Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants from Wastewater: Current Status and Perspectives

Abstract

Human activities and rapid modernization have affected the ecological and economical aspects worldwide resulting in alarming situations such as global warming and the accumulation of waste disposal and toxic contaminants in water. Contaminants in water are toxic and carcinogenic, posing a serious threat to the environment. Water is a precious and limited resource and hence it is highly imperative to utilize effective remediation strategies for the removal of pollutants. Several competitive remediation techniques have been proposed due to their distinctive features including ease of operation, inexpensiveness and universal nature. The present chapter highlights the potential of ecofriendly biomass-derived biochars as adsorbents for the effective removal of toxic contaminants. This includes biochars derived from industrial solid wastes, agricultural wastes, clays minerals and municipal wastes. Biomass-derived biochars are found to be highly efficient, alternative and carbon-neutral precursors and provide a new approach to the modular adsorption process. The present chapter also includes conversion of waste materials into efficient bio-adsorbents followed by their applications for the purification of wastewater. Besides, attempts are made to discuss the techno-economic and future perspectives of eco-friendly and low-cost biochars for the treatment of wastewater.

1. Introduction

Freshwater is a basic demand for human activities including industrial, agricultural and domestic activities. These activities produce a huge amount of contaminated water resulting from the discharge of undesirable toxic and carcinogenic contaminants (inorganic/organic/biological agents/radioactive wastes) into water bodies that impose a serious concern on the environment and living species. According to World Health Organization (WHO) and the literature available [1, 2], the majority of water on earth is salty, requiring treatment before it can be used. The rest of the freshwater is in glaciers and underground reservoirs. Industrial activities (automobile manufacturing, textile, dyeing, paint, paper and pulp, tannery and leather industry) and Agricultural activities (excess use of fertilizers and pesticides, antibiotics, processed wastes of crop plantation) and unwanted environmental changes (damages to sewer system due to high rainfall, soil runoff, use of pesticides and fertilizers) are the major cause of water pollution [3, 4, 5]. Hence, the preservation of freshwater, as well as the quality improvement of contaminated water (decontamination of pollutants from water), is a growing challenge.

Literature reports several feasible and popular conventional separation techniques for the treatment of polluted water such as chemical precipitation, adsorption, ion-exchange, flotation, coagulation and flocculation, ultrafiltration, nanofiltration, reverse osmosis, electrochemical process, evaporation and photo-catalysis [6, 7]. Each technique is effective in its own way and offers several advantages for one process but at the same time imposes several restrictions on other processes. However, amongst these popular conventional separation techniques, the chemical and electrochemical treatment processes are ineffective even at very low pollutant concentrations, due to excessive amount of chemical usage, sensitivity towards variable wastewater input and producing a large amount of sludge that needs further treatment before releasing it to the environment [8]. Other processes such as ultrafiltration, nanofiltration, reverse osmosis and ion exchange are the most expensive to treat a large amount of wastewater adding demerits to explore at the industrial scale.

Safe drinking water demand at a reasonably low price with an effective and sustainable treatment approach is a prime focus of industrialists and academicians. Currently, adsorption is used for wastewater treatment and is gaining wide attention due to its effectiveness and feasibility. In this regard, biomass-derived biochars are gaining attention due to their high potential, sustainability, carbon neutrality, low cost, mobile capability and wide availability in nature. Synthesis of biochars from inexpensive matters (living and non-living biomass) leads to significant cost reduction in waste disposal [9, 10]. These biochars can be obtained from various sources including industries and agricultural activities, plant wastes, fruit wastes, naturally occurring inorganic materials and living and dead biomass [11]. Literature reports a wide variety of biomass-derived biochars for effective wastewater treatment such as date pits, vermiculite plants, coconut shell and husk, bamboo waste, rice husk, ground nutshell, shells of almond, wheat bran and Heveabrasiliensis seed coat [12, 13]. These waste materials not only balance the environmental problems but an unutilized and a potential resource is also managed during the process. Also, utilization of these biochars resolves several major challenges associated with up-scaling technology including pollutant selectivity, regeneration, sludge formation and pollutant recovery and also exhibits excellent adsorption ability. Many factors affect the adsorption capacity of these biochars including physical and chemical properties of pollutants (i.e. molecular weight, oxidation state and ionic radius), characteristics of biochar and the process parameters (i.e., quantity of bio-sorbent, pH, temperature and sorbate concentration). Besides influencing the dissociation of pollutant sites and solution chemistry, pH plays a crucial role in the speciation and biosorption affinity of pollutants. Other factors include the composition of biomass (cellulose, hemicellulose, lignin and extractives), pore structure, surface charge and heteroatom content in the biochar. The adsorption capacity of biochar is highly dependent on the chemical compositions and carbohydrate contents of biomass which may vary from source to source and species to species. The biomass exists in diverse forms and has distinctive physical and chemical compositions, carbohydrate and lignin fractions as summarized in Table 1. In comparison to other wastes, agricultural and forestry wastes have high percentage composition of carbohydrates and lignin [14, 15]. Utilization of such biochars not only enhances the removal efficiency of various pollutants but simultaneously helps in the reduction of atmospheric carbon dioxide via the processing of waste biomass for a wide range of applications such as the synthesis of biofuels (i.e. biobutanol, bioethanol, and biomethanol), energy storage and soil refinement [16]. Apart from biochars, hydrochars have gained significant importance. Hydrochar is a char which is made by hydrothermal carbonization (a process where biomass is heated to a temperature range of 200–300°C in the presence of water), and is comprised of two phases: liquid and solid. Hydrochars offer advantages like low oxygen and ash content, zero hazardous chemical waste generation, high production yield (approximately 30-60 wt%), mild temperature processing (180-250°C), large surface areas and porosity. These materials offer several applications in many areas including soil amelioration, energy storage and water purification.

