Effects of UV Light Treatment on Functional Group and Its Adsorption Capacity of Biochar

Abstract

This study aimed to investigate the impact of UV treatment on the surface functionality and adsorption capacity of biochar, with the goal of enhancing its effectiveness as an adsorbent for toluene. The surface and near-surface functionality and structure of biochar were studied to evaluate the impact of UV treatment by utilizing X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) techniques. Biochar was generated by pyrolyzing wood chips at 900 °C without any oxidant injection in order to increase their carbon content. To boost biochar’s adsorption capability, UV irradiation on the biochar is utilized before and during the penetration process. Toluene was selected as the target absorbing material. The equilibrium adsorption capacity and rate were simulated using the Wheeler equation. It was found that the adsorption capacity of biochar increased significantly after pretreatment with ultraviolet light irradiation with a wavelength of 254 nm and an intensity of 280 μW/cm² and reached a saturated state after 15 h. SEM and XPS showed that the UV-biochar modification technology not only improved the pore structure of biochar, but also increased the content of -O-containing functional groups on the surface of biochar and improved the adsorption capacity of biochar. The experimental results for sample M50_Uu demonstrated significant improvement in adsorption performance. The adsorption saturation time increased by 80%, and the equilibrium adsorption capacity rose from 12.80 mg/g to 54.60 mg/g. The main reason for the adsorption capacity increase by UV treatment is functional group formation, of which rate linearly increases with pretreatment energy until 11 W·hr/g_biochar, after which the increase rate is slow.

1. Introduction

Biochar is produced by pyrolyzing or gasifying biomass raw materials, such as wood, crop waste, and plant waste under the conditions of anoxic and high temperature (200–900 °C) [1,2]. Biochars, such as charcoal, bamboo charcoal, straw charcoal, and rice husk charcoal, are widely used in various industries, such as water and air purification, microbial reactors, and agriculture. The chemical composition of biochar includes aromatic hydrocarbons and elemental carbon or graphite, which contains more than 60% carbon, as well as H, O, N, S, and trace elements [3,4].

Although activated carbon derived from biochar is commonly used as a pollution control agent, its production involves an energy-intensive activation process that emits pollution, making it environmentally unfriendly [5,6]. In contrast, biochar can be produced without activation, making it an environmentally friendly alternative to activated carbon. Biochar has several fundamental properties, including low water solubility, high carboxylation, aromatic structure, porosity, electrical conductivity, and specific surface area, resulting in high adsorption capacity, oxidation resistance, and decomposition resistance [7,8,9,10,11]. In addition, biochar contributes to long-term carbon sequestration in soil, making it a unique natural carbon sequestration method [12,13,14]. Therefore, biochar has the potential to replace activated carbon in various industries, such as pollution control, material production, and CO₂ storage.

Biochar is a remarkably versatile adsorbent that has proven effective in addressing a wide range of pollutants, including heavy metals, dyes, and organic compounds [15,16,17,18]. The adsorption performance of biochar is directly influenced by its inherent properties, such as surface area, porosity, and functional groups, which can be customized through various treatments, including pyrolysis, activation, and chemical modification [19,20]. The adsorption mechanisms of biochar are intricate, involving a combination of physical and chemical interactions, such as Van der Waals forces, hydrogen bonding, and π-π interactions [21,22,23]. Numerous factors, including pH, temperature, pollutant concentration, and the presence of co-pollutants, can significantly impact the efficiency of biochar as an adsorbent [24,25]. While biochar has displayed promising outcomes in the treatment of wastewater and contaminated soils, further research is necessary to optimize its performance and explore its potential applications in air pollution control [26,27,28].

It is important to note that the characteristics of biochar exhibit substantial variation depending on the specific production method employed [12,29]. In addition to the conventional charcoal kiln, biochar can be generated through alternative methods, such as torrefaction, pyrolysis, and gasification. Biochar produced through torrefaction and low-temperature pyrolysis techniques retains its aromatic components but possesses a lower carbon content, making it better suited as a fuel substitute rather than an activated carbon material [28,29,30]. On the other hand, during the high-temperature gasification process, most of the hydrogen components volatilize, leaving behind primarily carbon and mineral components [31]. The production of general activated carbon from biochar typically involves activation using substances, like KOH or water vapor, followed by material dissolution using strong acids [32]. Considering the growing demand for environmentally friendly carbon, there is an urgent need for technologies that can increase the carbon content of biochar without resorting to environmentally destructive activation processes [33].

Thermal decomposition under anoxic conditions above 800 °C has been tested in a small research reactor and is a technology that can increase the carbon content of biochar but has not been commercialized due to low energy efficiency and lack of uniform heating technology in actual application [34]. In this study, we employed a biochar oven that utilizes the advantages of flameless combustion technology, offering uniform temperature distribution and high heat transfer performance. The continuous production of biochar at temperatures exceeding 900 °C was achieved, as illustrated in Figure 1. The heating energy source for this system is pyrolysis gas, enabling surplus energy generation while meeting the oven’s own energy requirements [35].

Volatile organic compounds (VOCs) are a group of organic chemicals that easily evaporate into the air at room temperature, and they are known to have negative effects on human health and the environment [36]. Toluene, a common VOC, is a hazardous air pollutant that can cause respiratory irritation, headaches, and even damage to the central nervous system [37,38]. Therefore, the removal of toluene from indoor and outdoor air is a significant concern for environmental and public health [39,40].

There are nitrogen and oxygen functional groups attached to the surface of biochar which have a great influence on the adsorption capacity of biochar [41]. Some studies have shown that the adsorption mechanism of biochar is based on physical adsorption and chemical functional groups, which depend on biochar’s surface pore size, specific surface area, pyrolysis conditions, and oxygen-containing functional groups [42,43]. UV radiation has been demonstrated in recent research to increase polar groups, such as carboxyl groups on the material’s surface, activating monomers on the polymer surface to initiate graft polymerization [44,45,46,47,48]. This suggests that UV has the potential to boost biochar’s adsorption ability. However, the previous research did not thoroughly investigate its mechanism and how to use UV for the optimized process of biochar upgrading. For industrial utilization of biochar as an alternative of activated carbon, an economic UV treatment process based on surface reforming reaction theory should be introduced.

In this study, toluene, a typical volatile organic pollutant, was selected as the adsorbate, the biochar was modified by ultraviolet light, and the adsorption performance of biochar before and after modification was compared and analyzed. The properties of biochar before and after modification were analyzed by BET, SEM, XPS, and other techniques, and the reason why the adsorption capacity of biochar was improved was discussed. The performance of biochar adsorption capacity was evaluated using the Wheeler model, and the equilibrium adsorption capacity and adsorption rate of biochar were evaluated to determine the pathways to improve its adsorption capacity. Our findings can provide a theoretical basis for the potential application of biochar in various environments, especially in the search for economical activation pretreatment under the influence of UV irradiation intensity and time.

2. Experimental Conditions and Methods

2.1. Production of Biochar and UV Treatment

Figure 1 depicts the lab-scale biochar production furnace, which has a biomass feeding rate of 10 kg/h for wood pellet. The main reactor consists of a combustion chamber and a pyrolysis reactor. The combustion chamber utilizes flameless technology to achieve a uniform temperature distribution in the pyrolysis reactor at 900 °C and to reduce NOx emissions [46,47,48]. The produced biochar was crushed and sieved at 30–50 mesh, 50–100 mesh, and 100–200 mesh to produce 0.350 mm (M50), 0.154 mm (M100), and 0.074 mm (M200) samples, respectively. Subsequently, the M50, M100, and M200 samples were exposed to radiation for 6 h at an intensity of 280 μW/cm² (International Light Technology ILT1400 A photometer) using ultraviolet UV at a wavelength of 254 nm. These UV-pretreated samples were denoted as M50_U, M100_U, and M200_U, respectively, and were prepared for subsequent biochar adsorption reaction experiments. The cases in which the UV LED was turned on during the penetration experiment have the tail name of ‘u’ at the end. The sample classification is shown in

Table 1. Experimental biochar samples’ name depending on preparation condition.

Name of Sample		UV Source
		None	Pre-UV Treatment	In Situ-UV Treatment	Both UV Treatments
Mesh size	30–50 mesh (0.350 mm)	M50	M50_U	M50_u	M50_Uu
	50–100 mesh (0.154 mm)	M100	M100_U	M100_u	M100_Uu
	100–200 mesh (0.074 mm)	M200	M200_U	M200_u	M200_Uu

.

2.2. Adsorption Penetration Experiment

Figure 2 shows the flow diagram of the adsorption experiment. The biochar samples were then placed in a biochar adsorption reactor. The outlet concentration was measured under ambient conditions, 1 ATM and 25 °C. As shown in Figure 2 the adsorption penetration experimental system is divided into three parts. The first part (region ①) is the toluene feeding system, which uses air as the carrier. The first air stream is the main air stream, while the second air stream enters the toluene bottle to provide a controlled concentration of toluene. The third air stream is used to control humidity. The second part (region ②) consists of a mixing bottle and adsorption reactor. The third part (region ③) of the gas detection section uses a Tiger2000 (Core Tech Korea Business) sensor to measure the toluene concentration in the inlet and outlet of the reactor.

The total length of the reactor is 20 cm with an inner diameter of 5 cm (region ④). The distance between the biochar and the in situ UV lamp in the reactor is 4 cm, and the UV irradiation light source in the adsorption experiment is an LED lamp with an intensity of 136 μW/cm² and a wavelength of 280 nm. The biochar samples used during the adsorption experiments were labelled as M50_u, M100_u, and M200_u. Both the pre-UV-treated and modified biochar utilized in the adsorption experiments were denoted as M50_Uu, M100_Uu, and M200_Uu, as indicated in Table 1. The experiment was carried out under the conditions of an airflow rate of 3.6 L/min and a toluene inlet concentration of 105 ppm through a cold toluene vaporization and dilution system (region ①). In order to control toluene concentration, the toluene bottle was placed in an ice bucket, the temperature of which was maintained at 0 °C.

Before the start of the experiment, it was ensured that the toluene concentration was stable. Then 2g of each biochar sample was put into the reactor. While the total air flow rate of 3.6 L/min was flowing in the reactor, the transient outlet concentration changes was recorded until the outlet mass concentration equaled the inlet mass concentration. When the adsorption was considered saturated, the experiment was terminated. In order to reduce measurement errors, the data were acquired 3 times repeatedly, and the average value was taken. The absorptive capacity of unit biochar was calculated by the Wheeler equation, for which the formula is as follows, and which was mostly used to compute the saturated adsorption capacity and adsorption rate of biochar [49]:

$$t_b=\left(\frac{W_{e}W}{C_{0}Q}\right)-\left(\frac{W_e{\rho }_{\beta }}{K_v{C}_0}\right)\ln \left(C_0-C_t\right)$$

(1)

$$ln\left(\frac{C_0-C_t}{C_x}\right)=\left(\frac{K_{v}W}{Q{\rho }_{\beta }}\right)-\frac{K_v{C}_0}{W_e{\rho }_{\beta }}{t}_b$$

(2)

where $t_b$ is the breakthrough time (min), $C_t$ is the outlet concentration (g/m³), $C_0$ is the inlet concentration (g/m³), $Q$ is the volumetric flow rate (m³/min), $W$ is the weight of the biochar adsorbent (g), $W_e$ is the equilibrium adsorption capacity (g/g_biochar), ${\rho }_{\beta }$ is the packed bed density (g/cm³), and $K_v$ is the absorption rate (min⁻¹).

2.3. Surface Functional Group Analysis Using XPS

Functional groups, such as -COOH, can be generated at the biochar surface by reaction with oxygen and moisture in the air. An XPS (X-ray photoelectron spectrometer) is utilized to measure oxygen content, which is a usual method to detect formation of functional groups [50]. To conduct a UV-biochar modification experiment, M100 was sprinkled evenly on a glass fiber filter paper with a 5 cm diameter. An ultraviolet lamp (5 W, 254 nm) was placed 3 cm above the sample and set at a power of 280 µW/cm². The UV irritation was carried out for 30 min, 1 h, 2 h, 4 h, and 6 h, respectively.

3. Results and Discussion

3.1. Physical Properties and Elemental Analysis of Biochar Samples

The microstructures of raw and UV-treated biochar samples were studied using scanning electron microscopy (JEOL LTD (JAPAN)), as shown in Figure 3. The raw biochar samples formed submicron particles with a uniform size and high density, as well as abundant pores. After UV pretreatment, the surface of the biochar became smoother due to the oxidation of biochar carbon under the action of UV radiation. The analysis of the elements in

Table 2. BET analysis and elemental analysis of biochar.

Samples		BET Surface Area (m2/g)		Micropore Volume (cm3/g)		Pore Size (nm)
M50		147.268		0.0831		3.48
M100		176.166		0.0996		3.16
M200		145.620		0.1174		1.87
M50_		168.190		0.0568		2.65
M100_U		188.836		0.0852		2.97
M200_U		157.030		0.1152		1.85
Element	N	C	H	O	S	Ash	Sum
M100	0.30%	87.10%	0.42%	2.80%	0%	9.38%	100%
M100_U	0.25%	84.97%	0.38%	3.15%	0%	11.25%	100%

including the characteristics of the biochar also shows that the oxygen content in the UV-pretreated biochar increased while the carbon content decreased, indicating that the biochar underwent surface oxidation of the carbon base by UV treatment.

As shown in Figure 4a, the UV-pretreated biochar samples exhibited steeper N₂ adsorption curves compared with the untreated samples. This indicates that the modified biochar has a larger surface area and stronger adsorption capacity [51]. The shape of the adsorption curve for a particular substance can provide insight into the characteristics of the surface onto which the substance is adsorbed. In this case, the adsorption curve shows the adsorption of a substance to a surface under conditions that increase gradually with increasing pressure or concentration of the substance in the surrounding environment [51,52].

Figure 4b shows that the pore volume of the biochar treated with UV radiation becomes smaller. This effect is especially pronounced in the M50 sample with larger grains. This may be due to the formation of small micropores due to UV exposure. The presence of these micropores may have an important effect on the ability of biochar to adsorb and retain certain substances, as well as its total surface area and porosity [53]. Elemental analysis also showed that the carbon content in the biochar decreased, and the oxygen content increased after UV treatment, suggesting that oxygen-containing functional groups may contribute to the adsorption capacity of the biochar [8,54]. Under ultraviolet (UV) irradiation, an oxidation reaction occurs on the surface of biochar, resulting in the increase in oxygen-containing functional groups, such as carboxyl groups [55,56]. These acidic oxygen functional groups are more likely to react with aromatic hydrocarbons, and oxygen-containing functional groups can also form electron–electron complexes with aromatic hydrocarbons, thereby improving the adsorption capacity of toluene [57].

3.2. Effect of Biochar Particle Size on Penetration Curve

Figure 5 illustrates the penetration results to measure the adsorption capacity of biochar with different particle sizes for toluene. A finer biochar particle size results in the adsorption curve being shifted to the right, which shows stronger adsorption capacity. The calculated results of its equilibrium adsorption capacity are 12.8 mg/g, 19.3 mg/g, and 29.1 mg/g for M50, M100, and M200, respectively, as shown in Table 2. This is because fine particles have a stronger absorption capacity; this is well-known due to the increased surface area and the reduced space between particles, increasing binding capacity between biochar and VOCs.

3.3. Effect of UV Treatment on Adsorption Capacity

Figure 6 illustrates the penetration curves of biochar with different particle sizes under four different conditions.

Table 3. Fitting parameters of the Wheeler equation for a toluene permeation curve under different conditions.

Sample	Size (mm)	Initial Condition			Breakthrough Time [min−1]	Parameter Estimate			R2
		Q [cm3·min−1]	W [g]	Co [ppm]		We [mg. g−1]	Std De	K v [min−1]
M50	0.35	3600	2	105	95	12.8	0.000132288	7,048,672.2	0.98
M100	0.154	3600	2	105	104	19.3	0.000301386	9,661,361.4	0.98
M200	0.074	3600	2	105	100	29.4	0.000264575	16,332,800.4	0.93
M50_u	0.35	3600	2	105	140	18.2	0.000964365	5,309,208	0.94
M100_u	0.154	3600	2	105	144	28.5	0.000251661	11,624,371.2	0.94
M200_u	0.074	3600	2	105	130	31.4	0.000929157	11,177,280	0.92
M50_U	0.35	3600	2	105	141	50.8	0.000503322	28,627,808.4	0.91
M100_U	0.154	3600	2	105	146	51.5	0.000321455	28,998,055.8	0.95
M200_U	0.074	3600	2	105	153	50.4	0.000550757	25,777,602	0.94
M50_Uu	0.35	3600	2	105	156	54.6	0.000264575	30,150,713	0.91
M100_Uu	0.154	3600	2	105	157	54.8	0.000793725	27,132,847	0.93
M200_Uu	0.074	3600	2	105	180	59.2	0.000737111	24,254,697.6	0.92

contains the results of the calculations for the parameters of the Wheeler equation. It can be seen that all biochar samples exhibit the best adsorption effect for toluene under the both UV-treatments condition, and the increase in adsorption capacity due to UV pretreatment is the most pronounced, which improves the adsorption capacity of M50 biochar particles by an average of 3.27 times. The adsorption could be due to the fact that biochar still reacts with a low-power UV (136 μw/cm²) lamp while adsorbing toluene. Additionally, the adsorption saturation time is increased by the UV treatment: M50 from 95 min to 156 min, M100 from 104 min to 157 min, and M200 from 100 min to 180 min. The relative effect of UV treatment seems stronger with larger particles, which may be because UV reacts more strongly with the larger biochar particles. It is currently thought that the larger size allows more room for ultraviolet light to interact deeply within the biochar, resulting in an increase in surface area and the production of more functional groups that can adsorb toluene. M200 (the smallest particle) always has the best adsorption capacity, demonstrating that the size of the particles and the gaps between the particles are crucial factors in biochar adsorption capacity. In conclusion, the M200_Uu biochar sample showed the best adsorption capacity, and the M50 biochar sample had the largest increase rate in adsorption capacity after UV pretreatment. To ensure more accurate results, all measurements were performed three times on each particle and the average was taken as the measurement result.

3.4. Effect of UV Pretreatment Duration on Adsorption Capacity

Figure 7 shows that the adsorption capacity of biochar increases with the decrease in particle size and the increase in UV pretreatment duration. At a pretreatment time of 0, the adsorption capacity depending on the particle size has a multiple times difference; meanwhile, the UV-pretreated samples’ difference is not so significant. However, it is natural that the smaller particle has a higher adsorption capacity. Most of all, UV pretreatment time is related with process cost, which is preferred to be as low as possible. After one hour of pretreatment, it has biggest increase, while the increase by the duration is slower as the treatment time increases. Hence, focused study has been carried out in the next section using XPS to measure the surface functional group.

3.5. X-ray Photoelectron Spectroscopy

The surface functional groups generated on the biochar surface by the exposure to UV lamp during different time periods were analyzed by high-resolution XPS. Figure 8 displays the XPS survey spectra of unmodified and UV-modified biochar samples, which all exhibit a strong C 1s peak at 284.5 eV. This position mainly contains carbon–carbon and carbon–hydrogen bonds [58]. The 530 eV (O 1s) peak indicates that the main oxidation functional group is the carboxyl group (–COOH) or alcoholic hydroxyl group (–OH) [55]. The O 1s peak is also present in all samples, with a significant intensity around 530 eV. However, the intensity of the O 1s peak gradually increases as the UV pretreatment time increases. It is possible that during the biochar modification process, some groups on the biochar surface, such as benzyl, are oxidized to carboxyl groups under the conditions of UV light and oxygen. Additionally, UV treatment may increase surface oxides, such as O₃, which further enhances the oxygen content of the surface and improves its hydrophilicity and adsorption capacity [56,59]. The possible chemical reactions involved are as follows [60]:

$$\begin{array}{cc}\hfill & \hfill {\mathrm{O}}_2\stackrel{h\nu \left(\lambda =184.9 \mathrm{n}\mathrm{m}\right)}{\to }2\left[\mathrm{O}\right]\\ \hfill & \hfill \end{array}$$

(3)

$$\left[\mathrm{O}\right]+{\mathrm{O}}_2\to {\mathrm{O}}_3$$

(4)

$${\mathrm{O}}_3\stackrel{h\nu \left(\lambda =253.7 \mathrm{n}\mathrm{m}\right)}{\to }\left[\mathrm{O}\right]+{\mathrm{O}}_2$$

(5)

$${\mathrm{C}}_s+\left[\mathrm{O}\right]/{\mathrm{O}}_3\stackrel{h\nu \left(\lambda =253.7 \mathrm{n}\mathrm{m}\right)}{\to }\mathrm{C}{\mathrm{O}}_x$$

(6)

where [O] stands for atomic oxygen, s stands for surface and near surface, and x is the number of oxygen atoms in the functional group. Figure 9 shows that the O/C ratio of UV-treated biochar increases linearly with treatment time. The pretreatment UV energy (W·hr/g_biochar) was determined by integration of UV light intensity during the pretreatment duration to obtain the total irradiation energy and dividing it by the mass of biochar applied. The O/C ratio of biochar gradually increased and tended to be saturated with the increase in ultraviolet irradiation treatment time. Meanwhile, the increase rate is slow at 6 h pretreatment, representing 16.5 W·hr/g_biochar. Error bars represent 90% confidence intervals for the mean. These results indicate that UV treatment can alter the surface chemistry of biochar, and that the effectiveness of UV treatment is directly related to the treatment time. The economic UV pretreatment duration and energy could be around 4~6 h, which is 11~16.5 W·hr/g_biochar.

4. Conclusions

Biochar is an upcoming carbon source alternative to fossil carbon to mitigate climate change. As biochar contains carbon originated from the atmospheric CO₂, its industrial utilization will have a positive effect on climate change mitigation as well as environmental problem solutions. Biochar is a low-capacity material compared to activated carbon but stays at the same level prior to the activation process, so represents an eco-friendly carbon source satisfying industrial carbon needs, such as pollution control, material manufacturing, and climate change mitigation.

The biochar produced from the present biochar oven which operates pyrolysis at 900 °C shows higher carbon contents as well as BET surface area compared to existing biochar technology. The results of the study showed that UV pretreatment increases the adsorption capacity of biochar significantly by 3.27 times for M50 samples, as well as reaction duration. The effect on the adsorption capacity of biochar with larger particles was more pronounced than biochar with smaller particles. The influence of UV pretreatment duration as well as in situ UV irradiation in the reactor were explored. The following are the specific outcomes:

• The raw biochar particles adsorption experiment showed that smaller particles have higher adsorption capability in the absence of UV irradiation.
• The UV-biochar modification technology does not only change the pore structure of biochar, but also make the -O-containing functional groups more abundant on the surface and improves the adsorption capacity of VOCs. The adsorption effect of UV treatment on biochar with a particle size of M50 (0.350 mm) is the most noticeable, with a 3.27 times increase in adsorption capacity.
• The dynamic adsorption process of the modified biochar to toluene conforms to the Wheeler model.
• UV pretreatment is demonstrated to be an effective method for enhancing the adsorption capability of biochar. Additionally, simultaneous irradiation during the penetration process further increases the adsorption capability. These findings suggest that UV treatment holds great potential for improving the adsorption performance of biochar.
• The main reason for the adsorption capacity increase by UV treatment is functional group formation, the rate of which shows a linear increase with pretreatment energy until 11 W·hr/g_biochar, after which the increase rate is slow.