Photodegradation of Phenolic Compounds from Water in the Presence of a Pd-Containing Exhausted Adsorbent

applied sciences

Article Photodegradation of Phenolic Compounds from Water in the Presence of a Pd-Containing Exhausted Adsorbent

Lavinia Lupa 1 , Laura Cocheci 1,* , Bogdan Trica 2, Adina Coroaba 3 and Adriana Popa 4

1 Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, 6 V. Parvan Blv, RO-300223 Timisoara, Romania; [email protected] 2 National Institute for Research & Development in Chemistry and Petrochemistry—ICECHIM, 202 Spl. Independentei, RO-060021 Bucuresti, Romania; [email protected] 3 Centre of Advanced Research in Bionanoconjugates and Biopolymers, “Petru Poni” Institute of Macromolecular Chemistry Iasi, Grigore Ghica Voda Alley, No.41 A, 700487 Iasi, Romania; [email protected] 4 Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Blv., RO-300223 Timisoara, Romania; [email protected] * Correspondence: [email protected]

Received: 18 October 2020; Accepted: 22 November 2020; Published: 26 November 2020

Abstract: A closed-cycle technology regarding the use of an exhausted Pd-based adsorbent as a photocatalyst in the degradation process of phenol is presented. Pd (II) represents a precious metal of great economic importance. Its obtained from natural sources become more difficult to achieve. Therefore, also considering the regulations of the “circular economy,” its recovery from secondary sources turn out to be a stringent issue in the last years. Pd(II) ions are removed from aqueous solution through adsorption onto Florisil (an inorganic solid support—magnesium silicate) impregnated with Cyphos IL 101 (trihexyl tetradecyl phosphonium chloride). It was observed that the presence of the ionic liquid (IL) in the adsorbent structure doubles the adsorption efficiency of the studied materials. The newly obtained Pd-based photocatalyst was exhaustively characterized and was used in the degradation process of phenol from aqueous solutions. The phenol degradation process was studied in terms of the nature of the photocatalyst used, time of photodegradation and solid: liquid ratio. It was observed that both the presence of IL and Pd lead to an increase in the efficiency of the phenol degradation process. The new Pd-based photocatalyst could be efficiently used in more cycles of phenol photodegradation processes. When is used as a photocatalyst the Florisil impregnated with IL and loaded with 2 mg/g of Pd, a degree of mineralization of 93.75% is obtained after 180 min of irradiation of a phenol solution having a concentration of 20 mg/L and using a solid:liquid ratio = 1:1.

Keywords: palladium; adsorption; ionic liquid; phenol; photodegradation

1. Introduction Palladium (Pd) is one of the precious metals of great economic importance. Its specific physical and chemical properties make it useful in many fields such as electrical and electronic industries, corrosion-resistant materials, jewelry or as catalysts in various chemical processes [1,2]. Due to its importance and the rare ore deposits which make a high price for this metal, it is essential to develop efficient recovery methods for this element, especially from water containing trace amounts of Pd. There are numerous methods for palladium recovery from diﬀerent waste, such as solvent extraction [3–5], ion exchange [6–8], electrometallurgy [9] and adsorption [10–12]. From all the mentioned methods, adsorption proved to be the most eﬃcient for the recovery of precious metals from aqueous solutions

Appl. Sci. 2020, 10, 8440; doi:10.3390/app10238440 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8440 2 of 15 due to easy operation, the existence of versatile adsorbents (such as activated carbon, chitosan, calcium alginate, graphene oxide, titanate nanotubes, etc.) and high efficiency even in case of solutions containing trace amounts of metal ions [11–17]. Since the ionic liquids (IL) were successfully used in the liquid–liquid extraction of precious metals [18,19], in the last years, new attention has been given to the use of ionic liquids for obtaining different adsorbents for palladium removal from aqueous solutions [20–22]. The immobilization of the ionic liquids in suitable solid support prevents the drawbacks of the liquid–liquid extraction, such as the loss of the ionic liquid in water and a possible occurrence of an ion-exchange between the metal ion and the cation of the ionic liquid. Through impregnation of the ionic liquid onto a suitable solid support, higher efficiency in the removal process of metal ions from aqueous solutions is achieved because it favorably combines the properties of ILs with the advantages of the solid supports [23,24]. In a previous study, the adsorption performance of the Florisil impregnated with Cyphos IL 101 (trihexyl tetradecyl phosphonium chloride) in the removal process of Pd(II) ions from residual solutions was studied [25]. It was observed that the impregnated Florisil with Cyphos IL-101 develops a maximum adsorption capacity in the removal process of Pd(II) ions from an aqueous solution of 126 mg/g, which is two times higher than the adsorption capacity developed by the raw Florisil [25]. The Circular Economy Action Plan of EU intends to close the loop of product lifecycles through sustainable consumption and production by improving waste management with greater recycling and reuse in order to create benefits for the environment, the economy and the society [26]. In all the adsorption studies, the major problem is, “What happened with the exhausted adsorbent?”. Very few papers answer this question. In order to respond to the European Union targets and to create benefits for the environment, economy and society and to respond to this frequently asked question in adsorption processes treatment, the aim of this paper is to study the possible use as a photocatalyst of an exhausted adsorbent from the removal process of Pd(II) ions from aqueous solutions. Many Pd-based composites are synthesized and studied as photocatalysts in various degradation processes [27–30]. However, in all cases, the obtaining method is very complicated, involving several steps of synthesis and using expensive resources. By using a waste resulted from another process as a photocatalyst, the Pd ions from secondary sources were capitalized. Consequently, the use of Pd ions from prime materials is avoided, which led to a minimization of the cost of treatment. The method proposed in this paper represents a closed cycle technology, and it is advantageous from both economic and environmental protection perspectives. The Pd-based photocatalysts obtained through Pd adsorption onto Cyphos IL 101 impregnated Florisil have been used in the degradation process of phenol from aqueous solutions. The phenolic compounds deserve the entire attention because it is found in many industrial wastewaters and even in some water resources due to the fact that it is used in several industrial branches. At concentrations higher than 0.001 mg/L, it is very toxic and carcinogenic [28–31]. The technologies utilized for removing phenols from industrial wastewater could be classified as destructive or non-destructive. The non-destructive technologies (distillation, adsorption, extraction, membrane separation) are feasible when the main concern is the recovery of the phenolic compounds. Otherwise, the destructive technologies (chemical or electrochemical oxidation, advanced oxidation processes, biological treatment) are more feasible due to the fact that not implies the transport of pollutants from one phase to another. Each of these technologies has advantages and drawbacks, and the feasibility of utilization one or another removal process of phenolic compounds must take into account the initial concentration, the pH and salinity of wastewater and the costs of treatment. Advanced oxidation processes (ozonation, wet air oxidation, Fenton oxidation, photo-assisted catalytic oxidation) are feasible in the tertiary process of water treatment in order to remove recalcitrant organic pollutants from water and to reach the physicochemical characteristics required by the regulations. Appl. Sci. 2020, 10, 8440 3 of 15

