Greener and size-specific synthesis of stable Fe-Cu oxides as earth-abundant adsorbents for malachite green

Abstract

A greener and sustainable pathway to the assembly of Fe, Cu -based adsorbent is described using Virginia creeper (Parthenocissus quinquefolia) leaf extracts in presence of oxalic acid which avoids the use of toxic chemicals. Characterization of the synthesized mixed Fe, Cu oxides are carried out by SEM, TEM, XRD, FT-IR, XPS, and BET techniques; SEM and TEM results disclosed particle size ranging from 160 nm to 1 μm in presence of varying oxalic acid amounts of 0 and 0.1 mol/L. The X-ray photoelectron spectroscopy studies revealed that the sample comprised of Fe, Cu-based hybrid oxides and oxalates. The ensuing results from altered operational parameters namely initial pH, initial malachite green (MG) concentration, the sample dosage and the reaction temperature suggest that the MG adsorption capacity of synthesized materials could be well structured by simply varying the amount of oxalic acid. The optimal sample (S₃ sample) has a remarkably high maximum adsorptive capacity (~1399 mg/g) for aqueous MG removal at 303 K and natural pH (~ 6.58), which is superior to recently documented sorbents. The results demonstrate that the adsorption is spontaneous (i.e., ∆G < 0) via an endothermic process wherein the synthesized adsorbent displayed excellent characteristics: 1) maintained a high adsorption capacity under a wide range of pH conditions; 2) remained chemically stable under ambient storage environments to allow for extended stowage; and 3) portrayed high reusability with no waning effect after 4 adsorption/desorption cycles.

1. Introduction

With the rapid pace of industrialization, contamination of air, water, soil, and groundwater has become a major challenge for modern societies [1–3]. Pollutants ranging from petroleum hydrocarbons to heavy metals require remediation efforts in order to protect the environment and human health [4–6]. Especially the discharge of colored wastewater from printing companies and pharmaceutical plants has inflicted immeasurable harm to the ecological environment and humans [7, 8]with more than 7×10⁵ tons of dyes being released to the environment per annum [9, 10]. Consequently, it is urgent to explore economic and effective techniques for minimizing the dye contamination [11]. Research to date have shown that dye-laden effluents can be remediated by multiple approaches, including advance oxidation [12], membrane filtration [13, 14], adsorption [15, 16] and coagulation/flocculation [17], among others. Amongst these methods, adsorption has gained prominence in the decolourization of dye wastewater based on its simple design, easy operation and high efficiency [15, 18, 19].

Nanomaterials, as a representative of efficient adsorbents, have garnered attention due to their well-defined shape, large surface area, and easy-to-control porosities [20, 21]. To date, many smart nanomaterials including carbon nanotubes [22], graphene [23], and metal oxides [24] have been explored for the efficient removal of dyes. However, current studies show that the challenges remain for the mass production, primarily due to the complexity of the synthesis process and the use of hazardous reagents [25, 26]. On the other hand, environmental remediation activities can often result in secondary environmental impacts, such as energy and material consumption, air and water pollution during the manufacturing process, etc. [27–30]. In recent years, environmental professionals are increasingly interested in more sustainable means of environmental remediation [31–33]. Hence, it is imperative that greener, economic and efficient protocols for the preparation of nano-adsorbents need to be developed for application in dye removal.

Green nanotechnology predicates the development of novel and environmentally friendly method for the preparation of nanomaterials [34–36]. Considerable efforts have been expanded to adopt the phyto-synthesis methods for the rapid fabrication of nanomaterials, which are quick and devoid of secondary pollution thus ensuring the possibility of large-scale production of the nanomaterials [37–39]. The assembly of noble metal nanoparticles, such as Ag, Au, and Pt, using the plant biomass or extracts have been reported [36]; gold nanoparticle synthesis via plant extract-mediated preparation is comparable to that of chemical methods [40]. Previous studies have indicated that iron nanoparticles with different morphologies (e.g. spheres, platelets, and nanorods) could be obtained instantaneously in aqueous tea extracts [41–43]. In addition, metal oxide nanomaterials, such as iron oxide [37, 44], copper oxide [34, 45], manganese dioxide [46], zinc oxide [47], palladium oxide [48], could be synthesized using the extracts from tea, coffee, and sorghum. Interestingly, it has been found that the plant extracts, rich in polyphenols or related antioxidants, could serve as both reducing and capping agents [49, 50]. Interestingly, Makarov et al. [44] found that the iron nanoparticles synthesized by the plant extract with high concentrations of organic acids exhibited higher stability. Oxalic acid, a low molecular weight organic acid, is a metabolic product of organisms and is widely distributed in plants, animals, and fungi. To our knowledge, the performance of oxalic acid in the assembly of nanomaterials has been largely overlooked [51], and such a study using this smallest dicarboxylic acid may provide necessary stimulation for researchers to develop an economic, safer and efficient adsorbent.