Type of waste	Cellulose	Hemicellulose	Lignin
Forestry waste
Softwood
Douglas fir	39	23	28
Scoots pine	40	25	28
Scandinavian pine	44	26	29
Scandinavian spruce	43	27	29
Hardwood
Babool	48.5	18.3	20.9
Catalpa	54	16.6	15.94
Indian rosewood	39	25	25
Chinaberry	40	21	30
Oil palm frond	49.8	17	20.5
Hardwood barks	22-40	20-38	30-55
Softwood barks	18-28	15-33	30-60
Canola plant species
C. caperata	46	16.9	15
B. adusta	44.28	15.61	13.83
F. gilva	46.04	16.99	15.75
P. tuberregium	44.99	16.59	11.09
Agricultural waste
Olive husk	24	23.6	48.4
Cotton seed hairs	80-95	5-20	0
Corn cob	50.5	31	15
Stored refuse	60	20	20
Corn stover	37-42	20-28	18-22
Coastal Bermuda grass	25	35	6.4
Swine waste	6	28	0
Solid cattle manure	1.6-4.7	1.4-3.3	2.7-5.7
Municipal waste
Pineapple leaf	73.4	25	10.5
Banana steam	60.6	12.4	18.6
Coconut shell	44.2	56.3	32.8
Hazelnut shell	28.8	30.4	42.9
Newspaper	40-55	25-40	28-30
Primary wastewater solids	8-15	NA	25-29
Industrial waste
Sugar beet waste	26.3	18.5	2.5
Sugarcane baggase	26-50	24-34	10-26
Rice straw	32-41	15-24	10-18
Sawdust	45.1	28.1	24
Cotton flax	80-95	5-20	NA
Sorghum straw	32	24	13

Table 1.

Percentage Composition in Biomass (Cellulose, Hemicellulose, Lignin).

2. Remediation techniques employed for the removal of contaminants from wastewater

Several techniques are commercially available, to remove various contaminants including (inorganic and organic chemicals in dissolved and non-dissolved forms, biologically active agents, radioactive substances, polychlorinated biphenyls and pesticides) from wastewaters and some are summarized in Table 2. The following Table 2 lists the most common commercially available techniques used in different sectors for pollutant removal due to their distinctive characteristics such as low-cost operation, flexibility and design simplicity.

2.1 Ion exchange

The ion exchange reaction is a reversible chemical reaction that involves the removal of dissolved ions from a solution and their replacement with other ions of the same or similar electrical charge. This process uses an insoluble matrix (or support structure) which is in the form of small microbeads (0.25–1.43 mm radius), usually white or yellowish, and are fabricated from an organic polymer substrate. This process has been widely employed for the separation of ionic dyes and heavy metal ions from aqueous streams. The widely used materials for this process are ion-exchange resins that can be natural or synthetic having the ability to exchange their cations with the solutes present in the aqueous streams. Several parameters affecting the ion-exchange process are temperature, solution pH, initial metal concentration, contact time and ionic charges. Zeolites (silicate minerals) are most abundant in nature and have been extensively used to separate heavy metal ions from aqueous streams under different conditions [16, 18]. Although natural Zeolites show good performance in a few cases scale-up of the process at an industrial level is still restricted. In contrast, synthetic resins show high efficiency in comparison with natural resins. Literature reports that macroporous anion exchangers (MP62, weak basic and S6328a, strong basic) are more effective with higher affinity and adsorptive capacity to separate pollutants from wastewaters originating from textile industries [19, 20].

