Contributions of Various Cd(II) Adsorption Mechanisms by Phragmites australis-Activated Carbon Modified with Mannitol.

Due to its high toxicity, persistence, and bioaccumulation in the food chain, controlling cadmium (Cd) pollution in wastewater is urgent. Activated carbon is a popular material for removing Cd. To improve the Cd(II) adsorption efficiency by increasing the number of oxygen-containing functional groups, Phragmites australis-activated carbon (PAAC) was modified with mannitol at a low temperature (150 °C). The textural and chemical characteristics of PAAC and modified PAAC (M-PAAC) were analyzed by surface area analysis, elemental analysis, Boehm's titration, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Batch adsorption experiments were conducted to investigate the influence of Cd(II) concentration, contact time, ionic strength, and pH on Cd(II) adsorption. The main adsorption mechanisms of Cd(II) on activated carbon were quantitatively calculated. The results showed that mannitol modification slightly decreased the S BET (5.30% of PAAC) and increased the content of carboxyl, lactone, and phenolic groups (total increase of 43.96% with PAAC), which enhanced the adsorption capacity of PAAC by 58.59%. The adsorption isotherms of PAAC and M-PAAC were described well using the Temkin model, while the intraparticle diffusion model fitted the Cd(II) adsorption kinetics best. Precipitation with minerals was a crucial factor for Cd(II) adsorption on activated carbon (50.40% for PAAC and 40.41% for M-PAAC). Meanwhile, the Cd(II) adsorption by M-PAAC was also dominated by complexation with oxygen-containing functional groups (33.60%). This research provides a method for recovering wetland plant biomass to prepare activated carbon and efficiently treat Cd-containing wastewater.

Introduction

As typical pollution, heavy metals have attracted extensive attention.[1] Among the potential heavy metal pollutants, cadmium (Cd) is one of the most concerned heavy metals due to the high toxicity and persistence and bioaccumulation in the food chain.[2,3] Long-term exposure to Cd can seriously damage the kidneys, lungs, and bones.[4] The maximum concentration of Cd in drinking water specified by the World Health Organization (WHO) is 0.003 mg/L.[5] Cd is widely used in the electroplating industry, manufacture of nickel–cadmium batteries, fertilizers, pesticides, pigments, and dyes, and textile operations[6] and mainly discharged into the environment through human activities. To control the Cd concentration in the environment and reduce the possibility of Cd exposure, Cd-containing wastewater must be treated before it is discharged.

Cd removal from wastewater is usually accomplished by several techniques, such as precipitation,[7] ion exchange,[8] solvent extraction,[9] membrane separation,[10] and adsorption.[11] Compared with other techniques, the advantages of adsorption include a high efficiency, low material and process costs, availability, and ease of operation;[4] therefore, adsorption methods are promising methods for removing Cd. Many kinds of adsorbents have been shown to improve the treatment efficiency, including organic-based adsorbents (natural organic sorbents or synthetic polymers) and inorganic-based adsorbents (zeolites, clays, SiO2, Al2O3, and other oxides).[4]arene modified silica resin for the adsorption of metal ions: Equilibrium, thermodynamic and kinetic modeling studies. J. Mol. Liq.. 2021 ">12] Owing to its well-developed pore structure, high specific surface area, high adsorption capacity, low cost, nontoxicity, and high stability, activated carbon is an excellent adsorbent for removing heavy metal pollutants from wastewater.[13]

Generally, activated carbon can be produced by either physical activation or chemical activation.[14] Physical activation methods often use oxygen-containing gases (such as CO2, O2, and H2O) to improve the porosity of activated carbon, while chemical activation methods use reagents (such as KOH, ZnCl2, and H3PO4) to introduce abundant functional groups and a high surface area.[15] Because it produces less environmental pollution and has a low activation temperature, H3PO4 is one of the most important activators.[16] H3PO4 can be dehydrated and partly condensed into pyrophosphoric acid (H4P2O7) at 213 °C, which plays an effective role during activation processes.[17] Because it is a larger molecule, pyrophosphoric acid activation generates more mesopores than phosphoric acid activation;[18] thus, to obtain a high mesopore content and large specific surface area, pyrophosphoric acid was chosen to prepare activated carbon in this study.

Apart from physical properties, surface functional groups also play a significant role during adsorption processes, particularly oxygen-containing functional groups such as −COOH and −OH.[19,20] A previous study reported that oxygen-containing functional groups on the adsorbent surface can serve as adsorption sites via ion exchange, electrostatic interactions, and metal ion complexation.[3] Increasing the number of oxygen-containing functional groups on the surface of activated carbon can enhance its adsorption capacity toward metal ions.[11] Organophosphorus compound is an ester of phosphoric acid and alcohol. It has been proved to be a novel activator for developing activated carbon from lignocellulosic materials with high surface oxygen-containing groups and heavy metal-ion adsorption capacity.[21] Polyhydric alcohols, containing multiple hydroxyl groups, can condense with H3PO4 via esterification to form phosphates with low molecular weight, which can be easily decomposed into radicals and phosphorus oxides.[22] Mannitol (C6H14O6) has six hydroxyl groups and is categorized as a polyalcohol or sugar alcohol. Accordingly, mannitol was chosen as an activated carbon modifier.

The aims of this study were to modify Phragmites australis-activated carbon (PAAC) with mannitol (M-PAAC) and investigate its Cd(II) removal performance and mechanisms. The specific targets were to (1) evaluate the physiochemical properties of PAAC and M-PAAC, (2) investigate the isotherms and kinetics of Cd(II) adsorption using PAAC and M-PAAC, (3) explore the effect of ionic strength and pH on Cd(II) adsorption, and (4) quantitatively determine the main Cd(II) adsorption mechanisms. This study provides a new adsorbent for removing Cd(II) and is also beneficial for recycling P. australis biomass from wetlands.

