Removal of cadmium and lead from aqueous solutions using nitrilotriacetic acid anhydride modified ligno-cellulosic material

Abstract

Cadmium (Cd²⁺) and lead (Pb²⁺) posed severe health risks worldwide. To remove these contaminants from aqueous solution, a nitrilotriacetic acid anhydride (NTAA) modified ligno-cellulosic material (NTAA-LCM) was prepared and characterized. Batch sorption experiments were performed to evaluate the influences of various factors such as contact time, pH, ionic strength, temperature and initial metal concentration on the sorption of metals. Results from elemental analysis and FTIR suggested that ester bonds and amine groups were successfully introduced into NTAA-LCM. Fast adsorption rates were observed, and the maximum sorption capacities of NTAA-LCM for Cd²⁺ and Pb²⁺ reached 143.4 and 303.5 mg g⁻¹ at 298 K, respectively. Both the pseudo-second-order model and the Langmuir model described the adsorption extremely well. Thermodynamic analysis showed that the sorption was endothermic but spontaneous. The sorption was a chemical process involving surface chelation and ion exchange; this was reflected in the metal/NTA ratio. Additionally, NTAA-LCM retained high metal sorption capacity after seven cycles of regeneration by HNO₃.

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1 Introduction

Heavy metal pollution inflicts adverse effects on organisms and humans and is considered a global environmental issue.¹ Heavy metals such as cadmium (Cd²⁺) and lead (Pb²⁺) are among the most common pollutants found in industrial effluents. Conventional methods employed for the removal of heavy metals from effluents mainly include chemical precipitation, ion exchange, membrane separation, reverse osmosis, and electrolysis. Most of these methods are costly or ineffective. Adsorption is considered to be one of the most convenient and cost-efficient methods.^2–5 Activated carbon,^6,7 clay minerals,⁸ synthetic inorganic materials,⁹ synthetic nano-particles¹⁰ and others have been utilized as adsorbents.¹¹ Most of these methods are costly, ineffective, or both. Thus, novel adsorbents with high adsorption capacity, abundant availability are increasingly needed for the removal of heavy metals.

Recently, natural materials have attracted increasing attention as adsorbents for heavy metals because of their renewable and biodegradable properties. For example, wheat straw has been treated with citric acid to remove copper ion,¹² and biodegradable chitosan-coated sand has been developed for removal of Cu(II) and Pb(II) in aqueous solution.¹³ Surface modification with anhydrides received great interest for its fast kinetics, high adsorption capacity and chemical stability among various modification methods.^5,14 This kind of modification introduces carboxylic acid functional groups by esterification reaction. Succinic, maleic, phtalic, EDTA, and DTPA are examples of cyclic anhydrides that have been used for modification of ligno-cellulosic materials.^5,14–16

Corn is one of the largest grain crops in the world, especially in China and USA. As one of the common agricultural residues, cornstalk was usually burnt directly in the field without effective utilization, which causes serious air pollution. However, these residues have been proved to be potential adsorbents in the treatment of pollutants.^15,17–20 The main components of cornstalk are cellulose (38.92%), hemicellulose (20.87%), and lignin (21.52%).²¹ Cellulose has primary and secondary hydroxyl groups, and lignin has hydroxyl phenolic groups. Therefore, ligno-cellulosic materials such as cornstalk can, through their hydroxyl functions, react chemically and produce new materials with novel properties. To date, there have been several studies reported on the preparation of cornstalk materials with different modifiers. For example, in Zheng's work, the maximum adsorption capacity of acrylonitrile-modified cornstalk for Cd(II) was three times higher than that of the unmodified cornstalk.¹⁹ However, until now, there is no study with respect to the removal of heavy metals with the utilization of chelator modified cornstalk material. Nitrilotriacetic acid (NTA) is a powerful chelating agent with three carboxyl groups, which has shown excellent metal binding properties. As a low-cost and biodegradable chelating agent which was primarily reported as magnetic nanoparticles modifier to anchor proteins,^22–24 NTA can be used to synthesize NTA anhydride (NTAA) and further modify ligno-cellulosic materials.²⁵ Additionally, NTAA is a liquid anhydride which can thoroughly react with the material, and the use of NTAA as a modifier, which to the best of our knowledge, is reported for the first time.

