Biochar is a type of charcoal produced through the pyrolysis or thermal decomposition of organic materials, typically biomass like wood, crop residues, or organic waste, in a low-oxygen (anaerobic) environment. The resulting material is rich in carbon and has a high surface area, making it suitable for various agricultural, environmental, and industrial applications. Here is A comprehensive overview on Biochar (green adsorbent), its preparation, factors affecting, uses and its modification:

Authors:

Izaz Ul Islam

LinkedIn: Click here to see Izaz’s profile

A portion of Bachelor’s research project

1. Rice straw biochar as an adsorbent

1.1. Rice straw

Approximately 60 % of the world’s rice is produced and consumed in South East Asian and South Asian countries such as Pakistan, Bangladesh, Sri lanka, and India. Along with such a huge amount of rice production the generation of rice waste such as rice straw and rice husk is also considerably large (Chandra et al., 2019). According to the studies rice crops produce approximately 8 × 10¹¹ kg of rice straw annually (Goodman et al., 2020). In underdeveloped countries, a significant portion of rice straw was used for burning. Such an open burning results in severe environmental problems that badly affect all the components of the environment. Rice straw is extremely abundant in Pakistan, low in cost, and easily available. So rice straw can be utilized as a substantially effectual adsorbent for the elimination of heavy metals from wastewater.

2. Biochar (A comprehensive overview on Biochar)

The introduction of biochar as an emergent technology strongly influences every sector of life such as energy, water, habitats, health, sanitation, agriculture, livelihoods, carbon sequestration, and the environment because of its high effectiveness and multidimensional properties. Biochar is an alternative name for charcoal that is employed for some specific purposes other than combustion. It is carbonaceous material that is produced from various biomasses. Due to its unique physical and chemical properties biochar having a wide range of applications in numerous fields (Reddy et al., 2012).

2.1. Preparation of biochar

The methods involved in the preparation of biochar include Pyrolysis, Microwave carbonization, and Hydrothermal carbonization. The chemical and physical properties include the structure of pores, ash, capacity for cation exchange, specific surface area, functional group number, and type, and yield. The biochar prepared through the hydrothermal carbonization results in a high yield and involves no drying step as compared to pyrolysis. On contrary, the biochar prepared through microwave carbonization is a controllable process with high energy, efficiency, quick heating, and no hysteresis (Yang et al., 2019).

A comprehensive overview on Biochar

2.2. Pyrolysis

For the preparation of biochar, the most frequent method is Pyrolysis that involves thermal decomposition in an oxygen deficient environment. Basically, the pyrolysis involves the use of high-temperature medium or electric heating for the decomposition of biomass in the temperature range of 400 – 900 Ċ. The parameters that influence the biochar production as a result of pyrolysis involve the rate of heating, atmosphere, and temperature of the reaction, time of residence in the pyrolysis chamber, and nature of raw materials (Yang et al., 2019).

2.2.1. Raw material effect on pyrolysis

A large number of biomasses have been consumed as a raw material in the pyrolysis for the preparation of biochar. That includes rice straw, rice husk, corn cab, corn straw, peanut hull, date pits, banana peel, etc. As the cellulose, hemicellulose, lignin and silicon content of various biomasses are different therefore the ash content and elemental composition of biochar are also different. According to the reported studies, the ash content of other biomasses are lower as compared to rice straw that was higher due to the higher content of silicon (Ender et al., 2012).

2.2.2. Reaction temperature effect on pyrolysis

Biochar prepared in the temperature range of 400 – 900ċ are effective. Increasing the pyrolysis temperature beyond 900ċ the quantity of acidic functional group and yield of biochar decreases. On the other hand’s pH, ash content, and basic functional group increase. In addition, the pyrolysis temperature also affects specific surface area and pore volume (Yang et al., 2019).

2.2.3. Effect of heating rate on pyrolysis

The rate of heating in the pyrolysis may be fast pyrolysis or slow pyrolysis. In fast pyrolysis, the minute particles of organic materials in the pyrolysis chamber are blown out thus unmasking it as a result in a millisecond to second heat transfer occur in it (Laird et al., 2009). On the other hand in slow pyrolysis involves the gradual transfer of heat to the organic material existent in the pyrolysis chamber in the complete lack of oxygen. The advantage of pyrolysis is that all the biomass in the pyrolysis chamber is completely pyrolyzed. While in Fast pyrolysis some fraction of biochar remained unpyrolyzed (Mohan et al., 2006; Bruun et al., 2012).

2.2.4. Effect of residence time on pyrolysis

Residence time is inversely proportional to the yield of biochar at the same temperature of pyrolysis. More the time of residence less will be the yield of biochar and vice versa. Previous studies indicated that biochar prepared from orange peel at 700ċ of pyrolysis temperature and 6h of residence time the yield of biochar is only 5.93% (Chen at al., 2019). Up to a certain limit, the increase in residence time leads to an increase of pores and specific surface area. Lu et al (1995) reported that when the time of residence raised from 2 to 3h the pores and specific surface area is decreased. This is because an increase in the time of residence is effective for the development of pores in biochar. But it leads to the damage of pore structure if the residence time is too prolonged (Tay et al., 2001).

