Degradation of sodium isopropyl xanthate from aqueous solution ...

Journal of Environmental Management 211 (2018) 225e237

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Research article

Degradation of sodium isopropyl xanthate from aqueous solution using sonocatalytic process in the presence of chalcocite nanoparticles: Insights into the degradation mechanism and phytotoxicity impacts Alireza Khataee*, Rana Honarnezhad, Mehrangiz Fathinia Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 October 2017 Received in revised form 12 January 2018 Accepted 19 January 2018

In the present work, the sonocatalytic degradation of sodium isopropyl xanthate (SIPX) was investigated in the presence of Cu2S nanoparticles. Cu2S nanoparticles were produced by means of a high-energy planetary mechanical ball milling method within the processing times of 0.5, 1.5, 3 and 4.5 h. The physical and chemical characteristics of Cu2S particles were studied before and after ball milling process using various analytical techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) coupled Energy-dispersive X-ray spectroscopy (EDX), atomic absorption spectroscopy (AAS) and nanoparticles size distribution (NSD). The XRD pattern of the samples confirmed the presence of tetragonal and cubic crystalline phases of Cu2S. In addition, the results of SEM and NSD analysis showed that the increase in the ball milling time from 0.5 to 4.5 h notably decreased the size of nanoparticles to the range of 20e40 nm. Furthermore, AAS result showed that the concentration of Cuþ ions was much lower than that of the accepted value in the aqueous media (0.009 mg/L) after 60 min of the sonocatalysis. The study on the effects of the main key parameters showed that 93.99% of SIPX (10 mg/L) was removed during 60 min of the sonocatalytic process under the optimum conditions: pH of 7.3, Cu2S concentration of 1.5 g/L, and ultrasonic power of 150 W. The sonocatalytic degradation mechanism was thoroughly examined in the presence of different organic and inorganic scavenger compounds, including ethanol, EDTA, NaCl and Na2SO4. The obtained results confirmed OH and holes (hþ) as the dominant oxidizing species in Cu2S catalyzed sonolysis. In order to get the benefits of the integrated sonocatalytic process, different rate enhancing compounds were 2 introduced into the system. For the first time, the S2O2 8 and Cu2S catalyzed sonolysis (US/Cu2S/S2O8 ) system was introduced as an efficient and novel sonocatalytic system for fast degradation of SIPX. Moreover, the phyto-toxicological assessments proved the reduction in the toxicity of the sonocatalytictreated SIPX solution by increase in the reaction time, from 20 to 60 min. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Ball milling Chalcocite Degradation pathways Persulfate Sodium isopropyl xanthate Sonocatalysis

1. Introduction Mining industries play an important role in the massive production of raw materials. Though, the various stages of metal extraction and separation introduce hazardous organic and inorganic contaminants into the environment. Chemical collectors such as sodium xanthate salts are among the most widely used

* Corresponding author. E-mail address: [email protected] (A. Khataee). https://doi.org/10.1016/j.jenvman.2018.01.054 0301-4797/© 2018 Elsevier Ltd. All rights reserved.

compounds in mining industries at selective separating of sulfide minerals by froth flotation. Sodium xanthate salts are also utilized in some other processes, such as cellulose synthesis, pesticide manufacturing, and as a corrosion inhibitors in engine oil additives (Lotter and Bradshaw, 2010; Molina et al., 2013). The wide application of sodium xanthate salts, especially in mining industry, has caused its continuous occurrence in mining wastewater. These compounds are harmful to biota and have an extended inhibitory effect on nitrifying bacteria. Hence, it is necessary to remove sodium xanthate salts from mining wastewater before its discharge to the environment (Lotter and

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Bradshaw, 2010; Molina et al., 2013). The ability for degrading a wide range of organic compounds is the most prominent feature of the advanced oxidation processes (AOPs) (Bustillo-Lecompte and Mehrvar, 2016; Kumar and Rao, 2017; Orge et al., 2012). Heterogeneous sonocatalytic process (HSP) using solid nanocatalysts is one of the recently-introduced AOPs which is recently used for treatment of polluted effluents such as halogenated hydrocarbons, pesticides, and dyes from aqueous solutions (Khataee et al., 2015). Most of the shortcomings accompanied sonolysis alone can be overcome by addition of suitable nanocatalyst to the process (Banerjee et al., 2012; Wang et al., 2011; Zhang et al., 2018). Moreover, nanocatalysts provide more active sites, which lead to the generation of higher quantities of reactive radicals, thus increasing the degradation rate of the pollutant. The remarkable degradation efficiency, complete mineralization of the target compounds, low power consumption, easy separating and reusability of the catalyst and simple operation of the system are a number of the merits of HSP application (Khataee et al., 2015; Wang et al., 2014). A large number of nanomaterials of different morphologies have been used in HSP for degradation of various organic pollutants from water bodies. Nevertheless, the utilization of semiconductor materials in this process was hardly reported (Shimizu et al., 2007; Wang et al., 2008). Recently, copper sulfide (CuxS) compounds have received particular attention (Grozdanov and Najdoski, 1995) for optical and electrical applications. Among the CuxS compounds, such as chalcocite (Cu2S), djurleite (Cu1.9375S), anilite (Cu1.75S), and covellite (CuS), Cu2S is of great interest owing to its special properties and potential applications. Cu2S is a p-type semiconductor, with a direct band gap of 1.2 eV (Gorai et al., 2004). The availability of Cu2S nanostructures with well-defined morphologies and dimensions enables new type of applications and/or enhancing the performance of the currently existing photoelectric devices due to the quantum size effects (Du et al., 2006). So far, Cu2S nanoparticles were prepared in diverse morphologies, but by utilizing different costly materials and through complex methods. Moreover, there is no report on the production of Cu2S nanomaterials with a highenergy planetary ball milling method. This method is simple, cost-effective and can efficiently generate fine and uniform nanostructures during a short processing time. These advantages cannot be achieved using conventional chemical synthesis methods for production of large amount of nanomaterials (De Carvalho et al., 2013; Xu et al., 2013). For example, Farhadi and Siadatnasab (2016) synthesized Cu2S nanoparticles via thermal decomposition of Cu(II) diethyldithiocarbamate complex using an electric furnace at 220 C for 0.5 h. The size of the produced nanoparticles was in the range of 40e60 nm. In another work (Mousavi-Kamazani et al., 2013), Cu2S nanoparticles were synthesized via an ultrasonicassisted method by employing Na2SO3 as a reducing agent at 80 C for 5 h. The average crystallite diameter of the produced nanoparticles was about 25 nm. Obviously, in the above synthesis methods of the nanomaterials, the processes require higher operating temperatures with relatively costly substrates. In the present work, for the first time, the degradation of sodium isopropyl xanthate (SIPX) was investigated via heterogeneous sonocatalytic process in the presence of Cu2S nanoparticles. These nanoparticles were produced in large scale by mechanical highenergy planetary ball milling method using natural Cu2S sample, at different time sets of ball milling namely, 0.5, 1.5, 3 and 4.5 h. In order to examine the physical and chemical characteristics of the unmodified sample and the produced nanoparticles, the X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy EDX, nanoparticles size distribution (NSD) and atomic