Therefore, the NTAA-modified cornstalk has the potential to be an excellent novel material because it is inexpensive and biodegradable in comparison to other chelating materials. In this study, dewaxed cornstalk (LCM) was chemically modified with NTAA, and the characteristics of the novel material were analyzed. Also, the adsorption behavior of NTAA-LCM for Cd²⁺ and Pb²⁺ from aqueous solutions was evaluated by batch tests, and the interaction mechanisms between metals and NTAA-LCM were explored.

2 Materials and methods

2.1 Materials

The cornstalk was collected from Hunan Province, Southeast China. NTA, acetic anhydride, pyridine and N,N′-dimethylformamide (DMF) were purchased from Aladdin Shanghai/China and used without further purification. Sodium hydroxide (NaOH), acetone, sodium hydrogen carbonate (NaHCO₃), 95% ethanol, n-hexane, Cd(NO₃)₂·4H₂O, Pb(NO₃)₂ and nitric acid were of analytical reagent grade and purchased from Kermel Tianjin/China. Milli-Q ultrapure water (18.2 MΩ cm) was used throughout the study.

2.2 Preparation of LCM

After being cut to 2–4 cm, the cornstalk was washed with ultrapure water, dried at 80 °C in an oven for approximately 24 h, and then the cornstalk was milled through a 60-mesh sieve. Then, 10 g of dried powder was washed in a Soxhlet apparatus with 60 mL of n-hexane at 50 °C for 4 h and then dried at 80 °C in an oven and stored in a desiccator prior to use. The obtained material was LCM.

2.3 Synthesis of NTAA and NTAA-LCM

In order to improve the synthesis yield of NTAA-LCM, we optimized the modification time and the LCM/NTA ratio by two steps.

Step 1: effect of modification time.

6 mL of pyridine, 6 mL of DMF, and 4 g of NTA were placed in a 150 mL Erlenmeyer flask, and then 6 mL of acetic anhydride was added dropwise. The mixtures were agitated strongly at 65 °C for 24 h and NTAA was obtained.^25,26 Then 500 mg of LCM and 6 mL of DMF were added to the mixture solution containing NTAA, and the suspension was shaken and heated at 75 °C for 20 h.

The same procedure was carried up for reaction times of 10 and 30 h. After filtration, the obtained new material were washed with acetic anhydride, DMF, deionized water, saturated sodium bicarbonate solution, deionized water, ethanol 95%, acetone, and then dried in an vacuum at 80 °C for 2 h and left to cool in a desiccator. After cooling, the mass percent gains were calculated.

Step 2: effect of the LCM/NTA ratio.

3 mL of pyridine, 3 mL of DMF, and 2 g of NTA were placed in a 150 mL Erlenmeyer flask, and then 3 mL of acetic anhydride was added dropwise. The mixtures were agitated strongly at 65 °C for 24 h. Then 500 mg of LCM and 3 mL of DMF were added to the mixture solution, and the suspension was shaken and heated at 75 °C for 20 h.

The same procedure was carried out at constant anhydride/solvent ratio. The amounts of NTA and volumes of each organic reagent were 3 g/4.5 mL, 4 g/6 mL, and 6 g/9 mL, respectively. After modification, the materials were treated as described above, and the mass percent gain were also calculated. Air was isolated with plastic wraps during each shock process to prevent water from entering into the flasks.

2.4 Characterization of NTAA-LCM

NTAA-LCM was characterized by weight gain, elemental analysis, FTIR and EDAX. Elemental content was analyzed by an Elementar Vario ELIII elemental analyzer with ± 0.3% accuracy (Germany). The Fourier transform infrared spectrophotometer (Nico-let, Nexus-670 FTIR) spectra of the materials were recorded in the range of 4000–800 cm⁻¹. Energy-dispersive X-ray analysis (EDAX) of the materials were conducted by an FEI Quanta 200 environmental scanning electron microscope coupled with EDAX genesis xm-2 EDS. Nitrogen adsorption measurements at 77 K were performed using an TRISTAR II 3020 V1.03 volumetric adsorption analyzer. BET analyzes were used to determine the surface area.

