The accumulation of Cu2+ in water is a potential threat to human health and environment. Dicarboxylic nanocellulose (DNC) with rich carboxyl groups was prepared through the NaIO4–NaClO2 sequential oxidation method to efficiently remove copper ions, and the Cu2+ adsorption properties and cost were studied. The maximum adsorption capacity reached 184.2 mg/g at pH 6 and an adsorbent dose of 5 g/L. Theoretically, the maximum adsorption capacities of monocarboxylic nanocellulose (MNC), DNC, and tricarboxylic nanocellulose (TNC) with carboxyl groups as the main adsorption sites were calculated to be 228.7, 261.3, and 148.1 mg/g, respectively. The Cu2+ adsorption costs of MNC, DNC, and TNC were calculated and compared with those of powdered activated carbon (PAC). The Cu2+ adsorption capacity of DNC is higher than that of PAC, and the adsorption cost is close to or lower than that of PAC, demonstrating that the DNC prepared by sequential oxidation of NaIO4–NaClO2 has competitive adsorption capacity and cost in the treatment of wastewater containing Cu2+.
The rapid development of industrial and agricultural products since the 20th century has led to the gradual accumulation of heavy metal ions in soil and water. Heavy metal ions accumulate in the human body through the transmission of the food chain, causing irreversible harm [
Adsorption is one of the most common approaches for removing heavy metal ions from aqueous solutions. This method mainly exploits the porosity, high specific surface area, and high surface activation energy of the adsorbent to remove heavy metal ions from water by surface complexation, hydrogen bonding, and electrostatic attraction. Common adsorbents include activated carbon, ion-exchange fibers, modified cellulose, and other materials [
Biomass is a renewable resource, so the use of biomass to prepare various biomass materials instead of non-renewable materials has become a research hotspot [
Studies have shown that the increase in the carboxyl content of nanocellulose enhances its interaction with positively charged heavy metal ions; thus, nanocellulose with higher carboxyl content has a strong adsorption capacity for heavy metal ions [
In addition, the adsorption capacity and cost of adsorbent are the two most important factors to optimize for the adsorption process to be economically feasible. Currently, activated carbon is still the main metal-ion adsorbent commonly used in the market. Granular activated carbon (GAC) and powdered activated carbon (PAC) are the two forms of activated carbon that are usually applied. Carboxylic nanocellulose is usually powdered, so it is fair to compare the adsorption cost of carboxylic nanocellulose with that of PAC. In 1995, the prices of PAC were around $0.80–2.00 per kg in the USA and $0.70–1.50 per kg in Europe [
In this study, microcrystalline cellulose (MCC) was used as the raw material, and DNC was prepared using the NaIO4–NaClO2 sequential oxidation method. The effects of initial pH, initial Cu2+ concentration, and adsorbent dose on the adsorbed amount were investigated. The adsorption kinetics and isotherm models are discussed. In addition, the theoretical maximum adsorption capacities of Cu2+ onto MNC, DNC, and TNC were calculated, and the Cu2+ adsorption costs of MNC, DNC, and TNC were compared with those of PAC. This study can provide a scientific basis for the development of renewable nano-adsorbents with high adsorption capacity and low cost.
All chemicals used in this study were of reagent grade and used as received without further purification. All test water was deionized, and all chemicals were purchased from Chemical Reagent Co., Ltd., China National Medicine Group, China.
DNC was prepared using a previously reported method [
One gram of the dried precipitate was suspended in 50 mL water and 2.93 g NaCl; NaClO2 and H2O2 (with molarities twice that of the aldehyde content of the dried precipitate) were added to this mixture. The mixture was stirred at RT for 24 h, and the pH was maintained at 5.0 by adding NaOH (0.5 M). After the reaction, the oxidized product was washed with deionized water by centrifugation at 8500 g for 5 min. After the conductivity of the supernatant dropped to less than 30 μS/cm, the dried DNC was obtained by drying in a vacuum freeze dryer.
The morphology of DNC was observed and analyzed using transmission electron microscopy (TEM; JEM-2100, Japan Electronics Corporation, Japan) and field-emission scanning electron microscopy (FE-SEM; JSM-7401F, JEOL, Ltd., Japan), respectively. Fourier-transform infrared spectroscopy (FTIR; Nicolette 6700, Nicolette, USA) was used to acquire the infrared spectrum of the DNC. X-ray diffraction analysis (XRD; D/max-2200VPC type, Nippon Science Co., Ltd., Japan) was used to characterize the nanocellulose in terms of crystal form and crystallinity. The diffraction patterns were background corrected to remove any signal from the sample holder. Data from a blank run including the sample holder were subtracted from the data of experimental samples [
where [
where I200 is the maximum diffraction intensity at 2θ between 22° and 23°; Iam is the minimum diffraction intensity at 2θ between 18° and 19° [
Batch adsorption experiments were conducted to explore the effects of the initial pH value (1–6), adsorbent dose (2.5–25 g/L), initial Cu2+ concentration (51.6–1014.0 mg/L), and contact time (2 h) on the adsorption capacities of Cu2+ ions. Specifically, a designated amount of adsorbent was introduced into a 100 mL beaker containing a 50 mL Cu2+ solution of known concentration. Then, the initial pH was adjusted precisely to the desired value with 0.1 M HCl or 0.1 M NaOH. The beaker was covered with aluminum foil during adsorption to prevent evaporation of the solution. After continuous stirring for 2 h at 23 ± 2°C, the adsorbent was filtered out with a 0.22 μm needle filter, and the remaining Cu2+ concentration in the filtrate was measured. The results were recorded as the averages of three parallel tests.