2. Materials and Methods

2.1. Obtaining and Characterization of the Pd-Based Photocatalyst The Pd-based photocatalyst was obtained through Pd(II) adsorption from residual solution onto the Florisil impregnated with Cyphos IL-101. The Florisil and the Cyphos IL-101 were purchased from Sigma–Aldrich, Darmstadt, Germany. The adsorption of Pd (II) ions was studied in a previous work, and the dependence of the maximum adsorption capacity of the impregnated Florisil function of solid:liquid (S:L) ratio, pH, contact time and initial concentration of the Pd(II), was established [25]. It was put in evidence that the ionic liquid (Cyphos IL 101) enhance the adsorption capacity of Florisil in the removal process of Pd(II) ions from aqueous solutions. The adsorbent was obtained through impregnation of Florisil with trihexyl tetradecyl phosphonium chloride using the ultrasonication method of impregnation. Then, 0.1 g of IL was impregnated onto 1 g of Florisil. The sample was entitled F-IL). The maximum adsorption capacity developed by F-IL, of 133.3 mg/g, was obtained in the following conditions: solid:liquid ratio (S:L) of 2 g/L, pH = 3 and stirring time of 60 min [25]. Therefore, in the present paper, the same conditions were used to obtain the Pd-based photocatalyst. Were prepared higher quantities of photocatalyst containing different amount of Pd ions (1 mg Pd/g of F-IL—the sample being entitled F-IL-Pd1 and 2 mg Pd/g of F-IL—the sample being entitled F-IL-Pd2, respective). The used photocatalyst was analyzed by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which was coupled with energy-dispersive X-ray analysis (EDX) and by XPS analysis. The FTIR spectra (KBr pellets) of the samples were recorded on a Shimadzu IR Prestige-21 FTIR 1 spectrophotometer (Shimadzu, Kyoto, Japan) in the range 400–4000 cm− . SEM micrographs were recorded using a Quanta FEG 250 microscope (Thermo Fisher Scientific, Waltham, MA, USA). TEM micrographs were obtained using an FEI Tecnai F20 G2 TWIN Cryo-TEM (Thermo Fisher Scientific, Waltham, MA, USA). Initially, the samples were ground to reduce the size of the particles so as to make them compatible with TEM analysis. The powdered samples were placed over lacey carbon copper grids. After removing the excess powder, the samples were analyzed. The scanning transmission electron microscopy (STEM) feature of the microscope, together with the EDX detector (X-MaxN 80T model from Oxford Instruments, Abingdon, UK), allowed identifying elements present in the sample. X-ray photoelectron spectroscopy (XPS) was performed on a KRATOS Axis Nova (Kratos Analytical, Manchester, UK), using AlKα radiation, with 20 mA current and 15 kV voltage (300 W). The XPS survey spectra for the Pd-based photocatalyst sample were collected in the range of 10–1200 eV with a − resolution of 1 eV and a pass energy of 160 eV. The high-resolution spectra for all the elements identified from the survey spectra were collected using a pass energy of 20 eV and a step size of 0.1 eV. Data were analyzed using the Vision Processing software (Vision2 software, Version 2.2.10). The binding energy of the C1s peak was normalized to 285 eV.

2.2. Application of the Pd-Based Photocatalyst in the Degradation Process of Phenol The obtained Pd-based photocatalyst was used in the degradation process of phenol from aqueous solutions, using a photoreactor PhotoLAB B400-700 Basic Batch-L (provided by Peschl-Ultraviolet, Mainz, Germany). The photocatalytic effect of the obtained Pd-based photocatalyst containing different amount of Pd loaded (F-IL-Pd1—impregnated Florisil with IL and 1 mg Pd/g and F-IL-Pd2—impregnated Florisil with IL and 2 mg Pd/g) was compared with the effect of raw Florisil (F) and impregnated Florisil with Cyphos IL-101 (F-IL). A well-defined quantity of the photocatalysts (0.7 g) was treated with 700 mL of phenol solution containing 20 mg/L phenol. Initially, the phenol solution was stirred for 30 min with the photocatalyst in the dark in order to achieve the adsorption–desorption equilibrium between the phenol and the surface of the photocatalyst. After this time, the sample was irradiated for 3 h with a UV-vis lamp type TQ 150. The samples were collected at different time intervals (5, 15, 30, 45, 60, 120 and 180 min), and the photodegradation profiles of phenol were Appl. Sci. 2020, 10, 8440 4 of 15 monitoring by realizing the UV-vis absorption spectra using a Varian Cary 100 spectrophotometer (AgilentAppl. Sci. 2020 Technologies,, 10, x FOR PEER Santa REVIEW Clara, CA, USA). The degradation degree of phenol was determined4 from of 15 the absorbance recorded at λ = 270 nm, which is one of the two maximum local excitation bands of hydroxyl-monosubstitutedusing a Varian Cary 100 spectrophotometer phenyl: (Agilent Technologies, Santa Clara, CA, USA). The degradation degree of phenol was determined from the absorbance recorded at λ = 270 nm, which is 270 270 one of the two maximum local excitation bands of hydroxyl-monosubstitutedAi At phenyl: Degradation degree (%) = − 100 A270 Degradation degree (%) = i 100 where 𝐴270 is the initial absorbance of the phenol solution, recorded at 270 nm270 and 𝐴 is the where Ai is the initial absorbance of the phenol solution, recorded at 270 nm and At is the absorbance ofabsorbance the solution of the after solution time t, recordedafter time at t, 270recorded nm. at 270 nm. ToTo determine determine both both the the degree degree of phenol of phenol degradation degrada andtion the and degree the degree of mineralization, of mineralization,total organic total carbonorganic (TOC) carbon was (TOC) measured was measured by using by a TOCusing analyzer a TOC analyzer (provided (provided by Shimadzu, by Shimadzu, Kyoto, Kyoto, Japan). Japan). ExperimentsExperiments werewere triplicate,triplicate, andand thethe resultsresults areare presentedpresented asas a a mean. mean. TheThe phenolphenol degradationdegradation eefficiencyfficiency waswas determineddetermined asas aa functionfunction ofof thethe naturenature ofof thethe usedused photocatalystphotocatalyst andand thethe S:LS:L (solid:liquid)(solid:liquid) ratio.ratio.

