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Biochar in Remediation of Organic Pollutants in Water
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This entry is adapted from the peer-reviewed paper 10.3390/catal12020210

Biochar is a biological material for environmental remediation due to its low-cost precursor (waste), low toxicity, and diversity of active sites, along with their facile tailoring techniques. Due to its versatility, biochar has been employed as an adsorbent, catalyst (for activating hydrogen peroxide, ozone, persulfate), and photocatalyst. Biochar could also be applied in remediation of organic pollutants in water.

biochar adsorption H2O2 activation O3 activation peroxymonosulfate activation peroxydisulfate activation photocatalysis organic pollutants

1. Introduction

Over the past few years, the rapid growth of various industries has led to the release of toxic pollutants to the environment. In particular, various refractory pollutants such as antibiotics, phenolic compounds, dyes, and heavy metals are detected in numerous water bodies and soil [1][2][3][4][5]. In particular, pollution due to refractory pollutants in water bodies is of emerging concern due to its greater mobility compared to soil. While these pollutants pose a threat to public health, they also disrupt the microbial ecosystem [6][7]. For instance, the release of trace antibiotics will lead to the development of multidrug-resistant strains, which will result in less effective clinical treatment effects of conventional antibiotics [7]. Phenolic compounds such as chlorophenols are toxic and may induce carcinogenic and mutagenic effects. In order to alleviate this problem, methods such as adsorption [8], membrane separation [9], and advanced oxidation processes (AOPs) [10] can be used to treat the wastewater before it is discharged into the environment. Generally, adsorption and membrane separation involve the separation of pollutants and water without destroying the pollutants, while AOPs utilizes reactive oxygen species (ROS) generation for organics mineralization.
Recently, biochar has gained attention in various environmental remediation applications. Biochar can be obtained by the direct carbonization of organic wastes in an oxygen-deficient environment. Compared with other carbon-based materials, some of the advantages of using biochar for environmental remediation include low production cost (cheap raw materials), relatively simple preparation methods (facile pyrolysis of biomass), and eco-friendly (waste recovery). Furthermore, the properties of biochar can be engineered more easily (compared with other carbon allotropes) by changing the synthesis parameters (e.g., synthesis temperature and duration) [11][12], careful selection of biomass as a precursor [13], and functionalization (e.g., heteroatom doping, oxygen tuning, and defects) [14][15].
Numerous reviews on the use of biochar in specific environmental applications are available [16][17][18][19][20][21][22], howbeit, with modest emphasis on comparing biochar performance and the mechanism of treating organic pollutants under different systems for finding superior applications of biochar and to fully realize its potential. For instance, Dai et al. [21] discussed the employment of biochar as an adsorbents for organic pollutants, however, without discussing biochar potential in other pollutant remediation systems. Similarly, Zhou et al. [19] and Zhao et al. [23] reviewed biochar as AOPs catalyst and persulfate activator, respectively, without comparison against other systems (e.g., adsorption, O3 activation).

