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Chemosphere 206 (2018) 502e512

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Efficient removal of dyes from aqueous solutions using a novel hemoglobin/iron oxide composite* Matthew Essandoh*, Rafael A. Garcia United States Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Biobased and Other Animal Coproducts Research Unit, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA

h i g h l i g h t s  A novel magnetized hemoglobin (Hb/Fe3O4) was synthesized.  Hb/Fe3O4 was tested as an adsorbent for the removal of six different dyes.  Dye removal was dominated by electrostatic interaction.  The Hb/Fe3O4 was effective over a wide pH (4e10) range.  The stability of the Hb/Fe3O4 composite was demonstrated.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 March 2018 Received in revised form 27 April 2018 Accepted 29 April 2018 Available online 10 May 2018

Magnetic particles entrapped in different matrices that display high thermal stability, low toxicity, interactive functions at the surface, and high saturation magnetization are of great interest. The objective of this work was to synthesize a novel hemoglobin/iron oxide composite (Hb/Fe3O4) for the removal of different dyes (indigo carmine, naphthol blue black, tartrazine, erythrosine, eriochrome black T and bromophenol blue) from aqueous solutions. The Hb/Fe3O4 composite was characterized using scanning electron microscopy (SEM), laser diffraction particle size analysis, FT-IR spectroscopy, isoelectric point determination and thermogravimetric analysis (TGA). The Hb/Fe3O4 composite showed high removal efficiency toward all the different classes of dyes studied and the mechanism of adsorption was dominated by electrostatic interaction. Adsorption was found to follow pseudo-second order kinetic model and Langmuir isotherm. The Langmuir monolayer adsorption capacities for all the dyes range from 80 to 178 mg/g. The Hb/Fe3O4 composite possesses extra advantage of being easily isolated from aqueous suspension using an external magnet. The stability of the prepared Hb/Fe3O4 composite was also demonstrated. © 2018 Elsevier Ltd. All rights reserved.

Handling Editor: Y Yeomin Yoon Keywords: Organic dyes Wastewater treatment Removal Iron oxide composite (Fe3O4) Adsorption Mechanism

1. Introduction Dyes are used in a variety of industries including paper, paint, textile, and leather manufacture (Hashem et al., 2007). Dyecontaminated wastewater from textile plant is recognized as one of the most polluting industrial wastewater, especially considering the amount of water used and the content of the their discharge

* Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. * Corresponding author. E-mail address: fl[email protected] (M. Essandoh).

https://doi.org/10.1016/j.chemosphere.2018.04.182 0045-6535/© 2018 Elsevier Ltd. All rights reserved.

effluent (Chequer et al., 2013). Wastewater from these industries show a lot of variations in wastewater characteristics such as biochemical oxygen demand (BOD), color intensity, chemical oxygen demand (COD). It has been reported that a significant amount of synthetic dyes (~12%) used in the manufacturing and processing operations are lost, and roughly 20% of these lost dyes find their way into industrial wastewaters (Hema and Arivoli, 2007; Weber and Stickney, 1993). Even low concentration of dyes in effluent from the dye industries significantly decrease the clarity of water and are highly undesirable (Nigam et al., 2000). The dyes in effluents are of primary concern because of their harmful effect in the environment and also to humans (Robinson et al., 2001). Unfortunately, because of their high stability to temperature, detergents, light-just to name a few, they elude most conventional treatment