Materials and Methods

Reagents and Materials

All chemicals used in this study were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All solutions were prepared with deionized water. The preparation of 500 mg/L Cd(II) stock solution was conducted by dissolving an appropriate amount of cadmium chloride in deionized water. The working solution was prepared by further diluting the stock solution. Mannitol was used to modify activated carbon. P. australis was obtained from Weishan Lake (116.92°E, 34.97°N, Shandong Province, China). After collection, P. australis was washed five times with deionized water and dried at 105 °C for 48 h to a constant weight. Then, P. australis was sliced into 0.45–1.0 mm pieces before use.

Preparation of Adsorbents

P. australis was mixed with pyrophosphoric acid at an impregnation ratio of 1:1.2 (w: w, P. australis/pyrophosphoric acid) and impregnated in a beaker with a lid at 20 ± 2 °C for 12 h. Then, the sample was moved into a crucible and placed in a muffle furnace, and pyrolysis was conducted at 500 °C for 1 h with a 10 °C/min heating rate. After pyrolysis, activated carbon was cooled to ambient temperature. The obtained activated carbon was washed with deionized water to a constant pH and then dried at 105 °C to a constant weight and sieved through a 100 mesh sieve (0.154 mm). The activated carbon was saved in a desiccator for further experiments and labeled PAAC.

The activated carbon modification conditions were optimized by orthogonal experiments (Table S1). PAAC (5 g) was mixed with 0.025 mol phosphoric acid and 0.005 mol mannitol dissolved in deionized water. The PAAC sample was impregnated in a covered crucible at 20 ± 2 °C for 12 h. Then, the crucible was transferred into a muffle furnace and heated from ambient temperature (20 ± 2 °C) to 150 °C at a rate of 10 °C/min. The PAAC sample was maintained at 150 °C for 3 h. After cooling, the modified activated carbon was washed with deionized water to a constant pH. The obtained modified activated carbon (labeled M-PAAC) was dried at 60 °C to a constant weight.

Characteristics of Adsorbents

The pore structure characteristics of PAAC and M-PAAC were analyzed by N2 adsorption/desorption at 77 K using a surface area analyzer (Quanta Chrome Corporation, USA). The average pore diameter (Dp) was determined using eq

The total pore volume (Vtotal) was determined at P/P0 = 0.99. The surface area (SBET) was calculated by the Brunauer–Emmett–Teller (BET) method. The micropore volume (Vmic), micropore surface area (Smic), and external surface area (Sext) were calculated by the t-plot method. The pore size distribution was determined by the density functional theory (DFT) method. Elemental analysis (C, O, N, and H) of PAAC and M-PAAC was performed using a Vario EI III elemental analyzer (USA). The surface functional groups were identified by Fourier-transform infrared (FTIR) spectroscopy (20 XS, Nicolet, America) in the wavenumber range of 400–4000 cm–1 using 50 scans at a 2 cm–1 resolution. In an infrared drying oven, the activated carbon (5 mg) and KBr salt (100 mg, Sigma-Aldrich, USA) were fully ground and mixed using an agate mortar. Then, the mixture was pressed to a small thin disc with a disc pressing machine. The disc sample was transferred to a sample rack for FTIR analysis. Boehm’s titration method[23] was conducted to quantify the content of acidic and basic functional groups on the PAAC and M-PAAC surfaces. X-ray photoelectron spectroscopy (XPS, PHI 550 ESCA/SAM, PerkinElmer, USA), using Mg Kα irradiation (1486.71 eV photons) as the X-ray source, was used to determine the surface binding state and elemental speciation of PAAC and M-PAAC before and after Cd(II) adsorption. All binding energies were referenced to the 284.6 eV C 1s peak to correct for surface charging. The point of zero charge (pHPZC) of PAAC and M-PAAC was measured by the pH drift method.[24]

Batch Adsorption Experiments

Batch adsorption experiments were performed by agitating a measured amount of the adsorbent in 50 mL of Cd(II) solution in a 100 mL Erlenmeyer flask using a digital thermostatic shaker (200 rpm, 20 ± 2 °C). For adsorption isotherm experiments, Cd(II) solutions with different initial concentrations (20–80 mg/L) were prepared. Mixtures of activated carbon (0.6 g/L) (Figure S1) and Cd(II) solution were shaken for 48 h to reach adsorption equilibrium. After equilibrium, the solutions were filtrated with a 0.45 μm membrane, and the Cd(II) concentration was measured with inductively coupled plasma optical emission spectrometry (ICP–OES, PQ 9000, Analytik-Jena, Germany). The Cd(II) adsorption isotherm experiments were performed at three different ionic strengths (I = 0, 100, and 1000 mM NaCl) to explore the impact of ionic strength (I) on Cd(II) adsorption after the above steps.

For adsorption kinetics experiments, activated carbon with a concentration of 0.6 g/L was added to 50 mL of 30 mg/L Cd(II) solution. The samples were shaken in a digital thermostatic shaker (200 rpm, 20 ± 2 °C) and then were taken at pre-determined times (0–15 h), filtered, and analyzed. The influence of pH on the Cd(II) removal was investigated within a pH range of 2.0–7.0 at 30 mg/L initial Cd(II) concentration using a 0.6 g/L adsorbent. The initial pH of Cd(II) solutions was adjusted using 0.10 M HCl or 0.10 M NaOH. The pH value was tested with a pHS-3C pH meter (China). After equilibrium, the solutions were filtered and measured by ICP–OES to determine their Cd(II) concentration.