2. Applications of Biochar in Environmental Remediation

2.1. Adsorption

Adsorption is a common technique to remove organic pollutants. Due to its relatively low operational cost, adsorption is widely utilized for water remediation. Table 1 presents an overview of the performance of biochar as an adsorbent for various organic pollutants. The criteria for an effective adsorbent include having high SSA and porosity along with the abundance of active sites. Generally, the adsorption process relies on the liquid–solid intermolecular attraction between the adsorbate and the adsorbent, which leads to the accumulation of solute molecules on the adsorbent surface [24]. The adsorption mechanisms between the biochar and organic pollutant occur through physical and chemical interactions, including H-bonding, hydrophobic interactions, electrostatic attraction, π-π EDA interactions, complexes adsorption, Lewis acid–base interactions, pore filling, partition uncarbonized fraction, dipole–dipole interactions, Coulombic attraction, spectrometer exchange, and acceptor interactions [22]. The adsorption process undertaken is controlled by the nature of the adsorbate, biochar properties, and operational condition (i.e., pH, pressure of water matrix, rate). The adsorption process is divided into three stages. First, there is external mass transfer of organic pollutants from the aqueous solution to the biochar surface (external diffusion), which is followed by the diffusion of organic molecules into the pores of biochar (internal diffusion) and ending with the adsorptive interactions (Figure 1) [14][25].
Figure 1. Mechanism of phenol adsorption by N-doped biochar. Reprinted with permission from Li et al.[14]. Copyright 2019 American Chemical Society.
Table 1. An overview of biochar preparation and performance in adsorption of organic pollutants.
Table
Biochar Precursor and
Synthesis		 Performance Removal Mechanism Ref
  • Peanut shells
  • Pyrolysis: 700 °C
  • N, S co-doping: Thiourea + biochar, pyrolysis: 350 °C, 1 h
  • [DEP] = 20 mg L−1, [Biochar] = 1 g L−1, pH = 7
  • Qe of 3.60 ± 0.20 mg g−1 and k2 of 0.04 ± 0.05 g mg−1 min−1 in 4 h
  • Oxidized S and pyridinic N were crucial in the adsorption process
  • Removal efficiency remained ≥80% after 5 cycles (used biochar was washed with methanol)
 [26]
  • Hickory sawdust + boric acid
  • Pyrolysis: 700 °C, 2 h
  • [SMX] = 50 mg L−1, [Biochar] = 0.5 g L−1, pH = 5
  • Qe of 94.76 mg g−1 in 240 min
  • Enhanced porosity and SSA by boric acid activation promoted better adsorption capacity
  • Adsorption capacity and SSA were in well agreement (R2 = 0.9091), indicating that adsorption followed the pore-filling process
  • Adsorption capacity decrease from 94.76  to 64.62 mg g−1 after 5 cycles (NaOH treatment of used catalyst after every cycle)
 [27]
  • Tapioca peels + S doping
  • Pyrolysis: 800 °C, 3 h, N2 atmosphere
  • [RhB] = 25 mg L−1, [Biochar] = 2 g L−1, pH = 8
  • 92.6% removed in 120 min
  • Biochar had homogeneous surface interactions with RhB via monolayer adsorption system
  • Adsorption was controlled by H-bonding, surface interaction, and electrostatic attraction
  • Adsorption efficiency decreased with each cycle (NaOH treatment of used catalyst after every cycle)
 [28]
  • Livistona chinensis
  • Pyrolysis: 500 °C, 4 h, N2 atmosphere
  • [MG] = 150 mg L−1, [Biochar] = 1 g L−1, pH = 7 ± 0.5
  • 99.7% removed in 24 h
  • Adsorption was best described by Langmuir isotherm model (R2 = 0.97)
 [29]
  • Grapefruit peel
  • Pyrolysis: 400 °C, 2 h
  • Modification with FeSO4 and FeCl3
  • [BPA] = 150 mg L−1, [Biochar] = 1 g L−1, pH = 7 ± 0.5
  • Saturated adsorption capacity 9.7098 mg g−1 after 150 min
  • Chemical adsorption is rate control step in adsorption with non-uniform adsorbent and evenly distributed energy of surface adsorption
  • Adsorption mechanism was controlled by a variety of forces (i.e., π-π EDA interaction, H-bond, etc.)
 [30]
  • Sawdust + Zn and Fe loading
  • Pyrolysis: 600 °C, 2 h, N2 atmosphere
  • [TET] = 150 mg L−1, [Biochar] = 1 g L−1, pH = 6
  • Adsorption process was controlled by chemisorption
  • H-bond between OH of TET and OFGs biochar along with the π-π EDA interactions promote fast sorption
  • After desorption (using NaOH treatment), the adsorption decrease from 91.6% to 89% after 3 cycles
 [31]
  • Rice straw
  • Pyrolysis: 700 °C, 2 h
  • H3PO4 treatment
  • [TC] = 120 mg L−1, [Biochar] = 2/3 g L−1
  • Qe of 166.3 mg g−1 in 192 h
  • Π-π EDA interactions and H-bonding are ascribed as primary sorption mechanisms
 [32]
  • Pomelo peels
  • Pyrolysis: 450 °C, 15 min
  • Chitosan treatment for chitosan–biochar hydrogel beads
  • [CIP] = 50 mg L−1, [Biochar] = 0.1 g L−1, pH = 3–10
  • Sorption capacity of 36.72 mg g−1
  • Adsorption efficiency remained similar at pH 3–10 but significantly decreases at higher pH
  • Adsorption mechanisms encompassed π-π EDA interaction, H-bonding, and hydrophobic interaction
  • OFGs groups such as -OH and -COOH on biochar make it more hydrophilic for hydrophobic interaction with CIP
  • F and N heteroaromatic ring of CIP allow it to act as π electron acceptor
 [33]

