**2. Biochar properties for wastewater treatment**

Biochar is a carbon-rich solid material produced from biomass through a thermochemical process called pyrolysis. During pyrolysis, lignin, cellulose, hemicellulose, fat, and starch in the feedstock are thermally broken down forming three products: biochar (solid), bio-oil (partly condensed volatile matter), and non-condensable gases (CO2, CO, CH4, and H2) [26, 27]. The bio-oil and gases can be captured to produce energy and depending on the feed valuable coproducts like wood preservatives, food flavoring, adhesive, or biochemical compounds [28]. The yield of biochar and the properties, however, depends on the pyrolysis condition. Slow pyrolysis at moderate temperature (350–500°C) and slow heating rate results in higher yield (30%) of biochar than around 10% or less yield with fast pyrolysis (600–700°C and fast heating rate) or gasification (temperature 700°C or above) [29]. The feedstock type and pyrolysis condition used during the production of biochar notably change the physiochemical properties such as surface area, polarity, atomic ratio, pH, and elemental composition [25, 30, 31]. These properties determine the effectiveness of biochar in wastewater treatment.

Biochar has wide applications in water and wastewater treatment because of its distinctive characteristics, for example, adsorption capacity, specific surface area, microporosity, and ion exchange capacity [30, 32]. The removal mechanisms of different pollutants are governed by their interactions with various attributes of biochar, which depends on pyrolysis temperature and feedstock type [33]. Pyrolysis temperature greatly affects the properties of biochar. The increase in pyrolysis temperature results in higher carbon content, hydrophobicity, aromaticity, surface area, and microporosity in biochar [34]. Similarly, the pH of the biochar increases with increasing pyrolysis temperature due to enrichment of ash content in the biochar [35, 36]. High-temperature (>500°C) biochar has low polarity and acidity due to loss of O- and H-containing functional groups [34]. Lower pyrolysis temperature (<500°C) facilitates partial carbonization, thus yielding biochar with smaller pore size, lower surface area, and high O-containing functional groups [36]. Lower temperature biochar contains a higher content of dissolved organic carbon, relatively low polarity and C/N ratio [30, 34, 37].

Biochar often compromises of both positively and negatively charged surfaces (zwitterionic) [34, 35]. The negatively charged functional groups contribute to cation exchange capacity (CEC) whereas anion exchange capacity (AEC) is also exhibited by O-containing functional groups (oxonium heterocycles) in biochar [36, 38]. Oxygen (O) containing alcohol, carbonyl, and carboxylate functional groups are generally believed to contribute to biochar cation exchange capacity because they carry a negative charge and serve as Lewis bases for the sorption of cations. Whereas, it is believed that oxonium functional groups contribute to pHindependent anion exchange and that both pyridinic functional groups and nonspecific proton adsorption by condensed aromatic rings contribute to pH-dependent anion exchange capacity in biochars [38].

Biochar derived from woody biomass and crop residues has a higher surface area compared to that of solid municipal wastes and animal manure [30]. Apart from the usual pyrolysis method, different engineering methods have been developed and used to expand biochar's applications. Engineered biochar is the derivative of biochar that is modified by physical, chemical, and biological methods to improve its physical, chemical, and biological properties (e.g., specific surface area, porosity, cation exchange capacity, surface functional group, pH etc.) and its adsorption capacity [37, 39, 40]. Some of the modification includes anaerobic digestion of feedstock before pyrolysis, steam/gas activation, pyrolysis using microwave heating, ball milling, magnetic modification, chemical modification using hydrogen

peroxide, alkali or acid, and impregnation/coating with chemicals [41]. The detail about the modified biochar for wastewater treatment will be discussed in the following sections.

### **2.1 Biochar modification**

Researchers have discussed several methods for modifying the properties of biochar [42]. These methodologies include treatments with steam, acids, bases, metal oxides, carbonaceous materials, clay minerals, organic compounds, and biofilms [43].