In the recycling experiments, the pH of Cd(II) solution, Cd(II) concentration, and dosage of activated carbon were 7, 30 mg/L, and 0.6 g/L, respectively. The Cd(II)-loaded activated carbon was desorbed in 0.5 M HCl solution for 6 h. Also, the recovered activated carbon was used again in the next adsorption process. Five recycling rounds were performed.

The adsorption amounts of Cd(II) on PAAC and M-PAAC (qe (mg/g)) were calculated as follows (eq )where C0 and Ce are the initial and equilibrium Cd(II) concentrations (mg/L), respectively, V is the solution volume (L), and M is the adsorbent mass (g).

Contribution of Main Mechanisms

The potential Cd(II) adsorption mechanisms were summarized into the following four types:[11,19,25] (i) ion exchange with metal ions (Qe), (ii) precipitation with minerals (Qp), (iii) complexation with oxygen-containing functional groups (Qf), and (iv) coordination with π-electrons (Qπ). The method used to quantitatively calculate the contribution of each mechanism to Cd(II) adsorption with activated carbon was as follows.[2,26,27] In this part, the pH of Cd(II) solution, Cd(II) concentration, and dosage of activated carbon were 7, 30 mg/L, and 0.6 g/L, respectively.where QK, QNa, QCa, and QMg are the adsorption capacity contributions of K+, Na+, Ca2+, and Mg2+ exchange, respectively; mK, mNa, mCa, and mMg (mg/g) are the concentrations of K+, Na+, Ca2+, and Mg2+ released from activated carbon into the solution, respectively; and MK, MNa, MCa, MMg, and MCd are the molecular weights of K, Na, Ca, Mg, and Cd, respectively.where Qmin is the adsorption amount caused by interactions with minerals, Qt is the total adsorption capacity, and Qdmin is the adsorption capacity of demineralized activated carbon.

Ion exchange with metal ions (Qe): ion exchange between Cd(II) and metal ions on activated carbon was calculated by the difference in the amount of exchangeable cations (K+, Na+, Ca2+, and Mg2+) in solution before and after Cd(II) adsorption (eq ).

Precipitation with minerals (Qp): minerals in activated carbon can be removed by washing with HCl solution.[25] To obtain demineralized activated carbon, activated carbon was treated with 1 M HCl for 5 h and then washed with deionized water to a constant pH (6.12 and 4.58 for PAAC and M-PAAC, respectively). The adsorption of Cd(II) on minerals resulted from ion exchange and mineral precipitation. Qp was determined using eq

Complexation with oxygen-containing functional groups (Qf): the oxygen-containing functional groups on activated carbon can react with Cd(II) in the solution via eqs and 6, which decreased the pH before and after Cd(II) adsorption; therefore, Qf was calculated from the amount of released H+.

Coordination with π-electrons (Qπ): Cd(II) adsorption by demineralized activated carbon occurred via coordination with π-electrons and complexation with oxygen-containing functional groups; therefore, Qπ can be obtained using eq

Statistical Analysis

The ionic distribution of Cd(II) over a specific pH range in aqueous solutions was simulated using ver. 3.0 Visual MINTEQ (https://vminteq.lwr.kth.se/). The peak fitting of XPS data was performed with software CasaXPS (http://www.casaxps.com/casaxps2315.htm). Batch experiments (isotherms, kinetics, and the influence of ionic strength and pH) were conducted in triplicate. The experimental data are reported as the mean ± standard deviation. Blank experiments were performed under the same experimental conditions.

Results and Discussion

Textural and Chemical Characterization of Adsorbents

The specific surface area and pore size distributions are crucial physical factors for metal adsorption on activated carbon.[4] The textural parameters of PAAC and M-PAAC are displayed in Table S2. The N2 adsorption/desorption isotherms for PAAC and M-PAAC (Figure S2) belonged to type IV isotherms according to the IUPAC classification, which indicates a mesoporous structure.[28,29] These results were confirmed from the pore size distributions, which showed that the pore size was mainly in the range of 2–15 nm for PAAC and M-PAAC. The Vmes/Vtot of PAAC and M-PAAC was 59.60 and 78.64%, respectively, also indicating that the pores were mainly mesopores (2–50 nm). Cui et al.[20] reported similar results, in which mesopores were dominant (47–66%) in activated carbon derived from wetland plants. The PAAC possessed a larger BET surface area (SBET, 856.08 m2/g) and total pore volume (Vtot, 0.55 cm3/g) than M-PAAC (810.70 m2/g, 0.50 cm3/g). The SEM micrographs exhibit different pore structures of PAAC and M-PAAC (Figure S3). Modification with mannitol blocked the pores of PAAC, which decreased SBET and Vtot. The pore volume and specific surface area decreased due to partial pore blockage after introducing more functional groups via modification.[30]

Elemental analysis (Table S3) showed that PAAC contained 65.46% C, 31.41% O, 1.89% H, and 1.06% N, while the proportions of the corresponding elements in M-PAAC were 53.22, 42.25, 2.19, and 1.17%, respectively. After modification, the C content of M-PAAC decreased by 12.24% due to an increase in the relative O content. The relatively higher value of O/C for M-PAAC (0.81) compared with that for PAAC (0.48) indicated that more oxygen existed on the M-PAAC surface in various functional groups. In addition, the retention of oxygen-containing functional groups on activated carbon retained the hydrophilicity of its surface.[20] The N-doping of activated carbon also improved its hydrophilicity.[31] The (O + N)/C ratio of M-PAAC was 0.84, which is higher than that of PAAC (0.50), which demonstrated that the surface of M-PAAC had a high hydrophilicity and polarity.[32] Hydrophilicity and polarity are conducive to the adsorption of polar water-soluble pollutants, such as Cd(II).[20]

