coatings

Article

Surface Functionalization of Mesoporous Carbon forthe Enhanced Removal of Strontium andCesium Radionuclides

Munzir Hamedelniel Suliman, Mohammad Nahid Siddiqui * and Chanbasha Basheer

Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia;g201304270@kfupm.edu.sa (M.H.S.); cbasheer@kfupm.edu.sa (C.B.)* Correspondence: mnahid@kfupm.edu.sa; Tel.: +966-138-602-529

Received: 24 August 2020; Accepted: 23 September 2020; Published: 25 September 2020�����������������

Abstract: Mesoporous carbons (MPC) and modified mesoporous carbons (MMPC) were preparedfrom asphalt for the adsorption of several metal ions from the aqueous solution. In this study,we investigated the adsorption efficiency of Cesium (Cs+) and Strontium (Sr2+) ions using mesoporousand modified mesoporous carbons. The optimum conditions for the removal of Cs+ and Sr2+ wereat 10.0 pH, 1.00 ppm (1000.0 µg/L) concentration, 20.0 min contact time, 0.20 g/L adsorbent dose,25.0 ◦C temperature with more than 95.0% removal of Cs+ and Sr2+ ions using MMPC. The limit ofdetection (LOD) was found to be 0.030 ppb and 10.00 ppb for Sr2+ and Cs+ metals ions, respectively,while the Limit of quantification (LOQ) was calculated to be 0.10 ppb for Sr2+ and 35.00 ppb for Cs+

metals ions. The functionalization of the MPC was performed using potassium permanganate toget MMPC, which were characterized by FT-IR spectroscopy. The nature of the X-ray diffractionpeaks suggests that the MPC and MMPC carbons are amorphous and semi-crystalline materials.The scanning electron microscope (SEM) and transition electron microscope (TEM) studies showedthe changes in the morphology due to the adsorption on the surface of the carbons. The TEM analysisclearly showed that the metal ions blocked most of the pores on the surface. The surface area, by N2

adsorption isotherm (BET), of MPC and MMPC were 937 and 667 m2·g−1, respectively. Among the

adsorption isotherms, Langmuir isotherm showed the best linearity. The Langmuir isotherm indicatesthat the adsorption is monolayer and homogeneous with a finite number of ions. Adsorption kineticsshowed better linearity with pseudo-second-order plots and obeys this order. This process indicatesthat the chemical interaction, such as covalent or ionic bonding, took place between the metal ionsand the carbon adsorbents.

Keywords: surface functionalization; surface modification; mesoporous carbon; adsorption;metal ions

1. Introduction

Clean water resources are vital to humans, animals, and plants. However, the extensive use ofthese resources, especially in the industrial process, leads to lower water quality and the availabilityof clean water [1–3]. The wastewater treatment for the removal of organic substances and heavymetals (such as As, Zn, Ni, Cu, Hg, Sr, Cs, and more) is considered as a significant and challengingtask [4]. The contamination of water with strontium (Sr2+) and cesium (Cs+) ions especially theradioactive isotopes 90Sr and 137Cs represent a significant concern due to their long half-life (t1/2 = 29.0and 30 years respectively) and high solubility [5]. These species are produced from the nuclear fusionand remains with nuclear wastes and emit beta and gamma radiations [6]. Thus, the development ofefficient, simple, and cost-effective methods to remove these dangerous ions from wastewater andproduce clean water. Different removal methods were investigated, such as liquid extraction [7],

Coatings 2020, 10, 923; doi:10.3390/coatings10100923 www.mdpi.com/journal/coatings

Coatings 2020, 10, 923 2 of 13

photo-degradation [8], electrochemical techniques [9], coprecipitation [10] and adsorption usingseveral adsorbents. Adsorptive removal of Sr and Cs ions is the preferred approach in terms ofadsorption capacity, selectivity, simplicity, and cost-effectiveness. The development of low-costadsorbent draws excellent attention, such as red mud [11], zeolite [12], bentonite [13], and eggshell [14].Carbon nanomaterials have been wildly investigated in water treatment due to the unique propertiesof these materials. These materials have shown an excellent adsorption ability to organic and inorganicpollutants through a different mechanism, physiosorption and chemisorption with different adsorptioncapacity depend chiefly on adsorbents’ active sites and available surface area [15]. Several carbonaceousadsorbent materials, including carbon nanotubes (CNT), carbon fibers, activated carbons and morehave been extensively examined for the treatment of polluted and wastewater [16–18]. The adsorptioncapacity of these carbonaceous adsorbents was significantly enhanced by the treatment of acids/bases,which leads to the functionalization of the carbonaceous material. This treatment introduced severalrelevant functional groups including some of the prominent negative functionalities such as carboxylic(–COOH), carbonyl (–C=O), and hydroxyl (–OH) which are capable of facilitating the interaction ofthe adsorbent with the cationic species in the aqueous medium, due to the replacement of the weakinteraction (van der Waals) with stronger electrostatic interactions [19]. The most used standardfunctionalization system is the mixture of potassium permanganate with nitric and sulfuric acidson the surface of several carbon adsorbents [20–22]. In another study, functionalized CNT showeda substantial enhancement in their adsorption capacity compared to other non-functionalized CNTfor the removal of Cu2+

, which showed an excellent removal by 94.5% at pH of 5.0 for 101 min [23].A similar comparison was reported for oxidized CNT for the removal of several ions (Cd2+, Zn2+, Ni2+,Cu2+, and Pb2+), which showed higher removal efficiency than the pristine. This enhancement could beexplained by the interaction of the metal ion with the functionalities mentioned above [24]. The removalof both Cs+ and Sr2+ was investigated by using sodium cobalt hexacyanoferrate encapsulated inalginate vesicle with CNT [25]. Generally, the adsorption capacity of carbon absorbent increased bythe enlargement of the surface area. The introduction of porosity on the carbon also increases thesurface area.

In this study, a simple approach to prepare high surface area functionalized mesoporous carbonfrom an inexpensive petroleum matrix source, asphalt, was adopted and was tested for the adsorptionof Sr2+ and Cs+ metal ions from aqueous solution. Asphalt is very inexpensive and widely accessibleheavy crude oil fraction obtained after all boiling fractions have been removed and there is no scopeto upgrade further. Asphalt contains the highest portion of carbon (85%–90%) with approximately10%–12% of hydrogen, therefore, economically not many useful fractions except is being used inroads and roofs. Asphalt has unique physicochemical properties, which afforded a very high surfacearea porous carbon (MPC) that can be easily tailored to suit many applications. The performanceof MPC and oxidatively modified MMPC nanoporous carbon materials were tested as nanoporousfilters to remove radioactive metal ions and polynuclear aromatic hydrocarbons (PAHs) from water.Compared with other porous activated carbon materials, the MMPC performance was much better andthe production cost was much cheaper than other reported carbon materials. Thines et al. [18] reportedthat the mesoporous functionalized carbons may induce exceptional metal-carbon interaction, causinga significant effect on several critical processes, including the interfacial equilibrium, desorption,and adsorption of highly reactive classes and their interaction with the surface. The mesoporousfunctionalized carbons were characterized using several spectroscopic techniques. The adsorptionparameters, kinetic (pseudo-first and second orders), and the adsorption isotherms (Langmuir,Freundlich, and Temkin) were investigated and optimized. The thermodynamic parameters such asentropy (∆S), enthalpy (∆H), and the Gibbs free energy of the processes were calculated and confirmedthat the metal ions removal process is exothermic and spontaneous.