Flame atomic absorption spectrometry (900 H, Perkin Elmer Company, USA) was employed to detect the Cu2+ concentration in the solution before and after adsorption. First, the concentration of the test solution was diluted to 1.0–5.0 mg/L with 10% HCl. Thereafter, 3 mL of the diluted solution was used to measure the Cu2+ concentration before and after adsorption. Subsequently, the concentrations of the original Cu2+ solutions were calculated according to the dilution factor. Finally, the adsorbed amount (qe) was calculated using the following equation:
where
To investigate the reusability of DNC after the adsorption of Cu2+ ions, adsorption and desorption experiments were carried out at the initial pH of 6, the adsorbent dose of 5 g/L, the initial Cu2+ concentration of 1014.0 mg/L, and the contact time of 2 h. After the adsorption reaction, the solution was centrifuged at 3000 rpm for 5 min to obtain a blue precipitate, which is characteristic of the presence of Cu2+. Then, 50 mL HCl (0.5 M) was added and placed on a magnetic stirrer for continuous stirring. Although the blue precipitate disappeared immediately after adding HCl, the desorption reaction was carried out for 2 h to ensure complete desorption. After desorption was completed, the desorbed adsorbent was obtained by centrifugal separation and used again to adsorb copper ions. The adsorption and desorption experiments were repeated five times, and the adsorbed amounts were measured each time.
To analyze the adsorption rates of Cu2+ onto nanocellulose, pseudo-first-order and pseudo-second-order kinetic models were tested [
where
The pseudo-second-order kinetic equation is as follows:
where
Adsorption isotherms are predominantly used to study the relationship between the concentration of heavy metal ions adsorbed on the adsorbent and the equilibrium concentration of the solution and can help elucidate the equilibrium adsorption mechanism between solid substances and solutions. The Langmuir and Freundlich models are the most frequently used isotherm models.
The Langmuir adsorption model is assumed for monolayer adsorption, and the active adsorption sites are uniformly distributed on the surface of the adsorbent. The equation is as follows [
where
The Freundlich model assumes heterogeneous multilayer adsorption, which means that the distribution of the adsorption sites on the surface of the adsorbent is not fixed, and the equation is as follows [
where
XRD is used to analyze the crystal form and crystallinity of DNC. The XRD patterns of MCC and DNC are shown in
Steric hindrance and repulsion forces prevent hydrated metal ions from utilizing all the active sites afforded by the carboxylic groups accumulated on the surface of nanocellulose. A large amount of the appropriate functionalities should exist on the surface of the adsorbent for the adsorption of Cu2+; furthermore, a relatively high surface area is required. The content of carboxyl groups on DNC is 3.4 mmol/g, which is approximately double the carboxyl content of MNC prepared by TEMPO-mediated oxidation [
The effects of the initial pH value, adsorbent dose, initial Cu2+ concentration, and contact time on the Cu2+ adsorption capacities of DNC were studied. The experimental design and results are presented in
No. | Factor | DNC dose (g/L) | Cu2+ concentration (mg/L) | Initial pH | Mean adsorbed amount ± S.D. (mg/g) |
---|---|---|---|---|---|
1 | pH | 5.0 | 1171.20 | 1 | 63.60 ± 10.46 |
2 | 5.0 | 1171.20 | 2 | 71.84 ± 8.13 | |
3 | 5.0 | 1171.20 | 3 | 95.64 ± 9.67 | |
4 | 5.0 | 1171.20 | 4 | 123.24 ± 7.35 | |
5 | 5.0 | 1171.20 | 5 | 143.94 ± 8.89 | |
6 | 5.0 | 1171.20 | 6 | 157.56 ± 4.83 | |
7 | DNC dose | 2.5 | 1219.20 | 6 | 176.64 ± 13.15 |
8 | 5.0 | 1219.20 | 6 | 101.76 ± 10.09 | |
9 | 10.0 | 1219.20 | 6 | 41.92 ± 4.70 | |
10 | 15.0 | 1219.20 | 6 | 30.29 ± 3.02 | |
11 | 20.0 | 1219.20 | 6 | 20.64 ± 2.15 | |
12 | 25.0 | 1219.20 | 6 | 14.50 ± 1.55 | |
13 | Cu2+ concentration | 5.0 | 51.60 | 6 | 7.42 ± 13.15 |
14 | 5.0 | 127.50 | 6 | 22.36 ± 2.11 | |
