The oyster shells used in the laboratory were obtained from an agricultural product market. The surfaces of the clam shells were cleaned using detergent and a cleaning ball. The surfaces were polished until clean and then soaked in 0.5% dilute hydrochloric acid for 0.5 h to remove excess impurities. After cleaning with pure water, these pretreated shells were dried in a drying oven at 65°C; the shells were crushed to a powder with sealed sample pulverizer, and the shell powder was sieved through 200 mesh. Finally, the sieved powder was calcined in a muffle furnace at 900°C for 2 h, producing the modified shell powder, which was sealed and kept until further use.

The Msp/CeNT composite photocatalyst was prepared using the sol-gel method. We mixed 30 mL of anhydrous ethanol with a certain amount of modified shell powder via ultrasonic vibration, and we added 8.5 mL of tetra-butyl titanate to produce solution A. We combined 20 mL of anhydrous ethanol, 1.5 mL of ultrapure water, and a certain amount of hydrochloric acid, which we fully mixed. We calculated the molar amounts of doped cerium and nitrogen. The cerium and nitrogen sources were cerium nitrate hexahydrate and urea, respectively, which were added into the mixed solution. The resulting solution was mixed to produce solution B. We combined 20 mL of anhydrous ethanol, 1.5 mL of ultrapure water, and a certain amount of hydrochloric acid, which we fully mixed. We calculated the molar amounts of doped cerium and nitrogen. The cerium and nitrogen sources were cerium nitrate hexahydrate and urea, respectively, which were added into the mixed solution. The resulting solution was mixed to produce solution B. Under vigorous stirring without bubbles, solution B was slowly dropped into solution A, after which the mixture was continuously stirred for 1.5–2 h to prepare the composite photocatalyst gel, which was dried in a vacuum-drying oven at 65°C for 12 h, and then ground and crushed with a mortar. Then, the modified shell powder/Ce-N-TiO₂ was prepared via calcination at 600°C for 2 h in a muffle furnace. Ce-N-TiO₂ was prepared without adding the modified shell powder using the same method.

This experiment was performed using a xenon lamp as the light source and a double-layer beaker with a jacket as the reaction container. Phytic acid (100 mL) at a mass concentration of 50 mg/L was added to a double-layer beaker; 10 mg of modified shell powder, 90 mg of Ce-N-TiO₂, and 100 mg of modified shell powder/Ce-N-TiO2 composite photocatalyst were added to the phytic acid. The phytic acid degradation rates of the modified shell powder, CeNT, and composite photocatalysts via adsorption and photocatalysis were investigated. Before the degradation test, the mixed modified shell powder/Ce-N-TiO₂ solution was placed in a dark room, and an adsorption–desorption equilibrium test was conducted for 2 h in the dark. A magnetic stirrer was used under irradiation with a xenon lamp. Under the xenon lamp illumination, a small amount of liquid was measured with a sampler at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4 h, which was then centrifuged and filtered for the subsequent determination of the mass concentration of phosphate. The effects of the reaction temperature, initial pH, stirring speed, composite photocatalyst dosage, and light intensity on the synergistic effect were further analyzed using this method.

Ammonium molybdate spectrophotometry was used for phosphate determination. We pipetted 1 mL of the solution to be measured into a 25 mL colorimeter tube, and we fixed the volume to 25 mL with distilled water. Ascorbic acid (1 mL) was added and shaken well, and 2 mL of the molybdate solution was added dropwise after 30 s. The solution was allowed to stand for 15 min and then measured colorimetrically at 700 nm using a UV–visible spectrophotometer. The phosphate concentration was determined according to the standard phosphate curve, and the degradation rate of the phytic acid solution was calculated as follows: η = C t C 0 × 100 % where, η is degradation rate (%) of organic phosphorus in phytic acid solution; C_t is the content of PO₄ ^3- in the phytic acid solution at moment t; C₀ is the total phosphorus content in phytic acid at the initial moment.

Figure 1A shows a scanning electron micrograph of the modified shell powder at ×400 magnification, indicating a loose, porous, layered surface structure, which is suitable for photocatalyst loading. Figure 1B shows a scanning electron micrograph of the Ce-N co-doped TiO₂ loaded with modified shell powder with a 1 μm scale bar. The modified TiO₂ after doping was uniformly loaded in a thin layer on the surface of the modified shells. Figure 1C depicts the loaded Ce-N co-doped TiO₂ in a 200 nm scanning electron microscope image, which shows that the particle distribution and size were relatively uniform, and loading on the surface of modified shell was uniform. Some areas showed particle agglomeration, which may have been caused by the high-temperature calcination. The results of the pore size analysis of a composite photocatalyst sample is shown in Figure 1D, which was analyzed using ImageJ. The particle size was 30.3787 nm, which is classified as mesoporous. Mesoporous structures are favorable for the diffusion of phytic acid in a catalyst and for the combination of phytic acid molecules with the catalyst, which promotes the photocatalytic reaction.