2.2 Advanced oxidation processes (AOP)

Advanced Oxidation Process (AOP) is a treatment technology designed to remove organic matter from wastewater by oxidation through a reaction with hydroxyl radicals. As opposed to direct oxidation, AOPs usually consume less energy. In AOPs, a sufficient amount of hydroxyl radicals are produced that impact water purification. Hybrid advanced oxidation processes such as photocatalytic fenton, photo-fenton, H₂O₂/O₃/photocatalysis and photo-electrocatalysis have drawn the attention of industrialists and academicians due to their efficiency and cost-effectiveness [18, 19]. Currently, several nano-particle supported AOPs have been discovered for the remediation of several contaminants from wastewater such as methyl orange, methylene blue, 2,4-dichlorophenol and pentachlorophenol. Recently, research has also been carried out to explore the activity of photo-Fenton and/or heterogeneous Fenton catalysts for the simultaneous removal of multiple contaminants from waste streams. TiO₂ photocatalyst mixed with fly ash has also been employed for simultaneous separation of Cd⁺² ion and methyl orange dye from an aqueous stream (removal efficiencies of Cd⁺²: 88% and methyl orange: 70%) [21]. Similarly, heterogeneous catalyst Fe^IIFe₂^IIIO₄ nanoparticles supported on activated carbon have been utilized in a photo-Fenton process for remediation of pollutants (aniline and benzotriazole) and maximum removal efficiency for aniline was found to be 70.4% and benzotriazole to be 99.5% [22]. Although, the studies based on nano-particle supported AOPs proved to be promising at the pilot-scale, this process has no valid evidence to prove its cost-effectiveness and its eco-friendly operation due to the toxicity of nanoparticles. Also, there is no reliable information available about the commercialization of AOPs for the simultaneous treatment of multi-component pollutant systems.

2.3 Flotation

The flotation technique has been extensively used to remove inorganic heavy metal ions from aqueous streams. Flotation is a separation process that works on the introduction of gas bubbles as the transport medium. Suspended particulate matter, being hydrophobic or adhering to gas bubbles and move towards the water solution surface—i.e., contrary to the direction of gravity. In this technique, heavy metal ions are made hydrophobic by the use of some hydrophobic agents such as surfactants (surface-active chemicals) and separated with the assistance of air bubbles. The surface-active agents consist of a hydrophilic head (water-loving part, polar) and hydrophobic carbon chains (non-polar, water-hating part). The air bubbles loaded by solutes float over the water surface and are separated as a metal-rich froth [17]. This process is highly effective for the removal of sulfide minerals. Despite several advantages (i.e. almost all minerals can be removed, surface properties are highly governed and controlled by flotation agents used), this process has some disadvantages such as high cost and complex. According to recent research, open tank settling clarifiers are currently used as primary, secondary and tertiary clarifiers. This is primarily due to their reluctance to embrace new technologies in the development of dissolved air flotation (DAF), especially in paper mills. The specific clarification is limited to 0.5 GPM per square foot. Chemical treatment improves specific load and transparency while the residence time of settling is still 60-200 minutes.

2.4 Adsorption

Adsorption is a well-established separation process used widely for the removal of inorganic and organic compounds from wastewater. This process is proved to be superior as compared to other remediation techniques due to its ease of operation. It is simple and flexible in design, capable to treat dye wastewater effectively even at higher concentrations and also insensitive to the toxicity of contaminants [3, 5, 23]. The adsorption technique is dependent upon the affinity of contaminants towards the adsorbing materials. It is influenced by many other factors such as specific surface area of adsorbent, interactions between pollutant and sorbent, particle size distributions, solution pH, system temperature and contact time. The proper selection criteria of any adsorbent for separation are based on several characteristics such as adsorption capacity of adsorbent, selectivity, regeneration power, mechanical strength and low cost. Several adsorbents have been extensively utilized and show high sorption capacity for simultaneous removal of organic and inorganic solutes from wastewater as shown in Table 3. For instance, Fly ash has been successfully utilized for the separation of heavy metals and dyes from a multi-component aqueous solution; Ca(PO₃)₂-modified carbon can be used for the separation of heavy metal ions and dye (acid blue 25); Nano-particles (TiO₂) for removal of organic dye, copper and silver heavy metals; Graphene oxide nano-composite can be used for separation of cadmium and ionic dyes; Magnetic metal–organic frameworks composite i.e. (Cu-MOFs/Fe₃O₄) have been used for separation of malachite green dye and lead ions; Zr-based magnetic Composites i.e. Zr-MFCs and Amino-decorated for separation of lead and methylene blue [17, 24]. The use of this technology for the treatment of textile wastewaters is still limited due to excessive maintenance cost, high regeneration cost, issues regarding proper disposal of used adsorbents and the requirement of pretreatment to reduce suspended solids into feed for acceptable operational range. Thus, the adsorption technique shows promising outcomes at a commercial scale and resolves several challenges associated with waste disposal and regeneration.