2.2. H2O2 Activation

Chemical oxidants such as H2O2, persulfate (PS), and O3 are capable of treating organic effluents. However, due to their relatively mild oxidative potentials, catalytic activation using biochar to generate reactive species is desirable. The activation process follows an electron transfer regime between the activator (biochar) and the oxidant, and these activation mechanisms are influenced by the properties of the peroxide (O-O) bond (i.e., bond distance and dissociation energy). Among the common oxidants, H2O2, with a redox potential of 1.76 V vs. NHE [34], is highly preferable in water treatment because its decomposition products are only oxygen and hydrogen, which is considered environmentally friendly to water treatment [35]. The H2O2 structure encompasses an O-O bond distance of 1.460 Å with a dissociation energy of 377 kJ/mol. In a biochar/H2O2 system, the electron transfer process from biochar to H2O2 molecules induces the breakage of the O-O or O-H bond for subsequent ROS formations. Table 2 overviews the performance and mechanism of H2O2 activation by biochar catalysts.
Table 2. An overview of biochar preparation and performance in H2O2 activation for organic pollutants removal.
Table
Biochar Precursor and
Synthesis		 Performance Removal Mechanism Ref
  • Maize straw + Fe-impregnation
  • Pyrolysis: 700 °C, 2 h, N2 atmosphere
  • [SMX] = 10 μM, [Biochar] = 1 g L−1, [H2O2] = 3 mM, pH = 5
  • 100% removed in 2 h
  • C-OH activates H2O2 to produce •OH, HO2•, and alkyl radicals to degrade SMX
 [36]
  • Sewage sludge
  • Pyrolysis: 500 °C, 4 h, N2 atmosphere
  • Acid (HNO3) treatment
  • [CIP] = 10 mg L−1, [Biochar] = 1 g L−1, [H2O2] = 10 mM, pH = 7.4
  • 93% removed in 24 h
  • Single-electron transfer from biochar PFRs to H2O2 induced •OH generation
  • Consumption of sp.3 and sp.2 carbons indicated and electron transfer regime from amorphous carbon to graphitic carbon, as the latter serves as a reactive site
  • C=O, pyridinic N, and pyridinic N were active sites for the activation by electron transfer to H2O2
 [37]
  • Wheat straws
  • Pyrolysis: 700 °C, 2 h, N2 atmosphere
  • [SMT] = 13.7 μM, [Biochar] = 1 g L−1, [H2O2] = 3 mM, pH = 5
  • 100% removed in 2 h
  • SSA and porosity were crucial in SMT adsorption and catalytic •OH generation
  • Biochar surface acidity was negatively correlated with catalytic activation, whereas surface basicity was positively correlated with catalytic activation
  • Excessive SMT adsorption prior to activation hindered H2O2 activation
 [38]
  • Biogas residue + KHCO3 activation
  • Pyrolysis: 800 °C, 3.5 h, N2 atmosphere
  • Acid (HCl) washing
  • [Benzene] = 10 μM, [Biochar] = 1 g L−1, [H2O2] = 3 mM, pH = 5
  • OFGs, C=C, pyridinic N, and graphitic N allowed electron transfer for •OH, •O2−, and 1O2 formation
  • 1O2 generation dominated the catalytic reaction
 [39]
  • Sewage sludge
  • Pyrolysis: 600 °C, 30 min
  • Pyrolysis + kaolin: 1100 °C, 30 min, N2 atmosphere
  • [CIP] = 10 mg L−1, [Biochar] = 0.2 g L−1, [H2O2] = 10 mM, pH = 4
  • <30% removed after 1 h of adsorption and 80% removed after 20 min from H2O2 addition
  • Large SSA and inorganic groups enhance CIP adsorption
  • ∙O2− is formed by reactions of •OH and H2O2
 [40]
  • Pig manure
  • Pyrolysis: 500 °C, 2 h, N2 atmosphere
  • [TC] = 30 mg L−1, [Biochar] = 0.5 g L−1, [H2O2] = 5 mM, pH = 7.4
  • 100% removed in 240 min
  • Electron transfer pathway from PFRs to H2O2 was responsible for •OH formation
  • TC adsorption can block the active site and minimize reactions between PFRs and H2O2, lowering activation performance
  • Removal efficiency decreased from 100% to 74.5% in 4th run
 [41]
  • Sugarcane residues + Fe impregnation
  • Pyrolysis: 600 °C, 4 h
  • [OG] = 0.1 g L−1, [Biochar] = 0.5 g L−1, [H2O2] = 0.075 g L−1, pH = 5.5
  • 99.7% removed within 2 h
  • HO2• and •OH are a function of H2O2 concentration
  • Catalytic degradation preferred lower pH due to the higher oxidative potential of HO2• and •OH, slower decomposition of H2O2 to H2O and O2, and formation of inner-sphere complexes of Fe oxides and OG
 [42]
  • Sugarcane bagasse + steel pickling waste liquor
  • Pyrolysis: 400 °C, 2 h, N2 atmosphere
  • [MNZ] = 40 mg L−1, [Biochar] = 0.3 g L−1, [H2O2] = 5 mM, pH = 5.61
  • 100% removed within 2 h
  • Surface-bounded •OH were the major ROS responsible for degradation
 [43]