Figure 2 shows the N₂ adsorption–desorption isothermal curves and pore size distribution curves of the CeNT and Msp/CeNT. The adsorption isothermal curves of both catalysts contain hysteresis loops inside the relative pressure region of 0.8–1.0, which indicates the fourth type of adsorption isothermal model and that the composite catalysts had a mesoporous structure (Lei et al., 2020). The average pore size of the composite photocatalyst decreased from 15.3216 nm before modification to 12.0106 nm after load modification. A composite photocatalyst with a mesoporous structure has a large specific surface area, which enables the increased mass transfer of pollutants in which the pollutants are subjected to less diffusion resistance, thus effectively increasing photocatalytic efficiency (Li et al., 2022). The specific surface area of Msp/CeNT increased from 45.5875 to 56.3615 m²/g after doping with the modified shell powder, an increase of 23.6%. The loaded modified shell helped enrich the phytic acid on the surface of the composite photocatalyst, which facilitated the synergistic adsorption and photocatalysis effects.

Figure 3 shows the Raman spectra of the different samples. Six peaks (A_1g+2B_1g+3E_g) can be observed in the Raman activity pattern of anatase TiO₂ (Wang et al., 2015). The spectra of both samples showed strong vibrations at four bands: 145.18 cm⁻¹ (Eg), 399.8 cm⁻¹ (B_1g), 520.28 cm⁻¹ (A_1g+B_1g), and 641.14 cm⁻¹ (Eg), indicating that the TiO₂ in the composites was present as the anatase type (Ohsaka et al., 1978). The two characteristic peaks at 399.8 and 520.28 cm⁻¹ represent the tensile vibration of the Ti-O bond; the peaks at 145.14 and 640 cm⁻¹ indicate the bending vibration of the Ti-O bond. The peak value of the prepared sample shifted to a higher wavenumber, which may have been because of the presence of oxygen vacancies (Zhang et al., 2023). No Raman spectral peaks representing the rutile phase were found, indicating that no rutile TiO₂ was present or that the content of the rutile phase in the sample was relatively low and did not reach the detection limit. Overall, the findings indicated that the composite catalyst was thoroughly crystallized and mainly consisted of anatase TiO₂, which is suitable for photocatalysis.

The sizes of the particles and the defects in a nanomaterial are the two most important factors that affect the material’s intensity, peak width, and position of the Raman spectral peaks. Figure 3 shows a strong peak enhancement and a larger peak in the Raman spectra of the Ce and N co-doped composite catalysts compared with those of the pure TiO₂ sample (Choudhury et al., 2013). Moreover, the Raman vibrational peak of the composite catalyst at 145.18 cm⁻¹ (Eg) shifted toward a higher wavenumber compared with that of pure TiO₂. This occurred because the ionic radius of Ce³⁺ is larger than that of Ti, and Ce³⁺, when doped in lattice interstitials, distorts the lattice and generates a large number of oxygen vacancy defects, which shifts the Eg mode toward a higher wavenumber (Zhang et al., 2023). The formation of O-Ti-N bonds via N doping decreased the strength of the Ti-O bonds and changed the intensity and width of the vibrational peaks. The Raman spectral peaks of the co-doped catalysts were stronger and wider than those of the single TiO₂ Raman spectral peaks, suggesting that more oxygen vacancies were present on the surface, which strengthened photocatalytic performance.

The XRD pattern shows that the composite catalyst sample was successfully doped with cerium and nitrogen (Zhang et al., 2023). The crystal size of the modified TiO₂ sample was reduced because of the doping with metallic Ce and the nonmetallic B, which inhibited the phase transition of TiO₂, thus maintaining the anatase phase, which is more conducive to photocatalysis. According to the calculations, the crystal size of the sample was notably smaller than that of the undoped sample.

The adsorption of phytic acid using the modified shell powder, the photodegradation of phytic acid using CeNT, and the degradation of phytic using by composite photocatalysts were investigated under an initial pH of 6, a light intensity of 400W, a catalyst dosage of 1 g/L, a stirring speed of 300 rpm, and at ambient temperatures of 15, 25, or 35°C. The experimental results are shown in Figure 5.