absorption spectroscopy (AAS) were fulfilled. Subsequently, the effects of the main operational parameters including milling time, pH, catalyst concentration, reusability and stability, ultrasonic power and SIPX initial concentration were investigated on its removal efficiency. Afterwards, the presence of different compounds such as enhancers and scavengers were studied on the removal efficiency. The efficiency of the degradation system in the presence of ethanol was assessed under different ultrasonic power. Moreover, the degradation mechanism was proposed using the data obtained from the degradation steps. Finally, the phyto-toxicological effects of the treated and untreated SIPX samples during the degradation process were examined on an aquatic species. 2. Experimental 2.1. Chemicals Sodium isopropyl xanthate (SIPX) was obtained from Yantai Lunshun Huitong Biotechnology Co (China). The properties of SIPX are presented in Table S1. As it is given in Table S1, its molecular weight (g/mol) and lmax (nm) is 158.22 and 300, respectively. NaOH, H2O2, K2S2O8, EDTA, KIO4, NaCl, Na2SO4 and ethanol were obtained from Merck Co (Germany). 2.2. Preparation of Cu2S nanoparticles Chalcocite ore was extracted from Sungun copper mine and then crushed by jaw and cone crushers. The obtained sample was crushed further by rod and ball milling to reduce the size of Cu2S sample. Finally Cu2S microparticles were subjected to a highenergy planetary ball mill (RETSCH - PM400, Germany) at a rotation speed of 320 rpm for 0.5, 1.5, 3 and 4.5 h to prepare nanostructured Cu2S. The ball milling process was performed under ambient conditions (25 C and 1 atm). Different balls with sizes of 1 and 2 mm (10 balls with the size of 1 mm and 5 balls with the size 2 mm) were used in the milling procedure. Both the balls and bowl of the ball milling process were made from stainless steel. The ratio of ball-mass to powder-mass was chosen 10:1. 2.3. Sonocatalytic procedure The sonocatalytic degradation of SIPX was done in ultrasonic machine Ultra-8060D-H, operating at an ultrasonic frequency of 36 KHz and output power of 150 W. Ice cubes were used to adjust the bath water temperature at 25 C. For each test, particular dosage of sonocatalyst particles was added to the Erlenmeyer flask containing 100 ml of SIPX solution at specified concentration. The original pH of SIPX solution was 7.3, and the experiments were conducted at this pH value without any pH adjustment. All experiments were conducted in dark to prevent photo excitation of chalcocite nanoparticles. At a defined time interval, a small portion (2 ml) of the reaction solution was taken out and then 2 ml ethanol was added in it to prevent OH radicals oxidation. After sedimentation of sonocatalyst particles, the top clear solution was used to examine SIPX concentration. Solution absorbance was measured at l ¼ 301 nm by using UVevisible spectrophotometer (Analytic Jena, Specord 250, Germany). The removal efficiency was calculated by this equation: (A0-At)/A0 100%, where A0 and At are the SIPX absorbance before and after sonocatalytic process. To investigate the influence of some inorganic and organic ions, they were added separately to the SIPX solution. 2.4. Characterization of the catalyst In the present work, the following analyses were performed to


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thoroughly investigate the physical and chemical properties of the samples. XRD analysis was used to study the crystalline phase of the samples using an X-ray powder diffraction (Siemens, D5000, Germany) in the range of 10 2q 80 at accelerating voltage of 40 kV and an applied current of 30 mA. SEM coupled with EDX microanalysis (Mira3 FEG SEM, Tescan, Czech Republic) was utilized for morphological studies. Digimizer v4.1.1.0 software was applied for determining the nanopowder particle size distribution. FT-IR patterns of the samples were recorded using FT-IR spectrometer (Tensor 27, Bruker, Germany) from 4000 cm1 to 400 cm1 with KBr pellet technique. AAS (Novaa 400, Analytikjena, Germany) was used to measure the dissolved copper ion concentration in the solution. 2.5. Phyto-toxicological evaluations 2.5.1. Plant growth conditions and phyto-toxicity experiments Lemna minor (L. minor) fronds, used as plant species for phytotoxicological experiments, that were obtained from Alijan Village located 40 km from Bostanabad (East Azarbaijan Province). The plants were disinfected by sodium hypochlorite, washed by distilled water and cultured in Steinberg media (Fathinia and Khataee, 2015; Zezulka et al., 2013). All the phyto-toxicity experiments were performed in a glass beaker under static conditions at 25 C under 16/8 (light/dark) photoperiod (Fathinia and Khataee, 2015). Subsequently, the toxicological effects of four solutions: a) initial SIPX solution before sonocatalytic treatment, SIPX solution after b) 20 min c) 40 min and d) 60 min of sonocatalytic degradation process were assessed on L. minor. For this aim, a 100 mL of 10 mg/L of SIPX solution before and after 20, 40 and 60 min of sonocatalytic treatment process were added to 200 mL beakers, separately. Then, 30 mature fronds of L. minor were added to the above solution. 10 mL of growth medium was added to each beaker. Finally, after one week the fronds were counted in each solution for calculating the relative frond numbers according to Eq. (1).

