Photocatalytic Degradation of Dichlorvos in Visible Light by Mg^𝟐+-TiO_𝟐 Nanocatalyst

Abstract

Photocatalytic activity of TiO₂ was studied by doping with magnesium (Mg²⁺-TiO₂) with varying magnesium weight percentages ranging from 0.75–1.5 wt%. The doped and undoped samples were synthesized by sol-gel method and characterized by X-ray diffraction (XRD), N₂ adsorption-desorption (BET), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), and scanning electron microscopy (SEM). The XRD data has shown that anatase crystalline phase in Mg²⁺-TiO₂ catalysts, indicating that Mg²⁺ ions did not influence the crystal patterns of TiO₂. The presence of magnesium ions in TiO₂ matrix has been determined by XPS spectra. DRS spectra showed that there is a significant absorption shift towards the visible region for doped TiO₂. The SEM images and BET results showed that doped catalyst has smaller particle size and highest surface area than undoped TiO₂. The photocatalytic efficiency of the synthesized catalysts was investigated by the photocatalytic degradation of aqueous dichlorvos (DDVP) under visible light irradiation, and it was found that the Mg²⁺-doped catalysts have better catalytic activity than undoped TiO₂. This can be attributed that there is a more efficient electron-hole creation in Mg²⁺-TiO₂ in visible light, contrary to undoped TiO₂ which can be excited only in UV irradiation. The effect of dopant concentration, pH of solution, dosage of catalysts, and initial pesticide concentration has been studied.

1. Introduction

Advanced oxidation process (AOP) is an alternative way of treating undesirable organic pollutants, including pesticides. According to the literature [1, 2] among AOPs, heterogeneous photocatalysis seems to be an attractive method and has been successfully employed for the degradation or transform into less harmful substances of various families of pollutants [3–5].

The photoinduced redox reactions have received an attention after the discovery of first photoelectrochemical splitting of water on titanium dioxide electrode by Fujishima and Honda [6]. Later, many novel redox reactions of organic and inorganic substrates have been induced by band-gap irradiation of a variety of semiconductor particles [7, 8]. There are a lot of different semiconductor materials which are readily available (e.g., TiO₂, ZnO, Fe₂O₃, CdS, and ZnS), but only a few are suitable for sensitizing the photomineralization of a wide range of organic pollutants. A sensitizer for reaction must be [9, 10] (i) photoactive, (ii) able to utilize visible and/or near the UV light, (iii) biologically and chemically inert, and (iv) photostable.

Among various semiconductor photocatalysts available, TiO₂ has attracted a great deal of study for its high photocatalytic activity, nontoxic property, chemical stability, and highest oxidation rate of many photoactive metal oxides investigated [11]. TiO₂ is capable of decomposing a wide range of organic, inorganic, and toxic pollutants [12].

Although TiO₂ is superior to other semiconductors for many practical uses, two types of defects limit its photocatalytic activity. Firstly, TiO₂ has high bandgap energy (3.2 eV), which is only 4-5% of the overall solar spectrum. Thus this restricts the use of visible light. Secondly, the high rate of electron-hole recombination at TiO₂ particles results in low photoquantum efficiency [13]. In order to overcome these limitations of TiO₂, many attempts have been made to prepare different types of TiO₂ by depositing of noble metals, mixing metal oxides, surface derivatization, and doping selective metal ions into TiO₂ [14, 15], and also Graphene-based semiconductor photocatalysts are prepared [16].

The addition of low percentage of metal by doping was often proposed to improve the photocatalytic activity of TiO₂. The advantage of doping the metal ions into TiO₂ is the temporary trapping of the photogenerated charge carriers by the dopant and the inhibition of their recombination during migration from the inside of the material to the surface, by modifying the bandgap of the photocatalyst. A wide range of metal ions have been studied for doping TiO₂ and their effects on the properties of TiO₂ and on its photocatalysis [17, 18], but reports on alkaline-earth metal ions doping into TiO₂ and their photocatalytic properties have seldom been seen.

