Non-equilibrium atmospheric pressure plasma assisted degradation of the pharmaceutical drug valsartan: influence of catalyst and degradation environment

This paper investigated the degradation of the pharmaceutical drug Valsartan (VS) using non-equilibrium atmospheric pressure plasma (NEAPP) with various operating conditions. The heterogeneous photocatalyst ZnO nanoparticles (NP's) were synthesized using a hydrothermal process. The morphology, chemical composition and structure of as-synthesized ZnO NPs were examined by Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) analysis. Then, VS degradation was examined in three subsequent treatment conditions including plasma treatment alone, the combination of plasma with as-prepared ZnO NPs and various environments (air, oxygen and hydrogen peroxide) at fixed plasma operating potential and treatment time. The degradation efficiency of plasma-treated VS by various conditions was observed using UV-visible spectroscopy. Optical Emission Spectrometry (OES) was used to characterize the distribution and emission intensity of various reactive species (OH˙, N2-SPS and O) during the degradation processes which plays a vital role in the degradation of VS. The role of OH˙ and H2O2 during the degradation process was further examined by chemical dosimetry and spectroscopic techniques. Furthermore, pH, conductivity and TOC of the untreated and plasma-treated VS were also investigated. The results on the degradation of VS showed that plasma treatment combined with ZnO NP's has a significant effect on degradation of molecules of VS than degradation processes carried out by other experimental conditions due to the formation of higher concentrations of various reactive oxygen and nitrogen species during the degradation processes.


Introduction
For the past few decades, industrialization and increase in population has been adversely affecting the human health and environment. The WHO and other health organizations reported that population growth and pollution are the main causes for the increasing number of human diseases worldwide. 1 Hence, the pharmaceutical industry plays a signicant role in developing medication for reducing the chronic diseases. The drugs like carteolol, amlodipine, metformin, aspirin, terbutaline and mitomycin are common to treat the diseases like heart disease, hypertension (high blood pressure), diabetes, stroke, asthma and cancer respectively. Valsartan (VS) (3-methyl-2-[pentanoyl-[[4-[2-(2H-tetrazol-5-yl)phenyl]phenyl] methyl]amino]-butanoic acid) is an Angiotensin II receptor blockers (ARBs), 2 used to treat hypertension, cardiovascular diseases and diabetic kidney diseases. 3 According to WHO report (2013), more than 1.5 billion people suffer from cardiovascular diseases 4 and hence worldwide consumption of valsartan is signicantly increased. Prolonged consumption of this drug is reported to produce a severe allergic reaction, difficulty in breathing, depression and vision problems. On the other hand, valsartan given/prescribed for the treatment of hypertension, is not completely metabolised (>96% excreted unchanged) 5 and eliminated as a biologically active compound to the environment which adversely affects aquatic life. 6 Recent studies about waste water (WW)/sewage treatment plant (STP) reported that, valsartan is one of the most existing contaminations compared with other commonly used drugs. 7 For instance, around 6.0 mg L À1 of VS in WW/STP, 8 19.8 mg L À1 VS in hospital effluent 7 and 0.09 ng L À1 VS in marine and estuarine water (San Francisco Bay, in the USA) has found. Even though, the presence of pharmaceutical compounds in very few extent in the range of nanogram level, it can produce acute and chronic effects on human health and aquatic life. 9 For these reasons, various attempts have been made to remove pharmaceuticals compounds from waste water. Various conventional methods such as adsorption, coagulation and ltration has been involved to degrade the pharmaceutical compound containing waste water. However, Conventional treatment method cannot completely degrade the pharmaceutical compounds 10,11 and it generates the secondary pollutants. Advanced Oxidation Process (AOP's) is one of the very active methods to overpower the limitation of conventional treatment methods. [12][13][14] In AOP, various reactive oxygen and nitrogen species (ROS and RNS) such as OHc, O, NO 2À , NO 3À , and H 2 O 2 during the degradation process which powerfully attack the toxic organic pollutants and convert them into nontoxic substance such as CO 2 and H 2 O. 15,16 Nevertheless, in many cases, instead of complete oxidation, it produce partial oxidation products and it again generate secondary pollutants to the environment.
Recently, non-equilibrium atmospheric pressure plasma (NEAPP) have great potential of complete oxidation of pharmaceutical compounds present in waste water via the production of various primary ðAr * ; O * 2 ; e *À ; N * 2 and H 2 O * Þ and secondary reactive species (OHc, O, O 3 and H 2 O 2 ) at gas phase and gas-water interface. These primary and secondary reactive species can efficiently degrade the organic pollutants present in the waste water. 17,18 Jean Marie Herrmann 19 reported that, heterogeneous photocatalyst (TiO 2 ), totally degrade and mineralize large variety of organic and inorganic aqueous pollutant into CO 2 and harmless inorganic anions.
Hence, we attempt to degrade the VS compounds via NEAPP at various operating conditions such as Ar plasma, the combination of Ar plasma with ZnO NP's and various environments (air, oxygen and hydrogen peroxide). The choice of ZnO is on the basis of its cost effectiveness, thermal stability at room temperature, high electron mobility, high photochemical reactivity, stability to photo-corrosion and low toxicity than other catalyst. 20,21 Furthermore, it was synthesized by hydro-thermal method and was characterized by various techniques such as Field-Emission Scanning Electron Microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) studies. The inuence of various operating conditions on degradation of VS aqueous solution was examined comprehensively. Then, VS degradation was examined in three subsequent treatment conditions including plasma treatment alone, the combination of plasma with as-prepared ZnO NP's and various environments (air, oxygen and hydrogen peroxide) at xed plasma operating potential and treatment time.
2 Experimental procedure

