Novel magnetic lignin composite sorbent for chromium(VI) adsorption

Abstract

A novel magnetic lignin composite was prepared and modified with diethylenetriamine. The properties of the composite were characterized by SEM, FTIR, XRD, TGA and VSM. Then, the adsorption of chromium(VI) from aqueous solutions using this magnetic lignin composite was investigated. Chromium(VI) removal is pH dependent and the optimum adsorption was observed at pH 2.0. The pseudo-second-order model and the Langmuir adsorption isotherm model were applied to describe the adsorption kinetics and adsorption isotherm, respectively, for the chromium(VI) adsorption. Thermodynamic parameters were calculated, which revealed the adsorption process to be spontaneous and exothermic. Regeneration of the magnetic lignin composite was achieved by using 0.4 M NaCl and 0.2 M NaOH, more than 87% efficiency retention was obtained after 5 cycles.

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1. Introduction

In recent years, environmental contamination caused by toxic metals has become a worldwide concern because of the potential environmental and biological problems.^1–4 A common heavy metal ion contaminant, chromium, is widely used in many industries including tanning, electroplating, paint, textile, and metal finishing.^5–7 Two stable oxidation states of chromium, the hexavalent Cr(VI) and trivalent Cr(III) states, are present in the environment. The former is known to be more toxic, teratogenic, carcinogenic, mutagenic and mobile than the latter. Cr(VI) is a highly toxic ion affecting aquatic life and human health at relatively low concentrations (US Environmental Protection Agency (EPA), 1998). Accumulation of Cr(VI) in the human body can cause either the alteration or loss of biological function. Thus, there is great need to prevent further Cr(VI) contamination.^8–10 At present, chemical precipitation, adsorption, ion-exchange, membrane separation, reverse osmosis, oxidation/reduction and electroflotation are conventional methods applied in their removal Cr(VI) from wastewater.^11–15 However, high capital costs for the above processes limit their applicability to removing Cr(VI). Therefore, it is especially important to develop a low-cost adsorbent from natural abundant materials (i.e. bio-sorbents).

Lignin, the second most abundant renewable material next to cellulose in biomass, is a very promising raw material available at low cost and low toxicity for the preparation of various functional materials.^16,17 Due to the presence of phenolic, hydroxyl, carbonyl, methoxy and aldehyde groups, lignin and its derivatives have been proven to be an excellent adsorbent for removal of heavy metal ions (e.g., Hg(II), Cr(VI), Cu(II), Pb(II))^18–20 and toxic dyes (e.g., Procion Blue MX-R dye, reactive dye Brilliant Red HE-3B, Congo red, Eriochrome blue black R)^21–23 from industrial effluents. However, after adsorption, all of the lignin and its derivatives are difficult to recover from the aqueous solution using traditional separation methods filtration and sedimentation.²⁴ Furthermore, commonly spent adsorbents are discarded generating secondary pollution.²⁵ Thus, the difficulties encountered in regenerating lignin and its derivatives limit its applications in many fields. Recently, to solve this problems, the application of magnetic adsorbent technology has been received considerable attention.^26,27 So, the preparation of magnetic lignin-based adsorbents would be a good choice to solve this problem. To our knowledge, there is no report regarding the removal of Cr(VI) using magnetic lignin-based adsorbents.

We prepared a novel diethylenetriamine modified epichlorohydrin crosslinked magnetic lignin adsorbent (M-lignin-ECH-DETA) and used it to remove Cr(VI) from aqueous solutions. The obtained functionalized adsorbent M-lignin-ECH-DETA was characterized by SEM, FTIR, XRD, TGA and VSM. In adsorption studies, the effects of pH, initial Cr(VI) concentration, contact time, temperature and reusability were tested in batch experiments.

