# Adsorption of Nitrate onto ZnCl2-Modified Coconut Granular Activated Carbon: Kinetics, Characteristics, and Adsorption Dynamics

Authors: Lingjie Liu; Min Ji; Fen Wang
Journal: Advances in Materials Science and Engineering
 (2018)

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## Abstract

Coconut granular activated carbon (CGAC) was modified by impregnating with ZnCl2 solution to remove nitrate from aqueous solutions. Sorption isotherm and kinetic studies were carried out in a series of batch experiments. Nitrate adsorption of both ZnCl2-modified CGAC and CGAC fitted the Langmuir and Freundlich models. Batch adsorption isotherms indicated that the maximum adsorption capacities of ZnCl2-modified CGAC and CGAC were calculated as 14.01 mgN·g−1 and 0.28 mgN·g−1, respectively. The kinetic data obtained from batch experiments were well described by pseudo-second-order model. The column study was used to analyze the dynamic adsorption process. The highest bed adsorption capacity of 1.76 mgN·g−1 was obtained by 50 mgN·L−1 inlet nitrate concentration, 20 g adsorbents, and 10 ml·min−1 flow rate. The dynamic adsorption data were fitted well to the Thomas and Yoon–Nelson models with coefficients of correlation R2 > 0.834 at different conditions. Surface characteristics and pore structures of CGAC and ZnCl2-modified CGAC were performed by SEM and EDAX and BET and indicated that ZnCl2 had adhered to the surface of GAC after modified. Zeta potential, Raman spectra, and FTIR suggested the electrostatic attraction between the nitrate ions and positive charge. The results revealed that the mechanism of adsorption nitrate mainly depended on electrostatic attraction almost without any chemical interactions.


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## Body

## 1. Introduction

Nitrate is a serious contaminant, which is mainly from agriculture, municipal, and industrial wastewaters [1]. Some conventional techniques are used to remove nitrate from water including biological processes (nitrification, denitrification) [2], zero-valent iron (Fe0) [3, 4] or magnesium (Mg0) [5], electrokinetic denitrification [6], reverse osmosis [7], catalytic denitrification [8], ion exchange [9, 10], and chemical reduction [11].Among several technologies for nitrate removal, adsorption process was widely used because of its convenience, low consumption of energy, and ease of operation. Adsorption, in general, is the removal process of soluble substances that are in solution on a suitable interface [1]. Activated carbon has gained wide attention as an efficient adsorbent which can adsorb various pollutants in aquatic phase especially organic pollutants [12]. However, it shows poor adsorption towards anionic pollutants. As reported in Table 1, most of the papers found in the literature review were devoted to the nitrate adsorption with modified and various synthesis materials.Table 1
Literature information on various adsorbents for nitrate removal from water.
Adsorbent
Contact time
q
m (mg·g−1)
Reference
Untreated coconut granular activated carbon
2 h
1.7
[13]
ZnCl2-treated coconut granular activated carbon
2 h
10.2
[13]
Commercial activated carbon modified by C13H42NBr
120 min
21.51
[12]
Modified anion exchange resin (Dowex 21K XLT) by iron
180 min
333.47
[14]
Modified sugarcane bagasse biochar
60 min
28.21
[15]
Sugar beet bagasse by chemical activation
—
9.14–27.55
[16]
Amine-grafted corn cob/coconut copra
24 h
220.99/261.29
[17]
Pilot-scale-produced wheat/cotton stalk resins
60 min
43.88/33.35
[18]
Lab-scale-produced wheat/cotton stalk resins
60 min
50.24/39.15
[18]
ALR-AE resin
10 min
44.61
[19]
Carbon-silicon nano/bulk composites
60 min
11.343/5.495
[20]
Polyethylene glycol/chitosan
40 min
50.68
[21]
Polyvinyl alcohol/chitosan
40 min
35.03
[21]
Scrap tire chips
3 d
2.92
[22]
Anion exchange resin (Dowex 21K XLT)
180 min
122.23
[14]
New polymeric resin modified with amino and quaternary ammonium groups
24 h
221.8
[23]
Cross-linked organic-inorganic hybrid biocomposites (chitosan/bentonite)
180 min
35.68
[24]
Cross-linked organic-inorganic hybrid biocomposites (chitosan/titanium oxide)
180 min
43.62
[24]
Cross-linked organic-inorganic hybrid biocomposites (chitosan/alumina)
180 min
45.38
[24]
Mg-Al-Cl hydrotalcite-like compound
40 min
19.12
[25]In addition, studies showed the nitrate adsorption due to the different mechanisms [26–30]. The mechanism of nitrate adsorption by magnetic amine-cross-linked biopolymer includes electrostatic attraction, ion exchange, and surface complex formation [27]. Soybean was modified with calcium chloride and hydrochloric acid and by calcination, and results confirmed that the mechanism of nitrate adsorption was mainly ion exchange [29]. Nitrate was adsorbed by modified biochar (amine-cross-linked reaction that amine group played the main role in nitrate uptake) depending on the electrostatic attraction mechanism [28, 30]. In addition, nitrate was adsorbed by magnetic multiwalled carbon nanotubes depending on the magnetic reaction to the desired separation [26].Although studies have examined nitrate adsorption by modified activated carbon using ZnCl2, little research examined nitrate adsorption mechanisms by modified activated carbon and the adsorption dynamics. Therefore, the main objectives are as follows: (1) nitrate adsorption properties by ZnCl2-modified coconut granular activated carbon (ZnCl2-modified CGAC), including the kinetics and isotherms, in batch experiments; (2) physicochemical properties confirmed by SEM, BET, DFT, zeta potentials, and Raman spectra to fully understand the adsorption mechanisms; and (3) column adsorption experiments with different parameters, including adsorbent bed depth, initial concentration and flow rate, and data analysis from column study using Thomas and Yoon–Nelson models.


## 2. Materials and Methods

### 2.1. Materials and Specimen Preparation

Coconut granular activated carbon (CGAC) was gained from Huansheng Company, Henan Province, China. It was sieved through a 2 mm sieve, washed with deionized water, and dried at room temperature for 48 h for future use. CGAC was mixed with ZnCl2 solution. In this study, 200% impregnation ratio was used, which meant that 10 g coconut granular activated carbon was added into the ZnCl2 solution containing 20 g ZnCl2 and 100 ml deionized water. The mixture was stirred for 1 h at 80°C. Then, the sample was dried in an oven for 24 h at 106°C. The resulting sample was placed in the furnace and carbonized at 500°C for 1 h. The product was washed with 0.5 M HCl sequentially and then washed by deionized water repeatedly until the pH of the solution reached about 5.5. After that, the samples were dried at 106°C and stored in a desiccator for further use. All the chemicals including ZnCl2, KNO3, and HCl were of analytical grade. The nitrate stock solutions (100 mgN·L−1) were prepared by dissolving 0.7218 g KNO3 in 1000 ml deionized water.


### 2.2. Batch Experiments

Batch experiments were carried out at different NO3−-N concentrations (5, 10, 15, 20, 50, 100 mgN·L−1). For adsorption equilibrium studies, experiments were conducted in 500 ml conical flasks containing 2.0 g of adsorbent, 200 ml nitrate solution, and 0.01 M NaCl to keep ion strength at 20°C. The pH was not adjusted during the experiments. The mixtures were shaken at 170 rpm for 12 h and then filtered using a 0.22 μm membrane filter. The concentration of residual NO3−-N was measured by ion chromatograph (Thermo Scientific Aquion IC 1100). The amount of NO3−-N adsorbed by per mass unit of adsorbent was calculated by the following equation:(1)qe=C0−CeVW,where qe (mgN·g−1) is the adsorbent capacity, C0 and Ce (mgN·L−1) are the concentrations of NO3−-N at initial and at equilibrium, respectively, V is the volume of the solution (L), and W is the mass of adsorbent used (g).For batch kinetic studies, the same procedure was followed, but the aqueous samples were taken at preset time intervals. The NO3−-N uptake at any time, qt (mgN·g−1), was calculated by the following equation:(2)qt=C0−CtVW,where Ct (mgN·L−1) is the NO3−-N concentration at any time.