Synthesis and characterization of ZnO nanoparticles
For the synthesis of ZnO NP's, 0.1 M of zinc acetate solution was prepared by adding 1.1 g of zinc acetate in 50 ml of methanol under continuous stirring. Subsequently, 0.2 g of NaOH was dissolved in 25 ml of methanol, the mixed solution was added drop wise into the precursor solution to maintain the pH between 8 to 11. The obtained solution was further transferred into teon-lined sealed autoclave and heated at 120 C for 24 hours under autogenous pressure. It was then allowed to cool naturally to room temperature. Aer the reaction was complete, the resulting white solid product was washed with methanol at several times, ltered and then dried in a hot air oven at 60 C. Furthermore, the dried white precipitate was calcinated at 600 C for 3 hours. 22 The morphology of synthesized ZnO NP's were examined by FE-SEM (ZEISS SIGMA FESEM, Germany). The chemical composition of the ZnO NP's was analysed by FT-IR (FT/IR-4700 typeA). Aer the degradation process the ZnO NP's was further ltered by Whatman-40 lter paper and washed several times with DI water followed by ethanol. The obtained ZnO NP's was preheated at 60 C and calcined at 600 C for 3 h. Thereaer, the chemical compositions of the recycled ZnO NP's were further conrmed by FTIR. The phase and crystallinity was observed by X-ray Diffraction (XRD) (X'pert PRO X-ray Diffractometer, PANalytical, Netherland) with Cu-Ka (l ¼ 1.5406 A and 2q range from (10-90 ) radiation.

NEAPP reactor
The NEAPP effluent treatment reactor was used for this treatment is shown in Fig. 1. The details of the plasma reactor are explained elsewhere. 23 The major components of the system are plasma torch and high frequency and high voltage power supply. The plasma torch consists of rod-type live and ring shape ground electrode which made of copper. The live electrode is encapsulated by quartz tube in order to avoid arcing and was connected to an AC power supply which is operated with maximum voltage and frequency of 40 kV and 50 kHz. The ground ring electrode was situated just below the live electrode and the distance between them was 2.5 cm. The electrode assembly was further covered by a Teon jacket in order to prevent electric induction during the experiment (Fig. 1). The system has a separate provision for gas inlet which is controlled by mass ow controller (AALBORG GFC37).