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technologies. Several techniques are available for the removal of dyes from wastewater including heterogeneous photocatalysis (Jiang et al., 2014), coagulation/flocculation (Chenna et al., 2016), nanofiltration (Kebria et al., 2015), and forward osmosis (Zhao et al., 2015). However, adsorption is commonly employed because of low cost and simplicity. Recently, various authors have used different adsorbent to remove organic dyes from aqueous solutions with varying degrees of success (Dhananasekaran et al., 2016; Gautam et al., 2015; Robati et al., 2016). Most of the commonly used adsorbent are very high-priced, hard to recover and recycle, and above all suffers from high activation and reactivation cost. However, hemoglobin (Hb), a globular protein, is a substance that is underutilized and may have good properties that will cause dyes to adsorb. Hb has both positive and negative charges on its surface and therefore may serve as a good candidate for it attachment or attraction to other compounds. The uneven circulation of ionic groups on the surface of a protein can trigger electrostatic interactions between itself and adsorbent, even when the protein net charge has the same sign as that of the adsorbent (Lesins and Ruckenstein, 1988). Some authors have shown how hemoglobin can be immobilized on [email protected] magnetic nanoparticles by using glutaraldehyde, a homobifunctional reagent (Chen et al., 2012). The synthesized adsorbent was successful in isolating plasmid DNA. Also, hemoglobin attached to amino-functionalized magnetic particles has been develop (Tang et al., 2011). This adsorbent was successful for the removal of bisphenol A. Annually, approximately 2 million tons of animal blood are produced in the US as a by-product from slaughterhouses (Del Hoyo et al., 2007). Most of this blood is used in relatively low value animal feed applications or the blood may end up polluting the water that are being discharge into the wastewater stream. However, hemoglobin (Hb) can easily be isolated from blood (Morrison and Hisey, 1937) and used as a starting material for the preparation of hemoglobin/iron oxide composite, thereby minimizing agricultural waste. This will also serve as a good way of converting waste into useful product. The syntheses of hemoglobin/iron oxide composite is very important. Magnetic particles (particles which show response to magnetic field gradients) exist in different sizes and shapes. Among the various magnetic particles, iron oxide magnetic particles have received considerable attention, and currently are the only magnetic particle type approved for clinical use in the United States (Neuberger et al., 2005). Magnetic particles have found a lot of applications including biosensing (Diez et al., 2012), magnetic storage media (Reiss and Hutten, 2005), and biomedical applications such as drug delivery and multi-imaging (Lee et al., 2013). A nanohybrid, consisting of magnetite attached to exfoliated silica platelets have been developed for attracting bacteria in microbiological media (Liu et al., 2016). The magnetite attached to silicate platelets greatly help in the capturing and destruction of the bacterial cells, and subsequently removing them using an external magnet. These broad applications of magnetic nanoparticles are mainly as a result of their non-toxicity, biodegradability, and ease of synthesis (Wiogo et al., 2012). Magnetized compounds utilized in magnetic separation offers a unique advantage when it comes to the recovery of the spent separating agent because of the ease of separation and re-use. Despite advances in magnetic nanoparticles, not all sectors have come to appreciate the importance and other potential applications of magnetic substances. To the best of our knowledge, there are no reports for various dye removal using Hb/Fe3O4. In this study, we report a one-step approach for the synthesis of Hb/Fe3O4 composite. This is a simple and inexpensive method for the preparation of Hb/Fe3O4. Our work also include the morphology, thermal stability, and


adsorption properties of the novel hemoglobin/iron oxide composite (Hb/Fe3O4) and it application for the removal of different dyes from aqueous solutions. The dyes studied were diverse. The six dyes investigated fall into three structural classes; these included indigoid dyes (e.g. indigo carmine), xanthene dyes (e.g. erythrosine) and arylmethane dyes (e.g. bromophenol blue). Also of these six dyes, three of them were azo dyes (naphthol blue black, tartrazine and eriochrome black T). Further, the studied dyes are more representative of the dyes likely to be encountered on industrial scale, as about 60% of commercial dyes are azo dyes. 2. Materials and methods 2.1. Materials All chemicals were of analytical or ACS grade and were used without further purification. Iron (II) sulfate heptahydrate, iron (III) sulfate hydrate, hemoglobin (from bovine blood, lyophilized powder), and the anionic and neutral dyes [indigo carmine (pKa ¼ 12.2), naphthol blue black, tartrazine (pKa ¼ 9.4), erythrosine (pKa ¼ 4.1), eriochrome black T (pKa ¼ 6.6 and 11.6), bromophenol blue (pKa ¼ 4)] were purchased from Sigma Aldrich (Saint Louis, MO). Structures of these dyes are shown in Fig. 1. 2.2. Synthesis of hemoglobin/iron oxide composites (Hb/Fe3O4) An aqueous solution containing ferric and ferrous sulfate in 2:1 M ratio was stirred continuously for about 15 min. Hb was then added to this aqueous solution, followed by dropwise addition of 6 M NaOH to raise the solution to a pH ~9, under nitrogen atmosphere. The suspension was allowed to stand overnight. Next, Hb/ Fe3O4 composite was recovered by centrifugation at 5000  g for 15 min. It was then washed repeatedly with water to bring the composite to a near neutral pH. The composite which was dark brown in color was found to be attracted to an external magnet. FreeZone 1 L benchtop freeze dry system (Labconco Corporation, Kansas City, MO) was used to lyophilized the Hb/Fe3O4 composite. 2.3. Particle size distribution (PSD), FTIR and surface area analysis The particle size and size distribution information including the span, surface weighted mean size (D[3,2]) and the volume weighted mean size (D[4,3]) of the Hb/Fe3O4 composite were determined by using laser diffraction particle size analysis (Mastersizer 3000, Malvern Instruments, Worcestershire, UK). The Mastersizer 3000 instrument is capable of measuring particles from 0.01 to 3500 mm. The PSD was calculated using an absorbance of 0.2 and a refractive index of 2.42. The infrared spectra for Hb/ Fe3O4 was determined using a Thermo Nicolet 6700 FT-IR (Thermo Electron Corporation, Madison, WI) spectrometer. Samples were ground with KBr and a hydraulic press was used to produce a KBr pellet of Hb/Fe3O4. The samples were run using a total of 64 scans from 4000 to 500 cm1. The instrument used for the surface area analysis was the TriStar II Plus (Micromeritics, Norcross, GA). The isotherm was obtained at 195.85  C. The surface area from the nitrogen adsorption was calculated based on the well-known Brunauer-Emmett-Teller (BET) method. The pore volume distribution as a function of pore size was calculated using the BarretJoyner-Halenda (BJH) method. 2.4. Scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) Samples were first placed on stubs before coating with gold (EMS 150R ES, EM Sciences, Hatfield, PA). The topography and


