Removal of methylene blue (MB) by bimetallic- metal organic framework

Introduction

Dyes exist where there is civilisation. They are used to colour products, and employed in various industries, such as the food, paper, carpet, rubber, plastic, cosmetic, acrylic, wool, nylon, silk and textile industry [1-3]. Cationic methylene blue MB (tetramethylthionine chloride) is a basic thiazine dye as shown in Figure 1 [4]. As a basic dye, MB is not strongly hazardous, but may cause some harmful effects on humans and aquatic lives. It is also resistant to biological degradation [5].

fig-c4bafe9627a3f4eb15cf36c7a00f6dd3 Figure 1. Chemical structure of MB <xref id="xref-ea34e607261a40b128ce37f1b24950f3" ref-type="bibr" rid="ref-7b893ae5b66561b8f94a25228c1ac80f">[1]</xref>

Water is a precious resource for all living creatures on earth. A significant environmental challenge is the removal of dye pollutants from fabric and textile wastewater [6]. Use of dyes to colour products consumes significant volumes of water; consequently, a substantial amount of coloured wastewater can be generated [7]. Many approaches to dye removal have been proposed to treat the industrial wastewater [8,9]. The techniques are classified into three main types: physical, chemical and biological treatments. These techniques include coagulation, membrane, separation process, adsorption process, filtration, softening , reverse osmosis, electrochemical processes, chemical oxidation, and aerobic and anaerobic microbial degradation [10].

The adsorption process is the simplest technique for dye removal due to its low cost, easy availability, simplicity of design, high efficiency, ease of operation [11,12]. While activated carbon is presently believed to be the most operative adsorbent, its high cost means its production and regeneration remain uneconomical [13,14]. This limitations of using activated carbons have led researchers to seek low-priced dye sorbents, such as coal, fly-ash, silica gel, wool waste, agricultural waste, wood waste, and clay materials [11,15]. In recent years, research and development in the field of design and synthesis of MOFs has led to a rapid growth in practical and conceptual developments [16-21]. An extensive class of crystalline materials has become available because of metal organic framework (MOF) chemistry, which has superior characteristics such as high stability, tuneable metrics, organic functionality and porosity [16].

Its exceptional porousness means that MOFs have potentially numerous applications; their demonstrated applications in gas storage, separations, catalysis, energy technology fuel cells, supercapacitors and catalytic conversions has made them objects of intensive study, industrial-scale production and application [22-25].

The unique characteristics of MOF-type substances that make them the focus of much worldwide research are their pore geometry and high porosity [26,27], their central metals [28,29], open metal sites [30,31], functionalised linkers [32,33] and their loading of active species [34,35]. All these characteristics have been scientifically employed to successfully improve interactions between the sorbates and MOFs. Specifically, these characteristics distinguish MOFs from other porous material in the field of adsorption processes for the effective removal of hazardous compounds [36]. Accordingly, MOFs are superior adsorbents because of their various host–guest interactions, acid-base [37,38], π-complexation [39], H-bonding [40,41] and coordination with open metal sites [31,36]. The pore size has a prime effect on the adsorption capacity of MOFs therefore exceptionally high dye uptake was demonstrated by mesoporous MOFs [42]. Very few studies was made based on bimetallic MOF [43]. UiO-66 has attractive characteristics as an adsorbent to toxic chemicals from wastewater because it has a higher hydrothermal stability among other MOFs [44]. Bimetallic-UiO-66 was recently used to significantly adsorb anionic dyes [45].

In this study, based on batch adsorption experiments, UiO-66 and UiO-66-Ca samples were used as sorbents to remove MB from an aqueous solution. Equilibrium and kinetic adsorption models were used to represent the experimental data. The equilibrium study was undertaken using Langmuir and Freundlich isotherms. The kinetics study was conducted using pseudo first-order and pseudo second-order models as well as intraparticle diffusion.

Materials and Methods Synthesis and Activation

All chemicals were supplied by Sigma-Aldrich (Australia) without further purifications.

UiO-66 was synthesised successfully using a scaled-up procedure of a previously reported method [46]. Specifically, 2.27 mmol of ZrCl4 and 2.27 mmol 1,4-benzenedicarboxylic acid (BDC) were mixed with 405.38 mmol of N, N-dimethylformamide (DMF) in an autoclave and heated in a preheating oven at 393 K for 24 h. The produced UiO-66 was immersed in chloroform for 5 days to remove unreacted precursors. Then, the crystalline product was filtered and dried under vacuum at 463 K for 48 h.

UiO-66-10%Ca was synthesised by mixing ZrCl4 (1.5 g) with BDC (1.1 g) in 73 mL of DMF. After mixing for 15 min, 0.15 g of Ca (NO3)2.4H2O was added and followed by the addition of 2 mL of H2O to the mixture. The solution is mixed for approximately 30 min; then transferred into a 125-mL Teflon-lined autoclave, which is tightly sealed and placed in a preheated oven at 132 °C for 1 d. UiO-66-30%Ca was synthesised by mixing ZrCl4 (1.5 g, 6.44 mmol) with BDC (1.3 g, 7.82 mmol) in 70 mL of DMF. The solution was mixed for 30 min, then Ca (NO3)2.4H2O (0.45 g, 2.86 mmol, 99%; Sigma-Aldrich) was added to the mixture. After that, 5 mL of deionised water was added into the mixture. Eventually, the mixture was transferred to a Teflon-lined autoclave which was tightly sealed and moved into a preheating oven at 430 K. The products were then filtered, dried and immersed in absolute methanol (100%, Sigma-Aldrich) for 5 d, after that it was dried and heated under vacuum at 473 K overnight before use as adsorbents.

 Characterisation

The thermal stability of UiO-66, UiO66-10%Ca and UiO66-30%Ca were assessed by a thermogravimetric analysis (TGA) instrument (TGA/DSC1 STARe system; Mettler-Toledo). The samples were loaded into a pan and heated to 1173 K at a rate of 5 K/min. The air flow rate was maintained at 50 mL/min. FTIR spectra (Spectrum 100 FT-IR spectrometer, PerkinElmer, Waltham, USA) were obtained to assess the stability of the functional groups on the organic ligands. The spectra were scanned from 600 to 4000 cm−1 with a resolution of 4 cm−1 using an attenuated total reflectance technique. X-ray powder diffraction and patterns were obtained with an X-ray diffractometer (D8 Advance, Bruker AXS) using Cu Kα radiation (λ = 1.5406 Å) with accelerating voltage and current of 40 kV and 40 mA respectively. Autosorb-1(Quantachrome, instruments) was used to determine N2 adsorption/desorption isotherms as well as the pore size and surface area of the MOFs. The samples were initially evacuated at 473 K for 24 h. Then, the sample was analysed to determine surface area, pore size and pore volume.

 Adsorbtion Study

An aqueous stock solution of MB (1000 ppm) was prepared by dissolving MB (C16H18ClN3S, ≥95%, Sigma-Aldrich) in deionised water. Aqueous solutions with different concentrations of MB (5–100 ppm) were prepared by successive dilution of the stock solution with water. After obtaining the UV spectra of the solutions with a spectrophotometer (UV spectrophotometer), the MB concentrations were determined using absorbance at 668 nm wavelength of the solutions. A calibration curve was obtained from the spectra of the standard solutions (5–100 ppm).

Prior to adsorption, the adsorbents were dried overnight under vacuum at 373 K. Several glass containers were cleaned, dried and filled to 20 mL with MB of different concentrations ranging from 5 to 50 ppm. An exact amount of the MOF adsorbent (20 mg) was then put in each container.

The dye solutions containing the adsorbents were mixed well with a magnetic stirrer and maintained for a period from 5 min to 24 h at 298 K. Samples for analysis were collected by a syringe filter at different sampling intervals. UV spectrometer was used to investigate the dye content in the supernatant.

