Experimental Study on the Influence of Initial pH, Ionic Strength, and

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Experimental Study on the Influence of Initial pH, Ionic Strength, and Temperature on the Selective Adsorption of Dyes onto Nanodiamonds Hossein Molavi, Alireza Pourghaderi, and Akbar Shojaei*

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/02/19. For personal use only.

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran 11155-9465, Iran S Supporting Information *

ABSTRACT: In the current work, the performance of untreated nanodiamonds (UNDs) and thermally oxidized nanodiamonds (ONDs), as adsorbents for selective adsorption of methylene blue (MB) and methyl orange (MO) from aqueous media, was examined. The adsorption isotherm, initial pH, ionic strength, and thermodynamic study were investigated in batch experiments. The equilibrium adsorption data were analyzed by Langmuir and Freundlich isotherm models, which indicated that the isotherms were well fitted with the Langmuir model for both dyes. Thermodynamic parameters indicated that the adsorption operation was a feasible, spontaneous, and physisorption process in experimental conditions. Meanwhile, the adsorption of MO onto the UND was exothermic and determined by enthalpy change, while the adsorption of MB onto the OND was endothermic and an entropy-controlling process. The adsorption selectivity study exhibited that OND can rapidly and selectively separate cationic MB dye from the MO and MB mixture. On the basis of the negatively charged surface of the OND with highly negative zeta potential, the fact that the OND opted to adsorb MB over MO was firmly dependent on the initial pH value and ionic strength, which suggested that the possible mechanism of adsorption such as electrostatic interaction, hydrogen bonding, and π−π stacking might be important. Finally, based on the mentioned considerations, thermally OND was an attractive candidate for rapid adsorption and separation of cationic dyes from aqueous solution.

1. INTRODUCTION Adsorption separation is worth noticing as an attractive technique for removal of organic dyes from wastewater because of its many advantages such as low cost, simplicity of design, simple operation, operation at ambient temperature and pressure, insensitivity to toxic pollutants, and environment-friendly nature.1−3 Recently, many kinds of solid adsorbent such as graphene, graphene oxide, zeolite, metal− organic frameworks, carbon nanotube, layered double hydroxides, and mesoporous materials have been reported in the field of dye-selective adsorption.1,4−12 Nanodiamonds (NDs) are rather a new class of dyeadsorbent materials, which is produced by the detonation process, have an as-synthesized diameter of 4−10 nm, constructed from an inert diamond core and a graphene-like shell with a large number of functional groups including, carboxyl, anhydride, hydroxyl, ether, ester, ketone, and aldehyde.13,14 Because of its outstanding features such as good physical and chemical stability, excellent resistance to radiation and harsh environments, low cytotoxicity, high surface area, facile surface modification, and high affinity to organic dyes, the ND could be a suitable candidate for selective-dye adsorption.1,4 In recent years, selective removal of organic dyes from wastewater has been the subject of many studies, so various adsorbents have been introduced and their efficiencies have been examined. However, the experimental works on the ND © XXXX American Chemical Society

are relatively new and the reports in this area are superficial and sparse. For instance, in our previous work, the untreated ND (UND) and a set of NDs oxidized at different oxidation times were used as adsorbents for anionic (methyl orange, MO) and cationic (methylene blue, MB) dyes. According to the obtained results, the UND exhibited high adsorption capacity for anionic MO dye as a result of hydrogen bond, the formation of this robust bond via the sulfonate groups of MO dye and the oxygen-containing functional groups on the exterior surface of UND.1 Besides, the similar adsorption mechanism was accounted by Wang et al., when using the raw ND for adsorption of acid orange 7 from wastewater.15 In contrast to UNDs, oxidized NDs (ONDs) presented more tendencies for the adsorption of cationic MB dye because of great electrostatic interactions of MB (cationic dye) with ONDs (which had negative surface charges). Additionally, the adsorption capacity of MB onto ONDs and the adsorption selectivity against MO escalated by rising the thermal oxidation time because of higher electrostatic attraction caused by highly negative zeta potential of ONDs.1 The current study is aimed to investigate the adsorption selectivity of the UND and thermally OND against cationic and anionic dyes. Anionic MO and cationic MB dyes were Received: November 18, 2018 Accepted: March 13, 2019