Coatings 2020, 10, 923 3 of 13

2. Materials and Methods

Asphalt was procured from the Ras Tanura refinery through the Research and Development Centerof Saudi Aramco at Dhahran, Kingdom of Saudi Arabian. The following reagents were purchasedand used in this work; which includes potassium hydroxide (85%, Sigma Aldrich, Louis, MO, USA),hydrochloric acid (37%, Merck, Kenilworth, NJ, USA), sulfuric acid (95%–97%, Merck), potassiumpermanganate (99.5%, Sigma Aldrich). 1.0 and 0.1 M solution were prepared by using nitric acid,ACS reagent 70% from Sigma-Aldrich Company. 1.0 and 0.1 M solution were prepared by usingsodium hydroxide pellets from Fluka. Strontium standard 1000 ug/mL from ULTRA SCIENTIFICAnalytical Solution. Cesium chloride salt from Puratronic brand with Johnson Matthey ChemicalsLimited (London, UK) as manufacturer.

2.1. Preparation of Activated and Mesoporous Carbons

Mesoporous carbon (MPC) was prepared sequentially by heating the asphaltic material in atubular furnace (MTI Corporation, Richmond, CA, USA) at 525 ◦C for 7 h under a continuous streamof nitrogen gas. The resulted mixture was further carbonized by treating with potassium hydroxide(KOH) powder in a fixed mass ratio of 1:4 (carbon:KOH) at a higher temperature around 750 ◦C undera continuous stream of nitrogen gas. The afforded mesoporous carbon was washed several times withdistilled water for extended times, followed by diluted hydrochloric acid (HCl) and, again washed bydistilled water to remove all possible traces of NaCl or HCl. The MPC product was further drenchedin distilled water, filtered, dried in the oven, and stored.

2.2. Functionalization of Mesoporous Carbon

The mesoporous carbon (MPC) was treated with potassium permanganate (KMnO4), an oxidativeagent, to prepare functionalized mesoporous carbon (MMPC) which possesses various oxygenfunctional groups. Around 10.0 g of MPC was transferred into a 1000 mL Pyrex beaker containing150 g of 88.6% of H2SO4 (82 mL Conc. H2SO4 + 12 mL H2O) and 6.0 g of KMnO4 and let to restfor about one hour. Another 6.0 g dose of KMnO4 was added to reaction mixture and left for 7 h tocomplete the reaction. Finally, 500 mL of distilled water was added to the resultant solution and waited1 h to completely quench the reaction. The product obtained was functionalized mesoporous carbon(MMPC) which was washed thoroughly several times with distilled water until the pH value of 5 ofthe washings was obtained. The MMPC product was filtered, dried in the oven and stored [1].

2.3. Characterization of MPC and MMPC

The characteristic structure and surface morphology of the mesoporous carbon and functionalizedmesoporous carbon were studied by several analytical techniques including Scanningelectronmicroscopy (SEM) for the morphology were obtained using FESEM/FIB/GIS (Tescan Lyra-3,Brno-Kohoutovice, Czech Republic) operated at 20.0 kV. X-ray diffraction (XRD) analysis wasperformed on the Rigaku Ultima IV X-ray diffractometer (RIGAKU, Tokyo, Japan). The FT-IRspectral measurements were carried out on Nicolet 6700 FT-IR spectrometer (Thermo Scientific,Waltham, MA, USA) in the range 400–4000 cm−1. The Brunauer–Emmett–Teller (BET) analysis wasperformed using a micromeritics (Tristar II PLUS, Norcross, GA, USA) instrument under the continuousadsorption conditions to determine the surface area. Transmission electron microscopy (TEM) of bothmesoporous carbon samples were carried out using JEM2100F multipurpose (Jeol, Tokyo, Japan),200 kV FE (Field Emission) analytical electron microscope.

2.4. Adsorption Experiments

Initially, adsorption experiments were carried out in a 50-mL Erlenmeyer flask using both MPCand MMPC under varying parameters such as solution pH, concentration (ppm), adsorption time (min),catalyst dosage (mg), solution volume (mL), and temperature (◦C), the concentration of metal ions was

Coatings 2020, 10, 923 4 of 13

one ppm with 200 rpm stirring speed. The experiments were performed in triplicate. After adsorption,with a syringe and PTFE filter disc, samples were analyzed using ICP-OES with a Model PlasmaQuant®

PQ 9000 (Analytik Jena, Jena, Thuringia Land, Germany) with ppt detection limits. The optimumremoval parameters were at 10.0 pH, 1.00 ppm (1000.0 µg/L) concentration, 20.0 min contact time,0.20 g/L adsorbent dose, 25.0 ◦C temperature. This reaction method was a very precise and accuratewhich also afforded limits of detection (LOD) and limits of quantification (LOQ). The LOD was foundto be 0.030 ppb for Sr2+ and 10.00 ppb for Cs+ metals ions. The LOQ was calculated to be 0.10 ppbfor Sr2+ and 35.00 ppb for Cs+ metals ions. After each extraction, the sorbent was washed with acid(1 M HCl) for 20 min and then heated at 150 ◦C to regenerate the sorbent.

3. Results and Discussion

To understand this study properly, the results and discussion is being divided into two parts:(i) characterization of MPC and MMPC, and (ii) removal of Sr2+ and Cs+ metal ions by adsorption.

3.1. Characterization of MPC and MMPC

The prepared MPC and MMPC were characterized using several spectroscopic techniquesincluding XRD, FT-IR, SEM, TEM, and BET to study the phase, surface, the microstructural, and variousphysico-chemical properties of the adsorbents. Various properties of MPC, and MMPC were compared,however, the characteristic properties of MPC and MMPC were found to be of a great significance anddiscussed in detail. Activated carbon (AC) was the main precursor of the MPC and MMPC withoutany adsorptive capabilities.

FT-IR spectra in Figure 1 shows the surface modification of the MPC carbon surface onpermanganate oxidation in MMPC carbon. Several vibration peaks were observed corresponding tothe C=O, COOH, and OH groups. The most significant peaks observed in MMPC carbon were at3431.8, 1738.8, 1629, 1457.4, 1392.2, and 1054 cm−1, while in MPC two most prominent peaks observedwere at 3413 and 1629 cm−1 only. In the FTIR spectra, the 2356 cm−1 is belonging to trapped CO2.The bands at 2924 cm−1 are related to the C–H aldehyde group. The 1738 and 1629 cm−1 belongs tothe carbonyl, C=C double bonds, respectively. The secondary absorptions at 1457 and 1392 cm−1 arepeaks related to –COOH and –CH=O moieties [1]. Which makes MMPC very effective and efficientadsorbent for removing metal ions and other pollutants.