Figure 5 shows that the actual phytic acid degradation efficiency of the composite photocatalyst at different reaction temperatures was higher than the sum of the theoretically superimposed degradation efficiency of phytic acid via adsorption using modified shell powder alone and photocatalytic degradation with CeNT alone, which proves the synergistic effects of adsorption and photocatalysis in the degradation of phytic acid of the composite photocatalyst. Figure 5 also shows that temperature affects the synergistic effect: the composite photocatalytic degradation efficiency increased as the temperature increased, reaching a maximum at 25°C, after which the degradation rate gradually decreased. The main reason for this finding is that the movement of phytic acid molecules accelerates with the increase in temperature, which increases the effective collision efficiency between the active groups on the composite photocatalyst and phytic acid molecules. In addition, the process of the adsorption of phytic acid on the composite photocatalyst is an exothermic reaction; as such, an increase in temperature is favorable for the desorption of pollutants and phytic acid degradation intermediates from the composite catalyst (Zhang et al., 2022). As the reaction proceeded, the concentration of phytic acid in the system gradually decreased, and the intermediate products generated on the surface of the catalyst gradually desorbed with increasing temperature, providing more active sites for phytic acid molecules, which in turn increased their chance of being oxidized via photocatalysis. Phytic acid degradation rate decreased as the temperature further increased, which may have been because of the evaporation of water from the solution system. Part of the catalyst attached to the inner wall of the reaction vessel, which reduced the chance of contact with the catalyst and decreased the phytic acid degradation efficiency. The COD removal efficiency in phytic acid is also affected by temperature, and the degradation efficiency of COD is lower than that of phytic acid because phytic acid has six phosphate groups when COD is in the last stage of degradation.

The adsorption of phytic acid using modified shell powder alone, the photodegradation of phytic acid by CeNT alone, and the synergistic adsorption and photodegradation of phytic acid using the composite photocatalyst were investigated at an initial pH of 5, a light intensity of 500W, a temperature of 25°C, and a stirring speed of 300 rpm, and with different composite catalyst dosages of 0.8, 1, and 1.2 g/L. The phytic acid removal results are shown in Figure 6.

Figure 6 shows that the actual degradation efficiency of phytic acid was larger than the sum of the theoretically superimposed phytic acid degradation rates of the modified shell powder adsorption alone and CeNT photocatalysis alone for the different dosage conditions. The results indicated that the composite photocatalyst had a synergistic effect through the adsorption–photocatalytic degradation of phytic acid. Analyzing Figure 6 demonstrates the effect of dosage on this synergistic effect: the phytic acid degradation rate gradually increased with an increase in the photocatalyst dosage and reached a maximum value at a dosage of 1 g/L. More phytic acid was adsorbed onto the surface of the catalyst as the catalyst dosage increased, which increased the opportunity for contact with the active groups on the catalyst and thus the photocatalytic efficiency (Gusain et al., 2020). Moreover, the probability of photon absorption by CeNT increased with the gradual increase in the concentration of the composite catalyst in the solid–liquid system, which generated more photogenerated electrons and holes, leading to an increase in the phytic acid oxidation efficiency. However, the catalytic efficiency no longer increased but gradually decreased with further increases in the composite catalyst dosage, which was attributed to the suspended particles of the high-concentration catalyst having a scattering and masking effect on the light. In this situation, the photocatalysts could not be sufficiently excited by light energy, which ultimately weakening the effect of the photocatalyst (Nishiyama et al., 2018). Although the number of active sites in the solution increased with increasing photocatalyst dosage, the tendency for particle-to-particle agglomeration also increased when the photocatalyst dosage was too high, which gradually led to a decrease in the number of active sites and thus to a decrease in the photodegradation rate (Soto-Vazquez et al., 2016).

The adsorption of phytic acid using modified shell powder alone, the photocatalytic degradation of phytic acid using CeNT alone, and the degradation of phytic acid using the composite photocatalyst were investigated at pH 3, 5, 7, and 9 under an initial temperature of 25°C, a light intensity of 500 W, a catalyst dosage of 1 g/L, and a stirring speed of 300 rpm. The results are shown in Figure 7.

