Nickel-Modified Biocarbons from Mixed Wood Sawdust: Multitechnique Study of Structure and Photocatalytic Activity

TL;DR

This study explores how nickel-modified biocarbons can be used to remove pollutants by examining their structure and photocatalytic activity.

Contribution

The novel contribution is the investigation of how synthesis conditions affect the structure and photocatalytic properties of nickel-modified biocarbons.

Findings

Pyrolysis temperature significantly affects biocarbon structure and properties.

Nickel addition enhances photocatalytic activity by forming redox centers.

Higher pyrolysis temperatures increase thermal stability and alkalinity.

Abstract

The increasing environmental pollution with persistent organic compounds demands the development of sustainable materials capable, among others, of simultaneous adsorption and catalytic degradation of pollutants. In this study, nickel-modified biocarbons were obtained in the process of biomass pyrolysis at the temperatures of 500 and 800 °C, with the Ni content of 5 and 10% by weight, in order to determine the effect of synthesis conditions on the structure, surface chemistry and functional properties of materials. A wide range of research methods was used to analyze structural parameters, elemental composition, surface morphology, functional groups as well as adsorption and photocatalytic properties. The results indicate that the pyrolysis temperature is the main factor determining the evolution of biocarbons, leading to a decrease in the specific surface area and microporosity, an increase in carbon aromatization, a reduction in oxygen groups, and an increase in alkalinity and thermal stability. The addition of nickel promotes formation of crystalline Ni phases and redox centers, while partially blocking micropores. As a result, the materials obtained at 800 °C are characterized by an increased photocatalytic activity. The paper provides mechanistic insights into the structure–property–function relationships and practical guidance for the design of biocarbons with optimized adsorption and photocatalytic properties.

Figures

Full text

1. Introduction

Contamination of surface water and industrial wastewater with difficult-to-degrade organic compounds (including azo dyes) remains one of the key environmental challenges, as many of these substances exhibit persistence, toxicity, and the ability to inhibit biological treatment processes. At the same time, there is an increasing pressure on the circular economy, where the biomass waste is treated as a raw material for the production of functional materials. Biocarbon, i.e., a carbon material obtained in thermochemical processes (e.g., pyrolysis) from biomass/waste, at present is broadly described as a “platform” for environmental remediation owing to its expandable porosity and structural stability as well as the ability to shape surface chemistry [1,2,3].

In the literature, it is emphasized that the properties of biocarbons are largely controlled by the conditions of the production process, and among them, the pyrolysis temperature plays a superior role, determining aromatization of the carbon matrix, pore distribution (micro-/meso-), content of heteroatoms, and the number and type of surface functional groups [2,4]. These changes translate directly into the adsorption mechanisms (pore filling, electrostatic interactions, hydrogen bonding, complexing, π–π interactions) and can affect a trade-off between the sorption capacity resulting from microporosity and the mass transport kinetics supported by the mesopores network [4,5,6,7]. For example, a decrease in the sorption capacity with an increase in the pyrolysis temperature in the dye–biocarbon systems was found to be associated with changes in functionalization and/or availability of the active surface [7,8,9].

In order to overcome the limitations of pure biocarbon, especially with regard to the degradation of pollutants but not only their adsorption, the trend of hybrid materials is intensively developing: biocarbon as a carrier/conductive system and adsorbent couples with the catalytic phases (oxides, sulfides, metals, semiconductors), obtaining the effect of adsorption–catalysis/photocatalysis synergy [4,5,10]. As follows from the reviews, biocarbon supports photocatalysis by (1) increasing the accessibility of impurities near the active centers (pre-adsorption), (2) facilitating the separation of e^−^/h^+^ pairs due to defects and conductivity of the carbon matrix, and (3) stabilization and dispersion of catalytic nanophases, which translates into a greater activity and better separability of materials [5,6,10,11].

The systems with nickel (Ni/NiO and derivatives) are of particular interest because nickel can act as redox centers, electron traps and a promoter of oxidant activation, and at the same time, depending on the content and conditions of the process, it affects the remodeling of the carbon structure and the accessibility of pores. The experimental work indicates that the biocarbon–Ni/NiO composites can remove dyes (e.g., methyl orange) effectively by both adsorption and oxidation/photocatalysis processes, with the function of nickel phase dispersion and micro-/mesopore balance being essential for obtaining large efficiency and stability of action [12,13,14,15]. In parallel, the literature on biocarbon modification and its applications in advanced oxidation processes (AOP) shows that the material design requires simultaneous changes in texture (transport/kinetics), surface chemistry (charge/polarity/complexation points) as well as the type and availability of active phases (ROS generation, radical and non-radical pathways) [16,17,18,19,20].

In this context, it is particularly justified to determine the optimal conditions for the synthesis of biocarbons modified with Ni ions (e.g., by changing the pyrolysis temperature and nickel content), as these parameters can have an adverse effect on the key performance characteristics: temperature is conducive to aromatization and deoxygenation, and thus to changes in hydrophilicity and the number of functional groups, while the Ni content can improve photocatalytic activity, however, it can also block some micropores or lead to coalescence nanophases, reducing the adsorption capacity. Therefore, an approach based on extremely broad material characteristics (porosity/PSD, carbon structure, phase and crystallinity identification, functional group and surface composition analysis, thermal properties and morphology) is crucial for reliably uniting the manufacturing conditions with adsorption mechanisms and the efficiency as well as stability of photocatalysis in the dye systems.

The novelty of the presented research consists of a systematic, two-factor approach to the design of nickel-modified biocarbons, in which the pyrolysis temperature and the content of Ni ions were simultaneously controlled. Then these parameters were united with the complete chain of relationships of pore structure—carbon structure—surface chemistry—function. An important contribution is the exceptionally rich, complementary characteristics of the structural parameters, elemental composition, surface morphology, and functional groups as well as adsorption and photocatalytic properties, which enables reliable explanation to why the temperature increase shifts the material from the role of an adsorbent with large microporosity and a number of functional groups towards a stable carbon–nickel composite containing an ordered carbon matrix, Ni redox centers and greater photocatalytic potential. This provides not only application data, but also practical guidance for determining optimal synthesis conditions for the materials capable of efficient removal of contaminants in the processes combining adsorption and photocatalysis.

