Photocatalytic oxidation of arsenic by ultraviolet radiation

Abstract

The present invention discloses a process of photooxidation of solutions containing As(lll), in which the solution containing As(lll) "a" in a first stage "i" is sent to a photoreactor, to which in a second stage "ii" a photocatalyst "b" and an oxidizing agent "e" are fed, and to which subsequently in a third stage "iii" UV light "c" is provided, to later pass to a fourth stage of solid-liquid separation "iv", wherein in a fifth stage "v" the photocatalyst that is recirculated to stage "ii" is recovered, and wherein the oxidized arsenic solution is sent to a sixth stage "vi" of arsenic removal where an arsenic residue "d" is generated and sent for final disposal.

Description

[0001] PHOTOCATALYTIC OXIDATION OF ARSENIC BY ULTRAVIOLET RADIATION

[0002] The invention discloses a process for the photocatalytic oxidation of arsenic (III) solutions (As(lll)) by using ultraviolet (UV) radiation in the presence of titanium IV oxide (TiOs) and an oxidizing agent.

[0003] TECHNICAL FIELD

[0004] The handling of materials containing arsenic (As) and its derivatives has led to a considerable effort in research and technological development for industrial applications, seeking process sustainability based on scale-ups focused on optimizing variables such as time, energy impact, and water resource consumption. Regardless of the technology chosen for As removal, a significant number of these require an oxidative transformation stage of the metal.Considering the last two decades, within the universe of research related to As oxidation processes, the distribution of the number of works is widely inclined to conventional oxidation processes, through direct treatment with oxidizing agents in aqueous phase, having in second place, hybrid processes in which said oxidizing agents are combined with solid materials, giving rise to oxidation processes in heterogeneous medium, where the adsorption of As in the solid phase plays a fundamental role.A smaller proportion of works, although increasing significantly, considers a series of processes known as “advanced oxidation processes” (AOP), these processes contemplate the oxidation of As by the generation of electron flows mediated by: a) the application of a potential difference in an electrochemical system in order to generate adequate redox reactions or b) irradiation with light of specific wavelengths to induce the transfer of electrons from the conduction band to the valence band in semiconductor materials, where these are transferred to substances capable of generating reactive species responsible for oxidizing the metal.

[0005] For the POA associated with photocatalytic oxidation of As, research is focused on the development of complex photocatalytic materials that allow the use of radiation of varied wavelength (UV, Visible light) optimizing the transfer of electrons resulting from the absorption of radiation and minimizing the recombination of the hole / electron pair, prioritizing the generation of reactive species responsible for the oxidation of As. Similarly, the development of systems that allow efficient irradiation of the reaction medium is also sought, optimizing the use of energy from the radiation sources, as well as systems that allow the greatest exposure to incident radiation of the suspensions containing As.

[0006] GENERAL BACKGROUND

[0007] Arsenic (As) contamination in aquatic environments is a challenge currently faced by more than 70 countries. This poses a serious health problem. The International Agency for Research on Cancer (IARC) has classified arsenic as a human carcinogen based on epidemiological and occupational studies. The agency has linked the ingestion of arsenic in water to skin and lung cancer. As a result of research into arsenic remediation, the upper concentration limit for arsenic in water for human consumption has been decreasing over time. Given the toxicity of As to various human organ systems, the QMS and the United States Environmental Protection Agency (USEPA) have suggested a concentration of 10 pg / L of As in drinking water as a safe limit for human consumption.However, even the 10 pg / L As standard in drinking water would not be safe for human consumption and could induce various types of toxicity.

[0008] Although both natural and anthropogenic sources are responsible for As contamination of surface and groundwater, the main sources of water contamination that pose risks to larger populations worldwide are of natural origin. Arsenic contamination of water under natural environmental conditions is significantly influenced by geological, hydrological, and geochemical factors. Anthropogenic processes responsible for the release of As into different environmental matrices include industrial activities, extensive mining operations, and the use of As-based preservatives and pesticides, resulting in accumulation in atmospheric and terrestrial ecosystems. As contamination affects agriculture, crops, soil characteristics, and bacterial community structure.The population's heavy dependence on freshwater reserves makes it imperative to develop suitable treatment technology for the removal of As in order to provide drinking water free of this element.

[0009] In general, depending on the extent and depth of As contamination, various remediation measures have been developed. These can be classified into at least two categories: biological remediation processes and physicochemical removal techniques.

[0010] Among the biological processes for arsenic removal, bioremediation stands out. This group of processes involves the degradation, transformation, stabilization, or volatilization of contaminants from the source with the help of microorganisms or plants. Another process known as microbial bioremediation is used primarily in soils and utilizes fungi, bacteria, and microalgae for the decontamination of freshwater.

[0011] Physicochemical techniques for arsenic removal correspond to processes in which the preferential interaction of As with substances of various nature allows the physicochemical behavior of the metal to be modified in solution, making its physical removal possible. Among the most common processes are filtration, adsorption, coagulation, and flocculation. The filtration process allows the separation of a solid phase from a liquid phase, and this process can occur through at least two different processes: i) A filter material (e.g., activated carbon, zeolites, clays, etc.)) is capable of interacting with soluble As, through preferential interactions that allow the adsorption of the metal on the filter material, allowing the passage of the liquid free of the contaminant, i) the filter material is capable of retaining insoluble forms of the metal, which, due to their particle size, are retained by the filter material, giving way to the liquid with little presence of the filtered solid.

[0012] Among the aforementioned physical separation processes, modern versions of these processes include electrochemical treatment procedures or techniques, which have attracted considerable attention due to their flexibility and environmental compatibility. Electrochemical separation methods are classified into three types: electrosorption, electrodialysis, and electrocoagulation.

[0013] Regardless of the physical separation process used for arsenic removal, the oxidation state of arsenic is a key factor to consider for its removal from aqueous media. In general, the available technologies are more efficient in removing arsenate (As +5 ) compared to the removal of arsenite (As +3). Studies show that arsenite has no charge at a pH below 9.2 and because of this charge difference between both species, trivalent arsenic cannot be removed simply by methods such as precipitation, adsorption or ion exchange. Oxidation processes for the removal of arsenic from water involve the conversion of As (III) to As (V), that is, from a soluble form to an insoluble form with a high affinity for adsorbent phases and therefore less mobile, so it can be precipitated and removed from the system. Oxidation processes are crucial for the removal of arsenic in groundwater due to anaerobic conditions and the predominance of As (III) in them. For this reason, the oxidation of arsenite to arsenate is essential before executing other removal methods, and for this purpose various physicochemical oxidation methods are used.

[0014] The most common consideration for the classification of oxidation processes is related to the complexity of the process of generation of the reactive species responsible for the oxidative modification of arsenic. Conventional oxidation processes (also known colloquially as "chemical oxidation") are processes derived from the direct treatment of As with chemical agents, whether natural or synthetic, whose decomposition and / or transformation generates the species responsible for oxidizing arsenic, species among which hydroxyl radicals (OH) are a relevant actor, followed by peroxide ( 2 2 ) and superoxide ( 2 2). As(III) can be oxidized by most of the conventional oxidants in current use. Chemical oxidation commonly shows simple kinetic processes, with high reaction rates, however, the efficiency of chemical oxidation is invariably affected by other substances present in the water, so the choice of oxidants to be used is fundamental. Among the most commonly used oxidizing agents are molecular oxygen (O2), ozone (O3), chlorine (CI2), chlorine dioxide (CIO2), hydrogen peroxide (H2O2), hypochlorite (HCIO4) and permanganate (MnO4).

[0015] Conventional oxidation processes can take place in a homogeneous phase (generally in an aqueous medium or gas phase), but they can also occur in a heterogeneous medium, in which the presence of a solid phase can induce modifications in the oxidation process associated, either individually or jointly, with: i) localized concentration of arsenic and / or oxidizing agents by adsorption at the solid / liquid interface, increasing its thermodynamic activity, inducing the displacement of the reaction towards the formation of products, favoring oxidation.

