Effective removal of arsenic (V) from aqueous solutions using efficient CuO/TiO2 nanocomposite adsorbent European Journal of Chemistry 13 (3) (2022) 284-292 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2022 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.13.3.284-292.2283 European Journal of Chemistry View Journal Online View Article Online Effective removal of arsenic (V) from aqueous solutions using efficient CuO/TiO2 nanocomposite adsorbent Saima Farooq 1, Asima Siddiqa 2,*, Sobia Ashraf 3, Sabtain Haider 4, Saiqa Imran 5, Shabnam Shahida 3 and Sara Qaisar 2 1 Department of Biological Sciences and Chemistry, College of Arts and Sciences, University of Nizwa, Nizwa 616, Oman 2 Nanosciences and Technology Department, National Centre for Physics, Islamabad, 44000, Pakistan 3 Department of Chemistry, The University of Poonch Rawalakot, Azad Kashmir, Pakistan 4 Department of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan 5 Pakistan Council of Research in Water Resources, Ministry of Science and Technology, Islamabad, Pakistan * Corresponding author at: Nanosciences and Technology Department, National Centre for Physics, Islamabad, 44000, Pakistan. e-mail: asima.siddiqa@ncp.edu.pk (A. Siddiqa). 10.5155/eurjchem.13.3.284-292.2283 Received: 05 May 2022 Received in revised form: 07 June 2022 Accepted: 11 June 2022 Published online: 30 September 2022 Printed: 30 September 2022 The groundwater is one of the biggest natural resources for providing drinking water to millions of people all around the globe. However, the presence of large amount of arsenic(V) in water causes serious health hazards to the consumers which necessitates the development of cost-effective remediation. The CuO/TiO2 nanocomposites were prepared by the precipitation-deposition method for the removal of the arsenate ion (AsO43-) from water. The prepared samples were characterized by powder X-ray diffraction, Fourier transform infrared, and scanning electron microscopy to examine crystallite size and structure, material purity, textural features, morphology, and surface area. The effect of different operating parameters such as pH, contact time, initial concentration of arsenic(V) and nanocomposite dose on the removal rate of arsenic(V) was examined to optimize the adsorption performance of the CuO/TiO2 nanocomposite. In addition, the adsorption mechanism was studied by employing Langmuir and Freundlich adsorption isotherms to gain better understanding of the adsorption mechanism. The Freundlich adsorption isotherm fits well with the experimental data and the maximum adsorption capacity of the Langmuir model was found to be 90 mg/g for arsenic(V). The CuO/TiO2 nanocomposite shows remarkable adsorption performance for the treatment of arsenic(V) contaminated water samples. This study provides a cost-effective solution for the safe use of groundwater contaminated with arsenic. TiO2 CuO/TiO2 Adsorption Arsenate ion removal Hybrid nanocomposite Water purification system Cite this: Eur. J. Chem. 2022, 13(3), 284-292 Journal website: www.eurjchem.com 1. Introduction Arsenic (As) is the third most toxic element that comprises approximately 1.8 ppm of the Earth’s crust [1]. It is the major constituent of 200 mineral species having formula MAsS and MAs2 (where M = Fe, Co, Ni) such as arsenopyrite (FeAsS), orpiment (As2S3), and realgar (AsS) [2]. The natural activities (such as weathering of rocks, forest fires and volcanic events) and anthropogenic sources (like pesticides, herbicides, mining, copper smelting, chemical warfare agents and fossil fuel bur- ning) are held responsible for the release of arsenic in the air, water and soil [2-4]. In the environment, arsenic exposure to human system may occur through inhalation in the air, absorption through the skin, or ingestion of contaminated food and water. However, contaminated water is the most lethal route. In 2012, almost 200 million people throughout the world were exposed to drinking water having As concentration above 50 ppb [5]. This situation is even worser in Asian countries specifically in Bangladesh, China, India, and Pakistan. Human exposure to arsenic in these countries is constantly increasing, due to the excessive release of As-rich untreated industrial effluent, inadequate wastewater treatment processes, poor health facilities, lack