Removal of Molybdenum from Contaminated Groundwater Using Carbide-Derived Carbon

: In the present work, the removal of Mo from aqueous solutions and real groundwater by using the novel high-surface-area adsorbent carbide-derived carbon (CDC) was performed. The adsorbent was characterized using X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), Brunauer, Emmett, and Teller (BET) surface area analysis, and Fourier-transform infrared spectroscopy (FTIR). The effect of the operational parameters (contact time, CDC loading, Mo concentration, and pH) on the adsorptive performance of the sorbent in the batch adsorption mode was studied. The experimental work revealed that the adsorption of Mo onto CDC is a very fast process and provides 99% Mo removal in less than 30 min. The adsorption process was pH-dependent, achieving the maximum adsorptive removal at a pH range of 3–5. The highest adsorption capacity corresponded to 16.24 mg/g at a Mo concentration of 10 ppm, adsorbent loading of 0.6 g/L, and pH 3. Four models were used to analyze the adsorption isotherms of Mo onto CDC, which were Freundlich, Langmuir, Temkin, and Sips. The obtained adsorption results were also processed using four adsorption kinetic models: intra-particle diffusion, Elovich, second-order, and pseudo-ﬁrst-order. The adsorption of Mo onto CDC was found to ﬁt the Freundlich isotherm model, as conﬁrmed by the highest R 2 values (0.9118) and lowest SSE (0.4777), indicating the heterogeneous multilayer adsorption of Mo onto CDC. Likewise, the experimental adsorption data were found to be more consistent with the pseudo-second-order model. The main adsorption mechanisms contributing to Mo adsorption were found to be electrostatic interactions and ligand–ligand exchange, in addition to surface complexation or ion exchange between Mo ions and oxygen-containing groups on the CDC’s surface. Moreover, the removal efﬁciency under acidic conditions (pH: 3) was found to be stable and high (>99%), regardless of the Mo concentration (0.5–10 ppm) due to the characteristic PZC corresponding to CDC (pH 9.9). A performance test of the CDC using both real groundwater and GW spiked with 570 µ g/L Mo showed an almost complete removal of Mo from GW. The regeneration tests conﬁrmed that adsorbed Mo can be recovered from CDC by pH adjustment and the regenerated CDC can be reused.


Introduction
Molybdenum is an important microelement for human beings but might cause serious health effects when people are exposed to elevated levels of Mo in food and drinking water.According to the WHO, some of the potential adverse effects associated with the consumption of or exposure to high levels of Mo are tremors, fatigue, weakness, and joint pain [1].The World Health Organization (WHO)'s recommended value for Mo in drinking water is 70 µg/L [2].
The reported Mo levels in groundwater samples collected from some parts of the world were found to exceed the WHO recommended value [3].For instance, the Mo level in groundwater samples collected from La Pampa province in Argentina was as high as Water 2023, 15, 49 2 of 28 89.2 µg/L [4].Likewise, the survey study conducted by the United States national waterquality-assessment program on 3063 groundwater samples reported a maximum Mo level of 4700 µg/L.Similarly, the analysis of the Mo level in groundwater in Maryland (USA) reported a maximum Mo level of 143 µg/L [5].In Qatar, the field sampling and analysis of 205 GW samples have shown elevated levels of molybdenum, with a mean of 26.9 µg/L and a maximum level of 103 µg/L [6].The hydrogeochemical analysis of the samples found that the Mo level in Qatar reached 293 µg/L, 40 groundwater samples exceeded the US Environmental Protection Agency guidelines for short-term use for irrigation water, and 18 exceeded the USEPA guidelines for long-term use for irrigation water [7].Industry is the main anthropogenic source of groundwater contamination in Qatar with molybdenum.Molybdenum is an important element that is used in manufacturing steel alloys, as it is added to increase their tensile strength and toughness, in addition to their corrosion resistance [8].Moreover, the gigantic scale of the oil and gas industry in Qatar is also responsible for contaminating groundwater, as it uses Mo as a catalyst for the desulfurization process [6,9].Mo is also used in many other industries, such as fertilizers, pigments, corrosion inhibitors, and lubricating oils [10][11][12].
Due to the shallow GW aquifers in Qatar (max 150 m deep), the elevated levels of Mo in GW can be attributed to anthropogenic sources, such as industry and agriculture [6].The overuse of fertilizers (rich in Mo) in agriculture was found to be responsible for GW contamination with Mo.The other major anthropogenic source of GW contamination with Mo is the oil and gas industry.Mo is widely used as a catalyst to speed up the removal of sulfur from oil and gas [9].The gigantic-scale production facilities for natural gas in Qatar, which make it the world's largest producer of natural gas, might be one of the causes of the elevation in the Mo level in the GW of Qatar.
One of the most widely adopted techniques for groundwater treatment is adsorption due to the high simplicity of operation, removal efficiency, and cost-effectiveness.However, the common sorbents used for water treatment, such as activated carbon (AC), are not highly efficient for Mo removal from water.For instance, Xiong [13] compared the removal efficiency of Mo from aqueous solutions using AC and crosslinked persimmon tannin (CPT) adsorption gel.The experimental testing of both adsorbents showed that AC could not be used to remove Mo efficiently.Likewise, the performance testing of AC to remove Mo ions from water by the incorporation of oxygen-containing groups, such as Al 2 O 3 , onto AC showed that neither AC nor AC-Al 2 O 3 were able to notably remove Mo from water [14].The inability of AC and AC-Al 2 O 3 to remove Mo despite the introduction of oxygen-containing groups was attributed to the fact that AC has a low point of zero charge (PZC) (2.73), which makes AC negatively charged at all pH values above 2.73.Even when AC was modified with oxygen-containing groups, AC-Al 2 O 3 failed to adsorb Mo, as the PC of the AC-Al 2 O 3 was 6.62, which was lower than the operating conditions of the reported adsorption experiments.Therefore, the search for novel sorbents with improved properties and high sorption capacities toward Mo is still an ongoing field of research.
Carbide-derived carbon (CDC) is a new carbon-based material.Owing to its favorable properties, such as its high surface area, electrical conductivity, and tunable nano-porous structure, CDC has caught the interest of material scientists around the world in numerous applications, such as supercapacitors, carbon capture, gas storage, etc. CDC was reported to have a higher surface area when compared with other carbon-based materials, such as CNTs and AC [15].Previously, CDC was used for hydrogen capture [16,17], methane storage [18], electrolyte adsorption [19], and phosphate adsorption.The adsorptive removal of ibuprofen and phosphate from water was reported and showed that CDC achieved fast and efficient adsorptive removal of these pollutants from a model aqueous solution and treated sewage effluent [20,21].
The aim of this work is to, for the first time, investigate the adsorption of Mo from water by using CDC under various conditions, such as different pH levels, temperatures, CDC dosages, agitation speeds, and Mo feed concentrations.

Materials
The Mo solutions were prepared from a 1000 mg/l Mo standard solution (traceable from NIST), which was purchased from Merck (Darmstadt, Germany).Distilled water was used for the preparation of the Mo solutions.
The CDC (type: nanopore U) used in the current study was obtained from Carbon-Ukraine Company (Kiev, Ukraine).The CDC was synthesized from titanium carbide (TiC) powder, which was used as a precursor.The TiC powder was first heated to 800 • C in a stream of flowing argon gas-this step was followed by the treatment of the TiC with chlorine and hydrogen gases at 800 • C for 6.5 h and at 600 • C for 2 h, respectively.

