Modeling of adsorption behavior of the amine-rich GOPEI aerogel for the removal of As(III) and As(V) from aqueous media

Abstract

In the present study, a PEI cross-linked graphene oxide aerogel (GOPEI) was prepared. The interaction of GO and PEI was investigated by FT-IR and XPS analysis and after that further characterization was conducted using SEM, EDX, XRD, Raman spectroscopy and BET surface area measurement. The prepared GOPEI aerogel was utilized for the treatment of As(V) and As(III) contaminated water. The maximum uptake capacity, 4.80 ± 0.27 mg g⁻¹ for As(V) and 4.26 ± 0.24 mg g⁻¹ for As(III), was obtained at an initial As(V)/As(III) concentration of 3 mg L⁻¹, GOPEI dose 0.6 g L⁻¹, and ambient temperature (30 °C). The adsorption process was found to be pH sensitive where the optimum pH was 4 for As(V) and 7 for As(III) whereas at pH 6 significant uptake capacity was observed for both As(V) and As(III), which is close to the pH of drinking water. Therefore, GOPEI can be used for the adsorption of As(V) as well as As(III) at a common pH and ambient temperature without much changing the pH of drinking water. In order to use the GOPEI aerogel as an adsorbent in a continuous column for the treatment of arsenic contaminated water, the practicability was tested by conducting detailed kinetics, isotherm and thermodynamics studies. The adsorption, which occurs on a monolayer on the heterogeneous surface of the GOPEI aerogel, was found to be thermodynamically feasible and follows pseudo-second-order kinetics.

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1. Introduction

The presence of arsenic in ground and surface water is considered a serious worldwide environmental problem because it is ubiquitous in the Earth’s crust.¹ However, combustion of fossil fuels, mining and use of arsenic containing pesticides and herbicides are the anthropogenic sources, but arsenic contamination in natural water bodies are of geogenic origin.² Arsenic can show toxicity to human health even at low concentration because it has a tendency to accumulate in living tissues. The two oxidation states i.e. arsenite [As(III)] and arsenate [As(V)] of inorganic arsenic, are mainly present in the ground as well as surface water as a harmful contaminant. As(III) is more toxic than As(V) because it undergoes facile intake by living cells and has strong bonding affinity toward proteins.^3,4 The intake of As(V) and As(III) contaminated drinking water causes various detrimental consequences on human health such as blackfoot disease, nervous disorders, cardiovascular disorders and cancers of different organs.⁵ For the removal of As(V) and As(III) from water several methods such as precipitation,⁶ coagulation,⁷ osmosis,⁸ reverse osmosis,⁹ ion-exchange,¹⁰ ultrafiltration¹¹ and adsorption¹² have been reported. Among them, adsorption is efficient and economically viable and has the possibility of metal recovery.¹³ Even though researchers have demonstrated numerous adsorbents for the removal of As(V) as well as As(III), their practical application suffer due to a lack of detailed kinetics, isotherms and thermodynamics studies which are essential to design column studies.¹⁴

In the past decades, porous materials such as silica gel,¹⁵ zeolites¹⁶ and metal–organic frameworks¹⁷ have been extensively used as adsorbents for the removal of environmental pollutants. Aerogels are a porous material, therefore they deserve special attention in the respect of porosity and high surface area. Aerogels have been used as electrically conducting and non-conducting materials, adsorbents and supporting materials for catalysis due to their unique structure comprised of open and interconnected pores in a continuous solid matrix.^18,19 Generally, metal oxides or inorganic aerogels are prepared by a sol–gel process followed by freeze drying for the removal of solvent leaving a porous structure whereas, polymeric aerogels are prepared by freeze-drying the cross-linked hydrogel of molecular precursors.²⁰ Recently, enormous interest in graphene for the preparation of aerogels has grown due to its high theoretical surface area, remarkable mechanical properties and chemical stability.^21,22 The preparation of graphene aerogels is performed in three consecutive steps: preparation of a graphene oxide (GO) hydrogel, freeze drying and reduction.^23–27 The intrinsic modification of aerogels with foreign atoms such as nitrogen and sulfur modulates the physicochemical properties, especially the surface chemistry of the graphene aerogel.²⁸ Liu et al. prepared a nitrogen doped graphene aerogel through the thermal treatment of a hybrid assembly of GO and o-phthalonitrile.²⁹ Tingting et al. obtained a nitrogen and sulfur doped graphene aerogel using molecular precursors, GO and thiourea, at 90 °C followed by thermal reduction at 200 °C.³⁰ In the present study, the surface chemistry of the aerogel was tailored for the adsorption of As(V) and As(III) by keeping the aqueous chemistry of arsenic in mind. In connection to this, polyethyleneimine (PEI) was selected as an amine group-rich cross-linker for the preparation of a nitrogen-doped aerogel because the amine groups of PEI can easily interact with the oxygen containing functional groups of GO and recently, it has also been used for environmental applications.³¹ The simple mixing of PEI into a GO suspension forms a cross-linked hydrogel without heat treatment and is converted into the GOPEI aerogel by freeze drying. The obtained GOPEI aerogel was utilized for the adsorption of As(III) and As(V) in an aqueous medium. The parameters affecting the adsorption of As(V) as well as As(III) on the GOPEI aerogel were optimized by batch mode experiments. The detailed kinetics, isotherm and thermodynamics studies of adsorption of As(III) and As(V) on the GOPEI aerogel were conducted. These batch mode experiment data will be useful in the next step of our GOPEI based column study which will be helpful to design a pilot plant for the treatment of As(V) and As(III) contaminated water.