Adsorbed amounts of MB by the Zr-MOFs at each time interval of time, the equilibrium and percentage removal of MB were computed according to the following equations:

\begin{equation} q_{t}=\left ( C_{0}-C_{t} \right )\frac{V}{m} \tag{1} \end{equation}

\begin{equation} q_{e}=\left ( C_{0}-C_{e} \right )\frac{V}{m} \tag{2} \end{equation}

\begin{equation} R \%=\frac{\left ( C_{o} - C_{t}\right )}{C_{o}}\times 100 \tag{3} \end{equation}

Where:

qt: the amount of MB adsorbed per unit weight of MOF at any time t (mg/g)

qe: the amount of MB adsorbed per unit weight of MOF at equilibrium (mg/g)

C0: the initial concentration of the MB solution at time zero (mg/L)

Ct: the concentration of the MB solution at time t (mg/L)

Ce: the concentration of the MB solution at equilibrium (mg/L)

V: volume of the MB solution in batch adsorption process (L)

R%: percentage removal of MB [5]

m: Zr-MOF mass used in adsorption batch process (g) [1,47,5].

Adsorption mechanism and rate of diffusion were estimated using three kinetic models: pseudo first-order, pseudo second-order [48-50] and intraparticle diffusion model [51,52]. The adsorbents’ adsorption behaviours were simulated using the Freundlich and Langmuir adsorption isotherms [48-50,53].

<bold id="bold-81986e73365d23ce7722108a88464b52">Kinetics study</bold>

Adsorption mechanism and rate of diffusion were estimated using three kinetic models: pseudo first-order, pseudo second-order [48-50] and intraparticle diffusion model [51,52]. The nonlinear form of the Lagergren pseudo first-order kinetic equation can be written as follows [54,55]:

\begin{equation} \frac{dq}{dt}=k_{1}\left ( q_{e}-q_{t} \right ) \tag{4} \end{equation}

The linear form of the pseudo first-order kinetic equation can be expressed as follows:

\begin{equation} ln\left ( q_{e}-q_{t} \right )=ln\left ( q_{e} \right )-k_{1}t \tag{5} \end{equation}

The nonlinear form of the pseudo second-order kinetic equation can be written as follows [56]:

\begin{equation} \frac{dq}{dt}=k_{2}\left ( q_{e}-q_{t} \right )^{2} \tag{6} \end{equation}

The linear form of the pseudo second-order kinetic equation can be written as follows:

\begin{equation} \frac{t}{q_{t}}=\frac{1}{k_{2}q_{e}^{2}}+\frac{1}{q_{e}}t \tag{7} \end{equation}

Where:

qe: the amount of MB adsorbed per unit weight of MOF at equilibrium (mg/g)

qt: the amount of MB adsorbed per unit weight of MOF at any time t (mg/g)

k1: pseudo first-order rate constant (min–1)

t: time (min)

k2: pseudo second-order rate constant (g/mg min).

A linear plot of the pseudo first-order model (ln [qe – qt]) against time provides the values for the kinetics sorption parameters, such as rate constant (k1), equilibrium adsorption capacity (qe) and the linear regression coefficient (R2). Likewise, a linear plot of the pseudo second-order model (t/qt) against time also provides the rate constant (k2), equilibrium adsorption capacity (qe) and the linear regression coefficient (R2).

As a result of the limitations of the pseudo first-order and pseudo second-order kinetic equations, the lack of an identified adsorption mechanism and the rate-limiting steps in the adsorption process, Weber and Morris established intraparticle diffusion model [117]. In general, the migration of sorbate molecules in bulk to the surface of a solid sorbent by intraparticle diffusion process is what controls the rate of most liquid/solid sorption systems. The analysis using Weber and Morris’s intraparticle diffusion model is as follows [51,52]:

\begin{equation} q_{t}=k_{p}t\tfrac{1}{2}+C \tag{7a} \end{equation}

Where:

qt: the amount of MB adsorbed per unit weight of MOF at any time t (mg/g)

kp: intraparticle diffusion rate constant (mg/g min0.5)

t: time (min)

C: constant represents the surface adsorption [57-59].

 <bold id="bold-76a69928df41258264e73c466a84b8d3">Equilibrium study</bold>

The adsorbents’ adsorption behaviours were simulated using the Freundlich and Langmuir adsorption isotherms [48-50,53].

The nonlinear form of the Langmuir isotherm can be expressed as:

\begin{equation} q_{e}=\frac{q_{m}k_{L}C_{e}}{\left ( 1+k_{L}C_{e} \right )} \tag{8} \end{equation}

while the linear form can be written as [60]:

\begin{equation} \frac{C_{e}}{q_{e}}=\frac{1}{q_{m}}C_{e}+\frac{1}{k_{L}q_{m}} \tag{9} \end{equation}

Where:

qm: Langmuir maximum loading capacity (mg/g)

kL : Langmuir constant related to the energy of adsorption and affinity of binding sites (L/mg) [61]

Ce: Equilibrium concentration of dye in solution (mg/L)

qe: Amount of dye adsorbed at equilibrium per unit mass of sorbent (mg/g).

The equilibrium experimental data were fitted using the linear form of the Langmuir isotherm equation (Equation 9). Specifically, the Langmuir parameters qm, KL, and R2 were obtained from the plot of (Ce/qe) against Ce.

The dimensionless constant separation factor, RL, is vital to the Langmuir isotherm, and can be found in the following equation [62-65]:

\begin{equation} R_{L}=\frac{1}{\left ( 1+k_{L}C_{0} \right )} \tag{10} \end{equation}

Where C0 is the initial concentration of adsorbate (mg/L) and KL (L/mg) is the Langmuir constant.

The shape of the isotherm depends on RL, because this factor indicates the adsorption process as:

Unfavourable (RL > 1)

Linear (RL = 1)

Favourable (0 < RL < 1)

Irreversible (RL = 0).

The nonlinear form of the Freundlich isotherm is written as:

\begin{equation} q_{e}=k_{F}C_{e}^{\frac{1}{n}} \tag{11} \end{equation}

Whereas the linear form of the Freundlich isotherm equation can be written as [60,66]:

\begin{equation} ln\left ( q_{e}\right )=ln\left (k_{F} \right )+\frac{1}{n} ln\left ( C_{e}\right ) \tag{12} \end{equation}

Where KF is the calculated Freundlich equilibrium constant ([mg/g] [L/mg] 1/n) and is an indicator of adsorption capacity, and n is a measure of the deviation from linearity of adsorption (g/L).

 Results and Discussion Characterisation

Figure 2 shows the N2 adsorption/desorption isotherms for UiO-66-Ca samples and UiO-66. Hysteresis in the desorption isotherm was distinguishably demonstrated by UiO-66- 10% Ca which had a sharp increase in adsorption at relative pressures close to 0.999. This observation is strong evidence that the mesopore and macropore sizes were enhanced [67].

fig-7a9057e216c47da3b22d17498f087a88 Figure 2. N2 adsorption/Desorption Isotherm of UiO-66 and UiO-66(Zr)-Ca

table-wrap-ba6b26fa10f393f5ac7ea1444f82c32d Table ‎1. Textural properties of the adsorbents based on N2 adsorption/isotherm

Table caption

Adsorbents Specific surface area (SBET) (m2g–1) Pore volume (cc g–1) Pore diameter (nm)
UiO-66 1585.5 0.82 1.04
UiO-66-10%Ca 918.115 1.10 2.39
UiO-66-30%Ca 557.681 0.25 0.91

In addition, Table 1 presents the textural properties of all adsorbents, according to the calculations of the N2 adsorption isotherm. The specific surface area (SBET) decreased with increasing content of a second metal. BET surface area in UiO-66 was 1585.50 m2 g–1 and then decreased to 918.115 and 557.68 m2 g–1 in UiO-66-10%Ca and UiO-66(Zr)-30%Ca respectively due to increasing the content of Ca in the synthesis process. The current BET values are acceptable when they compared with that in previous studies [68].