Sol-gel process is one of the versatile, simple, and easy means of synthesizing nanosize materials in contrast to the other conventional preparation methods which do not usually produce homogeneous, high-surface-area materials. The main advantage of the sol-gel method in preparing catalytic materials is its excellent control over the properties of the product via a host of parameters that are accessible in all four key processing steps: formation of a gel, aging, drying, and heat treatment. Although titania catalysts have been extensively studied, the procedure of preparing photocatalytic metal-doped titania is still of great interest.

Waste waters generated from the agricultural application of dichlorvos contain the residue of dichlorvos which has been evaluated in a wide range of toxicology assays including bioassays for carcinogenicity and mutagenicity (genotoxicity) [19, 20]. It is also more likely to contaminate surface and ground waters due to its high solubility in water. This could have an adverse effect on the aquatic environment, and procedure has to be found for its degradation. Electron transfer process occurring between pesticide and semiconductor, especially TiO₂, has been examined and found to have practical potential [21].

The present paper has been focused to develop a visible light response catalyst via magnesium-doped titanium dioxide in varying percentages of metal. Characterization has been carried out using XRD, BET, XPS, DRS, and SEM. The photocatalytic activities of the synthesized samples have been evaluated by the degradation of a representative model pesticide pollutant, dichlorvos.

2. Experimental

2.1. Preparation of Photocatalysts

Titanium tetra-n-butoxide [Ti(O-Bu)₄] and magnesium nitrate were obtained from E. Merck (Germany) and used as titanium and magnesium sources for preparing anatase TiO₂ and Mg²⁺-TiO₂ photo catalysts. Aqueous dichlorvos solution was used as a model compound for degradation. All other chemicals used in this work are of analytical grade, and doubly distilled water was used for the solution preparation.

Mg²⁺-TiO₂ samples were prepared by sol-gel method in which initially a solution containing 40 mL of absolute alcohol, 6 mL of H₂O, and magnesium nitrate with required percentages (0.75 wt.%, 1.00 wt.%, 1.25 wt.% and 1.50 wt.%) was prepared (solution I). Another solution was prepared by taking 21 mL of Ti(O-Bu)₄ in 40 mL of absolute alcohol with stirring for 10 minutes, and then 3 mL of HNO₃ was added dropwise under continuous stirring for 30 minutes (solution II). Solution I was added to solution II slowly from the burette with vigorous stirring at room temperature until the transparent sol was obtained, and the resulting sol was further stirred for 1 h. The gel was prepared by aging the solution for 48 h at room temperature. The derived gel was dried at 100°C in an oven and ground. The catalyst powder was calcined at 400°C in furnace for 2 h. Undoped TiO₂ is also prepared with same procedure without metal nitrate. The doping concentrations are expressed as weight percentage of titanium atom. The powders are stored in black coated air-tight glass containers.

2.2. Characterization of Photocatalysts

The crystal phase composition of the prepared photocatalysts (TiO₂, Mg²⁺-TiO₂) was determined by X-ray diffraction measurement carried out at room temperature using a PANalytical, D/Max-III A diffractometer with CuK_α radiation (𝜆=0.15148nm) with a liquid nitrogen gas-cooled germanium solid-state detector. The accelerating voltage of 35 kV and emission current of 30 mA were used and studied in the range of 2°–90° 2θ with a step time of 2° min⁻¹. The Brunauer-Emmett-Teller (BET) surface area was determined from the N₂ adsorption-desorption isotherm at 77.3 K by using a Quantachrome Nova 2200 E system. The sample was outgassed for 3 h at 300°C prior to the adsorption. The specific surface area was determined by using the standard BET method. To study the valence state of the photocatalysts, X-ray photoelectron spectroscopy (XPS) was recorded with the PHI quantum ESCA microprobe system, using the AlK_α line of a 250 W X-ray tube as a radiation source with the energy of 1486.6 eV, 16 mA × 12.5 kV, and a working pressure lower than 1 × 10⁻⁸ Nm⁻². As an internal reference for the absolute binding energies, the C 1s peak of hydrocarbon contamination was used as reference to 284.8 eV. The fitting of XPS curves was analyzed with MultiPak 6.0 A software. The DRS were recorded with a Shimadzu 3600 UV-visible NIR spectrophotometer equipped with an integrating sphere diffuse reflectance accessory, using BaSO₄ as reference scatter. Powder samples were loaded into a quartz cell, and spectra were recorded in the range of 200–900 nm. The morphology and size of particles was characterized using SEM (JSM-6610 LV) spectrophotometer and operated at 20 kV. A Horiba Jobin FluoroMax-4 instrument has been used for the PL, spectral data analysis, with a PMT voltage was 150 V and with slit set both at 2.5 nm.