2.3. O3 Activation

O3 is widely used for wastewater disinfection due to its high redox potential (E° = 2.08 V). Ozone may also directly oxidize organic pollutants with the targeting of C=C or N=N bonds [44]. However, direct ozonation is limited by selective interactions with organic molecules under acidic conditions. Additionally, O3 direct reaction with some organics such as saturated carboxylic acids and inactivated aromatics is slow, making it difficult to achieve complete mineralization [45]. Nonetheless, enhanced ozonation can be achieved by catalytic activation for escalated ROS generation and faster O3 decomposition. Table 3 provides an overview on the performance and mechanism of biochar in O3 activation.
Table 3. An overview of biochar preparation and performance in O3 activation for organic pollutants removal.
Table
Biochar Precursor and
Synthesis		 Performance Removal Mechanism Ref
  • Coking wastewater treatment sludge
  • Pyrolysis: 700 °C, 2 h, N2 atmosphere
  • [Phenol] = 0.2 g L−1, [Biochar] = 1 g L−1, [O3] = 14 ± 1 mg L−1, 1.0 L min−1
  • 95.4% removed in 30 min
  • C-O and C=O served as active sites for ozone catalytic decomposition
  • C=O adsorbed dissolved O3 and initiated its rapid decomposition for ∙O2− generation
  • C=O was oxidized into O-C=O after reaction
  • Presence of HCO3− promoted •OH and HCO3• formation, reacting with O3 for •O2− generation
  • Removal efficiency decreased from 95.4% to 59.3% in 4th cycle
 [46]
  • Activated petroleum waste sludge
  • Pyrolysis: 850 °C, 1 h, N2 atmosphere
  • [Benzoic acid] = 100 mg L−1, [Biochar] = 1 g L−1, [O3] = 20 mg min−1, pH = 3.8–10.0
  • ≥95% removed in 30 min
  • Functional C groups, Si-O groups, metallic oxides of Zn, Al, Fe, and Mg are expected as the active sites
  • Reaction was suggested to be mediated by •OH
  • Excessive adsorption under acidic conditions leads to competition with ozonation process
  • Removal efficiency fell to 79.1% in 5th cycle
 [47]
  • Peanut shell
  • Pyrolysis: 600 °C, 4 h, N2 atmosphere
  • [KET] = 2 mg L−1, [Biochar] = 500 mg L−1, [O3] = 0.5 L min−1, pH = 6 ± 0.1
  • 99.9% removed in 3 min
  • O3 direct or indirect attack on C=O, C=C, and -OH on biochar initiated radical chain reactions
  • Delocalized π-electrons react with H2O to form hydroxide OH− and H3O+ that will yield HO2, •O2−, and •OH after reaction with O3
  • 1O2, •O2−, and •OH were the main species responsible for degradation
  • Removal efficiency decreased to 94.8% in 5th cycle
 [48]

