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Research Article pubs.acs.org/journal/ascecg

Chemical and Pathogenic Cleanup of Wastewater Using SurfaceFunctionalized CeO2 Nanoparticles Savita Chaudhary,*,† Priyanka Sharma,‡ Diksha Singh,§ Ahmad Umar,*,∥,⊥ and Rajeev Kumar‡ †

Department of Chemistry and Centre of Advanced Studies in Chemistry, ‡Department of Environment Studies, and §Centre for Nanoscience and Nanotechnology, University Institute of Emerging Area in Science and Technology, Panjab University, Chandigarh 160014, India ∥ Department of Chemistry, College of Science and Arts and ⊥Promising Centre for Sensors and Electronic Devices (PCSED)Najran University, Najran 11001, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: In this paper, we report the simple synthesis, detailed characterization, and total wastewater cleanup by adsorption using bare and surfactant-functionalized cerium oxide (CeO2) nanoparticles. The synthesis of CeO2 nanoparticles was performed by a facile aqueous solution process and characterized by a diverse range of techniques, which confirmed that the nanoparticles are well-crystalline, possessing good structural and optical properties. The competence of the prepared nanoparticles was further explored to determine the dye removal efficiency. The developed nanoparticles have also provided chlorine-free disinfection of water. The observed results revealed that the synthesized nanoparticles efficiently lower the pollutant concentrations, reduced turbidity, and exhibited significant reductions in total dissolved solids, chemical and biochemical oxygen demands, and pathogenic load. Interestingly, the surfactant-functionalized nanoparticles revealed that they possess the ability to remove approximately 99% of dye (at a specific set of conditions) from a wastewater system. Further, the dye removal efficiencies of functionalized nanoparticles varied from 112.4 to 171.3 mg/g of dye, which is superior as compared to those of other nanoadsorbents studied for the same dye. The excellent dye adsorption performance was mainly due to the higher available surface area of the functionalized CeO2 nanoparticles. The regeneration of both dyes as well as nanoparticles further strengthens the importance of functionalized nanoparticles to utilize them for the next dye removal cycle. The in vitro antimicrobial and antimold activities of functionalized nanoparticles further revealed their disinfective nature, which is a crucial step in wastewater treatment. The presented work demonstrates that simply synthesized surface-functionalized CeO2 nanoparticles can be used efficiently for other potential environmental remediation applications. KEYWORDS: Cerium oxide, Dye removal, Nanoadsorbent, Antimicrobial, Surface functionalization

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INTRODUCTION Recently, environmental pollution generated by global industrialization and its corresponding manufacturing and processing has created copious amounts of pollution including volatile organic compounds (VOCs), green-house gases (GHGs), particulate matter, nitrogen oxide (NOx), sulfur oxide (SOx), sludge, toxic effluents, and so on.1,2 The air pollutants generated from these industries settle due to gravity or rainfall3 and mix with soil or surface water to ultimately contaminate the environment.4 Thus, the ultimate sink for air and land pollutants is water, i.e., either surface or groundwater. In addition, a massive quantity of water is used in various industrial functionalities, including as a coolant and in cleaning, rinsing, processing, cutting, washing, and so forth5 that is then released into natural water resources. The industrial effluent is considered a dangerous source of environmental problem due to its adverse effects on water bodies such as retardation of light © 2017 American Chemical Society

penetration, which affect the photosynthetic activity and acute toxicity of aquatic organisms.6−8 Additionally, if any of the contaminant enters the food chain, it can cause severe damage to humans by affecting the functioning of vital organs including the heart, kidney, reproductive system, liver, and central nervous system.9,10 The corresponding high chemical oxygen demand (COD), biochemical oxygen demand (BOD), suspended solids, pathogenic load, and intense color also make these effluents a major concern with respect to environmental safety.11 Conventional steps in effluent treatment include coagulation and flocculation,12 filtration,13 ion exchange,14 reverse osmosis,15 electrical,16 thermal desalination,17 ozonization,18 ultrasonication,19 photodegradation,20 Received: April 5, 2017 Revised: May 27, 2017 Published: June 13, 2017 6803

DOI: 10.1021/acssuschemeng.7b01041 ACS Sustainable Chem. Eng. 2017, 5, 6803−6816

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ACS Sustainable Chemistry & Engineering rectification,21 and adsorption.22,23 After these purification steps, the leftover sludge needs to be disposed off either by thermal destruction or by burying it deep into the land or oceans.24 Biological processes being cheaper and more energy efficient25 always possess the risk of their direct or indirect entry into the food chain and the environment. The chemicals, risk factor of endemics, and time taken to complete the steps are so large that they make the water purification process a very costly and tedious process. For all of these concerns, water treatment is considered a major issue in the field of environmental protection. To resolve these issues in fewer steps without using external energy and sludge production, we need efficient and advanced materials that alone can clear all chemical and biological contaminants from the wastewater. The remediating agent should be highly effective at low dosages yet nontoxic to humans and other nonpathogenic living forms. Being the most skilled for these criteria, nanoparticles are a promising class of adsorbents for the removal of harmful pollutants from wastewater resources.26,27 The higher exterior surface area and superior adsorption capacities of nanoparticles make them proficient adsorbents.28 To date, numerous studies have presented the use of a diverse range of nanoparticles in pollutant removal applications.29−32 Herein, we have considered wastewater treatment using surface-functionalized cerium oxide nanoparticles (CeO2 nanoparticles) by targeting organic dyes as a model organic pollutant. The choice of CeO2 was based on the unusual property of cerium having two interchangeable oxidation states. This property of CeO2 causes oxygen vacancies over the surface and localized electrons (polarons), which facilitates the diffusion of oxygen and oxygen-containing compounds over the nanoparticles. Cerium is also the most abundant in earth’s crust among the rare earth metals, hence making it suitable for use at the commercial level. Furthermore, the toxicity threshold of pure cerium is very high, making the adsorbent less toxic when released into the environment. CeO2 is commonly used as an oxidizing agent, polishing powder, colorant in glasses and ceramics, and as a catalyst in many reactions.33 Available literature has documented several stabilizing agents for various nanoparticles that amend the size, surface area, and adsorption properties of the nanoparticles.29−32,34−36 In this regard, surface functionalization of the CeO2 nanoparticles with various surfactants has been performed, and its relative effect on wastewater treatment has been evaluated. In previous studies, cerium was commonly used as a doping material in the nanoparticles for wastewater treatment; in these methods, restoration of the adsorbent is not emphasized.37,38 The use of pure CeO2 nanoparticles in wastewater treatment makes the piece of work odd and novel among the other nanoparticlebased methods. Biocidal properties of CeO2 nanoparticles39−41 provided the idea of utilizing the leftover nanoparticles for disinfection of wastewater after the adsorption of the pollutants, which is otherwise a major area of concern.42 The comparative results of bare CeO2 nanoparticles have also been introduced to verify the role of surface covering in the adsorption of dye molecules. Eriochrome Black T (EBT, an azo dye) was chosen as model pollutant as compounds containing azo and sulfur groups, which are highly toxic to the biota.36 EBT is significantly used in dyeing fabrics of wool, silk, and nylon and biological staining.38 It is also used as an indicator in complexometric titrations for the determination of a water harness in terms of calculating total Ca2+, Mg2+, and Zn2+ ions