2.3. Photocatalytic Activity of the Catalysts

Photocatalytic degradation studies of DDVP were carried out in a modified photoreactor system, in which 100 mL of reaction mixture illuminated with 400 W high-pressure mercury vapor lamp (Osram, India). The distance between the light source and the reaction beaker was 20 cm. Prior to irradiation, the solution with catalyst was stirred in the dark for 45 min to ensure establishment of adsorption-desorption equilibrium of dichlorvos on catalyst surface. Aliquots of the samples were withdrawn from the solution by using Millipore syringe (0.45 μm) at certain time intervals and analyzed for dichlorvos concentration. The quantitative determination of DDVP was performed by measuring the absorption of phosphate ion formed at 465 nm with a Milton-Roy spectronic-1201 UV-visible spectrometer. The IR (>700 nm) and UV (<360 nm) radiations were filtered by water and UV filters, respectively.

Previous studies have shown that the photocatalytic degradation of pesticides by irradiating with UV light source and using TiO₂ or ZnO as catalyst leads to the formation of H⁺, Cl⁻, PO₄³⁻, and CO₂ as final products, and their formation during the progress of the degradation of DDVP was confirmed by simple qualitative analysis tests. PO₄³⁻ was determined calorimetrically by the molybdenum blue method:CH₃O₂POOCHCCl₂+92O₂⟶PO₄³⁻+4CO₂+2Cl⁻+5H⁺+H₂O. (1) Hence, in order to study the extent of mineralization of DDVP over illuminated Mg²⁺-doped TiO₂, the absorbance measurements of PO₄³⁻ were carried out. The percentage of degradation was calculated from the following equation:%Phosphateionformation=𝐴_𝑡𝐴₀×100, (2) where 𝐴₀ is calculated concentration of phosphate ion of the corresponding concentration and 𝐴_𝑡 is absorbance at time 𝑡.

3. Results and Discussions

3.1. XRD Diffraction Study and BET Results

The crystalline phases of the synthesized TiO₂ and Mg²⁺-TiO₂ were examined by XRD, and the diffractograms are given in Figure 1. The XRD patterns of calcined (400°C) TiO₂ and the samples of Mg²⁺-TiO₂ (Mg/Ti = 0.75, 1.0 and 1.25 wt.%) show only anatase form, indicating that Mg²⁺ ions in TiO₂ did not influence the crystal patterns of TiO₂ particle. Further, examination of XRD patterns of both pure and Mg²⁺-doped TiO₂ illustrates the existence of peaks at 25.3, 37.7, 47.7, and 54.2 (2θ). The intensity of anatase TiO₂ peaks increased due to doping, and peaks corresponding to MgNO₃ and MgCO₃ were not detected. This may be due to the well dispersion of Mg²⁺ content in TiO₂ particles.

Figure 1 

XRD patterns of (a) undoped TiO2, (b) 0.75 wt.% of Mg2+-doped TiO2, (c) 1.0 wt.% of Mg2+-doped TiO2, and (d) 1.25 wt.% of  Mg2+-doped TiO2.