2.5 Adsorption experiments

2.5.1 Adsorption kinetics. Metal ion adsorption capacities were investigated as a function of time to determine the adsorption equilibrium time. The equilibrium time was one of the parameters for economical wastewater treatment plant applications. 50 mg of NTAA-LCM were placed in 150 mL Erlenmeyer flasks containing 50 mL of 200 mg L⁻¹ Cd²⁺ or Pb²⁺ solution. The experiments were done at pH 4.0 for Cd²⁺ and 3.8 for Pb²⁺ by adding dilute HNO₃ or NaOH. The flasks were shaken at constant speed (150 rpm) at 25 °C, and the time intervals used were from 1 to 120 min. At each adsorption time, samples were subjected to centrifugation at 4000 rpm for 10 min and the suspensions were filtered, diluted, and measured using atomic absorption spectroscopy (Analyst 700, Perkin Elmer, USA). All adsorption experiments were performed in duplicate, and the relative errors were about 5%. The amount of metals adsorbed by NTAA-LCM in the period of time was calculated using the following equation.where q_t (mg g⁻¹) is the amount of metal ion adsorbed on the NTAA-LCM at any time, m (g) is the dry weight of NTAA-LCM, V (mL) is the volume of metal solution, C₀ (mg L⁻¹) is the initial metal concentration. When t equals to the equilibrium contact time, q_t represents the amount of metal ion adsorbed at equilibrium.

2.5.2 Effects of pH and ionic strength on metals adsorption. The effects of pH and ionic strength on metals adsorption was studied by a series of experiments at varying concentrations of CaCl₂ solutions. 50 mg of NTAA-LCM were added to 150 mL Erlenmeyer flasks containing 50 mL of 200.0 mg L⁻¹ Cd²⁺ or Pb²⁺ solution with various concentrations of CaCl₂. Solubility product constants (7.2 × 10⁻¹⁵ for Cd(OH)₂ and 1.43 × 10⁻²⁰ for Pb(OH)₂) and concentrations of Cd²⁺ and Pb²⁺ were used to calculate the maximum pH where these ions may not occur as hydrolyzed species.¹⁴ Calculated maximum pH for Cd²⁺ and Pb²⁺ were 8.3 and 5.5, respectively. This procedure was necessary to ensure that only Cd²⁺ or Pb²⁺ would be adsorbed. The pH value was adjusted to 2.0–8.0 and 1.0–6.0 for Cd²⁺ and Pb²⁺, respectively. The flasks were shaken at constant speed (150 rpm) at 25 °C for 60 min. After filtration and diluted, the metal ion concentration was determined using atomic absorption spectroscopy (Analyst 700, Perkin Elmer, USA).

2.5.3 Adsorption isotherms of NTAA-LCM. To evaluate the effect of initial metal ion concentration on the adsorption of metals onto NTAA-LCM, 50 mg each material were added to 150 mL Erlenmeyer flasks with 50 mL of metal ion solution at specific concentration (from 20 to 500 mg L⁻¹ for Cd²⁺, and from 20 to 600 mg L⁻¹ for Pb²⁺). The flasks were agitated at 150 rpm for 60 min (pH = 4.0) at different temperatures (298, 303, and 313 K). The metal ion concentration was determined using atomic absorption spectroscopy (Analyst 700, Perkin Elmer, USA).

2.6 Regeneration studies

To evaluate the reusability, regeneration of NTAA-LCM was studied. 1 g of adsorbent was loaded by 100 mL of 200 mg L⁻¹ Cd²⁺ or Pb²⁺ solution. After attaining equilibrium, the exhausted adsorbent was centrifuged and filtered. Then metal ions were eluted by 200 mL of 1.0 M HNO₃ solution at contact time of 60 min. After desorption, adsorbent was then rinsed twice with distilled water and was reemployed in metal ion removal. This adsorption/desorption cycle was repeated 7 times to test the reusability of the adsorbent. The regeneration efficiency (RE) of the material was calculated as follows:where q₀ and q_n (mg g⁻¹) are the adsorption capacities of the material before and after regeneration, respectively.

3 Results and discussion

3.1 Synthesis of NTAA-LCM

Fig. 1 illustrated the synthesis route used to prepare NTAA-LCM and a suggested mechanism for Cd²⁺ or Pb²⁺ adsorbed to NTAA-LCM. Before the modification, the powdered cornstalk sample was washed thoroughly with n-hexane to remove the hydrophobic botanic wax on the surface.²⁷ Treatment with n-hexane and modification with NTAA improved the hydrophilicity of the adsorbent, so the adsorbent could fully contact with heavy metals in the adsorption process. Additionally, the dewaxing process exposed the functional groups such as phenolic, hydroxyl, carboxyl, benzyl alcohol, methoxyl, and aldehyde groups.² As shown in Fig. 1, pyridine was used as an activating agent to introduce NTA through formation of an interval carboxylic acid anhydride. NTAA molecular could be broken by DMF, and then could react with hydroxyl groups of LCM.¹⁴ Saturated NaHCO₃ solution was used to convert the carboxylic group into carboxylate function for a better chelating function.