Relative frond numberðRFNÞ ¼ ðfrond N7 frond N0 Þ ð1=ðfrond N0 ÞÞ

(1)

where N0 and N7 are the number of fronds on 0 and 7 days, respectively (Khataee et al., 2016). 3. Results and discussion 3.1. Characterization of the samples 3.1.1. XRD and FT-IR analysis The crystal structure of the initial chalcocite sample (un-milled) and nanostructured chalcocite after 0.5, 1.5, 3 and 4.5 h of ball milling process were identified by X-ray diffraction (Fig. 1). As can be seen in Fig. 1, XRD pattern of the samples contain diffraction peaks at 2q values of 23.45 , 31.62 , 32.49 , 35.35 , 38.9 , 39.77, 45.26 , 46.03 , 48.2 and 61.35 which are related to the orientations of (101), (110), (111), (112), (104), (113), (200), (201), (202), and (214) planes of the Cu2S tetragonal phase (JCPDS Card No. 72-1071). Moreover, the diffraction peaks observed at 2q values of 27.3 , 31.59 , 56.6 , 66.35 could be indexed to the (111), (200), (222) and (400) planes of the Cu2S cubic phase (JCPDS Card No. 84-1770) (Farhadi and Siadatnasab, 2016; Zhang et al., 2017). It should be noted that since the Cu2S was extracted from a natural ore, identifying two different crystalline phases in the sample structure was quite normal. In addition, from XRD analysis data it can be concluded that the crystalloid nature of the samples was gradually destroyed by increasing the milling time from 0.5 to 4.5 h. Furthermore, the average crystalline size of the Cu2S nanoparticles

Fig. 1. XRD patterns and of un-milled Cu2S and Cu2S nanoparticles, curve a: un-milled; after curve b: 0.5 h; curve c: 1.5 h; curve d: 3 h and curve e: 4.5 h of ball milling process.

were determined using the DebyeeScherrer equation (Farhadi and Siadatnasab, 2016; Zhang et al., 2017). The calculated size of the produced nanocrystallites were found to be 23, 19, 17.5 and 17 nm for the produced nanoparticles at different milling times of 0.5, 1.5, 3 and 4.5 h, respectively. As it can be observed, the size of Cu2S nanocrystallites was decreased by increasing of the ball milling time. However since the particles tend to accumulate with each other, the total size of the nanoparticles were increased as presented in Fig. 2. FT-IR technique is usually applied to study the type of the functional groups on the surface of the powder samples. Fig. S1 shows the results of FT-IR analysis for the un-milled (Spectrum a) and nanostructured samples (Spectra b to e). In these spectra, the absorption bands at 1070e1220 cm1, 1624 cm1 and 3500e3600 cm1 (Mousavi-Kamazani et al., 2013) show the stretching vibrational bands of S¼O, C¼O and O-H species, respectively. The presence of similar functional groups in the nanostructured samples confirmed that the surface properties of the samples remained unchanged after ball milling process. 3.1.2. SEM, EDX and NSD analysis The morphology, type of the elements and mean size of the particles in the un-milled and nanostructured samples produced in different milling periods were investigated by SEM, EDX and NSD analyses, respectively. The results are shown in Figs. 2e4. The obtained images from SEM analysis in different magnifications are presented in Fig. 2a to j. Fig. 2a and b illustrate the SEM images of un-milled Cu2S sample. From the images, the surface of the initial sample contained aggregated coarse particles in the micrometer size. Fig. 2c and d shows that after 0.5 h of ball milling Cu2S particles were produced in nanoscale with a spherical morphology. In addition, a relatively regular distribution of the nanoparticles could be observed in these figures. Fig. 2e to j shows the decreased size of the particles by increasing the milling time. Meanwhile, the particles tended to accumulate and form aggregated assemblies with micro scale. To further investigate the size of the particles produced at different milling time, nanoparticles size distribution (NPS) curves were drawn (Fig. 3a to d). Fig. 3a shows that after 0.5 h of milling process, 53.33% particles are in the range of 20e40 nm. In addition, as can be seen from Fig. 3b to d, the frequency of the particles in the range of 20e40 nm is significantly decreased by increasing the milling time from 1.5 to 4.5 h. Accordingly, the increase in the milling time was not beneficial for producing Cu2S particles in nanoscale.

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EDX is a chemical microanalysis technique, which is used to characterize the elemental composition of the samples. In this work, it was used to identify the main elements of Cu2S before and after ball milling process. The EDX spectrum of un-milled Cu2S sample (Fig. 4a) shows that Cu and S existed on its surface as the main chemical elements. The EDX spectra of 0.5, 1.5, 3 and 4.5 h milled samples are also presented in Fig. 4b to e. These figures illustrate that the chemical composition of Cu2S was not changed after ball milling process and Cu and S presented in the chemical matrix of the sample as the main compositional elements. It is worth to mention that in addition to the main elements, C, O and Mg, Al, S, K, Fe were also appeared in the EDX spectra of the primary (un-milled sample) and the other milled chalcocite samples. The presence of C and O might be related to several factors: (i) the organic pollutants from air could be adsorbed on to the sample during the grinding and milling steps; (ii) the ball milling instrument could be polluted with organic compounds and (iii) other materials such as Al (as impurities) might be involved in the sample extracted from natural ore. 3.2. Degradation of SIPX by sonolysis and sonocatalysis

Fig. 2. SEM images of (a,b): un-milled Cu2S; (c,d): Cu2S nanoparticles after 0.5 h, (e,f): 1.5 h, (g,h): 3 h and (i,j): 4.5 h of ball milling process.