3.3. Results and Discussion

3.1.3.1. Characterization ofof thethe Pd-BasedPd-Based PhotocatalystPhotocatalyst TheThe FTIR,FTIR, SEM,SEM, TEMTEM andand XPSXPS analysis were usedused to prove the fact that the studied solid support, Florisil,Florisil, waswas impregnatedimpregnated withwith thethe studiedstudied IL,IL, CyphosCyphos IL-101.IL-101. ThisThis structuralstructural characterizationcharacterization waswas usefuluseful toto understandunderstand what what happened happened with with the the solid soli supportd support during during functionalization functionalization and and which which is the is reasonthe reason for itsfor e itsﬃciency efficiency increasing, increasing, and and how how IL and IL and Pd ionsPd ions are are bounded bounded onto onto the the Florisil Florisil surface surface in orderin order to beto surebe sure of theof the photocatalyst photocatalyst stability. stability. TheThe FTIRFTIR spectra spectra of theof the studied studied photocatalysts photocatalysts are presented are presented in Figure in1 .Figure From the1. FTIRFrom spectrum,the FTIR thespectrum, impregnation the impregnation is conﬁrmed is by confirmed the appearance by the in appe the spectrumarance in ofthe the spectrum solid support of the of solid the vibrationalsupport of 1 bandsthe vibrational characteristic bands for characteristic the ionic liquid, for namely:the ionic theliquid, intense namely: bands the between intense 2950 bands and between 2800 cm 2950− occur and due2800 to cm –CH−1 occur3 (methyl) due to and –CH –CH3 (methyl)2– (methylene) and –CH vibrations.2– (methylene) vibrations.

FigureFigure 1.1. FTIRFTIR spectraspectra ofof thethe studiedstudied materials.materials. The SEM images are presented in Figure2. From the SEM images, it can be observed that the The SEM images are presented in Figure 2. From the SEM images, it can be observed that the Florisil particles before impregnation are in various sizes and shapes, with many microcracks and Florisil particles before impregnation are in various sizes and shapes, with many microcracks and network defects. After the impregnation, the surface of the Florisil particles is much more uniform. network defects. After the impregnation, the surface of the Florisil particles is much more uniform. It It can be observed the distribution of the ionic liquid particles on their surface by ﬁlling the microcracks. can be observed the distribution of the ionic liquid particles on their surface by filling the microcracks. After Pd adsorption, the particle surface is very compact; Pd (II) adsorption is observed as a coating on the impregnated Florisil particles. Appl. Sci. 2020, 10, 8440 5 of 15

AfterAppl. Sci. Pd 2020 adsorption,, 10, x FOR thePEER particle REVIEW surface is very compact; Pd (II) adsorption is observed as a coating5 of on 15 theAppl. impregnated Sci. 2020, 10, x FOR Florisil PEER particles. REVIEW 5 of 15 (a) (b) (a) (b)

FigureFigure 2.2.SEM SEM imagesimages of:of: ((aa)) Florisil;Florisil; ((bb)) FlorisilFlorisil afterafter impregnationimpregnation withwith CyphosCyphos IL-101;IL-101; (c(c)) Florisil Florisil afterFigureafter impregnationimpregnation 2. SEM images and and of: Pd(II) Pd(II) (a) adsorption.Florisil; adsorption. (b) Florisil after impregnation with Cyphos IL-101; (c) Florisil after impregnation and Pd(II) adsorption. InIn FigureFigure3 3are are presented presented the the bright-ﬁeld bright-field transmission transmission electron electron microscopy microscopy (BFTEM) (BFTEM) images images of of Florisil after impregnation with Cyphos IL-101 and the Florisil after impregnation and Pd(II) adsorption. FlorisilIn Figureafter impregnation3 are presented with the bright-fieldCyphos IL-101 transm andission the electronFlorisil microscopyafter impregnation (BFTEM) and images Pd(II) of It can be observed the Florisil particles with the uniform distribution of the IL in the pore structure Florisiladsorption. after It impregnationcan be observed with the FlorisilCyphos particles IL-101 wiandth the uniformFlorisil distributionafter impregnation of the IL andin the Pd(II) pore (the black spots represent the IL particles which ﬁll the pores of the Florisil) (Figure3a). After Pd adsorption.structure (the It canblack be spotsobserved represent the Florisil the IL particles particle wis whichth the uniformfill the pores distribution of the Florisil)of the IL (Figure in the pore 3a). adsorption, the surface is very compact due to its coating with Pd ions (Figure3b). structureAfter Pd adsorption,(the black spots the surface represent is very the compacIL particlet dues which to its coatingfill the withpores Pd of ionsthe Florisil)(Figure 3b).(Figure 3a). After Pd adsorption, the surface is very compact due to its coating with Pd ions (Figure 3b).