of water quality surveillance, and public ignorance [6,7]. Arsenic causes skin lesions, respiratory diseases, mutagenic changes, and tumours depending on the proportion and duration of exposure [8,9]. As a result, the International Agency for Research on Cancer (IARC) has classified it as a class 1 human carcinogen substance. In 1993, the United States Environmental Protection Agency (US-EPA) and World Health Organization (WHO) reduced the permissible arsenic level in drinking water from 50 to 10 ppb [3]. This drastic decrease sets an alarming condition for the regulatory authorities in every corner of the world. In our environment, arsenic exists in either organic or inorganic form. In groundwater, arsenic occurs mainly in inorganic form (as oxyanions) such as arsenite As(III) and arsenate As(V) which are much more noxious than organic form [10]. However, As(V) is less poisonous in comparison to As(III) but it is thermodynamically more stable due to its preponderate as a major arsenic constituent of groundwater. Therefore, it is ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.13.3.284-292.2283 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.13.3.284-292.2283 mailto:asima.siddiqa@ncp.edu.pk http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.13.3.284-292.2283&domain=pdf&date_stamp=2022-09-30 Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 285 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 the need to develop an economical and effective method for arsenate removal [11]. Researchers have employed conven- tional methods for the treatment of As(V) present in water like advanced oxidation process, bioremediation, adsorption, membrane filtration technologies, electrochemical methods and ion-exchange process [3,4,6,12-14]. Among those, adsorp- tion has been considered as an efficient method owing to its prominent features i.e., high efficiency, availability, profita- bility, cost-effectiveness, regeneration of adsorbent and ease in operation. The process simply involves the segregation of toxins from water and accretion of them on the adsorbent surface by physical or chemical interaction. This technique is easy to operate and equally effective in the removal of toxic pollutants even at low concentrations [15]. Several adsorbents such as dry plants, red mud, zeolites, hydrotalcite, rice husk and natural clays have been employed for the removal of arsenic [16,17]. However, most of these adsorbents have shown poor adsorption capacity for As(V) and also face difficulty in regeneration. The adsorption capacity is highly dependent on the nature, texture, size, and shape of the adsorbent. Therefore, research- hers have turned their attention toward the development of hybrid nanomaterial composites for environmental remedia- tion owing to their enhanced surface area, more active sites, higher selectivity, and ease in regeneration [18]. In recent years, TiO2 has been extensively investigated for the removal of As due to its low toxicity, higher chemical and thermal stability, low generation cost and high selectivity for As. However, low surface area and higher agglomeration have limited TiO2 performance [19]. Recently, researchers have found that TiO2 based nanocomposites come with a large surface area and a higher adsorption rate. Feng et al. [20] reported an improved removal efficiency of As(III) and As(V) over Fe3O4@SiO2@TiO2 from wastewater released during the gold cyanide process. Sagharloo et al. [21] studied the removal of arsenic from contaminated water using immobilized ZnO/TiO2 AC. Along with TiO2, CuO nanoparticles have gained much attention for improving the adsorption efficiency of different nanosorbents for arsenic removal by making hybrid composites. Malwal et al. [22] reported the increased removal efficiency of As(III) over the CuO/ZnO nanocomposite. Sharma et al. [23] prepared a CuO/ZnO nanocomposite with an enhanced adsorption rate for Cr(VI) and Pb(II) along with improved photocatalytic degra- dation of basic violet and reactive yellow. Bhattacharya et al. [24] reported the removal of ciprofloxacin over CuO/TiO2 nanocomposite. Furthermore, to our knowledge, the CuO/TiO2 nanocomposite has not yet been used to remove As(V) from aqueous media. In the present work, CuO/TiO2 nanocomposites were prepared by the precipitation-deposition method. The charac- terization of prepared samples was done by employing X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), and Scanning electron microscopy (SEM). The adsorption potential of the CuO/TiO2 nanocomposite toward As(V) under different parameters such as pH, contact time, arsenic ion concentration, and the adsorbent dose was investigated. The isothermal studies and regeneration capacity of nanocomposite were also studied. The result of this work helps develop a cost- effective solution for the removal of arsenic from groundwater. 