Fig. 1 Synthesis route of NTAA-LCM.

The mass percent gain (mpg) was calculated using the following equation.

where m_b (mg) and m_f (mg) are masses of the adsorbent before and after the modification, respectively.

The effect of modification time was evaluated to verify the minimum reaction time necessary to introduce the defined amount of NTA into NTAA-LCM. For this, time intervals of 10, 20, and 30 h were used. The mass percent gains were calculated and used to evaluate the synthesis yield. The obtained results were shown in Table 1. As shown in Table 1, the largest mass percent gain (28.3%) was obtained at the reaction time of 20 h. Between 20 and 30 h a mass percent loss was noticed. This result was possible due to crosslink reaction between the carboxyl function of NTA and non-esterified hydroxyl groups from LCM.¹⁴

Table 1 Effect of reaction time and the amount of NTA on the mass percent gain of NTAA-LCM

Synthesized material	Reaction time (h)	NTA (g)	mpg (%)
NTAA-LCM	10	4	24.5
0.5 g	30		26.3
	20		28.3
		2	20.4
		3	26.9
		4	28.3
		6	28.3

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The effect of NTA amount was evaluated to determine the minimum amount of NTA necessary to obtain the largest mass percent gain at the reaction time of 20 h. In order to perform the study, NTA amounts of 2, 3, 4, and 6 g were chosen with fixed NTA/solvent ratio which can be seen from the Section 2.3. The NTA/solvent ratio is considered to be a very important variable and its variation may affect the synthesis yield. As shown in Table 1, the mass percent gain reached a plateau when at least 4 g NTA was used in the modification of LCM. In this reaction condition the mpg was found to be 28.3%. Therefore, the optimized reaction condition was prepared by using 4 g NTA and reaction time of 20 h.

3.2 Characterization of NTAA-LCM

A quantitative analysis of carbon, hydrogen, and nitrogen was performed by an elemental analyzer for the unmodified and best obtained material. The NTA-groups on NTAA-LCM was calculated from the difference of nitrogen content in LCM (1.015%), NTAA-LCM (1.975%) and NTA (7.320%) by the following equation:¹

W_NTAA-LCM = W_NTA × NTA% + W_LCM(1 − NTA%) (4)

where W_NTAA-LCM, W_NTA, and W_LCM are the nitrogen content of NTAA-LCM, NTA, and LCM, respectively; NTA% is the content of the NTA groups in NTAA-LCM. The calculated result showed that the content of the NTA groups in the modified material was 15.23%, i.e., 152.3 mg (0.797 mmol) of NTA was present in 1 g of the resultant NTAA-LCM. The calculated content of NTA in the material was 0.797 mmol g⁻¹ (Table 2).

Table 2 Results of elemental analysis for dewaxed cornstalk before and after NTAA-modification

Sample	Elemental content (wt%)			NTA% (wt%)	Loading of NTA (mmol g^−1)
	C	H	N
LCM	45.79	7.029	1.015	0	0
NTAA-LCM	43.34	6.107	1.975	15.23	0.797

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The spectra of LCM, NTAA-LCM and NTAA-LCM-Cd were shown in Fig. 2. A strong band at 1734 cm⁻¹ appeared in the spectrum of NTAA-LCM. The new peak indicated that ester bond and carboxyl were introduced into NTAA-LCM after modification with NTAA,⁵ which corresponded to the elemental analysis results. The peak at 1734 cm⁻¹ in the spectrum of NTAA-LCM-Cd was much weaker than that of NTAA-LCM, which could be explained by the fact that carboxyl groups and heavy metals participated in the chelating reaction.²⁸ The bands at 3455 and 1061 cm⁻¹ were characteristics of cellulose or hemi-cellulose.⁵ The peaks at 1642 and 897 cm⁻¹ were indicative of hemicelluloses,²⁹ whereas the peak at 1421 cm⁻¹ was attributed to asymmetric and symmetric axial deformations of carboxylate ions.¹⁶

Fig. 2 FTIR spectra of LCM, NTAA-LCM and NTAA-LCM-Cd.

