Biochar for volatile organic compound (VOC) removal: Sorption performance and governing mechanisms

Mitigation of indoor air pollution: A review of recent advances in adsorption materials and catalytic oxidation

Indoor air pollution with toxic volatile organic compounds (VOCs) and fine particulate matter (PM2.5) is a threat to human health, causing cancer, leukemia, fetal malformation, and abortion. Therefore, the development of technologies to mitigate indoor air pollution is important to avoid adverse effects. Adsorption and photocatalytic oxidation are the current approaches for the removal of VOCs and PM2.5 with high efficiency. In this review we focus on the recent development of indoor air pollution mitigation materials based on adsorption and photocatalytic decomposition. First, we review on the primary indoor air pollutants including formaldehyde, benzene compounds, PM2.5, flame retardants, and plasticizer: Next, the recent advances in the use of adsorption materials including traditional biochar and MOF (metal-organic frameworks) as the new emerging porous materials for VOCs absorption is reviewed. We review the mechanism for mitigation of VOCs using biochar (noncarbonized organic matter partition and adsorption) and MOF together with parameters that affect indoor air pollution removal efficiency based on current mitigation approaches including the mitigation of VOCs using photocatalytic oxidation. Finally, we bring forward perspectives and directions for the development of indoor air mitigation technologies.

Metal chloride-loaded biochar for phosphorus recovery: Noteworthy roles of inherent minerals in precursor

Phosphorus (P) is a valuable resource, while it is vastly lost with wastewater causing eutrophication. In this study, to recover P, composite biochars were prepared by pyrolyzing biowaste impregnated with FeCl₃ or MgCl₂. It was found that inherent mineral profiles in the biowastes played important roles in interacting with metal chlorides and determined P sorption and precipitation. Specifically, two biowastes containing distinct mineral contents, sawdust and sediment, were selected as model components, being alone or mixed at 1:1 (w/w) to prepare biochars with low, moderate and high mineral contents. Results showed that biochar itself could not absorb P, while loading FeCl₃ or MgCl₂ achieved P recovery rates of approximate 60-100% and 50-100%, respectively, via electrostatic attraction or ligand exchange of PO₄^3- with -OH/-COOH, which was attributed to the enhanced positive charges and -OH/-COOH on the materials by these metal chlorides. Inherent minerals inhibited FeCl₃ transforming into Fe₃O₄ in pyrolysis and promoted generation of Fe₄(PO₄)₃(OH)₃ in P sorption, thus high-mineral content was more appropriate for FeCl₃ loading; however, precursors with low-mineral content was suitable for MgCl₂ loading, since the bulk-C in biochar acted as porous structure to support MgO crystals with high superficial area (∼255.85 m² g^-1). Besides, FeCl₃ and MgCl₂ both drove dissolution of inherent minerals significantly, while inherent minerals inhibited release of soluble Fe and Mg²⁺ into solution, which minimized secondary pollution. This study implied that in constructing composite biochar for catching P, the type of metal chloride should match the inherent minerals in biowastes to maximize P recovery and minimize secondary pollution.

Air oxidation coupling NH3 treatment of biomass derived hierarchical porous biochar for enhanced toluene removal

In this study, hierarchical porous biochar was prepared from poplar sawdust by air oxidation coupling with NH₃ treatment for the removal of toluene. The results showed that the mesopore volume of the sample with air oxidation (PS‒O₂) increased significantly to 0.263 cm³/g from the blank sample (PS, 0.053 cm³/g). This could be attributed to the selective removal of the lignin carbon by air oxidation to develop mesopores in biochar. Following further NH₃ treatment (PS‒O₂‒NH₃), the basic surface chemistry on biochar was improved due to increased basic N-containing groups and decreased acidic O-containing groups, together with the micropore volume also increased to 0.231 cm³/g from 0.186 cm³/g of PS‒O₂. The formation mechanism of hierarchical porous structure of biochar was also discussed. The adsorption capacity of PS‒O₂‒NH₃ for toluene reached 218.4 mg/g at the initial concentration of 820 mg/m³, which was 383.2% higher than that of PS. The adsorption isotherm study indicated that the adsorption process of toluene was monolayered and the maximal adsorption capacity of PS‒O₂‒NH₃ for toluene could reach as high as 476.2 mg/g. The results demonstrated that air oxidation coupling NH₃ treatment is a highly effective method for the preparation of hierarchical porous biochar for enhancing toluene adsorption performance.

Ball milling biochar with ammonia hydroxide or hydrogen peroxide enhances its adsorption of phenyl volatile organic compounds (VOCs)

Pristine biochar (CN600), ball-milled biochar (CN600-BM), H₂O₂ modified BM-biochar (CN600-O), and NH₄OH modified BM-biochar (CN600-N) derived from corn stalk were applied to adsorb phenyl volatile organic compounds (VOCs). H₂O₂ and NH₄OH modification of BM-biochar significantly improved its physicochemical characteristics and adsorption abilities. The specific surface area of CN600-O increased 2.05 and 1.23 times compared to CN600 and CN600-BM, respectively; while CN600-N increased 2.41 and 1.45 times, respectively. In addition, the ball milled biochars, especially CN600-O, showed higher acidity and polarity than CN600. The VOC adsorption amount onto biochars was 10.96-130.21 mg/g. CN600-O and CN600-N had high uptake of the VOCs and reached 100.07-111.79 mg/g and 110.49-130.21 mg/g, respectively. CN600-N showed the best performance with P-xylene adsorption up to 130.21 mg/g. VOC adsorption onto the CN600-O and CN600-N were mainly governed by surface adsorption and associated with morphology characteristics of the biochars as well as VOC properties such as boiling point and molecular size. Five cycles of adsorption-desorption experiments showed that CN600-O and CN600-N had good reusability with the reuse efficiencies of 88.01 %-92.21 % and 92.19 %-95.39 %, respectively. The results indicate that O- and N-doped ball-milled biochars are promising in adsorption for effective and sustainable VOC removal.