2. Experimental 2.1. Reagents All chemicals were analytical grade and were used without further purification. Commercial titania powder (anatase TiO2, 99.7%), copper nitrate trihydrate (Cu(NO3)2·3H2O, 99%), hydrochloric acid (HCl, 37%), ethanol (C2H5OH, 99.8%), sodium hydroxide (NaOH, 98%), and sodium arsenate dibasic hepta hydrate (Na2HAsO4·7H2O, 98%) were purchased from Sigma- Aldrich, USA. Distilled water was used throughout the study. 2.2. Synthesis of CuO/TiO2 nanocomposites CuO/TiO2 nanocomposites were synthesized by the previously reported precipitation-deposition method [25]. First, the required amount (0.3, 0.5 and 0.9 g) of titania was dissolved separately in 20 mL of ethanol and stirred in each suspension for 24 hours to completely disperse the TiO2 in the solvent. Second, 20 mL of different molar solutions (i.e., 0.02, 0.04 and 0.06 M) of copper nitrate were prepared in distilled water. Then, add 20 mL of 0.02 M copper nitrate solution in 0.3 g/20 mL ethanol TiO2 suspension and left on stirring for an hour to obtain a homogenous mixture. Subsequently, the pH of the mixture was adjusted to 10 by adding 1 M sodium hydroxide solution dropwise and stirring until black precipitates formed. The precipitates were extracted by centrifugation and rinsed several times with distilled water to neutralize the pH. Finally, the precipitates obtained were dried in an oven at 90 °C for 8 hours and then calcined for 3 hours at 400 °C. In this way, the nanocomposite of CuO/TiO2 was obtained. A series of nano- composites (1, 3 and 5 wt % CuO/TiO2) were synthesized by the same procedure by simply varying concentrations of Cu(NO3)2·3H2O solution and TiO2 suspension. 2.3. Characterization The crystal structure, phase, and size of the prepared nanocomposite materials were analysed using a powder X-ray diffractometer (Bruker D8 Advance X-ray diffractometer) employing CuKα radiation in the 2θ range of 20-80° at a scan rate of 0.02°/sec. The functional group and phase purity of all samples were analysed using Fourier transform infrared spectra recorded on a Jasco 1106 spectrophotometer in the range of 500-4000 cm-1 using a KBr disk. The microstructure and morphology were studied by scanning electron microscopy (SEM LEO 1430 VP, Germany) on gold-coated surfaces. 2.4. Batch adsorption experiments The stock solution (1000 ppb) of As(V) was prepared by dissolving the stoichiometric amount of sodium arsenate dibasic heptahydrate in distilled water and the required dilution was made in the same solvent. A series of batch experiments were performed to study the effect of pH of solution, contact time, arsenic ion concentration and adsorbent dose on the removal of As(V) from aqueous solution using the prepared CuO/TiO2 nanocomposite. These experiments were carried out in 250 mL glass beaker containing 100 mL of varying initial concentration (100-1000 ppb) of aqueous As(V) solution at different pH (1-9) and nanocomposite dosages (0.05 to 0.30 g). All these experiments were carried out at room temperature and solutions were stirred for different time intervals (1-6 h) in dark. After adsorption, collected samples were centrifuged and filtered to separate nanocomposite. Finally, the filtrate was analysed by using hydride generated atomic absorption spectrophotometer (Vario 06, Germany) to obtain optimized nanocomposite dosage and As(V) concent- ration. This instrument can measure arsenic even at a very low concentration as it employed sodium borohydride (as a reducing agent) which converts arsenic into an arsine gas. The concentration of arsenic is detected as a function of amount of arsine gas produced [26]. The adsorption capacity of nanocomposite (Qe) and percentage removal efficiency (%R) was calculated by employing Equations (1 and 2) given below [27]: 𝑄𝑄e = (C0−Ce) M × 𝑉𝑉 (1) 286 Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 Figure 1. XRD spectra of (a) TiO2, (b) 1wt. % CuO/TiO2, (c) 3wt. % CuO/TiO2, (d) 5wt. % CuO/TiO2 and (e) CuO. %𝑅𝑅 = (C0−Ce) C0 × 100 (2) where Qe is the adsorbate amount adsorbed per gram of adsorbent at equilibrium (mg/g), C0 and Ce are the initial and equilibrium concentration of adsorbate (mg/L), V is the volume of solution (L), M is the mass of adsorbent (g) and %R represents the overall removal of adsorbate based on the ratio of adsorbent dosage to adsorbate volume at a particular adsorbate concentration. 