FigureFigure 3. 3.Bright-field Bright-field transmission transmission electron electron microscopy microscopy (BFTEM) (BFTEM) images of images (a) Florisil of after (a) impregnationFlorisil after with Cyphos IL-101; (b) Florisil after impregnation and Pd(II) adsorption. Figureimpregnation 3. Bright-field with Cyphos transmission IL-101; (b ) electronFlorisil after microscopy impregnati (BFTEM)on and Pd(II)images adsorption. of (a) Florisil after impregnation with Cyphos IL-101; (b) Florisil after impregnation and Pd(II) adsorption. The impregnation of Florisil with trihexyl tetradecyl phosphonium chloride is proven by the EDXThe spectra impregnation (Figure 4), ofwhere Florisil besides with the trihexyl specific tetradecyl peaks of phosphoniumFlorisil (Mg, O, chloride Si), the specificis proven peaks by thefor EDX spectra (Figure 4), where besides the specific peaks of Florisil (Mg, O, Si), the specific peaks for Appl. Sci. 2020, 10, 8440x FOR PEER REVIEW 6 of 15 the used ionic liquid (C, Cl and P) can be observed. For the sample obtained after Pd adsorption The impregnation of Florisil with trihexyl tetradecyl phosphonium chloride is proven by the EDX characteristicAppl. Sci. 2020 peak, 10 , ofx FOR this PEER element REVIEW is observed, evidencing its adsorption onto the studied material.6 of 15 spectra (Figure4), where besides the specific peaks of Florisil (Mg, O, Si), the specific peaks for the used ionic liquidthe used (C, ionic Cl and liquid P) can(C, beCl and observed. P) can Forbe observ the sampleed. For obtainedthe sample after obtained Pd adsorption after Pd adsorption characteristic peak ofcharacteristic this(a) element peak isobserved, of this element evidencing is observed, its adsorption evidencing its onto adsorption the studied onto material.the studied material.

(a)

(b) (b)

FigureFigure 4. Energy-dispersive 4. Energy-dispersive X-ray X-ray analysis analysis (EDX) (EDX) spectra spectra of (a )of Florisil (a) Florisil after after impregnation impregnation with with Cyphos FigureCyphos 4. Energy-dispersive IL-101; (b) Florisil X-rayafter impregnation analysis (EDX) and Pd(II) spectra adsorption of (a) . Florisil after impregnation with IL-101;Cyphos ( bIL-101;) Florisil (b) after Florisil impregnation after impregnation and Pd(II) and adsorption. Pd(II) adsorption. In order to establish the elemental composition of the sample that will be used as a photocatalyst In order to establish the elemental composition of the sample that will be used as a photocatalyst In(Florisil order toimpregnated establish the with elemental Cyphos IL-101composition and loaded of the with sample Pd ions), that the will XPS be wideused scanas a photocatalystwas used (Florisil impregnated with Cyphos IL-101 and loaded with Pd ions), the XPS wide scan was used (Florisil(Figure impregnated 5a and Table with 1). Cyphos IL-101 and loaded with Pd ions), the XPS wide scan was used (Figure5a and Table1). (Figure 5a and Table 1). 250000 (a)

250000 O 1s 200000 (a) O 1s

200000 150000

150000 100000 Intensity(cps) Mg 1s Mg Pd 3d Na 1s

10000050000 P 2p C 1s C Si 2p Si Intensity(cps) Cl 2p Cl Mg 1s Mg Pd 3d Na 1s

50000 0 P 2p C 1s C 1400 1200 1000 800 600 400 2002p Si 0 Cl 2p Cl Binding Energy(eV) 0 1400 1200 1000 800 600 400 200 0 Binding Energy(eV) Figure 5. Cont. Appl. Sci. 2020, 10, 8440 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15

6000 12000

5000 (b) (c) 10000

Caliphatic 4000 8000

3000

6000 2000 Intensity(cps) Intensity(cps)

Chetero 4000 1000

0 2000

294 292 290 288 286 284 282 280 278 276 355 350 345 340 335 330 Binding Energy(eV) Binding Energy(eV) FigureFigure 5. 5. XPSXPS characterization characterization of of the the stud studiedied Pd-based Pd-based photocatalyst: photocatalyst: (a (a) )XPS XPS wide wide scan; scan; (b (b) )C C 1s 1s and and (c(c) )Pd Pd 3D 3D high-resolution high-resolution spectra. spectra.

Table 1. Elemental concentration resulted from the XPS wide scan spectrum of the studied Pd- Table 1. Elemental concentration resulted from the XPS wide scan spectrum of the studied Pd-based based photocatalyst. photocatalyst. Atomic Mass Element Atomic Mass Element Conc. (%) Conc. (%) Conc. (%) Conc. (%) C 20.25 11.91 C 20.25 11.91 Pd 2.76 14.39 OPd 2.7653.68 14.39 42.06 SiO 53.6819.40 42.06 26.68 MgSi 19.401.85 26.68 2.20 MgP 1.850.32 2.20 0.48 Cl 0.52 0.90 P 0.32 0.48 Na 1.22 1.38 Cl 0.52 0.90 Na 1.22 1.38 The XPS wide scan spectrum proves that the Florisil (which is a magnesium silicate with trace concentrationThe XPS wide of sodium) scan spectrum was impregnated proves that with the the Flor studiedisil (which IL (Cyphos is a magnesium IL-101—trihexyl silicate (tetradecyl)with trace concentrationphosphonium of chloride) sodium) andwas alsoimpregnated it could be with observed the studied the presence IL (Cyphos of the IL-101—trihexyl Pd ions. (tetradecyl) phosphoniumBy analyzing chloride) the high-resolutionand also it could spectra be observed of the the elements, presence XPS of allowsthe Pd ions. the identiﬁcation of the chemicalBy analyzing structure. the The high-resolution high-resolution spectra XPS spectra of the for elements, C 1s and XPS Pd 3dallows were the analyzed identification in detail of for the the chemicalPd-based structure. photocatalyst The high-resol (Figure5b,c).ution The XPS signal spectra corresponding for C 1s and to Pd C 1s 3d was were divided analyzed into in two detail distinct for thecomponents Pd-based correspondingphotocatalyst to(Figure the Caliphatic 5b,c). atThe 284.5 signal eV andcorresponding Chetero at 286.6 to C eV. 1s The was signal divided corresponding into two 2+ distinctto Pd 3d components was decomposed corresponding into two distinctto the componentsCaliphatic at 284.5 corresponding eV and Chetero to the at Pd286.6at eV. 337.2 The eV signal and to 4+ correspondingthe Pd at 338.5 to Pd eV 3d (Table was2 ).decomposed into two distinct components corresponding to the Pd2+ at 337.2 eV and to the Pd4+ at 338.5 eV (Table 2). Table 2. Experimental binding energies in eV for the Pd-based photocatalyst (* reference peak).