2. Experimental

2.1. Chemicals

Lignin was purchased from Sigma. Epichlorohydrin, diethylenetriamine, ethylene glycol, ethanol, sodium acetate, sodium dodecanesulphonate, FeCl₃·6H₂O, K₂Cr₂O₇, Na₂CO₃, NaCl, HCl and NaOH were obtained from the Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were analytical grade and used as received without further purification. An aqueous solution of Cr(VI) was prepared by dissolving K₂Cr₂O₇ in double distilled water. A stock solution with a concentration of 1000 mg L⁻¹ of Cr(VI) was prepared and subsequently diluted. All solutions were prepared using double distilled water.

2.2. Preparation of Fe₃O₄

Fe₃O₄ was synthesized by a solvothermal reaction method.²⁸ FeCl₃·6H₂O (2.50 g) was dissolved in ethylene glycol (50.0 mL) to form a clear solution. Then a sodium acetate (7.50 g)–ethylene glycol (25.0 mL) solution was added dropwise into the aforementioned mixture, where sodium acetate could provide elemental oxygen for the formation of Fe₃O₄ and ethylene glycol served as a reductant to favor the formation of Fe₃O₄, instead of Fe₂O₃. After vigorous stirring for 30 min, the resulting homogeneous mixture was sealed in a Teflon-lined stainless steel autoclave (100 mL). The autoclave was heated to 200 °C, maintained for 8 h, and allowed to cool to room temperature. After magnetic separation, the precipitations were washed several times with ethanol and water, and then dried at 60 °C in vacuum for 6 h.

2.3. Preparation of M-lignin-ECH

Coating of the Fe₃O₄ with lignin was achieved by a reverse phase suspension method. 0.60 g Fe₃O₄ particles were dispersed in 120 mL paraffin, containing 1.50 g sodium dodecanesulphonate in a three-neck flask. Then 30.0 mL of a lignin solution in 12.0% (wt%) NaOH with a concentration of 10.0% w/v were added. The suspension was stirred at 800 rpm with a mechanical stirrer for 30 min. Then 12.0 mL epichlorohydrin solution was added and the suspension was stirred for 3 h at 60 °C. The resultant magnetic lignin (M-lignin-ECH) was collected using a permanent magnet and dried at 60 °C in a vacuum oven.

2.4. Modification of M-lignin-ECH with diethylenetriamine

3.0 g M-lignin-ECH and 75.0 mg sodium carbonate anhydrous were added into a three-neck flask, and then 30.0 mL of diethylenetriamine were dropped into the reaction system was kept at 70 °C for 3 h. After the reaction was completed, the product (M-lignin-ECH-DETA) was isolated by magnetic separation and thoroughly washed with distilled water. The precipitate was dried at 60 °C in a vacuum oven.

Lignin-ECH-DETA was synthesized approximately following the method described in 2.3 and 2.4, except for the absence of Fe₃O₄.

2.5. Characterization

FT-IR spectra of the lignin-ECH-DETA and M-lignin-ECH-DETA were recorded using a Nicolet iS50 FT-IR spectrometer in the range of 4000–400 cm⁻¹. The samples were analyzed in the form of KBr pellets. Thermogravimetric analysis was performed on a TG/DSC1/1100 Mettler Toledo thermogravimetric analyzer under a nitrogen atmosphere from room temperature to 800 °C at a heating rate of 10 °C min⁻¹. X-ray powder diffraction (XRD) measurements were obtained by D8 Advance (Bruker AXS), using CuKα radiation (λ = 1.5406 Å) in the range of 2θ = 10°–70°. The magnetization curve of the products were obtained using a LakeShore 7307 vibrating sample magnetometer (VSM) with an applied field between −10 000 and 10 000 Oe at room temperature. The samples were also analyzed with scanning electron microscopy (SEM; Hitachi S-4800). The XPS study of the samples with adsorbed Cr species was performed on an X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe).

