Green catalytic process for in situ oxidation of Arsenic(III) in concentrated streams using activated carbon and oxygen gas

Arsenic(III) oxidation is a critical pre-treatment step for overall arsenic immobilization in concentrated industrial arsenic streams. Activated carbon (AC) catalysis is a green, economical and efficient method to oxidize As(III) from waters with high arsenic concentration prior to its removal through precipitation or adsorption. This research investigates AC-catalyzed oxidation process for oxidizing aqueous solutions of As(III) and proposed the possible reaction pathway. Batch tests were performed and efficient oxidation of 2.0 g/L acidic As(III) solution have been induced on AC surfaces in the presence of oxygen. The in-situ formation of reactive oxygen species on carbon surfaces and arsenic adsorption onto AC play important roles in As(III) oxidation. The kinetics of adsorption and catalyzed oxidation has been studied and the samples were characterized using ICP-OES, Zeta potential, TEM coupled with EDX and XPS techniques. A systematic reaction pathway was proposed, and reusability of AC has confirmed the economic viability of the proposed green process. This study offers a promising and facile solution for As(III) oxidation from waste water, mining and metal industrial waste streams under ambient conditions for arsenic immobilization.

Intensification of heterogeneous TiO2 photocatalysis using the NETmix mili-photoreactor under microscale illumination for oxytetracycline oxidation

This study focuses on the intensification of heterogeneous TiO₂ photocatalysis for the removal of a contaminant of emerging concern (CEC), oxytetracycline (OTC), as a polishing step of urban wastewaters, using an innovative NETmix mili-photoreactor under UVA-LEDs illumination. The effect of catalyst coated surface per reactor volume and the illumination mechanism, back-side (BSI) or front-side (FSI) irradiation, on OTC oxidation were evaluated. For that, a thin film of photocatalyst was uniformly deposited on the front borosilicate slab (BS) (BSI mechanism; 333 m²_catalyst m^-3_reactor) or on the network of channels and chambers imprinted in the back stainless-steel slab (SSS) (FSI mechanism; 989 m²_catalyst m^-3_reactor) using a spray system. OTC removal was also assessed as a function of TiO₂ film thickness immobilized on both slabs. The photocatalyst reactivity in combination with photoreactor was significantly enhanced (3.4 times) from 0.64 to 2.19 mmol_OTC m^-3_{illuminated reactor volume} s^-1, when considering the BSI and FSI mechanisms, respectively. In addition, the influence of UVA-LEDs intensity on OTC oxidation rate was investigated. UVA-LEDs plates were placed on the top of the NETmix borosilicate window. Moreover, the effect of water matrix was assessed using a secondary effluent from an urban wastewater treatment plant fortified with OTC. OTC oxidation rate was only inhibited in about 1.3 times in the presence of the real matrix, showing the ability of the NETmix to overcome matrix effects due to its unique characteristics. Catalyst film stability over four consecutive reaction cycles was evaluated using synthetic and real matrices fortified with OTC.

Overcoming limitations in photochemical UVC/H2O2 systems using a mili-photoreactor (NETmix): Oxytetracycline oxidation

This study focuses on the intensification of a photochemical UVC/H₂O₂ system using a mili-photoreactor (NETmix) for a better and faster elimination of oxytetracycline (OTC) from urban wastewater. This mili-photoreactor comprises a network of small cylindrical chambers and prismatic transport channels sealed by a UVC transparent quartz slab allowing unique properties. Since light has a profound effect on the photochemical process, UVC photons distribution over the reaction medium was investigated using a multiple UVC lamp design (4, 6 or 11 W) allocated in parallel or perpendicular to the solution movement. In addition, the effect of other operating variables, such as oxidant dosage (100-900 mg L^-1), oxidant feed configuration (single entry or continuous multi-injection) and flow rate (50-100 L h^-1) was studied. A kinetic model able to describe the OTC oxidation by the UVC/H₂O₂ photochemical system in the mili-photoreactor was also developed. Moreover, matrix effect was evaluated by spiking OTC in a secondary effluent from an urban WWTP. In this case, OTC degradation was inhibited in about 2 to 3 times due to the presence of organic/inorganic substances (soluble and particulate), inherent to the real matrix, that act as scavenger of oxidant species and as UVC light filter. The NETmix mili-photoreactor presented high photochemical space time yield (PSTY) values when compared with a conventional tubular photoreactor. This highlights the NETmix capacity to enhance UVC/H₂O₂ processes through an homogeneous light distribution over the entire reaction medium.

As(III) and Cr(VI) oxyanion removal from water by advanced oxidation/reduction processes-a review

Water pollution by human activities is a global environmental problem that requires innovative solutions. Arsenic and chromium oxyanions are toxic compounds, introduced in the environment by both natural and anthropogenic activities. In this review, the speciation diagrams of arsenic and chromium oxyanions in aqueous solutions and the analytical methods used for their detection and quantification are presented. Current and potential treatment methods for As and Cr removal, such as adsorption, coagulation/flocculation, electrochemical, ion exchange, membrane separation, phyto- and bioremediation, biosorption, biofiltration, and oxidative/reductive processes, are presented with discussion of their advantages, drawbacks, and the main recent achievements. In the last years, advanced oxidation processes (AOPs) have been acquiring high relevance for the treatment of water contaminated with organic compounds. However, these processes are also able to deal with inorganic contaminants, mainly by changing metal/metalloid oxidation state, turning these compounds less toxic or soluble. An overview of advanced oxidation/reduction processes (AO/RPs) used for As and Cr removal was carried out, focusing mainly on H₂O₂/UVC, iron-based and heterogeneous photocatalytic processes. Some aspects related to AO/RP experimental conditions, comparison criteria, redox mechanisms, catalyst immobilization, and process intensification through implementation of innovative reactors designs are also discussed. Nevertheless, further research is needed to assess the effectiveness of those processes in order to improve some existing limitations. On the other hand, the validation of those treatment methods needs to be deepened, namely with the use of real wastewaters for their future full-scale application. Graphical abstract ᅟ.