2. Materials and Methods

2.1. Biocarbons Preparation

Biocarbons were obtained on the basis of mixed wood sawdust, washed, dried (T = 100 °C, 24 h), ground and fractionated; a fraction in the range of 1–2 mm was used for the tests. Sawdust was immersed in solutions of nickel(II) hexahydrate nitrate (Ni(NO_3_)2·6H_2_O). The concentrations were selected so that the nickel content in the biocarbons, after taking into account the degree of thermal burning, was 5% and 10%. The suspensions were left in the impregnation solutions for 7 days, stirred from time to time, and then the materials were dried and pyrolyzed. Pyrolysis was made in one stage, in an atmosphere of inert gas N_2_ (200 mL/min) at a temperature increase of 10 °C/min, to a temperature of 500 or 800 °C. The duration of the isothermal stage at the maximum temperature was 1 h, after which the system was cooled in a neutral atmosphere to room temperature.

2.2. Methods

2.2.1. Low-Temperature Nitrogen Adsorption/Desorption

The structural characteristics of magnetic biocarbons were determined based on the nitrogen adsorption/desorption isotherms (−196 °C; ASAP 2405 analyzer, Micromeritics, Norcross, GA, USA). Before the analysis, the samples were degassed at 200 °C for 6 h to remove physically adsorbed water. Based on the obtained data, the specific surface areas (SBET) were determined. The desorption data were used to calculate the pore size distributions (PSD, fv(Rp)~dVp/dRp) based on the self-regularization procedure (fv(Rp) ≥ 0 for any Rp) with the regularization parameter α = 0.01. The slit-shaped pores model was used. The fV(Rp) functions were applied to calculate the share of micropores (Smi and Vmi, Rp < 1 nm), mesopores (Sme and Vme, 1 nm ≤ Rp ≤ 25 nm), and macropores (Sma and Vma, Rp > 25 nm). The total pore volumes Vp were calculated as the sum of Vmi, Vme and Vma. The fit of the slit pore model was evaluated based on the parameter  $[eqn]$ . The procedure of PSD calculation is given in [21,22].

2.2.2. Thermal Analysis (TGA)

Thermal analysis was performed using a derivatograph (Derivatograph C, Magyar Optikai Művek, MOM, Budapest, Hungary). TG, DTG and DTA curves were recorded. Measurements were made in the air (sample mass ~30 mg, 20–1200 °C; heating rate 10 °C/min) or in the N_2_ (20–900 °C; temperature rise: 10 °C/min) atmosphere. The volatile matter content (VM%) was estimated from the TGA data (atmosphere N_2_; 150–900 °C). The ash content (A%) was defined as the residue of the complete thermal decomposition of organic substances in the air atmosphere. The fixed carbon content (FC%) was calculated from the following relationship: FC% = 100 − (A% + VM%). The thermostability index was estimated on the basis of the relationship: Cthermo = %FC/(%FC + %VC) [23]. All parameters were determined regarding the dry weight of the sample.

2.2.3. Raman Spectroscopy

The degree of ordering of the biocarbons structure was determined on the basis of Raman spectra (Raman Station 400 F, Perkin Elmer, Waltham, MA, USA) with a thermoelectrically cooled CCD detector and a diode laser. The wavelength of the laser was 785 nm. Before the measurements, the materials were dried.

2.2.4. X-Ray Diffraction Analysis (XRD)

To identify and quantify crystalline phases in biocarbons, the XRD analysis was made by means of the Empyrean diffractometer (Empyrean, PANalytical, Almelo, The Netherlands) in the angular range 0–90° (2Theta).

2.2.5. Attenuated Total Reflectance-Fourier-Transform Infrared Spectroscopy (FTIR-ATR)

FTIR-ATR spectra were recorded using the Perkin-Elmer Spectrum 400 FT-IR/FT-NIR spectrometer (Perkin-Elmer, Waltham, MA, USA) with a diamond chamber with enhanced total reflection (ATR). Before the measurements, the samples were dried and powdered. All spectra were recorded in the range of 4000–650 cm^−1^ with a resolution of 4 cm^−1^.

2.2.6. High-Resolution X-Ray Photoelectron Spectroscopy (XPS)

The high-resolution X-ray photoelectron spectroscopy (XPS) studies were carried out using the multi-chamber UHV system (PREVAC, PREVAC sp. z o.o., Gdańsk, Poland). Spectra were collected using a hemispherical Scienta R4000 electron analyzer and a Scienta SAX-100 X-ray source (Al Kα, 1486.6 eV, 0.8 eV band) equipped with the XM 650 X-ray Monochromator (Scienta Omicron AB, Uppsala, Sweden) (0.2 eV band). The pass energy of the analyzer was set to 200 eV for the survey spectra (with 1000 meV step), and 50 eV for the regions (high-resolution spectra). The base pressure in the analysis chamber was 5 × 10^−9^ mbar [24].

2.2.7. Surface pH

The pH value of the biocarbon surface was investigated using a modified procedure described in [25]. Biocarbons samples were dried, weighed ~0.1 g each, and poured with 5 cm^3^ of redistilled water. Then the samples were placed in a shaker (Grant Instruments Ltd., Shepreth, UK, 25 °C, 140 rpm, 24 h) and the pH of the solutions was measured.

2.2.8. Scanning Electron Microscopy/Energy Dispersion (SEM/EDS)

To study the morphology of the biocarbons, a scanning electron microscope Quanta 3D FEG FEI, Field Electron and Ion Co., Hillsboro, OR, USA) was used. The measurements were performed under the small vacuum conditions at a voltage of 5 kV. In order to perform qualitative and quantitative analyses, X-ray spectroscopy with the energy dispersion SEM/EDS (EDAX, Mahwah, NJ, USA) was used. The measurements were made at an acceleration of 20 kV.

2.2.9. Transmission Electron Microscopy (TEM/STEM)

The selected samples were examined by transmission electron microscopy using a Titan G2 60–300 kV microscope (FEI Company, Hillsboro, OR, USA) at the electron beam acceleration voltage of 300 kV.

