Sorptive removal of Remazol Brilliant Blue R from aqueous solution by diethylenetriamine functionalized magnetic macro-reticular hybrid material

Abstract

Chitosan (natural polymer), glycidyl methacrylate (synthetic polymer) and magnetite are combined to produce novel magnetic macro-reticular hybrid synthetic–natural materials. The amino group concentration in the obtained materials was enriched through two different modification routes to generate effective sorbents for Remazol Brilliant Blue R (RBBR) ions. These materials showed a higher affinity towards the uptake of RBBR ions from aqueous media: maximum sorption capacity reached 0.153 mmol g⁻¹ at pH 2.0 and at 25 °C. The nature of the interaction of RBBR with the sorbents was identified. The uptake kinetics and sorption isotherms were best described by the pseudo-second order rate equation and the Langmuir equation, respectively. The distribution coefficient was calculated at different temperatures and the thermodynamic parameters have been calculated: the sorption reaction is endothermic, spontaneous and increases the entropy of the system. Alkaline solution (0.5 M NaOH) was used for desorbing RBBR from loaded sorbents: a regeneration efficiency as high as 90–99% was obtained.

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1. Introduction

The textile industry is a large sector and is considered the greatest creator of liquid dye effluents in the form of pollutants. Along with many industries such as textile, rubber, leather, paper, cosmetics, plastics, food, and pulp they use synthetic organic dyes and water to color their products.¹ Textile effluents are ranked among the pollutants most harmful to the environment due to the high discharge volume and diversity in their composition, associated with the quantities of dye lost during the dying process.² The water contaminated by dye at even a concentration of 1.0 mg L⁻¹ would cause a change in color and make it improper for human usage.³ Remazol Brilliant Blue R (RBBR) is one of the most important identified contaminants among the various dye pollutants of industrial effluents. Due to its good solubility, reactivity, high toxicity and possible accumulation in the environment, RBBR is a common water pollutant and it may frequently be found in trace quantities in industrial wastewater.^4,5 Large volumes of water are consumed by textile industries for wet processing of textiles.⁶ Discharging dyes into the aquatic resources can cause several problems as the presence of noisome synthetic dyes in the aquatic sources and food chain causes severe health risks on the living organisms, flora as well as fauna.⁷ The removal of noxious dyes from wastewater is currently based on a wide variety of physicochemical procedures, which include flocculation combined with flotation, membrane filtration, electro-kinetic coagulation, ozonation, oxidation, precipitation, photocatalytic degradation and sorption.⁸ However, most of the dyes used in the textile industry are difficult to remove by conventional physicochemical and biological treatment methods, since they are stable against the light and oxidizing agents and resist biodegradation.⁹ Sorption is the most widespread process that has been used in the field of industrial wastewater treatment.¹⁰ Recently, numerous studies proposed a variety of sorbent materials for the removal of dyes from wastewater.^11–13 Among these a natural biopolymer, chitosan, which originates from shells of crustaceans obtained by the deacetylation of chitin has been recently investigated as a sorbent for the removal of both organic and inorganic contaminants.^14–17 The hydrophilic nature, biocompatibility, biodegradability, antibacterial properties, nontoxicity, low-cost, fast sorption kinetics, and renewable nature have increased the application of chitosan in sorption processes.¹⁸ However its application on the industrial scale is limited by its insufficient mechanical strength and acid resistance.¹⁹ To overcome these disadvantages, chitosan-based composites have been prepared and proven to have resistance to acidic environments with better sorption.²⁰ The selection, modification and elaboration of new composite sorbents for various pollutants reduction are important criteria in the development of wastewater treatment technologies. Nowadays composite polymeric materials play an essential role in the field of environmental technology.^21,22 The increased interest of the scientific community in the polymeric macro-reticular materials composites is due to their properties as well as of the various requirements of the environmental field.²³ Glycidyl methacrylate (GMA) is an attractive vinyl monomer because of its good mechanical strength, high tensile strength, resistance to acid and alkaline media, resistance to attrition, porous structure, low toxicity, and lower cost compared with other acrylic monomers.²⁴ This polymer is a potent sorbent for various transition metals. Recently, we reported on the use of magnetic sorbents in the removal of some metals from aqueous solutions. These methods are cheap and often highly scalable. Moreover, techniques employing magnetism are more amenable to automation.^25,26 Previously, we studied the sorption behavior of acidic dyes onto GMA, magnetic GMA and magnetic chitosan sorbents under different experimental conditions.^27–29

In a continuation of our research on magnetic chitosan, a magnetic macro reticular-material based on a synthetic polymer (GMA) and a natural polymer (chitosan) was developed for the efficient removal of RBBR dye from aqueous solution. This modification aims to enhance the mechanical strength and acid resistance of magnetic chitosan along with increasing the sorption capacity of magnetic chitosan by the further grafting of amino groups in magnetic chtiosan–GMA sorbent. The concentration of amino groups in the obtained materials was increased through the treatment with diethylenetriamine or epichlorohydrine followed by diethylenetriamine. The development of a sorbent and profitable operation of the sorption process requires the kinetic, equilibrium and thermodynamic data. Kinetic parameters such as the specific sorption rate constant, and sorption equilibrium play an important role in characterizing the sorption process. Isotherm results, kinetic and thermodynamic parameters for the sorption process of RBBR from aqueous solutions onto the prepared hybrid materials were obtained. These valuable data are dearly needed in the design and scale up to a pilot and commercial scale of any type of sorbent. The effect of several parameters, including the initial pH of the solution, the sorbent dose, and the effect ionic strength were studied. The regeneration of the loaded materials for repeated use was carried out.

2. Experimental

2.1. Chemicals

All chemicals used were of analytical-reagent grade and distilled water was used for the preparation of all aqueous solutions. Glycidyl methacrylate (GMA), N,N′ methylenebisacrylamide (MBA) and benzoyl peroxide (Bz₂O₂) were Fluka Analytical products. Chitosan, epichlorohydrine, diethylenetriamine (DETA) and Remazol Brilliant Blue R (RBBR) were Sigma-Aldrich products; Isopropyl alcohol was from Carlo Erba reagents. All other chemicals were Prolabo products and were used as received.