The functional groups on activated carbon play a significant role in metal adsorption. To determine the surface functional groups of PAAC and M-PAAC, FTIR spectra were obtained (Figure a). The broad peak at 3434–3408 cm–1 corresponded to the stretching vibration of the hydroxyl groups of alcohols, phenols, and carboxylic acids.[33] This peak increased significantly after the modification of PAAC. The peak at 2922 cm–1 was due to the C–H stretching vibration of lignin, hemicellulose, and cellulose,[34] while the peak of olefinic C–H appeared at 690 cm–1.[20] The peak around 1575 cm–1 was assigned to the aromatic C=C or the C=O of ketones or carbonyls.[35] The peak of C–O for ethers was located from 1039 to 1169 cm–1.[36] After modification, the types of FTIR peaks were basically the same and the intensity of the peaks changed obviously. Boehm’s titration was performed to further quantify the surface functional groups. The total acidic groups of M-PAAC, including carboxyls, lactones, and phenols, were 2.7669 mmol/g due to an increase in carboxyl groups, which were much higher than those of PAAC (1.9221 mmol/g) (Figure b). More acidic groups, especially carboxyls, are favorable for the removal of metal ions in aqueous solutions.

Figure 1

(a) FTIR spectra of the PAAC and M-PAAC before and after the adsorption of Cd(II) and (b) surface functional groups of PAAC and M-PAAC measured by Boehm’s titration.

Cd(II) Adsorption Isotherms

The effect of the initial Cd(II) concentration on the adsorption capacity of PAAC and M-PAAC was investigated (Figure ). Upon increasing the Cd(II) concentration, the Cd(II) adsorption amount on PAAC and M-PAAC increased. For M-PAAC at an ion strength of 0 mM, when the initial concentration of Cd(II) increased from 20 to 60 mg/L, the Cd(II) adsorption amount increased significantly from 23.16 to 48.70 mg/g, while the Cd(II) adsorption amount increased slowly from 48.70 to 52.52 mg/g after the initial Cd(II) concentration exceeded 60 mg/L. This phenomenon was caused by the fixed number of vacant sites and active groups on the surface of a fixed amount of activated carbon.[37] At a low initial Cd(II) concentration (20–60 mg/L), the small transfer resistance between the liquid and solid phases was conducive to Cd(II) adsorption, resulting in significantly increased Cd(II) adsorption. When the initial Cd(II) concentration increased (≥60 mg/L), the transfer resistance between the solid and liquid phases also increased, which caused the Cd(II) adsorption amount to increase slowly.[1]

Figure 2

Adsorption isotherms of Cd(II) fitted with the Langmuir model (solid lines) (a,b), Freundlich model (dash lines) (a,b), and Temkin model (c,d) for PAAC (a,c) and M-PAAC (b,d). Also, the effect of ion strength (0, 100, and 1000 mM) for the adsorption of Cd(II) on PAAC and M-PAAC.

Adsorption isotherms describe the adsorption behavior, illustrate the surface properties of activated carbon, and characterize relationships between the Cd(II) concentration and Cd(II) adsorption amount.[38,39] The adsorption isotherms of Cd(II) on PAAC and M-PAAC were fitted with the Langmuir, Freundlich, and Temkin models (Figure S2). The related parameters were calculated from three isotherms (Table ). According to the RMSE value, the fitting data of the Temkin model were closest to the experimental data. The adsorption data were well fitted using the Temkin isotherm model (R2 ≥ 0.96 and RMSE ≤1.49 × 10–3), indicating that the energy released during adsorption decreased upon increasing the coverage of the activated carbon surface.[40] The Langmuir constant (KL) for PAAC and M-PAAC was 0.054 and 0.101 L/mg, respectively, suggesting that M-PAAC had better adsorption affinity to Cd(II) than PAAC.[41,42]

Table 1

Isotherm Adsorption Parameters of Cd(II) on PAAC and M-PAAC Fitted Using Langmuir, Freundlich, and Temkin Isotherm Models

			PAAC			M-PAAC
model	parameters	unit	0 mM	100 mM	1000 mM	0 mM	100 mM	1000 mM
Langmuir	qmax	mg/g	55.866	49.261	37.736	63.694	60.606	40.323
	KL	L/mg	0.054	0.050	0.049	0.101	0.037	0.052
	R2		0.986	0.897	0.984	0.992	0.995	0.995
	RMSE		0.141	0.009	0.038	0.464	0.002	0.071
Freundlich	KF	mg(1-1/n) L1/n/g	8.196	6.544	4.896	12.760	5.256	5.731
	1/n		0.414	0.431	0.436	0.384	0.520	0.418
	R2		0.962	0.978	0.951	0.969	0.976	0.982
	RMSE		0.030	8.3 × 10–5	0.020	0.024	0.012	0.027
Temkin	AT	L/mg	0.497	0.482	0.403	1.036	0.288	0.459
	b	J/mol	192.318	223.662	273.099	177.422	165.754	262.214
	R2		0.973	0.965	0.961	0.971	0.993	0.986
	RMSE		9.35 × 10–4	5.05 × 10–4	1.56 × 10–4	1.49 × 10–3	6.07 × 10–5	8.9 × 10–5

The maximum Cd(II) adsorption capacity (qm) of M-PAAC was higher than that of PAAC, but PAAC possessed a higher SBET and Vtot than M-PAAC. This phenomenon was attributed to the physicochemical properties of the materials, especially the surface functional groups. M-PAAC exhibited a higher qm (63.70 mg/g) than some other adsorbents, such as pyromellitic dianhydride-rice straw biochar (9.9 mg/g),[43] proanthocyanidin-functionalized Fe3O4 magnetic nanoparticles (20.9 mg/g),[44] magnetic marine algae biochar (34.9 mg/g),[45] bentonite–Fe3O4–MnO2 (35.5 mg/g),[46] and magnetic sodium alginate–alkaline residue aerogel (38.83 mg/g).[47] Some reported adsorbents have a better Cd(II) adsorption capacity,[32,42] but M-PAAC was prepared from harvested wetland plants, which is a good method of resource reuse.