2.2.10. Analysis CHNS

The elemental analysis (CHNS) was determined using a Vario Micro Cube analyzer (Elemental, Langenselbold, Germany). Before the measurements, the samples were dried to a constant mass. The ash content was estimated as Ash_estimated_ = Ni + Σ(Mg, Si, P, S, K, Ca, Fe); then, oxygen was determined indirectly using the formula O = 100 − (C + H + N + S + Ash_estimated_). This procedure reflects the standard practice of oxygen amount determination “by difference”, taking into account the inorganic fraction (ash).

2.2.11. Adsorption and Photocatalysis

In order to assess the adsorption capacity of materials and determine the time of equilibrium determination, adsorption studies of methyl orange (MO) were carried out. The experiment was conducted in the dark, adding 0.5 g of shredded biocarbon to 500 cm^3^ of the 50 mg/dm^3^ MO solution. The mixture was placed on a magnetic stirrer (IKA Werke GmbH & Co. KG, Staufen im Breisgau, Germany, 400 rpm); samples were taken every 15 min and then filtered using syringe filters (Chemland, Stargard, Poland, PTFE 30 mm, 0.45 μm). Concentrations of solutions were measured using a UV-Vis spectrophotometer (Helios Gamma, Spectro-Lab, Łomianki, Poland) at λ = 464 nm. The experiment was conducted for 150 min at 25 °C.

The photocatalytic activity of BC/Ni biocarbons was investigated in the process of photocatalytic decomposition of MO applying the UV light. In each experiment, 0.5 g of crushed biocarbon was added to a 500 cm^3^ MO solution at a concentration of 50 mg/dm^3^. The mixture was placed in a photocatalytic reactor equipped with a UV lamp (14.5 W) and a magnetic stirrer (400 rpm). In the first stage, there was a 15 min adsorption in the dark, after which the UV lamp was switched on. The concentration of MO after the adsorption step in the dark was taken as the initial concentration in the photocatalytic degradation experiments, and the extent of removal was calculated with respect to this value [26].

To calculate the extent of removal (R%), Equation (1) was used:

$$[eqn]$$

where C0 is the initial concentration of the dye [mg/dm^3^] and C is the concentration of the dye after some time [27].

The photocatalytic process was conducted for 150 min at the autogenous pH. Samples taken every 15 min were filtered with syringe filters (PTFE 30 mm, 0.45 μm), and the concentrations of solutions were measured spectrophotometrically (Helios Gamma, Spectro-Lab, Poland). The discoloration was determined by measuring the absorbance drop observed at the wavelength λ = 464 nm.

Photocatalytic studies were carried out also for the initial BC-800 biocarbon, obtained in the same way as BC/Ni biocarbons, without the impregnation stage. Moreover, in order to assess the amount of substance removed from the solution by UV light alone, the MO photolysis was investigated.

Detailed studies of the kinetics of the photocatalysis process were carried out using the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic equations (Equations (2) and (3)):

$$[eqn]$$ $$[eqn]$$

where C—the concentration of the dye at any time (t) [mg/dm^3^]; C0—the initial concentration of dye [mg/dm^3^]; k1—the PFO reaction rate constant [min^−1^]; k2—the PSO reaction rate constant [g·mg^−1^·min^−1^] [28,29].

The time required to decompose 50% of the initial amount (t1/2) of impurity (MO) for the pseudo-first-order kinetics (Equation (4)) was also determined:

$$[eqn]$$

This is a practical kinetic parameter describing the efficiency of photocatalytic degradation of pollutants as it directly determines the time it takes to remove 50% of the initial concentration of the compound. It allows for unambiguous comparison of the activity of different photocatalysts (a smaller t1/2 means a faster process), and allows for a preliminary assessment of the material suitability for technological applications. Moreover, changes in the t_1/2_ values can indirectly reflect the effect of catalyst modification on the generation of reactive oxygen species, the separation of charge carriers, as well as the number of accessible active sites.

In the process of heterogeneous photocatalysis, holes (h^+^), hydroxyl radicals (•OH) and superoxide anion radicals (•O_2_^−^) play a major role in the decomposition of organic compounds [30]. The supply of electrons and holes to the system is due to irradiation with photons with an energy equal to or exceeding that of the catalyst gap. When the light energy gets the nanoparticles (NP), the electrons (e^−^) from the valence band (VB) are excited to the conduction band (CB), forming positively charged holes (h^+^) in the valence band (Equation (5)). The electrons in the conduction band (e^−^) react with O_2_ molecules as electron acceptors, resulting in formation of a strong oxidizer, the peroxide radical anion (•O_2_^−^) (Equation (6)). Similarly, the holes (h^+^) in VB react with H_2_O molecules to generate a hydroxyl radical (•OH) (Equation (7)). Both radicals are responsible for the degradation of dyes [31,32].

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$

Moreover, positively charged holes (h^+^) are characterized by intensive oxidizing properties. They can react with water adsorbed on the surface of the photocatalyst or hydroxyl groups to form active hydroxyl radicals (•OH), which possess strongly oxidizing properties and allow for complete degradation of dyes [33].

In order to determine the active species involved in the photocatalytic reaction, an experiment was conducted to capture holes (h^+^), hydroxyl radicals (•OH) and superoxide radicals (•O_2_^−^) using Na_2_EDTA, isopropanol (IPA) and parabenzoquinone (BQ), respectively. Each of the active species scavengers was added individually during the photocatalytic reaction in the following amounts: 15.3 mL of Na_2_EDTA/500 mL, 5.9 mL of IPA/500 mL and 0.8 mg of BQ/500 mL [34]. The method of the active species capture experiment was the same as in the case of photocatalytic studies. The addition of various scavengers causes the capture of various active forms formed as a result of irradiation, leading to the inhibition of the photodegradation process. This makes it possible to assess which of the active forms is the main contributor to the photocatalytic process.