Fe₃O₄ particles were synthesized by co-precipitation of ferric and ferrous salts according to a previously reported method.²⁶

2.2. Preparation of the magnetic-macro-reticular hybrid material

Step 1: Magnetic chitosan particles were prepared by a co-precipitation method; 5 g chitosan powder was dissolved in 200 mL CH₃COOH solution 8% (w/v). 6.479 g of FeCl₃·6H₂O and 3.334 g of FeSO₄·7H₂O were dissolved into 200 mL of distilled water, then 3.0 mL of 3.0 M HCl was added with continuous stirring for 20 min. After complete dissolution of the iron salts, it was poured on the dissolved chitosan. Then chemical precipitation was achieved at 40 °C under vigorous stirring by the drop wise addition of 20 mL of NaOH solution (50%, v/v). The product was isolated by filtration and washed with distilled water followed by ethanol then air dried. The product was ground to fine particles and is named MC.

Step 2: The magnetic-chitosan–glycidyl methacrylate macro-reticular hybrid material was prepared through the polymerization of GMA in the presence of MC particles (prepared in step 1). GMA (5.0 g) corresponding to a mass ratio of 50% (w/w, referred to chitosan amount) ​was used for GMA grafting. 0.4 g of MBA was used as a cross-linking agent. 0.15 g Bz₂O (initiator) was added to the mixture with stirring. Five milliliters isopropyl alcohol and 40 mL of cyclohexane were mixed and then added to the former solution. All the contents were poured into a flask containing 200 mL (0.5%) polyvinyl alcohol and heated in a water bath at 75–80 °C with continuous stirring for 4 h. The product was filtered off then washed repeatedly with distilled water and acetone, then dried in air. The product is named MCGMA. The suggested chemical structure of MCGMA is shown in Scheme 1.

Scheme 1 The suggested chemical structure of chitosan–glycidyl methacrylate–N,N′ methylenebisacrylamide (macro-reticular hybrid material).

2.3. Chemical modification of MCGMA

2.3.1. Modification with diethylenetriamine. MCGMA was suspended in dimethylformamide (100 mL) and then treated with 5 mL DETA. The reaction mixture was stirred at 75 °C for 12 h. The product was washed with water followed by acetone. The product was dried in air and named MCGMA-I.

2.3.2. Modification with epichlorohydrine followed by diethylenetriamine. MCGMA was then suspended in 75 mL isopropyl alcohol. Then 7 mL epichlorohydrine (62.5 mmol) dissolved in 100 mL acetone/water mixture (1 : 1 v/v) was added. The above mixture was stirred for 24 h at 60 °C. The solid product was filtered off and washed several times with water. The obtained product was treated with DETA according to the method in Section 2.3.1. The product was dried in air and is referred to by MCGMA-II.

2.4. Estimation of the amine content

The amine content in the obtained resin was estimated using the volumetric method of HCl according the early reported method.²³ Fifty milliliters of HCl (0.05 M) was added to 0.5 g of MCGMA-II or MCGMA-II and conditioned for 15 h on a shaker at 20 °C. The residual concentration of HCl was measured through the titration against 0.05 M standardized NaOH. The number of moles of HCl interacted with by the amino groups and consequently the amino group concentration was calculated using the following equation:where M₁ and M₂ are the initial and final concentrations of HCl.

2.5. Preparation of solutions

Distilled water was used for the preparation of all aqueous solutions. A stock solution (1.0 mM) of RBBR dye was prepared in distilled water. The desired concentrations were then obtained by dilution. HCl (0.01–0.05 M) and NaOH (0.01–0.05 M) were used to control the pH of the medium. 0.5 M of NaOH was used for elution of RBBR dye bonded to the sorbent.

2.6. Characterization of the sorbents

In order to confirm the functionalization of the sorbents, MCGMA-I and MCGMA-II materials were examined in dried KBr powder by recording the infrared spectra over the range of 4000–400 cm⁻¹ using a Fourier transform infrared (FTIR) spectrophotometer (FT/IR-4100typeA). The morphology and the elemental distribution of RBBR dye in the magnetic-macro reticular materials were analyzed using a Scanning Electron Microscope coupled with an Energy Dispersive X-ray analysis system (Quanta 250-FEI).

The magnetic properties of the sorbents were measured on a vibrating-sample magnetometer (VSM) (Lake Shore 730T, Westerville OH, USA) at room temperature.

2.7. Sorption experiments

A stock solution (1 × 10⁻³ M) of RBBR dye was prepared in distilled water. The other solutions were obtained by dilution of the stock solution with distilled water just prior to the experiments. For the study of the pH effect 20 mL of 1 × 10⁻⁴ M of RBBR solutions at different pH values (in the range 1–9) were mixed with 50 mg of sorbent (dried weight) for 3 h, and the stirring speed was maintained at 40 rpm using a reciprocal agitator, Rota bit, J.P. Selecta (Spain). The pH values were adjusted by addition of 0.01–0.05 M HCl and 0.01–0.05 M NaOH solutions and measured by using a pH meter (Aqualytic AL15). Samples were collected and filtrated through magnetic separation and the filtrate was analyzed for residual RBBR dye concentration using a Perkin-Elmer AA800 spectrophotometer (Model AAS) at a wavelength of 595 nm. The pH was not controlled during the sorption but the final pH was systematically recorded.

For sorption isotherms 50 mg of sorbent (m) was mixed with 20 mL (V) of RBBR dye solutions at different initial concentrations (C₀, ranging between 1 × 10⁻⁴ and 1 × 10⁻³ M) for 3 h. The pH of the solutions was initially set at 3. After solid/liquid separation, the residual concentration (C_e, mmol RBBR L⁻¹) was determined by a UV-vis spectrophotometer and the sorption capacity (q_e, mmol g⁻¹) was determined by the mass balance equation:

where C₀ and C_e are the initial and equilibrium concentration of metal ions in solution (M), respectively, V is the volume of solution (L) and m the mass of sorbent (g).

