Biochar in Remediation of Organic Pollutants in Water

Biochar in Remediation of Organic Pollutants in Water: Comparison

Please note this is a comparison between Version 4 by Vicky Zhou and Version 3 by Vicky Zhou.

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].

Table 1. An overview of biochar preparation and performance in adsorption of organic pollutants.

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 Q_e 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 Q_e 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

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.

can reduce O₂ to generate ^•O₂⁻ whereas h_vb⁺ 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.

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]
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)
Poplar woodchips Pyrolysis: 300 °C, 3 h Ball milling	^[	²⁸	^]
	[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]	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.

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.

Biochar Precursor and Synthesis	Performance

. 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.

Biochar Precursor and Synthesis	Performance	Removal Mechanism	Ref
Removal Mechanism	Ref
Biochar Precursor and Synthesis	Performance	Removal Mechanism
Maize straw + Fe-impregnation Pyrolysis: 700 °C, 2 h, N
Ref
Coking wastewater treatment sludge ₂ atmosphere	Pyrolysis: 700 °C, 2 h, N₂ atmosphere
Food waste digestate Pyrolysis: 800 °C, 2 h, N₂ atmosphere	[SMX] = 10 μM, [Biochar] = 1 g L⁻¹, [H₂O₂] = 3 mM, pH = 5 100% removed in 2 h	[Phenol] = 0.2 g L⁻¹, [Biochar] = 1 g L⁻¹, [O₃] = 14 ± 1 mg L⁻¹, 1.0 L min	[X-3B] = 1 g L⁻¹, [Biochar] = 0.5 g L⁻¹⁻¹ , [PDS] = 1.5 mM, pH = 3.78 95.4% removed in 30 min 85.72% removed in 30 min C-OH activates H₂O₂ to produce ^•OH, HO₂^•, and alkyl radicals to degrade SMX	C-O and C=O served as active sites for ozone catalytic decomposition C=O adsorbed dissolved O	sp.² carbon served to produce radicals by PDS activation in an electron-accepting process ₃ and initiated its rapid decomposition for ∙O₂⁻ generation C=O was oxidized into O-C=O after reaction Graphitic and pyridinic N facilitated ROS generation C=O contributed to ¹O₂ formation Presence of HCO ^[36]
₃	⁻	promoted ^• OH and HCO ₃^• formation, reacting with O₃ for ^•O₂⁻ generation Removal efficiency decreased from 85.72% to 32.53% at 30 min in 2nd cycle Removal efficiency decreased from 95.4% to 59.3% in 4th cycle	^[46]
^[	⁴⁹	^]	Sewage sludge	Activated petroleum waste sludge Pyrolysis: 500 °C, 4 h, N₂ atmosphere Pyrolysis: 850 °C, 1 h, N₂ atmosphere
Peanut shells Pyrolysis: 900 °C, 2 h, N₂ atmosphere Acid (HNO₃) treatment	[CIP] = 10 mg L⁻¹, [Biochar] = 1 g L⁻¹, [H₂O₂] = 10 mM, pH = 7.4 [Benzoic acid] = 100 mg L⁻¹, [Biochar] = 1 g L 93% removed in 24 h	⁻¹, [O₃] = 20 mg min⁻¹, pH = 3.8–10.0	[SMT] = 40 μM, [Biochar] = 0.2 g L⁻¹, [PMS] = 1 mM, pH = 6 ≥95% removed in 30 min Single-electron transfer from biochar PFRs to H	98.3% removed in 120 min ₂O₂ induced ^•OH generation Functional C groups, Si-O groups, metallic oxides of Zn, Al, Fe, and Mg are expected as the active sites 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 Reaction was suggested to be mediated by ^•OH COOH groups promoted SO₄^•− and Excessive adsorption under acidic conditions leads to competition with ozonation process Removal efficiency fell to 79.1% in 5th cycle	Sieving to small-sized biochar (0–75 µm) enhanced PDS activation due to more graphitic and aromatic carbon, COOH content, and higher SSA ^•OH formation C=O, pyridinic N, and pyridinic N were active sites for the activation by electron transfer to H₂O₂	^[47]
		Biochar mediated electron transfer from SMT to PDS Electron-rich active sites transfer electrons to PDS for SO₄^•−, ^•OH, ¹O₂, and ^•O₂⁻	^[37]
generation			^[50]	Wheat straws	Peanut shell Pyrolysis: 700 °C, 2 h, N₂ atmosphere	Pyrolysis: 600 °C, 4 h, N
Spent tea leave ₂ atmosphere	Pre-oxidation: 250 °C, 0.5 h, air environment Pyrolysis: 500 °C, 1 h, N₂ atmosphere [SMT] = 13.7 μM, [Biochar] = 1 g L⁻¹, [H₂O₂] = 3 mM, pH = 5 [KET] = 2 mg L⁻¹, [Biochar] = 500 mg L⁻¹, [O₃] = 0.5 L min 100% removed in 2 h	⁻¹, pH = 6 ± 0.1	[CTC] = 50 mg L⁻¹, [Biochar] = 0.1 g L⁻¹ 99.9% removed in 3 min	, [PDS] = 1 g L⁻¹, pH = 6.0 97.4% removed in 90 min SSA and porosity were crucial in SMT adsorption and catalytic ^•OH generation O₃ direct or indirect attack on C=O, C=C, and -OH on biochar initiated radical chain reactions Biochar surface acidity was negatively correlated with catalytic activation, whereas surface basicity was positively correlated with catalytic activation Delocalized π-electrons react with H₂O to form hydroxide OH⁻ and H₃O⁺ that will yield HO₂, ^•O₂⁻, and ^•OH after reaction with O₃ Excessive SMT adsorption prior to activation hindered H₂O₂ activation	¹ O₂, ^•O₂⁻, and ^•OH were the main species responsible for degradation Removal efficiency decreased to 94.8% in 5th cycle ^[38]
			^[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).