Methods
The CDC microstructure and surface morphology were inspected by a field-emission scanning electron microscope (FE-SEM, QUANTA FEG 650, Thermo Fisher Scientific, Billerica, MA, USA) connected to an energy-dispersive spectrometer (EDS, Bruker Xflash 6l60, Germany), which was used for the elemental analysis of the CDC before and after Mo adsorption.The TEM images were acquired by a high-resolution transmission electron microscope (HR-TEM) (FEI Talos 200X, Thermo Fisher Scientific, Billerica, MA, USA) functioning at 200 kV.For TEM analysis, the CDC powder was sonicated in isopropyl alcohol for 5 min; then, 20 µL of the homogenized solution was placed over a 300-mesh lacey/copper carbon grid and dried at room temperature before imaging.Fourier-transform infrared spectroscopy (FT-IR, type; Thermo Fisher Scientific Nicolet iS10, USA) was utilized to analyze the developed chemical groups on the CDC's surface, the spectra range was varied between 600 and 4000 cm −1 .A Brunauer-Emmett-Teller (BET) surface area analyzer (Micromeritics, ASAP 2020, Norcross, GA, USA) was utilized to determine the surface area of the CDC.Barrett, Joyner, and Halenda (BJH) was utilized for the pore-size-distribution evaluation by the BET equipment.The BET was degassed at a temperature of 250 • C for 4 h, then N 2 gas at 77 K was used as an adsorbate for the BET and BJH analyses.The zeta potential measurements with CDC particles were conducted using a Zetasizer (type; Nano ZS 90, Malvern Instruments Ltd., UK) functioning with an internal laser of 4.0 MW; the zeta potential values were evaluated at room temperature (25 • C) within a pH range of 3-10.

Mo Batch Adsorption Experiments
The adsorption testing was accomplished via batch adsorption tests.In all adsorption experiments, the CDC was added to 100 mL of the Mo solution in 250 mL conical flasks and shaken under the required conditions by using a Grant OLS Aqua Pro temperaturecontrolled shaker (Model OLS26, UK).While adsorption was taking place, samples for analysis were collected over time, filtered through a 0.22 micron PTFE membrane filter, and their Mo content was analyzed.
The determination of the level of Mo in water samples was conducted using inductively coupled plasma-optical emission spectrometry (5800 ICP OES, Agilent Technologies, Santa Clara, CA, USA) in accordance with an ISO-certified procedure (ISO 11885:2007).The calibration curve of the ICP-OES was created by using Mo solutions at different concentrations prepared from 1000 ppm Mo standard stock calibration solution (Agilent Technologies, USA).
The Mo adsorption capacity q e (mg of Mo/g of CDC) and removal efficiency from water were estimated according to Equations ( 1) and (2); where C 0 and C e are the initial and final Mo concentrations (mg/L), respectively, V is the volume of the Mo solution (L), and m is the mass of the CDC (g).Each adsorption test was examined three times and the mean values of these trials were illustrated.The effect of CDC dosage was tested using a CDC dosage ranging between 0.2 and 1.2 g/L.The effect of the pH level on the adsorption of Mo was investigated within a pH range of 3-10 by adjusting the pH of the Mo solution by adding aqueous HCl and NaOH solutions.The agitation speed range studied in the present work was 100-200 rpm.The effect of temperature in addition to the adsorption thermodynamics were tested over a temperature range of 25-45 • C (298.15-318.25 K).Moreover, the adsorption kinetics experiments were conducted by evaluating the equilibrium concentration of Mo after time intervals ranging between 1 min and 24 h.
The efficiency of CDC in removing Mo from multicomponent aqueous solutions containing heavy metals (As, Be, Cd, Cr, Mo, Ni, Pb, Se, and Ti), as well as at different ionic strengths of the aqueous solutions (0, 0.001, and 0.01 M NaCl) was investigated.
Finally, performance testing of the CDC using real GW was conducted by spiking GW with 250 and 500 µg/L molybdenum.The adsorption experiments and determination of the Mo level were conducted following the same procedure described above.

Adsorption Kinetics
The kinetics of Mo adsorption onto CDC were assessed by identifying the adsorption uptake as well as the rate-limiting step.The obtained adsorption results were processed as per the following adsorption kinetic models: intra-particle diffusion, Elovich, second-order, and pseudo-first-order [21].Table 1 lists the mathematical expressions in addition to the linearized correlations of the used kinetic models.

Mathematical Expression Linearized Form XY
Pseudo-first-order dq t dt = K 1 (q e − q t ) ln(q e − q t ) = ln(q e ) − K 1 t ln(q e − q t ) vs. t Pseudo-second-order q e t t q t vs. t Elovich dq t dt = α exp(−βq t ) q t = 1 β ln(αβ) + 1 β ln(t) q t vs. ln t

Intra-particle diffusion
where q t is the adsorption capacity (in mg/g) at any given time (t, min) and q e is the equilibrium adsorption capacity (in mg/g).K 1 and K 2 represent the rate constants of the first-and second-order models in min −1 and g/mg/min, respectively.α and β, are constants related to the Elovich and intra-particle diffusion models.
A and β are the initial rate of adsorption (in g/mg) and the desorption (mg/g/min) constant, respectively, whereas K IPD and C stand for the intraparticle intercept constant (mg/g/min −0.5 ) and the intraparticle intercept (mg/g), respectively.
The half-life (t 1/2 ) or time required to adsorb half of the adsorbate in the solution was calculated using Equation (3): (3)

Adsorption Isotherms
The adsorption isotherm experiments were carried out at various Mo concentrations over a concentration range of 1-100 mg/L.The experiments were conducted for 6 h at pH 6, ambient temperature of 25 • C, and agitation speed of 150 RPM.

Isotherm Model Non-Linear Expression Parameters
Langmuir q e = q m K L C e 1+K L C e q m and K L stand for the maximum adsorption capacity of the monolayer and Langmuir constant Freundlich 1 n e n is the heterogeneity factor and K F is the Freundlich constant (mg/g)/(dm 3 /mg) n Temkin q e = RT ln(K t C e ) b K t is the equilibrium binding constant (in L/mg) and b is a parameter representing the variation in the adsorption energy values (in kJ/mol) Sips 1 n e K s and n are Sips's constant and the model exponent, respectively The Langmuir model is used for the monolayer adsorption of an adsorbate onto an adsorbent's surface, and assumes that all of the available adsorption sites are energetically homogeneous and the sorption activation energy for any adsorbate molecule is the same [25].
The separation factor or the Langmuir model's dimensionless constant can be determined from Equation (4): where R L represents the adsorption favorability; if R L > 1, the adsorption is unfavorable; if R L = 0, it is irreversible; if R L = 1, it is linear; and for favorable adsorption, R L is in the range 0 to 1 [10].
The Freundlich model w used to describe heterogeneous, reversible, multi-layered, and non-ideal adsorption.The heterogeneity of the Freundlich model indicates that the adsorption energy varies according to the adsorption site within the adsorbent [26].The Sips model, on the other hand, is reported to fit heterogeneous adsorption systems [27].
In the present work, a Microsoft Excel solver was used to fit the above-mentioned isotherm models by reducing the residual sum of squares of errors (RSS).RSS is used to determine the amount of discrepancy or variance between an experimentally measured adsorption capacity and the value predicted by isotherm models (through linear regression).Equation ( 5) was used to determine RSS.
The total sum of squares (TSS) is similar to RSS (Equation ( 5)); however, in TSS, q i (model) is replaced with q i (mean).The standard deviation (SD) is used to measure the amount of dispersion associated with a particular group of data points.Equation ( 6) was used to estimate the value of SD, while Equation ( 7) was used to determine the value of the coefficient of determination (or R 2 ).