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DOI: 10.1021/acs.jced.8b01091 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2.4. Analysis. The zeta potential of the UND and OND in pure water was determined by a zeta potential analyzer (Zetasizer, Malvern Instruments, Worcestershire, UK, model Nano ZS) at various pH values.

employed as sample ionic dyes to explore the adsorption performance of ND-derived adsorbents. Equilibrium adsorption isotherms and thermodynamics studies have been performed and the obtained results were analyzed with conventional models like Langmuir and Freundlich isotherm models. To study the spontaneous adsorption of both dyes, thermodynamic parameters such as ΔG (Gibbs free energy change), ΔH (enthalpy change), and ΔS (entropy change) were calculated. Additionally, a comparative study of both UND and OND adsorbents for selective adsorption was carried out at different adsorption conditions including temperature, initial pH value, and ionic strength. Finally, plausible adsorption mechanisms based on different interactions between dyes and adsorbents were also postulated.

3. RESULTS AND DISCUSSION The UND and OND were fully characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy, and zeta potential analyses, the results of which are shown in Figures S1−S5, and more details are described elsewhere.1,2 Briefly, the results obtained from FTIR spectroscopy (Figure S1), zeta potential (Figure S5a), and Boehm titration (Figure S5b) indicated that the amount of carboxylic acid functional groups on the surface of the thermally OND increases with increasing the thermal oxidation duration. This contributes to the improvement of the adsorption capacity of MB and adsorption selectivity over MO which is due to increment of the interaction via cationic MB dye and adsorbents with negative charges.1 The desorption of MO from the UND and MB from the OND was also studied, and so, the results indicated that the desorption of both anionic and cationic dyes in ethanol is slightly more than that in acetone. The driving force of MB and MO desorption from adsorbents is mainly due to disruption of the electrostatic interaction between dyes and the adsorbent and debonding the hydrogen bonds between MO and UND in presence of ethanol molecules. Additionally, as can be inferred from Figure S7, the adsorption capacity of the OND for MB and the UND for MO slightly decreased after three adsorption−desorption cycles, but they are still reasonably high, indicating their excellent potential as reusable adsorption nanomaterials.1 3.1. Adsorption Isotherms. Adsorption isotherm is one of the main parameters which can be used to evaluate the adsorption properties of adsorbents and also depict how the dye molecules interact with the adsorbent surface.19 Therefore, to describe the adsorption mechanism thoroughly, two wellknown isotherm models like Langmuir and Freundlich were used. The Langmuir adsorption model assumes that adsorbates adsorb through a homogeneous and monolayer manner on the surface of the adsorbent and there is no transmission of the adsorbate between adsorption sites because every adsorption site have the same adsorption energy with no interactions between adsorbed molecules. While the Freundlich model is formed on the multilayer and heterogeneous adsorption on the surface of the adsorbent because of the difference in the energy of active sites.4,20 The linear form of both Langmuir and Freundlich models are given as follows, respectively5

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Detonated ND (diameter of 4−6 nm, purity 98−99% wt, and SBET = 282 m2/g) was purchased from NaBond Technologies Co., China. MO, MB, potassium chloride (KCl 99% wt), and sodium chloride (NaCl 99.5% wt) were purchased from Merck and Sigma-Aldrich. Distilled water was used in all solution experiments. 2.2. Thermal Oxidation of NDs. The thermally OND at 425 °C for 4.5 h oxidation time was prepared according to the procedure reported in our previous work.1,16 2.3. Batch Adsorption Experiments. To investigate the adsorption isotherm, 10 mg of the UND or OND was added into 25 mL of the aqueous media with different concentrations of MO or MB (10−50 ppm) at 25, 35, and 45 °C. After adsorption equilibrium, the adsorbents were removed from the dye solution by 10 min centrifugation at 4000 rpm, and then, dyes’ concentration was determined using UV−vis spectroscopy. Finally, the maximum adsorption capacity of each dye was determined using the Langmuir adsorption isotherm. The equilibrium adsorption capacity qe (mg/g) was calculated according to the following equation17,18 qe =

(C0 − Ce)V W

(1)