### 2.3. Characterization

Surface properties of CGAC and ZnCl2-modified CGAC were observed through a scanning electron microscope (SEM) (HITACHI s4800). The distribution of elements on the surface or in the pores of carbon particles was determined by the same SEM together with energy dispersive X-ray spectroscopy.Pore structure characteristics of CGAC and ZnCl2-modified CGAC were determined by nitrogen adsorption at 77 K (Quantachrome Autosorb iQ2). Prior to gas adsorption measurements, the carbon was degassed at 170°C in a vacuum condition for a period of at least 10 h. The BET surface area was determined by application of the Brunauer–Emmett–Teller (BET) equation [16]. The pore size distributions of the CGAC and ZnCl2-modified CGAC were determined by the density functional theory (DFT) method [16].Zeta potential measurements of CGAC, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC were carried out by the microelectrophoresis (Malvern Zetasizer Nano ZS). The samples were grounded in an agate mortar and sieved through a 74 μm sieve. 100 mg of powder samples was mixed with 1 L deionized water by ultrasonic cleaner (KQ-300DE, China) for 10 min at 20°C. After that, the samples were settled for 30 min. The suspension was collected to determine the zeta potential at several pH values from 2.0 to 12.0 using 0.01 M of NaOH and HCl to adjust pH. The pHPZC was the corresponding pH value when the zeta potential was 0 mV.To make an intensive study on the interaction mechanisms of nitrate onto ZnCl2-modified CGAC, Raman spectroscopic analysis was performed. In the Raman analysis, 0.1 g ZnCl2-modified CGAC was placed in 50 ml of nitrate solution with concentration of 0.5 mol·L−1. The pure solid samples of KNO3, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC were analyzed by Raman spectroscopy (DXR Microscope). The laser wavelength used in Raman measurement was 1050 nm.FTIR spectroscopy (Thermo Nicolet 6700 Spectrometer, USA) was done to identify the chemical functional groups presented on ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid samples of KNO3. The saturated samples after adsorption were prepared by mixing the ZnCl2-modified CGAC (2 g) with solution (200 ml) containing 500 mgN·L−1 of nitrate. Samples of particle size <45 μm were first dried for 24 h at a temperature of 383 K. The dried samples were mixed with finely divided KBr at a ratio of 1 : 100. The spectrum was scanned from 400 to 4000 cm−1.


### 2.4. Column Studies

In column experiment, a glass column (30 cm height and 2.3 cm inner diameter) was filled with ZnCl2-modified CGAC on glass wool support. In a typical experiment, a synthetic NO3−-N solution was fed into the column from the top at a desired flow rate using a peristaltic pump. A known quantity of the prepared ZnCl2-modified CGAC was packed in the column to yield the desired bed height of the adsorbent 39 mm, 78 mm, 117 mm (equivalent to 10 g, 20 g, and 30 g of ZnCl2-modified CGAC) at flow rate of 10 ml·min−1 and initial NO3−-N concentration of 20 mgN·L−1. The effect of NO3−-N concentration on the adsorption capacity was studied using initial NO3−-N concentrations of 10, 20, and 50 mgN·L−1 with column flow rate of 10 ml·min−1 and 20 g ZnCl2-modified CGAC. The effect of different flow rates on the adsorption capacity was studied at 5, 10, and 20 ml·min−1 with 20 mgN·L−1 initial NO3−-N concentration and 20 g adsorbents. Samples were collected from the bottom of the column at regular time intervals and analyzed for residual nitrate concentrations. The studies were conducted at room temperature (20 ± 2°C), and natural pH of solutions was about 6.5. The flow of the column was continued until the effluent concentration (Ct) approached the influent concentration (C0), Ct/C0=0.95.The equilibrium NO3−-N uptake per unit mass of adsorbent (q0) was calculated by the following equation:(3)Vtotal=Qttotal.The value of the total mass of NO3−-N adsorbed, qtotal (mg), could be calculated from the area under the breakthrough curve:(4)qtotal=∫0VtotalC0−CedV,where Vtotal is the effluent volume at equilibrium, Q is the volumetric flow rate (ml·min−1), and ttotal is the total flow time [28, 31].Equilibrium metal uptake or maximum capacity of the column,qeq (mgN·g−1), in the column was calculated by the following equation:(5)qeq=qtotalm,where m is the mass of the adsorbent (g).Total amount of NO3−-N ion entering column (mtotal) was calculated by the following equation [32]:(6)mtotal=C0Qttotal1000.


## 2.1. Materials and Specimen Preparation

Coconut granular activated carbon (CGAC) was gained from Huansheng Company, Henan Province, China. It was sieved through a 2 mm sieve, washed with deionized water, and dried at room temperature for 48 h for future use. CGAC was mixed with ZnCl2 solution. In this study, 200% impregnation ratio was used, which meant that 10 g coconut granular activated carbon was added into the ZnCl2 solution containing 20 g ZnCl2 and 100 ml deionized water. The mixture was stirred for 1 h at 80°C. Then, the sample was dried in an oven for 24 h at 106°C. The resulting sample was placed in the furnace and carbonized at 500°C for 1 h. The product was washed with 0.5 M HCl sequentially and then washed by deionized water repeatedly until the pH of the solution reached about 5.5. After that, the samples were dried at 106°C and stored in a desiccator for further use. All the chemicals including ZnCl2, KNO3, and HCl were of analytical grade. The nitrate stock solutions (100 mgN·L−1) were prepared by dissolving 0.7218 g KNO3 in 1000 ml deionized water.


## 2.2. Batch Experiments

Batch experiments were carried out at different NO3−-N concentrations (5, 10, 15, 20, 50, 100 mgN·L−1). For adsorption equilibrium studies, experiments were conducted in 500 ml conical flasks containing 2.0 g of adsorbent, 200 ml nitrate solution, and 0.01 M NaCl to keep ion strength at 20°C. The pH was not adjusted during the experiments. The mixtures were shaken at 170 rpm for 12 h and then filtered using a 0.22 μm membrane filter. The concentration of residual NO3−-N was measured by ion chromatograph (Thermo Scientific Aquion IC 1100). The amount of NO3−-N adsorbed by per mass unit of adsorbent was calculated by the following equation:(1)qe=C0−CeVW,where qe (mgN·g−1) is the adsorbent capacity, C0 and Ce (mgN·L−1) are the concentrations of NO3−-N at initial and at equilibrium, respectively, V is the volume of the solution (L), and W is the mass of adsorbent used (g).For batch kinetic studies, the same procedure was followed, but the aqueous samples were taken at preset time intervals. The NO3−-N uptake at any time, qt (mgN·g−1), was calculated by the following equation:(2)qt=C0−CtVW,where Ct (mgN·L−1) is the NO3−-N concentration at any time.


## 2.3. Characterization

Surface properties of CGAC and ZnCl2-modified CGAC were observed through a scanning electron microscope (SEM) (HITACHI s4800). The distribution of elements on the surface or in the pores of carbon particles was determined by the same SEM together with energy dispersive X-ray spectroscopy.Pore structure characteristics of CGAC and ZnCl2-modified CGAC were determined by nitrogen adsorption at 77 K (Quantachrome Autosorb iQ2). Prior to gas adsorption measurements, the carbon was degassed at 170°C in a vacuum condition for a period of at least 10 h. The BET surface area was determined by application of the Brunauer–Emmett–Teller (BET) equation [16]. The pore size distributions of the CGAC and ZnCl2-modified CGAC were determined by the density functional theory (DFT) method [16].Zeta potential measurements of CGAC, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC were carried out by the microelectrophoresis (Malvern Zetasizer Nano ZS). The samples were grounded in an agate mortar and sieved through a 74 μm sieve. 100 mg of powder samples was mixed with 1 L deionized water by ultrasonic cleaner (KQ-300DE, China) for 10 min at 20°C. After that, the samples were settled for 30 min. The suspension was collected to determine the zeta potential at several pH values from 2.0 to 12.0 using 0.01 M of NaOH and HCl to adjust pH. The pHPZC was the corresponding pH value when the zeta potential was 0 mV.To make an intensive study on the interaction mechanisms of nitrate onto ZnCl2-modified CGAC, Raman spectroscopic analysis was performed. In the Raman analysis, 0.1 g ZnCl2-modified CGAC was placed in 50 ml of nitrate solution with concentration of 0.5 mol·L−1. The pure solid samples of KNO3, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC were analyzed by Raman spectroscopy (DXR Microscope). The laser wavelength used in Raman measurement was 1050 nm.FTIR spectroscopy (Thermo Nicolet 6700 Spectrometer, USA) was done to identify the chemical functional groups presented on ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid samples of KNO3. The saturated samples after adsorption were prepared by mixing the ZnCl2-modified CGAC (2 g) with solution (200 ml) containing 500 mgN·L−1 of nitrate. Samples of particle size <45 μm were first dried for 24 h at a temperature of 383 K. The dried samples were mixed with finely divided KBr at a ratio of 1 : 100. The spectrum was scanned from 400 to 4000 cm−1.


## 2.4. Column Studies

In column experiment, a glass column (30 cm height and 2.3 cm inner diameter) was filled with ZnCl2-modified CGAC on glass wool support. In a typical experiment, a synthetic NO3−-N solution was fed into the column from the top at a desired flow rate using a peristaltic pump. A known quantity of the prepared ZnCl2-modified CGAC was packed in the column to yield the desired bed height of the adsorbent 39 mm, 78 mm, 117 mm (equivalent to 10 g, 20 g, and 30 g of ZnCl2-modified CGAC) at flow rate of 10 ml·min−1 and initial NO3−-N concentration of 20 mgN·L−1. The effect of NO3−-N concentration on the adsorption capacity was studied using initial NO3−-N concentrations of 10, 20, and 50 mgN·L−1 with column flow rate of 10 ml·min−1 and 20 g ZnCl2-modified CGAC. The effect of different flow rates on the adsorption capacity was studied at 5, 10, and 20 ml·min−1 with 20 mgN·L−1 initial NO3−-N concentration and 20 g adsorbents. Samples were collected from the bottom of the column at regular time intervals and analyzed for residual nitrate concentrations. The studies were conducted at room temperature (20 ± 2°C), and natural pH of solutions was about 6.5. The flow of the column was continued until the effluent concentration (Ct) approached the influent concentration (C0), Ct/C0=0.95.The equilibrium NO3−-N uptake per unit mass of adsorbent (q0) was calculated by the following equation:(3)Vtotal=Qttotal.The value of the total mass of NO3−-N adsorbed, qtotal (mg), could be calculated from the area under the breakthrough curve:(4)qtotal=∫0VtotalC0−CedV,where Vtotal is the effluent volume at equilibrium, Q is the volumetric flow rate (ml·min−1), and ttotal is the total flow time [28, 31].Equilibrium metal uptake or maximum capacity of the column,qeq (mgN·g−1), in the column was calculated by the following equation:(5)qeq=qtotalm,where m is the mass of the adsorbent (g).Total amount of NO3−-N ion entering column (mtotal) was calculated by the following equation [32]:(6)mtotal=C0Qttotal1000.


## 3. Results and Discussion

### 3.1. Adsorption Kinetics

The adsorption kinetics described the uptake rate of nitrate ion on the ZnCl2-modified CGAC, which controlled the equilibrium time [33]. The kinetic studies were helpful for predicting the adsorption rate with time and explaining the dynamic interactions of nitrate ions with adsorbents, which gave important information for designing and modeling the processes [27, 33]. The experimental data were fitted by three different kinetic models: the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model, and the results were discussed below.The linear equation for pseudo-first-order kinetic model, widely used to predict sorption kinetics, was given by Langergren and Svenska [33], defined as follows:(7)lnqe−q=lnqe−k1t.Pseudo-second-order kinetic model, based on equilibrium adsorption, was expressed as follows:(8)tq=1k2qe2+1qet.Intraparticle diffusion model described by Weber and Morris [34] was widely used to explain the rate-limiting step:(9)qt=kdift+C,where qe and qt (mg·g−1) are the amount of adsorbate adsorbed at equilibrium and at any time, t (min), respectively, and k1 (min−1), k2 (g·mg−1·min−1), and kdif (mgN·(g·min1/2)−1) were the adsorption rate constant of pseudo-first-order model, pseudo-second-order model, and intraparticular diffusion model, respectively.The nitrate adsorption by ZnCl2-modified CGAC increased with the increase in initial nitrate concentration as shown in Figure 1(a), and the linear plots of different kinetic models are shown in Figures 1(b)–1(d). Moreover, kinetic constants of different kinetic models for the nitrate adsorption are shown in Table 2.Figure 1
Linear plots of adsorption kinetics: (a) the variation of adsorption capacity with adsorption time at various initial nitrate concentrations; (b) pseudo-first-order adsorption plot; (c) pseudo-second-order adsorption plot; (d) intraparticle diffusion adsorption plot.
(a)
(b)
(c)
(d)Table 2
Parameters for different adsorption kinetic models.
C
0 (mgN·L−1)
q
e
,
exp (mgN·g−1)
Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
Intraparticle diffusion model
q
e
,
cal (mgN·g−1)
k
1 (h−1)
R
2
q
e
,
cal (mgN·g−1)
k
2 (g·(mgN·h)−1)
R
2
k
dif (mgN·(g·min1/2)−1)
c
R
2
10
0.47
0.33
0.040
0.995
0.51
0.202
0.995
0.044
0.117
0.966
20
0.93
0.46
0.014
0.413
0.72
0.380
0.977
0.056
3.175
0.593
50
1.81
1.64
0.049
0.910
2.04
0.037
0.981
0.190
0.293
0.966
100
2.49
1.31
0.018
0.664
2.32
0.057
0.985
0.184
0.769
0.793The correlation coefficients (R2) for the pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion model were 0.413–0.995, 0.977–0.995, and 0.593–0.966, respectively. Meanwhile, according to the calculated value (qe,cal) and experimental uptake value (qe,exp), the pseudo-second-order model was considered as the best-fit model in describing the nitrate adsorption from aqueous solution [29, 35]. Pseudo-second-order model has been frequently invoked to describe adsorption of inorganic pollutants on GAC-based materials [29], and a similar study reported the nitrate adsorption onto magnetic amine-cross-linked biopolymer fitted with pseudo-second-order model as well [27].


### 3.2. Adsorption Isotherms

The adsorption isotherm was conducted to determine the maximum adsorption capacities and expressed the relationship between the amount of sorption and residual nitrate concentration at equilibrium [36]. The adsorption isotherm indicated how the adsorption molecules distributed between the liquid phase and the solid phase when the adsorption process reached an equilibrium state [37]. In order to optimize the design of an adsorption system, three adsorption isotherms, namely, the Langmuir, Freundlich, and Temkin isotherm models in their linear forms were applied to the equilibrium data to find the suitable model that could be used for design purpose [37]. Figure 2 typically shows the nitrate adsorption isotherms on the ZnCl2-modified CGAC and CGAC. All the correlation coefficient, R2 values, and the parameters obtained for the models are summarized in Table 3. The adsorption data fitted satisfactorily to both Langmuir (R2 = 0.970) and Freundlich (R2 = 0.982) models, better than Temkin model (R2 = 0.828), are shown in Table 3 and Figure 2. Application of the Langmuir model for ZnCl2-modified CGAC and CGAC allowed the determination of the maximum equilibrium adsorption capacity [36], which were 14.01 mgN·g−1 and 0.28 mgN·g−1, respectively. The nitrate adsorption capacity of modified adsorbents was much higher than that of raw adsorbents. The maximum adsorption capacity obtained for ZnCl2-modified CGAC was higher than the corresponding values assumed by others for polyethylene glycol/chitosan and polyvinyl alcohol/chitosan (11.44 mgN·g−1 and 7.91 mgN·g−1) [21]. In addition, the constant 1/n in the range of 0-1 showed the favorable conditions for adsorption [38].Figure 2
Adsorption isotherms for nitrate adsorption onto CGAC and ZnCl2-modified CGAC: (a) experimental points; (b) Langmuir adsorption isotherm plots; (c) Freundlich adsorption isotherm plots; (d) Temkin adsorption isotherm plots.
(a)
(b)
(c)
(d)Table 3
Langmuir, Freundlich, and Temkin model constants for nitrate adsorption.
Adsorbent
LangmuirCe/qe=1/QKL+Ce/Q
Freundlichlnqe=lnKf+1/nlnCe
Temkinqe=BlnKT+BlnCe
Q (mgN·g−1)
K
L (L·mg−1)
R
2
K
f (mgN·g−1)·(L·mg−1)1/n
1
/
n
R
2
K
T (L·mg−1)
B (J·mol−1)
R
2
ZnCl2-modified CGAC
14.01
0.0058
0.970
0.13
0.813
0.982
0.35
1.0055
0.828
CGAC
0.28
0.0294
0.817
0.00851
0.746
0.962
0.24
0.0707
0.745
C
e is the equilibrium concentration (mg·L−1); Q is the monolayer saturation adsorption capacity; KL is the Langmuir constant (L·mg−1); Kf is the adsorbent-adsorbate relative affinity in the adsorption process ((mgN·g−1)·(L·mg−1)1/n); KT is equilibrium blinding constant (L·mg−1); and B is the Temkin constant related to the heat of adsorption.