Before investigating the effects of the main operating parameters on the removal efficiency of SIPX, the effect of the milling time was studied on the catalytic efficiency of Cu2S nanoparticles during sonocatalytic process. The obtained results are presented in Fig. 5. In these experiments, the initial concentration of SIPX was 10 mg/L and the pH of the solution was the original pH of SIPX solution (7.3) and was not adjusted during the reaction. Fig. 5, shows that SIPX removal efficiency via sonolysis alone was about 8.38% after 40 min of the process. However, the presence of 1.5 g/L of Cu2S microparticles (un-milled sample) in the process enhanced the SIPX removal efficiency up to 63.68% (Fig. 5) within the same reaction time. Moreover, the SIPX removal efficiency was significantly enhanced up to 86.4%, 94.3% and decreased to 48.5% and 41.7% when 1.5 g/L of 0.5, 1.5, 3 and 4.5 h milled Cu2S nanoparticles were added into the reaction bulk, respectively. It should be pointed out that the physical adsorption of SIPX onto the surface of un-milled and 1.5 h milled Cu2S particles was less than 15%. The control experiments were accomplished with Cu2S nanoparticles in the absence of ultrasonic irradiations and in the absence of any oxidant. This shows that only a small percent of SIPX was removed through physical adsorption process and the major part of degradation was completed by sonocatalysis. To explain the reason and the mechanism behind the observations in Fig. 5, it should be stated that the degradation of the pollutant via heterogeneous sonocatalytic process is accomplished via two pathways. In one pathway, the degradation is accomplished without interfering Cu2S nanoparticles in the homogeneous phase of the reaction under ultrasonic irradiations, which is called sonolysis alone. In this pathway the degradation of the SIPX is performed mainly by OH and OOH radicals which are produced exclusively from pyrolysis reactions of the bulk solvent (H2O) based on the acoustic cavitation theory at the interface of the collapsing bubble and the solvent, according to Eqs. (S1) to (S7) (Adewuyi, 2001; Minero et al., 2005). The other pathway runs in the presence of solid catalyst particles, Cu2S particles in the present work, which promote the efficiency of the process (Banerjee et al., 2012; Wang et al., 2011). Solid catalyst can increase the degradation efficiency of the pollutant via various mechanisms. For example, it can increase the generation rate of the cavitation bubbles due to the additional nuclei. This phenomenon increases the rate of pyrolysis reaction of H2O, which subsequently increases the formation of free radicals. Moreover, the solid nanocatalyst can provide a surface at nano scale for H2O2


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Fig. 3. Nano particle size distribution of Cu2S nanoparticles after (a): 0.5 h, (b): 1.5 h, (c) 3 h and (d): 4.5 h of ball milling process.

molecules to be decomposed to OH radicals that increases the degradation of the adsorbed pollutant on the surface of the catalyst. In the case of heterogeneous nanocatalyst, such as Cu2S, two additional mechanisms could also be involved in the process. The successful and rapid degradation of SIPX via sonocatalytic process using Cu2S nanoparticles can be related to the sono-luminescence phenomena. Since Cu2S is a visible light semiconductor, it can be easily excited by the sono-luminescence and hot spot phenomenon (Farhadi and Siadatnasab, 2016; Soltani et al., 2016). This mechanism occurs because of the acoustic cavitation created from the collapse of the cavitation bubbles. Sono-luminescence spectrum involves a wide range of wavelengths, including visible light. Consequently, Cu2S nanoparticles are excited by both hot spots and sono-luminescence spectrum. This leads to the formation of various oxidizing species such as holes (hþ) and extra OH radicals (Qiu et al., 2014; Zhang et al., 2016). It should be mentioned that both OH and hþ can directly attack the pollutant molecules on the surface of the catalyst. Besides, the produced electrons can efficiently decompose H2O2 molecules to OH, which subsequently increases the concentration of the oxidant species in the system (Zhang et al., 2016). On the other hand, the simultaneous utilization of Cu2S and sonolysis due to the formation of H2O2 leads to the integration of heterogeneous Fenton-like process with sonolysis, which results in their synergistic effects in the system. Accordingly, the transformation of Cu2þ to Cu1þ is increased as given in Eqs. (2) and (3) (Bokare and Choi, 2014). (2) Cu1þ þ H2O2/ Cu2þþ OHþOH

(3)

Furthermore, the reduction of Cu2þ and regeneration of Cu1þ is accelerated due to the presence of ultrasonic irradiations, which lead to the simultaneous synergistic effect and surplus OH in the

system. Consequently, the removal efficiency of SIPX was improved through the heterogeneous sonocatalytic process (Friedrich et al., 2012). As stated earlier, the higher SIPX removal efficiency was obtained in the case of 1.5 h milled Cu2S sample compared with that of 0.5, 3 and 4.5 h milled Cu2S nanoparticles. This difference in SIPX removal efficiency could be related to the variations in the size of the particles. Based on the results, (see Figs. 2 and 3), the increase in the ball milling duration developed the aggregated assemblies at micro scale, which reduced the number of the active sites on the surface of the catalyst and decreased the removal efficiency. On the other hand, since there was no significant difference in the removal efficiency of SIPX in the presence of both 0.5 and 1.5 h milled Cu2S nanoparticles, further experiments were performed with 0.5 h milled Cu2S nanoparticles. 3.3. The influencing factors in sonocatalytic process 3.3.1. Effect of pH The pH of the solution is one of the most important factors since it determines both the surface properties of catalyst and nature of the pollutant in the solution (Devi and Kumar, 2012; Jamalluddin and Abdullah, 2014). In this study, the catalytic performance of Cu2S nanoparticles was investigated by varying the pH of the SIPX solution from 7.3 to 8.5 and 10, to clarify the effect of the initial pH of the solution. It has been reported that acidic pH values lead to the hydrolysis and formation of different forms of SIPX in aqueous solution such as xanthic acid, which can be decomposed as well (Iwasaki and Cooke, 1958; Sihvonen, 2012). The formation of the above mentioned species under the acidic conditions change the UVeVis spectrum of SIPX. More precisely, acidic condition changes the intensity and the location of lmax in UVeVis spectrum (See Fig. S2). Since in the present work, the removal efficiency of SIPX was monitored by recording its UVeVis pattern. Accordingly, the