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Fig. 1. Structures of the dyes.

composition of Hb and Hb/Fe3O4 composite were studied using scanning electron microscope, FEI Quanta 200 F (Hillsboro, OR, USA) with an accelerating voltage of 10 kV. The samples were observed in high vacuum. Samples elemental composition and interpretation were done using an Oxford Xmaxn 80 mm2 detector (Oxford Instruments, United Kingdom) and AZtec software version 3.1 (Oxford Instruments, United Kingdom), respectively. 2.5. Thermogravimetric analysis (TGA) Thermal properties of the samples were studied using a TA Instruments Q500 thermal analyzer (TA Instruments, Delaware, USA). The samples (~10 mg) were placed in aluminum pans and heated from ambient to 1000  C at a rate of 20  C/min. The nitrogen balance and sample purge flow utilized in the study were 40 and 60 mL/min, respectively. 2.6. Adsorption test In all the adsorption studies, an aqueous solution of dye with suspended adsorbent was gently mixed using a rotating mixer with excess equilibration time (24 h). For kinetics experiments (mass of adsorbent ¼ 50 mg, volume of solution ¼ 50 mL), aliquots were taken from 50 to 100 mg/L dye solutions at predefined time points. In isotherm studies (mass of adsorbent ¼ 40 mg, volume of solution ¼ 20 mL), varying dye concentrations (20e1000 mg/L) were used. All experiments were done at their natural pH unless otherwise specified and were found to be 5.90, 6.34, 5.91, 6.01, 5, and 6.41 for indigo carmine, naphthol blue black, tartrazine, erythrosine B, eriochrome black T and bromophenol blue, respectively. For pH

experiments, 20 mg of the adsorbent and 20 mL of a dye solution were employed. The pH of the solutions were varied from 4 to 9 using MMT buffer (Garcia et al., 2013). The concentration of dye remaining in the supernatant was determined at its wavelength of maximum absorption. Preliminary experiments (not shown for brevity) indicate that the adsorption capacity of iron oxide and pristine Hb are lower than the Hb/Fe3O4 composite. The adsorption capacity was calculated using Eq. (S1). All experiments were carried out in triplicate and the average results are presented unless otherwise stated. 2.7. Stability and regeneration of Hb/Fe3O4 composite A leaching test under conditions used in our study was done. The composite (20 mg) was dispersed in water (30 mL) and gently swirled using a rotating mixer for 24 h. After magnetic separation, Thermo Scientific iCAP Q ICP-MS (Thermo Fisher Scientific, Bremen, Germany) was used for the analysis of iron in the supernatant. Further, the amount of hemoglobin in the supernatant obtained after equilibration was measured by using the alkaline heamatin D575 method with some slight modifications (Zander et al., 1984). In summary, alkaline haematin detergent, commonly called AHD reagent, was prepared. This was done by dissolving 25 g of Triton-X 100 to a 1 L NaOH (0.1 M). Then, to 3 mL of the AHD reagent was added 20 mL of the supernatant. The mixture was allowed to sit undisturbed for 6 min after vortexing, before taking the absorbance at 575 nm. The concentration of hemoglobin was determined from a calibration graph prepared using a chlorohematin standard. Recyclability of the prepared composite was demonstrated following a recent protocol with slight modification (Essandoh