in the water. Because of its structure, EBT is found to show skin and eye irritation in mammals. The adsorption of pollutants is emphasized in this work as the degradation may release products that are more toxic than the parent pollutant. For instance, EBT on degradation releases naphtaquinone, which is found to be carcinogenic.36 The higher absorption capacity (qe) of surface-functionalized CeO2 nanoparticles as compared to that in the literature further signifies the supremacy and uniqueness of the work among the significant number of studies performed in this vein (Table S1).26,29,36,43−45 The influence of operational parameters including adsorbent dosage, initial concentrations of dye, contact time, pH, and salt effect on adsorption efficiency has also been carried out for as-formed CeO2 nanoparticles. The quality of treated water has been assessed in terms of lowering of COD, BOD, and total dissolved solids (TDS) of the treated water. Disinfection of wastewater using the CeO2 nanoparticles is based upon its in vitro antimicrobial and antimold activities. The unused and left over traces of the nanoparticles and escaped surfactant have the tendency to bind with the bacteria and other biological moieties in water, and this is the major reason for disinfective behavior of the as-prepared CeO2 nanoparticles. Besides the absorption kinetics, different absorption isotherms have also been employed to understand the mechanistic aspect of dye removal. For the usability of the functionalized CeO2 nanoparticles to be authenticated, the respective system has also been extended to real water samples from tap, canal, tube well, and hand pump sources. The recovery of CeO2 nanoparticles as adsorbent has also been carried out and formulates the above process more economical and appropriate for industrial purposes. Hence, the proposed work advances the conventional wastewater treatment process. The multistep procedure of wastewater cleanup can be reduced to fewer steps and leads to the generation of new environmental remediation practices. The applicability of the as-described methodology in real samples has opened immense potential for the use of CeO2 nanoparticles in practical water treatment applications.

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EXPERIMENTAL DETAILS

Materials. Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99%), hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), cetylpyridinium bromide hydrate (CPB), and cetylpyridinium chloride (CPC) were purchased from Sigma-Aldrich (Figure S1a−d). Muller Hinton (MH) agar was provided by Himedia. Ammonia solution (NH4OH) was purchased from Merck; 99.9% absolute ethanol was procured from Changshu Yangyuan Chemical, China. Malt extract agar (MEA) and metal salts for ion effect testing, namely, aluminum nitrate, barium acetate, cadmium acetate, calcium chloride, copper chloride, magnesium acetate, nickel acetate, nickel chloride, sodium carbonate, sodium sulfate, and zinc sulfate, were purchased from Qualigens, India. Requisite chemicals for the determination of COD, i.e., potassium dichromate, sulphamic acid, silver sulfate, sulfuric acid, ferrous ammonium sulfate ferroin indicator, and mercuric sulfate were also obtained from Qualigens, India. Salts for BOD, i.e., potassium dihydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, ferric chloride, magnesium sulfate, manganous sulfate, sodium azide, potassium iodide, sodium thiosulfate, and starch were purchased from Sigma-Aldrich, India. The different dyes, such as methyl orange (MO), victoria blue (VB), rhodamine 6-G (R6G), direct red (DR), brilliant blue, (BB) congo red (CR), fast green (FG), and eriochrome black-T (EBT), were obtained from Sigma-Aldrich, India with purity >80%. All of the chemicals were used as obtained without any further processing and purification. 6804


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ACS Sustainable Chemistry & Engineering Synthesis of Surface-Functionalized CeO2 Nanoparticles. The synthesis of the CeO2 nanoparticles was employed in surfactant media to coat the nanoparticles with respective surfactants. For preparing bare CeO2 nanoparticles, no surfactant was used during the synthesis. As the surface of bare CeO2 nanoparticles was anionic in nature, cationic surfactants were thus used for better adsorption over the surface of nanoparticles and for controlling the size of the prepared nanoparticles. The variable head groups, chain lengths, and counterions in CTAB, CTAC, CPC, and CPB has further enhanced the scope of the chosen surfactant during the synthesis of CeO2 nanoparticles. Synthesis of the surface-functionalized CeO2 nanoparticles were performed by the aqueous precipitation method in the respective surfactant media, as explained elsewhere.46 For a concise explaination, 0.03 M Ce(NO3)3·6H2O was added to the 0.05 M solution of the respective surfactant in 50 mL of ethanol followed by the dropwise addition of 0.25 M NH4OH. The pH of the solution was adjusted before the addition of NH4OH, i.e., kept at pH 7.33. The resultant mixture was then consequently stirred at 40 °C for 24 h to obtain a yellow-colored solution of CeO2 nanoparticles. The obtained nanoparticles were separated from the mother solution, washed, and dried in an oven maintained at 50 °C overnight to obtain powdered particles. The corresponding concentration of CeO2 nanoparticles in the respective surfactant medium was estimated by using the equation

c=

CeO2 nanoparticles. These conditions were taken to obtain the maximum dye adsorption potential of the nanoparticles. The resultant mixture was equilibrated for 24 h with constant stirring (200 rpm) at 25 °C. After equilibration, the dispersion was centrifuged and subjected to UV−vis analysis. The concentration of each dye was then estimated from the calibration curve obtained from the absorbance measurement for different amounts of dissolved dyes. The control experiment for each type of dye was performed in the absence of nanoparticles. The adsorption behavior of dyes was further studied over a wide range of pH values, i.e., pH 2, 4, 5, 6, 8, 10, and 12 at 25 °C. The pH values were varied over the full scale to analyze the influence of dispersion media over the adsorption behavior of the dye using CeO2 nanoparticles. Batch mode adsorption experiments were conducted with initial dye concentration = 50 ppm and nanoparticles dosage = 100 ppm. The removal efficacy of each type of dye was calculated using the equation

⎛ C0 − Cf ⎞ dye removal efficiency (%) = ⎜ ⎟× 100 ⎝ C0 ⎠

where C0 and Cf correspond to the original and final concentrations of the dye solutions before and after the adsorption treatment over surface-functionalized CeO2 nanoparticles, respectively. For the effectiveness of as-prepared surface-functionalized CeO2 nanoparticles to be verified, the dye adsorption studies were also carried out in different water sources such as water from the tap, well, tube well, hand pump, and canal from Village Kiloi, District Rohtak, Haryana, India. The various water sources was utilized as industries do not use distilled or deionized water, and this may mimic the real wastewater sample after the addition of model pollutant. Such an approach will help in estimating the behavior of the nanoparticles in real effluent conditions. During the analysis, all of the water sources were spiked with 50 ppm nanoparticles and 50 ppm dye and stirred for 24 h. The absorption spectra were taken for all of the samples to determine the concentration of adsorbed dye. The water samples were also analyzed in terms of TDS, electrical conductivity (EC), salt concentration, dissolved oxygen (DO), COD, and BOD before and after treatment with surface-functionalized CeO2 nanoparticles by using multiparamter PCSTestr 35 from Eutech Instruments, Oakton. For the water analysis, the respective dye solution of 50 ppm was adsorbed using 50 ppm nanoparticles at pH 5 for a time period of 120 min. The COD value was calculated using the equation

