The Removal and Mitigation Effects of Biochar on Microplastics in Water and Soils: Application and Mechanism Analysis

Abstract

Due to their widespread distribution, microplastics (MPs) are endangering the soil ecological environment system, causing water pollution and altering the soil’s physicochemical and microbiological features. Because of its unique pore structure and strong stability, biochar is widely used as an adsorbent. However, the effects of MP–biochar interactions in water and soil environment are still unclear. This review outlines the application and mechanism of biochar as an adsorbent in a water environment for the removal of MPs. Also, biochar serves as remediation material for MPs in soils as it mitigates the adverse effects of MPs on soil properties, enzyme activities and soil microbial community. It was found that woody biochar had the highest yield and was more effective in adsorbing MPs. Further research should focus on the combined effects of biochar and MPs, the environmental risks of biochar, the modification of biochar application of MP-removal technologies, the characterization of MP properties, the remediation of combined contamination of MPs and other pollutants, and the transportation of MPs.

1. Introduction

1.1. Biochar

1.1.1. Preparation, Types, and Properties of Biochar

Compared to biological degradation [1], catalysis, and chemical oxidation/reduction processes [2] for removing pollutants [3,4], adsorption is considered highly efficient, cost-effective, and environmental-friendly [5]. The sorbents frequently utilized are activated carbon [6], zeolites [7], clay [8], biochar [9], coffee grounds, layered double oxides, and magnetic particles. Biochar has the advantages of an extensive specific surface area, a wealth of functional groups, high stability, strong adsorption capacity, low price, and good pore structure [10]; therefore, it has been used for the removal of pollutants for a long time.

As shown in Figure 1, the predominant techniques employed for the fabrication of biochar in environmental remediation are pyrolysis and hydrothermal carbonization [11]. Pyrolysis is the process of thermally decomposing biomass into solids, gases, and condensed liquids at temperatures between 300 °C to 900 °C [12]. The solid material produced is known as biochar. Pyrolysis can be categorized into two main types: slow pyrolysis and fast pyrolysis. This distinction is made based on variations in factors such as temperature, pressure, residence time, and heating rate [11]. Biochar produced by slow pyrolysis has the advantages of a greater yield and structural stability compared to that produced by fast pyrolysis [13]. Kim et al. investigated the use of the fast pyrolysis of pine as a raw material for biochar production [14]. The study found that the yield of high-temperature biochar decreased by almost 50% at lower temperatures. The preparation of biochar using pyrolysis is an environmentally friendly approach, which is simpler and more cost-effective than traditional methods of producing other carbon materials, such as graphene.

Biochar can be prepared from multiple biomass feedstocks, e.g., sewage sludge, kitchen waste, rice straw, rice bran, and wood. Biochar has distinct physical and chemical properties due to differences in biomass feedstocks, temperatures, and production processes (

Table 1. Feedstocks and approaches for the preparation of biochar.

Feedstock	Preparation Condition	Carbon Content	Application of Biochar	Reference
Bamboo	Pyrolysis, 500 °C, -	83.6%	Removal of norfloxacin from water	[18]
Corncob	Pyrolysis, 500, 700 °C, 2 h	-	Adsorption of neutral organic compounds	[19]
Peanut straw	Pyrolysis, 400 °C, 3 h	-	Optimization of biochar performance	[20]
Wood	Gasification, 750 °C, 0.25 h	48.4%	Optimization catalyst performance	[21]
Leaf	-	59.3%
Bark	-	50.4%
Wood chips	Gasification, 1200 °C, 0.5–0.75 h	80.6%	Elimination of copper and cadmium	[22]
Wheat straw	Gasification, 750 °C	46.8%	Lead removal adsorbent	[23]
Sewage sludge	180 °C, 1.25 h	29.8%
Peanut hull	Hydrothermal carbonization, 300 °C, 5 h	-
Switchgrass	Hydrothermal carbonization, 300 °C, 0.5 h	70.5%	Pyrolysis of organic wastes for biochar	[24]
Banana peels	Hydrothermal carbonization, 230 °C, 2 h	71.38%	Soil remediation of MP contamination	[25]
Pig manure	Hydrothermal carbonization, 240 °C, 2 h	23.9%	Soil remediation of MP contamination	[26]

) [15]. Biochar generated at elevated temperatures typically exhibits a greater surface area and carbon content, predominantly attributed to the elimination of volatile organic compounds during the high-temperature pyrolysis process, increasing in micropore volume [16]. Alongside that, biochar production decreases with increasing temperature [17]. Therefore, a balance of biochar yield and adsorption capacity is required.

1.1.2. Application of Biochar as a Remediation Material

As shown in Table 2, biochar is widely used in water environments and soil remediation. Due to its heterogeneous carbon composition, it can react with soil organic matter to adsorb polycyclic aromatic hydrocarbons, antibiotics and other organic contaminants [27].

In addition, transforming waste biomass into biochar and subsequently incorporating it into soils provides an effective strategy for waste management and sustainable agricultural advancement. The approach not only mitigates the issues associated with waste disposal, but also contributes to the improvement of soil fertility [28,29].

