Room-Temperature Thermal Cycling Driven Pyro-Catalysis over g-C₃N₄/ZnO Composites for Efficient Dye Degradation

Abstract

A highly efficient pyro-catalytic system based on a g-C₃N₄/ZnO composite has been developed for dye degradation under near-room-temperature thermal cycling (25–60 °C). This system integrates pyroelectric charge generation with electrochemical redox reactions. The g-C₃N₄/ZnO for pyro-catalytic Rhodamine B (RhB) dye decomposition with 95.6% efficiency in the dark, whereas pristine g-C₃N₄ reached only approximately 60.1% under identical conditions. The degradation mechanism is primarily driven by the in situ generation of superoxide (•O₂⁻) and hydroxyl (•OH) radicals, as verified by radical quenching experiments. The formation of the composite facilitates the efficient spatial separation of pyroelectric-induced charges, thereby endowing g-C₃N₄/ZnO with a significantly enhanced pyro-catalytic performance compared to g-C₃N₄ alone. This study demonstrates the promising application of g-C₃N₄/ZnO as a high-performance pyro-catalyst under mild thermal conditions, offering a sustainable and light-independent strategy for wastewater treatment by utilizing ambient temperature fluctuations.

1. Introduction

The ubiquitous release of persistent dye contaminants in industrial wastewater presents a major environmental and health threat, associated with acute ecological damage and potential carcinogenicity, thereby driving an urgent demand for effective remediation strategies. These compounds can lead to acute problems for aquatic life and humans [1,2,3]. For environmental remediation purposes, wastewater treatment has been implemented via a diverse array of physical, chemical, and biological methodologies [4]. Physical treatment methods essentially involve the transfer of dyes from aqueous solutions to solid phases, a process that is prone to inducing secondary pollution [5,6]. Chemical treatment approaches, such as photocatalytic degradation and Fenton oxidation, are capable of effective and relatively benign pollutant degradation. Nevertheless, these methods are still plagued by drawbacks, including sluggish reaction kinetics, high operational costs, and stringent reaction condition requirements [7,8,9,10,11]. Notably, semiconductor-based photocatalysis relies heavily on sufficient light irradiation (especially UV or visible light) to generate electron-hole pairs, which fundamentally limits its application in light-deficient environments such as deep wastewater tanks, nighttime operations, or turbid industrial effluents [1,7]. Additionally, the inefficient utilization of photogenerated carriers and the low utilization efficiency of full-spectrum solar energy remain major technical bottlenecks [8]. The major limitation of biological treatment processes lies in their limited effectiveness for diverse dye-containing effluents, since the high stability of synthetic dyes impedes biodegradation, and their inherent toxicity often compromises the viability and metabolic functions of microbial consortia essential for pollutant degradation [12]. Hence, there is an essential need to engineer novel and eco-compatible protocols for wastewater purification to meet the increasingly stringent environmental standards. dye-laden effluents

Thermal energy, as a prevalent and readily available source, is most easily obtained in nature [13]. At present, pyro-catalysis is mainly realized through the organic material pyrolysis, a process that is strictly confined to a high-temperature range of 400–1000 °C [14]. High temperature limits the practical application of the pyrolysis of organic materials. Up to now, systematic studies focusing on pyro-catalysis operating under room-temperature conditions have been extremely limited in existing literature.

Near room temperature, pyroelectric materials demonstrate remarkable thermoelectric conversion capabilities when subjected to a temperature gradient. The pyroelectricity enables the generation of positive-negative charge pairs, which further induce the production of strong oxidant free radicals to drive the oxidative degradation of dye wastewater, thus holding substantial potential for practical dye treatment under the cyclic cold-hot conditions during night and day [15,16]. Crucially, unlike photocatalysis, pyro-catalysis is driven solely by temperature fluctuations and operates independently of light. This attribute confers a distinctive advantage for continuous wastewater treatment in the dark or under opaque conditions, seamlessly bridging the day-night cycle and weather-imposed interruptions in solar-based processes. Moreover, in contrast to photovoltaic materials, which typically exhibit conversion efficiencies below 20%, pyroelectric materials demonstrate significantly better energy conversion efficiencies of 40% to 45% [17]. Theoretically, owing to its distinct working mechanism, pyro-catalysis may offer a significant efficiency advantage over photocatalytic processes that rely on photoelectrochemical pathways. Therefore, pyroelectric materials exhibit unique substantial potential for the efficient degradation of dye contaminants driven by the mild day-night cold-hot alternation in the natural environment.