3. Results and Discussion

Figure 1 presents the low-temperature adsorption/desorption isotherms of N_2_ obtained for BC/Ni biocarbons (Figure 1a). The curves differ from each other first of all depending on the pyrolysis temperature. Isotherms of biocarbons obtained at 500 °C (BC-500-5%, BC-500-10%) are typical of disordered materials with a poorly developed surface. Their shape is similar to type I isotherms (according to the classification IUPAC [35]), which indicates the microporous structure of these materials. The intensive increase in adsorption in the low relative pressure range p/p0 (<0.1) is due to the significant content of micropores. On the other hand, the shape of the isotherms obtained for the materials pyrolyzed at 800 °C (BC-800-5%, BC-800-10%) is similar to those of type IV ones, indicating a small content of micropores, which were oxidized during the thermochemical conversion and transformed into mesopores. The hysteresis loops belonging to type H2 indicate the presence of an evolved system of connected or bottle-shaped pores [36]. The isotherms of BC-500-5% and BC-500-10% are located much higher compared to those of the materials obtained at 800 °C (BC-800-5% and BC-800-10%), which translates into their larger specific surface area. In the case of an increase in the nickel content (5% vs. 10%), a decrease in the specific surface area (Table 1) is visible due to micropores being blocked by the Ni crystallites and a larger proportion of mesopores in the materials [37]. The pore volume distribution curves (Figure 1b) indicate that an increase in the pyrolysis temperature results in the transformation of a microporous monomodal structure (BC-500-5%, BC-500-10%, Rav~0.8 nm) into the bimodal structure materials (BC-800-5%, BC-800-10%) with developed micropores (Rav~0.8 nm) and the formation of additionally narrow mesopores (Rav~1.1 nm, Figure 1b). Figure 2 presents the graphical course of changes in the proportion of surface area (%S, Figure 2a) and volume (%V, Figure 2b) of micro- and mesopores in the studied biocarbons. The observed changes (Figure 2) confirm the effect of an intensive reduction in the surface area and volume of micropores with a simultaneous increase in the surface area and volume of mesopores caused by the intensification of thermochemical processes at higher temperatures.

The structural parameters obtained from the N_2_ adsorption isotherms are presented in Table 1. A gradual reduction in the surface area and microporosity of the investigated biocarbons is observed, affected by the increase in both temperature and nickel content, with the influence of temperature being more significant. The materials obtained at the temperature of 500 °C are characterized by SBET~412 and 400 m^2^/g and Vp~0.63 cm^3^/g; these values decrease slightly with the increasing nickel content. For the biocarbons obtained at 800 °C, the materials are characterized by SBET~149 and 124 m^2^/g and Vp~0.23 and ~0.19 cm^3^/g (for BC-800-5% and BC-800-10%), with the reduction in these values being much more intensive than for the low-temperature biocarbons. Modification of surface and porosity with the increasing pyrolysis temperature can be caused also by the oxidizing effect of NO_x_ formed during the decomposition of nickel nitrate at temperatures above 500 °C [38]. During such reactions, the intensive oxidation of carbon and collapse of thin walls of micropores takes place, causing transformation of the surface structure from the micro- to mesoporous one (parameters included in Table 1; Figure 1 and Figure 2). Another factor influencing the change in porosity is the increase in the nickel content (5–10%). Nickel is present in an amorphous form in the materials obtained at 500 °C, and at a higher temperature is transformed into the form of crystallites (Figure 3a) characterized by a dimension typical of mesopores (~6–50 nm, Table 2), which can block the entrances of pores, causing a reduction in their surface area and volume. Small values of the Δw parameter indicate a good fit of the slit pore model. The area and volume of the macropores are negligible (parameters are not included in Table 1).

The XRD spectra (Figure 3a) of biocarbons show an evident dependence of the degree of crystallinity on the pyrolysis temperature. In the BC-500-5% and BC-500-10% samples, the characteristic reflections assigned to the ordered carbon structures (Table 2) are not observed; they are amorphous. The reflections attributed to the nickel compounds are very faintly visible, suggesting the presence of Ni in the form of largely dispersed, amorphous or very fine nanoregions with the sizes below the XRD detection threshold (approx. 3–5 nm). This phenomenon is consistent with the mechanism of thermal decomposition of Ni(NO_3_)2·6H_2_O, which transforms into unstable nickel nitrates, leading to the formation of NiO/Ni(OH)2 [39], whereby at 500 °C these products can remain weakly crystalline and strongly bound to the carbon surface.

For the samples obtained at 800 °C (BC-800-5% and BC-800-10%), diffraction peaks are observed at 2θ ≈ 26.2° and 42.1°, characteristic of ordered graphene planes [40,41]. The diffractogram analysis also confirms the presence of crystalline forms of nickel, with nickel being mainly found as metallic Ni^0^ in terms of sensitivity of the XRD method. In the BC-800-5% sample, the Ni^0^ signals are weak, indicating a greater dispersion of small crystallites of the nickel phases in the carbon matrix. In the BC-800-10% sample, an intense reflection is observed at 2θ ≈ 44.5°, attributed to the Ni^0^ (111) plane [37], as well as weaker peaks at ~51.8° and ~76.3°, corresponding to reflections (200) and (220). The large metal content promotes coalescence and growth of nickel crystallites (Table 2) during pyrolysis, leading to more intense diffraction reflections [17,42]. The presence of metallic nickel is a result of the reducing nature of the carbon matrix, which reduces NiO to Ni^0^ at a high temperature, as broadly described in the literature [43,44,45,46]. The absence of reflections characteristic of Ni(OH)2 and Ni_2_O_3_/NiOOH indicates that most of the oxidized forms of nickel were reduced to Ni^0^. This indicates that at 800 °C, the organic matter acts as an effective reducing agent, and the final stable phases of nickel are metallic Ni^0^ and possibly trace amounts of NiO (below the XRD detection threshold).