Table 2. Experimental binding energies in eV for theBinding Pd-based photocatalystRelative (*reference peak). Element Assignment Energy (eV) Conc. (%) Binding Relative Element AssignmentC 285 * 98.25 C 1 s aliphatic Energy (eV) Conc. (%) Chetero 286.6 1.75 Caliphatic 285* 98.25 C 1 s Pd2+ 337.2 86.27 Pd 3d Chetero 286.6 1.75 Pd4+ 338.5 13.73 Pd2+ 337.2 86.27 Pd 3d Pd4+ 338.5 13.73 3.2. Application of the Pd-Based Photocatalyst in the Degradation Process of Phenol 3.2. ApplicationThe characterized of the Pd-Based materials Photocatal wereyst used in asthe photocatalysts Degradation Process in the of degradation Phenol process of phenols fromThe aqueous characterized solutions materials through UV-viswere used irradiation. as photocatalysts From the characterizationin the degradation section, process it was of observedphenols fromthat theaqueous Florisil solutions support afterthrough IL impregnation UV-vis irradiation. presented From a higher the availablecharacterization active site, section, which it led was to a observed that the Florisil support after IL impregnation presented a higher available active site, which led to a higher adsorption degree in the removal process of Pd ions from aqueous solutions Appl. Sci. 2020, 10, 8440 8 of 15 higher adsorption degree in the removal process of Pd ions from aqueous solutions [25]. The exhausted adsorbentAppl. Sci. resulted 2020, 10, x afterFOR PEER Pd adsorptionREVIEW presented a more compact surface. This means that the8 of sample 15 with Pd content presents higher stability, and it is expected to present also a higher photocatalytic activity.[25]. TheThe photocatalyticexhausted adsorbent activity resulted of the after studied Pd adso samplerption was presented determined a more in compact the degradation surface. This process of phenolmeans fromthat the aqueous sample solution with Pd throughcontent presents UV-vis high irradiation.er stability, and it is expected to present also a higher photocatalytic activity. The photocatalytic activity of the studied sample was determined in 3.2.1.the Inﬂuence degradation of the process Photocatalyst of phenol from Nature aqueous solution through UV-vis irradiation.

3.2.1.UV-vis Influence molecular of the spectrometry Photocatalyst allows Nature both qualitative and quantitative analysis. The correlation between the UV absorption bands and the structure of the compounds (both for phenols and for the UV-vis molecular spectrometry allows both qualitative and quantitative analysis. The productscorrelation resulting between from the the UV oxidation absorption of bands phenols), and sothe the structure qualitative of the analysiscompounds is a (both fast andfor phenols economical identiﬁcationand for the technique.products resulting The shiftingfrom the oxidation of the absorption of phenols), maxima so the qualitative at a certain analysis wavelength is a fast and gives informationeconomical about identification the functional techniqu groupse. The that shifting appear of the or disappearabsorption atmaxima a certain at a timecertain of wavelength reaction. givesThe UV-visinformation spectra about of phenolthe functional at a di ﬀgroupserent timethat appear of the photodegradationor disappear at a certain process time for of all reaction. the studied materialsThe are UV-vis presented spectra in Figureof phenol6. Initially,at a different the phenoltime of solutionsthe photodegradation were stirred process for 30 for min all with the the photocatalyststudied materials in the darkare presented to achieve in theFigure adsorption–desorption 6. Initially, the phenol equilibrium solutions were between stirred thefor phenol30 min and the surfacewith the of photocatalyst the photocatalysts. in the dark to achieve the adsorption–desorption equilibrium between the phenol and the surface of the photocatalysts.

(a) (b)

(e)

FigureFigure 6. UV-vis 6. UV-vis spectra spectra of of the the photodegradation photodegradation process of of phenol: phenol: (a) ( photolysis;a) photolysis; and andphotocatalysis photocatalysis in thein presencethe presence of (ofb) (b F;) (F;c )(c F-IL;) F-IL; (d ()d) F-IL-Pd1; F-IL-Pd1; ((ee)) F-IL-Pd2. Appl.Appl. Sci. Sci. 20202020, ,1010, ,x 8440 FOR PEER REVIEW 99 of of 15 15