Graphitic carbon nitride (g-C₃N₄), a structural analog of graphite, has garnered significant attention in material science owing to its favorable narrow bandgap (e.g., 2.72 eV) that confers remarkable applicability in diverse advanced processes, including renewable energy conversion, value-added chemical synthesis, and environmental remediation [18,19,20]. In recent years, two-dimensional materials, such as MoS₂ and graphitic materials, have been of great interest owing to their pronounced piezoelectric and ferroelectric performance [21,22]. As a general rule, carbon nitrides possess a centrosymmetric crystal structure, a feature that endows them with neither piezoelectric nor ferroelectric properties [23]. Nevertheless, the nanosheets demonstrate anomalous piezoelectric and ferroelectric properties, which arise from the interplay between structural symmetry and nanoscale dimensional effects. Meanwhile, the performance of carbon nitride-based materials is influenced by energy consumption associated with internal electric polarization under strain gradient conditions [24,25]. Ferroelectric materials typically show significant pyroelectric capabilities [26,27]. Therefore, 2D g-C₃N₄ nanosheets possess pyroelectricity in theory.

Despite its advantages, the pyroelectricity of bare g-C₃N₄ is inherently limited, primarily due to the rapid recombination of photogenerated electron–hole pairs, which severely hinders the efficient separation and interfacial migration of charge carriers [28,29,30]. Notably, despite these intrinsic limitations in the pyro-catalytic performance of pristine g-C₃N₄, its inherent matrix is endowed with abundant active sites that enable the coordination with diverse compounds or metallic species, thereby furnishing a facile route to achieve high-reactivity surface functionalization of g-C₃N₄ and laying a robust foundation for subsequent performance enhancement through rational modification strategies [19,31]. The pyro-catalytic performance of g-C₃N₄ can be enhanced through diverse strategies, such as doping modification and composite construction with compounds like transition metals, metal oxides, and metal sulfides, which effectively suppress charge carrier recombination [32]. ZnO is a direct bandgap semiconductor. Its bandgap is approximately 3.37 eV. It has a relatively high excitonic binding energy of approximately 60 meV. As a result, it can achieve efficient absorption and emission of ultraviolet light at room temperature [33,34,35]. More importantly, when hybridized with g-C₃N₄, ZnO can form a type-II (staggered) composite due to their well-aligned band structures. This architecture facilitates the directional migration of photogenerated carriers across the heterojunction interface, markedly suppressing radiative recombination and prolonging carrier lifetime [36]. Consequently, the integration of ZnO with g-C₃N₄ is expected to construct a robust composite that not only improves photo-pyroelectricity but also offers enhanced stability and recyclability. Thus, decorating g-C₃N₄ with ZnO presents a predictable and effective strategy for achieving markedly improved thermo-pyro-catalytic performance.

This study demonstrates a high pyro-catalytic efficiency of probably 95.6% for RhB dye degradation using g-C₃N₄/ZnO composites via a mild thermal cycle (from 25 °C to 60 °C), highlighting its potential for environmental remediation by utilizing low-grade thermal energy from sources including industrial waste heat and geothermal reservoirs [37,38].

2. Materials and Methods

2.1. Materials

All reagents used in the experiments were of analytical grade. To maintain consistency and avoid introducing additional variables, these materials were used directly as supplied by the manufacturer, without undergoing any further purification processes. Melamine (C₃N₃(NH₂)₃), Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), Sodium hydroxide (NaOH), Polyethylene glycol 400 (PEG400), Rhodamine B (RhB) and Absolute Ethyl alcohol (CH₃CH₂OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Synthesis of Pyro-Catalysts

A simple and scalable strategy was adopted for the fabrication of g-C₃N₄ nanosheets [39,40]. In a typical procedure, a certain amount of C₃N₃(NH₂)₃ was followed by grinding in a mortar for 20 min. The precursor was calcined in a muffle furnace at 520 °C for 4 h under a heating ramp of 10 °C min⁻¹, followed by natural cooling to ambient temperature. The product was subsequently ground in a mortar for 20 min and collected as the final material.