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Fig. 4. EDX spectrum of (a): un-milled Cu2S; (b): Cu2S nanoparticles after 0.5 h, (c): 1.5 h, (d): 3 h and (e): 4.5 h of ball milling process.

removal efficiency of SIPX was only examined under alkaline conditions, as given in Fig. 6a. From the figure, there was no significant difference in the removal efficiencies under different pH values. The slight increase in the degradation levels under pH 8.5 and 10 could be attributed to the facilitated formation of various oxidizing species due the higher amounts of adsorbed OH groups on the surface of the catalyst. Based on the results, the original pH of SIPX was selected as the optimum pH for next experiments.

3.3.2. Effect of catalyst dosage, reusability and stability From economical point of view, the nanocatalyst dosage is among the most important parameters in sonocatalytic process that should be taken into account. In this work, in order to optimize the amount of the nanocatalyst, 0.5e2 g/L of 0.5 h milled Cu2S nanoparticles were used at the operating ultrasonic power of

150 W for degradation of 10 mg/L SIPX at pH of 7.3, during 60 min of reaction (Fig. 6b). The removal efficiency of SIPX was continuously enhanced by increasing the catalyst concentration up to 1.5 g/L. The reason was that the increased amount of the nanocatalyst formed surplus nucleation sites for the cavitation process (Khataee et al., 2017b). Consequently, the amount of oxidant species such as hþ and OH are increased, leading to the enhanced removal efficiency. However, further enhancement in the catalyst dosage up to 2 g/L did not significantly alter the removal efficiency due to the following reasons: (a) decreased active sites in the aggregated assemblies at higher nanocatalyst concentrations; (b) decreased occurrence of sono-luminescence, hot spot phenomena and reduced amounts of cavitation bubbles due to


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result, the catalyst produced during 0.5 h of ball milling time was used for further experiments as a stable sonocatalyst. On the other hand, the stability of the optimum catalyst employed in four repeated runs was investigated by XRD analysis and the results are reported in Fig. 6d. As it can be observed in this figure, the XRD pattern which illustrates the crystallite nature of the catalyst was not significantly changed after sonocatalytic process. This can be another proof for the stability of the Cu2S nanoparticles during the sonocatalytic process.

Fig. 5. Removal efficiency of SIPX in curve a: adsorption process, curve b: sonolysis process and sonocatalysis process in the presence of Cu2S nanoparticles after: curve c: 4.5 h; curve d: 3 h; curve e: un-milled Cu2S; curve f: 0.5 h; curve g: 1.5 h ball of ball milling process. Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min.

the adsorption of the available ultrasonic irradiations at higher nanocatalyst dosages (Hapeshi et al., 2013; Min et al., 2012); Similar results were also reported in other studies (Liu et al., 2016). Accordingly, 1.5 g/L of Cu2S was selected as the optimum amount for the next tests. One of the key factors for development and application of a catalyst at industrial scale is the stability and reusability of the catalyst (Khataee et al., 2017a). Therefore, the reusability of the catalyst was evaluated during four cycles of the sonocatalytic process under the identical operating conditions. After each run, the Cu2S nanoparticles were isolated from the solution, washed, dried, and applied for the next experiment. The degradation efficiency of SIPX during the four successive runs is presented in Fig. S3. As shown in Fig. S3, the degradation efficiency reduced from 93.9 to 87.5% after four repeated runs. The results confirmed the durability of 0.5 h ball milled Cu2S. On the other hand, the AAS was used to measure the concentration of dissolved Cu ions. This test was carried out to control the stability of the catalyst and to check whether the catalyst is dissolved in the solution media or is stable under the applied conditions (Fig. S4). The figure shows that the extended contact time of the catalyst with distilled water (in the absence of SIPX) gradually increased the leaching of Cu ions to the solution. The maximum amount of leached Cu ions in the solution was 0.009 mg/L after 60 min that was much lower than the accepted value for Cu in the aqueous media (1.3 mg/L) (Council, 2000). The obtained results established the acceptable reusability and durability of the Cu2S nanoparticles produced during 0.5 h of ball milling process under the operated conditions. On the other hand, Figs. 2 and 3 show that the size of the particles was considerably decreased to nanoscale by increasing the ball milling time and the particles tended to agglomerate which leads to the generation of microparticles. Consequently, the effects of the ball milling time and size of the produced nanoparticles were investigated on the amount of the leached Cu ions into the solution. The obtained results are presented in Fig. 6c. This figure shows that the increase in the ball milling time from 0.5 to 4.5 h gradually increased the concentration of Cu ions in the solution from 0.0086 to 0.18 mg/L, respectively. Besides, the reported amounts in Fig. 6c were lower than that of the accepted value for Cu ions in aqueous media. However, the ball milling time influenced the stability of the produced particles. As a