Preparation of Valsartan solution and degradation process
Initially, the stock solution having 10 À4 mole concentration of VS was prepared for degradation process. In the rst treatment condition, VS aqueous solution was poured into reaction ask and was positioned 5 mm distance below the nozzle orice. Subsequently, argon plasma forming gas was allowed between the electrodes and high voltage was applied till the homogeneous jet was expelled via nozzle exit. The plasma plume was discharged from the nozzle exit and was allowed to pass into VS aqueous solution and treatment was carried out at optimized operating potential and treatment time of 35 kV and 10 min. In the second treatment condition, air was allowed into the reaction ask which leads to produce air bubbles in the aqueous solution and the same was stirred continuously using magnetic stirrer. Subsequently, the air bubbles containing aqueous solution was treated by Ar plasma at the operating parameters which was optimized for plasma treatment. Similarly VS solution was treated using O 2 bubbles and also by adding H 2 O 2 (1.6 Â 10 À5 M). In the nal condition of degradation processes, 20 mg of ZnO NP's was dispersed into the aqueous solution containing reaction ask and consequently ZnO containing VS solution was further treated by Ar plasma. The typical parameters of plasma assisted VS aqueous solution treatment are given in Table 1.

Characterization
2.5.1 Identication of reactive species. The information of the reactive species produced in plasma jet during the degradation of VS was examined by an optical emission spectroscopy (OES) (Ocean Optics, HR 4000CG UV-NIR, 1 nm). The emission spectra were recorded in the wavelength region of 200-1100 nm. An optical ber cable (QP400-2-SRBX) was used to collect the optical signals and was connected to the collimating lens in order to conne the eld of view, increase the collection efficiency and the spatial resolution of spectrometer. The optical ber setup was placed directly near the plasma aqueous solution treatment region using glass feed through. The spectral measurement was further diagnosed by Ocean view soware.
2.5.2 Detection of OH radicals and H 2 O 2 . Terephthalic dosimetry is one of the chemical methods to quantify the OH radicals produced during the degradation process. Terephthalic Acid (TA) is an excellent OH radical scavenger at the pH range of 10-11. TA does not react with other reactive species such as H 2 O 2 , O 2À etc. TA molecule is reacted with OH radical to form 2hydroxy terephthalic acid (HTA) which can be detected by uorescent method. UV light (l ¼ 310) is irradiated in the aqueous solution containing TA and HTA. The HTA molecule alone emits the radiation at l ¼ 425 nm in UV region, whereas TA molecule does not emit. The concentration of HTA molecules are proportional to the concentration of OH radicals which directly related with the intensity of uorescence emission. 24,25 Potassium titanium(IV) oxalate (K 2 TiO (C 2 O 4 ) 2H 2 O) method is used to determine the concentration of H 2 O 2 by spectrophotometric method. Initially, 3.5 g of K 2 TiO (C 2 O 4 ) 2H 2 O was added to mixed solution of concentrated H 2 SO 4 (27.2 ml) and deionised water (30 ml). The titanium reagent solution was further made up to 100 ml by using DI water. For analysis, 5 ml of titanium reagent and 5 ml of VS solution were taken into a calibrated ask and made up to 25 ml by adding DI water. The solution was treated by above mentioned operating conditions and the production of H 2 O 2 was measured using spectrophotometer at a wavelength of 420 nm. 26,27 The concentration of H 2 O 2 (mol L À1 ) was calculated as follow where A bt and A at are the absorbance (at l max ) of the untreated and plasma treated solutions respectively. x and l are the volume of the solution and the path length (cm) of the spectrophotometer cuvette respectively. 2.5.3 Investigation of degradation processes. The degradation efficiency of plasma treated VS aqueous solution with different treatment condition was observed using UV-Vis spectrophotometer (OCEAN Optics HR4000) equipped with Deuterium Halogen light source. The % of degradation of VS aqueous solution due to plasma treatment was calculated by the following formula. 28 where, C o and C t are the initial and nal (aer plasma) concentration of VS solution. The conductivity and the pH of the degraded VS solution were measured using a digital electrical conductivity meter-611 (Elico Ltd, India) and a digital pH meter (pHep, HANNA Instruments, USA) respectively. Total organic carbon analyzer (Shimadzu TOC-LPH) was used to where, TOC in and TOC  are the initial and nal concentrations of TOC in VS aqueous solution respectively.