Pre-adsorption was beneficial to the subsequent catalytic degradation


	Delocalized π-electrons and Fe facilitated SO	₄ ^•−, ^•OH, and ¹O₂ generation	^[51]
Biogas residue + KHCO₃ activation
Dairy manure Pyrolysis: 800 °C, 3.5 h, N₂ atmosphere Anaerobic digestion: 37 °C, 35 days Acid (HCl) washing	Pyrolysis: 800 °C, 2 h, N₂ atmosphere [Benzene] = 10 μM, [Biochar] = 1 g L⁻¹, [H₂O₂] = 3 mM, pH = 5	[SMX] = 15 mg L⁻¹, [Biochar] = 1 g L⁻¹, [PMS] = 2.5 mM, pH = 5.56 90.2% removed in 60 min OFGs, C=C, pyridinic N, and graphitic N allowed electron transfer for ^•OH, ^•O₂⁻, and ¹O₂ formation	Defects were positively correlated with degradation potential (R² = 0.92) ¹O₂ generation dominated the catalytic reaction	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 ^[39]
	^[	⁵²	^]	Sewage sludge
Straw Pyrolysis: 600 °C, 30 min Pyrolysis: 700 °C, 2 h, N₂ atmosphere Pyrolysis + kaolin: 1100 °C, 30 min, N₂ atmosphere	N doping: Thiourea + biochar, pyrolysis: 800 °C, 2 h, N ₂ atmosphere [CIP] = 10 mg L⁻¹, [Biochar] = 0.2 g L⁻¹, [H₂O₂] = 10 mM, pH = 4	[TC] = 20 mg L⁻¹, [Biochar] = 200 mg L⁻¹, [PDS] = 2 mM, pH = 7 <30% removed after 1 h of adsorption and 80% removed after 20 min from H₂O₂ addition	97% removed in 60 min and 100% in 120 min Large SSA and inorganic groups enhance CIP adsorption ∙O₂⁻ is formed by reactions of ^•OH and H₂O₂	Reaction mainly relied on nonradical electron transfer between TC and PDS due to enhanced graphitization degree Radicals had no effect on TC degradation ^[40]
		Removal efficiency decreased from 100% to 55% in 3rd cycle	^[53]	Pig manure Pyrolysis: 500 °C, 2 h, N₂ atmosphere
Rice husk	Pyrolysis: 850 °C, 1 h [TC] = 30 mg L⁻¹, [Biochar] = 0.5 g L⁻¹, [H₂O₂] = 5 mM, pH = 7.4 100% removed in 240 min	[SMX] = 500 μg L⁻¹, [Biochar] = 100 mg L⁻¹, [PDS] = 500 mg L⁻¹ 96% removed in 120 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	Reaction controlled by either electron transfer/¹O₂ control surface-bound radicals ^[41]
^[	⁵⁴	^]	Sugarcane residues + Fe impregnation Pyrolysis: 600 °C, 4 h
Wheat straw + B doping Pyrolysis: 900 °C, 2 h, N₂ atmosphere KOH activation: 600 °C, 2 h, N₂ atmosphere	[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	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 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	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 ^[42]
		^[	^55]	Sugarcane bagasse + steel pickling waste liquor Pyrolysis: 400 °C, 2 h, N₂ atmosphere
Wood shavings + N and S co-doping Pyrolysis: 800 °C, 2 h	[MNZ] = 40 mg L⁻¹, [Biochar] = 0.3 g L⁻¹, [H₂O₂] = 5 mM, pH = 5.61	[MB] = 8 mg L⁻¹, [Biochar] = 0.2 g L⁻¹, [PMS] = 0.4 g L⁻¹, pH = 3–4 100% removed within 2 h	>95% removed in 30 min 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]




