Abstract

Activated mineral adsorbent (AMA) was prepared via double salts (Na2SO4 and CaCO3) heat treatment activation of solid-state potassium feldspar. Adsorption performance of AMA for Cd(II) and Pb(II) was investigated by batch mode and factors affecting adsorption including pH value, initial concentration of adsorbate, contact time, adsorbent dosage and temperature on adsorption performance for Cd(II) and Pb(II) were studied. The results indicated that the adsorption process was pH dependent, endothermic and spontaneous. When the adsorption process of Cd(II) and Pb(II) on AMA reached equilibrium, the maximum saturated adsorption capacities were 263.16 and 303.03 mg/g for Cd(II) and Pb(II) ions, respectively, showing higher adsorption removal efficiency. The Langmuir adsorption isotherm and pseudo second kinetic equation could well fit the adsorption process of Cd(II) and Pb(II) by AMA. Besides, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were also performed to further reveal the adsorption mechanism. The results indicated that ion exchange, precipitation and adsorption played an important role in adsorption process. From the investigation, it was concluded that AMA was an excellent adsorbent with the advantages of environment-friendly, inexpensive, facile preparation and higher adsorption capacity of toxic Cd(II) and Pb(II) ions.

HIGHLIGHTS

  • An activated mineral adsorbent (AMA) was prepared via double salts heat treatment activation of a solid-state potassium feldspar.

  • The maximum saturated adsorption capacities were 263.16 and 303.03 mg/g for Cd(II) and Pb(II) ions, respectively.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

With the rapid development of modern industry, heavy metals have caught extensive attention due to their toxicity, carcinogenicity, and non-biocompatibility in aqueous solution (Tan et al. 2012). Unfortunately, they can be accumulated in organisms through the food chain and eventually cause a tremendous threat to human health (Ozay et al. 2009; Hu et al. 2011; Yan et al. 2012). Among the heavy metals, Pb and Cd are nonessential elements for living organisms and could lead to a long-term risk to ecological and human health even in parts per billion (ppb or μg/L) (Agrafioti et al. 2014; Christou et al. 2017; Xiao et al. 2017). They are usually found in wastewater from industrial activities including mining, metal smelting, leather tanning, electroplating, and petrochemical production (Parab et al. 2010). A survey of the concentration of heavy metals in wastewater, which is the majority of industrial wastewater and groundwater pollution, indicated that the concentration of Pb and Cd was approximately 50–200 mg/L (Yang et al. 2014). The World Health Organization (WHO) recommended guidelines for drinking water are 0.05 mg/L for Pb and 0.005 mg/L for Cd, respectively. The urgent problem motivates the scientific community to study various methods to remove Pb and Cd from wastewater in efficient and economically practicable approaches.

So far, various methods, such as ion exchange (Mahmoud & Hoadley 2012), chemical precipitation (Ali et al. 2013), filtration (Cotte et al. 2015), electrochemical deposition (Venkatasubramanian et al. 2011), membrane separation (Cheng et al. 2010; Gao et al. 2014), and flotation (Mahmoud et al. 2015; Taseidifar et al. 2017) have been investigated for heavy metals removal from wastewater. However, many of these strategies present several disadvantages, such as high cost, high reagent and energy requirements, generation of a large amount of sludge and secondary pollution (Fu & Wang 2011; Purkayastha et al. 2014). Compared with other methods, adsorption, particularly using low-cost absorbents, has been considered to be the promising one because of its simple operation, economic efficiency and low cost (He et al. 2016a, 2016b, 2017a). That being the case, an adsorbent with excellent adsorption performance has a significant role in the adsorption method. Moreover, as the environmental safety of new materials has been paid more and more attention, environmentally friendly adsorbent has become the mainstream (Zhao et al. 2015; Lessa et al. 2018).

Clay minerals have been recognized as the materials of ‘greening the 21st century material world’ owing to possessing excellent physicochemical properties such as environmental friendliness, biocompatibility, biodegradability and lamellar structure (Zarghami et al. 2016). Many publications have focused on the adsorption performance of secondary clay minerals, such as bentonite, biotite, chlorite, montmorillonite, lennilite and kaolinite, which are mainly the transitional weathering product of the primary clay minerals (Soratto & Crusciol 2008; Al-Jabri 2010; Khan et al. 2012; Mangwandi et al. 2014; Abad-Valle et al. 2016; Harja et al. 2016), while primary clay minerals have seldom been studied. Due to the primary clay minerals being rich in reserves and cheap, they are naturally more available than secondary clay minerals. Besides, China is abundant in primary clay mineral resources, which can provide vast mineral materials for the remediation of heavy metal pollution. Potassium feldspar, a primary clay mineral, has a curved double chain structure connected by Si-O tetrahedron and Al-O tetrahedron, which is extremely stable (Guo et al. 2015). The application of potassium feldspar is limited due to its inherent structure. Therefore, some modification methods should be taken to improve the porous structure and adsorption capacity of potassium feldspar. It is reported that common modification methods including heat treatment, hydrothermal method, organic modification, inorganic pillared, acid treatment, polymer intercalation and complex modification have been applied to modify the primary clay minerals. Cao et al. (2004) modified the sepiolite by heat treatment and acid treatment. It was found that the specific surface area, ion exchange capacity and adsorption performance of modified sepiolite were highly improved.

In previous studies, researchers commonly chose one kind of additives heat treatment, and the adsorption performance of modified clay mineral is limited. The double salts heat treatment is adopted to modify to activate the K and Si in potassium feldspar, which contributes to improving its structure and adsorption performance. Na2SO4 and CaCO3 are selected as double salts. Na2SO4 with a low melting point (884 °C) is used as a co-solvent to lower the melting temperature of the system, which can break the stable structure of Si-Al-O bond in potassium feldspar (Kumar et al. 2018). At the same time, the structure of potassium feldspar is changed from order to disorder. With the goal of enhancing homeomorphism, CaCO3 is selected to enhance the activity of K, Al and Si in potassium feldspar, which can make it more prone to ion exchange, substitution and replacement and increase its replacement capacity. Moreover, the melting point of CaCO3 and potassium feldspar is 1,300 °C. In order to make CaCO3 fully melt activated potassium feldspar in the melting state and give consideration to energy saving, this paper selected the roasting temperature at 1,300 °C and the roasting time at 1 h (Zhu 2003; Han & Cao 2004).

In this study, an activated mineral adsorbent (AMA) was prepared via double salts (Na2SO4 and CaCO3) heat treatment activation and applied to the Cd(II) and Pb(II) removal. The effect of adsorbent dosage, initial pH value, initial concentration of metal ions, adsorption temperature and contact time on the adsorption efficiency of AMA was investigated by batch experiments. Adsorption kinetics, isotherm and thermodynamics were also studied in detail. In addition, the interaction mechanism of Cd(II) and Pb(II) with AMA was also systematically studied.

