Removal of heavy metals from aqueous solutions by modified waste silk

Abstract

In order to reuse the waste fabric and develop a novel textile-based adsorbent for heavy metal removal, the waste silk fabric was modified by tannic acid (TA) and the prepared adsorbents were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analysis. The removal of Cd(II), Cu(II), and Ni(II) ions from aqueous solutions was investigated using the modified silk fabric (TA-SF) under various conditions and the adsorption behavior of heavy metal ions was compared with the original silk fabric. The results showed that the maximum metal adsorption amounts reached 100% in 10 min. The adsorption isotherm models were demonstrated using Langmuir, Freundlich, and Temkin isotherm models, and the adsorption of TA-SF to Cd(II), Cu(II), and Ni(II) were well fitted with Langmuir isotherm than Freundlich and Temkin isotherm model. Moreover, the adsorption kinetics was well fitted with to the pseudo-first-order and pseudo-second-order kinetic models, and adsorption kinetics indicated that the former model was better suited than the latter. For 60 mg/L initial concentrations of Cd(II), Cu(II), and Ni(II) at pH 9.0, the adsorbents' maximum adsorption capacity was 8.03 mg g⁻¹, 7.42 mg g⁻¹, and 7.47 mg g⁻¹, respectively. Within 5 min, the absorbed metal ions on TA-SF could mostly be desorbed. Moreover, TA-SF can adsorb heavy metal ions from dyeing aqueous solution, showing its capability of simultaneous removal of heavy metal and waste dyes. The results suggest that the lower-cost TA-SF could be an effective adsorbent for removing heavy metals from industrial wastewater.

Keywords

adsorption desorption heavy metals tannic acid waste silk

Heavy metal pollution causes more and more public problems in modern society because of its long-term accumulation in animal and human tissues.^1,2 Metal toxicology concerns 80 elements and their compounds, including relatively simple ionic salts and complex structures composed of ligand structures and organometallic compounds.³ Environmental and human exposure to metal element pollution may be caused by natural factors (such as erosion of metal mineral surface deposits) or human activities (mining, smelting, fossil fuel combustion, and metal applications in the industry).³ Heavy metals often enter the food chain through aquatic and terrestrial pathways, causing harm to animals and humans.⁴

Heavy metals in water can come from various natural sources, including volcanoes, bedrock, mineral weathering, erosion, and anthropogenic activities such as mining, industries, wastewater drainage, and agricultural activities.³ The modern chemical industry is mainly based on catalysts, heavy metals, or heavy metal compounds, mostly as heat stabilizers.^5,6 Similarly, heavy metals also have other industry applications, such as electroplating and lubricant manufacturing.⁷ However, electroplating is the primary source of pollution because almost all of these processes require water for cleanings, such as washing off spills, replenishing process fluids, cooling and lubrication quenching, assembly, and testing.³

In industrial wastewater, many harmful heavy metals are often released into the environment, causing significant soil and water contamination.³ Wastewater discharged directly into natural water bodies is greatly detrimental to aquatic ecosystems.⁸ Ten heavy metals are currently of great concern to public health: lead, arsenic, cadmium, chromium, cobalt, copper, mercury, manganese, zinc, and nickel. Their high water solubility makes them readily ingested by aquatic living organisms, allowing them to accumulate and reach harmful concentrations in the ecosystem. Toxic metals such as cadmium, copper, and nickel can cause living beings to suffer from severe illnesses.^9,10 Heavy metal exposure is associated with developmental retardation, multiple diseases, kidney damage, autoimmunity, and even death in extreme cases. Therefore, water needs to be separated from polluted water before discharge.³ Removing of metals from waste streams is essential because of their toxicity and the possibility of reusing metals in different industrial applications.

Numerous approaches to extracting heavy metals from water have recently been developed, including ion exchange, chemical precipitation, and reverse osmosis. Some of them are considered too costly and insufficient to treat water contaminants.³ Adsorption is considered an attractive alternative method to reduce the amount of heavy metals in wastewater due to the availability of low-cost, eco-friendly, and efficient adsorbents.^11,12 Novel adsorbents have also been established to improve the adsorption capacity, selectivity, and reusability for a wide range of applications of traditional adsorbents. In contrast to traditional adsorbents such as starch, chitosan, protein, pea fly ash, aluminum phosphates, perlite, brown marine macroalgae, biofilms, and associated minerals are used to promote the removal of heavy metals due to their ability to increase adsorption performance at a low cost.³

Textiles worldwide are increasing day by day and are expected to triple by 2050.^13,14 As the global demand for textile products grows, so does textile production.³ Textiles are made from different fibers, including natural protein fibers, natural and generated cellulose fibers, and synthetic fibers. Textiles are primarily used as clothes, bedding, and decorative clothing. They are have limited applications in other fields, such as civil engineering, packaging, agriculture, medical aircraft, aircraft, electronics, etc.^15,16 As the world's largest textile exporter and producer, China plays an important role in the global textile supply chain.¹⁷ Approximately 45% of textiles produced in China are wasted, ranging from pre-consumer waste composed of fabrics, materials, and clothing processing, to post-consumer waste when discarding used clothing.³ As for pre-consumer waste, textile waste production in China is expected to exceed 100 million tons. Silk has become the most common and high-grade textile material due to its attractive luster, soft feeling, good moisture absorption, good mechanical properties, and air permeability. Among all the natural fibers, silk fiber is one of the most expensive fibers, and waste silk from the textile industries could be used as a reinforcing material to substitute for silk fiber.³ Moreover, the waste silk is inexpensive and has almost similar physical properties, contributing to an overall reduction in composite manufacturing costs.³ Therefore, the need to assess the success and potential challenges of waste management in Chinese textile industry cannot be overemphasized.