Adsorption Thermodynamics
The evaluation of the adsorption thermodynamics can be achieved by calculating the thermodynamic parameters at various temperatures, as the adsorption process is temperature-dependent [28].The importance of determining the thermodynamic parameters is to provide insight into the spontaneity of the adsorption process.In the present work, the adsorption thermodynamics parameters were evaluated over a temperature range of Water 2023, 15, 49 6 of 28 25-45 • C. The Gibbs free energy (∆G 0 ) at any temperature was estimated using Nernst's equation [29]: where R is the universal gas constant (≈8.314J. K −1 .Mol −1 ), T is temperature (in Kelvin), and K eq is the adsorption equilibrium constant, which can be estimated from Equation (9).
Moreover, the Gibbs free energy (∆G 0 ) can be estimated from the Gibbs-Helmholtz equation, which enables the calculation of the Gibbs free energy, given the enthalpy and entropy of the system.
∆G 0 = ∆H 0 − T.∆S 0 (10) Substituting Equations ( 9) and ( 10) into the Gibbs-Helmholtz equation produces the following equation: The simplified version of the Gibbs-Helmholtz equation can be used to calculate the changes in entropy (∆S 0 ) and enthalpy (∆H 0 ) by plotting 1  T on the x-axis and lnk eq on the y-axis.The enthalpy change can be obtained from the slope of the plot (or ∆H 0 R ) and the entropy change can be calculated from the y-intercept (or ∆S 0 R ).The activation energy of the adsorption of Mo onto CDC was calculated from Equation ( 12): where k 0 stands for the rate constant at temperature T and k ae is the pseudo-second-order model constant.

Regeneration of CDC Adsorbent
The regeneration of the CDC adsorbent was conducted by placing Mo-saturated CDC samples in aqueous solutions at pH 11 and shaking the suspensions for 2 h.The desorption of the Mo ions from the CDC was evaluated by analyzing the Mo content in water samples following the same procedure described in Section 2.3 and by the XPS spectra of the CDC samples before and after the regeneration procedure.

Results and Discussion
The schematic presentation of the experimental work carried out in this study is shown in Figure 1.In the first stage, CDC material will be extensively characterized by using XPS, XRD, SEM, TEM, EDS, BET, and FTIR techniques, followed by the evaluation of the effects of operational parameters, such as the contact time, CDC loading, Mo concentration, and feed pH, on the adsorption process.Next, the adsorption isotherms and kinetics of Mo adsorption on CDC will be studied, followed by employing CDC for the treatment of real groundwater contaminated with Mo.Finally, the evaluation of the possibility of the regeneration of the used CDC by pH adjustment will be performed.

CDC Characterization
The SEM images were acquired in order to evaluate the size and shape of the CDC. Figure 2 presents the SEM images of the CDC particles at low and high magnifications.It can be observed that the CDC particles had irregular shapes with a wide range of sizes ranging between 200 nm and 2 µm.The CDC particles formed aggregates of irregular shapes.The TEM image is presented in Figure 3 and illustrates that a highly porous carbon material has been developed.

CDC Characterization
The SEM images were acquired in order to evaluate the size and shape of the CDC. Figure 2 presents the SEM images of the CDC particles at low and high magnifications.It can be observed that the CDC particles had irregular shapes with a wide range of sizes ranging between 200 nm and2 µ m.The CDC particles formed aggregates of irregular shapes.The TEM image is presented in Figure 3 and illustrates that a highly porous carbon material has been developed.

CDC Characterization
The SEM images were acquired in order to evaluate the size and shape of the CDC. Figure 2 presents the SEM images of the CDC particles at low and high magnifications.It can be observed that the CDC particles had irregular shapes with a wide range of sizes ranging between 200 nm and2 µ m.The CDC particles formed aggregates of irregular shapes.The TEM image is presented in Figure 3 and illustrates that a highly porous carbon material has been developed.

CDC Regeneration
Water 2022, 14, x FOR PEER REVIEW 9 of 31    The pore size distribution of the CDC particles was studied by the BJH method, and the results are displayed in Figure 4a.As seen, the CDC contained mainly micropores with a significant peak at 2 nm.It can also be seen that most of the pores were between 1 and 4 nm.The mean pore size according to the BJH adsorption/desorption results was 2.8-3.1 nm.The estimated BET surface area of CDC in the present work was 1,120 m 2 /g.The BET surface area, along with the pore size, of the CDC were found to be in good agreement with other work reported in the literature, such as Xing [30], who reported a BET surface area and average pore size of pristine CDC of 1216 m 2 /g and 2.13 nm, respectively.The N2 adsorption-desorption curve of CDC is illustrated in Figure 4b.According to the IUPAC classification, N2 adsorption by CDC follows the type Ia sorption isotherm suggesting an ultra-highly microporous material.Similar curves were obtained for the CDC produced at 800 °C using TiC as a precursor.
Figure 5 depicts the XRD pattern of CDC, which was obtained by a Bruker D8 Advance spectrometer with a scan area of 3-90 2q degs.As seen, the absence of a strong peak indicates the amorphous structure of the CDC.The pore size distribution of the CDC particles was studied by the BJH method, and the results are displayed in Figure 4a.As seen, the CDC contained mainly micropores with a significant peak at 2 nm.It can also be seen that most of the pores were between 1 and 4 nm.The mean pore size according to the BJH adsorption/desorption results was 2.8-3.1 nm.The estimated BET surface area of CDC in the present work was 1,120 m 2 /g.The BET surface area, along with the pore size, of the CDC were found to be in good agreement with other work reported in the literature, such as Xing [30], who reported a BET surface area and average pore size of pristine CDC of 1216 m 2 /g and 2.13 nm, respectively.The N 2 adsorption-desorption curve of CDC is illustrated in Figure 4b.According to the IUPAC classification, N 2 adsorption by CDC follows the type Ia sorption isotherm suggesting an ultra-highly microporous material.Similar curves were obtained for the CDC produced at 800 • C using TiC as a precursor.
Figure 5 depicts the XRD pattern of CDC, which was obtained by a Bruker D8 Advance spectrometer with a scan area of 3-90 2q degs.As seen, the absence of a strong peak indicates the amorphous structure of the CDC.
The zeta potential of the CDC is depicted in Figure 6.The zeta potential vs. pH plot shows that, with the exception of highly alkaline conditions (around pH 10), the zeta potential values of CDC were totally positively charged, with a decreasing overall positive charge along with pH.The point of zero charge (PZC) or point at which the net surface charge of CDC was zero occurred at 9.9.The surface charge of the CDC at the studied pH range in addition to the PZC was found to agree with results reported in the literature [20,21,31,32].
The elemental composition of the CDC before and after the adsorption experiments was evaluated by XPS and EDS analyses.Using the XPS technique, the main constituents of CDC before adsorption were carbon (87.3%), oxygen (6.8%), nitrogen (4.7%), chlorine (0.43%), and silicon (0.36%).No Mo was detected in the CDC sample before adsorption.On the other hand, the Mo content in the CDC sample after adsorption was 1.87%.The confirmation of the presence of Mo in samples using XPS was reported in the literature within the same range of binding energies [33][34][35][36][37]. EDS analysis of the CDC was also conducted prior to and after the adsorption process.Figure 7 shows that carbon is the main constituent of the CDC, besides oxygen and chloride.In addition to these elements, Mo was detected in the CDC samples collected after the adsorption process, as depicted in Figure 6.The SEM elemental mapping (Figure 7) also confirmed Mo adsorption on the CDC.The zeta potential of the CDC is depicted in Figure 6.The zeta potential vs. pH plot shows that, with the exception of highly alkaline conditions (around pH 10), the zeta potential values of CDC were totally positively charged, with a decreasing overall positive charge along with pH.The point of zero charge (PZC) or point at which the net surface charge of CDC was zero occurred at 9.9.The surface charge of the CDC at the studied pH range in addition to the PZC was found to agree with results reported in the literature [20,21,[31][32].The elemental composition of the CDC before and after the adsorption experiments was evaluated by XPS and EDS analyses.Using the XPS technique, the main constituents of CDC before adsorption were carbon (87.3%), oxygen (6.8%), nitrogen (4.7%), chlorine (0.43%), and silicon (0.36%).No Mo was detected in the CDC sample before adsorption.On the other hand, the Mo content in the CDC sample after adsorption was 1.87%.The confirmation of the presence of Mo in samples using XPS was reported in the literature within the same range of binding energies [33][34][35][36][37]. EDS analysis of the CDC was also conducted prior to and after the adsorption process.Figure 7 shows that carbon is the main constituent of the CDC, besides oxygen and chloride.In addition to these elements, Mo was detected in the CDC samples collected after the adsorption process, as depicted in Figure 6.The SEM elemental mapping (Figure 7) also confirmed Mo adsorption on the CDC.The corresponding functional groups on the CDC were determined by FTIR analysis.In the FTIR spectrum illustrated in Figure 8, a broad band was detected in the region of 3300-3500 cm −1 , which belonged to the O-H stretching vibration, pointing out to the presence of adsorbed water molecules.Two more peaks positioned between 3000 and 2800 cm −1 belonged to the stretching vibration of C-H bonds, while the peaks at 1241 and 1574 cm −1 were linked to C-O-C and C=C stretching bonds, respectively [20,21,[38][39][40].Likewise, the presence of Mo in the FTIR spectra of the CDC samples after adsorption can be observed in Figure 8.The peaks at 597 and 898 cm −1 were reported in the literature to correspond to the stretching vibrations of Mo-O and Mo=O, respectively [41,42].The Mo The corresponding functional groups on the CDC were determined by FTIR analysis.In the FTIR spectrum illustrated in Figure 8, a broad band was detected in the region of 3300-3500 cm −1 , which belonged to the O-H stretching vibration, pointing out to the presence of adsorbed water molecules.Two more peaks positioned between 3000 and 2800 cm −1 belonged to the stretching vibration of C-H bonds, while the peaks at 1241 and 1574 cm −1 were linked to C-O-C and C=C stretching bonds, respectively [20,21,[38][39][40].Likewise, the presence of Mo in the FTIR spectra of the CDC samples after adsorption can be observed in Figure 8.The peaks at 597 and 898 cm −1 were reported in the literature to correspond to the stretching vibrations of Mo-O and Mo=O, respectively [41,42].The Mo peaks in the FTIR spectra confirm the adsorption of Mo onto CDC.