The Raman spectra (Figure 3b) of all biocarbons contain the dominant bands D (~1310–1323 cm^−1^) and G (~1580–1591 cm^−1^), characteristic of carbons with a turbostratic structure and varying degrees of sp^2^ phase order. The materials obtained at 500 °C show wider bands (especially D, 1308–1338 cm^−1^) and lower ID/IG values (~1.083–1.087, Table 2), indicating a large proportion of amorphous structures and heterogeneous aromatic domains typical of lower pyrolysis temperatures [47,48]. Increasing the pyrolysis temperature to 800 °C shifts the G-band towards higher wavenumbers (~1577–1584 cm^−1^) and significantly increases the ID/IG ratio (~1.3, Table 2), while narrowing the D-band, indicating increased aromatization and reorganization of the sp^2^ network towards more graphitized fragments. The effect of nickel content is most evident for the biocarbons obtained at 800 °C; an increase in the Ni content from 5% to 10% results in a slight narrowing of the D-band, suggesting a more homogeneous sp^2^ structure and a possible catalytic contribution of Ni to the ordering of the biocarbon structure at a higher temperature. For the low-temperature biocarbons (500 °C), the positions and widths of the Raman bands are similar, which indicates that the effects of incomplete carbonization and large dispersion of carbon structures dominate in this stage [49,50,51]. It is important that structural defects in biocarbons can create new adsorption sites, improving the sorption capacity of materials [52].

The FTIR-ATR spectra of biocarbons (Figure 4) reflect the synergistic influence of both the development of the carbon structure and the formation of various forms of nickel and remaining inorganic residues. The spectra in 3000–2800 cm^−1^ range, especially for pure biocarbons BC-500 and BC-800 (without Ni), are characteristic of the asymmetric stretching vibrations of C–H bonds of alkyl groups (–CH_3_, –CH_2_). In the samples obtained at 500 °C, there is evident presence of oxygen groups, which is characteristic of low-temperature biocarbons with a less condensed aromatic structure. The strong band at approx. 1579 cm^−1^ is associated with the skeletal vibrations C=C in aromatic rings coupled with carbonyl groups and the asymmetric tensile vibrations COO^−^ [53,54]. In the samples pyrolyzed at 800 °C, the maximum shifts to approx. 1541 cm^−1^ and becomes noticeably narrower, which confirms a larger degree of condensation of graphene domains and a reduction in the number of oxygen groups. A similar relationship was described for the biocarbons with a large degree of aromaticity [55]. The wide band around 1170 cm^−1^ corresponds to C–O vibrations in phenols, alcohols and aryl ethers as well as in the lignin fragments (C–O, C–O–C), which is consistent with the reports on biocarbons rich in lignin oxygen groups [56]. The intensity of this band decreases significantly with the increase in the pyrolysis temperature and the increase in nickel content, which indicates that nickel catalyzes the processes of dehydroxylation, decarboxylation and dealkoxylation, leading to the depletion of the surface oxygen groups. This is consistent with its documented action in hydrothermal and pyrolytic reactions, in which the presence of nickel accelerates removal of oxygen from the organic matter [57].

The presence of nickel compounds modifies the interpretation of FTIR spectra significantly. Residual nitrate ions formed from Ni(NO_3_)2 precursors generate characteristic bands of asymmetric stretching N–O around 1380 cm^−1^ and deformation vibrations NO_3_^−^ around 820 cm^−1^, described in detail for nitrate salts and surfaces containing sorbed NO_3_^−^ [58]. These bands can be accompanied by overlapping C–O and aromatic C–H signals in the regions of 1200–900 cm^−1^ and 900–650 cm^−1^, which explains their greater complexity in the samples with a larger nickel content. The peaks at 2172 and 1975 cm^−1^, present in the spectra of materials obtained at 500 °C, rarely observed in the pure lignocellulosic matter, can be attributed to the vibrations of carbonyl bonds in –COOH groups [53] and those of C=C=O in ketenes [59]. However, in the presence of nickel, these bands can also have the contribution of Ni–CO vibrations, typical of the transient carbonyl metal complexes formed during pyrolysis (range 2100–1900 cm^−1^) [55]. Their disappearance at 800 °C indicates the disintegration of these complexes and the transition of nickel mainly to metallic forms (also confirmed by XRD, Figure 3a, Table 2). In the range of 900–650 cm^−1^, the observed bands include deformation vibrations of aromatic C–H [60].

The FTIR spectra indicate that the presence of nickel affects the behavior of the material in two ways: it modifies the spectrum due to the presence of NO_3_^−^ ions and Ni–CO complexes as well as intensifies the process of deoxygenation of the carbon matrix which results in a reduction in the number of carboxyl, phenolic and hydroxyl groups. As a result, systematic attenuation of signals in the range of 1600–1000 cm^−1^ and dominance of the aromatic carbon structure in the samples pyrolyzed at higher temperatures and with a larger proportion of nickel are observed. Thus, the spectra confirm interactions of the mechanisms of thermal degradation of functional groups and the catalytic effect of nickel on the structure of biocarbon.

The results of thermal analysis (Figure 5) indicate that the course of thermal decomposition of biocarbons depends significantly on the temperature of their production and nickel content. The materials obtained at 500 °C show an earlier and more rapid mass loss (Figure 5a) due to a larger oxygen group content and a larger volatile carbon fraction content (%VC, Table 3), which is consistent with the results of FTIR, XRD and XPS. In the case of biocarbons pyrolyzed at 800 °C, intensive decomposition begins only at temperatures above 450 °C, and the mass loss is much milder, which confirms a larger degree of aromatization (%FC, Table 3) and a greater thermal stability of these materials (Cthermo, Table 3). The addition of nickel causes that for the materials, especially those with a nickel content of 10%, where a large content of inorganic residues is observed (%A, Table 3) and the thermal decomposition of these materials is shifted towards higher temperature values, which suggests a synergistic effect of high pyrolysis temperature and the presence of a catalyst. The DTG curves (Figure 5b) complement these observations, showing a shift in the maximum degradation rates from the temperature range of 300–600 °C for BC-500 biocarbons to 450–750 °C for BC-800 materials. This shift confirms greater thermal stability associated with the condensation and reorganization of carbon domains during pyrolysis at a higher temperature. The multiple minima on the DTG curves observed for the materials with a larger Ni content indicate the processes of more intensive degradation and secondary cracking of aerobic biomass fragments due to the catalytic action of nickel. Thus, the presence of nickel not only modifies the surface and decomposition pattern but also affects the architecture of the carbon matrix. The analysis of the DTA curves (Figure 5c) confirms the differences between low- and high-temperature biocarbons. The materials obtained at 500 °C are characterized by pronounced thermal effects in the temperature range of ~300–600 °C, associated with the decomposition of residual, unprocessed organic fractions and restructuring of less ordered carbon domains. In the samples obtained at 800 °C, a diminution shift in energy effects towards higher temperatures and their significant weakening are observed, which confirms a larger degree of ordering and suggests a gradual reduction in the content of surface functional groups.