2.6. Cr(VI) uptake experiments

The pH effect experiments were performed at room temperature (25 °C) by adjusting the initial pH in the range of 1.0–6.0 for Cr(VI) using 0.1 M HCl. A dose of 40.0 mg M-lignin-ECH-DETA was mixed with 50.0 mL of 100 mg L⁻¹ Cr(VI) solution in several 100 mL Erlenmeyer flasks. The resulting suspension was stirred for 12 h. The sorbent was recovered through magnetic separation after adsorption and the residual solution was analyzed by the atomic absorption spectrophotometer (TAS-990, Beijing Purkinje General Instrument Co. Ltd., China) to get the concentration of Cr(VI).

Kinetic studies for Cr(VI) were conducted using 40.0 mg M-lignin-ECH-DETA in 50.0 mL of the adsorbate solution (100, 150 mg L⁻¹) in 100 mL Erlenmeyer flasks. The suspensions were stirred at regular intervals for 10, 20, 30, 45, 60, 75, 120, 180, 240 min.

Isotherm tests were performed to determine the maximum adsorption capacity (q_max) for Cr(VI). M-lignin-ECH-DETA (40 mg) was mixed with 50 mL solutions in 100 mL Erlenmeyer flasks containing adsorbates with different initial concentrations such as 80, 100, 120, 150, 200, 250, 300 mg L⁻¹ of Cr(VI). The suspensions were stirred for 12 h and the temperature was kept at 25 °C.

Thermodynamic parameters were measured to evaluate the effect of temperature on the Cr(VI) sorption on magnetic lignin and to understand the nature of sorption. A series of Erlenmeyer flasks (100 mL) containing 50.0 mL Cr(VI) solutions with initial concentrations of 100 mg L⁻¹ and 40.0 mg M-lignin-ECH-DETA were shaken at 25, 30, 35, 40 °C for 12 h.

The amount of adsorption (q) was defined by the following equation:

where q_e is the amount of Cr(VI) adsorbed onto the bioadsorbents (mg g⁻¹), c_o and c_e are the initial and equilibrium concentrations of Cr(VI) (mg L⁻¹), respectively. V is the volume of Cr(VI) solution (L), and m is the weight of the bioadsorbents (g).

2.7. Regeneration experiments

For regeneration experiments, 0.10 g of M-lignin-ECH-DETA were loaded with Cr(VI) using 125.0 mL (100 mg L⁻¹) metal ion solution at 25 °C, pH 2 and contact time of 12 h. M-lignin-ECH-DETA was collected, and gently washed with distilled water to remove any unabsorbed metal ions. M-lignin-ECH-DETA was then suspended with 100.0 mL alkaline eluents (0.4 M NaCl + 0.2 M NaOH) and stirred.

The regeneration efficiency was defined as:

where q₁ is the first time the amount of Cr(VI) adsorbed onto the bioadsorbents (mg g⁻¹), q_n is the nth time the amount of Cr(VI) adsorbed onto the bioadsorbents (mg g⁻¹). All adsorption experiments were repeated at least twice to ensure accuracy of the obtained data. The average uncertainties were <4%.

3. Results and discussion

3.1. Structure and properties of M-lignin-ECH-DETA

SEM images of Fe₃O₄ and M-lignin-ECH-DETA composites are presented in Fig. 1. It is observed that the surface of M-lignin-ECH-DETA composites was non-uniform and the interfacial adhesion that binded the Fe₃O₄ on M-lignin-ECH-DETA surface was very clear.

Fig. 1 SEM images of (a) Fe₃O₄ and (b) M-lignin-ECH-DETA.