Effect of Adsorption Time, CDC Dose, and Shaking Speed
The adsorption experiments with CDC were conducted in order to study the effect of contact time on the removal efficiency.To that end, the adsorption time was varied from 0.5 to 24 h while maintaining a feed Mo concentration of 1 ppm and CDC loading of 0.6 g/L. Figure 9 shows the Mo removal efficiency of CDC vs. adsorption time.As seen, the removal efficiency of CDC was found to exceed 99%, even after 0.5 h of adsorption.This can be attributed to the high surface area of CDC and fast adsorption kinetics.

Effect of Adsorption Time, CDC Dose, and Shaking Speed
The adsorption experiments with CDC were conducted in order to study the effect of contact time on the removal efficiency.To that end, the adsorption time was varied from 0.5 to 24 h while maintaining a feed Mo concentration of 1 ppm and CDC loading of 0.6 g/L. Figure 9 shows the Mo removal efficiency of CDC vs. adsorption time.As seen, the removal efficiency of CDC was found to exceed 99%, even after 0.5 h of adsorption.This can be attributed to the high surface area of CDC and fast adsorption kinetics.

Effect of Adsorption Time, CDC Dose, and Shaking Speed
The adsorption experiments with CDC were conducted in order to study the effect of contact time on the removal efficiency.To that end, the adsorption time was varied from 0.5 to 24 h while maintaining a feed Mo concentration of 1 ppm and CDC loading of 0.6 g/L. Figure 9 shows the Mo removal efficiency of CDC vs. adsorption time.As seen, the removal efficiency of CDC was found to exceed 99%, even after 0.5 h of adsorption.This can be attributed to the high surface area of CDC and fast adsorption kinetics.The next adsorption experiments carried out were focused on the effect of the loading rate of CDC on the removal efficiency.Figure 10 shows the removal efficiency of CDC at loading rates ranging between 0.2 and 1.2 g/L.As seen, the removal efficiency of CDC was found to be higher than 99%, even with the loading rate being as low as 0.2 g/L.
Water 2022, 14, x FOR PEER REVIEW The next adsorption experiments carried out were focused on the effect of the l rate of CDC on the removal efficiency.Figure 10 shows the removal efficiency of C loading rates ranging between 0.2 and 1.2 g/L.As seen, the removal efficiency of CD found to be higher than 99%, even with the loading rate being as low as 0.2 g/L.Thereafter, the effect of the Mo concentration on the removal efficiency and a tion capacity was studied.Figure 11 depicts the removal efficiency and adsorption ity at different Mo concentrations in the feed solutions (1, 5, and 10 ppm).As se removal of Mo from water and the adsorption capacity clearly decreased with incr Mo concentration due to the saturation of the adsorption sites available on the CDC face.In the present work, the reported removal efficiency and adsorption capacit found to outperform those of other adsorbents cited in the literature for Mo remov water (Table 1).For instance, Verbinnen [43] developed a zeolite-supported ma adsorbent for the removal of Mo from water.Despite having a lower Mo concen (0.9 ppm) and higher adsorbent loadings (1-20 g/L), the reported adsorption cap (18-23 mg/g) after a 24 h residence time were lower than those in the present wo [44] prepared a novel biosorbent made from carbonized pomelo peel modified w noscale zero-valent iron and cetyl-trimethyl ammonium bromide for the removal from water.Despite using a higher loading of the biosorbent (1 g/L), the reported a tion capacity (37-48 mg/g) was lower than the values observed in the present stud [45] modified activated drinking water residuals by heat treatment at 600 °C follow HCl treatment in order to increase their adsorption capacity for Mo in water.Th duced residuals under the optimum conditions were reported to increase the adso capacity from 18.44 mg/g to 39.52 mg/g.Tu [44] synthesized crystalline ZnFe2O4 na ticles to remove Mo from water.Although the Mo concentrations reported (2.8-2.8 were lower than the ones used in the present work and the adsorbent loading was (2 g/L), the removal efficiency was as low as 2.5% and the adsorption capacity w mg/g.Thereafter, the effect of the Mo concentration on the removal efficiency and adsorption capacity was studied.Figure 11 depicts the removal efficiency and adsorption capacity at different Mo concentrations in the feed solutions (1, 5, and 10 ppm).As seen, the removal of Mo from water and the adsorption capacity clearly decreased with increasing Mo concentration due to the saturation of the adsorption sites available on the CDC's surface.In the present work, the reported removal efficiency and adsorption capacity were found to outperform those of other adsorbents cited in the literature for Mo removal from water (Table 3).For instance, Verbinnen [43] developed a zeolite-supported magnetite adsorbent for the removal of Mo from water.Despite having a lower Mo concentration (0.9 ppm) and higher adsorbent loadings (1-20 g/L), the reported adsorption capacities (18-23 mg/g) after a 24 h residence time were lower than those in the present work.Tu [44] prepared a novel biosorbent made from carbonized pomelo peel modified with nanoscale zero-valent iron and cetyl-trimethyl ammonium bromide for the removal of Mo from water.Despite using a higher loading of the biosorbent (1 g/L), the reported adsorption capacity (37-48 mg/g) was lower than the values observed in the present study.Lian [45] modified activated drinking water residuals by heat treatment at 600 • C followed by HCl treatment in order to increase their adsorption capacity for Mo in water.The produced residuals under the optimum conditions were reported to increase the adsorption capacity from 18.44 mg/g to 39.52 mg/g.Tu [44] synthesized crystalline ZnFe 2 O 4 nanoparticles to remove Mo from water.Although the Mo concentrations reported (2.8-2.87 ppm) were lower than the ones used in the present work and the adsorbent loading was higher (2 g/L), the removal efficiency was as low as 2.5% and the adsorption capacity was 1.4 mg/g.