Tannic acid (TA) is a plant polyphenol found in almost all aerial plant tissues. TA is astringent polyphenolic biomolecule that binds with proteins and other chemicals and precipitates them, including amino acids and alkaloids.¹⁸ TA comprises five pyrogallol classes and five catechol groups that can interact with substances through multiple binding actions such as hydrogen bond, ionic bond, coordinate action, and hydrophobic effect. TA has valuable properties such as antioxidant, antibacterial, and biodegradability. In recent years, the expansion of different bonding modes of TA in materials science has been booming.

There are a few studies on textile fibers as heavy metal adsorbents, but as far as we know, there is no study on silk fabric modification with TA for this purpose. The present work aims to evaluate the removal of Cd(II), Cu(II), and Ni(II) ions from aqueous solutions by TA modified silk fabric (TA-SF) adsorbent. The three heavy metal ions are widely used in mining and smelting industry. The effects of the initial concentration of metal ions, contact time, and solution pH on removal were discussed. However, the adsorption isotherms and kinetics were investigated. Furthermore, the ability of TA-SF and SF as adsorbents to remove heavy metals was compared. The result indicates TA-SF is an effective, stable, and regenerable adsorbent for heavy metal removal. This work can provide a basis for application of waste silk in heavy metal removal in the future.

Experimental

Materials

The waste silk fabric (48 g/m²) used as adsorbent was provided by Huajia Silk Group, China. Analytical grade TA (98.5%), sodium perborate (NaBO₃ · 4H₂O), ferric chloride hexahydrate (FeCl₃ · 6H₂O) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Furthermore, stock solutions of 1000 mg/L were prepared by dissolving appropriate amounts of CdN₂O₆ · 4H₂O, Cu(NO₃)₂ · 3H₂Om and Ni(CH₃COO)₂ · 4H₂O reagents in deionized water without additional purification. Then the stock solutions were further diluted to the concentrations required. Finally, the pH adjustment was performed using 0.1 mol/L NaOH or 0.1 mol/L HNO₃. The metal ions were measured using the inductively coupled plasma optical emission spectrometry (ICAP6300 DUO, Thermo Fisher Scientific Co., Ltd, England).

Preparation of TA-SF

The waste silk fabric was synthesized by using TA (see in Scheme 1). In general, the waste silk was pre-treated with deionized water. The waste silk fabric (0.8 g) was immersed into a 250 mL flask containing a 150 mL 0.5 mmol/L TA and 2 mmol/L ferric chloride (FeCl₃ · 6H₂O) at 50°C in a water shaking bath. After 20 min, 12 mmol/L of sodium perborate (NaBO₃ · 4H₂O) was added and the solution was stirred for 40 min. Finally, the modified silk fabric (TA-SF) was withdrawn, washed, and dried in an oven at 60°C.

Scheme 1.

Preparation of adsorbent (TA-SF) for removal of heavy metal ions.

Characterization studies

Fourier transform infrared (FTIR)

The original silk fabrics (SF) and modified silk fabrics (TA-SF) were characterized by FTIR with a Nicolet-5700 Fourier transform infrared spectrometer (Thermo Electron Scientific Instruments Co., Ltd, USA)

X-ray diffraction (XRD)

X-ray diffraction analysis (XRD) was carried out by X’Pert Pro MPD X-ray powder diffractometer (Panalytical, Holland) with a source of filtered CuKα radiation.

Scanning electron microscopy (SEM)

The surface morphologies of SF and TA-SF were observed using an S-4800 scanning electron microscopy (SEM, Hitachi Ltd, Japan) under different magnifications.

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) spectra of silk fabrics before and after adsorption were analyzed by ESCALAB 250XI X-ray photoelectron spectroscopy (Thermo Fisher Scientific Co., Ltd, USA) to determine the element content of each sample.

Mechanism of tannic acid (TA) grafted with silk

TA was absorbed on the surface of silk fibers, and hydroxyl and amino groups of TA can combine with amino acids of silk fibers through hydrogen bonds; phenol hydroxyls of TA can combine with aromatic amino acids of silk fibers through π–π stacking interaction. Moreover, the phenol hydroxyls of TA can complex with metal ions and absorb more metal ions. Silk fibroin can chelate with metals ions, which is thought to be due to the coordination bond between Cd(II), Cu(II), and Ni(II) ions and the surface functional groups (phenolic hydroxyl groups and amino groups) of TA-SF. As a result, TA provides more adsorption sites, explaining the adsorption mechanism of TA-SF adsorbent for metal ions. Moreover, the adsorption occurs through negative charges on the SF surface with metal ions under neutral conditions (Scheme 2).

Scheme 2.

Schematic of the mechanism of TA grafted silk.

Metal adsorption procedure analysis

CdN₂O₆ · 4H₂O, Cu(NO₃)₂ · 3H₂O, and Ni(CH₃COO)₂ · 4H₂O were dissolved in distilled water and stirred well, and then transferred to 1000 mL volumetric flasks to prepare standard stock solution with a concentration of 1000 mg/L Cd(II), Cu(II), and Ni(II) ions. To optimize the adsorption parameters, 0.4 g of adsorbent and 50 mL of metal solutions containing Cd(II), Cu(II), or Ni(II) were put together at room temperature and concentrations ranging from 20 mg/L, 40 mg/L, 60 mg/L to 80 mg/L and 100 mg/L for each metal ions. This experiment was carried out for 20 min in a water-bath shaker and the solution was filtered through 0.45 m Millipore filter paper at the end of the specified time for the adsorption process, and the concentration of filtered metal ions was measured using the inductively coupled plasma optical emission spectrometry (Thermo iCAP 6300 DUO). The difference between the initial concentration (C₀) and the equilibrium concentration (C_e) of metal ion was determined by the amount of metal ion adsorbed on the TA-SF. The adsorption capacity q_e (mg/g) and the percentage removed R (%) from the solution of Cd(II), Cu(II), and Ni(II) ions were calculated by using the following equations:

q_{e} = \frac{V (C_{0} - C_{e})}{m}

(1)

R % = \frac{(C_{0} - C_{e})}{C_{0}} \times 100

(2)

where C₀ is the initial concentrations (mg/L) and C_e is the equilibrium concentration of metal ion (mg/L), while m and V is the weight of the adsorbent (g) and the volume of the metal ion solution (L), respectively.