2.4. PS Activation

Recently, sulfate radical (SO4•−)-based AOPs (SR-AOPs) have gained attention for organic pollutant degradation. Generally, SO4•− can be achieved through the activation of PS such as peroxymonosulfate (PMS) and peroxydisulfate (PDS). Table 4 provides an overview on biochar performance in PS activation to remove organic pollutants.
Table 4. An overview of biochar preparation and performance in PS activation for organic pollutants removal.
Table
Biochar Precursor and
Synthesis		 Performance Removal Mechanism Ref
  • Food waste digestate
  • Pyrolysis: 800 °C, 2 h, N2 atmosphere
  • [X-3B] = 1 g L−1, [Biochar] = 0.5 g L−1, [PDS] = 1.5 mM, pH = 3.78
  • 85.72% removed in 30 min
  • sp.2 carbon served to produce radicals by PDS activation in an electron-accepting process
  • Graphitic and pyridinic N facilitated ROS generation
  • C=O contributed to 1O2 formation
  • Removal efficiency decreased from 85.72% to 32.53% at 30 min in 2nd cycle
 [49]
  • Peanut shells
  • Pyrolysis: 900 °C, 2 h, N2 atmosphere
  • [SMT] = 40 μM, [Biochar] = 0.2 g L−1, [PMS] = 1 mM, pH = 6
  • 98.3% removed in 120 min
  • Sieving to small-sized biochar (0–75 µm) enhanced PDS activation due to more graphitic and aromatic carbon, COOH content, and higher SSA
  • COOH groups promoted SO4•− and •OH formation
  • Biochar mediated electron transfer from SMT to PDS
  • Electron-rich active sites transfer electrons to PDS for SO4•−, •OH, 1O2, and •O2− generation
 [50]
  • Spent tea leave
  • Pre-oxidation: 250 °C, 0.5 h, air environment
  • Pyrolysis: 500 °C, 1 h, N2 atmosphere
  • [CTC] = 50 mg L−1, [Biochar] = 0.1 g L−1, [PDS] = 1 g L−1, pH = 6.0
  • 97.4% removed in 90 min
  • Pre-adsorption was beneficial to the subsequent catalytic degradation
  • Delocalized π-electrons and Fe facilitated SO4•−, •OH, and 1O2 generation
 [51]
  • Dairy manure
  • Anaerobic digestion: 37 °C, 35 days
  • Pyrolysis: 800 °C, 2 h, N2 atmosphere
  • [SMX] = 15 mg L−1, [Biochar] = 1 g L−1, [PMS] = 2.5 mM, pH = 5.56
  • 90.2% removed in 60 min
  • Defects were positively correlated with degradation potential (R2 = 0.92)
  • Electron-rich C=O promoted •O2− and 1O2 generation
  • Graphitic N accelerated electron transfer from carbon to O2 for 1O2 formation
  • Pyridine N was not an active site
  • Graphitic structure facilitated electron transfer between SMX and PMS
  • Removal efficiency decreased from 90.2% to 62.5% in 5th cycle
 [52]
  • Straw
  • Pyrolysis: 700 °C, 2 h, N2 atmosphere
  • N doping: Thiourea + biochar, pyrolysis: 800 °C, 2 h, N2 atmosphere
  • [TC] = 20 mg L−1, [Biochar] = 200 mg L−1, [PDS] = 2 mM, pH = 7
  • 97% removed in 60 min and 100% in 120 min
  • Reaction mainly relied on nonradical electron transfer between TC and PDS due to enhanced graphitization degree
  • Radicals had no effect on TC degradation
  • Removal efficiency decreased from 100% to 55% in 3rd cycle
 [53]
  • Rice husk
  • Pyrolysis: 850 °C, 1 h
  • [SMX] = 500 μg L−1, [Biochar] = 100 mg L−1, [PDS] = 500 mg L−1