Adsorption Isotherms
In the present work, Langmuir, Freundlich, Temkin, and Sips isotherm models were applied to the experimental data.The isotherm models were compared by determining the correlation coefficient (R 2 ) and the sum of the square error (SSE) values by fitting the experimental data to non-linear forms of the isotherm models.Figure 12 shows the experimental adsorption results fitted into Langmuir, Freundlich, Temkin, and Sips isotherm models.The isotherm parameters corresponding to the applied isotherm models are provided in Table 4.As seen from the statistical coefficient values (Table 4), the adsorption of Mo onto CDC can be better represented by the Freundlich isotherm model, as confirmed by the highest R 2 values (0.9118) and lowest SSE (0.4777), indicating that heterogeneous multilayer adsorption of Mo occurred on the CDC.The Langmuir model also showed good compliance with the experimental data, with R 2 of 0.831, indicating that the homogeneous monolayer adsorption clearly also took place (i.e., the adsorptive removal of Mo by CDC was a combination of a physical monolayer adsorption and a chemical adsorption process).Similar findings were reported for the adsorption of phosphate and surfactants on CDC [20,21,32].
applied to the experimental data.The isotherm models were compared by determi the correlation coefficient (R 2 ) and the sum of the square error (SSE) values by fitting experimental data to non-linear forms of the isotherm models.Figure 12 shows the ex imental adsorption results fitted into Langmuir, Freundlich, Temkin, and Sips isoth models.The isotherm parameters corresponding to the applied isotherm models are vided in Table 2.As seen from the statistical coefficient values (Table 2), the adsorptio Mo onto CDC can be better represented by the Freundlich isotherm model, as confir by the highest R 2 values (0.9118) and lowest SSE (0.4777), indicating that heterogen multilayer adsorption of Mo occurred on the CDC.The Langmuir model also sho good compliance with the experimental data, with R 2 of 0.831, indicating that the ho geneous monolayer adsorption clearly also took place (i.e., the adsorptive removal o by CDC was a combination of a physical monolayer adsorption and a chemical adsorp process).Similar findings were reported for the adsorption of phosphate and surfact on CDC [20,21,32].
The favorability of the adsorption process can be anticipated by calculating the v of the separation factor (RL) using Equation ( 4).In the present work, the calculated s ration factor was within the range of 0.00003-0.0014,which was between 0 and 1, ind ing a favorable adsorption process [48].The favorability of the adsorption process can be anticipated by calculating the value of the separation factor (R L ) using Equation ( 4).In the present work, the calculated separation factor was within the range of 0.00003-0.0014,which was between 0 and 1, indicating a favorable adsorption process [48].

Adsorption Kinetics
The effect of contact time on the adsorptive removal of Mo by CDC is shown in figure .As seen in Figure 13, almost complete removal of Mo was achieved after 30 min of adsorption, indicating a very rapid adsorption process.The experimental data of Mo removal were evaluated by four different kinetic models: the pseudo-first-order, pseudosecond-order, Elovich, and intra-particle diffusion models.Table 5 lists the estimated correlation parameters in addition to the coefficient of determination (R 2 ) for each of the studied models.It was found that the experimental adsorption data were more consistent with the pseudo-second-order model, as indicated by the highest correlation coefficient (R 2 ) of 0.9998 for this model.On the contrary, the intra-particle diffusion and pseudofirst-order models exhibited relatively low correlation coefficients of about 0.6268 and 0.4269, respectively, which indicated the unsuitability of these models to fit the studied adsorption system.It seems that the Elovich model (R 2 > 0.9) was in better agreement with the experimental data when compared with the intra-particle diffusion and pseudo-firstorder models.The number of available adsorption sites in CDC can be estimated from the 1  β value, which was 0.024 in the present work.By referring to the Elovich model in Table 5, the value of the y-intercept (or 1  β ln(αβ)) can be used to estimate the adsorption capacity (q e ) at the start of the adsorption experiment, which was found to be 0.881 mg/g.This value was found to agree with the experimental adsorption capacity value at t = 1, which corresponded to 0.878 mg/g.The half-life (t 1/2 ), or time required to adsorb half of the adsorbate in the solution, was calculated using Equation ( 3).The half-life required for the CDC to adsorb half of the Mo in water was estimated to be 1.7 min.The fast kinetics of Mo adsorption by CDC might be beneficial for Mo removal from water, as it would reduce the required residence/contact time and eventually lower the operation cost.
moval were evaluated by four different kinetic models: the pseudo-first-or second-order, Elovich, and intra-particle diffusion models.Table 3 lists the e relation parameters in addition to the coefficient of determination (R 2 ) for eac ied models.It was found that the experimental adsorption data were more co the pseudo-second-order model, as indicated by the highest correlation coef 0.9998 for this model.On the contrary, the intra-particle diffusion and pseud models exhibited relatively low correlation coefficients of about 0.6268 and 0 tively, which indicated the unsuitability of these models to fit the studied ad tem.It seems that the Elovich model (R 2 > 0.9) was in better agreement wit mental data when compared with the intra-particle diffusion and pseudo-firs els.The number of available adsorption sites in CDC can be estimated from which was 0.024 in the present work.By referring to the Elovich model in value of the y-intercept (or 1  ()) can be used to estimate the adsorption at the start of the adsorption experiment, which was found to be 0.881 mg/ was found to agree with the experimental adsorption capacity value at t=1, sponded to 0.878 mg/g.The half-life (t1/2), or time required to adsorb half of in the solution, was calculated using Equation ( 3).The half-life required fo adsorb half of the Mo in water was estimated to be 1.7 min.The fast kinet sorption by CDC might be beneficial for Mo removal from water, as it wou required residence/contact time and eventually lower the operation cost.