2.5. Isothermal analysis The adsorption and desorption isotherms were also investigated by using the well-known Langmuir and Freundlich isotherm model. The Langmuir isotherm is described by using the Equation (3) as follows; 1 𝑄𝑄e = 1 𝑄𝑄max + 1 𝐾𝐾Ce𝑄𝑄max (3) where Qe is the adsorption capacity of the adsorbent (mg/g), and Ce is the equilibrium concentration of adsorbate (mg/L). K is the Langmuir constant (L/mg) which is related to adsorption energy and Qmax is the optimum adsorption capacity of adsorbent (mg/g) was calculated from the slope (1/K) and intercept values (1/Qmax) of Langmuir plot [28]. The important characteristics of the Langmuir isotherm are expressed in terms of the dimensionless equilibrium parameter (RL). 𝑅𝑅L = 1 1+𝐾𝐾𝑄𝑄max (4) The RL value defines adsorption as irreversible (RL = 0), favourable (1 > RL > 0), linear (RL = 1), or unfavourable (RL > 1) [29]. The Freundlich isotherm is expressed in terms of the following Equation (5); 𝐿𝐿𝐿𝐿𝐿𝐿𝑄𝑄e = 1 𝑛𝑛 𝑙𝑙𝐿𝐿𝐿𝐿𝐶𝐶e + 𝑙𝑙𝐿𝐿𝐿𝐿𝐾𝐾f (5) where Qe is the adsorption capacity of the adsorbent (mg/g), Ce stands for the equilibrium concentration of adsorbate (mg/L). The Kf and n represent the adsorption strength and intensity respectively, and their values were calculated from the intercept and slope of Freundlich plot [28,29]. 2.6. Desorption of arsenic The regeneration capacity of the CuO/TiO2 nanocomposite was investigated by performing a desorption experiment. For this purpose, 0.2 g of arsenic loaded nanocomposite was added to 0.5 M NaOH solution that is used as a desorbing agent and stirred in the solution for half an hour. The concentration of arsenic in the solution was determined after desorption. The desorption efficiency of the nanocomposite was calculated using the following Equation (6); 𝐷𝐷 = Cd−Ce C0−Ce × 100 (6) where D is the desorption efficiency and Cd is the concentration of adsorbate after desorption. C0 and Ce represented the initial and equilibrium concentration of adsorbate, respectively. The regenerated nanocomposite was used again for the adsorption of As(V) and these desorption-adsorption experiments were repeated several times. 3. Results and discussion 3.1. Material characterization 3.1.1. XRD analysis The XRD pattern of pure TiO2, CuO and CuO/TiO2 (1, 3 and 5 wt. %) nanocomposites is shown in Figure 1. The XRD spectra of TiO2 represented in Figure 1a was compared with our previously reported work [30] which confirmed all of the peaks having 2θ value of 25.32, 37.8, 48.2, 53.9, 55.3, 62.9, and 69.01° matches the reflection in the (101), (004), (200), (105), (211), (204) and (116), respectively, crystal planes were presenting tetragonal geometry and anatase phase of TiO2. The XRD spectra of CuO presented in Figure 1e was compared with the literature [31] which clearly showed that all the peaks having 2θ value of 32.31, 35.33, 38.74, 48.84, 53.43, 58.28, 61.43, 66.02, and 67.99° were corresponding to (110), (-111), (111), (- 202), (020), (202), (-113), (022), and (113), respectively, crystal planes of the monoclinic phase of Tenorite CuO. Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 287 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 Table 1. Crystallite size and lattice parameter of the synthesized materials. Material Crystallite size (nm) d-spacing (Å) a (Å) b (Å) c (Å) TiO2 41.98 3.5122 3.7880 3.7880 9.5076 CuO 43.01 2.7368 4.6201 3.4232 5.0571 1wt. % CuO/TiO2 42.09 3.5131 3.7878 3.7878 9.5172 3wt. % CuO/TiO2 42.11 3.5136 3.8006 3.8006 9.5096 5wt. % CuO/TiO2 41.75 3.5141 3.8046 3.8046 9.5252 Figure 2. FTIR spectra of (a) TiO2, (b) 1wt. % CuO/TiO2, (c) 3wt. % CuO/TiO2, (d) 5wt. % CuO/TiO2 and (e) CuO. The diffractograms of 1, 3 and 5 wt. % CuO/TiO2 nanocom- posites are