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Environmental contamination caused by oxyanions like nitrate, selenate, chromate, and other related functional groups in water and wastewater is identified as a serious threat to human health. Among the different techniques attempted to remove or convert these compounds, reduction with the use of microorganisms has gained importance due to high selectivity and efficiency. Hydrogen-based membrane bioreactor (HMBR) is identified as a suitable method due to the non-toxic properties of hydrogen as an electron-donor, reduced secondary pollution, and easy generation of hydrogen through cost-effective novel methods. This research article is focused on the principle and mechanism of removal of anions, organic compounds, heavy toxic polyatomic metal ions, and antibiotics using hydrogen mediated microbial reduction process. Denitrification studies in HMBR at different operating conditions, namely membrane type, pH, nitrate concentration, and hydrogen partial pressure, are reviewed and the commonly reported efficiency ranges between 70 and 100%. The removal efficiencies attained by HMBR are as follows: perchlorate 30–100%, selenate 92–100%, phenol 87–99%, arsenate 60–68% and halogenoalkanes 45–99%. The mechanism of reduction in HMBR was explained with reference to nitrate, arsenate, chlorophenol, and chromate. [Display omitted] • Mechanism of Hydrogen based Membrane Reactor and applications for removal of pollutants are reviewed. • The working principle is discussed with schematic illustration. • The influence of key operating variables is discussed. • Scope for future research and research gap is presented. [ABSTRACT FROM AUTHOR]

The considerable increase in world energy consumption owing to rising global population, intercontinental transportation and industrialization has posed numerous environmental concerns. Particularly, in order to meet the required electricity supply, thermal power plants for electricity generation are widely used in many countries. However, an annually excessive quantity of waste fly ash up to 1 billion tones was globally discarded from the combustion of various carbon-containing feedstocks in thermoelectricity plants. About half of the industrially generated fly ash is dumped into landfills and hence causing soil and water contamination. Nonetheless, fly ash still contains many valuable components and possesses outstanding physicochemical properties. Utilizing waste fly ash for producing value-added products has gained significant interests. Therefore, in this work, we reviewed the current implementation of fly ash-derived materials, namely, zeolite and geopolymer as efficient adsorbents for the environmental treatment of flue gas and polluted water. Additionally, the usage of fly ash as a catalyst support for the photodegradation of organic pollutants and reforming processes for the corresponding wastewater remediation and H 2 energy generation is thoroughly covered. In comparison with conventional carbon-based adsorbents, fly ash-derived geopolymer and zeolite materials reportedly exhibited greater heavy metal ions removal and reached the maximum adsorption capacity of about 150 mg g−1. As a support for biogas reforming process, fly ash could enhance the activity of Ni catalyst with 96% and 97% of CO 2 and CH 4 conversions, respectively. [Display omitted] • Waste fly ash is converted into value-added materials via different methods. • The utilization of fly ash-derived materials has been comprehensively discussed. • Fly ash-based materials are effectively used as adsorbents and catalysts. [ABSTRACT FROM AUTHOR]