Desorption procedure

Adsorption experiments were performed by shaking 0.4 g of TA-SF at room temperature with 50 mL of 60 mg/L Cd(II), Cu(II), and Ni(II) solutions for 10 min. The solution was filtered after the adsorption of the metal ions on the TA-SF, and the filtrate was used to calculate the percentage of adsorption for each metal ion. For the desorption study, a metal-loaded TA-SF sample was treated with 50 mL of 0.1 mol/L HNO₃ solutions in a volumetric conical flask and stirred for 5 to 30 min at 30°C in an orbital shaker. The amount of desorbed metal ion was measured by the difference between the amount of fully loaded metal ion on the adsorbent materials and the same solution. The desorbed efficiency (percentage of desorption) of Cd(II), Cu(II), and Ni(II) ions were calculated by equation (3).

Percent (%) of desorption

= \frac{amount o f C d (I I), C u (I I) and N i (I I) d esorbed}{amount o f C d (I I), C u (I I) and N i (I I) adsorbed} \times 100

(3)

Experiment on simultaneous removal of dyes and heavy metal ions

To investigate the removal effect of modified silk fabric on both dyes and heavy metal ions, 0.4 g of adsorbent (TA-SF) and 50 mL of solutions containing Methylene Blue and heavy metal ions (Cd(II) or Cu(II) or Ni(II)) were performed together at room temperature. The concentration of Methylene Blue was 20 mg/L, and the metal ions concentration was 50 mg/L for each metal. This experiment was carried out for 30 min in a water-bath shaker and the solution was filtered through 0.45 m Millipore filter paper at the end of the specified time for the adsorption process and the concentration of filtered metal ions was measured using the inductively coupled plasma optical emission spectrometry (ICAP 6300 series).

Results and discussions

Characterization of TA-SF

Fourier transform infrared (FTIR) analysis

FTIR spectroscopy was used to analyze the functional groups of waste silk fabrics before and after treatment, as shown in Figure 1. It can be seen from Figure 1 that the absorption peaks at 3282 cm⁻¹ and 3071 cm⁻¹ belong to the stretching vibration of -OH and C-H of polyphenols in waste silk, respectively. In the 1624 cm⁻¹ and 1617 cm⁻¹ zones, high absorption peaks are assigned to the -C=O stretching of amide I. The peaks at 1507 cm⁻¹ and 1515 cm⁻¹ are attributed to the N-H stretching vibration of amide II, and those at 1230 cm⁻¹ and 1223 cm⁻¹ are derived from the O-H stretching of amide III.^19,20 At 2161 cm⁻¹, there is an additional absorption peak for TA-SF corresponding to the C=O groups. In particular, the strong absorption of amide I band at 1624 cm⁻¹ (SF) and 1617 cm⁻¹ (TA-SF) is also used to measure the crystallinity index of SF protein. SF and TA-SF exhibited high absorption peaks around 1066 cm⁻¹ and 1061 cm⁻¹, corresponding to C-O-C bending and stretching vibration, respectively. It is observed that the specific absorption peaks of SF and TA-SF do not overlap. Therefore, SF with high TA content shows vast absorption peaks due to the accumulation of a large amount of TA on the SF surface, The secondary structure of SF can be further studied by using the amide I region of 1624 to 1617 cm⁻¹. These investigations demonstrate that the surface of the TA-SF has been chemically activated for adsorption procedures.²¹

Figure 1.

Fourier-transform infrared spectroscopy (FTIR) spectra of SF and TA-SF.

X-ray diffraction (XRD) analysis

The crystal phases present in the samples were identified by XRD with diffraction angle (2θ) from 10° to 90° and a scan rate of 5°/min. Figure 2 shows XRD pattern of silk before and after modification. The results demonstrate that SF and TA-SF show typical crystalline peaks respectively. The amorphous existence of these fibers is characteristic of the giant peaks of SF and TA-SF occurring at 2θ of 20.5° and 20.3°. This observation indicates the dominance of the c-axis direction as the main ascending direction. Furthermore, it was also observed that the intensity of the typical peaks of SF and TA-SF were different.^22,23 However, the XRD diffraction peaks emerging in TA-SF indicate the presence of TA surface coating materials on SF.

Figure 2.

X-ray diffraction (XRD) patterns of SF and TA-SF.

Scanning electron microscope (SEM) analysis

Scanning electron microscope (SEM) imaging was used to study the morphology of the SF and TA-SF, as shown in Figure 3(a). The representative images of SF and TA-SF are shown in Figure 3(a)–(c) and Figure 3(d)–(f), respectively. It was observed that there were significant differences in the surface morphology of waste silk before and after modification. As shown in Figure 3, the surface morphology of SF is very smooth, while on the other hand, TA-SF is clustered, convex, and rough. With the increase of surface roughness, the actual surface area of the adsorbent increased, so there are many places on the surface for metal ion adsorption.^24,25 Overall, after modification of waste silk, SEM analysis showed different surface morphologies.

Figure 3.

Scanning electron microscope (SEM) image at different magnifications of SF (a)–(c) and TA-SF (d)–(f).

