Investigating the synergistic effect of UV/PS/TiO 2 and UV/PI/TiO 2 processes on paraquat herbicide degradation in media aqueous: statistical optimization, kinetic study, and estimation of electrical energy consumption

In this paper, the performance of UV/PS/TiO 2 and UV/PI/TiO 2 as hybrid AOPs for degradation of paraquat (PQ) herbicide in aqueous solution has been studied. The effect of several factors such as UV irradiation, initial oxidant concentration, nano-TiO 2 (TiO 2 NPs) dosage, and pH on the degradation efficiency was investigated. Process optimization was performed by Central Composite Design (CCD) and response surface methodology (RSM) for 30 mgL -1 of herbicide at 25 ˚C and 40 min. Based on the results, for UV/PS/TiO 2 process a degradation efﬁciency of 83% was obtained in the optimum condition of initial PS concentration of 400 mgL -1 , initial TiO 2 NPs concentration of 150 mgL -1 , and pH=6.3. Also for UV/PI/TiO 2 process, 87% degradation efﬁciency was achieved in the optimum condition of initial PI concentration of 88 mgL -1 , initial TiO 2 NPs dosage of 125.5 mgL -1 , and pH of 7.5. Mineralization efficiency of the PQ solution by using PS and PI were about 47.5% and 57%, respectively after 80 min. Kinetic studies showed that both process follow pseudo-first-order kinetic model and their kinetic constants were 0.0299 min -1 for PS process and 0.0604 min -1 for PI process. Electrical energy consumption was estimated about 481.60 kWh/m 3 for PS process and 238.41 kWh/m 3 for PI process.


Introduction
Paraquat is a nonselective contact herbicide to control or suppress a broad spectrum of emerged weeds. It is the most toxic herbicide, the third most widely used herbicide in the world (Bromilow, 2004). The United States Environmental Protection Agency (USEPA), classified paraquat dichloride as a restricted use pesticide due to high acute toxicity to animals and people from intentional or inadvertent exposure with the acute oral toxicity of 4,4-bipyridyl with an LD50 value of 40-200 mg/kg body weight. It has life-threatening effects on the gastrointestinal tract, kidney, liver, heart and other organs (Agency U.S.E.P, 1997; Watts, 2011).
Heterogeneous photocatalytic degradation in presence of nanostructure catalysts attained good efficiencies in degradation of organic compounds among the various AOPs (Sillanpää and Matilainen, 2010;Znad et al, 2018). Heterogeneous photo-catalysis involves the use of a suspension of semiconductor powder (usually transition metal oxides) as a catalyst for AOPs.
The process is photo-induced and requires irradiation for the activation of the catalyst (Hodges et al., 2018;Medynska, 2018;Moshe et al. 2009;Atalay and Ersöz, 2016). For example, metal-oxide nanoparticles of TiO2 have been extensively utilized in photocatalytic oxidation processes, due to high photocatalytic activity and nontoxic properties (Wang, 2016;Jain and Vaya, 2017;Razali et al., 2013;Sillanp, 2018). In the its photocatalytic activity process photo electrons in the conduction band and highly oxidative holes in the valence band are produced where a reaction occurs with the adsorbed water (i.e. surface hydroxyl) to form the highly reactive hydroxyl radicals according eq. 1 (Binas, 2017).
Thus, heterogeneous photo-catalysis technique considers particularly due to its ability to completely oxidize many organic compounds without the formation of hazardous by products (Zhu and Wang, 2017;Aramyan andMoussavi, 2017, Gupta, 2012) .
Inorganic oxidants such as ClO 3 − , BrO 3 − , H 2 O 2 , S 2 O 8 2− ,and IO 4 − used for removal and mineralization of various organic pollutants from aqueous solutions due to the synergistic effect in generated different highly reactive radicals in Hybridizing photo-oxidation process of various inorganic oxidants and be having better results in comparison to the individual processes Eskandarloo et al., 2015;Wang and Hong, 1999;Elddine, 2015;Jafarinejad, 2017, Ali andHassan, 2008. They can activate by ultrasonic waves, heating, UV irradiation and also it can chemically activate using Fe 0 , Fe 2+ , Zn 2+ , etc Sharma et al., 2015;Sahoo, 2013. Undergoing photolysis or thermolysis in aqueous solution, S 2 O 8 2− decomposes to generate the reactive radicals (eq. 2-4) (Cao, 2010;Chia et al, 2004 Periodate as an inorganic oxidant can oxidize a wide range of organic compounds quickly due to generate highly reactive radicals and non-radical intermediates under photolysis in aqueous solution (eq.5-12) (Lia, 2016;Cantavenera, 2007)..
The aim of this work is comparative study of the performance of UV/PS/TiO2 and UV/PI/TiO2 as hybrid AOPs on removal of paraquat herbicide from aqueous solution. The process was modeled and optimized by response surface methodology (RSM). Also, kinetic and electrical cost estimation has been assessed.