Figure 7 shows that the actual degradation phytic acid rate was always higher than the sum of the theoretical values of phytic acid adsorption using the modified shell powder alone and phytic acid degradation using CeNT alone under different pH conditions, indicating that the composite photocatalyst synergistically affected the adsorption and photodegradation of phytic acid. Figure 7 also shows that when the initial pH of the solution increased from 3.0 to 9.0, the synergistic efficiency of the composite photocatalyst increased and then decrease. The maximum phytic acid degradation rate was reached at an initial pH of 5, potentially because the photocatalytic activity of TiO₂ is higher under acidic conditions. The positively charged catalyst surface facilitates the process of photogenerated electron transfer to the catalyst surface, which favors the generation of reactive radicals, such as ⋅O₂ ⁻ and ⋅OH, in the presence of oxygen in aqueous media and slows the complexation of photogenerated electrons with holes. The degree of protonation on the surface of CeNT and the number of positive charges carried increase under lower pH conditions, which benefits the transfer of photogenerated electrons to the catalyst in the photocatalytic reaction, thus enhancing the adsorption–photodegradation efficiency. The degradation efficiency of phytic acid decreased as the pH of the reaction system was further increased. This could be attributed to the increase in the amount of hydroxide in water, which consumed the photogenerated vacancies generated on the catalyst surface and reduced the number of active groups accessible to phytic acid, thus degrading the catalytic efficiency. The COD removal efficiency in phytic acid was also affected by the pH, and the maximum removal rate was achieved at pH 5, which first increased and then decreased.

The phytic acid degradation rates using adsorption alone, the photocatalysis using Ce-N-TiO₂ alone, and the synergistic adsorption and photodegradation using the composite photocatalyst were investigated under an initial pH of 5, a temperature of 25°C, a catalyst dosage of 1 g/L, and light intensity of 500W while varying the stirring speed: 100, 200, 300, or 400 rpm. The results are shown in Figure 8.

Figure 8 shows that the phytic acid degradation efficiency was higher using the composite photocatalyst than the sum of the adsorption of phytic acid using the modified shell powder alone and the photodegradation of phytic acid using CeNT alone under different stirring speeds, which indicated that the composite photocatalyst has a synergistic effect on the adsorption and photodegradation of phytic acid. The maximum synergistic efficiency of phytic acid adsorption and photocatalysis as well as the COD removal efficiency reached a maximum when the stirring speed was 300 rpm. The stirring speed affected the synergistic effect of the composite catalyst as follows: the phytic acid degradation rate increased and then decreased with increasing magnetic stirring rate, mainly because the rotational speed of the magnetic stirrer affected the dissolved oxygen content in the aqueous phytic acid solution. The higher the rotational speed, the larger the amount of dissolved oxygen, which is capable of reacting with photogenerated electrons to produce more superoxide radicals and increases the oxidative phytic acid degradation rate. Cerium ions use dissolved oxygen to generate superoxide radicals. In addition, the accelerated rotational speed of the magnetic stirrer increases the probability of collision between particles, which is conducive to the adsorption and desorption processes. However, too fast a rotational speed leads to the appearance of a central vortex in the container, resulting in powder photocatalysts being nonuniformly dispersed in the phytic acid solution, thus reducing the contact with the concentration of phytic acid. This results in a decrease in the degradation efficiency.

The phytic acid degradation rate was investigated for the adsorption using the modified shell powder alone, via photocatalysis using Ce-N-TiO₂ alone, and the synergistic photodegradation and adsorption using the composite photocatalyst under an initial pH of 5, a temperature of 25°C, a catalyst dosage of 1 g/L, and a stirring speed of 300 rpm, for various illumination intensities of 300, 400, and 500. The phytic acid degradation rate via adsorption and photodegradation with the composite photocatalyst is shown in Figure 9.

Figure 9 shows that the actual phytic acid degradation efficiency with the composite catalyst was higher than the sum of degradation efficiency of phytic acid phytic acid adsorbed by modified shell powder alone and degraded by CeNT light alone under different light intensities. We noted a synergistic effect of adsorption and photodegradation of phytic acid by Msp/CeNT under the different light intensity conditions. Moreover, Figure 9 shows that the phytic acid photodegradation efficiency increased with increasing light intensity, reaching a maximum when the light intensity was 500 W. This occurred because the light intensity determined the degree of light absorption of the Msp/CeNT composite photocatalyst at a given wavelength, and the rate of photogenerated electron–hole formation in the photocatalytic reaction depended on the light intensity. The higher the light intensity, the more photons are produced per unit time and area. This leads to an increase in the number of photogenerated carriers on the catalyst surface, resulting in the production of more photogenerated electrons and holes. The COD removal efficiency of phytic acid showed the same trend with increasing light intensity.