However, the pore volume and average pore size were enhanced in the MOFs with the lowest content of the second metal. The highest pore volume and pore size were seen in UiO-66-10%Ca, which were 1.10 cc g–1 and 2.39 nm, respectively. The results indicate that the addition of low concentrations of the second metal in the single-pot synthesis, followed by the activation process using the solvent exchange method, enhanced the pore volume and pore size replacing the second metal by methanol molecules which were discarded by the heating in the second stage of the activation process [69].

fig-a260e59ad4558f36cf03f43649662b01 Figure 3. Characterisation of UiO-66 samples. (a) PXRD patterns, (b) FTIR spectra and (c) TGA profiles of the UiO-66 samples

Figure 3(a) compares the XRD patterns of UiO-66-Ca with that of UiO-66. The results demonstrate that the integrity of the structure was maintained in activated samples, which indicates that the synthesis and activation procedures succeeded reliably without suspected impurities of a metal oxide inside the pores. The XRD patterns of activated samples are similar to the XRD pattern of UiO-66 in previous studies [46,70,71]. The patterns of the samples after using in the adsorption experiments in Figure 3a shows that UiO-66-30% Ca demonstrated higher stability than other samples because the pattern of this sample displayed all peaks as same as those of activated samples. However, other samples were distinguished by the main characteristic peak in 2 theta of 7° while other peaks were significantly reduced.

Figure 3(b) shows that the spectra of all samples, including that of UiO-66, exhibit the same vibration bands with slight deviations in the position of some peaks with increases in the content of a second metal. In addition, the peaks in the mixed-metal samples were broader than the peaks in the single-metal (Zr) sample, which indicates a difference in the dipole between ground state and excited state in the mixed-metal UiO-66 as a result of incorporating a second metal in the metal centre [72,73]. The vibration band of 1615–1580 cm–1 was attributed to C=C-C stretching in the aromatic ring of terephthalate salts; however, this band extended from 1590 to 1525 cm–1 in the mixed-metal UiO-66 [74]. Further, the bands at 1500 and 1390 cm–1 were attributed to the stretching vibrations of symmetric COO– and asymmetric COO– in coordinated organic linkers, as shown in the spectrum of UiO-66.

Moreover, the weak bands at 881, 812 and 785 cm–1 were assigned to Zr-O whereas the peak at 730 cm–1 in the UiO-66 spectrum was assigned to the stretching vibration of C-H and out-of-plane bending of aromatic ring in the main skeleton of UiO-66; this peak was shifted to 744 cm–1 in the spectra of bimetal UiO-66 [73,75]. In addition, the band at 1017 cm–1 belonged to C-H stretching in the MOF.

Figure 3(c) presents the results of thermogravimetric analysis for all adsorbents in this study. All samples appear to have the same thermal stability, with structural stability at increasing temperatures up to 725 K.

Kinematic Modelling Study

Figure 4 (a) to (f) describe the adsorption kinetics of MB by single-metal UiO-66 and bimetal UiO-66(Zr)-Ca. This Figure shows the amount of dye adsorbed (mg/g) on the adsorbents during different time periods (min) for various initial concentrations of MB. For all MB concentrations, MB uptake at the commencement of the adsorption process is very rapid; after an initial period of time, it proceeds at a slower rate until the saturation is attained [76-78].

fig-68dafabf47d05de6dc7947b9a43897d6 Figure 4. Fitting of experimental data by first-order and second-order kinetic models of MB adsorption onto UiO-66 (a, b), UiO-66-30%Ca (c, d) and UiO-66-10%Ca (e, f)

This phenomenon can be explained thus: the first available MB molecules are favourably adsorbed onto the most active sites of the single-metal and bimetal Zr-MOF, and the high initial MB uptake is possible because of the accessibility of many active sites. A longer contact time between the MOFs and MB results allows to increase the removal of MB until equilibrium adsorption capacity is reached [78]. Another explanation is that a higher initial concentration of MB provides more MB molecules and greater driving force of the aqueous phase (MB) against the solid phase (MOFs) to overcome mass-transfer resistance. This fact gives rise to increase collisions between MB molecules and active sites on the adsorbent [79,80]. For instance, the adsorption capacity for MB onto UiO-66 at equilibrium increased from 2.151 to 14.837 mg. g–1 with increase in MB concentration from 5 to 50 mg. L–1. Also, the adsorption capacity in UiO-66-Ca 10% and UiO-66-30%Ca was higher than that of UiO-66. It was reported that vacant metal sites in bimetallic MOFs are enhanced after removing the second metal by the solvent exchange activation [81]. Therefore, removing of Ca from UiO-66-Ca increased the active sites and consequently enhanced the adsorption capacity of MB [81].

table-wrap-f0275517efc2428e02475e687f4ebfc0 Table 2. Calculated kinetics constant (k<sub id="sub-8e586c640ad9c9b6481ed5fcd14a9529">2</sub>) and correlation coefficient (R<sup id="sup-6209caa94be1337fa453eb2a4904100c">2</sup>) for Ci = 5, 15, 30 and 50 ppm

Table caption

Adsorbent Adsorbate Pseudo second-order kinetics constant k2 (g/(mg.min))
5 ppm 15 ppm 30 ppm 50 ppm
k2 R2 k2 R2 k2 R2 k2 R2
UiO-66 MB 0.01050 0.9989 0.00546 0.9992 0.00273 0.9992 0.00147 0.999
UiO-66-10% Ca MB 0.86348 0.9999 0.07616 0.9963 0.04628 0.9999 0.02259 0.9998
UiO-66-30% Ca MB 0.00498 0.9991 0.00212 0.9984 0.00167 0.9992 0.00217 0.9996

The pseudo first-order and pseudo second-order model were employed for the adsorption of MB onto UiO-66, UiO-66-10%Ca and UiO-66-30%Ca. The linear regression correlation, R2, was calculated to identify the model of best fit; higher R2 values mean a better fit for the experimental data. The results of the correlational analysis of the amount of adsorbed dye (mg/g) against contact time, for the various initial concentrations of MB (5, 15, 30 and 50 ppm) are shown in Figure 4. The results indicate that the amount of dye loading (qt [mg/g]) increases with contact time at each level of MB concentration. In addition, the amount of MB adsorbed increased with increasing in the initial MB concentration [47].

table-wrap-3e75e00812cbdbbf18af3ac89726f970 Table ‎3. Calculated kinetics constant (k<sub id="sub-30354ab2fd9f11b14e52eceb367bb836">1</sub>) and correlation coefficient (R<sup id="sup-f091854ed26c3f9f70cbe78d6ee2c0ea">2</sup>) for Ci = 5, 15, 30 and 50 ppm

Table caption

Adsorbent Adsorbate Pseudo first-order kinetics constant k1 (min–1)
5 ppm 15 ppm 30 ppm 50 ppm
k1 R2 k1 R2 k1 R2 k1 R2
UiO-66 MB 0.0101 0.9888 0.0119 0.9920 0.0120 0.9925 0.0113 0.9813
UiO-66-10% Ca MB 0.2669 0.9716 0.0913 0.9731 0.0395 0.9886 0.0370 0.9920
UiO-66-30% Ca MB 0.0105 0.9930 0.0161 0.9636 0.0118 0.9927 0.0144 0.9960

The kinetics of the adsorption process in the laboratory-based on batch experiments enables the prediction of the rate at which a pollutant is removed from bulk solutions, which informs the design of adsorption treatment plant columns [82]. However, the physical and chemical properties of the adsorbent significantly affect its adsorption kinetics, which in turn, affects the sorption mechanism. Statistics from kinetics studies of pseudo first-order and pseudo second-order kinetics model equations have been investigated for fit with contact time data [76]. Table 2 and Table 3 below present their main characteristics as calculated kinetic constants (k1, k2) and correlation coefficients (R2) for Ci = 5, 15, 30 and 50 ppm.