2.4.1 Utilization of biochar as an adsorbent

All the above processes show their advantage and disadvantage concerning process efficiency, high costs (capital or operational), adsorbents, process conditions and removal percentage of pollutants. In this regard, biochars are receiving increasing attention and are highly recommended as a bio-adsorbent since they can both mitigate climate change by capturing carbon dioxide from the atmosphere into soil and increase the removal of organic pollutants. Biochar is defined as a carbon-rich material produced during the pyrolysis process that is a thermochemical decomposition of biomass with a temperature of about ≤700°C in the absence or limited supply of oxygen. As it is having a high-carbon content (approximately 60–90%), the application of biochar for the removal of a wide variety of contaminants from wastewater is considered a significant and long-term approach to sink atmospheric CO₂ in terrestrial ecosystems. Several kinds of biomass can be used as sources of biochar, such as wood chips, animal manure, and crop residues. Biochars have the ability to enhance the recycling of agricultural and forestry wastes. Biochar adsorbents are relatively cost-effective, environment-friendly and will be a beneficial tool for environmental remediation. Thus biochar research is gaining attention.

2.4.1.1 Characteristics of Biochar

The properties of biochar are determined by the pyrolysis temperature, the residence time, the feedstock considered, and the technology used for conversion. These factors influence the effectiveness of contamination removal. It was found that the amount of carbonized matter, the surface area, the pores, and the hydrophobicity of biochar increased with increasing temperature, consequently increasing the affinity of organic pollutants for adsorption. The presence of a high amount of carbonized matter in biochar favours the adsorption of contaminants, especially for the compounds having oxygen and hydrogen functional groups. According to research, activated carbon derived from wheat residue at 500-700°C was well carbonized and had a high surface area (>300 m²/g), whereas charcoal made at 300-400°C was partially carbonized and had a lower surface area (<200 m²/g) [25]. Hence, the former material exhibits high sorption capability for the removal of organic pollutants. Biochar can be made of diverse materials exhibiting different properties. The change of properties of biochar can be correlated to their function. Additionally, improving the adsorption capability of biochar through different treatments, such as chemical activation and surface modifications are found to be effective in improving its properties. This may be due to the enhanced porous structure and sorption properties that occurs after activation process [25]. Apart from activation of biochar, magnetization is also a useful method to improve biochar property. Tables 4 and 5 summarizes the different preparation methods of biochar under different operating conditions.

Type of wastewater	Removed pollutant	Type of adsorption	Adsorbents		Performance		References
			Novel	Commercial	Novel (adsorption capacity)	Commercial (adsorption capacity)	[4]
Industrial	Fluoride ions (7.5 PPM)	Adsorption	Carbon slurry	Powder Activated carbon	4.86 mg/g	1.10 mg/g	[13]
Industrial (Textile Wastewater)	Direct Blue 85 (450 PPM)	Oxidation	Metal oxide/hydroxide sludge	Powder Activated carbon	339 mg/g	7.69–18.7 mg/g	[16]
Industrial (Medical discharge, Surface treating wastewater and automotive discharge)	Chromium (VI) (55 PPM)	Adsorption	Calcinated cereal and other crops by-product	Powder Activated carbon	90.37%	89.85%	[8]
Industrial (Textile Wastewater)	Safranin-T (30 PPM)	Coagulation, adsorption, flocculation and reverse osmosis	Chemically activated rice and wheat husks	Powder Activated carbon	0.014 mol/g	0.526 mol/g	[17]

Table 2.

Performance summary of different adsorption techniques using different wastewater.

Preparation methods	Reaction time	Heating rate	Temperature (°C)	Yield (%)			References
				Solid	Liquid	Gas
Fast pyrolysis	Seconds	Fast	<1000	10	70	20	[3, 5]
Hydro-carbonization	Minutes to hours	Slow	< 350	50–80	—	—	[20]
Flash pyrolysis	Seconds	Faster	775–1025	10–15	70–80	5–20	[19]
Slow pyrolysis	Hours	Slow	< 700	35	30	35	[12]
Gasification	Seconds to Minutes	Faster	700–1500	10	5	85	[3, 5]

Table 3.

Summary of adsorption capacity of various biomass-derived biochar with different operating conditions.

Material	Process type	Concentration range	Adsorbate	Contact time	Adsorption capacity	Percentage adsorption	Reference
Juniper wood	Fast Pyrolysis	—	Cd (II)	30 min	24.8–28.3 μ molg^−1	—	[26]
Charfines, bituminous coal and lignite coal	Slow Pyrolysis	50 mg L^−1	Direct brown	60 min	6.4, 2.04 and 4.1mg g^−1	—	[27]
Lignite-lignin	Slow Pyrolysis	—	Cu (II), Ni (II) and Pb (II)	Cu (II) and Ni (II): 40–70 min; Pb (II): 10–30 min	17.8, 13.0 and 56.7 mg g^−1	67%	[28]
Peat	Slow Pyrolysis	100–500 mgdm^−3	Pb	4 h	27–106 mg g^−1	—	[29]
Pink bark	Fast Pyrolysis	≤ 400 mg L^−1	Cu (II), Ni (II) and Cd (II)	24 h	0.149, 0.107 and 0.126, mmolg^−1	—	[25]
Sphagnum peat moss	Fast Pyrolysis	35–210, 10–100 and 25–200 mg L^−1	Pb, Ni and Cu	—	24.6, 7.5 and 14.3 mg g^−1	—	[24]
Starch graft copolymer	Slow Pyrolysis	—	Cu (II) and Pb (II)	2 h	2.12 and 2.09 mmol g^−1	—	[17]
Bentonite	Gasification	100 μg ml^−1	Cu (II)	180 min	4.75 mg g^−1	85%	[25]
Chitosan bead (Chemically crosslinked)	Fast Pyrolysis	—	Reactive blue 2, reactive yellow reactive yellow 2 and Reactive red 2	5 days	86; 1911; 2498,; 2436 and; 2422; mg g^−1	—	[29]
R. arrhizus and C.vulgaris	Fast Pyrolysis	1996.2 and 387 mg L^−1	Iron (III)-cyanide complex	—	612.2 and 387 mg g^−1	—	[24]
Anodonta shell	Gasification	—	Reactive green 12 and direct green	15 days	260.436 and11.3 mg g^−1	—	[29]
Pinus sylvestris bark	Slow Pyrolysis	5–20 mg L^−1	Cr (III)	24 h	9.77 mg g^−1	≥9	[30]
Natural clay	Fast Pyrolysis	10–50 ppm	Ni (II)	45 min	12.5 mg g^−1	—	[30]
Saw dust: walnut	Gasification	50–1000 and 50–500 mg L^−1	Methylene blue and Acid blue 25	60–180 min	59.17, 36.98 mg g^−1	—	[25]
Peanut hull	Fast Pyrolysis	≤ 1000 mg L^−1	Pb (II), Zn (II), Cu (II) and Cd(II)	4 h	30, 9, 8 and 6 mg g^−1	—	[31]
Sawdust	Slow Pyrolysis	1–50 mg L^−1	Cu (II)	60 min	4.40–0.16 mg g^−1	—	[25]
Peanut hull carbon	Gasification	10–20 mg L^−1	Hg (II)	5–180 min	109.89 mg g^−1	—	[31]
Kraft lignin	Fast Pyrolysis	5–200 mg L^−1	Cu (II)	3 h	3.38 mg g^−1	—	[24]
Alkali-treated straw	Gasification	—	Cr (III)	60 min	3.91 mg g^−1	—	[17]
Orange peel	Fast Pyrolysis	—	Direct red 23 and Direct red 80	15 min	10.72 and 21.05 mg g^−1	—	[31]
Hazelnut shell	Slow Pyrolysis	0.1–2.0 mmoL^−1	Cd^2+, Cr (IV), Zn^2+ and Cr (III)	5 h	5.42, 3.99, 1.78 and 3.08 gKg^−1	92.4, 97.8, 87.9 and 94.6	[31]

Table 4.

