One-Step-Modified Biochar by Natural Anatase for Eco-Friendly Cr (VI) Removal

Abstract

The global serious pollution situation urgently needs green, efficient, and sustainable development methods to achieve heavy metal pollution control. The photocatalytic properties of anatase are sufficient to achieve pollution control by providing photoelectrons to harmful heavy metals. However, since natural anatase particles tend to agglomerate and deactivate in water, most studies have been conducted to prepare TiO₂–biochar nanocomposites using chemical synthesis methods. In the present study, we utilized pyrolytic sintering to load natural anatase onto biochar to obtain natural anatase–biochar (TBC) composites. Characterization tests, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), showed that anatase was uniformly partitioned into the surface and pores of biochar without destroying the lattice structure. Due to its photocatalytic properties, TBC degraded Cr (VI) by 99.63% under light conditions. This is 1.58 times higher than the dark condition. Zeta potential showed that the surface of the TBC was positively charged under acidic conditions. The charge attraction between TBC and chromium salt was involved in the efficient degradation of Cr (VI). Different sacrificial agents as well as gas purge experiments demonstrated that photoelectrons (e⁻) and superoxide radicals (${\mathrm{O}}_2^{-}$) dominated the degradation of Cr (VI). TBC has the characteristics of high efficiency, stability, and sustainability. This may provide a new idea for the preparation of photocatalytic materials and the realization of environmental protection and sustainable development through heavy metal pollution control.

1. Introduction

With the swift advancement of the economy and the deepening of urbanization, human activities are exerting a greater negative influence on natural environmental systems [1,2,3]. One of the primary concerns pertains to the high levels of toxicity, long-term persistence, and the slow degradation of heavy metal pollution. This type of contamination not only causes damage to the ecological environment but also poses a significant risk to human health through direct ingestion, inhalation of air, and skin contact [4,5,6]. Chromium (Cr) is frequently utilized as mordants and impregnators in the impregnation industry. Nevertheless, an excessively high concentration of chromium can result in immeasurable damage to the human nervous system [7,8]. Hence, extensive research has been dedicated to the development of techniques for eliminating Cr (VI) from industrial wastewater [9]. These methods include adsorption, electrocoagulation, ion exchange process, membrane filtration, and reverse osmosis [10,11,12,13]. Nevertheless, the limitations associated with these technologies, such as poor efficacy, high expenses, excessive energy consumption, and substantial by-product contamination, severely hinder their practical utility [14]. Consequently, it is crucial to devise a cost-effective and environmentally friendly approach for removing Cr (VI) from wastewater [15,16].

Photocatalytic degradation is a technology known for its environmental friendliness, wide applicability, and absence of secondary pollution among the array of available technologies [17,18]. As one of the advanced oxidation processes, the photocatalytic abatement of pollutants has attracted considerable interest from both academic and industrial societies. To date, numerous photocatalysts such as Fe_NPs@Fe_SAsNC, ZnIn₂S₄/CdS, CoO@MnCo₂O₄, and CeO₂/BiOX have been developed [19,20,21,22]. A previous study synthesized (CeGdHfPrZr)O₂ nanoparticles by hydrothermal synthesis method and studied the photocatalytic degradation activity of (CeGdHfPrZr)O₂ nanoparticles [23]. Many studies have effectively synthesized new photocatalysts in different ways and successfully used them for Cr (VI) degradation, such as flower-like C/Bi/BiOI heterojunction, (CeGdSmYZr)O₂ high-entropy oxide nanoparticles, and S-scheme CuInS₂/ZnIn₂S₄ heterostructures [24,25,26]. Titanium dioxide (TiO₂) is distinguished among photocatalysts for its non-toxicity, cost-effectiveness, environmental benignity, and robust chemical stability, garnering widespread attention from researchers [27]. The application of TiO₂ as a semiconductor-based catalyst for the photocatalytic reduction of Cr (VI) has become a central area of investigation in numerous research endeavors [28]. Researchers synthesized HC/TiO₂ composites using a simple one-step hydrothermal method and studied the photocatalytic reduction performance of Cr (VI) [29]. However, the application of pure TiO₂ is limited by its wide bandgap, small specific surface area, poor adsorption performance, and weak response to visible light. The practical use of TiO₂ is also restricted by its propensity to agglomerate, which makes its dispersal and reuse difficult [30,31,32,33].