### 3.3. Characteristics

#### 3.3.1. SEM Analysis

The scanning electron microscopy images of CGAC and ZnCl2-modified CGAC are shown in Figures 3(a) and 3(b), respectively. Compared with the surface of CGAC (Figure 3(a)), ZnCl2-modified CGAC had crystal on the surface (Figure 3(b)). The elemental composition of ZnCl2-impregnated activated carbon determined by EDAX is shown in Table 4. After modified by ZnCl2, the carbon content decreased from 74.75% to 37.61% and the zinc contents and the chloride contents increased to 53.61% and 0.49%, respectively.Figure 3
SEM images of CGAC and ZnCl2-modified CGAC.
(a)
(b)Table 4
EDAX data for CGAC and ZnCl2-modified CGAC.
CGAC
ZnCl2-modified CGAC
Element
Wt.%
Element
Wt.%
C
74.75
C
37.61
O
6.14
O
0.9
Al
3.29
Al
7.38
Mg
15.21
Cl
0.49
Ca
0.61
Zn
53.61


#### 3.3.2. BET Analysis

The surface area and pore characteristics are shown in Table5. The BET specific surface area of CGAC was 876.752 m2·g−1, and the surface area of ZnCl2-modified CGAC decreased to 567.524 m2·g−1. It was obvious that the pore width decreased after impregnation with ZnCl2, from 0.518 nm to 0.492 nm. The decrease of surface area was ascribed to blockage of pore openings by the ZnCl2 that prohibited access of adsorbing gas molecules [29]. This indicated that the ZnCl2 had adhered to the surface of the CGAC and the result was confirmed by the SEM analysis as shown in Figure 3. It had been proved that activated carbon with large amounts of pore structures was inefficient for adsorption capacity [4, 28, 39]. As a result, micropore adsorption was absent for the potential nitrate adsorption by activated carbon [28].Table 5
Pore characteristics of CGAC and ZnCl2-modified CGAC.
Adsorbent
Surface area (m2·g−1)
Pore width (nm)
Pore volume (cm3·g−1)
CGAC
876.752
0.518
0.404
ZnCl2-modified CGAC
567.524
0.492
0.253


#### 3.3.3. Zeta Potentials

Zeta potentials of original activated carbon, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC as a function of pH are shown in Figure 4. All the samples of the zeta potentials became more negative with the increase in pH, probably because of the deposition of more OH− on the adsorbent surface [40]. Zeta potentials of CGAC were in the range of +12.55 to −30.50 mV as the initial pH of the suspensions increased from 2.01 to 11.99. After modified by ZnCl2, zeta potentials of ZnCl2-modified CGAC increased slightly (+14.60 to −29.65 mV) in designed pH range. Point of zero charge pH (pHpzc) of CGAC was located at 2.39. After the ZnCl2 modification, pHpzc of modified GAC had a slight increase to 2.56. This suggested that ZnCl2 loaded on the surface of activated carbon increased positive charge on activated carbon surface. After adsorption, zeta potentials had an apparent decrease at the pH of 5.3∼12, this illustrated that nitrate ions had been adsorbed on the surface of ZnCl2-modified CGAC, and the adsorption mechanism of ZnCl2-modified CGAC for nitrate was based on electrostatic attraction [28].Figure 4
Zeta potentials of CGAC, ZnCl2-modified CGAC, and ZnCl2-modified CGAC after nitrate adsorption.


#### 3.3.4. Raman Spectra

Figure5 shows the Raman spectra of ZnCl2-modified CGAC, pure KNO3, and nitrate-loaded ZnCl2-modified CGAC. Two peaks at 1332.5 cm−1 and 1586.1 cm−1 in the Raman spectrum of ZnCl2-modified CGAC were the characteristic peaks of polycrystalline graphites, namely, G (graphite) band and D (disorder) band, which explicitly appear at about 1580 and 1360 cm−1, respectively. The pure KNO3 crystal had a characteristic peak at 1042.3 cm−1. After the adsorption process, the nitrate on ZnCl2-modified CGAC illustrated the Raman peak at 1039.4 cm−1, which was almost overlapped with the peak of KNO3. The results indicated that nitrate ions were adsorbed onto the surface of ZnCl2-modified CGAC through electrostatic attraction between the free nitrate ions and the positively charged ions of ZnCl2 sites [28], which corresponded well to the decrease of zeta potentials after nitrate adsorption onto ZnCl2-modified CGAC.Figure 5
Raman spectra of CGAC, ZnCl2-modified CGAC, and anion-laden ZnCl2-modified CGAC.


#### 3.3.5. FTIR Spectra

Figure6 shows the FTIR spectra of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3 samples. The FTIR spectra analysis of these samples illustrated the change of functional groups.Figure 6
FTIR pattern of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3.In FTIR spectra of ZnCl2-modified CGAC and nitrate-loaded ZnCl2-modified CGAC, the bands at about 3430 cm−1 were ascribable to ν (O-H) vibrations in hydroxyl groups. The low-frequency values for these bands suggested that the hydroxyl groups were involved in hydrogen bonds and the position of the band due to nonbonded OH groups was usually above 3500 cm−1 for alcohols, phenols, and carboxylic acids [41]. The O-H stretching vibrations occurred within a broad range of frequencies indicating the presence of “free” hydroxyl groups and bonded O-H bands of carboxylic acids. The bands observed at 1800–1000 cm−1 were presumed to be associated with oxygenated C=O and C-O-R structures as the range was reported to reflect the presence of moieties of different C=O (amide, esters, carboxylic acids, quinines, etc.) and C-O-R (aryl and alkyl esters, carboxylic) structures depending on the extent of coalification [42].Nitrate curve in Figure6 and the pure KNO3 crystal had a characteristic peak at 1384.66 cm−1 in FTIR spectra. Compared with the curve of ZnCl2-modified CGAC, a new peak at 1384.66 cm−1 (NO3−) was observed in the curve of nitrate-laden ZnCl2-modified CGAC which was assigned to the special vibration of nitrate. Meanwhile, few special vibrations could be shown in FTIR spectra, so that the main mechanism of adsorption nitrate based on electrostatic attraction almost without any chemical interactions existed.


### 3.4. Column Studies

To investigate the adsorption ability to the continuous nitrate removal from solution onto ZnCl2-modified CGAC, dynamic column adsorption tests and the dynamic models were conducted to evaluate the performance of a continuous system.


#### 3.4.1. Effect of the Column Depth on the Breakthrough Curves

As shown in Figure7(a) and Table 6, when the mass of adsorbent was 10 g, 20 g, and 30 g, the breakthrough points occurred at the time of 200 min, 320 min, and 650 min, respectively. The saturated adsorption capacities of ZnCl2-modified CGAC (10 g, 20 g, and 30 g) in column were about 0.25 mgN·g−1, 0.20 mgN·g−1, and 0.17 mgN·g−1, respectively. The breakthrough time increased with the increase in mass which might be due to the more contact time. The increase in NO3−-N uptake capacity with the increasing bed depth in the column may be due to increased adsorbent surface area, which provided more binding sites for the column adsorption [43, 44].Figure 7
Breakthrough curve: (a) the effect of column depth on NO3−-N adsorption (Q = 10 ml·min−1, C0 = 20 mgN·L−1); (b) the effect of flow rate on NO3−-N adsorption (m = 30 g, C0 = 20 mgN·L−1); (c) the effect of influent concentration on NO3−-N adsorption (m = 30 g, Q = 10 ml·min−1).
(a)
(b)
(c)Table 6
Parameters in fixed-bed column for nitrate adsorption by ZnCl2-modified CGAC.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
t
total (min)
m
total (mg)
q
total (mg)
q
eq (mgN·g−1)
V
eff (ml)
10
10
20
200
40.13
2.52
0.25
2000
20
10
20
320
64.21
4.03
0.20
3200
30
10
20
650
133.63
5.05
0.17
6500
20
20
20
290
112.51
5.38
0.27
5800
20
5
20
520
47.82
3.63
0.18
2600
20
10
10
640
65.03
2.31
0.12
6400
20
10
50
260
129.16
5.21
0.26
2600


#### 3.4.2. Effect of Flow Rates on the Breakthrough Curves

As shown in Figure7(b) and Table 6, the breakthrough curve generally occurred faster with higher flow rate at 20 ml·min−1. With the increases in flow rate, the adsorption capacities were 0.18 mgN·g−1, 0.20 mgN·g−1, and 0.27 mgN·g−1, respectively. As indicated in Figure 7(b), the breakthrough curve became steeper as the flow rate increased. This was due to that, at a high rate of influent, NO3−-N did not have enough time to contact with ZnCl2-modified CGAC [43]. At a low rate of influent, NO3−-N maybe had more time to be in contact with adsorbent [44]. Breakthrough time reaching saturation was increased significantly with a decrease in the flow rate.


#### 3.4.3. Effect of Influent Concentration on the Breakthrough Curves

As shown in Figure7(c) and Table 6, it is illustrated that the adsorption process reached saturation faster and the breakthrough time decreased with increasing influent NO3−-N concentration. The adsorption capacities of ZnCl2-modified CGAC for nitrate were 0.12 mgN·g−1, 0.20 mgN·g−1, and 0.26 mgN·g−1, respectively. The maximum capacity at 50 mgN·L−1 was higher than those at 10 mgN·L−1 and 20 mgN·L−1. This might be attributed to high influent NO3−-N concentration providing higher driving force for the transfer process to overcome the mass transfer resistance [45]. This result indicated that the change of concentration gradient affected the saturation rate and breakthrough time [46, 47].


#### 3.4.4. Application of Thomas Model and Yoon–Nelson Model

Thomas model was widely used to describe the performance of adsorption process in a fixed-bed column. This model assumed plug flow behavior in the bed and used Langmuir isotherm for equilibrium and the second-order reversible kinetics [48]. The model was described by the following equation:(10)lnC0Ct−1=Kthq0mQ−KthC0t,where Kth (L·min−1·mg−1) is the Thomas rate constant, q0 (mgN·g−1) is the maximum sorption capacity, m (g) is the mass of adsorbent, and Q (ml·min−1) is the flow rate.A simple theoretical model developed by Yoon and Nelson was also tested to investigate the breakthrough behavior of nitrate onto ZnCl2-modified CGAC [49, 50]. The linearized model for a single component system was expressed as follows:(11)lnCtC0−Ct=KYNt−τKYN,where KYN (min−1) is the rate constant of model and τ (min) is the time required for 50% adsorbate breakthrough.Among Thomas and Yoon–Nelson models, the parameters listed in Tables7 and 8, both of them provided good fit (R2 > 0.834) to the experimental data at various conditions. In a comparison of values of R2 and breakthrough curves, both Thomas and Yoon–Nelson models could be used to describe the behavior of the nitrate adsorption in a fixed-bed column. The results indicated that the external and internal diffusions would not be the limiting step [44].Table 7
Thomas parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
Th (ml·(min·mgN)−1) × 10−4
q
0 (mgN·g−1)
R
2
10
10
20
22.00
0.24
0.834
20
10
20
6.97
0.94
0.955
30
10
20
3.39
1.55
0.955
20
20
20
12.10
1.15
0.947
20
5
20
4.65
0.94
0.986
20
10
10
7.60
1.06
0.975
20
10
50
4.61
1.79
0.956Table 8
Yoon–Nelson parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
YN  × 10−2 (min−1)
τ (min)
R
2
10
10
20
2.20
24.00
0.833
20
10
20
1.40
93.73
0.955
30
10
20
0.68
232.82
0.955
20
20
20
2.35
59.30
0.947
20
5
20
0.86
203.73
0.986
20
10
10
0.77
208.80
0.975
20
10
50
2.29
72.25
0.956


## 3.1. Adsorption Kinetics

The adsorption kinetics described the uptake rate of nitrate ion on the ZnCl2-modified CGAC, which controlled the equilibrium time [33]. The kinetic studies were helpful for predicting the adsorption rate with time and explaining the dynamic interactions of nitrate ions with adsorbents, which gave important information for designing and modeling the processes [27, 33]. The experimental data were fitted by three different kinetic models: the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model, and the results were discussed below.The linear equation for pseudo-first-order kinetic model, widely used to predict sorption kinetics, was given by Langergren and Svenska [33], defined as follows:(7)lnqe−q=lnqe−k1t.Pseudo-second-order kinetic model, based on equilibrium adsorption, was expressed as follows:(8)tq=1k2qe2+1qet.Intraparticle diffusion model described by Weber and Morris [34] was widely used to explain the rate-limiting step:(9)qt=kdift+C,where qe and qt (mg·g−1) are the amount of adsorbate adsorbed at equilibrium and at any time, t (min), respectively, and k1 (min−1), k2 (g·mg−1·min−1), and kdif (mgN·(g·min1/2)−1) were the adsorption rate constant of pseudo-first-order model, pseudo-second-order model, and intraparticular diffusion model, respectively.The nitrate adsorption by ZnCl2-modified CGAC increased with the increase in initial nitrate concentration as shown in Figure 1(a), and the linear plots of different kinetic models are shown in Figures 1(b)–1(d). Moreover, kinetic constants of different kinetic models for the nitrate adsorption are shown in Table 2.Figure 1
Linear plots of adsorption kinetics: (a) the variation of adsorption capacity with adsorption time at various initial nitrate concentrations; (b) pseudo-first-order adsorption plot; (c) pseudo-second-order adsorption plot; (d) intraparticle diffusion adsorption plot.
(a)
(b)
(c)
(d)Table 2
Parameters for different adsorption kinetic models.
C
0 (mgN·L−1)
q
e
,
exp (mgN·g−1)
Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
Intraparticle diffusion model
q
e
,
cal (mgN·g−1)
k
1 (h−1)
R
2
q
e
,
cal (mgN·g−1)
k
2 (g·(mgN·h)−1)
R
2
k
dif (mgN·(g·min1/2)−1)
c
R
2
10
0.47
0.33
0.040
0.995
0.51
0.202
0.995
0.044
0.117
0.966
20
0.93
0.46
0.014
0.413
0.72
0.380
0.977
0.056
3.175
0.593
50
1.81
1.64
0.049
0.910
2.04
0.037
0.981
0.190
0.293
0.966
100
2.49
1.31
0.018
0.664
2.32
0.057
0.985
0.184
0.769
0.793The correlation coefficients (R2) for the pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion model were 0.413–0.995, 0.977–0.995, and 0.593–0.966, respectively. Meanwhile, according to the calculated value (qe,cal) and experimental uptake value (qe,exp), the pseudo-second-order model was considered as the best-fit model in describing the nitrate adsorption from aqueous solution [29, 35]. Pseudo-second-order model has been frequently invoked to describe adsorption of inorganic pollutants on GAC-based materials [29], and a similar study reported the nitrate adsorption onto magnetic amine-cross-linked biopolymer fitted with pseudo-second-order model as well [27].


## 3.2. Adsorption Isotherms

The adsorption isotherm was conducted to determine the maximum adsorption capacities and expressed the relationship between the amount of sorption and residual nitrate concentration at equilibrium [36]. The adsorption isotherm indicated how the adsorption molecules distributed between the liquid phase and the solid phase when the adsorption process reached an equilibrium state [37]. In order to optimize the design of an adsorption system, three adsorption isotherms, namely, the Langmuir, Freundlich, and Temkin isotherm models in their linear forms were applied to the equilibrium data to find the suitable model that could be used for design purpose [37]. Figure 2 typically shows the nitrate adsorption isotherms on the ZnCl2-modified CGAC and CGAC. All the correlation coefficient, R2 values, and the parameters obtained for the models are summarized in Table 3. The adsorption data fitted satisfactorily to both Langmuir (R2 = 0.970) and Freundlich (R2 = 0.982) models, better than Temkin model (R2 = 0.828), are shown in Table 3 and Figure 2. Application of the Langmuir model for ZnCl2-modified CGAC and CGAC allowed the determination of the maximum equilibrium adsorption capacity [36], which were 14.01 mgN·g−1 and 0.28 mgN·g−1, respectively. The nitrate adsorption capacity of modified adsorbents was much higher than that of raw adsorbents. The maximum adsorption capacity obtained for ZnCl2-modified CGAC was higher than the corresponding values assumed by others for polyethylene glycol/chitosan and polyvinyl alcohol/chitosan (11.44 mgN·g−1 and 7.91 mgN·g−1) [21]. In addition, the constant 1/n in the range of 0-1 showed the favorable conditions for adsorption [38].Figure 2
Adsorption isotherms for nitrate adsorption onto CGAC and ZnCl2-modified CGAC: (a) experimental points; (b) Langmuir adsorption isotherm plots; (c) Freundlich adsorption isotherm plots; (d) Temkin adsorption isotherm plots.
(a)
(b)
(c)
(d)Table 3
Langmuir, Freundlich, and Temkin model constants for nitrate adsorption.
Adsorbent
LangmuirCe/qe=1/QKL+Ce/Q
Freundlichlnqe=lnKf+1/nlnCe
Temkinqe=BlnKT+BlnCe
Q (mgN·g−1)
K
L (L·mg−1)
R
2
K
f (mgN·g−1)·(L·mg−1)1/n
1
/
n
R
2
K
T (L·mg−1)
B (J·mol−1)
R
2
ZnCl2-modified CGAC
14.01
0.0058
0.970
0.13
0.813
0.982
0.35
1.0055
0.828
CGAC
0.28
0.0294
0.817
0.00851
0.746
0.962
0.24
0.0707
0.745
C
e is the equilibrium concentration (mg·L−1); Q is the monolayer saturation adsorption capacity; KL is the Langmuir constant (L·mg−1); Kf is the adsorbent-adsorbate relative affinity in the adsorption process ((mgN·g−1)·(L·mg−1)1/n); KT is equilibrium blinding constant (L·mg−1); and B is the Temkin constant related to the heat of adsorption.


## 3.3. Characteristics

### 3.3.1. SEM Analysis

The scanning electron microscopy images of CGAC and ZnCl2-modified CGAC are shown in Figures 3(a) and 3(b), respectively. Compared with the surface of CGAC (Figure 3(a)), ZnCl2-modified CGAC had crystal on the surface (Figure 3(b)). The elemental composition of ZnCl2-impregnated activated carbon determined by EDAX is shown in Table 4. After modified by ZnCl2, the carbon content decreased from 74.75% to 37.61% and the zinc contents and the chloride contents increased to 53.61% and 0.49%, respectively.Figure 3
SEM images of CGAC and ZnCl2-modified CGAC.
(a)
(b)Table 4
EDAX data for CGAC and ZnCl2-modified CGAC.
CGAC
ZnCl2-modified CGAC
Element
Wt.%
Element
Wt.%
C
74.75
C
37.61
O
6.14
O
0.9
Al
3.29
Al
7.38
Mg
15.21
Cl
0.49
Ca
0.61
Zn
53.61


### 3.3.2. BET Analysis

The surface area and pore characteristics are shown in Table5. The BET specific surface area of CGAC was 876.752 m2·g−1, and the surface area of ZnCl2-modified CGAC decreased to 567.524 m2·g−1. It was obvious that the pore width decreased after impregnation with ZnCl2, from 0.518 nm to 0.492 nm. The decrease of surface area was ascribed to blockage of pore openings by the ZnCl2 that prohibited access of adsorbing gas molecules [29]. This indicated that the ZnCl2 had adhered to the surface of the CGAC and the result was confirmed by the SEM analysis as shown in Figure 3. It had been proved that activated carbon with large amounts of pore structures was inefficient for adsorption capacity [4, 28, 39]. As a result, micropore adsorption was absent for the potential nitrate adsorption by activated carbon [28].Table 5
Pore characteristics of CGAC and ZnCl2-modified CGAC.
Adsorbent
Surface area (m2·g−1)
Pore width (nm)
Pore volume (cm3·g−1)
CGAC
876.752
0.518
0.404
ZnCl2-modified CGAC
567.524
0.492
0.253


### 3.3.3. Zeta Potentials

Zeta potentials of original activated carbon, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC as a function of pH are shown in Figure 4. All the samples of the zeta potentials became more negative with the increase in pH, probably because of the deposition of more OH− on the adsorbent surface [40]. Zeta potentials of CGAC were in the range of +12.55 to −30.50 mV as the initial pH of the suspensions increased from 2.01 to 11.99. After modified by ZnCl2, zeta potentials of ZnCl2-modified CGAC increased slightly (+14.60 to −29.65 mV) in designed pH range. Point of zero charge pH (pHpzc) of CGAC was located at 2.39. After the ZnCl2 modification, pHpzc of modified GAC had a slight increase to 2.56. This suggested that ZnCl2 loaded on the surface of activated carbon increased positive charge on activated carbon surface. After adsorption, zeta potentials had an apparent decrease at the pH of 5.3∼12, this illustrated that nitrate ions had been adsorbed on the surface of ZnCl2-modified CGAC, and the adsorption mechanism of ZnCl2-modified CGAC for nitrate was based on electrostatic attraction [28].Figure 4
Zeta potentials of CGAC, ZnCl2-modified CGAC, and ZnCl2-modified CGAC after nitrate adsorption.


### 3.3.4. Raman Spectra

Figure5 shows the Raman spectra of ZnCl2-modified CGAC, pure KNO3, and nitrate-loaded ZnCl2-modified CGAC. Two peaks at 1332.5 cm−1 and 1586.1 cm−1 in the Raman spectrum of ZnCl2-modified CGAC were the characteristic peaks of polycrystalline graphites, namely, G (graphite) band and D (disorder) band, which explicitly appear at about 1580 and 1360 cm−1, respectively. The pure KNO3 crystal had a characteristic peak at 1042.3 cm−1. After the adsorption process, the nitrate on ZnCl2-modified CGAC illustrated the Raman peak at 1039.4 cm−1, which was almost overlapped with the peak of KNO3. The results indicated that nitrate ions were adsorbed onto the surface of ZnCl2-modified CGAC through electrostatic attraction between the free nitrate ions and the positively charged ions of ZnCl2 sites [28], which corresponded well to the decrease of zeta potentials after nitrate adsorption onto ZnCl2-modified CGAC.Figure 5
Raman spectra of CGAC, ZnCl2-modified CGAC, and anion-laden ZnCl2-modified CGAC.


### 3.3.5. FTIR Spectra

Figure6 shows the FTIR spectra of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3 samples. The FTIR spectra analysis of these samples illustrated the change of functional groups.Figure 6
FTIR pattern of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3.In FTIR spectra of ZnCl2-modified CGAC and nitrate-loaded ZnCl2-modified CGAC, the bands at about 3430 cm−1 were ascribable to ν (O-H) vibrations in hydroxyl groups. The low-frequency values for these bands suggested that the hydroxyl groups were involved in hydrogen bonds and the position of the band due to nonbonded OH groups was usually above 3500 cm−1 for alcohols, phenols, and carboxylic acids [41]. The O-H stretching vibrations occurred within a broad range of frequencies indicating the presence of “free” hydroxyl groups and bonded O-H bands of carboxylic acids. The bands observed at 1800–1000 cm−1 were presumed to be associated with oxygenated C=O and C-O-R structures as the range was reported to reflect the presence of moieties of different C=O (amide, esters, carboxylic acids, quinines, etc.) and C-O-R (aryl and alkyl esters, carboxylic) structures depending on the extent of coalification [42].Nitrate curve in Figure6 and the pure KNO3 crystal had a characteristic peak at 1384.66 cm−1 in FTIR spectra. Compared with the curve of ZnCl2-modified CGAC, a new peak at 1384.66 cm−1 (NO3−) was observed in the curve of nitrate-laden ZnCl2-modified CGAC which was assigned to the special vibration of nitrate. Meanwhile, few special vibrations could be shown in FTIR spectra, so that the main mechanism of adsorption nitrate based on electrostatic attraction almost without any chemical interactions existed.


## 3.3.1. SEM Analysis

The scanning electron microscopy images of CGAC and ZnCl2-modified CGAC are shown in Figures 3(a) and 3(b), respectively. Compared with the surface of CGAC (Figure 3(a)), ZnCl2-modified CGAC had crystal on the surface (Figure 3(b)). The elemental composition of ZnCl2-impregnated activated carbon determined by EDAX is shown in Table 4. After modified by ZnCl2, the carbon content decreased from 74.75% to 37.61% and the zinc contents and the chloride contents increased to 53.61% and 0.49%, respectively.Figure 3
SEM images of CGAC and ZnCl2-modified CGAC.
(a)
(b)Table 4
EDAX data for CGAC and ZnCl2-modified CGAC.
CGAC
ZnCl2-modified CGAC
Element
Wt.%
Element
Wt.%
C
74.75
C
37.61
O
6.14
O
0.9
Al
3.29
Al
7.38
Mg
15.21
Cl
0.49
Ca
0.61
Zn
53.61


## 3.3.2. BET Analysis

The surface area and pore characteristics are shown in Table5. The BET specific surface area of CGAC was 876.752 m2·g−1, and the surface area of ZnCl2-modified CGAC decreased to 567.524 m2·g−1. It was obvious that the pore width decreased after impregnation with ZnCl2, from 0.518 nm to 0.492 nm. The decrease of surface area was ascribed to blockage of pore openings by the ZnCl2 that prohibited access of adsorbing gas molecules [29]. This indicated that the ZnCl2 had adhered to the surface of the CGAC and the result was confirmed by the SEM analysis as shown in Figure 3. It had been proved that activated carbon with large amounts of pore structures was inefficient for adsorption capacity [4, 28, 39]. As a result, micropore adsorption was absent for the potential nitrate adsorption by activated carbon [28].Table 5
Pore characteristics of CGAC and ZnCl2-modified CGAC.
Adsorbent
Surface area (m2·g−1)
Pore width (nm)
Pore volume (cm3·g−1)
CGAC
876.752
0.518
0.404
ZnCl2-modified CGAC
567.524
0.492
0.253


## 3.3.3. Zeta Potentials

Zeta potentials of original activated carbon, ZnCl2-modified CGAC, and nitrate-loaded ZnCl2-modified CGAC as a function of pH are shown in Figure 4. All the samples of the zeta potentials became more negative with the increase in pH, probably because of the deposition of more OH− on the adsorbent surface [40]. Zeta potentials of CGAC were in the range of +12.55 to −30.50 mV as the initial pH of the suspensions increased from 2.01 to 11.99. After modified by ZnCl2, zeta potentials of ZnCl2-modified CGAC increased slightly (+14.60 to −29.65 mV) in designed pH range. Point of zero charge pH (pHpzc) of CGAC was located at 2.39. After the ZnCl2 modification, pHpzc of modified GAC had a slight increase to 2.56. This suggested that ZnCl2 loaded on the surface of activated carbon increased positive charge on activated carbon surface. After adsorption, zeta potentials had an apparent decrease at the pH of 5.3∼12, this illustrated that nitrate ions had been adsorbed on the surface of ZnCl2-modified CGAC, and the adsorption mechanism of ZnCl2-modified CGAC for nitrate was based on electrostatic attraction [28].Figure 4
Zeta potentials of CGAC, ZnCl2-modified CGAC, and ZnCl2-modified CGAC after nitrate adsorption.


## 3.3.4. Raman Spectra

Figure5 shows the Raman spectra of ZnCl2-modified CGAC, pure KNO3, and nitrate-loaded ZnCl2-modified CGAC. Two peaks at 1332.5 cm−1 and 1586.1 cm−1 in the Raman spectrum of ZnCl2-modified CGAC were the characteristic peaks of polycrystalline graphites, namely, G (graphite) band and D (disorder) band, which explicitly appear at about 1580 and 1360 cm−1, respectively. The pure KNO3 crystal had a characteristic peak at 1042.3 cm−1. After the adsorption process, the nitrate on ZnCl2-modified CGAC illustrated the Raman peak at 1039.4 cm−1, which was almost overlapped with the peak of KNO3. The results indicated that nitrate ions were adsorbed onto the surface of ZnCl2-modified CGAC through electrostatic attraction between the free nitrate ions and the positively charged ions of ZnCl2 sites [28], which corresponded well to the decrease of zeta potentials after nitrate adsorption onto ZnCl2-modified CGAC.Figure 5
Raman spectra of CGAC, ZnCl2-modified CGAC, and anion-laden ZnCl2-modified CGAC.


## 3.3.5. FTIR Spectra

Figure6 shows the FTIR spectra of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3 samples. The FTIR spectra analysis of these samples illustrated the change of functional groups.Figure 6
FTIR pattern of ZnCl2-modified CGAC, nitrate-loaded ZnCl2-modified CGAC, and solid KNO3.In FTIR spectra of ZnCl2-modified CGAC and nitrate-loaded ZnCl2-modified CGAC, the bands at about 3430 cm−1 were ascribable to ν (O-H) vibrations in hydroxyl groups. The low-frequency values for these bands suggested that the hydroxyl groups were involved in hydrogen bonds and the position of the band due to nonbonded OH groups was usually above 3500 cm−1 for alcohols, phenols, and carboxylic acids [41]. The O-H stretching vibrations occurred within a broad range of frequencies indicating the presence of “free” hydroxyl groups and bonded O-H bands of carboxylic acids. The bands observed at 1800–1000 cm−1 were presumed to be associated with oxygenated C=O and C-O-R structures as the range was reported to reflect the presence of moieties of different C=O (amide, esters, carboxylic acids, quinines, etc.) and C-O-R (aryl and alkyl esters, carboxylic) structures depending on the extent of coalification [42].Nitrate curve in Figure6 and the pure KNO3 crystal had a characteristic peak at 1384.66 cm−1 in FTIR spectra. Compared with the curve of ZnCl2-modified CGAC, a new peak at 1384.66 cm−1 (NO3−) was observed in the curve of nitrate-laden ZnCl2-modified CGAC which was assigned to the special vibration of nitrate. Meanwhile, few special vibrations could be shown in FTIR spectra, so that the main mechanism of adsorption nitrate based on electrostatic attraction almost without any chemical interactions existed.


## 3.4. Column Studies

To investigate the adsorption ability to the continuous nitrate removal from solution onto ZnCl2-modified CGAC, dynamic column adsorption tests and the dynamic models were conducted to evaluate the performance of a continuous system.


### 3.4.1. Effect of the Column Depth on the Breakthrough Curves

As shown in Figure7(a) and Table 6, when the mass of adsorbent was 10 g, 20 g, and 30 g, the breakthrough points occurred at the time of 200 min, 320 min, and 650 min, respectively. The saturated adsorption capacities of ZnCl2-modified CGAC (10 g, 20 g, and 30 g) in column were about 0.25 mgN·g−1, 0.20 mgN·g−1, and 0.17 mgN·g−1, respectively. The breakthrough time increased with the increase in mass which might be due to the more contact time. The increase in NO3−-N uptake capacity with the increasing bed depth in the column may be due to increased adsorbent surface area, which provided more binding sites for the column adsorption [43, 44].Figure 7
Breakthrough curve: (a) the effect of column depth on NO3−-N adsorption (Q = 10 ml·min−1, C0 = 20 mgN·L−1); (b) the effect of flow rate on NO3−-N adsorption (m = 30 g, C0 = 20 mgN·L−1); (c) the effect of influent concentration on NO3−-N adsorption (m = 30 g, Q = 10 ml·min−1).
(a)
(b)
(c)Table 6
Parameters in fixed-bed column for nitrate adsorption by ZnCl2-modified CGAC.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
t
total (min)
m
total (mg)
q
total (mg)
q
eq (mgN·g−1)
V
eff (ml)
10
10
20
200
40.13
2.52
0.25
2000
20
10
20
320
64.21
4.03
0.20
3200
30
10
20
650
133.63
5.05
0.17
6500
20
20
20
290
112.51
5.38
0.27
5800
20
5
20
520
47.82
3.63
0.18
2600
20
10
10
640
65.03
2.31
0.12
6400
20
10
50
260
129.16
5.21
0.26
2600


### 3.4.2. Effect of Flow Rates on the Breakthrough Curves

As shown in Figure7(b) and Table 6, the breakthrough curve generally occurred faster with higher flow rate at 20 ml·min−1. With the increases in flow rate, the adsorption capacities were 0.18 mgN·g−1, 0.20 mgN·g−1, and 0.27 mgN·g−1, respectively. As indicated in Figure 7(b), the breakthrough curve became steeper as the flow rate increased. This was due to that, at a high rate of influent, NO3−-N did not have enough time to contact with ZnCl2-modified CGAC [43]. At a low rate of influent, NO3−-N maybe had more time to be in contact with adsorbent [44]. Breakthrough time reaching saturation was increased significantly with a decrease in the flow rate.


### 3.4.3. Effect of Influent Concentration on the Breakthrough Curves

As shown in Figure7(c) and Table 6, it is illustrated that the adsorption process reached saturation faster and the breakthrough time decreased with increasing influent NO3−-N concentration. The adsorption capacities of ZnCl2-modified CGAC for nitrate were 0.12 mgN·g−1, 0.20 mgN·g−1, and 0.26 mgN·g−1, respectively. The maximum capacity at 50 mgN·L−1 was higher than those at 10 mgN·L−1 and 20 mgN·L−1. This might be attributed to high influent NO3−-N concentration providing higher driving force for the transfer process to overcome the mass transfer resistance [45]. This result indicated that the change of concentration gradient affected the saturation rate and breakthrough time [46, 47].