X-ray photoelectron spectroscopy (XPS) analysis

XPS was adopted to investigate the chemical composition and functional groups of SF and TA-SF. The XPS spectra of silk before and after adsorption of Cd(II), Cu(II), and Ni(II) metal ions, are shown in Figure 4. The percentage of such surface elements of silk samples obtained by the XPS wide scan spectrum is shown in Table 1. The results showed that the surface elements of C and N of the treated fabric are significantly smaller than the untreated sample. The surface element of O of the untreated fabric is smaller than the treated sample. In addition, on the other hand, after adsorption of Cd(II), Cu(II), and Ni(II) metal ions on TA-SF, the surface elements of metal ions increased significantly. As shown in Figure 4(a), peaks of C, N, O elements appear on the XPS spectra of SF and TA-SF/Fe samples, and the corresponding peak of Cd 3d, Cu 2p, and Ni 2p elements appear on the TA-SF/Cd, TA-SF/Cu, and TAA-SF/Ni surface spectrum. Figure 4(b)–(c) shows the C1s spectra of SF and TA-SF, and the C1s spectra are fitted to the absorption peaks at 284.6, 285.5, 286.4, and 288.1 eV, corresponding to C-C, C-N, C-OH, and O-C-O bonds, respectively. For more comprehensive analysis, after the adsorption of Cd(II), Cu(II), and Ni(II), Figure 4(d)–(f) shows the core-level of Cd3d, Cu2p, and Ni2p on the surface of TA-SF, and the characteristic peaks are attributed to Cd 3d_3/2 (412.1 eV), Cd 3d_5/2 (405.8 eV), Cu 2p_1/2 (952.8 eV), Cu 3p_3/2 (952.8 eV), Ni 2p_1/2 (874.1 eV), and Ni 2p_3/2 (856.1 eV), respectively.³ Therefore, the XPS results confirm that the waste silk was modified with TA and Cd(II), Cu(II), and Ni(II) were successfully adsorbed by TA-SF.

Figure 4.

X-ray photoelectron spectroscopy (XPS) spectra of wide scan spectra (a) SF, TA-SF and after adsorption of Cd(II), Cu(II), and Ni(II); high resolution C1s of (b) SF; (c) TA-SF; (d) TA-SF/Cd; (e) TA-SF/Cu, and (f) TA-SF/Ni.

Table 1.

Surface elemental compositions (atomic weight percentage) of SF, TA-SF and after adsorption of TA-SF/Cd, TA-SF/Cu, and TA-SF/Ni

Elements (%)	Sample
Elements (%)	SF	TA-SF	TA-SF/Cd	TA-SF/Cu	TA-SF/Ni
C	70.75	65.34	63.64	66.61	64.61
O	17.84	27.12	27.22	29.35	30.89
N	10.94	5.82	8.59	3	3.59
Cd	0	0	0.55	0	0
Cu	0	0	0	1.04	0
Ni	0	0	0	0	0.91

Adsorption

Effect of different sample

The adsorption performance of Cd(II), Cu(II), and Ni(II) ions by SF and TA-SF were investigated. It can be seen from Figure 5 that different samples have a significant influence on metal adsorption, and TA-SF is more effective for the removal of heavy metal ions. The experiments were conducted with the initial concentrations of Cd(II), Cu(II), and Ni(II) solution at 60 mg/L. The adsorbent (SF and TA-SF) weight was 0.4 g and was placed in 150 mL of conical flask containing 50 mL of metals solution respectively at solution pH, which was incubated in a water shaking bath at 30°C for 30 min. As shown in Figure 5, the removal percentage of SF is 31%, 32%, and 24% for Cd(II), Cu(II), and Ni(II) ions, respectively. On the other hand, the removal percentage of TA-SF is 100% for Cd(II), 87% for Cu(II), and 78% for Ni(II) metals. At the same time, TA-SF also has higher adsorption capacity(q_e) than SF sample. Accordingly, TA-SF have better adsorption ability for Cd(II), Cu(II), and Ni(II) ions, and it might be a consideration for further investigation.

Figure 5.

Evaluation of the metals adsorption behavior of SF and TA-SF.

Effect of the concentration of heavy metal ions

According to the sources of pollution, the concentration of metal ions found in the polluted area varies. Therefore, the effects of various concentrations of heavy metal ions on their adsorption by modified TA-SF are exciting to research. The removal of Cd(II), Cu(II), and Ni(II) ions on TA-SF is shown in Figure 6. As a function of the initial metal ion concentration (20 mg/L, 40 mg/L, 60 mg/L, 80 mg/L, and 100 mg/L), 0.4 g of adsorbent was contained in a 150 mL conical flask and 50 mL of Cd(II), Cu(II), and Ni(II) solution were transferred to the flask at 30°C for 30 min at solution pH. The results showed that TA-SF absorbed almost 100% of Cu(II) and Ni(II) ions at metal ion concentrations of 20 mg/L and 40 mg/L. Further increases in the concentration of metal ions to 60 mg/L, 80 mg/L and 100 mg/L, the metal adsorption capacity decreased to 87%, 70%, and 56% for Cu(II), and 79%, 63%, and 50% for Ni(II). For Cd(II) ions, increasing the concentration of metal ions from 20 mg/L to 60 mg/L did not affect the performance of adsorption (100%), but increasing the concentration of metal ions to 80 mg/L and 100 mg/L reduced the removal percentage to 97% and 87%, respectively. According to the increase in adsorption capacity, adjusting the adsorption efficiency to 60 mg/L concentration could be a suitable parameter for Cd(II) ion. In addition, with an increase in adsorption capacity for Cu(II) and Ni(II) ions, the investigation showed that 60 mg/L was the maximum adsorption capacity, and there was no further increase in adsorption capacity with increasing initial ion concentration due to the attainment of dynamic equilibrium between the adsorbent and adsorbate.³ The initial concentration of 60 mg/L can explain the efficient removal of Cd(II), Cu(II), and Ni(II) ions from aqueous solutions.

Figure 6.

The effect of initial concentration of Cd(II), Cu(II), and Ni(II) ions adsorption.