The energy dispersive X-ray analysis (EDXA) spectra of NTAA-LCM before and after interaction with Cd²⁺ or Pb²⁺ were shown in Fig. 3. The spectra showed the presence of various elements and their atom weight percentage. The major elements present in NTAA-LCM were C, O, Na⁺, N, Si, respectively (Fig. 3a). The interaction between NTAA-LCM with Cd²⁺ (Pb²⁺) was depicted in Fig. 3b (Fig. 3c), indicating that metals reacted with carboxyl groups of NTAA-LCM. Na⁺ was replaced by Cd²⁺ or Pb²⁺ after adsorption, suggesting that adsorption took place by surface chelation and ion exchange.

Fig. 3 EDAX spectra of (a) NTAA-LCM before metal adsorption; (b) NTAA-LCM after Cd²⁺ adsorption; (c) NTAA-LCM after Pb²⁺ adsorption.

Nitrogen adsorption/desorption isotherms were recorded to investigate the surface and pore texture of the materials. The isotherm curve of NTAA-LCM showed an unclosed hysteresis loop in the P/P₀ range 0.05–0.2, indicating that the shape and size of pores of the present material was uneven. This unevenness was further supported by several pore size distributions centers. New peaks were not founded after LCM modified with NTA (Fig. 4). The BET surface areas of NTAA-LCM and LCM were 1.8006 and 2.4470 m² g⁻¹, respectively. And the pore sizes of NTAA-LCM and LCM were 8.9278 and 11.7910 nm, respectively. The decrease of BET surface area and pore size might attribute to the introduction of NTA groups.

Fig. 4 Nitrogen adsorption/desorption isotherms and pore size distribution of LCM and NTAA-LCM.

3.3 Adsorption kinetics

Adsorption experiments were performed to investigate the effect of contact time on metal ion uptake and determine adsorption equilibrium time for NTAA-LCM. Fig. 5 showed the adsorption kinetics of Cd²⁺ and Pb²⁺ on the modified material. It could be observed that the adsorption equilibrium was reached within 5 min for Cd²⁺ and Pb²⁺ (Fig. 5a). Therefore, the time was set to 60 min for pH and concentration-dependent experiments. During the first minute, the adsorption of Cd²⁺ and Pb²⁺ sharply increased, with more than 95% of the total ions being adsorbed by NTAA-LCM. The adsorption amounts of Cd²⁺ and Pb²⁺ were 98.80 and 159.91 mg g⁻¹, respectively.

Fig. 5 Adsorption of Cd²⁺ and Pb²⁺ by NTAA-LCM as a function of time (a); pseudo-second-order adsorption kinetics of Cd²⁺ and Pb²⁺ (b). Reaction conditions: 200 mg L⁻¹ Cd²⁺ and Pb²⁺ solution, NTAA-LCM dosage = 1 g L⁻¹ at 25 °C, pH 4.0 for Cd²⁺ and 3.8 for Pb²⁺, respectively.

The pseudo-second-order model, based on the assumption that the rate-limiting step might be chemical sorption or chemisorption involving valency forces,³⁰ was applied to get further insights into the mechanisms of sorption reactions. Kinetic data were calculated with the following equation:

h = kq_e² (6)

where q_t (mg g⁻¹) is the adsorption amount at time t (min), q_e (mg g⁻¹) is the calculated value of the adsorption at equilibrium, k (g mg⁻¹ min⁻¹) is the rate constant of pseudo-second-order, h (mg g⁻¹ min⁻¹) is the initial sorption rate.

The value of k, q_e, and h could be calculated from the slope and the intercept of the straight line (Fig. 5b), and the values were listed in Table 3. The results showed that the calculated q_e was equal to the experimental value of adsorption capacity (q), demonstrating that pseudo-second-order model fitted perfectly the experimental data. Therefore, the adsorption of Cd²⁺ and Pb²⁺ by NTAA-LCM was a chemical process.^5,31 The initial adsorption rate of Cd²⁺ and Pb²⁺ were 684.0 and 699.3 mg g⁻¹ min⁻¹, respectively, suggesting an initial fast adsorption. The k values for Cd²⁺ and Pb²⁺ adsorption were calculated to be 0.0701 and 0.0275 k g mg⁻¹ min⁻¹, respectively. The low value of rate constant (k) suggested that the adsorption rate decreased with the increase in time and the adsorption rate was proportional to the number of unoccupied sites.