In the present study, FTIR spectra of M-lignin-ECH-DETA and lignin-ECH-DETA are illustrated in Fig. 2. According to the reported literature,²⁹ the appearance of signals at 3000–3500 cm⁻¹ was assigned to the stretching of –OH, –NH groups. The absorption peak at 2919 cm⁻¹ was attributed to C–H stretching vibration of either methyl or methylene group. The peak at 1643 cm⁻¹ confirmed the –NH scissoring of the primary amine. The peak at 1510 cm⁻¹ corresponded to the aromatic skeleton vibrations. The peak at 1457 cm⁻¹ was originated from C–N stretching mode of amino groups.²⁰ The band around 1221 cm⁻¹ indicated the appearance of aromatic phenyl C–O.³⁰ The biosorption band 1130 cm⁻¹ was mainly due to the secondary amino groups.³¹ The biosorption band 1026 cm⁻¹ displayed the stretching vibration of a C–O bond.³² The presence of absorption at 854 cm⁻¹ for M-lignin-ECH-DETA was assigned to C–H aromatic out-of-plane deformation.¹⁷ Compared to lignin-ECH-DETA, the main changes to the FTIR spectrum of M-lignin-ECH-DETA was the appearance of a new band at 585 cm⁻¹ which was ascribed with Fe–O groups. This indicated that Fe₃O₄ particles were successfully embedded in M-lignin-ECH-DETA.

Fig. 2 FT-IR spectra of M-lignin-ECH-DETA and lignin-ECH-DETA.

Fig. 3 shows the XRD patterns of Fe₃O₄ and M-lignin-ECH-DETA. Fe₃O₄ was pure with a spinel structure, as can be seen from six characteristic diffraction peaks (2θ = 30.1°, 35.5°, 43.1°, 53.4°, 57.0°, and 62.5°).³³ The XRD pattern of M-lignin-ECH-DETA was very similar to that of the Fe₃O₄, implying that the crystal Fe₃O₄ did not change and Fe₃O₄ have been coated in M-lignin-ECH-DETA.

Fig. 3 XRD pattern of Fe₃O₄ and M-lignin-ECH-DETA.

The thermogravimetric (TG) curves obtained for the Fe₃O₄, M-lignin-ECH-DETA, lignin-ECH-DETA under N₂ atmosphere are presented in Fig. 4. The weight loss could be divided into three stages for M-lignin-ECH-DETA, lignin-ECH-DETA. The first stage was below 200 °C, which was ascribed to the volatilization of free water. The second stage, in the range 200–500 °C, corresponded to a major weight loss of the main organic component. The last stage was found from 500 to 800 °C, ascribing to further decomposition of the aromatic rings of the lignin composite. The thermal stability of M-lignin-ECH-DETA was higher than lignin-ECH-DETA, indicating Fe₃O₄ particles have been embedded in M-lignin-ECH-DETA. It was in agreement with the above-mentioned discussion. The curve for Fe₃O₄, M-lignin-ECH-DETA, lignin-ECH-DETA the residue yield was about 97.8 wt%, 44.4 wt% and 35.8 wt% at 800 °C, respectively. From these numbers the mass of Fe₃O₄ in M-lignin-ECH-DETA was estimated to be about 14 wt%.

Fig. 4 TGA curves of Fe₃O₄, M-lignin-ECH-DETA, lignin-ECH-DETA.

The magnetic hysteresis loop of Fe₃O₄ and M-lignin-ECH-DETA are demonstrated in Fig. 5. As can be seen, the saturation magnetization of Fe₃O₄ and M-lignin-ECH-DETA was about 79.9 and 10.6 emu g⁻¹, respectively. The result indicated that M-lignin-ECH-DETA possessed a sensitive magnetic responsiveness, which can be easily separated with the help of the external magnetic field.

Fig. 5 Magnetization curves of Fe₃O₄ and M-lignin-ECH-DETA.