It is well known that the adsorptive properties of nanomaterials have a strong relation with their morphology and the surface area; samples with a higher surface area tend to have more adsorption sites. Nevertheless, the goal of obtaining materials with controlled size and morphology is still a challenging endeavour in the realm of green technology development. With this objective, an unprecedented attempt has been made here to fabricate Fe,Cu-based materials using leaf extract of Virginia creeper (Parthenocissus quinquefolia), and the effect of oxalic acid, as an additive, on the morphology of ensuing green synthesized hybrid oxides. Herein, we describe a simple, greener and inexpensive method for the assembly of hybrid oxides emanating from earth-abundant materials with the inherent advantage of wider flexibility in terms of controlling the morphology and their potential for broader applications. The prepared nanocomposites are tested for their potential for the removal of malachite green (MG) dye, and the optimal sample (S₃ sample) shows the maximum adsorption capacity of ~1399 mg/g at T of 303 K and at natural pH (~6.58), which is superior to most of the reported MG adsorption values. The optimized sample displays excellent stability and reusability in the removal of MG dye and we envisage that with aforementioned advantages the present greener synthetic route to the Fe, Cu-based adsorbent may have great potential for practical and sustainable textile dyeing wastewater treatment.

2. Experimental Materials and Methods

2.1. Chemicals and materials

All the used reagents, Ferric chloride hexahydrate (FeCl₃·6H₂O), Cupric chloride dihydrate (CuCl₂·2H₂O) were purchased from Beijing Chemical Reagents Company (China) and used directly without further purification.

2.2. Preparation methods

2.2.1. Preparation of Virginia creeper leaf extracts

Virginia creeper leaves (VC) were collected locally (from the Tsinghua University campus), washed with deionized water three times to eliminate surface dust, and dried at room temperature until their weight became constant under ambient conditions and then cut into pieces. The extract was prepared by boiling dry VC leaves in deionized water with a concentration of 60 g/L at 353 K for 90 minutes. Thereafter, the extracts were vacuum-filtered and stored at 277 K for further use.

2.2.2. Synthesis of the adsorbents

First, a specified quantity of oxalic acid (C_{oxalic acid}=0, 0.0125, 0.025, 0.05, 0.1 mol/L) was added to 200 mL extract of virginia creeper leaf under vigorous stirring. After stirring for 30 minutes, premixed 100 mL solution of FeCl₃ and CuCl₂ (0.12 mol/L) was added dropwise with vigorous stirring for 12 hours at ambient temperature. The formation of the adsorbent was indicated by the appearance of dark black precipitate. The sample was separated by centrifugation (4000 r/min for 10 minutes) and then washed with deionized water thrice. Finally, the product was dried for 12 hours at 333 K prior to its further use. The ensuing samples by varying the amount of oxalic acid at 0, 0.0125, 0.025, 0.05, and 0.1 mol/L, were labeled as S₁, S₂, S₃, S₄, and S₅, respectively.

2.3. Characterization

The morphology of all the samples obtained were examined using transmission electron microscopy (TEM, TECNAI G2). The Fourier transform infrared spectroscopy (FTIR, Nicolet IS10) was employed to identify the functional groups of the samples. The powder X-ray diffraction (XRD) patterns of the samples were acquired using a Rigaku D/Max-RB diffractometer with Cu/Kα radiation (k=0.15406 nm, 35 kV, 40 mA). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Escalab 250Xi Spectrometer with a monochromated Al K excitation, which was used to determine the surface chemical elemental composition and its chemical changes. The Brunauer–Emmett–Teller (BET) specific surface area and the pore size distributions were analysed by nitrogen adsorption–desorption measurements via an Autosorb-1 specific surface area analyser (Quantachrome Instruments, USA).

2.4. Adsorption experiments

The MG adsorption capacity of the prepared samples was investigated by a series of batch experiments. Some experimental parameters, such as adsorbent dosage, solution pH, contact time, initial concentration and temperature, were taken into consideration. For a typical adsorption test, a certain dose (0.3 g/L) of sample was added to 20 mLs of a 50 mg/L MG aqueous solutions at 303 K, After shaking for 12 hours until an adsorption equilibrium was reached, the mixture was separated by centrifugation (4000 r/min for 5 minutes). The effect of dosage was investigated by changing the added amount of the sample (0.05–0.7 g/L) at natural pH of circa 6.6; pH values could be adjusted using HCl or NaOH solutions. The concentrations of the MG solution were in the range of 50 to 500 mg/L. The batch adsorption experiments focusing on the effect of temperature on the adsorption capacity were carried at 303, 313, and 323 K, respectively. Finally, the concentration of MG in the supernatant was measured by a UV–Vis spectrophotometer (Shimadzu UV-1800 spectrophotometer, Japan) at a wavelength of 619 nm. The equilibrium adsorption capacity Q_e (mg/g) and the removal efficiency of MG were determined by the following equations:

$$Q_e=\frac{\left(C_0-C_e\right)V}{m}$$ 1

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{\%}\right)=\frac{100(C_0-C_e)}{C_0}$$ 2

where C₀ (mg/L) and C_e (mg/L) are the MG concentrations in the solution at t₀ and t_e. The time(t_e) required to reach the equilibrium could be determined by Fig. S2, and the adsorption time(12 h) in the test of adsorption equilibrium capacity Q_e was set longer than t_e. V (L) is the total volume of the suspension, and m (g) is the mass of the adsorbent.

2.5. Desorption and regeneration of adsorbent

The experimental regeneration of the adsorbent was carried out in the four successive cycles. In each cycle, 6 mg of the adsorbent was added into 20 mLs of a 50 mg/L MG solution followed by shaking for 12 hours (303 K). Then, the MG-loaded adsorbent was obtained through centrifugation and washing with anhydrous ethanol several times until no green color was evident. Finally, the regenerative adsorbent was placed in an oven at 343 K overnight, and then utilized for the next adsorption-desorption cycle. The adsorption capacity of the regenerated sample was calculated by the aforementioned method.

3. Results and discussion

3.1. Characterization of samples

To unravel the distribution of elements on the surface of the adsorbent samples and their valence states, the XPS measurements were carried out and the spectra are shown in Figure 2. As displayed in Fig 2 A, the peaks of the XPS full spectra indicated that C, O, and Fe were the predominant elements in S₁-S₅ samples. The peak for the copper element was not obvious in the S₁-S₄ samples due to its low content or its high dispersibility. Fig. 2 B presents the fitting curves of C 1s peaks in the S₁, S₃, and S₅ samples; the peaks are obviously different from samples prepared by chemical method [51]. The three peaks located at binding energies of 284.6, 286.7, and 289.1 eV can be assigned to the C-H(C-C, C=C), C-O, and O=C-OH functional groups[7, 61], respectively. The presence of multiple C peaks implies that some biomolecules are attached to the surface of the adsorbent samples [62]. In Fig. 2 C, the two peaks located at 531.6 and 532.8 eV suggested the presence of the lattice oxide oxygen in the metal oxides (Fe-O/Cu-O) of samples [63] and O=C-OH/C-OH functional groups [64], respectively. For S₁ sample without oxalic acid, the C and O peaks may be ascribed to polyphenols, sugars, or other related phytochemicals present in the virginia creeper leaf extract [64]. Similar results have been obtained for materials by Wang et al.[65, 66], and they were classified as iron complex nanoparticles. Moreover, combined with the characteristics of Fig. 2 B and 2 C, one can see that, with the increasing addition of oxalic acid, the intensity of the peaks located at 289.1 and 532.8 eV has increased, indicating the enrichment of O=C-OH functional groups on the surface of the adsorbent samples [67]. Figure 2 D shows the spectra of Fe and Cu in sample S₃. The Fe 2p3/2 and Fe 2p1/2 core levels are located at 711.3 and 725.0 eV, respectively, which are consistent with those reported for iron oxide previously [51, 64, 68]. The two main peaks located at 935.3 and 953.7 eV belong to Cu2+ in CuO [51]. Based on these results, it can be concluded that a process of enrichment of carboxylate groups on the surface of iron and copper hybrid oxides occurs with the oxalic acid addition ranging from 0 to 0.1 mol/L. Furthermore, these results also provide strong evidence for the shell being mainly composed of biomolecules, metal oxides and oxalate on their surfaces.

Fig 2.

(A) XPS survey spectrum of S₁-S₅ samples; (B) C 1s spectra of S₁, S₃, and S₅ samples;(C) O 1s spectra of S₁, S₃, and S₅ samples;(D) Fe 2p and Cu 2p spectrum of S₃ sample

The XRD pattern of the S₁-S₅ samples in the 2θ range of 2–80° is shown in Fig. 3. In this analysis, no obvious peaks for the S₁-S₃ samples were observed, indicating that the greener protocol produced amorphous iron-copper oxides, which was similar to the previously published data on synthetic nanoparticles using tea and eucalyptus extractives [69]. Furthermore, with the addition of the oxalate at more than 0.05 mol/L during the synthesis, the XRD pattern of S₄ and S₅ samples showed diffraction patterns at 2θ values of 22.9, 36.2, 36.9, 42.4, 46.8, and 52.1° corresponding to the [110], [120], [210], [111], [220], and [130] planes of the orthorhombic CuC₂O₄· nH₂O lattice, respectively. This obviously indicated the existence of oxalate in the S₄ and S₅ samples, which is consistent with the results of XPS spectra.

Fig 3.

XRD patterns of the S₁-S₅ samples

The FT-IR measurement was employed to gain detailed information regarding the surface group of the green synthetic samples. As shown in Fig. 4 A, the characteristic peaks of all samples at around 3436.7 cm−1 could be pertinent to O-H stretching vibration due to the water present on the surfaces of the samples [70, 71]. The absorption bands located at 1614 cm−1 correspond to the C=C aromatic ring stretching vibration, which may be related to polyphenols or sugars/glycosides in plant extracts [62, 72]. The peaks at 1461 cm−1 are attributed to the O–H bending of the phenolic group [73]. The absorption band at around 1069 cm−1 is ascribed to the C=O of carboxylic acid whereas bands at around 546 cm−1 refer to metal-oxygen stretch in metal oxides [74]. All of the above peaks revealed that the basic bonding types of all samples were practically similar. Moreover, the peaks for S₄ and S₅ samples at 1320 and 820 cm−1 are assigned to the C-O and C-C stretching vibrations of coordinated oxalic acid; the presence of oxalate was also confirmed from the corresponding peaks at 1412 cm−1 for oxalate coordination [51]. Moreover, the corresponding peak intensity of the S₅ sample is higher than that of S₄, indicating surface enrichment of the carboxylate group. This is consistent with the conclusions of the adsorbent characterization via XPS and XRD.

Fig 4.

FTIR spectra for (A) S₁-S₅ samples, (B) MG, S₃ sample before and after adsorption (S₃+MG)

The FTIR spectrum of the S₃ sample before and after MG adsorption on the S₃ sample are depicted in Fig. 4 B. The peak at 1069 cm−1 decreased on MG-adsorbed sample and this could be pertinent to electrostatic interactions between the –COO- group of the S₃ sample and the positively charged nitrogen centre of the MG dye [75, 76]. The appearance of a new peak at 1237 cm−1 indicated that a certain number of MG molecules were adsorbed on the surface of the adsorbent material [53, 75]. In addition, the peak at 1461 cm−1 in the S₃ sample shifted relatively to lower wavenumbers and its intensity decreased after MG adsorption, revealing that the interactions could take place between the –OH groups and the MG dye molecules [75]. Based on the above analysis, it can be deduced that the MG adsorption process materialized via electrostatic interactions.

TEM, SEM

The TEM images of samples without (S₁ sample) or with the oxalic acid (S₂, S₃, S₄, and S₅ samples) are shown in Fig. 5. It can be observed that the samples obtained by the greener process consist of particles with diameters ranging from 160 nm to 1 μm. Interestingly, the particles with a diameter of about 400 nm were present in the S₁ sample, after the addition of oxalic acid in the protocol, whereas the diameter of the ensued S₂ sample decreased to 160 nm. However, as the amount of oxalic acid was further increased to 0.1 mol/L, the diameters of the prepared sample (S₅ sample) increased to 1 μm. It is well known that the adsorption capacity of any adsorbent has a very strong correlation with their surface active sites; smaller the particle size of the adsorbent, the larger the surface area they possess as more active sites are available. Combined with the results of TEM images, the diameters of the prepared samples can be adjusted and controlled in the range from ∼ 160 nm to ∼ 1 μm, which is beneficial for the alteration of the adsorption capacity of MG. The SEM image of S₃ sample and its corresponding elemental mapping are shown in Figure S1; green, blue and red images indicate the O-, Fe- and Cu-enriched areas of the sample, which implied that the elements are well dispersed on the surface of the S₃ sample. In conjunction with the results of XPS and XRD, it is obvious that this greener synthetic method of material preparation need to be studied due to its simplicity and the utilization of low-cost earth-abundant raw materials.

Fig 5.

TEM images of (A) S₁ sample, (B-C) S₂ sample, (D) S₃ sample (E) S₄ sample (F) S₅ sample

The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution of S₁, S₃, and S₅ samples were obtained next via N₂ adsorption-desorption measurements at 77 K. According to Figure 6 A, the type-Ⅳ behaviour with a hysteresis loop is observed, and this was ascribed to capillary condensation. This phenomenon indicates the existence of mesopores in the samples [28]. The Brunauer −Emmett − Teller (BET) surface areas increased from 13.664 m2 · g−1 for S₁ sample to 19.214 m2 · g−1 for S₃ sample, and then reduced to 13.084 m2 · g−1 for S₅ sample, which affirmed the important role of the oxalic acid in the synthesis. Generally, the higher surface area can provide more active sites and provide sufficient interaction between the adsorbents and the dye molecules [77]. The distribution of pore sizes (Fig. 6 B) in the range of 2.1–4.0 nm further confirmed the existence of mesopores on the surface of the adsorbents. These results indicate that both, the presence of oxalic acid and its amount used significantly impact on the textural properties of the prepared samples.

Fig 6.

(A)The nitrogen adsorption-desorption isotherms (B) pore size distributions of the S₁, S₃, and S₅ samples

3.2. Effects of operating conditions on MG adsorption

3.2.1. Effects of the addition of oxalate on MG removal and adsorption kinetics

In the process of pollutant removal, the removal efficiency of adsorbents is closely related to their composition. Firstly, the MG adsorption capacity of Fe alone without oxalic acid and Fe, Cu-based material without oxalic acid were investigated (natural pH, temperature 303 K, 20 mL 50 mg/L MG solution, dosage 0.3 g/L) and the results indicated that the values of latter(Q_e: 110.15 mg/g ) was better than that of former (Q_e: 32.69 mg/g). In order to further enhance their performance in environmental remediation, the effect of oxalate on the adsorption capacity of MG was investigated. As is observed in Fig. S2A, the MG adsorption capacity of all samples initially increased rapidly, and then became constant. The fast adsorption phase may be attributed to the high concentration gradient and numerous surface sites, which benefited the transfer and adsorption of the MG molecule. Then the effect of these favourable adsorption factors would decline with an increase in contact time, thereby resulting in the slower adsorption rate [78]. Moreover, the S₃ and S₄ samples reached the adsorption equilibrium in the first 30 minutes of contact time; adsorption capacity constants of these samples being in the order: S₂ (167.02 mg/g) > S₃ (166.57 mg/g) > S₄ (164.86 mg/g) > S₁ (110.15 mg/g) > S₅ (37.48 mg/g). Combined with the results of the XPS, FTIR and BET analysis, the high capacity of S₂-S₄ samples may be attributed to their size, high surface area and their oxalate-enriched surface, which could provide more active sites and electrostatic attraction for MG dye molecules. Considering the cost, the removal rate and the efficiency of the adsorbent, the S₃ sample was selected as the main research candidate for further experiments.

Later, to understand the essence of the adsorption process, the pseudo-first-order kinetics model, pseudo-second-order kinetics model, and intraparticle diffusion model were adopted to analyse the adsorption kinetics [8, 78, 79]. These models can be expressed as follows:

$$\ln (q_e-q_t)=\ln q_e-k_{1}t$$ 3

$$\mathrm{t}/q_t=1/{(k}_2{q}_e^2)+t/q_e$$ 4

$$q_t=k_{pi}{t}^{1/2}+C_i$$ 5

where q_e and q_t (mg/g) are the amounts of MG dye adsorbed at equilibrium and given time, respectively. k₁ (min−1), k₂ (g/mg/min) and k_pi (mg g−1 min−0.5) represent the rate constants of pseudo-first-order, pseudo-second-order, and intraparticle diffusion model, respectively. C_i is a constant (mol/g), which can be used to determine whether the controlling step is intraparticle diffusion [52]. If the q_t is linear with t1/2 and the value of C_i is zero, the adsorption process is controlled by the intraparticle diffusion only; if the results of curve fitting contain multi-line plots, then there exists two or more steps in the adsorption process [52]. The line plots and the corresponding kinetic parameters of the models are shown in Fig. S2 and Table 2.

Table 2.

Kinetic parameters of models for different samples

Samples	Qe,exp (mg/g)	Pseudo-first-order			Pseudo-second-order
		Qe,1	K1×102 (min−1)	R2	Qe,2	K2×104 (g/mg min)	R2
S1	110.15	46.31	0.382	0.865	114.28	2.38	0.998
S2	167.02	50.08	0.394	0.952	172.12	2.46	0.998
S3	166.57	15.59	0.400	0.955	169.49	8.44	0.999
S4	164.86	9.090	0.497	0.431	165.84	15.9	0.997
S5	37.480	27.68	0.304	0.985	42.50	2.63	0.985

By contrasting the correlation coefficient of the different models, it was found that the adsorption of MG onto samples fitted well with the pseudo-second-order model with higher R2 values (> 0.99) than with the pseudo-first-order kinetics model (R2 < 0.99). The equilibrium adsorption capacity values calculated from the pseudo-second-order model are close to the experimental values, indicating that the pseudo-second-order kinetic model is more suitable for describing the adsorption process of MG onto the adsorbent samples [53, 80]. Additionally, it can be noted from Fig. S2D that the plots were non-linear over the entire time, and the values of intercepts were not zero, suggesting that the dye adsorption process was controlled by multiple processes [52].

3.2.2. Effects of solution pH

The pH value is an important factor affecting the adsorption process, because it influences the surface charge and the ionic state of the adsorbent. Therefore, the effects of initial pH on the MG removal were investigated. As illustrated in Fig. 7, the MG adsorption capacity of S₃ sample increased gradually with the solution pH range from 3.13 to 9.23; similar phenomenon has been reported for the adsorption of MG on activated carbons and rattan sawdust [56]. MG is a cationic dye and the high concentration of H+ at lower pH values may compete with dye cations for the available adsorption sites of the sample, thus a lower adsorption capacity was observed [59]. In addition, the carboxylate groups on the surface of the adsorbent sample can be transformed to –COOH groups at lower pH values (pH < 4.70) [57], which is unfavourable for the interaction between the MG dye molecule and the S₃ sample, thus leading to a decrease in MG adsorption capacity. In contrast, at higher pH values, the surface of sample became negatively charged and exhibited a high MG adsorption capacity. In summary, the as-prepared sample showed a high adsorption capacity over a wide pH range, indicating its promising and excellent adsorption performance in the field of dye wastewater remediation.

Fig. 7.

The effect of initial pH on the adsorption capacity of S₃ sample. pH = 3.13–9.23; T = 303 K; the amount of adsorbent 0.006 g/20 mL; Initial MG concentration 50 mg/L; contact time = 12 hours.

3.2.3. Effect of adsorbent dose

The relationship between the dosage and the MG adsorption capacity of samples was investigated next. Different amounts of adsorbent (0.05–0.7 g/L) were added to 20 mLs of the dye solution with an initial MG dye concentration of 50 mg L−1, and then shaken at 303 K, natural pH of circa 6.6 for 12 hours. As shown in Fig. 8, with the increase of dosage, the MG removal efficiency of sample increased rapidly, while the equilibrium adsorption capacity of the sample decreased. The improvement in the removal efficiency can be attributed to the enhanced surface area and the more available active surface sites [81]. Additionally, the decreased adsorption capacity at high dosages is due to the reduced amount of MG molecules adsorbed per unit mass of the adsorbent, thus causing the decrease in Q_e values with increasing adsorbent mass concentrations [81].

Fig. 8.

The effect of dosage (0.05–0.7 g/L) on the MG adsorption capacity of the S₃ sample. Natural pH of circa 6.6; T = 303 K; Initial MG concentration 50 mg/L; contact time = 12 hours.

3.2.4. Effect of initial concentration, temperature and adsorption isotherms

The effect of initial MG concentration (from 50 to 500mg/L) and temperature (from 303 to 323 K) on the adsorption capacity of the S₃ sample was carried out. As shown in Fig. S3, the adsorption capacity of the adsorbent sample increased with the increase of MG concentration when the temperature was kept constant. This may be explained by the strong driving force, which exists in the solution with a high concentration gradient, thus benefitting the transfer of the dye molecules between the solid phase and the liquid phase, and causing more dye molecules to be adsorbed. Besides, it is clearly observed that the adsorption capacity showed the incremental trend at high temperatures, implying that the process of adsorbing MG molecules was endothermic in nature.

To better illustrate the adsorption behaviour between the adsorbate and the adsorbent in the solution, the Langmuir, Freundlich, and Dubinin-Radushkevish (D-R) isotherm models were used to describe the adsorption processes[82]. The Langmuir isotherm model is based on the hypothetical premise that monolayer adsorption occurs at the specific homogeneous sites within the adsorbent [61]. The Freundlich adsorption isotherm model is founded on the assumption that the interactive behavior is related to multiple-layer adsorption occurring on the heterogeneous surface with different adsorption energies and affinities [61]. The Dubinin-Radushkevish (D-R) isotherm model is usually adopted to distinguish whether the adsorption is physical or chemical [83]. The three model equations can be described as follows:

$$C_e/q_e=1/(q_m{K}_L)+C_e/q_m$$ 6

$$\ln q_e=\ln K_F+\frac{1}{n}\ln C_e$$ 7

$$\mathrm{l}\mathrm{n}{q}_e=ln{q}_m-k{\epsilon }^2$$ 8

where C_e is the equilibrium concentration of MG (mg L− 1), q_e is the quantity of MG adsorbed per gram of sample after equilibrium (mg g− 1), q_m represents the maximum adsorption capacity (mg g− 1), K_L is the Langmuir adsorption constant (L mg− 1), K_F and 1/n are the proper constants of the Freundlich isotherm, which are respectively represented by the adsorption capacity and adsorption intensity [61]. Finally, k (mol2∙kJ− 2) is the D-R isotherm constant ($\mathrm{E}=\frac{1}{\sqrt{2k}}$, when E < 8 kJ mol−1, it is physisorption and if it is between 8–16 kJ mol−1, it is chemical in nature), and ε represents Polanyi potential ( ε = RTln(1+1/C_e)).

The isotherm parameters of each model are summarized in Table 3. It has been observed that the correlation coefficient (R2>0.99) of the Langmuir model was higher than that of the Freundlich (R2:0.857–0.935) and the D-R model (R2:0.738–0.914), and this verified that the MG adsorption behavior of the S₃ sample could be better expressed by the Langmuir equation. Moreover, it was found that the maximum adsorption capacity of sample increased from 1399 to 1977 mg/g when the temperature increased from 303 to 323 K, which was higher than adsorbents previously reported (Table 1). In addition, the values of 1/n obtained from the Freundlich model are smaller than 1, suggesting that the removal condition is favourable for the adsorption reaction. In the D-R model, the values of free adsorption energy (E = (2k)− 0.5) were lower than 8 kJ/mol, thus the physical adsorption process played the main role in the adsorption of MG onto the S₃ sample.

Table 3.

Isotherm constants and values of R2 for S₃ sample.

T(K)	Langmuir isotherm			Freundlich isotherm			D-R isotherm
	Qm(mg /g )	KL (L/mg )	R2	KF (L/g)	n	R2	Qm(mg/g)	E(kJ/mol)	R2
303	1399	0.125	0.996	237	2.389	0.857	980	1.166	0.914
313	1476	0.143	0.998	249	2.254	0.893	942	1.502	0.834
323	1977	0.123	0.990	238	1.603	0.935	967	1.679	0.738

Table 1.

Comparison of adsorption capacities of MG adsorbed by various adsorbents.

Adsorbents	Experimental conditions				qmc (mg/g)	References
	Dosage (g/L)a	pH	Temp.(K)	C0b (mg/L)
CO2-activated porous carbon	0.4	Undefined	313	40–100	284	[52]
PVP@CNTs-Cu2O	2.0	Undefined	293	50–3000	1423	[53]
Fe-Cu bagasse composite	1.0	Undefined	298	1–150	61	[54]
chitosan–surfactant–core–shell (CSCS) beads	0.8	7.00	298	10–400	360	[55]
AC-H3PO4/Steam	1.0	Undefined	298	500–1200	769	[56]
granular composite hydrogel(AA–IA–APT5)	0.5	Undefined	303	200–1800	2433	[57]
NiO flowerlike nanoarchitectures	0.3	Undefined	Ambient	50	142	[58]
Gx-cl-P(AA-co-AAm)/Fe3O4 hydrogel nanocomposite	0.2	neutral pH	298	100–500	497	[59]
bivalve shell-Zea mays L. husk leaf	2.5	6.00	303	10–200	82	[60]
Fe-Cu based adsorbent	0.3	6.58	303	50–500	1399	This work
Fe-Cu based adsorbent	0.3	6.58	313	50–500	1476	This work

^a:the mass of the adsorbent contained in each volume of MG aqueous solution

^b:the initial concentration of MG in the aqueous solution

^c:the maximum adsorption capacity of adsorbent (calculated from the Langmuir model)

Besides, the Langmuir isotherm is also expressed in terms of a dimensionless constant called a separation factor (R_L), which is defined by Eq. 9:

$$R_L=1/(1+{\mathrm{K}}_L{C}_0)$$ 9

where C₀ (mg L− 1) is the initial MG concentration. The R_L value indicates the isotherm is favourable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1), or irreversible (R_L = 0) [79]. In this study, the calculated R_L values are between 0 and 1, indicating that the adsorption of MG on the S₃ sample is favourable [79].

3.2.5. Adsorption thermodynamics

Thermodynamic parameters, such as the Gibbs free energy (ΔG), the enthalpy changes (ΔH), and the entropy change (ΔS), can provide further information about the inherent energetic changes and feasibility of the adsorption reaction. The values of ΔG, ΔH and ΔS can be obtained by the following equations:

$$K_d=\frac{q_e}{C_e}$$ 10

$$\mathrm{\Delta }{G}_0=\mathrm{\Delta }{H}_0-\mathrm{T}\mathrm{\Delta }{S}_0$$ 11

$$\ln K_d=-\Delta H_0/\left(RT\right)+\mathrm{\Delta }{S}_0/R$$ 12

where K_d is the equilibrium constant (mL/g), q_e is the adsorption capacity (mg/g), and C_e is the MG concentration at equilibrium(mg/L). R is the universal gas constant (8.314 J /(mol·K)) and T is the absolute temperature (K). The thermodynamic parameters ΔH and ΔS can be obtained from the slope and the intercept of the linear plot of ln K_d vs. 1/T and are given in Fig. S4 and Table 4. The positive and a smaller value of ΔH (less than 40 kJ mol−1) for the adsorbent indicates that the adsorption process is endothermic and governed by physical adsorption [52, 81], respectively. This is consistent with the earlier conclusion that the higher temperature improved the adsorption capacity. The positive value of ΔS indicated a randomness enhancement in the process of adsorption [81]. The negative ΔG values (−10.647 to −13.026 kJ/mol) established that the MG adsorption process was feasible and spontaneous [81]. Besides, the ΔG values decreased with the increase of temperature, indicating the enhanced feasibility of the adsorption via physical forces [52]. Thus, higher temperatures were favourable to the adsorption of MG, which is in good agreement with the adsorption isotherms.