Table 3 Pseudo-second-order kinetic and intra-particle diffusion constants for the adsorption of ions by NTAA-LCM

Ions	Pseudo-second-order model					Intra-particle diffusion model
	k (g mg^−1 min^−1)	qe (mg g^−1)	q (mg g^−1)	h (mg g^−1 min^−1)	R^2	ki (mg g^−1 min^−1/2)	L (mg g^−1)	R^2
Cd^2+	0.0701	98.81	98.80	684.0	1	0.6544	152.89	0.931
Pb^2+	0.0275	159.49	159.91	699.3	0.999	0.3701	95.75	0.938

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Because the above equation could not give definite mechanism, the intra-particle diffusion model was tested. Weber and Morris³² indicated that the fraction of solute adsorbed can be expressed in terms of the square root of time. A plot of fraction of solute absorbed against t^1/2 may be used to estimate the intra-particle diffusion rate in the linear range. The equation was listed as follows:

q_t = k_it^0.5 + L (7)

where k_i (mg g⁻¹ min^−1/2) is the intra-particle diffusion rate constant and L (mg g⁻¹) is a constant related with the thickness of boundary layer. As shown in Fig. 6, the relationship between q_t and t^0.5 was linear over the whole time range, but the linear plots did not pass through the origin, which indicated that intra-particle diffusion may not be the controlling factor in determining the kinetics of the process.³² The k_i values for Cd²⁺ and Pb²⁺ adsorption were calculated to be 0.6544 and 0.3701 mg g⁻¹ min^−1/2, respectively. These results indicated that the adsorption took place in two stages: a very rapid surface adsorption and slow intra-particle diffusion.

Fig. 6 Linear fit of experimental data for intra-particle diffusion model.

3.4 Effects of pH and ionic strength on metals adsorption

Solution pH was one of the most important variables affecting the adsorption process due to the pH affected the aqueous chemistry and surface binding-sites of the adsorbent. It was also well known that industrial wastewater contained not only pollutants but also high concentrations of salts, which might affect the removal of pollutants. The effects of pH and ionic strength on Cd²⁺ or Pb²⁺ uptake were investigated by conducting batch experiments at different concentrations of CaCl₂, and the results were shown in Fig. 7. The solution pH influenced on the adsorption because it affected the activity of functional groups (carboxyl, amine groups), as well as the competition of metal ions for the free binding sites. The adsorption of Cd²⁺ or Pb²⁺ on NTAA-LCM increased slowly with the increased pH in the absence and presence of CaCl₂. At lower pH values, functional groups of NTAA-LCM were closely associated with hydronium ions (H₃O⁺) and restricted the approach of metals as a result of the repulsive force. While the pH increased, the adsorption surface became deprotonated, thus increasing the amount of metals adsorbed.¹⁴ Over the pH range from 6.5 to 1.5, the adsorption capacities for Cd²⁺ and Pb²⁺ decreased slowly from 103.2 and 157.3 mg g⁻¹ to 85.5 and 143.0 mg g⁻¹ in the absence CaCl₂, respectively. Most published literatures had reported that the adsorption of metal ions changed greatly at various pHs.^15,33 Compared with these results, the adsorption capacity of NTAA-LCM was changed little over the pH range from 6.5 to 1.5, which indicating that the complex between metal ions and the carboxyl and amine groups of the adsorbent was stable.¹⁶ And the advantage of NTAA-LCM material was that NTAA-LCM could be promising in the elimination of heavy metals from acidic or neutral wastewater. However, the increases in the concentrations of CaCl₂ from 0 to 0.01 mol L⁻¹ resulted in a significant decrease of the Cd²⁺ or Pb²⁺ adsorption onto NTAA-LCM, which might be attributed to the following reasons: (i) the ionic strength of solution influenced the activity coefficient of Cd²⁺ or Pb²⁺, which limited their transfer to the NTAA-LCM surface;³⁴ (ii) competition of Ca²⁺ with the Cd²⁺ or Pb²⁺ for adsorption sites of NTAA-LCM resulted in the observed decrease in the uptake capacities with increasing CaCl₂ concentration. Rosenberg and his coworkers³⁵ have reported that NTA anhydride modified silica polyamine composite materials (BP-NT and WP-NT) had the ability to remove divalent metals from low pH aqueous solutions, and the selectivity for metals was Cu²⁺ > Ni²⁺ > Zn²⁺ > Co²⁺. These results were consistent with our reports of NTAA-LCM.

Fig. 7 Effects of pH and ionic strength on metals adsorption. Reaction conditions: 200 mg L⁻¹ Cd²⁺ and Pb²⁺ solution, NTAA-LCM dosage = 1 g L⁻¹ at 25 °C, within 60 min, C_0(CaCl2) = 0, 0.001, and 0.01 M.