15 | 5.0 | 201.60 | 6 | 37.30 ± 7.26 | |
16 | 5.0 | 252.10 | 6 | 47.18 ± 6.68 | |
17 | 5.0 | 607.00 | 6 | 118.50 ± 13.72 | |
18 | 5.0 | 1014.00 | 6 | 184.20 ± 14.74 |
As shown in
Adsorption experiments were performed by varying the initial Cu2+ concentration from 50 to 1000 mg/L to investigate the effect of the initial Cu2+ concentration on the adsorbed amount (
As shown in
The adsorption capacity of DNC was taken as 100% when it first adsorbed Cu2+, and after the first desorption and re-adsorption, the adsorbed amount decreased to 96% (
Pseudo-first-order and pseudo-second-order kinetic models were used to investigate the kinetics of Cu2+ removal by DNC (
Pseudo-first-order model | Pseudo-second-order model | ||||
---|---|---|---|---|---|
k1 (min−1) | qe (mg/g) | R2 | k2 (g/(mg min)) | qe (mg/g) | R2 |
0.0356 | 1.56 | 0.6187 | 0.297 | 183.48 | 0.9998 |
Langmuir and Freundlich isotherm models were used to obtain the isotherm parameters for the adsorption of Cu2+ ions onto DNC (
Langmuir model | Freundlich model | ||||
---|---|---|---|---|---|
qm (mg/g) | KL (L/mg) | R2 | KF (L/mg) | n | R2 |
217.4 | 0.2541 | 0.9991 | 1.318 | 1.9627 | 0.6187 |
The adsorption of Cu2+ by carboxylic nanocellulose was mainly achieved through the adsorption of Cu2+ by carboxyl and hydroxyl sites, and carboxyl groups were found to be more effective in adsorbing Cu2+ than hydroxyl groups [
According to
where
Because one five-membered stable chelate is formed by every four hydroxyl groups and one Cu2+ ion, the theoretical maximum adsorption amount of hydroxyl groups can be calculated as follows [
where
The theoretical maximum adsorption capacity of carboxylic nanocellulose (
The glucose rings in the molecular chain structure of cellulose are alternately distributed, i.e., the functional groups at C2, C3, and C6 are alternately distributed [
According to our previous study [
For MNC, only the hydroxyl groups at the C6 site are oxidized to carboxyl groups, and the four hydroxyl groups at the C2 and C3 sites can capture one Cu2+ ion [
For DNC, the hydroxyl groups at the C2 and C3 sites are oxidized to carboxyl groups, and it is difficult for the one hydroxyl group remaining at the C6 site to capture copper ions alone [
For TNC, the hydroxyl groups at the C6, C2, and C3 sites are oxidized to carboxyl groups. The length of cellobiose, the basic unit of cellulose, is 1.03 nm [
Maximum adsorption capacity of calculated theoretically (mg/g)a | Experimental results from references (mg/g) | Reference | |||
---|---|---|---|---|---|
I | II | III | |||
MNC | 182.8 | 243.8 | 228.7 | 145.0 |
[ |
DNC | 334.2 | 449.1 | 261.3 | 185.0 |
[ |
TNC | 472.9 | 630.5 | 148.1 | 97.3 |
[ |
PAC | _ | _ | _ | 95.0 |
[ |
Note: aCalculated method for theoretical maximum adsorption capacity: (I) determined by Maekawa’s method [
The PAC price of $0.8–2.0 per kg collected from Chinese manufacturers in 2020 was used to analyze the adsorption cost of PAC (
Adsorbent | Price range ($/kg) | Theoretical maximum adsorption capacity (mg/g) | Cycles |
---|---|---|---|
PAC | $0.8–2.0 | 127.0 [ |
1 |
MNC | $77.2–231.7 | 228.7 (this study) | 5 |
DNC | $77.2–231.7 | 261.3 (this study) | 5 |
TNC | $77.2–231.7 | 148.1 (this study) | 5 |
Note: PAC = powdered activated carbon; MNC = monocarboxylic nanocellulose; DNC = dicarboxylic nanocellulose; TNC = tricarboxylic nanocellulose.
The adsorption costs of Cu2+ ions for MNC, DNC, TNC, and PAC are shown in
DNC was prepared from MCC using the NaIO4–NaClO2 sequential oxidation method. The higher carboxyl content of 3.4 mmol/g and high specific surface area of 500 m2/g endow DNC with a strong adsorption capacity of 184.2 mg/g for Cu2+ ions. The theoretical maximum adsorption capacities of Cu2+ for MNC, DNC, and TNC were calculated to be 228.7, 261.3, and 148.1 mg/g, respectively. The Cu2+ adsorption cost of DNC is close to or lower than that of PAC, demonstrating that DNC prepared by NaIO4–NaClO2 sequential oxidation has competitive adsorption capacity and cost for the treatment of wastewater containing Cu2+.