Effect of contact time

The effect of contact time on TA-SF adsorption of Cd(II), Cu(II), and Ni(II) metal ions from aqueous solutions is shown in Figure 7. The experiments were investigated at various contact times ranging from 1 to 20 min at 30°C with an adsorbent dose of 0.4 g and 60 mg/L of metal ions at solution pH. Due to the presence of numerous active sites on the adsorbent surface, adsorption was very fast,99% for Cd(II), 82% for Cu(II), and 77% for Ni(II) were adsorbed within 5 min. As shown in Figure 7, after 10 min of contact time, equilibrium adsorption is established and the maximum TA-SF uptake for all metal ions are reached: 100% (8.03 mg g⁻¹) for Cd(II), 87% (6.51 mg g⁻¹) for Cu(II), and 78% (5.79 mg g⁻¹) for Ni(II). After reaching equilibrium, the contact time no longer affected the metal-ion adsorption and removal efficiency and adsorption capacity.³ For the subsequent analysis, an adsorption time of 10 min was used.

Figure 7.

The effect of contact time of Cd(II), Cu(II), and Ni(II) ions adsorption.

Effect of pH

The pH of the aqueous solution has a significant impact on the adsorption process, especially for heavy metal ions like Cd(II), Cu(II), and Ni(II), which occur at different pH levels in distinct species. The effect of the solution pH on the adsorption of Cd(II), Cu(II), and Ni(II) metal ions by TA-SF at different pH values from 3 to 11 was investigated, and the results are presented in Figure 8. The results show that the removal of Cd(II), Cu(II), and Ni(II) metal ions in TA-SF is highly dependent on the pH of the aqueous solution. The amount of adsorbed heavy-metal ions demonstrates that the removal rate of TA-SF for Cd(II) increased from 72% to 100%, for Cu(II) from 34% to 100%, and for Ni(II) from 5% to 100% while the pH increased from 3.0 to 11.0, The maximum removal rates of Cd(II), Cu(II), and Ni(II) were 100% (8.03 mg g⁻¹, 7.42 mg g⁻¹, and 7.47 mg g⁻¹, respectively) at pH 9.0. The adsorption of Cd(II), Cu(II), and Ni(II) ions decreased dramatically with the decrease of solution pH, particularly when pH > 3, the removal rate of each heavy metal ion decreased to less than 5%. It is generally believed that metal ion adsorption increases with the increase of pH as the solution becomes less stable for the metal functional groups.^26,27 Nonetheless, the adsorption capacity decreased at higher pH values (11.0), which may be due to the precipitation of the Cd(II), Cu(II), and Ni(II) ions.

Figure 8.

The effect of solution pH of Cd(II), Cu(II), and Ni(II) ions adsorption.

Adsorption isotherm

Adsorption is a complex process generally defined by isotherms, calculated by fitting experimental data with mathematical models. The impact of the initial metal ion concentration was investigated to establish the adsorption isotherms of Cd(II), Cu(II), and Ni(II) ions using TA-SF. The adsorption equilibrium between the adsorbent and the aqueous heavy metal ions was determined using Langmuir, Freundlich, and Temkin adsorption isothermal models.

Langmuir isotherm

The Langmuir model is a good monolayer adsorption isotherm, which relates the equilibrium concentration of adsorbent to the capacity of adsorbent. This model provides a strong relationship with a wide range of experimental data, and the linearized form of Langmuir is expressed in the equation (4):

\frac{C_{e}}{q_{e}} = \frac{C_{e}}{q_{m}} + \frac{1}{k_{L} q_{m}}

(4)

where the amount of adsorbed metal ions is q_e (mg g⁻¹), C_e (mg L⁻¹) is the equilibrium concentration of metal ions, q_m (mg g⁻¹) is the maximum quantity of adsorbed metal ions, and k_L is the Langmuir constant correlated with the affinity of the active site. The Langmuir adsorption isotherm model plots are shown in Figure 9, and the results are examined in Table 2. The result shows that the Langmuir isotherm model fits the experimental data well, and the equation correlation coefficients demonstrate that metal ion adsorption by the TA-SF can be successfully described by this model (R² > 0.9). And the adsorption capacity q_m (mg g⁻¹) for Cu(II) is 6.55 mg g⁻¹, for Ni(II) is 6.05 mg g⁻¹, and the maximum adsorption capacity for Cd(II) ions is 10.62 mg g⁻¹. Comparative values of coefficient of determination from three isothermal models indicate that Langmuir has better performance for representing the equilibrium data than Freundlich and Temkin.^28,29

Figure 9.

Langmuir adsorption isotherm for Cd(II), Cu(II), and Ni(II) ions

Table 2.

Langmuir adsorption isotherm models parameters of metals ions on TA-SF.

Type of metal ions	q_m (mg g⁻¹)	K_L (L mg⁻¹)	R²
Cd(II)	10.62	9.66	0.9998
Cu(II)	6.55	71.01	0.9996
Ni(II)	6.05	8.96	0.9998

Freundlich isotherm

The Freundlich isothermal adsorption model was used to analyze the adsorption strength of adsorbent. It ensures that the heterogeneous surface of the adsorbent has non-ideal adsorption. Thus instrumental is Freundlich isothermal, which is given by the linearized form at equation (5):

Log q_{e} = (\frac{1}{n}) L og C_{e} + Log k_{f}

(5)

where q_e is the number of metal ions adsorbed per unit weight of the adsorbent (mg g⁻¹), C_e is the equilibrium of the concentration of metal ions (mg L⁻¹), k_f (mg g⁻¹) is the maximum adsorption capacity-related constant of Freundlich, and n is a surface homogeneity factor-related constant. It is possible to calculate the Freundlich parameters and obtain n and k_f from the linear isothermal plots of log q_e versus log C_e curves. These values are given in Figure 10 and Table 3. The Freundlich isothermal parameters also provide sufficient evidence for the feasibility of TA-SF adsorption. The n > 1 value demonstrates the adsorption of all three metal ions on TA-SF is spontaneous and reveals that the adsorption process was favorable.^30,31 The correlation coefficient values of the equations indicate that this model can well explain the adsorption of metal ions.