Cd(II) Adsorption Kinetics

The influence of adsorption time on the adsorption capacity of PAAC and M-PAAC was studied (Figure ). The equilibrium adsorption capacity of Cd(II) by M-PAAC (35.00 mg/g) was much higher than that of PAAC (22.07 mg/g), demonstrating that modification improved the adsorption capacity; however, the adsorption equilibrium times and rates of PAAC and M-PAAC for Cd(II) were similar. In the first 60 min, the Cd(II) adsorption amount of PAAC and M-PAAC increased sharply. During the initial phase of adsorption, Cd(II) was mainly adsorbed on the external surface and there were many free adsorption sites on the material’s surface.[3] From 60–200 min, slow growth in the Cd(II) adsorption amount was observed. Upon increasing the Cd(II) adsorption capacity, the adsorption sites on the surface of PAAC and M-PAAC became occupied. When the number of adsorption sites decreased, the adsorption rate became more dependent on the Cd(II) transfer rate from external surfaces to the interior of activated carbon. This led to the slow increase in the adsorption capacity during the later stage (60–200 min).[27,48]

Figure 3

Adsorption kinetics of Cd(II) on PAAC and M-PAAC, (a) adsorption kinetic curves, (b) pseudo-first-order model-fitted plots, (c) pseudo-second-order model-fitted plots, and (d) intraparticle diffusion model-fitted plots.

The adsorption kinetics (Figure ) were studied to understand the adsorption efficiency and determine the rate-controlling steps during adsorption.[12] The kinetic parameters were calculated and are summarized in Table . Compared with the pseudo-first-order model, the pseudo-second-order model better represented the adsorption kinetics of Cd(II) by PAAC and M-PAAC. Liu et al.[2] reported similar results, in which the Cd(II) adsorption kinetics of blue algae-derived biochar were fitted better with the pseudo-second-order model. According to the RMSE, the intraparticle diffusion model fitted well with the kinetic data. For the intraparticle diffusion model, the C2, C3, k, k, and ki3 of M-PAAC were higher than those of PAAC, implying that modification improved the diffusion rate and increased initial adsorption.[2,49]C1 was zero, indicating that the adsorption rate was regulated by intraparticle diffusion, while C2 and C3 were not zero, suggesting that the adsorption process was limited by multiple steps.[50] From the fitting of the intraparticle diffusion model (Figure d), the adsorption process of PAAC and M-PAAC for Cd(II) removal can be divided into three phases. In the first stage, Cd(II) diffused to the adsorbent surface through the boundary layer and was rapidly adsorbed.[51] In the second stage, Cd(II) diffused into the activated carbon through pores on the surface, and the adsorption rate was slow. In the third stage, Cd(II) adsorption by activated carbon nearly reached equilibrium;[1] therefore, during Cd(II) adsorption by PAAC and M-PAAC, the rate-controlling steps included external mass transfer and intraparticle diffusion.

Table 2

Kinetic Parameters of the Pseudo-First-Order Model, Pseudo-Second Order Model, and Intraparticle Diffusion Model for the Adsorption of Cd(II) by PAAC and M-PAAC

kinetic models	parameters	unit	PAAC	M-PAAC
experimental	qe,exp	mg/g	22.067	35.000
pseudo-first-order	qe,cal	mg/g	3.805	6.623
	k1	1/min	0.005	0.003
	R2		0.791	0.761
	RMSE		18.862	27.930
pseudo-second-order	qe,cal	mg/g	22.573	34.722
	k2	g/(mg·h)	0.009	0.004
	R2		1.000	1.000
	RMSE		1.658	2.951
intraparticle diffusion	ki1	mg/(g·h1/2)	6.271	8.766
first stage	C1		0.000	0.000
	R12		0.977	0.993
	RMSE1		2.164	1.637
	ki2	mg/(g·h1/2)	0.368	0.759
second stage	C2		17.961	25.939
	R22		0.947	0.886
	RMSE2		0.497	0.966
	ki3	mg/(g·h1/2)	0.073	0.157
third stage	C3		20.535	30.167
	R32		0.982	0.985
	RMSE3		0.067	0.133