This study employed a straightforward solvothermal approach to synthesize ZnO nanomaterials [41]. 1.1 g Zn(CH₃COO)₂·2H₂O and 4.0 g NaOH were dissolved in 30 mL CH₃CH₂OH and 7.5 mL Polyethylene glycol 400 (PEG400) with vigorously stirring for 30 min. The precursor solution was subsequently sealed in a 50 mL Teflon-lined autoclave and subjected to a hydrothermal treatment at 120 °C for 12 h. The autoclave was subsequently allowed to cool naturally to room temperature in ambient air. The obtained products were collected by centrifugation, followed by repeated washing with deionized water and absolute ethanol. The washed solids were subsequently dried at 100 °C in air for 4 h to obtain a powdered precursor for composite fabrication.

The g-C₃N₄ and ZnO powders were combined and ground thoroughly in a mortar with a pestle for a duration of 20 min to ensure uniform mixing. Subsequently, the mixed g-C₃N₄/ZnO precursor was transferred to an alumina crucible and calcined in a muffle furnace at 500 °C for 2 h, with the temperature ramped up at a heating rate of 10 °C min⁻¹. Following natural cooling to ambient temperature, the obtained composite products were subjected to additional grinding in a mortar for 20 min to ensure uniform particle size distribution.

2.3. Characterization

The crystalline phases of the samples were analyzed by powder X-ray diffraction (XRD, Philips PW3040/60, Royal Dutch Philips Electronics Ltd., Eindhoven, The Netherlands) equipped with a Scintag Pad V detector. Surface morphology was characterized by scanning electron microscopy (SEM, Nova NanoSEM X30, Thermo Fisher Scientific, Winsford, UK). For SEM characterization, the sample preparation was carried out in a sequential manner. First, the as-prepared products were dispersed in absolute ethanol and sonicated for 10 min to obtain a homogeneous suspension. Subsequently, several drops of the resulting suspension were pipetted onto conductive tape. The loaded conductive tapes were then air-dried at room temperature to remove residual ethanol before SEM measurements were performed.

2.4. Evaluation of Pyro-Catalytic Performance

The pyro-catalytic activity of the g-C₃N₄/ZnO composites was evaluated based on the degradation of RhB under periodic temperature variations. For a typical catalytic degradation experiment, 50 mg of the g-C₃N₄/ZnO composite was dispersed into 50 mL of an aqueous RhB solution (5 mg·L⁻¹) contained in a glass reactor. A heated magnetic stirrer (RCT-B-S25, IKA-Werke GmbH & Co. KG, Staufen, Germany) was employed to ensure continuous mixing and uniform heating of the reaction mixture. Before the initiation of thermal cycles, the catalyst-RhB suspension was subjected to continuous stirring under dark conditions for 2 h, which allowed the system to reach adsorption–desorption equilibrium and eliminate the interference of adsorption on subsequent catalytic performance evaluation. The solution was exposed to the same thermal cycles, ranging from 25 °C to 60 °C degrees Celsius, as illustrated in Figure 1. A systematic evaluation of the pyro-catalytic performance was conducted by monitoring the degradation kinetics of the RhB dye solution. The concentration change in RhB over time, which served as the direct indicator of catalytic activity, was precisely monitored with a UV–Vis spectrophotometer (UV2300II, Techcomp, Shanghai, China). From this time-dependent concentration data, the degradation efficiency was calculated to quantify the catalytic output. The essential pyro-catalytic process was initiated by subjecting the reaction mixture to repeated thermal cycles, which provided the cyclic temperature variation required to activate the pyroelectric effect in the catalyst and thereby drive the degradation reaction. At intervals of every 6 thermal cycles, 2 mL of the reaction mixture was withdrawn. The sample was subsequently centrifuged immediately at 8000 rpm for 5 min. To ensure a clear liquid phase for subsequent analysis, the centrifuged solution was subjected to filtration to remove any residual solid catalyst. With bandgaps of approximately 2.72 eV and 3.37 eV, both pristine g-C₃N₄ and ZnO fall within the visible-light spectrum. In addition, to ensure that the observed degradation was solely attributable to pyro-catalysis rather than any photochemical processes, all experiments, including blank controls, were conducted in complete darkness following the identical procedure.