[0016] (i) Modification of the oxidation mechanism, either by modifying the pathway by which the oxidizing species are formed or by carrying out the oxidation of the metal by an alternative pathway to that observed in a homogeneous medium. The most common solid phases used in heterogeneous oxidation processes correspond to phases with high porosity and surface area to volume ratio, for example, metal oxides or highly structured complex minerals, in order to maximize the interaction with the metal and the oxidizing agents, either by affinity or mechanical retention. Photocatalytic processes (photochemical oxidation) involve the use of ultraviolet radiation in the presence of oxygen for the conversion of As(III) to its more soluble form, i.e., As(V), by the generation of hydroxyl ions from Fe(III) species.The use of this methodology generates an additional benefit over oxidation in the aqueous or heterogeneous phase due to the low or non-existent generation of toxic sludge and the low energy required. Another efficient way to generate hydroxyl radicals through photocatalysis is by using titanium dioxide (TiO2) as a catalyst. Heterogeneous photocatalysis has become an efficient alternative for the degradation of many pollutants. This technique uses radiant energy, visible and / or ultraviolet light from artificial light sources or directly from the sun, which, when interacting with a catalyst (semiconductor), generates a charge separation through transfer processes, leading to the formation of reactive oxygen (hydroxyl radicals, superoxide anion, hydrogen peroxide, etc.), species necessary for the oxidation of organic and inorganic pollutants.

[0017] One of the most efficient semiconductors, both in terms of cost and photocatalytic properties, is titanium dioxide (TiO2), a semiconductor material with interesting photophysical properties. Briefly, the ability of a semiconductor like TiO2 to be activated by photon absorption is associated with the energy difference, or bandgap (Eg), that separates its valence band (VB) electrons from their conduction band (CB) counterparts, where Eg is typically less than 5 eV in common semiconductors. Upon excitation light, electrons move from VB to CB, leaving a hole (h+) in the VB. Furthermore, the transferred electrons can participate in the generation of reactive oxygen species (ROS), leading to photocatalytic degradation.The use of TiO2 is limited by the fact that the light absorption corresponding to Eg by crystalline forms of TiO2 falls in the ultraviolet range, which is a minor component of the solar spectrum. This disadvantage of the semiconductor is due to its high Eg value, being 3.2 eV for anatase and 3.02 eV for rutile (variants of crystalline TiO2). The Eg value of a photocatalytic system can be controlled in several ways, the most trivial being the use of different semiconductor materials; however, it is also possible to achieve this by combining semiconductors of different Eg values ​​in structural combinations known as heterojunctions, for example, by coprecipitation or the inclusion of metallic and / or non-metallic elements.

[0018] Photocatalytic oxidation of arsenic

[0019] Photocatalytic arsenic oxidation refers to the process of using light and a photocatalyst to oxidize arsenic compounds in water or soil. The photocatalyst is usually a material such as titanium dioxide (TiO2) or zinc oxide (ZnO) that absorbs light and generates electron-hole pairs, which can then be used to oxidize arsenic compounds. During the photocatalytic oxidation process, arsenic compounds absorb light, and electrons in the valence band of the photocatalyst are excited to the conduction band. This creates a hole in the valence band, which acts as a powerful oxidizing agent. The photocatalytic oxidation process can be performed with different photocatalysts and light sources. The choice of photocatalyst and light source can affect the efficiency of the process and the conversion rate of arsenic species.

[0020] Overall, photocatalytic arsenic oxidation is an effective method for treating arsenic-contaminated water and soil, allowing for efficient removal of contaminants and conversion to less toxic forms. However, it also has its own limitations and drawbacks, such as the need for high-intensity light sources and the potential for the formation of byproducts that can be harmful to the environment.

[0021] PRIOR ART

[0022] The photocatalytic activity for the oxidation of arsenic has been evaluated in the presence of iron and organic complexing agents (Hug, S.J., Canónica, L., Wegelin, M., Gechter, D., and von Gunten, U. Solar Oxidation and Removal of Arsenic at Circumneutral pH in Iron Containing Waters, Environmental Science & Technology 2001 35 (10), 2114-2121 ), as a simple method of water purification in areas with low access to drinking water. Thermal and photochemical oxidation assays of As(lll) on a time scale of hours, in water containing 500 pg / L As(lll), 0.06-5 mg / L Fe(l 1 , 111) and 6 mM 4-bicarbonate at pH 6.5-8.0, in the presence of high concentrations of citrate, showed that more than 90% of the As(lll) could be photochemically oxidized in 2-3 h by illumination with 90 W / m³ UV-A light. 2.Similarly, the oxidation of As(lll) has been studied in irradiated ferrioxalate solutions as a function of pH (3-7), As(lll), Fe(lll), and 2-propanol concentration. (Kocar, BD and Inskeep, WP, Photochemical Oxidation of As(lll) in Ferrioxalate Solutions, Environmental Science & Technology 2003 37 (8), 1581 -1588) The oxidation rates of As(lll) (0.5-254 pM h-1 ) were first order at varying As(lll) and Fe(lll) concentrations and increased with decreasing pH.

[0023] The combination of iron and oxidizing agents has also been evaluated in photocatalytic As oxidation processes in an iron(II l) / sulfite system under visible light using sunlight or a light-emitting diode lamp (Xu, J., Ding, W., Wu, F., Mailhot, G., Zhou, D., and Hanna, K. (2016). Rapid catalytic oxidation of arsenite to arsenate in an iron (II l) / sulfite system under visible light. Applied Catalysis B: Environmental, 186, 56-61.), observing a high As(lll) oxidation efficiency at near-neutral pH, while 93% of As(lll) was removed from the solution by centrifugal treatment after 30 min of irradiation. The As(lll) oxidation pathways at circumneutral pH involve free radicals (H O, SO 4- * and SO 5 *) and charge transfer from the ligand to the metal between As(lll) and colloidal ferric hydroxide. The sequential addition of sulfite improves the oxidation efficiency of As(lll) (66.7 pM).

[0024] In a similar line, bifunctional mesoporous TiO s composites (meso-TiO sj / a-FesO s, synthesized by Fe impregnation 3+on meso-TiOs followed by calcination at 300 °C, (Zhou, W., Fu, H., Pan, K., Tian, ​​C., Qu, Y., Lu, P., & Sun, C.C. (2008). Mesoporous T¡O2 / a-Fe2O3: bifunctional composites for effective elimination of arsenite contamination through simultaneous photocatalytic oxidation and adsorption. The Journal of Physical Chemistry C, 112(49), 19584-19589.) showed synergy of the photocatalytic ability of meso-TiOs for the oxidation of As(III) to As(V) and the adsorption performance of a-FesOs for As(V). The meso-TiOs / a-FesOs composites oxidized As(III) to As(V) with high efficiency at various pH values. At the same time, As(V) was effectively removed by adsorption onto the surface of the composites. As(V) can be easily desorbed from the composites by heat treatment in an alkaline solution. After several reuses, the composites exhibited catalytic and adsorption performance comparable to that of the initial use.

[0025] Mixed magnetite-TiOs nanoparticles synthesized, (Wu, S., Hu, W., Luo, X., Deng, F., Yu, K., Luo, S., ... & Zeng, G. (2013). Direct removal of aqueous As (III) and As (V) by amorphous titanium dioxide nanotube arrays. Environmental technology, 34(15), 2285-2290) showed that under UV light, As (III) can be oxidized to As (V) by O2 dissolved in y-FesOs-TiOs nanoparticle suspensions at various pH values. At the same time, As (V) was removed by adsorption on the nanoparticle surface.

[0026] Enhanced As(lll) removal by simultaneous photooxidation and scavenging process using layered double hydroxide (LDH) supported by TiOs nanoparticles (TiOs / LDH) has also been studied (Lee, SH, Kim, KW, Choi, H., & Takahashi, Y. (2015). Simultaneous photooxidation and sorptive removal of As (III) by TiOs supported layered double hydroxide. Journal of Environmental Management, 161 , 228-236).

[0027] UV radiation enhanced the removal rate of As(III), based on the continuous conversion of As(lll) to As(V), and that the removal rate was faster under alkaline conditions than under acidic and neutral conditions due to the abundance of oxidants and negatively charged As(lll) species (pKa: 9.2).

[0028] Complex TiOs-based materials have been considered to optimize the photocatalytic oxidation of As using sunlight as the energy source. Palladium-modified nitrogen-doped titanium oxide nanoparticles (TiON / PdO) (Li, QI, Easter, NJ, & Shang, JK (2009). As(III) removal by palladium-modified nitrogen-doped titanium oxide nanoparticle photocatalyst. Environmental science & technology, 43(5), 1534-1539) showed a high degree of As(III) removal under visible light illumination in photocatalysts.

[0029] DESCRIPTION OF FIGURES

[0030] Figure 1. Shows the concentration of total arsenic and arsenic (III) as a function of irradiation time in an internal irradiation system with UV-C lamps, in a borosilicate glass reactor containing a suspension of TiO24 g / L. 2 y Ace to tai = 1 g / L; H2SO4 = 20 g / L; pH = 2.66, Temperature = 30°C. Air flow: No flow (Top); With flow, dissolved O2: 5.2 - 9 mg O2 / L (Bottom). Number of UV-C (254 nm) 25 W lamps = 4; Suspension volume = 8 L.

