Water contamination is one of the most pressing environmental issues of the present. There is a significant amount of interest in the slow pyrolysis of biomass to produce biochar, a solid byproduct that is stable and rich in carbon. Adsorbents manufactured from hydrochars, sometimes referred to as hydrochar created by hydrothermal methods, have been tested for the removal of possible contaminants from wastewater. The hydrothermal processes of hydrothermal carbonization (HTC) and liquefaction (HTL) yield hydrochars, a distinct category of biochar. Because of its peak efficiency, large surface area, large size of pore and capacity to regenerate, hydrochar is an acceptable option for the rehabilitation of a range of pollutants. The formation, activation, identification, and use of biochar and hydrochar were highlighted in this review. The physiochemical properties of the char produced by the two processes are very different, which has an impact on their potential uses in areas like wastewater pollution remediation, soil improvement, greenhouse gas emission and carbon sequestration among others.

A critical review on biochar for environmental applications
    Abstract
    1 Introduction
    2 Literature survey
    3 Categories of ECs
        3.1 Pharmaceuticals
        3.2 Endocrine disrupting chemicals
        3.3 Pesticides
        3.4 Other pollutants
            3.4.1 Heavy metals
            3.4.2 Dyes
    4 Biochar
    5 Biochar activation
    6 Production of biochar
        6.1 Pyrolysis
            6.1.1 Slow pyrolysis
            6.1.2 Fast pyrolysis
            6.1.3 Flash pyrolysis
        6.2 Torrefaction
        6.3 Gasification
    7 Biochar application
    8 Hydrochar
    9 Production of hydrochar
        9.1 Hydrothermal carbonization
        9.2 Hydrothermal liquefaction (HTL)
        9.3 Hydrothermal gasification
    10 Elemental changes brought about by HTC
    11 Structural changes in hydrochar
    12 Hydrochar properties
    13 Hydrochar activation
        13.1 Physical activation
            13.1.1 Steam activation
        13.2 Chemical activation
            13.2.1 Acid activation
            13.2.2 Alkali activation
    14 Benefits of Hydrochar
    15 Difference between biochar and hydrochar
    16 Applications of hydrochar
    17 Future scope
    18 Conclusion
    References

• R. Sivaranjanee(Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, Tamil Nadu 603110, India)
• P. Senthil Kumar(Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, Tamil Nadu 603110, India, Centre of Excellence in Water Research (CEWAR), Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, Tamil Nadu 603110, India)
• Gayathri Rangasamy(University Centre for Research and Development, Department of Civil Engineering, Chandigarh University, Gharuan, Mohali, Punjab 140413, India, School of Engineering, Lebanese American University, Byblos, Lebanon, Department of Sustainable Engineering, Institute of Biotechnology, Saveetha School of Engineering, SIMATS, Chennai 602105, India)