For uptake kinetics 250 mg of sorbent was mixed with 100 mL of RBBR solution (C₀: 5 × 10⁻⁴ M) at pH 3. Samples (5 mL) were collected (the sorbent being magnetically separated) at fixed times and the residual concentrations were determined using a UV-vis spectrophotometer. The agitation speed was set at 100 rpm while the temperature was maintained at 25 ± 1 °C. The sorbed amount of RBBR per unit weight of the sorbent at time t (q(t), mmol RBBR g⁻¹), was calculated from the mass balance equation (taking into account the decrement in the volume of the solution) according to:

where C(t)_(i) (M) is the RBBR concentration of the withdrawn sample number i at time t and C(t)₍₀₎ = C₀, V(t)_(i) (mL) is the volume of the solution in the flask at sample number i and time t, and m is the mass of the sorbent in the flask. Here V(t)_(i) − V(t)_(i−1) equals 5 mL (the sample volume).

The effect of temperature on the sorption of RBBR was tested out in 20 mL of dye solutions (5 × 10⁻⁴ M, pH 3) with 0.05 g of sorbent for 100 min at various temperatures.

The effect of sorbent dose on RBBR dye removal was tested out in 20 mL of dye solution (5 × 10⁻⁴ M, pH 3) at 25 ± 1 °C for 100 min.

Regeneration experiments were performed by contact of 0.3 g of the sorbent with 20 mL of 5 × 10⁻⁴ M RBBR at pH 3 for 100 min. The amount of dye sorbed (and the sorption capacity) was determined by the mass balance equation (eqn (1)). The solution was magnetically decanted and the sorbent was washed by distilled water. The loaded sorbent was mixed with 20 mL of 0.5 M NaOH for 30 min. The regenerated sorbent was magnetically decanted, then carefully washed by distilled water for reuse in the second run. The regeneration efficiency (RE, %) was calculated according to the following equation:

3. Results and discussion

3.1. Sorbents characterization

FT-IR spectrometry was used to characterize the structure of the sorbents (Fig. SM1, see ESI†). The two sorbents have similar FT-IR profiles: the same bands appear on the two spectra, the only differences observed are small shifts in some of these bands. The large band in both sorbents around 3291–3299 cm⁻¹ is generally attributed to the combination of stretching vibrations of OH and NH₂ groups, while a series of –CH vibrations are reported around 2940–2942 cm⁻¹. These bands are present in a wide number of compounds and they bring poor information on the specific properties of the sorbents. More interesting and representative are the different bands identified in the range 1800–400 cm⁻¹. The bands at 1265–1267, 1161–1162 and 988–989 cm⁻¹ are representative of the saccharide ring in both sorbents. The bands at 1652–1654 cm⁻¹ is associated to the CONH₂ group³⁰ in MCGMA-I and MCGMA-II sorbents, while the bands at 1561–1562 cm⁻¹ is probably assigned to the in-plane bending vibration of free amine groups and/or the asymmetric carboxylate stretching vibration in both sorbents.^30,31 The grafting of diethylenetriamine, via glycidylmethacrylate, can be identified by the strong peaks observed around 1724–1722 cm⁻¹ (assigned to C=O stretching vibration).³² Additional methylene (–CH₂–) groups can probably be associated to the bands observed at 1469–1475 cm⁻¹.³³ The bands at 595–588 cm⁻¹ in both sorbents were attributed to γ-Fe₂O₃.

The surface morphologies of MCGMA-I (before and after sorption) and MCGMA-II (before and after sorption) are shown in Fig. 1 and 2, respectively. The SEM micrographs reveal that the surface of both sorbents is porous, which can be clearly seen in the magnified view of the SEM micrographs, and the sorption of dye changed the surface structures of both MCGMA sorbents.

Fig. 1 Scanning electron micrographs of; (above) unloaded MCGMA-I, (below) loaded MCGMA-I.

Fig. 2 Scanning electron micrographs of; (above) unloaded MCGMA-II, (below) loaded MCGMA-II.

The Energy Dispersive X-ray analysis (EDX) of the MCGMA sorbents before and after sorption demonstrated that the surfaces of the sorbents contain various elements such as carbon, oxygen, nitrogen, and iron. It is observed that the sulfur is found on the sorbents’ surfaces after sorption process as shown in Fig. 3b and 4b, therefore it is revealed that the sorption of RBBR has ensued on the surface of MCGMA sorbents. The compositions of various elements in weight percentage (weight %) and atom percentage (atomic %) of the sample are illustrated in the graphical representation of EDX shown in Fig. 3 and 4. The level of the sulfur signal observed was sufficient for providing a qualitative idea of the homogeneous distribution of RBBR element at the surface of the sorbent: the percentage (in mass) of sulfur was 2.63% and 1.63% for MCGMA-I and MCGMA-II, respectively.

Fig. 3 Energy dispersive X-ray spectroscopy (EDX) of; (a) unloaded MCGMA-I, (b) loaded MCGMA-I.

Fig. 4 Energy dispersive X-ray spectroscopy (EDX) of; (a) unloaded MCGMA-II, (b) loaded MCGMA-II.

The amine content in MCGMA, MCGMA-I and MCGMA-II is 3.12, 5.42 and 5.86 mmol g⁻¹, respectively. This confirms the enrichment of both MCGMA-I and MCGMA-II by amino groups. On the other hand, the amine content obtained by the HCl method for MCGMA-I and MCGMA-II is consistent with that obtained from EDX analysis (nitrogen mass percents are 7.93% (5.66 mmol g⁻¹) and 8.02% (5.72 mmol g⁻¹) for MCGMA-I and MCGMA-II, respectively). This indicates the success of the modification. The suggested chemical structures of MCGMA-I and MCGMA-II are shown in Scheme 2.

Scheme 2 The suggested chemical structures of MCGMA-I and MCGMA-II.

The magnetic properties of the materials were determined using VSM (vibrating sample magnetometry). Fig. 5 shows their typical magnetization loops. There was no remanence and coercivity, contrary to certain supported-magnetite materials.³⁴ This means that MCGMA sorbents are paramagnetic. The saturation magnetizations of MCGMA-I and MCGMA-II were found to be about 19.1 and 17.9 emu g⁻¹, respectively. These values are much smaller than the levels obtained for bulk phase magnetite (i.e., 49.21 emu g⁻¹). The slight decrease in the magnetization in case of MCGMA-II may be due to the grafting of DETA via epichlorohydrin, which increases the mass of organic material and decreases the relative percentage of magnetite in the sorbent. The reported decrease in saturation magnetization can be explained by several factors including size effects and particle crystallization,³⁵ and obviously by the fact that only a fraction of the sorbent (about 50%) is constituted by the magnetic core. A similar decrease in magnetization was observed for other chitosan–magnetite composites.³⁶ Therefore, MCGMA sorbents can be readily separated with the help of an external magnetic field. This may be very helpful for solid/phase separation or for handling the material in hazardous environments.

Fig. 5 Magnetization curves for MCGMA-I and MCGMA-II sorbents.

3.2. Sorption of RBBR

3.2.1. Effect of pH. The pH of the aqueous solution is a key parameter that controls the sorption. Hence, the effect of pH on the sorption of RBBR dye onto the prepared macro-reticular hybrid materials was investigated in an initial pH range of 1–10. The range of pH to be tested also depends on the stability of the sorbent; in the case of hybrid magnetite macro-reticular hybrid materials, the stability of both the biopolymer and the magnetic core should be considered. The cross-linking treatment contributes to improving the stability of the chemically modified polymer; the main limitation in terms of pH stability is thus related to the stability of the magnetic core. At pH below 1.5–2, Fe₃O₄ may partially and progressively dissolve (especially after long contact times). Fig. 6a gives information on the optimum equilibrium pH for this sorption process. It can be observed from Fig. 6a that the sorption of RBBR is affected by the pH of the solution. The minor change in sorption equilibrium at varying pH values may be attributed to the low initial concentration of the dye (0.1 mM) compared with the available active sites on both MCGMA-I and MCGMA-II sorbents (5.42 and 5.86 mmol g⁻¹). The removal % of RBBR from solution was 97–100%, the 100% removal was achieved at pH_eq 3.4 and 5.7 for MCGMA-I; and at pH_eq 3.5 and 7.0 for MCGMA-II (Fig. SM2, see ESI† ). It can be said that the relatively higher uptake of RBBR in acidic medium may be due to the presence of HCl in the medium which result in the protonation of amine groups on the beads of both sorbents. The RBBR anion (Scheme 3) could be able to exchange through the Cl⁻, which is electrostatically attached to the sorbents, according to the following reaction:

MCGMA-NH⁺Cl⁻ + RBBR-SO₃⁻Na⁺ ↔ MCGMA-NH⁺⁻O₃S-RBBR + Cl⁻

Fig. 6 (a) Effect of pH on the adsorption of RBBR onto MCGMA-I and MCGMA-II; (b) the recorded initial and equilibrium (final) pH: (T: 25 ± 1 °C; C₀: 5 × 10⁻⁴ M).

Scheme 3 Chemical structure of Remazol Brilliant Blue R.

MCGMA sorbents exhibited an almost similar sorption capacity at all pH_eq levels (pH 1–8.36) in this study, and its sorption capacity decreased slightly from 0.033 to 0.32 mmol g⁻¹ and from 0.04 to 0.039 mmol g⁻¹ for MCGMA-I and MCGMA-II, respectively, with an increase in the pH_eq from 1 to 8.36. The pK_a (10.45) of diethylenetriamine is higher than that of chitosan (6.5), and the higher the pK_a value of the ligand (DETA) grafted onto MCGMA sorbents, the wider is the effective pH range for anionic dye sorption.³⁷

The effluent from textile industry is usually at an alkaline pH, and therefore MCGMA sorbents are suitable for the removal of RBBR from alkaline industrial effluent.

Fig. 6b shows the pH variation during RBBR sorption. At pH 1, the equilibrium pH is not significantly changed: the amount of protons exchanged due to protonation of the sorbent is negligible compared to the total amount of protons in the solution. In the range of initial pH 2–7, the equilibrium pH strongly increases up to 3.5–7.9 (depending on the sorbent): protons are bound to sorbent matrix. In the range of initial pH 8–10 the equilibrium pH decreases and the equilibrium pH tends to stabilize around 7.8–8.3: the pH values are close to the pK_a value of amine groups of the ligand DETA,³⁷ other types of inter-molecular forces, such as van der Waals force may be playing a role in RBBR sorption; this may explain the wide pH range for RBBR sorption on MCGMA sorbents.

3.2.2. Kinetics. The time required for reaching the equilibrium has been evaluated by the plot of the sorption capacity (obtained by mass balance) as a function of time, under selected experimental conditions (reported in the caption of Fig. 7). The sorption process can be described by three steps: (a) the initial step lasting for a few minutes (less than 5 min) that counts for 69.6% and 91.2% of total sorption for MCGMA-I and MCGMA-II, respectively; (b) a second step, standing between 5 and 60 min that corresponds to the progressive saturation of the sorbents; and (c) the saturation plateau with a negligible residual sorption (which counts for less than 3% of total sorption). This last section is probably associated with the progressive and slow swelling of biopolymer layer with limited effects of resistance to intraparticle diffusion. The fast sorption (compared with other sorption systems based on chitosan flakes or glycidyl methacrylate) especially in the initial stage, can be attributed to the greater concentration gradient and more available sites for sorption. The second stage is probably controlled by the combination of reaction rate and the swelling effect of the MCGMA layer. As the sorption sites become progressively covered, the rate of sorption decreased. Basically, 80 min of contact appears to be sufficient for roughly achieving equilibrium.

Fig. 7 RBBR uptake kinetics using MCGMA-I and MCGMA-II sorbents (pH 3; T: 25 ± 1 °C; C₀: 5 × 10⁻⁴ M).

Actually, most sorption reactions take place through a series of mechanisms such as:³⁸ (i) external film diffusion, (ii) intra-particle diffusion, and (iii) reaction between sorbate and active site (physisorption and/or chemisorption). The contribution of resistance to film diffusion to kinetic control can be easily reduced by providing a sufficient agitation; with an agitation speed of 100 rpm the effect of film diffusion can be neglected. This is confirmed by the steep initial sorption; film diffusion is usually considered the most significant during the first minutes of contact. The uptake kinetics has been modelled using four models: the so-called pseudo-first and pseudo-second order rate equations, the simplified model of resistance to intraparticle diffusion and the Elovich model.³⁸ These models and their linear forms are reported in Table SM1 (see ESI†) where k₁ is the pseudo first order rate constant (min⁻¹) of sorption and q_e and q_t (mmol g⁻¹) are the amounts of RBBR sorbed at equilibrium and time t, respectively, k₂ is the pseudo second order rate constant (g mmol⁻¹ min⁻¹), K_i is the intraparticle diffusion rate (mmol g⁻¹ min^−0.5), α the initial sorption rate (mmol g⁻¹ min⁻¹) and β the desorption constant (g mmol⁻¹). The validity of each model is checked by the correlation coefficient associated to linear fit. Accordingly, and as shown in Table 1, the analysis of their correlation coefficients shows that the PSORE gives a best fit of experimental data for RBBR sorption on both MCGMA-I and MCGMA-II sorbents. The solid lines in Fig. 7 show the modeling of kinetic profiles with the PSORE: the model fits well the kinetic data, though some discrepancies can be observed in the curvature zone. The poor fit of experimental data with the intraparticle diffusion model (Weber & Morris equation) confirms that the resistance to intraparticle diffusion is not the rate controlling step for uptake kinetics. As expected, designing magnetic macro-reticular hybrid particles contributes to significantly reduce the impact of resistance to intraparticle diffusion.

Table 1 Kinetic parameters for RBBR sorption on the modified MCGMA sorbents^a

Sorbent	qe (a)	PFORE			PSORE			Weber and Morris model				Elovich equation
		k1 (b)	qe,calc (a)	R^2	k2 (c)	qe,calc (a)	R^2	Ki (d)		X	R^2	α (e)	β (f)	R^2
a Units: (a) mmol g^−1; (b) min^−1; (c) g mmol^−1 min^−1; (d) mmol g^−1 min^0.5; (e) mmol g^−1 min^−1; (f) g mmol^−1.
MCGMA-I	0.142	0.034	0.056	0.815	1.455	0.146	0.998	Ki1	0.0048	0.094	0.744	5.59	77.13	0.940
								Ki2	0.0028	0.111	0.677
								Ki3	0.0005	0.134	0.092
MCGMA-II	0.150	0.007	0.022	0.746	1.501	0.148	0.997	Ki1	0.0027	0.111	0.770	121.36	100.45	0.915
								Ki2	0.0020	0.119	0.913
								Ki3	0.0011	0.129	0.506

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Results of the intra-particle Weber–Morris diffusion model showed a multi-linear plot (Fig. SM4a, see ESI† Section), i.e. three linear sections of the plot q_t versus t^0.5, with fast kinetics in first step followed by gradual attainment of equilibrium for both sorbents. The multi-linear plot does not pass through the origin suggesting that intra-particle diffusion is not the sole rate-limiting step, but another step, e.g. film diffusion, could be process of highest resistance in a definite period of time. Significant differences of K_i values (Table 1) indicate the availability of the functional groups is primary controlled by diffusional transport through the pores system in the second and third steps; the structure of the porous sorbents, i.e. pores network, consists of macropores which extend into the particle interior and branch into a tree-like system of meso and micropores. The RBBR ions must diffuse through whole pore systems to reach the total surface area within the particles, where the intra-particle diffusion, resistance due to diffusional transport inside pores, slows down the overall process contributing to formation of a time-dependent concentration gradient due to fast kinetic process at surface, until saturation of all available surface sites was achieved. X represents the boundary layer diffusion effects (external film resistance). As the value of X decreases the effect of boundary layer diffusion on the reaction rate decreases. The obtained values of X (Table 1) indicate that the boundary layer diffusion effect (external film resistance) has a significant effect on the diffusion rate.

Elovich’s equation is another rate equation based on the sorption capacity. In 1934 the kinetic law of chemisorptions was established though the work of Zeldowitsch.³⁹ It has commonly been called the Elovich equation; this Elovich equation was applied to the sorption of RBBR by the modified MCGMA sorbents. The values of α and β were determined from the intercept and slope, respectively, of the linear plot of q_t vs. ln t (Fig. SM4b, see ESI†). The values of α for the sorption of RBBR ions on the modified sorbents are 5.59 and 121.36 (mmol g⁻¹ min⁻¹) for MCGMA-I and MCGMA-II, respectively. These values indicate that the initial sorption rate of RBBR ions is relatively high compared with the previous studies of Elwakeel et al.;^40,41 in that work, different modified chitosan sorbents were grafted with 3-amino-1,2,4 triazole,5-thiol or melamine to obtain efficient sorbents for dye recovery. The high initial sorption rate of RBBR on MCGMA sorbents may be attributed to the high concentration of active sites on the sorbent surface allowed for reacting with RBBR ions. The values of β are found to be 77.13 and 100.45 g mmol⁻¹ for MCGMA-I and MCGMA-II, respectively; β is a factor associated with the degree of surface coverage and activation energy for chemisorptions. These results further support the assumption that the sorption is due to chemisorption.

3.2.3. Equilibrium sorption isotherm. Fig. 8 shows RBBR sorption isotherm at an initial pH of 3.0 (initial pH shifted to 5.7 ± 0.2 and 7 ± 0.2 after metal sorption for MCGMA-I and MCGMA-II, respectively) using the magnetic macro-reticular hybrid materials. The sorption isotherm is characterized by a steep initial slope; the sorption capacity reaches a sorption capacity close to 0.1 and 0.12 mmol RBBR g⁻¹ for MCGMA-I and MCGMA-II, respectively (i.e., about 67% and 74% of the maximum sorption capacity) for a residual dye concentration of 0.11 and 0.03 mM. The sorption capacity then progressively increases with increasing metal concentration up to 0.153 and 0.174 mmol RBBR g⁻¹ for a residual concentration close to 0.32 and 0.26 mM for MCGMA-I and MCGMA-II, respectively. Taking into account the content of inactive magnetic core in the sorbent that represents about 30% of the total sorbent mass the sorption capacity tends to 0.22 and 0.19 mmol RBBR g⁻¹ for MCGMA-I and MCGMA-II, respectively.

Fig. 8 RBBR sorption isotherms onto MCGMA-I and MCGMA-II sorbents (solid lines are simulated from the Langmuir equation) (pH 3; T: 25 ± 1 °C).

The equilibrium sorption isotherms are some of the most important data for determining the mechanism of RBBR sorption on the modified MCGMA sorbents. Several isotherm models have been used to describe experimental data for sorption isotherms. However, among these, three different appropriate isotherm models were selected for this work for the analysis of the equilibrium data: the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models.⁴² These models and their linear forms are reported in Table SM2 (see ESI†), where q_e is the adsorbed value of RBBR at equilibrium concentration (mmol g⁻¹), q_L is the maximum sorption capacity or the saturation of the monolayer (mmol g⁻¹) and K_L is the Langmuir binding constant which is related to the energy of sorption (L mmol⁻¹), C_e is the equilibrium concentration of RBBR in solution (mmol L⁻¹). K_F (mmol g⁻¹) (L mmol⁻¹)^1/n and n are the Freundlich constants related to the sorption capacity and intensity, respectively. K_DR (J² mol⁻²) is a constant related to the sorption energy, Q_DR (mmol g⁻¹) is the theoretical saturation capacity, ε (J² mol⁻²) is the Polanyi potential, T is the temperature where the sorption occurs, A_T (L mmol⁻¹) is the Temkin isotherm constant, b_T (J mol⁻¹) is Temkin constant in relation to heat of sorption.

The Langmuir model is the simplest theoretical model for monolayer sorption onto a surface with a finite number of identical sites. It was originally developed to represent chemisorption on a set of distinct sorption sites. The values of q_L and K_L were determined from the slope and intercept, respectively, of the linear plot of C_e/q_e vs. C_e (Fig. SM5a, see ESI†) of the Langmuir equation and are given in Table 2. The asymptotic shape of the sorption isotherm (Fig. 8) suggests that the Langmuir equation is probably more appropriate for fitting experimental data than the other equations (e.g., the exponential trend of the Freundlich equation is not consistent with the profile of the curve). The Langmuir model is based on the assumption that sorption sites are identical and energetically equivalent, and that sorption occurs at monolayer coverage. The Langmuir parameters can be used to predict the affinity between the sorbate and sorbent using the dimensionless separation factor R_L

where K_L is the Langmuir equilibrium constant and C_o is the initial concentration of RBBR ions. Values of 0 < R_L < 1 indicate the “suitability” of the process. In this study, the values of R_L of the magnetic macro-reticular hybrid materials for the sorption of RBBR ions lie between 0.01 and 0.12 for both sorbents and all concentrations at 25 °C. This means much smaller than 1.0, and that the sorption is very favorable whatever the concentrations range.

Table 2 Parameters of the sorption isotherm models^a

Sorbent	Langmuir model				Freundlich model			Dubinin–Radushkevich (D–R) model				Temkin model
	qmax,exp (a)	qm,L (a)	KL (b)	R^2	n	KF (c)	R^2	QDR (a)	KDR (d)	Ea (e)	R^2	AT (f)	bT (e)	R^2
a Units: (a) mmol g^−1; (b) L mol^−1; (c) mmol g^−1 (L mmol^−1)^1/n; (d) J^2 mol^−2; (e) kJ mol^−1; (f) L mol^−1.
MCGMA-I	0.153	0.139	68.424	0.965	0.153	8.053	0.861	0.1308	2.07 × 10^−9	15.548	0.896	1089.21	240.500	0.850
MCGMA-II	0.174	0.171	124.45	0.988	0.224	5.509	0.851	0.173	3.39 × 10^−9	12.150	0.900	53.240	140.818	0.915

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The Freundlich isotherm model is applicable to highly heterogeneous surfaces. The values of n and K_F were determined from the slope and intercept, respectively, of the linear plot of log q_e vs. log C_e (Fig. SM5b, see ESI†) of the Freundlich equation and are given in Table 2. A larger value of n implies strong interactions between the sorbent and dye anion. When n equals one it indicates linear sorption leading to identical sorption energies for all the active sites.

Another popular equation for the analysis of isotherms is that proposed by Dubinin and Radushkevich (Table SM2, see ESI†). This isotherm was developed to account for the effect of the porous structure of the sorbents, and the energy involved in the process. The Polanyi potential (ε) is given as eqn (6):⁴³

R is the universal gas constant (8.314 J mol⁻¹ K⁻¹) and T is the absolute temperature (K). The slope of the plot of ln q_e vs. ε² (Fig. SM6a, see ESI† Section) gives K_DR and the intercept yields Q_DR (Table 2). The D–R constant (K_DR) can give the valuable information regarding the mean energy of sorption by eqn (7):where E_a (J mol⁻¹) is the mean sorption energy; in the range 1–8 kJ mol⁻¹ E_a describes a physical sorption, while chemical sorption corresponds to a mean energy higher than 8 kJ mol⁻¹.⁴³ The value of the mean sorption energy (E_a) is in the range of 8–16 kJ mol⁻¹ (Table 2) indicating that the sorption is predominantly a chemisorption process. Furthermore, this is consistent with the strong interaction between the RBBR dye and the sorbent active sites.