Several preparation methods of biochar under different operating conditions.

Synthesis method	Process summary	Merits	Demerits	Reference
Slow pyrolysis	Prolonged residence time ranging from 1 to 6 hr. of lignocellulosic biomass with low process temperature (<700°C) at atmospheric pressure	Equal fractions of products (liquid, solid & gas).; Large pore size, high ash and lignin content.; Cost effective, robust and modular	Highly endothermic	[26]
Fast pyrolysis	Fast heating rate with temperature (<1000°C) with shorter reaction time ranging from few seconds to minutes.	Major products are non-condensable gases; Produced bio-oil can be utilized as a feedstock for the production of energy.	Low biochar yield	[30]
Sol–gel method	Heating the solution of 0.5 mol of Citric acid and 0.5 M Aluminium nitrate at 65°C for 120 min at 180 rpm to form gel	High adsorption efficacy; High surface area and available active site; High thermal stability	High cost of the feedstock; Large shrinkage of volume and heavy cracking during drying; Produces non-uniform crystal defects	[29]
Ball milling method	Mixing of 3.30 g raw material with 330 g of agate spheres with 60 g of distilled water at 300 rpm for 12 h. Direction of mixing may be changed every 3 h. Finally, the solution can be centrifuged for 5 min at 9000 rpm. The resultant solid biochar is dried at 80°C for 12 h.	Larger pore volume; Efficient adsorbent	Loud noise & strong vibration during its working process; Time consuming	[22]
Co-precipitation method	Immersed the raw material with a solution of Magnesium Chloride and Aluminium Chloride (3:1) with uniform stirring for 12 h. Resultant suspension can be added to a beaker and stabilized at 60°C for 12 hr. with uniform stirring at 400 rpm. Finally, filter the resultant and washed thoroughly with distilled water and dried overnight at 80°C.	Promotes surface adsorption; Generates high-capacity adsorbent	Some impurities get precipitated with the product; Expensive	[28]
Hydrothermal synthesis method	Solution A → Dissolve 0.01 mole Iron nitrate and 0.02 mole Magnesium nitrate in 50 mL ultra-pure water.; Solution B → Dissolve 0.01 mol Sodium carbonate and 0.03 mol Sodium hydroxide in 30 mL aqueous solution.; Add 2 g raw material in Solution B followed by Solution A with uniform stirring for 30 min. Transfer the solution to Teflon-lined high-pressure reactor and aged for 6 hr. at 120°C. Finally, filter the resultant product and washed with distilled water and dried for 8 h at 70°C.	Good magnetic properties	Unable to see the growing crystals; Expensive	[24]
Solvothermal method	Mix Iron chloride, PEG 4000 and Sodium acetate in 80 mL of Ethylene glycol with constant stirring for 30 min followed by the addition of raw material. Autoclave the resultant solution for 8 hr. at 200°C and quench the mixture to room temperature. Collect the obtained black precipitate using magnet followed by washing with Ethanol and distilled water and kept the sample in oven for 8 h at 70°C.	Uniformly dispersed magnetic nanoparticles with controllable particle size synthesized; High product purity	Unable to see the growing crystals	[17]
Succinylation	Add 5 g raw material to 500 mL of xylene, 14 mL of trimethylamine & 10 g of succinic anhydride and heat the solution for 8 h at 120°C for Succinylation. Filter the resultant solution using micro syringe and washed with acetone several times to remove the residues of xylene.	Production of efficient biochar	Low stability at high temperatures	[20]

Table 5.

Overview of synthesis of biochar-based sorbents.

2.4.1.2 Biochar adsorption mechanism

Adsorption is a surface phenomenon with a common mechanism for the removal of organic and inorganic pollutants. When a solution containing an adsorbent solute comes into contact with a solid with a very porous surface structure, the intermolecular attractive force between the liquid and the solid causes some of the solute molecules to concentrate from the solution or deposit on the solid surface. The mechanisms for the removal of organic pollutants with biochar involves surface sorption, cation/ion exchange, electrostatic interactions, precipitation and complexation [28]. All these mechanism as an individual or together plays important role and show great effect on adsorption capacity.

Surface sorption: In this process, metal ions diffuse into the pores of the sorbent to form chemical bonds. The pore volume and the surface area of the sorbent (biochar) depend upon the carbonization temperature.

Electrostatic interaction: It is a mechanism that uses electrostatic interaction between the charged biochar particles and the metal ions to prevent metal ion mobilization.