Materials and instruments
Standard solution of Paraquat (PQ, 42%) with chemical name of 1,1dimethyl-4,4bipyridinium dichloride that its molecular structure is shown in supplementary Fig. 1 Hydrochloric acid and sodium hydroxide were used to adjust solution pH and in all of the experiments deionized water was utilized to prepare solutions. All the chemicals were Merck and Fluka products.

Concentration of the PQ was measured by an UV-Vis spectrophotometer (Double beam
Rally UV-2601). Total organic carbon (TOC) analysis was carried out by a multi N/C 3100 (Germany) instrument.

Photochemical reactor
An UVC lamp (Philips, 150 W and λmax = 254 nm) was used as light source and fixed into quartz tube and located in the center of the reactor. A cylindrical pyrex container with volume of 500 mL which was equipped with a cooling jacket to control the temperature was used as reactor vessel. The reactor content was stirred by magnetic stirrer. The photo-reactor schematic is shows in Fig. 4.

Procedure
In each run of the process, 400 mL of the PQ solution with desired initial concentration and pH value was transferred into the reactor. A certain amount of the TiO2 Nano-powder and inorganic oxidant were added, and after well mixing, the UV lamp was switched on to initiate the process. Samples at regular time intervals were withdrawn and degradation studies were carried out by measuring the absorbance at λmax = 258 that corresponds to C=C bands in pyridinium ring with the help of a UV-Vis spectrophotometer ( Fig. 5) (Sahoo, 2012). Also, mineralization study was carried out by measuring the TOC of the samples (supplementary Fig. 6).
The percentages of degradation and mineralization are calculated according to the following equations: Where, C0 and Ci are the concentration of the PQ before and after treatment.
Where, TOCt is the TOC at time 't' (Sahoo, 2012). CCD is used to optimize the values of significant variables and obtaining the best quantitative response. Also, it reduces the effect of uncontrolled variables by the experiments are randomly examined (Vahidian et al.,2016).
The total number of experiments (N) can be determined as follow (Martins, 2017): (Raissi and Farsani, 2009) where k, 2 k , 2k and N0 are the number of factors, the terms of cubic points, axial points, and center points, respectively.
So, CCD is able to model and optimize related operational factors of the AOPs and can specify the possible interaction between them (Martins, 2017).
In this study, the three important factors, i.e. initial pH, TiO2 nanoparticle dosage, and inorganic oxidant concentration were optimized based on obtained degradation efficiency (DE) of the PQ as the response via the CCD method.

Experimental design
In order to design of the experiments, the effective operational parameters such as persulfate and periodate concentration, initial pH, and    After regression analysis of the data, a second order polynomial equation was suggested by software to predict the response of the processes of UV/PS/TiO2 and UV/PI/TiO2.
The significance of the model and its terms was evaluated by analysis of variance (ANOVA) such that p-values less than 0.05 and greater than 0.10 indicate the model terms are significant and not significant, respectively. The terms of TiO 2 NPs, pH 2 , PS, PS × pH in process, and the terms of PI 2 , pH 2 , TiO 2 NPs in PI process, were significant. The ANOVA output for the reduced quadratic models (eq. 16 and 17) is demonestrated in  The "Pred R-Squared" of 0.93 and "Adj R-Squared" of 0.97 represent that the model predicts the response as well, and the "Adeq Precision" of 38.26 indicates an adequate signal to noise (a ratio greater than 4 is desirable). The R 2 of 0.97 implies that the model can predict the UV/PS/TiO2 process performance. Also, in the case of UV/PI/TiO2 process, the "Pred R-Squared" and "Adj R-Squared", and "Adeq Precision" were 0.78, 0.91, and 19.73 respectively. The R 2 of 0.94 implies that the model can predict the UV/PI/TiO2 process performance. The adequacy of the models was graphically evaluated and approved by diagnostic plots (supplementary Fig. 7a-b).