According to the R2 values obtained, they have been consistent and closer to unity for the pseudo second-order kinetic equation than for the pseudo first-order kinetic equation. Therefore, based on R2 values, the sorption kinetics of MB removal using single-metal and bimetal Zr-MOF were well described by the pseudo second-order kinetic equation. Further, the calculated equilibrium adsorption capacity agreed with the experimental equilibrium adsorption capacity, further indicating that the sorption of aqueous MB onto single-metal and bimetal Zr-MOF perfectly obeyed pseudo second-order kinetics that indicates strong interactions happened between MB and active sites in UiO-66 [83]. Specifically, the sorption of MB by single and bimetal Zr-MOFs occurred through chemisorption (the exchange or sharing of electrons between the sorbate and sorbent via covalent forces and ion exchange) [56,83].

Based on the mechanism underlying pseudo second-order kinetics, the effects of the initial concentration on the adsorption kinetics of MB onto the three MOFs (i.e., all the sorbent systems) were similar over time. UiO-66-10% Ca was taken to be a representative adsorbent and was used to explain the effects of the initial concentration on the rate of adsorption. Precisely, Table 2 shows that the adsorption rate constants (k2) of Pseudo second -order model on UiO-66-10% Ca were higher than those on other adsorbents. Specifically, k2 on UiO-66-10% Ca was 0.86348, 0.07616, 0.04628 and 0.02259 g mg−1 min−1 at initial MB concentrations of 5, 15, 30 and 50 mg L−1, respectively, signifying a decrease in adsorption rate at higher initial concentrations of MB. Reductions in the amount adsorbed at higher initial concentrations may be due to MB molecules having to enter the pores through a longer diffusion path. On the other hand, with less amounts of MB adsorbed, MB molecules tend to be rapidly adsorbed into the open pores of MOFs, which eventually increases the adsorption rate (K2). Table 3 shows the kinetic constant of Pseudo first- order model (K2) and R2. K1 of UiO-66-10% Ca was also higher than that of UiO-66 and UiO-66-30% Ca. For instance, it was 0.0101, 0.2669 and 0.0105 respectively at initial concentration of 5 ppm. The lower rate constant for MB adsorption onto the UiO-66-30% was tentatively ascribed to MB diffusion into the micropores of the MOF [84].

 Intraparticle diffusion modelling study

A multistep adsorption process consists of the mass transfer of MB from the solution to the surface of single-metal and bimetal UiO-66; this transfer determines the extent of reaction throughout the whole adsorption process [85]. Adsorption process mechanism of MB onto MOFs can be arranged into the following three stages:

1. Film diffusion: the initial stage of rapid adsorption

2. Successive intraparticle diffusion: the second stage of the process during which the adsorption rate slows

3. The final stage: the adsorption attains equilibrium and lasting constant.

Film diffusion is very fast because of the rapid sorption of MB to the surface of the MOF. This stage is featured by quick surface mass transfer caused by a large differential which acts as a driving force. This stage is when the most is adsorbed by adsorbents, according to Weng et al. [86]. Such a finding establishes MOF-MB systems as entailing a fast adsorption process. Consequently, these adsorbent systems are favourable alternatives for removing cationic dyes from wastewater effluent. The second stage, intraparticle diffusion, is slower because the occupation of MB molecules on many of the available external sites in the first step slows the diffusion of MB molecules into the pore spaces of the MOF [85].

The mechanism of MB sorption on the surface of MOF was investigated using contact time data. Specifically, experimental data were fitted to the intraparticle diffusion model (Equation 13) and the outcomes interpreted by plotting qt versus t1/2 in Figure 5. The most important aspects of the intraparticle diffusion plot are first, the linear portion and the intercept of the plot (c), which indicates the effects of the boundary layer on the adsorption process.

fig-f75aef52b66a45763618274b2d867bd2 Figure 5. Fitting of experimental data using intraparticle diffusion models of MB adsorption onto UiO-66 (a), UiO-66-30%Ca (b) and UiO-66-10%Ca (c)

The second linear portion of the plot can be used to interpret intraparticle diffusion. The plot can be used to derive values for parameters, such as kp (the diffusion rate), C and R2, as presented in Table 4. The kp can be determined from the slope of the plot. The slope can be used to estimate the driving force of diffusion, which plays a critical role in the adsorption reaction. Experimental data analysis demonstrated that the kp values increased from 0.0991 to 0.638 mg g–1min–(1/2) with increases in the initial MB concentration from 5 to 50 mg L–1. Therefore, higher initial concentrations of MB increase the driving force and subsequently increase the MB diffusion rate. Further, increasing initial MB concentrations over a similar range led to increases in the intercept value (C) from 0.5017 to 3.8144 mg g–1, suggesting that an initial high concentration of basic dye is associated with a stronger boundary layer effect in the sorption process. In addition, an increase in the intercept value (C) can indicate the availability of MB on the boundary layer of UiO-66.

table-wrap-5038cca18291c644fef7c8428b1138aa Table 4. Calculated kinetics constant (kp), C and correlation coefficient (R<sup id="sup-723196be6171ecca69d22e3575ba25c0">2</sup>) for Ci = 5, 15, 30 and 50 ppm.

Table caption

Adsorption mechanism
Intraparticle diffusion model
Adsorbent Initial concentration of MB solution (mg L–1) kp (mg g–1min–(1/2)) C (mg g–1) R2
UiO-66 5 0.0991 0.5017 0.9999
15 0.22 2.1455 0.9918
30 0.3955 3.5763 0.9884
50 0.638 3.8144 0.9959
UiO-66-10%Ca 5 0.4851 3.4862 0.9981
15 0.9352 9.927 0.9883
30 0.7581 23.867 0.9655
50 0.6585 39.364 0.9040
UiO-66-30%Ca 5 0.218 1.3901 0.9983
15 0.5641 2.4216 0.9951
30 0.7423 6.186 0.9999
50 0.727 11.013 0.9928

Equilibrium Modelling Study

Recent research has revealed that initial concentration of MB has a detrimental effect on adsorption process. Initial concentration of MB plays a role in determining removal efficiency of MB (R %) and equilibrium adsorption capacity (qe); indeed, the initial concentration of MB has profound consequences for R% and qe. The initial concentration of MB positively affected qe and negatively affected R% [60,78]. The observed decrease in MB removal (R% values of 43.03% to 29.67%) by UiO-66 samples was representative of the adsorption process in all systems and confirmed the occupation of all accessible active sites on the UiO-66 above a certain concentration of MB. However, the increase in equilibrium adsorption capacity (qe) from 2.15 to 14.83 mg/g can be attributed to the higher adsorption rate and the use of all available active sites on UiO-66 samples for sorption at higher concentrations of MB.

Equilibrium isotherms were examined using the Langmuir and Freundlich isotherms. The assumption of the Langmuir isotherm is monolayer coverage of sorbate over a sorbent with homogenous surface [5,63]. It assumes that the adsorption process occurs at specific homogenous sites over the adsorbent; that is, when an MB molecule occupies a specific site, additional sorption cannot happen again at the same site. Successful implantation of the Langmuir adsorption isotherm has been undertaken to explain the adsorption of basic dyes such as MB from aqueous solutions [66].

table-wrap-1cfc05d50a65d2b14003639d035b2b50 Table 5. Calculated equilibrium constants (kL, kF, qm, n and correlation coefficient (R<sup id="sup-8469372578a830af1344af0c1bc7d393">2</sup>)) of MB adsorption onto UiO-66, UiO-66-30%Ca and UiO-66-10%Ca for Ci = 5, 15, 30 and 50 ppm.

Table caption

Adsorbent Adsorption isotherm model Parameter Value R2
UiO-66 Langmuir qm (mg/g) 31.74 0.9889
KL (L/mg) 0.02447
Freundlich KF ([mg/g] [L/mg]1/n) 0.98157 0.9979
n (g/L) 1.2918
UiO-66-10%Ca Langmuir qm (mg/g) 50.2512 0.9951
KL (L/mg) 39.8
Freundlich KF ([mg/g] [L/mg]1/n) 47.9855 0.9973
n (g/L) 5.0150
UiO-66-30%Ca Langmuir qm (mg/g) 23.7529 0.9821
KL (L/mg) 0.4982
Freundlich KF ([mg/g] [L/mg]1/n) 8.0164 0.9926
n (g/L) 3.0911

According to Freundlich model, the favourability of adsorption can be estimated by the magnitude of the exponent (1/n), which predicts the feasibility of the adsorption process. The values of n must be greater than one for conditions to be favourable for an adsorption process [5,87]. The constant n values of UiO-66, UiO-66-10%Ca and UiO-66-30%Ca have been calculated to be 1.29, 5.01 and 3.09, respectively. These values confirm the favourability of adsorption of MB onto single-metal and bimetal Zr-MOF. The results of the correlational analysis for KF, n and the linear regression coefficient (R2) for the plot of the linear form of the Freundlich model are presented in Table 5.