The predominant carbon content, moderate oxygen content and trace amounts of nickel on the surface of biocarbons are confirmed by the XPS survey spectra. The exemplary spectra for BC-500-5% biocarbon are presented in Figure 6. The results of XPS analysis for other materials are included in the Supplementary Information (Figure S1). In all survey spectra (Figure 6a and Figure S1), the C 1s signal (~284–285 eV) is the strongest, typical of carbonaceous materials with a large degree of carbonization. The peak of O 1s (~531–533 eV) indicates the presence of oxygen groups, the intensity of which decreases with the increasing pyrolysis temperature and the nickel addition, which correlates with the results of thermal (Figure 5) and FTIR (Figure 4) analyses. In the spectra of the BC-500-5% (Figure 6a) and BC-500-10% (Figure S1) samples, there are observed peaks of Ni 2p_3/2_ and Ni 2p_1/2_ (852–872 eV), indicating a trace surface nickel content. In the materials obtained at 800 °C, these signals are weaker, pointing to more profound incorporation of nickel into the carbon matrix or a decrease in its content on the surface due to the catalytic remodeling of the carbon structure. The reduced intensity of the O 1s and Ni 2p bands in BC-800-5% and BC-800-10% biocarbons (Figure S1) confirms the advanced desorption of surface oxygen groups and the increase in surface aromatization as well as smaller accessibility of surface nickel in the carbon structure formed at higher temperatures.

The high-resolution spectra C 1s (Figure 6b and Figure S2) of all biocarbons are dominated by the C–C sp^2^ signal (284.5 eV) and, to a lesser extent, the C–C/C–H (285.0 eV) signal, which is typical of semi-ordered, graphite-like carbon structures. The C–C sp^2^ component share (Figure 6b and Figure S2) increases with the pyrolysis temperature, from approx. 69% in BC-500-5% to approx. 86% in BC-800-5%/10%, confirming the progressive aromatization and condensation of carbon domains at 800 °C. This trend is consistent with the results of XRD (Figure 3a, Table 2), indicating the development of graphene-like structures and the presence of ordered inorganic phases (Ni^0^). The C–O/C–O–C component (286.2–286.4 eV) is clearly reduced, taking into account the materials obtained at 500 and 800 °C, which confirms the intense dehydroxylation of the surface during pyrolysis. The decrease in its share corresponds to the FTIR spectra (Figure 4), in which the 1170 cm^−1^ band attributed to C–O groups disappears. The COOR component (~288.8 eV), corresponding to carboxyl groups and esters, also decreases compared to biocarbons obtained at 500 °C and 800 °C, which confirms thermal decarboxylation. This phenomenon is consistent with the disappearance of FTIR bands (1700–1300 cm^−1^) for the high-temperature samples. The presence of a π–π shake-up peak (~290.4 eV) is indicative of aromatic structures and conjugated π systems typical of graphite materials or high-temperature biocarbons, and can also reflect interactions with metallic nickel (Ni^0^). Such an interpretation is consistent with the literature data [61,62,63]. The increase in the intensity of this signal in biocarbons obtained at higher temperatures confirms the increase in the number of conjugated aromatic structures and the advanced graphitization of the carbon matrix.

The high-resolution spectra O1s (Figure 6c and Figure S3) for biocarbons show three main components of oxygen, whose positions and relative proportions reflect transformation of surface chemistry with the increasing pyrolysis temperature and nickel content. The component with the smallest bond energy, with the maximum at 530.6–530.9 eV, can be assigned to the aromatic carbonyl groups of the quinone/aryl-C=O type, with possible share of O bonds in the vicinity of inorganic phases (Ni–O, although the nickel fraction is small). The main component at ~532.3 eV corresponds mainly to oxygen in C–O bonds in ether groups and aliphatic alcohols and partially to oxygen in ester groups, while the component with the largest bond energy of 533.7–533.9 eV is characteristic of oxygen in –O–(C=O)–C– and phenolic Ar–OH groups. Comparison of the surface fractions of individual components indicates that the samples obtained at 500 °C are dominated by oxygen forms associated with polar C–O and Ar–OH groups (large bond energies), which indicates the presence of numerous phenols, ethers and lignocellulosic residues, also confirmed by the FTIR (Figure 4). With the increase in temperature to 800 °C, the total intensity of the O 1s band decreases, and the relative share of high-energy components (C–O, Ar–OH) decreases markedly in favor of the low-energy quinone/aromatic signal C=O. This indicates that during the high-temperature pyrolysis catalyzed additionally by nickel, dehydroxylation processes occur, leading to removal of the most labile oxygen species (C–O, Ar–OH). At the same time, conjugated carbonyls and quinones (surface groups of an alkaline nature) and fragments of Ni–O are stabilizing at the edges of the increasingly condensed sp^2^ domains. Such an image is fully consistent with the analysis of the C 1s (increase in C=C sp^2^ and π–π* shake-up signals, Figure 6b and Figure S2) and XRD (carbon matrix aromatization, Ni^0^ crystallization, Figure 3a) spectra and confirms that an increase in the pyrolysis temperature leads to strong deoxygenation of the biocarbon surface, while favoring more stable, conjugated carbonyl forms [64,65,66].