Cation/ion exchange: The major principle of this mechanism is the exchange between protons and ionized cations on the surface of the biochar. As a result, its ability to remove heavy metals depends on the size of the contaminated surface and the surface functional groups of the biochar.

Precipitation: It is one of the main mechanisms that can be used to remove inorganic pollutants from biochar. As a result, mineral precipitates are formed either within the solution or on the surface of the sorbing material. In particular, this occurs for biochar produced from pyrolysis of cellulose and hemicelluloses with a temperature exceeding 300°C and with an alkaline property.

Complexation: Metal complexation involves the formation of multi-atom structures through the interaction of specific metal ligands. Due to the oxygen-containing functional groups present in low-temperature biochar such as phenolic, lactonic, and carboxyl, it can bind with heavy metals. The oxygen content of the biochar can lead to an increase in surface oxidation and metal complexation.

Biochar’s remediation effect is achieved by these mechanisms as shown in Figure 1 and the nature of bonding working together, rather than acting separately. The nature of the bonding depends on the type of species interaction while the adsorption process is usually classified as physisorption (characteristic of weak Van Der Waals forces) or chemisorption (characteristic of covalent bonding) [29].

Figure 1.

Properties and mechanism of biochar functioning.

3. Development of economic and sustainable biomass derived biochar

3.1 Preparation methods of biochar

Several techniques including pyrolysis, gasification, hydro carbonization have been used for the synthesis of biochar affecting the adsorption capacity and are discussed in Table 4. Pyrolysis of biomass is found to be the most widely used technique and can be carried out in the absence of oxygen at high temperature. Pyrolysis process can be classified as slow, fast and flash depending on the temperature and residence time. A slower heating rate and a lower pyrolysis temperature can result in the high yield of solid products [27]. It was found that slow pyrolysis results in the formation of ~35% solid yield indicating the effectiveness of the process among other three-pyrolysis techniques. Hydrothermal carbonization (HTC) is another important technique used for the synthesis of biochar. Biochar obtained from HTC exhibit superior adsorption properties with zero production of toxic substances. The main limitations of this method are requirement of high pressure, reactor cost and high temperature that limits the practical applications. Recent literature shows that the treatment of sewage sludge is found to be more effective and feasible using HTC as compared to other thermochemical processes due to low energy consumption and high thermal and mechanical stability of biochar. In addition to slow pyrolysis and HTC, other methods such as rapid pyrolysis, flash pyrolysis and gasification are also efficient and cost effective. However, such methods have low product yield and are typically used to produce bio-oil or gaseous materials.

There is a strong relationship between the preparation method and the physicochemical properties of biochar as shown in Figure 2. Biochar can be produced from wide range of biomass such as municipal, agricultural, aquatic or forestry having different physical, chemical and structural properties. There are several factors affecting the physicochemical properties of biochar including type of the raw material, source of biomass, pyrolysis type (slow, rapid or flash), duration of pyrolysis, size of the substrate, temperature and heating rate [26]. These operating parameters results in the number of surface functional groups including hydroxyl, carbonyl, methyl and carboxyl. In addition, several factors affect the structure of biochar including oxygen-containing aromatic functional groups, high carbon content, surface area and high porosity. These factors significantly favour the adsorption of pollutants onto the surface of biochar.

Figure 2.

Biochar preparation methods and its applications.

3.2 Biochar’s properties influencing its activity

As discussed above, properties of biochar are influenced by pyrolysis temperature, residence time, feedstock, and the thermal conversion technology. The variations in these parameters results in the variation in the removal efficiency of pollutants as shown in Figure 3. The selection of biochar for a specific purpose depends on several factors such as mechanical strength, adsorption efficiency, cost, regeneration, ease of synthesis, selectivity for different pollutants, reusability and rate of adsorption and desorption. Due to high porosity, sorption ability of biochar is highly dependent on the surface area. The high surface area enhances the ability to adsorb the pollutants on the surface of biochar. This can be done either by physical modification (such as purging of steam and gas) or by chemical activation using various chemical reagents (concentrated or diluted). In addition to porosity, several other factors including pH, temperature, adsorbent dose, and agitation speed affects the adsorption process [20]. pH is the most crucial parameter that affects the dissociation of functional groups and the charge on the active sites, thereby affecting the adsorption capacity. Another significant parameter is the biochar dosage. Significant increase in the adsorption of pollutants has been found with increase in the biochar dosage due to the availability of sufficient active sites on the surface of biochar. While further increase in the biochar dose than the optimal dosage declines the adsorption of pollutants due to the saturation or blockage of active sites. Generally, adsorption processes are endothermic in nature thus on increasing the temperature, increase in the adsorption of pollutants was observed. It has been observed that high temperature leads to the degradation of molecules that results in the decline of adsorption capacity [18]. Thus, maintaining an optimum temperature is highly essential. Agitation speed is another critical parameter that influences the adsorption capacity and reaction mechanism. With increase in the agitation speed, gradual increase in the adsorption capacity has been observed. This may be due to the increase in the turbulence and reduction in the thickness of the boundary layer around the biochar that improves the interaction between adsorbate and adsorbent. According to the literature, the boundary layer and intraparticle diffusion are the controlling steps for the adsorption mechanism, and the optimum speed for adsorption process is usually in the range of 120 rpm to 200 rpm.