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of the MO or MB solution, respectively, V (L) is the volume of the dye solution, and W (g) is the mass of the adsorbent. To investigate the effect of solution pH on the adsorption capacity, the pH of the dye solution was adjusted to 1−13 with 0.1 M NaOH or 0.1 M HCl. To attain the thermodynamic parameters of the adsorption procedure such as ΔG (Gibbs free energy change), ΔH (enthalpy change), and ΔS (entropy change), the adsorption experiment was performed at 25−45 °C. Finally, the effect of ionic strength on the adsorption capacity of the dyes was investigated in the presence of different salts such as NaCl and KCl in the scope of 0.01−0.5 mg/L. The ideal adsorption selectivity index (S) of a pair of dyes onto the UND and OND can be calculated using the following equation1 S=

Ce C 1 = + e qe KLqmax qmax ln qe = ln KF +

(4)

where qmax is the maximum adsorption capacity (mg/g), KL is the Langmuir constant connected to affiliation of the adsorbate and adsorbent which determines the energy of adsorption, KF and nF are Freundlich isotherm characteristic constants related to adsorption capacity per unit concentration and adsorption intensity of the adsorbents, respectively.4,20

qi /Ci qj /Cj

1 ln Cq nF

(3)

(2)

where q is the equilibrium adsorption capacity of the dye at all concentrations (C) of the dye. B



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Figure 1. Model fit of Langmuir isotherms with adsorption results of MB onto the OND (a) and MO onto the UND (b).

the UND is an exothermic process, while maximum capacity for the adsorption of MB increases with increasing temperature, demonstrating that the adsorption of this dye onto the OND is an endothermic process. The influence of temperature on the selective-adsorption behavior of the OND toward MO/MB dyes was investigated by UV−vis spectroscopy and photo-imaging, and the results are depicted in Figure 2. As shown in Figure 2, the initial color

Here, the experimental adsorption isotherms of both MO and MB dyes were conducted at 25, 35, and 45 °C and fitted with both Freundlich and Langmuir models. As can be observed from Figure 1, the adsorption of MO onto the UND and MB onto the OND are fitted well to the Langmuir model at all temperatures examined here. Thus, the results indicate that adsorption is homogeneous and monolayer which a single molecule lonely bonded to a single active site on the surface of adsorbent and all active sites have the same affinity for dye molecules and no any interactions between bonded dye molecules.21 To determine the favorability of the adsorption process, the equilibrium parameter (RL) is calculated using the following equation22 RL =

1 1 + (KLC0)

(5)

where KL (L/mg) is the Langmuir constant and C0 (mg/L) is the initial concentration of dyes.2,22 The calculated RL value indicates whether the adsorption systems show linear adsorption (RL = 1), irreversible adsorption (RL = 0), favorable adsorption (0 < RL < 1), or unfavorable adsorption (RL > 1). All the calculated values of RL were found to be in the range of 0−1, which indicates the favorable adsorption of MO onto the UND and MB onto the OND, respectively. As shown in Table 1, the qmax values obtained from the Langmuir model for MO decrease by increasing the temperature, which suggested that the adsorption trend of MO onto

Figure 2. Adsorption selectivity of MB over MO onto the OND and the corresponding photo-images as a function of temperature.

of the solution is forest green, as was expected because of the mixture of orange and blue colors. However, after the adsorption process, the color of the solution changes to yellow green, demonstrating the reduction of blue color which is caused by removing MB dye from the solution. This indicates that the OND selectively adsorbs MB over MO, which could be due to the different ionic charges leading to various electrostatic interactions between these dyes and the OND. Therefore, it is evidenced that the OND is able to separate the cationic MB dye efficiently and quickly from the anionic MO in the mixture. It is evidenced that selectivity of the OND for MB/MO increases considerably from 50 to 130 by increasing the temperature from 25 to 45 °C, and accordingly, the color of the solution becomes lighter green and darker yellow. This behavior might be the result of different adsorption mechanisms of these dyes at various temperatures.1,23 Therefore, it deserves to be investigated in more detail, as would be given in later sections.