Synergy between S and N moieties altered surrounding electron density



^•	OH, SO	₄ ^•−, 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 (e_cb⁻) migrate to CB, positive holes (h_vb⁺) 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, e_cb⁻ will react with the electron acceptors adsorbed on the catalyst surface to reduce it, whereas h_vb⁺ 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 e_cb⁻

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.

References

Anh, H.Q.; Le, T.P.Q.; Da Le, N.; Lu, X.X.; Duong, T.T.; Garnier, J.; Rochelle-Newall, E.; Zhang, S.; Oh, N.H.; Oeurng, C.; et al. Antibiotics in surface water of East and Southeast Asian countries: A focused review on contamination status, pollution sources, potential risks, and future perspectives. Sci. Total Environ. 2021, 764, 142865.
Briffa, J.; Sinagra, E.; Blundell, R. Heavy metal pollution in the environment and their toxicological effects on humans. Heliyon 2020, 6, e04691.
Choong, Z.-Y.; Lin, K.-Y.A.; Lisak, G.; Lim, T.-T.; Oh, W.-D. Multi-heteroatom-doped carbocatalyst as peroxymonosulfate and peroxydisulfate activator for water purification: A critical review. J. Hazard. Mater. 2021, 426, 128077.
Offiong, N.-A.O.; Inam, E.J.; Etuk, H.S.; Ebong, G.A.; Inyangudoh, A.I.; Addison, F. Trace Metal Levels and Nutrient Characteristics of Crude Oil-Contaminated Soil Amended with Biochar–Humus Sediment Slurry. Pollutants 2021, 1, 119–126.
Offiong, N.A.O.; Inam, E.J.; Etuk, H.S.; Essien, J.P.; Ofon, U.A.; Una, C.C. Biochar and humus sediment mixture attenuates crude oil-derived PAHs in a simulated tropical ultisol. SN Appl. Sci. 2020, 2, 1930.
Bharagava, R.N.; Saxena, G.; Mulla, S.I.; Patel, D.K. Characterization and Identification of Recalcitrant Organic Pollutants (ROPs) in Tannery Wastewater and Its Phytotoxicity Evaluation for Environmental Safety. Arch. Environ. Contam. Toxicol. 2017, 75, 259–272.
van Duin, D.; Paterson, D.L. Multidrug-Resistant Bacteria in the Community: Trends and Lessons Learned. Infect. Dis. Clin. N. Am. 2016, 30, 377–390.
Rathi, B.S.; Kumar, P.S. Application of adsorption process for effective removal of emerging contaminants from water and wastewater. Environ. Pollut. 2021, 280, 116995.
Fonseca Couto, C.; Lange, L.C.; Santos Amaral, M.C. A critical review on membrane separation processes applied to remove pharmaceutically active compounds from water and wastewater. J. Water Process Eng. 2018, 26, 156–175.
Kanakaraju, D.; Glass, B.D.; Oelgemöller, M. Advanced oxidation process-mediated removal of pharmaceuticals from water: A review. J. Environ. Manag. 2018, 219, 189–207.
Suliman, W.; Harsh, J.B.; Abu-Lail, N.I.; Fortuna, A.M.; Dallmeyer, I.; Garcia-Perez, M. Influence of feedstock source and pyrolysis temperature on biochar bulk and surface properties. Biomass Bioenergy 2016, 84, 37–48.
