Advanced reduction processes (ARPs) based on catalytic hydrogenation for wastewater pollutants degradation - a special focus on process efficiency and mechanisms – a review

Catalytic hydrogenation receives significant attention for producing raw materials in the pharmaceutical and chemical industries, environmental remediation, etc. NaBH 4 , LiAlH 4, and gaseous hydrogen are used as reagents, while dithionate has high potential for future applications. Environmental and chemical engineering applications based on advanced reduction processes (ARPs) are accelerated using catalysts based on nanomaterials containing noble or non-noble metals, composite materials, or catalysts deposited on suitable supports. The process, achieves high conversion efficiency with low energy demands, under mild conditions, primarily using noble metal catalysts mainly platinum, palladium, and gold. Meanwhile, catalysts based on silver, cobalt, nickel, or copper also show high potential and economic feasibility. Priority wastewater pollutants such as nitrite and nitrate, nitrophenols, dyes (rhodamine B, methylene blue, etc.), and oxyanions are extensively reduced using the catalytic hydrogenation processes. Inorganic supports like silica gel, activated alumina, titanium dioxide, and organic supports like graphitic carbon nitride, graphene oxide, and chitosan, were extensively studied and revealed high applicability. Herein, this review summarises the recent progress in catalytic hydrogenation process, type of catalysts, their synthesis, and a detailed discussion of their application in wastewater treatment. Stability of different catalysts, evaluation of hydrogenation methods, and their mechanisms using different catalysts for various pollutant degradation are discussed as well. The prospects for improving the catalytic hydrogenation process for pollutant degradation are also discussed. Strategies for catalysts preparation include impregnation (wet or dry), deposition-precipitation, sol-gel, hydrothermal, co-precipitation and colloidal methods. Most important aspects for development of effective process relate to design of catalyst providing effective adsorption of hydrogen source and pollutant, ability to provide high dissociation and spillover of hydrogen (fast hydrogen radical migration). Catalyst composite design should focus on obtaining desired alloy effect and strong interactions between the active metal and support. From technical and application point of view, design strategy must include control of particle size and morphology as well as poisoning resistance. Main challenges relate to achieving reproducibility, economic viability, and the production of stable catalytic materials. Optimised hydrogenation processes are rapid – best processes provide above 90 % degradation in less than 10 min of the treatment. Main advantage of ARPs comparing to Advanced Oxidation Processes (AOPs) relay on ability to perform selectively the degradation of target pollutant. Reasonably planned catalytic reduction fulfils the concepts of circular economy towards waste valorization by formation of useful chemicals such as aromatic amines (relevance for production of agrochemicals, dyes, cosmetics, pesticides), aminophenols (for pesticides, dyes, pharmaceuticals), furfuryl alcohol (components of biofuels). Multicriterial analysis of hydrogenation processes in respect to source of hydrogen (including molecular hydrogen, hydrogen donors and ionic hydride donors) reveals that preference should be given to solutions based on molecular hydrogen. Optimizing the reductant consumption, synthesising catalysts with higher stability, and eliminating process toxicity by enriching hydrogenation products into chemicals useful for other processes would be a key enhancement of catalytic hydrogenation in environmental remediation. • Review of the synthesis of noble and non-noble metal catalysts. • Progress of catalytic hydrogenation process for environmental remediation is reviewed. • The mechanisms of catalytic hydrogenation using different catalysts are summarized. • Domestic and industrial effluents treatment by reactions with reductive radicals. • Discussion of by-products formation and methods performance ranking.