Figure 10.

Freundlich adsorption isotherm for Cd(II), Cu(II), and Ni(II) ions.

Table 3.

Freundlich adsorption isotherm model parameters of metals ions on TA-SF.

Type of metal ions	n	K_f	R²
Cd(II)	5.893	7.988	0.9045
Cu(II)	8.501	4.718	0.9178
Ni(II)	10.315	4.476	0.9105

Temkin adsorption isotherm

The purpose of the Temkin adsorption isotherm is taken into consideration in the adsorbate–adsorbent interaction. It is assumed that the adsorption energy evolution and the adsorbent surface coverage are linear. The model of the Temkin isotherm is presented in equation (6):

q_{e} = \frac{R T}{b_{T}} {\ln A}_{T} + \frac{R T}{b_{T}} {\ln C}_{e}

(6)

where b_T (J mol⁻¹) and A_T (L g⁻¹) are Temkin constants related to the maximum binding energy and heat of adsorption, respectively; T (K) is temperature and R is the gas constant (8.314 J K⁻¹ mol⁻¹). The q_e against the ln C_e plots for different metals are shown in Figure 11, and the parameters obtained are given in Table 4. Table 4 the result shows that the correlation coefficients of all metal ions obtained by Temkin adsorption isotherm model are very consistent (0.9615, 0.9477, and 0.9473) with the adsorption of Cd(II), Cu(II), and Ni(II) by TA-SF.^32,33

Figure 11.

Temkin adsorption isotherm for Cd(II), Cu(II), and Ni(II) ions.

Table 4.

Temkin adsorption isotherm models parameters of metals ions on TA-SF.

Type of metal ions	b_T	A_T	R²
Cd(II)	2.469	30.289	0.9615
Cu(II)	1.21	64.706	0.9477
Ni(II)	0.943	150.161	0.9473

Adsorption kinetics

The kinetic adsorption for the treatment of aqueous solutions is of great significance. The adsorption kinetics data were analyzed to understand the dynamics of the adsorption process in terms of the order of rate constants. The pseudo-first-order and pseudo-second-order models were applied to the adsorption kinetic results to investigate the behavior of the adsorption process of heavy metals on TA-SF.

Pseudo-first-order kinetics

The pseudo-first-order model, the earliest known equation to describe the adsorption rate based on the adsorption capacity, was used to describe the adsorption kinetic data. The differential model expressed in linearized form is shown in equation (7):

\ln (q_{e} - q_{t}) = \ln q_{e} - k_{1} t

(7)

where k₁ is the first-order absorption rate constant, q_e is the number of metals absorbed at equilibrium by the adsorbent (mg g⁻¹), and q_t (mg g⁻¹) is the number of metals adsorbed at any given time t (min). The pseudo-first-order model for many adsorption systems was found to match the experimental data in the early stage of adsorption, which was the case in this experiment. From the slopes of the straight lines, the values of the pseudo-first-order rate constants k₁ were obtained. Figure 12 and Table 5 present the k₁ values, the correlation coefficients, R², and the expected and experimental q_e values. The resulting parameters show that the pseudo-first-order model correlation coefficient values are satisfactory and fitted for the adsorption of Cd(II), and Cu(II) ions.³

Figure 12.

Pseudo-first-order plots for the adsorption of Cd(II), Cu(II), and Ni(II) ions.

Table 5.

Pseudo-first-order kinetics parameters for Cd(II), Cu(II), and Ni(II) ions on TA-SF/Fe

Type of metal ions	q_e (mg g⁻¹)	k₁ (g mg⁻¹ min⁻¹)	R²
Cd(II)	1.63	0.27	0.9238
Cu(II)	4.67	0.27	0.9824
Ni(II)	1.05	0.20	0.8965

Pseudo-second-order kinetics

The linearized form of the pseudo-second-order model is expressed by equation (8):

\frac{t}{q_{t}} = (\frac{1}{q_{e}}) t + \frac{1}{k_{2} q_{e}^{2}}

(8)

where k₂ (g mg⁻¹ min⁻¹) is the pseudo-second-order rate constant, q_e and q_t (mg/g) are the adsorption capacity at equilibrium and time t, respectively. As shown in Figure 13, the kinetic equation of pseudo-second-order can fit well in the full range of metal-adsorbent interaction time because the q_e value obtained from these plots has good correlation with the experimental measurement values, and the correlation coefficients R² > 0.99 (Table 6). The correlation coefficients indicate the strong connection between the parameters and explain the pseudo-second-order kinetics followed by the process.³ The kinetics analysis shows that the pseudo-second-order equation can better explain the adsorption of metal ions than the pseudo-first-order equation.

Figure 13.

Pseudo-second-order plots for the adsorption of Cd(II), Cu(II), and Ni(II) ions.

Table 6.

Pseudo-second-order kinetics parameters for Cd(II), Cu(II), and Ni(II) ions on TA-SF.

Type of metal ions	q_e (mg g⁻¹)	k₂ (g mg⁻¹ min⁻¹)	R₂
Cd(II)	8.21	0.65	0.9997
Cu(II)	7.40	0.26	0.9999
Ni(II)	6.51	0.82	0.9998

Desorption

Desorption performance is a significant variable for recycling and repeated availability of ideal adsorbents. An ideal adsorbent not only has greater adsorption capacities, but also exhibits better desorption performance. Better desorption effectively reduces the overall cost associated with the removal of adsorbates, while high costs usually limit their potential use. Therefore, the optimal conditions for the successful desorption of Cd(II), Cu(II), and Ni(II) from TA-SF were studied to evaluate them. Figure 14 shows the effect of time on the desorption of Cd(II), Cu(II), and Ni(II) in a 0.1 mol/L HNO₃ solution from the metal-loaded TA-SF samples. It can be noted that within 5 min, Cd(II), Cu(II), and Ni(II) exceeded the maximum desorption rate, and the amount of recovered metal ions slightly increased with the increase of specific time. It is evident from the figure that the desorption rate were 78%, 92%, and 83% for Cd(II), Cu(II), and Ni(II), respectively. TA-SF has a strong recovery ability for further uses.³

Figure 14.

