to iron oxide were performed. The XRD 10
Surfaces and Interfaces 18 (2020) 100439
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Fig. 13. Pseudo first order for adsorption of direct red 28 dye onto (a) Raw and (b) Calcined EC sludge adsorbents.
Fig. 14. Pseudo second order for adsorption of direct red 28 dye onto (a) Raw (b) Calcined EC ludge adsorbents.
plant for effective removal of dye in aqueous solution.
Table 3 List of calculated parameters from pseudo-first and second-order Adsorption Kinetic. Parameters
qe,exp (mg/g) qe (mg/g) R2 K1 (min−1) K2(g/mg*min)
Pseudo First Order
Pseudo Second Order
Raw EC Adsorbent
Calcined EC Adsorbent
Raw EC Adsorbent
Calcined EC Adsorbent
1.937 0.019 −0.18265 0.029 ———–
1.942 0.044 −0.13511 0.048 ———
—— 1.926 0.99996 ————– -4.147
——1.930 0.99996 ————– -4.134
Declaration of competing interest The author declares that he has no known competing for financial interests or personal relationships that could have appeared to influence the work reported in this work. Acknowledgments The authors would like to thank the Faculty of Chemical and Food Engineering, Bahir Dar Institute of Technology to do this research opportunity. Also, my thanks go to GIZ STEPS program collaboratively with Bahir Dar Institute of Technology university-industry linkage office giving me the three-month externship program from Bahir Dar Textile Factory, Bahir Dar, Ethiopia. 11
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Recovery of iron hydroxides from electro-coagulated sludge for adsorption removals of dye wastewater: Adsorption capacity and adsorbent characteristics Tadele Assefa Aragaw
T
⁎
Faculty of Chemical and Food Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar, Ethiopia
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro-coagulated Sludge Adsorbent DR28 Iron Hydroxide Iron Oxide Isotherm
The aims of this research work was to investigate the potentials of raw and calcined iron hydroxides/oxides sludge adsorption for DR28 dye removal. The adsorbent is prepared from electro-coagulated (EC) sludge in industrial wastewater treatment plant, as a Nobel adsorbent for decolorizing direct red 28 dye (DR28). The EC sludge adsorbent were prepared with soaking processes and calcination in a range of temperature. The surface properties of raw and calcined EC sludge adsorbent were examined using Zeta Potential, XRD and FTIR. Basic (effect of solution pH, temperature, initial dye concentration) operation parameters were examined for raw and calcined EC sludge adsorbent. The diffraction patter suggests that the crystalline hematite are produced during calcination and increases its intensity as temperature increases. Langmuir and Freundlich Equation were used to model the adsorption equilibrium; and pseudo first and second order Equation were used to model the adsorption kinetics. The results suggested that the adsorption of DR28 dye is chemisorption, the interaction between the adsorbent surfaces with dye is strong. The Langmuir adsorption capacity of DR28 dye with raw and calcined EC sludge adsorbent was calculated as 1.262 mg/g and 1.252 mg/g respectively with experimental conditions: mixing time=20 min, adsorbent dosage=0.5 grams, initial dye concentration= 20 mg/L and solution pH =2 at ambient temperature. The adsorption kinetics model data were consistent with the pseudosecond-order. The removal efficiency of 97% was recorded at pH 2, ambient temperature 20 mg/l concentration and 1 g/100 ml for 1 hour. High direct red 28 dye uptake capability and cost-effectiveness of sludge utilization from textile wastewater treatment plant make it potentially attractive for dye removal.
1. Introduction The textile factory is producing a huge amount of dye-containing wastewater which is non-biodegradable organic compounds. Recently, the environmental impact of dye wastewater from textile, paint, and leather industries are required special attention to the treatment of azo dye. To eradicate this problem and reuse the electro-coagulated (EC) sludge, adsorptions of this kind of wastewater are important into natural effluents. Different dye contaminant removal such as physical, chemical and biological methods have been developed for the last decays. So far, the treatment of textile wastewater in Ethiopia has been based mainly on aerobic biological processes followed by chemical coagulation. Chemical and biological methods are effective for removing dyes, but they require specialized equipment and are quite energy-intensive; also, a large amount of byproduct is often generated that creates a sludge
⁎
disposal problem [1]. With the high cost and secondary sludge production problems involved in the above methods, further investigation of different adsorbents, non-conventional, for the adsorption process including waste sludge utilization is attracting scholar's attention. The author of this work previously has been done promising low-cost none conventional adsorbent from Ethiopian kaolin and also synthesized zeolite with hydrothermal methods for dye and hardness removal respectively [2,3]. Also finding another good low-cost and non-conventional adsorbents contribute to the sustainability of the environment [4] and also offer promising advantages with sludge waste management for commercial purposes in the future. Carbon based, biochar, materials as an adsorbent for the removal of organic pollutants especially azo dyes attracts attention now a days due to its diversified functionalities and have high potentials for contaminant removal in the wastewater steams. A developments of biochar-based composite with a foreign materials like magnetic-biochar,
Corresponding author. E-mail address: [email protected].
https://doi.org/10.1016/j.surfin.2020.100439 Received 17 December 2019; Received in revised form 9 January 2020; Accepted 11 January 2020 Available online 13 January 2020 2468-0230/ © 2020 Elsevier B.V. All rights reserved.