Coatings 2020, 10, x FOR PEER REVIEW 4 of 13

PlasmaQuant® PQ 9000 (Analytik Jena, Jena, Thuringia Land, Germany) with ppt detection limits. The optimum removal parameters were at 10.0 pH, 1.00 ppm (1000.0 µg/L) concentration, 20.0 min contact time, 0.20 g/L adsorbent dose, 25.0 °C temperature. This reaction method was a very precise and accurate which also afforded limits of detection (LOD) and limits of quantification (LOQ). The LOD was found to be 0.030 ppb for Sr2+ and 10.00 ppb for Cs+ metals ions. The LOQ was calculated to be 0.10 ppb for Sr2+ and 35.00 ppb for Cs+ metals ions. After each extraction, the sorbent was washed with acid (1 M HCl) for 20 min and then heated at 150 °C to regenerate the sorbent.

3. Results and Discussion

To understand this study properly, the results and discussion is being divided into two parts: (i) characterization of MPC and MMPC, and (ii) removal of Sr2+ and Cs+ metal ions by adsorption.

3.1. Characterization of MPC and MMPC

The prepared MPC and MMPC were characterized using several spectroscopic techniques including XRD, FT-IR, SEM, TEM, and BET to study the phase, surface, the microstructural, and various physico-chemical properties of the adsorbents. Various properties of MPC, and MMPC were compared, however, the characteristic properties of MPC and MMPC were found to be of a great significance and discussed in detail. Activated carbon (AC) was the main precursor of the MPC and MMPC without any adsorptive capabilities.

FT-IR spectra in Figure 1 shows the surface modification of the MPC carbon surface on permanganate oxidation in MMPC carbon. Several vibration peaks were observed corresponding to the C=O, COOH, and OH groups. The most significant peaks observed in MMPC carbon were at 3431.8, 1738.8, 1629, 1457.4, 1392.2, and 1054 cm−1, while in MPC two most prominent peaks observed were at 3413 and 1629 cm−1 only. In the FTIR spectra, the 2356 cm−1 is belonging to trapped CO2. The bands at 2924 cm−1 are related to the C–H aldehyde group. The 1738 and 1629 cm−1 belongs to the carbonyl, C=C double bonds, respectively. The secondary absorptions at 1457 and 1392 cm−1 are peaks related to –COOH and –CH=O moieties [1]. Which makes MMPC very effective and efficient adsorbent for removing metal ions and other pollutants.

Figure 1. FT-IR spectra of MPC and MMPC.

X-ray diffraction analysis of MPC and MMPC were carried out to investigate the phases which are shown in Figure 2. In MPC and MMPC, two diffraction peaks at 2θ angles of 23.5° and 43.0° ascribed to the reflection of planes 002 and 101 respectively. The nature of the diffraction peaks confirms that the synthesized MPC and MMPC were semi-crystalline and amorphous materials.

500100015002000250030003500

Wavenumber (cm-1)

Tran

smis

sion

%

1392.21457.4

1629.0

1738.82356.1

2857.5

2924.2

PC-K

PC

3431.8

MMPC

MPC

Figure 1. FT-IR spectra of MPC and MMPC.

X-ray diffraction analysis of MPC and MMPC were carried out to investigate the phases which areshown in Figure 2. In MPC and MMPC, two diffraction peaks at 2θ angles of 23.5◦ and 43.0◦ ascribed

Coatings 2020, 10, 923 5 of 13

to the reflection of planes 002 and 101 respectively. The nature of the diffraction peaks confirms thatthe synthesized MPC and MMPC were semi-crystalline and amorphous materials.Coatings 2020, 10, x FOR PEER REVIEW 5 of 13

Figure 2. X-ray diffraction of MPC and MMPC.

Figure 3 shows the microstructure and the morphology of the MPC (a) and MMPC (b) investigated by FESEM. The figures revealed the rough surface of agglomerated semi sphere particles with pores. These particles assemble in layers like morphology. SEM image was recorded after the adsorption of the cation. The porosity is relatively reduced due to adsorption of Sr2+ metal ions as shown in Figure 3c.

Figure 3. SEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

TEM was also carried out to augment the findings of FESEM. Figure 4a shows porous layers of in the case of MPC. MMPC sample Figure 4b also revealed more homogenous pores and thinner layers of carbon. This layer could be attributed to the oxidation process which occurs on the surface and can reduce the stacking of the layers. Similarly, a TEM was investigated after the adsorption. Small clusters of metal ions were deposited on the surfaces indicating great influence of the modified surface. The pores were blocked after the adsorption of the Sr2+ metal ions, as shown in Figure 4c.

Figure 4. TEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

10 15 20 25 30 35 40 45 50 55 60

002

2θ (deg.)

PC

002

101

101 PC-KIn

tens

ity (a

.u.) MPC

MMPC

Figure 2. X-ray diffraction of MPC and MMPC.

Figure 3 shows the microstructure and the morphology of the MPC (a) and MMPC (b) investigatedby FESEM. The figures revealed the rough surface of agglomerated semi sphere particles with pores.These particles assemble in layers like morphology. SEM image was recorded after the adsorptionof the cation. The porosity is relatively reduced due to adsorption of Sr2+ metal ions as shown inFigure 3c.

Coatings 2020, 10, x FOR PEER REVIEW 5 of 13

Figure 2. X-ray diffraction of MPC and MMPC.

Figure 3 shows the microstructure and the morphology of the MPC (a) and MMPC (b) investigated by FESEM. The figures revealed the rough surface of agglomerated semi sphere particles with pores. These particles assemble in layers like morphology. SEM image was recorded after the adsorption of the cation. The porosity is relatively reduced due to adsorption of Sr2+ metal ions as shown in Figure 3c.

Figure 3. SEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

TEM was also carried out to augment the findings of FESEM. Figure 4a shows porous layers of in the case of MPC. MMPC sample Figure 4b also revealed more homogenous pores and thinner layers of carbon. This layer could be attributed to the oxidation process which occurs on the surface and can reduce the stacking of the layers. Similarly, a TEM was investigated after the adsorption. Small clusters of metal ions were deposited on the surfaces indicating great influence of the modified surface. The pores were blocked after the adsorption of the Sr2+ metal ions, as shown in Figure 4c.

Figure 4. TEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

10 15 20 25 30 35 40 45 50 55 60

002

2θ (deg.)

PC

002

101

101 PC-K

Inte

nsity

(a.u

.) MPC

MMPC

Figure 3. SEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

TEM was also carried out to augment the findings of FESEM. Figure 4a shows porous layersof in the case of MPC. MMPC sample Figure 4b also revealed more homogenous pores and thinnerlayers of carbon. This layer could be attributed to the oxidation process which occurs on the surfaceand can reduce the stacking of the layers. Similarly, a TEM was investigated after the adsorption.Small clusters of metal ions were deposited on the surfaces indicating great influence of the modifiedsurface. The pores were blocked after the adsorption of the Sr2+ metal ions, as shown in Figure 4c.