  • 96% removed in 120 min
  • Reaction controlled by either electron transfer/1O2 control surface-bound radicals
 [54]
  • Wheat straw + B doping
  • Pyrolysis: 900 °C, 2 h, N2 atmosphere
  • KOH activation: 600 °C, 2 h, N2 atmosphere
  • Acid (HCl) washing
  • [SMX] = 20 mg L−1, [Biochar] = 0.1 g L−1, [PDS] = 1 mM, pH = 6.3
  • 94% removed in 120 min
  • B species acted as Lewis acid sites enhancing PDS adsorption
  • Defects and sp.2-conjugated π-system facilitated electron transfer
  • B substitution in carbon matrix enhanced catalyst stability due to active sites reversible transformation during catalytic activation
  • Removal efficiency decreased from 92% to 90% in 5th cycle
 [55]
  • Wood shavings + N and S co-doping
  • Pyrolysis: 800 °C, 2 h
  • [MB] = 8 mg L−1, [Biochar] = 0.2 g L−1, [PMS] = 0.4 g L−1, pH = 3–4
  • >95% removed in 30 min
  • Synergy between S and N moieties altered surrounding electron density
  • •OH, SO4•−, and 1O2 participated in the catalytic reaction
  • Removal rate decreased gradually from 0.202 min−1 to 0.019 min−1 in 4th cycle
 [56]
  • Corncob + N doping
  • Pyrolysis: 700 °C, 2 h, N2 atmosphere
  • [SDZ] = 10 μM, [Biochar] = 3 g L−1, [PDS] = 1 mM, pH = 7
  • 96.2% removed in 10 min
  • Pyridinic and pyrrolic N at edge sites disturbed electron density to create active sites
  • Graphitic N was not well correlated with catalytic activity
  • PDS was reduced at electron-rich N while SDZ was oxidized around adjacent electron-deficient C
  • Reaction was dominated by an electron transfer regime that was unaffected by inorganic anions except NOM
  • Removal efficiency decreased from 96.5% to 83.0% after 3 cycles
 [15]
  • Wetland plants (reed) + N doping
  • Pyrolysis: 900 °C, 90 min, N2 atmosphere
  • [OG] = 50 mg L−1, [Biochar] = 0.2 g L−1, [PDS] = 2 mM, pH = 9.5 ± 0.8
  • 100% removed in 60 min
  • Quaternary N induces asymmetric spin density and low electron density to adjacent carbons, facilitating the chemical bonding with negatively charged O atoms of the O-O bond in PDS for free radicals formations
  • Large SSA and π−π interactions between OG aromatic rings graphitic carbon structure enhanced adsorption
  • Catalytic activity was governed by nonradical pathway of surface-activated PDS complexes and 1O2
  • Removal efficiency decreased from 98.3% to 45.0 ± 2.0% after 5 runs
 [57]
  • Spirulina residue + N doping
  • Pyrolysis: 900 °C, 90 min, N2 atmosphere
  • Acid (HCl) washing
  • [SMX] = 20 mg L−1, [Biochar] = 0.5 g L−1, [PDS] = 6 mM
  • 100% removed in 45 min
  • Biochar provided electron-mediating medium between SMX and PDS for nonradical degradation
  • N doping caused a redistribution of charge densities in graphitic carbons where PDS will then bond with positively charged O atoms adjacent to N dopants to enhance activation performance
  • Surface-activated metastable PDS/carbon complex participated in reaction
  • Removal efficiency decreased from 100% to 42.51% after 3 cycles
 [58]