Surfaces and Interfaces 18 (2020) 100439
T.A. Aragaw
2. Materials and methods
nanometallic oxides/hydroxides-biochar and layered nanomaterial coated-biochar; and also activation of biochar with different chemicals are well studied to produce a high removal capacities of the material with a range of organic contaminants [5]. Also a composites of graphite and layered hydroxides nanomaterial sorbents for water treatment have a higher capacities due to their distinctive surface and structural properties. This materials have special removal capacities for heavy metal ions, radionuclides and azo dyes due to their large surface area and ion exchangeability [6].Thus, the iron oxides/ hydroxides from the EC sludge, after treatment, can be used as hybrid with carbon based adsorbents or/and directly as metal oxides as an adsorbent for different contaminant removal. Electrocoagulation wastewater treatment systems have been broadly used for different technological applications [7]. Due to the non-biodegradable natures of the dye wastewater, the textile wastewater treatment plant uses the electrocoagulation unit process and has huge sludge production. The electrocoagulation treatment processes in dye-containing wastewaters from the textile industry is a promising technique for not easily biodegradable compounds [8] but sludge production is still a problem as solid waste management. Thus utilizing the electro-coagulated sludge for adsorptions of wastewater is promising research in sludge management as well as resource recycling point of view. The primary interphase by-products of wastewater treatment in electrochemically induced by iron/steel electrodes are mainly ferrous hydroxide Fe (OH)2 and ferric hydroxide Fe(OH) [9] and oxidative transformation with calcination produces iron oxides as shown in Fig. 1. Especially hematite at higher temperature which have a crystalline nature. In addition to the pure iron hydroxides, oxy-hydroxides (goethite, α-FeOOH, and lepidocrocite, γ-FeOOH,) can also be formed and removed as a sludge in the electrocoagulation processes [10,11]. Thus to utilize the EC sludge for dye wastewater removal and preparing an adsorbents is attractive and promising interest. Saying this, the present study forestalls to remove direct red 28 dye as a modal azo dye with recycled EC sludge adsorbent. The FTIR spectra, XRD diffraction and zeta potential properties of the adsorbent are well discussed. Effect of basic adsorption parameters (pH, temperature, contact time and initial dye concentration); and adsorption isotherms, kinetics, and thermodynamics were carried out. The main objectives of the research are to study the adsorptions capacity of EC sludge, raw and calcined, for direct red 28 dyes which considers as a modal azo dye in the textile industries and adsorbent characteristics.
2.1. Chemicals and equipment Hot air oven, jaw crusher, disk mill, sieves, electronic balance, muffle furnace, pH meter, hot plate with a magnetic stirrer, measuring cylinder, test tubes, pipette, what man filter paper and centrifuge were used during adsorbent preparation and batch adsorption experiment. Also powder of dyes, concentrated sulphuric acid, sodium hydroxide, and potassium bromide chemicals, without purification, were used. 2.2. Adsorbent and adsorbate preparation 2.2.1. Sample collection and physical treatment Sludge sample was collected from Bahir Dar Textile factory wastewater treatment plant, Bahir Dar, Ethiopia. It was dried with an oven at 70 °C, ground and milled to passes less than 200 µm sieve using jaw crusher and disc mill. The desired product was used for wet treatment and oversize products have been returned to the crusher. For the separation of solid sludge from grease, soluble salts, organics to meet the needs of dye removal adsorbent; the beneficiation process was used. Ground and milled sludge were dispersed in de-ionized water for 24 h and bunged to separate all floatable oils, specks of dust and dissolved solid particles. With decanting the supernatant, the slurry was washed until the unstable suspended particle removed. The suspension after beneficiation were dried at 70 °C to remove absorbed water. Forward after, the beneficiated and dried solids were ground and milled for thermal treatment. 2.2.2. Thermal treatment The high temperature oxidative transformation of iron oxides and hydroxides are followed a range of temperature dependent phase interchanges from magnetite to hematite or/and maghemite [12–16]. Temperature dependency oxidative transformation stages of iron oxides formations are as follows [9]: (1) Temperature range 200- 400°C; 4 Fe3O4 +O2 → 6 γ-Fe2O3 (2) Temperature range 375 -550°C; 6 γ-Fe2O3→α-Fe2O3 (3) 550°C and above 4 Fe3O4 +O2 → 6 α-Fe2O3 The wet treated and dried powder was used for thermal treatment at 100, 300, 500 and 800 °C for 3 h in a muffled furnace to remove moisture and organic from the sludge. Also to transform goethite (FeOOH), which is precipitated during electrochemical effluent treatment to hematite and maghemite, α-Fe2O3 and γ-Fe2O3 with thermal oxidation.
Fig. 1. Photographic representations of (a) raw (b) calcined at 500 °C electro-coagulated (EC) sludge. 2
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Fig. 2. Calibration curve for the standard solution of Direct Red 28 Dye.