Consequently, researchers globally have undertaken significant efforts to alter the properties of TiO₂ with the aim of enhancing its functionality. For instance, various studies have shown that methods such as noble metal doping, ion doping, composite semiconductor, and surface photosensitization can expand the photoresponse range of TiO₂ and mitigate the recombination rate of photogenerated electron–hole pairs [34,35,36]. Moreover, loading TiO₂ onto solid carriers has been found to improve its dispersion and recycling performance [37]. It is advantageous to incorporate TiO₂ into porous solid carriers with adsorption properties. This approach supports catalyst regeneration and mitigates the challenges associated with TiO₂ powder loss and difficult separation and recovery [38]. Additionally, it enhances the specific surface area of TiO₂, thereby increasing the efficiency of its photocatalytic reaction [39]. Carriers exhibiting exceptional adsorptive properties can effectively capture and concentrate surrounding pollutants, leading to a significant enhancement in mass transfer rate during the reaction [40].

Biochar, obtained from biomass, is a carbon-rich material that is highly accessible and preferred [41]. As a highly porous material with a large specific surface area, biochar presents numerous potential applications in the realms of adsorption and catalysis [42,43]. Biochar can be prepared via the pyrolysis or coking of low-cost and wide-source agricultural biomass waste such as straw and sawdust [44]. Due to its large surface area, hierarchical porosity, and abundant functional groups, biochar has excellent adsorption properties and can be used to remove organic and inorganic pollutants [45,46]. Of greater significance is the fact that the surface of biochar can be subject to modifications or combinations with catalytic substances, allowing for the creation of superior photocatalytic composite materials [47]. Therefore, biochar–TiO₂ composite materials can be developed to synergistically adsorb and photocatalytically reduce Cr (VI) to Cr (III) in wastewater.

Studies have demonstrated that the incorporation of biochar materials into pure semiconductors enhances their adsorption and photocatalytic properties [48,49]. Biochar has high chemical stability and semiconductor characteristics, which can make it an excellent platform for supporting TiO₂ [50]. Biochar-supported TiO₂ can absorb energy levels exceeding its bandgap when subjected to ultraviolet light of a certain intensity, leading to electron transfer and the generation of highly photoactivated electron (e⁻) and hole (h⁺) pairs [51]. The e⁻ generated from the photoreaction mentioned above exhibits a high level of reducing capability and is capable of converting Cr (VI) to Cr (III) [30]. The electron-conductive nature of biochar can reduce the quick recombination of photo-induced electron–hole pairs during photocatalysis, thereby improving the removal efficiency of pollutants [52,53].

Currently, the preparation of TiO₂–biochar photocatalytic composite materials often utilizes nano-TiO₂ as a raw material, which undoubtedly increases the cost of material preparation [54,55]. Additionally, most methods for binding TiO₂ with biochar involve chemical gel and hydrothermal carbonization techniques [56,57]. While these methods have yielded favorable preparation effects, they are associated with overly complex production processes and high costs. Compared to the aforementioned methods, the oxygen-limited pyrolysis method for loading natural anatase onto biochar provides a cost-effective, convenient, and high-performance alternative for preparing novel photocatalytic composite materials. This approach holds significant potential for economic, environmental, and sustainable development. Furthermore, the degradation performance of photocatalytic composite materials is not only related to the properties of the carrier but also influenced by reaction conditions (such as pH and dosage). Therefore, optimizing both the material and photocatalytic reaction conditions and further investigating the potential mechanisms of photocatalytic reactions in treating chromium-contaminated wastewater are of significance.

In this study, we chose natural anatase to replace chemically synthesized TiO₂ nanoparticles. Natural anatase–biochar (TBC) composites were prepared by loading natural anatase onto corncob biochar using a one-step pyrolytic sintering method. Various characterization methods demonstrated that anatase binds well to the biochar without destroying the lattice structure. Subsequent comparisons of different materials further demonstrated the excellent Cr (VI) degradation performance of TBC. The mechanism of Cr (VI) degradation by combined charge and the photocatalysis of TBC was revealed by pH gradient experiments, zeta potential tests, free radical sacrifice experiments, and gas continuous blowing experiments. The cycling and dissolution experiments illustrated the stable and sustainable characteristics of TBC. We provide a natural, low-cost, efficient, and sustainable idea for the preparation of photocatalytic carbon-based materials.

2. Materials and Methods

2.1. Preparation of Biochar and TBC Composites

TBC composites were prepared from corncob biochar and anatase by oxygen-limited pyrolysis. The raw material for corncob was sourced from Lanzhou, Gansu Province, China. After a 5-day sun-drying period, it was pulverized into powder. The resulting powder was then subjected to an anaerobic pyrolysis environment with controlled nitrogen intake at a flow rate of 0.5 L/min. Following this, the powder underwent heating in a muffle furnace at a rate of 5 °C/min and was maintained at a temperature of 350 °C for 2 h to yield corncob biochar. The rate of temperature rise was measured using a muffle furnace system. The biochar, after preparation, was pulverized into fine powder, sieved through a 200-mesh screen, and then stored in a dry glass container.

Natural anatase was procured from the Bayan Obo Mining area in the Inner Mongolia Autonomous Region, China. Natural anatase was ground into a fine powder and sieved through a 200-mesh sieve. The TBC was prepared by incorporating 25% (by mass) anatase into biochar and subjecting it to oxygen-limited pyrolysis at 500 °C while maintaining other conditions consistent with the preparation of biochar. The resulting composite was further milled into powdered form and passed through a 200-mesh sieve before being stored in dry glass bottles for future use.

2.2. Characterization of TBC Composites

The crystalline phases of the TBC material powder were identified using multifunctional powder X-ray diffraction (XRD) (Ultima IV, Tokyo, Japan) with Cu kα radiation operated at 40 kV and 40 mA. The particle size and morphology of the TBC material powder were observed using a scanning electron microscope (SEM) (JSM-6510, Tokyo, Japan) operated at 0.5–30 kV and a working distance of 10 mm. Before SEM tests, gold was sprayed onto samples to ensure electrical conductivity. The elemental composition and valence state of the TBC material were determined by X-ray photoelectron spectroscopy (XPS), which involved using He lamp ultraviolet photoelectron spectroscopy (UPS) with 6 kV an argon (Ar) ion gun and Al/Mg double anode (Thermo, Waltham, MA, USA), with a maximum power of 600 W.

2.3. Electrochemical Characteristic Analysis of TBC Composites

The photoelectrochemical analysis of the TBC composites was conducted in a single-chamber system under illumination. This involved utilizing a TBC-modified FTO electrode as the working electrode, a platinum plate (2.5 cm × 2.5 cm) as the counter electrode, and a saturated calomel electrode (SCE, with an electrode potential of 2.24 V) as the reference electrode. The photogenerated current characteristics of the TBC composites were investigated through real-time amperometry measurements (I-t) at an applied potential of 0.8 V vs. SCE. In the alternate experiment, the photoanode surface was subjected to 30-second light and dark cycles directly from a light-emitting diode (LED) light source. The dark and light conditions were achieved using an external LED with a working wavelength range of 400–700 nm, which was close to visible light [58]. The light source was provided by a visible LED lamp bead. The illumination intensity was 60 mW/cm², as measured by a photosynthetic radiometer (FGH-1, Beijing Normal University Photoelectric Instrument Factory, Beijing, China). Electrochemical properties were assessed utilizing an electrochemical workstation (PGSTAT204, Herisau, Switzerland). Electrochemical impedance spectroscopy (EIS) was conducted in a traditional three-electrode system using 1 M Na₂SO₄ aqueous solution as the electrolyte, with a fluorine-doped tin oxide (FTO) electrode serving as the working electrode, SCE as the reference electrode, and Pt sheet as the auxiliary electrode, to investigate the charge transfer resistance of carriers on the surface of different materials. The electrochemical workstation was utilized to perform EIS measurements at the open-circuit voltage, with a frequency range spanning from 0.1 to 10 kHz. All the experimental procedures in this study were repeated at least three times to minimize experimental errors.

2.4. Analysis of Cr (VI) Degradation Property

To assess the impact of TBC composites on Cr (VI) degradation, a 50 mL shaker system was devised. Each vessel received 20 mg/L K₂Cr₂O₇ solution and 40 mg of the test material, with their concentration profiles meticulously monitored. Samples were taken at 1.0 h intervals after the start of the unit operation, with each sample volume being 1.5 mL. The degradation rates of anatase, biochar, and TBC composites were compared under both light and dark conditions. The lighting conditions were provided by the previously mentioned LED light source. The concentration of Cr (VI) was determined using a UV-Vis spectrophotometer and the 1,5-diphenylcarbazide method.

The formulas for calculating the removal percent of Cr (VI) (Formula (1)) and determining the degradation kinetics (Formula (2)) are as follows:

$$Removalpercent={\displaystyle \frac{C_0-C_f}{C_0}}\times 100\%$$

(1)

$$\ln \left({\displaystyle \frac{C_t}{C_0}}\right)=kt$$

(2)

where C₀ is the initial Cr (VI) concentration, C_f is the final Cr (VI) concentration, C_t is the Cr (VI) concentration at a specific time, k is the degradation rate constant, and t is the reaction time [59].

The adsorption rate of the adsorbate onto the adsorbent was determined by the kinetics of adsorption. To better assess the rate-controlling steps in the Cr (VI) adsorption process using different materials and to analyze the adsorption mechanism, first-order quasi-kinetic and second-order quasi-kinetic equations were employed to characterize the adsorption kinetics. The linear equations of the adsorption kinetics model are as follows:

First-order quasi-kinetic equation:

$$\ln \left(Q_e-Q_t\right)=\ln Q_e-k_{1}t$$

(3)

Second-order quasi-kinetic equation:

$$t/Q_t=1/(k_2\times Q_e^2)+t/Q_e$$

(4)

where Q_e is the equilibrium adsorption capacity, Q_t is the adsorption capacity at time, k₁ is the first-order quasi-kinetic rate constant, k₂ is the second-order quasi-kinetic rate constant, and t is the adsorption time.

2.5. pH Gradient Degradation

The zeta potentials of the TBC composites and biochar were measured using a nanoscale zeta potential analyzer (Stabino Zeta, Dusseldorf, Germany). The impact of the initial pH conditions on the adsorption of Cr (VI) by the TBC composite was investigated within a pH range of 2–11. Solutions containing 20 mg/L of Cr (VI) were pH-adjusted using dilute nitric acid or sodium hydroxide. A total mass of 40 mg of the TBC was mixed with 50 mL of the pH-adjusted Cr (VI) solution and transferred to a beaker. The beaker was placed on a magnetic mixer for 1 h to achieve balance. Subsequently, the supernatant was extracted by centrifugation at 4000 rpm for 3 min, and the Cr (VI) concentration was determined. The experiment was carried out under the conditions of LED light source as described above. All experiments were performed in triplicate to minimize error.

The effect of electron acceptor O₂ on photocatalytic degradation of Cr (VI) was studied by nitrogen/oxygen purging experiment. Two different conditions, namely oxygen-rich and anaerobic, were controlled, and the gas flow rate was 0.5 L/min.

2.6. Detection of Ti Leaching from TBC

Deionized water and the TBC were added to a 100 mL Erlenmeyer flask at a ratio of 0.8 mg/mL. The flask was then shaken at 25 °C in a thermostatic shaker at a rate of 150 rpm. Each oscillation lasted for 24 h, after which the solution was filtered using a 0.45 µm membrane to obtain the filtrate for analysis. The filtered TBC was then re-suspended in deionized water at the same ratio of 0.8 mg/mL in a 100 mL Erlenmeyer flask to start a new oscillation experiment. For the filtrate, 50 mL of the initial filtrate was discarded, and the pH was adjusted to below 2 by adding an appropriate amount of nitric acid. The titanium concentration in the solution was measured using inductively coupled plasma mass spectrometry (ICP-MS) (Thermo Fisher, XII). The ICP-MS tuning conditions were set as follows: Li > 5.0 × 10⁴ cps, In > 1.8 × 10⁵ cps, U > 2.0 × 10⁵ cps, oxide < 2%, doubly charged ions < 3%, with the Sc internal standard response between 70% and 130%. The RF power was set at 1550 W, with a sampling depth of 5 mm. The analysis was conducted in the standard mode (STD).

3. Results and Discussion

3.1. Characterization of TiO₂–Biochar (TBC)

3.1.1. Morphometric Analysis of TBC

The results of scanning electron microscopy (SEM) analysis are shown in Figure 1. The surface of the raw material of corn cob was relatively smooth and regular, with a multilayered bundle structure of cellulose (Figure 1a). After pyrolysis to become BC, the bundle structure partially collapsed. The release of volatile components and the overflow of gaseous substances formed pores (Figure 1b,d). SEM images of TBC verified the dispersion and effective immobilization of anatase particles on the biochar surface (Figure 1g,h). Compared with the BC, the surface of TBC was rough and densely distributed with fine particles, and part of the pore structure was filled with anatase. Energy-dispersive X-ray spectroscopy (EDX) analysis reveals the elemental distribution on the sample surfaces. As shown in Figure 1c, no Ti peaks are observed in pure biochar, whereas the TBC sample exhibits distinct Ti peaks (Figure 1i). A comparison of the EDX spectra between BC and TBC clearly indicates the presence of Ti on the TBC surface (Figure 1i). The combined SEM and EDX results confirm the successful attachment of anatase to biochar, demonstrating the successful synthesis of the anatase–biochar (TBC) composite.

3.1.2. Mineralogical Analysis of TBC

The XRD characterization results are shown in Figure 2. The XRD background noise of biochar was high. This is mainly because most of the organic carbon has an amorphous structure with a very low intensity close to the background intensity. Characteristic peaks of carbon complexes appeared at 26.84° and 28.06°, which is due to pyrolysis, leading to the appearance of crystalline carbon. Based on the XRD pattern of natural rutile, it can be determined that its main minerals are anatase (TiO₂) and calcite (CaCO₃). The peaks at 27.32°, 36.42°, 41.15°, 54.32°, 56.64°, 68.96°, and 69.82° in the XRD pattern correspond to the five characteristic diffraction crystal planes of anatase, namely (110), (101), (111), (211), (220), (301), and (112), respectively. The peaks at 23.12°, 29.13°, 39.48°, 43.17°, 47.63°, and 48.58° correspond to the characteristic diffraction crystal planes of calcite, namely (012), (104), (11-3), (202), (018), and (11-6), respectively. As can be seen from the figure, the peaks of anatase (TiO₂) are sharper and more intense, and this image indicates that anatase is better crystallized. The XRD pattern of biochar (BC) shows the characteristic peaks of crystalline facets of (012), (104), and (207) of calcite at 23.12°, 27.13°, and 69.17°, respectively. Similarly, 29.36°, 41.13°, and 49.24° also show characteristic peaks of KCl. These peak spectra of other elements indicate that there are still some components of corn cob biochar that are not pyrolyzed completely. Observing the XRD pattern of anatase–biochar composite (TBC), we found that the characteristic peaks of anatase (TiO₂) appeared at 36.42° and 56.72°. This indicates that rutile was successfully loaded onto rice husk biochar. At the same time, the pattern of TBC also showed peaks shaped like mixed oxides of Ti, Al, and Nb. This phenomenon indicates that some of the clay minerals were also successfully loaded onto the BC when TBC was prepared by pyrolysis.

3.1.3. Elemental Valence Analysis of TBC

XPS analysis can further reveal the elemental composition and valence states of the prepared materials. The full spectrum showed that the characteristic peaks of C 1s, O 1s, and Ti 2p in TBC appeared at 284.7 eV, 531.1 eV, and 458.9 eV, respectively (Figure 3a). The peaks of Ti 2p were not detected in BC, which is strong evidence of the successful loading of anatase into biochar. Figure 3b shows the fine spectrum of C 1s. Its spectral solution can be convolved into five carbon peaks. In addition to the oxidized carbon bonds of C=O (288.5 eV) and C–O (285.9 eV), satellite peaks (π–π*) located at 293.2 eV were found. The weak interactions between the aromatic rings are advantageous for the adsorption of substances such as Cr, atrazine, and tetracycline [60,61]. The appearance of C–F (293.2 eV) is usually attributed to the formation of biomass by its own pyrolysis process. It should be noted that no obvious Ti-C spectral peaks were observed near 281 eV, which implies that elemental C did not disrupt the lattice of TiO₂ and that anatase (TiO₂) in the TBC was adherent to BC [62]. In the O 1s fine spectrum (Figure 3c), the lattice oxygen (Ti–O–Ti) from anatase has a strong peak shape at 529.9 eV. BC mainly contributes to the hydroxyl oxygen O–H and C=O bonds [56]. As shown in Figure 3d, in the high-resolution XPS spectra of Ti 2p, the peaks at 459.0 eV and 464.7 eV belong to Ti 2p_3/2 and Ti 2p_1/2 of TiO₂, respectively. The binding energy difference (EB) between them is 5.7 eV, which satisfies the valence requirement of Ti⁴⁺. This further confirms the existence of Ti in the form of Ti⁴⁺ (TiO₂) [63].

3.2. Cr (VI) Degradation of Different Materials

As shown in Figure 4a, the degradation effect of pure anatase was very weak. The degradation rate was less than 8% with or without light. Despite the photocatalytic performance of anatase, anatase particles tended to agglomerate and deactivate in aqueous environments, which led to the insignificant degradation performance of pure anatase under light conditions. The degradation performance of BC under dark and light conditions was very close to each other, reaching a degradation efficiency of 47%. This also indicates that the biochar itself does not possess photocatalytic properties, which is similar to a previous study [64]. The TBC composites showed a degradation rate of 62% under dark conditions, with a maximum adsorption of 15.78 mg/g. The best performance was the degradation process of the TBC under light conditions. Under light conditions, TBC showed the highest photocatalytic activity by completing almost all the Cr (VI) degradation in 2 h. Its adsorption amount was 24.0 mg/g, which was 1.83 times higher than that of the TBC under dark conditions (

Table 1. Kinetic fitting parameters for degradation of Cr (VI) by TBC, BC, and anatase.

Adsorbent	Pseudo-First Order			Pseudo-Second Order
	Qe (mg/g)	k1 (min−1)	R2	Qe (mg/g)	k2 (min−1)	R2
Light + TBC	24.40	0.03823	0.9259	27.86	0.0358	0.9984
Light + BC	11.72	0.02348	0.9809	14.63	0.0683	0.9955
Light + TiO2	7.24	0.03014	0.9974	8.96	0.1116	0.9474
Dark + TBC	15.78	0.03265	0.9488	19.11	0.0523	0.9920
Dark + BC	11.24	0.03039	0.9932	13.90	0.0719	0.9919
Dark + TiO2	2.87	0.01377	0.9946	4.11	0.2431	0.9475

). The photocatalytic degradation experiments showed that the biochar structure could significantly improve the photocatalytic efficiency of TiO₂. The anatase can be uniformly distributed on the surface of biochar, which greatly slows down the agglomeration phenomenon of anatase. Meanwhile, with the advantage of its own adsorption and high surface area, the biochar can further adsorb Cr (VI) from the environment to the vicinity of the material. This provides more active sites for Cr (VI), which is conducive to the contact of Cr (VI) with the material to exchange electrons.

The adsorption process was fitted using the primary adsorption kinetic equation and the secondary adsorption kinetic equation. As shown in Figure 4b, the fit of pure anatase is even better than that of BC and TBC (R² > 0.99). However, even for anatase under light, the reaction rate k₁ was only 0.03014 min⁻¹. This implies that pure anatase does not have the potential to be applied for the direct degradation of Cr (VI). The quasi-secondary kinetic equations for both TBC and BC had R² > 0.99, whereas for anatase under darkness, the R² was only 0.9475 (Figure 4c,d). It can be said that the adsorption processes of TBC and BC fit the secondary adsorption kinetic model better. Their adsorption processes are not controlled by a single factor of substance concentration only. The light-illuminated TBC had a theoretical maximum adsorption capacity of 27.86, which was 3.84 times higher than that of the light-illuminated anatase (Table 1). The excellent degradation process of the TBC may have the combined effect of various adsorption pathways, such as photocatalytic and physical adsorption (van der Waals force, electrostatic force, pore adsorption, etc.).

3.3. Effect of pH

We verified the effect of initial pH on the degradation performance of TBC in the range of pH = 2–11. For TBC, the initial pH in the environment had a strong influence on the adsorption of Cr (VI). With the increase in pH, the degradation rate of the TBC gradually decreased from 99.41% to 28.48%. The magnitude of this change was close to 70% (Figure 5a). pH change affects the surface charge of the adsorbent and the morphology of the arsenic compounds, which in turn affects the adsorption of arsenic. In order to understand the role played by initial pH, we measured the surface potential points of TBC, BC, and anatase with initial pH change (Figure 5b). It was found that the pH corresponding to the PZC (point of zero charge) of TBC, BC, and anatase was less than 7.00, with the pH_pzc of TBC = 6.87. This means that the surface of TBC is positively charged when pH < pH_pzc. When pH > pH_pzc, the TBC surface is positively charged. Typically, TBC exists in solution in the following states [65]:

$$\mathrm{T}\mathrm{i}-\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}\to \mathrm{T}\mathrm{i}-{\mathrm{O}\mathrm{H}}_2^{+}, \mathrm{pH}<{\mathrm{pH}}_{\mathrm{pzc}}$$

(5)

$$\mathrm{T}\mathrm{i}-\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{T}\mathrm{i}-{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}, \mathrm{pH}>{\mathrm{pH}}_{\mathrm{pzc}}$$

(6)

Under acidic conditions, Cr (VI) exists mainly as ${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}$, $\mathrm{H}\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{-}$, and $\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}$. At this time, due to pH < pH_pzc, TBC produces more positively charged adsorption sites by removing hydroxyl ions. The negatively charged Cr (VI) in the environment and the positively charged TBC achieve efficient degradation through charge mutual attraction. As pH increases above pHpzc, TBC is negatively charged. In an alkaline environment, Cr (VI) mainly exists as ${\mathrm{C}\mathrm{r}\mathrm{O}}_4^{2-}$, and the charge repulsion effect made the degradation of TBC decrease. Moreover, Cr (III) produced by degradation forms Cr (OH)₃ precipitates under alkaline conditions. These precipitates attached to the surface of TBC will further prevent Cr (VI) from being degraded by TBC.

3.4. Influence of Sacrificial Agents on the Photocatalytic Process of TBC

When anatase is photoexcited to produce electron–hole pairs, the exact process at play remains to be explored. The degradation of Cr (VI) is dependent on the conversion of electrons to Cr (III). It is possible that photoelectrons and free radicals with strong oxidizing abilities are involved in the degradation process. In order to deeply explore the mechanism of Cr (VI) removal by TBC, we added different sacrificial agents to the reaction system (Figure 6a). Among them, potassium persulfate (K₂S₂O₈) was used to capture electrons (e⁻), superoxide dismutase (SOD) was used to disproportionate superoxide radicals (${\mathrm{O}}_2^{-}$), tert-Butanol (TBA) was used to burst hydroxyl radicals (·OH), and EDTA-2Na was used to scavenge holes (h⁺). The greatest effect was observed for K₂S₂O₈, where the degradation of TBC was reduced to 53%. This suggests that photoelectrons (e⁻) produced by photocatalysis will be the main Cr (VI) contributor. The addition of SOD scavenges some of the superoxide radicals, which leads to the reduction of ${\mathrm{O}}_2^{-}$ participating in the reduction of Cr (VI). The degradation efficiency of SOA-TBC was also only 74.69%. The protons (H⁺) produced by holes (h⁺) and H₂O favored the degradation of Cr (VI), so the other sacrificial agents also had a weak effect.

The specific contributions of photoelectrons (e⁻) and superoxide radicals (${\mathrm{O}}_2^{-}$) were further verified using N₂ blowdown experiments and O₂ blowdown experiments (Figure 6b). The final Cr degradation efficiencies of both TBC composites without gas purge and TBC composites subjected to nitrogen continuous purge experiments were close to 100%. However, the nitrogen continuously purged TBC had a faster degradation rate in the first 120 min. The TBC with O₂ purge was the least effective, with only 80.14% degradation efficiency. This is because more O₂ will compete with Cr (VI) for photoelectrons (e⁻) produced by photocatalysis. This phenomenon strongly proves that although both photoelectrons (e⁻) and superoxide radicals (${\mathrm{O}}_2^{-}$) can reduce Cr (VI), the photoelectrons (e⁻) directly delivered to Cr (VI) through photocatalysis are the most dominant factor. After 120 min, as the Cr (VI) degradation efficiency in the flask approached 100%, the adsorption capacity under nitrogen purging and without gas purging both stabilized at 25 mg/g.

3.5. Photoelectrochemical Properties of TBC

In order to evaluate the separation and transfer of photogenerated electrons and holes in the TBC composites, transient photocurrent analysis and electrochemical impedance analysis (Figure 7) were performed on different samples. The photocurrent density responses of the detected materials were reproducible with alternating light and dark conditions. It should be noted that TBC and anatase are favorable due to their photocatalytic properties. Biochar provides a channel for photoelectrons to shuttle electrons. TBC presented a much higher peak current than anatase at 0.44 μA/cm², which is 1.62 times higher than anatase. This difference is understandable (Figure 7a). TBC disperses anatase in a porous carbon skeleton, which greatly mitigates the agglomeration of anatase in the water column. Thanks to this, the separation efficiency of photogenerated electrons and photogenerated holes is improved, the lifetime is extended, and the recombination probability is greatly reduced. In addition, the matrix of the TBC is a stabilized biochar material, which leads to a low bandgap. The active sites formed by these electron–hole pairs compete with Cr (VI), which in turn leads to the efficient reduction of Cr (VI).

The Nyquist plots reflect the charge transfer resistance process and the separation of electron–hole pairs at the interface of different materials under light. The equivalent circuit diagram was fitted using NOVA 2.1 and is shown in Figure 7b. The solution internal resistance (Rs) values are similar across the conditioned experimental systems (TBC: 27.4 Ω, BC: 27.0 Ω, anatase: 27.9 Ω) due to the strict control of the different materials in the same solution for the experiments (

Table 2. Fitted values of electrochemical impedance for TBC, BC, and anatase.

Conditions	Rs	Rp	CPE
Light + TBC	27.4	103.0 Ω	238 μF
Light + BC	27	141.3 Ω	217 μF
Light + TiO2	27.9	432.0 Ω	176 μF

). This indicates that the electrode distances and solution properties did not change significantly during the experiment. Rp is the charge transfer resistance at the electrode interface. In general, the smaller the semicircle diameter is, the lower the interface resistance, the faster the charge transfer, and the more effective the electron–hole pair separation is. Among all the samples, TBC had the smallest radius (Rp = 103.0 Ω), indicating the largest conductivity. This represents excellent electronic compatibility and interfacial separation efficiency. Compared with the TiO₂ sample, the impedance of BC was only 32.70% of that of anatase. This indicates that BC has some electron transfer ability and can be used as an electron transfer channel. The Rp of anatase was 4.19 times that of TBC. We believe that although anatase produces photoelectron–photoholes in the presence of light, it can provide a channel for electrons. However, there is no specific substance in the environment to assist in photoelectron (e⁻) and photohole (h⁺) depletion, which allows for rapid electron–hole complexation. Moreover, anatase is heavily agglomerated in water. The irradiation of light can only excite the surface anatase to produce a response, and the internal anatase cannot produce an electronic pathway, and it is difficult to transfer e^- produced by excitation to the electrode interface, which leads to a large Rp value of anatase and a low efficiency of Cr (VI) degradation.

3.6. Cyclicity and Stability of TBC

Three cleaning solutions, H₂O, HCl, and NaOH, were used to evaluate the cycling reliability of TBC. Overall, the cycling reliability was ranked as H₂O > HCl > NaOH. There was not much difference in the cycling performance of the TBC under H₂O and HCl conditions. The degradation efficiency of about 80% was maintained in the first three cycles. In the fourth cycle, the adsorption amount was 17.38 mg/g, which meets the expectation. The NaOH cleaning left OH^- on the surface of TBC, which also led to the local pH increase during the degradation process, affecting the reduction of Cr (VI).

In order to observe the stability of the TBC, the detachment of Ti elements from the TBC was investigated. We carried out four oscillation experiments lasting 24 h and detected the concentration of Ti in them. It was found that the maximum release rate of Ti was 0.0483%. This indicates that TBC has good stability and is an excellent and stable adsorbent (Figure 8).

3.7. Mechanism of Cr (VI) Degradation in TBC

Based on our experimental results, we believe that there is a mechanism of Cr (VI) degradation by TBC in which multiple pathways work together (Figure 9). First, TBC has an abundant specific surface area and pore structure, which gives it an excellent physical adsorption capacity. The adsorption of positively charged TBC with negatively charged chromate through charge attraction is the main factor. This effect was confirmed in pH gradient experiments. The Cr (VI) degradation performance of TBC was significantly enhanced when light intervened. This was attributed to the photocatalytic effect of anatase. TBC with adsorption was better able to aggregate Cr (VI) from the environment in the vicinity of the material. The photoelectrons (e⁻) generated by anatase under light were directly transferred to Cr (VI), realizing efficient reduction. In addition, superoxide radicals (${\mathrm{O}}_2^{-}$) generated by the acceptance of electrons by O₂ in water also play a role. Although it competes with Cr (VI) for photoelectrons (e⁻), ${\mathrm{O}}_2^{-}$ is also reductive and can be involved in the reduction of Cr (VI), which requires protons (H⁺), meaning that the performance of TBC is better in a neutral acidic environment or a neutral environment (pH < 8). Photocatalytically generated holes (h⁺) can react with water, which also provides more electrons for Cr (VI) [55].

4. Conclusions

Overall, the preparation of natural anatase–biochar (TBC) composites by pyrolysis is a novel and effective method. Compared with the traditional sol–gel method for synthesizing TiO₂–biochar composites, this method is simpler and cheaper. During the pyrolysis process, anatase did not break the C-chemical bonds of the biochar but was uniformly distributed on the surface and in the pores of the biochar. This phenomenon well avoids the defect of deactivation of anatase by agglomeration in water. The TBC showed excellent Cr (VI) degradation performance, which was caused by a combination of multiple pathways, such as photocatalysis and charge interaction. The abundant pore structure and high specific surface area provide the TBC with more exposure to Cr (VI). In the photocatalytic degradation pathway, photoelectrons (e⁻) and superoxide radicals (${\mathrm{O}}_2^{-}$) were jointly involved in Cr (VI), and three photoelectrons (e⁻) were transferred from the one-step transfer of the conduction band of the TBC to Cr (VI) for direct reduction to Cr (III) with the highest efficiency. The method proposed in this paper can be expected to provide a reference for the use of natural anatase as an alternative to chemically synthesized TiO₂ nanoparticles, and the preparation of TBC composites using the sintering method is characterized by stability, high efficiency, and sustainability.