Biochars as media for air pollution control systems: Contaminant removal, applications and future research directions

Biochars are low-cost and renewable biomaterials with several applications, including soil amendment, mitigation of greenhouse gas emissions, and removal of both inorganic and organic contaminants in aqueous systems. An increasing body of recent evidence indicates that biochars can also remove gaseous chemical contaminants, such as those occurring in industrial flue gases. However, unlike other applications such as in agroecosystems, soil amendments, and aquatic systems, comprehensive reviews on biochar applications in the field of air pollution control are still lacking. The current paper examined existing evidence to understand the nature of contaminants, particularly the gaseous ones, potential applications, constraints, and future research needs pertaining to biochar applications in air pollution control. The preparation of biochars and their functionalized derivatives, and the properties influencing their capacity to remove gaseous contaminants are summarized. The removal capacity and mechanisms of various organic and inorganic gaseous contaminants by biochars are discussed. Evidence shows that biochars effectively remove metal vapours, particularly elemental mercury (Hg⁰), acidic gases (H₂S, SO₂, CO₂), ozone, nitrogen oxides (NO_x), and organic contaminants including aromatic compounds, volatile organic compounds, and odorous substances. The mechanisms for the removal of gaseous contaminants, including; adsorption, precipitation, and size exclusion were presented. Potential industrial application domains include remediation of gaseous emissions from incinerators, waste-to-energy systems, kilns, biomass and coal-fired boilers/cookers, cremation, smelters, wastewater treatment, and agricultural production systems including livestock husbandry. These industrial applications, coupled with the renewable, low-cost and sustainable nature of biochars, point to opportunities to further develop and scale up the biochar technology in the air pollution control industry. However, the biochar-based air filter technology still faces several challenges, largely stemming from constraints and several knowledge gaps, which were highlighted. Hence, further research is required to address these constraints and knowledge gaps before the benefits of the biochar-based air filters are realized.

Comparative investigation of characteristics and phosphate removal by engineered biochars with different loadings of magnesium, aluminum, or iron

Engineered biochars (EBCs) loaded with metal oxides/hydroxides have been used as sorbents to remove and recycle phosphate (P) from wastewater. However, P removal by EBCs made with different types and loading of metals have rarely been compared in a single study. Thus, in this study, EBCs were synthesized through pyrolysis of bamboo or hickory wood chips (25 g) pretreated with four amounts (25, 50, 75, and 100 mmol) of magnesium (Mg), aluminum (Al), or iron (Fe) salt solutions (Mg-EBC, Al-EBC, and Fe-EBC, respectively). The resulting EBCs were loaded with metal oxides/hydroxides that served as P adsorption sites. Al-EBCs showed the highest aqueous stability with little metal dissolution, which can be attributed to the low level of residual (unconverted) metal salt as well as the extremely low solubility of loaded Al metal oxyhydroxide. After the leaching/washing, the metal loading efficiencies of the Al- and Mg-EBCs were similar (50-60%) and stable metal loadings increased with pretreatment salt amounts, indicating that the amount of the two metal oxides/hydroxides in the EBCs can be controlled during pretreatment. However, stable iron oxide on the Fe-EBCs remained almost the same for all the four levels of pretreatment, reflecting saturation of the biochar surface. All the EBCs showed increasing P adsorption with increasing metal loading. At low initial P concentrations of 31 mg/L, Fe- and Al-EBCs removed up to 68% and 94% of P, likely through an electrostatic interaction mechanism. At high P concentrations, Mg-EBC had the largest P adsorption capacity (119.6 mg P/g), mainly through the combination of surface precipitation and electrostatic interaction mechanisms. This study demonstrates that metal oxide/hydroxide-loaded EBCs are promising sorbents that can be designed to meet specific needs for the removal of aqueous P in various applications.

Mitigation of Gaseous Emissions from Swine Manure with the Surficial Application of Biochars

Environmental impact associated with odor and gaseous emissions from animal manure is one of the challenges for communities, farmers, and regulatory agencies. Microbe-based manure additives treatments are marketed and used by farmers for mitigation of emissions. However, their performance is difficult to assess objectively. Thus, comprehensive, practical, and low-cost treatments are still in demand. We have been advancing such treatments based on physicochemical principles. The objective of this research was to test the effect of the surficial application of a thin layer (¼ inches; 6.3 mm) of biochar on the mitigation of gaseous emissions (as the percent reduction, % R) from swine manure. Two types of biochar were tested: highly alkaline and porous (HAP) biochar made from corn stover and red oak (RO), both with different pH and morphology. Three 30-day trials were conducted with a layer of HAP and RO (2.0 & 1.65 kg∙m−2, respectively) applied on manure surface, and emissions of ammonia (NH3), hydrogen sulfide (H2S), greenhouse gases (GHG), and odorous volatile organic compounds (VOCs) were measured. The manure and biochar type and properties had an impact on the mitigation effect and its duration. RO significantly reduced NH3 (19–39%) and p-cresol (66–78%). H2S was mitigated (16~23%), but not significantly for all trials. The phenolic VOCs had relatively high % R in most trials but not significantly for all trials. HAP reduced NH3 (4~21%) and H2S (2~22%), but not significantly for all trials. Significant % R for p-cresol (91~97%) and skatole (74~95%) were observed for all trials. The % R for phenol and indole ranged from (60~99%) and (29~94%) but was not significant for all trials. The impact on GHGs, isobutyric acid, and the odor was mixed with some mitigation and generation effects. However, larger-scale experiments are needed to understand how biochar properties and the dose and frequency of application can be optimized to mitigate odor and gaseous emissions from swine manure. The lessons learned can also be applicable to surficial biochar treatment of gaseous emissions from other waste and area sources.

A facile synthesis of nitrogen-doped porous carbons from lignocellulose and protein wastes for VOCs sorption

This paper reported a sustainable method for preparation of N-doped porous carbon from co-hydrothermal carbonization (co-HTC) of lignocellulose (as a carbon skeleton) and sludge protein (as a nitrogen dopant). The HTC pretreatment could improve the yield of porous carbon. Although a low mass ratio (1:1) of biochar and KOH was employed, large specific surface areas of the porous carbons could be achieved. The porous carbon from rice husk (RH) by the HTC (260 °C) combined with the KOH activation (750 °C) had a largest BET surface area up to 1396.5 m²/g. Meanwhile, its adsorption capacity on toluene could reach 394.2 mg/g. It was attributed to its large specific surface area and hierachical porous structure with higher meso-porosity. The porous carbons with N-doping also improved the adsorption capacity. Furthermore, thermal desorption at around 300 °C had a high potential for regeneration of the saturated porous carbons.

Surface modified and activated waste bone char for rapid and efficient VOCs adsorption

In this work, very efficient VOCs adsorbent was developed from waste bovine bone. After pyrolysis at 450 °C, the bone char was treated by H₃PO₄ for surface modification and activated by K₂CO₃ respectively. The prepared materials were characterized by N₂ adsorption isotherms, SEM, FT-IR, and XPS. Adsorption/desorption and regeneration behavior of VOCs were also studied. Results showed that H₃PO₄ modification can effectively accelerate the adsorption process and after K₂CO₃ activation, a new hierarchical pore structure was found with an ultrahigh total pore volume of 2.807 cm³/g. The specific adsorption capacity for typical VOC reached ∼13.03 mmol/g which is much higher than literature data under the same condition. Static toluene adsorption test on the prepared activated bone-char revealed that the hierarchical structure has provided abundant adsorption sites and the adsorption behavior can be well described by the pseudo-second-order model. The dynamic/static adsorption ratio increased from 70.31% to 78.62% due to less mass transfer resistance by surface modification.

Adsorption of tetracycline hydrochloride onto ball-milled biochar: Governing factors and mechanisms

Pristine and ball milled wheat stalk biochars pyrolysed at 300 °C, 450 °C, 600 °C were studied for tetracycline hydrochloride (TCH) adsorption from aqueous solution. Surface characteristics of ball milled biochar (BM-biochar) were significantly enhanced over their pristine counterparts. TCH adsorption occurred largely on external surface and by filling pores of biochars as evidenced by strong positive correlation between adsorption and external specific surface area (SSA), total pore volume, or mesoporous volume. A two-stage intra-particle diffusion model, limited by the TCH diffusion through the boundary liquid layer, well described TCH adsorption. Maximum TCH adsorption occurred at about pH = 6-8. While solution cations including Na⁺, K⁺ and Mg²⁺ subdued TCH adsorption as they competed for adsorption sites, Ca²⁺ promoted TCH adsorption due to formation of tetracycline-Ca²⁺ complexes. The best performing BM-biochar was the one pyrolysed at 600 °C with TCH adsorption amount of 84.54 mg/g. Therefore, this BM-biochar has the potential for TCH removal from aqueous solutions. And the research shed light on the management of organic contaminants in real wastewater by BM-biochar.

A non-edible waste as a potential sorptive media for removal of herbicide from the watershed

A non-edible waste, from biodiesel processing industry is being turned to carbonaceous material (biochar) using slow pyrolysis. The material was found to be amorphous with hydroxyl, methyl, carbonyl and carboxyl functional groups onto the surface. The influencing parameters, namely adsorbate concentration (0.05-5 mg/l), biochar loading (0.02-0.4 g), pH(3-12) and particle sizes (0.03-0.13 mm) were studied to observe the effect on the sorption of simazine using biochar. A multivariate optimization using central composite design in response surface methodology was performed employing desirability function. The optimized biosorption efficiency (B%) and capacity q_e was found to be 91.98 % and 0.83 mg/g respectively with the optimized parameters as 3.76 mg/l of adsorbate concentration, 0.12 g of biochar loading, pH of 5.26 and 0.0535 mm of particle size. The simazine adsorption phenomena were found to be multilayer heterogeneous sorption based on Langmuir and Freundlich models. The kinetics investigation shows that chemisorption was involved for the transfer of simazaine to the surface of biochar with three distinct intra particulate diffusional zones. An adsorption process requires activation energy of 11.27 kJ/mol and the negative magnitude of  ΔH^* indicates the exothermicity involved in the process.

Utilization of metal-organic frameworks for the adsorptive removal of an aliphatic aldehyde mixture in the gas phase

Considerable efforts have been undertaken in the domain of air quality management for the removal of hazardous volatile organic compounds, particularly carbonyl compounds (CCs). In this study, the competitive sorptive removal of six CCs (namely, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) was assessed using selected metal-organic frameworks (MOFs: MOF-5, MOF-199, UiO-66, and UiO-66-NH₂) and inexpensive commercial activated carbon as a reference sorbent. The sorption experiments were conducted using a mixture of the six CCs (formaldehyde and acetaldehyde at ∼1 Pa and propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde at ∼0.2 Pa) together with 15 Pa water and 2.6 Pa methanol in 1 bar nitrogen. For all of the carbonyl compounds other than formaldehyde, MOF-199 showed the best 10% breakthrough performance ranging from 34 L g^-1 and 0.14 mol kg^-1 Pa^-1 for acetaldehyde to 1870 L g^-1 and 7.6 mol kg^-1 Pa^-1 for isovaleraldehyde. Among all the sorbents tested, UiO-66-NH₂ exhibited the best 10% breakthrough performance metrics towards the lightest formaldehyde which remains to be one of the most difficult targets for sorptive removal (breakthrough volume: 285 L g^-1 and partition coefficient: 1.1 mol kg^-1 Pa^-1). Theoretical density functional theory (DFT)-based computations were also conducted to provide better insights into the adsorbate-adsorbent interactions. Accordingly, the magnitude of adsorption energy increased with an increase in the CC molar mass due to an enhancement in the synergetic interaction between C[double bond, length as m-dash]O groups (in adsorbate molecules) and the MOF active centers (open metallic centers and/or NH₂ functionality) as the adsorbent. Such interactions were observed to result in strong distortion of MOF structures. In contrast, weak van der Waals attraction between the hydrocarbon "tail" of CC molecules and MOF linkers were seen to play a stabilizing role for the sorbent structure. The presence of the NH₂ group in the MOF structure was suspected to play a key role in capturing lighter CCs, while such an effect was less prominent for heavier CCs. Overall, the results of this study provided a basis for the establishment of an effective strategy to enhance the sorption capacity of MOFs against diverse carbonyl species.

Removal of methylene blue from aqueous solutions by biochar prepared from the pyrolysis of mixed municipal discarded material

This paper investigates the adsorption of organic compounds in aqueous solution to biochar adsorbent, using methylene blue as an indicator for adsorption. Biochar was produced by the pyrolysis of mixed municipal discarded material in an innovative heat pipe reactor, the pyrolysis temperature was held at 300°C for 12 h. Biochar produced under these conditions was found to have oxygen containing functional groups that are beneficial to the adsorption of methylene blue as well as graphitic structures suggesting potential sites for π-π interactions with methylene blue. Methylene Blue followed the pseudo second order kinetic model with higher R² values than both the pseudo first order kinetic and intraparticle diffusion models. The adsorption also closely fit the Langmuir isotherm rather than the Freundlich model, suggesting monolayer adsorption rather than multilayer adsorption. Maximum adsorption capacity was observed at 7.2 mg/g for initial concentration of 100 mg/l Methylene blue in aqueous solution. The amount of Methylene blue adsorbed increased with increasing initial concentration as expected. The adsorption mechanisms are likely π-π interactions between methylene blue and the graphitic structures in the biochar which are shown to be present in Raman spectroscopy, as well as electrostatic attraction and ionic bonding between negatively charged surface sites on the char and the positive charge on the dissolved methylene blue molecules. The results show that biochar obtained from mixed waste could be employed as a low-cost and effective tool in water treatment for the removal of basic dyes and potentially other organic impurities.

Adsorption of acetone and cyclohexane onto CO2 activated hydrochars

Most of the volatile organic compounds (VOCs) are toxic and harmful to human health and environment. In this study, hydrochars activated with CO₂ were applied to remove VOCs. Two typical VOCs, acetone and cyclohexane, were used as the 'model' adsorbates to evaluate hydrochars' performance. Specific surface areas of pristine hydrochars were small (<8 m²/g), whereas activated hydrochars showed much higher values (up to 1308 m²/g). As a result, the adsorption of VOCs onto the pristine hydrochars (13.24-24.64 mg/g) was lower than that of the activated ones (39.42-121.74 mg/g). The adsorption of the two VOCs onto the hydrochars was exothermal. In addition, there were significant correlations (R² > 0.91) between the VOC removal and hydrochars' specific surface area. These results suggest that the governing mechanism was mainly physical adsorption. Increasing experimental temperature (80-139 °C) desorbed the VOCs from the hydrochars. Due to its higher boiling point, cyclohexane desorption required a higher temperature than acetone desorption. The reusability of the activated hydrochars to the two VOCs was confirmed by five continuous adsorption-desorption cycles. The overall results indicated that hydrochars, particularly after CO₂ activation, are sufficient for VOC abatement.

In-situ biodegradation of volatile organic compounds in landfill by sewage sludge modified waste-char

VOCs are the major harmful pollutants released from MSW landfills, which are toxicity to human health. In order to in-situ biodegradation of VOCs released from landfill, two novel laboratory-scale biocovers, including waste-char obtained from MSW pyrolysis (WC), and sewage sludge modified the WC (SWC), are used to degradate VOCs. The removal performances of VOCs as well as the bacterial community in the WC and SWC are investigated in a simulated landfill systems with the contrast experiment of a landfill cover soil (LCS) for 60 days. Meanwhile, the adsorption-biodegradation of VOCs model compounds over the LCS, WC, and SWC are also tested in fixed-bed adsorption reactor and in-situ FTIR. The VOCs removal efficiencies by the SWC are maintained above 85% for a long-term, much higher than that of the LCS and WC. The higher removal efficiencies and long-term stability for VOCs degradation in SWC are attributed to a strongly positive synergistic between adsorption and biodegradation that the gaseous VOCs released from MSW is effectively adsorbed by the SWC due to its higher VOCs adsorption capacity, and then the adsorbed-VOCs is converted into CO₂ and H₂O by the microorganisms that consuming the adsorbed-VOCs as energy and carbon sources. Subsequently, the decrease of the adsorbed-VOCs in SWC would also promote the transformation of the gaseous VOCs into the adsorbed VOCs and accelerate the growth of microorganisms by taking the adsorbed-VOCs as the energy and carbon source, resulted in a higher adsorption rate and degradation rate for VOCs.

MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants

A facile ball-milling method was developed to synthesize MgO/biochar nanocomposites as a dual-functional adsorbent. The physicochemical properties of the synthesized nanocomposites indicated that the composites achieved nano-scaled morphologies and mesoporous structure with MgO nanoparticles, which is approximate 20 nm and dispersed uniformly on the surface of the biochar matrix. Batch sorption experiments yielded 62.9% removal of phosphate, an anion, and 87.5% removal of methylene blue, a cationic organic dye, at low adsorbent dosages of 1.0 g L^-1 and 0.2 g L^-1, respectively. This work indicates that ball milling, as a facile and promising method for synthesis of carbon-metal oxide nanocomposites, lends the advantage of operational flexibility and chemical adjustability for targeted remediation of diverse environmental pollutants.

Novel biosorbents synthesized from fungal and bacterial biomass and their applications in the adsorption of volatile organic compounds

Adsorption is an efficient and low-cost technology used to purify volatile organic compounds (VOCs). In the current study, novel microbial adsorbents were synthesized using cells of lyophilized fungi (Ophiostoma stenoceras LLC) or bacteria (Pseudomonas veronii ZW) that were modified by aminomethylation. Based on the adsorption performance and structural characterization results, the modified fungal biosorbent was the best. Its maximum adsorption capacities for ethyl acetate, α-pinene, and n-hexane were 620, 454, and 374 mg·g^-1, respectively, which were much higher than those of other synthesized biosorbents. The specific surface area of the fungal biosorbent was 20 m²·g^-1, and most of the components were hydrocarbon compounds and polysaccharides. The VOC adsorption process on these synthesized biosorbents was in accordance with the Langmuir isothermal model and the pseudo-first-order kinetic model, thereby suggesting that physical adsorption was the dominant mechanism. The fungal biosorbent could be used for five consecutive VOC sorption-desorption cycles without any obvious decrease in adsorption capacity.

Preparation and characterization of a new waste-derived mesoporous carbon structure for ultrahigh adsorption of benzene and toluene at ambient conditions

In this work, a series of nanoporous carbon materials were synthesized using Iranian asphaltene as a low-cost carbon source and modified by melamine as a new nitrogen-rich promoter (M-IANC). The adsorption capacity of benzene and toluene on the synthesized M-IANCs was measured at low and high concentrations by an in-house built apparatus. The results demonstrated that the addition of melamine remarkably increased the mesoporous volume (up to 1.61 cm³/g) in the nanoporous carbon structure and, subsequently, created a large surface area (2692 m²/g) and pore volume (1.71 cm³/g). The resulting M-IANC-C nanostructure (melamine:PIA mass ratio of 1:2) depicted 228.18 wt.% and 82.08 wt.% adsorption capacity for benzene and toluene, respectively, which were 19.4 and 2.8 times higher than commercial activated carbon. In addition to the distinguished adsorptive behavior for benzene and toluene removal, M-IANC-C exhibited higher cyclic adsorption capacity than those of unmodified IANC sample after four consecutive cycles. The adsorption mechanism and the role of melamine groups in the adsorption of benzene and toluene were also studied by the density functional theory (DFT) calculations. Besides the inexpensive cost of the carbon source (asphaltene), results also indicate that the M-IANC can be a suitable candidate for VOC adsorption.

Sorption of volatile organic compounds on non-activated biochar

This work dealt with the determination of the suitability of sorption of Volatile Organic Compounds (VOCs) on biochars prepared from neem, sugarcane and bamboo feedstocks. Six different VOCs namely benzene, toluene, methyl chloride, xylene, chloroform and carbon tetrachloride were used in a laboratory-scale set-up on non-activated biochars prepared via slow pyrolysis (350-550 °C). Although all the chars showed considerable sorption but amongst them N3 (neem-based biochar) showed the maximum removal efficiency (65.5 mg g^-1 for toluene). Variation in pyrolysis temperature and feedstock type showed significant change in the porosity and specific surface area of the biochar, which is favorable for VOC sorption efficiency. With higher surface area and contact time, the sorption capacity of char enhanced. However, the extent of sorption capacity of biochars differed with changing VOC type. Pseudo-Second-Order model fitted well with the results obtained from VOC sorption kinetics.

Preparation of high-performance toluene adsorbents by sugarcane bagasse carbonization combined with surface modification

A series of activated carbons were prepared by carbonizing sugarcane bagasse combined with surface modification, which showed an excellent performance of adsorbing toluene (522 mg g⁻¹ at 30 °C).

Interaction with low molecular weight organic acids affects the electron shuttling of biochar for Cr(VI) reduction

Biochar can act as "electron shuttle" in soil redox reactions. It is possible that biochar accepts the electrons from low molecular weight organic acids (LMWOAs) in soil and then transfer them to the acceptors, e.g., Cr(VI). This study evaluated the interaction between seven soil LMWOAs and peanut shell biochar (BC) as well as its effect on the electron shuttling of biochar for Cr(VI) reduction. Both redox reactions and sorption process occurred during the interaction of biochar and LMWOAs, which altered the contents of Cr(VI) reduction-relevant groups (i.e., CO and CO) on the surface of biochar. The redox reactions were more important to the electron transfer between biochar produced at 400℃ (BC400) and LMWOAs due to the repeated cycle of reduction-oxidation of surface functional groups. The reduction rate of Cr(VI) by LMWOAs mediated by BC400 was 1.10-7.09 × 10^-3 h^-1, among which tartaric acid had the best reduction efficiency due to its highest reducing capability. For biochar produced at 700℃ (BC700), the sorption process of LMWOAs was the key factor to the direct electron shuttling process through the conjugated structure of biochar. The reduction rate of Cr(VI) by LMWOAs mediated by BC700 was significantly higher and ranged 7.40-864 × 10^-3 h^-1, with the oxalic acid having the best reduction efficiency due to its highest sorption capacity by BC700. The results obtained from this study can help to establish the linkage between biochar and LMWOAs in soil electron network, which better explains the multifunctional roles of biochar during the redox processes such as Cr(VI) reduction in soil.

Application of cyanated asphaltenes in gas-phase adsorption processes for removal of volatile organic compounds

The paper presents an innovative, chemically modified (methylcyanated) asphaltene-based adsorbent that can be an interesting low-cost alternative for traditional adsorbents. Adsorption properties of adsorbents were examined by inverse gas chromatography technique, adsorption isotherms, and breakthrough curves. A significant increase in retention volume for pyridine, 2-pentanone, nitropropane, toluene, and 1-butanol was observed. Rohrschneider–McReynolds constants revealed an increase in strength of interactions as a result of the modification, especially in strong proton–acceptor interaction (by a factor of 4.6). The surface-free energy of asphaltene adsorbents increased from 136.71 to 169.95 mJ m−2 after modification. It is similar to the surface-free energy of silica or alumina. Moreover, modified adsorbent shows very high adsorption potential for pyridine. Adsorption isotherms revealed that monolayer adsorption capacity for pyridine increased 1.5 times after modification. Breakthrough curves of pyridine indicate that chemical modification increased the adsorption capacity, removal efficiency, and throughput. Scale-up calculations revealed that adsorption column packed with modified asphaltene adsorbent would be almost two times smaller compared to a column packed with unmodified one.

Graphic abstract

Biochar reduces volatile organic compounds generated during chicken manure composting

The efficiency of biochar for reducing the levels of volatile organic compounds (VOC) was investigated in a composting mixture containing 90% poultry manure and 10% straw (with and without 3% biochar addition) at three different stages of the process. The use of a low application rate of biochar reduced the concentration of VOC during the thermophilic phase. Biochar significantly reduced the levels of nitrogen volatile compounds, which are the most abundant VOC family, originated from microbial transformation of the N-compounds originally present in manure. The most efficient VOC reduction was observed in oxygenated volatile compounds (ketones, phenols and organic acids), which are intermediates of organic matter degradation, whereas there was no effect on other VOC families (aliphatic, aromatic and terpenes). These results suggest the importance of not only the sorption capacity of biochar but also its impact in the composting progress as main drivers for VOC reduction.

Mesoporous graphene adsorbents for the removal of toluene and xylene at various concentrations and its reusability

As novel technologies have been developed, emissions of gases of volatile organic compounds (VOCs) have increased. These affect human health and are destructive to the environment, contributing to global warming. Hence, regulations on the use of volatile organic compounds have been strengthened. Therefore, powerful adsorbents are required for volatile organic compounds gases. In this study, we used graphene powder with a mesoporous structure to adsorb aromatic compounds such as toluene and xylene at various concentrations (30, 50, 100 ppm). The configuration and chemical composition of the adsorbents were characterized using scanning electron microscopy (SEM), N₂ adsorption-desorption isotherm measurements, and X-ray photoelectron spectroscopy (XPS). The adsorption test was carried out using a polypropylene filter, which contained the adsorbents (0.25 g), with analysis performed using a gas detector. Compared to graphite oxide (GO) powder, the specific surface area of thermally expanded graphene powder (TEGP800) increased significantly, to 542 m² g⁻¹, and its chemical properties transformed from polar to non-polar. Thermally expanded graphene powder exhibits high adsorption efficiency for toluene (92.7–98.3%) and xylene (96.7–98%) and its reusability is remarkable, being at least 91%.

Microbial Targeted Degradation Pretreatment: A Novel Approach to Preparation of Activated Carbon with Specific Hierarchical Porous Structures, High Surface Areas, and Satisfactory Toluene Adsorption Performance

Hierarchical porous carbon shows great potential for volatile organic compounds (VOCs) removal due to its high surface area and abundant porous framework. However, current fabrication protocols are complex and cause secondary pollution, limiting their application. Here, as a novel strategy, microbial lignocellulose decomposition as a pretreatment was introduced to fabricate hierarchical porous carbon (M-AC) from crude biomass substrate. The M-AC samples had high specific surface areas (maximum: 2290 m²·g^-1) and surfaces characterized by needle-like protrusions with a high degree of disorder attributed to hierarchical porous structures. Dynamic toluene adsorption indicated that the carbon materials with microbial pretreatment had much better adsorption performances (maximum: 446 mg/g) than activated carbon without pretreatment. The M-AC material pretreated with a cellulose-degrading microbe showed the best adsorption capacity due to well-developed micropores, whereas the M-AC material pretreated with a lignin-degrading microbe showed excellent transport diffusion due to well-developed mesopores. Therefore, this simple and effective approach using microbial decomposition pretreatment is promising for the development of hierarchical porous carbons with adjustable pore structures and high specific surface areas to remove target VOCs in practical applications.

One-step pyrolysis of lignin and polyvinyl chloride for synthesis of porous carbon and its application for toluene sorption

The aim of this work aims to synthesize the high performance porous carbons from lignin and rice husk (RH) via one-step pyrolysis with KOH and PVC. And those porous carbons were evaluated for toluene sorption. The unactivated biochars had ultralow specific surface area (S_BET). By co-pyrolysis of biomass with KOH, the S_BET of bio-carbon was significantly improved. The addition of PVC can further enhance the S_BET. Additionally, the S_BET of lignin-carbons was higher than that of RH-carbons. It was attributed to the destruction of pore structures in the RH chars with high salts by the washing process. Compared with the RH carbons, the lignin carbons had a higher micro-porosity. As for toluene sorption, the maximum breakthrough time (2195 min) and sorption capacity (263.4 mg g^-1) were achieved using LPKC. The textural properties (e.g., pore volume, micro-porosity, and hierarchically porous structure) can take significant effects on the sorption process.

Synthesis of high-performance hierarchically porous carbons from rice husk for sorption of phenol in the gas phase

Phenol as a semi-volatile organic compound (SVOC) extensively presents in industrial wastewater. Moreover, it is a main compound of tar existing in the vapor phase from biomass pyrolysis or gasification. So far, most of works on the phenol adsorption by activated carbons have been conducted in the liquid phase. However, the adsorption of phenol in the gas phase has not been reported. This work aims to synthesize the hierarchically porous carbons from the unaltered and pelletized rice husk (RH) via a facile pyrolysis followed by the ball-milling-assisted KOH activation. Herein, the silica nanoparticles in RH acted as a self-template to remarkably increase specific surface areas and pores, thereby giving rise to the formation of hierarchically porous carbons, which showed a relatively high adsorption capacity (maximum value: 1919 mg/g) of phenol in the vapor phase. Generally, the process of phenol adsorption onto porous carbons in the gas phase followed with various interactions, including pore filling, electrostatic interaction, hydrophobic effect, and functional groups effect (e.g., π-π interaction). And the pseudo-second-order model could well describe the adsorption kinetic. It is noted that the pelletized RH was more favorable to develop the porous carbons with the hierarchically meso-microporous structures that could enhance the transfer of the phenol molecules via the outer layer and subsequent uptake by the adsorption sites on the inner layer. Further, the SVOC phenol was hard to volatilize under ambient conditions due to its relatively higher boiling point (181.7 °C), so the thermal desorption was a potential way to regenerate the spent activated biochars.

Facile synthesis of porous carbons from silica-rich rice husk char for volatile organic compounds (VOCs) sorption

This work reported a facile synthesis of porous carbons from the silica-rich rice husk biochar via a ball-milling-assisted KOH activation for sorption of tar compounds and volatile organic compounds (VOCs) (i.e., toluene, phenol). The textural properties of activated biochars can be greatly influenced by the mass ratio of KOH and biochar. The high-performance biochar with a large specific surface area (S_BET: 1818 m²/g) was produced as the mass ratio was 3. This activated biochar exhibited a hierarchically meso-microporous structure, which benefited for the adsorption process. Particularly, it had long breakthrough time of 2784 min and high adsorption capacity of 264 mg/g for toluene, while it had short breakthrough time of 724 min and low adsorption capacity of 6.53 mg/g for phenol. Significantly, the mixed VOCs of toluene and phenol can be effectively adsorbed. Further, thermal desorption will be an alternative route for regeneration of waste activated biochar.

The adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review

In recent years, with the continuous development of industry and agriculture, the content of organic pollutants in the environment has been increasing, which has caused serious pollution to the environment. Adsorption has proven to be an effective and economically viable method of removing organic contaminants. Since biochar has many advantages such as various types of raw materials, low cost, and recyclability, it can achieve the effect of turning waste into treasure when used for environmental treatment. This paper summarizes the source and production of biochar, points out its research status in the removal of organic pollutants, expounds its adsorption mechanism on organic pollutants, introduces the relevant adsorption parameters, summarizes its regeneration methods, studies its application of engineering, and finally analyses of benefits and describes the development prospects.

Ball-milled biochar for galaxolide removal: Sorption performance and governing mechanisms

The environmental risk of galaxolide (HHCB) spurs the need to develop efficient and economical removal technology. Although sorption is one of the best removal approaches, studies on sorption of HHCB by biochar were limited. With the purpose of combining the advantages of ball-milling and sorption technologies, six ball-milled biochars (BM-biochars) varied with biomasses and pyrolysis temperature were produced, characterized, and tested for HHCB removal from aqueous solution. At an initial HHCB concentration of 2 mg L^-1, the unmilled and BM-biochars adsorbed 330-746 and 609-2098 mg kg^-1 of HHCB, respectively. The increase in sorption capacities (about 3-fold increase) was mainly ascribed to the increase in BM-biochar's external and internal surface area, pore volume and pore size, and the exposure of the graphitic structure. The removal of HHCB by the BM-biochars increased with increasing pyrolysis temperature. For lower temperature biochar (300 °C wheat straw biochar, WS300), hydrophobic partitioning played a major role in HHCB sorption onto unmilled biochar (log K_oc/log K_ow value of WS300 was 0.772 at a C_e of 1 mg L^-1). Ball milling reduced the hydrophobicity of 300 °C biochar, which diminished the HHCB sorption. However, increased surface area, pore volume, pore size, and graphitic structure provided additional sorption sites, resulting in enhanced HHCB uptake (log K_oc/log K_ow value of BMWS300 was 1.23 at a C_e of 1 mg L^-1). For higher temperature biochars (500 and 700 °C), ball milling mainly enhanced HHCB sorption onto high temperature biochars via surface adsorption, π-π interaction, and pore filling. For WS500, 77.9% of HHCB removal was due to surface adsorption. Ball milling increased this percentage to 96.7% for BMWS500. This work highlighted the potential of ball milling as an excellent engineering method to improve biochar's sorption properties.

Biochar as low-cost sorbent of volatile fuel organic compounds: potential application to water remediation

Pyrolysis of waste materials to produce biochar is an excellent and suitable alternative supporting a circular bio-based economy. One of the properties attributed to biochar is the capacity for sorbing organic contaminants, which is determined by its composition and physicochemical characteristics. In this study, the capacity of waste-derived biochar to retain volatile fuel organic compounds (benzene, toluene, ethylbenzene and xylene (BTEX) and fuel oxygenates (FO)) from artificially contaminated water was assessed using batch-based sorption experiments. Additionally, the sorption isotherms were established. The results showed significant differences between BTEX and FO sorption on biochar, being the most hydrophobic and non-polar contaminants those showing the highest retention. Furthermore, the sorption process reflected a multilayer behaviour and a relatively high sorption capacity of the biochar materials. Langmuir and Freundlich models were adequate to describe the experimental results and to detect general differences in the sorption behaviour of volatile fuel organic compounds. It was also observed that the feedstock material and biochar pyrolysis conditions had a significant influence in the sorption process. The highest sorption capacity was found in biochars produced at high temperature (> 400 °C) and thus rich in aromatic C, such as eucalyptus and corn cob biochars. Overall, waste-derived biochar offers a viable alternative to be used in the remediation of volatile fuel organic compounds from water due to its high sorption capacity.

Chemically activated hydrochar as an effective adsorbent for volatile organic compounds (VOCs)

Hydrochars derived from hickory wood and peanut hull through hydrothermal carbonization were activated with H₃PO₄ and KOH to improve their performance as a volatile organic compound (VOC) adsorbent. Polar acetone and nonpolar cyclohexane were used as representative VOCs. The VOC adsorptive capacities of the activated hydrochars (50.57-159.66 mg⋅g^-1) were greater than that of the nonactivated hydrochars (15.98-25.36 mg⋅g^-1), which was mainly caused by the enlargement of surface area. The significant linear correlation (R² = 0.984 on acetone, and R² = 0.869 on cyclohexane) between BET surface areas of hydrochars and their VOC adsorption capacities, together with the obvious adsorption exothermal peak of differential scanning calorimetry curve confirmed physical adsorption as the dominating mechanism. Finally, the reusability of activated hydrochar was tested on H₃PO₄ activated hickory hydrochar (HHP), which had higher acetone and cyclohexane adsorption capacities. After five continuous adsorption desorption cycles, the adsorptive capacities of acetone and cyclohexane on HHP decreased by 6.2% and 7.8%, respectively. The slight decline in adsorption capacity confirmed the reusability of activated hydrochar as a VOC sorbent.

The contribution of oxygen-containing functional groups to the gas-phase adsorption of volatile organic compounds with different polarities onto lignin-derived activated carbon fibers

Lignin-based activated carbon fibers (LCFK) were prepared by electrospinning method and evaluated in adsorption of volatile organic compounds (VOCs). Batch adsorption experiments for various component were carried out in a fixed-bed reactor. The molecular polarity of VOCs plays a pivotal role in the monocomponent dynamic adsorption. As a result, the adsorption capacity of toluene was larger than that of methanol or acetone. In the various multicomponent atmospheres (without water), the components interact with each other and competitive adsorption phenomenon occurs, resulting in the adsorption capacity of each component decreased significantly. Also, the samples before and after adsorption were characterized via Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Boehm titration. The results reveal that methanol and acetone, controlled by physical adsorption, prefer to be adsorbed on polar groups on the surface of LCFK through the dipole-dipole interactions (i.e., van der Waals' forces). Differently, the adsorption of toluene onto LCFK was controlled by physical and chemical processes, and the lactone groups have a positive contribution to the adsorption of toluene. It was also observed that water vapor can enhance the negative effect on the adsorption of VOCs, especially for toluene. The results from this study will be valuable for explaining the mechanisms of competitive adsorption among each component in the various multicomponent atmospheres and understanding the contribution of chemical functional groups on the surface of LCFK in the adsorption process.

Acetaldehyde removal and increased H2/CO gas yield from biomass gasification over metal-loaded Kraft lignin char catalyst

Acetaldehyde removal tests were performed to compare the catalytic activity of the Kraft lignin char (KC), KOH-treated Kraft lignin char (KKC), and activated carbon (AC) along with their impregnation with Mn in a plasma reactor. The gasification characteristics (syngas content, and H₂/CO ratio) of yellow poplar were investigated using nickel catalysts supported on KC, KKC, AC, and γ-Al₂O₃ in a U-type quartz reactor. KKC and Mn/KKC improved significantly the surface area and contents of O and N functional groups over the raw char. In particular, Mn/KKC showed the highest acetaldehyde-removal efficiency. The catalytic activity of Ni-impregnated KC, KKC, AC, and γ-Al₂O₃ decreased in the order of Ni/KKC > Ni/AC > Ni/KC > Ni/γ-Al₂O₃ for the gas yield and Ni/γ-Al₂O₃ >Ni/KC > Ni/AC >Ni/KKC for the oil yield, respectively. The Ni/KKC provides a more conducive environment for gasification, resulting in larger amounts of syngas (H₂ and CO) in the product gases. Moreover, Ni impregnated with char may be the most inexpensive and effective solution for achieving maximum tar reduction and syngas generation.

Biochar amendment improves crop production in problem soils: A review

Problem soils are referred to as those with poor physical, chemical, and biological properties that inhibit or prevent plant growth. These poor properties may be a result of soil formation processes but are largely due to inappropriate farming practices or anthropogenic pollution. The world has lost a third of its arable land due to erosion and pollution in the past 40 years. Thus, there is an urgent need for improving and remediating problem soils. As a novel multifunctional carbon material, biochar has been widely used as a soil amendment for improving soil quality. Previous reviews have summarized the characteristics of biochar, the interactions with various soil contaminants, and the effects on soil quality, soil productivity, and carbon sequestration. Relatively limited attention has been focused on the effects of biochar amendment on plant growth in problem soils. As a result, a comprehensive review of literature in the Web of Science was conducted with a focus on the effects of biochar amendment on plant growth in problems soils. The review is intended to present an overview about problem soils, biochars as functional materials for soil amendment, how amended biochars interact with soils, soil microbes, and plant roots in remediation of problem soil and improve plant growth. Additionally, existing knowledge gaps and future directions are discussed. Information gathered from this review suggests that biochar amendment is a viable way of improving the quality of problem soils and enhancing crop production. It is anticipated that further research on biochar amendment will increase our understanding on the interactions of biochar with components of problem soils, speed up our effort on soil remediation, and improve crop production in problem soils.

Dynamic adsorption of toluene on amino-functionalized SBA-15 type spherical mesoporous silica

Amino-functionalized spherical mesoporous silicas were successfully prepared via a convenient treatment method by using APTES, which was used for the adsorption treatment of toluene gas, showing obvious advantages.