*5.3.2 Bio-char as adsorbent for wastewater treatment*

Inadequate water supply globally has given significant attention to the production of sustainable and eco-friendly bio-char based adsorbent for treatment of wastewater containing heavy metals, emerging contaminants and dye effluent from textile industry [7]. Properties of bio-char, such as cation exchange capacity, high surface area, high carbon content, amorphous and hydrophilic nature aid its applicability. Previous research works have established bio-char as suitable material for treatment of wastewater. Jabar and Odusote [3, 6] used bio-char obtained from water lily and oil palm empty bunch fiber for treatment of water contaminated with malachite green and cibacron blue 3G-A (CB) dye respectively. Thue et al. [46] used shell of cashew of Para for treatment of wastewater containing emerging contaminants. Ozsin et al. [47] used chemically activated carbon from agricultural waste of chickpea and its application for heavy metal adsorption. Wood-based bio-char was used as adsorbent for removing toxic elements from wastewater by Shaheen et al. [48], while Gwenzi et al. [49] showed that designed water treatment plant based on biomass derived char was a prospective low cost sustainable technology for producing dirt free water.

## *5.3.3 Bio-char as energy and energy storage device*

Use of bio-char as source of energy for heating was old as human existence. It was popularly used as energy source for blast furnace in production of iron from iron ore called hematite and in petroleum refinery plant for separation of factions using fractional distillation. Researchers are currently focusing on the use of bio-char in production of supercapacitor called electrochemical capacitor [50]. Supercapacitor is an energy storage device that stores and releases electrical energy within a few seconds [51]. It stores energy as electric charge at electrolyte porous carbon electrode interface more than ten times conventional capacitor does [52]. It has to its advantages, high power density and satisfactory energy density [53]. Properties of bio-char, such as excellent electrochemical performance, high surface area, fast electron transport and abundant functional moieties make it suitable for production of supercapacitor [54]. Several researchers have prepared and activated bio-char for production of supercapacitor in electrochemistry. One of these researchers is Vinayagam and his co-researchers [55] who prepared activated carbon from *Sapindus trifoliatus* nut shell for high performance symmetric supercapacitor applications.