Where Co and Ct (mg/L) is the initial and concentration at time t of the dye in the solution, respectively.
2.2.3. Adsorbate preparation and analytical methods Modal dye, direct red 28, which can be used from the dye industry were taken from the commercial market. Stock solutions were prepared with 0.5 grams of powdered dye into 1 liter of double distilled water making 500 mg/L solution. Working and standard solutions (10, 20, 40, 60, 80 and 100 mg/L) were prepared with analytical methods. To know the maximum wavelength, λmax, and absorbance of the dye was taken and scanned from 200 to 700 nm using UV/VIS spectrometer (PerkinElmer Lambda 35) and it was found that 498 nm. To calculate the final dye concentration from each batch adsorption experiment, a calibration curve was prepared. The linear calibration curve as shown in Fig. 2 was used as a basis for determining the dye concentration variation as a result of the dye adsorption process for each experiment.
qe =
V(Co − Ce) m
(2)
qt =
V(Co − Ct ) m
(3)
Where qe and qt are, the amounts adsorbed (mg/g) at the equilibrium and at any time t, respectively. Co,Ce and Ct are the concentration of the dye in the solution (mg/L) at the initial, equilibrium and at time t, respectively; V is the volume of the solution (L), and m is the mass of the adsorbent (g).
2.3. Experimental design and description 2.4. Adsorbent characterization In a conical flask of 250 ml, working solutions with the desired concentration and 1 gram of EC sludge adsorbent were added and agitated with a magnetic stirrer on a digital hot plate at 200 rpm. All batch adsorption experiments were conducted using raw and calcined (500 °C) EC sludge adsorbent. Because calcination at 500 and 800 °C have a similar feature as confirmed from XRD diffraction patterns, I prefer only one calcined EC adsorbent throughout the experiment. The pH of the solution was adjusted with inorganic acidic/basic solution (1 molarity of HCl and NaOH) before adding the adsorbent. The batch adsorption experiments were performed as initial dye concentration (20, 40, 60, 80 mg/L), temperature (ambient, 25 ± 2, 40, 60 and 80 °C), solution pH (2, 4, 6, 8, 10) and mixing time (10, 20, 40, 60, 80 and 100 min) at 0.5 grams of adsorbent. At the end of each experiment, 15 ml of the solutions were withdrawn at a predetermined time and separate the clear supernatant using centrifuge (SIGMA 3-18 KS) at 4000 rpm for 10 min. The absorbance after adsorption was measured using UV/Vis spectrometer (PerkinElmer lambda 35) for each run. The final dye concentration was calculated from the calibration curve and the removal efficiency was calculated as Eq. (1) [17]. The equilibrium state concentration (loading) of adsorbate in the solid phase (qe, mg/g) and concentration (loading) of adsorbate in the solid phase at any time (qt, mg/g) were determined as Eqs. (2) and (3) respectively [18].
Removal Efficiency (%) =
(Co − Ct ) ×100 Co
FTIR Spectroscopy was used to determine the functional groups in the raw and calcined (500 °C) EC sludge adsorbent and also after DR28 dye adsorption. The analysis was done at a wave number range of 400–4000 cm−1 using JASCO 6600 typeA spectrophotometer. 1:100 gram sample to KBr ratio were grounded uniformly using mortar and pestle, and pelletized and absorption spectra was recorded. Zeta Potential Instrument (Malvern Zetasizer Nano ZS series) was used at different pH values to determine the surface charge nature of raw and calcined (500 °C) EC sludge adsorbent. The values were recorded at (acidic media) pH 2, pH 4, pH6 and (basic media) pH 8 and pH 10. The crystalline and phase change of the raw and calcined (100, 300, 500 and 800 °C) EC sludge were measured using X-ray diffraction (SHIMADZU, MAXima_X XRD-7000) with CuK radiation, voltage of 40.0 kV, with continuous scanning range of 10-80.
2.5. Isotherm, kinetics and thermodynamics study 2.5.1. Adsorption isotherm model Langmuir and Freundlich isotherm models were used to determine the relationship between dye ions from the solution adsorbed on EC adsorbents and remaining in the solution. The adsorption isotherm with Langmuir and Freundlich model was governed as the equation in Eqs. (4) and (5), respectively [19,20].
(1) 3
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Ce C 1 = e + qe qm qm KL
logq e = logkf +
thermal treatment, after adsorption with the calcined EC sludge is due to adsorbed carboxyl and amine function group in the dye molecule. The bands appearing at 450–650 cm−1 after calcination belong to the Fe-O stretching vibration from Fe2O3 [23], indicating that iron oxides (hematite or/and maghemite) are formed. The band corresponding to Fe-O stretching mode of Fe2O3 is shown at 576 cm−1 having a sharp (high) intensity in calcined than raw sludge suggesting that the FeOOH were oxidative transformed to iron oxide in a calcination treatment. The band appears at 1097 cm−1 is typically metal hydroxyl (M-OH) stretching vibration from raw EC sludge but disappears from calcined one due to oxidative transformation [26,27]. and Most Azo N=N stretching bands occur between 1580 and 1400 cm−1 which are shown in raw EC but disappeared from thermally treated. Also absorption bands at 3424 cm−1 which may be attributed to the -NH2 group, which overlaps with the stretching vibration of the OH groups. Also, the presence of the peak located around 2900 cm−1 corresponds to C-H vibrations in the raw EC sludge tells that there is adsorbed organic carbon from the sludge but disappear from calcined one [28,29]. Thus, thermal treatment of EC sludge is important for good adsorptions by removing the organic carbon which was embedded during electrocoagulation treatments of wastewater. Also, the peaks at 1430 cm−1 are disappeared and at1622 and 2340 cm−1 are insignificant in the calcined EC sludge. This is due to organic carbons in alkane; carboxyl and ester functional groups have decomposed during thermal treatment at 500 °C and the electrostatic interaction mechanisms with DR28 dye increases. This leads the adsorbent has good adsorption capacity as compared to raw EC sludge with having a large surface area and surface charge nature. As the pH of the reaction from the system decrease, EC sludge expected to have iron oxide and hydroxide appears positively charged which favors the adsorptions of direct red 28 and have higher zeta potentials which is anionic dye molecule. This is due to the electrostatic attraction between the positively charged adsorbent surfaces with the anionic dye molecule. As a result, at high pH, the uptake of direct red 28 dye, anionic dye, was low as shown in Fig. 4. Calcined EC sludge has high zeta potential value at pH 2 than raw EC sludge. This is because raw EC sludge has organic contaminant from the textile effluent as well as there is no transformed iron oxide which has high charge and magnetic nature. This can also be confirmed raw EC sludge is black but the calcined EC sludge is a red pigment powder as can be seen in Fig. 1. The red powder is an indication of Fe2O3 that has been produced by oxidative transformation of FeOOH during calcination. The x-ray diffraction patterns recorded for raw and calcined (100, 300, 500 and 800 °C) EC sludge from electrochemical unit processes of textile wastewater treatment plant are depicted in Fig. 5. The major diffraction intensity corresponds to α-FeOOH recorded at 2Ɵ = 21.3°, 30.2° and 35.5° in raw and calcined at 100 °C EC sludge. The XRD pattern of the this nanomaterial displays five major characteristic peaks situated at 2θ values of 30.1°, 33.3°, 35.7°, 40.7°, 43°, 49.6°, 54.1° and 62.5° that indicates the inverse spinel iron ferrite (Fe3O4) structure [30,31]. A sharp peak shown at 2Ɵ = 24.1° for calcined at 800 °C EC sludge tells that a crosstie pattern. Alternatively, the weak peaks at 2Ɵ = 32.2 and 33.3°, 40.7°, 49.6°, 54.1°, 57.7°, 62.5° and 64.2° are the characteristics to α-Fe2O3 structure and the intensity to this peaks becomes sharpen as the treatment temperature increases from 300 to 800 °C due to oxidative formations of α-Fe2O3. Dominantly, the hematite type of iron oxide peaks are high and sharp which are similar reports by [32]. The XRD diffractgram in Fig. 5 obtained after calcination with a range of temperatures illustrate that iron oxy-hydroxide, goethite (FeOOH), is disappeared after 300 °C and completely transformed to hematite up to 800 °C. But the presence of short peak at 2Ɵ = 57.7° is corresponds with maghemite and magnetite, suggests that it were not fully crystalline up to 800 °C. As the temperature increases, the hematite iron oxides have intense counts were recorded which thus confirmed the absence of any impurities. Also from the electro-coagulated metal hydroxide sludge in the
(4)
1 logCe n
(5)
Where qm is sorption capacity (mg/g), KL is sorption energy (L/g), Kf and n are the Freundlich model constants and indicating the relationship between sorption capacity and sorption intensity, respectively. If n = 1, n > 1, and n < 1, then the sorption process would be the linear, physical, or chemical in its nature, respectively. 2.5.2. Adsorption kinetic model The kinetic model was used to determine reaction kinetics from batch adsorption experiments of adsorption mechanism as a dependency of mixing time. Adsorption kinetics for pseudo-first and second-order models have been governed as a linear equation in Eqs. (6) and (7) respectively.
log(QE− qt) = logq e − t = qt
1 + k2q e2
k1 t 2.303
(6)
1 t qe
(7)
2.5.3. Thermodynamic behavior The thermodynamic property can be used for exothermic and endothermic reactions in the adsorption processes in terms of Gibb's free energy, enthalpy, and entropy. The value of standard change Gibbs free energy, enthalpy, and entropy can be determined with Eqs. (8) and (9) [19]. An exothermic reaction in thermodynamics of the adsorption process articulates as an increase of the atomic or molecular species release from adsorbent solid surface and re-enters to the liquid phase of adsorbate [21,22]. (8)
ΔG0=ΔH0 −TΔS0 lnK c =
−ΔG0 = ΔS0/R − ΔH0/RT RT
(9)
K c = q e/Ce
(10) −1
Where ΔG =standard change free Gibbs energy (kJ mol ), ΔH0= standard change enthalpy(Jmol-1), ΔS0= standard change entropy (J mol-1K-1) and R = universal gas constant (8.314 J mol−1K−1). Kc, the equilibrium constant, represents the ability of the adsorbent to retain the adsorbate and extent of movement of the adsorbate within the solution. Kc is the ratio of the equilibrium concentration of the dye (qe) adsorbed to adsorbent compared to the van't Hoff equation as equilibrium dye concentration in solution (Ce) [23], and can be deduced from Eq. (10). 0
3. Result and discussion 3.1. Surface and crystalline properties of the adsorbent FTIR spectra of raw and calcined (500 °C) EC sludge and after adsorption of DR28 dye with spectral scanning range 4000-400 cm−1 are depicted in Fig. 3 (a) and (b) respectively. Bands appearing at 3424, 2900, 1622, 1430, 1097 and 872 cm−1 were the stretching vibrations of eOH, which is assigned to surface OH groups of Fe3O4, C-H stretching, the asymmetric and symmetric bending vibration in C=O stretching in the carboxyl functional group [24], N=N stretching bands, C-O stretching and N-H deformation in the amines respectively from the raw Electro-coagulated (EC) sludge. The adsorption peaks at 3439 cm and 1632 cm−1 after heat treatment is also attributed to the water of hydration; the bending frequency of surface hydroxyl group was observed at 1392 cm−1 which are similar with the reports by [25]. A small peak appeared at 881 cm−1 and 1050 cm−1, which where disappear in 4
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Fig. 3. FTIR spectral analysis of (a) raw and calcined at 500 °C EC adsorbent (b) before and after direct red 28 dye adsorption.
calcined EC sludge at 500 °C were used due to calcination at 500 °C and 800 °C have similar surface and crystalline properties. A preliminary experiments on the effect of adsorbent was carried out at 0.1, 0.5, 1.0, 1.5, and 2.0 grams at solution pH of 2, ambient temperature, mixing time of 20 min and initial dye concentration of 20 mg/L and the optimum values were recorded at 0.5 g/ 100 L having 97.67% and 92.1% for calcind and raw EC adsorbent respectively. Thus 0.5 g/100 L adsorbent dosage were used as a constant throughout the experiments This is because examining the adsorbent dosage that can be enough in the available sorption surface for the desired adsorbate concentration is very important [34,35]. As can be seen in Fig. 6 (a) and (b) as initial dye concentrations are increased from 20 to 80 mg/L, the percentage of dye removed is being decreased with mixing time from 10 to 100 min at a solution pH of 2, ambient temperature (25 ± 2 °C) and adsorbent dosage of 0.5 g/100 ml of calcined and raw EC adsorbent respectively. This is due to the saturation of the available active sites of the adsorbents. At a low concentration, there will be unoccupied active sites on the surface of the adsorbent, and when the initial dye concentration increases, the active sites required adsorption of the dye molecules will be occupied [36]. Comparatively, the calcined EC sludge adsorbent have more efficiency (97%) than raw EC sludge adsorbent (92%) due to their surface and crystalline properties as described from Section 3.1. The effect of solution pH on calcined and raw EC sludge adsorbent was investigated at pH 2, 4, 6, 8 and 10 at ambient temperature (25 ± 2 °C), 20 mg/L dye concentration and 0.5 g/100 ml with different mixing time as shown in Fig. 7 (a) and (b) respectively. It has been observed that the removal efficiency is higher at pH 2 (97.6%) for Calcined and (92%) for raw EC sludge adsorbent at 20 min mixing time. Almost a similar percentage removal efficiency were recorded in solution pH from 4 to 8 but dramatically decreases at pH value of 10 for both raw and calcined EC sludge adsorbent. The maximum efficiency of the adsorbent to remove direct red 28 dye at pH 2 is due to released hydrogen ion in the solid surface to attract anionic natures of direct red 28 dyes solution together with positive charge nature of the adsorbents makes it a high electrostatic interactions in the solid-liquid phase system [37]. But, in the basic media, released hydroxyl ion has an electrostatic repulsion with the anionic direct red 28 dye makes it the solid-liquid phase in the adsorption process disturbs. Effect of temperature (ambient, 40, 60 and 80 °C in the adsorption
Fig. 4. surface charge and Zeta potentials of raw and calcined (500 °C) EC sludge adsorbent as a functions of pH.
electrocoagulation processes, different elements will be detected and dominantly were iron element about 83% as reported by [33]. 3.2. Effects of operation parameters The preliminary adsorption efficiency of the adsorbent with a different type of dyes, from cationic dye; Basic yellow 28, from anionic dye; direct red 28 and from reactive dyes; reactive red were tested without pH adjustment. The studied adsorbent was investigated as more effective for direct red 28 dyes removal. All the experiments were studied using direct red 28 dyes to examine the effectiveness of the raw and calcined electro-coagulated sludge as a potential adsorbent. For all effects of operation parameters, the calcined as compared with raw EC sludge has recorded better removal efficiency. This is due to the raw EC sludge surface occupied with different contaminants contained in the effluent and the calcined adsorbent is transformed in to iron oxides (hematite and maghemite) which are more stable than iron hydroxides or/ and oxy-hydroxides. For all batch adsorption experiments, the 5
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Fig. 5. X-ray diffraction pattern at different calcination temperatures and raw electro-coagulated (EC) sludge adsorbent.
from 10 to 100 min. As can be seen in Figs. 6-8 both in calcuned and raw EC sludge adsorbent, the percentage removal of direct red 28 dye is dramatically increased with increasing mixing time from 10 to 20 min. The further increase from 20 to 60 min, the percentage of dye removal is attained saturation of their performance. But further increase of mixing time after 60 min, the percentage removal decreases. The maximum removal efficiency was recorded as 97.5%, and 92% for calcined and raw EC sludge adsorbents respectively. It can be conclude that 10 minute is not enough for adsorbents to uptake the dye molecule and 20 to 60 min can be an equilibrium mixing time for the studied experimental conditions. Similar studies were reported for adsorptions of anionic dye, methyl orange, in the acidic media using mesoporous carbon materials as an adsorbent [28].
process was investigated at solution pH of 2, initial dye concentration of 20 mg/L and adsorbent dosage of 0.5 g/100 ml with different mixing time for both calcined and raw EC sludge adsorbent. As can be seen in Fig. 8 (a) and (b), with increasing reaction temperature, the percentage removal of dye shows that a slight decreases both in calcined and raw EC sludge adsorbents respectively. Even if the temperature effect is slight, recorded decreasing is due to the hole (active site) of the adsorbent at the surface become relaxed with high temperature in such a way that the adsorbed dye molecules can be released from the surface of the adsorbent and dissolved back to the liquid phase [21,22]. In the adsorption-desorption process, examining the exact required time is very important for the determinations of adsorbent performance. Thus six mixing time points were used for all batch experiments
Fig. 6. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent of with different initial concentration at 0.5 g adsorbent dosage, solution pH value 2 and at a temperature of 25 ± 2 °C. 6
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Fig. 7. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent with different solution pH value at 0.5 g dosage, 20 mg/L initial concentration and at a temperature of 25 ± 2 °C.
Fig. 8. Effect of mixing time on (a) calcined (b) raw EC sludge adsorbent with different adsorption temperature at 0.5 g adsorbent dosage, 20 mg/L initial concentration and solution pH 2.
3.3. Isotherm, kinetics and thermodynamics study
UV Vis spectra analysis before and after adsorptions at different pH values of direct red 28 dye were studied as shown in Fig. 9. The absorption peaks at 350 nm and 498 nm were disappeared at pH of 2, 4, 6 and 8 after adsorption of dye solutions which indicates that the color responsible chromosphere and azo functional groups are adsorbed on EC sludge adsorbent. But, at the far basic media at pH 10, the peak is similar to before adsorption of direct red 28 dye solutions even if the absorbance value has reduced as compared to before adsorption concentration. This is also confirmed from effect of initial dye concentration examination which has percentage removals below 25% at pH 10.
3.3.1. Thermodynamic study Effects of adsorption temperature on the uptake of dye solutions were studied. The thermodynamic parameters elucidate the feasibility, spontaneity and the nature of adsorbate-adsorbent were calculated with as the mathematical Eqs. (8) and (9). The values of ΔH° (J mol−1) and ΔS° (J mol−1K−1) were calculated from the slope and intercept of a linear plot lnKc versus 1/T as shown in Fig. 10 (a) and (b) for raw and calcined EC sludge adsorbent 7
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Fig. 9. UV-Vis spectra of before and after (pH from 2 to 10) direct red 28 dye adsorption on to calcined EC sludge adsorbent.
respectively. Calculated parameters for the adsorption of direct red 28 dye on to EC sludge adsorbent are shown in Table 1. Change in Enthalpy (ΔH°) for the adsorption process is positive tells that the process is recorded as endothermic. Also changes in entropy (ΔS°) is positive, confirms that there is an increase in the disordered nature at the adsorbent-adsorbate interaction during the process. The randomness at the solid-liquid interface could result from the higher translational entropy acquired by the displaced water molecules as compared to that loss as a result of dye uptake [38]. ΔH° values obtained for the adsorptions of dye molecules onto EC sludge adsorbent recorded to be less than 40 kJ/mole which is 19.7 and 19.8 for calcined and raw respectively. This indicates that the physical sorption process takes place in the system. ΔG° values calculated were negative as shown in Table 1 suggests that the adsorption process is spontaneous and higher negative values, -7.572 and -7.326, for calcined and raw EC sludge adsorbent respectively at the higher
Table 1 Thermodynamic parameters for the adsorption of direct red 28 dye on to raw and calcined EC sludge adsorbent. Adsorbents
ΔG0 [KJ/KmoL]
ΔH0 [KJ/ KmoL]
ΔS0 [KJ/ Kmol
19.712 19.862
0.077 0.076
Temperature [K] 298 Calcined Raw
−4.482 −4.246
313
333
353
−6.027 −5.786
−7.572 −7.326