2. Material and methods

2.1. Materials

Graphite powder (mesh size 150 μm), sulfuric acid (H₂SO₄, 99.99%), phosphoric acid (H₃PO₄, 99.00%) and potassium permanganate (KMnO₄, >99.00%) were purchased from Sigma-Aldrich. Branched polyethyleneimine (PEI) (99%, M.W. 1800), arsenic(III) oxide (As₂O₃, 99.5%) and sodium hydrogen arsenate heptahydrate (Na₂HAsO₄·7H₂O, 98%) were purchased from Alfa Aesar. Hydrogen peroxide (H₂O₂, 30%), hydrochloric acid (HCl, 35.4%), sodium hydroxide (NaOH, 97%) and nitric acid (HNO₃, 72%) were procured from SD Fine Chemicals Pvt. Ltd. All the chemicals were of analytical grade and used without further purification. The deionized double distilled water (DW) was used for making the standards and other solutions throughout the experiments. A stock solution of 1000 mg L⁻¹ As(V) was prepared by dissolving 4.164 g of Na₂HAsO₄ in 1000 mL of DW. 1.320 g of As₂O₃ was dissolved in the 10 mL of 5 M NaOH solution. Then it was neutralized with 5 M HCl solution and diluted up to 1000 mg L⁻¹ As(III) concentration. The working solutions of desired concentration were prepared from the stock solution for each experimental run.

2.2. Preparation of the GOPEI aerogel

Graphene oxide (GO) was prepared by a modified Hummers method.³² Briefly, 3 g graphite powder and 15 g KMnO₄ were mixed into a 9 : 1 (360 : 40 mL) mixture of concentrated H₂SO₄ and H₃PO₄. The thus obtained mixture was kept stirring at 50 °C for 12 h. Thereafter, the reaction mixture was cooled to room temperature and poured slowly into the ice bath. Then, 3 mL of 30% H₂O₂ was added into the reaction mixture with continued stirring for an additional 2 h. Further, the reaction mixture was centrifuged at 10 000 rpm for 15 min. Thus obtained solid material was collected and washed three times with 5% HCl and subsequently with DW until the pH became around 7. For each wash, the material was suspended through ultrasonication and recollected through centrifugation at 10 000 rpm for 15 min. Then, the final washed material was vacuum dried for 24 h.

The prepared GO was used for the preparation of the GOPEI aerogel. 0.05 g of GO was suspended into 50 mL water using a probe ultrasonicator (Sonics, Vibra Cell). Then, 0.1 g PEI was added into the GO suspension which immediately formed the hydrogel and excluded excessive solvent. Thus obtained hydrogel was frozen at −50 °C for 6 h and lyophilized for 6 h to obtain the GOPEI aerogel.

2.3. Characterization of the GOPEI aerogel

The Fourier transform-infrared (FT-IR) spectra of GO and the GOPEI aerogel were recorded in the range 4000–400 cm⁻¹ with KBr pellets using a Spectrum 100, PerkinElmer spectrometer. The type of constituent atoms and their bonding were explored through X-ray Photoelectron Spectroscopy (XPS) analysis using Mg Kα (1253.6 eV) radiation as the X-ray source with AMICUS, Kratos Analytical A Shimadzu, serial number-Cb40143/01. Scanning Electron Microscopy (SEM) (Quanta 200 F) equipped with Energy-Dispersive X-ray Spectroscopy (EDS) was used to observe the morphology and elemental analysis of the GO and the GOPEI aerogel. The Raman spectra were recorded at room temperature with a Renishaw inVia Raman spectrometer equipped with a diode pumped solid state laser of wavelength 532 nm. The Raman scattered light was collected in back-scattering geometry with a slit width of 50 μm. The structural phases of prepared GO and GOPEI aerogel were observed by X-ray diffraction (XRD) patterns recorded in a 2θ range of 5–40° using a Rigaku HR-XRD (SMART Lab, 9 kW power type) with Cu Kα (λ = 1.5406 Å) radiation and Ni filter. The X-ray scan rate was 2° min⁻¹ with a step size of 0.02°. The specific surface area (SSA) of the GOPEI aerogel was measured by a Brunauer–Emmett–Teller (BET) method through a nitrogen adsorption–desorption isotherm conducted using a Micrometrics, USA, ASAP 2020 Model.

2.4. Determination of pH of zero point charge

The pH of zero point charge (pH_zpc) is the pH at which the surface of the adsorbent becomes electrically neutral. The pH_zpc of the GOPEI aerogel was investigated for the determination of surface charge behavior with the changing solution pH. To determine the pH_zpc, six Erlenmeyer flasks (100 mL) containing 10 mL (0.01 N) NaCl solution were taken. The initial pH of the solutions was adjusted between 2 and 12 using a 0.1 N solution of HCl/NaOH. Thereafter, 0.01 g GOPEI aerogel was suspended in the NaCl solution of each flask and capped as well. The suspensions thus formed were stirred at 30 °C for 5 h. Afterwards, the GOPEI aerogel was separated from the suspensions through centrifugation and the supernatants were collected for the measurement of final pH. Then the pH_zpc was calculated by a plot of initial pH vs. final pH.

2.5. Batch adsorption studies

The batch adsorption experiments were conducted in 100 mL Erlenmeyer flasks. The calculated amount of GOPEI aerogel was added into the Erlenmeyer flasks containing 10 mL solution of As(III)/As(V) of desired concentration and well sealed. Afterwards, the flasks were kept on the thermostatic orbital shaker at 100 rpm for a predefined time. After adsorption, the GOPEI aerogel was separated by centrifugation at 5000 rpm for 5 min and the supernatant was collected for the analysis of the remaining As(III)/As(V). The concentration of As(III)/As(V) in the samples was measured using Atomic Absorption Spectrophotometry (AAS, Shimadzu AA-6300) connected to a hydride vapor generator (HVG, Shimadzu HVG-1) for the conversion of As(III)/As(V) into their hydrides. A hollow cathode lamp was used as a light source with a wavelength of 193.47 nm and the deuterium lamp was used for the background correction. The lamp-current was 12 mA whereas slit width was 0.7 nm. Acetylene (98%) was used as a fuel at a pressure of 0.9 kg cm⁻² and flow rate of 4.0 L min⁻¹ with the compressed air at pressure 3.5 kg cm⁻² and flow rate of 17.5 L min⁻¹. The pure argon gas (99.99%) was used at a flow rate of 70.0 mL min⁻¹ and pressure 3.2 ± 0.2 kg cm⁻² for purging purposes.

The uptake capacity (q) of the GOPEI aerogel for As(III)/As(V) was calculated using equation given below.³³

where C_i is the initial As(III)/As(V) concentration (mg L⁻¹), C_t is the residual As(III)/As(V) concentration (mg L⁻¹) in solution after adsorption, V is the volume (L) of working solution and W is the weight (g) of the GOPEI aerogel added to the working solution.