presented in Figures 1b-d, clearly showing that all nanocomposite samples contain peaks similar to the anatase TiO2. In addition, the diffraction peaks belonging to (-111) and (111) of the CuO crystal planes also appeared in spectra of 3 and 5 wt. % CuO/TiO2 nanocomposites and become more promi- nent with 5 wt. % composition. There was no other peak appeared belonging to Na2Ti3O7 or any other impurity which indicates that all of the nanocomposites primarily consist of tenorite CuO and anatase TiO2 and the addition of CuO to TiO2 did not cause any structural damages [32]. The average crystallite size calculated by the Debye-Scherrer equation and lattice parameters are given in Table 1. There was no significant change appearing in average crystallite size which also suggests that copper is present in the form of CuO rather than a metallic dopant. 3.1.2. FT-IR analysis FT-IR spectra of calcined samples were presented in Figure 2. The FT-IR spectra of all CuO/TiO2 nanocomposites showed a pattern similar to that of TiO2. Each spectrum contains a broad region below 1000 cm-1 which represents the symmetric and asymmetric stretching and bending vibrational modes of M-O. The FTIR spectra of all nanocomposites contain characteristic peaks at 530 and 657 cm-1 which occur due to the stretching and bending modes of Ti-O-Ti that confirmed the formation of TiO2 [33]. While peaks observed at 525 and 555 cm-1 were due to Cu- O stretching mode [31,34]. The peak that appeared at 1649 cm- 1 represents the H-O-H flexion which may be due to either water or ethanol occluded in pores structure or due to hydroxyl group attached at the surface of TiO2 structure (Ti-O-H) [35]. There was no other peak that appeared related to any impurity which signifies the purity of all calcined samples. 3.1.3. SEM analysis The surface morphology and structural features of TiO2, CuO, and 5 wt. % CuO/TiO2 nanocomposite have been investigated by SEM as shown in Figure 3a-c, respectively. The SEM micrograph of TiO2 shows that the nanoparticles are monodisperse and have a spherical geometry [36]. The SEM analysis of CuO presented in Figure 3b shows some agglomeration of particles due to a large surface area [37]. The homogeneous distribution of CuO over TiO2 was observed in the SEM image of the nanocomposite, as illustrated in Figure 3c. The appearance of black colour on the surface of the nanocomposite was due to the presence of copper oxide nanoparticles. The average particle size of the nanocomposite was slightly higher than that of the TiO2 nanoparticles due to some agglomeration of CuO with TiO2, which confirmed the formation of the nanocomposite [36,38]. Due to the small size of CuO, there was no change appeared in the spherical geometry of nanocomposite. 3.2. Optimization of operating parameters for As(V) adsorption On the basis of characterization results, we have selected 5 wt. % CuO/TiO2 composite for optimization of different reaction parameters such as pH, contact time, adsorbent dose, and initial As(V) concentration due to its better composition, phase purity and textural features. 3.2.1. Effect of pH The pH of the solution always plays a vital role in influencing the adsorption capacity of an adsorbent. The effect of pH on adsorption efficiency is much more pronounced than any other parameter because it directly concerns the surface charge of an adsorbent. It always increases adsorption by developing electrostatic attraction between oppositely charged surfaces of the adsorbent and pollutants. In addition to this, the type of arsenic species in the water will also vary with changes in pH [39]. The effect of pH on As(V) adsorption rate had been studied by varying in range of 1 to 9 (Operating conditions: C0 = 100 ppb; 5 wt. % CuO/TiO2 dose = 0.2 g; contact time = 2 hours; temperature = 298 K). 