MATERIALS AND METHODS

Chemicals and reagents

The potassium feldspar (SiO2:67wt%) was obtained from Tonghua, Jilin province, China. After grinding and sieving with 100-mesh, the potassium feldspar was oven-dried at 80 °C before use. Na2SO4, CaCO3, Cd(NO3)2·4H2O, Pb(NO3)2, HNO3, HCl, NaOH and other chemicals were purchased from Honghua Reagent Co. Ltd (Changsha, China). All chemicals and reagents were analytical grade except for HNO3, which was guaranteed grade. The standard stock solutions of Cd(II) and Pb(II) (500 mg/L) were prepared by dissolving Cd(NO3)2·4H2O and Pb(NO3)2 in distilled water. Initial concentrations of heavy metals for adsorption were prepared by diluting the standard stock solution. The initial pH of heavy metal solution was adjusted with 0.01M HCl or NaOH as required.

Preparation of AMA

To prepare the AMA with different Ca/Si molar ratios of potassium feldspar, calcium carbonate and sodium sulfate were accurately weighed. Then the mixtures were evenly mixed and calcinated for 1 h at different temperatures in a muffle furnace. Finally, the resultant products were cooled down to room temperature and stored in polyethylene bags. The specific weighing quality of feldspar, calcium carbonate and sodium sulfate and calcination conditions are shown in Table 1.

Table 1

The specific weighing quality of feldspar, calcium carbonate and sodium sulfate and calcination conditions of activated mineral adsorbents

NumberPotassium feldspar (g)Calcium carbonate (g)Sodium sulfate (g)Ca/Si molar ratiosCalcination temperature (°C)Holding time (h)
AMA-1 100 179 1.6 1,250 
AMA-2 100 201 1.8 1,250 
AMA-3 100 220 2.0 1,250 
AMA-4 100 179 1.6 1,300 
AMA-5 100 201 1.8 1,300 
AMA-6 100 220 2.0 1,300 
AMA-7 100 179 1.6 1,350 
AMA-8 100 201 1.8 1,350 
AMA-9 100 220 2.0 1,350 
AMA-10 100 167 1.5 1,250 
AMA-11 100 167 1.5 1,300 
AMA-12 100 167 1.5 1,350 
AMA-13 100 234 2.1 1,250 
AMA-14 100 234 2.1 1,300 
AMA-15 100 234 2.1 1,350 
NumberPotassium feldspar (g)Calcium carbonate (g)Sodium sulfate (g)Ca/Si molar ratiosCalcination temperature (°C)Holding time (h)
AMA-1 100 179 1.6 1,250 
AMA-2 100 201 1.8 1,250 
AMA-3 100 220 2.0 1,250 
AMA-4 100 179 1.6 1,300 
AMA-5 100 201 1.8 1,300 
AMA-6 100 220 2.0 1,300 
AMA-7 100 179 1.6 1,350 
AMA-8 100 201 1.8 1,350 
AMA-9 100 220 2.0 1,350 
AMA-10 100 167 1.5 1,250 
AMA-11 100 167 1.5 1,300 
AMA-12 100 167 1.5 1,350 
AMA-13 100 234 2.1 1,250 
AMA-14 100 234 2.1 1,300 
AMA-15 100 234 2.1 1,350 

Where Ca/Si is the molar ratio of calcium in calcium carbonate to silicon in potash feldspar.

Characterization of AMA

The surface morphologies of AMA before and after Cd(II) and Pb(II) adsorption were carried out on the scanning electron microscopy (SEM) measurement (JSM-6360LV, Japan). The specific surface area of AMA was determined by N2 adsorption/desorption isotherm at 77 K on a surface area and porosity analyzer (Gemini VII 2390, USA). Particle size of AMA was measured by laser particle size analyzer (Mastersizer 3000E, UK). The functional groups of AMA before and after Cd(II) and Pb(II) adsorption were determined by Fourier transform infrared (FT-IR) spectroscopy (Nicolet 6700, USA) in the range of 4,000–400 cm−1 using the KBr pellet technique. The surface chemical composition and binding energy change of AMA before and after Cd(II) and Pb(II) adsorption were confirmed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, USA). The crystal structure of AMA before and after Cd(II) and Pb(II) adsorption was analyzed by X-ray diffraction (XRD) analysis using an automated diffractometer (Bruker D8 Venture, Germany) with monochromatic Cu Kα radiation and the wavelength of 1.5406 Å. The Zeta potential of AMA was measured at different pH using a Zeta potential analyzer (JS94H, China).

Batch adsorption experiments

Accurately weighed 0.02 g AMA was added into 50 mL Erlenmeyer flask with 40 mL of Cd(II) or Pb(II) solution in batch adsorption experiments. The flasks were shaken in a temperature-controlled shaker (SHA-B, China) with shaking speed of 130 rpm for 16 h until the adsorption equilibrium was reached. The residual concentration of Cd(II) or Pb(II) was determined by the atomic absorption spectrometer (361MC, China).

The effect of adsorbent dosage on the adsorption capacity and removal efficiency of AMA was studied at adsorbent dosage from 0.25 g/L to 2.0 g/L. The effect of initial solution pH was investigated in the range of 2–8 for Cd(II) and Pb(II). The adsorption kinetic experiments were studied from 5 to 120 min for Cd(II) and Pb(II) solutions. The adsorption isotherm experiments of Cd(II) and Pb(II) were conducted with initial Cd(II) and Pb(II) concentrations varying from 25 to 300 mg/L. The adsorption thermodynamic experiments of Cd(II) and Pb(II) were carried out at four different temperatures, including 288, 298, 308 and 318 K. The adsorption capacity and removal efficiency of Cd(II) or Pb(II) on the AMA were calculated by the following equations.
formula
(1)
formula
(2)
where C0 and Ce (mg/L) are the initial and equilibrium concentration of metal ions, respectively. V (L) is the volume of adsorbate solution and m is the mass of the AMA.

RESULTS AND DISCUSSION

Different adsorbents on Cd(II) and Pb(II) removal

0.02 g AMA prepared at different calcination conditions was added to 40 mL Cd(II) or Pb(II) solutions (100 mg/L) to compare with the adsorption capacity of different AMA. The adsorption capacities of Cd(II) or Pb(II) on different AMA are displayed in Figure 1. At the Ca/Si molar ratio of 1.8, the AMA-2, AMA-5 and AMA-8 showed higher adsorption capacity toward Cd(II) than other AMA. It can be obviously observed that the AMA-5 showed the highest adsorption capacity toward both Cd(II) and Pb(II). Therefore, the AMA-5 was chosen for the characterization analysis and adsorption study.

Figure 1

The adsorption capacity of Cd(II)/Pb(II) on different adsorbents (adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, pH = 6, T = 298 K, Time = 16 h).

Figure 1

The adsorption capacity of Cd(II)/Pb(II) on different adsorbents (adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, pH = 6, T = 298 K, Time = 16 h).

Characterization of AMA

The morphological and structural properties of the materials were investigated by SEM. Potassium feldspar (Figure 2) is a complete particle with large particle size and smooth surface. Figure 3 shows the AMA has a porous lamellar structure with a small pore diameter. More channels inside the AMA might be opened due to the double salts heat treatment activation in the preparation process, which increased the specific surface area and pore volume of the clay mineral. Besides, with the increase of calcination temperature and Ca/Si molar ratio, the size of the clay mineral was more uniformly distributed.

Figure 2

The SEM images of the potassium feldspar.

Figure 2

The SEM images of the potassium feldspar.

Figure 3

The SEM images of (a) AMA-1, (b) AMA-2, (c) AMA-3, (d) AMA-4, (e) AMA-5, (f) AMA-6, (g) AMA-7, (h) AMA-8 and (i) AMA-9.

Figure 3

The SEM images of (a) AMA-1, (b) AMA-2, (c) AMA-3, (d) AMA-4, (e) AMA-5, (f) AMA-6, (g) AMA-7, (h) AMA-8 and (i) AMA-9.

The physicochemical properties of potassium feldspar and AMA are also listed in Table 2. Potassium feldspar has an average particle size of about 68.3 μm. With the increase of calcination temperature, the average particle size of AMA turns to smaller size. So, the dispersion of AMA was enhanced and it is easier to spread, which is beneficial to adsorption. It was found that Brunauer–Emmett–Teller (BET) surface area and average pore diameter of potassium feldspar was 2.29 m2/g and 11.54 nm, respectively. BET surface area and pore diameter of AMA were all more than that of potassium feldspar. Among them, AMA-5 presented the highest surface area (12.77 m2/g) and biggest pore diameter (14.97 nm). This may be due to the thermal activation of potassium feldspar, which opened up some small closed pores and produced CO2 from carbonate decomposition, causing AMA to form a porous structure (Murnandari et al. 2017). High surface area and elevated values of pore diameter for AMA are an excellent indicator that the adsorbent will be efficient for the removal of Cd(II) and Pb(II).

Table 2

The physicochemical characteristics of potassium feldspar and AMA

MaterialBET surface area (m2/g)Average pore diameter (nm)Average particle size (μm)
Potassium feldspar 2.29 11.54 68.3 
AMA-1 7.12 11.72 19.4 
AMA-2 10.25 12.21 19.8 
AMA-3 7.32 10.33 19.2 
AMA-4 7.23 10.51 18.5 
AMA-5 12.77 14.97 18.8 
AMA-6 8.27 10.89 18.4 
AMA-7 8.25 10.11 17.3 
AMA-8 9.56 11.75 17.7 
AMA-9 7.83 10.88 17.6 
MaterialBET surface area (m2/g)Average pore diameter (nm)Average particle size (μm)
Potassium feldspar 2.29 11.54 68.3 
AMA-1 7.12 11.72 19.4 
AMA-2 10.25 12.21 19.8 
AMA-3 7.32 10.33 19.2 
AMA-4 7.23 10.51 18.5 
AMA-5 12.77 14.97 18.8 
AMA-6 8.27 10.89 18.4 
AMA-7 8.25 10.11 17.3 
AMA-8 9.56 11.75 17.7 
AMA-9 7.83 10.88 17.6 

Adsorption study

The effect of adsorbent dosage on Cd(II) and Pb(II) adsorption

The adsorbent dosage is an important parameter which influences removal of metal ions from aqueous solution; thus, different amounts of AMA-5 were added individually to 40 mL Cd(II) or Pb(II) solutions (100 mg/L). Figure 4 displays the effect of AMA-5 dosage in the range of 0.25 to 2.0 g/L on the adsorption capacity and removal efficiency of Cd(II) or Pb(II). The adsorption capacity diminished with the increase of adsorbent dosage, while the removal efficiency presented the opposite tendency, which might be due to more quantity of the adsorption active sites at higher adsorbent dosage. When the equilibrium state was reached, the removal efficiency would not see any significant change even with the continuously rising adsorbent dosage. Meanwhile, the removal efficiency reached 99.87% and 99.64% for Cd(II) and Pb(II), respectively. When the adsorbent dosage was 0.5 g/L, both the adsorption capacity and removal efficiency were relatively high for adsorption Cd(II) and Pb(II) on AMA-5. To ensure the optimization of cost benefits, we selected 0.5 g/L as the optimum adsorption dosage in the following experiments.

Figure 4

The effect of adsorbent dosage on the Cd(II) and Pb(II) adsorption (C0 = 100 mg/L, pH = 6, T = 298 K, Time = 16 h).

Figure 4

The effect of adsorbent dosage on the Cd(II) and Pb(II) adsorption (C0 = 100 mg/L, pH = 6, T = 298 K, Time = 16 h).

The effect of initial solution pH on Cd(II) and Pb(II) adsorption

The effects of initial pH are on not only the species of heavy metal ions but also the surface charge of the adsorbent. Figure 5(a) shows the adsorption capacity of Cd(II) and Pb(II) on AMA-5 at solution pH of 2–8. It was obvious that the adsorption capacity of Cd(II) and Pb(II) on AMA-5 increased as the pH increased, and eventually reached equilibrium. The influence of pH on Cd(II) and Pb(II) removal was due to the electrostatic interaction and distribution of metal ions species (Yang et al. 2007). At low pH, H+ can compete with the positive metal ions on the AMA-5 surface sites resulting in low adsorption efficiency. With the pH value increased, the competitive adsorption was weakened for the decrease of H+ and the electrostatic between AMA-5 and Cd(II) and Pb(II) was enhanced, thus improving the adsorption capacity of Cd(II) and Pb(II) in acid conditions (Yan et al. 2014). However, as the pH value increased to weak basic conditions, Cd and Pb species were changed with the pH of the solution. The distribution of Cd and Pb species as a function of pH at 100 mg/L was calculated by Visual MINTED and is depicted in Figure 5(b) and 5(c).

Figure 5

(a) The effect of initial solution pH on the Cd(II) and Pb(II) adsorption(adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, T = 298 K, Time = 16 h). (b) Distribution of cadmium species as a function of pH calculated by Visual MINTED. (c) Distribution of lead species as a function of pH calculated by Visual MINTED.

Figure 5

(a) The effect of initial solution pH on the Cd(II) and Pb(II) adsorption(adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, T = 298 K, Time = 16 h). (b) Distribution of cadmium species as a function of pH calculated by Visual MINTED. (c) Distribution of lead species as a function of pH calculated by Visual MINTED.