Table 2. Biochar in water environment/soil remediation.

Biochar Feedstock	Pollution	Removal Efficiency	Reference
Biomass pyrolysis (waste wood, pig manure, or straw)	VOCs	50%	[30]
Poplar and conifer	Phenanthrene and pentachlorophenol	Greatly reduced	[31]
Antibiotic	Penicillin	44.1%	[32]
Oil palm frond	Tannic acid	67.4%	[33]
	Phenol	62.9%
Corn straw, bamboo	PAHs in carrot root	Greatly reduced	[34]
Straw powders	Tetrabromobisphenol A	97%	[35]
Chicken dung biochar	Cu	45.3%	[36]
Casuarina biochar	Cd, Co, Cr, Cu, Ni	Varied reduction	[37]
Bur cucumber	Sulfamethazine	Up to 86% reduction	[38]
Aminocyclopyrachlor and bentazone	Wood pellet biochar	Varied reduction	[39]
Peanut shell	Atrazine and nisulfuron	Greatly reduced	[40]
Straw, willow	PAHs	70.3% (biochar–straw), 29.3% (biochar–willow)	[41]
Muffle furnace	Dissolved organic matte	Increase of 5.3–17.7%	[42]
Sheep bone	Zn	57%	[43]

Table 2. Biochar in water environment/soil remediation.

Biochar Feedstock	Pollution	Removal Efficiency	Reference
Biomass pyrolysis (waste wood, pig manure, or straw)	VOCs	50%	[30]
Poplar and conifer	Phenanthrene and pentachlorophenol	Greatly reduced	[31]
Antibiotic	Penicillin	44.1%	[32]
Oil palm frond	Tannic acid	67.4%	[33]
	Phenol	62.9%
Corn straw, bamboo	PAHs in carrot root	Greatly reduced	[34]
Straw powders	Tetrabromobisphenol A	97%	[35]
Chicken dung biochar	Cu	45.3%	[36]
Casuarina biochar	Cd, Co, Cr, Cu, Ni	Varied reduction	[37]
Bur cucumber	Sulfamethazine	Up to 86% reduction	[38]
Aminocyclopyrachlor and bentazone	Wood pellet biochar	Varied reduction	[39]
Peanut shell	Atrazine and nisulfuron	Greatly reduced	[40]
Straw, willow	PAHs	70.3% (biochar–straw), 29.3% (biochar–willow)	[41]
Muffle furnace	Dissolved organic matte	Increase of 5.3–17.7%	[42]
Sheep bone	Zn	57%	[43]

1.2. Occurrence, Distribution, and Breakdown of Microplastics

Since the 1950s, plastic has emerged as one of the most widely used materials. However, once it enters the environment, it gradually accumulates, undergoes weathering, and breaks down into fragments. These fragments, which are smaller than 5 mm, are referred to as microplastics (MPs) [44,45]. Because of their strong stability, little particle size, and high abundance [46,47], MPs stay in the environment for long periods and are more prone to release toxic substances, including polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) [48]. Additionally, the extensive specific surface area of MPs allows them to adsorb various other contaminants, including heavy metals, organic compounds and pesticides, resulting in the formation of new composite pollutants, leading to uncertain toxic effects [49]. For example, PAHs could be transferred to MPs, which, after fragmentation into NPs, can be easy interiorized by aquatic organisms [50], and MPs are harmful to the organisms they are directly exposed to and those that consume them [51]. Also, previous study shows that when MPs interact with toxic heavy metals, their toxicity will be significantly enhanced [52].

Physical processes such as abrasion, heating/cooling, freezing/thawing, and humidification/drying can result in smaller MP particles. Complex processes lead to changes in the structure or content of chemicals (

Table 3. Breakdown methods for MPs.

Microplastic	Breakdown Method	Catalytic Material	Duration of Breakdown	Reference
PE	Natural weathering	-	-	[55]
HDPE	Natural weathering	-	3 years	[56]
PET	UV	-	1 year	[57]
PS, PE	K2S2O8 to oxidize	-	30 days	[58]
PS, PE, PVC	Heat treatment	-	2000 h	[59]
PE, PP	Gamma irradiation	-	-	[60]
PET (black)	Photooxidation	-	10 months	[61]
PET (white)	Photooxidation	-	10 months	[61]
PS (1 mm)	Photooxidation	-	24 h	[62]
LDPE	Photooxidation	-	175 h	[63]
PVC	Electrocatalysis	TiO2	6 h	[64,65]
PET	Biocatalysis	T. fusca	-	[64]
PCL	Biocatalysis	Keratinaes	-	[66]
PET	Biocatalysis	Thermobifida fusca	-	[67]

) [53], including photodegradation (assisted by natural ultraviolet light or catalysts) or biodegradation (with bacteria, fungi, algae, etc.) [54].