2.5. Photocatalysis

Photocatalytic degradation of organic pollutants has been extensity studied in the last few decades. Typically, metals oxides such as TiO2, ZnO, and BiVO4 have been commonly used as a photocatalyst for environmental application. During photocatalytic reactions, the absorption of electromagnetic radiation by the photocatalyst is crucial to activate the photocatalyst and produce ROS, which can be used to degrade organic pollutants. The energy of the absorbed light irradiation must be greater or equal to the bandgap energy (Eg) of that photocatalyst in order to excite the electrons from the conduction band (CB) to the valence band (VB) [59]. When the excited electrons (ecb−) migrate to CB, positive holes (hvb+) will be formed in VB. Then, the electron/hole pair will migrate to the surface of the semiconductor to react with the adsorbed substrates. Generally, ecb− will react with the electron acceptors adsorbed on the catalyst surface to reduce it, whereas hvb+ will react with electron donors on the catalyst surface to oxidize it [60][61]. Dissolved O2 and OH− ions in water can interact as electron acceptors and electron donors, respectively. The ecb− can reduce O2 to generate •O2− whereas hvb+ can react with the OH− to generate •OH. Nonetheless, •O2− can only be generated when the CB potential is more negative than the reduction potential of O2/•O2− (−0.33 eV). As for •OH formation, the VB potential edge of the photocatalyst must be sufficiently positive to oxidize hydroxide ions or adsorbed H2O to form •OH (OH−/•OH: +2.38 eV, H2O/•OH: +2.72 eV) [62][63][64].
Recently, biochar has been found to have a semiconductor-like structure and serves as an active carbonaceous photocatalyst. Table 5 provides the performance and mechanism of biochar as a photocatalyst to remove organic pollutants.
Table 5. An overview of biochar preparation and performance as photocatalyst activation for organic pollutants removal.
Table
Biochar
Precursor and Synthesis		 Performance Removal Mechanism Ref
  • Pine needles
  • Pyrolysis: 500 °C, 2 h
  • [DEP] = 20 mg L−1, [Biochar] = 0.2 g L−1, pH = 7, 100 W Hg lamp (350–450 nm)
  • 72.3% removed within 120 min
  • •OH was dominate species contributing to 76.7–82.8% DEP removal
  • UV irradiation excited quinone-like moieties to singlet states and then rapidly went through an intersystem crossing to yield excited triplet states that further reacted with dissolved oxygen by energy transfer to form 1O2
  • PFRs can mediate electron transfer to O2 for •OH formation
  • Electron transfer from VB to CB and electron–hole pairs formation allows electrons transfer to redox-active functional groups defects to form relatively stable PFRs
 [65]
  • Wheat straw
  • Pyrolysis: 500 °C, 2 h
  • [DEP] = 20 mg L−1, [Biochar] = 0.2 g L−1, pH = 7, 100 W Hg lamp (350–450 nm)
  • 60.9% removed within 120 min
  • •OH was dominate species contributing to 76.7–82.8% DEP removal
  • UV irradiation excited quinone-like moieties to singlet states and then rapidly went through an intersystem crossing to yield excited triplet states that further reacted with dissolved oxygen by energy transfer to form 1O2
  • PFRs can mediate electron transfer to O2 for •OH formation
  • Electron transfer from VB to CB and electron–hole pairs formation allows electrons transfer to redox-active functional groups defects to form relatively stable PFRs
 [65]
  • Poplar woodchips
  • Pyrolysis: 300 °C, 3 h
  • Ball milling
  • [EFA] = 20 mg L−1, [Biochar] = 0.2 g L−1, pH = 7, 500 W Xe lamp (UV cut-off filter), pH = 6.8
  • 80.2% removed after 1.5 h under dark and 2.5 h under illumination
  • More amounts of phenolic structures and defects on ball-milled biochar aided in ∙O2− generation
  • OFGs, particularly O-C=O, was important in ROS generation
  • OFGs acted as CB and defects acted as VB, forming a semiconductor-like structure
  • Excited electrons after light exposure react with O2 to form O2−
  • h+ on the defects with strong oxidative and O2− will attack EFA
 [66]