Effect of operational parameters and process optimization
The effect of operating factors on the process was assessed by three dimensional surface graphs. Figs. 8 and 9 show variation of the the degradation efficiency as a function of the initial pH and dose of TiO 2 NPs and oxidant dosage (PS and PI), while the PQ initial concentration is 30 mgL -1 at all tests. Fig. 8 shows the PI process efficiency at neutral condition is more than the alkaline and acidic conditions and it decreases at acidic conditions intensitly, while activity of PS independent from pH variations approximately. Fig. 9 shows the degradation efficiency is increased by increasing the PI, PS and TiO 2 NPs concentrations for both processes, in constant pH. This increasing is very intensive for PI due to production of more radicals (eqs. 5-12). So, it is a stronger and more active oxidizer. The operation parameters were optimized numrically based on the proposed reduced models (eqs. 16 and 17) using related numerical facilities of the applied software. To this aim, goals of the three variables and the model response were set at "in the range" and "maximizing" respectively. Desirability ramps for the numerical optimization of the UV/PS/TiO2 and UV/PI/TiO2 processes have been shown in supplementary Fig. 10

The process details and kinetic study
The degradation kinetic of the PQ was tested separately under their optimum conditions for both processes. The linear relationship between the investigated results for both the PS a a b a and PI processes shows that they follow the first-order kinetics which the fitting is shown in Fig. 12. Plotting variation of the logarithmic concentration ratio versus the irradiation time forms a straight line with a slope equal to the kapp. Kinetic constant was 0.0299 min -1 for PS process and 0.0604 min -1 for PI process.

Mineralization
Mineralization is a process of the complete oxidative degradation of organics and relevant intermediates to CO2, H2O and other mineral oxides (Saien, 2017;Marien, 2016

Electrical energy consumption
The electrical energy consumption (EEC) is one of the important criteria in the photochemical process. The figure-of-merit is the electrical energy per order, defined as the number of KWh of electrical energy required for reducing the concentration of a pollutant by 1 order of magnitude (i.e. 90% degradation), in 1 m 3 of contaminated water, and can be calculated as (Bolton, 2001): where P is the electrical power (kW) of the light source in the photochemical system, V is the volume (L) of the treated solution, and t is the time of irradiation (min). According to the first-order kinetic for the photocatalytic process, the constant ratio of log ([PQ]0/[PQ])/t represents the rate constant, k (in unit of min -1 ), and therefore, eq.18 can be rewritten as: Hence, under the optimum conditions of the photocatalytic PS and PI processes and considering to the rate constant of 0.0299 min -1 for PS process and 0.0604 min -1 for PI process, 150 W light source and 0.4 liter of treated PQ solution, the EEC is calculated as 481.60 kWh/m 3 for PS process and 238.41 kWh/m 3 for PI process after 60 min.

Conclusion
In this study a photocatalytic process using TiO2 nanoparticles (TiO2NPs), persulfate (PS) and periodate (PI) oxidizers was applied to degrade of the paraquat (PQ) molecules as a high toxic herbicide. The experiments were designed based on CCD method and also, the processes were modeled. Concisely, the following outstanding information can be briefly stated from this study: ➢ A reduced second order equation could model the photocatalytic degradation efficiency as a function of the initial pH, the PQ, PS, and PI concentration, and the TiO2NPs dosage.
➢ The operating parameters were optimized based on the models as: the initial pH = 6.3, [PS] = 400 mg/L, and [TiO2NPs] = 150 mg/L for PS process and the initial pH = 7.5, [PI] = 88 mg/L, and [TiO2NPs] = 125 mg/L for PI process were obtained. Under the conditions, the models predicted efficiency about 77% for UV/PS/TiO2 and 87% for UV/PI/TiO2 which they were confirmed empirically with only 6 and 3% error.
➢ The photocatalytic PQ degradation for both processes was well fitted by a pseudo first order kinetic model with rate constant of 0.0299 min -1 for UV/PS/TiO2 process and 0.0604 min -1 for UV/PI/TiO2 process.
➢ Under the optimum conditions, the PQ molecules were mineralized about 47.5 and 57 % after 80 min for UV/PS/TiO2 and UV/PI/TiO2 processes, respectively.