[0031] Figure 2. (Above) Shows the concentration of total arsenic and arsenic (III) as a function of irradiation time in an internal irradiation system with UV-C lamps, in a borosilicate glass reactor. totai = 1 g / L; H2SO4 = 25 g / L; pH = 0.5; Temperature = 30°C. Dissolved O2: 4.5 - 8.8 mg O2 / L Number of UV-C lamps (254 nm) 25 W= 4; Suspension volume = 8 L. (Below) Shows the relationship between arsenic concentration at a given time and initial arsenic concentration (Aso) as a function of irradiation time at pH = 0.6 (P8 table 1 ) and pH = 2.5 - 3 (P7 table 1 ).

[0032] Figure 3. Shows the concentration of total arsenic and arsenic (III) as a function of irradiation time in an internal irradiation system with UV-C lamps, in a borosilicate glass reactor. to tai = 2.8 g / L; Fe to tai = 8.2 g / L; Cu to tai = 19.8 g / L; H2SO4 = 25 g / L; pH = 0.5; Temperature = 30°C, dissolved O2: 4.0 - 7.5 mg O2 / L Number of UV-C lamps 25 W= 4; Suspension volume = 8 L.

[0033] Figure 4. Shows the relationship between arsenic concentration at a given time and initial arsenic concentration (Aso) as a function of irradiation time with UV-C light (254 nm) for different radiation doses. to tai = 2.0 g / L; Fe to tai = 5 g / L; Cu to tai = 11 g / L; H2SO4 = 75 g / L; pH = 0.5; Temperature = 40°C, dissolved O2: 4.0 - 7.5 mg O2 / L P11 - P14 Suspension volume = 8 L. P15 Suspension volume = 1.5 L.

[0034] Figure 5. Shows the percentage of oxidized arsenic at a fixed irradiation time as a function of the radiation dose applied to TiO2 and PLS suspensions, determined from data in Figures 1 to 3.

[0035] Figure 6. Shows the time required to reach 80% oxidation of As(III) as a function of the UV-C radiation dose required to achieve this consumption. Figure 7. Shows the relationship between arsenic concentration at a given time and initial arsenic concentration (Aso) as a function of irradiation time for different UV-C radiation doses (254 nm). As to tai = 10 g / L; Fe to tai = 20 g / L; Cu to tai = 39 g / L; H2SO4 = 75 g / L; pH = 0.5; Temperature = 40°C, dissolved O2: 5.1 - 8.3 mg O2 / L. Suspension volume = 1.5 L.

[0036] Figure 8 shows the time required to reach 80% As(III) oxidation as a function of the UV-C radiation dose required to achieve this consumption. Data taken from the profiles in Figure 7.

[0037] Figure 9. Shows the relationship between arsenic concentration at a given time and initial arsenic concentration (Aso) as a function of irradiation time with UV-C light (254 nm) at a fixed radiation dose (66 W / L). to tai = 2.0 g / L; Fe to tai = 7 g / L; Cu to tai = 1 1 g / L; H2SO4 = 75 g / L; pH = 0.5; Temperature = 40°C, dissolved O2: 2.0 - 5.0 mg O2 / L. Suspension volume = 1.5 L. Curve for oxidation of As in the presence of TiO2 4 g / L (■); Curve for oxidation of As in the presence of TiO2 (A); Curve corresponding to the difference between the oxidation curves in the absence and presence of TiO2 (•).

[0038] Figure 10. Shows the oxidation profiles of As (III) at constant UV-C radiation doses (254 nm) (66 W / L) at different photocatalyst concentrations.

[0039] Figure 11. Shows the elemental contents of As, Fe, and Cu in the photocatalytic material (TIO2) recovered in TIO2 / UV-C (254 nm)-mediated photocatalytic oxidation tests of Arsenic (III). Elemental determination was performed on the dry material obtained after acid washing and neutral washing of the solid.

[0040] Figure 12. Shows the X-ray diffraction pattern for TiO2 Aeroxide - P25, used as a lead solid in the photocatalyzed oxidation process of As (III). Phase designation: TiO2a = Anatase; TiO2r = Rutile.

[0041] Figure 13. Shows the X-ray diffraction pattern for T¡Ü2 recovered after the photocatalyzed oxidation process of As (III) according to test P20. (As to tai = 10 g / L; Fe totai = 20 g / L; Cutotai = 39 g / L; H2SO4 = 75 g / L; pH = 0.5; Temperature = 40°C. Dissolved O2: 5.1 - 8.3 mg O2 / L Suspension volume = 1.5 L; Radiation dose = 33 W / L. (Below) Mineralogical phase composition estimated by Rietveld analysis of the presented diffraction data.

[0042] Figure 14. (Top) Shows the As oxidation profiles photocatalyzed by TiCWV-C (254 nm) for successive oxidation cycles carried out with TiO2 recycled between tests. Irradiation dose = 66 W / L; As = 3 g / L - Fe = 5 g / L - Cu = 10 g / L; pH = 0.5; H2SO4 = 75 g / L. (Bottom) Shows the photocatalyst activity, expressed as percentage of As(III) oxidation at a defined irradiation time for the catalytic cycles considered. Figure 15. (Top) Shows the As(III) oxidation kinetics in PLS+TiO2 suspensions irradiated with UV-C light (254 nm) in a semi-continuous flow system at a fixed radiation dose (As=10 g / L). (Below) During the course of the irradiation process, no changes are observed in the total concentration of As, Fe and Cu.

[0043] Figure 16. Shows the photooxidation of As (III) in a stirred photoreactor. (A) P1 - 240 L O2 / hr - 1000 W (100 W / L) - 4 g / LT¡O2; (•) P2 - 240 L O2 / hr - 1000 W (100 W / L) - Without T¡O2; (■) P3 - 30 L O2 / hr - 500 W (50 W / L) - 4 g / LT¡O2.

[0044] Figure 17. Shows the photooxidation of As (III) in a stirred photoreactor. Process carried out with a change in the intensity of irradiation with UV-C light (254 nm), with a power variation from 500 to 1000 W P3 - 30 L O2 / hr - 500 W (50 W / L) - 4 g / LT¡O2; P4 - 30 L O2 / hr - 1000 W (100 W / L) - 4 g / LT¡O2.

[0045] Figure 18 shows the photooxidation of As(III) in a stirred reactor. Constant irradiation power (1000 W); radiation dose 50 W / L. (•) P1 - 250 L O2 / hr; (■) P6 - 150 L / hr.

[0046] Figure 19. Shows the oxidation of arsenic(lll) using a radiation dose of 16.6 W / L UV-C (254 nm) and a hydrogen peroxide to As(lll) ratio of 0.5 mol H202 / mol As(lll), at a dissolved oxygen concentration between 5-8 mg / L.

[0047] Figure 20. Shows the oxidation of arsenic (III) using a radiation dose of 33.3 W / L with a medium pressure mercury lamp with UV-C radiation (254 nm) and a ratio of hydrogen peroxide to arsenic (III) between 0.2 and 0.5 mol H202 / mol As(III), at a dissolved oxygen concentration between 5-8 mg / L.

[0048] Figure 21. Shows the oxidation of arsenic(lll) using a radiation dose of 10 W / L with a LED light-emitting diode lamp with UV radiation wavelength 365 nm and a hydrogen peroxide to arsenic(lll) ratio of 0.5 to 1.0 mol H202 / mol As(lll), at a dissolved oxygen concentration between 10-15 mg / L.

[0049] Figure 22. Shows the oxidation of arsenic(lll) using a radiation dose of 16 W / L of UV-C radiation and a sodium metabisulfite (MBS) to arsenic(lll) ratio of 0 to 1.0 mol / mol.

[0050] Figure 23. Shows the oxidation of arsenic(lll) using a radiation dose of 16 W / L of UV-C radiation and a sodium peroxydisulfate (SDP) to arsenic(lll) ratio of 0 to 1.0 mol / mol.

[0051] Figure 24. Shows the photooxidation process of arsenic, where the solution containing As(lll) “a” in a first stage “i” is sent to a photoreactor, to which in a second stage “i” a photocatalyst r “b” and an oxidizing agent “e” are fed, and to which subsequently in a third stage “iii” UV light “c” is provided, to subsequently move to a fourth stage of solid-liquid separation “iv”, where in a fifth stage “v” the photocatalyst is recovered and recirculated to stage “i”, and where the oxidized arsenic solution is sent to a sixth stage “vi” of arsenic abatement where an arsenical residue “d” is generated which is sent for final disposal.