Chandra, S.; Bhattacharya, J. Influence of temperature and duration of pyrolysis on the property heterogeneity of rice straw biochar and optimization of pyrolysis conditions for its application in soils. J. Clean. Prod. 2019, 215, 1123–1139.
Meng, H.; Nie, C.; Li, W.; Duan, X.; Lai, B.; Ao, Z.; Wang, S.; An, T. Insight into the effect of lignocellulosic biomass source on the performance of biochar as persulfate activator for aqueous organic pollutants remediation: Epicarp and mesocarp of citrus peels as examples. J. Hazard. Mater. 2020, 399, 123043.
Li, Y.; Xing, B.; Wang, X.; Wang, K.; Zhu, L.; Wang, S. Nitrogen-Doped Hierarchical Porous Biochar Derived from Corn Stalks for Phenol-Enhanced Adsorption. Energy Fuels 2019, 33, 12459–12468.
Wang, H.; Guo, W.; Liu, B.; Wu, Q.; Luo, H.; Zhao, Q.; Si, Q.; Sseguya, F.; Ren, N. Edge-nitrogenated biochar for efficient peroxydisulfate activation: An electron transfer mechanism. Water Res. 2019, 160, 405–414.
Do Minh, T.; Song, J.; Deb, A.; Cha, L.; Srivastava, V.; Sillanpää, M. Biochar based catalysts for the abatement of emerging pollutants: A review. Chem. Eng. J. 2020, 394, 124856.
Liang, L.; Xi, F.; Tan, W.; Meng, X.; Hu, B.; Wang, X. Review of organic and inorganic pollutants removal by biochar and biochar-based composites. Biochar 2021, 3, 255–281.
Wang, J.; Wang, S. Preparation, modification and environmental application of biochar: A review. J. Clean. Prod. 2019, 227, 1002–1022.
Zhou, X.; Zhu, Y.; Niu, Q.; Zeng, G.; Lai, C.; Liu, S.; Huang, D.; Qin, L.; Liu, X.; Li, B.; et al. New notion of biochar: A review on the mechanism of biochar applications in advannced oxidation processes. Chem. Eng. J. 2021, 416, 129027.
Luo, K.; Pang, Y.; Wang, D.; Li, X.; Wang, L.; Lei, M.; Huang, Q.; Yang, Q. A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs). J. Environ. Sci. 2021, 108, 201–216.
Dai, Y.; Zhang, N.; Xing, C.; Cui, Q.; Sun, Q. The adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review. Chemosphere 2019, 223, 12–27.
Gasim, M.F.; Lim, J.-W.; Low, S.-C.; Lin, K.-Y.A.; Oh, W.-D. Can biochar and hydrochar be used as sustainable catalyst for persulfate activation? Chemosphere 2022, 287, 132458.
Zhao, Y.; Yuan, X.; Li, X.; Jiang, L.; Wang, H. Burgeoning prospects of biochar and its composite in persulfate-advanced oxidation process. J. Hazard. Mater. 2021, 409, 124893.
Nageeb, M. Adsorption Technique for the Removal of Organic Pollutants from Water and Wastewater. In Organic Pollutants—Monitoring, Risk and Treatment; InTech: London, UK, 2013.
Duan, Q.; Li, X.; Wu, Z.; Alsaedi, A.; Hayat, T.; Chen, C.; Li, J. Adsorption of 17β-estradiol from aqueous solutions by a novel hierarchically nitrogen-doped porous carbon. J. Colloid Interface Sci. 2019, 533, 700–708.
Guo, R.; Yan, L.; Rao, P.; Wang, R.; Guo, X. Nitrogen and sulfur co-doped biochar derived from peanut shell with enhanced adsorption capacity for diethyl phthalate. Environ. Pollut. 2020, 258, 113674.