The surface area and the type of porosity of the synthesized MPC and MMPC adsorbents weredetermined by the N2 adsorption isotherm (BET). The surface area of the MPC has been calculated tobe 937 m2

·g−1. The calculated value revealed a significant drop as a result of carbon functionalization.The MMPC surface area was 667 m2

·g−1. Figure 5 shows the BET isotherms of MPC and MMPCwhere Figure 5a compares the N2 adsorption-desorption isotherms of MPC and MMPC; whereasFigure 5b comapres the pore size distribution of MPC and MMPC. The sorbent consists of bothmicro and mesoporous structure. However, the majority of the pores are classified as mesoporous.The adsorption isotherm in Figure 5 confirms there were two types of pores meso (36.6 Å for MPC and

Coatings 2020, 10, 923 6 of 13

33.6 Å for MMPC) and microporosity (<2 nm) and the mesopores volume were 0.62 cm3·g−1 for MPC

and 0.16 cm3·g−1 for MMPC. MPC and MMPC were grained manually to obtain fine powder form.

We may expect to have a particle size of about 100 µm size.

Coatings 2020, 10, x FOR PEER REVIEW 5 of 13

Figure 2. X-ray diffraction of MPC and MMPC.

Figure 3 shows the microstructure and the morphology of the MPC (a) and MMPC (b) investigated by FESEM. The figures revealed the rough surface of agglomerated semi sphere particles with pores. These particles assemble in layers like morphology. SEM image was recorded after the adsorption of the cation. The porosity is relatively reduced due to adsorption of Sr2+ metal ions as shown in Figure 3c.

Figure 3. SEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

TEM was also carried out to augment the findings of FESEM. Figure 4a shows porous layers of in the case of MPC. MMPC sample Figure 4b also revealed more homogenous pores and thinner layers of carbon. This layer could be attributed to the oxidation process which occurs on the surface and can reduce the stacking of the layers. Similarly, a TEM was investigated after the adsorption. Small clusters of metal ions were deposited on the surfaces indicating great influence of the modified surface. The pores were blocked after the adsorption of the Sr2+ metal ions, as shown in Figure 4c.

Figure 4. TEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

10 15 20 25 30 35 40 45 50 55 60

002

2θ (deg.)

PC

002

101

101 PC-K

Inte

nsity

(a.u

.) MPC

MMPC

Figure 4. TEM of (a) MPC, (b) MMPC, and (c) MMPC after adsorption of Sr2+ ions metal ions.

Coatings 2020, 10, x FOR PEER REVIEW 6 of 13

The surface area and the type of porosity of the synthesized MPC and MMPC adsorbents were determined by the N2 adsorption isotherm (BET). The surface area of the MPC has been calculated to be 937 m2·g−1. The calculated value revealed a significant drop as a result of carbon functionalization. The MMPC surface area was 667 m2·g−1. Figure 5 shows the BET isotherms of MPC and MMPC where figure 5a compares the N2 adsorption-desorption isotherms of MPC and MMPC; whereas figure 5b comapres the pore size distribution of MPC and MMPC. The sorbent consists of both micro and mesoporous structure. However, the majority of the pores are classified as mesoporous. The adsorption isotherm in Figure 5 confirms there were two types of pores meso (36.6 Å for MPC and 33.6 Å for MMPC) and microporosity (<2 nm) and the mesopores volume were 0.62 cm3·g−1 for MPC and 0.16 cm3·g−1 for MMPC. MPC and MMPC were grained manually to obtain fine powder form. We may expect to have a particle size of about 100 µm size.

(a) (b)

Figure 5. BET isotherm of MPC and MMPC: (a) N2 adsorption-desorption isotherm; (b) Pore size distribution.

3.2. Optimization of Adsorption Parameters

MPC and MMPC were used as adsorbents for the removal of Sr2+ and Cs+ ions in aqueous solution. Several adsorption parameters were investigated and optimized to achieve the maximum adsorption capacity, such as adsorbent dosage, the time of the adsorptions, and the pH of the solution.

3.3. Dosage of the Adsorbent

The dosage of adsorbent is considered a critical parameter in the wastewater treatment process. Different amounts of MPC and MMPC (5, 10, 20, 30, and 40 mg) were studied in the removal of Sr2+ and Cs+ ions as shown in Figure 6. The adsorption took place in 30 mL aqueous solution with a pH of 10.0, MPC showed maximum adsorption efficiency for Sr with 42.8% at a dosage of 5 mg, while showed 26.7% for Cs when 10 mg used as a dosage amount. Remarkable enhancement was observed in the case of 10 mg of MMPC, which showed 93.3% and 85.6% adsorption for Sr2+ and Cs+ ions respectively. One interesting aspect of the dosage of adsorbent study is that the more adsorbent is used, the fewer cations it retains. This could be due to a hydrophobic effect that leads to the adsorbent particles’ agglutination, leading thus to a diminished total available surface.

Figure 5. BET isotherm of MPC and MMPC: (a) N2 adsorption-desorption isotherm; (b) Poresize distribution.

3.2. Optimization of Adsorption Parameters

MPC and MMPC were used as adsorbents for the removal of Sr2+ and Cs+ ions in aqueoussolution. Several adsorption parameters were investigated and optimized to achieve the maximumadsorption capacity, such as adsorbent dosage, the time of the adsorptions, and the pH of the solution.

3.3. Dosage of the Adsorbent

The dosage of adsorbent is considered a critical parameter in the wastewater treatment process.Different amounts of MPC and MMPC (5, 10, 20, 30, and 40 mg) were studied in the removal of Sr2+

and Cs+ ions as shown in Figure 6. The adsorption took place in 30 mL aqueous solution with a pH of10.0, MPC showed maximum adsorption efficiency for Sr with 42.8% at a dosage of 5 mg, while showed26.7% for Cs when 10 mg used as a dosage amount. Remarkable enhancement was observed in thecase of 10 mg of MMPC, which showed 93.3% and 85.6% adsorption for Sr2+ and Cs+ ions respectively.One interesting aspect of the dosage of adsorbent study is that the more adsorbent is used, the fewercations it retains. This could be due to a hydrophobic effect that leads to the adsorbent particles’agglutination, leading thus to a diminished total available surface.

Coatings 2020, 10, 923 7 of 13Coatings 2020, 10, x FOR PEER REVIEW 7 of 13

Figure 6. Effect of adsorbent dosage for the removal of (a) Sr2+ and (b) Cs+.

3.4. Optimization of Time

The optimum removal time for the ions using MPC adsorbent was 45 min to remove only 11.0% and 1.5% of the Sr2+ and Cs+, respectively. The adsorption took place in 30 mL aqueous solution with a pH of 10.0, the concentration of metal ions was one ppm and 5 mg adsorbent dosage. The removal time and the adsorption capacity were enhanced significantly upon the use of MMPC in only 5 min to 85.5% and 81.0% of Sr2+, and Cs+ respectively were removed, and the optimum removal time was found to be 15 min (after 20 min, the maximum adsorption is already attained. Prolonged times would lead to increase desorption), which showed removal efficiency of 97.5% and 83.0% for Sr2+ and Cs+ respectively. The effect of time on the adsorption of Sr2+, and Cs+ metal ions on MPC and MMPC is given Figure 7.

Figure 7. Effect of removal time for the removal of (a) Sr2+ and (b) Cs+ ions.

3.5. Optimization of the pH

Figure 8 shows the effect of pH on the adsorption phenomenon of MPC and MMPC. The pH was adjusted using both 0.1 M and 1.0 M nitric acid and 0.1 M and 1 M sodium hydroxide solutions. The pH of the solution has a significant influence on the adsorption process. This effect can be attributed to the effect of the pH on the functionalities on the adsorbents, which will affect the adsorption process. At high pH, basic functionalities such as (–COO−) could enhance the binding with the cations. Several experiments took place in pH range between 3 and 12. The removal efficiency increased with increasing the pH up to 10.0, which was considered the optimum pH. Although pH 10.0 showed the highest adsorption capacity, pH 7.0 and 9.0 (normal water pH) revealed a comparable removal efficiency with pH 10.0, which means this process is suitable for industrial samples. In Figure 8b, it can be observed that there is a sharp drop in the adsorption efficiency of Cs+ ions. It has been reported that at higher pH values the Cs+ ions may form hydroxides leading to lower adsorption [26,27].

Figure 6. Effect of adsorbent dosage for the removal of (a) Sr2+ and (b) Cs+.

3.4. Optimization of Time

The optimum removal time for the ions using MPC adsorbent was 45 min to remove only 11.0%and 1.5% of the Sr2+ and Cs+, respectively. The adsorption took place in 30 mL aqueous solution witha pH of 10.0, the concentration of metal ions was one ppm and 5 mg adsorbent dosage. The removaltime and the adsorption capacity were enhanced significantly upon the use of MMPC in only 5 min to85.5% and 81.0% of Sr2+, and Cs+ respectively were removed, and the optimum removal time wasfound to be 15 min (after 20 min, the maximum adsorption is already attained. Prolonged times wouldlead to increase desorption), which showed removal efficiency of 97.5% and 83.0% for Sr2+ and Cs+

respectively. The effect of time on the adsorption of Sr2+, and Cs+ metal ions on MPC and MMPC isgiven Figure 7.

Coatings 2020, 10, x FOR PEER REVIEW 7 of 13

Figure 6. Effect of adsorbent dosage for the removal of (a) Sr2+ and (b) Cs+.

3.4. Optimization of Time

The optimum removal time for the ions using MPC adsorbent was 45 min to remove only 11.0% and 1.5% of the Sr2+ and Cs+, respectively. The adsorption took place in 30 mL aqueous solution with a pH of 10.0, the concentration of metal ions was one ppm and 5 mg adsorbent dosage. The removal time and the adsorption capacity were enhanced significantly upon the use of MMPC in only 5 min to 85.5% and 81.0% of Sr2+, and Cs+ respectively were removed, and the optimum removal time was found to be 15 min (after 20 min, the maximum adsorption is already attained. Prolonged times would lead to increase desorption), which showed removal efficiency of 97.5% and 83.0% for Sr2+ and Cs+ respectively. The effect of time on the adsorption of Sr2+, and Cs+ metal ions on MPC and MMPC is given Figure 7.

Figure 7. Effect of removal time for the removal of (a) Sr2+ and (b) Cs+ ions.

3.5. Optimization of the pH

Figure 8 shows the effect of pH on the adsorption phenomenon of MPC and MMPC. The pH was adjusted using both 0.1 M and 1.0 M nitric acid and 0.1 M and 1 M sodium hydroxide solutions. The pH of the solution has a significant influence on the adsorption process. This effect can be attributed to the effect of the pH on the functionalities on the adsorbents, which will affect the adsorption process. At high pH, basic functionalities such as (–COO−) could enhance the binding with the cations. Several experiments took place in pH range between 3 and 12. The removal efficiency increased with increasing the pH up to 10.0, which was considered the optimum pH. Although pH 10.0 showed the highest adsorption capacity, pH 7.0 and 9.0 (normal water pH) revealed a comparable removal efficiency with pH 10.0, which means this process is suitable for industrial samples. In Figure 8b, it can be observed that there is a sharp drop in the adsorption efficiency of Cs+ ions. It has been reported that at higher pH values the Cs+ ions may form hydroxides leading to lower adsorption [26,27].

Figure 7. Effect of removal time for the removal of (a) Sr2+ and (b) Cs+ ions.

3.5. Optimization of the pH

Figure 8 shows the effect of pH on the adsorption phenomenon of MPC and MMPC. The pH wasadjusted using both 0.1 M and 1.0 M nitric acid and 0.1 M and 1 M sodium hydroxide solutions. The pHof the solution has a significant influence on the adsorption process. This effect can be attributed to theeffect of the pH on the functionalities on the adsorbents, which will affect the adsorption process. Athigh pH, basic functionalities such as (–COO−) could enhance the binding with the cations. Severalexperiments took place in pH range between 3 and 12. The removal efficiency increased with increasingthe pH up to 10.0, which was considered the optimum pH. Although pH 10.0 showed the highestadsorption capacity, pH 7.0 and 9.0 (normal water pH) revealed a comparable removal efficiency withpH 10.0, which means this process is suitable for industrial samples. In Figure 8b, it can be observedthat there is a sharp drop in the adsorption efficiency of Cs+ ions. It has been reported that at higherpH values the Cs+ ions may form hydroxides leading to lower adsorption [26,27].

Coatings 2020, 10, 923 8 of 13Coatings 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 8. Effect of the solution pH for the removal of (a) Sr2+ and (b) Cs+ ions.

3.6. Adsorption Kinetics

Adsorption kinetics (pseudo-first-order and pseudo second-order) were investigated for MPC and MMPC for the removal of Sr2+ and Cs+ using Equations (1) and (2). Where (qe) is the adsorption capacity at the equilibrium, (qt) is the adsorption capacity at certain time, k the rate of adsorption and t is the time of adsorption. log 𝑞 − 𝑞 = log𝑞 − 𝑘𝑡2.0303 (1) 𝑡𝑞 = 1ℎ + 𝑡𝑞 (2)

As per Figure 9, better linearity was observed in case of pseudo-second-order plots (Figure 9c,d), which is suggest that the adsorption obey that order, which indicates the interaction between the ions and the adsorbent takes place through chemical interaction such as covalent or ionic bond [28].

Figure 9. Pseudo-first-order plot for MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+ and pseudo-second-order plot for MPC and MMPC for the removal of (c) Sr2+ and (d) Cs+ ions.

Figure 8. Effect of the solution pH for the removal of (a) Sr2+ and (b) Cs+ ions.

3.6. Adsorption Kinetics

Adsorption kinetics (pseudo-first-order and pseudo second-order) were investigated for MPCand MMPC for the removal of Sr2+ and Cs+ using Equations (1) and (2). Where (qe) is the adsorptioncapacity at the equilibrium, (qt) is the adsorption capacity at certain time, k the rate of adsorption and tis the time of adsorption.

log(qe − qt) = log qe −kt

2.0303(1)

tqe

=1h+

tqe

(2)

As per Figure 9, better linearity was observed in case of pseudo-second-order plots (Figure 9c,d),which is suggest that the adsorption obey that order, which indicates the interaction between the ionsand the adsorbent takes place through chemical interaction such as covalent or ionic bond [28].