Every year, large amounts of MPs are released into the water environment through surface runoff, rainfall, and sewage, resulting in a serious MP pollution of water bodies, which can negatively impact ecosystems. Researchers have found MPs in the equatorial Atlantic, polar regions, surface waters of the oceans, and the deep sea [68]. Zhao et al. conducted the first quantitative study of suspended MPs in the East China Sea and found that the concentration of MPs in seawater was 0.167 ± 0.138 particles/m³, and it was confirmed that rivers are one of the main sources of MPs in the ocean [69]. Compositional analyses of MPs showed that the main types of MPs in some sections of Yangtze River ((1660.0 ± 639.1)–(8925 ± 1591) particles/m³) were polyethylene terephthalate (PET) and polypropylene (PP) [70]. From the viewpoint of MP distribution and chemical composition, the enrichment of MPs in freshwater is generally larger than that in the ocean, which is mainly closely related to human activities, and more closely related to the level of regional industrial development.

While MP pollution was initially discovered in marine environments, recent studies have shown that it is more prevalent in soils. This suggests that land may be a significant sink for MP accumulation. MPs on land are estimated to be roughly 4 to 23 times more abundant than what is discovered in aquatic environments [71], with roughly 79% of the plastic garbage accumulating in landfills [72]. In the regions of Europe and North America, the projected annual deposition of MPs within agricultural territories is calculated to be in the range of 63,000–430,000 t and 44,000–300,000 t, respectively. This data underscore the significant accumulation of MPs in these agricultural ecosystems, highlighting the need for further academic investigation into the dynamics and impacts of these materials in the soil environment [73]. MPs can reach the soil environment via the decomposition of larger plastic fragments, such as plastic mulches and greenhouse films, as well as the use of soil amendments like PS flakes and polyurethane (PU) foam [74,75]. Furthermore, the application of sewage sludge as a soil amendment in agricultural practices contributes to the infiltration of MPs into the soil matrix [76]. It should be noted that an overwhelming majority, exceeding 90% of MPs, are retained within sewage sludge following the wastewater treatment process [77]. Estimation based on the current trend of plastic production suggests that approximately 24,000 t of plastic waste may infiltrate natural ecosystems by the year 2050 [78]. When MPs reach the soil system, they can interact with soil particles, affecting the soil’s chemical and physical characteristics. The change influences the framework of the microbial group [79]. In recent years, there has been a growing interest in biodegradable plastics as a sustainable alternative to conventional plastics, given their potential to decompose more readily in the environment. However, the rapid breakdown of biodegradable plastics might pose new dangers to the soil ecosystem, as they may be degraded into secondary products with undefined toxicity at a faster rate than conventional plastics, thereby posing an additional hazard to the aquatic environment. Therefore, the use of biodegradable plastics should be carefully considered [80].

The co-existence of MPs with biochar is commonly found in the environment, especially in soils. Feng et al. reported that in the presence of MPs and biochar, the volatilization of NH₃ increased owing to the increased number of functional genes involved in nitrogen transformation [81]. Shang et al. found that the combination of biochar and polar/nonpolar MPs showed different adsorption capacities for phenanthrene in soil [82]. Interestingly, Khalid et al. demonstrated that polyvinyl chloride (PVC) inhibited crop yields, the soil activity of enzymes, and microorganisms [83]; however, the adverse effects can be mitigated by biochar.

However, relatively few studies have investigated the interaction between MPs and biochar. This study introduces the environmental application of biochar in MP pollution, summarizes the effects of biochar on soil physicochemical properties and biology, and analyzes the adsorption mechanism of biochar for MPs. The impacts of biochar on MP composite pollution are also presented. The stability of biochar and its negative impact on the environment are also mentioned.

2. Biochar as an Adsorbent for MPs in Water

Only a few studies have developed remediation materials for the removal of microplastics from the water environment, and some of these materials include metal-organic framework adsorbents, hydrogels/aerogels, sponges, superhydrophobic materials, and multifunctional materials [84]. Similar to other conventional contaminants, MPs can also be adsorbed by biochar. Therefore, biochar is usually used for the removal of MPs, particularly in the water environment. Figure 2 shows the removal process of MPs. Some examples of biochar and MP interactions are shown in Table 4. Generally, biochar is prepared by pyrolysis, whereby the biochar is placed in a reactor and heated to 180 °C to 700 °C. The main MP types were polystyrene (PS) and polyethylene (PE), which were generally removed with straw-based and wood-based biochar, all with removal efficiencies above 90% and up to 100%. Compared to other materials, biochar is more effective in removing MPs. For example, the removal efficiency of MPs via photocatalysis (ultraviolet) was only 89.3% for PET and 6.4% for PE [61,85]. Also, microorganisms are reported to degrade MPs; however, the biodegradation rate is not satisfactory. Müller et al. demonstrated that the removal efficiency of Thermobifida fusca for PET was approximately 50% [65]. The weight losses of PE, PET, and PS degraded by Bacillus cereus were 1.6–7.4%, and the weight losses of PE, PET, PP, and PS degraded by Bacillus subtilis were 3.0–6.2% [86].

The removal efficiency does not vary significantly with particle size and MP type. For instance, Hsieh et al. [89] investigated the adsorption efficiency of woodchip biochar for PSMPs, with a removal rate of almost 100%. Also, biochar is effective for the removal of smaller particles, such as nanoplastics (NPs). A study using rice-straw biochar demonstrated a removal efficiency of up to 99.6% for PSNPs [88].