Effect of Ionic Strength on Cd(II) Adsorption

The effect of ionic strength (0, 100, and 1000 mM NaCl) on the adsorption of Cd(II) by activated carbon was examined to distinguish non-specific and specific adsorption.[52] The adsorption amount of Cd(II) on PAAC and M-PAAC decreased upon increasing the ionic strength (Figure ). When the ionic strength varied from 0 to 1000 mM at 30 mg/L initial Cd(II) concentration, the Cd(II) adsorption capacity of PAAC and M-PAAC decreased from 33.23 to 24.33 mg/g and 48.70 to 28.31 mg/g, respectively. The qm calculated using the Langmuir model (Table ) also decreased upon increasing the ionic strength. The decreased Cd(II) adsorption can be explained by two reasons. First, at a higher ionic strength, more vacancies were occupied with Na+,[53] which resulted in stronger competitive adsorption between Na+ and Cd(II). Second, the adsorption of Na+ on activated carbon influenced the potential of the interfaces and reduced the number of negative charges on the surface.[14] Inner-sphere complexes are related to strong covalent or ionic bonding and are only weakly affected by the ionic strength in the aqueous phase, while outer-sphere complexes involving only electrostatic interactions were strongly influenced by the ionic strength.[54] The strong effect of ionic strength on Cd(II) adsorption showed that ion exchange and outer-sphere complexation contributed to Cd(II) adsorption on PAAC and M-PAAC; thus, when the NaCl concentration was 1000 mM, Cd(II) continued to be adsorbed, indicating the occurrence of inner-sphere complexation between Cd(II) and surface functional groups on activated carbon.

Effect of pH on Cd(II) Adsorption

The solution pH had an obvious effect on metal-ion adsorption because it influenced the surface electric charge density of activated carbon and Cd(II) speciation.[2] pHPZC represents the pH point at which the net charge on a material is zero. When the external environment pH > pHPZC, the material will be deprotonated and negatively charged. Inversely, if the external environment pH < pHPZC, the material will be protonated and positively charged.[42]Figure a shows that the pHPZC of PAAC and M-PAAC were 6.30 and 3.00, respectively. After modification, the pHPZC decreased due to the presence of more acidic functional groups on the sample surface. Meanwhile, the lower pHPZC of M-PAAC was consistent with the higher surface acidity of M-PAAC (Figure b). Cd(II) exists as Cd2+, Cd(OH)2 (aq), [Cd2(OH)]3+, [Cd(OH)]+, [Cd(OH)3]−, and [Cd(OH)4]2– at different pH values (Figure b). At pH < 7.5, the predominant species of Cd(II) was Cd2+. At pH > 7.5, [Cd(OH)]+ appeared. Consequently, the adsorption experiment for investigating the influence of pH on Cd(II) adsorption was performed at pH values from 2.0 to 7.0 to avoid interference from some Cd(II) species such as Cd(OH)2 (aq).

Figure 4

(a) pHPZC of PAAC and M-PAAC, (b) species distributions of Cd(II) vs pH at a Cd(II) concentration of 30 mg/L, (c) effects of pH on Cd(II) adsorption by PAAC and M-PAAC, and (d) final pH after Cd(II) adsorption by PAAC and M-PAAC.

To investigate the effect of initial pH on Cd(II) adsorption, the equilibrium Cd(II) adsorption capacity and the finial pH were tested (Figure c,d). The initial pH strongly influenced the adsorption. When the initial pH was 2.0–4.0, the Cd(II) adsorption sharply increased upon increasing the initial pH. At a pH of 2.0, the adsorption amount of PAAC (5.55 mg/g) was higher than that of M-PAAC (1.25 mg/g). At a pH < 3.0, the surface of M-PAAC was positive with a pHPZC of 3.00. Because M-PAAC had fewer alkaline groups and more acidic groups than PAAC, the electron repulsion to Cd2+ and ionic competition between H+ and Cd2+ were stronger for M-PAAC than PAAC,[3] which led to lower adsorption by M-PAAC. At pH 3.0–4.0, the surface of M-PAAC was negatively charged, while the surface of PAAC was positively charged. The adsorption amount of M-PAAC increased and was higher than that of PAAC. Positively charged Cd(II) species were repelled by the positively charged functional groups on the carbon surface, which resulted in a low equilibrium adsorption capacity of Cd(II). When the initial pH was 4.0–7.0, the adsorption capacity of Cd(II) stabilized. Changes in the final pH after adsorption equilibrium indicated that −COOH and −OH were protonated and deprotonated during Cd(II) adsorption.[25] The influence of pH indicates the role of electrostatic interactions and ion exchange during Cd(II) adsorption.

Adsorption Mechanisms

Ion Exchange with Metal Ions (Qe)

Previous studies have suggested that many metal ions (K+, Na+, Ca2+, and Mg2+) are retained on activated carbon through electrostatic attraction, complexation with functional groups, or precipitation.[55] The P. australis-derived activated carbon contained 42.16 g/kg K, 0.25 g/kg Na, 3.34 g/kg Ca, and 1.33 g/kg Mg.[20] These ions on activated carbon played a significant role during adsorption and could exchange with Cd(II). Ion exchange with metal ions is a crucial and common mechanism for heavy metal adsorption on activated carbon.[25] To quantify the Cd(II) amount adsorbed by ion exchange with metal ions, the net release of metal ions (K+, Na+, Ca2+, and Mg2+) during adsorption was calculated by testing the metal-ion concentrations in the solution before and after Cd(II) adsorption. Among the four metals, K+ was released in the greatest amount (2.76 mg/g for PAAC and 2.59 mg/g for M-PAAC), while less Na+, Ca2+, and Mg2+ were released (Table S4). This phenomenon may be due to the high content of K in the P. australis-derived activated carbon. Monovalent cations (K+ and Na+) mainly formed outer-sphere complexes through electrostatic attraction and were retained on the activated carbon surface, while divalent cations (Ca2+ and Mg2+) precipitated and complexed with oxygen-containing functional groups and were retained on the inner-sphere surface.[26,56]

Precipitation with Minerals (Qp)