Zhang, X.; Gang, D.D.; Zhang, J.; Lei, X.; Lian, Q.; Holmes, W.E.; Zappi, M.E.; Yao, H. Insight into the activation mechanisms of biochar by boric acid and its application for the removal of sulfamethoxazole. J. Hazard. Mater. 2021, 424, 127333.
Vigneshwaran, S.; Sirajudheen, P.; Karthikeyan, P.; Meenakshi, S. Fabrication of sulfur-doped biochar derived from tapioca peel waste with superior adsorption performance for the removal of Malachite green and Rhodamine B dyes. Surf. Interfaces 2021, 23, 100920.
Giri, B.S.; Sonwani, R.K.; Varjani, S.; Chaurasia, D.; Varadavenkatesan, T.; Chaturvedi, P.; Yadav, S.; Katiyar, V.; Singh, R.S.; Pandey, A. Highly efficient bio-adsorption of Malachite green using Chinese Fan-Palm Biochar (Livistona chinensis). Chemosphere 2022, 287, 132282.
Wang, J.; Zhang, M. Adsorption Characteristics and Mechanism of Bisphenol A by Magnetic Biochar. Int. J. Environ. Res. Public Health 2020, 17, 1075.
Zhou, Y.; Liu, X.; Xiang, Y.; Wang, P.; Zhang, J.; Zhang, F.; Wei, J.; Luo, L.; Lei, M.; Tang, L. Modification of biochar derived from sawdust and its application in removal of tetracycline and copper from aqueous solution: Adsorption mechanism and modelling. Bioresour. Technol. 2017, 245, 266–273.
Chen, T.; Luo, L.; Deng, S.; Shi, G.; Zhang, S.; Zhang, Y.; Deng, O.; Wang, L.; Zhang, J.; Wei, L. Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure. Bioresour. Technol. 2018, 267, 431–437.
Afzal, M.Z.; Sun, X.F.; Liu, J.; Song, C.; Wang, S.G.; Javed, A. Enhancement of ciprofloxacin sorption on chitosan/biochar hydrogel beads. Sci. Total Environ. 2018, 639, 560–569.
Lim, J.; Hoffmann, M.R. Substrate oxidation enhances the electrochemical production of hydrogen peroxide. Chem. Eng. J. 2019, 374, 958–964.
Li, W.; Bonakdarpour, A.; Gyenge, E.; Wilkinson, D.P. Production of Hydrogen Peroxide for Drinking Water Treatment in a Proton Exchange Membrane Electrolyzer at Near-Neutral pH. J. Electrochem. Soc. 2020, 167, 044502.
Zhang, X.; Sun, P.; Wei, K.; Huang, X.; Zhang, X. Enhanced H2O2 activation and sulfamethoxazole degradation by Fe-impregnated biochar. Chem. Eng. J. 2020, 385, 123921.
Luo, K.; Yang, Q.; Pang, Y.; Wang, D.; Li, X.; Lei, M.; Huang, Q. Unveiling the mechanism of biochar-activated hydrogen peroxide on the degradation of ciprofloxacin. Chem. Eng. J. 2019, 374, 520–530.
Huang, D.; Wang, Y.; Zhang, C.; Zeng, G.; Lai, C.; Wan, J.; Qin, L.; Zeng, Y. Influence of morphological and chemical features of biochar on hydrogen peroxide activation: Implications on sulfamethazine degradation. RSC Adv. 2016, 6, 73186–73196.
Sun, P.; Hua, Y.; Zhao, J.; Wang, C.; Tan, Q.; Shen, G. Insights into the mechanism of hydrogen peroxide activation with biochar produced from anaerobically digested residues at different pyrolysis temperatures for the degradation of BTEXS. Sci. Total Environ. 2021, 788, 147718.
Li, J.; Pan, L.; Yu, G.; Xie, S.; Li, C.; Lai, D.; Li, Z.; You, F.; Wang, Y. The synthesis of heterogeneous Fenton-like catalyst using sewage sludge biochar and its application for ciprofloxacin degradation. Sci. Total Environ. 2019, 654, 1284–1292.
Huang, D.; Luo, H.; Zhang, C.; Zeng, G.; Lai, C.; Cheng, M.; Wang, R.; Deng, R.; Xue, W.; Gong, X.; et al. Nonnegligible role of biomass types and its compositions on the formation of persistent free radicals in biochar: Insight into the influences on Fenton-like process. Chem. Eng. J. 2019, 361, 353–363.
Park, J.H.; Wang, J.J.; Xiao, R.; Tafti, N.; DeLaune, R.D.; Seo, D.C. Degradation of Orange G by Fenton-like reaction with Fe-impregnated biochar catalyst. Bioresour. Technol. 2018, 249, 368–376.
Yi, Y.; Tu, G.; Eric Tsang, P.; Fang, Z. Insight into the influence of pyrolysis temperature on Fenton-like catalytic performance of magnetic biochar. Chem. Eng. J. 2020, 380, 122518.
Cuerda-Correa, E.M.; Alexandre-Franco, M.F.; Fernández-González, C. Advanced Oxidation Processes for the Removal of Antibiotics from Water. An Overview. Water 2019, 12, 102.
Mehrjouei, M.; Müller, S.; Möller, D. A review on photocatalytic ozonation used for the treatment of water and wastewater. Chem. Eng. J. 2015, 263, 209–219.
Zhang, F.; Wu, K.; Zhou, H.; Hu, Y.; Preis, S.V.; Wu, H.; Wei, C. Ozonation of aqueous phenol catalyzed by biochar produced from sludge obtained in the treatment of coking wastewater. J. Environ. Manag. 2018, 224, 376–386.
Chen, C.; Yan, X.; Xu, Y.Y.; Yoza, B.A.; Wang, X.; Kou, Y.; Ye, H.; Wang, Q.; Li, Q.X. Activated petroleum waste sludge biochar for efficient catalytic ozonation of refinery wastewater. Sci. Total Environ. 2019, 651, 2631–2640.
Li, H.; Liu, S.; Qiu, S.; Sun, L.; Yuan, X.; Xia, D. Catalytic ozonation oxidation of ketoprofen by peanut shell-based biochar: Effects of the pyrolysis temperatures. Environ. Technol. 2020.
Liu, J.; Huang, S.; Wang, T.; Mei, M.; Chen, S.; Li, J. Peroxydisulfate activation by digestate-derived biochar for azo dye degradation: Mechanism and performance. Sep. Purif. Technol. 2021, 279, 119687.
Jin, Z.; Xiao, S.; Dong, H.; Xiao, J.; Tian, R.; Chen, J.; Li, Y.; Li, L. Adsorption and catalytic degradation of organic contaminants by biochar: Overlooked role of biochar’s particle size. J. Hazard. Mater. 2022, 422, 126928.
Chen, Y.P.; Zheng, C.H.; Huang, Y.Y.; Chen, Y.R. Removal of chlortetracycline from water using spent tea leaves-based biochar as adsorption-enhanced persulfate activator. Chemosphere 2022, 286, 131770.
Wang, Y.; Song, Y.; Li, N.; Liu, W.; Yan, B.; Yu, Y.; Liang, L.; Chen, G.; Hou, L.; Wang, S. Tunable active sites on biogas digestate derived biochar for sulfanilamide degradation by peroxymonosulfate activation. J. Hazard. Mater. 2022, 421, 126794.
Zhong, Q.; Lin, Q.; He, W.; Fu, H.; Huang, Z.; Wang, Y.; Wu, L. Study on the nonradical pathways of nitrogen-doped biochar activating persulfate for tetracycline degradation. Sep. Purif. Technol. 2021, 276, 119354.