Desorption of Cd(II), Cu(II), and Ni(II) ions in the presence of 0.1 mol/L HNO3 solution as a function of time from the metal-loaded TA-SF.

Simultaneous removal of dyes and heavy metal ions

Residual dyes and heavy metal ions often coexist in dye wastewater, which has higher biological and cellular toxicity. The adsorption of heavy metal and Methylene Blue on modified silk fabric was studied, and the results are listed in Table 7 and Figure 15. It can be seen that TA-SF had excellent removal to Methylene Blue, and the color removal percentage reached 99% after 30 min. The modified silk fabric also shows good removal of heavy metal ions, especially Cd(II). Thus, this trial experiment provides the potential for removing dyes in the presence of heavy metals from industrial wastewater.

Table 7.

Adsorption performance of Cd(II), Cu(II), and Ni(II) ions from dyeing wastewater

Samples	Cd(II)		Cu(II)		Ni(II)
Samples	q_e (mg g⁻¹)	R (%)	q_e (mg g⁻¹)	R (%)	q_e (mg g⁻¹)	R (%)
SF	1.62	20	1.08	14	2.09	24
TA-SF	8.04	100	6.89	87	6.19	77

Figure 15.

Dye degradation images before and after removal of dyes.

As shown in Table 8, the experimental data of this study is comparable to the existing results and be better than many research. Additionally, it can be observed in Table 8 that the adsorption capacities of Cd(II), Cu(II), and Ni(II) ions on TA-SF reached a high value within 10 min, which is better than the previous studies on the adsorption of Cd(II), Cu(II), and Ni(II) ions from various adsorbents. Moreover, the order of q_m and removal percentage of TA-SF to three metals ions are: Cd(II) > Cu(II) > Ni(II). The reason is that the coordination constant of hydroxyl with Cd(II), Cu(II), and Ni(II)Cd is 4.17, 7.0, and 4.97, respectively. However, Cd has the highest atomic weight, and the atomic weight of Cu and Ni are similar. In summary, TA-SF has the strongest coordination capability and adsorption effect to Cd(II), and the worst adsorption effect to Ni(II), which also presents the same adsorption trend with the materials listed in Table 8.

Table 8.

Comparison of adsorption capacities with previous studies of different adsorbents for the adsorption of Cd(II), Cu(II,) and Ni(II) ions

Metal Ions	Adsorbent	q_m (mg g⁻¹)	Time	Reference
	Mercerized nanocellulose	2.06	120	16
Cd(II)	Microfibrillated cellulose (MFC I)	4.20	12 h	18
	minerals (sepiolite)	8.30	3 h	80
	Kaolinite clay	0.88	1 h	83
	Amberlite IR-120	0.90	75 min	86
	TA-SF	8.03	10 min	This study
	Mercerized nanocellulose	1.90	120	16
Cu(II)	Microfibrillated cellulose (MFC I)	3.15	12 h	18
	Modified oak sawdust	3.22	4 h	76
	Kaolinite clay	1.22	1 h	83
	Amberlite IR-120	0.34	90 min	86
	TA-SF	7.42	10 min	This study
	Mercerized nanocellulose	0.74	120	16
Ni(II)	Microfibrillated cellulose (MFC I)	2.73	12 h	18
	Modified oak sawdust	3.29	4 h	76
	Kaolinite clay	0.9	1 h	83
	Amberlite IR-120	0.82	90 min	86
	TA-SF	7.47	10 min	This study

Conclusion

The waste silk fabric was modified by TA and was used as an adsorbent for the removal of Cd(II), Cu(II), and Ni(II) from aqueous solutions. The modified silk fabric was characterized by FTIR, XRD, SEM, and XPS to assess the functional groups, structure morphologies, and element content. The modified silk (TA-SF) had a higher adsorption affinity for Cd(II), Cu(II), and Ni(II) metal ions than SF. The effects of adsorption conditions, such as metal ions concentrations, contact time, and pH of the solution, were studied and optimized. The Langmuir model fit better than Freundlich and Temkin isotherm models for the adsorption of Cd(II), Cu(II), and Ni(II) ions. Additionally, the pseudo-second-order equation is well fitted for studies of kinetics. In addition, the absorbed metal ions on TA-SF can be easily desorbed, and TA-SF can simultaneously adsorb heavy metal and dyes, indicating it can also be applied in the dye removal treatment of textile dyeing & printing contaminated water and its application prospect in industrial wastewater treatment.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51973144, 51741301); the Major Program of Natural Science Research of Jiangsu Higher Education Institutions of China (18KJA540002); the Natural Science Foundation of Jiangsu Province (BK20201181); Jiangsu Province “333” project (BRA2020324) and the Foundation of Jiangsu Engineering Research Center of Textile Dyeing and Printing for Energy Conservation, Discharge Reduction and Cleaner Production(Q811580621).

ORCID iD

Tieling Xing

References

Amirnia

Madhumita

Argyrios

Heavy metals removal from aqueous solutions using Saccharomyces cerevisiae in a novel continuous bioreactor–biosorption system. Chem Eng J 2015; 264: 863–872.

Kumrić

Anđelka

Tatjana

MTP

, et al. Simultaneous removal of divalent heavy metals from aqueous solutions using raw and mechanochemically treated interstratified montmorillonite/kaolinite clay. Ind Eng Chem Res 2013; 52: 7930–7939.

Egorova

Valentine

PA.

Which metals are green for catalysis? Comparison of the toxicities of Ni, Cu, Fe, Pd, Pt, Rh, and Au salts. Angew. Chem Int Ed 2016; 55: 12150–12162.