CMMCeO2 4 3 r πρN0 3

(1)

where CM denotes the molar concentration of Ce(NO3)3·6H2O, MCeO2 is the molar mass of CeO2, r is the average radius of CeO2 nanoparticles obtained from TEM and XRD of as-functionalized nanoparticles,41 ρ is the density of CeO2 (7.65 g cm−3), and N0 is Avogadro’s number. The content of surfactant over nanoparticles was ascertained in terms of its density over nanoparticles and density provided in the reaction mixture. The density of the surfactant molecules was calculated using the simple formula ρ=

MS VS

(3)

(2)

where ρ is the density of the surfactant around the nanoparticle surface, Ms is the total mass of surfactant molecules around nanoparticles and total volume taken by the surfactant molecules in each nanoparticle-surfactant cluster (Figure S2). The mass of surfactant molecules in each cluster was calculated from TGA plots39 provided elsewhere.46 Characterization Techniques. The structural and morphological characterization of as-formed nanoparticles was carried out by a diverse range of techniques. The corresponding UV−vis spectra of CeO2 nanoparticles and the respective concentrations of different types of dye solutions were analyzed using a Thermoscientific UV−vis spectrophotometer. Fourier transform infrared measurements of surfactant-functionalized CeO2 nanoparticles before and after dye loading were carried out on a PerkinElmer (RX1) FTIR spectrophotometer. The X-ray diffractometer from Panalytical D/Max-2500 was used to obtain information on the crystalline nature and size of nanoparticles. The morphological analysis was carried out on a Hitachi (H-7500) TEM and Hitachi (SU8010) scanning electron microscope. The loading of dye was further confirmed using a Nikon fluorescent optical microscope. Raman analysis of as-synthesized nanoparticles was carried with a Renishaw in-via reflex micro-Raman spectrometer. The pH measurements were performed on a Mettler Toledo digital pH meter. Dye Adsorption Experiments. In the current study, we chose eight dyes, i.e., MO, VB, R6G, DR, BB, CR, FG, and EBT, for investigating the adsorption efficiency of CeO2. The choice of these dyes was based on covering the diverse range of anionic, cationic, and azo nature of the water pollutants for adsorption studies. The respective solution of 50 ppm of each dye was added to 100 ppm of

COD (mg/l) =

(a − b)N × 8000 sample volume

(4)

where a is the volume of Fe(NH4)2(SO4) required for blank in ml, b is the volume of Fe(NH4)2(SO4) required for the sample in ml, and N is the normality of Fe(NH4)2(SO4). The respective values of BOD were estimated using

BOD (mg/l) = (initial DO − DO5) × dilution factor

(5)

where DO is the dissolved oxygen of the sample before incubation and DO5 is the dissolved oxygen of the sample after 5 days incubation at 20 °C. The respective desorption of dye from the surface of nanoparticles was carried out to make the process more economical. In brief, the dye molecules were desorbed by washing dye-loaded CeO2 nanoparticles with 1 M NaOH. The process was repeated 2−3 times followed by 2− 3 washings with deionized water. The desorbed dye was subjected to UV−vis analysis to determine the identity of the dye. The desorbed nanoparticles were analyzed by XRD and further used for another cycle of dye removal. The dye desorption process favors the reuse of adsorbed dye and the nanoparticles. The spent reagents, desorbed dye, and nanoparticles may be separated by gravity settling followed by ion exchange units. This step of desorption and reuse will overcome the flaw of having an expensive lanthanide-based adsorbent and significantly reduce the net cost of wastewater treatment. The recovery will also prevent the entry of the nanoparticles into the environment, 6805


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ACS Sustainable Chemistry & Engineering making the whole process more eco-friendly. The CeO2 nanoparticles here are reported to resolve almost all wastewater treatment-related problems with the recovery of adsorbent and adsorbate, which will make the process much cheaper as compared to the costly conventional methods. Disinfective Behavior of CeO2 Nanoparticles. The wastewater also contains biological contaminants such as pathogenic bacteria, fungi, and algae. In vitro antibacterial activity of the nanoparticles was determined to test the levels of bacterial contaminant removal using the surface-functionalized CeO2 nanoparticles. For the analysis, bacterial strains of Staphylococcus aureus and Escherichia coli were treated with all of the functionalized nanoparticles by employing the agar well-diffusion method.37 The choice of the chosen bacterial strains was determined for studying the disinfective nature of the nanoparticles. E. coli and S. aureus are commonly found Gram-negative and -positive bacteria in water bodies, respectively. Both of these strains have the tendency to resist under harsh environmental conditions. Therefore, studying the influence of CeO2 over them has clearly shown the disinfective nature of these nanoparticles against pathogens and enhances the scope of the nanoparticles. Pure cultures were obtained from the Microbiology Department of Panjab University Chandigarh and were regrown in nutrient broth at 37 °C for 24 h. The inoculated plates with fungal disks were allowed to grow at 27 °C for 72 h. Around 25 mL of autoclaved molten MH agar was poured into sterilized Petri dishes and allowed to solidify. Then, 200 μL of bacterial culture was inoculated over agar with the help of a spreader, and wells of 7 mm diameter were punctured. To each well was added 50 μL of the as-synthesized CeO2 nanoparticles at different concentrations, and the plates were incubated at 37 °C for 24 h. Sterile distilled water was used as a control during the analysis. The inhibition zone formed around each well was measured, and the effective zone of inhibition was estimated for all of the samples. The experiments were performed in triplicate. Antimold studies were carried out on Echinodontium taxodii isolated from Kinnaur, Himachal Pradesh, India. The fungus is very sensitive toward environmental changes and is thus considered good for antifungal activity of the nanoparticles. The antimold analysis of the nanoparticles was performed with different concentrations of nanoparticles. For the experiments, the nanoparticles were mixed with autoclaved MEA media and poured into sterile petri dishes. Afterward, the fungal discs from a pregrown fungal petri dish were placed on the sample containing media. The plates were allowed to grow at 27 °C for 72 h. Then, the growth diameter of the fungal colony was measured. The experiment was repeated in triplicate, and the mean was taken for growth index calculations. Growth index for Echinodontium taxodii over media containing nanoparticles was measured using the equation growth index (%) =

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Figure 1. Typical (a) FTIR and (b) Raman spectra of as-synthesized CeO2 nanoparticles.