Effects of Feed Temperature and Adsorption Thermodynamics
The effect of temperature on the adsorption of Mo onto CDC was investigated by evaluating the adsorption capacity at different feed solution temperatures (25,35, and 45 • C) and at pH values of 6 and 9.As shown in Figure 14, increasing the temperature was observed to increase the Mo adsorption capacity of CDC.For instance, at pH 6, the adsorption capacity increased from 5.51 to 6.04 mg/g by elevating the feed solution temperature from 25 to 45 • C. The increase in the adsorption capacity of CDC with temperature can be attributed to the higher mass transfer rate of Mo ions, which resulted in faster and more efficient binding with CDC.The increase in the adsorption capacity of CDC with feed temperature in the present work was found to be in good agreement with other work reported in the literature [49][50][51].0.6

Effects of Feed Temperature and Adsorption Thermodynamics
The effect of temperature on the adsorption of Mo onto CDC was inve evaluating the adsorption capacity at different feed solution temperatures (25 °C ) and at pH values of 6 and 9.As shown in Figure 14, increasing the temp observed to increase the Mo adsorption capacity of CDC.For instance, at pH sorption capacity increased from 5.51 to 6.04 mg/g by elevating the feed solut ature from 25 to 45 °C .The increase in the adsorption capacity of CDC with t can be attributed to the higher mass transfer rate of Mo ions, which resulted i more efficient binding with CDC.The increase in the adsorption capacity o feed temperature in the present work was found to be in good agreement with reported in the literature [49][50][51].The thermodynamic parameters of adsorption, such as the Gibbs free energy (∆G 0 ), entropy (∆S 0 ), and enthalpy (∆H 0 ) were evaluated by using the Nernst Equation ( 8) in Section 2.6.Table 6 shows the thermodynamic parameters for the adsorption of Mo onto CDC at pH 6 and 9 and at a temperature range of 25-45 • C (298.15-318.25 K).At pH 6, the values of the Gibbs free energy changes were found to be −2.759,−3.243, and −3.727 kJ/mol at 25, 35, and 45 • C, respectively.The negative value associated with the Gibbs free energy at the studied temperatures indicated the spontaneity of the adsorption, whereas the increase in the amount of Gibbs free energy with temperature indicated that the favorability of the adsorption process increased with temperature, which was found to agree with other work in the literature [52].The range of the Gibbs free energy observed in the present work was between 0 and −20 kJ/mol, which was reported in the literature to account for physisorption, which is a form of adsorption in which the interactions between Mo and CDC are mainly controlled by Van der Waals forces [53].Conversely, Gibbs free energy values below −40 kJ/mol were reported to account for chemisorption (or chemical adsorption), which is an adsorption process taking place between Mo and particular active sites at the CDC's surface [54].On the other hand, the Gibbs free energy change associated with adsorption from basic media (pH 9) was found to demonstrate positive values (3.43-3.55kJ/mol), indicating the unfavourability of the adsorption process.
Moreover, the entropy change associated with the studied adsorption process was found to correspond to 48.44 and −6.4 J/mol.K at pH 6 and 9, respectively.In general, a positive entropy change in adsorption signifies an increase in the degree of disorder/randomness in the system, as well as the higher affinity of the sorbate towards the sorbent [55][56][57].The overall signs corresponding to the entropy change at pH 6 and 9 can be used as further evidence to support the experimental data, which showed a highly favorable adsorption process at pH 6 and unfavorable adsorption process at pH 9.
Finally, the estimated enthalpy change was positive and corresponded to 11.685 kJ/mol.The positive enthalpy value here supported the experimental finding that the adsorption process of Mo onto CDC was endothermic due to the existence of more energy in the system after adsorption compared with the system before adsorption [58].On the other hand, exothermic reactions have been reported to have a negative reaction enthalpy difference due to the existence of less energy in the products compared with reactants as a result of the release of more energy than that absorbed [59].

Effect of pH
Due to the wide range of oxidation states of Mo that exist in natural waters, the formation of various Mo species in water is highly pH-dependent (Figure 15).Equations ( 13)- (19) show some of the main reactions involving Mo species that take place in natural water bodies and affect the overall charge of Mo ions [61]. MoO2 Mo + 4H + ➔ H2MoO2 + H2O ( 16) Figure 16 shows the removal efficiency vs. feed Mo concentration at three pH levels (3, 6, and 9).As seen, the adsorption of Mo by CDC was found to be pH-dependent.The removal efficiency under acidic conditions (pH: 3) was found to be stable and high (>99%), regardless of the Mo concentration (0.5-10 ppm).This can be attributed to the fact that, under acidic conditions, the surface of CDC was protonated by the available H+ ions in acidic water, which resulted in the formation of the positive charge on the sorbent's surface that attracted negatively charged Mo species via attractive coulombic forces.This can be confirmed by the zeta potential curve of CDC displayed in Figure 6, which shows positive zeta potential values of the sorbent in the acidic feed solution.Figure 6 shows that, with increasing pH, the zeta potential values of CDC were found to decline until reaching the PZC at 9.9.Such zeta potential behavior of CDC correlated with the reduction in the Mo removal efficiency of CDC at pH 6 and 9.The decrease in the overall positive charge of CDC under neutral and basic pH values was caused by the continuous deprotonation of the sorbent's surface by the addition of OH -ions, which reduced the attractive coulombic forces between CDC and negatively charged Mo ions; hence, the adsorption process became less favorable.
This behavior was found to agree with other work published in the literature due to the variation in the speciation of Mo in water at different pH levels [58,60,61].For in- Equations ( 13)- (19) show some of the main reactions involving Mo species that take place in natural water bodies and affect the overall charge of Mo ions [61].
Figure 16 shows the removal efficiency vs. feed Mo concentration at three pH levels (3, 6, and 9).As seen, the adsorption of Mo by CDC was found to be pH-dependent.The removal efficiency under acidic conditions (pH: 3) was found to be stable and high (>99%), regardless of the Mo concentration (0.5-10 ppm).This can be attributed to the fact that, under acidic conditions, the surface of CDC was protonated by the available H+ ions in acidic water, which resulted in the formation of the positive charge on the sorbent's surface that attracted negatively charged Mo species via attractive coulombic forces.This can be confirmed by the zeta potential curve of CDC displayed in Figure 6, which shows positive zeta potential values of the sorbent in the acidic feed solution.Figure 6 shows that, with increasing pH, the zeta potential values of CDC were found to decline until reaching the PZC at 9.9.Such zeta potential behavior of CDC correlated with the reduction in the Mo removal efficiency of CDC at pH 6 and 9.The decrease in the overall positive charge of CDC under neutral and basic pH values was caused by the continuous deprotonation of the sorbent's surface by the addition of OH − ions, which reduced the attractive coulombic forces between CDC and negatively charged Mo ions; hence, the adsorption process became less favorable.
gen-containing groups on the CDC's surface, might also contribute to Mo adsorption [45 The presence of oxygen-containing groups on the CDC's surface was confirmed by XP analysis in this study, as well as in previous work.For example, Xing [30] reported a average oxygen content in the CDC sample of 8.7%. Figure 17  This behavior was found to agree with other work published in the literature due to the variation in the speciation of Mo in water at different pH levels [58,60,61].For instance, Al-Gaashani [14] tested the adsorption performance of AC modified with aluminum oxide in removing Mo ions from water.The adsorption tests, which were conducted under acidic, neutral, and alkaline conditions, demonstrated that the highest removal occurred under acidic conditions (specifically at pH 2) and that the removal efficiency was found to decrease along with pH until a very low removal efficiency was reached at pH 8. Al-Gaashani [14]  .Similarly, Goldberg [62] reported the maximum adsorption capacity of some natural adsorbents, such as clay and soils, for Mo ions to exist at low pH levels.It was observed that the adsorption capacity decreased suddenly at pH levels above 5, reaching zero at pH 8.
Interestingly, as seen in Figure 16, at pH 9, the adsorption of Mo was still taking place, despite the repulsive interactions between Mo ions and the CDC's surface.Clearly, other adsorption mechanisms, such as complexation or the ion exchange of Mo ions with oxygencontaining groups on the CDC's surface, might also contribute to Mo adsorption [45].The presence of oxygen-containing groups on the CDC's surface was confirmed by XPS analysis in this study, as well as in previous work.For example, Xing [30] reported an average oxygen content in the CDC sample of 8.7%. Figure 17