M. Essandoh, R.A. Garcia / Chemosphere 206 (2018) 502e512

et al., 2015). An adsorbent dose of 2 g/L was stirred gently with 20 mg/L of the dye solution. After equilibration, the Hb/Fe3O4 composite was removed from the suspension with an external magnet and then washed with two to three 10 mL of methanol. The supernatant was analyzed by UVevis spectrophotometry to determine the concentration of dye left in the solution. The Hb/ Fe3O4 composite obtained was used for subsequent adsorption experiment to determine the recyclability of the prepared composite.


3.2. SEM-EDX

3. Results and discussion

Scanning electron microscope (SEM) was used to study the morphology of Hb before and after magnetization. SEM images of Hb and Hb/Fe3O4 composite are shown in Fig. 2. Smooth, flat plates are seen with the pure Hb sample (Fig. 2a and b). In the case of Hb/ Fe3O4 composite, iron oxides are clearly seen on the surface of flat plates of Hb. The Hb surfaces were apparently occupied by newborn iron oxides, which were formed during the chemical coprecipitation. The micrographs also show significant differences in the distribution of iron oxide intercalated on the smooth flat layers of the Hb sample. Iron weight percent analyzed by SEM-EDX also reveal that the amount of iron are quite different in the native Hb (0.16%) and Hb/Fe3O4 (9.50%).

3.1. Synthesis of hemoglobin/iron oxide composite (Hb/Fe3O4)

3.3. Particle size distribution, FTIR and surface area analysis

Several routes are available for the production of magnetic iron oxide. In this study, magnetic iron oxide intercalated with Hb was done using chemical coprecipitation. This technique may be the simplest technique and large amounts of the iron oxide can be produced (Laurent et al., 2008). The interaction between iron oxide (Fe3O4) and Hb are non-covalent interactions. The Hb molecules binds to the surface of the iron oxide through van der Waals interaction, hydrogen bonding and hydrophobic interaction. The overall binding of the Hb on the iron oxide is a process that depends on several factors including the structure of the iron oxide, the medium and the Hb molecules (Saptarshi et al., 2013). It is worth noting that the composite particles were strongly magnetic and they could be removed easily from the suspension using an external magnet.

Fig. S1 depicts the particle size distribution of the synthesized adsorbent. The volume weighted mean diameter of the adsorbent was found to be 92.57 mm. The span, which describes the distribution width was 6.05. The D (v, 0.1), D (v, 0.5) and D (v, 0.9) indicates that 10, 50, and 90% of the sample mass distribution lies below this value and were to found to be 7.89, 41.77, 260.33 mm, respectively. The FTIR spectra of Fe3O4, Hb/Fe3O4 and Hb is shown in Fig. S2. The peak around 570 cm1 is due to FeeO bonds in the lattice of magnetite which is displayed in both iron oxide and the composite. The band at 1100 cm1 arises from CeO single bond stretching. The maximum at 1690 cm1 corresponds to the C]O stretch while the band at 3300 cm1 is due to the existence of NeH stretch. The weak peak around 2959 indicates the presence of eCH2 stretching vibrations. These results are consistent with a material

Fig. 2. SEM images of native Hb at (a) 1000 and (b)10000 magnification. The images of Hb/Fe3O4 are shown in (c) 1000 and (d) 10000.