Dudka

Miller

WP.

Accumulation of potentially toxic elements in plants and their transfer to human food chain. J Environ Sci Health B 1999; 34: 681–708.

Hua

Shujuan

Bingcai

, et al. Heavy metal removal from water/wastewater by nanosized metal oxides: a review. J Hazard Mater 2012; 211: 317–331.

Dastyar

Abdul

Jun

, et al. Biofuel production using thermochemical conversion of heavy metal-contaminated biomass (HMCB) harvested from phytoextraction process. Chem Eng J 2019; 358: 759–785.

Jacob

Chinnannan

Rijuta

, et al. Biological approaches to tackle heavy metal pollution: a survey of literature. J Environ Manage 2018; 217: 56–70.

Grantham

Miguel

Isabelle

, et al . A mesocosm approach for detecting stream invertebrate community responses to treated wastewater effluent. Environ. Pollut 2012; 160: 95–102.

Suopajärvi

Henrikki

Mikko

, et al. Lead adsorption with sulfonated wheat pulp nanocelluloses. J Water Process Eng 2015; 5: 136–142.

10.

Ansari

Marr

Tariq

Heavy metals in marine pollution perspective-a mini review. Res J Appl Sci 2004; 4: 1–20.

11.

Ojedokun

Olugbenga

SB.

Sequestering heavy metals from wastewater using cow dung. Water Resour Ind 2016; 13: 7–13.

12.

Oladipo

Mustafa

Samaneh

Adsorption of anthraquinone dye onto eco-friendly semi-IPN biocomposite hydrogel: equilibrium isotherms, kinetic studies and optimization. J Taiwan Inst Chem Eng 2014; 45: 653–664.

13.

Tumpa

Syed

, et al. Barriers to green supply chain management: An emerging economy context. J Clean Prod 2019; 236: 117617.

14.

Rissman

Chris

Eric

, et al. Technologies and policies to decarbonize global industry: Review and assessment of mitigation drivers through 2070. Appl Energy 2020; 266: 114848.

15.

Mia

Biaobiao

Xiaowei

, et al. Dopamine grafted iron-loaded waste silk for Fenton-like removal of toxic water pollutants. Polymers 2019; 11: 2037.

16.

Kirmasha

Mohaiman

Zulkiflle

, et al. Mechanical performance of unstitched and silk fiber-stitched woven kenaf fiber-reinforced epoxy composites. Materials 2020; 13: 4801.

17.

Mônica

Felipe de

Francisco de

, et al. A comparative understanding of corporate social responsibility of textile firms in Brazil and China. J Clean Prod 2012; 20: 119–126.

18.

Chen

Chung

ZT.

Chung, Mutagenicity and antimutagenicity studies of tannic acid and its related compounds. Food Chem. Toxicol. 2000; 38:1–5.

19.

Tas

Sarit

BB.

Rapid coating of Ti6Al4V at room temperature with a calcium phosphate solution similar to 10× simulated body fluid. J Mater Res 2004; 19: 2742–2749.

20.

Zhang

Sanjana

Kai

, et al. Fabrication of collagen films with enhanced mechanical and enzymatic stability through thermal treatment in fluorous media. ACS Appl Mater Interfaces 2020; 12: 6590–6597.

21.

Liu

Qianying

Qiang

, et al. Intelligent silk fibroin ionotronic skin for temperature sensing. Adv Mater Technol 2020; 5: 2000430.

22.

Moreno

Alvaro

, et al . Removal of heavy metal water pollutants (Co2+ and Ni2+) using polyacrylamide/sodium montmorillonite (PAM/Na-MMT) nanocomposites. ACS omega 2019; 4: 10834–10844.

23.

Satilmis

Uyar

Electrospinning of ultrafine poly (1-trimethylsilyl-1-propyne) [PTMSP] fibers: highly porous fibrous membranes for volatile organic compound removal. ACS Appl Polym Mater 2019; 1:787–796.

24.

Ming

Zhi

Shiyu

, et al. Novel silk fibroin films prepared by formic acid/hydroxyapatite dissolution method. Mater Sci Eng C 2014; 37: 48–53.

25.

Colnot

Tamara

Tissue interactions in long bone development. In Bone and development. London: Springer, 2010, pp. 25–37.

26.

Meng

Hui

, et al. Effective removal of heavy metal ions Cd2+, Zn2+, Pb2+, Cu2+ from aqueous solution by polymer-modified magnetic nanoparticles. J Hazard Mater 2012; 211: 366–372.

27.

Aklil

Mouflih

Sebti

Removal of heavy metal ions from water by using calcined phosphate as a new adsorbent. J Hazard Mater 2004; 112: 183–190.

28.

Phuengprasop

Jarinya

Fuangfa

Removal of heavy metal ions by iron oxide coated sewage sludge. J Hazard Mater 2011; 186: 502–507.

29.

Gopalakrishnan

Rajasekar

Sakthivel

, et al. Removal of heavy metal ions from pharma-effluents using graphene-oxide nanosorbents and study of their adsorption kinetics. J Ind Eng Chem 2015; 30: 14–19.

30.

Xin

Qin

Jian

, et al. Highly efficient removal of heavy metal ions by amine-functionalized mesoporous Fe₃O₄ nanoparticles. Chem Eng J 2012; 184: 132–140.

31.

Joo

Hasmukh

Mogens

, et al. Selective removal of heavy metal ions by disulfide linked polymer networks. J Hazard Mater 2017; 332: 140–148.

32.

Argun

Sukru

Celalettin

, et al. Heavy metal adsorption by modified oak sawdust: Thermodynamics and kinetics. J Hazard Mater 2007; 141: 77–85.

33.

Sadeek

Nabel

Hassan

, et al. Metal adsorption by agricultural biosorbents: adsorption isotherm, kinetic and biosorbents chemical structures. Int J Biol Macromol 2015; 81: 400–409.