Influence of co-existing ions on the adsorption of Mo by CDC
Water treatment in real processes or industry usually involves the treatment of complex multicomponent solutions composed of different ions that might simultaneously adsorb to the available sorption sites on a sorbent's surface.For instance, some cations, anions, and heavy metal ions available in groundwater might adversely affect the adsorption efficiency and, consequently, the adsorption capacity of the sorbent [63][64][65].Hence, the effect of competing ions on the Mo adsorption capacity of CDC was studied.Mixed solutions of As, Be, Cd, Cr, Mo, Ni, Pb, Se, and Ti ions with different concentrations (2 and 5 ppm) and pH values (2 and 4) were used during these adsorption experiments.Figure 18 shows the adsorption capacity (mg/g) of CDC toward the studied pollutants after adsorption for 1 h.As expected, the presence of the competing pollutant ions was observed to

Influence of co-existing ions on the adsorption of Mo by CDC
Water treatment in real processes or industry usually involves the treatment of complex multicomponent solutions composed of different ions that might simultaneously adsorb to the available sorption sites on a sorbent's surface.For instance, some cations, anions, and heavy metal ions available in groundwater might adversely affect the adsorption efficiency and, consequently, the adsorption capacity of the sorbent [63][64][65].Hence, the effect of competing ions on the Mo adsorption capacity of CDC was studied.Mixed solutions of As, Be, Cd, Cr, Mo, Ni, Pb, Se, and Ti ions with different concentrations (2 and 5 ppm) and pH values (2 and 4) were used during these adsorption experiments.Figure 18 shows the adsorption capacity (mg/g) of CDC toward the studied pollutants after adsorption for 1 h.As expected, the presence of the competing pollutant ions was observed to reduce the Mo adsorption capacity of CDC from 163 mg/g (in the single Mo solution) to 128 mg/g (in the mixed multicomponent solution) under the same experimental conditions.This can be attributed to the competition of the different metal species for the available adsorption spots inside the CDC.
to that of Molybdate (1.4 Å ) than that of Pb (1.8 Å ) [66].The removal of Mo by CDC in the presence of the co-existing metals was found to outperform some of the work published in the literature.For instance, Verbinnen [43] conducted the simultaneous removal of Mo (0.8 mg/L), Sb (1.6 mg/L), and Se (0.208 mg/L) from industrial wastewater by using magnetite.It was found that higher adsorbent loading (20 g/L) and a longer residence time (24 h) were required to treat the wastewater, which was reflected in the drastic reduction in the adsorption capacity of magnetite down to 0.04 mg/g due to the competition of coexisting metals for the adsorption sites.

Effect of ionic strength of feed solution
The effect of the salinity of the aqueous solution on the adsorption of Mo was also studied by carrying out adsorption experiments at different NaCl concentrations in the feed solution.Figure 19 shows the Mo ion removal efficiency of CDC from 0, 0.001, and 0.01 M NaCl solutions.It was found that the higher the salinity level, the lower the removal efficiency of CDC.These findings can be explained by the reduction in electrostatic interactions between the Mo ions and adsorption sites on the CDC's surface due to the shielding effect of the NaCl electrolyte.These findings correlate well with previous data on the impact of the ionic strength of solutions on the adsorption capacity of adsorbents reported in the literature [67][68][69].It might be concluded that the competition between Mo and other species greatly depends on their ionic radii.As seen in Figure 19, the adsorption of Mo was more affected in the presence of As than Pb due to the fact that the atomic radius of As (1.1 Å) was closer to that of Molybdate (1.4 Å) than that of Pb (1.8 Å) [66].The removal of Mo by CDC in the presence of the co-existing metals was found to outperform some of the work published in the literature.For instance, Verbinnen [43] conducted the simultaneous removal of Mo (0.8 mg/L), Sb (1.6 mg/L), and Se (0.208 mg/L) from industrial wastewater by using magnetite.It was found that higher adsorbent loading (20 g/L) and a longer residence time (24 h) were required to treat the wastewater, which was reflected in the drastic reduction in the adsorption capacity of magnetite down to 0.04 mg/g due to the competition of co-existing metals for the adsorption sites.

Testing CDC in Groundwater
A performance test of CDC using real groundwater collected from the northern part of Qatar was conducted in the present work.The concentrations of the main elements in

Effect of ionic strength of feed solution
The effect of the salinity of the aqueous solution on the adsorption of Mo was also studied by carrying out adsorption experiments at different NaCl concentrations in the feed solution.Figure 19 shows the Mo ion removal efficiency of CDC from 0, 0.001, and 0.01 M NaCl solutions.It was found that the higher the salinity level, the lower the removal efficiency of CDC.These findings can be explained by the reduction in electrostatic interactions between the Mo ions and adsorption sites on the CDC's surface due to the shielding effect of the NaCl electrolyte.These findings correlate well with previous data on the impact of the ionic strength of solutions on the adsorption capacity of adsorbents reported in the literature [67,68].

Testing CDC in Groundwater
A performance test of CDC using real groundwater collected from the northern part of Qatar was conducted in the present work.The concentrations of the main elements in the GW samples were as follows: Ba (109 ppb), Ca (446 ppb), Cu (6.55), Mg (153 ppm), Mo (70 ppb), and Na (688 ppm).Figure 20 shows the removal efficiency of CDC from the unspiked GW sample along with samples spiked with 250 and 500 µg/L at pH 3. As seen, CDC was found to completely remove Mo from the unspiked GW sample (70 ppb).Moreover, the removal efficiencies for the CDC from GW samples spiked with 250 and 500 µg/L were 98.6% and 97.2%, respectively.The adsorption capacity for the removal of CDC from real groundwater had a range of 0.1-0.9mg/g.These values were found to be higher than the adsorption capacities reported in the literature for the removal of Mo from synthetic water using zeolite-supported magnetite [43], acrylamide triazine polymer [46], and activated carbon [14].

Regeneration of CDC Adsorbent
The regeneration of the CDC adsorbent was conducted by placing the exhausted sorbent in an aqueous solution at pH 11 and shaking the solution with the sorbent for 2 h.The analysis of the aqueous solution after regeneration by using the ICP-OES method showed that more than 95% of the Mo adsorbed on CDC was released into the water.In addition, Figure 21 shows the XPS spectra of the exhausted CDC sample before and after regeneration.As seen, the peaks related to molybdate ions were lacking in the XPS spectra of the CDC sample after regener-