Catalysis characterization
The morphology of synthesized ZnO NP's was observed by FE-SEM which exhibits that the morphology of ZnO NP's has crystalline structure comprising of aggregated spherical particles (Fig. 2a). Fig. 2b   . The average crystallite size of the synthesized ZnO Np catalyst was $20 nm calculated using the Debye-Scherer's formula D ¼ (0.94l)/(b cos q), (where l is the wavelength of X-ray (Cu-Ka radiation), b is the full width at half maximum intensity and q is the Bragg's angle.

FTIR result
The chemical composition of ZnO NP's was studied by FTIR spectra as displayed in Fig. 3a. It exhibits major peaks corresponding to stretching vibration mode of ZnO bond around 493 cm À1 . The presence of peaks at 3432.6 cm À1 and 1628.6 cm À1 attributed to O-H stretching vibration and the adsorption of water on the particles surface. 31,32 FTIR results conrm that the major dominance of functionalities in the NP's was ZnO bond. Fig. 3b, depicted the FTIR spectrum of recycled  ZnO NP's aer degradation processes. It implies that aer three cycles, no signicant change was observed in the chemical composition of ZnO NP's which conrms the retention of functional groups before and aer recycling process. Fig. 4 implies the OES spectra of argon plasma jet during degradation of VS for various operating conditions. It was observed that OES spectra of Ar plasma jet (before degradation) exhibits various spectral lines due to argon excited species (Ar*) (4s ) 4p lines) (690-900 nm), OH radicals (309 nm), atomic oxygen (3s 5 S ) 3p 5 P) (772 and 842 nm) 33,34 and nitrogen second positive system (N 2 -SPS) (B 3 P g ) C 3 S u ) (336, 354 and 380 nm). The formation of identied reactive species such as OHc, N 2 -SPS and O (reactive nitrogen species (RNS) and reactive oxygen species (ROS)) in plasma jet may be attributed to the diffusion of ambient gas molecule and moisture in the atmosphere into the plasma jet causes the formation of ROS and RNS via following reaction mechanism 35,36 Ar* + H 2 O / Ar + OHc +H (R1) e À * + O 2 / 2Oc + e À (R2)

Determination of reactive species present in various environments: OES analysis
The OES spectrum of Ar plasma jet during degradation of VS aqueous solution exhibits two new peaks due to H line of the Balmer series such as Ha (at 655.35 nm) and Hb (at 588.31 nm). The formation of Ha and Hb may be attributed to dissociation of water molecule (H 2 O + e À / OHc+ H + + 2e À ) during the processes. However, the intensities of spectral lines due to OHc, O and N 2 -SPS were found to be increased slightly when compared with that of spectral lines observed in Ar plasma alone. A similar spectral lines was observed when degradation of VS aqueous solution carried out by the Ar plasma at various environment such as air, O 2 and H 2 O 2 . However, the intensity of spectral lines found to be increased in the order of P + air < P + O 2 < P + H 2 O 2 due to the formation of higher concentration of ROS and RNS during the degradation processes with respect to the environment of degradation. When Ar plasma degradation processes combined with ZnO NP's the OES exhibits various new spectral lines at 483, 434, 517 and 590 nm corresponds to Zn and Zn 2+ . 37 The formation of species may be due to the interaction of plasma species with ZnO NP's cause's excitation of Zn during the degradation of VS which plays a signicant catalytic role during degradation processes and stimulates oxidation reactions. Furthermore, the intensity of spectral line due to OHc, O and N 2 -SPS were increased substantially compared with the degradation processes carried out at various environment due to the formation of higher concentration of reactive species than the other treatment condition which will be discussed more detail in section 3.4.  aqueous solution exhibited major absorbance characteristic peak at 260 nm. Aer, plasma treatment alone, the intensity of the absorbance peak was found be decreased signicantly which indicating that 24% of the VS molecules was decomposed during the processes (Fig. 5b). Aer plasma degradation carried out at various environment, the intensity of the absorption peak decreased in the order of P > P + air > P + O 2 > P + H 2 O 2 which indicates an increase in the degradation percentage of VS aqueous solution. The above changes may be correlated to the oxidative degradation of VS molecules in aqueous solution due to the formation of primary ðAr * ; O * 2 ; e *À ; N * 2 and H 2 O * Þ and secondary reactive species (OH . , O, O 3 and H 2 O 2 ) during the plasma degradation processes. Finally, the plasma treatment combined with ZnO NP's, obtained lower intense of absorbance peak with maximum degradation percentage of 49% compared to other treatment conditions which mainly caused by the formation of higher concentration of various reactive species during this synergetic process by following facts. Interaction of electron and excited species in plasma with molecules of aqueous solution leads to produce various ROS