For the distribution of Cd species in Figure 5(b), at pH <6, the predominant species was Cd2+. At pH 6–12, the main species were Cd(OH)+, Cd2(OH)3+ and Cd(OH)2. At pH >12, the main cadmium species that existed were Cd(OH)3 and Cd(OH)42−. The precipitation constant of Cd(OH)2(s) was 2.5 × 10−14 (298 K) and Cd began to form precipitation at pH 8.42. For the distribution of Pb species in Figure 5(c), at pH <6, the predominant species was Pb2+. The main species at pH 7–12 were Pb(OH)+, Pb(OH)2, Pb2(OH)3, Pb3(OH)4+2 and Pb4(OH)4+4. At pH range of 12–14, the lead species existing was Pb(OH)3. The precipitation constant of Pb(OH)2(s) was 1.2 × 10−15 (298 K) and Pb began to form precipitation at pH 8.20 (Xu et al. 2008). The formation of hydroxide complexes of Cd(II) and Pb(II) could affect the adsorbent effectiveness. Therefore, the solution pH was maintained at 7.0 in all the following experiments to ensure the optimum adsorption property as well as to avoid the precipitation of Cd(II) and Pb(II).

The effect of contact time and adsorption kinetics

The effect of contact time was studied to evaluate the adsorption behavior of Cd(II) and Pb(II) on AMA-5 and the results are presented in Figure 6(a). A rapid adsorption rate was observed for Cd(II) and Pb(II) in the first 2 h due to a large number of available adsorption sites on AMA-5 surfaces. The adsorption equilibrium was achieved after reaction for 7 h. The adsorption capacity by AMA-5 was in order of Pb(II) > Cd(II), which indicated that in a single heavy metal system, AMA-5 showed higher removal efficiency of Pb(II) with adsorption capacity of 196.9 mg/g at the initial Pb(II) concentration of 100 mg/L, while a lower removal efficiency of Cd(II) was obtained with adsorption capacity of 186.3 mg/g.

Figure 6

(a) The effect of contact time on the Cd(II) and Pb(II) adsorption on AMA-5 (adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, pH = 7, T = 298 K). (b) Linear fit of pseudo-first-order model of kinetic data. (c) Linear fit of pseudo-second-order model of kinetic data. (d) Intra-particle diffusion model plot of kinetic data.

Figure 6

(a) The effect of contact time on the Cd(II) and Pb(II) adsorption on AMA-5 (adsorbent dosage = 0.5 g/L, C0 = 100 mg/L, pH = 7, T = 298 K). (b) Linear fit of pseudo-first-order model of kinetic data. (c) Linear fit of pseudo-second-order model of kinetic data. (d) Intra-particle diffusion model plot of kinetic data.

Adsorption kinetics is commonly applied to estimate the adsorption rate and offer some valuable information about the adsorption process. Hence, the experimental adsorption kinetic data were analyzed by using the pseudo-first-order and pseudo-second-order model. The equation of the pseudo-first-order model is given as follows (Lagergren 1898):
formula
(3)
By contrast, the pseudo-second-order model is represented by the following (Ho & McKay 1999):
formula
(4)
where Qt(mg/g) is the adsorption capacity at time t(min), Qe is the adsorption capacity at equilibrium and k1(1/min) and k2(g/(mg·min)) are the pseudo-first-order and pseudo-second-order kinetic constant, respectively. The linear fitting curves are displayed in Figure 6(b) and 6(c) and the relevant fitting parameters are listed in Table 3. The correlation coefficient (R2) values of Cd(II) and Pb(II) in pseudo-second-order were 0.9983 and 0.9982, respectively, which were much higher than those of pseudo-first-order. Moreover, the Qe calculated from pseudo-second-order showed a good agreement with the experimental Qe values, which illustrated that the adsorption process favored a pseudo-second-order process.
To better understand the adsorption rate controlling steps, the Weber Morris intraparticle diffusion model was used to fit the experimental data (Chabani et al. 2007; Chaari et al. 2015). The intra-particle diffusion model is expressed by the following equation.
formula
(5)
where kpi is the rate constant and Ci is the constant obtained from the intercept and reflects the thickness of the boundary layer. In general, the larger the intercept, the thicker the boundary layer effect.
Table 3

Constant and correlation coefficients for the kinetic model of Cd(II) and Pb(II) on AMA-5

Heavy metalsQe,expPseudo-first-order
Pseudo-second-order
(mg/g)k1 (1/min)Qe,cal (mg/g)R2k2*10−4 (g/mg·min)Qe,cal (mg/g)R2
Cd(II) 186.37 0.0069 115.32 0.9890 1.4003 192.31 0.9983 
Pb(II) 196.97 0.0096 144.07 0.9843 1.5610 200.00 0.9982 
Heavy metalsQe,expPseudo-first-order
Pseudo-second-order
(mg/g)k1 (1/min)Qe,cal (mg/g)R2k2*10−4 (g/mg·min)Qe,cal (mg/g)R2
Cd(II) 186.37 0.0069 115.32 0.9890 1.4003 192.31 0.9983 
Pb(II) 196.97 0.0096 144.07 0.9843 1.5610 200.00 0.9982 
Table 4

Intra-particle diffusion model parameters of adsorption Cd(II) and Pb(II) on AMA-5

Heavy metalsFilm diffusion
Intra-particle diffusion
Equilibrium stage
kp1C1R2kp2C2R2kp3C3R2
Cd(II) 9.758 35.68 0.9773 5.085 84.34 0.9957 0.870 163.1 0.9596 
Pb(II) 9.846 48.50 0.9793 5.045 98.097 0.9806 0.6002 181.4 0.9797 
Heavy metalsFilm diffusion
Intra-particle diffusion
Equilibrium stage
kp1C1R2kp2C2R2kp3C3R2
Cd(II) 9.758 35.68 0.9773 5.085 84.34 0.9957 0.870 163.1 0.9596 
Pb(II) 9.846 48.50 0.9793 5.045 98.097 0.9806 0.6002 181.4 0.9797 

The fitting curves of intra-particle diffusion model are depicted in Figure 6(d) and the relevant fitting parameters are listed in Table 4. The plots obtained from the adsorption kinetic data did not yield a straight line, which suggested that the intra-particle diffusion was not the only rate-controlling step. The fitting curves exhibited three distinct regions. The first linear region could be attributed to the transport of the heavy metals from the solution to the surface of AMA-5, which was related to boundary layer diffusion. The second linear region was the gradual adsorption step and the intra-particle diffusion was the rate-control step. The third linear region was ascribed to the final adsorption equilibrium where a very low concentration of heavy metals remained in the solution. Therefore, the adsorption of Cd(II)/Pb(II) on AMA-5 was controlled by both boundary layer diffusion and intra-particle diffusion.

The effect of initial adsorbate concentration and adsorption isotherm

The effect of initial concentration on Cd(II) and Pb(II) on AMA-5 is depicted in Figure 7. It can be observed that in the same adsorption temperature, the adsorbent capacity of AMA-5 for Cd(II) and Pb(II) increases with increasing initial concentration as the concentrations of heavy metal ions were less than 100 mg/L. Once beyond 100 mg/L, the increase of equilibrium adsorption capacity slowed down. The adsorption capacity of AMA-5 for Cd(II) and Pb(II) also increased with adsorption temperature, implying the adsorption was an endothermic reaction and higher adsorption temperature would benefit the adsorption process.