3.5 Adsorption isotherms

The Langmuir model (L type, based on monolayer adsorption of solute, eqn (8)) and the Freundlich model (F type, developed for heterogeneous surfaces, eqn (10)) are widely used in literatures.³⁶ These two models were used to evaluate the adsorption capacity of the modified and unmodified material for Cd²⁺ and Pb²⁺ (Fig. 8). The contact time used was 60 min. The adsorption studies were conducted at pH 4.0 for Cd²⁺ and 3.8 for Pb²⁺, respectively. The initial concentration of metal ions were from 20 to 500 mg L⁻¹ for Cd²⁺ and 20 to 600 mg L⁻¹ for Pb²⁺, respectively.where q_m (mg g⁻¹) is the maximum adsorption capacity, b (L mg⁻¹) is a Langmuir constant related to the bond energy of the sorption reaction between metal ions and adsorbents, and q_e (mg g⁻¹) is the equilibrium adsorption capacity. R_L is the equilibrium parameter which can be applied to predict whether the adsorption is favorable.where K_f (L g⁻¹) and n are Freundlich constants representing the adsorption capacity and the adsorption intensity, respectively.³⁷

Fig. 8 Adsorption isotherms for Cd²⁺ and Pb²⁺ onto NTAA-LCM at various temperatures. Reaction conditions: Cd²⁺ from 20 to 500 mg L⁻¹, pH 4.0; Pb²⁺ from 20 to 600 mg L⁻¹, pH 3.8; NTAA-LCM dosage = 1 g L⁻¹ at 25 °C, within 60 min.

The relative parameters calculated from the Langmuir and Freundlich isotherm models were listed in Table 4. Adsorption isotherms were better fitted by the Langmuir model rather than the Freundlich model with relatively higher correlation coefficients, which suggested that metals sorption on NTAA-LCM was similar to monolayer adsorption.¹⁵ Generally, the isotherm-fitting between the experimental data and models was merely a mathematical model and could not provide any strong evidence of the actual sorption mechanism.¹⁵ Nonetheless, parameters such as q_m, R_L, and b were important to optimize sorption system design. The maximum adsorption capacities (q_m) calculated by the Langmuir model were 143.4 and 303.5 mg g⁻¹ for Cd²⁺ and Pb²⁺ at 298 K, respectively. Both b and q_m increased with the increase of temperature, indicating that the bond energy between the surface sites and metal ions was larger at higher temperature and sorption on the modified material is promoted at higher temperature. Additionally, the adsorption was favorable at different temperatures with the value of R_L range from 0.048 to 0.816 (Table 4). Compared with other cornstalk materials, NTAA-LCM exhibited better performance (Table 5). The adsorption mechanism of metals on NTAA-LCM might mainly include chelation and ion exchange. Simple calculations showed that the maximum adsorption capacities of NTAA-LCM reached 1.276 and 1.465 mmol g⁻¹ for Cd²⁺ and Pb²⁺ at 298 K, respectively (Table 4), and the content of the NTA-groups introduced to the surface of NTAA-LCM was 0.797 mmol g⁻¹ (Table 2). These results demonstrated that the ratio between the amount of Cd²⁺ (Pb²⁺) adsorbed and that of introduced NTA was 1.6 : 1 (1.8 : 1). As shown in Fig. 1, some metals were coordinated via 2 carboxyl groups and 1 amine nitrogen, and the others were adsorbed by carboxyl groups through ion exchange. These results demonstrated that NTAA-LCM, which has porous structure and high functional sites, was a promising adsorbent for the removal of metal ions from waste water.

Table 4 Isotherm constants for metal ions onto NTAA-LCM at various temperatures

Metal	T (K)	Langmuir model				Freundlich model
		qm (mg g^−1)	b (L mg^−1)	RL	R^2	Kf (L g^−1)	n	R^2
Cd^2+	298	143.44	0.011	0.151–0.816	0.998	8.540	2.221	0.967
	303	151.71	0.016	0.111–0.758	0.999	12.573	2.458	0.952
	313	162.99	0.024	0.076–0.673	0.995	19.649	2.815	0.956
Pb^2+	298	303.52	0.015	0.098–0.766	0.990	23.749	2.401	0.983
	303	309.28	0.021	0.072–0.701	0.992	30.418	2.572	0.977
	313	325.60	0.033	0.048–0.601	0.991	42.433	2.828	0.954

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Table 5 Adsorption capacities of cornstalk modified by different methods