Avramiotis, E.; Frontistis, Z.; Manariotis, I.D.; Vakros, J.; Mantzavinos, D. Oxidation of Sulfamethoxazole by Rice Husk Biochar-Activated Persulfate. Catalysts 2021, 11, 850.
Liu, B.; Guo, W.; Wang, H.; Si, Q.; Zhao, Q.; Luo, H.; Ren, N. B-doped graphitic porous biochar with enhanced surface affinity and electron transfer for efficient peroxydisulfate activation. Chem. Eng. J. 2020, 396, 125119.
Oh, W.-D.; Jannah Zaeni, J.R.; Lisak, G.; Andrew Lin, K.-Y.; Leong, K.-H.; Choong, Z.-Y. Accelerated organics degradation by peroxymonosulfate activated with biochar co-doped with nitrogen and sulfur. Chemosphere 2021, 277, 130313.
Zhu, S.; Huang, X.; Ma, F.; Wang, L.; Duan, X.; Wang, S. Catalytic Removal of Aqueous Contaminants on N-Doped Graphitic Biochars: Inherent Roles of Adsorption and Nonradical Mechanisms. Environ. Sci. Technol. 2018, 52, 8649–8658.
Ho, S.H.; Chen, Y.-d.; Li, R.; Zhang, C.; Ge, Y.; Cao, G.; Ma, M.; Duan, X.; Wang, S.; Ren, N.-q. N-doped graphitic biochars from C-phycocyanin extracted Spirulina residue for catalytic persulfate activation toward nonradical disinfection and organic oxidation. Water Res. 2019, 159, 77–86.
Abdellah, M.H.; Nosier, S.A.; El-Shazly, A.H.; Mubarak, A.A. Photocatalytic decolorization of methylene blue using TiO2/UV system enhanced by air sparging. Alex. Eng. J. 2018, 57, 3727–3735.
Byrne, J.A.; Dunlop, P.S.M.; Hamilton, J.W.J.; Fernández-Ibáñez, P.; Polo-López, I.; Sharma, P.K.; Vennard, A.S.M. A review of heterogeneous photocatalysis for water and surface disinfection. Molecules 2015, 20, 5574–5615.
Mahlambi, M.; Ngila, C.; Mamba, B. Recent Developments in Environmental Photocatalytic Degradation of Organic Pollutants: The Case of Titanium Dioxide Nanoparticles-A Review. J. Nanomater. 2015, 2015, 790173.
Arimi, A.; Günnemann, C.; Curti, M.; Bahnemann, W.D. Regarding the Nature of Charge Carriers Formed by UV or Visible Light Excitation of Carbon-Modified Titanium Dioxide. Catalysts 2019, 9, 697.
Pirhashemi, M.; Habibi-Yangjeh, A. Preparation of novel nanocomposites by deposition of Ag2WO4 and AgI over ZnO particles: Efficient plasmonic visible-light-driven photocatalysts through a cascade mechanism. Ceram. Int. 2017, 43, 13447–13460.
Rashid, J.; Parveen, N.; Iqbal, A.; Awan, S.U.; Iqbal, N.; Talib, S.H.; Hussain, N.; Akram, B.; Ulhaq, A.; Ahmed, B.; et al. Facile synthesis of g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01) nanosheets for DFT supported visible photocatalysis of 2-Chlorophenol. Sci. Rep. 2019, 9, 10202.
Fang, G.; Liu, C.; Wang, Y.; Dionysiou, D.D.; Zhou, D. Photogeneration of reactive oxygen species from biochar suspension for diethyl phthalate degradation. Appl. Catal. B Environ. 2017, 214, 34–45.
Xiao, Y.; Lyu, H.; Tang, J.; Wang, K.; Sun, H. Effects of ball milling on the photochemistry of biochar: Enrofloxacin degradation and possible mechanisms. Chem. Eng. J. 2020, 384, 123311.