Initially, the effect of process parameters was investigated in a wide range such as contact time 0–90 min, initial As(III)/As(V) concentration 0.5–4 mg L⁻¹, pH 2.0–11.0, GOPEI dose 0.1–1 g L⁻¹ and temperature 10–50 °C by varying one parameter at a time while keeping other parameters constant. Then, only the significant effective ranges of process parameters were discussed in the study. Each adsorption experiment was repeated three times and the average value was taken into account for further analysis.

2.6. Kinetics studies

The time dependent kinetics of the adsorption process is influenced by the physical and chemical properties of the adsorbent. Therefore, the kinetics studies of the adsorption process are important to help us understand the mechanism of adsorption and to design a treatment system. In order to investigate the kinetics of As(III)/As(V) adsorption on the GOPEI aerogel, various kinetics models such as pseudo-first order, pseudo-second order, mass transfer, intraparticle diffusion and Richenberg were used.

2.6.1. Pseudo-first order. The linear equation of the pseudo-first-order model can be given as:³⁴where k_s represents the equilibrium rate constant which can be calculated from the slope of the plot log(q_e − q_t) vs. t (min). q_t and q_e are the uptake capacity (mg g⁻¹) at time t and at equilibrium respectively.

2.6.2. Pseudo-second order. The linear equation of the pseudo-second-order model can be written as follows:³⁵where k′₂ represents the equilibrium rate constant for the pseudo-second-order kinetic model.

2.6.3. Mass transfer study. The adsorption process is carried out by the mass transfer of adsorbate from the liquid phase to the adsorbent surface. The mass transfer of adsorbate from liquid phase to adsorbent surface involves four consecutive steps as follows:¹⁴

(1) The transport of adsorbate from the bulk solution to the boundary film.

(2) Diffusion of adsorbate from the boundary film to the adsorbent surface i.e. external diffusion.

(3) Transfer of the adsorbate from the adsorbent surface to the active sites present in interparticle space and pores i.e. intraparticle diffusion.

(4) Adsorption and desorption of the adsorbate on the active sites of the adsorbent.

Step (1) cannot be a rate limiting step because the adsorption process is performed under continuous shaking conditions which inhibit a concentration gradient of adsorbate being established between the liquid phase and boundary film. Step (4) is considered as a quasi instantaneous mechanism. Therefore, either intraparticle diffusion or external diffusion would be the rate controlling step as the rate of adsorption usually depends on the rate of the slowest step. The contribution of these two steps in the overall rate of adsorption can be predicted through Mckay et al.’s model which has been used in its linear form:^14,36

where C_i (mg L⁻¹) denotes the initial adsorbate concentration, C_t (mg L⁻¹) represents the adsorbate concentration after time t (min), k is the Langmuir constant calculated by Langmuir isotherm and m (g L⁻¹) is the mass of adsorbent used per unit volume of the working solution. S_s (cm⁻¹) is the specific surface area of adsorbent present per unit volume of reaction mixture and β_t (cm² s⁻¹) is the external mass transfer coefficient.

2.6.4. Intraparticle diffusion. The slow intraparticle diffusion of the adsorbate is often present during adsorption on porous materials which play an important role in the rate of adsorption. The Weber–Morris model of intraparticle diffusion was used to identify the contribution of intraparticle diffusion on the overall rate of the current adsorption process. The linear form of the Weber–Morris model can be represented as follows:³⁷

q_t = k_idt^0.5 + C (v)

where k_id (mg g⁻¹ min^−0.5) represents the intraparticle diffusion rate constant which can be calculated from the slope of the plot of q_t vs. t^0.5 and C represents the intercept.

2.6.5. Richenberg model. In order to investigate the contribution of different adsorption mechanisms such as mass transfer and intraparticle diffusion on the actual rate of adsorption the Richenberg model was employed, which can be presented as follows:³where G = q_e/q_t and B_t is the mathematical function of G which can be calculated for each value of G by the following equation.

B_t = −0.4977 ln(1 − G) (vii)

The calculated B_t was plotted against t.

2.7. Isotherm studies

Isotherm studies are useful to measure the distribution of adsorbate between the liquid and solid phase at equilibrium. The isotherm parameters explain the characteristics of the adsorbent’s surface and adsorption affinity of adsorbent at fixed temperature and pH. Therefore, the selection of a suitable adsorption isotherm is essential to design a practical adsorption system.³⁸ In the present study the equilibrium data were applied to the most commonly used isotherm models such as Langmuir, Freundlich and the Dubinin–Radushkevich (D–R) model.

2.7.1. Langmuir isotherm. This isotherm model assumes a monolayer adsorption of adsorbate on the adsorbent surface where no interaction exists between two neighboring adsorbate molecules. The linear form of the Langmuir isotherm model can be given as follows:³⁹where q⁰ is the maximum uptake capacity possible after complete monolayer adsorption on the adsorbent surface and b is Langmuir parameter of binding energy.

2.7.2. Freundlich isotherm. The Freundlich isotherm assumes a multilayer adsorption and heterogeneous distribution of binding sites on the adsorbent surface. Its linear form can be written as follows:⁴⁰where k_f is the Freundlich isotherm constant and n represents the Freundlich parameter of adsorption intensity. k_f and n can be calculated from the intercept and slope respectively of the plot log C_e vs. log q_e.

2.7.3. D–R isotherm. Dubinin and Radushkevich have proposed a more general D–R isotherm on the basis of the adsorption potential theory of Polanyi rather than an assumption on the homogeneous surface or constant adsorption potential.^41,42 The D–R isotherm is employed using equilibrium data for the investigation of adsorption free energy and nature of the adsorption. Its mathematical equation can be written in linear form as follows:⁵

ln q_m = ln X_m − βF² (x)

where X_m is the maximum adsorption capacity (mmol g⁻¹) and q_m is the adsorbate adsorbed (mmol g⁻¹). β is an isotherm constant which can be calculated from the slope of the plot of ln q_e vs. F². F is the Polanyi potential which can be calculated from the following equation:where R is the gas constant and T is the temperature of the adsorption equilibrium.

The isotherm constant β is related to the adsorption free energy (E) by the following relationship.