Since Mg²⁺ is more electropositive, the electronic cloud in each TiO₂ might be loosely held, favoring the formation of less dense anatase phase. In other words, the tight packing arrangement required for rutile phase formation is fully suppressed by the addition of magnesium nitrate in water which enhances the polarity of water, thus facilitating the formation of anatase phase exclusively. In addition, the presence of residual alkyl groups can also reduce the rate of crystallization of TiO₂ which favored the formation of less dense anatase phase.

The BET surface area results have shown that there is a much increase in surface area of doped catalysts than undoped one. With the incorporation of Mg²⁺ dopant during the sol-gel preparation technique, there was crystal growth suppression, favoring the formation of smaller TiO₂ crystallite. This effect may be attributed to the enhanced lattice strain in the doped TiO₂ network and then decrease grain growth rate.

3.2. Diffuse Reflectance Spectroscopy

The DRS of Mg²⁺-doped TiO₂ and undoped TiO₂ samples are given in Figure 2. The spectra of undoped TiO₂ showed an absorption peak at 388 nm in the UV region. The absorption peak of the doped TiO₂ with different percentages of Mg²⁺ at 0.75 wt.%, 1.0 wt.%, 1.25 wt.%, and 1.50 wt.% shows considerable shift towards the visible region at around 400–800 nm for all the samples. The tailing absorption peaks can be considered as the extra tail states in the bandgap because Mg²⁺ was added to the TiO₂ matrix. The extension of absorption edge to longer wavelengths for Mg²⁺/TiO₂ indicates the existence of good contact between TiO₂ and the metal (Mg²⁺) ion grain.

Figure 2 

The DRS-UV-Vis spectra of Degussa P25, undoped, and doped TiO2 with different % of Mg2+.

The UV-Vis absorption edge and bandgap energies of the samples were determined from the reflectance [F(R)] spectra using the Kubelka-Munk (KM) formalism and the Tauc plot method [22]. The extrapolated lines for Mg²⁺-doped TiO₂ have been given in Figure 3 and used to determine the bandgap energies for Mg²⁺-doped TiO₂, undoped TiO₂, and Degussa P25 samples. The calculated bandgap energies are given in Table 1. The largest reduction gap is observed for catalyst containing 1.25 wt.% of Mg²⁺-doped TiO₂. This reduction bandgap may be attributed to the doping of Mg²⁺ as impurity into the host structure (TiO₂) and create extra energy levels situated within the bandgap [23]. The extra energy level created within the bandgap due to shifting of the Fermi energy level of Mg²⁺ towards valence band since metal create P-type semiconductor.

Table 1 

Crystallite size, BET surface area, and bandgap energy of each catalyst.

Figure 3 

Plots of transformed Kubelka-Munk functions [𝐹(𝑅)⋯ℎ𝑣]1/2 versus h𝑣 for Degussa P25, undoped TiO2,  and Mg2+-doped TiO2 samples.

3.3. BET Surface Areas and Pore Distribution

The BET surface areas of the as-prepared Mg²⁺-TiO₂ sample have been given in Table 1, and nitrogen adsorption and desorption isotherms and corresponding pore size distribution curves (inset) are given in Figure 4. It is observed that at relatively high pressures between 0.075 and 0.15 (𝑃/𝑃_𝑜), the curve exhibit a hysteresis loop indicating the presence of mesopores. The shape of the hysteresis loop is of type H3, associated with particle giving rise to narrow slit shaped pores [24–27]. From Figure 4 the inset figure indicated the pore size distribution range from 20 to 35 nm. The pore volume of the as-prepared catalyst is 0.135 m³/g. However, the existing poresarise maybedue to removal of nitrate ion and organic moiety from the metal precursors (magnesium and titanium). Due tothis,organized porous structure within the bulk sample of the catalyst may helpful in photocatalysis.

Figure 4 

Nitrogen adsorption-desorption isotherms and corresponding pore size distribution. Curves (inset) of the Mg2+-TiO2 prepared at 1.25 wt.% of Mg2+.