2.2.5. Effect of pyrolysis atmosphere on pyrolysis

The pyrolysis must be carried out in such an environment that is dominated by the presence of inert gases such as Ar, N₂ that is employed for the isolation of oxygen. The physical activation also called gas activation involves the preparation of biochar in the presence of O₃, NH₃, CO_2, and H₂O. These gases are involved in the decomposition of biochar non – structural components that result in the opening of internal pores and leads to the expansion in the pore volume and specific surface area (Jimenez-Cordero et al., 2015; Cha et al., 2011).

2.3. Uses of biochar

The uses of biochar are mainly divided into two categories that are the use of biochar in soil and the non-soil use of biochar. But here we will mainly focus on the non-soil use of biochar. The figure summarizes the uses of biochar as;

Various uses of biochar: A comprehensive overview on Biochar

2.3.1. Non- soil use of biochar

2.3.1.1. Use of biochar in the removal of environmental pollution

Environmental pollution that includes the befouling of air, water, and soil is a global problem. Over time, the intensity of environmental pollution is increasing day by day due to hasty industrialization and swift commercial and anthropogenic activities. Biochar belongs to such a class of materials that are involved in the remediation of environmental pollution i.e., both organic and inorganic pollutants via degradative and adsorption processes (Baratoli et al., 2020).

2.3.1.1.1. Removal of inorganic pollutants

The presence of dissolved metal species in water results in the contamination of water that is a major problem in developed and underdeveloped countries. For safety and human health, the solution of this problem is in critical demand. The utilization of biochar for the exclusion of water pollution is a very affordable solution. Various studies reported that the use of filtering materials that are based on biochar significantly minimize the total chemical demands and the concentrations of various metal ions such as Cr⁶⁺, Cr³⁺, Pb²⁺, Cu²⁺, Zn²⁺, As³⁺, PO^3-_4, and NH⁺₄ (Bartoli et al., 2020).

2.3.1.1.2. Removal of organic pollutants

Pharmaceuticals, residues of polymers, and dyes introduce a considerable amount of organic molecules in wastewater. For these organic pollutants removal, a large number of carbonaceous materials are used that include carbon nanotubes, graphenes, and carbon dots. These carbonaceous materials not only detect the organic pollutants but are also involved in their removal. However its high-cost limit it uses for the removal of organic materials. As compared to carbonaceous materials biochar that represents a low-cost material with high efficiency is currently used for the elimination of organic pollutants from wastewater (Bartoli et al., 2020).

2.3.1.1.3. Removal of gaseous pollutants

The purification of the gaseous mixture is one of the basic concerns of industries. The frequently used method for gaseous pollutants removal is the use of selective membranes. Currently, for the purification of the gaseous mixture, the adsorber based on biochar had been employed. Various studies reported the CO_2, SO_2, and H₂S gases have successfully been adsorbed by the use of biochar-based adsorbers (Bartoli et al., 2020).

2.3.1.2. Use of biochar for energy storage

The 21^st century’s greatest challenge for scientists is the storage of energy. For this purpose, technology has been introduced that includes the use of fuels and solar cells, super capacitors, and batteries with maximum efficiencies. A fuel cell is used for the generation of electrical energy by the supply of fuels such as Carbon, Hydrogen, Methane and Oxygen, and Hydrogen Peroxide. Similarly, batteries and super capacitors are used for electrical energy storage. The unique chemical and physical properties of biochar make it the best material for the storage of energy by using it in batteries, fuel cells, and super capacitors (Bartoli et al., 2020).

2.3.1.3. Biochar other uses

Besides the above application biochar has its application in every field of life. Currently, biochar is used for the preparation of biochar-based composites, plastics, as a catalyst, electrochemical measurement device, and in biological procedures.

2.4. Biochar modification

To enhance the adsorption process of biochar various methods are adopted to modify the biochar and study the effect as a result of modification. The modification involves the use of chemical and physical methods for the activation of biochar to acquire the corresponding results. The aspects that disturb the course of modification are activator nature, time of activation, time of soaking, and the temperature at which the activation occurs (Yang et al., 2019). The various modifications methods are;

2.4.1. Chemical reduction       

Chemical reduction entangles the use of a reducing agent to diminish the functional group present on the biochar surface and hence it’s non-polar nature. It is also called alkali modification. As a consequence of chemical reduction, the biochar capability of adsorption for the pollutants increased. The reducing agents that are frequently used for chemical reduction include KOH, NH₄OH, and NaOH respectively (Yang et al., 2019).

2.4.2. Chemical oxidation (A comprehensive overview on Biochar)

Chemical oxidation involves the enhancement of oxygen-containing functional groups i.e.,   -OH, COOH, etc., via the biochar surface oxidation. This enhancement of the functional group increased the hydrophilicity of the biochar. The chemical oxidation results in the change of biochar structure and pore size and hence the adsorption capacity of the biochar for the polar adsorbate is increased. HNO_3, H₂O_2, H₃PO_4, and HCl are the frequently used oxidant in chemical oxidation (Yang et al., 2019).

2.4.3. Metal impregnation

The adsorption of metal ions onto the biochar pores and surface is known as metal impregnation. Metal impregnation lead to the increase of the specific surface area. Similarly, these metal ions also play the role of adsorbent and combine with the adsorbate and enhance the process of adsorption. Frequently use metal ions for metal impregnation include magnesium, silver, zinc, iron, etc. (Yang et al., 2019).

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