### 3.4.4. Application of Thomas Model and Yoon–Nelson Model

Thomas model was widely used to describe the performance of adsorption process in a fixed-bed column. This model assumed plug flow behavior in the bed and used Langmuir isotherm for equilibrium and the second-order reversible kinetics [48]. The model was described by the following equation:(10)lnC0Ct−1=Kthq0mQ−KthC0t,where Kth (L·min−1·mg−1) is the Thomas rate constant, q0 (mgN·g−1) is the maximum sorption capacity, m (g) is the mass of adsorbent, and Q (ml·min−1) is the flow rate.A simple theoretical model developed by Yoon and Nelson was also tested to investigate the breakthrough behavior of nitrate onto ZnCl2-modified CGAC [49, 50]. The linearized model for a single component system was expressed as follows:(11)lnCtC0−Ct=KYNt−τKYN,where KYN (min−1) is the rate constant of model and τ (min) is the time required for 50% adsorbate breakthrough.Among Thomas and Yoon–Nelson models, the parameters listed in Tables7 and 8, both of them provided good fit (R2 > 0.834) to the experimental data at various conditions. In a comparison of values of R2 and breakthrough curves, both Thomas and Yoon–Nelson models could be used to describe the behavior of the nitrate adsorption in a fixed-bed column. The results indicated that the external and internal diffusions would not be the limiting step [44].Table 7
Thomas parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
Th (ml·(min·mgN)−1) × 10−4
q
0 (mgN·g−1)
R
2
10
10
20
22.00
0.24
0.834
20
10
20
6.97
0.94
0.955
30
10
20
3.39
1.55
0.955
20
20
20
12.10
1.15
0.947
20
5
20
4.65
0.94
0.986
20
10
10
7.60
1.06
0.975
20
10
50
4.61
1.79
0.956Table 8
Yoon–Nelson parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
YN  × 10−2 (min−1)
τ (min)
R
2
10
10
20
2.20
24.00
0.833
20
10
20
1.40
93.73
0.955
30
10
20
0.68
232.82
0.955
20
20
20
2.35
59.30
0.947
20
5
20
0.86
203.73
0.986
20
10
10
0.77
208.80
0.975
20
10
50
2.29
72.25
0.956


## 3.4.1. Effect of the Column Depth on the Breakthrough Curves

As shown in Figure7(a) and Table 6, when the mass of adsorbent was 10 g, 20 g, and 30 g, the breakthrough points occurred at the time of 200 min, 320 min, and 650 min, respectively. The saturated adsorption capacities of ZnCl2-modified CGAC (10 g, 20 g, and 30 g) in column were about 0.25 mgN·g−1, 0.20 mgN·g−1, and 0.17 mgN·g−1, respectively. The breakthrough time increased with the increase in mass which might be due to the more contact time. The increase in NO3−-N uptake capacity with the increasing bed depth in the column may be due to increased adsorbent surface area, which provided more binding sites for the column adsorption [43, 44].Figure 7
Breakthrough curve: (a) the effect of column depth on NO3−-N adsorption (Q = 10 ml·min−1, C0 = 20 mgN·L−1); (b) the effect of flow rate on NO3−-N adsorption (m = 30 g, C0 = 20 mgN·L−1); (c) the effect of influent concentration on NO3−-N adsorption (m = 30 g, Q = 10 ml·min−1).
(a)
(b)
(c)Table 6
Parameters in fixed-bed column for nitrate adsorption by ZnCl2-modified CGAC.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
t
total (min)
m
total (mg)
q
total (mg)
q
eq (mgN·g−1)
V
eff (ml)
10
10
20
200
40.13
2.52
0.25
2000
20
10
20
320
64.21
4.03
0.20
3200
30
10
20
650
133.63
5.05
0.17
6500
20
20
20
290
112.51
5.38
0.27
5800
20
5
20
520
47.82
3.63
0.18
2600
20
10
10
640
65.03
2.31
0.12
6400
20
10
50
260
129.16
5.21
0.26
2600


## 3.4.2. Effect of Flow Rates on the Breakthrough Curves

As shown in Figure7(b) and Table 6, the breakthrough curve generally occurred faster with higher flow rate at 20 ml·min−1. With the increases in flow rate, the adsorption capacities were 0.18 mgN·g−1, 0.20 mgN·g−1, and 0.27 mgN·g−1, respectively. As indicated in Figure 7(b), the breakthrough curve became steeper as the flow rate increased. This was due to that, at a high rate of influent, NO3−-N did not have enough time to contact with ZnCl2-modified CGAC [43]. At a low rate of influent, NO3−-N maybe had more time to be in contact with adsorbent [44]. Breakthrough time reaching saturation was increased significantly with a decrease in the flow rate.


## 3.4.3. Effect of Influent Concentration on the Breakthrough Curves

As shown in Figure7(c) and Table 6, it is illustrated that the adsorption process reached saturation faster and the breakthrough time decreased with increasing influent NO3−-N concentration. The adsorption capacities of ZnCl2-modified CGAC for nitrate were 0.12 mgN·g−1, 0.20 mgN·g−1, and 0.26 mgN·g−1, respectively. The maximum capacity at 50 mgN·L−1 was higher than those at 10 mgN·L−1 and 20 mgN·L−1. This might be attributed to high influent NO3−-N concentration providing higher driving force for the transfer process to overcome the mass transfer resistance [45]. This result indicated that the change of concentration gradient affected the saturation rate and breakthrough time [46, 47].


## 3.4.4. Application of Thomas Model and Yoon–Nelson Model

Thomas model was widely used to describe the performance of adsorption process in a fixed-bed column. This model assumed plug flow behavior in the bed and used Langmuir isotherm for equilibrium and the second-order reversible kinetics [48]. The model was described by the following equation:(10)lnC0Ct−1=Kthq0mQ−KthC0t,where Kth (L·min−1·mg−1) is the Thomas rate constant, q0 (mgN·g−1) is the maximum sorption capacity, m (g) is the mass of adsorbent, and Q (ml·min−1) is the flow rate.A simple theoretical model developed by Yoon and Nelson was also tested to investigate the breakthrough behavior of nitrate onto ZnCl2-modified CGAC [49, 50]. The linearized model for a single component system was expressed as follows:(11)lnCtC0−Ct=KYNt−τKYN,where KYN (min−1) is the rate constant of model and τ (min) is the time required for 50% adsorbate breakthrough.Among Thomas and Yoon–Nelson models, the parameters listed in Tables7 and 8, both of them provided good fit (R2 > 0.834) to the experimental data at various conditions. In a comparison of values of R2 and breakthrough curves, both Thomas and Yoon–Nelson models could be used to describe the behavior of the nitrate adsorption in a fixed-bed column. The results indicated that the external and internal diffusions would not be the limiting step [44].Table 7
Thomas parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
Th (ml·(min·mgN)−1) × 10−4
q
0 (mgN·g−1)
R
2
10
10
20
22.00
0.24
0.834
20
10
20
6.97
0.94
0.955
30
10
20
3.39
1.55
0.955
20
20
20
12.10
1.15
0.947
20
5
20
4.65
0.94
0.986
20
10
10
7.60
1.06
0.975
20
10
50
4.61
1.79
0.956Table 8
Yoon–Nelson parameters at different conditions using linear regression analysis.
m (g)
Q (ml·min−1)
C
0 (mgN·L−1)
K
YN  × 10−2 (min−1)
τ (min)
R
2
10
10
20
2.20
24.00
0.833
20
10
20
1.40
93.73
0.955
30
10
20
0.68
232.82
0.955
20
20
20
2.35
59.30
0.947
20
5
20
0.86
203.73
0.986
20
10
10
0.77
208.80
0.975
20
10
50
2.29
72.25
0.956


## 4. Conclusions

(1)
Nitrate adsorption behavior of CGAC and ZnCl2-modified CGAC was described successfully by both Langmuir and Freundlich models, and the maximum adsorption capacity was predicted to be 14.01 mgN·g−1 and 0.28 mgN·g−1, respectively. The kinetic data indicated that the adsorption process obeyed the pseudo-second-order model.
(2)
The characteristics (SEM and EDAX, surface area, pore structure, zeta potential, Raman spectra, and FTIR) indicated that the mechanism of nitrate adsorption mostly depended on the electrostatic attraction between the free nitrate ions and the positively charged ions, almost without any chemical interactions.
(3)
In column study, the breakthrough curves were strongly dependent on the mass of adsorbents, flow rate, and initial NO3−-N concentration. Both Thomas and Yoon–Nelson models were found to be in good agreement with the experimental data and could be used for prediction of the experimental results as well.



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*Source: 1939032-2018-06-03.xml*