Adsorbent	Modifying method	Metal	Qmax (mg g^−1)	Equilibrium time (min)	Optimal pH	Source
Maize	Polyurethane	Pb^2+	12.5	—	4.0	17
Corn stalk	Acrylonitrile	Cd^2+	12.73	—	7.0	19
Zea mays	NaCl solution	Cd^2+	62.96	200	4.0	20
Sponge		Pb^2+	65.82	100
Maize stalk sponge	—	Pb^2+	80.0	120	6.0	18
Corn stalk	Nitrolotriacetic acid anhydride	Cd^2+	143.4	5	5–7	In this study
		Pb^2+	303.5		3.5–5.5

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3.6 Thermodynamic study

In our study, three important thermodynamic parameters, ΔG⁰, ΔH⁰, and ΔS⁰ were taken into account to determine the spontaneous nature of the processes. The Gibbs free energy ΔG⁰ can be calculated from the following equation:

ΔG⁰ = −RT ln K (11)

where R (8.314 J mol⁻¹ K⁻¹) and T (K) are the universal gas constant and the temperature in Kelvin, respectively. The sorption equilibrium constant K was calculated by plotting ln K_d versus C_e and extrapolating C_e to zero.³⁸ As shown in Table 6, the negative ΔG⁰ values indicated that the sorption of Cd²⁺ and Pb²⁺ on NTAA-LCM was spontaneous under the experimental conditions. The decrease in ΔG⁰ with increasing temperature demonstrated more efficient sorption at higher temperatures. Additionally, enthalpy ΔH⁰, entropy ΔS⁰ were calculated by the following equation:

Table 6 Thermodynamic parameters for the adsorption of metal ions onto NTAA-LCM at various temperatures

Metal	T (K)	ΔG^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)	ΔH^0 (kJ mol^−1)	R^2
Cd^2+	298	−0.368	35.910	10.320	0.990
	303	−0.582
	313	−0.913
Pb^2+	298	−2.085	27.250	6.029	0.994
	303	−2.237
	313	−2.497

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ΔH⁰ and ΔS⁰ could be calculated from the slope and intercept of ln K versus 1/T. The positive value of ΔH⁰ indicated that metals sorption on NTAA-LCM was an endothermic process. Meanwhile, the positive value of ΔS⁰ suggested an increased randomness during the adsorption process.

3.7 Regeneration studies

Regeneration experiment is necessary to evaluate the stability of the material and recover valuable metals from wastewater. Desorption was carried out with HNO₃ solution and the process repeated 7 times. The results were shown in Fig. 9. The N content of NTAA-LCM remained relatively stable, suggesting that 1 M HNO₃ had little influence on ester bond between LCM and NTA. HNO₃ could effectively regenerate the adsorbent with regeneration efficiencies ranging from 85% to 99%. These results demonstrated that NTAA-LCM could be effectively regenerated by HNO₃ solution. Rosenberg and his coworkers³⁵ have also reported that BP-NT and WP-NT could be effectively regenerated by acid solution without evident amide bond hydrolysis.

Fig. 9 Content of N (obtained from elemental analysis) and reusability efficiency of NTAA-LCM after 7 cycles of regeneration. Reaction conditions: 200 mg L⁻¹ Cd²⁺ and Pb²⁺ solution, pH 4.0 for Cd²⁺ and 3.8 for Pb²⁺, NTAA-LCM dosage = 1 g L⁻¹ at 25 °C, within 60 min.

4 Conclusions

A novel bio-adsorbent, NTAA-LCM, was successfully prepared through a simple procedure. This material exhibited fast adsorption for metals; the adsorption equilibrium was reached within 5 min for both Cd²⁺ and Pb²⁺. The maximum adsorption capacities of NTAA-LCM for Cd²⁺ and Pb²⁺ reached 143.4 and 303.5 mg g⁻¹ at 298 K, respectively. Results from elemental analysis and the presence of a strong band at 1734 cm⁻¹ in the spectrum of NTAA-LCM confirmed the introduction of NTA-groups into the cornstalk; specifically, 0.797 mmol g⁻¹ of NTA was introduced. Competition of Ca²⁺ with the Cd²⁺ or Pb²⁺ for adsorption sites of NTAA-LCM resulted in the observed decrease in the uptake capacities with increasing CaCl₂ concentration. The regeneration efficiency of NTAA-LCM remained relatively high after seven cycles of regeneration. On the whole, this cost-effective material could be promising in the elimination of heavy metals from acidic or neutral wastewater.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant no. 51278464 and 51478172) is highly appreciated.

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