288 Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 (a) (b) (c) Figure 3. SEM images of (a) TiO2, (b) CuO, (c) and 5wt. % CuO/TiO2. Figure 4. Effect of solution pH on As(V) adsorption rate on 5 wt. % CuO/TiO2. Figure 4 represents the variations in the % removal efficiency as a function of the pH of the solution. The result shows that the removal efficiency increased when the pH increased from 1 to 3. This is because, at pH = 3, H2AsO4-1 is the only predominant As(V) species that exist in water and the surface of the nanocomposite is also protonated in an acid medium leading to a positively charged surface. This results in a strong electrostatic attraction between negatively charged arsenic species (H2AsO4-1) and positively charged nanocom- posite surfaces [40]. Further increase in pH leads to a decrease in removal efficiency. The possible reason for this is the increased electrostatic repulsion between negatively charged As(V) species (H2AsO4-1 or HAsO4-2) and the negatively charged nanocomposite surface developed in basic medium. Therefore, the removal efficiency of the nanocomposite decreased at higher pH values [41]. The optimum value pH for As(V) removal was found to be 3. 3.2.2. Effect of contact time The effectiveness of the adsorbent is also measured in terms of contact time. The dependence of % removal efficiency on contact time was investigated by varying time from 1 to 5 hours (Operating conditions: C0 = 100 ppb; 5 wt. % CuO/TiO2 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 9 10 Re m ov al e ffi ci en cy (% ) pH Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 289 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 Figure 5. Effect of contact time on As(V) adsorption rate on 5 wt. % CuO/TiO2. Figure 6. Effect of adsorbent dose on As(V) adsorption rate on 5 wt. % CuO/TiO2. dose = 0.2 g; pH = 3; temperature = 298 K) as shown in Figure 5. It was observed that the adsorption rate of As(V) increased sharply by the varying shaking time from 1 to 2 hours and become almost constant when the shaking time was further increased from 2 to 5 hours. It was concluded that equilibrium was established between the adsorbent and the adsorbate within the initial 2 hours. An increased shaking time did not show any effect on the equilibrium of arsenate adsorption on the nanocomposite surface [42]. This is because the surface area of the nanocomposite becomes rapidly saturated and no more sites are available for the adsorption of As(V) [43]. This result demonstrated the efficient affinity of the CuO/TiO2 nanocomposite for As(V). 3.2.3. Effect of the nanocomposite dose Studying the impact of adsorbent dose on adsorption capacity is way more pronounced in economic terms. The effect of nanocomposite dosage on As(V) removal rate had been studied by varying amounts of 5 wt. % CuO/TiO2 nanocom- posite from 0.05 to 0.3 g (Operating conditions: C0 = 100 ppb; pH = 3; contact time = 2 hours; temperature = 298 K) as shown in Figure 6. The adsorption rate increased rapidly with an increase in the adsorbent dose to 0.2 g. It has been reported [36] that increased nanocomposite dose provides more active sites for As(V) adsorption which result in an enhanced adsorption rate. However, an additional increase in the dose of nanocom- posites causes agglomeration of particles, as a result, decreasing the adsorption efficiency. Since the adsorption performance had not been improved with further increase in nanocomposite dose, therefore 0.2 g was considered as an optimized nanocomposite dose. 3.2.4. Effect of As(V) initial concentration It is necessary from an industrial point of view to study the effect of the initial concentration of As(V) initial concentration on the adsorption rate. The effect of the initial concentration on its adsorption rate had been studied by varying the concentration of solution from 10 to 1000 ppb (Operating conditions: 5 wt. % CuO/TiO2 dose = 0.2 g; pH = 3; contact time = 2 hours; temperature = 298 K) as shown in Figure 7. The result demonstrates that the adsorption of As(V) was not much affected by the increase in initial concentration. One of the possible reasons is that the presence of CuO in the nanocomposite enhanced the surface area, which results in higher adsorption capacity [44]. Moreover, the nanocomposite was found to be a promising candidate for the removal of As(V) at both very high and low concentration levels and can be employed for the treatment of contaminated drinking water where As(V) present at very low concentrations (10 ppb). 3.3. Isothermal studies The adsorption isotherm developed an equilibrium relationship between the concentration of adsorbate in solution and at the surface of adsorbent at a constant temperature [45]. For this purpose, the initial As(V) concentration was varied from 10-1000 ppb (Operating conditions: 5 wt. % CuO/TiO2 dose = 0.2 g; pH = 3; contact time = 2 hours; temperature = 298 K) to study the mechanism of As(V) adsorption on the surface of the nanocomposite. The well-known Langmuir and Freundlich adsorption isotherms were used to investigate the adsorption capacity of 5 wt. % CuO/TiO2 nanocomposite. 