Table 1. Langmuir Isotherm Constant and Thermodynamic Adsorption Parameters for MO onto the UND and MB onto the OND Langmuir constants T (°C)

qmax (mg/g)

KL (L/mg)

25 35 45

62.309 34.771 28.920

0.065 0.058 0.063

25 35 45

64.952 75.845 89.641

0.272 0.515 0.848

R2

ΔG0 (kJ/mol)

MO on UND 0.977 −1.706 0.962 −1.340 0.981 −0.973 MB on OND 0.985 −3.519 0.931 −3.879 0.943 −4.238

ΔH0 (kJ/mol)

ΔS0 (J/mol K)

−12.624

−36.620

7.201

35.956

C



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Figure 3. Effect of initial pH on MO adsorption on the UND and MB on the OND (a) and the zeta potential of NDs as a function of pH (b).

However, the positive value of ΔS0 for this adsorption process is favorable for impulsive adsorption which might be due to desorption of many adsorbed water molecules during the adsorption of one MB molecule. This observation suggests that the spontaneous adsorption of MB onto the OND is controlled by entropy change. Based on ΔG0 values (in the range of −10 to 0 kJ/mol) and ΔH0 values (lower than 84 kJ/mol) for both dyes, it can be postulated that the adsorption of MO and MB onto the UND and OND, respectively, is a physical adsorption process.19,25−27 3.3. Effect of Initial pH on the Adsorption of the Dyes. The pH of the dye media is one of the main important parameters, which controls the adsorption process and the adsorption capacity.24 The pH of the dye solutions mainly affects the electrostatic interaction between the adsorbent and dyes by changing the zeta potential of the adsorbent which depends on the functional groups on the surface of the adsorbent.6 Therefore, the effect of initial pH on the adsorption capacity of MO and MB onto the UND and OND, respectively, was studied in the range of pH values from 1.6 to 13.0 at room temperature and dyes concentration of 20 ppm. As displayed in Figure 3a, the adsorption capacity of MO onto the UND increases rapidly by increasing the pH up to 4 and then shows a relative decrease with further increase of pH, which could be due to increasing of the electrostatic repulsion via the anionic MO dye and the UND with negative charges. To determine the adsorption mechanism, the zeta potential of the UND and OND as a function of initial pH was also investigated, and the results are shown in Figure 3b. It is clear that the zeta potential of both adsorbents decreases by increasing the initial pH. This behavior might be due to the presence of large number of oxygen-containing functional groups like carboxyl on the surface of NDs. Additionally, as reported in our previous work,1 the content of carboxyl groups on the exterior surface of the ND increases by thermal oxidation time, which results in highly negative zeta potential which extremely changes by increasing the initial pH. Therefore, it can be concluded that the electrostatic interaction is a key factor that controls the adsorption process. It can be understood that the UND had the maximum adsorption capacity for MO in the pH = 3.3, which could be due to its neutral form in this pH. Because the pKa value of MO is equal to 3.47,5 it stands chiefly in the neutral form at pH = 3.3, which results in disappearance of the electrostatic repulsion between negative charge of the UND (−2.1 mV) and

3.2. Adsorption Thermodynamics. To explore the impact of temperature on the adsorption of MO onto the UND and MB onto the OND, the basic thermodynamic parameters such as Gibbs standard free energy (ΔG0, kJ/mol), standard enthalpy (ΔH0, kJ/mol), and standard entropy (ΔS0, J/mol K) were calculated using following equations9,10 ΔS 0 ΔH 0 − R RT

(6)

ΔG 0 = ΔH 0 − T ΔS 0

(7)