Apart from the effectiveness of biochar for removing MPs, biochar’s main raw material, and agricultural and forestry waste are inexpensive and easily accessible compared to other materials. Recent advancements in production technology and equipment have enhanced the efficiency of biochar production, leading to a lower energy consumption and reduced costs. Furthermore, the production and application of biochar effectively mitigate greenhouse gas emissions, potentially generating additional revenue through carbon credit trading [90].

Table 4. Adsorption of MPs on biochar.

Biochar Feedstock	Preparation Parameter	Microplastic	Removal Efficiency	Reference
Corn straw and hardwood feedstock	Pyrolysis, heated 300 °C, 400 °C, 500 °C	PSMPs	>95% (packed column)	[69]
Corncob biochar	Pyrolysis, heated 500 °C, 2 h, 5 °C/min	PSNPs	90%	[91]
Jujube biomass	Pyrolysis, heated 700 °C, 3 h, 5 °C/min	PE pellets	>99%	[92]
Woodchips	Pyrolysis, 700 °C	PSMPs (1 μm)	100%	[89]
Rice straw	Pyrolysis, heated 5 °C/min to 700 °C	PSNPs (300 nm)	99.96%	[88]
Pinewood sawdust	Pyrolysis, heated at 550 °C for 2 h in a tube furnace with a heating rate of 5 °C/min	PSMPs (1 µm)	94.81%	[93]
Sugarcane bagasse	Pyrolysis, heated at 750 °C. After the synthesis, biochar was pulverized using a ball mill at 25 hertz/min oscillating frequency for 3 min	PSNPs (<500 nm)	99%	[94]
Contaminated corncobs	Pyrolysis, heated 180 °C for 6 h	PSNPs (100 nm)	100%	[95]

Table 4. Adsorption of MPs on biochar.

Biochar Feedstock	Preparation Parameter	Microplastic	Removal Efficiency	Reference
Corn straw and hardwood feedstock	Pyrolysis, heated 300 °C, 400 °C, 500 °C	PSMPs	>95% (packed column)	[69]
Corncob biochar	Pyrolysis, heated 500 °C, 2 h, 5 °C/min	PSNPs	90%	[91]
Jujube biomass	Pyrolysis, heated 700 °C, 3 h, 5 °C/min	PE pellets	>99%	[92]
Woodchips	Pyrolysis, 700 °C	PSMPs (1 μm)	100%	[89]
Rice straw	Pyrolysis, heated 5 °C/min to 700 °C	PSNPs (300 nm)	99.96%	[88]
Pinewood sawdust	Pyrolysis, heated at 550 °C for 2 h in a tube furnace with a heating rate of 5 °C/min	PSMPs (1 µm)	94.81%	[93]
Sugarcane bagasse	Pyrolysis, heated at 750 °C. After the synthesis, biochar was pulverized using a ball mill at 25 hertz/min oscillating frequency for 3 min	PSNPs (<500 nm)	99%	[94]
Contaminated corncobs	Pyrolysis, heated 180 °C for 6 h	PSNPs (100 nm)	100%	[95]

As shown in Figure 3, the adsorption mechanism in water mainly includes physical adsorption and chemical adsorption (chemical bonding/complexation) [56,96]. The mechanisms by which biochar can remove MPs from water include physical adsorption, electrostatic interactions [93], pore filling, hydrogen bonding, hydrophobic adsorption, and π–π interactions [97].

Among these, physical adsorption is a dominant mechanism [98]. The carbonized portion of the biochar acts as an adsorbent, attracting contaminants through electrostatic forces and non-polar interactions, while the non-carbonized portion acts as a partitioning mediator. Ahmad et al. identified the primary mechanisms of interaction between jujube-waste-derived biochar and PE, including entrapment in the pores (pore filling), entanglement with flaky structures, and electrostatic interaction. MP particles can enter into the pores located on the surface of porous adsorbents such as biochar, and are trapped within the pore. The entrapment mechanism could be dominant in biochar-based adsorbents due to the porous structure [92]. The pyrolysis temperature primarily determines the two adsorption mechanisms. At low pyrolysis temperatures (<400 °C), partitioning is the main mechanism, whereas at high pyrolysis temperatures (>500 °C), adsorption is the primary mechanism.

An electrostatic interaction is defined as the weak attractive or repulsive contact between biochar and functional groups containing oxygen on the outside of MPs that have opposing or identical charges [94]. Electrostatic adsorption occurs when the biochar surface carries a charge, enabling it to attract and adsorb MPs with opposite charges through electrostatic forces. The screening effect of counterions in high-ionic-strength environments can neutralize the charge on the biochar surface, diminishing the attractive forces between the biochar and the charged pollutants. Studies have shown that MPs exhibit strong hydrophobicity and tend to accumulate at high concentrations [97]. The polarity of biomass weakens during high-temperature pyrolysis, leading to an increase in its hydrophobicity. This is also a significant factor contributing to hydrophobic adsorption. Physical interception involves filtration and pore filling. Pore filling, hydrophobic interaction, and hydrogen bonding may all be involved in PSNP adsorption to biochar [91].

A hydrophobic interaction might have played a more important role in PSNP adsorption to fresh biochar, whereas the oxygen-containing surface groups could be more involved in the PSNP adsorption to oxidized biochar. When plastic particles are relatively large, they can reduce the interference of surface interaction effects. Wang et al. described the mechanism of action as “stuck”, “trapped”, and “entangled” when simulating the removal of PS (10 µm) by corn stover and hardwood biochar filter columns [93,99]. The spruce bark biochar prepared by Siipola et al. using steam activation has a larger and higher porosity, allowing for PE (10 µm) to easily aggregate and settle in the pores [100]. Furthermore, the adsorption of MPs on biochar is also influenced by π–π electron pairs and hydrogen bonding. The π electron cloud of biochar may be incoherent with the π electron cloud of anions, cations, proton donor functional groups, and aromatic MPs. Valence interactions, π–π bonds, are key to the adsorption of highly aromatic MPs, especially for plastic particles containing benzene rings [101].

The biochar adsorption of MPs is a promising way to reduce the negative ecological impacts of MPs. However, it is not a final solution for MP remediation, as adsorbed MPs may remain in the environment and have long-term harmful consequences [102]. Functional biochar can be prepared to increase its ability to adsorb or remediate organic matter, for example, by improving the original biochar through chemical and physical modification, or by adding specific microorganisms to increase its ability to adsorb MPs [103].

3. Combined Effects of Biochar and MPs on Soil Environment

Differently from the direct adsorption of MPs on biochar in the water environment, biochar in the soil environment can mitigate the adverse effects of MPs via altering soil properties, enzyme activities, and microbial community.

Table 5. Biochar interacts with MPs.

Microplastic	Biochar Feedstock	Preparation Condition	Duration of Cultivation	Interaction Mechanism	Combined Effect	Reference
LDPE	Oilseed rape straw (OSR), soft wood pellet (SWP)	550 °C, 700 °C	100 days	Soil properties	Reduced plant-available P (23–86%) and elevated pH (0.15–0.46 units), EC (0.14–0.38 ds m−1), TN (63–120%), K (12–41%), and FDA activity (27–280%)	[104]
LDPE	Oilseed rape (OSR), soft wood pellet (SWP)	550 °C, 700 °C	100 days	Soil properties	Improved plant growth, soil microbes, and enzyme activity	[109]
PE	Straw biochar, manure biochar	500 °C, 4 h	45 days	Reducing greenhouse gas emissions	Reduced CH4 emissions by 35.8%, lowed N2O, CO2 and CH4 emissions by 24.8%, 6.2%, and 65.2%	[110]
PVC	Cotton stalks	650–750 °C	21 days	Soil properties	Shoot dry be increased matter production in PVC-MPs contaminated soil	[83]
PE	Wheat (Triticum aestivum) straw, cow manure	500 °C	45 days	Changing greenhouse gas emissions	Increased N2O emissions by 37.5% but decreased CH4 emissions by 35.8%	[90]
PVC	Corncob	500 °C, 3 h	14 days	Enzyme activities	Decreased the contents of H2O2 and MDA	[106]
PS	Food waste	500 °C, 20 min	5 weeks	Soil properties	Changed in available nitrogen (NO3-N: 325.5 mg kg−1, NH4+-N: 105.2 mg kg−1), soil electrical conductivity (EC, 2.04 ds m−1), available phosphorus (88.4 mg kg−1), and total exchangable cations (18.6 c mol (+) kg−1)	[105]
PS	Peanut	450 °C, 4 h	80 days	Soil microorganisms	Reinstate the microbial communities’ variety that was impacted by the pollution of MPs and increase the proportion of bacteria that are associated with resistance to pathogens	[107]
PVC	Corncob biochar	-	20 days	Plants	significantly reduced the contents of H2O2 and MDA in lettuce sprouts but increased the content of H2O2 in roots	[111]
PE	Corn stalk biochar	600 °C	300 days	Soil microorganisms	significantly increased the abundance of Subgroup_10 for the 16S rRNA gene and treatments with MPs alone significantly increased the relative abundance of Streptomyces for the phoD gene compared to CK	[79]
PS	Wheat straw and cow dung	500 °C, 600 °C, 700 °C	24 h	Soil properties	The removal efficiencies of MPs exceeded 86% for all biochar	[112]

summarized the combined effects of biochar and MPs on the soil environment. MPs, e.g., LDPE and PS, can change the physical and chemical properties of soils, increasing pH, electrical conductivity (EC), total nitrogen (TN), K, and enzyme activity, and decreasing plant available phosphorus (AP) (23–86%) [104,105]. The interaction of biochar with PE could alter greenhouse gas emissions in soil, reduce CO₂ content, and increase N₂O emissions [90]. In addition, the application of biochar can increase the dry weight of plant stems, reduce the content of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and alleviate the adverse effects on plants [106]. Biochar altered the diversity of microbial communities inhibited by MP pollution, promoting the correlation between relative bacterial abundance and pathogen resistance [107]. Furthermore, the integration of biochar into agricultural soils not only mitigates the detrimental effects of MPs, but also enhances soil fertility through its nutrient-rich composition. This dual action underscores the potential of biochar as a multifunctional amendment, capable of simultaneously addressing soil contamination and promoting plant health [108].

3.1. Combined Effects of Biochar and MPs on Soil Properties

Dissanayake et al. reported that the form and kind of MPs alter soil pH, although the mechanism remains uncertain [109]. Palansooriy et al. found that both rapeseed straw and cork particle biochar could enhance the pH of soils contaminated by LDPE [104]. This is mainly because the added biochar is alkaline and could neutralize the decreased pH caused by MPs. However, given that many factors affect soil pH, further research is required on the mechanism by which biochar regulates the impact of MPs on soil pH.

The impacts of biochar on EC and AP differ according to the kind of biochar and soil characteristics. Cork particles can reduce electrical conductivity, while rape biochar has the opposite effect, as rape straw contains more mineral components than coniferous wood. Kloss also indicated that biochar derived from straw possesses a higher electrical conductivity (EC) and ash content compared to that from wood. This characteristic endows rapeseed biochar with a greater capacity to enhance soil electrical conductivity [113].

The addition of MPs can reduce the amount of rapid available phosphorus (AP) [79], while the incorporation of biochar can enhance the availability of AP in soils contaminated with MPs. Jiang et al. discovered that using biochar enhanced the number of accessible phosphorus by increasing a lot of soil organic matter [114]. Additionally, recent research has demonstrated that the available phosphorus present in biochar will enhance the Olsen-P content in the soil, but it relies on the amount of biochar applied [115]. Biochar regulates soil pH, adsorbs complexes of soil phosphorus and metals, and acts directly as a carbon source for soil microorganisms, enhancing the biological transformation rate of soil phosphorus. Nevertheless, certain research has indicated that the influence of biochar on phosphorus concentrations in alkaline soils exhibits a degree of complexity [116], and will adversely affect plant AP in saline alkaline soils, leading to phosphorus precipitation and adsorption [117]. The increase in AP in acid soils is beneficial for the growth of soil microorganisms, which will help soil bacterial groups perform their functions better [118]. It is speculated that the application of biochar may alter the internal environment of MP-contaminated soils by altering the activity of soil enzymes and associated bacterial groups.

3.2. Combined Effects of Biochar and MPs on Enzyme Activities

Soil enzymes are dynamic biochemical catalysts that participate in the transformation of nutrients and the cycling of elements within the soil ecosystem [119]. They are highly responsive to ecological shifts and human activities, making them valuable bioindicators for evaluating soil health and quality [120]. Consequently, the measurement of soil enzyme activity serves as a prevalent method for evaluating the physical and chemical attributes, microbial community composition, and overall biological functioning of soil systems [121,122].

3.2.1. Combined Effects of Biochar and MPs on Fluorescein Diacetate Hydrolase (FDAse) Activity

Soil FDAse activity is intimately connected with the microbial biomass and the overall metabolic processes within the soil, serving as a reliable indicator for monitoring short-term fluctuations in soil quality [123]. Palansooriya et al. found that after the addition of kitchen waste biochar, FDAse activity decreased by 46.2% compared to the blank group [104]. Furthermore, Egamberdieva et al. found an improvement in FDAse activity after biochar treatment compared to no biochar treatment under drought stress [124]. It is speculated that this may be due to the different fundamental properties of biochar. The studies have shown a significant relationship between MP concentration and microbial biomass [125], with reduced microbial activity under MP exposure. Liu et al. reported that under polypropylene MP-contaminated loess, MPs are an important factor influencing soil microbial activity [126]. Therefore, the interaction between biochar application and MPs may increase the transformation of soil nutrients and change the metabolic function of the soil, thereby effectively improving the activity of soil microorganisms.

3.2.2. Combined Effects of Biochar and MPs on Urease Activity

Urease activity, integral to the nitrogen cycle, facilitates the hydrolysis of nitrogenous organic compounds. This enzymatic activity is extensively employed as a bioindicator of soil health, particularly for scrutinizing the alterations induced under conditions of environmental stress, such as the deleterious impacts of heavy metal contamination on soil microbial communities [127]. Ghani et al. discovered that the application of biochar to the soil where eggplant was grown increased soil urease activity [128]. The changes in soil nitrogen content under biochar treatment, when compared with soil contaminated by MPs, and the integration of biochar may serve to mitigate the detrimental impacts of MPs, thereby preserving the integrity and stability of the soil ecosystem. The presence of biochar can potentially buffer the negative impacts on soil microbial communities and enzymatic activities by reducing the bioavailability of MPs and providing a more conducive environment for microbial activity.

3.2.3. Combined Effects of Biochar and MPs on Phosphatase Activity

Acid phosphatase is an enzymatic catalyst that mediates the hydrolysis of phosphate monoesters and phosphodiesters within the soil matrix. This biochemical process facilitates the transformation of organic phosphorus compounds into inorganic forms, thereby rendering them accessible for plant uptake and assimilation [129]. Soil microbial activity, soil carbon, nitrogen, phosphorus content, and pH mainly affect acid phosphatase activity. Studies have indicated that using biochar reduces acid phosphatase activity. In soil contaminated with LDPE, after adding rapeseed biochar straw under drought and sufficient water conditions, the acid phosphatase activity decreased, which could be attributed to the elevation in pH value. Yang et al. observed that the application of bamboo biochar resulted in a decrease in acid phosphatase activity, which was attributed to an increase in soil pH [130]. Chen et al. reported a reduction in acid phosphatase activity and enhancement in alkaline phosphatase activity following biochar application [131]. Excess nutrients, including nitrogen, phosphorus, and potassium, can also reduce soil acid phosphatase activity [132,133].

3.3. Combined Effects of Biochar and MPs on Soil Organisms

3.3.1. Combined Effects of Biochar and MPs on the Diversity of Soil Microorganisms

Chen et al. investigated the impact of adding 2% dose of PLA-MPs on the diversity of microorganisms in the soil [134]. The research reported that the experimental soil bacterial α diversity index was not significantly changed. Other studies have reported that the addition of bioplastic polyhydroxyalkanoate (PHA) increased α diversity and altered soil bacterial communities at various taxonomic levels [135]. Several researchers have found that MPs have a slight impact on the α diversity of soil bacterial communities. This may be due to the inherent ability of soils to resist various disturbances. Compared to the MP treatment alone, the incorporation of biochar led to an increase in both the Chao1 and Shannon indexes. This indicates that biochar application can restore and even enhance bacterial species richness.

3.3.2. Combined Effects of Biochar and MPs on Soil Microbial Community Composition

A high relative abundance of Firmicutes, Proteobacteria, Actinobacteria, and Reticulobacteria was observed in the soil contaminated with MPs after the application of biochar [104]. As Bacillus belongs to the phylum Firmicutes, which consists mainly of spore-producing bacteria, Bacillus are more resistant to a variety of unfavorable environmental conditions, such as aridity [136]. Actinobacteria and Proteobacteria, like Firmicutes and Proteobacteria, are capable of surviving severe environmental conditions, including arid stress and heavy metal contamination [137]. They can survive in soil polluted with MPs. Ye et al. reported that seawater contaminated with PSMPs had an increase in Proteobacteria [138]. Similarly, Zhu et al. found that Proteobacteria was the most abundant phylum in both MP- and arsenic-contaminated soil [139]. In soil contaminated with MPs such as PVC and PP, Proteobacteria was the dominant phylum, followed by Firmicutes and Actinobacteria [140]. In addition, it has been reported that Proteobacteria and Actinobacteria can degrade plastic mulch in farmland, so they are also called plastic-associated bacteria [141].

The introduction of biochar modified the bacterial community within the soil that has been contaminated with MPs. This change in relative abundance mitigates the threat of MPs to the framework of the soil bacterial community. The Lysobacter genus has been found to enhance the antagonistic effect of MP-contaminated soil against plant pathogenic fungi and Gram-positive bacteria. It also inhibits the growth and invasion of pathogenic bacteria, reduces the incidence of plants in MP-contaminated soils, and improves soil stability. This finding underscores the fact that the implementation of biochar serves to attenuate the disruptive effects of MPs on the taxonomic and structural integrity of soil bacterial assemblages, concurrently augmenting the ecological stability of soils with MP pollution.

3.4. Combined Effects of Biochar and MPs on Plants

Research has consistently demonstrated that the introduction of MPs exerts detrimental effects on the growth of plants. Scholarly research has delineated that exposure to PE can impede the developmental processes of wheat. This inhibition is attributed to the perturbation of antioxidant enzymatic activities and the photosynthetic apparatus within these plants [142,143]. On the side, plants can absorb MPs through their roots and transport them into their bodies, potentially affecting human health through the food chain. Li et al. discovered that corncob biochar can alleviate the inhibitory effects of PVC-MPs on lettuce yield at appropriate concentrations. Single PVC-MPs make H₂O₂ and MDA content increase, causing oxidative damage to lettuce [111]. The addition of corncob biochar significantly reduced the contents of H₂O₂ and MDA in lettuce sprouts but increased the content of H₂O₂ in roots. Kaur et al. stated that MPs reduced the mitotic index [144]. Furthermore, a majority of MPs have been found to induce cytotoxic effects and nuclear aberrations, primarily through the disruption of spindle assembly and the induction of micro-nucleated cells [145]. Elbasiouny et al. found that biochar mitigated the negative impact of MPs [110]. Upon the introduction of MPs, there was a marked decrease in the mitotic index, specifically by 9.39%, when contrasted with the control group. However, the incorporation of biochar resulted in a smaller reduction in the mitotic index (1.5%) compared to the control group. Although the primary mechanism underlying the genotoxic effects of MPs remain elusive, it is plausible that their accumulation within root tissues could obstruct cellular wall pores, thereby impeding the translocation of essential nutrients [146].