The SEM studies (Figure 7) indicate that the obtained materials are characterized by a layered structure with a visibly preserved carbon precursor skeleton. The surface of biocarbon is rough, containing scaly, irregular structures with visible cracks and cavities of various shapes and sizes. This indicates a developed specific surface of the materials, which is consistent with the results of structural studies (Table 1). Additionally, the higher pyrolysis temperature affected widening of the existing pores, increasing heterogeneity of the carbon structure. Numerous bright dots visible in the images (Figure 7g,h) can be interpreted as nickel particles deposited and well dispersed on the surface of carbon materials, with a size of nanometers. Similar structures were observed in the paper by Li et al. [67], where nickel in the metallic form was deposited on the carbon surface. For biocarbons obtained at 800 °C, these particles are much larger, indicating a larger nickel crystallite size, which is consistent with the results of XRD (Table 2).

The elemental characteristics of biocarbons (Table 4) confirms the significant influence of both pyrolysis temperature and nickel content on their chemical composition and degree of carbonization. Both EDS data and CHNS results indicate that an increase in the pyrolysis temperature from 500 °C to 800 °C leads to strong deoxygenation. A reduction in the surface oxygen content (Table 4, EDS) from 8.28 to 7.06 wt% at 500 °C to 3.04–1.85 wt% at 800 °C indicates disappearance of oxygen groups and restructuring of the surface towards more reduced aromatic structures. At the same time, the observed decrease in the oxygen content (CHNS, e.g., a decrease to 0.80 wt% for BC-800-5% and 1.56 wt% for BC-800-10%) indicates that the deoxygenation process concerns not only the surface but also the volume structure of the material. These results are consistent with the typical effects of high-temperature biomass carbonization, including aromatization, ring condensation and reduction of oxygen functional groups in the carbon structure, which results in the increased hydrophobicity, thermal stability and alkaline surface character (pH > 11 for the materials pyrolyzed at 800 °C).

An increase in the nickel content from 5% to 10% results in a marked increase in its content on the surface, from about 3.4 percent to 5.3 percent (at 5%) to approx. 9.0–10.9 wt% (at 10%). These data confirm that Ni is largely concentrated on the surface of biocarbons, and in the case of the samples obtained at 800 °C, its share is further increased due to greater mobility and remodeling of the carbon matrix. This is accompanied by an increase in the inorganic fraction, which directly affects the calculation of oxygen by the method of difference: the more Ni and other mineral elements, the smaller the fraction of the mass that can be interpreted as oxygen.

The O/C ratios confirm advanced deoxygenation of the samples obtained at 800 °C (O/C = 0.009–0.018), while biocarbons obtained at 500 °C show higher values (O/C ≈ 0.068–0.137). At the same time, a drastic decrease in the hydrogen content is observed in CHNS (from 2.34 to 1.67 wt% at 500 °C to 0.33–0.29 wt% at 800 °C) and low H/C values (0.004–0.003) testify to the dominance of sp^2^ structures and the practical disappearance of aliphatic fragments. Both O/C and H/C are classic indicators of the degree of aromatization of biocarbons [68], and their values for the materials obtained at 800 °C are typical of largely carbonized structures.

The surface pH of biocarbons (Table 4) was ~7.2–7.4 and ~11.2 for the materials obtained at 500 and 800 °C, respectively. The acidic groups include mainly carboxyls, lactones and anhydrides, which dissociate readily, producing protons and decreasing the pH value of the aqueous environment, while alkalinity is associated with phenolic, ether, carbonyl and more stable π-electron centers of graphene edges which act as Lewis bases, capable of donating electron pairs. Phenolic groups are a special case because they have a dual character: in the Brønsted’s sense, they are weakly acidic, but on the surface of carbon materials, they can exhibit Lewis alkalinity, depending on the electron environment of the carbon matrix [69,70,71]. Their acid-base properties depend on the presence of electron-acceptor or electron-donor groups in the vicinity, the level of graphitization and the pyrolysis temperature of biocarbons, which determines the degree of condensation of aromatic domains. As a result, biocarbon is a complex acid-base material in which acidic oxygen groups dominate in low-temperature biocarbons, while basic π centers and phenolic Lewis groups become crucial at large carbonization, shaping the sorption and reactive properties of the surface depending on pH and environmental conditions. The pH of biocarbons increased with the increasing pyrolysis temperature, which is associated with the loss of surface functional groups of an acidic nature [72] and was confirmed, among others in the FT-IR (Figure 4) and XPS (Figure 6) spectra.

The presence of nickel nanoparticles in the composites is confirmed by the results of TEM and STEM analyses. As shown in the TEM images (Figure 8), biocarbons contain homogeneous nanometer-sized Ni particles (black spots) largely dispersed in the carbon matrix. Increasing the nickel content in materials results in an increase in the size of crystallites and denser surface coverage with nickel nanoparticles without forming large aggregates. The STEM images (Figure 9) confirm the uniform dispersion of nickel crystallites on the surface of the carbon matrix.

Figure 10a (adsorption) demonstrates the contrasting adsorption capacities of the materials pyrolyzed at 500 °C and 800 °C. BC-800-5% and BC-800-10% exhibit MO uptake during the initial 15 min (the total MO removal, R%, was ~28 and ~21%, respectively), whereas BC-500 samples showed nearly flat profiles, confirming negligible adsorption (R% ~1). This observation is consistent with the dominance of ultramicropores in BC-500 materials, which are too small to be accessible to MO molecules because the size of the MO molecule is 1.31 × 0.55 × 0.18 nm^3^ [73]. On the contrary, BC-800 samples possess more accessible mesopores, enabling an effective contact between MO and catalytic sites.

Figure 10b presents the results of the assessment of the photocatalytic properties of the tested materials. All photocatalytic studies were preceded by a 15 min adsorption stage in the dark. As a result of only the photolysis process, ~71% of the dye degraded after 150 min of UV irradiation. When BC-500-5% and BC-500-10% are used, R% is 69% and 71%, respectively. The curves C/C0 = f(t) (Figure 10b) for these materials coincide with the MO photolysis curve itself, which allows the conclusion that removal of MO in the presence of these materials occurs only as a result of direct degradation of MO under the influence of UV, but not photocatalytic reactions. As a result of the photocatalysis process with the addition of BC-800-5% and BC-800-10% biocarbons, complete discoloration of the solutions was observed after 135 min and 105 min for BC-800-5% and BC-800-10%, respectively (Figure 10b). More intensive and faster removal of MO is noticeable with the use of a composite with a larger nickel content. The increased steepness of the BC-800-10% curve reveals greater catalytic efficiency originating from improved charge separation and larger active site availability.

To examine the potential effect of nickel on photocatalytic reactions, the experiment was also conducted for the nickel-free BC-800 biocarbon (Figure 10b). After 150 min of exposure, the R% was ~73%. This is a value very similar to the degradation of MO as a result of photolysis (~71%). This indicates that the carbon material did not exhibit photocatalytic properties, and the degradation resulted from the presence and content of nickel in the ordered structure of biocarbons. Similar results were obtained in the paper by Is Fatimah et al. [16] where, during photocatalytic oxidation in the presence of H_2_O_2_ on the composites Ni@BC, complete discoloration of the solution was achieved after 90 min.

The comparison of the course of individual processes taking place in the case of the BC-800-10% material is presented in Figure 10c. After 105 min (dotted line, Figure 10c), adsorption contributes ~20%, photolysis ~51%, and photocatalysis ~29% (green oval, Figure 10c). The figure visually highlights that photocatalysis constitutes the decisive contribution in the MO degradation. This confirms the synergistic role of Ni species and carbon conductivity in facilitating ROS formation.

In order to determine which species are most involved in the photocatalytic degradation of MO, studies were carried out with the use of radical scavengers: Na_2_EDTA-hole uptake (h^+^), parabenzoquinone (BQ)-superoxide radical uptake (•O_2_^−^) and isopropanol (IPA) uptake of hydroxyl radicals (•OH). The selected scavengers capture active radicals, causing inhibition of the photocatalysis process [74]. The greater the process inhibition (weaker removal), the greater the contribution of a specific active species in the photocatalysis process. The results of the experiment for the BC-800-10% biocarbon are presented in Figure 10d.

The data indicate that each of the applied radical scavengers affected inhibition of the photocatalytic degradation of MO in a different way. The largest inhibition was observed after the addition of IPA—after 105 min (the time at which complete MO degradation took place in the control system, dotted line, Figure 10d), the R% value was only ~47%. For Na_2_EDTA, the degradation process was inhibited to a slightly lesser degree, with R% reaching ~56%. The weakest inhibition was observed in the presence of BQ. After 105 min, R% remained high at ~89%, which indicates that superoxide radicals (•O_2_^−^), scavenged by BQ, do participate in the photocatalytic reaction, but they are not the primary active species. The results indicate that the main contribution to the photocatalytic degradation of MO comes from formation of hydroxyl radicals (•OH) and hole–electron interactions (h^+^/e^−^).

The analysis of kinetics of photocatalytic processes is a key element enabling evaluation of the reaction mechanism and comparison of the activity of the studied photocatalysts. For the biocarbons BC-800-5% and BC-800-10%, the experimental data were matched to the two most commonly used models: pseudo-first-order kinetics (PFO) and pseudo-second-order kinetics (PSO) (Figure S4). The resulting matching parameters are presented in Table 5. The high values of the determination coefficient for PFO kinetics (R^2^ = 0.9421–0.9594) indicate that degradation of MO on both materials proceeds according to the pseudo-first-order mechanism. On the contrary, the PSO model shows much smaller compliance (R^2^ = 0.5102–0.6067), indicating that the process is not controlled by chemisorption or surface reactions requiring two-molecule interactions typical of PSO. Compliance with PFO kinetics means that the rate of degradation is proportional to the instantaneous concentration of the dye, which is characteristic of photocatalytic processes in which radical reactions (•OH, h^+^, •O_2_^−^) generated on the catalyst surface predominate. The reaction rate constant k_1_ increased from 0.0349 to 0.0415 min^−1^, with a simultaneous decrease in the t_1/2_ parameter from 19.86 to 16.70 for a material containing a larger nickel content in the structure (BC-800-10%), indicating better photocatalytic properties compared to the BC-800-5% material. The increased degradation is due to the larger accessibility of active sites, allowing effective interactions between the pollutant molecules and the photocatalyst [75,76].

4. Conclusions

The integrated analysis of the results obtained by all applied test methods shows that the pyrolysis temperature is a key factor controlling the evolution of the structure, surface chemistry and functionality of nickel ion-modified biocarbons, while the Ni content (5 and 10%) plays a modulating role, manifested mainly in the accessibility of pores and the nature of active centers. An increase in the temperature from 500 to 800 °C leads to a fundamental transformation of the material; there is a strong decrease in the specific surface area and pore volume, combined with a transition from a strictly microporous structure to a system dominated by narrower mesopores, as confirmed by the N_2_ adsorption isotherms and PSD distributions. This transformation improves the transport properties of the system, which are important from the point of view of catalytic processes. In parallel, the results of XRD and Raman spectroscopy indicate a progressive aromatization and ordering of the carbon structure with temperature, which is accompanied by crystallization and coalescence of nickel phases in the samples obtained at 800 °C. On one hand, this promotes partial blocking of narrow pores, especially at 10% Ni content; however, it also leads to the formation of stable redox centers and electron traps, which can play a key role in photocatalytic mechanisms. At the same time, the analyses of FTIR-ATR, XPS, EDS and CHNS indicate that the increase in temperature causes a profound rearrangement of the surface chemistry from materials rich in polar-acid oxygen groups at 500 °C to largely deoxidized, more graphite and basic surfaces at 800 °C. The processes of dehydroxylation, decarboxylation and dealkoxylation result in a decrease in the number of sites capable of electrostatic interactions and hydrogen bonds, but at the same time, increase the proportion of π–π interactions as well as chemical and thermal stability of materials, which is confirmed by the results of TGA and proximate analyses. As a consequence, biocarbons obtained at 800 °C exhibit a significant adsorption potential in relation to methyl orange and act as stable carbon–nickel composites suitable for photocatalytic applications, in which the ordered structure of carbon, the presence of crystalline Ni phases, the alkaline nature of the surface and favorable mass transport conditions are crucial.