3. Results and Discussion

3.1. Characterization Analysis

The X-ray diffraction (XRD) is conducted to determine the crystalline structure and phase purity of g-C₃N₄ and ZnO in the synthesized composites. The crystalline phases identified by XRD are presented in Figure 2. The recorded diffraction peaks within 20.0–70.0° correspond to their respective phase structures and indicate high crystallinity. The characteristic diffraction peak observed at 27.4° is assigned to the (002) crystallographic plane of graphitic carbon nitride, reflecting the periodic interlayer stacking of its conjugated aromatic systems. This assignment is consistent with the reference pattern for g-C₃N₄ provided in JCPDS Card 87-1526. No discernible impurity peaks are detected in the patterns, which confirms the high phase purity of the as-synthesized products. Similarly, the diffraction features shown in the upper section of Figure 2 are characteristic of the wurtzite structure, confirming the successful formation of ZnO nanorods via the solvothermal method. All observed peaks match well with the hexagonal wurtzite phase of ZnO as referenced in JCPDS Card No. 36-1451 [33]. And in the g-C₃N₄/ZnO composite, the diffraction peaks corresponding to the ZnO phase exhibit no detectable shift, and no additional crystalline phases are observed. The narrow and intense diffraction peaks of the ZnO phase indicate the high crystallinity of the synthesized nanomaterials.

The morphological and microstructural characteristics of the g-C₃N₄ and the g-C₃N₄/ZnO materials are characterized by scanning electron microscopy (SEM), as presented in Figure 3. All images share the same scale bar of 500 nm. As shown in Figure 3a, the fabricated g-C₃N₄ exhibits a characteristic sheet-like morphology with a relatively smooth surface. The morphology of pristine ZnO is composed of aggregated structures (granular/rod-like) featuring distinctly defined particle boundaries, as presented in Figure 3b. The g-C₃N₄/ZnO composite, as visualized in Figure 3c,d, exhibits a closely integrated morphology wherein ZnO particles distinguished by their brighter contrast are anchored onto or embedded within the g-C₃N₄ sheets, thereby confirming effective hybridization. A higher-magnification view reveals intimate interfacial contact between the two components. This close heterojunction interface is vital for facilitating charge transfer, which suppresses electron-hole recombination and enhances pyroelectricity. These results demonstrate the successful synthesis of a structurally well-integrated g-C₃N₄/ZnO composite. This favorable architecture not only increases the available adsorption sites on the catalyst surface but also enhances its overall pyroelectric performance.

3.2. Pyro-Catalytic Performance of g-C₃N₄/ZnO Composites

The temporal evolution of the RhB absorbance spectra under pyro-catalytic conditions is directly illustrated in Figure 4, highlighting the degradation efficiency of the g-C₃N₄/ZnO catalyst across thermal cycles. The characteristic absorption peak of RhB at approximately 554 nm diminishes rapidly in intensity over time and eventually levels off, indicating the near-complete degradation of the dye. Inside Figure 4, there is a photograph showing the change in the color of RhB. Obviously, a progressive fading of the RhB solution from pink to colorless is observed throughout the thermal cycling process, culminating in a transparent solution after 42 cycles.

The decomposition ratio (D) for RhB is defined as Equation (1):

$$D={\displaystyle \frac{C_0-C}{C_0}}\times 100\%$$

(1)

where C₀ and C denote the RhB concentration at the time of adsorption equilibrium and the actual concentration after different thermo-cycle times, respectively [42].

An exponential decrease in the normalized concentration (C/C₀) with increasing thermal cycles is exhibited by g-C₃N₄/ZnO in Figure 5a, illustrating its rapid pyro-catalytic efficiency. The pyro-catalytic decomposition ratio of RhB over the g-C₃N₄/ZnO composite catalyst is calculated to be approximately 95.6% after 42 thermal cycles. Kinetic modeling is employed to fit the experimental data, from which the reaction rate was determined. As shown in Figure 5b, a linear fit is obtained for the plot of ln(C/C₀) against thermal cycle time, demonstrating that the pyro-catalytic degradation of RhB on g-C₃N₄/ZnO adheres to pseudo-first-order kinetics.

The expression of the kinetic equation is possible by Equation (2): [43]

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

(2)

The quantities denoted by C and C₀ are equivalent to those in Equation (1). The variable t represents the cumulative thermal cycle count, and k is the apparent pseudo-first-order rate constant obtained from the linear fitting of ln(C/C₀) against t.

The superior RhB decomposition performance of the g-C₃N₄/ZnO composites under room-temperature thermal cycling is demonstrated in Figure 6 through a comparative study with the catalyst-free system and g-C₃N₄ alone. It is obvious to observe that the g-C₃N₄/ZnO composite exhibits superior pyroelectric performance relative to the other two samples. The apparent rate constant (k) of the g-C₃N₄/ZnO composite, derived from the pseudo-first-order kinetic model (Equation (2)), is 6.78 × 10⁻² min⁻¹, confirming the applicability of this kinetic model to the degradation process. As indicated in Figure 5, the degradation of RhB proceeds at a relatively slow rate when catalyzed by pristine g-C₃N₄. The g-C₃N₄/ZnO composite exhibits a rate constant approximately three times higher than that of pristine g-C₃N₄, owing to the formation of a composite that suppresses charge carrier recombination. From Equation (1), the decomposition ratio of RhB with g-C₃N₄/ZnO is found to reach approximately 95.6% after 42 thermal cycles, while pure g-C₃N₄ is estimated to reach 60.1% after the same number of cycles. This once again demonstrates the significantly higher pyro-catalytic decomposition ratio of g-C₃N₄/ZnO compared to pure g-C₃N₄. A control experiment without any catalyst showed only minimal RhB decomposition under pyro-catalytic conditions. In contrast, the significant degradation achieved with g-C₃N₄/ZnO demonstrates a genuine catalytic effect, excluding mere physical adsorption.

3.3. The Active Substances Analysis

To identify the active species involved in the pyro-catalytic degradation of RhB, different kinds of scavengers are added to the pyro-catalytic experiments. As shown in Figure 7, the degradation efficiency of RhB using the g-C₃N₄/ZnO composite catalyst decreases to varying degrees when different scavengers are used. The lower degradation efficiency of RhB signifies a more pronounced inhibitory effect on the pyro-catalytic performance of the catalyst by the scavenger. Ethylene diamine tetra-acetic acid (EDTA), benzoquinone (BQ), and tert-butyl alcohol (TBA) are used as scavengers for holes (h⁺), superoxide radicals (•O₂⁻), and hydroxyl radicals (•OH), respectively [44,45]. Therefore, the pyro-catalytic reaction proceeds primarily through the generation of holes, •OH, and •O₂⁻. Among these, •O₂⁻ is deduced to be the most critical active species, as evidenced by the lowest degradation efficiency observed for the g-C₃N₄/ZnO composites in the presence of the •O₂⁻ scavenger (BQ).

3.4. Analysis on Pyro-Catalysis Mechanism of the g-C₃N₄/ZnO

Pyroelectricity, induced by a temperature gradient (ΔT) across the material, facilitates the decolorization of RhB, whereas constant temperature changes are ineffective. This observation further confirms the presence of pyroelectricity in the g-C₃N₄/ZnO composites. The working mechanism of pyroelectric materials under thermal cycles involves the following steps. At initial thermodynamic equilibrium, screening of the bound polarization charges is achieved by compensation carriers [46,47]. When a thermal gradient is applied, the change in polarization creates a transient imbalance between these bound charges and their screening counterparts, leading to the development of a surface potential that enables charge transfer [48]. When the accumulated surface charge density surpasses the bound polarization charge density, the resulting breakdown in charge screening establishes an electrostatic potential sufficient to drive redox reactions of adsorbed species. These reactions generate •O₂⁻ and •OH radicals, which subsequently lead to the oxidative decolorization of RhB [2].

On the basis of the information detected above, the proposed mechanism is shown schematically in Figure 8. The favorable band alignment between g-C₃N₄ and ZnO accounts for the enhanced pyroelectricity of the composites. As widely recognized, •O₂⁻ and •OH radicals play critical roles in dye degradation. Their generation requires redox potentials of −0.33 eV and +1.763 eV respectively [44]. The overall mechanism involving electronic transitions and the subsequent dye degradation can be summarized as follows in Equations (3)–(6),

g-C₃N₄/ZnO + ΔT → q⁻ + q⁺

(3)

O₂ + q⁻ → •O₂⁻

(4)

OH⁻ + q⁺ → •OH

(5)

•O₂⁻ (or •OH) + RhB → dye decomposition

(6)

The decolorization mechanism is attributed to a pyro-catalytic process activated by thermal cycling. Temperature fluctuations (ΔT) across the pyroelectric g-C₃N₄ nanosheets create a time-varying polarization, resulting in the accumulation of uncompensated positive and negative surface charges (Equation (3)). These induced charges serve as active sites for subsequent electrochemical reactions. Specifically, the negative charges facilitate the one-electron reduction in adsorbed O₂ to yield superoxide radical anions (•O₂⁻), while the positive charges promote the oxidation of OH⁻ to generate hydroxyl radicals (•OH) (Equations (4) and (5)). The formed •OH and •O₂⁻ radicals are potent oxidants that non-selectively attack the chromophoric structure of RhB, resulting in its cleavage and decolorization (Equation (6)) [44,45,48].

The thermal energy supplied per cycle is calculated using Equation (7),

ΔQ = c·m·ΔT

(7)

where c denotes the specific heat capacity, and m is the mass of the RhB dye solution. From a mechanistic perspective, the key to continuous catalysis lies in the kinetic mismatch under non-equilibrium conditions. In a static thermal state, polarization charges are electrically screened, yielding no net catalytic activity. Under dynamic temperature changes, however, the slower relaxation of screening charges compared to the rapid change in polarization produces a transient, unscreened surface potential. When the density of these accessible surface charges exceeds that of the bound polarization charges, they become energetically capable of initiating interfacial redox reactions with solution-phase reactants (OH⁻,•O₂), thereby sustaining the production of radicals for pollutant degradation [49].

3.5. The Recyclability and Stability of the g-C₃N₄/ZnO Composite

Three consecutive catalytic cycles of RhB decomposition under identical conditions demonstrated the reusability and structural integrity of the g-C₃N₄/ZnO composite. After each cycle, the g-C₃N₄/ZnO catalyst was separated by centrifugation, washed, and dried for reuse. For the next cycle, the catalyst was added to a freshly prepared RhB solution with the same initial concentration. Figure 9 demonstrates the reusability of the g-C₃N₄/ZnO composite, as evidenced by its consistent RhB degradation efficiency over three successive cycles. Only a slight decrease in the decomposition ratio was observed after three catalytic cycles, yet the pyro-catalytic performance remained robust, indicating remarkable stability for RhB degradation [50]. The minor decline in activity after three cycles may be due to incomplete catalyst recovery during the recycling process [51]. The combined advantages of high stability and recyclability endow the g-C₃N₄/ZnO composites with strong potential for practical dye degradation applications.

4. Conclusions

In this study, under mild thermal cycling (between 25 °C and 60 °C), the g-C₃N₄/ZnO composites achieved a high pyro-catalytic decomposition efficiency of probably 95.6% for RhB, far surpassing the approximately 60.1% efficiency of pure g-C₃N₄. The superior performance originates from the integrated pyroelectric and electrochemical effects, which maximize charge carrier utilization for redox reactions in the absence of light. The formation of superoxide and hydroxyl radicals as reactive intermediates was confirmed during pyro-catalysis. These radicals subsequently cause the oxidative decolorization of RhB. The composites between g-C₃N₄ and ZnO facilitate efficient separation of photogenerated charge carriers, leading to superior pyro catalytic performance compared to pristine g-C₃N₄. This study presents a novel g-C₃N₄/ZnO composite-based pyro-catalytic system that achieves efficient dye degradation driven by mild temperature fluctuations (25–60 °C). The approach harnesses ubiquitous natural day-night temperature variations as the sole energy input, demonstrating the feasibility of utilizing low-grade thermal energy for environmental remediation. Practical applications include off-grid wastewater treatment, industrial waste heat recovery, and integration with geothermal or solar thermal sources. Future efforts may focus on developing flexible pyro-catalytic films, coupling with solar photothermal conversion systems, and scaling toward prototype reactors for real water treatment. Ultimately, this work exemplifies an energy harvesting environmental technology that transforms dissipative heat into a resource for purification, offering a sustainable pathway for low-carbon water treatment.