3.4. Analysis of Hydroxyl Radicals (OH^•)

The analysis of hydroxyl radicals has been carried out by photoluminescence technique [28]. 0.1 g of Mg²⁺-TiO₂ in 10 ppm of coumarin solution was illuminated with visible light. The fluorescence emission spectrum excited at 428 nm from the coumarin solution was measured at every 15 min of illumination. Figure 5 shows the induction of fluorescence from 10 ppm coumarin solution. As shown in the figure, gradual increase in the fluorescence at about 500 nm for 7-hydroxycoumarin which is obtained due to reaction of OH radicals and coumarin was observed by continuous illumination of visible light on the Mg²⁺-TiO₂ solution. The trend observed in the figure indicated that formation of the OH radicals is proportional to the irradiation time.

Figure 5 

PL spectral changes with irradiation time on the undoped TiO2 and 10 ppm coumarin.

3.5. XPS Measurements

X-ray photoelectron spectroscopy (XPS) analysis of magnesium- (II) doped sample was performed, and high-resolution scans are shown in Figures 6(a), 6(b), and 6(c). The XPS observations show that only Mg, Ti and O elements were detected from the samples in spectrum analysis. The binding energy of Mg 2p was found to be 50.4 eV (Figure 4(a)) which is a typical of Mg²⁺ that bonded to oxygen and was assigned to the compound MgO. The binding energies of Ti 2p_3/2 and Ti 2p_1/2 were found to be 458.4 and 464.0 eV, and these bands could belong to Ti⁴⁺ (Figure 6(b)) and there was no fitting peak for Ti³⁺. Hence the XPS-spectra confirmed the chemical composition of magnesium-doped sample to be MgO and TiO₂.

(a)

(b)

(c)

Figure 6 

(a) High resolution of Mg 2p spectrum, (b) high resolution of Ti 2p spectrum, and (c) high resolution of O 1s spectrum.

3.6. Scanning Electron Microscopy

The SEM images of TiO₂ and metal-doped TiO₂ catalysts are shown in Figures 7(a) and 7(b). The image of samples containing 1.25 wt.% of Mg²⁺ ion shows the morphological changes induced by the addition of alkaline earth cation. The catalysts are found to contain irregular-shaped particles which are again aggregates of tiny crystals. Figure 7(a) shows anatase (undoped TiO₂) produced with no addition of magnesium ion. The sample appears as large blocks of coarse material of dimensions from around 1 to 20 μm. Figure 7(b) shows sample containing 1.25 wt.% of Mg²⁺, with reduction in the particle size to 1–10 μm range. This clearly illustrates the altered morphology of the catalyst powders, which consists of a large portion of submicron-sized particles.

(a)

(b)

Figure 7 

SEM images of (a) undoped TiO2 and (b) 1.25 wt.% of Mg-doped TiO2.

3.7. FT-IR Spectral Study

FT-IR spectra of undoped TiO₂ and 1.25 wt.% of Mg²⁺-TiO₂ given in Figures 8(a) and 8(b) have shown peaks corresponding to stretching vibrations of O–H and bending vibrations of adsorbed water molecules around 2322–3317 cm⁻¹ and 1613 cm⁻¹, respectively. The intensity of the peaks became less in Mg²⁺-TiO₂, indicating the removal of large portion of adsorbed water from TiO₂.

(a)

(b)

Figure 8 

FT-IR spectra of (a) undoped TiO2 and (b) 1.25 wt.% Mg+2-TiO2.

The broad band below 456.06 cm⁻¹ of undoped TiO₂ is due to Ti–O–Ti vibration. This peak has been shifted to 470.63 cm⁻¹ in Mg-doped TiO₂ which may be because of Ti–O–Mg vibration. Hence the FT-IR spectral study along with XPS established the substitution of Mg²⁺ into TiO₂ lattice.