These variations were associated with the generation of residual stress in the presence of external template over the surface of the nanoparticles. The difference in density and coefficient of thermal expansion between the external coating and substrate could further enhance this residual stress in the presence of surfactant. The differential behavior of CPB-coated CeO2 nanoparticles could also be related to their better adsorption property as compared to other nanoparticles from the Raman analysis. The additional broad Raman peak at 598 cm−1 is attributed to the defect spaces in nanoparticles. The intensity of this peak is found to decrease in the presence of surfactant moieties over the surface of nanoparticles. This reduction is mainly asymmetric due to the enhanced phonon lifetime in the nanocrystalline regime. Therefore, we can conclude that the sensitivity of this peak is due to the disorder in the oxygen sublattice of the nanoparticles, which was further affected by the presence of external coating, thermal variations, and changes in the obtained size of nanoparticles with the external coating.48 This particles size variation has also led to differences in phonon relaxation; therefore, the highest intensity of 598 cm−1 peak was obtained for the bare CeO2 nanoparticles as compared to its surfactant-coated counterparts.49 The surfactant-stabilized CeO2 nanoparticles have shown well-dispersed morphology in aqueous solution. The presence of cationic capping agents has not only avoided aggregation but also enhanced the adsorption properties of CeO2 nanoparticles. The content of surfactants around each nanoparticle has been ascertained and tabulated in Table 1. The results clearly indicate the maximum density of CPB around the CeO2

growth area in presence of nanoparticles × 100 growth area in control

(6)

RESULTS AND DISCUSSION Characterization of Surfactant-Coated Nanoparticles. The FTIR spectra of the as-synthesized nanoparticles are shown in Figure 1(a). The C−H bending vibrations between 1300 and 1400 cm−1 correspond to the C−H bonds in the surfactant moieties over the nanoparticles, whereas the bands around 1650 and 1500 cm−1 are assigned to νas and νs stretching vibrations of N−H bonds present in ammonium head groups of the surfactants. The results are in accordance with previous findings.46 The Raman spectra of the nanoparticles have been shown in Figure 1(b). All of the spectra displayed an intense Raman active T2g mode at around 450 cm−1, which could be assigned to the symmetric Ce−O vibration of the CeO2 lattice.47 Moreover, the peak position has shown significant variation after functionalization with surfactants as compared to that of bare CeO2 nanoparticles. 6806


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Table 1. Obtained Size of CeO2 Nanoparticles, Concentration, Mass, Volume, and Density of Surfactant Templates around Prepared Nanoparticles nanoparticles 1. bare 2. CTABCeO2 3. CTACCeO2 4. CPB-CeO2 5. CPC-CeO2

r (nm)

c (×10−4 M)

mass of surfactant molecules (Ms) (g)

volume occupied by surfactants (Vs) (cm3)

density of surfactant molecules around nanoparticles (ρ) (g/cm−3)

3.5 2.1

0.0652 0.289

70.8 × 10−26

16.81 × 10−20

0.421 × 10−5

2.25

1.37

2.7 × 10−26

13.98 × 10−20

0.02 × 10−5

2.63 2.42

0.147 0.19

24.7 × 10−26 11.9 × 10−26

1.65 × 10−20 2.48 × 10−20

1.495 × 10−5 0.48 × 10−5

This was mainly explained on the basis of partial absorption efficiency of as-synthesized nanoparticles toward these dyes, whereas lower adsorption efficiency of ∼35−38% for aqueous solutions of VB, DR, MO, and R6G has suggested the ineffectual adsorption behavior of CeO2 nanoparticles toward these dyes (Figure S3C). Generally, the wastewater from dye industries often contains a large mixture of different types of dyes. Hence, testing the sensitivity of the as-prepared nanoadsorbent toward a mixture of dyes was also carried out. The combination of different mixtures of dyes was prepared by mixing all of the understudied dyes. A batch of all of the dyes mentioned above was mixed in such concentrations that the resulting solution has a total of 50 ppm concentration. The absorption maxima have shown significant decreases in the presence of nanoparticles (Figure S3D and E). The fast removal ability of as-formed CeO2 nanoparticles has further opened a new path for utilizing these materials for removing multianalytes and acting as a better environmental remediating agent. The influence of surfactant coating on dye removal efficiency of CeO2 nanoparticles has also been carried out by measuring their spectroscopic analyses. The influence of pure surfactants over the dye removal was also determined in absence and presence of nanoparticles (Figure S4A and B). From the spectra, it was found that, for all of the surfactants, there was a significant decrease in the absorption intensity at the isosbestic point (λ = 520 nm) and corresponding enhancement with red-shifted peak at around 640 nm for a chosen model dye, i.e., EBT. These results clearly suggest that surfactant coating over the surface of nanoparticles enhances the complex formation rate of dye molecules with surfactant monomers and hence acts as a better adsorbent.42 Effect of Operational Parameters. Effect of pH. The adsorption behavior of EBT was studied over a wide range of pH values, i.e., pH 2, 4, 5, 6, 8, 10, and 12 at 25 °C. The whole range of pH values was selected to cover all possible conditions in real water samples. The batch mode adsorption experiments were conducted with dye:nanoparticles at 1:2 (initial dye concentration = 50 ppm and nanoparticle dosage = 100 ppm) to obtain perfect adsorption of the EBT. It was found that EBT equilibrium adsorption efficiency of bare CeO2 and surfactantloaded CeO2 nanoparticles was higher at pH 2 and 4. The results were explained on the basis of the zero point charge (zpc) of CeO 2 nanoparticles. The zpc value of CeO 2 nanoparticles lies in the pH range of 6.8−8.3 (Figure S4c). The surface charge is positive at pH values lower than zpc. The positive surface charge was mainly due to the accumulation of H+ ions. On the other hand, the presence of excessive OH− ions makes the surface negatively charged at pH values higher than zpc. In the current case, EBT being anionic in nature is adsorbed more efficiently at low pH values and enhances the removal efficiency of CeO2 nanoparticles (Figure 2a). In