Fig. 10. Thermodynamic study for direct red 28 dye adsorption on to (a) Raw and (b) Calcined EC sludge adsorbents. 8
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studied adsorbents were 1.262 mg/g and 1.252 for raw and calcined EC sludge adsorbent respectively. The adsorption capacity of different hybrid and pure iron oxides/hydroxides for direct red 28 dye removal have been reported with different scholars. Such as using coated FeOOH porous thin films adsorption capacity is 14.4 mg/g [41], using magnetic nanopoweder is 54.46 mg/g [42], using Zn0.3Fe0.45O3 nanoparticle is 333.33 mg/g [43]. Also using non hybrid Fe3O4 spheres with loose structure is 22.64-28.46 mg/g [43], using pretreated Fe3O4 is 21.28 mg/g [44] and using Calcined magnesite is 0.2271 mg/g [45]. In this study, DR28 dye removal efficiency is more than 97% though the adsorption capacity is low as compared with hybrid and nanocomposite iron oxide adsorbents. This is due to the initial concentration and solid/liquid ratio in the adsorption process; the number of adsorption sites and available specific surface area; tendency of adsorbate to adsorb compared to stay in solution. Also the process depends on the adsorbate ionic nature, electronegativity, free energy of hydration and affinity of adsorbate-adsorbent. Volume of dye solution and mass of adsorbent plays an important role. In addition, increasing adsorption density of a direct red 28 dye on EC sludge adsorbent can have a capability to reduce the further tendency of adsorbent to adsorb owing to increasing competition among solute ions and a decrease in the number of vacant sites available for further adsorption. For the removal of dyes on to EC sludge adsorbent, it is also important knowing with what tenacity the solute is binding with the adsorbent. That is whether the retention is through physical or chemical sorption take place in the adsorption processes. Thus, it can be conclude that higher adsorption efficiency will have low adsorption capacity due to experimental condition, nature of the adsorbent and the adsorbent-adsorbate interaction in the adsorption phenomenon.
Table 2 List of calculated parameters from the Langmuir and Freundlich adsorption isotherm models for raw and calcined EC sludge adsorbents. Parameters
N Kf (mg/g) qm (mg/g) R2 KL (L/mg) RL
Langmuir Isotherm
Freundlich Isotherm
Raw EC Adsorbent
Calcined EC Adsorbent
Raw EC Adsorbent
Calcined EC Adsorbent
——— ——– 1.262 0.9504 14.40 1.05*10–2
——– ——– 1.252 0.9186 17.75 1.01*10–2
1.275 1.114 ———– 0.9844 ————– ————–
5.847 0.905 —————0.7156 ————– ———–
temperature (353.15 K) suggesting that the process at higher temperature was more spontaneous. 3.3.2. Adsorption isotherm model To this research adsorption isotherms was studied with Langmuir and Freundlich isotherm models. Thus, these models were examined by calculating the relationship between the amount of solute (dye) adsorbed onto the solid phase and the equilibrium concentration of the solute in solution at ambient temperature. Langmuir [39] and Freundlich [40] isotherm models were studied to understand the degree and extent of favorability of dye adsorption onto EC sludge adsorbents, respectively. These models, Langmuir and Freundlich, are given by the Eqs. (4) and (5), respectively. The main feature of the Langmuir equation is the separation factor (RL) that is defined as the Eq. (11). RL indicates of isotherm shape, to favorable adsorption 0< RL<1, for unfavorable adsorption RL>1, for linear adsorption RL=1, and irreversible adsorption RL=0 [40].
RL =
1 1 + KL Co
3.4.3. Adsorption kinetics Reaction kinetic studying is important to know the information about reaction mechanisms in the adsorption process. Pseudo-firstorder [46] and pseudo-second-order [47] were selectively used to analyze kinetic modeling of the processes. These two models are governed with mathematical Equations given by (13) and (14) respectively:
(11)
Where KL is the Langmuir constant related to the energy of adsorption (L/mg) and Co is the highest initial dye concentration (mg/L). As can be seen in Table 2, Freundlich isotherm is best described the uptake of direct red 28 dye onto raw EC sludge adsorbent (R2=0.9844) which indicates that dominantly multilayer adsorption (physical adsorption) take place. But Langmuir isotherm fits for calcined EC sludge adsorbent (R2 = 0.9186) which is dominated by monolayer adsorption (chemisorption) role in the dye solution uptake which is also confirmed with pseudo-second-order kinetic model. The calculated values shows that RL value is 1.01*10–2 and 1.05*10–2 for calcined and raw EC sludge adsorbent respectively which exists between 0 and 1 while the value of n was greater than 1. This indicates that the adsorption process was favorable. Graphically, the Langmuir isotherm model is presented in Fig. 11 (a) and (b) for raw and calcined EC sludge adsorbents respectively. The applicability of the Freundlich adsorption isotherm was analyzed with the same set of experimental data with that of the Langmuir model which shows in Fig. 12 (a) (b) for raw and calcined EC sludge adsorbent respectively. The linearized Equation of the Freundlich model can be mentioned as Eq. (12).