3.8. Photocatalytic Degradation of Dichlorvos

To understand the photocatalytic activity of the prepared catalysts, degradation of dichlorvos pesticide under the irradiation of visible light has been carried out, in the presence and absence of catalysts. The percentage of DDVP degradation is significantly less in the absence of catalyst (0.05% for 9 hours irradiation) when compared to that of undoped or doped titanium dioxide. A blank experiment in the absence of irradiation along with catalysts demonstrated that no significant change in the DDVP concentration was observed. The efficiency of photocatalytic degradation process depends on various experimental parameters such as effect of dopant concentration, pH, catalyst dosage, and initial concentration of pollutant. Hence, it is essential to optimize these parameters to achieve higher degradation efficiency of photocatalyst.

3.8.1. Effect of Mg Dopant Concentration on Photocatalytic Degradation of DDVP

Figure 9 shows the results of the photocatalytic degradation of DDVP by irradiation of visible light over Degussa P25, undoped TiO₂ catalyst, and Mg-doped TiO₂ photocatalyst with different doping concentrations of Mg in TiO₂. Among various concentrations of Mg, 1.25 wt.% of Mg-TiO₂ catalyst exhibits the highest photocatalytic activity. This may be attributed to that increase in dopant concentration leads to increase in number of trapped charge carriers per particle [15, 29] and is the highest in 1.25 wt.% of Mg²⁺-TiO₂ which is the optimum concentration showing maximum photocatalytic activity.

Figure 9 

Effect of dopant concentration on the degradation of dichlorvos. Catalyst dosage = 0.10 g, pH = 3, and [DDVP] = 150 ppm.

The results of the experiment show that the rate of phosphate ion formation increased with increase in the concentration of magnesium ions, and it was optimum at 1.25 wt.%. However, magnesium ions’ doping became detrimental when the dopant concentration was above 1.25 wt.%. It also demonstrates that, even though the concentration of the doped magnesium ions is small, it still gives much influence on the photocatalytic activity of TiO₂ particles.

3.8.2. Effect of pH

The adsorption properties of catalysts can be greatly changed at different pH values; hence, the catalyst-assisted photodegradation of DDVP has also been monitored by in situ measurements of pH of the aqueous solution with irradiation time, at a fixed weight of catalyst and DDVP pesticide concentration at different pH values. Solution pH influences adsorption and desorption of the substrate, catalyst surface charge, oxidation potential of the valence band, and other physicochemical properties.

The effect of pH on the photocatalytic degradation of DDVP is shown in Figure 10. The degradation plot for DDVP has been plotted between rate constant and pH of the solution. Based on the results, it is observed that the rate of degradation decreased with increase in pH of the solution. The optimum pH of the solution for the degradation of DDVP is 2. Generally in acidic pH the surface of the TiO₂ is in the form of TiOH₂⁺. The photogenerated electrons can be captured by the adsorbed H⁺ to form H^•_ads. At higher pH, the surface of catalyst has a net negative charge as TiO₂⁻, and hence the degradation rate was found to be lower:TiOH+H⁺⟶TiOH₂⁺,pH<6.25,TiOH⟶TiO⁻+H⁺,pH>6.25. (3)

Figure 10 

The effect of pH on the degradation of dichlorvos by Mg2+-doped TiO2. Catalyst dosage = 0.10 g, [DDVP] = 150 ppm.

3.8.3. Effect of Catalyst Dosage

The effect of the amount of catalyst on the photodegradation rate was investigated at a fixed pH and initial concentration of the dichlorvos, experiments were performed with varying amounts of Mg²⁺-TiO₂ from 0.025 g to 0.3 g in 100 mL DDVP pesticide aqueous solution. The degradation pattern from such experiments has been depicted in Figure 11, which reveals that the rate of degradation increases linearly with increase in the amount of catalyst up to 0.1 g and then decreases. This may be explained by two main reasons. The first one is due to the presence of foreign metal ions (Mg²⁺) as dopant that helps in inhibiting electron-hole pair recombination and enhancing interfacial charge transfer reactions, which leads to increased catalytic activity.

