Potential Application of Ferrate-Modified Commercial Biochar to Control Ammonia and Hydrogen Sulfide

Abstract

Biochar produced from natural organic waste has attracted considerable attention as a promising resource for odor control due to its porous structure. In this study, the potential application of commercially available biochar as an adsorbent for ammonia (NH₃) and hydrogen sulfide (H₂S) removal was investigated. Among the tested samples, HT800 biochar pyrolyzed at 800 °C exhibited the highest adsorption capacity, showing superior performance compared to conventional activated carbon. Surface modification produced contrasting effects on the adsorption efficiencies of activated carbon and biochar, with ferrate-modified biochar demonstrating the highest adsorption performance. Fourier transform infrared spectroscopy (FT-IR) analysis confirmed that ferrate modification significantly enhanced Fe–O functional groups on the biochar surface. These findings suggest that the improved ammonia removal performance may be attributed not only to acid–base interactions associated with surface functional groups, but also to the direct oxidation of odor-causing compounds by iron oxide species.

1. Introduction

As demand for waste-derived organic resources is continuously growing, biogas plants using biological digestion of livestock manure are also increasing [1,2,3]. The residual manure discharged after biogas production is either converted into liquid fertilizer or treated through wastewater purification processes. However, during this process, incomplete degradation of residual organic matter—such as proteins, fermentable carbohydrates, and lipids—can occur, which serves as a primary source of odor emissions [4,5]. In animal manure, proteins act as precursors for odor-causing compounds such as sulfides, indoles, and phenolic compounds, while volatile fatty acids, ammonia, and volatile amines are also major contributors to malodor [6].

In particular, the leachate generated from biogas plants poses significant odor issues during the liquefaction process, as it contains high concentrations of residual nitrogenous and sulfur compounds. Ammonia (NH₃) and hydrogen sulfide (H₂S) are the dominant odorants emitted from the leachate storage and application stages, and their effective control remains a major environmental challenge [7,8,9].

Activated carbon, a widely used adsorbent, has demonstrated high adsorption efficiency for various odorous compounds, including ammonia and hydrogen sulfide [10]. Nevertheless, with growing concern over climate change and increasing emphasis on sustainable resource utilization, research has steadily expanded into developing eco-friendly odor adsorbents derived from naturally occurring waste materials such as waste wood, fruit peels, and herbaceous biomass [11,12,13].

Biochar generally exhibits a porous structure formed during pyrolysis, enabling its use as an adsorbent [14]. It has been shown to be effective in reducing emissions of volatile substances such as ammonia and methane and can be used to reduce greenhouse gases by absorbing carbon dioxide [15]. However, because of its lower specific surface area [16], its adsorption capacity for odorants is relatively lower than that of activated carbon. To address this limitation, efforts have been made to enhance the adsorptive performance of biochar through modification [17].

Modification, such as acid treatment with sulfuric acid or phosphoric acid or treatment with iron salts, increases the surface area of the biochar [18], thereby improving its adsorption capacity compared to the biochar before modification [19,20]. However, because the biochar resources vary from wood waste to livestock manure and pyrolysis temperature for biochar production significantly influences its surface area, porosity, pH, cation exchange capacity, carbon yield, and carbon stability [21,22], the adsorption performance of biochar can vary. In particular, the specific surface area and functional groups associated with the adsorption of odorous compounds in biochar vary widely depending on the feedstock and pyrolysis conditions. This variability makes it difficult to define the adsorption characteristics of biochar in a consistent manner and, consequently, to assess its potential as a substitute for activated carbon.

Therefore, this study aimed to evaluate the odor adsorption capacity of commercially available biochar and to compare the obtained results with those reported in previous studies in order to identify generalizable adsorption characteristics of biochar as an adsorbent. In addition, the effects of surface modification of commercial biochar on the adsorption of ammonia and hydrogen sulfide were investigated to determine whether changes in surface characteristics and functional group properties could enhance its odor adsorption performance. This assessment is essential for advancing the practical application and commercialization of biochar as an adsorbent.

2. Materials and Methods

2.1. Biochar and Activated Carbon

As a control adsorbent, 2~3 mm sized coal-based activated carbon was purchased from commercial seller (Shinkwang Chemical Industry Co., Ltd., Gyeonggi-do, Republic of Korea). Five commercial biochar products which were pyrolyzed at different temperatures with various source materials in Republic of Korea were selected: pyrolyzed biochar at 325 °C with oak tree (LT325, Korea Bichar Agricultural Corporation Co., Ltd., Jeonbuk Special Autonomous Province, Republic of Korea), pyrolyzed biochar at 700 °C with wood (HT700, Sanglim Agricultural Corporation Co., Ltd., Jeonbuk, Republic of Korea), at 800 °C with wood pellets (HT800, K-Agro Co., Ltd., Daegu Metropolitan City, Republic of Korea), and at 350 °C and 500 °C with forest byproduct (LT350 and MT500, Gyeongdong Development Co., Ltd., Chungcheongbuk-do, Republic of Korea). Characteristics of the biochar and activated carbon are summarized in

Table 1. Characteristics of used biochar and activated carbon.

Adsorbent	Sample Name	Source Material	Pyrolysis Temperature (°C)	Shape	Surface Area (m2/g)	Pore Size (nm)
Activated carbon	AC800	coal	800	2~3 mm granules	1006	2.02
Biochar	LT325	oak	325	Fragment	12.7	5.25
	HT700	wood	700	Coarse granules	0.22	4.78
	LT350	forest byproducts	350	Fragment	0.382	4.17
	MT500	forest byproducts	500	Small size fragment	205.7	2.10
	HT800	Wood pellets	800	Coarse granules	135.9	2.64

.

2.2. Odor Adsorption by Commercial Biochar

To compare the effect of pyrolysis temperature on odor adsorption capacity, a complex odor test was conducted with five collected commercial biochars. As an odor source, pig manure sludge was collected from a pig manure treatment plant (Icheon-si, Gyeonggi-do, Republic of Korea) and stored in a dark refrigerator at 4 °C during the whole test period. A total of 0.3 L/min of ambient air was blown into 300 mL of sludge contained in 1 L glassware to generate odorous emission and was passed through the column filled with 500–1000 mL of adsorbent and through the blank column to measure initial odor level.

In the preliminary investigation of odor generation, odorants produced from pig manure sludge were classified and quantified as ammonia, hydrogen sulfide, and volatile organic compounds (VOCs). However, due to the low concentration of VOCs, only ammonia and hydrogen sulfide were measured in this experiment. Real-time measurements with GC based analysis and an oxidative electro sensor were conducted at 1 min intervals at both the inlet and outlet of the adsorbent reactor using a portable gas detector (Model: Sky2000-M3, SafeGas, Shenzhen, China). The tests were terminated when the concentration at the outlet surpassed the initial levels.

2.3. Surface Modification of Adsorbents

For surface modification, low-temperature biochar (LT325) and high-temperature biochar (HT700), which showed relatively high efficiency in odor adsorption experiments (HT800 was excluded as no breakthrough was observed), were selected and modified, and the adsorption performance after modification was compared with that of activated carbon (AC800).

A 5% phosphoric acid and a 5% sulfuric acid solution were prepared by diluting reagent-grade solutions purchased from Sigma-Aldrich (St. Louis, MO, USA) (phosphoric acid, CAS number: 7664-38-2, purity 85% and sulfuric acid, CAS number: 7664-93-9, purity 95%). The ferrate solution was prepared by the wet chemical method, in which ferric chloride (CAS No.: 7705-08-0, purity 58%, Daemyung Chemical Co., Ltd., Gyeonggi-do, Republic of Korea) was oxidized with hypochlorous acid (CAS No.: 7681-52-9, purity 12–15%, Samchun Pure Chemical Co., Ltd., Gyeonggi-do, Republic of Korea) under alkaline conditions.

For surface modification, the ferrate solution was mixed with the adsorbent at a volume ratio of 1:1 and immersed for 3.5 h, followed by filtration and air drying.

For sulfuric acid and phosphoric acid impregnation, the adsorbent was immersed in same volume of the prepared 5% (w/w) sulfuric acid solution and 5% (w/w) phosphoric acid solution for 1 h and filtered and then air dried, respectively.

2.4. NH₃ and H₂S Adsorption Test with Modified Biochars

The adsorption performance of the modified biochar was evaluated in a fixed-bed column system using a gas mixture prepared from NH₃ (1000 ppm), H₂S (100 ppm), and O₂ (100%) standard gases controlled by mass flow controllers (MFCs). To simulate ambient atmospheric conditions, the oxygen concentration in the gas stream was adjusted to 21% by regulating the flow rates of the individual gases. The resulting inlet concentrations of NH₃ and H₂S were measured as 445 and 40 ppm, respectively. The gas mixture was passed through a column packed with 15 g of adsorbent, with a bed height of 40–60 mm, at a total flow rate of 0.4 L/min. The concentrations of NH₃ and H₂S in the effluent gas were continuously monitored at the column outlet using a portable gas detector (POLI MP400, mPower, Shanghai, China). Concentration measurements were recorded at 1 min intervals, and the breakthrough point was defined as the time at which the outlet concentration reached 10% of the inlet concentration (C/C₀ = 0.1).

2.5. Analysis of Adsorbent Characteristics

The pore characteristics of the adsorbents were determined from N₂ adsorption–desorption isotherms measured at 77 K using an Autosorb iQ analyzer (Quantachrome Instruments, Boca Raton, FL, USA). Prior to analysis, the samples were degassed to remove moisture and impurities by heating from room temperature to 100 °C at a rate of 5 °C/min, followed by degassing at 100 °C for 240 min. The specific surface area was calculated using the multipoint Brunauer–Emmett–Teller (BET) method, while the pore volume and average pore diameter were obtained from the N₂ adsorption–desorption data.

The surface functional groups were analyzed by Fourier transform infrared spectroscopy (FT-IR; INVENIO, Bruker, Bremen, Germany). For FT-IR analysis, samples were mixed with KBr at a mass ratio of 1:100, ground, and compressed into pellets using a hydraulic press. The spectra were recorded over the range of 8000–350 cm⁻¹.

The surface morphology of the adsorbents was observed using scanning electron microscopy (SEM; JSM-IT800SHL, JEOL, Tokyo, Japan) at an accelerating voltage of 5–10 kV and magnifications ranging from 500× to 5000×. Prior to SEM observation, the samples were coated with a 7 nm-thick platinum layer using an ion sputter coater at a current of 20 mA. The elemental composition and distribution on the surface of the modified biochar were analyzed by energy-dispersive X-ray spectroscopy (EDS) combined with elemental mapping.

3. Results

3.1. Odor Adsorption of Biochar and Effect of Pyrolysis Temperature

In the removal of malodorous compounds generated from pig manure, ammonia adsorption by the five biochars was generally low. The lowest adsorption capacity was observed for LT350 (0.08 mg/g), while the other biochars—excluding HT800—showed similar capacities in the range of 0.13–0.17 mg/g. These values were markedly lower than those of activated carbon (AC800), which exhibited an adsorption capacity of 116 mg/g. Notably, HT800 demonstrated the highest adsorption performance, with no breakthrough observed over 96 h.

Biochar produced from various feedstocks under different pyrolysis temperatures exhibits significant variations in structural characteristics and surface chemical properties. In general, specific surface area, pore volume, and pore size are widely recognized as key parameters governing adsorption performance. To investigate the effects of pore size and pore volume developed during pyrolysis as a function of temperature and feedstock type, previously reported data were collected and comparatively analyzed (

Table 2. Physiochemical characteristics and adsorption capacity of nonmodified biochar.

Pyrolysis Temp (°C)	Catagory	Raw Material	Surface Area (m2/g)	Total Pore Volume (cm3/g)	Pore Size (nm)	Odor Compound	Adsorption Capa (mg/g)	Reference
250	Herbal	corn stover	3.083	0.011	13.838	tri-phosphate	0.8837	[23]
300	Fruits	apple	14.4	0.015	4.323	n.a.	n.a.	[24]
300	Woody	date palm	2.040	0.0047	n.a.	n.a.	n.a.	[25]
325	Woody	oak tree	0.22	0.0003	5.49	manure odor	0.13 (NH3)	This study
350	Woody	forest byproduct	0.382	0.0004	4.17	manure odor	0.08 (NH3)	This study
350	Herbal	corn stover	14.284	0.020	5.688	tri-phosphate	1.0147	[23]
400	Woody	Pinewood	14.2	n.a.	3.64	n.a.	n.a.	[26]
400	Woody	oak	11.5	n.a.	3.24	n.a.	n.a.	[26]
400	Woody	date palm	5.535	0.0055	n.a.	n.a.	n.a.	[25]
400	Herbal	Rice straw	10.6	0.03	12.8	SO2	11.1	[27]
500	Herbal	parm mesocarp fiber	19.8	0032	6.5	formaldehyde	n.a.	[28]
500	Woody	Pinewood	25.5	n.a.	4.52	n.a.	n.a.	[26]
500	Woody	oak	28.4	n.a.	5.32	n.a.	n.a.	[26]
500	Woody	date palm	123.625	0.0209	n.a.	n.a.	n.a.	[25]
500	Herbal	corn stover	215.937	0.135	2.492	tri-phosphate	1.1567	[23]
500	Woody	forest byproduct	205.7	0.1078	2.1	manure odor	0.17 (NH3)	This study
500	Fruits	Leftover rice	2.76	n.a.	n.a.	H2S	12.11	[29]
500	Organic waste	Cow dung	7.01	n.a.	n.a.	H2S	29.81	[30]
500	Fruits	Coconut husk	0.18	n.a.	n.a.	H2S	30.44	[30]
500	Woody	Maple wood	161	n.a.	n.a.	H2S	6.1	[31]
600	Herbal	parm mesocarp fiber	8.59	0.013	5.9	formaldehyde	n.a.	[28]
600	Woody	Pinewood	142.7	n.a.	6.35	n.a.	n.a.	[26]
600	Woody	oak	138.4	n.a.	6.04	n.a.	n.a.	[26]
600	Woody	date palm	221.230	0.0317	n.a.	n.a.	n.a.	[25]
600	Organic waste	Food waste digestate	123.41	0.0938	4.0492	H2S	12.5	[32]
700	Herbal	parm mesocarp fiber	6.89	0.013	7.1	formaldehyde	n.a.	[28]
700	Fruits	apple	314.7	0.138	1.299	n.a.	n.a.	[24]
700	Woody	Pinewood	167.4	n.a.	6.87	n.a.	n.a.	[26]
700	Woody	oak	154.2	n.a.	6.49	n.a.	n.a.	[26]
700	Woody	Saw dust	524	0.45	n.a.	H2S	0.24	[33]
700	Woody	date palm	249.130	0.0308	n.a.	n.a.	n.a.	[25]
700	Herbal	corn stover	435.573	0.258	2.368	tri-phosphate	2.2574	[23]
700	Woody	lumber	12.67	0.0153	4.78	manure odor	0.14 (NH3)	This study
725	Woody	Wood logs	385	0.2002	n.a.	H2S	18	[34]
800	Herbal	parm mesocarp fiber	5.48	0.01	7.1	formaldehyde	n.a.	[28]
800	Woody	Pinewood	154.4	n.a.	6.24	n.a.	n.a.	[26]
800	Woody	oak	152.4	n.a.	6.05	n.a.	n.a.	[26]
800	Woody	wood pellet	135.9	0.0898	2.64	manure odor	n.d.	This study
838	Woody	Wood logs	517	0.2888	n.a.	H2S	17.8	[34]

n.a.: not available, n.d.: not detected.

).

In this study, MT500, derived from forest byproducts, exhibited a larger specific surface area and an adsorption capacity approximately twice that of LT350, indicating that the increased surface area developed at higher pyrolysis temperatures positively influences adsorption performance. However, among the wood-based biochars, HT800—produced at the highest pyrolysis temperature—demonstrated the greatest adsorption efficiency. Previous studies have reported that date palm biochar shows a marked increase in specific surface area at 500 °C [25], whereas pine biochar exhibits a rapid increase at 600 °C followed by stabilization at 700 °C, suggesting that the optimal pyrolysis temperature varies depending on the feedstock [26].

At relatively low pyrolysis temperatures (<350 °C), herbaceous biomass exhibited higher specific surface areas than woody biomass. At intermediate temperatures (400–600 °C), the specific surface area varied depending on the feedstock; however, in the case of corn stover, the specific surface area increased by approximately 15-fold from that at 350 °C, reaching 215.9 m²/g at 500 °C [23]. At higher temperatures (>700 °C), substantially greater specific surface areas were observed compared to those at lower temperatures, regardless of the feedstock type.

However, even if pyrolyzed at similar temperatures or the same raw materials are used, an increase in BET surface area does not lead to a proportional increase in odor gas adsorption capacity. This phenomenon was also observed in this study. When comparing adsorption capacities at different pyrolysis temperatures based on ammonia adsorption capacity, oak biochar pyrolyzed at 325 °C exhibited lower surface area (0.22 m²/g) than forest-byproduct biochar pyrolyzed at 350 °C (0.382 m²/g), yet showed a higher adsorption capacity (0.13 vs. 0.08 mg/g). Lumber biochar produced at 700 °C showed a surface area of 12.67 m²/g (≈58-fold higher than oak biochar), but its adsorption capacity (0.14 mg/g) remained similar. For forest-residue biochar, increasing the temperature from 350 to 500 °C increased the surface area from 0.382 to 205.7 m²/g (≈540-fold), whereas the adsorption capacity increased only from 0.08 to 0.17 mg/g (≈2.1-fold). These results indicate that surface structure such as surface area, pore volume and size is not directly correlated with adsorption capacity.

3.2. Adsorption by Surface-Modified Biochar

Biochar surface modification has been extensively reported as an effective strategy to enhance adsorption performance. In particular, acid functionalization using sulfuric acid or phosphoric acid has been widely employed to improve the removal of basic malodorants. In this study, biochar was modified via an impregnation approach using ferrate—an oxidant with high oxidative potential—in addition to conventional sulfuric and phosphoric acid treatments. Previous studies reported that sulfuric acid treatment can prolong ammonia adsorption time, although high impregnation concentrations can reduce surface area due to pore collapse or blockage [35]. Other activation strategies, such as phosphoric acid combined with oxidants, have been shown to increase adsorption capacity and, in some cases, to increase surface area proportionally with impregnation concentration [17,33].

In this study, activated carbon modified with 5% sulfuric acid and 5% phosphoric acid exhibited enhanced ammonia adsorption efficiency, with no breakthrough observed during the experimental period (Figure 1a). In contrast, for biochar, breakthrough occurred within 1 min for both LT325 and HT700, before modification as well as after phosphoric acid and sulfuric acid modification, indicating that no significant improvement in adsorption performance could be achieved (Figure 1b,c).

The breakthrough curves were fitted to the Yoon–Nelson model using nonlinear regression analysis. Among the tested adsorbents, AC800, Fr-AC800, Fr-HT700, and Fr-LT325 exhibited breakthrough profiles suitable for model fitting. The remaining samples were excluded from model application because breakthrough was either not observed within the experimental monitoring period or occurred too rapidly to obtain reliable fitting results.

For the samples to which the model was successfully applied, the predicted time to 50% breakthrough, τ, increased slightly from 68.5 min for AC800 (R² = 0.97, SSE = 0.0008) to 71.0 min for Fr-AC800 (R² = 0.94, SSE = 0.017). In addition, ferrate modification of HT700 and LT325 increased the predicted 50% breakthrough time by 28.0 min (R² = 0.95, SSE = 0.0005) and 11.8 min (R² = 0.81, SSE = 0.001), respectively. These results suggest that ferrate modification may improve the adsorption performance of selected biochar adsorbents, although it has limitations in value credibility because of extrapolation in model prediction.

A similar trend was observed for ferrate modification: the adsorption efficiency of activated carbon decreased, whereas that of biochar increased. These results suggest that the surface modifications induce different, and in some cases opposite, effects on adsorption behavior in biochar compared to activated carbon.

For hydrogen sulfide removal, activated carbon exhibited markedly superior adsorption performance compared to biochar. Specifically, activated carbon successfully removed all incoming hydrogen sulfide throughout the entire observation period, regardless of its modification status, without any breakthrough occurring. In contrast, biochar demonstrated minimal adsorption capacity in its original, sulfuric acid-modified, and phosphoric acid-modified forms, with breakthrough observed immediately after start of the experiment, preventing the estimation of adsorption capacity. Although ferrate modification resulted in a slight improvement in H₂S adsorption efficiency, breakthrough was still observed within the first 5 min in all cases. This behavior is consistent with previous findings on CO₂-activated biochars, in which CO₂ activation did not always enhance H₂S adsorption and, in some cases, even decreased the adsorption capacity [30]. These observations indicate that surface activation or modification of biochar does not universally improve adsorption performance for all odorants but may exert compound-specific effects depending on the properties and adsorption mechanisms of individual odor compounds.

As summarized in

Table 3. Surface physical characteristics and adsorption capacity of tested adsorbents.

Adsorbents	Surface Area (m2/g)		Pore Volume (cc/g)			Pore Size (nm)	Adsorption Capacity (mg/g)
	Multi-Point	Micropore	Total	Meso	Micro	Average	-
AC800	1006	857.6	0.5075	0.0879	0.3563	2.02	0.504
Fr-AC800	706	570.2	0.3761	0.0901	0.2364	2.13	0.322
SA-AC800	821.2	685.4	0.4435	0.1109	0.2842	2.16	1.036
PA-AC800	866.5	724.7	0.4410	0.0868	0.3002	2.04	1.135
HT700	12.7	12.1	0.0153	0.0105	0.0051	4.78	0.016
Fr-HT700	13.76	-	0.0376	0.0359	0.0010	10.94	0.237
SA-HT700	13.29	-	0.0335	0.0299	0.0005	10.07	-
PA-HT700	10.31	-	0.0521	0.0503	0.0002	20.2	-
LT325	0.22	-	0.0110	0.0104	0.0002	5.25	0.006
Fr-LT325	3.88	-	0.0294	0.0278	0.0005	2.86	0.124
SA-LT325	7.90	-	0.0225	0.0190	0.0002	9.62	-
PA-LT325	6.93	-	0.0205	0.0167	0.0001	9.65	-

, the observed trend may be partly attributed to reductions in BET surface area and total pore volume, particularly the loss of micropore volume, following ferrate impregnation, which likely contributed to the decreased adsorption capacity of activated carbon. However, because sulfuric acid and phosphoric acid impregnation also reduced surface area and pore volume while concurrently enhancing adsorption capacity, the overall correlation between textural properties and adsorption performance appears weak. This discrepancy may be associated with a shift in pore-size distribution, wherein micropore volume decreased whereas mesopore volume increased, thereby lowering the relative micropore fraction.

For biochar, surface modification generally increased specific surface area and pore volume, yet adsorption performance did not improve except in the case of ferrate modification. The differential effects of ferrate modification between Fr-LT325 and Fr-HT700 can be attributed to the combined influence of specific surface area, pore structure, and surface functional groups. In the case of Fr-LT325, ferrate modification resulted in an approximately 17-fold increase in specific surface area and a ~20-fold enhancement in adsorption capacity compared to the unmodified sample. This improvement in adsorption performance is supported by the distinct Fe–O functional group features observed in the FTIR spectra, indicating successful incorporation of iron-related functionalities.

However, the specific surface area of Fr-LT325 remained approximately one-third that of Fr-HT700, leading to an overall adsorption capacity that was about half that of the high-temperature biochar. In contrast, for high-temperature biochar, although the increase in specific surface area was limited, the adsorption capacity improved by approximately 15-fold. These results suggest that enhancing the adsorption capacity of biochar requires consideration not only of physical adsorption governed by pore structure but also of chemical adsorption associated with surface functional groups which are involved in surface chemistry and the introduction of Fe-containing surface species that can provide additional reactive sites (particularly for sulfur-containing gases) [31].

3.3. Surface Chemical Characteristics Induced by Modification

EDS analysis confirmed that impregnation increased the surface concentrations of sulfur, phosphorus, and iron depending on the modifying agent, indicating successful surface modification (Figure 2). Phosphoric acid impregnation increased the P content to 0.45, 0.34, and 0.62 at% for AC800, HT700 biochar, and LT325 biochar, respectively. Sulfuric acid impregnation increased the S content to 1.05 at% for AC (approximately twofold higher than unmodified AC) and to 0.65 and 0.75 at% for HT700 and LT325 biochars. Ferrate impregnation increased Fe content to 3.6 at% for AC and to 0.35 and 0.85 at% for HT700 and LT325 biochars, respectively.

SEM images further showed that the modification route dictated the type of surface heterogeneity generated (Figure 3). Activated carbon exhibited limited morphological change after modification, whereas biochar showed pronounced structural alterations. Ferrate treatment produced a rougher and more heterogeneous surface with fine-scale deposits, whereas acid treatments predominantly created larger etched features and macropores. Although surface roughness was qualitatively assessed from SEM images, samples exhibiting greater surface heterogeneity tended to show better adsorption performance, suggesting that the distribution and accessibility of adsorption sites is as important as the total surface area.

The FT-IR spectra of biochar and activated carbon showed similar functional group characteristics, although different trends in peak changes were observed after modification (Figure 4). In activated carbon, the band near 3418 cm⁻¹, corresponding to O–H functional groups, exhibited increased transmittance after modification, implying a reduction in O–H groups relative to the raw material. In other regions, however, decreased transmittance was observed after modification, indicating an increase in C=C, C–H, and C–O related functionalities. These changes may be associated with the improved adsorption performance resulting from phosphoric acid or sulfuric acid treatment.

In biochar, the changes in the peaks around 675 and 615 cm⁻¹ following ferrate modification are likely attributable to Fe–O groups, indicating that Fe₂O₃ was successfully immobilized on the biochar surface [36,37]. As NH₃ is a basic gas, it can react with acidic oxygen-containing functional groups, including –COOH, phenolic –OH, and lactone-type groups, via Brønsted or Lewis acid–base interactions, forming surface-bound NH₄⁺ species and promoting chemisorption [19,38]. Therefore, the variations in the O–H, C–O, and C=O related bands after modification may indicate the introduction or increased exposure of oxygen-containing functional groups on the adsorbent surface.

However, although ferrate modification increased the number of surface functional groups and was accompanied by an improvement in adsorption capacity, the enhancement cannot be explained solely by changes in functional groups. In particular, the phosphoric acid- and sulfuric acid-modified samples also showed changes in functional groups, but these did not lead to a corresponding increase in adsorption efficiency. Therefore, the improved removal of malodorous substances after ferrate modification is more likely attributable to the oxidative activity of ferrate species bound to the adsorbent surface than to adsorption by the adsorbent itself.

4. Discussion

4.1. Effect of Pyrolysis Temperature on Textural Parameters of Biochar

The adsorption of malodorous substances is greatly affected by the specific surface area, pore volume, and pore size of the adsorbent, and micro-sized pores are known to contribute to increasing adsorption efficiency. Therefore, activated carbon that matches these characteristics is utilized to remove malodorous substances from various sources. In terms of organic resource recycling, biochar produced through pyrolysis of wood or natural byproducts is being studied as a potential substitute for activated carbon. To increase its usability, various methods such as surface modification to increase the specific surface area during biochar production are also being studied. In particular, it has been reported that a higher pyrolysis temperature has a positive effect on pore formation, and therefore a high pyrolysis temperature is preferred in the production of biochar.

However, the pyrolysis temperatures of biochar reported in the literature range widely from 250 to 800 °C, and the feedstocks also vary, including herbaceous and woody biomass, food waste, and animal manure. Accordingly, the specific surface area varies significantly depending on the feedstock. At pyrolysis temperatures of 250–350 °C, the specific surface area ranges from 0.22 to 14.4 m²/g, while at 400–600 °C, it increases to a broader range of 0.18–221 m²/g. At higher temperatures (700–800 °C), an even wider distribution is observed, ranging from 5.48 to 517 m²/g.

Correlation analysis between pyrolysis temperature and specific surface area indicates that, for a given feedstock, the specific surface area generally increases with increasing pyrolysis temperature. However, the overall correlation remains weak when different feedstocks are considered together. This finding suggests that pore development and the formation of specific surface area are strongly affected by feedstock composition, particularly by the content and characteristics of volatile components released during pyrolysis. Accordingly, the thermal stability of adsorbents is also an important factor in evaluating their practical applicability, especially in terms of regeneration and repeated use. Previous studies have used thermogravimetric analysis (TGA) to examine the thermal stability of biochar-based adsorbents and to relate it to adsorption–desorption behavior or regenerable performance. For instance, the thermal stability and repeated adsorption/regeneration performance of magnetic tea biochar were evaluated [39], and the thermal stability of pristine and magnetic olive-stone biochar was assessed together with their adsorption–desorption performance over six cycles [40]. In the present study, however, regeneration and reuse experiments for the modified biochar were beyond the scope of the work; therefore, the lack of direct evaluation of re-generability and repeated-use performance is acknowledged as a limitation.

Biochar produced at higher thermal decomposition temperatures generally exhibits enhanced carbonization, resulting in a larger specific surface area compared to low-temperature biochar [41]. An increase in specific surface area is typically associated with improved adsorption capacity. However, as discussed previously, low-temperature biochar does not necessarily demonstrate inferior adsorption performance relative to high-temperature biochar. Notably, although the specific surface area of low-temperature biochar was approximately 1/57 that of high-temperature biochar, the difference in adsorption capacity was only about 1/2.7. This discrepancy indicates that adsorption behavior cannot be sufficiently explained solely by differences in pore volume or pore size. It can be inferred that the optimal pyrolysis temperature, which influences adsorption performance, is contingent upon the chemical composition inherent to the material, as the temperature also affects the formation of surface functional groups [42].

4.2. Influence of Surface Modification on Adsorption Capacity and Textural Properties

Surface modification of activated carbon with acidic agents, such as sulfuric acid and phosphoric acid, is widely recognized as an effective approach for enhancing ammonia removal performance, and a similar trend was observed in the present study. In contrast, in the case of biochar, modification with sulfuric acid or phosphoric acid resulted in a decrease in specific surface area and an increase in pore size, which consequently led to a decline in adsorption performance. Ferrate modification, however, improved the removal efficiency, suggesting that the enhanced performance may be attributed to the oxidation of malodorous compounds by iron oxides formed on the surface.

The contrasting responses of activated carbon and biochar indicate that identical impregnation chemistries can modify the dominant adsorption mechanisms in different ways, depending on the nature of the parent carbon matrix. Microporous activated carbon appears to be more vulnerable to performance deterioration when its micropores are blocked or constricted, whereas biochar may derive greater benefit from the introduction of reactive functional groups or metal-containing species than from moderate changes in surface area alone. This matrix-dependent behavior has been frequently reported in studies on biochar modification, suggesting that improvements in adsorption performance are governed not only by increases in surface area, but also by the engineering of surface functional groups.

Iron-based modification has been widely reported as an effective strategy for improving the adsorption performance of biochar toward various contaminants. For example, FeCl₃-modified biochar has been shown to enhance ammonium adsorption by increasing the abundance of oxygen-containing functional groups, such as carboxyl and hydroxyl groups, and by altering the surface properties of biochar [20]. In addition, Long et al. reported that metal modification changed the surface potential of biochar, and FeCl₃-modified biochar exhibited the highest positive surface charge due to the formation of Fe₂O₃ on the biochar surface, which enhanced adsorption performance [43]. These findings suggest that iron-based modification can influence adsorption not only through changes in pore structure but also through changes in surface functional groups and surface charge.

However, regarding H₂S removal, in contrast to the results of this study, there were also results from previous studies showing that iron impregnation of biochar increased the H₂S adsorption capacity by up to 3.9 times and that the conversion of Fe₃O₄ to FeSO₄ after H₂S adsorption indicated the involvement of reactive chemisorption [31]. Fe-based biochar composites have shown high gaseous NH₃ adsorption capacity, with the adsorption mechanism being attributed to both physical and chemical adsorption processes [44]. Ferrate-based modification has also been reported to generate iron oxides, oxygen-containing functional groups, and porous structures on biochar, thereby improving adsorption performance for target contaminants [45,46]. Therefore, the enhanced NH₃ adsorption observed in the present study can be interpreted as being associated with ferrate-induced changes in surface functional groups and Fe-related active sites, as supported by the FTIR results showing Fe–O-related bands after modification. However, the contribution of surface charge, including zeta potential, should be interpreted as literature-based evidence because zeta potential was not directly measured in this study.

4.3. Adsorption Enhancement by Iron-Oxide and Its Surface Chemistry

Although biochar modification generally increased BET surface area and pore volume, adsorption performance did not improve except after ferrate modification. This indicates that improved textural properties alone were not sufficient to enhance odor gas adsorption on biochar. Instead, chemical adsorption associated with surface functional groups likely plays a more explanatory role.

Fe-containing surface species may provide additional reactive sites, which could be particularly important for the adsorption of sulfur-containing gases. This interpretation is consistent with previous findings for FeOOH/activated carbon composites [47], as well as with broader studies showing that metal- or metal oxide-modified biochar can enhance adsorption performance by introducing new chemisorption pathways and redox-active sites [17,20]. FT-IR analysis further suggested that oxygen-containing functional groups, including O–H, C=O, and C–O, were introduced or exposed during the modification process. Notably, only ferrate modification, unlike conventional acid impregnation, improved the performance of biochar, implying that the incorporation of Fe-containing species together with their associated surface redox activity may play a critical role in the capture of odorous gases in this system.

Previous studies have shown that chemical modification of biochar can alter surface functional groups and surface charge properties, including zeta potential, thereby affecting adsorption behavior. Metal modification changed the surface potential of corncob biochar, and FeCl₃-modified biochar exhibited a highly positive surface charge due to the formation of Fe₂O₃ on the biochar surface [43]. Another research demonstrated that FeCl₃/HCl modification of wheat straw biochar increased ammonium adsorption capacity by at least 14%, mainly due to increases in –OH and O–C=O functional groups, specific surface area, and Fe³⁺/Fe²⁺ redox coupling [20]. Furthermore, if the biochar is negatively charged then it showed electrostatic attraction to NH₄⁺-N [48].

From previous findings, it is implied that ferrate impregnation may influence adsorption performance through changes in surface functional groups and surface charge properties to enhance Lewis acid–base interactions between NH₃ and Fe-associated or oxygen-containing surface sites.

5. Conclusions

This study evaluated the potential use of commercially available biochar as an adsorbent for controlling NH₃ and H₂S, which differentiates it from many previous studies that mainly focused on laboratory-prepared biochar. The main conclusions are as follows.

(1)

Among the activated carbon and five commercially available biochars evaluated, HT800 exhibited superior ammonia adsorption performance compared to activated carbon despite possessing only one-tenth of its specific surface area. This result demonstrates the high efficiency of HT800 for ammonia removal and highlights its potential as an effective alternative to activated carbon without requiring additional modification.

(2)

Acid and ferrate modifications did not significantly affect the specific surface area of the adsorbents; however, they exhibited contrasting trends in ammonia adsorption performance. The adsorption efficiency of activated carbon increased following acid modification, whereas biochar showed improved ammonia removal efficiency after ferrate modification. These findings suggest that the effectiveness of surface modification for enhancing adsorption performance may vary depending on the intrinsic properties of the adsorbent, indicating the need for further investigation into adsorbent-specific modification strategies.

(3)

FTIR analysis confirmed that ferrate modification significantly enhanced the formation of iron-based surface species and oxygen-containing functional groups on the biochar surface compared to activated carbon. These results suggest that the improved ammonia removal performance may not only be attributed to the roles of surface functional groups, such as carboxyl and hydroxyl groups introduced through surface modification, but also to the direct oxidative interactions of iron oxide species with ammonia during the deodorization process.

Various studies have identified specific surface area, pore structure characteristics, and surface modification as important factors for the application of biochar as an adsorbent. In this study, ferrate modification demonstrated promising potential for enhancing the adsorption performance of biochar, and commercially available biochar, in particular, showed the possibility of serving as a viable alternative adsorbent. However, to further expand the practical application of biochar, additional integrated studies considering regeneration performance, reusability, and long-term durability will be necessary.