log q e = log kf +
1 log Ce n
dqt = k1(qe− qt) dt
(13)
dqt = k2 (q e − qt) dt
(14)
Where, qe and qt are the amounts of adsorbate adsorbed (mg.g−1) at equilibrium and at any time t, and k1 (min−1), k2 (g.mg−1min−1) are the pseudo-first and second-order rate constants, respectively. The pseudo-first-order rate has been expressed in terms of Eq. (13) and applying boundary conditions, t = 0 to te and qt = 0 to qt; with Eq. (6). The values k1 and qe were obtained from the slope and intercepts of the plot of log (qe – qt) versus contact time respectively. Results of Pseudo first order for adsorption has been shown in Fig. 13 (a) and (b) for the raw and calcined EC sludge adsorbents respectively. Also, the plot of the pseudo-second-order model is shown in Fig. 14 (a) and (b) for raw and calcined EC sludge adsorbent respectively which expresses a positive linear equation. The correlation coefficient as shown in Table 3 of the pseudosecond-order model (R2=0.99996) for both raw and calcined EC sludge adsorbents suggesting that the adsorption process is dominated with chemisorption mechanisms of adsorption, where the removal of dye from a solution is due to physicochemical interactions between solidliquid phases [48,49]. and
(12)
Freundlich isotherm model plotted as log (qe) versus log (Ce) and the parameters' values are mention in Table 2. The favorability of isotherm models characterized in terms of the magnitude exponent (n). If the value of sorption intensity (n) is in the range of 2 to 10 represent well, 1 to 2 moderately difficult, and less than 1 poor adsorption characteristics [3]. Thus, the values of n were 5.847 and 1.275 for calcined and raw EC sludge adsorbents respectively which indicate the favorability of adsorption is well fitted in calcined EC adsorbent and moderately fitted in raw one. The adsorption capacity (qe) of the
Conclusion Utilization of EC sludge for the preparations of adsorbent in development of dye removal technology is low-cost and bifunctional as environmental management. Adsorbents were successfully prepared 9
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Fig. 11. Langmuir isotherm for direct red 28 dye removal onto (a) raw (b) Calcined EC sludge adsorbents.
Fig. 12. Freundlich isotherm for direct red 28 dye removal onto (a) raw (b) Calcined EC sludge adsorbents.
diffraction patterns tells that the crystalline and pure hematite type of iron oxide are produced at a temperature of 500 °C and above. From 20 to 60 min adsorption, more than 97% and 92% of direct red 28 dyes are removed at the initial dye concentration of 20 mg/L, pH 2, adsorbent dosage 0.5 g/100 ml at ambient temperature for calcined and raw EC sludge adsorbents. The absorption kinetics study for direct red 28 dye was fitted with a pseudo-second-order model suggesting that the adsorption process is chemisorption. The Langmuir adsorption capacity of DR28 dye with raw and calcined EC sludge adsorbent was calculated as 1.262 mg/g and 1.252 mg/g respectively with experimental conditions: mixing time=20 min, adsorbent dosage=0.5 grams, initial dye concentration= 20 mg/L and solution pH =2 at ambient temperature. It can be deduce that EC adsorbent can be recycled from wastewater sludge having electrochemical processes in their effluent treatment
from EC sludge and effectively utilized for direct red 28 dye removal in aqueous solution. Surface and crystalline characteristics of prepared adsorbent suggesting that dominantly iron oxides (hematite and maghemite) is contained and it has a good adsorptive and magnetic nature as observed during batch experiment. The recycled EC sludge as an adsorbents performance for effective direct red 28 dye removal have been successfully tested. Regarding the preparation parameters of adsorbent, removal efficiencies of direct red 28 dye both in raw and calcined EC sludge are mainly affected by solution pH in the adsorption process. FTIR functional groups identified iron oxide vibration sharp peaks after heat treatment, and after adsorption of direct [28] dye is favored. Also iron hydroxide functional group peaks disappeared after heat treatment which tells that transformation of iron hydroxide to iron oxide were performed. The XRD 10
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Fig. 13. Pseudo first order for adsorption of direct red 28 dye onto (a) Raw and (b) Calcined EC sludge adsorbents.
Fig. 14. Pseudo second order for adsorption of direct red 28 dye onto (a) Raw (b) Calcined EC ludge adsorbents.
plant for effective removal of dye in aqueous solution.
Table 3 List of calculated parameters from pseudo-first and second-order Adsorption Kinetic. Parameters
qe,exp (mg/g) qe (mg/g) R2 K1 (min−1) K2(g/mg*min)
Pseudo First Order
Pseudo Second Order
Raw EC Adsorbent
Calcined EC Adsorbent
Raw EC Adsorbent
Calcined EC Adsorbent
1.937 0.019 −0.18265 0.029 ———–
1.942 0.044 −0.13511 0.048 ———
—— 1.926 0.99996 ————– -4.147
——1.930 0.99996 ————– -4.134
Declaration of competing interest The author declares that he has no known competing for financial interests or personal relationships that could have appeared to influence the work reported in this work. Acknowledgments The authors would like to thank the Faculty of Chemical and Food Engineering, Bahir Dar Institute of Technology to do this research opportunity. Also, my thanks go to GIZ STEPS program collaboratively with Bahir Dar Institute of Technology university-industry linkage office giving me the three-month externship program from Bahir Dar Textile Factory, Bahir Dar, Ethiopia. 11
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