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containing both iron oxide and protein (Ai et al., 2011; Gupta and Gupta, 2005; Kong and Yu, 2007). The Hb/Fe3O4 composite exhibits the characteristic of both the pure Fe3O4 and Hb. FTIR data indicates that the composite contains oxygen-containing functional groups, which can acts as reactive sites, enabling it interaction with other compounds. Detailed properties of the adsorbent including surface area, total intrusion volume, pore size and other characteristics are shown in Table 1 while a N2-adsorption isotherm is shown in Fig. S3. The composite shows an unrestricted multi-layer formation. The synthesized composite is both macroporous and mesoporous in nature. However, macropore and mesopore adsorbents typically show type II and type IV adsorption isotherms, respectively, although we are unable to tell if there is hysteresis or not with only the adsorption branch data (Fig. S3). The ratio of the total pore volume to the apparent volume of the composite was also found to be 66.13%. The median pore diameter in terms of volume and area were 13.38 and 0.0065 mm, respectively. 3.4. Thermogravimetric analysis (TGA) Fig. 3 shows the TGA-DTA (thermal gravimetric analysis and differential thermal analysis) curve from ambient to 1000  C. Pure Hb (TGA curve) lost about 10% of its weight up to 200  C, then exhibited approximately 65% weight loss from 200 to 500  C, followed by just 5% reduction in weight from 500 to 1000  C. The Hb/ Fe3O4 composite also showed about 10% decrease in weight at 200  C followed by a gradual decrease in weight up to 1000  C. The total weight loss at 1000  C was 80 and 60%, for pure Hb and the Hb/Fe3O4 composite, respectively. These results indicate that the thermal stability of the Hb/Fe3O4 composite was higher than the native Hb. DTA curve showed a derivative peak loss at a temperature less than 100 and at 327  C due to the evaporation of physically adsorbed water and the decomposition of Hb sample, respectively. A derivative peak was seen at 298 and around 740  C for Hb/Fe3O4 composite sample, which was ascribed to hematite-type impurity present in the magnetite sample and the reduction of magnetite (Fe3O4) to metallic iron or to lower oxidation state oxide, respectively (Essandoh et al., 2017; Jozwiak et al., 2007). 3.5. Adsorption kinetics Fig. 3. TGA-DTA curves of (a) unmodified Hb and (b) Hb/Fe3O4 composite.

Amount of dye adsorbed onto the adsorbent increases with time (Fig. 4). The increase in adsorption capacity is small after 6 h of equilibration. Excess equilibration time (24 h) was used to ensure that there is a complete saturation of the adsorbent with the dye. When different dye concentrations were studied with fixed amount of adsorbent, the adsorption capacity was found to increase with increasing concentration of dye (Fig. 5). At 20 and 60 mg/L concentration, all the dyes were removed from aqueous solution with adsorption capacities of 10 and 30 mg/g, respectively. Although the amount of dye adsorbed varied at higher dye concentrations (e.g.

Table 1 Properties of Hb/Fe3O4 composite. Parameter


BET surface area Single point adsorption total pore volume of pores Single point desorption total pore volume of pores BJH adsorption cumulative volume of pores Average pore size Total intrusion volume Total pore area Average pore diameter Porosity

12.43 m2/g 0.048 cm3/g 0.046 cm3/g 0.051 cm3/g 12.08 nm 1.252 mL/g 36.50 m2/g 0.137 mm 66.13%

Fig. 4. Amount of dye adsorbed onto Hb/Fe3O4 at different time in aqueous solution. Experimental conditions [initial dye concentration ¼ 100 mg/L, temperature ¼ ambient, adsorbent dose ¼ 2 g/L]. Experiments were carried out in triplicate and average results are shown.

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Fig. 5. Amount of dye adsorbed onto Hb/Fe3O4 at different dye concentration in aqueous solution. Experimental conditions [time ¼ 24 h, temperature ¼ ambient, adsorbent dose ¼ 2 g/L]. Experiments were carried out in triplicate and average results are shown.

300 mg/L), the adsorption capacity was found to increase with increasing concentration. To understand the adsorption kinetics, both the pseudo-first and second order models were investigated using Eqs. (S2) and (S3), respectively. Correlation coefficients, experimental and calculated qe values obtained when the experimental data are fitted to these Eqs. (S2) and (S3) are shown in Table 2. Experimental and calculated qe values vary widely, an indication that the data does not follow the pseudo-first order kinetics. However, there is a close agreement between the experimental and calculated qe values, coupled with high correlation coefficients ranging from 0.994 to 1 in the case of pseudo-second order model. The results clearly show that the pseudo-second order model best fits the experimental data. 3.6. Adsorption isotherm Adsorption isotherm typically describes the equilibrium