Investigation of plasma treated VS solution with different environment-UV analysis
In spite of reactive species, high intense of UV photon in plasma has also contributed to produce various reactive species by photocatalysis mechanism. The ZnO NP's in aqueous solution absorbs UV photons and forms an electron and hole pairs (e À CB + h + VB ). This photo induced electron-hole pair migrate to the surface of ZnO undergoes redox reaction to produce large number of OHc radicals, super oxide anion radicals (O 2 c À ) and hydroperoxy radicals (HO 2 c) via following reaction.
In this way higher concentration of various reactive species are produced in the plasma degradation processes combined with ZnO NP's which are facilitated to decompose VS in the aqueous solution. Comparison of degradation of VS by this technique with other treatment methods was given in Table 2. It is evident that combination of NEAPP with ZnO NP's yield higher degradation efficiency within shorter treatment time compared with other AOP due to the formation of higher concentration of reactive species.

Detection of OHc and H 2 O 2 spectroscopy analysis
In radical chemistry and radiation chemistry OH radical plays a vital role due to their high oxidative potential (2.8 V). The quantity of OHc radicals during the plasma process was analysed by UV-Visible spectroscopy. The UV light can be easily detected with terephthalate, which react with OHc radicals to form hydroxyl terephthalic acid (uorescent product) which has uorescent emission at 425 nm. The intensity of uorescent emission is proportional to the production of OHc radicals. The amount of OH radicals formed during the degradation of VS aqueous solution under different environment such as plasma, air, oxygen, H 2 O 2 and ZnO Np's (at constant applied potential of 35 kV and treatment time of 10 min) is reported in Fig. 6. In plasma, the excited Ar reacts with aqueous solution and H 2 O 2 to produce OH radicals which exhibits the uorescence emission intensity of 263. During the degradation of VS aqueous solution with different environment, the production of OH radicals considerably increased and the uorescence emission intensity follows the order: plasma < P + air < P + O 2 < P + H 2 O 2 < P + ZnO at 35 kV and 10 minutes of treatment time. Comparing with various treatment conditions, ZnO NP's shows highest uorescence intensity of 2123 due to the higher production of OH radicals during the reaction.
Amount of H 2 O 2 formed during the degradation of VS in presence of various treatment conditions is depicted in Fig. 7. Generally; high energy electron dissociates the water molecule to produce H 2 O 2. It is clear from Fig. 7       The mineralization of VS aqueous solution by various plasma treatment condition were examined by measuring elimination of TOC in aqueous solution (Fig. 9). It was observed that 5.43% of TOC was removed from the VS aqueous solution by Ar plasma treatment alone and was found to be increased by the degradation processes carried out by the following order P < P + air < P + O 2 < P + H 2 O 2 < P + ZnO, this may be due to formation of various reactive species during the processes. The obtained reactive species further dissociate/oxidized the carbon network of an organic compounds into small fragments of carbon molecules which may be further converted into various acid and nally to carbon dioxide (CO 2 ). The variation in pH, electrical conductivity and TOC removal percentage clearly implies that the mineralization of VS aqueous solution was achieved by plasma treatment. 44

Conclusion
The NEAPP in an aqueous solution containing pharmaceutical drug VS with ZnO NP's showed higher degradation (%) compared with other treatment conditions such as plasma alone, air, O 2 and H 2 O 2 due to the formation of higher concentration of ROS conrmed by spectroscopic analysis. Moreover, a decrease in pH, increase in conductivity and percentage of TOC removal results supports that substantial degradation of VS aqueous solution in the order of P < P + air < P + O 2 < P + H 2 O 2 < P + ZnO. Finally, we conclude that VS degradation carried out by the combination of plasma with catalyst exhibited signicant efficacy than other treatment conditions.

Conflicts of interest
There are no conicts to declare.