3.3.3. Effect of ultrasonic power The ultrasonic power is among the important parameters that notably influence the SIPX removal efficiency. Fig. 6e demonstrates the sonocatalytic degradation of SIPX as a function of time using 0.5 h milled nanostructured Cu2S and ultrasonic powers of 150, 300 and 400 W, respectively. From Fig. 6e, the removal efficiency of SIPX was gradually increased by increasing the ultrasonic power. From the literature review, increasing the ultrasonic power improves the pollutant removal efficiency due to enhanced cavitation elements at higher ultrasonic power values (Khataee et al., 2016). It should be noted that, the cavitation at lower ultrasonic power is more violent than the cavitation at higher intensities (Khataee et al., 2015, 2016). Accordingly, higher localized temperatures and pressures are produced at the cavitation sites formed under low powers. However, the optimum ultrasonic power is specific for each system and it depends on both photon energy and photon number generated due to the hotspot and sono-luminescence phenomena (Khataee et al., 2015, 2016). On the other hand, at lower ultrasonic powers, the photon energy is enhanced while, the number of photon are increased at higher ultrasonic powers (Zhou et al., 2014). It is believed that in the presence of a narrow band gap semiconductor like Cu2S (1.2 eV), the cavitation elements are increased at higher ultrasonic intensities, which lead to the increased amount of electron and hole pairs and enhanced degradation efficacies (Farhadi and Siadatnasab, 2016; Khataee et al., 2016; MousaviKamazani et al., 2013; Zhang et al., 2017). Moreover, the diffusion of mass transfer is greatly increased at higher ultrasonic powers that is beneficial for degradation of SIPX (Khataee et al., 2017a). Although, the increased ultrasonic power increases the number of cavitation elements and degradation rate, the system turbulence increases at high ultrasonic powers and requires more electric power to generate cavitation (Park et al., 2017). Therefor 150 W was chosen as the suitable ultrasonic power from economic point of view in the present work. 3.3.4. Effect of SIPX initial concentration Fig. 6f shows the effect of SIPX concentration on its sonocatalytic removal efficiency at different dosages of 10, 15, 20 and 25 mg/L, where the other parameters were used at their optimum values. From the figure, the SIPX removal efficiency at the initial concentration of 10 mg/L was larger than that of 15, 20 and 25 mg/L. This was because the number of SIPX molecules on the surface of the catalyst was increased as its concentration increased, thus reducing the amount of the adsorbed H2O and OH on the active sites. Consequently, the amount of the available OH radicals was significantly decreased. Simply, due to the increased concentration of SIPX and unchanged amount of the produced OH under the constant operating conditions, the removal efficiency was decreased. 3.4. Elucidating the degradation mechanism of the sonocatalytic process 3.4.1. The role of scavengers One simple way to study the type of the determinative oxidant species is to quench them with the use of an appropriate inorganic

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Fig. 6. (a): Effect of initial pH on SIPX sonocatalytic removal efficiency; Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, US Power ¼ 150 W, Time ¼ 60 min; (b): Effect of the initial catalyst dosage on SIPX sonocatalytic removal efficiency; Experimental conditions: [SIPX]0 ¼ 10 mg/L, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min; (c): Dissolved concentration of copper ions of different nanoparticles produced during 0.5, 1.5, 3 and 4.5 h of ball milling process in reaction bulk after 60 min; Experimental conditions: [Catalyst] ¼ 1.5 g/L, US Power ¼ 150 W, Time ¼ 60 min; (d): XRD patterns of 0.5 h ball milled Cu2S nanoparticles curve a: after and curve b: before its utilization in sonocatalytic process;.(e): Effect of the ultrasonic power on SIPX sonocatalytic removal efficiency; Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, pH ¼ 7.3, Time ¼ 60 min; (f): Effect of the initial SIPX concentration on SIPX sonocatalytic removal efficiency; Experimental conditions: [Catalyst] ¼ 1.5 g/L, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min.

or/and organic scavenger. The kinetic rate constant of the reaction of these compounds with the target oxidant species is higher than that of the pollutant molecules (Adewuyi, 2001; Neta et al., 1988). In this work, two organic and two inorganic compounds including ethanol, EDTA, NaCl and Na2SO4 were used to identify the main radical species. Table S2 shows the kinetic rate constant of these compounds with the specific ROSs (Buxton et al., 1988; Neta et al., 1988). Generally, the obtained data helps for proposing the possible

degradation mechanism of the pollutant. From the obtained data presented in Fig. 7a, the addition of EDTA significantly retarded the sonocatalytic degradation of SIPX. The decreased degradation level can be attributed the elimination of the adsorbed sono-generated holes by EDTA molecules, due to the spatial effects of EDTA molecules, as given in Table S2. This implies the major role of the sono generated hþ both as an oxidizing agent and as a source of OH radicals in the sonocatalytic process. This outcome was also


233

Fig. 7. (a): Effect of scavenging compounds on SIPX sonocatalytic removal efficiency; Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, [Radical scavenger] ¼ 0.06 mM, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min; (b): Effect of ethanol on SIPX sonocatalytic removal efficiency. Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, [Ethanol] ¼ 0.06 mM, pH ¼ 7.3, Time ¼ 60 min. (c): The changes in the UVeVis spectrum of 10 mg/L of SIPX in the absence and (d): in the presence of ethanol. Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, [Ethanol] ¼ 0.06 mM, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min; (e): Effect of enhancing compounds on SIPX sonocatalytic removal efficiency in the absence of US irradiation; curve a: H2O2, curve b: K2S2O8, curve c: KIO4 and curve d: without enhancer and in the presence of US irradiation; curve e: H2O2, curve f: K2S2O8 and curve g: KIO4. Experimental conditions: [SIPX]0 ¼ 10 mg/L, [Catalyst] ¼ 1.5 g/L, [Enhancer] ¼ 0.06 mM, pH ¼ 7.3, US Power ¼ 150 W, Time ¼ 60 min.

reported in the similar studies present in this area (Farhadi and Siadatnasab, 2016). Fig. 7a demonstrates that the removal efficiency of SIPX was significantly diminished in the presence of SO2 4 ions from 93.99% to 24.04% during 60 min. The SO2 4 ions are also a unique scavenger for both adsorbed hþ and OH radicals. Consequently, the

degradation levels considerably decreased due to the bilateral impact of SO2 4 ions, as given in Eqs. (4) and (5) (Burns et al., 1999; Yuan et al., 2013). (4)

234


(11)

