# Evaluation of Different Clinoptilolite Zeolites as Adsorbent for Ammonium Removal from Highly Concentrated Synthetic Wastewater

^{1}

^{2}

^{*}

## Abstract

**:**

_{0}= 1000 mg NH

_{4}-N/L) such as pH, temperature, or contact time. All three CLIs tested were effective at adsorbing ammonium. In a pH range from 2 to 8, all CLIs were able to eliminate ammonium equally well. Furthermore, a contact time of 60 min was sufficient to achieve 84–88% of the maximum load. At a temperature of 34 °C, the highest loading was achieved (investigated range: 10–34 °C), indicating that the adsorption process of all CLIs was exergonic and exothermic. Especially for wastewater streams with high ammonium concentrations such as sludge water from wastewater treatment plants, CLI proved to be suitable to adsorb ammonium.

## 1. Introduction

_{3}) for fertilizer production, ensures the nutrition of half the world’s population [3]. However, the production of NH

_{3}requires a high amount of energy [4]. Dawson and Hilton [5] calculated that 1.1% of the world’s energy consumption can be attributed to the production of fertilizers; 90% of this is due to the production of nitrogen fertilizers. Thus, partial substitution of the increasing demand for NH

_{3}may be achieved by recovering ammonium (NH

_{4}

^{+}) from wastewater. For instance, sludge water from anaerobic digesters is a possible source of ammonium due to high ammonium-N (NH

_{4}-N) concentrations of around 1000 mg/L and low volume flows which allow a partial flow treatment.

_{4}

^{–}and SiO

_{4}tetrahedral, which are connected by a common oxygen atom. The micropores formed by this structure are fine enough to allow cations and water molecules to enter and be exchanged [7]. This ability is based on the substitution of SiO

_{4}by AlO

_{4}

^{−}, creating a negative charge, which has to be compensated by exchangeable cations such as Na

^{+}, K

^{+}, Ca

^{2+}, and Mg

^{2+}[8].

_{4}-N concentration), which is why a direct comparison of the investigated CLI, e.g., in terms of adsorption properties of ammonium from highly concentrated wastewater, is difficult. An overview of published data about CLI adsorption of ammonium is given in Table 1, showing the large variety of parameters examined, such as particle size (0.063–1.43 mm), contact time (20 min–24 h), and initial NH

_{4}-N concentration ranges (e.g., 25–150 mg/L vs. 10–5000 mg/L). However, it cannot be deduced from these investigations to what extent each parameter has an effect on the adsorption capacity. Hence, it becomes apparent that there is still a lack of detailed comparisons regarding the adsorption properties of CLI.

## 2. Materials and Methods

#### 2.1. Zeolite Samples, Solutions, and Chemicals

_{4}-N/L was prepared from ammonium chloride with an arbitrary pH of 5.3. NH

_{4}Cl (p.a.), NH

_{4}Ac (ammonium acetate, p.a.), KCl (p.a.), NaOH (p.a.), HF (47–51%, p.a.), HCl (32%, p.a.), and ethanol (99.5%) were obtained from VWR International (Radnor, Pennsylvania, USA). HNO

_{3}(67–69%, p.a.) was obtained from Bernd Kraft GmbH (Duisburg, Germany).

#### 2.2. Experimental Design

#### 2.2.1. Cation Exchange Capacity (CEC)

_{4}Ac solution for 30 minutes in a rotator (uniROTATOR2, LLG Labware, Meckenheim, Germany) at 120 rpm and 22 °C (no pH adjustment). After centrifugation at 4000 rpm for 10 min, the supernatant was decanted and collected. The remaining CLI in the centrifuge tube was again mixed with fresh 30 mL of 1 M NH

_{4}Ac solution and treated according to the described procedure. In total, the procedure was carried out three times and the arising three supernatants were mixed and collected. Subsequently, the mixtures were analyzed for desorbed alkali and alkaline earth metals, which correspond to the cations exchanged. Afterwards, the CLI was washed with 30 mL 80% ethanol solution three times to remove unadsorbed ammonium, following the same procedure until no ammonium in the supernatant was detected. The loaded CLI was then regenerated three times with 30 mL 1 M KCl solution using the same procedure. The resulting supernatant was collected and analyzed for ammonium. The cation exchange capacity corresponds to the cations that were exchanged by ammonium.