nanoparticles as it occupies the least volume. The obtained data further shows the most packed and close interaction of CPB with the CeO2 nanoparticle surface. The lower value of densities and corresponding large volumes of CTAB and CTAC around nanoparticles have explained their less packed interactions over the surface of nanoparticles. The large volumes of CTAB, CTAC, and CPC can also be explained due to their bulkier size and multilayer-loose layering over the nanoparticle surface. The results could help in hypothesizing the statements that the CPB-coated nanoparticles may resist many adsorption−desorption cycles because of the high interaction of CPB with the CeO2 surface, whereas other surfactants may be washed away after few cycles of dye adsorption−desorption. Adsorption Characteristics of Different Dyes over CeO2 Nanoparticles. The feasibility of surface-functionalized CeO2 nanoparticles as an efficient nanoadsorbent for the removal of commercially available dyes was tested against MO, VB, R6G, DR, BB, CR, FG, and EBT (Figure S3A). The choice of these dyes was based on covering the diverse range of anionic, cationic, and azo nature of the commercially available dye for adsorption studies. Figure 3SB shows the digitally colored images and the corresponding variations in efficiency of the percentage of dye removal (Figure S3C). Moreover, upon interpretation of the results, it has also been found that, out of different types of chosen dyes, EBT can be completely adsorbed over CeO2 nanoparticles. This was mainly due to the superior ability of CeO2 nanoparticles toward EBT as compared to those of other dyes (Figure S3C). The enhanced adsorption ability of EBT was mainly associated with the complexation of Ce4+ ions with EBT in aqueous media. The electronegativity value (1.06) and radius of Ce4+ ions (0.107 nm) are appropriate for complex formation with the dye,40 contribute toward higher removal efficiency of the dye (Scheme S1), and thus EBT was chosen as the model system for adsorption studies. The net adsorption shown by bare CeO2 nanoparticles was mainly explained via the adsorption of H+ ions over the negatively charged nanoparticles. The higher available surface area and energy of CeO2 nanoparticles further leads to multilayer adsorption of dye (proven in adsorption isotherms). Additionally, ion-exchange and π−π dispersion interactions further support the formation of multilayers of dyes over the nanoparticle surface. The available functional groups in dye molecules further support the H-bonding and sulfur− oxygen bonding and hence result in the formation of a multilayer of dye molecules over the surface of the nanoparticles. The ionic complexation of EBT with surfactant-coated nanoparticles has further made the current system more advantageous relative to bare CeO2 nanoparticles. On the other hand, the aqueous solution of BB and CR become lighter but not fully adsorbed over CeO2 nanoparticles. 6807


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inappropriate to maintain in a real water system as more neutralizers will be needed to add after completion of the adsorption process. This will further enhance the chemical pollution in the treated water. Moreover, the real wastewater samples have a generally acidic nature varying between pH 5 and 7.50 Therefore, to keep the experimental working conditions close to those of real water samples, we have chosen pH 5 for further analysis. Effect of CeO2 Nanoparticle Dosage. The influence of nanoparticle dosage amounts on dye removal efficiency of surface-functionalized CeO2 nanoparticles is illustrated in Figure 2b. Nanoparticle concentrations of 25 (1:0.5), 50 (1:1), 100 (1:2), 250 (1:5), 500 (1:10), and 1000 (1:20) ppm against 50 ppm of EBT at pH 5 and 25 °C for the time interval of 60 min were chosen to find the appropriate dye:nanoparticle dosage at that particular pH. The data has clearly illustrated that the surface-functionalized nanoparticles have higher removal efficiency (>97% at 1000 ppm dosage amount) as compared to that of bare nanoparticles (∼75%). In particular, CTACfunctionalized CeO2 nanoparticles possess 99.5% dye removal efficiency as compared to 98.5% in CTAC, 97.6% in CPB, and 98.7% in CPC-functionalized CeO2 nanoparticles. The enhancement of around 22% in comparison to bare nanoparticles clearly signifies the importance of surface functionalization in wastewater treatment. The greater available surface area due to restricted growth of nanoparticles might be the probable reason for enhanced removal efficiency in the presence of surfactant. The percentage removal of EBT has further increased with increasing the concentration of adsorbent (Figure 2b) due to greater availability of the adsorbent surface area for adsorption. Effect of Initial Dye Concentration. The effect of varying the dye dosage amount, i.e., 10, 20, 30, 50, 70, and 100 ppm, was also investigated by keeping the concentration of nanoparticles fixed at 50 ppm with pH 5 at 25 °C (Figure 2c). From the data, it has been anticipated that the dye removal efficiency of nanoparticles has a negative impact on the initial dye concentration. With an increase in the initial dye concentration, there was a tremendous decrease in the removal efficiency of adsorbed dye. It has been found that, for bare nanoparticles, the dye removal efficiency was 40% at 20 ppm [EBT] in the presence of 50 ppm of nanoparticles, whereas under similar conditions, the dye removal efficiency was

Figure 2. Effect of (a) pH, (b) nanoparticle dosage amount, and (c) initial dye concentration on the removal efficiency of EBT in different surface-functionalized CeO2 nanoparticles.

addition, the presence of cationic surfactants over the surface of nanoparticles has further augmented the absorption efficiency of EBT. These surfactant templates possess −NH2 as a functional group, which are helpful in the complexation with dye molecules. Similarly, the surface of nanoparticles carry a negative charge in the higher pH range, which attracts cationic dyes. This pH-dependent adsorptive behavior of as-functionalized nanoparticles has significantly enhanced the scope of CeO2 nanoparticles for wastewater treatment by simply varying the pH of the solution. Although the percentage removal is maximum at pH ≤4 (Figure 2a), such conditions are

Figure 3. (a) FTIR and (b) XRD spectra of pure EBT before and after adsorption over bare, CTAB-, CTAC-, CPB-, and CPC-coated CeO2 nanoparticles. 6808


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Figure 4. TEM images of (a) bare CeO2 nanoparticles in the absence of EBT. TEM images of (b) bare, (c) CTAB-, (d) CTAC-, (e) CPB-, and (f) CPC-functionalized CeO2 nanoparticles after the adsorption of EBT dye.

spectra of surface-modified CeO2 nanoparticles have shown ample adsorption sites (available −OH, −NH, and Ce−O groups) over the surface of nanoparticles due to the presence of surfactant coatings (see Figure 1a). In presence of dye, the FTIR spectra of nanoparticles have shown major differences in the region of 1200−1000 cm−1. Moreover, viable differences were also observed in the region of 1600−1400 cm−1. These variations were associated with the excellent adsorption of EBT dye over the nanoparticle surface. The electrostatic interactions among the −NN- groups of EBT and −OH groups available over the surface of nanoparticles enhances the adsorption of dye. The spectra that peak at 3402 cm−1, which is the −OH stretching mode, have a shown significant blue shift to 3385 cm−1, suggesting participation of the −OH group in dye adsorption. The surfactant further augments the adsorption of azo dyes as compared to cationic dyes (repulsion among dyes and surfactant) and hence enhances the selectivity of the system for organic toxins. In addition, the presence of cationic surfactants over the surface of nanoparticles enhances the available surface area of nanoparticles due to the inhibition of the growth of nanoparticles. The comparison of the XRD spectra of surfactant-coated CeO2 nanoparticles in the presence and absence of dye molecules further confirms that the identity of the nanoparticles was not lost during dye adsorption (Figure 3b). These results pointed toward the physisorption of dye molecules over the nanoparticle surface. In addition, the accumulation of dye molecules over the exterior surface of the CeO2 nanoparticles has the tendency to bring the nanoparticles together due to intermolecular attraction and hence varies the size of the dye-adsorbed nanoparticles as compared to those of pure nanoparticles (Table S1). The size of the dye adsorbed nanoparticles was calculated using the Debye Schrerrer equation of the peaks at 28.6°, 33.2°, 47.5°, and 56.2°, and the average of these values is considered for comparison.