Figure 6 illustrates the experimental equilibrium data and the predicted theoretical isotherms for the adsorption of MB onto single-metal and bimetal Zr-MOFs. It is apparent, from Figure 6 and the R2 values in Table 5, that there is closer fit between the experimental data and Freundlich isotherm compared to that with the Langmuir isotherm, at higher values of R2.

fig-a184da19754a50c4520a6ce5bcbe93d5 Figure ‎6. Fitting of experimental data using Langmuir and Freundlich models of MB adsorption onto UiO-66 (a), UiO-66-30%Ca (b) and UiO-66-10%Ca (c).

Analyses and calculations of the Langmuir and Freundlich plots revealed that the values of the linear regression correlation coefficient (R2) for the Langmuir model are 0.9889, 0.9951 and 0.9821, and for the Freundlich model 0.9979, 0.9973 and 0.9926, for UiO-66, UiO-66-10%Ca and UiO-66-30%Ca, respectively.

Further, Freundlich constants (KF) related to the bonding energy of MB molecules with single-metal and bimetal Zr-MOFs were greater than Langmuir constants which were related to the affinity of MB molecules to single-metal and bimetal Zr-MOF in all cases. As a result, the adsorption of MB onto single-metal and bimetal Zr-MOF occurred as multilayer adsorption on a heterogeneous surface. The calculated maximum monolayer adsorption capacity (qm) of Zr-MOF for MB is 50.25 mg/g for UiO-66-10%Ca, a relatively satisfactory adsorption capacity (see Table 4). According to Langmuir isotherm, the calculated results for the separation factor (RL) are (0.89–0.44), (0.005–0.0005) and (0.28–0.03) for UiO-66, UiO-66-10%Ca and UiO-66-30%Ca, respectively. RL values for the sorption of MB onto single-metal and bimetal UiO-66 are in the range of 0 < RL < 1, indicating that the adsorption was favourable. Further, higher initial MB concentrations in the adsorption process can make it irreversible [88].

Table 6 lists the maximum adsorption capacity (qm) of the UiO-66 adsorbents in this study for MB, relative to those reported in the literature for different adsorbents of MB. The performance of UiO-66 in MB removal is relatively effective by comparison.

table-wrap-986a62b69ed91b3f071c793fdacfda68 Table ‎6. Comparison of monolayer equilibrium capacity for methylene blue onto different adsorbents.

Table caption

Adsorbent qm (mg/g) Reference
Untreated coffee husks 90.1 [89]
Sewage sludge from agrifood industry wastewater treatment plant 86.957 [90]
Raw date pits 80.29 [91]
Calcined pure clay 56.31 [92]
UiO-66-10%Ca 50.25 This study
UiO-66-30%Ca 23.75 This study
UiO-66 14.52 This study
Luffa cylindrica fibres 47 [57]
Carbon nanotubes 46.2 [93]
Rice husk 40.59 [94]
Garden grass 31.4 [95]
Raw clay 27.49 [92]
Jute processing waste 22.47 [96]
Fe (III)/Cr (III) hydroxide 22.8 [60]
Banana peel 20.8 [97]
Orange peel 18.6 [97]
Activated date pits (T = 900 °C) 17.27 [91]
Fly-ash 13.42 [98]
Calcined raw clay 13.44 [92]
Activated date pits (T = 500 C) 12.94 [91]
Zeolite 12.7 [99]
Clay 6.3 [100]
Fly-ash 1.3 [99]

Conclusion

The main goal of the current study was to assess the adsorption capacity of MB in UiO-66, UiO-66-10%Ca and UiO-66-30%Ca. The Zr-MOFs were prepared according to a single pot solvothermal methods with a modification using trace additives of Ca. Compared with the UiO-66 without the modification; the textural properties of the modified UiO-66 were enriched while their performances were enhanced to remove MB from wastewater. The kinetics of MB sorption onto UiO-66, UiO-66-10% Ca and UiO-66-30% Ca were fitted by the pseudo first and second-order models. The second model offered the best fit for the experimental data for all systems studied. The mechanism of MB sorption onto the surface of MOFs was investigated using contact time data. Specifically, the fitting of experimental data to the intraparticle diffusion model identified three stages in the sorption process.

Langmuir and Freundlich plot analyses and calculations revealed that the values of the linear regression correlation coefficient (R2) for the Freundlich model were greater than those for the Langmuir model, for UiO-66, UiO-66-10%Ca and UiO-66-30%Ca. As a result, the adsorption of MB onto single-metal and bimetal Zr-MOFs was considered to occur as multilayer adsorption on a heterogeneous surface.

Langmuir maximum loading capacity (qm) was compared with other reported adsorbents in previous studies. The values of the separation factor (RL) indicated that the adsorption was a favourable process. Using the Freundlich linear model, constant n values for UiO-66, UiO-66-10%Ca, and UiO-66-30%Ca were found to be more than one (i.e., n > 1). These values confirm the favourability of MB adsorption onto single-metal and bimetal Zr-MOF. This study can suggest the bimetallic UiO-66 as an attractive adsorbent to remove dyes from wastewater.