E is the energy required for the adsorption of one mole of adsorbate on the adsorbent surface. The value of E can classify the nature of the current adsorption system as either physical or chemical.

2.8. Thermodynamics studies

The proper investigation of thermodynamics parameters could provide us with the energy change during the adsorption and thermodynamic feasibility of the adsorption process. The thermodynamics parameters, such as change in Gibbs free energy (ΔG⁰), entropy (ΔS⁰) and enthalpy (ΔH⁰) of adsorption, were calculated from the following equations.⁴³

ΔG = −RT ln k_c (xiv)

where C_Ae (mg L⁻¹) is the concentration of adsorbate on the adsorbent at equilibrium, C_e (mg L⁻¹) is the concentration of adsorbate remaining in the liquid phase at equilibrium, R denotes the universal gas constant (8.314 J mol⁻¹) and T (K) represents the absolute temperature.

3. Results and discussion

3.1. Characterization of the GOPEI aerogel

In the present study, two properties of different materials i.e. high surface area of GO and high nitrogen content of PEI are combined to prepare the aerogel. For the primary confirmation, the FT-IR spectra of GO, PEI and GOPEI aerogel were recorded (Fig. 1). The FT-IR spectrum of GO showed peaks at 3147.29, 1714.56, 1614.43 and 1029.59 cm⁻¹ which were attributed to ν_s(O–H), ν_s(C=O), ν_s(C=C) and ν_s(C–O) respectively. The FT-IR spectrum of the GOPEI aerogel shows peaks at 1646.37 and 1516.45 cm⁻¹ attributed to ν_s(C=O) and ν_b(N–H) respectively, which were due to the formation of the amide bond between GO and PEI. The peak at 3280.79 cm⁻¹ in the FT-IR spectrum of PEI represents amine groups that shift to higher wavenumber in the GOPEI aerogel and become distinguishable into the different degree of amine groups. The peaks at 3843.30 and 3743.69 cm⁻¹ from the GOPEI aerogel appeared due to the ν_s(N–H) and ν_as(N–H) of primary amines, respectively. Whereas, the peak at 3619.23 cm⁻¹ were attributed to the ν_s(N–H) of the secondary amine. Moreover, the FT-IR spectra of the GOPEI aerogel exhibited some of the same peaks as PEI such as 2923.42, 2841, 1456.36, 1264.51, 1041.82 cm⁻¹.

Fig. 1 FT-IR spectra of GO, PEI and the GOPEI aerogel.

The XPS analysis of GO and the GOPEI aerogel was conducted to determine the chemical structure and different possible bonding between them. The wide range XPS spectra of GO and GOPEI aerogel are presented in Fig. 2a. The XPS spectrum of GO has peaks corresponding to the bonding energy of C 1s and O 1s electrons whereas, the GOPEI aerogel showed one extra peak corresponding to the bonding energy of the N 1s electron with the C 1s and O 1s electrons. Thus, the prepared GOPEI aerogel consists of GO and PEI. The core level C 1s spectrum (Fig. 2b) of GO showed Gaussian peaks at 285.01, 287.11 and 288.79 eV which correspond to C=C or C–C, C–OH and COOH respectively. Whereas, the core level C 1s spectrum (Fig. 2c) of the GOPEI aerogel possessed Gaussian peaks at 285.01, 285.92, 287.10 and 287.40 eV which corresponded to the C=C or C–C, C–N, C–O and O=C–N respectively. Hence, the COOH group of the GO reacted with the NH₂ group of PEI and formed the amide bond. The N 1s core level spectrum (Fig. 2d) of the GOPEI aerogel represented three Gaussian peaks at 399.4, 400.8 and 401.4 eV corresponding to C–N–C, N–C=O and C–NH₂ respectively, which also clearly confirmed the covalent interaction i.e. amide bond formation between GO and GOPEI.

Fig. 2 XPS spectra (a) wide scan spectra of GO and the GOPEI aerogel, (b) core level C 1s spectrum of GO, (c) core level C 1s spectrum of the GOPEI aerogel and (d) core level N 1s spectrum of the GOPEI aerogel.

The morphology of GO and the GOPEI aerogel was examined by SEM. The SEM image (Fig. 3a) of GO showed a flaky wrinkled sheet like morphology. The SEM image (Fig. 3b) of the GOPEI aerogel showed non-uniform and interconnected pores which facilitate the diffusion of adsorbate inside the aerogel. The random assembly of PEI cross-linked GO sheets left pores between them. The surface elemental analysis of GO and GOPEI aerogel was also conducted by recording the EDS spectra which revealed that GO (Fig. 3c) contained only carbon and oxygen atoms whereas, the GOPEI aerogel (Fig. 3d) exhibited a significantly high nitrogen content, about 41.64 atomic% on the surface.

Fig. 3 SEM images of (a) GO and (b) the GOPEI aerogel. EDS spectra of (c) GO and (d) the GOPEI aerogel.

Further, the evidence of interaction of GO and PEI was observed by Raman spectroscopy. The Raman spectra of GO and the GOPEI aerogel are shown in Fig. 4a. Usually, GO exhibits two prominent Raman modes; one of them is a D band at 1348 cm⁻¹ due to the breathing mode of the k-point of A_1g symmetry and another is a G band at 1587 cm⁻¹ due to the E_2g phonon mode of the sp² carbon atoms.^44,45 Although, no significant change was observed in the position of D and G bands of GO after the interaction of PEI but the ratio of the intensity of the D band (I_D) and G band (I_G) can be used for the determination of the structural change on the GO sheet after the formation of the GOPEI aerogel. I_D/I_G was found to be 0.911 for GO and 1.007 for the GOPEI aerogel. Therefore, the increased ratio for GOPEI reflected the increase in structural disorder of GO sheets after covalent bonding with PEI.

Fig. 4 (a) Raman spectra of GO and GOPEI aerogel. (b) XRD patterns of GO and GOPEI aerogel.