decreased to 3% for 100 ppm of EBT concentration. The corresponding decrease of 98.5 to 83.2% for CTAB, 99.8 to 53.1% for CTAC, 96 to 65.7% for CPB, and 96.6 to 92% in the case of CPC-functionalized nanoparticles for 20 to 100 ppm [EBT] was observed. This behavioral change clearly signifies the importance of surface functionalization under different studied conditions. With these surface modulations, we can easily composite design our system for a higher percentage removal of dye in real wastewater samples. At a fixed nanoparticle dosage, the percentage removal efficiency of adsorbed dye was decreased with the increase in the initial concentration of dye in the solution. At a low concentration of dye, the ratio of the initial number of dye moles to the available adsorption sites over CeO2 nanoparticles is low, and its removal efficiency was high. Effect of Contact Time. The effect of contact time on the adsorption of EBT was also studied to determine the maximum time taken by CeO2 for the removal of EBT from solution at pH 5. Briefly, 100 ppm of bare and surfactant-loaded CeO2 was added to 50 mL of 50 ppm EBT solution. The corresponding absorbance of the solution at 520 nm wavelength was observed as a function of time up to 300 min for monitoring the EBT concentration (Figure S5 A−C). Figure S5A showed the influence of time on dye removal at pH 5. From the spectra, it was clear that surface-functionalized nanoparticles have higher removal capacity as compared to that of bare nanoparticles. Almost all of the EBT became adsorbed within 60 min (Figure S5 B and C). Further equilibrating for a longer amount of time does not enhance the removal efficiency of the system. Therefore, an agitation time of 60 min was set as the optimum contact time for the adsorption reaction. CeO2 Nanoparticle−Dye Interactions. The adsorption behavior of the chosen model dye was further verified by recording the FTIR spectra as functionalized nanoparticles in the presence and absence of EBT dye (Figure 3a). The IR 6809


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ACS Sustainable Chemistry & Engineering The difference in the headgroup of the employed surfactant has also shown a significant influence on the adsorption of the dye. The presence of a larger pyridinium ring in the cases of CPC- and CPB- as compared to those of CTAB- and CTACfunctionalized nanoparticles have the tendency to use large available space and hence affect the adsorbing power of nanoparticles. The CPC- and CPB-modulated particles have a loose arrangement of surfactant monomers over the exterior surface of CeO2. As a result, the dye adsorption over the active site on nanoparticles is directly influenced by the effective encounter of dye molecules on these loose surfactant arrangements. A schematic illustration showing the adsorption of EBT over the CPB-functionalized nanoparticles is depicted in Scheme S2. Microscopic Validation of Dye Adsorption over CeO2 Nanoparticles. Figure 4 shows contrasting TEM images of dye-loaded surfactant-functionalized CeO2 nanoparticles. As evident from the images, the dye-adsorbed CeO2 nanoparticles possessed large agglomerates with numerous dark points at the center. It may also be noted that the size of bare CeO2 nanoparticles is greater than that of surfactant-coated nanoparticles,46 and this order remains the same even after the adsorption of dye. The average size of bare CeO2 nanoparticles was enhanced from 3.5 to 3.9 nm, whereas for CTAB, CTAC, CPB, and CPC nanoparticles, the average size increased from 2.1, 2.25, 2.63, and 2.42 nm to 3.42, 2.9, 3.5, 3.2 nm, respectively, after the adsorption of EBT. This is mainly explained by the coating ability of surfactants over nanoparticles and their corresponding influence on the obtained sizes of nanoparticles in different surfactant media. The restricted and controlled sizes of nanoparticles in surfactant media provided higher available active sites on the exterior surface of surfactantcoated CeO2 nanoparticles for the adsorption of EBT molecules (confirmed by TEM images). The results were also verified by SEM and EDS spectra of dye-loaded surfactantfunctionalized nanoparticles (Figures S6 and S7). An inspection of dye-adsorbed nanoparticles has further revealed a distortion in spherical morphology due to enhanced adsorption of dye molecules over the surface of nanoparticles. Being fluorescent in nature, bare and surfactant-loaded CeO2 nanoparticles have shown fluorescence under a blue filter (excitation wavelength of 450−480 nm), and the dye has shown better fluorescence under a red filter (excitation wavelength of 510−560 nm). Therefore, images were taken under both the filters and then layered and converted to RGB mode to view the combined effect. The obtained results significantly pointed toward adsorption of EBT molecules over the nanoparticle surface. The representative optical fluorescent microscopic images of dye-loaded surface-functionalized CeO2 nanoparticles are depicted in Figure S8. Adsorption Kinetics and Isotherms. For the binding ability of dye molecules and their respective rate-determining steps to be understood, the kinetic data of dye adsorption over the surface of bare and functionalized nanoparticles were analyzed with Lagergren pseudo-first-order and Ho pseudosecond-order kinetics by linear method (Figure 5a and b). The respective equation of Lagergren pseudo-first-order equation is expressed in eq 7. ln

C0 = kt Ce

Figure 5. (a) Pseudo-first-order adsorption and (b) pseudo-secondorder adsorption kinetics of EBT over bare and surface-functionalized CeO2 nanoparticles.

where C0 and Ce are the initial and equilibrium concentrations of dye in the solution and k is the rate constant (min−1) in eq 6. The values for rate constant k and regression coefficient R2 were calculated from the plot of ln C0/Ce vs t (time) and tabulated in Table 2. The linear form of the pseudo-secondorder kinetic model is expressed in eq 8. t 1 1 = + qt qe kqe2 (8) where k (g mg−1 min−1) is the rate constant and qe is the amount of EBT adsorbed over the nanoparticle surface at equilibrium. Upon interpreting the data, it was found that, in contrast with the pseudo-first-order kinetic model, the corresponding pseudo-second-order kinetic model was more appropriate (with higher correlation coefficient R2 values, see Table 2) for surfacefunctionalized CeO2 nanoparticles, whereas bare CeO2 nanoparticles followed the pseudo-first-order kinetic model. Such behavioral variations were explained on the basis of different modes of adsorption mechanisms between adsorbent and adsorbate. In the case of bare nanoparticles, chemical interactions between EBT molecules and nanoparticles were mainly associated with adsorptive removal of dye; thus, it follows pseudo-first-order kinetics. On the other hand, the adsorption occurred over the external surface of the surfactantcapped CeO2 nanoparticles via diffusion, and a small amount of dye molecules were also transported within the pores of the adsorbent. Moreover, the obtained qe values in each case were in good agreement with the experimentally calculated qe value. The adsorption capacity of surface-functionalized nanoparticles was further estimated through adsorption isotherm studies by using four different models, namely, Langmuir,

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Table 2. Adsorption Kinetics and Adsorption Isotherms for the Adsorption of EBT Dye over Bare and Surfactant-Loaded CeO2 Nanoparticles kinetic model pseudo-first-order pseudo-second-order

Langmuir

Freundlich

Temkin

Dubinin−Radushkevich

parameters

bare CeO2 NPs

K1 R2 K2 (×10‑4) qe R2

0.019 0.9877 2.7 420.17 0.9429

qmax b R2 QF N R2 B (×10‑2) k (×10‑2) R2 ε R2

75.2 252.24 0.9999 71.29 1.09 0.9998 16.46 323.68 0.8662 −68.49 0.8409

CTAB-CeO2 NPs 0.079 0.830 3.76 735.29 0.9958 Isotherms 112.4 631.71 0.9998 78.09 1.06 0.9999 17.47 447.95 0.9338 −66.2479 0.933

Fruendlich, Temkin, and Dubinin−Radushkevich equations, for examining how the dye molecules allocate themselves over the surface of nanoparticles (details given in the Supporting Information). The plots of specific sorption Ce/qe against the equilibrium concentration, Ce are shown in Figure S9. The remaining plots for different isotherms are also plotted in Figures S10−12. The obtained values of the respective constants are illustrated in Table 2 for bare and surfactantfunctionalized CeO2 nanoparticles. By inferring the obtained values of R2, it was established that the Freundlich model fits better for the adsorption of EBT dye over the bare and surfactant-modulated CeO2 nanoparticles. Because the R2 value of the Freundlich model is highest among the four models applied over the data,22,26 the association of dye molecules by electrostatic attraction presents multilayer adsorption of EBT molecules over the exterior facade of CeO2 (Scheme S3). The highest obtained adsorption ability of CeO2 was further evaluated by comparing the qmax values for surfactantfunctionalized CeO2 nanoparticles. It has been found that CPB-functionalized CeO2 nanoparticles have shown a value of 171.3 mg/g as compared to 169.4 mg/g for CPC, 117.7 mg/g for CTAC, 112.4 mg/g for CTAB, and 75.2 mg/g for bare CeO2 nanoparticles. The data was further compared with the available literature for the adsorption of EBT dye over various adsorbent (Table S2).45,48,51−54 The relative qmax values are tabulated in Table 2. From the statistics, it has been predicted that the present synthetic approach to develop modified CeO2 nanoparticles as adsorbent provides higher values of qmax. Therefore, the present adsorbent has superior adsorption efficiency toward dye by strong electrostatic attraction. The dye adsorption studies were further carried out in different water sources such as water from tap, well, tube well, hand pump, and canal (Figure 6). All of the water sources were contaminated with 50 ppm EBT, to which 50 ppm nanoparticles were added and stirred for 24 h. The absorption spectra were taken in the case of all of the samples. The results illustrate that EBT has been successfully removed up to 80% in all samples, but removal efficiencies in the cases of water from the tube well and hand pump water were around 60−70% for all of the modified CeO2 nanoparticles. This could be due to

CTAC-CeO2 NPs

CPB-CeO2 NPs

CPC-CeO2 NPs

0.205 0.8934 2.55 877.19 0.9814

0.644 0.9799 8.58 961.54 0.9952

0.294 0.9645 3.438 934.58 0.9951

117.7 279.39 0.9997 72.09 1.08 0.9999 109.17 41.60 0.8398 −16.185 0.7155

171.3 263.56 0.9998 82.99 1.04 0.99998 169.79 67.52 0.9564 −8.243 0.8735

169.4 21.75 0.9989 49.66 1.29 0.9999 172.81 63.78 0.9194 −6.96 0.8414

Figure 6. Percentage removal efficiency of EBT dye with bare and surface-functionalized CeO2 nanoparticles in various types of spiked water.

the interference of salts of different metals that will interact with EBT and make it unavailable for adsorption. For this to be verified, the influence of different salts were also analyzed to determine the effectiveness of the surface-modulated CeO2 nanoparticles (Figure S13a and b). From the results, it has been anticipated that the presence of salts like nickel acetate, copper chloride, barium acetate, sodium carbonate, nickel chloride, sodium sulfate, and zinc sulfate have a very strong influence on the adsorption efficiency of bare CeO2 nanoparticles, i.e., adsorption limited to less than 50%. In the case of different surfactant functionalizations, CPB-capped CeO2 nanoparticles have not shown any kind of salt effect. The surfactant capping has restricted the interference of metal ions over the surface of nanoparticles. Such results have further enhanced the scope of CPB-capped CeO2 nanoparticles to work effectively in wastewater systems. The prepared nanoparticles were also tested to calculate the COD, BOD, and other water parameters including TDS, EC, and salt concentration of pure dye solutions in the presence and absence of nanoparticles. The treated water has shown significant reduction of 79% for COD and 87% for BOD 6811


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reduction of the method, dye desorption experiments were also carried out. The results make this approach more efficient and feasible in the context of industrial usage. In brief, the dye molecules were desorbed by washing dye-loaded CeO2 nanoparticles with 1 M NaOH. The process was repeated 2− 3 times followed by 1−2 washings with deionized water. The desorbed dye was subjected to UV−vis analysis to determine its identity, and the desorbed nanoparticles were analyzed using XRD (Figure 8). The EBT crystals in dried form show intense and sharp XRD peaks, which were lost when it was dissolved in water. This is because of the increased size of the nanoparticles after dye adsorption. The recovered nanoparticles have shown no extra peak or distortion as compared to the earlier peak, which confirms that recovered nanoparticles are pure and can be reused. The reusability of as-prepared nanoparticles was also tested against EBT dye. In the first recycle process, the removal efficacy was around 95%, followed by 91 and 89% in subsequent cycles. The NaOH and ethanol spent in the process can be regenerated by ion exchange units and distillation processes. The regeneration process offers recovery of the adsorbed dye, nanoparticles, and desorption reagents. Therefore, the results point out the efficacy of as-prepared nanoparticles in an economical way. Upon comparing it with the conventional method, it was found that, by using CeO2 nanoparticles, we have eliminated several steps in the water treatment process, such as coagulation, and removal of organic impurities (details provided in the Supporting Information). The system works at normal pH, i.e., pH 5. After the treatment with nanoparticles, there is no need for adding neutralizers. Moreover, the prepared CeO2 nanoparticles can effectively remove the pathogens from the water, which can cut the disinfection cost of the sample (Figure S16). Moreover, the

(Figure 7.). On the other hand, there was no variation in DO values, indicating that the nanoparticles have no effect on the dissolved oxygen content of water.

Figure 7. Percent reduction in COD, BOD, DO, TDS, EC, and salts concentration shown by bare CeO2, CTAB-, CTAC-, CPB-, and CPCcoated CeO2 nanoparticles.

Desorption Studies. In particular, the effluent treatment processes have involved multiple steps of filtration, coagulation, chemical treatment, disinfection, reverses osmosis, and so forth, which makes the treatment more costly. With the application of CeO2 nanoparticles, we can reduce the number of processes and hence reduce the cost of water treatment. For further cost

Figure 8. Schematic illustration showing the recovery of nanoparticles and dye molecules. 6812


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Figure 9. Antibacterial assay of functionalized CeO2 nanoparticles against S. aureus and E. coli by the agar well diffusion method.

The results further confirm that CeO2 nanoparticles are effective disinfectants depending upon the contaminant load. Moreover, it has also been found that a comparatively higher zone of inhibition was achieved for S. aureus as compared to that for E. coli, which is because of the higher sensitivity of S. aureus toward reactive oxygen species (ROS) generated by CeO2 nanoparticles.55 The antimicrobial activity of CeO2 nanoparticles for less sensitive bacteria such as E. coli has also favored because as nanoparticles it is nonselective and applicable to resistant strains in an effective manner. The bar graph in Figure S14 displays statistical data of the zone of inhibition by nanoparticles as a function of concentration for E. coli and S. aureus bacteria. The respective fungal colony diameter was also measured at regular time intervals up to 72 h (Figure S15). Different concentrations of the nanoparticles ranging from 25 to 2000 ppm were carried out to determine the minimal inhibitory concentration (MIC) of uncoated nanoparticles. The results clearly point out that CeO2 nanoparticles do not show an inhibitory effect below 500 ppm. Upon further increasing the concentration of the nanoparticles, there was a significant decrease in fungal growth. The growth inhibition property was found to be maximum for bare CeO2 nanoparticles as compared to that for surfactant-coated nanoparticles. The possible reason for the enhanced antifungal property of bare CeO2 nanoparticles could be related to their size. The bare nanoparticles possess bigger size as compared to that of surface-coated nanoparticles (confirmed from TEM and SEM studies). These larger size particles have the ability to clog the agar pores and reduce the nutrient flow toward growing fungi.56 The strong antibacterial and antimold behavior of asprepared nanoparticles has further been explained on the basis of their size, specific surface area, and charge surface of the as-formed nanoparticles. These surface-modified nanoparticles

used nanoparticles during water treatment can also be recovered in a single and easy step. By reducing of harmful chemicals used in the conventional method, CeO2 nanoparticles can largely resolve the problem of water treatment. Therefore, the development of a single tool, i.e., use of CeO2 nanoparticles for almost all of the steps in the water treatment operation will turn out to be more economically viable as compared to the available methods. A rough cost estimation per liter of water treatment has been framed in Table S3. As per the calculation, from the Langmuir isotherm (qmax), it was also found that 75.2, 112.4, 117.7, 171.3, and 169.4 mg of dye can be adsorbed by 1000 mg of bare, CTAB-, CTAC-, CPB-, and CPC-coated CeO2 nanoparticles. For the sake of comparison, if we consider a very high concentration of dye, i.e., 500 ppm, then we will require 6.7, 4.5, 4.2, 2.8, and 3 g of bare, CTAB-, CTAC-, CPB-, and CPC-coated CeO2 nanoparticles for 1 L volume of water (details provided in the Supporting Information). Disinfection of Wastewater. Wastewater has always possessed some biological pollutants along with organic contaminants. The removal of these biological pollutants further requires additional treatment processes and hence enhances the overall working cost of the environmental remediation process. Hence, in the current work, these modified CeO2 nanoparticles have the ability to work against these biological pollutants. The formed particles were tested for their antimicrobial and antifungal properties against S. aureus, E. coli, and E. taxodii fungi. Figure 9 shows the size of the zone of inhibition formed around each CeO2 nanoparticle with both test organisms (bacteria). The bacterial activity was also tested against different concentrations of nanoparticles varying from 25 to 200 ppm. A clear zone of inhibition started to appear at very low concentration of 25 ppm, which further increased with increasing concentration of the nanoparticles. 6813


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ACS Sustainable Chemistry & Engineering possess higher band gaps, i.e., to 3.7, 3.8, 3.98, and 3.96 eV for CTAB-, CTAC-, CPB-, and CPC-coated CeO2 nanoparticles (calculated using UV−vis absorption data) as compared to that of bare CeO2 nanoparticles (3.3 eV). The photoexcitation of these modified particles have the ability to form superoxide and hydroxyl radicals (•OH), which damage the cells and ultimately affect the bacterial growth. Second, electrostatic interactions between the negatively charged cell wall and positively charged nanoparticles due to the presence of a surfactant coating can also affect the exchange pumps and barriers of the cell wall. The resultant interaction has the tendency to inhibit cellular exchange and ultimately prevents bacterial and fungal growth and could be useful for their applications in efficient cleaning of wastewater. The toxic behavior of the nanoparticles is a significant barrier toward its use on a commercial scale as the release of the nanoparticles into the environment is highly undesirable.57 The nanoparticles can enter biological systems and thereby damage their vital functionalities by binding and blocking biomolecules.58 Thus, the risk of using nanobased systems always persists. This left a window for the development of various methods by which the release of nanoparticles to the environment can be restricted. Authors are working on the direction of developing retrievable packets containing nanoparticles that will overcome the problem of their release in the area under consideration.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; tel: +91 9417250377; fax: +91 172 2545074. *E-mail: [email protected]; tel: +966-534574597. ORCID

Ahmad Umar: 0000-0003-2673-4355 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.C. is grateful to DST Inspire Faculty award [IFA-CH-17] and Purse Grant II for financial assistance. P.S. is thankful to DST, India for providing an Inspire Senior research fellowship [IF140267]. A.U. acknowledges the Ministry of Higher Education, Saudi Arabia for granting Promising Centre for Sensors and Electronics Devices (PCSED) to Najran University, Saudi Arabia.

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CONCLUSIONS In summary, size-controlled CeO2 nanoparticles were synthesized using several cationic surfactants by simple solution process. The cationic surfactants could provide appropriate template for modulating the size and band gap of the prepared nanoparticles. The functionalized CeO2 nanoparticles not only possess better size control but also exhibit improved adsorption capacity for different dyes as compared to those of bare CeO2 nanoparticles. The adsorption efficiency of bare CeO2 nanoparticles was 90%. The cationic surfactant coatings enhance the adsorption removal to higher levels, i.e., ∼99% in the case of CPB-capped CeO2 nanoparticles. The detailed absorption kinetics and isotherm clearly reveals direct evidence for enhanced absorption efficiency of CeO2 nanoparticles. The current system is equally effective in different spiked water systems and works efficiently in the presence of interfering ions. The regeneration of both adsorbate and adsorbent have further highlighted the importance of the system. The functionalized CeO2 nanoparticles have also displayed strong antibacterial and antifungal activities against Gram-positive and -negative bacteria. The presented results afford numerous water remediation applications using CeO2 nanomaterials from laboratory research to realistic water treatment applications.

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calculated using the Debye−Scherer equation, SEM images of dye-adsorbed nanoparticles, fluorescent microscopic images of bare and dye-adsorbed nanoparticles, adsorption isotherms, schemes related to nanoparticle− dye interactions, table for the comparison of dye adsorption ability of various adsorbents, effect of different interfering ions on the dye removal efficiency, and antimicrobial and antifungal properties of the nanoparticles and cost estimation of the as-described method (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01041. Molecular structures of the dyes, methodology for calculating the content of the surfactant on the nanoparticle surface, nanoparticle−dye interactions and various parametric studies, zeta potential values of asprepared nanoparticles, table of nanoparticle size 6814


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