3.5. Biochar as a Remediation Material for MPs in Soils

Biochar can enhance soil characteristics. Research has shown that soil pollution caused by MPs can be remediated by biochar (Figure 4). MPs can reduce the activity of antioxidant enzymes and affect the photosynthesis of plants, thus reducing fruit yield and endangering human health. Biochar can alleviate the physicochemical properties of soil caused by MP pollution by increasing pH, AP, soil enzyme activity, microbial abundance, the chao1 index, and Shannon index, and increasing the abundance of Firmicutes, Proteobacteriaes, Actinobacteria, and Reticulobacteria.

Although biochar has demonstrated the capacity to enhance the physical and chemical characteristics of soil affected by MP pollution, it is crucial to highlight that adding biochar to soils with limited buffering capacity might have negative consequences, such as lowering soil phosphorus and other micronutrients. On the other hand, applying biochar will improve the immobilization of harmful substances in the soil. Therefore, caution is required when applying biochar, considering both the type of biochar and the soil’s chemical and physical characteristics. The use of biochar as a soil amendment is a complex process that requires a nuanced understanding of its interactions with soil properties and the potential impacts on plant growth.

4. Prospects and Challenges

The study found that due to the instability of environmental factors and the complexity and diversity of biological systems, current research is mainly conducted in laboratories. It is not yet clear whether the results are universally representative and whether they can be applied in real environments. The application of biochar in mitigating MP pollution in a realistic environment is influenced by factors including the types and shapes of MPs, the properties of biochar and their interactions with soil constituents, and other co-existing pollutants.

Currently, there are not many studies on the biochar remediation of MPs, especially the impacts of their combined effects on soil. Given the creation of MP–biochar particles, the co-occurrence of MPs and biochar could potentially influence their interactive dynamics within the soil matrix, leading to modifications in their surface properties and consequently affecting the adsorptive interactions with other soil-borne contaminants. Therefore, it is necessary to investigate the impact of the coexistence of biochar and MPs on soil, and research into how biochar and MPs interact with soil is essential.

The potential environmental risks of biochar cannot be ignored, even though its large specific surface area, refractory decomposition, and catalytic properties have made it widely used in environmental applications, including soil restoration, carbon sequestration, and the management of wastewater. However, the heavy metals included in biochar can be possibly released into water when biochar is used as remediation material for polluted water, leading to heavy metal pollution. The real environmental impacts of biochar, such as the possible emission of polycyclic aromatic hydrocarbons and the release of metallic ions into the environment, remain unclear, which needs further exploration.

Characterizing the alterations before and after removal, as well as the difficulties encountered is a crucial first step when talking about the advancement and use of MP-removal technologies. In recent years, researchers have been working hard to establish accurate measuring methods for characterizing MPs. MPs have been tracked in studies using optical microscopy for particle size distribution in terms of width and length, FTIR, and total organic carbon (TOC) analysis. The reduction in particle size, as determined by optical microscopy and particle size distribution in terms of width and length, is another way to verify the decomposition of MPs. Methods for the qualitative and quantitative study of MPs both before and after removal have been made possible by recent technical advancements.

Many variables can influence the transport of MPs in soil, including soil properties, soil biota, and human activities. Among these, soil texture is a key factor. The size of soil pores plays a crucial role and directly affects the migration of MPs. However, the mechanism of biochar in the migration and transformation of MPs is still unclear and needs further investigation.

To overcome the problem of poor treatment efficiency of some original biochar, innovative nanotechnology and biotechnology engineering techniques can be used to prepare biochar, such as using oxidants to enhance the groups that contain oxygen on the surface of biochar and increasing its magnetism through metal ion modification. This enhances the function of the biochar system in remediating MP pollution in soil. Also, it is essential to design multifunctional composite materials (such as BC-LDHs, carbon nanotube biochar composites) that can significantly improve the properties of the adsorbents. Plant–biochar–microorganism joint remediation can also be employed to achieve better treatment results.

Existing research focuses mainly on the treatment of single contaminants with biochar. There is relatively little research on the remediation of the combined contamination of MPs and other pollutants. However, the complex nature of environmental contamination, where the co-existence of MPs and other pollutants are often found, necessitates a deeper insight into the behavior of these contaminants in the presence of biochar.

5. Conclusions

This review summarizes the application and mechanisms of biochar on the removal of MPs in water and soil environments. Biochar serves as a remediation material for MPs in soils as it mitigates the adverse effects of MPs on soil property, enzyme activities, and soil microbial community. Further research should focus on the combined the combined effects of biochar and MPs, the environmental risks of biochar, the modification of biochar application of MP-removal technologies, the remediation of combined contamination of MPs and other pollutants, and the transportation of MPs. However, the widespread application of biochar in MP adsorption remains limited, facing challenges such as the recovery and regeneration of the adsorbent, secondary pollution from modification strategies, and the proper disposal of adsorbed biochar. In conclusion, while biochar adsorption offers a promising method for tackling the issue of MP pollution, continued research and innovation are crucial for optimizing its application, ensuring its economic feasibility, and evaluating its environmental impact.