Figure 3.

Factors affecting the properties of biochar.

4. Future perspectives

It is evident from the above studies that the biochars are potential and economical candidates for water purification. This study covers the advancement in the field of biochars followed by their utilization in various fields. However, detailed research is still required in terms of physical and chemical modifications to enhance porosity and surface area of biochar. Further, polymorphs of biomass for the production of biochar and their effect on multicomponent systems still needs exploration. A more underlying mechanistic approach is required to understand the role and performance of individual components i.e. cellulose, hemicellulose and lignin as these polymers provide heterogeneity to the biomass matrix. Differences in the magnitude of adsorption capacities using different biochars having the same origin and composition is an indicative of unexplained correlation between morphological patterns and molecular structure of biochars. Besides this, a critical investigation is required to determine the effectiveness of surface area, porosity and functional groups of biochars. Many studies cover the technical performance of biochars while the economic feasibility and environmental impact is neglected. Studies need to be carried out in detail to suggest an effective binding mechanism of several pollutants with biochars. Also, no study has been reported on the removal of anions, radionuclides and pesticides using biochars. Further, limited data is available for the competitive adsorption of contaminants especially on phenols and dyes. In addition, some biochars are incapable to perform under neutral conditions (pH 7.0) and at low concentrations (μg/mL), therefore it is essential to develop biochars which are effective at normal temperature and short residence time. Despite the limited price information and widespread utilization, scale-up technology of biochars is strongly recommended due to their engineering applicability, easy availability and techno-economic feasibility.

5. Drawbacks of biochar

Although there is a growing consensus on the benefits of biochar in various areas and at the same time different point of view exists. Several concerns have been raised on the sustainability and carbon neutrality in the utilization of biochar. Some of the challenges which limit the usage of biochar for scale-up production include (i) incompetence while supressing the emission of greenhouse gases (ii) effectiveness of biochar for all type of organic pollutants (iii) toxicity of biochar. For instance, production of biochar from different raw material may contain chlorinated organic compounds such as polyvinyl chloride or pentachlorophenol and may result in the formation of polychlorinated biphenyl-p-dioxins, PAH and furans. However, if there is a sudden increment in the level of such compounds in the biochar then it imposes threat to the environment and human health. Therefore, it is essential to suitably select the feedstock and synthesis conditions including temperature, residence time and technology that could control the concentrations of potentially toxic compounds in the desired biochar. Safe usage of biochar materials ensuring human health and environment benefit along with comprehensive life cycle analysis and environmental risk assessment is recommended.

6. Conclusion

This chapter attempts to cover wide range of low-cost biochars for the effective removal of toxic contaminants from wastewater. These materials offer several advantages including technical feasibility and engineering applicability and serves as a boon for the environmental scientists and government authorities. The suitable selection of biochars not only minimizes the cost inefficiency but also improves profitability and adds promising benefits for the scale-up technologies in future. In addition, some biomass derived materials with and without prior pretreatment can be used as biochars in non-industrialized sectors. The purpose is to implement sustainable development policies at local and national levels. With few exceptions, it appears from the literature that biochars having good carbon content are usually versatile adsorbents that can be successfully used to remove contaminants from wastewater. Besides the technological progression, some limitations that still need to be overcome are (i) low surface area of biochar (ii) critical balance between pH and operating temperature during adsorption (iii) relationship between composition and constituents of the biochars is essential. Last but not the least, exploration of the possibility of recovering or reusing adsorbed substances needs attention.

Acknowledgments

The authors acknowledge Guru Gobind Singh Indraprastha University, New Delhi, India.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter.