Paraquat (PQ) is a non-selective contact herbicide used to control or suppress a broad spectrum of emerged weeds. It is the most toxic herbicide, and the third most widely used in the world [1]. The United States Environmental Protection Agency (USEPA) has classified paraquat dichloride as a restricted-use pesticide due to its highly acute toxicity to animals and people from intentional or inadvertent exposure with acute oral toxicity of 4,4-bipyridyl with an LD50 value of 40–200 mg/kg of body weight. It has life-threatening effects on the gastrointestinal tract, kidneys, liver, heart, and other organs [2, 3].
In the recent years, advanced oxidation processes (AOPs) have been intensively studied as the most environmentally friendly and promising techniques for the degradation of recalcitrant organic pollutants in water by powerful oxidants, especially hydroxyl radicals and superoxide radicals [4,5,6,7,8,9,10].
Heterogeneous photocatalytic degradation in the presence of the nanostructure catalysts has attained good efficiencies in the degradation of organic compounds among the various AOPs [11,12,13,14,15,16,17,18]. In the photocatalytic activity process, the photoelectrons in the conduction band and highly oxidative holes in the valence band are produced, where a reaction occurs with the adsorbed water to form the highly reactive hydroxyl radicals according to Eqs. 1–8 [19].
$${\text{TiO}}_{{2}} \mathop{\longrightarrow}\limits^{{{\text{uv}}}}{\text{e}}_{{}}^{ - } + {\text{h}}_{{}}^{ + } ,$$
(1)
$${\text{TiO}_2}({{\text{h}}^ +)} + {{\text{H}}_2}{\text{O}_{\text{ad}}} \to {\text{TiO}_2} + {\text{HO}}\cdot + {{\text{H}}^+}$$
(2)
$${\text{Ti}}{{\text{O}}_{\text{2}}}{\text{(}}{{\text{e}}^{\text{ - }}}{\text{)}} + {{\text{O}}_{\text{2}}} \to {\text{Ti}}{{\text{O}}_{\text{2}}} + {\text{O}}_{\text{2}}^{\cdot{\text{ - }}},$$
(3)
$${\text{O}}_{\text{2}}^{ \cdot {\text{ - }}} + {{\text{H}}^ + } \to {\text{HO}}_{\text{2}}^ \cdot,$$
(4)
$${\text{O}}_{\text{2}}^{ \cdot {\text{ - }}} + 3{\text{HO}}_{\text{2}}^ \cdot \to {\text{H}}{{\text{O}}^ \cdot } + {\text{3}}{{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}} + {{\text{e}}^ - }$$
(5)
$${\text{2HO}}_{\text{2}}^ \cdot \to {{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$$
(6)
$${\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{TiO}}_{{2}} {\text{(e}}^{ - } {)} \to {\text{TiO}}_{{2}} + {\text{HO}}^{ - } + {\text{HO}} ,$$
(7)
$${\text{H}}{{\text{O}}^ \cdot } + {\text{Organics}} \to {\text{Intermediates}} \to ... \to {\text{C}}{{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}}$$
(8)
As another powerful AOP method, inorganic oxidants such as \({\text{ClO}}_{3}^{-},\) \({\text{BrO}}_{3}^{-},\) \({\text{H}}_{2}{\text{O}}_{2},\) \({\text{S}}_{2}{\text{O}}_{8}^{2-},\) and \({\text{IO}}_{4}^{-}\) are used for the removal and mineralization of various organic pollutants from aqueous solutions. They produce different highly reactive radicals and in the hybridizing oxidation processes they have synergistic effect which gives better results in comparison to the individual processes [20,21,22,23,24]. They have enhanced the rate of UV-induced decomposition of organic pollutants in the presence of photocatalysis. This enhancement is as a result of the reduction of electron/hole recombination because of the reaction of activated electron by active oxidant such as \({\text{S}}_{2}{\text{O}}_{8}^{2-}\) and \({\text{IO}}_{4}^{-}\) [25,26,27].
Undergoing photolysis or thermolysis in an aqueous solution, persulfate (PS) is decomposed to generate the reactive radicals (Eqs. 9–11) [28, 29].
$${\text{S}}_{2} {\text{O}}_{8}^{2 - } + {\text{UV }}\left( { < 270{\text{ nm}}} \right) \to 2{\text{SO}}_{4}^{ - } ,$$
(9)
$${\text{SO}}_{4}^{ - } + {\text{ H}}_{2} {\text{O }} \to {\text{SO}}_{4}^{2 - } + {\text{HO}}^{ \cdot } + {\text{ H}}^{ + } { }\left( {\text{at all pHs}} \right),$$
(10)
$${\text{SO}}_{4}^{ - } + {\text{ OH}}^{ - } { } \to {\text{SO}}_{4}^{2 - } + {\text{HO}}^{ \cdot } { }\left( {\text{at all pHs}} \right).$$
(11)
Periodate (PI), as an inorganic oxidant, can oxidize a wide range of organic compounds quickly due to the generation of highly reactive radicals and non-radical intermediates under photolysis in an aqueous solution (Eqs. 12–19) [28,29,30].
$${\text{IO}}_4^ - + {\text{hv~}} \to {\text{IO}}_3^ \cdot + {{\text{O}}^{ \cdot - }} ,$$
(12)
$${{\text{O}}^{ \cdot - }} + {{\text{H}}^ + }\rightleftarrows {\text{O}}{{\text{H}}^ \cdot } ,$$
(13)
$${\text{O}}{{\text{H}}^ \cdot } + {\text{IO}}_4^ - \to {\text{O}}{{\text{H}}^ - } + {\text{IO}}_4^ \cdot ,$$
(14)
$$2{\text{O}}{{\text{H}}^ \cdot } \to {{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}},$$
(15)
$$2{\text{IO}}_{4}^{ \cdot } { } \rightleftarrows {\text{ I}}_{2} {\text{O}}_{8},$$
(16)
$${\text{I}}_{2} {\text{O}}_{8} + {\text{ H}}_{2} {\text{O }} \to {\text{IO}}_{3}^{ - } + {\text{ IO}}_{4}^{ - } + 2{\text{H}}^{ + } + {\text{ O}}_{2},$$
(17)
$$2{\text{IO}}_{3}^{ \cdot } { } \rightleftarrows {\text{ I}}_{2} {\text{O}}_{6} ,$$
(18)
$${\text{I}}_{2} {\text{O}}_{6} + {\text{ H}}_{2} {\text{O }} \to {\text{IO}}_{3}^{ - } + {\text{IO}}_{4}^{ - } + 2{\text{H}}^{ + } .$$
(19)
AOPs have limitations. In general, one of the main limitations of AOPs is that they cannot be used for effluents with high pollutant content due to their high cost. Also, in the AOP methods, the safety aspects of using UV light should be considered in the design of the process and the relevant reactors, which is one of the limitations of process operating. Cantavenera et al. [31] investigated the photocatalytic degradation of PQ in the presence of polycrystalline TiO2 Degussa P25 irradiated by near-UV light. They observed an increase of both degradation and mineralization rates after an induction time of 45–60 min and the complete photocatalytic mineralization of PQ (20 mgL−1) after 3 h of irradiation using 0.4 g l−1 of catalyst at natural pH [31] such that both time and catalyst amount used were high. Ignace et al. [32] studied the photocatalytic degradation of PQ in a fixed bed photoreactor under UV irradiation at 368 nm. This contained ß-SiC alveolar foams coated with TiO2 P25. The results showed that under optimal operating conditions at natural pH = 6.7, [PQ] = 10 mgL−1), and flow (26 mL/min), degradation and mineralization obtained about 43% and 27% respectively, after about 70 min [32] and these results are low. Zahedi et al. [33] studied the photocatalytic degradation of paraquat herbicide in the presence TiO2 nanostructure thin films under visible and sunlight irradiation using continuous flow photoreactor. The results indicated that at optimum pH 5.8, maximum decomposition of 84.39% in 5 h occurred under visible irradiation with initial concentration of 10 mgL−1 and the amount of photocatalyst of 30.8 g [33] such that both time and used catalyst amount were high.
The aim of this work is comparative study of the performance of UV/PS/TiO2NPs and UV/PI/TiO2NPs as hybrid AOPs and synergistic effect of these hybrid processes for degradation of the paraquat herbicide in aqueous solution. The process was modeled and optimized by response surface methodology (RSM). Also, the kinetic and the electrical energy consumption were assessed. So far, researchers have not studied the electrical energy consumption for the hybrid photocatalytic/periodate and persulfate process of paraquat herbicide and, this assessment have been performed in this work for the first time.