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 R em ov al e ffi ci en cy (% ) Contact time (h) 0 10 20 30 40 50 60 70 80 90 100 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 R em ov al e ffi ci en cy (% ) Adsorbent dose (g) 290 Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 Table 2. Calculated Langmuir parameters for 5 wt. % CuO/TiO2 nanocomposite. Langmuir parameters % Removal efficiency Qmax 90 r2 0.9 RL 0.9 Table 3. Calculated Freundlich parameters for 5 wt. % CuO/TiO2 nanocomposite. Freundlich parameters % Removal efficiency Kf 0.82 N 1 r2 1 Table 4. Maximum adsorption capacity of various adsorbents for As(V) removal. Adsorbent Adsorption capacity (mg/g) References Nano-CuO & Nano-TiO2 36.39 & 29.24 [47] CuO 1.17 [48] Fe2O3-MnO2 16.6 [49] γ- Fe2O3-TiO2 33.03 [50] Fe2O3@CuO&GO 62.60 [51] Magnetic graphene oxide 69.44 [52] 5 wt. % CuO/TiO2 90 This study Figure 7. Effect of initial arsenic ion concentration on As(V) adsorption rate on 5 wt. % CuO/TiO2. 3.3.1. Langmuir adsorption isotherm The Langmuir isotherm presumed that monolayer adsorption takes place on the adsorbent surface having a finite number of indistinguishable sites [46]. The adsorption energy is not affected by the amount of surface coverage in this model. K and Qmax were determined using the slope (1/Qmax) and intercept (1/KQmax) values obtained from the plot of Ce/Qe against Ce. To ensure that As(V) is homogeneously adsorbed at the available nanocomposite sites, the RL value was calculated from Equation (4). The RL value and other calculated parameters are given in Table 2. The RL value indicates that the As(V) adsorption over 5 wt % CuO/TiO2 nanocomposite was favourable. 3.3.2. Freundlich adsorption isotherm The Freundlich adsorption isotherms assumed that multilayer adsorption occurs on the surface of the adsorbent. This isotherm suggested that the ratio of adsorbate concentration at adsorbent surface to solution does not remain constant at different concentrations of solution [46]. The linear equation of Freundlich isotherm was used to determine Kf and other n for nanocomposite by plotting log (Qe) against log(Ce). The calculated Freundlich parameters for 5 wt% CuO/TiO2 nanocomposites are displayed in Table 3. Based on the correlation coefficient value (R2), it has been clear that the Freundlich isotherm fits well to demonstrate the adsorption mechanism of As(V) on the nanocomposite surface. For favorable adsorption, the value of Freundlich’s constant “n” must be in the range of 1-10 [45]. As the value of “n” was found high enough to indicate the better separation of As(V) from the aqueous solution, so it was suggested that the physical adsorption occurs, and the nanocomposite possesses a high affinity for As(V). The value of Freundlich parameters demonstrated adsorption capacity of the 5 wt. % CuO/TiO2 nanocomposite has been found greater for As(V). 3.4 Regeneration of nanocomposite The low removal efficiency of As(V) at pH = 9 implied that adsorbed arsenic ion can be effectively removed from the surface of 5 wt. % CuO/TiO2 nanocomposite in basic medium. In a basic medium, the surface of the nanocomposite becomes negatively charged due to the increase in hydroxyl ions, which results in the desorption of negatively charged arsenate species (H2AsO4-1 or HAsO4-2) [41]. The presence of NaOH in solution causes rapid de-protonation of nanocomposite surface which causes a rapid increase in the desorption rate of As(V). For this purpose, As(V) loaded CuO/TiO2 nanocomposite was stirred for one hour in 0.5 M NaOH solution. The adsorption and desorption efficiency of the nanocomposite was calculated from Equations (1) and (6) and the values are presented in Figure 8. The As(V) adsorption and desorption rate on the surface of 5 wt. % CuO/TiO2 nanocomposite remained almost constant during each run of the desorption-adsorption cycles. 