Coatings 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 8. Effect of the solution pH for the removal of (a) Sr2+ and (b) Cs+ ions.

3.6. Adsorption Kinetics

Adsorption kinetics (pseudo-first-order and pseudo second-order) were investigated for MPC and MMPC for the removal of Sr2+ and Cs+ using Equations (1) and (2). Where (qe) is the adsorption capacity at the equilibrium, (qt) is the adsorption capacity at certain time, k the rate of adsorption and t is the time of adsorption. log 𝑞 − 𝑞 = log𝑞 − 𝑘𝑡2.0303 (1) 𝑡𝑞 = 1ℎ + 𝑡𝑞 (2)

As per Figure 9, better linearity was observed in case of pseudo-second-order plots (Figure 9c,d), which is suggest that the adsorption obey that order, which indicates the interaction between the ions and the adsorbent takes place through chemical interaction such as covalent or ionic bond [28].

Figure 9. Pseudo-first-order plot for MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+ and pseudo-second-order plot for MPC and MMPC for the removal of (c) Sr2+ and (d) Cs+ ions.

Figure 9. Pseudo-first-order plot for MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+ andpseudo-second-order plot for MPC and MMPC for the removal of (c) Sr2+ and (d) Cs+ ions.

Coatings 2020, 10, 923 9 of 13

Langmuir, Freundlich and Temkin adsorption isotherms were plotted using Equations (3)–(5) andrepresented in Figure 10. From the figure, Langmuir isotherm showed the best linearity. This isothermindicates that the adsorption is monolayer and homogeneous with a finite number of ions [29].

Ce

qe=

1kqm

+Ce

qm(3)

ln qe = ln k +1n

ln Ce (4)

qe =RTB

ln KT Ce (5)

Coatings 2020, 10, x FOR PEER REVIEW 9 of 13

Langmuir, Freundlich and Temkin adsorption isotherms were plotted using Equations (3)–(5) and represented in Figure 10. From the figure, Langmuir isotherm showed the best linearity. This isotherm indicates that the adsorption is monolayer and homogeneous with a finite number of ions [29]. 𝐶 𝑞 = 1𝑘𝑞 + 𝐶 𝑞 (3)

ln 𝑞 = ln𝑘 + 1𝑛 ln𝐶 (4)

𝑞 = 𝑅𝑇𝐵 ln𝐾 𝐶 (5)

3.7. Adsorption Thermodynamics

Thermodynamic parameters such as enthalpy change (∆H), entropy change (∆S), and change in the free energy (∆G) were calculated using the effect of three different temperatures on the adsorption process at 25, 35 and 45 °C and given in Figure 11. Equation (6) (Van’t-Hoff equation) was used to calculate ∆H and ∆S from the figure, and ∆G was calculated using Equation (7), all the values are summarized in Table 1. ln𝐾𝑐 = − ∆𝐻𝑅𝑇 + ∆𝑆𝑅 (6)

∆𝐺 = ∆𝐻 − 𝑇∆𝑆 (7)

Figure 10. Adsorption isotherms for MMPC for the removal of Cs+ (red line) and Sr2+ (black line) using Langmuir isotherm (a), Freundlich isotherm, (b) and Temkin isotherm (c) models.

The positive ∆H confirmed that the reaction is endothermic, while the positive ∆S suggests the increase of the disorder during adsorption as a result of the release of water molecules from the substantial hydration. The process was confirmed to be spontaneous from the negative values of Gibbs free energy.

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.2

0.4

0.6

0.8

1.0

Sr2+

Cs+

R2=0.994

Ce/q

e

Ce

R2=0.989

a

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5-0.3

-0.2

-0.1

0.0

Sr2+

Cs+

R2=0.100

ln(q

e)

ln (Ce)

R2=0.961

b

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.2

0.4

0.6

0.8

1.0

Sr2+

Cs+

R2=0.999

ln(C

e)

qe

R2=0.978c

Figure 10. Adsorption isotherms for MMPC for the removal of Cs+ (red line) and Sr2+ (black line)using Langmuir isotherm (a), Freundlich isotherm, (b) and Temkin isotherm (c) models.

3.7. Adsorption Thermodynamics

Thermodynamic parameters such as enthalpy change (∆H), entropy change (∆S), and change inthe free energy (∆G) were calculated using the effect of three different temperatures on the adsorptionprocess at 25, 35 and 45 ◦C and given in Figure 11. Equation (6) (Van’t-Hoff equation) was used tocalculate ∆H and ∆S from the figure, and ∆G was calculated using Equation (7), all the values aresummarized in Table 1.

ln Kc = −∆HRT

+∆SR

(6)

∆G = ∆H − T∆S (7)

The positive ∆H confirmed that the reaction is endothermic, while the positive ∆S suggests theincrease of the disorder during adsorption as a result of the release of water molecules from thesubstantial hydration. The process was confirmed to be spontaneous from the negative values of Gibbsfree energy.

Coatings 2020, 10, 923 10 of 13

Coatings 2020, 10, x FOR PEER REVIEW 10 of 13

Figure 11. Van’t Hoff relationship with temperature for Sr2+ and Cs+ ions.

Table 1. Thermodynamic parameters calculated from Van’t-Hoff equation.

Parameters ∆H (kJ·mol−1) ∆S (kJ K−1·mol−1) ∆G (kJ·mol−1) Cs+ 5.87 0.031 −3.37 Sr2+ 4.45 0.029 −4.19

3.8. Regeneration of Adsorbents

The reusability of the adsorbent is considered as a critical factor for the real-life application. The regeneration of MPC and MMPC adsorbents was investigated for the three-cycle for the removal of Sr2+ and Cs+ in the aqueous medium as reported in Figure 12. The MPC and MMPC showed excellent durability and reusability, with higher removal efficiency for MMPC with no significant drop in the removal percentage.

Figure 12. The regeneration of MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+.

4. Conclusions

Our study reports a straightforward, cost-effective, and efficient method to prepare mesoporous carbons (MPC) and modified mesoporous carbons (MMPC) from the asphalt. The MPC and MMPC were characterized using several spectroscopic techniques such as FT-IR, XRD, BET, SEM and TEM. Table 2 shows the comparison of the surface areas and pore sizes of recently developed mesoporous carbon adsorbents from asphalt, which shows that our MMPC adsorbent has better properties. In this study, we investigated the adsorption efficiency of Sr2+ and Cs+ ions using mesoporous and modified mesoporous carbons. Introduction of negative functionalities such as carboxylic (–COOH), carbonyl (–C=O), and hydroxyl (–OH) which are capable of facilitating the interaction of the adsorbent with the cationic species in the aqueous medium, due to the replacement of the weak interaction (van der Waals) with stronger electrostatic interactions. The optimum conditions for the removal of Cs and Sr

Figure 11. Van’t Hoff relationship with temperature for Sr2+ and Cs+ ions.

Table 1. Thermodynamic parameters calculated from Van’t-Hoff equation.

Parameters ∆H (kJ·mol−1) ∆S (kJ K−1·mol−1) ∆G (kJ·mol−1)

Cs+ 5.87 0.031 −3.37Sr2+ 4.45 0.029 −4.19

3.8. Regeneration of Adsorbents

The reusability of the adsorbent is considered as a critical factor for the real-life application.The regeneration of MPC and MMPC adsorbents was investigated for the three-cycle for the removal ofSr2+ and Cs+ in the aqueous medium as reported in Figure 12. The MPC and MMPC showed excellentdurability and reusability, with higher removal efficiency for MMPC with no significant drop in theremoval percentage.

Coatings 2020, 10, x FOR PEER REVIEW 10 of 13

Figure 11. Van’t Hoff relationship with temperature for Sr2+ and Cs+ ions.

Table 1. Thermodynamic parameters calculated from Van’t-Hoff equation.

Parameters ∆H (kJ·mol−1) ∆S (kJ K−1·mol−1) ∆G (kJ·mol−1) Cs+ 5.87 0.031 −3.37 Sr2+ 4.45 0.029 −4.19

3.8. Regeneration of Adsorbents

The reusability of the adsorbent is considered as a critical factor for the real-life application. The regeneration of MPC and MMPC adsorbents was investigated for the three-cycle for the removal of Sr2+ and Cs+ in the aqueous medium as reported in Figure 12. The MPC and MMPC showed excellent durability and reusability, with higher removal efficiency for MMPC with no significant drop in the removal percentage.

Figure 12. The regeneration of MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+.

4. Conclusions

Our study reports a straightforward, cost-effective, and efficient method to prepare mesoporous carbons (MPC) and modified mesoporous carbons (MMPC) from the asphalt. The MPC and MMPC were characterized using several spectroscopic techniques such as FT-IR, XRD, BET, SEM and TEM. Table 2 shows the comparison of the surface areas and pore sizes of recently developed mesoporous carbon adsorbents from asphalt, which shows that our MMPC adsorbent has better properties. In this study, we investigated the adsorption efficiency of Sr2+ and Cs+ ions using mesoporous and modified mesoporous carbons. Introduction of negative functionalities such as carboxylic (–COOH), carbonyl (–C=O), and hydroxyl (–OH) which are capable of facilitating the interaction of the adsorbent with the cationic species in the aqueous medium, due to the replacement of the weak interaction (van der Waals) with stronger electrostatic interactions. The optimum conditions for the removal of Cs and Sr

Figure 12. The regeneration of MPC and MMPC for the removal of (a) Sr2+ and (b) Cs+.

4. Conclusions

Our study reports a straightforward, cost-effective, and efficient method to prepare mesoporouscarbons (MPC) and modified mesoporous carbons (MMPC) from the asphalt. The MPC and MMPCwere characterized using several spectroscopic techniques such as FT-IR, XRD, BET, SEM and TEM.

Table 2 shows the comparison of the surface areas and pore sizes of recently developed mesoporouscarbon adsorbents from asphalt, which shows that our MMPC adsorbent has better properties.In this study, we investigated the adsorption efficiency of Sr2+ and Cs+ ions using mesoporous andmodified mesoporous carbons. Introduction of negative functionalities such as carboxylic (–COOH),carbonyl (–C=O), and hydroxyl (–OH) which are capable of facilitating the interaction of the adsorbent

Coatings 2020, 10, 923 11 of 13

with the cationic species in the aqueous medium, due to the replacement of the weak interaction(van der Waals) with stronger electrostatic interactions. The optimum conditions for the removalof Cs and Sr were at 10.0 pH, 1.00 ppm (1000.0 µg/L) concentration, 20.0 min contact time, 0.20 g/Ladsorbent dose, 25.0 ◦C temperature with more than 95.0% removal of Cs+ and Sr2+ ions using MMPC.Furthermore, MMPC revealed excellent durability for three regeneration cycles without a significantdrop in the removal efficiency. Additionally, normal water pH, comparable removal of Cs+ andSr2+ (around 80.0%) was achieved. The applied removal conditions and the cost-effective catalystare considered as promising for treating the real polluted samples and other industrial applications.The Langmuir adsorption isotherm showed the best linearity indicating the adsorption process ismonolayer and homogeneous with a finite number of ions. Adsorption kinetics showed better linearitywith pseudo-second-order plots and obeys pseudo-second-order. This process confirms that thechemical interaction, such as covalent or ionic bonding, took place between the metal ions and thecarbon adsorbents.

Table 2. Comparison of the surface area and pore size of the mesoporous carbon obtained from differentasphalt samples.

Carbon Surface Area (m2·g−1) Pore Size (nm) Ref.

Activated carbon (Jordan Asphalt) - - [30]Graphitic mesoporous carbon 479 7.53 [31]Oxidized meoporous carbon 333.6 3.93 [32]Hierarchical porous carbon 3581 2.20 [33]

MMPC 667 3.60 This work

Author Contributions: Conceptualization, M.N.S. and C.B.; funding acquisition, M.N.S. and C.B.;writing—original draft preparation, M.H.S.; writing—review and editing, M.H.S., M.N.S., and C.B.; supervision,M.N.S. and C.B. All authors have read and agreed to the published version of the manuscript.

Funding: Authors would like to thank Deanship of Scientific Research, King Fahd University of Petroleum andMinerals (KFUPM) for funding this work through project number IN171031.

Acknowledgments: Authors would like to acknowledge support provided by the Chemistry department,King Fahd University of Petroleum and Minerals (KFUPM) in carrying out this work.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Siddiqui, M.N.; Ali, I.; Asim, M.; Chanbasha, B. Quick removal of nickel metal ions in water usingasphalt-based porous carbon. J. Mol. Liq. 2020, 308, 113078. [CrossRef]

2. Alissa, F.M.; Mohammed, D.; Osman, A.M.; Chanbasha, B.; Siddiqui, M.N.; Al-Arfaj, A.A.; Suliman, M.H.Synthesis of highly efficient asphalt-based carbon for adsorption of polycyclic aromatic hydrocarbons anddiesel from emulsified aqueous phase. Carbon Lett. 2020, 30, 555–567. [CrossRef]

3. Basheer, A.A. New generation nano-adsorbents for the removal of emerging contaminants in water. J. Mol.Liq. 2018, 261, 583–593. [CrossRef]

4. Allen, H.E.; Perdue, E.M.; Brown, D.S. Metals in Groundwater; CRC Press: Boca Raton, FL, USA, 1993.5. Todd, T.A.; Todd, T.A.; Law, J.D.; Herbst, R.S. Cesium and Strontium Separation Technologies Literature Review;

INEEL/EXT-04-01895; Idaho National Engineering and Environmental Laboratory: Idaho Falls, ID, USA, 2004.6. Aguila, B.; Banerjee, D.; Nie, Z.; Shin, Y.; Ma, S.; Thallapally, P.K. Selective removal of cesium and strontium

using porous frameworks from high level nuclear waste. Chem. Commun. 2016, 52, 5940–5942. [CrossRef]7. Wilmarth, W.R.; Lumetta, G.J.; Johnson, M.E.; Poirier, M.R.; Thompson, M.C.; Suggs, P.C.; Machara, N.P.