#### 2.2.2. Isoelectric State of CLI and pH-Dependent Adsorption

#### 2.2.3. Isothermal Adsorption

_{0}(mg/L) cannot be changed without influencing its ionic composition, the sorbent mass m (g) was varied. Thus, different quantities ranging from 2 to 48 g sorbent were mixed with 200 mL adsorption solution V

_{p}(mL) and stirred at a constant temperature (10 °C, 22 °C, and 34 °C) on a magnetic stirrer at 400 rpm. After 20 h, the residual ammonium concentration c

_{eq}(mg/L) as well as pH in the filtrate were determined. Since the pH barely varied between the different dosages and a competition adsorption by Na

^{+}or H

_{3}O

^{+}cations as well as a dilution due to the pH adjustment was to be avoided, a pH correction was not conducted. One experimental approach without sorbent for each examined pH was used to determine unwanted ammonium elimination, e.g., by stripping or adsorption onto parts of the glass apparatus. The ammonium concentration in the filtrate of that approach is expressed as c

_{B}(mg/L). The loading q

_{eq}(mg/g) of the sorbent mass was determined by Equation (1).

#### 2.2.4. Adsorption Kinetics

_{4}Cl solution (1.5 L) at a specific sorbent ratio of 0.1 g CLI per mg NH

_{4}-N on a magnetic stirrer at 22 °C. At periodic intervals, an aliquot (10 mL) was taken and immediately membrane-filtered (0.45 µm pore size) to prevent further contact between sorbent and sample. Subsequently, the ammonium concentration was measured in the filtrate and the time-dependent loading of the sorbent q

_{t}(mg/g) was calculated. Since it is known from published studies that the adsorption kinetics strongly depend on the stirring speed [15,21,22], a high rotation frequency of 800 rpm was chosen to determine the maximum possible adsorption kinetic values. During the test, due to sampling, the total volume was continuously reduced. However, it can be assumed that during the sampling no change in the ratio of the sorbent mass to the volume of the adsorption solution occurred due to the homogeneously mixed conditions.

_{4}Cl-soution was 5.3. As a consequence of the contact with zeolite, the pH value immediately rose to 6.6 and increased to 7.1 with increasing contact time.

#### 2.3. Adsorption Models

#### 2.3.1. Freundlich Model

_{eq,F}(mg/g) can be calculated by exponentiation of the corresponding equilibrium concentration c

_{eq}(mg/L) with the factor 1/n (-), as described by Equation (2).

_{F}and 1/n with the help of nonlinear regression or linearization are given, e.g., by Ho et al. [24]. In this study, the linearization is done by plotting log q

_{eq}versus log c

_{eq}. The gradient of the graph corresponds to n, while the tenth power of the intercept represents K

_{F}.

#### 2.3.2. Langmuir Model

_{L}(L/mg) is the Langmuir constant and q

_{max}(mg/g) the maximum capacity.

_{eq}/q

_{eq}vs. c

_{eq}, 1/q

_{eq}vs. 1/c

_{eq}, q

_{eq}vs. q

_{eq}/c

_{eq}, or q

_{eq}/c

_{eq}vs. q

_{eq}, a linear relationship for Equation (3) can be deduced [26]. Table 3 lists the four possible linear forms for determining Langmuir constants. In this study, only the type of isotherm with the highest coefficient of determination r² is listed. The correlation coefficient r² of the nonlinear form of the Langmuir isotherm and the experimentally determined loads q

_{eq}and the arithmetical average loads $\overline{{\mathrm{q}}_{\mathrm{eq}}}$ was calculated according to Equation (4).

#### 2.3.3. Temkin Model

_{T}(1/mol), and A

_{T}(L/mg) the Temkin isothermal constants.

_{T}and A

_{T}, ln c

_{eq}vs. q

_{eq}is plotted; the slope represents the term RT/b

_{T}, the intersection with the ordinate the term RT ln(A

_{T})/b

_{T}.

#### 2.3.4. Thermodynamic Calculations

^{0}(kJ/mol) can be calculated according to the following Equation (7)

_{d}is the thermodynamic equilibrium constant, here the Freundlich constant (L/g). According to Milonjic [31], it should be noted that K

_{d}must be dimensionless. Therefore, the use of the temperature-dependent equilibrium constant K

_{F}must be corrected by a factor of 1000 g/L (density of water) in its dimensionless form. The relationship of the other thermodynamic parameters such as change in enthalpy ∆H

^{0}(kJ/mol) and change in standard entropy ∆S

^{0}(J/(mol K)) can be derived by means of the Gibbs–Helmholtz equation (8).

^{0}and free standard enthalpy ∆H

^{0}, is shown in a diagram in which the logarithmic equilibrium constant K

_{d}is plotted against the reciprocal value of the temperature 1/T (Van’t–Hoff diagram). Here, the gradient corresponds to the quotient of the negative change in the free standard enthalpy ∆H

^{0}and the universal gas constant R. Furthermore, the quotient of the change of the free molar standard entropy ∆S

^{0}and the universal gas constant can be read from the axis section.

^{0}, meaning energy is absorbed by the adsorption process. A negative value indicates exothermic adsorption, meaning energy is being released. A spontaneous (exergonic) adsorption is described by negative values of ∆G

^{0}, while negative values of ∆S

^{0}indicate a random adsorption behavior.

#### 2.4. Kinetic Models

#### 2.4.1. Intraparticle Diffusion

_{ID}(mg/(g min

^{0.5})) and the square root of the contact time t (min). McKay et al. [34] extend this model by the constant C (mg/g), which is proportional to the thickness of the boundary layer as well as the initial adsorption by it. The time-dependent loading of the sorbent q

_{t,ID}(mg/g) can be calculated by Equation (9).

_{ID}, q

_{t}versus t

^{0.5}is plotted. The slope of the resulting graph corresponds to k

_{ID}while the intersection with the ordinate corresponds to C. Sole intraparticle diffusion occurs when the graph intersects the origin (C = 0). If a multistage diffusion process is present, two or more partial lines passing into each other can be approximated to the existing empirical measuring points of q

_{t}.

#### 2.4.2. Pseudo-Second-Order

_{t}(mg/g) at any time t

_{2}is the pseudo-second-order rate (mg/(g min)) and q

_{e}(mg/g) the load at equilibrium. From the integration of Equation (9) with the boundary conditions q

_{t}= 0 at t = 0 and q

_{t}= q

_{t}at t = t, four different linear forms of the PSO model can be obtained (Table 4).

^{2}(Equation (4)) is listed.

#### 2.5. Analytical Methods

_{3}(65%), 4 mL HF, and 2 mL HCl. The mixture was digested by microwave (Start, MLS GmbH, Leutkirch, Germany) with a selected program run of 10 min at 110 °C, then 5 min at 140 °C, and finally 9 min at 190 °C. Together with the cooling phase, the digestion lasted 64 min. Heavy metals were analyzed by inductively coupled plasma mass spectrometry (Nexion 2000, Perkin Elmer, Waltham, MA, USA).

## 3. Results and Discussion

#### 3.1. Cation Exchange Capacity

#### 3.2. Isoelectric State and pH-Dependent Adsorption

_{4}Cl solution after pH adjustment. The increase of the pH values in the range of 2 to 7 can be attributed to the removal of NH

_{4}

^{+}leaving an increasing amount of OH

^{−}, as well as leaching of cations (e.g., K

^{+}, Na

^{+}, Ca

^{2+}, Mg

^{2+}). At higher pH values, especially in the alkaline range, no change in the pH could be observed. This can be attributed to the decrease in adsorption, since uncharged NH

_{3}which occurs at pH > 9 cannot be adsorbed by CLI and, therefore, no more cations that influence the pH can be released.

_{4}-N/g (loading not shown in the figure). The arbitrary pH value of the NH

_{4}Cl solution is indicated in the figure and amounted to 5.3. With pH values higher than 9, a significant decrease of elimination could be observed for all CLIs. CCP 20 eliminated 73% at pH 9, whereas at pH 10 only 30% of ammonium were removed. Within the same pH range, elimination by Micro 200 and EcoZeo 20 decreased from 69% to 45%. At higher pH values, with all three sorbents only a low elimination of 20% was achieved.

#### 3.3. Isothermal Adsorption

_{eq}and the associated loading of the CLI q

_{eq}after 20 h contact time of the ammonium solution at different temperatures. The lines represent the Freundlich isotherm, of which the coefficients of determination were the highest of all the isotherm models (Freundlich, Langmuir, Temkin) tested. From the high correlation with the Freundlich isotherm it can be deduced that CLI has a heterogeneous surface which allows a nonideal adsorption. With increasing load of the CLI, less adsorption of ammonium can be achieved. Table 6 lists the coefficients of the isothermal fit according to Freundlich, Langmuir, and Temkin as well as the coefficients of determination. The final pH was found to be between 6.4 and 7.3 whereby the latter value was reached with the largest amount of CLI.

_{eq}when a low c

_{eq}remained in the solution. At a temperature of 34 °C (307 K), all CLIs achieved the highest loading. The highest loading of q

_{eq}= 18.8 mg/g was achieved with CCP 20 and Micro 200, followed by EcoZeo 20 with q

_{eq}= 16.3 mg/g. Due to higher temperature, there is a lower viscosity of the solution. As a result, the NH

_{4}

^{+}cations can penetrate deeper into the lattice, resulting in higher loading of the CLI.

#### 3.4. Thermodynamic Properties

^{0}in the examined temperature range (283–307 K), the adsorption process of ammonium onto the investigated CLI was exergonic, i.e., a voluntary reaction. The free standard enthalpy ΔH

^{0}of all three sorbents was negative, indicating an exothermic reaction. The standard molar entropy ΔS

^{0}, which was positive for Micro 200 and EcoZeo 20, indicates that the ammonium adsorption is a directional process, decreasing slightly as the temperature increases. However, the negative molar standard entropy of CCP 20 indicates, that the sorption process was random.

^{0}ranged from −2.8662 to 0.224 kJ/mol [14], −0.79 to 1.63 kJ/mol [46], and −0.22 to 1.60 kJ/mol [19], respectively. In this study, the values of ΔG

^{0}range from −15 to −13.7 kJ/mol. The much lower values regarding ΔG

^{0}of this study can be attributed to the smaller particle size and therefore short diffusion pathways of cations into the CLI compared to the ones of Alshameri et al. [14], Gunay [46], and Karadag et al. [19], where zeolites with particle sizes of 0.063–0.074 mm, 0.3–0.6 mm, and 1.0–1.4 mm, respectively, were used. Furthermore, the values of ΔG

^{0}of the investigated CLIs decrease with increasing CEC, indicating that a high natural preload of the CLI with alkali and alkaline earth cations leads to a more exergonic adsorption. Similar to the results published by other researchers (ΔH

^{0}: −49.384, −22.34, −5.43, −15.38 kJ/mol [14,19,45,46]), which indicate that adsorption of ammonium is exothermic, a slightly exothermic adsorption was found for the CLIs tested in this study (ΔH

^{0}ranging from 4.7 kJ/mol (EcoZeo 20) to 16.9 kJ/mol (CCP 20)). Furthermore, results reported with negative values of ΔS

^{0}(−156.1, −74.42, −43.03, −49.34, J/(K mol) [14,19,45,46]) indicate decreasing ammonium uptake due to increasing randomness. In contrast to this, a strongly directed adsorption process, as indicated by positive ΔS

^{0}values ranging between 32.7 J/(K mol) (EcoZeo 20) and 17.6 J/(K mol) (Micro 200), was achieved with the investigated materials of this study.

#### 3.5. Kinetic Studies

_{t}as a function of the contact time) of the three sorbents in NH

_{4}Cl solution are depicted in Figure 4. Furthermore, the fit to the ID model, which achieved higher r² values compared to the PSO model, is represented as lines.

_{2}= 0.064 g/(mg min)), followed by EcoZeo 20 (k

_{2}= 0.048 g/(mg min)) and Micro 200 (k

_{2}= 0.046 g/(mg min)). Equilibrium loading q

_{e}of CCP 20 was also highest (q

_{e}= 6.99 mg/g), followed by Micro 200 (q

_{e}= 6.62 mg/g) and EcoZeo 20 (q

_{e}= 6.13 mg/g). According to the ID model, the intraparticle adsorption rate k

_{ID}of Micro 200 was highest (0.203 mg/(min

^{0.5}g)), followed by EcoZeo 20 (0.191 mg/(min

^{0.5}g)) and CCP 20 (0.159 mg/(min

^{0.5}g)). Initial adsorption C, on the other hand, was the highest of CCP 20 (5.25 mg/g), followed by Micro 200 (4.38 mg/g) and EcoZeo 20 (4.02 mg/g). Thus, although the adsorption rates of Micro 200 and EcoZeo 20 were higher, neither CLIs reached the loading of CCP 20, which remained unattained due to the large initial adsorption by its thick boundary layer.

_{e}but a slow adsorption rate k

_{2}[22,47,48]. Thus, the adsorption velocities found in this study were slower than those reported by Karadag et al. [47] (k

_{2}= 0.526 g/(mg min), c

_{0}= 100 mg/L) and Moussavi et al. [48] (k

_{2}= 0.096 g/(mg min), c

_{0}= 100 mg/L), but exceeded those of Erdogan and Ülkü [22] (k

_{2}= 0.6 × 10

^{−3}g/(mg min), c

_{0}= 300 mg/L) significantly.

_{2}with the smallest particles. Therefore, sorbent particles should always be as small as practicable in order to achieve a high adsorption rate.

## 4. Conclusions

^{+}and Na

^{+}cations of all CLIs tested. The isoelectric state of the CLIs was in the alkaline range, resulting from a large negative charge attracting cations as well as leaching of alkali and earth alkali cations influencing the pH. Furthermore, a contact time of 60 minutes was sufficient to achieve 84–88% of the maximum load. At a temperature of 34 °C, the highest loading was achieved (investigated range: 10–34 °C). The adsorption process of all CLIs was of exergonic and exothermic nature. Especially for those types of wastewater streams with high ammonium concentrations such as sludge water from anaerobic sludge digesters operated at mesophilic temperatures, CLI proved to be a suitable sorbent for the elimination of ammonium. Both the CEC determination and the determination of the isoelectric state are simple and fast methods to assess qualitatively the adsorption capacity of a CLI. In this study, CCP 20, which has the largest CEC and lowest isoelectric state, achieved the highest adsorption capacity. Depending on the research objective, the investigation of the CEC and the isoelectric point can provide sufficient information about the adsorption behavior of CLI.

## Author Contributions

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**pH values of the filtrates after 20 h contact with CCP 20, Micro 200, and EcoZeo 20 (c

_{0}= 1000 mg NH

_{4}-N/L, sorbent ration 0.1 g

_{CLI}/mg

_{NH4-N}) as a function of the initial pH values (adjusted pH values of the solution before contact with CLI).

**Figure 2.**Elimination of ammonium by CCP 20, Micro 200, and EcoZeo 20 after 20 h contact time from NH

_{4}Cl solution (c

_{0}= 1000 mg NH

_{4}-N/L, sorbent ration 0.1 g

_{CLI}/mg

_{NH4-N}) as well as speciation α of ammonium as a function of pH.

**Figure 3.**Equilibrium loading q

_{eq}and equilibrium concentration c

_{eq}as well as the fit according to the Freundlich model of CCP 20, Micro 200, and EcoZeo 20 after 20 h contact time with ammonium solution (c

_{0}= 1000 mg NH

_{4}-N/L, pH

_{Start}5.3, pH

_{End}6.5–7.3) at different temperatures.

**Figure 4.**Loading of CCP 20, Micro 200, and EcoZeo 20 as a function of the contact time (c

_{0}= 1000 mg NH

_{4}-N/L, pH

_{Start}5.3, pH

_{End}6.6–7.1, sorbent ratio 0.1 g

_{CLI}/mg

_{NH4-N}).

**Table 1.**Overview of constraints and coefficients of isotherms according to Langmuir and Freundlich in different studies investigating the adsorption of ammonium onto clinoptilolite.

Country (Area) | Particle Size | CLI | Initial | Contact Time | Langmuir | Freundlich | Reference | ||||
---|---|---|---|---|---|---|---|---|---|---|---|

d_{P} | Share | Concentration | K_{L} | q_{max} | r^{2} | K_{F} | 1/n | r^{2} | |||

[mm] | [%] | [mg NH_{4}-N/L] | [L/g] | [mg/g] | [-] | [L/g] | [-] | [-] | |||

Slovakia (-) | <0.2 | - | 10–5000 | over night | 0.006 | 33 | 0.99 | 1.84 | 0.36 | 0.97 | [10] |

China (Huludao) | 0.063–0.073 | 93 | 10–240 | 5 h | 0.1998 | 3.445 | 0.9772 | 1.252 | 0.5192 | 0.7363 | [14] |

China (Zhejiang) | 0.8–1.43 | 48 | 10–4000 | 24 h | 0.009 | 14.265 | 0.993 | 0.985 | 0.355 | 0.973 | [15] |

Bulgaria (-) | 0.2–1.0 | 83 | 50–250 | 1 h | 0.063 | 7.85 | 0.998 | 1.88 | 0.27 | 0.899 | [16] |

China (Zhejiang) | <0.074 | - | 50–250 | 8 h | 0.074 | 11.202 | 0.968 | 2.71 | 0.28 | 0.982 | [17] |

Turkey (Sivas) | <0.075 | - | 20–400 | - | 0.0548 | 9.64 | 0.965 | 0.93 | 0.488 | 0.965 | [18] |

Turkey (-) | 1.0–1.4 | 85 | 25–150 | - | 0.029 | 8.121 | 0.927 | 0.517 | 0.612 | 0.952 | [19] |

Turkey (Dogantepe) | 1.0–2.0 | 45 | 8.8–885 | 20 min | 0.018 | 25.77 | 0.9851 | 2.23 | 0.38 | 0.9847 | [20] |

Supplier | Labradorit | Zeocem | Zeocem |
---|---|---|---|

CLI | CCP 20 | Micro 200 | EcoZeo 20 |

Particle Size | 0−20 µm | 0−200 µm | 0−20 µm |

Element | % [wt/wt] | % [wt/wt] | % [wt/wt] |

Al | 5.4% | 5.3% | 5.3% |

Ca | 1.6% | 1.8% | 1.8% |

Mg | 0.3% | 0.3% | 0.3% |

Na | 0.4% | 0.1% | 0.1% |

K | 2.0% | 1.9% | 1.9% |

Si | 35.5% | 34.7% | 34.7% |

Pb | 0.001% | 0.001% | 0.001% |

Ba | 0.08% | 0.08% | 0.08% |

Fe | 1.0% | 0.9% | 0.9% |

Ti | 0.1% | 0.1% | 0.1% |

**Table 3.**Linear forms of the Langmuir isotherm (according to [27]).

Type | Linear form | Plot | K_{L} | q_{max} |
---|---|---|---|---|

I | $\frac{{\mathrm{c}}_{\mathrm{eq}}}{{\mathrm{q}}_{\mathrm{eq}}}\text{}=\text{}\frac{1}{{\mathrm{q}}_{\mathrm{max}}{\mathrm{K}}_{\mathrm{L}}}+\frac{{\mathrm{c}}_{\mathrm{eq}}}{{\mathrm{q}}_{\mathrm{max}}}$ | $\frac{{\mathrm{c}}_{\mathrm{eq}}}{{\mathrm{q}}_{\mathrm{eq}}}\text{}\mathrm{vs}{.\text{}\mathrm{c}}_{\mathrm{eq}}$ | slope/intercept | 1/slope |

II | $\frac{1}{{\mathrm{q}}_{\mathrm{eq}}}\text{}=\text{}\left[\frac{1}{{\mathrm{q}}_{\mathrm{max}}{\mathrm{K}}_{\mathrm{L}}}\right]\frac{1}{{\mathrm{c}}_{\mathrm{eq}}}+\frac{1}{{\mathrm{q}}_{\mathrm{max}}}$ | $\frac{1}{{\mathrm{q}}_{\mathrm{eq}}}\mathrm{vs}.\text{}\frac{1}{{\mathrm{c}}_{\mathrm{eq}}}$ | intercept/slope | 1/intercept |

III | ${\mathrm{q}}_{\mathrm{eq}}{\text{}=\text{}\mathrm{q}}_{\mathrm{max}}-\left[\frac{1}{{\mathrm{K}}_{\mathrm{L}}}\right]\frac{{\mathrm{q}}_{\mathrm{eq}}}{{\mathrm{c}}_{\mathrm{eq}}}$ | ${\mathrm{q}}_{\mathrm{eq}}\text{}\mathrm{vs}.\text{}\frac{{\mathrm{q}}_{\mathrm{eq}}}{{\mathrm{c}}_{\mathrm{eq}}}$ | 1/slope | intercept |

IV | $\frac{{\mathrm{q}}_{\mathrm{eq}}}{{\mathrm{c}}_{\mathrm{eq}}}{\text{}=\text{}\mathrm{q}}_{\mathrm{max}}{\mathrm{K}}_{\mathrm{L}}{-\mathrm{c}}_{\mathrm{eq}}{\mathrm{K}}_{\mathrm{L}}$ | $\frac{{\mathrm{q}}_{\mathrm{eq}}}{{\mathrm{c}}_{\mathrm{eq}}}\text{}\mathrm{vs}{.\text{}\mathrm{q}}_{\mathrm{eq}}$ | (–1) slope | intercept/slope |

**Table 4.**Linear forms of PSO model (according to [36]).

Type | Linear form | Plot | k_{2} | q_{e} |
---|---|---|---|---|

I | $\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}\text{}=\text{}\frac{1}{{\mathrm{k}}_{2}{\mathrm{q}}_{\mathrm{e}}{}^{2}}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$ | $\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}\text{}\mathrm{vs}.\text{}\mathrm{t}$ |
(slope)^{2}/intercept
| 1/slope |

II | $\frac{1}{{\mathrm{q}}_{\mathrm{t}}}\text{}=\text{}\left[\frac{1}{{\mathrm{k}}_{2}{\mathrm{q}}_{\mathrm{e}}{}^{2}}\right]\frac{1}{\mathrm{t}}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}$ | $\frac{1}{{\mathrm{q}}_{\mathrm{t}}}\text{}\mathrm{vs}.\text{}\frac{1}{\mathrm{t}}$ |
(intercept)^{2}/slope
| 1/intercept |

III | ${\mathrm{q}}_{\mathrm{t}}{\text{}=\text{}\mathrm{q}}_{\mathrm{e}}-\left[\frac{1}{{\mathrm{k}}_{2}{\mathrm{q}}_{\mathrm{e}}}\right]\frac{{\mathrm{q}}_{\mathrm{t}}}{\mathrm{t}}$ | ${\mathrm{q}}_{\mathrm{t}}\text{}\mathrm{vs}.\text{}\frac{{\mathrm{q}}_{\mathrm{t}}}{\mathrm{t}}$ | (−1)/(slope × intercept) | intercept |

IV | $\frac{{\mathrm{q}}_{\mathrm{t}}}{\mathrm{t}}{\text{}=\text{}\mathrm{k}}_{2}{\mathrm{q}}_{\mathrm{e}}{}^{2}{\text{}-\text{}\mathrm{kq}}_{\mathrm{e}}{\mathrm{q}}_{\mathrm{t}}$ | $\frac{{\mathrm{q}}_{\mathrm{t}}}{\mathrm{t}}\text{}\mathrm{vs}{.\text{}\mathrm{q}}_{\mathrm{t}}$ |
(slope)^{2}/intercept
| −intercept/slope |

CLI | Adsorbed | Desorbed | Exchanged Cations | |||
---|---|---|---|---|---|---|

NH_{4}^{+} | K^{+} | Na^{+} | Ca^{2+} | Mg^{2+} | CEC | |

[meq/100g] | [meq/100g] | [meq/100g] | [meq/100g] | [meq/100g] | [meq/100g] | |

CCP 20 | 136.9 | 49.3 | 21.0 | 65.6 | 3.3 | 139.2 |

Micro 200 | 125.0 | 47.4 | 3.4 | 67.4 | 4.3 | 122.5 |

EcoZeo 20 | 121.4 | 45.1 | 7.6 | 62.9 | 3.8 | 119.4 |

**Table 6.**Coefficients of the isothermal adaptation according to Freundlich of CCP 20, Micro 200, and EcoZeo 20 after 20 h contact time with ammonium solution (c

_{0}= 1000 mg NH

_{4}-N/L, pH

_{Start}5.3, pH

_{End}6.5–7.3).

CLI | Temperature | Freundlich | Langmuir | Temkin | ||||||
---|---|---|---|---|---|---|---|---|---|---|

T | K_{F} | 1/n | r² | K_{L} | q_{max} | r² | A_{T} | b_{T} | r² | |

[K] | [L/g] | [-] | [-] | [L/mg] | [mg/g] | [-] | [L/mg] | [J g/(mg mol)] | [-] | |

CCP 20 | 283 | 0.581 | 0.483 | 0.9239 | 0.003 | 19.90 | 0.5478 | 0.0371 | 584 | 0.8839 |

295 | 0.438 | 0.536 | 0.9349 | 0.003 | 21.58 | 0.5581 | 0.0311 | 524 | 0.9039 | |

307 | 0.331 | 0.594 | 0.9821 | 0.002 | 26.71 | 0.5840 | 0.0244 | 441 | 0.9450 | |

Micro 200 | 283 | 0.352 | 0.541 | 0.8584 | 0.002 | 21.24 | 0.5764 | 0.0245 | 568 | 0.8058 |

295 | 0.337 | 0.562 | 0.9043 | 0.002 | 23.11 | 0.5805 | 0.0236 | 513 | 0.8483 | |

307 | 0.260 | 0.612 | 0.9004 | 0.002 | 26.58 | 0.6037 | 0.0203 | 476 | 0.8442 | |

EcoZeo 20 | 283 | 0.325 | 0.532 | 0.8547 | 0.002 | 18.23 | 0.5619 | 0.0229 | 632 | 0.8159 |

295 | 0.343 | 0.553 | 0.9533 | 0.002 | 22.15 | 0.5770 | 0.0241 | 538 | 0.8939 | |

307 | 0.278 | 0.595 | 0.9698 | 0.002 | 23.87 | 0.5779 | 0.0207 | 500 | 0.9391 |

**Table 7.**Thermodynamic properties of CCP 20, Micro 200, and EcoZeo 20 after 20 h contact time with ammonium solution (c

_{0}= 1000 mg NH

_{4}-N/L, pH

_{Start}5.3, pH

_{End}6.5–7.3).

CLI | Temperature | Free Reaction Enthalpy | Free Standard Enthalpy | Molar Standard Entropy |
---|---|---|---|---|

T | ΔG^{0} | ΔH^{0} | ΔS^{0} | |

[K] | [kJ/mol] | [kJ/mol] | [J/(K mol)] | |

CCP 20 | 283 | −15.0 | ||

295 | −15.0 | −16.9 | −6.8 | |

307 | −14.8 | |||

Micro 200 | 283 | −13.9 | ||

295 | −14.3 | −9.1 | 17.6 | |

307 | −14.2 | |||

EcoZeo 20 | 283 | −13.7 | ||

295 | −14.3 | −4.7 | 32.7 | |

307 | −14.4 |

**Table 8.**Coefficients of adsorption kinetics according to the PSO and ID model of CCP 20, Micro 200, and EcoZeo 20 (c

_{0}= 1000 mg NH

_{4}-N/L, pH

_{Start}5.3, pH

_{End}6.6–7.1, sorbent ratio 0.1 g

_{CLI}/mg

_{NH4-N}).

CLI | Pseudo-Second-Order Kinetic Model | Intraparticle Diffusion Model | ||||
---|---|---|---|---|---|---|

k_{2} | q_{e} | r² | k_{ID} | C | r² | |

[g/(mg min)] | [mg/g] | [-] | [mg/(min^{0.5} g)] | [mg/g] | [-] | |

CCP 20 | 0.064 | 6.99 | 0.8319 | 0.159 | 5.25 | 0.8852 |

Micro 200 | 0.046 | 6.62 | 0.8101 | 0.203 | 4.38 | 0.9520 |

EcoZeo 20 | 0.048 | 6.13 | 0.8158 | 0.191 | 4.02 | 0.9473 |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Wasielewski, S.; Rott, E.; Minke, R.; Steinmetz, H.
Evaluation of Different Clinoptilolite Zeolites as Adsorbent for Ammonium Removal from Highly Concentrated Synthetic Wastewater. *Water* **2018**, *10*, 584.
https://doi.org/10.3390/w10050584

**AMA Style**

Wasielewski S, Rott E, Minke R, Steinmetz H.
Evaluation of Different Clinoptilolite Zeolites as Adsorbent for Ammonium Removal from Highly Concentrated Synthetic Wastewater. *Water*. 2018; 10(5):584.
https://doi.org/10.3390/w10050584

**Chicago/Turabian Style**

Wasielewski, Stephan, Eduard Rott, Ralf Minke, and Heidrun Steinmetz.
2018. "Evaluation of Different Clinoptilolite Zeolites as Adsorbent for Ammonium Removal from Highly Concentrated Synthetic Wastewater" *Water* 10, no. 5: 584.
https://doi.org/10.3390/w10050584