Figure 7

The adsorption isotherms of AMA-5 for Cd(II) and Pb(II) (adsorbent dosage = 0.5 g/L, pH = 7, T = 288∼318 K, Time = 16 h).

Figure 7

The adsorption isotherms of AMA-5 for Cd(II) and Pb(II) (adsorbent dosage = 0.5 g/L, pH = 7, T = 288∼318 K, Time = 16 h).

In order to understand the adsorption mechanism, two commonly used adsorption isotherm models named Langmuir and Freundlich were used in this study for adsorption isotherm data analysis. The Langmuir isotherm model proposes monolayer adsorption on a homogeneous surface and the adsorbates have no interaction with each other, which can be expressed as follows (Farghali et al. 2013):
formula
(6)
The Freundlich isotherm model assumes multilayer adsorption with a heterogeneous distribution of active sites, which could be represented as follows (Ho & McKay 1999):
formula
(7)
where Qe(mg/g) and Qm(mg/g) are the equilibrium adsorption ability and the maximum adsorption capacity, respectively, KL is the Langmuir constant and KF and n are the Freundlich constant related to the adsorption capacity and intensity, respectively.

The fitting plots of Langmuir and Freundlich isotherm models are presented in Figure 9 and the related fitting parameters are listed in Table 5. As shown in Figure 8, the experimental adsorption data of Cd(II) and Pb(II) on AMA-5 fitted well with the Langmuir adsorption model and the plots showed quite good linearity. The correlation coefficients R2 of Langmuir model were much larger than that of Freundlich model, which indicated that the Langmuir model was more suitable for describing the adsorption of Cd(II) and Pb(II) on AMA-5. The maximum adsorption capacities calculated by Langmuir model were 263.16 mg/g for Cd(II) and 303.03 mg/g for Pb(II) at 318 K.

Table 5

Isotherm parameters obtained from the Langmuir model and Freundlich model of Cd(II) and Pb(II) on AMA-5 at different temperatures

Metal ionT (K)Langmuir model
Freundlich model
Qm (mg/g)kL (L/mg)R2kF (mg/g (L/mg)1/n)nR2
Cd(II) 288 232.56 0.1401 0.9986 57.9743 3.4388 0.7666 
298 243.90 0.1653 0.9973 71.1863 3.7875 0.6715 
308 250.00 0.2381 0.9981 81.9574 4.1667 0.7278 
318 263.16 0.3220 0.9975 97.7292 4.6147 0.8715 
Pb(II) 288 277.28 0.3186 0.9978 99.2955 4.4883 0.8831 
298 285.71 0.4023 0.9989 101.1495 4.2662 0.8481 
308 294.18 0.5574 0.9984 122.9650 4.9092 0.9163 
318 303.03 0.7333 0.9985 140.1341 5.3967 0.9122 
Metal ionT (K)Langmuir model
Freundlich model
Qm (mg/g)kL (L/mg)R2kF (mg/g (L/mg)1/n)nR2
Cd(II) 288 232.56 0.1401 0.9986 57.9743 3.4388 0.7666 
298 243.90 0.1653 0.9973 71.1863 3.7875 0.6715 
308 250.00 0.2381 0.9981 81.9574 4.1667 0.7278 
318 263.16 0.3220 0.9975 97.7292 4.6147 0.8715 
Pb(II) 288 277.28 0.3186 0.9978 99.2955 4.4883 0.8831 
298 285.71 0.4023 0.9989 101.1495 4.2662 0.8481 
308 294.18 0.5574 0.9984 122.9650 4.9092 0.9163 
318 303.03 0.7333 0.9985 140.1341 5.3967 0.9122 
Figure 8

Linear fit of Langmuir isotherm model of AMA-5 for (a) Cd(II) and (c) Pb(II). Linear fit of Freundlich isotherm model of AMA-5 for (b) Cd(II) and (d) Pb(II).

Figure 8

Linear fit of Langmuir isotherm model of AMA-5 for (a) Cd(II) and (c) Pb(II). Linear fit of Freundlich isotherm model of AMA-5 for (b) Cd(II) and (d) Pb(II).

Figure 9

InKd vs 1/T for Cd(II) and Pb(II) on AMA-5.

Figure 9

InKd vs 1/T for Cd(II) and Pb(II) on AMA-5.

Based on the Langmuir adsorption isotherm model, a dimensionless separation constant RL could be applied to determine the favorability of the adsorption process, which is expressed as the following equation:
formula
(8)
where C0 (mg/L) is the initial concentration of adsorbate and b (L/mg) is the Langmuir constant. The value of RL ranges from 0 to 1, which indicates the adsorption process is favorable, once the value of RL is greater than 1, implying the unfavorability of the adsorption process. In this study, the RL values were calculated to be 0.0232–0.2221, 0.0198–0.1948, 0.0138–0.1438 and 0.0102–0.1105 for Cd(II) and 0.0104–0.1115, 0.0082–0.0904, 0.0059–0.0670 and 0.0045–0.0517 for Pb(II) with initial concentration ranging from 25–300 mg/L at 288, 298, 308, and 318 K, demonstrating that favorable process of AMA-5 for Cd(II) and Pb(II).

The removal capacities of AMA-5 for Cd(II) and Pb(II) were compared with other presented adsorbents to illustrate the excellent adsorption performance of AMA-5 (Table 6). It can be seen that AMA-5 could efficiently remove Cd(II) and Pb(II) compared with other adsorbents published in the previous literature.

Table 6

Comparison of the maximum adsorption capacity of Cd(II) and Pb(II) on AMA with various adsorbents published in literature

AdsorbentsQm(mg/g)
Reference
Cd(II)Pb(II)
Chitosan coated cotton fibers 14.14 86.09 Zhang et al. (2008)  
Magnetic graphene oxide/LDH 45.05 192.31 Huang et al. (2018)  
Activated carbon-chitosan complex (2:1) 69.4 125.4 Ge & Fan (2011)  
Fe3O4/cyclodextrin 27.7 64.5 Badruddoza et al. (2013)  
P-MCS 71.53 151.06 Zhao et al. (2017)  
Activated carbon 15.70 21.80 Rao et al. (2009)  
β-CD polymer 163.20 215.20 He et al. (2017b)  
potash feldspar 10.56 12.34 Saha et al. (2003)  
AMA-5 243.90 294.18 This study 
AdsorbentsQm(mg/g)
Reference
Cd(II)Pb(II)
Chitosan coated cotton fibers 14.14 86.09 Zhang et al. (2008)  
Magnetic graphene oxide/LDH 45.05 192.31 Huang et al. (2018)  
Activated carbon-chitosan complex (2:1) 69.4 125.4 Ge & Fan (2011)  
Fe3O4/cyclodextrin 27.7 64.5 Badruddoza et al. (2013)  
P-MCS 71.53 151.06 Zhao et al. (2017)  
Activated carbon 15.70 21.80 Rao et al. (2009)  
β-CD polymer 163.20 215.20 He et al. (2017b)  
potash feldspar 10.56 12.34 Saha et al. (2003)  
AMA-5 243.90 294.18 This study 

The effect of adsorption temperatures and adsorption thermodynamics

The effect of adsorption temperature on the adsorption of Cd(II) and Pb(II) on AMA-5 is shown in Figure 9. It can be obviously observed that the adsorption capacity of Cd(II) and Pb(II) on AMA-5 increased with the increasing of adsorption temperature. To investigate whether the adsorption process of Cd(II) and Pb(II) on AMA-5 was endothermic or exothermic, spontaneous or nonspontaneous, the thermodynamic parameters including standard free energy change (ΔG0), standard enthalpy change (ΔH0) and standard entropy change (ΔS0) were calculated from the following equations (Ghosh & Bhattacharyya 2002):
formula
(9)
formula
(10)
formula
(11)
where R (8.314 J/mol·K) is the universal gas constant and T is the adsorption temperature in Kelvin; Kd is the distribution coefficient. The linear regression of InKd versus 1/T for adsorption of Cd(II) and Pb(II) on AMA-5 at 288, 298, 308, and 318 K are given in Figure 10. By fitting each linear regression, we could calculate the value of ΔS0 and ΔH0 as shown in Table 7. The positive ΔH0 values suggested that the adsorption of Cd(II) and Pb(II) on AMA-5 was endothermic in nature. The negative ΔG0 values suggested that the adsorption of Cd(II) and Pb(II) on AMA-5 was feasible and spontaneous and higher adsorption temperature was beneficial for adsorption of Cd(II) and Pb(II) (Xie et al. 2015). The ΔS0 values indicated the increased randomness at the solid–solution interface (Yu et al. 2015).

Adsorption mechanism

In order to explore the mechanisms of metal sorption by AMA, FT-IR, XRD and XPS analysis were performed. The FT-IR spectra were recorded and the results are shown in Figure 10. The relatively intense and broad peaks at around 3,457 cm−1 corresponded to the -OH stretching bond, whereas the peak at 3,664 cm−1 was attributed to the intermolecular hydrogen -OH stretching bond. The peaks at 1,409 cm−1 and 1,460 cm−1 were assigned to the symmetric and asymmetric vibrations of carboxylic bonds, relatively. The strong absorbance peak appeared at 1,648 cm−1 owing to the stretching vibration of C = O. The peaks at 607 cm−1 were the bending vibration of Si-O. The peaks observed at 832 cm−1 and 1,111 cm−1 were correlated to the symmetric and asymmetric vibrations of Si-O-Si, respectively. The absorbance peaks at 983 cm−1 and 507 cm−1 were ascribed to the stretching vibration and bending vibration of Al-O, respectively. The carboxylate could donate the H+ and OH due to the presence of carbonyl and hydroxyl groups, and ligands were formed between metal ions and carboxylic groups by substituting H+ with metal ions; the bands of carboxyl groups shift when adsorption was over. Moreover, the -OH bond and Si-O-Si bond disappeared after adsorption, which indicated the involvement in metal adsorption. These changes revealed that the carboxyl, -OH and Si-O-Si are involved in formation of bonds with heavy metal ions.

Table 7

Thermodynamics parameters for adsorption of Cd(II) and Pb(II) on AMA-5

Heavy metalT (K)ΔG0 (kJ/mol)ΔH0 (kJ/mol)ΔS0 (J/mol)
 288 −6.67 27.36 118.15 
Cd(II) 298 −7.85 
 308 −9.03 
 318 −10.21 
 288 −7.81   
Pb(II) 298 −9.63 44.44 181.44 
 308 −11.44   
 318 −13.25   
Heavy metalT (K)ΔG0 (kJ/mol)ΔH0 (kJ/mol)ΔS0 (J/mol)
 288 −6.67 27.36 118.15 
Cd(II) 298 −7.85 
 308 −9.03 
 318 −10.21 
 288 −7.81   
Pb(II) 298 −9.63 44.44 181.44 
 308 −11.44   
 318 −13.25   
Figure 10

The FT-IR images of the AMA-5 and after adsorption of Cd(II) and Pb(II).

Figure 10

The FT-IR images of the AMA-5 and after adsorption of Cd(II) and Pb(II).

The XRD patterns of AMA-5 before and after adsorption of Cd(II) or Pb(II) are shown in Figure 11(a) and 11(b). The characteristic peaks of calcium silicate lead occurred, suggesting that it was the predominant lead silicate generated after adsorption of Pb by AMA-5. Cadmium silicate was also found in the residual AMA-5 solids after Cd adsorption. The XPS spectra of AMA-5 and Cd(II) or Pb(II) loaded AMA-5 are shown in Figure 12. The XPS spectra showed that the main peaks corresponding to O 1 s, Ca 2p, C 1 s, and Si 2p were around 529.8, 346.3, 285.3, and 102.7 eV, respectively. It can be seen that Cd(II) or Pb(II) loaded AMA-5 showed new strong peaks compared with AMA-5 spectra, thanks to Cd 3d peaks (Figure 12(b)) and Pb 4f (Figure 12(c)). The binding energies of 405.51, 412.03, 138.56 and 143.38 eV were assigned to Cd 3d5/2, Cd 3d3/2, Pb 4f7/2 and Pb 4f5/2, respectively, indicating that Cd and Pb had been adsorbed on the AMA-5 surface. Therefore, the component elements analyzed by the XPS spectrum were consistent with the XRD survey.

Figure 11

The XRD patterns of AMA-5 before and after adsorption of Cd(II) (a) and Pb(II) (b).

Figure 11

The XRD patterns of AMA-5 before and after adsorption of Cd(II) (a) and Pb(II) (b).

Figure 12

XPS wide scan showing surface elemental composition of AMA-5, Cd-AMA-5 and Pb-AMA-5 (a), high-resolution XPS spectra of Cd 3d (b) and Pb 4f (c) of AMA-5 after Cd2+ or Pb2+ adsorption.

Figure 12

XPS wide scan showing surface elemental composition of AMA-5, Cd-AMA-5 and Pb-AMA-5 (a), high-resolution XPS spectra of Cd 3d (b) and Pb 4f (c) of AMA-5 after Cd2+ or Pb2+ adsorption.

Based on the results obtained in the above investigation, the high adsorption capacity of Cd(II) and Pb(II) on AMA can be summarized as follows:

After activation, a mass of the potassium ions could be removed and replaced by exchangeable calcium ions, meanwhile has a large specific surface area. Therefore, AMA shows strong ion exchange ability to remove Cd2+ and Pb2+, which usually occurs in the surface, interlaminar domain and pore channels of AMA, and can be represented by the following equation:
formula
(12)
As the main components of AMA, Ca2A12Si7 and 2CaO·SiO2 are partially hydrolyzed after entering aqueous solution, and the reactions in Equations (13) and (14) promote the increase of OH in aqueous solution. Meanwhile, insoluble compounds such as aluminosilicate of cadmium/lead or hydroxide precipitated.
formula
(13)
formula
(14)
formula
(15)
formula
(16)
Cd2+ and Pb2+, on the other hand, cannot exist in the pH <4.0 (Duan & Su 2014). While the AMA in acidic conditions still showed a greater Cd2+ and Pb2+ removal ability, suggesting that AMA in acid environment conditions showed strong chemical adsorption, the adsorption process is as follows:
formula
(17)

This process is mainly attributed to the surface of AMA silanol sites (≡Si-OH) and aluminols (≡Al-OH) deionization (Arancibia-Miranda et al. 2014). In a word, removal of metal ions by AMA occurs mainly by cation exchange mechanism, followed by precipitation and adsorption.

CONCLUSION

In this work, AMA (at Ca/Si molar ratio of 1.8) was synthesized successfully via heat treatment at 1,300 °C, it contains potassium feldspar, Na2SO4 and CaCO3, which was a new efficient adsorbent for the removal of Cd(II) and Pb(II) from aqueous solution. The dosing quantity of AMA, the initial pH, contact time, and initial metal concentration were found to be important for the removal of Cd(II) and Pb(II). Furthermore, Cd(II) and Pb(II) adsorption thermodynamic parameters ΔG < 0, ΔH > 0 and ΔS > 0 indicated that the removal process was spontaneous with heat adsorption and entropy increase. The adsorption equilibrium was attained at 7 h and the maximum adsorption capacity of Cd(II) and Pb(II) was 303.03 and 263.16 mg/g at 318 K, respectively, showing outstanding removal efficiency. The Langmuir equation could well describe the isothermal adsorption of AMA on Cd(II) and Pb(II) solution and the adsorption mechanism obeys the quasi-second-order kinetic equation, and the adsorption rate constants are 0.3220 and 0.7333, respectively. The results of FT-IR, XRD, and XPS analysis showed the precipitation as calcium silicate lead occurred and cadmium silicate. These results showed that AMA was an effective and alternative adsorbent for the removal of Cd(II) and Pb(II) ions from aquatic ecosystems.

ACKNOWLEDGEMENTS

This work was financially supported by the Postgraduate Independent Exploration and Innovation Project of Central South University, China (No. 2020zzts413).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