3. Conclusions and Future Prospects

Thus far, biochar, as a carbonaceous material with various active sites, has been successfully utilized in the decontamination of organic pollutants from water by different applications. Among the numerous applications of biochar in wastewater treatment, adsorption, H2O2 activation, O3 activation, PS activation, and photocatalysis hold the superlative potential in the treatment of organic pollutants. These applications were compared with respect to performance, mechanism of removal with emphasis on pollutant–biochar interactions and biochar active sites’ involvement, tolerance to changes in water pH, stability of biochar after consecutive cycles of pollutant treatment, and economic factors. Each application has its own advantages and limitations. Adsorption is superior in terms of simplicity of operational procedures, ability to remove oxidation-resistant pollutants, and cost-wise. AOPs utilize more active sites of biochar and are able to mineralize organics pollutants or form less resistant organic by-products, so they can be treated by conventional methods. All AOPs have their own advantages. For instance, PS activation is effective in utilizing all of the biochar active sites along with having a monopoly on the capability to produce SO4•−. H2O2 is a safe oxidant with nonharmful decomposition products (hydrogen and oxygen). O3 is a strong oxidant with various activation pathways. Photocatalysis does not require the use of any oxidant. Nonetheless, limitations are present in each application. (i) The low durability of biochar active sites during AOPs is repeatedly reported. In addition, (ii) the performance of pristine biochar for environmental remediation of organic pollutants is often slow with low efficiency. (iii) The cost of light source during photocatalytic reactions is a drawback. (iv) Adsorption is unable to mineralize or degrade organic pollutants. (v) Biochar–pollutant and biochar–oxidant interactions are highly influenced by water pH changes, hindering its application in real wastewater. To enhance biochar performance in environmental remediation, several suggestions can be made:
  • The fabrication of robust biochar with high graphitization and aromaticity degree can prevent poor durability. In addition, compositing with polymers can protect the active site of biochar from cannibalistic reactions and can also simplify the separation of the catalyst.
  • Heteroatom (i.e., N, S, B, F, P) doping is often found to have a fruitful effect on the performance of biochar in environmental remediation. However, co-doping and triple-doping are rarely reported. Hence, more studies are needed on multi-doped biochar, with systematic investigations on the interactions between the multi-dopants within the biochar structure and its effect on the biochar performance. Moreover, as heteroatomic doping of biochar can be achieved by in situ and post-treatment methods, a comparison between the two techniques is needed to determine the most efficient method.
  • For photocatalytic applications, LED lamps can be used instead of conventional light sources to avoid an additional cost of electricity and better utilization of energy.
  • Synergistic removal by adsorption with other AOPs methods may provide constructive results. Nonetheless, the optimization of adsorption contribution is crucial to avoid undesirable competition over biochar active sites and hinder removal performance.
  • Specific tailoring of biochar active sites that are unlikely to be affected with pH changes and/or can produce species that are resistant to changing pH (i.e., nonradical pathways) can endow the biochar with better performance over different water matrixes.

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