 1 04 46-53 332 2009 10.1016/j.jcis.2008.12.012 Almeida C.A.P. Debacher N.A. Downs A.J. Cottet L. Mello C.A.D. Journal of Colloid and Interface Science Removal of methylene blue from colored effluents by adsorption on montmorillonite clay 1 04 51-59 65 2005 10.1016/j.dyepig.2004.06.016 Bhattacharyya K Sharma A Dyes and Pigments Kinetics and thermodynamics of Methylene Blue adsorption on Neem () leaf powder 1 04 187-198 163 2009 10.1016/j.jhazmat.2008.06.078 Tunç Özlem Tanacı Hacer Aksu Zümriye Journal of Hazardous Materials Potential use of cotton plant wastes for the removal of Remazol Black B reactive dye 8 08 1244-1245 38 2003 10.1016/s0022-3468(03)00278-1 Albert Matthew Lessin Marc S Gilchrist Brian F Journal of Pediatric Surgery Methylene blue: dangerous dye for neonates 2-3 01 753-759 161 2009 10.1016/j.jhazmat.2008.04.019 Hameed B.H. Journal of Hazardous Materials Spent tea leaves: A new non-conventional and low-cost adsorbent for removal of basic dye from aqueous solutions 6 11 5301-5313 8 2019 10.1016/j.jmrt.2019.08.051 Abdelrahman Ehab A. Hegazey R.M. El-Azabawy Ragaa E. Journal of Materials Research and Technology Efficient removal of methylene blue dye from aqueous media using Fe/Si, Cr/Si, Ni/Si, and Zn/Si amorphous novel adsorbents 1-3 05 70-80 177 2010 10.1016/j.jhazmat.2009.12.047 Rafatullah Mohd. Sulaiman Othman Hashim Rokiah Ahmad Anees Journal of Hazardous Materials Adsorption of methylene blue on low-cost adsorbents: A review 1740 7 1747 43 2004 10.1021/ie034218g Gupta V. K. Suhas Suhas Ali Imran Saini V. K. Industrial & Engineering Chemistry Research Removal of Rhodamine B, Fast Green, and Methylene Blue from Wastewater Using Red Mud, an Aluminum Industry Waste 3 05 247-255 77 2001 10.1016/s0960-8524(00)00080-8 Robinson Tim McMullan Geoff Marchant Roger Nigam Poonam Bioresource Technology Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative 25 3 08 946-952 136 2006 10.1016/j.jhazmat.2006.01.038 Wang S Zhu Z Journal of Hazardous Materials Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution 1-3 06 259-267 194 2006 10.1016/j.desal.2005.10.032 Bulut Yasemin Aydın Haluk Desalination A kinetics and thermodynamics study of methylene blue adsorption on wheat shells 14 10 3357-3366 35 2001 10.1016/s0043-1354(01)00056-2 Meshko V Markovska L Mincheva M Rodrigues A.E Water Research Adsorption of basic dyes on granular acivated carbon and natural zeolite 3 136 2006 Wang S Zhu ZH Journal of Hazardous Materials Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution 1 01 129-138 39 2005 10.1016/j.watres.2004.09.011 Wang Shaobin Boyjoo Y. Choueib A. Zhu Z.H. Water Research Removal of dyes from aqueous solution using fly ash and red mud 2 06 455-458 286 2005 10.1016/j.jcis.2005.01.036 Akbal Feryal Journal of Colloid and Interface Science Adsorption of basic dyes from aqueous solution onto pumice powder 6941 06 705-714 423 2003 10.1038/nature01650 Yaghi Omar M. O'Keeffe Michael Ockwig Nathan W. Chae Hee K. Eddaoudi Mohamed Kim Jaheon Nature Reticular synthesis and the design of new materials 8 08 474-484 31 1998 10.1021/ar970151f Yaghi Omar M. Li Hailian Davis Charles Richardson David Groy Thomas L. Accounts of Chemical Research Synthetic Strategies, Structure Patterns, and Emerging Properties in the Chemistry of Modular Porous Solids† 19 11 06 1460-1494 37 1998 10.1002/(sici)1521-3773(19980619)37:11<1460::aid-anie1460>3.0.co;2-z Batten Stuart R. Robson Richard Angewandte Chemie International Edition Interpenetrating Nets: Ordered, Periodic Entanglement 1 06 37-48 152 2000 10.1006/jssc.2000.8667 Férey Gérard Journal of Solid State Chemistry Building Units Design and Scale Chemistry 8 08 1739-1753 71 1998 10.1246/bcsj.71.1739 Kitagawa Susumu Kondo Mitsuru Bulletin of the Chemical Society of Japan Functional Micropore Chemistry of Crystalline Metal Complex-Assembled Compounds 1 06 1-2 152 2000 10.1006/jssc.2000.8733 Yaghi O.M. O'Keeffe M. Kanatzidis M. Journal of Solid State Chemistry Design of Solids from Molecular Building Blocks: Golden Opportunities for Solid State Chemistry 29 6149 08 1230444 341 2013 10.1126/science.1230444 Furukawa Hiroyasu Cordova Kyle E. O’Keeffe Michael Yaghi Omar M. Science The Chemistry and Applications of Metal-Organic Frameworks 26 2 01 673-674 112 2012 10.1021/cr300014x Zhou Hong-Cai Long Jeffrey R. Yaghi Omar M. Chemical Reviews Introduction to Metal–Organic Frameworks 7 626-636 16 2006 10.1039/b511962f Mueller U. Schubert M. Teich F. Puetter H. Schierle-Arndt K. Pastré J. J. Mater. Chem. Metal–organic frameworks—prospective industrial applications 25 34 08 13-16 86 2008 10.1021/cen-v086n034.p013 Jacoby Mitch Chemical & Engineering News Heading to market with MOFS 01 5990 07 424-428 329 2010 10.1126/science.1192160 Furukawa H. Ko N. Go Y. B. Aratani N. Choi S. B. Choi E. Yazaydin A. O. Snurr R. Q. O'Keeffe M. Kim J. Yaghi O. M. Science Ultrahigh Porosity in Metal-Organic Frameworks 51 12 17998-17999 127 2005 10.1021/ja0570032 Millward Andrew R. Yaghi Omar M. Journal of the American Chemical Society Metal−Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature 33 08 10870-10871 130 2008 10.1021/ja8036096 Caskey Stephen R. Wong-Foy Antek G. Matzger Adam J. Journal of the American Chemical Society Dramatic Tuning of Carbon Dioxide Uptake via Metal Substitution in a Coordination Polymer with Cylindrical Pores 25 07 8775-8777 131 2009 10.1021/ja901587t Hamon Lomig Serre Christian Devic Thomas Loiseau Thierry Millange Franck Férey Gérard Weireld Guy De Journal of the American Chemical Society Comparative Study of Hydrogen Sulfide Adsorption in the MIL-53(Al, Cr, Fe), MIL-47(V), MIL-100(Cr), and MIL-101(Cr) Metal−Organic Frameworks at Room Temperature 16 08 8620-8626 24 2008 10.1021/la800803w Karra Jagadeswara R. Walton Krista S. Langmuir Effect of Open Metal Sites on Adsorption of Polar and Nonpolar Molecules in Metal−Organic Framework Cu-BTC 1 01 190-197 90 2011 10.1016/j.fuel.2010.08.008 Blanco-Brieva G. Campos-Martin J.M. Al-Zahrani S.M. Fierro J.L.G. Fuel Effectiveness of metal–organic frameworks for removal of refractory organo-sulfur compound present in liquid fuels 17 6 07 755-762 14 2008 10.1007/s10450-008-9137-6 Arstad Bjørnar Fjellvåg Helmer Kongshaug Kjell Ove Swang Ole Blom Richard Adsorption Amine functionalised metal organic frameworks (MOFs) as adsorbents for carbon dioxide 1-3 09 535-542 181 2010 10.1016/j.jhazmat.2010.05.047 Haque Enamul Lee Ji Eun Jang In Tae Hwang Young Kyu Chang Jong-San Jegal Jonggeon Jhung Sung Hwa Journal of Hazardous Materials Adsorptive removal of methyl orange from aqueous solution with metal-organic frameworks, porous chromium-benzenedicarboxylates 31 07 13906-13917 113 2009 10.1021/jp902736z Peterson Gregory W. Wagner George W. Balboa Alex Mahle John Sewell Tara Karwacki Christopher J. The Journal of Physical Chemistry C Ammonia Vapor Removal by Cu3(BTC)2 and Its Characterization by MAS NMR 08 49-54 100 2012 10.1016/j.fuproc.2012.03.006 Khan Nazmul Abedin Jhung Sung Hwa Fuel Processing Technology Adsorptive removal of benzothiophene using porous copper-benzenetricarboxylate loaded with phosphotungstic acid 01 444-456 244-245 2013 10.1016/j.jhazmat.2012.11.011 Khan Nazmul Abedin Hasan Zubair Jhung Sung Hwa Journal of Hazardous Materials Adsorptive removal of hazardous materials using metal-organic frameworks (MOFs): A review 4 1306-1308 47 2011 10.1039/c0cc04759g Khan Nazmul Abedin Jun Jong Won Jeong Jong Hwa Jhung Sung Hwa Chem. Commun. Remarkable adsorptive performance of a metal–organic framework, vanadium-benzenedicarboxylate (MIL-47), for benzothiophene 01 123-129 129 2013 10.1016/j.apcatb.2012.09.020 Ahmed Imteaz Hasan Zubair Khan Nazmul Abedin Jhung Sung Hwa Applied Catalysis B: Environmental Adsorptive denitrogenation of model fuels with porous metal-organic frameworks (MOFs): Effect of acidity and basicity of MOFs 10 180-185 237-238 2012 10.1016/j.jhazmat.2012.08.025 Khan Nazmul Abedin Jhung Sung Hwa Journal of Hazardous Materials Low-temperature loading of Cu+ species over porous metal-organic frameworks (MOFs) and adsorptive desulfurization with Cu+-loaded MOFs 18 33 08 11623-11627 105 2008 10.1073/pnas.0804900105 Britt D. Tranchemontagne D. Yaghi O. M. Proceedings of the National Academy of Sciences Metal-organic frameworks with high capacity and selectivity for harmful gases 19 5 01 2047-2056 115 2011 10.1021/jp1092724 Hamon Lomig Leclerc Hervé Ghoufi Aziz Oliviero Laetitia Travert Arnaud Lavalley Jean-Claude Devic Thomas Serre Christian Férey Gérard De Weireld Guy Vimont Alexandre Maurin Guillaume The Journal of Physical Chemistry C Molecular Insight into the Adsorption of H2S in the Flexible MIL-53(Cr) and Rigid MIL-47(V) MOFs: Infrared Spectroscopy Combined to Molecular Simulations 07 4 12 1704350 30 2017 10.1002/adma.201704350 Mo Zong-Wen Zhou Hao-Long Zhou Dong-Dong Lin Rui-Biao Liao Pei-Qin He Chun-Ting Zhang Wei-Xiong Chen Xiao-Ming Zhang Jie-Peng Advanced Materials Mesoporous Metal-Organic Frameworks with Exceptionally High Working Capacities for Adsorption Heat Transformation 20 8 07 4861-4866 20 2020 10.1021/acs.cgd.0c00239 Xue Huan Huang Xin-Song Yin Qi Hu Xiao-Jing Zheng He-Qi Huang Ge Liu Tian-Fu Crystal Growth & Design Bimetallic Cationic Metal–Organic Frameworks for Selective Dye Adsorption and Effective Cr2O72– Removal 08 88-95 500 2017 10.1016/j.jcis.2017.04.001 Azhar Muhammad R. Abid Hussein R. Periasamy Vijay Sun Hongqi Tade Moses O. Wang Shaobin Journal of Colloid and Interface Science Adsorptive removal of antibiotic sulfonamide by UiO-66 and ZIF-67 for wastewater treatment 12 2 02 54-62 1 2020 10.31258/jamt.1.2.54-62 Al Amery Naser Abid Hussein Rasool Wang Shaobin Liu Shaomin Journal of Applied Materials and Technology Enhancing acidic dye adsorption by modified UiO-66 1 01 120-124 366 2012 10.1016/j.jcis.2011.09.060 Abid Hussein Rasool Pham Gia Hung Ang Ha-Ming Tade Moses O. Wang Shaobin Journal of Colloid and Interface Science Adsorption of CH4 and CO2 on Zr-metal organic frameworks 30 2-3 05 870-875 164 2009 10.1016/j.jhazmat.2008.08.084 Hameed B.H. Ahmad A.A. Journal of Hazardous Materials Batch adsorption of methylene blue from aqueous solution by garlic peel, an agricultural waste biomass 2-3 12 576-581 160 2008 10.1016/j.jhazmat.2008.03.028 Hameed B.H. Rahman A.A. Journal of Hazardous Materials Removal of phenol from aqueous solutions by adsorption onto activated carbon prepared from biomass material 5 07 451-465 34 1999 10.1016/s0032-9592(98)00112-5 Ho Y.S McKay G Process Biochemistry Pseudo-second order model for sorption processes 1 03 71-78 295 2006 10.1016/j.jcis.2005.08.006 Wang Shaobin Li Huiting Xu Longya Journal of Colloid and Interface Science Application of zeolite MCM-22 for basic dye removal from wastewater 31 2 60 89 1963 Weber Walter J. Morris J. Carrell Journal of the Sanitary Engineering Division Kinetics of Adsorption on Carbon from Solution 09 S707-S716 9 2016 10.1016/j.arabjc.2011.07.021 Gupta Neha Kushwaha Atul K. Chattopadhyaya M.C. Arabian Journal of Chemistry Application of potato (Solanum tuberosum) plant wastes for the removal of methylene blue and malachite green dye from aqueous solution 3 03 1336-1349 90 2009 10.1016/j.jenvman.2008.09.003 Lin Su-Hsia Juang Ruey-Shin Journal of Environmental Management Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: A review 1 05 53-60 164 2009 10.1016/j.jhazmat.2008.07.131 Uddin Md. Tamez Islam Md. Akhtarul Mahmud Shaheen Rukanuzzaman Md. Journal of Hazardous Materials Adsorptive removal of methylene blue by tea waste 15 2 1907 10.1007/BF01501332 Lagergreen S. Zeitschrift für Chemie und Industrie der Kolloide Zur Theorie der sogenannten Adsorption gelöster Stoffe 5 34 1999 Ho Y.S McKay G Process Biochemistry Pseudo-second order model for sorption processes 1-2 05 389-394 153 2008 10.1016/j.jhazmat.2007.08.070 Demir H. Top A. Balköse D. Ülkü S. Journal of Hazardous Materials Dye adsorption behavior of Luffa cylindrica fibers 1 01 88-96 72 2007 10.1016/j.dyepig.2005.08.002 Rattanaphani Saowanee Chairat Montra Bremner John B. Rattanaphani Vichitr Dyes and Pigments An adsorption and thermodynamic study of lac dyeing on cotton pretreated with chitosan 10 10 2182-2189 89 2011 10.1016/j.cherd.2010.11.024 Moussavi Gholamreza Khosravi Rasoul Chemical Engineering Research and Design The removal of cationic dyes from aqueous solutions by adsorption onto pistachio hull waste 3 02 207-215 74 2005 10.1016/j.jenvman.2004.08.016 Namasivayam C. Sumithra S. Journal of Environmental Management Removal of direct red 12B and methylene blue from water by adsorption onto Fe (III)/Cr (III) hydroxide, an industrial solid waste 1-2 01 37 1988 10.1007/bf00226476 Gupta G.S. Prasad G. Panday K.K. Singh V.N. Water, Air, and Soil Pollution Removal of chrome dye from aqueous solutions by fly ash 1 01 143-149 75 2007 10.1016/j.dyepig.2006.05.039 Hameed B.H. Ahmad A.L. Latiff K.N.A. Dyes and Pigments Adsorption of basic dye (methylene blue) onto activated carbon prepared from rattan sawdust 22 3 03 819-825 141 2007 10.1016/j.jhazmat.2006.07.049 HAMEED B DIN A AHMAD A Journal of Hazardous Materials Adsorption of methylene blue onto bamboo-based activated carbon: Kinetics and equilibrium studies 2 05 212-223 5 1966 10.1021/i160018a011 Hall K. R. Eagleton L. C. Acrivos Andreas Vermeulen Theodore Industrial & Engineering Chemistry Fundamentals Pore- and Solid-Diffusion Kinetics in Fixed-Bed Adsorption under Constant-Pattern Conditions 13 24 37 1988 Gupta G. S. Prasad G. Panday K. K. Singh V. N. Water, Air, and Soil Pollution Removal of chrome dye from aqueous solutions by fly ash 14 1 21 04 284 2005 10.1016/j.jcis.2004.09.063 Kumar K. Vasanth Ramamurthi V. Sivanesan S. Journal of Colloid and Interface Science Modeling the mechanism involved during the sorption of methylene blue onto fly ash 3089 10 3094 4 2012 10.1039/C2NR30244F Abid Hussein Rasool Ang Ha Ming Wang Shaobin Nanoscale Effects of ammonium hydroxide on the structure and gas adsorption of nanosized Zr-MOFs (UiO-66) 11 7 06 2152-2161 60 2015 10.1021/acs.jced.5b00229 Rada Zana Hassan Abid Hussein Rasool Sun Hongqi Wang Shaobin Journal of Chemical & Engineering