[0052] DESCRIPTION OF THE INVENTION

[0053] The invention discloses a photocatalytic oxidation process of As(lll) by using UV radiation.

[0054] More specifically, the present invention discloses a photocatalytic process with titanium IV oxide (TiOsj / ultraviolet light in arsenic (III) solutions.

[0055] One of the relevant aspects of the invention is that it demonstrates that it is possible to oxidize arsenic by irradiating titanium (IV) oxide (TiOs) suspensions with UV-C radiation, both in solutions containing only arsenic, as well as in complex solutions containing other metals in high concentrations. A relevant aspect with respect to the prior art is that the proposed process allows the photooxidation of arsenical solutions with As(III) concentrations greater than 1 g / L. The TiOs / UV-C photocatalyzed oxidation process takes place in processes of varying complexity, for example, in internal irradiation systems in a stirred reactor and irradiation in semi-continuous flow systems. There is a correlation between the arsenic oxidation rate and the radiation dose, expressed as the power of the radiant light source per liter of irradiated suspension (W / L).The increase in radiation intensity improves the activity of the photocatalyst, however, at the highest light intensities evaluated, secondary oxidation processes not dependent on the activity of the photocatalyst occur; these processes contribute to the higher rates of As oxidation observed.

[0056] Another relevant aspect of the invention is that for ideal solutions of As(lll) at a concentration of 1 g / L at a sulfuric acid concentration of 20 g / L it was possible to obtain oxidation yields close to 94%, at a power of 12.5 W / L. In synthetic solutions of the PLS type originating from the leaching of foundry dusts with an As(lll) concentration of 2 g / L and in the presence of ferrous ion and copper, at a power input of 34 W / L it was possible to obtain a 99% oxidation yield after 24 h of oxidation.

[0057] In order to study the oxidation kinetics at scalable power inputs, oxidation tests were performed in stirred photoreactors. The invention demonstrates that by selecting a stirred photoreactor, it is possible to oxidize As(lll) in solutions of sulfuric acid plant effluents (SAP) containing 11 g / L of arsenic, reaching oxidation rates of 1 g As / h. The optimization of the photooxidation process would be given by an improvement in the mass transport processes, due to much more efficient agitation, as well as radiation sources that allow a better photon flux in addition to an optimization of the irradiation interface between the lamp and the suspension to be irradiated.

[0058] The invention demonstrates that the photocatalytic material obtained after an As photooxidation process retains 80% of its activity after 5 consecutive oxidation cycles, where the solid retains its initial composition, in terms of the proportion of crystalline phases required in the material for the photocatalytic phenomenon to occur.

[0059] Additionally, the present invention shows that the addition of an oxidizing agent generates a catalytic effect in the photooxidation of arsenic(III), allowing the oxidation kinetics of the system to be accelerated.

[0060] Among the most commonly used oxidizing agents are sodium metabisulfite, sodium persulfate, molecular oxygen (O2), ozone (O3), chlorine (CI2), chlorine dioxide (CIO2), hydrogen peroxide (H2O2) and hypochlorite (HCIO). 4- ) and permanganate (MnO 4- ).

[0061] Furthermore, the present invention demonstrates that conventional UV-C radiation generation systems, such as medium-pressure mercury lamps, are useful for generating photons that generate oxidative capacity in arsenic solutions. Similarly, the use of light-emitting diode (LED) systems generates photons within the UV range that are also capable of generating arsenic(III) oxidation.

[0062] Specifically, the present invention discloses a photooxidation process of solutions containing As(lll), comprising the following steps:

[0063] 1. A photooxidation process for solutions containing As(lll), comprising the steps of: i. providing a first solution containing As(lll) in a photoreactor provided with a UV radiation system;

[0064] i. adding to the photoreactor of step (i) a titanium (IV) dioxide photocatalyst and an oxidizing agent to generate a first pulp; iii. turning on the energy source of the UV radiation system to begin the oxidation of As (III) to As (V) and form a second pulp; iv. transferring the second pulp formed in step (i) to a solid-liquid separation stage to generate a first oxidized arsenic solution and the photocatalyst; v. recirculating the photocatalyst from step (iv) to stage (i); vi. sending the first oxidized arsenic solution from step (iv) to an arsenic abatement process.

[0065] In a preferred option!, the first As(lll) solution has an As(lll) concentration of at least 1 g / L.

[0066] In a preferred option! Among the most commonly used oxidizing agents we can mention sodium metabisulfite, sodium persulfate, molecular oxygen (O2), ozone (O3), chlorine (CI2), chlorine dioxide (CIO2), hydrogen peroxide (H2O2) and hypochlorite (HCIO 4 ) and permanganate (MnO 4 ).

[0067] In a more preferred option! the oxidizing agent in step (i) is hydrogen peroxide in a molar ratio of 0.2 to 1.0 mol FhC^ / mol As(lll).

[0068] In another preferred option, the UV radiation system in stage (i) preferably generates UV-B or UV-C radiation.

[0069] In an even more preferred option!, the UV radiation system of stage (i) preferably generates UV-C radiation.

[0070] In an even more preferred option, the radiation system provides a radiation dose between 0.5 and 800 W / L.

[0071] In another preferred option, the radiation system of stage (i) is provided externally.

[0072] In another preferred option, the radiation system for stage (i) is provided internally.

[0073] In another preferred option, the photoreactor of stage (i) is a flow system.

[0074] In an even more preferred option, the photoreactor of step (i) is provided with stirring.

[0075] In another preferred option, the photocatalyst of stage (i) is titanium dioxide (IV).

[0076] In another preferred option, the titanium (IV) dioxide photocatalyst has a mixture of anatase and rutile in the proportion of 84% Anatase and 16% Rutile.

[0077] In an even more preferential option, the photocatalyst is Aeroxide P25.

[0078] In another preferred embodiment, the photocatalyst is added at a concentration of between 1 and 8 g / L, more preferably between 1 and 4 g / L, and most preferably 4 g / L. In another preferred embodiment, step (iii) is carried out at a temperature range of between 20 and 50°C.

[0079] In another preferred option, in step (iii) the concentration of dissolved oxygen in the solution must be maintained within the range of 1 to 10 mg / L, which can be provided by supplying oxygen or air.

[0080] In another preferred option, step (iii) is carried out in a pH range less than 3, more preferably in a pH range less than 1.

[0081] In another preferred option, stage (iii) has a residence time of 1 to 30 hours, more preferably 1 to 10 hours.

[0082] Alternatively, UV radiation is provided by medium-pressure mercury lamps or LED lamps.

[0083] In another preferred option, the solution produced by the photooxidation process is sent to an arsenic reduction process, which can even more preferably be selected from one of the forms of scorodite or calcium arsenate, without necessarily being limited exclusively to these last two methods.

[0084] APPLICATION EXAMPLES

[0085] Materials and assembly

[0086] As photooxidation test setup

[0087] An experimental setup was carried out at laboratory level that considered external irradiation systems, internal irradiation in a stirred reactor and internal irradiation in a flow system.

[0088] Materials

[0089] Thermocouple

[0090] 10 L Reactor - Schott Borosilicate Glass

[0091] 2 L reactor - Polypropylene

[0092] Teflon impeller and blades

[0093] Mechanical agitator

[0094] Drying oven

[0095] Glassco Vacuum Filtration System

[0096] 2,000 and 4,000 mL Kitasato flask.

[0097] Marprene® hoses. Watson Marlow 530 and 630 peristaltic pumps.

[0098] Filter paper

[0099] Whatman nylon filter paper.

[0100] Thermometer

[0101] Schott and SI Analytics brand pH electrode.

[0102] Solsim-Luzchem external irradiation system

[0103] Econizer UVnizer Ozone flow irradiation system without ozone generation system.

[0104] Hitachi UV-B lamps, nominal power 8 W.

[0105] UV-C lamps (G25WT5 / 4P-SE), nominal power 25 W.

[0106] UV-C lamps, nominal power 40 W.

[0107] Reagents sulfuric acid 98%

[0108] Distilled water

[0109] Arsenic (III) oxide, Sigma-Aldrich

[0110] Titanium (IV) oxide Aeroxide P25, Evonik

[0111] Iron (II) sulfate heptahydrate

[0112] Copper (II) sulfate pentahydrate

[0113] Example 1

[0114] Oxidation of As by external irradiation of As (III) + TiOs suspensions with UV-B light (280 - 315 nm).

[0115] Eight Hitachi FL8BL-B lamps (365 nm) with a nominal power consumption of 8 W are installed in the respective lateral positions, distributed equally on both sides of the photoreactor.

[0116] Introduce a magnetic stirrer in the central position of the photoreactor, in order to ensure adequate ventilation of the same, minimizing overheating of the sample.