Regeneration of CDC Adsorbent
The regeneration of the CDC adsorbent was conducted by placing the exhausted sorbent in an aqueous solution at pH 11 and shaking the solution with the sorbent for 2 h.The analysis of the aqueous solution after regeneration by using the ICP-OES method showed that more than 95% of the Mo adsorbed on CDC was released into the water.In addition, Figure 21 shows the XPS spectra of the exhausted CDC sample before and after regeneration.As seen, the peaks related to molybdate ions were lacking in the XPS spectra of the CDC sample after regeneration (Figure 21b).These findings indicate that the exhausted CDC adsorbent could be efficiently regenerated and Mo ions could be recovered from CDC by adjusting the pH of the aqueous solution.This study shows the potential of using CDC as an efficient sorbent for molybdenum removal from wastewater in the batch mode at the bench scale level; however, future studies should focus on (1) column adsorption studies and (2) the technoeconomic feasibility of employing CDC for the treatment of wastewater.The adsorbents used in industry were reported in the literature to employ the continuous mode due to its outstanding features, such as increased productivity, decreased cost, increased quality, etc.The aim of the present work was to test the proof of concept by investigating the effectiveness of CDC in removing Mo from contaminated GW, which was found to be successful.The focus of future work must be on the continuous mode using adsorption columns; hence, the potential of scaling up to a continuous mode must be carried out using an up-flow fixed bed column and the breakthrough parameters (such as the breakthrough time, exhaust time, breakthrough volume, exhaust volume, adsorption capacity, mass transfer zone height, and moving rate) must be evaluated.Furthermore, technoeconomic feasibility studies must also be conducted in order to compare the benefits of using CDC as an adsorbent for wastewater treatment against the incurred cost.This will enable us to assess whether using this material on a large scale will be economically justified and the returns will overcome the expenditure.This study shows the potential of using CDC as an efficient sorbent for molybdenum removal from wastewater in the batch mode at the bench scale level; however, future studies should focus on (1) column adsorption studies and (2) the technoeconomic feasibility of employing CDC for the treatment of wastewater.The adsorbents used in industry were reported in the literature to employ the continuous mode due to its outstanding features, such as increased productivity, decreased cost, increased quality, etc.The aim of the present work was to test the proof of concept by investigating the effectiveness of CDC in removing Mo from contaminated GW, which was found to be successful.The focus of future work must be on the continuous mode using adsorption columns; hence, the potential of scaling up to a continuous mode must be carried out using an up-flow fixed bed column and the breakthrough parameters (such as the breakthrough time, exhaust time, breakthrough volume, exhaust volume, adsorption capacity, mass transfer zone height, and moving rate) must be evaluated.Furthermore, technoeconomic feasibility studies must also be conducted in order to compare the benefits of using CDC as an adsorbent for wastewater treatment against the incurred cost.This will enable us to assess whether using this material on a large scale will be economically justified and the returns will overcome the expenditure.

Conclusions
For the first time, the adsorption of Mo from aqueous solutions using CDC was studied in a batch adsorption mode under different operating conditions, such as different adsorption times, CDC dosages, agitation speeds, Mo feed concentrations, pH values, and temperatures of the feed solutions.CDC samples before and after Mo adsorption were extensively characterized following SEM, TEM, XRD, XPS, BET, and FTIR methods.
The adsorption of Mo onto CDC was found to be better fit by the Freundlich isotherm model, as confirmed by the highest R 2 values (0.9118) and lowest SSE (0.4777), while the experimental adsorption data were found to be more consistent with the pseudo-secondorder kinetics adsorption model.Notably, the adsorption kinetics were very fast, and more than 99% removal of Mo from water was reached within a short adsorption time (less than 30 min).The main adsorption mechanisms contributing to Mo adsorption were suggested to be electrostatic interactions, ligand-ligand exchange, and surface complexation or ion exchange between Mo ions and oxygen-containing groups on the CDC surface.It should be mentioned that CDC almost completely removed Mo at a concentration of 1 ppm from water at an adsorbent loading of 0.6 g/L.
It was shown that Mo adsorption on CDC was pH-dependent, achieving the maximum adsorptive removal within a pH range of 3-5.The highest adsorption capacity corresponded to 16.24 mg/g at a Mo concentration of 10 ppm, adsorbent loading of 0.6 g/L, and pH 3.Moreover, the removal efficiency at acidic conditions (pH: 3) was found to be stable and high (>99%), regardless of the Mo concentrations (0.5-10 ppm).
The regenerability of the CDC was evaluated, and it was shown that about 95% of the Mo adsorbed could be recovered from CDC by regeneration at pH 11.Finally, the testing of the CDC using real groundwater showed that CDC could be used as an efficient adsorbent for the removal of Mo from groundwater; however, future work on the continuous fixedbed-adsorption removal of Mo and techno-economic feasibility analysis are required to prove the efficiency of CDC for water treatment on an industrial scale.

Figure 1 .
Figure 1.Schematic of the experimental work performed in this study.

Figure 1 .
Figure 1.Schematic of the experimental work performed in this study.

Figure 1 .
Figure 1.Schematic of the experimental work performed in this study.

Figure 2 .
Figure 2. SEM images of CDC particles at different magnifications.

Figure 2 .
Figure 2. SEM images of CDC particles at different magnifications.

Figure 2 .
Figure 2. SEM images of CDC particles at different magnifications.

Figure 6 .
Figure 6.Intensity of EDS spectra (counts per second) vs. energy (keV) for CDC samples before (red line) and after Mo adsorption (blue line).Figure 6. Intensity of EDS spectra (counts per second) vs. energy (keV) for CDC samples before (red line) and after Mo adsorption (blue line).

Figure 6 . 31 Figure 7 .
Figure 6.Intensity of EDS spectra (counts per second) vs. energy (keV) for CDC samples before (red line) and after Mo adsorption (blue line).

Figure 7 .
Figure 7. SEM elemental mapping of CDC before and after the adsorption of Mo.

31 Figure 8 .
Figure 8. FTIR spectra of CDC samples before (blue line) and after (red line) Mo adsorption.

Figure 8 .
Figure 8. FTIR spectra of CDC samples before (blue line) and after (red line) Mo adsorption.

31 Figure 8 .
Figure 8. FTIR spectra of CDC samples before (blue line) and after (red line) Mo adsorption.

Figure 12 .
Figure 12.Adsorption isotherm modeling of Mo removal by the CDC.pH 5.2, CDC dosage o g/L, and adsorption time of 1 h.

Figure 13 .
Figure 13.Nominal equilibrium concentration of Mo vs. time for the adsorption of Mo with CDC; CDC loading: 0.6 g/L; pH: 6; agitation speed: 150 RPM.
correlated the decrease in the removal efficiency with pH due to the presence of negatively charged Mo ions, such as Mo 7 O 24 6− , Mo 7 O 23 OH 5− , Mo 8

Figure 17 .
Figure 17.Schematic representation of the possible mechanisms of Mo adsorption by the CDC.

Figure 17 .
Figure 17.Schematic representation of the possible mechanisms of Mo adsorption by the CDC.

Figure 18 .
Figure 18.Adsorption capacity of CDC toward various metals in a multicomponent aqueous solution (the heavy metal concentrations were 2 and 5 ppm at pH 2 and 4; CDC loading: 0.6 g/L; agitation speed: 150 RPM.

Figure 18 .
Figure 18.Adsorption capacity of CDC toward various metals in a multicomponent aqueous solution (the heavy metal concentrations were 2 and 5 ppm at pH 2 and 4; CDC loading: 0.6 g/L; agitation speed: 150 RPM.

Figure 19 .
Figure 19.Mo removal efficiency and adsorption capacity of CDC at various NaCl concentrations in the feed solution.

Figure 19 .
Figure 19.Mo removal efficiency and adsorption capacity of CDC at various NaCl concentrations in the feed solution.

Water 2022 ,
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Figure 21 .
Figure 21.XPS (a,b) of CDC samples before and after Mo adsorption.

Figure 21 .
Figure 21.XPS (a,b) of CDC samples before and after Mo adsorption.

Table 1 .
Adsorption kinetic models used in the study.

Table 2 .
The adsorption isotherm models and adsorption parameters.

Table 1 .
Main findings reported in the literature for the removal of some heavy metals from water by adsorption onto various adsorbents, in addition to the present work.

Table 3 .
Main findings reported in the literature for the removal of some heavy metals from water by adsorption onto various adsorbents, in addition to the present work.

Table 2 .
Isotherm parameters, correlation factors, and SSE of the tested models on Mo remov the CDC.

Table 4 .
Isotherm parameters, correlation factors, and SSE of the tested models on Mo removal by the CDC.

Table 5 .
Kinetic model parameters and correlation factors of the tested models on Mo adsorption by the CDC.

Table 6 .
Gibbs free energy, entropy, and enthalpy of the adsorption process of Mo onto CDC at various temperatures and pH 6 and 9; CDC loading: 0.6 g/L; agitation speed: 150 RPM.