Data Bifunctionalized Metal Organic Frameworks, UiO-66-NO2-N (N = -NH2, -(OH)2, -(COOH)2), for Enhanced Adsorption and Selectivity of CO2 and N2 12 23-35 Volume 13 2018 10.2147/dddt.s182983 Al Haydar Muder Abid Hussein Rasool Sunderland Bruce Wang Shaobin Drug Design, Development and Therapy Multimetal organic frameworks as drug carriers: aceclofenac as a drug candidate 04 415-420 187 2012 10.1016/j.cej.2012.01.104 Abid Hussein Rasool Tian Huyong Ang Ha-Ming Tade Moses O. Buckley Craig E. Wang Shaobin Chemical Engineering Journal Nanosize Zr-metal organic framework (UiO-66) for hydrogen and carbon dioxide storage 72 1 82 4 2013 10.1080/19475411.2012.688773 Abid Hussein Rasool Shang Jin Ang Ha-Ming Wang Shaobin International Journal of Smart and Nano Materials Amino-functionalized Zr-MOF nanoparticles for adsorption of CO2 and CH4 7 07 823-830 58 2004 10.1366/0003702041389283 Di Marino Antonio Mendicuti Francisco Applied Spectroscopy Thermodynamics of Complexation of Dimethyl Esters of Tere-, Iso-, and Phthalic Acids with α- and β-Cyclodextrins 05 6 02 3295-3304 120 2016 10.1021/acs.jpcc.5b10123 Dhumal Nilesh R. Singh Manish P. Anderson James A. Kiefer Johannes Kim Hyung J. The Journal of Physical Chemistry C Molecular Interactions of a Cu-Based Metal–Organic Framework with a Confined Imidazolium-Based Ionic Liquid: A Combined Density Functional Theory and Experimental Vibrational Spectroscopy Study 10815 10837 2000 10.1002/9780470027318.a5606 Coates John Meyers R.A. Encyclopedia of Analytical Chemistry Interpretation of Infrared Spectra, A Practical Approach 1 6 2013 Aroke U O Abdulkarim A Ogubunka R O ATBU Journal of Environmental Technology Fourier-transform Infrared Characterization of Kaolin, Granite, Bentonite and Barite 822 4 827 76 1998 10.1002/cjce.5450760419 Ho Y. S. Mckay G. The Canadian Journal of Chemical Engineering The kinetics of sorption of basic dyes from aqueous solution by sphagnum moss peat 259 1 267 194 10.1016/j.desal.2005.10.032 Bulut Yasemin Aydın Haluk Desalination A kinetics and thermodynamics study of methylene blue adsorption on wheat shells 26 1 12 28-37 33 2012 10.1002/ep.11741 Senthil Kumar P. Senthamarai C. Durgadevi A. Environmental Progress & Sustainable Energy Adsorption kinetics, mechanism, isotherm, and thermodynamic analysis of copper ions onto the surface modified agricultural waste 1-3 11 134-140 262 2010 10.1016/j.desal.2010.05.059 Ai Lunhong Jiang J. Desalination Fast removal of organic dyes from aqueous solutions by AC/ferrospinel composite 3 09 1515-1524 192 2011 10.1016/j.jhazmat.2011.06.068 Ai Lunhong Zhang Chunying Chen Zhonglan Journal of Hazardous Materials Removal of methylene blue from aqueous solution by a solvothermal-synthesized graphene/magnetite composite 14 8130-8139 10 2020 10.1039/d0ra00305k Abid Hussein Rasool Rada Zana Hassan Li Yuan Mohammed Hussein A. Wang Yuan Wang Shaobin Arandiyan Hamidreza Tan Xiaoyao Liu Shaomin RSC Advances Boosting CO2 adsorption and selectivity in metal–organic frameworks of MIL-96(Al) via second metal Ca coordination 3 701-713 75 2007 10.1016/j.dyepig.2006.07.023 Doğan Mehmet Özdemir Yasemin Alkan Mahir Dyes and Pigments Adsorption kinetics and mechanism of cationic methyl violet and methylene blue dyes onto sepiolite 08 75-85 523 2018 10.1016/j.jcis.2018.03.084 Zaboon Sami Abid Hussein Rasool Yao Zhengxin Gubner Rolf Wang Shaobin Barifcani Ahmed Journal of Colloid and Interface Science Removal of monoethylene glycol from wastewater by using Zr-metal organic frameworks 81 1 87 16 2009 10.1016/j.jhazmat.2008.03.052 Ji Biyan Shao Fei Hu Guanjiu Zheng Shourong Zhang Qingmei Xu Zhaoyi Journal of Hazardous Materials Adsorption of methyl tert-butyl ether (MTBE) from aqueous solution by porous polymeric adsorbents 604 4 613 43 2012 10.1016/j.jtice.2012.01.008 Gupta Neha Kushwaha Atul Kumar Chattopadhyaya M.C. Journal of the Taiwan Institute of Chemical Engineers Adsorption studies of cationic dyes onto Ashoka (Saraca asoca) leaf powder 1 10 417-424 170 2009 10.1016/j.jhazmat.2009.04.080 Weng Chih-Huang Lin Yao-Tung Tzeng Tai-Wei Journal of Hazardous Materials Removal of methylene blue from aqueous solution by adsorption onto pineapple leaf powder 2 06 115-124 70 1998 10.1016/s0923-0467(98)00076-1 Ho Y.S. McKay G. Chemical Engineering Journal Sorption of dye from aqueous solution by peat 21 1 03 276-286 152 2008 10.1016/j.jhazmat.2007.06.107 Ponnusami V. Vikram S. Srivastava S.N. Journal of Hazardous Materials Guava (Psidium guajava) leaf powder: Novel adsorbent for removal of methylene blue from aqueous solutions 3 07 507-512 155 2008 10.1016/j.jhazmat.2007.11.093 Oliveira Leandro S. Franca Adriana S. Alves Thiago M. Rocha Sônia D.F. Journal of Hazardous Materials Evaluation of untreated coffee husks as potential biosorbents for treatment of dye contaminated waters 1 07 59-68 15 2003 10.1016/s1369-703x(02)00177-8 Otero M Rozada F Calvo L.F Garcı́a A.I Morán A Biochemical Engineering Journal Kinetic and equilibrium modelling of the methylene blue removal from solution by adsorbent materials produced from sewage sludges 2 10 193-202 39 2003 10.1016/s0032-9592(03)00065-7 Banat Fawzi Al-Asheh Sameer Al-Makhadmeh Leema Process Biochemistry Evaluation of the use of raw and activated date pits as potential adsorbents for dye containing waters 10 21 2001 Ghosh D Bhattacharyya K Indian Journal of Environmental Protection Removing colour from aqueous medium by sorption on natural clay: a study with methylene blue 9 05 3040-3046 101 2010 10.1016/j.biortech.2009.12.042 Yao Yunjin Xu Feifei Chen Ming Xu Zhongxiao Zhu Zhiwen Bioresource Technology Adsorption behavior of methylene blue on carbon nanotubes 1 06 90-100 286 2005 10.1016/j.jcis.2005.01.007 Vadivelan V. Kumar K. Vasanth Journal of Colloid and Interface Science Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk 1-2 07 214-226 146 2007 10.1016/j.jhazmat.2006.12.010 Kumar K. Vasanth Porkodi K. Journal of Hazardous Materials Mass transfer, kinetics and equilibrium studies for the biosorption of methylene blue using Paspalum notatum 17 11 1919-1928 96 2005 10.1016/j.biortech.2005.01.039 Banerjee Souvik Dastidar M.G. Bioresource Technology Use of jute processing wastes for treatment of wastewater contaminated with dye and other organics 10 3 06 263-274 92 2002 10.1016/s0304-3894(02)00017-1 ANNADURAI G JUANG R LEE D Journal of Hazardous Materials Use of cellulose-based wastes for adsorption of dyes from aqueous solutions 10 09 1401-1407 60 2005 10.1016/j.chemosphere.2005.01.091 Wang Shaobin Boyjoo Y. Choueib A. Chemosphere A comparative study of dye removal using fly ash treated by different methods 8 08 1159-1164 17 2002 10.1016/s0883-2927(02)00057-4 Woolard C.D Strong J Erasmus C.R Applied Geochemistry Evaluation of the use of modified coal ash as a potential sorbent for organic waste streams 2 01 310-314 269 2004 10.1016/j.jcis.2003.09.004 Gürses A Karaca S Doğar Ç Bayrak R Açıkyıldız M Yalçın M Journal of Colloid and Interface Science Determination of adsorptive properties of clay/water system: methylene blue sorption