concentration of adsorbate on adsorbent. The adsorption isotherms were obtained in batch mode by adding a fixed amount of adsorbent (40 mg) to varying concentrations of the dyes (20e1000 mg/L) at room temperature until equilibration (24 h) is achieved. The results obtained were fitted to the Freundlich and Langmuir isotherm models as shown in Eqs. (S4) and (S5). The isotherm parameters and correlation coefficients obtained from the linear analysis are shown in Table 3. The results show that the Langmuir adsorption isotherm fits the experimental data better compared to the Freundlich isotherm as exhibited by the high correlation coefficient (R2 > 0.990). The Langmuir adsorption isotherms and its linear analysis are also shown in Fig. 6 and Fig. S4, respectively. The maximum monolayer adsorption capacities were found to be 178.6, 104.2, 114.9, 80, 178.6, and 101 mg/g for eriochrome black T, indigo carmine, naphthol blue black, tartrazine, erythrosine and bromophenol blue, respectively. A number of studies have used different adsorbents to remove various hazardous organic dyes from aqueous solutions. Table 4 compares our Langmuir adsorption capacities in this study with other adsorbents that have been reported by different authors for the removal of dyes from aqueous solution. These results clearly depict that the amount of dye (mg) remove per gram of adsorbent employed in this study was better than most adsorbents that have been utilized to remove various dyes. The Hb/ Fe3O4 was very effective for all the tested dyes, although their adsorption capacities differ. Thus, the synthesized composite can be used as a suitable adsorbent for the removal of different dyes from industrial effluents. Hb makes a significant contribution to the adsorption capacity of the composite adsorbent. The adsorption capacity determined for only magnetite (Fe3O4) for the removal of naphthol blue black dye in aqueous solution was 33 mg/g (not shown for brevity). However, with the incorporation of Hb to the magnetite, the synthesized adsorbent delivered an adsorption capacity of approximately 114 mg/g for the removal of similar dye in aqueous solution. This indicates a substantial increase in adsorption capacity in the presence of Hb. The availability of different functional groups like

Table 2 Pseudo-first and second order kinetic parameters for dye adsorption onto Hb/Fe3O4 composite. Dye


Pseudo-first order parameters

Pseudo-second order parameters

Initial conc. (mg/L)

qe, exp. (mg/g)

qe, calc. (mg/g)

k1 (gmg1h1)


qe, exp. (mg/g)

qe, calc. (mg/g)

k2 (gmg1h1)


50 100 50 100 50 100 50 26 50 100 50 100

47.54 69.10 49.54 78.37 49.61 94.10 46.25 25.63 49.89 99.76 49.85 97.63

11.20 11.68 11.65 13.05 13.05 44.00 7.53 1.35 3.09 24.21 15.96 49.66

0.61 0.41 0.58 0.45 0.45 0.27 0.53 0.19 0.78 0.52 0.56 0.12

0.986 0.998 0.988 0.946 0.946 0.979 0.935 0.623 0.946 0.957 0.995 0.867

47.54 69.10 49.54 78.37 49.61 94.10 46.25 25.63 49.89 99.76 49.85 97.63

47.84 69.44 49.75 79.37 50.00 96.15 46.51 25.71 50.00 100 50.25 102

0.17 0.11 0.16 0.04 0.10 0.02 0.22 0.54 0.80 0.07 0.11 0.01

1.000 1.000 1.000 0.999 1.000 0.999 1.000 1.000 1.000 1.000 1.000 0.994

Table 3 Freundlich and Langmuir isotherm parameters for dye adsorption onto Hb/Fe3O4.



Kf (mg/g) 1/n R2 Qo (mg/g) b R2







56.60 0.130 0.988 101 0.839 0.999

35.92 0.298 0.680 178.6 0.272 0.999

35.28 0.236 0.627 114.9 0.551 0.999

23.01 0.224 0.752 80 0.360 0.995

21.44 0.315 0.682 104.2 0.166 0.999

25.16 0.478 0.828 178.6 0.122 0.992


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Fig. 6. Adsorption isotherm for (a) eriochrome black T (b) indigo carmine (c) naphthol blue black (d) tartrazine (e) erythrosine and (f) bromophenol blue in aqueous solution. Experimental conditions [mass of adsorbent ¼ 40 mg, volume of solution ¼ 20 mL, dye concentrations (20e1000 mg/L) at room temperature]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