[0117] Pour the suspension of As(III) solution and titanium(IV) dioxide Aeroxide P25 into a glass container. Insert a magnetic stirrer and place the container on the magnetic stirrer. Start stirring and wait at least 10 minutes before turning on the lamps to ensure the suspension is homogeneous.

[0118] Once the suspension is homogenized, close the reactor door and turn on the lamps, along with the air circulation cooling system. Once the lamps are turned on, the photocatalyzed As oxidation reaction begins.

[0119] Collect samples of the oxidizing suspension at the respective residence times, filter the suspension and determine the concentration of total As and As (III).

[0120] Example result 1

[0121] In the tests carried out on the concentration of total arsenic and arsenic (III) as a function of irradiation time in an external irradiation system with UV-B lamps, in a borosilicate glass reactor containing 200 mL of suspension of TiOs and As to tai = 1 g / L; H2SO4 = 20 g / L; pH = 2.66, Temperature = 30°C, TiOs concentration: 1 g / L and 4 g / L.

[0122] The results of the tests carried out on As(III) and TiO2 suspensions with different photocatalyst concentrations (1 g / L and 4 g / L), irradiated externally with UV-B ultraviolet light, with a total irradiation power of 64 W (8 lamps of 8 W), show considerable differences in the oxidation efficiency depending on the photocatalyst concentration. At the lowest photocatalyst concentration (1 g / L) tested, no differences were observed between the total arsenic (As(III) and TiO2) concentrations. totai) and As (III) in the system during the residence time considered to irradiate the samples, indicating zero oxidation of As in the system. By increasing the photocatalyst concentration to 4 g / L, maintaining the irradiation conditions, differences are observed between the concentration of As to tai and As(III), which become more pronounced as the irradiation time progresses, reaching an As(III) concentration of 0.8 g / L after 6 hours, corresponding to 20% oxidation of As(lll). From the decrease in the As(lll) concentration, it is possible to establish that the As(lll) oxidation rate corresponds to 0.03 g As(11 l) / h.

[0123] The low rate of As oxidation observed at the maximum photocatalyst concentration (4 g / L) corresponds to an acceptable result, taking into account the selected irradiation conditions, which allow providing the reaction mixture with a radiation dose equivalent to 0.5 W / L. Example 2

[0124] Irradiation process of Arsenic (III) + T1O2 suspensions with UV-C light sources (100 - 280 nm) immersed in a stirred reactor.

[0125] In a 10 L Schott Duran borosilicate glass reactor, place UV-C lamps (without electrical connection) in their corresponding quartz tubes inside the reactor, equidistant from each other along the entire length of the reactor's inner wall, to ensure homogeneous irradiation of the samples. Ensure the lamps are positioned at the same depth.

[0126] Add the suspension of As (II Ij+TiOs (8 L) using a peristaltic pump. Once inside the reactor, keep the suspension under constant stirring by using the blade rotor. Maintain stirring (minimum 500 rpm) for at least 20 min before turning on the lamps.

[0127] For air-bubbled reactions, a fritted glass cap is connected to a plastic tube connected to a peristaltic pump (Watson Marlow QDOS30, flow rate 500 ml / min).

[0128] Connect the UV-C lamps to their respective power sources. Once the lamps are turned on, the oxidation process begins simultaneously.

[0129] Example 3

[0130] Irradiation process in flow system for Arsenic (III) + T1O2 suspensions with UV-C light sources (100 - 280 nm).

[0131] In a stirred tank, add the As(III) + TiOs Aeroxide P25 suspension and pump it into the photoreactor flow system until the suspension moves freely through the system and is able to completely fill the irradiation segments. Recirculate the suspension for 10 to 25 minutes. Turn on 40 W UV-C radiation inside the photoreactor, thus starting the photooxidation process.

[0132] Collect samples of the oxidizing suspension at the respective residence times using a measuring cylinder, taking a volume of suspension directly from the circulation hose between the flow system and the stirred tank. Filter the suspension using a vacuum filtration system and 45 mm diameter nylon membranes with a pore size of 0.45 µM (Whatman). Determine the As concentration. to tai and As (III). Example 4

[0133] Protocol for irradiation of Arsenic (III) + T1O2 suspensions with UV-C light sources (100 - 280 nm) immersed in a stirred photoreactor.

[0134] Pour 10 L of the working solution and 80 g of photocatalyst into a container and transfer to the photoreactor using a peristaltic pump.

[0135] When the photoreactor is filled with the suspension to be oxidized, start the flow of oxygen / air from the cylinder and start the agitator operating at the final stirring speed.

[0136] Set the lamp to the target power (500-1000 W) and turn it on. This initiates the photooxidation process. Take samples every hour and analyze them to determine the As concentration. to tai and As (III). Once the established oxidation time has elapsed, turn off the lamp in order to stop the oxidation.

[0137] Results Examples 2 and 3

[0138] Oxidation of As by internal irradiation of As (III) + T1O2 suspensions with UV-C light (100 - 280 nm) in a stirred reactor with immersed lamps and flow system The test plan associated with the photocatalytic oxidation tests of As in stirred reactors with immersed lamps and in a flow system is detailed in Table 1.

[0139] Table 1. Summary of experimental test set for photocatalytic oxidation of As (III) in stirred reactors with immersed lamps.

[0140] Photooxidation of As (III) solutions

[0141] Based on the limited oxidation performance shown by the photocatalyst in the external UV-B light irradiation configuration, tests were developed by modifying two critical parameters for the photocatalytic process: i) Minimizing the distance from the light source to the reaction mixture, which is optimized by introducing the light source inside the reactor in direct contact with the photocatalyst suspension;

[0142] ¡i) By increasing the energy of the incident radiation, which is achieved by modifying or reducing the wavelength of the radiation emitted by the light source, in this case, by modifying the light source from a majority emission in the UV-B range (280 - 315 nm) with a maximum emission of the radiation source at 365 nm, to a light source with majority emission in the UV-C range (100 - 280 nm) with a maximum emission centered at 254 nm. Solutions of As (0.7 g / L) in the presence of T¡O2 (4 g / L) irradiated with 4 UV-C lamps (25 W nominal power) do not show a significant difference between the concentration of As (III) and the concentration of As to tai (Figure 1, P5 see Table 1), a trend observable over a residence time of 7 h.

[0143] For irradiation times longer than 7 h, a decrease in the As(III) concentration attributed to the occurrence of the photocatalyzed oxidation process can be observed, with an As(III) oxidation rate of 0.07 g of As(III) / h. A moderate decrease in the pH is observed. ?reaching a value of 2.3 (initial pH = 2.8). By carrying out the same oxidation process, but incorporating an air flow, (Figure 1 , P6 Table 1 ). The latter allows increasing the dissolved oxygen (DO) concentration from 2.3 mg / L to 5.2 - 9 mg / L at 30°C, the maximum temperature reached when irradiating the mixture at extended residence times. A progressive decrease in the As(III) concentration is observed at shorter residence times (1 .5 h). The decrease in the As(III) concentration is monotonic, with an oxidation rate of 0.04 g As(lll) / h. In relation to the pH, a decrease is observed, reaching a value of 2.2, comparable to that reached in the test without air flow. For both tests (P5 and P6 Table 1) no decrease in the concentration of Astotai is observed, ruling out a significant adsorption of arsenic in the photocatalytic material.

[0144] Another aspect considered corresponds to the control of the presence of amorphous material in TiOs, which should be composed only of crystalline phases, in a mass ratio of Rutile:Anatase of 25:75.

[0145] It is possible to remove the amorphous material potentially present in TiOs by calcining at 420°C for 2 h, which is known as “activation” of the photocatalyst.

[0146] In As(III) oxidation tests on activated TiO2 suspensions (4 g / L) irradiated with 100 W UV-C in the presence of air flow, a decrease in As(III) concentration similar to that observed for test P6 (Table 1 ) carried out under similar conditions but in the presence of unactivated catalyst was observed. In both cases, in the presence of activated / unactivated photocatalyst, the determined oxidation rate corresponds to 0.04 g As / h. The zero difference in As oxidation rate observed for activated / unactivated TiO2 indicates that the content of amorphous material in the solid is minimal (see Figures 1 and 2).

[0147] Photocatalytic oxidation of As below those tested (pH = 2 - 3) would be affected in its performance, with lower oxidation rates being expected. Figure 2 (top) shows that, under similar irradiation conditions (4 UV-C lamps 25 W, 4 g / L TiOs, DO = 4.5 - 8.8 mg O2 / L, T = 30°C), a decrease in the pH of As(III) + TiOs suspensions from pH 3.0 to 0.6 increases the oxidation rate up to 0.125 g As / h, a three-fold increase compared to the minimum value previously obtained for the As oxidation rate of 0.04 g As / h.

[0148] Under UV-C conditions at 100 W, 4 g / L TiO2, DO = 4.5 - 8.8 mg O2 / L, T = 30°C, it was observed that at a pH of 0.6 to 3, an oxidation rate of up to 0.125 g As / h was obtained, whereas at pH values ​​greater than 3, an As oxidation rate of 0.04 g As / h was observed. (See figure 2 below). Example 5

[0149] Photooxidation of arsenic in synthetic PLS solutions

[0150] The oxidation of As in TiO2 + synthetic PLS suspensions (2.8 g / L As(lll); 8.2 g / L Fe; 19.8 g / L Cu) irradiated under previously standardized conditions (TiO2 = 4 g / L; Total volume = 8 L; Temperature = 30°C; dissolved oxygen (DO) = 4.0 - 7.5 mg O2 / L; pH = 0.5) takes place with an oxidation rate equivalent to 0.06 g As(lll) / h comparable to that observed in previous tests. During the irradiation process and subsequent oxidation of As(III) there was no evidence of oxidation of the ferrous ions present in the suspension (Figure 3), which are found in a proportion four times greater than the arsenic concentration considered, with no significant differences observed between the Fe concentration and the Cu concentration. totai and Fe(II), ruling out significant adsorption processes between the element and the photocatalytic material. Similarly, the copper concentration remains unchanged throughout the photocatalytic process (see Figure 3). This means that oxidation using TiO2 is selective toward As, showing no effects on Fe or Cu.

[0151] In order to improve the oxidative process in PLS solutions, the increase in power per liter (W / L) irradiated to the photoreactor was evaluated.

[0152] Figure 4 shows the change in the oxidation profiles of PLS ​​solutions (1 g / L Cu; 5 g / L Fe(II); 2 g / L As(lll)). An increase in the oxidation rate is observed with increasing radiation dose (W / L). The increase in radiation dose modifies the arsenic oxidation rate from 0.05 g As / h (13 W / L) to 0.20 g As / h (67 W / L). This is an almost proportional increase in the oxidation rate (approximately 4-fold) with increasing light intensity (approximately 5-fold) (see Figure 4).

[0153] Despite the almost direct proportionality established between the As oxidation rate and the increase in radiation dose, the increase is not strictly linear. When comparing the percentage of oxidized arsenic (III) as a function of the radiation dose in the system (Figure 5), it is observed that energy increases to obtain radiation doses above 25 W / L at long irradiation times (10 h) improve the As oxidation rate more efficiently, accounting for the potential existence of secondary oxidation processes not necessarily dependent on the activity of the photocatalyst, but rather intrinsically generated by the interaction of high-intensity radiation with other relevant factors present in the heterogeneous reaction mixture under irradiation (see Figure 5).

[0154] From a projection (Figure 6) associated with the intensity of incident radiation in the reaction mixture and the time required to reach a given percentage (80%) of oxidized arsenic, it is possible to estimate the radiation dose required to oxidize said percentage of arsenic present in the irradiated PLS solution (11 g / L Cu; 5 g / L Fe (II); 2 g / L As (lll)) at the required irradiation time. To achieve total oxidation of As in 10 h, the required radiation dose would correspond to 48 W / L, a dose that increases to 130 W / L if the irradiation time is reduced to 1 h.Under similar irradiation conditions and characteristics of the reaction mixture, under the premise that the increase in the concentration of As (without modifying the other components of the PLS solution) does not significantly modify the molar extinction coefficient of the reaction medium to UV-C radiation, nor the ability of the photocatalyst to interact with the radiation by increasing the arsenic concentration to 11 g / L, the radiation dose required to oxidize 80% of As would correspond to 260 W / L, while reducing this time to 1 h increases this value to 800 W / L. (see figure 6).

[0155] Considering the projections discussed above, As oxidation tests in higher concentration PLS solutions (39 g / L Cu; 20 g / L Fe(II); 10 g / L As(lll)) irradiated with incremental radiation doses, resulted in As oxidation rates of 0.06 g As / h at 16 W / L, which increased to 0.25 g As / h at 50 W / L, with no significant improvement in the oxidation rate, with a value of 0.27 g As / h, when increasing the radiation dose to 66 W / L. At the highest radiation doses considered, 72 h are required to completely oxidize the arsenic present in the reaction volume considered (1.5 L of suspension). Similar to what was evaluated for dilute PLS solutions, the radiation dose required to oxidize the As present in the PLS solutions concentrated in As, Cu and Fe was estimated. Figure 8 shows the corresponding residence time profile to reach 80% As oxidation and the required radiation dose.To achieve 80% oxidation of As(III) in 1 hour in PLS solutions (As(III) = 10 g / L; Cu = 39 g / L; Fe(II) = 20 g / L) a radiation dose of 200 W / L would be required, a value well below that projected using the curve for 2 g / L of As (800 W / L). Consequently, this comparison shows that at higher arsenic concentrations there would be a more efficient use of the power delivered to the system to produce oxidation, (see figures 7 and 8).

[0156] Although, at similar radiation doses, the PLS solution with higher concentration of As requires a longer residence time for equivalent oxidation percentages (we know that the oxidation rate does not depend on the arsenic concentration but on the capacity to produce oxidizing species, which does depend on the radiation intensity), the curve obtained for solutions with higher concentrations of As, Cu and Fe shows, especially for the lower radiation doses evaluated, a greater efficiency in reducing the oxidation time with increasing radiation dose.These results would be associated with a previously discussed phenomenon, corresponding to the generation of oxidizing species by alternative pathways not associated with the activity of the photocatalyst itself. In the tests carried out at the highest light intensity achieved in the systems considered, photolytic processes would take place, for example, photolysis of the water molecule, with the consequent generation of hydroxyl radicals, or interaction between UV-C light and dissolved oxygen, giving way to the generation of ozone or superoxide anions.A comparison of the oxidative activity in PLS solutions in the absence and presence of photocatalyst (4 g / L; Figure 9), subjected to the highest radiation dose studied (66 W / L), shows that the contribution of the photocatalytic processes (mediated by TiO2) and photolytic processes (derived only from the interaction between light and the components of the aqueous phase) would contribute to the oxidation of arsenic in a percentage ratio of 60 / 40 respectively. The oxidation profile obtained for the tests in the absence of photocatalyst shows a progressive decrease in the arsenic (III) concentration due to the presence of photogenerated oxidizing species up to a residence time of 10 h (in As (III) = 2 g / L solutions); however, the oxidation of As (III) is limited at longer residence times (see Figure 9).

[0157] Increasing the photocatalyst concentration to 8 g / L above the standard concentration (4 g / L) showed no effect on the As oxidation rate, which is interpreted as photocatalyst saturation. (See Figure 10)

[0158] Example 6

[0159] T1O2 recovery and reuse

[0160] Photocatalyst recovery after the As oxidation process is possible with recovery percentages of 80-90%. The elemental characterization of the recovered TiO s depends on the nominal concentrations of the solutions with which the material was in contact during photocatalysis. Figure 11 shows the As, Fe, and Cu contents for a range of photooxidation tests performed. The arsenic content in the recovered material is, in relative terms, independent of the arsenic content in the treated solutions. When treating As(III) solutions at concentrations in the range of 1-11 g / L, the As content determined in the recovered TiO s is in the range of 1-6%, which for the standard dose of TiO s used in the oxidation tests (4 g / L) corresponds to arsenic masses between 0.04-0.24 g As / g TiO s.Similarly, the incorporation of Fe into the recovered material is even lower compared to that of As, obtaining, for Fe concentrations in solution between 5 - 20 g / L, an Fe content in the solid in the range between 0.6 - 2.3%, equivalent to 0.02 - 0.09 g Fe / g TiO s, evidencing a very low saturation of TiO s with the Fe in solution. In oxidations occurring in the presence of copper, at concentrations between 10 - 40 g / L, the Cu content presents a high variability, with values ​​between 0.15 - 50%. For oxidation tests carried out in the presence of 11 g / L of copper (P11 - P15 Table 1 ) a significant Cu content (24%) is only observed in the solid for the test carried out at the highest dose of UV-C radiation (P15; 67 W / L Table 1 ), for lower radiation doses (P11 - P14 see Table 1 ), the recovered copper content is minimal and relatively constant (0.15 - 0.35 %).By increasing the copper concentration up to 40 g / L (P19 -P22 see Table 1 ), the Cu content in the photocatalyst is highly fluctuating, where the highest percentage of Cu in the solid (50 %) is observed at a low radiation dose (P20; 33 W / L Table 1 ), while at the highest applied radiation dose (P21 ; 66 W / L (Table 1 ) the Cu content in the solid is relatively low, reaching 7.6% of Cu adsorbed in the material. The differences observed could be associated with the presence of a greater mass of mixture in the tests of irregular behavior (40 g / L Cu, 10 g / L As(lll) and 20 g / L Fe) which would make them more susceptible to the quality of the stirring in the suspensions and to their adequate oxygenation, which would be even more evident for these oxidations since they require even longer treatment times than those that presented a more regular behavior. (see figure 11).

[0161] Crystallographic characterization of the photocatalytic material was performed before and after As photooxidation. The X-ray diffraction (XRD) pattern for TiOs Aeroxide P25 (see Table 1) prior to As oxidation (Figure 12) shows the presence of the TiOs anatase and rutile crystalline phases in the proportion of 84% Anatase and 16% Rutile, in good correspondence with the ideal proportion of 75 / 25 respectively for each phase. For the material obtained after As oxidation, the corresponding XRD pattern (Figure 13) shows that the proportion between Anatase and Rutile is preserved, with a minor loss of the Anatase phase. The presence of metallic copper and iron is also observed in the recovered material, the latter in the form of austenite, as well as magnetite / maghemite and titanomaghemite. (see Figures 12 and 13)

[0162] The recovered photocatalytic material was subjected to successive oxidation cycles to evaluate the material's reusability with adequate retention of photocatalytic activity. After five cycles of reuse of the material, a moderate decrease in photocatalyst activity was observed (approximately 20%). The modification in activity did not occur progressively with increasing reuse cycles, but rather occurred in a seemingly random manner. In the absence of changes in the intrinsic composition of the catalyst, the differences in activity would be associated with changes in the proportion and nature of the adsorbed species (Figure 14). Example 7

[0163] Oxidation of As by internal irradiation of As (III) + T1O2 suspensions with UV-C light (100 - 280 nm) in a semi-continuous flow system.

[0164] In order to reduce the suspension volume in contact with the UV-C light sources, optimizing the irradiation surface and the exposure of the photocatalyst particles to radiation, As oxidation tests were performed on concentrated PLS solutions (10 g / L As(III); 38 g / L Cu; 18 g / L Fe) in a semi-continuous flow system, provided with 4 irradiation segments each equipped with a 40 W UV-C lamp and an irradiation volume of 4 liters per segment. Figure 15 (Above) shows the decrease curve in As(III) concentration when irradiating 25 L of PLS+T¡O2 suspension (4 g / L) for 120 h (5 days). During the course of irradiation time, a linear decrease in the concentration of As (III) is observed, reaching 60% As oxidation after 120 h of treatment, equivalent to an oxidation rate of 0.05 g As / h.By comparing the oxidation rate obtained for the flow system with those determined for the minimum radiation dose evaluated in the stirred reactor configuration (16 W / L; 0.06 g As / h), it is possible to establish a slight improvement in the oxidation process, taking into consideration that, per radiation segment, the radiation dose used in the process corresponds to approximately 10 W / L, which implies a lower requirement of radiant energy to oxidize a similar amount of As. During the course of the irradiation process, no changes are observed in the total concentration of As, Fe and Cu, (see Figure 15 (Below).

[0165] Tests were conducted in a stirred photoreactor using EPAS (sulfuric acid plant effluent) arsenical solutions. Six tests were performed, which considered parameters such as light radiation power, oxygen / air flow, and the absence and presence of photocatalyst. (Table 2)

[0166] Table 2

[0167] For test P1 (Table 2) (1000 W total power, 100 W / L applied radiation dose, Figure 16), the observed oxidation was higher than the results obtained for the stirred reactor system with immersed lamps, with an oxidation rate of 0.9 g As / h. The oxidative activity observed for the photoreactor allows a reduction of about 3 days to oxidize equivalent quantities of arsenic starting from solutions of 11.7 g / L of As, taking as reference oxidation rates obtained in previous As (III) photooxidation tests carried out in stirred reactors with immersed lamps (Table 2).

[0168] In the absence of photocatalyst (test P2 (table 2), Figure 16), under similar irradiation conditions and O2 feed, a minimum oxidation rate (0.15 g As / h) intrinsic to photolysis processes is observed, demonstrating that the activity of the photocatalyst is responsible for most of the As oxidation process (greater than 90%) and that it is the activity of this material that is improved in the reactor configuration and experimental conditions selected, and the observed improvement does not correspond to a simple increase in photolytic activity given the high intensity of radiant energy used.

[0169] Similarly, tests with lower radiation intensity (test P3 (Table 2); 500 W total power; 50 W / L applied radiation dose, (Figure 16) in addition to a lower O2 flow, show comparable oxidation rates, equivalent to 0.8 g As / h which is faster compared to oxidation tests in stirred reactors with immersed lamps, indicating that even with a 50% reduction in radiant intensity and a close to 90% reduction in oxygen flow, the system allows a very efficient oxidation of arsenical solutions. Under similar experimental conditions to test P4 (Table 2) (Figure 17), but increasing the irradiation time, it is possible to observe that the decrease in As (III) concentration becomes asymptotic around 21 hours of irradiation, with an oxidation rate that reaches values ​​of 0.8 g As / h.The increase in the power of the radiant energy upon reaching the low consumption zone of As (Ill) in the P4 test does not improve the oxidation rate, which indicates that over-illuminating the catalyst does not increase the generation rate of oxidizing species, and the decrease only depends on the availability of arsenical species to oxidize (see figures 16, 17).

[0170] In test P5 (Table 2), carried out in a stirred photoreactor without O2 / air flow (only with atmospheric oxygen), a color change to dark brown and precipitate formation were observed, probably due to copper precipitation due to lack of oxygen. The test was aborted due to the risk of arsine gas formation. The oxidation rate obtained (1.0 g As / h) was comparable to other tests.

[0171] Oxidation tests in a stirred photoreactor at the highest irradiation power evaluated (P6 Table 2; 1000 W, Figure 18, but with a higher air flow (150 L air / h) show an improvement in the oxidation rate with a value of 1.0 g As / h, which is 10% lower than that previously evaluated for P1 irradiated under similar conditions, but injected with a high flow of pure O2 (240 L / h), indicating that the system still works very efficiently despite the difference in the amount of dissolved oxygen in both oxidations (see Figure 18).

[0172] In general terms, when comparing the As oxidation rates obtained (Table 3), an improvement of up to 20% can be observed for the tests performed in the stirred photoreactor compared to all the configurations tested. This would be associated with improved agitation and improved irradiation for the required wavelengths.

[0173] Table 3. Oxidation rates for TiO2 / UV-C radiation-mediated As(III) oxidation tests in stirred reactors with immersed lamps and flow system

[0174] Example 8

[0175] Photooxidation in the presence of an oxidizing agent

[0176] Arsenic (III) oxidation experiments were carried out using a UV-C radiation dose of 16.6 W / L and a hydrogen peroxide to As (III) ratio of 0.5 mol HsO / mol As (III), at a dissolved oxygen concentration between 5-8 mg / L. The characteristic of the working solution was As (lll) = 10 g / L; H2SO4 = 130 g / L which was placed in a 10 L reactor. The concentration of Aeroxide P25 catalyst was 4 g / L. The results show that in the test with only the addition of hydrogen peroxide the oxidation yield reaches 37% and remains stable for at least 20 hours. In the case where only UV-C radiation is provided without the addition of hydrogen peroxide, the oxidation reaches 70% after 25 h of oxidation.When UV-C radiation was used in conjunction with the addition of hydrogen peroxide, the arsenic (III) oxidation yield reached 99% after 20 h, demonstrating a synergistic effect between UV-C radiation and hydrogen peroxide. (See Figure 19).

[0177] Example 9

[0178] Photooxidation in the presence of an oxidizing agent

[0179] Arsenic (III) oxidation experiments were performed using a radiation dose of 33.3 W / L with a medium pressure mercury lamp with UV-C radiation and a hydrogen peroxide dose with respect to arsenic (III) between 0.2 and 0.5 mol HsO / mol As(lll), at a dissolved oxygen concentration between 5-8 mg / L. The characteristic of the working solution was As = 10 g / L; H2SO4 = 130 g / L which was placed in a 10 L reactor. The concentration of Aeroxide P25 catalyst was 4 g / L. The results show that the addition of hydrogen peroxide accelerates the As (III) oxidation reaction significantly, reducing the time to reach 92% oxidation from 32 to 16 h, (see figure 20).

[0180] Example 10

[0181] Photooxidation in the presence of an oxidizing agent

[0182] Arsenic (III) oxidation experiments were carried out using a radiation dose of 10 W / L with a LED light-emitting diode lamp with UV radiation wavelength 365 nm and a hydrogen peroxide to arsenic (III) ratio of 0.5 to 1.0 mol HsO / mol As(lll), at a dissolved oxygen concentration between 5-8 mg / L. The characteristic of the working solution was As(lll) = 6 g / L; H2SO4 = 5 g / L which was placed in a 10 L reactor. The concentration of Aeroxide P25 catalyst was 4 g / L. The results show that the test with only the addition of hydrogen peroxide in a ratio of 0.5 mol HsO / mol As(lll) the oxidation yield reaches 43%, while in the ratio 1.0 mol HsO / mol As(lll) it reaches 82% and remains stable for at least 8 hours. In the case of adding UV radiation, an acceleration effect of the oxidation kinetics of As(lll) is observed, reaching an oxidation yield of 95% in 2 h, (see figure 21).

[0183] Example 11

[0184] Photooxidation in the presence of an oxidizing agent

[0185] Arsenic (III) oxidation experiments were carried out using a radiation dose of 16 W / L of UV-C radiation and sodium metabisulfite as the oxidizing agent, in a sodium metabisulfite (MBS) to arsenic (III) ratio of 0 to 1.0 mol MBS / mol As(lll). The characteristic of the working solution was As(l ll)0 = 10 g / L; H2SO4 = 130 g / L which was placed in a 1.5 L reactor. The concentration of Aeroxide P25 catalyst was 4 g / L. By adding MBS at a molar ratio of 0.5:1 to the arsenic concentration (10 g / L) present in the EPAS solutions, no significant improvements in As oxidation efficiency were observed, with 70% oxidation at 24 h under a radiation dose of 16 W / L (Figure 22) both in the absence and presence of MBS. Doubling the MBS concentration to a MBS / As molar ratio = 1 allows, in an equivalent irradiation time (24 h), to increase the As oxidation efficiency up to 97%.Example 12.

[0186] Photooxidation in the presence of an oxidizing agent

[0187] Arsenic (III) oxidation experiments were carried out using a radiation dose of 16 W / L of UV-C radiation and sodium persulfate as the oxidizing agent, at a sodium persulfate (DPS) to arsenic (III) ratio of 0 to 1.0 mol DPS / mol As(lll). The working solution characteristic was As(lll)o = 10 g / L; H2SO4 = 130 g / L, which was placed in a 1.5 L reactor. The Aeroxide P25 catalyst concentration was 4 g / L. As oxidation tests in EPAS, treated in UV-C / TiOs systems, irradiated at the lowest radiation dose possible in the reactor configuration considered in the tests (16 W / L), show considerable improvements in the oxidation efficiency when incorporating DPS to the system at PDS / As molar ratios = 0.2 - 1.0 (Figure 23).At a PDS / As ratio = 0.2, a minimal increase in the As oxidation rate is observed, going from 70% oxidation (24 h) in the absence of PDS, to 78% oxidation in 23 h after the addition of PDS. A further increase in the PDS concentration, up to a PDS / AS molar ratio = 0.5, allows an almost complete oxidation of As (96%) in a time interval of 12 h, with an As oxidation rate close to 0.8 g As / h. The most dramatic change in the oxidation efficiency of the UV-C / TiOs / PDS system is obtained at a PDS / As ratio = 1.0, with the total oxidation of As present in the EPAS solutions in an interval < 4 h, with an oxidation rate close to 2 g As / h.

[0188] Example 13

[0189] Precipitation of arsenic in the form of scorodite

[0190] An oxidized arsenic solution was taken according to the experiment in example 10 with 6 g / L Astotai (As(lll)=300 mg / L), to which a concentration of 5 g / L of ferrous ion arranged ferrous sulfate heptahydrate was added, and magnetite was added with a particle size of 75% passing #200 mesh so that the ferric ion supplied from the magnetite had a molar ratio of 0.75 mol Fe(l I l) / mol As(V). The pulp was kept under stirring for 24 h at 85°C. The results showed that the solution had a concentration of As to tai = 80 mg / L, and the precipitated solid had an As content of 30% w / w, with a scorodite content of 94% being detected in XRD analysis. Example 14

[0191] Precipitation of arsenic in the form of calcium arsenate

[0192] An oxidized arsenic solution was taken according to the experiment in example 10 with 6 g / L Astotai (As(l 11) = 300 mg / L), to which calcium hydroxide was added so that the pH reached a value of 10.5. The pulp was kept under stirring for 8 h at room temperature.

[0193] The results showed that the solution had a concentration of As to tai = 20 mg / L, and the precipitated solid had an As content = 35% w / w, with a calcium arsenate content of 80% being detected by DRX analysis.

Claims

CLAIMS 1. A photooxidation process for solutions containing As(lll), comprising the steps of: i. providing a first solution containing As(lll) in a photoreactor provided with a UV radiation system; i. adding to the photoreactor of step (i) a titanium (IV) dioxide photocatalyst and an oxidizing agent to generate a first pulp; iii. turning on the energy source of the UV radiation system to begin the oxidation of As (III) to As (V) and form a second pulp; iv. transferring the second pulp formed in step (iii) to a solid-liquid separation stage to generate a first oxidized arsenic solution and the photocatalyst; v. recirculating the photocatalyst from step (iv) to step (i); vi. sending the first oxidized arsenic solution from step (iv) to an arsenic abatement process.

2. The photooxidation process of claim 1, wherein the first As(lll) solution has an As(lll) concentration of at least 1 g / L.

3. The photooxidation process of claim 1, wherein the UV radiation system of step (i) generates UV-B or UV-C radiation.

4. The photooxidation process of claim 1, wherein the UV radiation system of step (i) generates UV-C radiation.

5. The photooxidation process of claim 1, wherein the UV radiation is provided by means of medium pressure mercury lamps or through LED type lamps.

6. The photooxidation process of claim 1, wherein the radiation system of step (i) is provided externally.

7. The photooxidation process of claim 1, wherein the radiation system of step (i) is provided internally.

8. The photooxidation process of claim 1, wherein the radiation system provides a radiation dose between 0.5 and 800 W / L.

9. The photooxidation process of claim 1, wherein the photoreactor of the step (i) is a semi-continuous flow system.

10. The photooxidation process of claim 1, wherein the photoreactor of step (i) is provided with stirring.

11. The photooxidation process of claim 1, wherein the titanium (IV) dioxide photocatalyst has a mixture of anatase and rutile in a proportion of 84% Anatase and 16% Rutile.

12. The photooxidation process of claim 1, wherein the photocatalyst is added at a concentration of between 1 and 8 g / L.

13. The photooxidation process of claim 1, wherein the photocatalyst is added at a concentration of between 1 and 4 g / L.

14. The photooxidation process of claim 1, wherein the photocatalyst is added at a concentration of 4 g / L.

15. The photooxidation process of claim 1, wherein the oxidizing agent is selected from sodium metabisulfite, sodium persulfate, molecular oxygen (O2), ozone (O3), chlorine (CI2), chlorine dioxide (CIO2), hydrogen peroxide (H2O2) hypochlorite (HCIO 4 ) and permanganate (MnO 4 ).

16. The photooxidation process of claim 1, wherein the oxidizing agent of step (i) is selected from sodium persulfate, sodium metabisulfite and hydrogen peroxide.

17. The photooxidation process of claim 1, wherein the oxidizing agent of step (i) is hydrogen peroxide H2O2 in a molar ratio of 0.2 to 1.0 mol H2O2 / mol As(lll).

18. The photooxidation process of claim 1, wherein step (iii) is carried out in a temperature range between 20 and 50°C.

19. The photooxidation process of claim 1, wherein step (iii) is carried out with mechanical agitation.

20. The photooxidation process of claim 1, wherein in step (iii) the concentration of dissolved oxygen in the solution must be maintained within the range of 1 to 10 mg / L, which can be provided by providing oxygen or air.

21. The photooxidation process of claim 1, wherein step (iii) is carried out in a pH range of less than 3.

22. The photooxidation process of claim 1, wherein step (iii) is carried out in a pH range of less than 1.

23. The photooxidation process of claim 1, wherein step (iii) has a residence time of 1 to 30 hours.

24. The photooxidation process of claim 1, wherein step (iii) has a residence time of 1 to 10 hours.

25. The photooxidation process of claim 1, wherein step (vi) consists of a process of reducing arsenic in the form of scorodite.

26. The photooxidation process of claim 1, wherein step (vi) consists of an arsenic reduction process in the form of calcium arsenate.

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