Table 4.

Thermodynamics parameters for MG adsorption on the S₃ sample.

Temperature (K)	∆G0 (kJ mol −1)	∆H0 (kJ mol −1)	∆S0 (J mol −1K −1)
303	−10.647	25.398	118.96
313	−11.837
323	−13.026

4. Reusability

In addition to the adsorption rate and adsorption capacity, the reusability of adsorbent is of paramount importance, an important consideration to evaluate the value of adsorbent’s practical applications. To date, the main eluents and processes used for the desorption of adsorbent comprise acids (HCl, HNO₃), bases (NaOH, NaHCO₃), organic solvents (ethanol, methanol, and acetone), ultrasound, and thermal regeneration [84, 85]. Usually, the ultrasound and thermal regeneration methods are limited in view of the expensive nature of devices. Compared with acids and bases, the low toxicity and low corrosivity of ethanol have certain advantages. In this study, anhydrous ethanol was selected as the desorbing agent and the adsorption-desorption experiment was conducted by adding the S₃ sample (0.3 g/L) to the MG solution (50 mg/L), then shaken oscillatively to adsorption equilibrium. Four cycles of adsorption–desorption were completed and the results are shown in Fig. 9. The adsorption capacity of the reused adsorbent remained at almost 162.78 mg/g for the four cycles, indicating that anhydrous ethanol can be regarded as a good desorption agent and the absorbent has a good recyclable performance thus rendering it a potentially effective adsorbent for the removal of MG.

Fig.9.

The MG adsorption capacity of original and regenerated S₃ sample with evolution of adsorption-regeneration cycles. Natural pH of 6.6; temperature 303 K; the amount of adsorbent 0.3 g/L; initial MG concentration 50 mg/L.

5. Stability

The stability of S₃ sample was evaluated by adding fresh (1 day) and aged (stored in air for 3, 5, 7, 14, 21 and 28 days) S₃ sample (0.3 g/L) to MG solution (50 mg/L), and then oscillating it to adsorption equilibrium (see Fig. 10); adsorption capacity remained consistent at ~ 162 mg/g, suggesting that the sample was highly stable. Earlier studies involving similar preparations with extracts corroborates the stabilizing aspects. Prasad et al. [86] prepared the iron oxide nanocrystals using Garlic Vine leaf extract, with the premise that the adhering organic biomolecule on the nanoparticles surface serve as steric stabilizer against their inherent agglomeration behavior, a typical trait of small particles. Makarov et al. [44] compared the amorphous iron oxide nanoparticles produced by Rumex acetosa extracts which were more stable than that produced by Hordeum vulgare extracts; the presence of organic acids (such oxalic or citric acids) presumably stabilized the iron nanoparticles. In the context of our work, the high stability of the S₃ sample may be related to the existence of the biomolecules and oxalate on the surface. Moreover, the high stability of this sample to air not only saves the storage cost of the adsorbent, but also bodes well for the potential applications in real world remediation scenarios involving transportation.

Fig. 10.

The MG adsorption capacity of S₃ sample stored in air for 1,3,5,7,14,21 and 28 days. Natural pH of 6.6; temperature 303 K; the amount of adsorbent 0.3 g/L; initial MG concentration 50 mg/L.

6. Conclusions

There is a growing need in the environmental remediation field which calls for greener remediation reagents deployed under more sustainable means [87–89]. In the present study, a green, ecofriendly method, devoid of hazardous materials (e.g. toxic reductants such as borohydrides or hydrazine, and solvents), was designed for the economical and efficient synthesis of Fe, Cu-based adsorbents. XPS analysis showed that the green synthetic material was mainly composed of Fe, Cu-based hybrid oxides and oxalic acid; the electronic attraction between the material and MG dye molecules being responsible for the adsorption process. In the dye removal test, the samples comprising an appropriate amount of oxalate exhibited an enhanced adsorption ability for MG dye, and the adsorption capacity was only slightly influenced by solution pH. Moreover, it is worth noting that the MG adsorption process followed the pseudo-second-order kinetic model and Langmuir adsorption isothermal model; maximum MG adsorption capacity of the selected sample (S₃) was calculated to be 1399 mg/g at near neutral pH (~6.58) and 303 K, which is higher than that of most recently reported adsorbents. In addition, the S₃ sample exhibited high stability and good reusability, which are important attributes for practical application. With these findings, this work not only promotes the use of natural, earth-abundant and renewable resources in synthesizing novel and efficient absorbents, but also provide a simple, convenient and cost-effective strategy for the purification of dye wastewater. This study may stimulate researchers to explore applications of this sustainable strategy for the remediation of other pollutants.

Fig.1.

Schematic illustration of the synthesis of Fe, Cu-based adsorbent