It is noted that in all the cases, the adsorption profile of phenol does not undergo any changes duringIt this is noted time. that After in allthe theirradiation cases, the starts, adsorption a hyperchromic profile of effect phenol and does a hypsochromic not undergo any (blue changes shift) effectduring of thisthe time.absorption After maximum the irradiation occur. starts, This a me hyperchromicans that the ephotodegradationffect and a hypsochromic process (blueof phenol shift) started,effect of and the the absorption intermediate maximum compounds occur. are This formin meansg that[32,33]. the At photodegradation higher irradiation process time occur of phenol the hypochromicstarted, and theeffect intermediate when impregnated compounds Florisil are formingand impregnated [32,33]. At Florisil higher and irradiation loaded with time occurdifferent the amountshypochromic of Pd eareffect used when as photocatalysts. impregnated Florisil and impregnated Florisil and loaded with different amountsThe degradation of Pd are used degree as photocatalysts. of phenol versus the photocatalyst nature at two different treatment time isThe presented degradation in Figure degree 7. ofIt is phenol noted versus that both the photocatalystthe presence of nature IL in atthe two catalyst different structure treatment and timethe Pdis presentedleads to an in Figureincrease7. Itin is the noted degree that bothof phenol the presence degradation of IL in from the catalyst aqueous structure solutions. and In the addition, Pd leads increasingto an increase the inamount the degree of Pd of from phenol the degradationcatalyst has froma positive aqueous influence solutions. on the In addition, process of increasing phenol degradation.the amount ofFurther Pd from studies the catalyst were performed has a positive using influence F-IL-Pd2 on as the a processphotocatalyst. of phenol Maybe degradation. a 10% increaseFurther studiesof efficiency were performeddoes not represent using F-IL-Pd2 a significant as a photocatalyst. increase, but Maybe it is a agreat 10% increase increase oftaking efficiency into accountdoes not the represent fact that a F-IL significant is a prime increase, material but itthat is a could great increasebe used for taking Pd intoadsorption, account and the factF-IL-Pd that is F-IL a wasteis a prime resulted material from thatanother could process. be used Furthermore, for Pd adsorption, if the photocatalyst and F-IL-Pd is contains a waste resultedPd ions, it from represents another muchprocess. higher Furthermore, stability in if the the degradation photocatalyst process contains of Pdphenols ions, itbecause represents Pd ions much are higher bounded stability on the in theIL and,degradation in this case, process is avoided of phenols the becausedegradation Pd ions of IL are from bounded the surface on the ILof and,Florisil in thisduring case, the is avoidedphotolysis the process.degradation If it ofis ILused from as the phot surfaceocatalyst of Florisil only during the F-IL the photolysisthe TOC co process.ncentration If it is increase used as photocatalyst during the treatmentonly the F-ILprocess, the which TOC concentration suggest that aincrease part of IL during from the the Florisil treatment surface process, is degraded which by suggest the UV-vis that a irradiation,part of IL fromso the the efficiency Florisil of surface this material is degraded decrease by thecontinuously, UV-vis irradiation, especially so when thee isffi usedciency in ofmore this cyclesmaterial of treatment decrease continuously,until it present, especially no photocatal whenytic is used activity. in more Furthermore, cycles of treatment the increased until efficiency it present, ofno the photocatalytic F-IL-Pd is underlined activity. Furthermore, by the fact that the increasedthis efficiency efficiency for the of phenol the F-IL-Pd degradation is underlined is obtained by the muchfact that quickly this e(atfficiency 120 min) for than the phenol in the case degradation of using isonly obtained F-IL as much photocatalyst quickly (atwhen 120 this min) efficiency than in the if obtainedcase of using only only after F-IL 180 as min photocatalyst of treatment. when This this me effiansciency that ifa obtainedshorter time only afteris needed 180 min for of treatment, treatment. whichThis means leads thatto decreasing a shorter timetreatment is needed costs. for treatment, which leads to decreasing treatment costs.

FigureFigure 7. 7. TheThe degradation degradation degree degree of of phenol phenol func functiontion of of the the photocatalyst photocatalyst nature. nature. 3.2.2. Influence of the S:L Ratio 3.2.2. Influence of the S:L Ratio From the economic point of view, it is desired to use as little photocatalyst amount as possible. From the economic point of view, it is desired to use as little photocatalyst amount as possible. For this reason, it is important to know which is the optimum amount of photocatalyst which could be For this reason, it is important to know which is the optimum amount of photocatalyst which could used with good results in the degradation process of phenol from aqueous solutions. The degradation be used with good results in the degradation process of phenol from aqueous solutions. The degree of phenol versus the S:L ratio is presented in Figure8. With the increase of the S:L ratio, degradation degree of phenol versus the S:L ratio is presented in Figure 8. With the increase of the S: the efficiency of the process of phenol degradation from aqueous solutions increase. Further increase of L ratio, the efficiency of the process of phenol degradation from aqueous solutions increase. Further the photocatalyst amount did not lead to a higher efficiency increasing; therefore, it is not recommended increase of the photocatalyst amount did not lead to a higher efficiency increasing; therefore, it is not to use an S:L ratio higher than 1:1. recommended to use an S:L ratio higher than 1:1. Appl. Sci. 2020, 10, 8440 10 of 15

Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 15

Figure 8. The degradationFigure 8. The degradation degree degree of of phenolphenol versus versus the solid:liquid the solid:liquid(S:L) ratio. (S:L) ratio. 3.2.3. Influence of the Photocatalyst Reuse 3.2.3. Influence of the PhotocatalystAn important factor that Reuse characterizes a photocatalyst is its stability. It is desired that after a treatment process, the photocatalyst to be recovered and reused in other treatment processes, with An important factorthe same efficiency that characterizes to minimize the treatment a photocatalyst costs. The photocatalyst is obtained its stability. by Pd adsorption It on is desired that after a IL impregnated Florisil showed stability in the sense that the Pd ions were not detached from the treatment process, thesolid photocatalyst support following a tophotocatalysis be recovered process. This and wasreused determined in by otheranalyzing treatment them in the processes, with the same efficiency to minimizesolution resulting the from treatment the photocatalysis costs. proce Thess when photocatalyst a concentration below obtained the detection bylimit Pd adsorption on IL was obtained. At the S:L = 0.5:1, after a photocatalytic process, the catalyst was reused in two new impregnated Florisildegradation showed processes stability of phenol in thefrom aqueous sense solu thattions. the It is Pd noted ions that werethe efficiency not of detached the from the solid photocatalyst decreases by 5% after the first cycle because a part of the IL impregnated onto Florisil support following aisphotocatalysis degraded by the UV-vis process. irradiation. In This the furt washer cycle, determined the efficiency of by phenol analyzing degradation them in the solution resulting from the photocatalysisremains constant (Figure process 9). These whenresults underlined a concentration the efficiency of using below as photocatalyst the detection of F-IL- limit was obtained. Pd compared with the sample of F-IL, where the probability of IL degradation from the Florisil At the S:L = 0.5:1, aftersurface is a much photocatalytic higher. process, the catalyst was reused in two new degradation processes of phenol from aqueous solutions. It is noted that the efficiency of the photocatalyst decreases by 5% after the first cycle because a part of the IL impregnated onto Florisil is degraded by the UV-vis irradiation. In the further cycle, the efficiency of phenol degradation remains constant (Figure9). These results underlined the efficiency of using as photocatalyst of F-IL-Pd compared with the sample Appl.of F-IL, Sci. 2020 where, 10, x the FOR probability PEER REVIEW of IL degradation from the Florisil surface is much higher. 11 of 15

Figure 9. Figure 9. TheThe degradation degradation degree degree of of phenol phenol versus versus the the photocatalyst photocatalyst reuse. reuse. Even if a part of IL is degraded from the surface of the photocatalyst, the eﬃciency of the Pd based Even if a part of IL is degraded from the surface of the photocatalyst, the efficiency of the Pd photocatalyst is not inﬂuenced only by 5% because the Pd ions are still coating the surface of Florisil. based photocatalyst is not influenced only by 5% because the Pd ions are still coating the surface of As it is mentioned in the literature, the photoirradiation led to the greater deposition of Pd ions onto Florisil. As it is mentioned in the literature, the photoirradiation led to the greater deposition of Pd ions onto the surface of the solid support [34–36]. This good deposition of Pd ions onto the surface of Florisil, even after one photodegradation process, can be seen from the BFTEM image and EDX spectrum of the reused photocatalyst presented in Figure 10. From the BFTEM images, it can be observed that a part of IL from the Florisil surface is degraded (the black spots are in a smaller amount), but there is still a very good coating with the Pd ions. This conclusion can be raised from the EDX spectrum where the peaks specific for the studied IL (C, P and Cl) are not so evident, but the Pd ions are very well-identified.

Figure 10. BFTEM image and EDX spectrum of the reused photocatalyst.

3.2.4. Mechanism of Phenol Degradation

Figure 11 presents the kinetic plot (C/C0 at λ = 270 nm vs. time) of phenol photodegradation. At different times of contact, the UV-vis absorption profiles of solutions together with the measured TOC concentrations were presented. The UV-vis absorption profiles of solutions allow making qualitative analysis for identification of the intermediate compounds of phenol oxidation. From the kinetic study and the absorption profiles at different time intervals, as well as by the correlation of our results with results reported in the literature, a mechanism of phenol degradation was proposed [32,33]. At the same time, the TOC was measured in order to determine the degree of Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15

Figure 9. The degradation degree of phenol versus the photocatalyst reuse.

Even if a part of IL is degraded from the surface of the photocatalyst, the efficiency of the Pd basedAppl. Sci. photocatalyst2020, 10, 8440 is not influenced only by 5% because the Pd ions are still coating the surface11 of of 15 Florisil. As it is mentioned in the literature, the photoirradiation led to the greater deposition of Pd ions onto the surface of the solid support [34–36]. This good deposition of Pd ions onto the surface of Florisil,the surface even of after the solid one supportphotodeg [34radation–36]. This process, good depositioncan be seen of from Pd ions the onto BFTEM the surface image ofand Florisil, EDX spectrumeven after oneof the photodegradation reused photocatalyst process, presented can be seen in fromFigure the 10. BFTEM Fromimage the BFTEM and EDX images, spectrum it can of thebe observedreused photocatalyst that a part presentedof IL from in Figurethe Florisil 10. From surface the BFTEMis degraded images, (the it canblack be spots observed are thatin a asmaller part of amount),IL from the but Florisil there surfaceis still a is very degraded good (thecoating black with spots the are Pd in ions. a smaller This amount),conclusion but can there be israised still a from very thegood EDX coating spectrum with thewhere Pd ions.the peaks This specific conclusion for the can studied be raised IL from (C, P the and EDX Cl) are spectrum not so whereevident, the but peaks the Pdspeciﬁc ions forare thevery studied well-identified. IL (C, P and Cl) are not so evident, but the Pd ions are very well-identiﬁed.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15

mineralization, not only the photodegradation degree of phenol. It can be observed that during the period of establishing the adsorption–desorption equilibrium between the phenol and the surface of the photocatalyst, it is adsorbed about 10% of the phenol present in the solution. At the start of the UV lamp, a change in the sample absorbance profile is observed due to the hyperchromic effect. This effect suggests that phenol passes into degradation intermediates: catechol, hydroquinone and p- benzoquinone [32,33]. Up to a 30 min irradiation time, there is no mineralization of these intermediate compounds. This can be seen from the TOC analysis, which shows that even after 30 min of irradiation, the TOC concentration is approximately equal to the initial TOC. Or it is possible that the TOC concentration to remain constant because a part of the IL is detached and degraded from the surface of the Florisil. After this time, in the sample spectrum appear a decrease in absorbance (hypochromic effect), so we can conclude that the process of mineralization has begun, besides the photodegradation process, also proven by theFigure TOC 10. concentration BFTEMBFTEM image image which and and EDX EDX after spectrum 60 min of has the reached reused photocatalyst.a value of 2.69 mg/L (Figure 11). 3.2.4. Mechanism of Phenol Degradation 3.2.4. ByMechanism further irradiating of Phenol the Degradation solution in the presence of the photocatalyst, the aromatic ring breaks withFigure the formation 11 presents of aliphatic the kinetic organic plot acids (C/C with0 at λth=e short270 nm molecule vs. time) (maleic of phenol acid, oxalic photodegradation. acid, formic Figure 11 presents the kinetic plot (C/C0 at λ = 270 nm vs. time) of phenol photodegradation. At Atacid) diff [32,33].erent times Its formation of contact, theis indicated UV-vis absorption by the hypsochromic profiles of solutions effect (a together blue shift) with by the displacing measured TOCthe different times of contact, the UV-vis absorption profiles of solutions together with the measured concentrationsmaximum adsorption were presented. from 270 Thenm to UV-vis 265 nm. absorption After 180 profiles min, the of solutionsTOC is 1.1 allow mg/L, making which qualitative means a TOC concentrations were presented. The UV-vis absorption profiles of solutions allow making analysis93.75% efficiency for identification of mineralization of the intermediate (Figure 11). compounds of phenol oxidation. qualitative analysis for identification of the intermediate compounds of phenol oxidation. From the kinetic study and the absorption profiles at different time intervals, as well as by the correlation of our results with results reported in the literature, a mechanism of phenol degradation was proposed [32,33]. At the same time, the TOC was measured in order to determine the degree of