The Temkin equation was also used to model the sorption isotherm (Table SM2, see ESI†). The Temkin isotherm takes into account the interactions between sorbent particles and sorbate assuming that the free energy of sorption is a function of the surface coverage.⁴⁴ The constant A_T reflects the initial sorption heat of the sorbent: the greater the A_T value the higher the sorption heat and the greater the affinity of the sorbent for the sorbate. The obtained values confirm the strong interaction between RBBR and the reactive groups at the surface of the MCGMA sorbents.

3.2.4. Influence of temperature. The sorption of RBBR onto the surface of the modified MCGMA sorbents reaches an equilibrium state then no further tendency is shown to change the concentrations of RBBR onto the sorbents surface and also in aqueous solution. At equilibrium, the rate of sorption of the RBBR dye onto the sorbents surface is equal to the rate of desorption of RBBR dye from the surface of the sorbents. The concentration of RBBR onto the sorbent surface and in aqueous solution becomes practically constant. The ratio of effective concentration of RBBR onto solid sorbents to the liquid aqueous solution is constant called the equilibrium constant, K_c of the sorption process.where C_s and C_e are equilibrium concentrations of RBBR onto the sorbents surface and in aqueous solution, respectively.

The classic van ’t Hoff reaction isotherm equation describes the correlation between the free energy change (ΔG), standard free energy change (ΔG^o) and equilibrium constant (K_c) at constant temperature (T) as:

ΔG = ΔG^o + RT ln K_c (9)

At equilibrium, the free energy change (ΔG) is zero; hence the eqn (9) reduces to

ΔG^o = −RT ln K_c (10)

This is the most important equation used in sorption thermodynamics to predict the feasibility of a sorption process. The standard free energy change is the free energy change of the sorption process at unit concentrations of sorbate and unit active sites of the sorbent.

To predict standard enthalpy change (ΔH^o) and standard entropy change (ΔS^o), eqn (10) can be rearranged in the form of equilibrium constant (K_c) as:

According to classic thermodynamics, the thermodynamic parameters standard free energy change (ΔG^o) is also related to standard enthalpy change (ΔH^o) and standard entropy change (ΔS^o) at constant temperature by the equation:

ΔG^o = ΔH^o − TΔS^o (12)

Therefore the van ’t Hoff equation becomes:

The values of standard enthalpy change (ΔH^o) and standard entropy change (ΔS^o) for the sorption of RBBR are determined from the slope and intercept for the plot of ln K versus 1/T and are listed in Table 3. The positive values of ΔH^o confirm the endothermic nature of sorption process. The negative values of ΔG^o (Table 3) indicate that the sorption reaction is spontaneous; the increase in the negativity of ΔG^o with increasing temperature confirms that the sorption is favorable at higher temperatures. Also the values of ΔG^o for MCGMA-II are more negative than that of MCGMA-I at low temperatures, which confirms that the sorption of RBBR ions is more favorable on MCGMA-II than MCGMA-I at low temperatures.

Table 3 Standard enthalpy, entropy and free energy changes for RBBR sorption

Sorbent	ΔH^o (kJ mol^−1)	ΔS^o (J mol^−1 K^−1)	T0 (K)	ΔG^o (kJ mol^−1)
				303 K	308 K	318 K
MCGMA-I	63.14	214.22	294.76	−1.76	−2.83	−4.97
MCGMA-II	20.99	78.49	267.47	−2.78	−3.18	−3.96

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In industry the optimum temperature at which the sorption is highly feasible and spontaneous is essential. The range of temperature can be predicted from the value of temperature at which the standard free energy is zero (T₀) for certain sorption processes. The zero standard free energy (T₀) of certain sorption processes can be evaluated from eqn (14) with the same sign of thermodynamic parameters standard enthalpy change (ΔH^o) and standard entropy change (ΔS^o)

This optimum temperature (T₀) at which the standard free energy is zero predicts the actual range of temperature at which the sorption process is feasible and may be used in wastewater sorption treatment in chemical engineering technology. The calculated values of zero standard free energy temperature (T₀) are 294.76 and 267.47 for MCGMA-I and MCGMA-II, respectively. The low T₀ for MCGMA-II indicates the feasibility of RBBR removal at low temperature.

3.2.5. Effect of sorbent dose. The sorption of RBBR on the modified MCGMA sorbents was studied by changing the quantity of sorbent in the range of 0.01 to 0.1 g for 20 mL of dye solution, with a dye concentration of 0.47 mM at 25 °C and pH of 3.0. The results in Fig. 9a show the RBBR sorption capacity as a function of sorbent amount. It has been found that the sorption capacity decreases from 0.41 mmol g⁻¹ to 0.09 when the mass of MCGMA-I and MCGMA-II increases from 0.01 to 0.1 g, respectively. Fig. 9b shows the effect of dose on the equilibrium concentration (C/C₀) of RBBR by the modified MCGMA sorbents. As the dose increases, the equilibrium concentration of RBBR is decreased, which is due to the increase in the sorbent surface area available for dye uptake. The results shown indicate that the removal efficiency increases up to C/C₀ = 0.0002 at sorbent dose of 0.1 g for 20 mL dye solution. At the low dose of the sorbents, all of the sites are exposed entirely and the sorption on the surface is saturated faster showing a higher sorption capacity. With an increase of sorbent dosage the amount of sorption sites is at an excess compared to present RBBR ions and the saturation of the sorption sites is not achieved. According to this result, the dose of 0.01 g for 20 mL dye solution will achieve the maximum loading capacity for the sorbent. On the other hand with a sorbent dosage of 0.1 g for 20 mL dye solution it is possible to remove about 99.98–100% of RBBR from the solution (achieving the environmental target). The sorbent dosage to be used depends on the target of the sorption process: concentration effect or maximum decontamination.