(5) SO2 4

ions, Fig. 7a displays that Cl ions had also Similar to negative effects on SIPX degradation efficiencies. The Cl ions are receptors of OH radials with a high kinetic constant, as given in Table S2. These ions react with OH radicals in different ways, leading to the formation of various anion radicals (Gr ci c et al., 2010; Khataee et al., 2017a). For example, HOCl can be generated as presented in Eq. (6). (6)

(7)

(8) Although, the radicals that are generated in the presence of SO2 4 and Cl ions, are also able to oxidize SIPX molecules to some extent (Eq. (4) to (8)), their oxidation potentials are much lower than that of OH radicals (Zhou et al., 2015) thus, reducing the degradation efficiency of SIPX. Ethanol is another well-known organic OH quencher (Khataee et al., 2017a). Fig. 7a illustrates that ethanol had a preventive role on the sonocatalytic removal efficiency of SIPX. This significant reduction in the removal efficiency can be assigned to the proprietary effect of ethanol as a hydroxyl radical receptor. To further investigate the importance of OH radicals in the studied system, the effects of ethanol on the efficiency of the sonocatalytic degradation of SIPX was assessed under different ultrasonic powers (Fig. 7b). It was observed that the addition of ethanol had a substantial suppressive effect on the degradation efficiency of SIPX under different ultrasonic powers. It can be concluded that the OH radicals had a crucial function in the sonocatalytic degradation of SIPX, in the present study. On the other hand, the sonocatalytic degradation of SIPX was evaluated in the presence and absence of ethanol using the UVeVis absorption spectra recorded during different reaction time (Fig. 7c and d). Fig. 7c shows the UVeVis absorption spectrum of SIPX in the absence of ethanol. A sharp maximum wavelength peak at lmax of 301 nm was observed in this figure that was related to SIPX. The rapid reduction in the peak intensity at 301 nm by proceeding the sonocatalytic process from 0 to 60 min can be related to the gradual decomposing of C¼S double bond in the SIPX molecules. This il lustrates that in the absence of ethanol as OH scavenger, OH radicals were completely consumed for degradation of SIPX molecules. However, the introduction of ethanol to the reaction bulk drastically retarded the intensity of SIPX maximum wavelength at 301 nm during the same reaction time span, as shown in Fig. 7d. This confirms the central role of OH radicals in the studied sono catalytic process. According to the obtained data, hþ and OH are verified to be the dominant reactive oxidizing species in the sonocatalytic degradation of SIPX in the presence of Cu2S nanoparticles. Subsequently, the sonocatalytic reaction mechanism is demonstrated in Eqs. (9)e(14) (Cao et al., 2017).

Cu2 S þ Ultrasonic irradiationsðLight=HeatÞ/Cu2 S e CB þ hþ VB

(9)

(10)

Cu2SOHþhþ/Cu2S OH

(12)

HeatþH2O2/ OHþ OH

(13)

(14) Eq. (9) shows that both light and hot spots (heat) which are produced from the cavitation phenomenon are able to efficiently excite Cu2S nanoparticles. The electron-hole pairs are produced on the surface of Cu2S nanoparticles due to the excitation by light and hotspots. The generated electrons are able to oxidize the reduced H2O2 molecules, leading to the formation of OH, as presented in Eq. (11). Moreover, the formed hotspots accompanied by high temperature (more than 1000 C) can thermally decompose the adsorbed H2O2 to OH radicals on the surface of Cu2S nanoparticles, as given in Eq. (13). Eq. (14) demonstrate that the generated holes are not only the main direct origin of OH formation on the surface of the catalyst, but also they are able to directly oxidize the SIPX molecules.

3.4.2. The role of the enhancers In order to increase the removal efficiency of the pollutant and to develop technical application of sonocatalytic process in various industries, several enhancers were used in this study. This led to the integration of the sonocatalytic process with other AOPs and producing various ROSs in the reaction bulk. This subsequently increased the degradation levels, due to the possible synergistic effects (Buxton et al., 1988; Neta et al., 1988). Fig. 7e depicts the influence of the employed enhancers, including potassium periodate (KIO4), potassium persulfate (K2S2O8), and H2O2 all of them with an initial concentration of 0.06 mM on the removal efficiency of SIPX. The removal efficiency of SIPX using of enhancing compounds was investigated both in the presence and in the absence of ultrasonic irradiations. By examining of curves a to c in Fig. 7e it can be proposed that without ultrasonic irradiations, enhancing compounds are not able to significantly increase SIPX removal efficiency. This suggests that these enhancing compounds need to be activated by a suitable activation source such as ultrasonic irradiations. However, curves e, f and g show that SIPX removal efficiency is remarkably increased from 63% to 78%, 84.33% and 98% in the case of H2O2, SO2 4 and IO4 ions during 10 min of the sonocatalytic reaction, respectively. Moreover, the sonocatalytic process had the highest degradation efficacy in the presence of IO 4 ions compared to the other studied systems. The highest effect of periodate on the removal efficiency could be depended on two factors: (i) capturing the generated electrons from Cu2S conduction band (Khataee et al., 2015) thus, reducing the recombination of electron-hole pairs, leaving hþ free for oxidizing reaction and increasing the generation of OH radicals; (ii) activation of periodate ions by ultrasonic irradiations. The generation of various active radicals such as periodyl radical (IO4) and iodyl radical (IO3) in the reaction of periodate ions with OH, H and H2O2 is shown in Eqs. (15)e(20). The produced active species are able to oxidize the SIPX molecules and improve the overall degradation rate. Our results are in consistent with the results of the other reported studies where the effect of different enhancers were investigated in detail (Hamdaoui and Merouani, 2017; Kumar and Devi, 2011). (15)

(16)


(17)

(18)

(19)

(20) S2O2 8

ions on sonocatalytic removal of The positive effect of SIPX was related to the formation of SO4 radicals with high redox potential (i.e. 2.6 V) which was comparable to OH radicals. SO4 radicals are produced from the activation of peroxydisulfate under ultrasonic irradiation and hot spots, as defined in Eqs. (21)e(25). Lastly, the removal efficiency of SIPX was enhanced due to the simultaneous existence of several ROSs such as OH, SO4 and holes. (21)