3.5. Comparison of adsorption capacity of 5 wt. % CuO/TiO2 nanocomposite with already reported adsorbents The maximum adsorption capacity of 5 wt. % CuO/TiO2 nanocomposite and other adsorbents for As(V) removal are summarized in Table 4 which compares the adsorption capacity of 5 wt. % CuO/TiO2 nanocomposite with that of other 95.0 95.5 96.0 96.5 97.0 97.5 98.0 98.5 99.0 99.5 100.0 0 200 400 600 800 1000 1200 R em ov al e ffi ci en cy (% ) Initial arsenic ion concentration (ppb) Farooq et al. / European Journal of Chemistry 13 (3) (2022) 284-292 291 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.3.284-292.2283 adsorbents reported in the literature, as calculated from Langmuir isotherm. The results in Table 4 demonstrate that 5 wt. % CuO/TiO2 nanocomposite provided higher adsorption capacity for As(V) removal than several other adsorbents reported in the literature. Figure 8. Desorption and adsorption efficiencies of 5 wt. % CuO/TiO2 nanocomposite. 4. Conclusion Cost-effective CuO/TiO2 nanocomposites (1, 3 and 5 wt. %) having enhanced surface area and high adsorption capacity were prepared by the precipitation-deposition method. Impor- tant parameters such as pH, contact time, arsenic ion concent- ration, and nanocomposite dose were optimized to enhance As(V) adsorption over the CuO/TiO2 nanocomposite. The results of Langmuir and Freundlich isotherms investigations indicate that Freundlich adsorption isotherm produces the best fit. The nanocomposite was successfully regenerated by treatment with NaOH solution and can be reused for three cycles without any prominent loss in adsorption efficiency. The CuO/TiO2 nanocomposite showed remarkable adsorption efficiency in arsenic decontamination. Acknowledgment The authors are highly thankful to Quaid-i-Azam University, Islamabad and National Centre for Physics, Islamabad Pakistan for providing the chemicals, laboratory facilities and sample characterization for this project. Disclosure statement Conflict of interests: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered. Sample availability: Samples of the compounds are available from the author. CRediT authorship contribution statement Conceptualization: Saima Farooq, Sobia Ashraf; Methodology: Asima Siddiqa, Sobia Ashraf; Validation: Shabnam Shahida, Sara Qaisar; Formal Analysis: Saiqa Imran, Sara Qaisar; Investigation: Sobia Ashraf, Sabtain Haider; Data Curation: Sabtain Haider; Writing - Original Draft: Saima Farooq, Sabtain Haider; Writing - Review and Editing: Saiqa Imran, Asima Siddiqa; Supervision: Shabnam Shahida; Project Administration: Asima Siddiqa. ORCID and Email Saima Farooq saima@unizwa.edu.om https://orcid.org/0000-0002-5037-0769 Asima Siddiqa a.sam.malik@gmail.com https://orcid.org/0000-0002-3087-8047 Sobia Ashraf sobiakhanashraf@gmail.com https://orcid.org/0000-0002-8744-6317 Sabtain Haider hsibtain04@gmail.com https://orcid.org/0000-0002-7405-4824 Saiqa Imran saiqa134@gmail.com https://orcid.org/0000-0002-0289-4736 Shabnam Shahida shabnamshahida@upr.edu.pk https://orcid.org/0000-0001-9861-4569 Sara Qaisar sara.qaisar@ncp.edu.pk https://orcid.org/0000-0002-3724-9665 References [1]. Shaji, E.; Santosh, M.; Sarath, K. V.; Prakash, P.; Deepchand, V.; Divya, B. V. Arsenic contamination of groundwater: A global synopsis with focus on the Indian Peninsula. Geosci. front. 2021, 12, 101079. [2]. Wongsasuluk, P.; Chotpantarat, S.; Siriwong, W.; Robson, M. Human biomarkers associated with low concentrations of arsenic (As) and lead (Pb) in groundwater in agricultural areas of Thailand. Sci. Rep. 2021, 11, 13896. [3]. Moreira, V. 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The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Reagents 2.2. Synthesis of CuO/TiO2 nanocomposites 2.3. Characterization 2.4. Batch adsorption experiments 2.5. Isothermal analysis 2.6. Desorption of arsenic 3. Results and discussion 3.1. Material characterization 3.1.1. XRD analysis 3.1.2. FT-IR analysis 3.1.3. SEM analysis 3.2. Optimization of operating parameters for As(V) adsorption 3.2.1. Effect of pH 3.2.2. Effect of contact time 3.2.3. Effect of the nanocomposite dose 3.2.4. Effect of As(V) initial concentration 3.3. Isothermal studies 3.3.1. Langmuir adsorption isotherm 3.3.2. Freundlich adsorption isotherm 3.4 Regeneration of nanocomposite 3.5. Comparison of adsorption capacity of 5 wt. % CuO/TiO2 nanocomposite with already reported adsorbents 4. Conclusion Acknowledgment Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: