1. Introduction
In the rapidly changing world, population growth is a constant factor. It has been estimated to increase to 9 billion and 11 billion by 2050 and 2100, respectively [
1]. With the growing population, the secure, sustainable, and safe availability of water, food, energy, and industrial raw materials has and will continue to be a challenge. Thus, the need for a transition to a sustainably-focused economy and society becomes increasingly important [
2]. This is a reason that many governments, business organizations, and educational institutions are opting for a bioeconomy. According to the German Bioeconomy Council, “Bioeconomy is the production and utilization of biological resources—including knowledge—to provide products, processes, and services in all sectors within the framework of a sustainable economy [
3].” Thus bioeconomic principles explore and exploit bioresources to formulate bioproducts of economic value [
4].
Biochar, made from the pyrolysis of forestry and agricultural residues, is considered to be a great opportunity to advance the bioeconomy [
4]. Biochar’s source material is often discarded biomas, and as it is formed, itlocks in residual carbon from this biomass into its porous solid structure, and thus prevents that the carbon from returning to the atmosphere in the form of carbon dioxide [
5]. Moreover, biochar can be utilized in different environmental applications such as wastewater treatment [
6,
7], energy storage [
8,
9], biofuel production [
10,
11], gas scrubbing [
12,
13], soil amendments [
14,
15], cattle feed [
16,
17], catalytic degradation of pollutants [
18,
19], and making cementitious materials [
20,
21]. Biochar’s potential as a carbon-negative technology made from waste which simultaneously contributes economic benefits makes it pertinent to achieving sustainable development goals. Hence, biochar has attracted the attention of scientists worldwide, as evidenced by an overwhelming number of research articles published that elucidate novel approaches to biochar preparation, modification, utilization, and insights into biochar structure.
Biochar without modifications, known as pristine biochar, exhibits poor properties for environmental applications, such as reduced porosity and surface area, and surface functional groups [
22]. Therefore, biochar needs to undergo modification, and depending on the chemicals and methods used, a wide range of designer biochars have been prepared for different environmental applications. Oxygen atoms are common on pristine biochar surfaces, which come from the biomass feedstock [
22]. Duan et al. [
23] reported that oxygen contents and their speciation should be carefully optimized as oxygen functionalities can influence the catalytic ability in carbonaceous materials. Oxygen-containing groups, such as carboxyl, carbonyl, hydroxyl, and phenolic, can help bind metal ions [
24]. Oxygen functional groups have also been reported to be effective in the photodegradation of antibiotic drugs, such as, enrofloxacin [
25]. Oxygen functional groups also enhanced supercapacitor activity in water hyacinth hydrochar [
8]. Thus, intentional oxygen modification of biochar has been studied with a number of oxidizing agents, such as potassium hydroxide, nitric acid, sulfuric acid, potassium permanganate, air, and ozone [
24,
26].
From a material design perspective, it is important to perform comparative studies which can form a baseline for determining the suitable oxidizing agent targeting specific properties in biochar. In our recent publication, we modified pine bark biochars with different nitrogen-containing chemicals and studied their effect on biochars’ physicochemical properties [
27]. It was clear that the resultant biochar’s properties are greatly influenced by these modifying chemicals. In this research, we studied the effects of five different oxidizing agents, namely sulfuric acid, nitric acid, ozone, hydrogen peroxide, and ammonium persulfate, on the surface properties of biochar. Pine bark-derived pristine biochar was saturated with oxidizing agents. The modified biochars were characterized using various instrumental and wet lab techniques to compare the biochar properties after modification.
2. Materials and Methods
Pine bark nuggets (6.65 ± 2.5 cm in length) were procured locally from Oldcastle Lawn and Garden Inc., GA. The nuggets were washed with deionized (DI) water, dried in a mechanical oven (Lindberg/Blue M MO1490SA-1) at 105 °C for 72 h, and stored. All the analytical grade chemicals were used to prepare and characterize the samples. Sulfuric acid (97.3% w/w, CAS: 7664-93-9), Nitric acid (69.0%, CAS: 7697-37-2), Hydrogen peroxide (31.4%, CAS: 7722-84-1), and Ammonium persulfate (CAS: 7727-54-0), Potassium hydroxide (CAS: 1310-58-3), Sodium hydroxide (CAS: 1310-73-2), Potassium nitrate (CAS: 7757-79-1), and Hydrochloric acid (25% v/v, CAS: 7732-18-5) were purchased from Fisher Chemical. Ozone was produced with the help of an ozone generator.
2.1. Sample Preparation
Pristine biochar was prepared by pyrolyzing the washed and dried pine bark nuggets in a Sentry 2.0 microprocessor box furnace at a heating rate of 10 °C/min under N2 flow of 5 L/min up to 400 °C and maintained for 4 h. The obtained sample was allowed to cool at room temperature and then ground and sieved to a particle size below 200 mesh. The powdered samples were stored in a sealed glass container and labeled BC. Subsequently, the modified samples were prepared from BC.
To prepare the modified biochar samples, BC samples were treated with several oxygen precursors.
Figure 1 shows a schematic of the different processing steps involved in sample preparation. For oxidation with ozone, 11 g/m
3 ozone (produced in a corona discharge ozone generator) (OL80, Yanco Industries, Burton, BC, Canada) was flown through a glass column packed with biochar. To ensure a uniform reaction, the column was flipped after 1.5 h. For liquid-phase oxidation, the biochar samples were reacted (wet impregnation) with 50% hydrogen peroxide, 18.25 M sulfuric acid, 15.8 M nitric acid, and 2 M ammonium persulfate (in 1 M sulfuric acid) for 3 h. Subsequently, the liquid oxidized biochars were washed several times under flowing water for 30 min. The washed samples were dried in an oven (Lindberg/Blue M MO1490SA-1) at 105 °C for 48 h to obtain oxygen-doped biochars and labeled as O
3-BC, H
2SO
4-BC, HNO
3-BC, APS-BC, and H
2O
2-BC depending on the oxygen treatment and stored for subsequent surface characterization studies.
2.2. Sample Characterization
2.2.1. Biochar pH
The pH values of the biochar samples were determined by equilibrating 0.4 g of biochar samples in 20 mL of DI water at 150 rpm for 24 h and then measuring the filtrate’s pH (Model: AB150; Manufactured by Fisher Scientific, Waltham MA, USA) [
28].
2.2.2. Point of Zero Charges (pHpzc) in Biochar Samples
Biochar samples’ pH
pzc was determined by mixing 0.2 g of samples with 40 mL 0.1 N potassium nitrate solution at 150 rpm for 24 h in a pH range of 2–14. The pH was adjusted by adding either 0.1 N nitric acid or potassium hydroxide solution. The samples were then filtered, and the pH of the filtrate was recorded. The final pH of the filtrates was plotted against their initial pH. The intersection point of the curve with the 45° straight line where pH
initial = pH
finalwas recorded as the pH
pzc of the biochar samples [
29].
2.2.3. Hydrophilicity of Biochar Samples
The hydrophilicity/hydrophobicity of the biochar samples was determined using the molarity of the ethanol droplet (MED) test, which is described in detail elsewhere [
30]. Briefly, the MED test uses varied quantities of ethanol to change the liquid’s surface tension. It is also referred to as the “percentage of ethanol” or “critical surface tension” test. This test quantifies biochar wettability by measuring the lowest ethanol concentration that permits a drop to penetrate within 3–5 s. It effectively determines how strongly a water drop is repelled and this property is best related to the degree of hydrophobicity. This approach is qualitative in nature. The hydrophobicity is correlated to the ethanol concentration in
Table 1. A higher value of the MED index is related to a higher degree of hydrophobicity. Thus, an index value of 0 is extremely hydrophilic, and seven is extremely hydrophobic. The surface tension of an ethanol solution decreases with the increasing concentration of ethanol solution. Thus, a droplet with lower surface tension (respectively, a higher concentration of an ethanol solution) will infiltrate into the porous solid material faster than a solution with high surface tension.
2.2.4. Concentration of Acidic and Basic Sites in Biochar
The concentration of acidic and basic sites on biochar surfaces was determined according to the procedure described elsewhere [
31]. Briefly, to determine the acidic site concentration, 0.2 g biochar samples were added to 25 mL of 0.02 M sodium hydroxide (NaOH) solution and stirred at 150 rpm for 48 h at room temperature. Then the solution was filtered, and the filtrate was titrated with 0.02 M hydrochloric acid (HCl) solution. The concentration of acidic sites was calculated by subtracting the moles of NaOH after titration from the initially present NaOH moles and dividing by the material mass. Similarly, the concentration of basic sites was determined by adding 0.2 g of biochar samples in 25 mL 0.02 M HCl and titrating the resulting filtrate with 0.02 M NaOH solution. pH (Model: AB150; Manufactured by Fisher Scientific, Waltham, MA, USA) was used to monitor the progress of the titration. All the values were measured in replicate.
2.2.5. Infrared (IR) Spectrum Analysis
A Bruker Platinum ATR spectrometer was used to obtain the IR spectrum for analyzing the surface functional groups in the biochar samples. Origin (2021b) software was used to construct and subtract the baselines of the obtained spectra.
2.2.6. X-ray Photoelectron Spectroscopy (XPS) Analysis
Biochar samples’ surface atomic composition was analyzed in a SPECS XPS system with a PHOIBOS 150 analyzer using Mg Kα radiation under a pressure of about 3 × 10−10 mbar. XPS Peak software (Version 4.1) was used to deconvolute the XPS spectra.
5. Conclusions
In this study, we compared the surface chemical properties of pine bark biochars modified with five different oxygen-enriching chemicals, namely, sulfuric acid, ozone, nitric acid, hydrogen peroxide, and ammonium persulfate. The results show that H2SO4-BC had the highest concentration of C=O bonds compared to all other tested samples. H2SO4-BC also had the highest percentage of esters and anhydrides, whereas HNO3-BC had the highest percentage of carbonyl and quinone-type oxygen atoms. Moreover, the pHpzc of biochar samples was negatively correlated to both the difference between the acidic and basic sites concentration on the biochar’s surface and the summation of O-1, O-2, and O-4 peaks. These results suggest that the structure of oxygen-enriched chemicals has a significant effect on the biochar properties. Furthermore, the oxygen functional groups on the biochar can influence different biochar properties, such as pHpzc, acidic, and basic sites’ concentration on biochar. Essentially, this study provides insight into the preparation strategy of application-specific carbon materials with oxygen-enriched chemicals and forestry wastes. Future studies should endeavor on studying the applications of biochars thus prepared with a focus on determining the biochar structure-activity relationship.