Photocatalytic Properties of Office-Paper-Waste-Derived Activated Carbon for Efficient Degradation of Organic Pollutants

Abstract

Sustainable waste recycling continues to be one of the most significant challenges in this century, especially for the office paper sector. On top of that, photocatalysis depends on solar radiation as an unlimited and environmentally friendly energy source for removing organic pollutants from contaminated water. The obtaining of AC from office paper waste was carried out with the help of the chemical activation method using ZnCl₂ as an activation agent, followed by heating the samples in adequate conditions. In the present research, we assessed the influence of the amount of ZnCl₂ activator on the properties of AC. In our experimental conditions, a part of ZnCl₂ was transformed into ZnO, deposited onto AC, and formed a composite. We attempted to minimize aggressive chemical agents through inexpensive technical solutions and experimental approaches. The properties of the obtained AC samples were evaluated by XRD, XPS, SEM/EDX, EPR, and surface area and porosity investigations. All of the samples exhibit photocatalytic activity toward Rhodamine B. The photocatalytic mechanism was evaluated considering the existence of reactive oxygen species (ROSs), as evidenced by spin-trapping experiments.

1. Introduction

Office paper is a typical everyday product that has a multitude of purposes for both people and organizations [1]. As a result, huge amounts of paper waste have been produced in the last few years. It is widely known that waste paper is a valuable source of carbon material, particularly because it contains significant amounts of cellulose [2,3,4].

Activated carbon is currently in high demand owing to its wide range of applications across various industries [5,6,7,8], owing to its excellent material with a large surface area which includes well-organized macro-, meso-, and micropores, as well as a wide range of chemical functional groups [9,10].

However, it is a critical issue to develop cost-effective activated carbon, because commercially accessible activated carbon is expensive. As a result, manufacturing low-cost activated carbon from various resources is a requirement for future generations. Recycling waste paper offers numerous advantages for the environment and the economy, such as conserving resources, saving energy, and reducing landfill waste [11,12,13,14]. Chemical engineering could play a considerable role in resource utilization, waste reduction, and accountable waste recycling to preserve the ecosystem from potentially harmful waste discharge [15].

Paper recycling involves a combination of physical and chemical methods, which can be challenging to manage depending on the type of paper waste used [16,17,18]. Several chemical compounds may transform carbonaceous raw materials, producing activated carbon with different textural properties. These serve as the subject of many research initiatives on carbon-based waste [4].

There are several types of waste paper, such as office paper, newsprint, cardboard, or mixed paper waste and the carbon yield potential depends on the type of waste paper. Different types of waste paper contain varying amounts of inorganic fillers (e.g., kaolin, calcium carbonate, titanium dioxide) and additives, which can significantly influence the properties of the resulting activated carbon [19]. Like other types of paper waste, office paper waste contains cellulose as the main component, it has a low lignin content, and the activated carbon that can be derived from it is highly absorptive, meaning it can be used for water and air purification applications. The variability in the composition of office paper waste, particularly the presence of inorganic fillers and additives, plays a significant role in determining the morphology and photocatalytic properties of activated carbon derived from such waste [20].

Chemical agents, including H₃PO₄, ZnCl₂, and K₂C₂O₄, are commonly used to activate waste paper [21,22,23,24]. Each of these has advantages and disadvantages relating to the textural features of activated carbon [25]. ZnCl₂-activated carbonaceous precursors can form pores and serve as a dehydrating agent, eliminating hydrogen and oxygen [26,27,28]. Zhao et al. [29] synthesized a series of activated carbon (AC) adsorbents from shaddock peel by using zinc chloride (ZnCl₂) as a pore-forming agent with various carbonization temperatures. This procedure shows potential for treating water pollution economically and efficiently. Hong et al. [26] observed that, during activation, a portion of ZnCl₂ is transformed into ZnO and loaded onto AC, thus considerably enhancing mercury removal. ZnCl₂ was employed to activate olive solid waste, which generated activated carbon with a significantly greater surface area and improved nitrate adsorption capability [30].

For our investigations, in this study we chose office paper waste because it is one of the most abundant paper waste sources.

The work focuses on obtaining activated carbon (AC) from waste office paper. Several kinds of AC were developed using office paper waste and ZnCl₂ as activators, followed by heating the samples in adequate conditions. The acquired AC-based samples were analyzed using relevant characterization approaches. The impact of the amount of ZnCl₂ activator on the properties of the AC was carefully studied. The ability of composites to act as photocatalysts for RhB degradation was investigated, along with the photocatalytic mechanism. Moreover, the selected chemical activation process of the office paper waste allowed us to obtain, in the same synthesis process, a composite based on activated carbon and ZnO.

The research conducted here constitutes a significant step toward a circular economy and sustainability.

2. Results and Discussion

The activated carbon (AC) samples were produced through the carbonization of waste paper under specific conditions. It is recognized that experimental parameters are critical to achieving the desired properties. Here, thermal analysis and its methodologies are a versatile tool for researching solid-phase processes, providing qualitative and quantitative data that can be used to set synthesis parameters for activated carbon products.

Understanding the temperature ranges at which paper decomposes allows the logical use of energy resources and the production of a more efficient end product. Figure 1 shows the non-activated office paper sample’s TG, DTA, and DTG thermal analysis curves between 25 and 1100 °C.

The thermal decomposition of the paper sample results in a total mass loss of Δm = 83.8%. The process occurs in four stages: dehydration and volatilization of low-molecular-weight substances, hemicellulose degradation (180–320 °C), cellulose degradation (300–400 °C), and lignin decomposition (200–500 °C), each accompanied by corresponding thermal processes. Mass reduces constantly in the temperature range of 500–1000 °C, with losses of less than 16%. In this interval, the carbon skeleton is formed due to condensation processes and compound rearrangement, providing the carbonized material additional rigidity. Considering these, three samples were produced and identified: AC0, AC1, and AC2. These varied from each other regarding the existence of activator and ZnCl₂ mass ratio on office paper waste. All samples were calcinated at 800 °C in a nitrogen (N₂) atmosphere. The general characteristics of the samples are presented in

Table 1. The composition and general characteristics of the samples.

Samples	Activator	Mass Ratio AC:ZnCl2
AC0	-	-
AC1	ZnCl2	1:1
AC2	ZnCl2	1:3

.

It is reported that the ZnCl₂ activator has a significant advantage over alkali metal activators in terms of ZnO loading [27,28].

Depending on the reaction conditions, several simultaneous reactions [24] can occur during the pyrolysis process, including the following:

$${\mathrm{ZnCl}}_2 + {\mathrm{H}}_2\mathrm{O}{\displaystyle \to }\mathrm{H}[{\mathrm{ZnCl}}_2(\mathrm{OH})]$$

(1)

$$\mathrm{H}[{\mathrm{ZnCl}}_2(\mathrm{OH})]{\displaystyle \to }\mathrm{ZnO} + 2\mathrm{HCL}\uparrow$$

(2)

$$2\mathrm{ZnO} + \mathrm{C}\stackrel{900 °\mathrm{C}, \mathrm{Ar}}{\to }2\mathrm{Zn} + {\mathrm{CO}}_2\uparrow$$

(3)

$$\mathrm{ZnO} + \mathrm{C}\stackrel{900 °\mathrm{C}, \mathrm{Ar}}{\to }\mathrm{Zn} + {\mathrm{CO}}_2\uparrow$$

(4)

2.1. Structural and Morphological Investigations of AC Samples

The samples’ crystalline structure was analyzed by XRD diffraction. In Figure 2, the diffractograms corresponding to the obtained samples are presented. The AC0 sample shows a diffraction peak located at 2θ = 22.5°, specific to cellulose’s (110) reflection plane [JCPD No 50-0926]. Another cellulose peak of lower intensity can be identified at 2θ = 16.4°, associated with the (002) plane. Moreover, the presence of a peak at 2θ = 42.0°, specific to the (100) plane of carbon, indicates that the thermal treatment changes the structure of the cellulose, giving it a structure specific to carbon. In addition to these peaks, the diffraction peaks specific to calcite are present. The calcite is an important component of the paper, making it shiny and opaque [31].

The AC1 and AC2 samples present the diffraction peaks of calcite and contain several intense peaks associated with ZnO (ICDD: 01-080-0075), probably due to the partial oxidation of ZnCl₂ used as an activator for the waste paper. As expected, the amount of ZnO generated correlates precisely to the amount of activator in the sample. Therefore, the AC2 sample, with a mass ratio of paper waste–ZnCl₂ of 1:3, supplies more ZnO, as evidenced by the distinctive ZnO peaks. The carbon peak is almost imperceptible in samples containing an activator, most likely due to ZnO shielding, while its level is below the device’s detection limit.

Table 2. Properties of main paper types.

Paper Type	Main Composition	Activated Carbon Characteristics	Recommended Applications	Ref.
Office Paper	High-purity cellulose, low lignin	High carbon yield, well-developed microporous structure	Water and air purification	[32]
Cardboard	Cellulose, hemicellulose, lignin, additives (adhesives, coatings)	Lower carbon yield, mesoporous structure formation	Adsorption of larger molecules, diverse uses	[33]
Newsprint	Mechanical pulp, high lignin content	Lower carbon yield, mixed micro- and mesoporous structure	Large-scale activated carbon production with broad pore size distribution	[34]
Mixed Paper Waste	Cellulose, lignin	Variable carbon yield and pore structure	Cost-effective activated carbon for various applications	[20]
Office paper	Cellulose, CaCO3	Lower carbon yield, mixed micro- and mesoporous structure	Water purification	This study

shows a comparative analysis of our work with already published studies, accounting for the different type of waste paper used, with some other elements that were used to produce activated carbon.

To obtain more detailed insights into the AC structure, the EPR investigations at room temperature were performed, and the obtained spectra are displayed in Figure 3. All spectra are composed of broad, intense lines which can be considered a contribution from paramagnetic species arising from impurities in the samples. Moreover, the samples contain a narrow EPR line with a Lorentzian line shape centered at g = 2.002 and a linewidth ∆H = 8G. This signal, observed in other carbon-based materials, was attributed to localized C-centered radicals, such as dangling bonds in defective carbon structures [30]. The intensity of this signal decreases with the amount of activator, indicating the reduction in C-centered radicals’ concentration.

SEM examined AC samples’ morphology at different magnitudes, as shown in Figure 4. It is observed that, overall, activated carbon exhibits an irregular sheet-like structure, having a different surface structure.

The surface morphologies shows that the AC samples without activators have a more compact surface, whereas the AC1 and AC2 samples have a more uneven and often rough surface structure due to the formation of ZnO particles following the activation process. It can be seen that the quantity of ZnO is proportional to the amount of activator in the sample.

The elemental composition of the AC2 sample was established and confirmed by EDS mapping, as illustrated in Figure 5. The images show the presence and distribution of the primary elements C, O, Zn, and Ca, typical of AC-ZnO composites.

The textural properties of the activated-carbon-based samples were revealed by measurements of the specific surface area and porosity, as presented in

Table 3. Textural properties of activated carbon samples obtained under different conditions.

Sample	SBET (m2/g)	Pores Volume (cm3/g)	Dpores (nm)
AC0	4.49	0.043	54.27
AC1	71.2	0.0612	33.93
AC2	318.45	0.043	5.35

.

The specific surface area varies from 4.49 m²/g to 318 m²/g depending on the calcination conditions and the manner of interactions with the activator.

The porosity analysis highlights the different behaviors of the AC samples depending on the synthesis conditions, and the results are shown in Figure 6. Including the activator causes cavities on the carbon surface because the activator evaporates, leaving space during carbonization. Furthermore, in the specific cases of the AC1 and AC2 samples, the heat treatment regime resulted in the partial oxidation of the activating agent, indicating the existence of ZnO. These micro/mesopores can enhance the specific area of the carbon material and supply more active sites for further chemical reactions for specific applications [28]. Thus, the AC0 sample (Figure 6a) shows type III isotherms that are characteristic of weak adsorbent–adsorbate interactions. The hysteresis shape formed by the two isotherms is of type H3, suggesting a material formed by flat particles or an adsorbent with crack-shaped pores. The pore volume distribution curve of the AC0 sample has a multimodal appearance with six maxima located in a wide range of pore sizes (12.5 nm–186 nm) and shows a predominance of macropores (D > 50 nm). The AC1 sample (Figure 6b) presents a type IV isotherm, characteristic of mesoporous adsorbents. This sample shows the H2 type of hysteresis, characteristic of interconnected pores with different sizes. The pore distribution curve shows maxima in the meso-domain (4.5, 15.7, 27.3 nm) and macro-domain (122.6 nm). Unlike the AC0 sample, mesopores with diameters of 4.5 nm are predominant. AC2 sample (Figure 6c) has a shape similar to type II isotherms, suggesting strong interactions between adsorbate–adsorbent and a non-porous adsorbent, a fact confirmed by the pore distribution curve that shows a predominance of the micropores (~2 nm) and small mesopores (4 nm).

2.2. Optical Properties

The samples’ optical properties were investigated by UV–Vis spectroscopy, and the obtained spectra measured in the 200–800 nm range are displayed in Figure 7. It can be observed that the AC0 sample shows a broad absorption spectrum. The presence of ZnO in the AC1 and AC2 samples drive to a narrow absorption spectrum, which is blue-shifted compared with that of the AC0 spectrum and has a maximum absorbance at 350 nm. Based on the UV–Vis absorption spectra, the band gap energy was calculated using the Tauc’s equation. The band gap energy was obtained by extrapolating to zero the linear part of (αhυ)² vs. hυ dependence. The ACO has a wide band gap of 4.21 eV. The band gap of the AC1 and AC2 samples is 3.25 and 3.30 nm, respectively.

2.3. Photocatalytic Properties

The sample’s photocatalytic activity was tested under visible light irradiation using a synthetic solution of RhB. The experiments were performed by dispersing 3 mg of samples in 10 mL of RhB solution, and before irradiation, the solution was held for 2 h in the dark to attain the adsorption/desorption equilibrium.

The sample’s photocatalytic activity was tested under visible light irradiation using a synthetic solution of RhB. The experiments were performed by dispersing 3 mg of the samples in 10 mL of RhB solution, and before irradiation, the solution was held for 2 h in the dark to attain the adsorption/desorption equilibrium. As an example, the UV–Vis absorption spectra of RhB aqueous solution in the presence of AC2 sample at different adsorption and irradiation time intervals are displayed in Figure 8a. It can be observed that the intensity of the absorption band of RhB located at 553 nm decreases with increases in the irradiation time.

As can be observed in Figure 8b, the samples have different adsorption capacities, ranging from 19% for the AC0 sample to 45% for the AC1 sample. Under visible light irradiation, the RhB removal rate increases. The best removal rate was obtained for the AC2 sample. To evaluate the photocatalytic activity of the samples, the first-order kinetic model was applied:

$$-\ln \left({\displaystyle \frac{{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}}\right)={\mathrm{k}}_{\mathrm{i}}\ast \mathrm{t}$$

where A_t and A₀ represent the absorbance of RhB at time t and after adsorption in the dark.

Linear dependence was obtained for all samples that fit well with the first-order kinetic model, with the fitting parameters displayed in the figure inset. The highest rate constant was obtained for the AC2 sample, sustaining the conclusions that possess the best photocatalytic activity.

Reactive oxygen species (ROSs) are responsible for decomposing various organic molecules [33,34,35,36,37]. To evidence the photocatalytic mechanism, the generation of reactive oxygen species was investigated using the EPR coupled with spin-trapping technique measurements, as shown in Figure 9. 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as spin-trapping agent, and dimethyl sulfoxide (DMSO) was used as the solvent.

After 10 min of irradiation, a complex EPR spectrum was obtained, as observed in Figure 8. The spectrum simulation indicates the presence of two spin adducts: •DMPO-OCH3 (aN = 13.2 G, aH = 8.17 G, g = 2.0105) and •DMPO-N (aN = 13.9 G). The •DMPO-OCH3 spin adduct appears due to DMSO interaction with the •OH species generated by sample irradiation. The second component was identified as a nitroxide-like radical (aN = 13.8 G, g = 2.0105) formed by the cleavage of N-C bonds and ring-opening of DMPO.

Based on the above results, we proposed the following degradation mechanism which is illustrated in Figure 10. It is widely recognized that photocatalytic activity depends upon phase structure, particle size, adsorption ability, and e⁻-h⁺ recombination rate [38,39]. Under visible light irradiation, e⁻-h⁺ pairs are formed by transferring e⁻ from the ZnO valence band (VB) to the conduction band (CB), leaving behind positively charged holes. Due to their wide band gaps, e⁻-h⁺ pairs cannot be generated in CaCO₃ [40,41]. The electrons from the CB of ZnO can be easily transferred to the CaCO₃ conduction band, preventing the recombination of the e⁻-h⁺ pair [41,42]. Moreover, the activated carbon plays a double role, contributing to pollutant molecules’ adsorption on the particle surface and acting as mediators for charge transfer due to their ability to capture electrons, contributing to the reduction in the charge carriers’ recombination [43]. Therefore, porous carbon on the photocatalyst surface enhances photocatalytic responses. Thus, the electron transferred to CaCO₃ will interact with the O₂ from water, generating •O₂^–, which forms •OH species by protonation. The holes from the ZnO VB will interact with the H₂O molecules or adsorbed hydroxyl ion (OH⁻), generating •OH.

One appealing feature of photocatalysts is their capacity to be reused more than once after removing a specific pollutant. The ability to reuse the AC2 sample was examined using the RhB pollutant. After five reaction cycles were performed in similar conditions, only a small decrease in the removal rate was observed. For each experimental protocol, the AC2 sample was washed with water and ethanol and dried for 12 h. Figure 11 shows the photocatalytic activity recorded during each of the five tests. Even after four measurements, the successful performance of the removal rate proved the photocatalyst’s stability.

3. Materials and Methods

3.1. Materials

The aforementioned chemical reagents were utilized in the synthesis process: H₂SO₄ (97% Alfa Aesar), NaOH (reagent grade, VWR Chemicals, Radnor, PA, USA), and zinc chloride–ZnCl₂ (ACS, Reag. Ph. Eur, VWR Chemicals, Radnor, PA, USA). Each of the chemicals was of analytical grade and was used according to instructions given, without additional purification.

3.2. Sample Preparation

Office A4 paper (Mondi SCP, Ružomberok, Slovakia) waste (5 g), printed on both sides, was cut into 0.5 cm wide strips, shredded, and treated with 1000 mL 0.1 M sulfuric acid under stirring. After 24 h, it was filtrated and rinsed with double-distilled water until the pH was neutral. The sediment was then dried in an oven at 65 °C for 12 h. The cleaned and dried paper pieces were impregnated with activated agent solution for activation. ZnCl₂ was dissolved in 100 mL H₂O and put over dried paper. Three samples were produced using varied dry paper and ZnCl₂ mass ratios: 1:0 (AC0), 1:1 (AC1), and 1:3 (AC2). After stirring for 1h for uniform impregnation, the mixture was kept in ZnCl₂ solution overnight. After this step, the sediment was separated by filtration and dried. The activated pieces were subsequently loaded onto an alumina boat, placed in the tube furnace’s quartz tube, and calcinated at 800 °C in N₂ flow. Finally, the carbon-based material was naturally cooled to room temperature, treated with HCl 3 M (10 mL in 100 mL of bi-distilled water) under sonication for 30 min to reduce the ash content and improve the porous properties of the samples. After separation by centrifugation, the samples were washed multiple times with distilled water, dried, and investigated to determine their specific features.

Finally, the carbon-based material was naturally cooled to room temperature, treated with HCl 3 M, dried, and investigated to determine its specific features.

To ensure that experimental conditions were reproducible, the synthesis process for each sample was repeated at least three times. The synthesis process is detailed in Figure 12.

3.3. Methods

The thermogravimetric analysis of the samples was analyzed with a Metttler Toledo TGA/SDTA851e (Greifensee, Switzerland) apparatus from ambient temperature up to 1200 °C in a platinum crucible, at a heating rate 20 °C/min, under a nitrogen flow of 60 mL/min.

The samples’ crystalline structure was further verified using X-ray diffraction (XRD), which was recorded with a Bruker D8 Advance X-ray diffractometer (Karlsruhe, Germany) set at 40 kV and 40 mA and fitted with a germanium monochromator in the incidence. The X-ray diffraction patterns were achieved in the 2θ range of 100–800 using Cu Kα1 radiation (λ = 1.54056 Å) in a step-scanning mode with increments of Δθ = 0.02°. The morphologies of the nanocomposites were investigated using transmission electron microscopy (TEM). The TEM experiments used a Hitachi SU8230 Transmission Electron Microscope (Hitachi, Tokyo, Japan) with a cold-field emission gun. The powder was combined with ethanol using a BANDELIN SonOREX (BANDELIN electronic GmbH & Co. KG, Berlin, Germany) homogenizer before spreading over a 400-mesh copper grid coated in carbon film.

The electron paramagnetic resonance (EPR) spectroscopy experiments were accomplished on a Bruker E-500 ELEXSYS spectrometer (Karlsruhe, Germany), using X-band (9.52 GHz) at room temperature, under identical conditions: microwave power, 0.635 mW; modulation frequency, 100 kHz. The porous structure of the materials was investigated using nitrogen adsorption–desorption isotherms. Total surface area (St) was calculated using the BET method, and porosity (pore volume, pore size) was determined using the Dollimore–Heal model. The isotherms were measured with Micromeritics, TriStar II 3020-Surface Area, and Porosity Analyzer (Norcross, GA, USA). The adsorption–desorption isotherms were measured close to the boiling point of nitrogen (77 K). Before any determination, the samples (about 0.2 g) were degassed at 200 °C for 4 h under 1Pa pressure to remove physisorbed pollutants from the sample’s surface.

The photodegradation of the RhB pollutant was performed using a homemade laboratory-visible reactor equipped with an ultrasonic bath and a 400 W halogen lamp (Osram, Munchen, Germania). The samples (3 mg) were dispersed in a 10 mL pollutant (RhB: 1.0 × 10⁻⁵ mol/L) solution, and then the obtained solution was stirred in the dark for 60 min to attain the adsorption equilibrium. Each photocatalytic experiment was continuously performed for 300 min. Every 60 min, a part of the resulting solution (3.5 mL) was examined using a UV–Vis spectrophotometer. The photocatalytic activity was evaluated by recording the pollutant-specific maximum absorbance and is calculated using the following equation:

Photocatalytic activity (%) = (1 − C_t/C₀) × 100

(5)

where C_t and C₀ represent the pollutant concentration at t and t = 0, respectively. A calibration curve was used to calculate the pollutant concentration corresponding to the measured absorbance

4. Conclusions

Recycling office paper is a convenient way to produce activated carbon (AC). In our experimental conditions, chemical activation with ZnCl₂ in various mass ratio waste paper activator allowed us to obtain AC loaded with ZnO. The produced composites’ structural, morphological, and photocatalytic properties have been established through relevant investigations. X-ray diffraction reveals the presence of ZnO in the samples containing the activator. Additionally, peaks related to cellulose, calcite from the paper component, and carbon can be identified in the samples. EPR spectroscopy proved the presence of carbon in the samples based on the existence of C-centered radicals. Surface morphology shows a more porous structure for samples with the activator. The specific surface area of samples varies from 4.49 m²/g to 318 m²/g and porosity analysis highlights different behavior depending on calcination conditions and the manner of interactions with the activator. The best photocatalytic activity toward RhB degradation was observed in the AC2 sample. Porous carbon on the photocatalyst surface, combined with ROS production, plays an important role in improving photocatalytic performance. This study suggests that discarded newspaper offers an alternative source of carbon. Using newspapers as a carbon source is economically viable, reduces pollution, and promotes long-term perspectives.