Figure 11 

The effect of catalyst dosage on the degradation of dichlorvos by Mg2+-doped TiO2, pH = 2, [DDVP] = 150 ppm.

Secondly, as the amount of catalyst increases, the number of photons absorbed and the number of DDVP molecules adsorbed are increased leading to the increase in DDVP degradation. At higher concentrations of catalyst, although more surface areas are available for constant DDVP molecules, the solution turbidity increases and it interferes with the penetration of light. The deactivation of activated molecules by collision with ground-state molecules may also hinder the photocatalytic efficiency [30]. Hence, above a certain level, additional catalyst amount is not involved in catalysis, and thus the rate levels off.

3.8.4. Effect of Initial Pesticide Concentration

At a fixed weight of Mg²⁺-TiO₂ and pH, variation of initial pesticide concentration on photo degradation was studied and shown in Figure 12. It was noted that the degradation rate increases with the increase of pesticide concentration up to a certain concentration (150 ppm), and a further increase leads to a decrease in the pesticide degradation rate. This is due to the reduction of generation of radicals (OH^•) on the catalyst surface since the active surfaces are covered by pesticide molecules.

Figure 12 

The effect of concentration of DDVP on the degradation of dichlorvos by Mg2+-doped TiO2, pH = 2, catalyst dosage = 0.10 g.

3.9. Photocatalytic Mechanism

Based on the experimental results and reports of the previous workers, the following mechanism would be proposed for the photocatalytic reactions of magnesium-doped TiO₂ [31–33].(a)Upon visible light illumination of photocatalyst, electrons are ejected from the valence band to the conduction band leaving positive holes in the valence band.(b)When the metal ion doped into TiO₂ lattice, it trapped the ejected electrons, holding up the recombination process.(c)The trapped electrons can be further scavenged by molecular oxygen, which is adsorbed on the TiO₂ surface to generate superoxide radical, and this in turn produces hydrogen peroxide (H₂O₂), hydroperoxy (HO₂^•), and hydroxyl (^•OH) radicals. (d)The positive holes in the valence band act as good oxidizing agents available for degradation of pesticide in the solution. Thus, the DDVP is attacked by the hydroxyl radicals formed both by trapped electrons and hole in the VB as given in the above equations, to generate phosphate ions, organic radicals, or other intermediates, which further undergo degradation.

4. Conclusions

The DRS analysis shows that doping of Mg²⁺ into TiO₂ shifts the absorbance band of TiO₂ from UV to visible region, and the bandgap energy is reduced for all doped catalysts (from 3.2 eV to 2.75 eV); the largest reduction gap was observed for doped catalyst of 1.25 wt.% of Mg. The incorporation of Mg²⁺ ions into the TiO₂ was evidenced by XPS analysis. The BET results also show that there is an increase in surface area of the catalyst, which enhances the photocatalytic degradation of DDVP in visible light. The present work imparts the significance of magnesium ion doping into titania, in dichlorvos degradation. It also establishes the doping of magnesium by sol-gel as a best alternative method. The experimental results show that the photocatalytic activity of undoped TiO₂ is lower than magnesium-doped TiO₂.

Magnesium doping causes better separation of electron and holes on the modified TiO₂ surface allowing more efficient channeling of the produced electrons into useful redox reactions, thus enhancing the DDVP degradation in the presence of visible light, when compared to undoped TiO₂ which can be excited only in less available UV light. The degradation of DDVP was maximum at 150 ppm DDVP concentration, 0.1 g of Mg²⁺-TiO₂ catalyst, and at a pH of 2 using 1.25 wt.% of magnesium-doped titania catalyst.

Acknowledgments

T. Rao and Mrs. T. Susmitha are thankful to UGC, New Delhi, for financial assistance as major research project to carry out research work and also thankful to Dr. Sridhar, Scientist, IICT, Hyderabad, for providing XPS spectral data. T. A. Segne is also thankful to the Ethiopian Government for their financial support for doing Ph.D. at the Andhra University, Visakhapatnam, India.