3.2. Adsorption properties

3.2.1 Effect of pH on Cr(VI) adsorption. One important factor in the Cr(VI) removal was the pH value of the aqueous solution, which can affect the adsorbent surface charge and Cr(VI) speciation.^5,34,35 The effect of pH on Cr(VI) adsorption efficiency by the M-lignin-ECH-DETA is shown in Fig. 6. When pH varied from 1.0 to 2.0, the adsorption percentage of Cr(VI) showed a significant increase. When pH exceeded 2.0, however, the adsorption percentage of Cr(VI) decreased sharply with the rising pH value. This adsorption behavior was attributed to the strong electrostatic attraction between negatively charged chromium species and protonated groups located on the adsorbent surface.³⁶ The Cr(VI) in aqueous solution exists in various forms, such as H₂CrO₄, HCrO₄⁻, Cr₂O₇²⁻, CrO₄²⁻, etc. As the pH increases, the dominant species of Cr(VI) are H₂CrO₄ (pH = 1), HCrO₄⁻ (pH = 2–4), Cr₂O₇²⁻ (pH = 4–6), CrO₄²⁻ (pH > 6).^8,35 When pH < 2.0, the adsorption was low owing to the strong competition for adsorption sites between H₂CrO₄ and protons.³⁷ When pH > 2.0, absorption of Cr(VI) on M-lignin-ECH-DETA decreased again as the pH increased. This behaviour could be explained by two reasons: first, the weakened protonation of M-lignin-ECH-DETA with rising pH which reduced the electrostatic interactions between Cr(VI) species and M-lignin-ECH-DETA;³⁸ second, at lower pH Cr(VI) exists as HCrO₄⁻ (the dominant species) requiring one sorption site on the M-lignin-ECH-DETA in order for the sorption to occur. However, at pH values > 4.0, the divalent forms of Cr(VI) species (Cr₂O₇²⁻, CrO₄²⁻) necessitated two adjacent sorption sites on the surface of M-lignin-ECH-DETA for the chromium to be firmly bound, making retention of the metal ions on the surface less likely.³⁹ Furthermore, the reduction of Cr(VI) to Cr(III) may also occur in the acidic condition.²⁶ To confirm this, XPS technique was employed to analyze composites after adsorption. XPS spectrum is exhibited in Fig. 7. Two energy bands at 586.1 eV and 576.3 eV suggested the existence of Cr(III). Therefore, according to the results of XPS, we can speculate that Cr(VI) adsorbed by M-lignin-ECH-DETA is partially reduced to Cr(III). Therefore, we choose pH = 2 as the optimal pH value for the subsequent experiments.

Fig. 6 Effect of initial pH on adsorption of Cr(VI) by M-lignin-ECH-DETA.

Fig. 7 XPS spectra of the M-lignin-ECH-DETA after Cr(VI) adsorption.

3.2.2 Adsorption kinetics. Fig. 8(a) shows the effect of contact time on the adsorption of Cr(VI) ions by M-lignin-ECH-DETA. It was obvious that the adsorption of Cr(VI) progressed rapidly during the initial 30 minutes. Then the uptake rate decreased noticeably until after 120 minutes a steady state was reached. The kinetics of adsorption provided insights about the mechanism of the adsorption process. In order to investigate it, the obtained adsorption/time data were plotted in pseudo-first-order model, pseudo-second-order and intraparticle diffusion model graphs as shown in Fig. 8(b)–(d).

Fig. 8 (a) Effect of contact time on the uptake of Cr(VI) by M-lignin-ECH-DETA. (b) Pseudo-first-order. (c) Pseudo-second-order kinetic models. (d) Intraparticle diffusion model.

The pseudo-first-order model equation is:

ln(q_e − q_t) = ln q_e − K₁t (3)

The pseudo-second-order model equation is always given as:

The intraparticle diffusion model is represented as follow:

q_t = K_it^0.5 + C (5)

where q_e and q_t (mg g⁻¹) are the amount of Cr(VI) adsorbed on M-lignin-ECH-DETA at equilibrium and at a given time t, respectively. K₁ is rate constant (min⁻¹) of pseudo-first-order model. K₂ (g mg⁻¹ min⁻¹) is the adsorption rate constant of pseudo-second-order model. K_i is the intraparticle diffusion. C is the thickness of the boundary layer.

The fitting parameters of adsorption kinetics are listed in Table 1. The correlation coefficients of the pseudo-second-order model (R²) were higher than that of the pseudo-first-order model (R²). Moreover, compared with pseudo-first-order model, adsorption values (q_e2,_cal) calculated by the pseudo-second order model were closer to the experimental results (q_e,_exp). This indicated that the adsorption process of Cr(VI) on M-lignin-ECH-DETA could be considered as a pseudo-second-order model process. This implied that the Cr(VI) uptake process was chemisorptions. Fig. 8(d) shows the plot of the amount of Cr(VI) adsorbed (q_t) versus the square root of time (t^0.5). It can be seen that the adsorption data are fitted by two separate straight lines. This revealed that the adsorption of Cr(VI) on M-lignin-ECH-DETA is a process involving external diffusion and final intraparticle diffusion.^26,27,40

Table 1 Kinetic parameters obtained through pseudo-first-order and pseudo-second-order for the adsorption of Cr(VI) onto M-lignin-ECH-DETA at initial concentration of 100 and 150 mg L⁻¹

Concentration Cr(VI) (mg L^−1)	Pseudo-first-order
	qe,exp (mg g^−1)	qe1,cal (mg g^−1)	K1	R^2
100	83.33	6.13	0.0397	0.952
150	100.89	19.48	0.0258	0.908

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	Pseudo-second-order
	qe,exp (mg g^−1)	qe2,cal (mg g^−1)	K2	R^2
100	83.33	85.54	0.0044	0.999
150	100.89	102.35	0.0032	0.999

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3.2.3 Adsorption isotherm. For interpretation of the interaction between the adsorbent and adsorbate, Langmuir, Freundlich and Temkin adsorption isotherm models were employed to analyze experimental data and describe the equilibrium of adsorption.

Langmuir equation is represented as follows:

the Freundlich isotherm is depicted by the following equation:

Temkin can be presented by the following equation:

q_e = B ln A + B ln c_e (8)

where q_e and c_e are the amount of Cr(VI) ions adsorbed (mg g⁻¹) at equilibrium and the adsorbate concentration in solution (mg L⁻¹), respectively. q_m (mg g⁻¹) and K_L (mL g⁻¹) are the Langmuir constants related to the saturated sorption capacity and sorption energy, respectively. K_F [(mg g⁻¹)(L mg⁻¹)1/n] is Freundlich constant which indicate the capacity of the adsorption. n is the heterogeneity factor. A is the equilibrium binding constant corresponding to the maximum binding energy and constant B is related to the heat of adsorption. The Cr(VI) adsorption isotherms for M-lignin-ECH-DETA are presented in Fig. 9 and isotherm parameters are summarized in Table 2.

Fig. 9 Isotherm curves of Cr(VI) adsorption on M-lignin-ECH-DETA.

Table 2 Parameters of the Langmuir isotherm, Freundlich isotherm, and Temkin models for Cr(VI) biosorption onto M-lignin-ECH-DETA

Freundlich isotherm			Langmuir isotherm			Temkin
KF	n	R^2	qm (mg g^−1)	KL	R^2	A	B	R^2
43.28	5.35	0.913	123	0.063	0.996	2.60	18.58	0.944

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As could be seen from Fig. 9, the adsorption of Cr(VI) would increase with the initial concentration. However, above a certain concentration, the adsorption capacity of Cr(VI) trended toward steady state, saturated, values. Langmuir, Freundlich and Temkin adsorption isotherm model fits show that the correlation coefficient of Langmuir isotherm (R² = 0.996) is higher than that of Freundlich isotherm (R² = 0.913) and Temkin isotherm (R² = 0.944). Hence, the adsorption isotherms are described well by the Langmuir isotherm models. The calculated maximum Cr(VI) uptake q_m is 123 mg g⁻¹. In other words, the adsorbed material M-lignin-ECH-DETA forms a monolayer on the surface with a finite number of identical sites that are homogeneously distributed across the adsorbent surface.