Fig. 9 Effect of sorbent dose (SD) on RBBR sorption using MCGMA-I and MCGMA-II sorbents: (a) sorption capacity vs. SD, (b) relative residual concentration (C/C₀) vs. SD (C₀: 5 × 10⁻⁴ M; T: 25 ± 1 °C; pH 2).

3.2.6. Effect of ionic strength (addition of NaCl). The effect of chloride ions on RBBR removal was examined by addition of increasing concentrations of NaCl (from 5 to 35 g L⁻¹; C₀: 0.5 3 M; sorbent dosage: 0.05 g 20 mL⁻¹) (Fig. 10). For the studied sorbents increasing the amount of NaCl hardly effected the sorption capacity: the sorption capacity decreases by 3.9–8.0%, when NaCl concentration reaches 35 g L⁻¹. This is probably due to the competitor effect of chloride anions against RBBR anions for interaction with the sorption sites; this indicates that even under these drastic conditions a high sorption capacity is maintained.

Fig. 10 Influence of NaCl on RBBR sorption onto MCGMA-I and MCGMA-II sorbents (C₀: 5 × 10⁻⁴ M; initial pH 2; T: 25 ± 1 °C; sorbent dosage: 0.05 g 20 mL⁻¹).

3.3. Comparison of sorption performance for RBBR ions with various sorbents

Table 4 shows a comparison of the maximum sorption capacities of materials found in the recent literature; the best operating conditions are also presented. A strict comparison is difficult since maximum sorption capacities were not obtained under similar experimental conditions; however, these data are sufficient to show that the modified MCGMA sorbents have a sorption capacity of the same order of magnitude as other sorbents (and in some cases better). The critical point that confirms the interest of these materials is the relatively fast sorption compared to other systems. The design of a magnetic core coated by a thin layer of chemically-modified MCGMA allows a reduction of the contribution of the resistance to intraparticle diffusion and makes kinetically very efficient sorbents for RBBR binding. The high sorption capacity of the modified MCGMA sorbents towards RBBR ions reveals that the sorbents could be promising for practical applications in RBBR ions removal from wastewater.

Table 4 Comparison of sorption performance for RBBR dye with various sorbents appeared in the literature

Adsorbent material	Initial pH	Contact time (min)	Temperature (°C)	Initial concentration (mg L^−1)	Sorbent dosage (g L^−1)	Sorption capacity (mg g^−1)	References
Polyurethane-foam	3.0	100	60	50	10	5.00	45
Chitosan–lignin composites	2.0	180	27 ± 2	300	2	95.93	46
Sodium alginate/poly(N-vinyl-2-pyrrolidone) blend hydrogel	1.2	600	25	250	1.3	55.28	47
Coconut shell based activated carbon	12.0	20	28.15	50	1.0	2.20	48
Alumina/carbon nanotube hybrid adsorbents	3.0	180	35	200	0.4	3.67	49
P. eryngii immobilized on Amberlite XAD-4	2.0	—	38.7	36.3	6.08	1.92	50
Functionalized multi-walled carbon nanotubes	2.5	20	25 ± 2	100	0.5	177.52	51
MCGMA-I	2.0	60	25 ± 1	313	2.5	95.98	This work
MCGMA-II	2.0	60	25 ± 1	313	2.5	109.14	This work

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3.4. Regeneration

Desorption of anionic dyes is generally operated by pH change. In most cases, desorption is performed under basic conditions. Regeneration of the MCGMA sorbents was carried out by contacting 20 mL of 0.5 M NaOH with the MCGMA sorbents for 30 min. Table 5 reports the evolution of regeneration efficiency for 4 sorption/desorption cycles. The sorbents show quite good stability in both sorption and desorption performance along the 4 sorption/desorption cycles. A slight decrease of sorption performance is observed but even at the fourth step the decrease in sorption efficiency is less than 5%. The hybrid magnetic macro-reticular materials can thus be used for repeated sorption/desorption cycles with high efficiency.

Table 5 Regeneration efficiency (%) of the modified MCGMA sorbents for RBBR removal

Cycle order	Sorbent
	MCGMA-I	MCGMA-II
1	99.9%	99.8%
2	99.4%	100.2%
3	95.9%	99.9%
4	98.6%	98.4%

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4. Conclusion

Magnetic macro-reticular hybrid synthetic-natural materials were prepared based on glycidyl methacrylate (synthetic polymer) and chitosan (natural polymer). SEM and EDX analyses show irregular objects with very porous surfaces and the presence of sulfur after RBBR sorption. The prepared sorbents are characterized by efficient and selective sorption towards RBBR ions from aqueous medium at approximately pH 2. The uptake kinetics are most closely described by the PSORE, while the distribution of the metal at equilibrium between the solid and the liquid is modeled by the Langmuir equation. Sorption isotherms (fitted by the Langmuir equation) show maximum sorption capacities close to 0.153 and 0.174 mmol g⁻¹ for MCGMA-I and MCGMA-II, respectively. The sorption mechanism is associated to an anion exchange/electrostatic attraction mechanism involving RBBR anions and charges of the sorbents. Increasing the concentration of NaCl hardly effects the sorption capacity. The nature of the sorption process is endothermic at the studied temperature range. The thermodynamic parameters show that the sorption is spontaneous and produces an increase in the “disorder” of the system. Finally, the RBBR anions can be efficiently desorbed from loaded sorbents with alkaline solution of 0.5 M NaOH and the sorbents can be reused for a minimum of 4 sorption/desorption cycles with a limited loss in efficiency (less than 5%) at the fourth sorption cycle.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26508h

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