Cd(II) can combine with minerals on activated carbon to form precipitates to achieve Cd(II) removal. Many studies have reported the indispensable contribution of precipitation with minerals to Cd(II) adsorption.[25,26] For example, precipitation with minerals dominated the Cd(II) adsorption on Canna indica-derived biochar[26] and blue algae-derived biochar,[2] accounting for 86.1–89.5 and 68.7–89.5% of Qt, respectively. The adsorption capacity of demineralized activated carbon (H-PAAC and H-M-PAAC) was significantly reduced from 22.07 mg/g (for PAAC) to 4.02 mg/g and from 35.00 mg/g (for M-PAAC) to 13.33 mg/g. Demineralization decreased the adsorption capacity by 81.78% for PAAC and 61.91% for M-PAAC. The lower reduction percentage of M-PAAC than that of PAAC may be due to the lower pHPZC of M-PAAC, which was not conducive to precipitation with minerals. Cd(II) can precipitate with anions released from activated carbon, such as PO43–, CO32–, SO42–, and OH–.[57] In the survey spectra of XPS (Figure S4), the P peak around 134 eV was observed. In the preparation of PAAC and the modification of M-PAAC, pyrophosphoric acid and phosphoric acid were used, which would introduce phosphorus into activated carbon. When phosphorus existed in the form of inorganic phosphate, it could combine with Cd(II) to form mineral precipitation. The formation of precipitates and ion exchange were not independent of each other and were influenced by the solution pH.[27] The solution pH was 4.55 for PAAC and 3.98 for M-PAAC after Cd(II) adsorption. Cd(II) may precipitate in pores, where the pH may be higher than that of the rest of the solution.[26]

Complexation with Oxygen-Containing Functional Groups (Qf)

The complexation between heavy metal and oxygen-containing functional groups (such as −OH, −R–OH, and −COOH) is considered to be the main mechanism of heavy metal adsorption by activated carbon.[19,20,26] Changes in the functional groups before and after Cd(II) adsorption were studied by FTIR and XPS to confirm the complexation with oxygen-containing functional groups during adsorption (Figure ). Comparing the FTIR spectra of PAAC and M-PAAC before and after Cd(II) adsorption, the peaks of C–O stretching vibration at 1139–1169 cm–1 and the carboxyl C=O at about 1560 cm–1 underwent an apparent shift and weakening, suggesting the complexation of Cd(II) with −COOH and −OH.[19] The new peak at 1398 cm–1 for PAAC-Cd and M-PAAC-Cd might be due to Cd complexes.[26]

Figure 5

C 1s and O 1s XPS spectra of (a,b) PAAC before Cd(II) adsorption, (c,d) PAAC after Cd(II) adsorption, (e,f) M-PAAC before Cd(II) adsorption, and (g,h) M-PAAC after Cd(II) adsorption.

In the XPS survey spectra (Figure S4), the intensity of O 1s peaks for M-PAAC was much higher than that of PAAC, indicating that M-PAAC contained more oxygen-containing groups. After modification, the percentage of O–C=O or C=O in the C 1s spectra increased from 10.01 to 18.05% (Figure ), also implying that oxygen-containing functional groups, especially carboxyl, were introduced on activated carbon by modification, in agreement with the results of Boehm’s titration (Figure b). Compared with the original PAAC and M-PAAC, the binding energy (BE) of −C=O in the O 1s spectra on the surface of PAAC-Cd and M-PAAC-Cd decreased from 530.82 and 530.95 to 530.70 and 530.78 eV, respectively. Contrarily, the C–O peaks of C 1s shifted slightly to a higher BE after Cd(II) adsorption. After Cd(II) was adsorbed on M-PAAC, the peak area percentage of C–O for C 1s increased from 18.68 to 38.33%, while the percentage of the O–C=O or C=O for C 1s peaks decreased from 18.05 to 7.04%. Changes in the oxygen-containing functional groups indicated that abundant −COOH and −OH were consumed during adsorption, which is consistent with previous research.[27,55] The H+ in the −COOH and −OH groups of activated carbon may have been replaced with cationic Cd(II) species, which decreased the solution pH.[25] The demineralization of activated carbon may eliminate the influence of minerals on solution pH, and the more apparent pH decrease of H-M-PAAC than that of H-PAAC (Table S6) suggested a greater contribution of functional group complexation for H-M-PAAC than H-PAAC. In addition, mannitol could form organophosphorus compounds by esterification and condensation with phosphoric acid. These compounds were easily decomposed into phosphorus oxides and free radicals, which was conducive to the binding of Cd(II) to activated carbon.[21]

Coordination with π-Electrons (Qπ)

Heavy metals can coordinate with the π-electrons provided by γ-CH and C=C of activated carbon.[58] Aromatic structures in activated carbon could donate π-electrons to heavy metals, which acted as π-electron acceptors.[59] Cd(II)−π interactions are considered to be an indispensable mechanism for activated carbon to remove Cd(II), which has been reported in many literature studies. π-Electron coordination contributed 8.50–16.84% of the total Cd(II) adsorption by camellia seed husk biochar,[11] while the contribution of Cd(II)−π interactions for C. indica-derived biochar was lower, accounting for 2.50–5.04%.[25] This indicates that activated carbon prepared from different raw materials and preparation conditions has different adsorption mechanisms. In the FTIR spectra (Figure a), the intensity of the γ-CH peak at 690 cm–1 for PAAC-Cd and M-PAAC-Cd was weakened, suggesting that Cd(II)−π interactions were involved in Cd(II) adsorption. The aromatic C=C or C=O stretching peaks at 1558–1613 cm–1 of PAAC-Cd and M-PAAC-Cd exhibited a shift and broadening compared with those of PAAC and M-PAAC. The changes in these bands in Cd(II)-loaded activated carbon explain the contribution of Cd(II)−π interactions. After Cd(II) adsorption on M-PAAC, the C=O peak of C 1s spectra shifted from 288.16 to 288.24 eV due to the coordination of Cd(II) with π electrons.[19] The higher C content and the lower H content (Table S3) imply that the aromatic structure of PAAC and M-PAAC was relatively developed.[27] After modification, the atomic ratio of H/C increased (Table S3), indicating that M-PAAC possessed lower aromaticity and more Cd(II)−π interactions.[19]