carboxyl, phenolic hydroxyl and amino groups-just to name a few, increases the composite chances of interacting with different pollutants (e.g. dyes) from aqueous solution. The use of hemoglobin (Hb) with magnetite (Fe3O4) also serves as a good route for converting waste into valuable product and leads to the generation of new composite materials with high adsorption capacity. 3.7. Effect of solution pH and adsorption mechanism The effect of solution pH on the adsorption of the different dyes are shown in Fig. 7. The isoelectric point (pI) for the Hb/Fe3O4 determined using a zeta potential instrument (Zetasizer Nano Z, Malvern, Westborough, MA) was found to be ~5.2 (see Fig. S5). Generally, the adsorption of the dyes decrease with increasing solution pH except eriochrome black T which is pH independent in

the range pH 5e9. At pH less than the pI of Hb/Fe3O4 (~5.2), the adsorbent surface is positively charged. This reduces any electrostatic repulsion between the Hb/Fe3O4 and the dyes (undissociated form), resulting in enhanced adsorption. It was therefore not surprising that the amount adsorbed (mg/g) within this region (pH 4e5) was high. These results are consistent with the work done by other reporters where by an increase in the percentage removal of dyes were observed at low solution pH (Al-Degs et al., 2008). However, at high pH values, greater than the pI of the Hb/Fe3O4, the adsorbent surface is negatively charged while the dyes are negatively charged (at pH > pKa). At these high pH values, the adsorption capacities are greatly reduced mainly as a result of electrostatic repulsion between the adsorbent and the dyes. Other authors have also observed similar trend whereby an increase in initial solution pH led to a decrease in the removal of dyes from

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Table 4 Comparison of adsorption capacities of different adsorbents for dye removal from aqueous solution. Adsorbent


Adsorption capacity (mg/g)


a-Chitin nanoparticle

Bromophenol blue Bromophenol blue Bromophenol blue Indigo carmine Indigo carmine Indigo carmine Indigo carmine Naphthol blue black Naphthol blue black Naphthol blue black Tartrazine Tartrazine Tartrazine Eriochrome black T Eriochrome black T Eriochrome black T Erythrosine Erythrosine Erythrosine

27.72 101 101.62 0.58 32.83 44.8 104.2 11.60 114.9 270.00 35.00 80 90.90 47.00 178.6 250.00 20.78 21.28 178.6

(Dhananasekaran et al., 2016) This study (Haider et al., 2011) (Fungaro et al., 2011) (Gutierrez-Segura et al., 2009) (Makarchuk et al., 2016) This study (Safarikova et al., 2005) This study (Galan et al., 2013) (Jebreil, 2014) This study (Gautam et al., 2015) (Moeinpour et al., 2014) This study (Saha et al., 2011) (Gupta et al., 2006) (Mittal et al., 2006) This study

Hb/Fe3O4 composite Evacuated granular charcoal Magnetic zeolite/iron oxide nanocomposite Fee Zeolitic tuff Iron oxide Hb/Fe3O4 composite Magnetic brewer's yeast Hb/Fe3O4 composite Mesoporous carbon Polyaniline iron oxide Hb/Fe3O4 composite Activated carbon biosorbent NiFe2O4 nanoparticles Hb/Fe3O4 composite Iron oxide nanoparticle Hen feathers Bottom ash Hb/Fe3O4 composite

Fig. 7. Effect of solution pH for the adsorption of eriochrome black T, indigo carmine, naphthol blue black, erythrosine and bromophenol blue onto Hb/Fe3O4 composite. Experimental conditions: mass of adsorbent (20 mg), volume of dye solution (20 mL), equilibration time (24 h) and concentration of dye solutions (83 mg/L for eriochrome black T, erythrosine and naphthol blue black; 48 mg/L for indigo carmine and bromophenol blue). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)


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aqueous solution (Robati et al., 2016). It is paramount for one to recognize that the surface charge of both the adsorbent and adsorbate dyes are complex functions of pH, and the adsorbate dyes also ionizes at different pH, and this can greatly contribute to the variation in the amount of dye adsorbed as the initial solution pH changes (Essandoh et al., 2015). Although various mechanisms (e.g. hydrogen bonding, electrostatic interactions, pep interactions, acidebase interactions and hydrophobic interactions) have been proposed for removal of dyes by adsorbent (Hasan and Jhung, 2015), the mechanism of dye adsorption onto the Hb/Fe3O4 surface is dominated by electrostatic interaction. Most of the dyes contain either one or two sulfonic acid groups which easily dissociates in aqueous medium to give sodium and sulfonate ions. At low solution pH, the sulfonate anions dominate its neutral form due to the extremely low pKa value (

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