3.2.4 Adsorption thermodynamics. Fig. 10 shows the adsorption of Cr(VI) biosorption onto M-lignin-ECH-DETA at different temperatures. The results found that the adsorption capacity decreased with increasing temperature. The thermodynamic parameters (ΔG^o, ΔS^o and ΔH^o) were calculated using the following equation:

ΔG = −RT ln(q_e/c_e) (9)

where q_e and c_e are the amount of Cr(VI) ions adsorbed (mg g⁻¹) at equilibrium and the adsorbate concentration in solution (mg L⁻¹), respectively. R is the universal gas constant (8.134 J (K⁻¹ mol⁻¹)), T is the temperature in Kelvin. The thermodynamic parameters for the adsorption of Cr(VI) are listed in Table 3.

Fig. 10 (a) Effect of temperature on the uptake of Cr(VI) using M-lignin-ECH-DETA. (b) Thermodynamic plot of ln(q_e/c_e) vs. 1/T.

Table 3 Thermodynamic parameters for the adsorption of Cr(VI) using M-lignin-ECH-DETA

ΔH^o (kJ mol^−1)	ΔS^o (J k^−1 mol^−1)	ΔG^o (kJ mol^−1)
		298 K	303 K	308 K	313 K
−14.1513	−39.9213	−2.2706	−2.0126	−1.8542	−1.6594

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The negative values of ΔG^o indicated the adsorption is spontaneous and feasibility of the adsorption of Cr(VI) on the M-lignin-ECH-DETA. The magnitude of the value of ΔG^o decreased from −2.2706 to −1.6594 kJ mol⁻¹ in the temperature range of 298–313 K, suggesting the adsorption is not favorable at higher temperatures.⁴⁰ The negative value of ΔH^o implied that the adsorption of Cr(VI) onto M-lignin-ECH-DETA was exothermic in nature and the adsorption of Cr(VI) was more effective at lower temperature. The negative value of ΔS^o reflected decreasing randomness at the solid/solution interface during the adsorption process.⁴¹

3.2.5 Regeneration experiments. The recycling of adsorbents plays an important role in evaluating the potential applicability of adsorbents. After adsorption of Cr(VI) onto the M-lignin-ECH-DETA, desorption experiments have been carried out using 0.4 M NaCl and 0.2 M NaOH. Then the regenerated M-lignin-ECH-DETA was reused to adsorb Cr(VI). The results are shown in Fig. 11. From this figure can be seen that the adsorption capacity was reduced with the number of regeneration times. In addition, the regeneration efficiency of the M-lignin-ECH-DETA after five times still could reach more than 87%. Therefore, M-lignin-ECH-DETA could be utilized repeatedly for the treatment of Cr(VI) effluents.

Fig. 11 Effect of recycling M-lignin-ECH-DETA on Cr(VI) adsorption.

4. Conclusions

In this study, a novel magnetic lignin composite was prepared and acted as a promising adsorbent for the adsorption of Cr(VI). The results showed that the optimal pH value was pH 2.0. Adsorption kinetics could be described well by a pseudo-second-order model. A Langmuir model represented the adsorption isotherm well and the maximum adsorption amount of Cr(VI) was calculated to be 123 mg g⁻¹. The obtained thermodynamic parameters revealed that the adsorption of Cr(VI) onto the adsorbent was an exothermic and spontaneous process. Additionally, magnetic lignin composite could be separated well using the magnetic properties of the composite. The composite showed good reusability losing only 13% of its capacity after 5 cycles. All these results demonstrated that the material is promising as sorbent in reducing pollution of Cr(VI) effluents.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (21206057), the Natural Science Foundation of Jiangsu Province, China (BK2012118) and (BK2012547), and MOE & SAFEA for the 111 Project (B13025) for financial support.

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