Figure 11. Degradation kinetics (C/C0 vs. time) of phenol photodegradation process, UV-vis absorption Figure 11. Degradation kinetics (C/C0 vs. time) of phenol photodegradation process, UV-vis profiles and total organic carbon (TOC) measurements of solutions at certain time intervals. absorption profiles and total organic carbon (TOC) measurements of solutions at certain time Fromintervals. the kinetic study and the absorption profiles at different time intervals, as well as by the correlation of our results with results reported in the literature, a mechanism of phenol degradation Comparing the performance of the studied materials with other photocatalysts reported in the literature (Table 3) could be concluded that the studied materials, which actually consist of a waste resulted from other processes, presents good photocatalytic activity in the degradation process of phenol from aqueous solutions.

Appl. Sci. 2020, 10, 8440 12 of 15 was proposed [32,33]. At the same time, the TOC was measured in order to determine the degree of mineralization, not only the photodegradation degree of phenol. It can be observed that during the period of establishing the adsorption–desorption equilibrium between the phenol and the surface of the photocatalyst, it is adsorbed about 10% of the phenol present in the solution. At the start of the UV lamp, a change in the sample absorbance profile is observed due to the hyperchromic effect. This effect suggests that phenol passes into degradation intermediates: catechol, hydroquinone and p-benzoquinone [32,33]. Up to a 30 min irradiation time, there is no mineralization of these intermediate compounds. This can be seen from the TOC analysis, which shows that even after 30 min of irradiation, the TOC concentration is approximately equal to the initial TOC. Or it is possible that the TOC concentration to remain constant because a part of the IL is detached and degraded from the surface of the Florisil. After this time, in the sample spectrum appear a decrease in absorbance (hypochromic effect), so we can conclude that the process of mineralization has begun, besides the photodegradation process, also proven by the TOC concentration which after 60 min has reached a value of 2.69 mg/L (Figure 11). By further irradiating the solution in the presence of the photocatalyst, the aromatic ring breaks with the formation of aliphatic organic acids with the short molecule (maleic acid, oxalic acid, formic acid) [32,33]. Its formation is indicated by the hypsochromic effect (a blue shift) by displacing the maximum adsorption from 270 nm to 265 nm. After 180 min, the TOC is 1.1 mg/L, which means a 93.75% efficiency of mineralization (Figure 11). Comparing the performance of the studied materials with other photocatalysts reported in the literature (Table3) could be concluded that the studied materials, which actually consist of a waste resulted from other processes, presents good photocatalytic activity in the degradation process of phenol from aqueous solutions.

Table 3. Comparison of the photocatalytic eﬃciency developed by various materials in the degradation process of phenol from aqueous solutions.

Phenol Initial Degradation Degree, Treatment Time, Photocatalyst References Concentration, mg/L % min Titanium dioxide-coated on the inner 30 50 300 [37] and outer quartz glass tubes Titanium dioxide (TiO ) Degussa P25 2 20 83.9 360 [38] (80% anatase, 20% rutile) ZnO 25 78.2 90 [39] 0.5% Pd-0.5% Au/TiO2 94.11 69 5520 [40] F-IL-Pd 20 78.6 180 Present work

4. Conclusions The exhausted adsorbent could be efficiently used as a Pd-based photocatalyst in the degradation process of phenol. The presence of the IL on solid support increases the adsorption efficiency of Pd ions from aqueous solutions. In addition, it was found that both the presence of ionic liquid and Pd (II) ions result in an increase in the efficiency of the phenol degradation process. Furthermore, increasing the amount of Pd (II) from the catalyst structure and increasing the S: L ratio has a positive effect on the degradation process. The obtained photocatalyst could be efficiently used in several cycles of degradation processes. A part of the IL from the Florisil surface is degraded during the first process of photodegradation, but at the same time, the Pd ions are more fixed on the solid support. The efficiency of the catalyst decreased only by 5%, and then the efficiency is maintained. These results underlined the efficiency and stability as photocatalyst of the sample F-Il-Pd compared with the sample F-IL in the degradation process of phenol from aqueous solutions. The results of the present manuscript underline the efficiency of waste resulted from an adsorbent process to be used as a photocatalyst. These results are in the frame of the closed cycle technology responding to the actual need of circular economy. Appl. Sci. 2020, 10, 8440 13 of 15

Author Contributions: Conceptualization, L.L. and L.C.; methodology, L.L. and A.P.; formal analysis, L.L., B.T. and A.C.; investigation, L.L.; writing—original draft preparation, L.L. and L.C.; supervision, A.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The research infrastructure developed at ICECHIM through the POS-CCE “AGRI-FLUX” project, nr. 645/18.03.2014, cod SMIS-CSNR 48695 was partially used to obtain the results presented in this study. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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