Waste-pretreatment technologies for remediation of legacy defense nuclear wastes. Solvent Extr. Ion Exch.2011, 29, 1–48. [CrossRef]

8. Kabra, K.; Chaudhary, R.; Sawhney, R.L. Solar photocatalytic removal of metal ions from industrial wastewater.Environ. Prog. 2008, 27, 487–495.

Coatings 2020, 10, 923 12 of 13

9. Chen, X.; Huang, G.; Wang, J. Electrochemical reduction/oxidation in the treatment of heavy metal wastewater.J. Metall. Eng. 2013, 2, 161–164.

10. Lakshtanov, L.Z.; Stipp, S.L.S. Experimental study of nickel (II) interaction with calcite: Adsorption andcoprecipitation. Geochim. Cosmochim. Acta 2007, 71, 3686–3697.

11. Milenkovic, A.; Smiciklas, I.; Bundaleski, N.; Teodoro, O.M.N.D.; Veljovic, D.; Vukelic, N. The role of differentminerals from red mud assemblage in Co (II) sorption mechanism. Colloids Surf. A Physicochem. Eng. Asp.2016, 508, 8–20.

12. Koshy, N.; Singh, D.N. Fly ash zeolites for water treatment applications. J. Environ. Chem. Eng. 2016, 4,1460–1472.

13. Tran, E.L.; Teutsch, N.; Klein-BenDavid, O.; Weisbrod, N. Uranium and cesium sorption to bentonite colloidsunder carbonate-rich environments: Implications for radionuclide transport. Sci. Total Environ. 2018, 643,260–269. [CrossRef] [PubMed]

14. Metwally, S.S.; Rizk, H.E.; Gasser, M.S. Biosorption of strontium ions from aqueous solution using modifiedeggshell materials. Radiochim. Acta 2017, 105, 1021–1103. [CrossRef]

15. Vipin, A.K.; Ling, S.; Fugetsu, B. Sodium cobalt hexacyanoferrate encapsulated in alginate vesicle with CNTfor both cesium and strontium removal. Carbohydr. Polym. 2014, 111, 477–484. [CrossRef]

16. AlSaadi, M.A.; AlMamun, A.; Alam, M.Z.; Amosa, M.K.; Atieh, M.A. Removal of cadmium from water byCNT-PAC composite: Effect of functionalization. Nano 2016, 11, 1650011. [CrossRef]

17. Ibrahim, R.K.; Hayyan, M.; AlSaadi, M.A.; Hayyan, A.; Ibrahim, S. Environmental application ofnanotechnology: Air, soil, and water. Environ. Sci. Pollut. Res. 2016, 23, 13754–13788. [CrossRef]

18. Thines, R.K.; Mubarak, N.M.; Nizamuddin, S.; Sahu, J.N.; Abdullah, E.C.; Ganesan, P. Application potentialof carbon nanomaterials in water and wastewater treatment: A review. J. Taiwan Inst. Chem. Eng. 2017, 72,116–133. [CrossRef]

19. Hu, C.; Xu, Y.; Duo, S.; Zhang, R.; Li, M. Non-covalent functionalization of carbon nanotubes with surfactantsand polymers. J. Chin. Chem. Soc. 2009, 56, 234–239. [CrossRef]

20. Lu, C.; Chiu, H. Chemical modification of multiwalled carbon nanotubes for sorption of Zn2+ from aqueoussolution. Chem. Eng. J. 2008, 139, 462–468. [CrossRef]

21. Mubarak, N.M.; Sahu, J.N.; Wong, J.R.; Jayakumar, N.S.; Ganesan, P.; Abdullah, E.C. Overview on theFunctionalization of Carbon Nanotubes. In Chemical Functionalization of Carbon Nanomaterials; CRC Press:Boca Raton, FL, USA, 2015; pp. 108–127.

22. Yang, S.; Li, J.; Shao, D.; Hu, J.; Wang, X. Adsorption of Ni (II) on oxidized multi-walled carbon nanotubes:Effect of contact time, pH, foreign ions and PAA. J. Hazard. Mater. 2009, 166, 109–116. [CrossRef]

23. Mubarak, N.; Daniel, S.; Khalid, M.; Tan, J. Comparative study of functionalize and non-functionalizedcarbon nanotube for removal of copper from polluted water. Int. J. Chem. Environ. Eng. 2012, 3, 314–317.

24. Rao, G.P.; Lu, C.; Su, F. Sorption of divalent metal ions from aqueous solution by carbon nanotubes: A review.Sep. Purif. Technol. 2007, 58, 224–231.

25. Suliman, M.H.; Adam, A.; Siddiqui, M.N.; Yamani, Z.H.; Qamar, M. Facile synthesis of ultrathin interconnectedcarbon nanosheets as a robust support for small and uniformly-dispersed iron phosphide for the hydrogenevolution reaction. Carbon 2019, 144, 764–771. [CrossRef]

26. Gutierrez, M.; Fuentes, H.R. A mechanistic modeling of montomorillonite contamination by cesium sorption.Appl. Clay Sci. 1996, 11, 11–24. [CrossRef]

27. Giannakopoulou, F.; Haidouti, C.; Chronopoulou, A.; Gasparatos, D. Sorption behavior of cesium on varioussoils under different pH levels. J. Hazard. Mater. 2007, 49, 553–556. [CrossRef] [PubMed]

28. Yadav, S.K.; Singh, D.K.; Sinha, S. Adsorptive removal of Hg (II) from synthetic and real aqueous solutionsusing modified papaya seed. J. Dispers. Sci. Technol. 2016, 37, 1613–1622. [CrossRef]

29. Ghasemi, M.; Naushad, M.; Ghasemi, N.; Khosravi-Fard, Y. A novel agricultural waste based adsorbent forthe removal of Pb (II) from aqueous solution: Kinetics, equilibrium and thermodynamic studies. J. Ind. Eng.Chem. 2014, 20, 454–461. [CrossRef]

30. Kandah, M.I.; Shawabkeh, R.; Al-Zboon, M.A. Synthesis and characterization of activated carbon fromasphalt. Appl. Surf. Sci. 2006, 253, 821–826. [CrossRef]

31. Zhan, X.; Zhong, C.; Lin, Q.; Luo, S.; Zhang, X.; Fang, C. Facile fabrication of graphitic mesoporous carbonwith ultrathin walls from petroleum asphalt. J. Anal. Appl. Pyrolysis 2017, 126, 154–157. [CrossRef]

Coatings 2020, 10, 923 13 of 13

32. Zhang, X.; Lin, Q.; Luo, Q.; Ruan, K.; Peng, K. Preparation of novel oxidized mesoporous carbon withexcellent adsorption performance for removal of malachite green and lead ion. Appl. Surf. Sci. 2018, 442,322–331. [CrossRef]

33. Pan, L.; Wang, Y.; Hu, H.; Li, X.; Liu, J.; Guan, L.; Tian, W.; Wang, X.; Li, W.; Wu, M. 3D self-assemblysynthesis of hierarchical porous carbon from petroleum asphalt for supercapacitors. Carbon 2018, 134,345–353. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).