The distribution of components PEI and GO in the GOPEI aerogel was also investigated by XRD. The obtained XRD patterns of GO and GOPEI aerogel are shown in the Fig. 4b. A single diffraction peak observed at 2θ = 11.3° in the XRD pattern of GO is due to the 002 diffraction of GO containing trapped water molecules between the sheets and oxygen rich functional groups on the sides of the sheets.^46,47 After the covalent combination with PEI, the XRD peak of GO was shifted towards a lower 2θ value (2θ = 6.4°) in the GOPEI aerogel. The shift in XRD peak is due to the increase in interlayer spacing, which can be attributed to the presence of PEI in the spacing between GO sheets. The same shift in XRD peak of GO after covalent functionalization was also achieved by Ramezanzadeh et al.⁴⁸ Moreover, the XRD peak of the GOPEI aerogel belonging to GO has very low intensity compared to pure GO, which suggests that almost all the GO sheets were exfoliated during the sonication process. Thereafter, the re-stacking of GO sheets during the formation of the GOPEI aerogel was disordered by the crosslinking through PEI.

The porous nature of the GOPEI aerogel was measured by nitrogen adsorption–desorption at 77 K, which is shown in Fig. 5a. The nitrogen adsorption–desorption isotherms were utilized in the Brunauer–Emmett–Teller (BET) model and the calculated SSA was found to be 357.67 m² g⁻¹. The pore size distribution shown in the inset of Fig. 5a was derived from the adsorption branch of the Barrett–Joyner–Halenda method which revealed a broad pore size distribution from 10 to 400 Å. The average pore size was found to be 54 Å.

Fig. 5 (a) N₂ adsorption–desorption isotherm of the GOPEI aerogel and pore size distribution inset. (b) pH_zpc of the GOPEI aerogel.

The type of charge present on the adsorbent surface is a significant characteristic of the adsorption process that depends on the solution pH. The relation between the charge on the adsorbent surface and solution pH depends on the pH_zpc value of the adsorbent. Therefore, the pH_zpc of the GOPEI aerogel was measured by the plot of initial pH vs. final pH (Fig. 5b). The plot showed a cross point at pH_i 7.5 which is considered to be the pH_zpc of the GOPEI aerogel.⁴⁹ The value of pH_zpc was found to be very close to the neutral pH of the water. Therefore, the adsorption of anions on the GOPEI aerogel would be favorable in a slightly acidic medium whereas, the adsorption of cations would be favorable in a slightly basic medium. Hence, the GOPEI aerogel may be an excellent adsorbent in an aqueous medium.

3.2. Effect of pH on the uptake capacity of GOPEI aerogels

The effect of pH on the uptake capacity of the GOPEI aerogel for As(V) as well as As(III) was investigated in the pH range from 2 to 11 at an initial As(V)/As(III) concentration 3 mg L⁻¹, GOPEI dose 0.6 g L⁻¹ and temperature 30 °C. The uptake capacities for As(V) and As(III) as a function of solution pH are shown in Fig. 6a. The uptake capacity for the As(V) initially increased with increasing pH which attained a maximum (4.74 ± 0.24 mg g⁻¹) at solution pH 4. After that it continuously decreased with further increase in pH from 4 to 11. This observed pH effect was due to the change in As(V) speciation as well as charge on the adsorbent surface with the change in pH. In the aqueous solution the existing species of As(V) depends on the solution pH as pH < 2.2: H₃AsO₄, 2.2 < pH < 6.9: H₂AsO₄⁻, 6.9 < pH < 11.5: HAsO₄²⁻, pH > 11.5: AsO₄³⁻ accordingly.⁵⁰ Initially, the pH from 2 to 4 was below the pH_zpc (7.5) of the GOPEI aerogel, therefore its positively charged surface was electrostatically favorable for the adsorption of anionic species i.e. H₂AsO₄⁻. After that the positive charge on the GOPEI aerogel decreased with a further increase in pH from 4 by which the surface became neutral around the pH_zpc which was unfavorable for the adsorption of anionic species of As(V). When the solution pH increased above the pH_zpc the surface of the GOPEI aerogel became negatively charged which was also unfavorable for the adsorption due to the repulsion between the negatively charged GOPEI aerogel surface and existing anionic species of As(V) i.e. HAsO₄²⁻.

Fig. 6 (a) Effect of pH on the uptake capacity of GOPEI. (b) Schematic representation of the interaction of As(V) and As(III) with the GOPEI aerogel.

Similarly, the uptake capacity of the GOPEI aerogel for As(III) increased with an increase in pH from 2 and achieved a maximum (4.37 ± 0.22 mg g⁻¹) at pH 7 followed by the continuous decrease with further increase in pH. This effect of pH on the uptake capacity for As(III) could also be explained on the basis of As(III) speciation and pH_zpc of the GOPEI aerogel. In the aqueous solution, As(III) is present as a non-ionic species i.e. H₃AsO₃ in the pH range from 2 to 9, which converts into an anionic species, H₂AsO₃⁻, above pH 9. When the pH starts to increase from 2, the surface of the GOPEI aerogel becomes positively charged which was not favorable for the adsorption of non-ionic species i.e. H₃AsO₃. But as the pH increased, the positive charge on the GOPEI aerogel surface decreased and the surface became neutral near the pH_zpc (7.5) which was favorable for the adsorption of nonionic species H₃AsO₃. Thereafter, on further increasing the pH above pH_zpc the surface of the GOPEI aerogel acquired a negative charge which again became unfavorable for the adsorption of H₃AsO₃. Above pH 9, the existing species, H₂AsO₃⁻, experienced a repulsive force with the negatively charged GOPEI aerogel surface, which was another factor for the decreased uptake capacity at higher pH.

Although, the optimum pH for the adsorption of As(V) and As(III) was 4 and 7, respectively, Fig. 6a clearly indicates that pH 6 is a common solution pH where the GOPEI aerogel has significant uptake capacity for both As(V) and As(III). Thus, when As(V) and As(III) exist simultaneously in water then pH 6 would be the optimum for the removal of As(V) and As(III) through the adsorption on the GOPEI aerogel. Based on the effect of pH, the adsorption of anionic species of As(V) and non-ionic species of As(III) on the GOPEI aerogel are schematically presented in Fig. 6b.

3.3. Effect of contact time and GOPEI dose on the uptake capacity

The effect of contact time on the uptake capacity of the GOPEI aerogel for As(V) and As(III) was investigated at GOPEI doses of 0.5, 0.6 and 0.7 g L⁻¹. The experiments were carried out at an initial As(V)/As(III) concentration of 3 mg L⁻¹, temperature 30 °C and pH 4 for As(V) and 7 for As(III). Fig. 7 represents the effect of contact time at GOPEI doses 0.5, 0.6 and 0.7 g L⁻¹ which indicated that initially, the rate of adsorption was very rapid because the active binding sites of the GOPEI aerogel were free and the concentration gradient of As(V)/As(III) was also high.⁵¹ But as the time advanced the rate of adsorption comparatively decreased due to the decreased extent of free binding sites and concentration gradient of As(V)/As(III). It was also observed that the equilibrium time did not depend on the GOPEI dose. Therefore, the equilibrium time was 35 min for As(V) and 50 min for As(III) at all GOPEI doses. The uptake capacity reached a maximum with a GOPEI dose of 0.5 g L⁻¹ for both As(V) and As(III), which did not decrease significantly on increasing the GOPEI dose to 0.6 g L⁻¹ but it decreased remarkably on increasing the GOPEI dose to 0.7 g L⁻¹. This is because the uptake capacity depends on the adsorbate-to-binding site ratio which decreased on increasing the GOPEI dose. That’s why As(V) and As(III) became insufficient at higher GOPEI doses to cover all the binding sites per unit weight.⁵²

Fig. 7 Effect of contact time on the uptake capacity at different GOPEI doses.

3.4. Kinetics studies

In order to investigate the suitable kinetics and mechanism for the adsorption of As(V) and As(III) on the GOPEI aerogel, various kinetics models were examined at three adsorbent doses i.e. 0.5, 0.6 and 0.7 g L⁻¹. The kinetics models were employed in their linear form and applicability of models was checked in terms of the correlation coefficient (R²).

3.4.1. Pseudo-first-order and pseudo-second-order kinetics models. The pseudo-first-order linear plots of log(q_e − q_t) vs. t for adsorption of As(V) and As(III) are shown in Fig. 8a. The pseudo-first-order model parameters and R² were calculated and are listed in Table 1. The experimental kinetics data were also employed in the pseudo-second-order linear plot t/q_t vs. t (Fig. 8b) and their slopes and intercepts were used for the calculation of model parameters and R² and are summarized in Table 1. The pseudo-second-order model showed reasonable linearity in the experimental data with the high value of R² and better approximation between calculated and experimental uptake capacity as compared to pseudo-first-order. Therefore, the adsorption of As(V) and As(III) on the GOPEI aerogel was elicited by pseudo-second-order kinetics, which suggested that the rate of adsorption for both As(V) and As(III) would be proportional to the (q_e − q_t)² term of pseudo-second-order which corresponds to the square of free binding sites present over the GOPEI aerogel. Moreover, the pseudo-second-order rate constant increased with the increasing GOPEI dose from 0.5 to 0.7 g L⁻¹ for both As(V) and As(III) adsorption due to the easy availability of binding sites at the higher dose.

Fig. 8 Kinetics of models plotted at different GOPEI doses: (a) pseudo-first-order and (b) pseudo-second-order.

Table 1 Kinetics parameters for the adsorption of As(V) and As(III) at different GOPEI doses

Model parameters	As(V)-0.5 g L^−1	As(V)-0.6 g L^−1	As(V)-0.7 g L^−1	As(III)-0.5 g L^−1	As(III)-0.6 g L^−1	As(III)-0.7 g L^−1
Pseudo-first-order
k1 (min^−1)	0.088	0.113	0.138	0.086	0.085	0.092
qe (exp.) (mg g^−1)	5.05 ± 0.26	4.82 ± 0.27	4.18 ± 0.27	4.66 ± 0.20	4.24 ± 0.23	3.82 ± 0.17
qe (cal.) (mg g^−1)	6.27	6.55	6.24	7.48	6.26	5.42
R^2	0.924	0.918	0.894	0.840	0.924	0.899
Pseudo-second-order
k′2	0.026	0.040	0.060	0.021	0.034	0.056
qe (mg g^−1)	5.62	5.44	5.16	4.93	4.75	4.52
R^2	0.997	0.990	0.991	0.992	0.991	0.992
Mass transfer
βt × 10^−5 (cm^2 s^−1)	4.56	5.92	5.77	3.12	3.92	3.35
R^2	0.998	0.990	0.991	0.997	0.991	0.994
Interparticle diffusion
kid (mg g^−1 min^−0.5)	1.06	0.97	0.77	0.82	0.73	0.55
C	−1.11	−0.62	−0.11	−0.92	−0.43	0.14
R^2	0.997	0.980	0.965	0.993	0.989	0.983

---

3.4.2. Mass transfer studies. The mass transfer profiles of Mckay et al.’s model for the adsorption of As(V) and As(III) on the GOPEI aerogel are presented in Fig. 9a. The plots have reasonable linearity with the experimental data for all the tested GOPEI doses with high values of R², which undoubtedly suggest that the external mass transfer existed in the adsorption process. The β_t for external mass transfer of As(V) and As(III) from the film boundary to the GOPEI aerogel surface was calculated for all GOPEI doses and listed in Table 1. The values of β_t [4.56 × 10⁻⁵, 5.92 × 10⁻⁵, 5.77 × 10⁻⁵ cm² s⁻¹ for As(V) and 3.12 × 10⁻⁵, 3.92 × 10⁻⁵, 3.35 × 10⁻⁵ cm² s⁻¹ for As(III)] were significantly high therefore, the external mass transfer of As(V) and As(III) to the adsorbent surface was rapid in the current adsorption system. Thus external mass transfer cannot be the rate controlling step.

Fig. 9 (a) Mass transfer plot at different GOPEI doses. (b) Intraparticle diffusion plot at different GOPEI doses. (c) Richenberg model plot at different GOPEI doses.

3.4.3. Intraparticle diffusion model. The consequence of diffusion of As(V) and As(III) into the pores and intraparticle spaces of GOPEI was examined using the Weber–Morris model, which is presented in Fig. 9b. The plot shows significant linearity with experimental data as its R² values were considerably high, and are listed in Table 1. Therefore, As(V) and As(III) diffused into the pores and intraparticle spaces during the adsorption. The values of k_id and C were calculated from the slope and intercept respectively which are also listed in Table 1. The Weber–Morris model assumes that if the straight line plot of q_t vs. t^0.5 passes through the origin, then the intraparticle diffusion process is a rate controlling step but if the plot does not pass through the origin then intraparticle or pore diffusion is not the sole rate controlling step.⁵³ The plot for the adsorption of As(V) and As(III) on the GOPEI did not pass through the origin thus, it is quite clear that the intraparticle diffusion was the not single rate controlling step. Therefore, more than one mechanism was involved in the adsorption of As(V) and As(III) on the GOPEI aerogel. The values of C clearly indicate the vital role of external mass transfer which may be another mechanism with intraparticle diffusion to dictate the rate of adsorption.^54,55

3.4.4. Richenberg model. Fig. 9c shows the plot B_t vs. t for the adsorption of As(V) and As(III) on the GOPEI aerogel surface which has considerable linearity with R² values 0.970, 0.983 and 0.975 for As(V) and 0.967, 0.984 and 0.968 for As(III) at GOPEI doses of 0.5, 0.6 and 0.7 g L⁻¹ respectively. But not a single line passed through the origin which again suggests that the intraparticle diffusion was not the sole rate controlling step which is consistent with the result of the Weber–Morris model.

3.5. Effect of initial concentration and temperature

In order to study the effect of initial concentration of As(V) and As(III) on the uptake capacity of the GOPEI aerogel, the experiments were carried out by varying the initial concentration from 0.5 to 4.5 mg L⁻¹. The effect was measured at three different temperatures, 20, 30 and 40 °C, while keeping the GOPEI dose at 0.6 g L⁻¹ and pH 4 for As(V) and 7 for As(III). The results presented in Fig. 10a clearly indicate that the uptake capacity increased with the increase in initial concentration up to 3 mg L⁻¹. After that, no significant increase was observed on the further increase in initial concentration. Therefore, 3 mg L⁻¹ was sufficient to cover all binding sites present on the 0.6 g L⁻¹ GOPEI dose. It was also observed from Fig. 10a that on increasing the temperature from 20 to 30 °C, the uptake capacity increased from 4.36 ± 0.24 to 4.80 ± 0.27 mg g⁻¹ for As(V) and from 2.86 ± 0.14 to 4.26 ± 0.24 mg g⁻¹ for As(III). Whereas, on further increasing the temperature up to 40 °C, the uptake capacity decreased to 4.51 ± 0.21 mg g⁻¹ for As(V) and 3.32 ± 0.16 mg g⁻¹ for As(III).

Fig. 10 Equilibrium studies at temperatures, 20, 30 and 40 °C: (a) effect of initial arsenic concentration on the uptake capacity, (b) Freundlich isotherm plot, (c) Langmuir isotherm plot, (d) D–R isotherm plot.

The uptake capacity of the GOPEI aerogel for As(V) and As(III) was found to be remarkably high compared to previously reported adsorbents. The comparison of the uptake capacity of the GOPEI aerogel with different adsorbents is summarized in Table 2.

Table 2 Comparison of the GOPEI aerogel with previously reported adsorbents for the removal of As(V) and As(III) in terms of uptake capacity

S. no.	Adsorbent	Uptake capacity (mg g^−1)	Ion removed	Method	Reference
1	Fe–MWCNT nanocomposite	1.723	As(III)	Adsorption	56
		0.189	As(V)
2	CNT/CuO nanocomposite	2.267	As(III)	Adsorption	14
		2.395	As(V)
3	Fe3O4–graphene composite	0.471	As(III)	Adsorption	57
4	Chitin hydrogel reinforced with TiO2 nanoparticles	3.10	As(V)	Adsorption	58
5	GOPEI aerogel	4.26 ± 0.24	As(III)	Adsorption	Present work
		4.80 ± 0.27	As(V)

---

3.6. Isotherm studies

The isotherm studies for the adsorption of As(V) and As(III) on the GOPEI aerogel were conducted by varying the initial As(V)/As(III) concentration from 0.5 to 3 mg L⁻¹ at three selected temperatures, 20, 30 and 40 °C, while keeping the GOPEI dose at 0.6 g L⁻¹ and pH 4 for As(V) and 7 for As(III). Thus, the obtained equilibrium data at different temperatures were employed in the linear form of Freundlich, Langmuir and D–R isotherms to investigate the most suitable isotherm for current adsorption system.

3.6.1. Freundlich isotherm. The linear plot of the Freundlich isotherm is presented in Fig. 10b which maintains linearity at all the tested temperatures with high values of R² (Table 3). The Freundlich isotherm parameters k_f and n were calculated from the intercept and slope respectively and listed in Table 3. The values of n for the adsorption of both As(V) and As(III) were found to be between 1 and 10 thus, the adsorption of As(V) and As(III) on the GOPEI aerogel would be a beneficial process.⁵⁹ It was observed that the value of k_f increased when initially increasing the temperature from 20 to 30 °C but decreased on a further increase in temperature up to 40 °C. Thus, 30 °C was a favorable temperature for As(V) as well as As(III).

Table 3 Parameters of Langmuir, Freundlich and D–R isotherms for the adsorption of As(V) and As(III) at different temperatures

Temperature	Freundlich parameters			Langmuir parameters			D–R parameters
	kf (mg g^−1)	n	R^2	q^0 (mg g^−1)	b (L mg^−1)	R^2	Xm (mmol g^−1)	E (kJ mol^−1)	R^2
As(V)
20 °C	8.66	1.34	0.99	4.47	2.37	0.99	0.0029	10.56	0.99
30 °C	16.13	1.97	0.98	5.88	9.60	0.99	0.0014	13.76	0.99
40 °C	8.73	1.84	0.99	5.79	8.87	0.97	0.0012	13.27	0.99
As(III)
20 °C	2.38	1.58	0.99	4.38	1.20	0.98	0.0008	10.69	0.99
30 °C	7.22	1.48	0.99	7.29	2.74	0.98	0.0022	11.18	0.99
40 °C	3.55	1.59	0.99	5.07	1.83	0.99	0.0011	11.59	0.98

---

3.6.2. Langmuir isotherm. Fig. 10c shows the linear plot of the Langmuir isotherm for the adsorption of As(V) and As(III) to possess considerable linearity with the high R² values (Table 3). The Langmuir isotherm constants b and q⁰ for the adsorption of As(V) and As(III) on the GOPEI aerogel at temperatures 20, 30 and 40 °C were calculated from the intercept and slope respectively and summarized in Table 3. q⁰ increased by increasing the temperature from 20 to 30 °C therefore, the high temperature was favorable for the adsorption of As(V) and As(III), but q⁰ decreased on further increasing the temperature up to 40 °C, which may be due to the increased randomness at the solid–liquid interface. Moreover, b also followed the same pattern as q⁰ with increased temperature which suggests that the bonding energy of the GOPEI aerogel for As(V) and As(III) increased with increasing temperature. Therefore, the adsorption of As(V) and As(III) was an endothermic process.

In order to investigate the favorability of the adsorption process for the removal of As(V) and As(III) with a GOPEI aerogel, the dimensionless separation factor (R_L) was calculated by the following equation³⁶

where b is the Langmuir constant and C₀ is the initial As(V)/As(III) concentration.

R_L, related to the adsorption energy, can predict the nature of adsorption whether irreversible (R_L = 1), favorable (0 < R_L > 1) or unfavorable (R_L > 1). The calculated R_L was found to be 0.033 for As(V) and 0.108 for As(III) suggesting the highly favorable adsorption by the GOPEI aerogel for the removal of As(V) and As(III) from water.

3.6.3. D–R isotherm. The data obtained from isotherm experiments were employed in the D–R isotherm to investigate the type of adsorption whether physical or chemical. The D–R isotherm plot (Fig. 10d) maintains linearity at all the tested temperatures for As(V) and As(III). The adsorption energy E (kJ mol⁻¹) calculated from the slope β (mol² kJ⁻²) and R² are listed in Table 3. The considerably high value of R² recommended the applicability of the D–R isotherm for the determination of the type of current adsorption process. A value of E between 8 and 16 kJ mol⁻¹ represents chemical adsorption whereas a value of E less than 8 kJ mol⁻¹ indicates a physical nature of adsorption.⁶⁰ The adsorption energies for adsorption of As(V) and As(III) on the GOPEI aerogel existed in the range of chemical adsorption.

All three applied isotherms present a linear fit to the experimental data with high values of R² (Table 3). The applicability of these models suggest a monolayer chemical adsorption of As(V) and As(III) on the energetically heterogeneous binding sites of GOPEI aerogel.

3.7. Thermodynamics studies

ΔG⁰ values for the adsorption of As(V) and As(III) on the GOPEI aerogel were calculated at 293, 298 and 303 K and are listed in Table 4. The negative value of ΔG⁰ suggested that the adsorption of As(V) as well as As(III) was a spontaneous process. The spontaneity of the process increased with the increase in temperature as the negative value of ΔG⁰ increased with increasing temperature. ΔH⁰ and ΔS⁰ were calculated from the slope and intercept of the Van’t Hoff plot (Fig. 11) for the adsorption of As(V) and As(III). The positive values of ΔH⁰ suggested the endothermic nature of the adsorption of As(V) and As(III) on the GOPEI aerogel which was consistent with the isotherm results. ΔS⁰ was also found to be positive for the adsorption of As(V) as well as As(III) on the GOPEI aerogel. It may be due to the fact that during the adsorption process, the subsequent desorption was also present, which increased the randomness at the solid–liquid interface.⁶¹

Table 4 Thermodynamic parameters for the adsorption of As(V) and As(III) on the GOPEI aerogel

Temperature (°C)	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (kJ mol^−1 K^−1)
As(V)
15	−3.25 ± 0.15
20	−4.68 ± 0.23
25	−6.28 ± 0.26	89.64	0.32
30	−8.10 ± 0.31
As(III)
15	−0.02 ± 0.001
20	−0.71 ± 0.03
25	−2.33 ± 0.12	98.68	0.29
30	−4.44 ± 0.19

---

Fig. 11 Van’t Hoff plot for the adsorption of As(III) and As(V) on the GOPEI aerogel.

4. Conclusions

The amine-rich GOPEI aerogel was prepared using GO and PEI which showed high specific surface area and remarkable affinity toward the adsorption of As(V) and As(III) from water. The maximum uptake capacity of the GOPEI aerogel was found to be 4.80 ± 0.27 mg g⁻¹ for As(V) and 4.26 ± 0.24 mg g⁻¹ for As(III) at an initial As(V)/As(III) concentration 3 mg L⁻¹, GOPEI dose 0.6 g L⁻¹, temperature 30 °C and pH 4 for As(V) and 7 for As(III). The adsorption of As(V) and As(III) was fast as the equilibrium was achieved within 35 min for As(V) and 50 min for As(III). The above fact is also supported by the kinetics studies because the adsorption process followed the pseudo-second-order kinetic model. The application of the intraparticle diffusion model and Richenberg model indicated that the intraparticle diffusion occurred in the adsorption process, but it was not the sole rate controlling step. The external mass transfer studies revealed that the removal of As(V) and As(III) was due to the adsorption process. It was further confirmed by isotherm studies using Langmuir, Freundlich and D–R isotherms. The equilibrium data showed a good fit with all these tested isotherm models which suggested a monolayer chemical adsorption of As(V) and As(III) on the energetically heterogeneous sites of the GOPEI aerogel. The Langmuir and Freundlich isotherms also revealed the optimum temperature, 30 °C, which is close to room temperature. Thus, the removal of As(V) and As(III) by the GOPEI aerogel would be energy saving. The negative values of ΔG⁰ reported the spontaneous process whereas the positive values of ΔH⁰ suggested the endothermic nature of the adsorption of As(V) and As(III) on the GOPEI aerogel. These batch mode results concluded that the GOPEI aerogel would be a potential adsorbent for the removal of As(V) and As(III) from drinking water. The obtained modeling results would be applicable to design continuous column studies for the treatment of As(V) and As(III) contaminated water using a GOPEI adsorbent.

Acknowledgements

Authors DKS, VK and VKS would like to acknowledge the MHRD, New Delhi, India for providing financial assistance. The authors are also thankful to the Director, Indian Institute of Technology (BHU), Varanasi, India for providing infrastructures and central instrumentation facilities centre (CIFC).

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