REFERENCES

Abadvalle
P.
Alvarezayuso
E.
Murciego
A.
Pellitero
E.
2016
Assessment of the use of sepiolite amendment to restore heavy metal polluted mine soil
.
Geoderma
280
,
57
66
.
Ali
H.
Khan
E.
Sajad
M. A.
2013
Phytoremediation of heavy metals – concepts and applications
.
Chemosphere
91
(
7
),
869
881
.
Al-Jabri
M.
2010
The utilizing of zeolite minerals as agriculture soil conditioner in relation to its standarization and increasing food crop
.
Jurnal Zeolit Indonesia
9
(
1
),
1
12
.
Arancibia-Miranda
N.
Escudey
M.
Pizarro
C.
Denardin
J. C.
García-González
M. T.
Fabris
J. D.
Charlet
L.
2014
Preparation and characterization of a single-walled aluminosilicate nanotube-iron oxide composite: its applications to removal of aqueous arsenate
.
Materials Research Bulletin
51
,
145
152
.
Badruddoza
A. Z. M.
Shawon
Z. B. Z.
Tay
W. J. D.
Hidajat
K.
Uddin
M. S.
2013
Fe3o4/cyclodextrin polymer nanocomposites for selective heavy metals removal from industrial wastewater
.
Carbohydrate Polymers
91
(
1
),
322
332
.
Cao
X.
Ma
L. Q.
Rhue
D. R.
Appel
C. S.
2004
Mechanisms of lead, copper, and zinc retention by phosphate rock
.
Environmental Pollution
131
(
3
),
435
444
.
Chaari
I.
Moussi
B.
Jamoussi
F.
2015
Interactions of the dye, CI direct orange with natural clay
.
Journal of Alloys and Compounds
647
,
720
727
.
Chabani
M.
Amrane
A.
Bensmaili
A.
2007
Kinetics of nitrates adsorption on Amberlite IRA 400 resin
.
Desalination
206
(
1–3
),
560
567
.
Christou
A.
Theologides
C. P.
Costa
C. N.
Kalavrouziotis
I. K.
Varnavas
S. P.
2017
Assessment of toxic heavy metals concentrations in soils and wild and cultivated plant species in Limni abandoned copper mining site, Cyprus
.
Journal of Geochemical Exploration
178
,
16
22
.
Cotte
L.
Waeles
M.
Pernet-Coudrier
B.
Sarradin
P. M.
Cathalot
C.
Riso
R. D.
2015
A comparison of in situ vs. ex situ filtration methods on the assessment of dissolved and particulate metals at hydrothermal vents
.
Deep Sea Research Part I: Oceanographic Research Papers
105
,
186
194
.
Farghali
A. A.
Bahgat
M.
El Rouby
W. M. A.
Khedr
M. H.
2013
Preparation, decoration and characterization of graphene sheets for methyl green adsorption
.
Journal of Alloys and Compounds
555
,
193
200
.
Fu
F.
Wang
Q.
2011
Removal of heavy metal ions from wastewaters: a review
.
Journal of Environmental Management
92
(
3
),
407
418
.
Ge
H.
Fan
X.
2011
Adsorption of Pb2+ and Cd2+ onto a novel activated carbon-chitosan complex
.
Chemical Engineering and Technology
34
(
10
),
1745
1752
.
Ghosh
D.
Bhattacharyya
K. G.
2002
Adsorption of methylene blue on kaolinite
.
Applied Clay Science
20
(
6
),
295
300
.
Guo
Y.
Li
C.
Lu
S. X.
Zhao
C.
2015
K2CO3-Modified potassium feldspar for CO2 capture from post-combustion flue gas
.
Energy and Fuels
29
(
12
),
8151
8156
.
Han
Z. H.
Cao
L.
2004
Preparation and characterization of caconanowires
.
Journal of Materials Ence
44
(
5
),
1198
1205
.
Harja
M.
Cimpeanu
S. M.
Dirja
M.
Bucur
D.
2016
Synthesis of Zeolite From Fly Ash and Their Use as Soil Amendment
.
Zeolites: Useful Minerals
,
Claudia Belviso, IntechOpen
, p.
43
.
He
J.
Cai
X.
Chen
K.
Li
Y.
Zhang
K.
Jin
Z.
Huang
X.
2016a
Performance of a novelly-defined zirconium metal-organic frameworks adsorption membrane in fluoride removal
.
Journal of Colloid and Interface Science
484
,
162
172
.
He
J.
Zhang
K.
Wu
S.
Cai
X.
Chen
K.
Li
Y.
Kong
L.
2016b
Performance of novel hydroxyapatite nanowires in treatment of fluoride contaminated water
.
Journal of Hazardous Materials
303
,
119
130
.
He
J.
Chen
K.
Cai
X.
Li
Y.
Wang
C.
Zhang
K.
Liu
J.
2017a
A biocompatible and novelly-defined Al-HAP adsorption membrane for highly effective removal of fluoride from drinking water
.
Journal of Colloid and Interface Science
490
,
97
107
.
He
J.
Li
Y.
Wang
C.
Zhang
K.
Lin
D.
Kong
L.
Liu
J.
2017b
Rapid adsorption of Pb, Cu and Cd from aqueous solutions by β-cyclodextrin polymers
.
Applied Surface Science
426
,
29
39
.
Ho
Y.
McKay
G.
1999
Pseudo-second order model for sorption processes
.
Process Biochemistry
34
(
5
),
451
465
.
Kumar
A.
Marcolli
C.
Luo
B. P.
Peter
T.
2018
Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – part 1: the k-feldspar microcline
.
Atmospheric Chemistry and Physics
18
,
7057
7079
.
Lagergren
S. K.
1898
About the theory of so-called adsorption of soluble substances
.
Kungliga Svenska Vetenskapsakademiens Handlingar
24
,
1
39
.
Mahmoud
M. R.
Lazaridis
N. K.
Matis
K. A.
2015
Study of flotation conditions for cadmium(II) removal from aqueous solutions
.
Process Safety and Environmental Protection
94
,
203
211
.
Mangwandi
C.
Albadarin
A. B.
JiangTao
L.
Allen
S.
Walker
G. M.
2014
Development of a value-added soil conditioner from high shear co-granulation of organic waste and limestone powder
.
Powder Technology
252
,
33
41
.
Murnandari
A.
Kang
J.
Youn
M. H.
Park
K. T.
Kim
H. J.
Kang
S. P.
Jeong
S. K.
2017
Effect of process parameters on the CaCO3 production in the single process for carbon capture and mineralization
.
The Korean Journal of Chemical Engineering
34
(
3
),
935
941
.
Ozay
O.
Ekici
S.
Baran
Y.
Aktas
N.
Sahiner
N.
2009
Removal of toxic metal ions with magnetic hydrogels
.
Water Research
43
(
17
),
4403
4411
.
Parab
H.
Joshi
S.
Sudersanan
M.
Shenoy
N.
Lali
A.
Sarma
U.
2010
Removal and recovery of cobalt from aqueous solutions by adsorption using low cost lignocellulosic biomass – coir pith
.
Journal of Environmental Science and Health Part A
45
(
5
),
603
611
.
Purkayastha
D.
Mishra
U.
Biswas
S.
2014
A comprehensive review on Cd(II) removal from aqueous solution
.
Journal of Water Process Engineering
2
,
105
128
.
Rao
M. M.
Ramana
D. K.
Seshaiah
K.
Wang
M. C.
Chien
S. C.
2009
Removal of some metal ions by activated carbon prepared from Phaseolus aureus hulls
.
Journal of Hazardous Materials
166
(
2–3
),
1006
1013
.
Taseidifar
M.
Makavipour
F.
Pashley
R. M.
Rahman
A. M.
2017
Removal of heavy metal ions from water using ion flotation
.
Environmental Technology and Innovation
8
,
182
190
.
Venkatasubramanian
R.
Jin
K. J.
Noshir
S. P.
2011
Use of electrochemical deposition to create randomly rough surfaces and roughness gradients
.
Langmuir
27.7
,
3261
3265
.
Xiao
R.
Wang
S.
Li
R.
Wang
J. J.
Zhang
Z.
2017
Soil heavy metal contamination and health risks associated with artisanal gold mining in Tongguan, Shaanxi, China
.
Ecotoxicology and Environmental Safety
141
,
17
24
.
Xie
M.
Zeng
L.
Zhang
Q.
Kang
Y.
Xiao
H.
Peng
Y.
Luo
J.
2015
Synthesis and adsorption behavior of magnetic microspheres based on chitosan/organicrectorite for low-concentration heavy metal removal
.
Journal of Alloys and Compounds
647
,
892
905
.
Xu
D.
Tan
X.
Chen
C.
Wang
X.
2008
Removal of Pb(II) from aqueous solution by oxidized multiwalled carbon nanotubes
.
Journal of Hazardous Materials
154
(
1–3
),
407
416
.
Yan
Y.
Dong
X.
Sun
X.
Sun
X.
Li
J.
Shen
J.
Wang
L.
2014
Conversion of waste FGD gypsum into hydroxyapatite for removal of Pb2+ and Cd2+ from wastewater
.
Journal of Colloid and Interface Science
429
,
68
76
.
Yang
H.
Yan
R.
Chen
H.
Lee D
H.
Zheng
C.
2007
Characteristics of hemicellulose, cellulose and lignin pyrolysis
.
Fuel
86
(
12–13
),
1781
1788
.
Yang
J.
Lei
M.
Chen
T.
Gao
D.
Zheng
G.
Guo
G.
Lee
D.
2014
Current status and developing trends of the contents of heavy metals in sewage sludges in China
.
Frontiers of Environmental Science and Engineering in China
8
(
5
),
719
728
.
Yu
S.
Zhai
L.
Wang
Y.
Liu
X.
Xu
L.
Cheng
L.
2015
Synthesis of magnetic chrysotile nanotubes for adsorption of Pb(II), Cd(II) and Cr(III) ions from aqueous solution
.
Journal of Environmental Chemical Engineering
3
(
2
),
752
762
.
Zhang
G.
Qu
R.
Sun
C.
Ji
C.
Chen
H.
Wang
C.
Niu
Y.
2008
Adsorption for metal ions of chitosan coated cotton fiber
.
Journal of Applied Polymer Science
110
(
4
),
2321
2327
.
Zhao
F.
Repo
E.
Yin
D.
Meng
Y.
Jafari
S.
Sillanpaa
M.
2015
EDTA-cross-linked β-cyclodextrin: an environmentally friendly bifunctional adsorbent for simultaneous adsorption of metals and cationic dyes
.
Environmental Science and Technology
49
(
17
),
10570
10580
.
Zhao
Q.
Zhao
H.
Yan
L.
Bi
M.
Li
Y.
Zhou
Y.
Jiang
T.
2017
Efficient removal of Pb(II) from aqueous solution by CoFe2O4/graphene oxide nanocomposite: kinetic, isotherm and thermodynamic
.
Journal of Nanoscience and Nanotechnology
17
(
6
),
3951
3958
.