Contribution Ratio of Four Adsorption Mechanisms

The potential Cd(II) adsorption mechanisms were summarized into the following four main categories:[11,19,25] (i) ion exchange with metal ions (Qe), (ii) precipitation with minerals (Qp), (iii) complexation with oxygen-containing functional groups (Qf), and (iv) coordination with π-electrons (Qπ). The contributions of these four Cd(II) adsorption mechanisms were calculated and are shown in Figure , and a schematic diagram is exhibited in Figure . After modification, the total Cd(II) adsorption capacity (Qt) on M-PAAC increased by 58.61% compared with that of PAAC (from 22.07 to 35.00 mg/g) (Table S7).

Figure 6

Estimated contribution of Cd(II) adsorption mechanisms on PAAC and M-PAAC, including ion exchange with metal ions (Qe), precipitation with minerals (Qp), complexation with oxygen-containing functional groups (Qf), and coordination with π-electrons (Qπ).

Figure 7

Schematic diagram of the main adsorption mechanisms of Cd(II) adsorption by PAAC and M-PAAC.

Compared with M-PAAC, the Cd(II) adsorption amount contributed by four mechanisms of PAAC all displayed an increase (Table S7). The released metal ions corresponded to 6.92 mg/g Cd(II) adsorption amount for PAAC and 7.53 mg/g for M-PAAC. Qe accounted for 31.37 and 21.50% of Qt for PAAC and M-PAAC, respectively. The different adsorption capacities of PAAC and M-PAAC due to ion exchange with metal ions might have been caused by pyrolysis during modification. After modification, the Qp contribution increased from 11.12 to 14.14 mg/g, while the Qp contribution percentage decreased from 50.40 to 40.41%. Qp was the major mechanism for Cd(II) adsorption by PAAC and M-PAAC, which was in agreement with the adsorption of Cd(II) on HCl-treated biochar.[11] In addition, the contribution of Qπ was 0.86 and 1.57 mg/g for PAAC and M-PAAC, respectively. Qπ had the lowest contribution among the four mechanisms, accounting for 3.90 and 4.49% of Qt, respectively. The Qf adsorption capacity of M-PAAC (11.76 mg/g) was higher than that of PAAC (3.16 mg/g), with a contribution increase from 14.32 to 33.60%, which revealed the role of oxygen-containing functional groups in Cd(II) adsorption by M-PAAC. The introduction of oxygen-containing functional groups was corroborated by the results of XPS and Boehm’s titration. According to quantitative calculations, ion exchange with metal ions and precipitation with minerals were the main factors influencing the Cd(II) adsorption on PAAC, while the adsorption of M-PAAC was dominated by precipitation with minerals and complexation with oxygen-containing functional groups.

Application and Environmental Implications

Activated carbon modified with mannitol at low temperatures improved the Cd(II) adsorption capacity. The improved adsorption performance by modification was mainly realized by introducing oxygen-containing groups. For Cd(II) removal by M-PAAC, the contribution percentages of complexation and precipitation mechanisms were 33.60 and 40.41%, respectively. Among the four main mechanisms, Cd(II) adsorbed by ion exchange was easily desorbed from activated carbon, resulting in secondary pollution.[60] The contribution of ion exchange decreased from 31.37 to 21.50% after modification, indicating that the adsorption of Cd(II) by M-PAAC was more stable. The recovery performance of activated carbon determines its application potential in the environment. As shown in Figure S5, the Cd(II) absorption capacity of PAAC and M-PAAC decreased during each cycle. After five cycles, the Cd(II) adsorption capacity of PAAC and M-PAAC were 62.02 and 76.62% of the initial adsorption capacity, respectively, which showed that PAAC and M-PAAC can be used as reusable adsorbents to remove Cd(II) from aqueous solution. The raw material was P. australis, which is widely used in wetlands and harvested in winter to prevent secondary eutrophication pollution. The method used in this study can convert waste biomass into activated carbon with excellent Cd(II) adsorption performance. This utilizes waste biomass, and the product can also be used to stably remove Cd(II) from wastewater.

Conclusions

This work demonstrated a new approach for the use of modified activated carbon for the adsorptive removal of Cd(II). The modification process was performed at a low temperature (150 °C). The modified activated carbon (M-PAAC) possessed a lower SBET than the original activated carbon (PAAC), but M-PAAC contained more acidic surface functional groups and displayed a higher adsorption capacity than PAAC. The adsorption isotherms were described well with the Temkin model, while the intraparticle diffusion model fitted the Cd(II) adsorption kinetics best. The rate-controlling steps of Cd(II) adsorption were external mass transfer and intraparticle diffusion. According to the contribution of the four different mechanisms, ion exchange with metal ions (31.37%) and precipitation with minerals (50.40%) were the main mechanisms that influenced Cd(II) adsorption on PAAC. Adsorption by M-PAAC was dominated by precipitation with minerals (40.41%) and complexation with oxygen-containing functional groups (33.60%). The results indicate the feasibility of using pyrophosphoric acid-activated carbon from P. australis modified with mannitol for removing Cd(II) from aqueous solutions.