(22)

(23)

(24)

(25) In the case of H2O2, the removal efficiency was enhanced owing to the formation of additional OH radicals from the dissociation of H2O2 under ultrasonic irradiations, as expressed in Eqs. (S8) to (S12) (Khataee et al., 2017a). Finally, by comparing the different enhancers employed in the present work, sonocatalytic degradation process in the presence of

235

Table 1 Relative frond number of L. minor exposed to different solutions, Experimental conditions: [Catalyst] ¼ 1.5 g/L, [SIPX]0 ¼ 10 mg/L, pH ¼ 7.33, US Power ¼ 150 W. Samples

Initial frond number

Relative frond number

Control SIPX alone 20 min sonocatalytic treated 40 min sonocatalytic treated 60 min sonocatalytic treated

30 30 30 30 30

0.6 0.1 0.2 0.3 0.4

persulfate ions (US/Cu2S/S2O2 8 ), which led to degradation of 98% SIPX during 10 min of reaction time, was identified as an innovative degradation system owing to the valuable influence of sodium persulfate. Moreover, potassium persulfate is a relatively inexpensive compound with high redox potential and high aqueous solubility, easy to maintain, and transport, and a very stable compound. The stability of the catalyst in the presence of potassium persulfate and the produced SO4 was determined to be 0.01 mg/L using AAS method. As aforementioned, the concentration of Cu ions in the sonocatalytic system in the absence of potassium persulfate was 0.0086 mg/L. The insignificant difference in the amount of Cu ions in the solution shows the stability of the employed sonocatalyst under the optimum operating conditions. Therefore, it can be proposed that the considerable enhancement in the removal efficiency by potassium persulfate is due to occurrence of reactions (21) to (25), and production of SO4. Furthermore, M. Cifuentes et al. (Molina et al., 2013) reported the degradation of SIPX by several expensive and complex oxidizing methods. In their work, 76%, 95% and 99% of 10 mg/L of SIPX was respectively removed with electrolysis, UV photolysis and photo-electro-oxidation processes. In comparison to this work, the sonocatalytic removal of SIPX in the presence of Cu2S nanoparticles and potassium persulfate can be considered as an innovative system for rapid and successful degradation of SIPX.

3.5. Phyto-toxicity assessments Phyto-toxicity is a mutual interaction of a plant species with a

Fig. 8. The visual changes of L. minor locate in (a): control sample, (b): SIPX alone solution and sonocatalytic treated samples for (c): 20 min (d): 40 min and (e): 60 min.

236


specific chemical compound in which the employed plant expresses an endpoint response to the toxicity of the compound in contact (Khataee et al., 2016). The phyto-toxicity results of the untreated and sonocatalytic treated solutions and Cu2S nanoparticles are presented in Table 1 and Fig. 8. Table 1 shows that the fronds number are significantly increased in the sonocatalytic treated samples. The visual changes in the number, color and integrity of the L. minor fronds is also presented in Fig. 8. These data demonstrate that the RFN was gradually improved from 0.2 to 0.4 by the progress in the reaction time from 20 to 60 min. This confirmed that SIPX was effectively degraded by sonocatalytic process and the further progress in the reaction time diminished the toxicity of the solution. Furthermore, the data given in Table 1 and Fig. 8 show that SIPX was toxic to the studied plant even at 10 mg/L, and it is necessary to completely remove from the contaminated effluents.

4. Conclusions In this study, heterogeneous sonocatalytic process in the presence of Cu2S nanoparticles was carried out for degradation of SIPX, as a model recalcitrant pollutant present in the mining wastewater. The results obtained from the XRD, FT-IR, SEM, EDX and NSD analyses showed that the high-energy planetary ball milling method was able to produce Cu2S nanoparticles without changing the chemical properties. In the one hand, the XRD results showed that by further increasing of the ball milling time the Cu2S crystallite size was decreased. In the other hand the SEM, NSD and AAS analyses showed that the obtained nanoparticles from higher ball milling durations tended to accumulate and form micro-assemblies with increased leaching of Cu ions inside the reaction bulk, respectively. Although, the concentration of leached Cu ions was increased by increasing of the ball milling time, the amount of Cu ions leached from the samples was much lower than that of accepted value for Cu ions in the aqueous media (0.009 mg/L). Furthermore, Cu2S nanoparticles produced during 0.5 h of milling process showed higher performance in sonocatalytic process compared with other samples produced at 1.5, 3 and 4.5 h of the process. The XRD pattern of the four-time used Cu2S nanoparticles and its constant performance in degradation of SIPX during repeatability tests confirmed the stability of the catalyst in sonocatalytic process. The results obtained from the optimization experiments showed that 93.9% of 10 mg/L of SIPX could be removed during 60 min of the reaction. Moreover, utilization of the different scavenger compounds such as ethanol, NaCl and Na2SO4 as OH þ radical scavengers and EDTA as a sono-generated h scavenger showed that SIPX removal efficiency was decreased when each of the mentioned scavengers were added to the reaction medium. This subject was also investigated and confirmed by monitoring the UVeVis spectrum of SIPX during sonocatalytic process in the presence of ethanol. Ultimately, the obtained data verified that OH radicals and hþ were the main oxidizing species for degradation of SIPX in the studied system. By combining the sonocatalytic process with various enhancers such as KIO4, K2S2O8, and H2O2, the role of different oxidizing species was discussed in the degradation process. Finally, considering the advantages and disadvantages of each employed enhancers, US/Cu2S/S2O2 8 system was introduced as a rapid and efficient degradation system for complete degradation of SIPX. At the end, the toxicity of Cu2S nanoparticles catalyzed sonolysis process was investigated on an aquatic plant, L. minor. The obtained data demonstrated a gradual reduction in the toxicity of the treated solutions by extending the reaction time from 20 to 60 min.

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