ln

qe Ce

=

where T is the absolute solution temperature (K), qe/Ce is the thermodynamic equilibrium constant, and R is the universal gas constant (8.314 J/mol K). The values of ΔH0 and ΔS0 were determined from the slope and intercept plot of ln qe/Ce against 1/T. Accordingly, the value of ΔG0 in the temperature range studied was calculated based on eq 7. The calculated thermodynamic parameters are listed in Table 1. The negative values of ΔG0 (see Table 1) indicate that the adsorption of both MO and MB onto the UND and OND, respectively, are favorable and spontaneous thermodynamically. As can be seen from Table 1, the absolute value of ΔG0 for adsorption of MO onto the UND gradually decreases with increasing temperature, while that for adsorption of MB onto the OND increases, which suggests that the adsorption of MB is more feasible and favorable at high temperatures.19,24 The negative ΔH0 value for MO, which is supported by decreasing the adsorption capacity by increasing the experimental temperature, shows that the adsorption of MO onto the UND is an exothermic process. The negative ΔH0 value for this adsorption process is favorable, while the negative ΔS0 value is unfavorable for spontaneous adsorption of MO onto the UND, which suggests that the randomness and the mobility of MO decrease after adsorption on the UND. Thus, the driving force of MO adsorption onto the UND is controlled by an enthalpy effect instead of an entropy impact.25 Contrary to MO adsorption, the positive value of ΔH0 for the adsorption of MB onto the OND confirms that the adsorption process is endothermic and the adsorption capacity increases with increasing temperature. The endothermic adsorption could be due to a weaker interaction between cationic MB dye and the OND in comparison with the interaction between the preadsorbed water and the OND. D



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Figure 4. Adsorption selectivity of MB over MO onto the OND and corresponding photo-images as a function of initial pH.

Figure 5. Effect of ionic strength on the adsorption capacity of MB onto the OND (a) and MO onto the UND (b).

neutral MO molecules.1 Therefore, here the gap between MO molecules and the UND decreases, which leads to the formation of strong hydrogen bonds between the oxygencontaining groups such as carboxyl, epoxy, and hydroxyl on the surface of the UND and the sulfonate groups of MO molecules. A similar observation was also reported in previous works.1,15 When pH becomes higher than the pKa value of MO, it dissociates to the sulfonate anions and sodium cations. Additionally, in this pH, the oxygen-containing functional groups such as hydroxylic and carboxylic groups on the surface of the UND are deprotonated to anionic forms and generate electrostatic repulsion between anionic MO molecules and high negative charge of the UND, which results in decreasing the adsorption capacity of MO onto the UND by increasing pH.28 By decreasing the initial pH of the UND/MO solution, the hydroxyl and carboxyl groups on the surface of the UND are protonated to the cationic form and the MO molecules exist in the neutral form. Thus, these phenomena prevent chances for the construction of hydrogen bonds via sulfonate groups of MO and the protonated hydroxylic and carboxylic groups on the surface of the UND which results in decreasing the adsorption capacity of MO onto the UND with decreasing pH. As it is demonstrated in Figure 3a, the trend of variation of adsorption capacity of MB onto the OND with pH is different from MO onto the UND, which might be attributed to different electrostatic forces between the OND and MB in comparison with the UND and MO. Accordingly, it is obvious that with increasing the initial pH, the electrostatic attraction between cationic MB molecules and the highly negatively

charged OND increases because of increment of the zeta potential of the OND (Figure 3b), leading to elevation of the adsorption capacity of the cationic MB dye.14 Thus, the increase in pH helped the selective adsorption of cationic dyes in the solution of both cationic and anionic dyes.29,30 Thus, to gain the selective adsorption and separation of a specific dye from the mixture of dyes, the experimental temperature is not the only factor to control the adsorption performance. The initial pH of dye solution, which determines the surface charge and zeta potential of the adsorbent, is another key factor to control the selective adsorption. Therefore, the selective adsorption behavior of OND toward both anionic MO and cationic MB dyes at different initial pH values were determined, and the results are shown in Figure 4. As it is seen, after the completion of the adsorption process, the solution color in any pH turned to a yellowish shade, in the light of blue color removal; and as can be seen by increasing the solution pH, this blue color removal becomes great and the adsorption capacity grows to a higher level. It can be found that the adsorption selectivity of MB over MO considerably increases by increasing the initial pH of the solution mixture and exhibits a maximum value at pH = 12.5. It can be assumed that at this pH, the adsorption selectivity is dominated by charge interactions. Here, the interfacial interaction via the MB cationic dye and the highly negative OND is electrostatic attraction, while this interaction for anionic MO is electrostatic repulsion, which results in increasing the adsorption capacity of MB and decreasing the adsorption capacity of MO, which consequently increases the adsorption selectivity of MB over MO on the OND. E



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process, while the adsorption of MB onto the OND was endothermic and controlled by entropy change. Furthermore, the equilibrium adsorption isotherms for both MO and MB dyes were well fitted to the Langmuir isotherm model. The adsorption selectivity study exhibited that the OND can rapidly and selectively separate the cationic MB dye from the MO/MB mixture. Besides, the adsorption selectivity of MB over MO onto the OND was strongly dependent on temperature, initial pH value, and ionic strength. The adsorption study indicated that the adsorption of anionic and cationic dyes on the surface of NDs was controlled by three mechanisms including electrostatic interaction, hydrogen bonding, and π−π stacking. Our work implies that the OND promises a big potential application for efficiently and quickly separating cationic dyes from wastewater and establishes a foundation for future studies in the field of selective adsorption.

Additionally, at the pH value lower than the pKa of MB, when the MB molecules exists in the neutral form, the capacity for adsorption of this dye onto the OND is equal to 25 mg/g which is almost a high value. Because in this condition there is no any electrostatic attraction between neutral MB molecules and negatively charged OND, the other mechanism like π−π stacking via the aromatic backbone of MB molecules and the graphene-like structure of the OND may happen. Therefore, the obtained results for dyes, both anionic and cationic, indicate that the adsorption on the surface of NDs is controlled by three mechanisms including, electrostatic interaction, hydrogen bonding, and π−π stacking, so depending on the adsorption condition, one of them dominates and controls the adsorption process. 3.4. Effect of the Ionic Strength on Adsorption of the Dyes. It is well known that there are several additives such as surfactants and salts in the industrials wastewater, which may retard or accelerate the adsorption process according to two effects, screening the electrostatic interaction between opposite charges, resulting in decreasing the adsorption capacity, and conversely, enhancing the dissociation of the dyes, which results in an increase in the adsorption capacity of dyes.2 Here, the effect of ionic strength (salt concentration) on the adsorption capacity of MB onto the OND and MO onto the UND is studied at neutral pH of each adsorbent with different salt concentrations adjusted by KCl and NaCl. As it is presented in Figure 5, the adsorption capacity of MO onto the UND decreases, while the adsorption capacity of MB onto the OND increases after addition of salt into dye solution and exhibits a maximum value at a certain concentration of salt (0.01 mol/L for both salts). The higher adsorption capacity of MB under these conditions can be attributed to the reduction of interactions between MB and water, which is caused by sodium and potassium cations. This phenomenon increases the affinity of MB to be adsorbed on the surface of the OND.31 Meanwhile, the adsorption capacity of the two dyes decreases by further increasing the concentration of salts. In the case of MB, this result could be due to the screening effect of Cl− ions which reduces the electrostatic attraction between MB and the OND.2,26,29,30 Accordingly, the different effects of ionic strength on the adsorption capacity of MO and MB can affect the adsorption selectivity of MB over MO. Therefore, the selective MB adsorption behavior of the OND against MO at different ionic strengths were investigated, so the outcomes indicated that the adsorption selectivity of MB over MO increased by the addition of salt into dye solution and exhibited a relative decrease in selectivity at higher salt concentrations. The maximum adsorption selectivity of MB over MO onto the OND at the critical concentration of NaCl and KCl (0.01 mol/ L for both salts) was found to be 117 and 95, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01091.

■

Procedure for studying the desorption and regeneration mechanisms of NDs. FTIR spectra of the UND and OND before and after adsorption of MO and MB, XRD patterns of the UND and OND before and after adsorption of MO and MB, field-emission SEM images of the UND and OND, zeta potential of the UND and OND, and the amount of carboxyl group content of adsorbents (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +98-2166166432. ORCID

Akbar Shojaei: 0000-0003-3708-4706 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support received from the Sharif University of Technology. REFERENCES

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4. CONCLUSIONS In the current work, the adsorption of anionic MO and cationic MB dyes onto NDs has been investigated to demonstrate the adsorption behavior of NDs against ambient conditions such as temperature, initial pH, and ionic strength. Because of the robust electrostatic attraction, the OND shows high affinity to the cationic MB dye, while the UND shows high affinity to the anionic MO dye which is attributed to the formation of the strong hydrogen bond. Based on the thermodynamic parameters, it was found that the adsorption of MO onto UND was exothermic and an enthalpy-controlling F



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Experimental Study on the Influence of Initial pH, Ionic Strength, and

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