Insights into the interaction between cadmium/tetracycline and nano-TiO2 on a zeolite surface

Titanium dioxide (TiO2) nanoparticles interact with organic–inorganic pollutants in the environment, and these interactions affect their environmental behavior. The mechanisms of the interaction between TiO2 and organic–inorganic pollutants on the surface of clay minerals are still unclear. In this work, isotherm adsorption was studied to explore the interactions between Cd2+/tetracycline (TC), TiO2 nanoparticles, and a zeolite (Zeo). SEM, FT-IR, and XPS were also used to reveal the interaction mechanism between organic–inorganic pollutants and TiO2 on their stability and mobility in the environment. Compared to the single systems, the adsorption of Cd2+ and TiO2 in the Cd + TiO2 composite system decreased by 3.43% and 9.90%, respectively; the TC and TiO2 adsorption in the TC + TiO2 composite system decreased by 14.39% and 45.47%, respectively. The antagonism between Cd2+ and TiO2 was due to Cd2+ and TiO2 competing for the electrostatic attraction (-OH) and hydrogen bonding sites (Si–O), and TC and TiO2 competing for the hydrogen bonding sites (-OH and C = O) on Zeo. The presence of TiO2 will increase the mobility of Cd2+ and TC on a clay surface, and this effect is more significant for organic pollutants TC. Compared with Cd2+, TC has a more significant boosting impact on the TiO2 mobility.


Introduction
In recent decades, the advantages of engineered nanoparticles have been widely recognized Romeo et al. 2020;Tortella et al. 2020). Moreover, their use and mass production have focused the attention on their environmental behavior. A previous study confirmed that nanoparticles in the environment interact with various pollutants (Ling et al. 2021), and these interactions can affect the environmental behavior of nanoparticles and pollutants. Zhang et al. (2019) demonstrated that GO enhanced Pb 2+ transport in sand by 5.5% by forming a complex with Pb 2+ (Zhang et al. 2019). Aggregated goethite particles may significantly reduce Cd 2+ transport from 87 to 27% in quartz sand (Chen et al. 2019). Furthermore, nanoparticles adsorb on the surface of solid particles, such as clay minerals, plant surfaces, and animal tissues, and change their surface structures Romeo et al. 2020). Titanium dioxide (TiO 2 ) nanoparticles have a wide range of applications, with an annual output of up to 10,000 t/year (Bour et al. 2015). Studies have confirmed that TiO 2 is found in different environments, such as soil, water, atmosphere, and landfills, owing to its massive production and use (Bour et al. 2015;Sun et al. 2014). Moreover, TiO 2 nanoparticles have a small particle size that can enter the microporous structure of solid particles, inducing changes in the specific surface area or pore size of the solid particles (Abbas et al. 2020). The interactions between TiO 2 , contaminants, and soil are complex and important.
In soil, TiO 2 becomes a carrier of pollutants, and the pollutant-induced changes in the surface properties of TiO 2 Responsible Editor: Guilherme L. Dotto will indirectly affect the adsorption behavior of TiO 2 on soil particles. On the other hand, the pollutants adsorbed on the surface and pores of soil particles also affect the adsorption behavior of TiO 2 on soil particles, thus affecting the migration and fixation of TiO 2 in the soil. Moreover, TiO 2 also affects the migration of pollutants in the soil. The surface of the nanoparticles contains various oxygen-containing functional groups, which can be adsorbed on the solid surface through hydrogen bonding , π-π/n-π EDA effect (Abbas et al. 2020), and electrostatic attraction (Bour et al. 2015). Such adsorptions change the number of functional groups on the solid surface. The adsorption process of pollutants is closely related to the solid surface structure (Chang et al. 2012).
Our previous study confirmed that the interaction between TiO 2 and tetracycline (TC) affects the stability of TC and TiO 2 on the surface of positively charged clay minerals. Moreover, this interaction enhances the mobilities of TC and TiO 2 in the environment (Wang et al. 2021). However, the interaction between organic-inorganic pollutants and TiO 2 on negatively charged clay minerals may be different from that on positively charged surfaces.
In this study, a zeolite (Zeo) and TiO 2 were chosen as the representatives of clay minerals and nanoparticles, respectively. The Zeo is a common clay with stable physical and chemical properties (Fibikar et al. 2010), while TiO 2 is a nanoparticle widely present in the environment (Bour et al. 2015). Furthermore, cadmium (Cd 2+ ) and TC were chosen as the representatives of inorganic and organic pollutants, respectively. Batch experiments were conducted to investigate the interactions between pollutants (Cd 2+ and TC), nanoparticles (TiO 2 ), and clay minerals (Zeo). Moreover, the adsorption of Cd 2+ /TC on the Zeo in the presence and absence of TiO 2 was further explored. Characterization techniques such as scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectrometry (XPS) were used to understand the interfacial interactions among Zeo, Cd 2+ /TC, and TiO 2 .
Single Cd solutions, single TC solutions, single TiO 2 , Cd + TiO 2 , and TC + TiO 2 mixed suspensions of different concentrations were prepared under dark conditions before the experiment to prevent possible photodegradation of TiO 2 . Then, the prepared solutions were sonicated for 30 min to obtain stable solutions and suspensions (Fathi Achachlouei and Zahedi 2018). The pH of all the solutions and suspensions was maintained at 6.0. Background ionic strength was 0.01 mol L −1 NaCl.

Adsorption experiments
All adsorption experiments were conducted under dark conditions to avoid photocatalysis between TiO 2 and pollutants. Moreover, the adsorption experiments were performed under stable adsorption conditions and in triplicates. Then, the obtained data were averaged over three values, and the standard of deviation was calculated.

Isotherm adsorption experiment
Isotherm adsorption was performed using the batch method. To study the difference between the adsorption processes of Cd 2+ , TC, and TiO 2 on the Zeo in single Cd, single TC, single TiO 2 , Cd + TiO 2 , and the TC + TiO 2 mixed system, we designed six groups for adsorption experiments: (1) single Cd adsorption (S-Cd) to investigate the adsorption of Cd 2+ on Zeo; (2) single TC adsorption (S-TC) to investigate the adsorption of TC on Zeo; (3) single TiO 2 adsorption (S-TiO 2 ) to investigate the adsorption of TiO 2 on Zeo; (4) Cd + TiO 2 adsorption, that is, C-Cd and C-TiO 2 (Cd), to investigate the adsorption of Cd 2+ and TiO 2 , respectively, on the Zeo in a Cd + TiO 2 composite suspension liquid; (5) TC + TiO 2 adsorption, that is, C-TC and C-TiO 2 (TC), to investigate the adsorption of both TC and TiO 2 , respectively, on the Zeo in a TC + TiO 2 composite suspension liquid; (6) no Zeo, that is, N-Cd and N-TC, to investigate the adsorption of Cd 2+ and TC on TiO 2 in Cd + TiO 2 and TC + TiO 2 composite suspension liquids, respectively.
To evaluate the effect of temperature on the adsorption of Cd 2+ , TC, and TiO 2 , the temperatures of the isotherm adsorption experiments were set to 10 °C, 25 °C, and 40 °C. Moreover, the pH and background ionic strength were 6 and 0.01 mol L −1 NaCl, respectively.
To evaluate the effect of pH on the adsorption of Cd 2+ and TiO 2 , the initial pH values of the solutions and suspensions at 20 mg·L −1 were adjusted between 3 and 8 using 0.1 mol·L −1 HCl and 0.1 mol·L −1 NaOH. For TC and TiO 2 , the initial pH values of the solutions and suspensions at 20 mg·L −1 were adjusted between 3 and 10 using 0.1 mol·L −1 HCl and 0.1 mol·L −1 NaOH. The experiments were conducted at 25 °C, and the background ionic strength was 0.01 mol·L −1 NaCl.
To a 50-mL polypropylene centrifuge tube, 0.2 g of the Zeo and 50 mL of a solution or a suspension were added. The centrifuge tubes were shaken horizontally (ZHICHENG ZWY-2012C, China) at 150 rpm and a certain temperature for 4 h in the dark (determined from the adsorption kinetic results Fig. S2).

Methods of analysis
For analyzing Cd 2+ and TC, 10 mL of supernatant was obtained and then centrifuged at 4000 rpm for 10 min after the adsorption equilibrium. The concentration of Cd 2+ was measured using an atomic absorption spectrometer (AAS, PE PinAAciie 900F), and that of TC was measured using an ultraviolet-visible (UV-vis) spectrophotometer (MAPADA UV-3200) at 358 nm. The adsorption of Cd. 2+ and TC were calculated by Eq. (1) where Q e (mg·g −1 ) is the adsorption amount of the adsorbent, C o and C e (mg·L −1 ) are the initial and equilibrium concentrations of the adsorbent in a solution, respectively, V (mL) is the volume of the solution, and m (g) is the amount of Zeo.
In the case of TiO 2 , the suspension stood for 5 min after the adsorption equilibrium was reached. Then, 10 mL of TiO 2 suspension liquid was obtained, and the suspension was added to the polytetrafluoroethylene crucible for digestion with sulfuric acid and hydrofluoric acid. The specific method is as follows: 10 mL of TiO 2 suspension was added to a polytetrafluoroethylene crucible; then, 5 mL of concentrated sulfuric acid and 3 mL of hydrofluoric acid were added to the polytetrafluoroethylene crucible, and the lid was closed. An electric hot plate was used to heat the crucible to 400 °C (30 min). Then, the lid of the polytetrafluoroethylene crucible was lifted to release the excess hydrofluoric acid, and further heating was conducted until white smoke was generated. After the solution in the crucible became colorless and transparent, the crucible was removed from the electric hot plate and cooled to room temperature (25℃). The crucible was rinsed with deionized water and the rinse solution was transferred to a 50-mL volumetric flask. The concentration of Ti was measured using a UV-vis spectrophotometer (MAPADA UV-3200) at 410 nm with hydrogen peroxide as the chromogenic agent. The TiO 2 adsorption on the Zeo in the single TiO 2 (S-TiO 2 ) and composite systems [C-TiO 2 (Cd) and C-TiO 2 (TC)] were calculated using Eq. (1), where Q e (mg·g −1 ) is the adsorption amount of TiO 2 , and C o and C e (mg·L −1 ) are the initial and equilibrium concentrations of TiO 2 in the suspension liquid, respectively.
Langmuir, Freundlich, and Sips models were used to fit the isotherm adsorption data. The equilibrium constant K S of the Sips model was used to calculate the thermodynamic parameters. The equations of the Langmuir, Freundlich, and Sips models are presented in Text S2.
The Zeo after the adsorption of Cd 2+ /TC and TiO 2 in the single and composite systems was dried at 60 °C and then characterized. The characterization methods such as SEM, FTIR, and XPS are discussed in Text S3.

Adsorption of Cd 2+ on the Zeo in the single and composite system
The batch experiment was conducted to investigate the effect of temperature and initial concentration on the adsorption of Cd 2+ on the Zeo in the single Cd and Cd + TiO 2 composite systems ( Fig. 1). The Sips model and calculated thermodynamic parameters are summarized in Table 1.
In the single Cd 2+ system, the Cd 2+ adsorption isotherms were nonlinear. It was found that the increasing temperature was conducive to the Cd 2+ adsorption, and the maximum adsorption capacities of Cd 2+ in the single Cd system were 2.43 mg g −1 (10 °C), 2.77 mg g −1 (25 °C), and 3.22 mg g −1 (40 °C). The adsorption of Cd 2+ on the Zeo in the single Cd system was a physical adsorption process with increased spontaneous endothermic entropy (Table 1). The effect of pH on the Cd 2+ adsorption in the single system is shown in Fig. 1d. The S-Cd adsorption gradually increased with increasing pH value (pH = 3-6), reaching the maximum adsorption capacity at pH = 6. Moreover, when pH was > 6, the S-Cd adsorption decreased owing to the precipitation of Cd 2+ under alkaline conditions (Đukić et al. 2015).
The maximum adsorption capacities of Cd 2+ in the Cd + TiO 2 composite system were 2.18 mg g −1 (10 °C),

3
Environmental Science and Pollution Research (2023) 30:18522-18534 S-TC represents a single TC system, C-TC represents a TC + TiO 2 composite system, and so on 2.77 mg g −1 (25 °C), and 3.09 mg g −1 (40 °C). On an average, the Cd 2+ adsorption in the Cd + TiO 2 composite system decreased by 3.43%. The effect of temperature on the adsorption of Cd 2+ in the Cd + TiO 2 composite system was the same as those in the single Cd 2+ systems. The changes in the Sips model fitting parameter Q ms were basically consistent with the result of the adsorption experiments. The adsorption affinity (K s ) of Cd 2+ in the Cd + TiO 2 composite system was increased and the adsorption spontaneity was enhanced compared to the single Cd system. The changes in the Cd 2+ adsorption with respect to pH in the Cd + TiO 2 composite systems were basically the same as that in the single Cd 2+ system (Fig. 1d). In the pH range of 3-8, the Cd 2+ adsorption in the composite system was always smaller than that of the single system.

Adsorption of TC on the Zeo in the single and composite systems
In the single TC system, the nonlinear adsorption isotherm lines were observed in TC adsorption. The TC adsorption decreased with increasing pH, and the maximum adsorption capacities of TC were 0.74 mg g −1 (10 °C), 0.69 mg g −1 (25 °C), and 0.63 mg g −1 (40 °C). The adsorption of TC in the single TC system was a physicochemical adsorption process with a spontaneous increase in thermal entropy (Table 1). The effect of the initial pH on S-TC is shown in Fig. 1e. It can be seen that S-TC increased with an increasing pH value in the range of 3-6, and S-TC decreased at the pH values > 6. In the pH range of 3-6, the surface of the Zeo was positively charged (pH ZPC = 5.2), while the charge on TC gradually changed from positive to neutral (Li et al. 2010), thereby decreasing the electrostatic repulsion between TC and the Zeo and increasing the TC adsorption. However, at pH > 5.5, TC (pH ZPC = 5.5) became anionic , and the electrostatic repulsion between the Zeo and TC increased, thereby decreasing the TC adsorption.
The isotherm adsorption isotherms of TC in the TC + TiO 2 composite systems are shown in Fig. 1b. The maximum adsorption capacities of TC in the TC + TiO 2 composite system were 0.66 mg g −1 (10 °C), 0.62 mg g −1 (25 °C), and 0.49 mg g −1 (40 °C). The TC adsorption in the TC + TiO 2 composite system decreased (by 14.39% on an average) with increasing temperature. In the TC + TiO 2 composite system, the affinity and spontaneity of the TC adsorption decreased compared to the single TC system. The changes in the TC adsorption with respect to pH in the TC + TiO 2 composite systems were basically the same as those in the single TC systems (Fig. 1e). In the pH range of 3-8, the TC adsorption in the composite system was always smaller than that in the single system.

Adsorption of TiO 2 on the Zeo in the single and composite systems
In the single TiO 2 system, the increasing temperature was conducive to the TiO 2 adsorption on Zeo, and the maximum adsorption capacities of TiO 2 were 0.70 mg g −1 (10 °C), 0.76 mg g −1 (25 °C), and 0.97 mg g −1 (40 °C). The adsorption of TiO 2 in the single TiO 2 system was a physicochemical adsorption process with a spontaneous increase in thermal entropy. The effect of initial pH on S-TiO 2 is shown in Fig. 1f. In the pH range of 3-5, S-TiO 2 increased with increasing pH. For pH > 6, the Zeo surface was negatively charged, and the electrostatic repulsion between the Zeo and TiO 2 increased, leading to a gradual decrease in S-TiO 2 . The maximum adsorption capacities of TiO 2 in the Cd 2+ + TiO 2 composite system were 0.63 mg g −1 (10 °C), 0.72 mg g −1 (25 °C), and 0.83 mg g −1 (40 °C). The TiO 2 adsorption in the Cd + TiO 2 composite system decreased by 9.90% on an average. Furthermore, the affinity (K s ) and spontaneity of the TiO 2 adsorption in the Cd + TiO 2 composite system increased compared to those in the single TiO 2 system. This was because of the electrostatic interaction between Cd 2+ and TiO 2 and their co-adsorption on the Zeo. The pH-dependent changes in the TiO 2 adsorption in the Cd + TiO 2 composite systems were basically the same as those in the single TiO 2 systems. In the pH range of 3-8, the TiO 2 adsorption in the Cd + TiO 2 composite systems was always lower than that in the single systems (Fig. 1f).
The maximum adsorption capacities of TiO 2 in the TC + TiO 2 composite system were 0.41 mg g −1 (10 °C), 0.43 mg g −1 (25 °C), and 0.47 mg g −1 (40 °C). The TiO 2 adsorption in the TC + TiO 2 composite system decreased by 45.47% on an average compared to that in the single TiO 2 system. Moreover, the affinity and spontaneity of the TiO 2 adsorption in the TC + TiO 2 composite system also increased. This may be because of the hydrogen bond formation between TiO 2 and TC in the composite system, resulting in increased adsorption affinity and spontaneity of TiO 2 . The changes in the TiO 2 adsorption with respect to pH in the TC + TiO 2 composite systems were basically the same as those in the single TiO 2 systems. In the pH range of 3-8, the TiO 2 adsorption in the TC + TiO 2 composite systems was always lower than that in the single TiO 2 systems (Fig. 1f).
The above results confirm that in the Cd + TiO 2 and TC + TiO 2 composite system, Cd 2+ /TC and TiO 2 were antagonists. The inhibition between TC and TiO 2 was more obvious compared to that between Cd 2+ and TiO 2 . The adsorption affinity and spontaneity of Cd 2+ in the Cd + TiO 2 composite system increased compared to those in the single Cd system. However, the adsorption affinity and the adsorption spontaneity of TC in the TC + TiO 2 composite system decreased. Moreover, the adsorption affinity and adsorption spontaneity of TiO 2 in the TC + TiO 2 and Cd + TiO 2 composite systems increased. The antagonism between Cd 2+ and TiO 2 and TC and TiO 2 existed in the pH range of 3-8.

SEM analysis
The SEM images of the Zeo before and after the adsorption are shown in Fig. 2. It can be seen that the Zeo surface was a rectangular parallelepiped with a smooth layered structure and porous structure (Fig. 2a). After the Cd 2+ adsorption in the single Cd system, the number of pores on the Zeo-Cd surface decreased and the layered structure became loose (Fig. 2b). After the TC adsorption in the single TC system, the porous structure on the Zeo-TC surface disappeared, and the surface became smoother and more rounded (Fig. 2c). After the TiO 2 adsorption in the single TiO 2 system, spherical particles (80-100 nm) appeared on the surface of Zeo-TiO 2 . The high-magnification images show that these spherical particles were formed by the accumulation of uniform spherical particles, thereby confirming that TiO 2 was successfully adsorbed on the Zeo surface (Fig. 2d).
After the simultaneous adsorption of Cd 2+ and TiO 2 in the Cd + TiO 2 composite system, they formed spherical particles (size = 10-80 nm) that were smaller than those formed after the simultaneous adsorption of Cd 2+ and TiO 2 in the single TiO 2 system (Fig. 2e). After the simultaneous adsorption of TC and TiO 2 in the TC + TiO 2 composite system, the Zeo surface and adsorbed TiO 2 became smoother and more rounded, and the particle size of the spherical particles decreased (20-60 nm) compared with a single TiO 2 system (Fig. 2f). The SEM images confirmed that the number of pore structures on the Zeo surface reduced after the Cd 2+ , TC, and TiO 2 adsorption. Compared to the single TiO 2 system, the size of the TiO 2 spherical particles on the Zeo surface in the composite system decreased.

CEC
The results of CEC calculations are summarized in Table 2. The CEC of the Zeo was 5.53 cmol kg −1 , which decreased by 7.96% after the adsorption of Cd in the single system. After the TC adsorption in the single TC system, the CEC decreased to 4.33 cmol kg −1 , indicating the adsorption of Cd 2+ on the Zeo through electrostatic attraction. After the adsorption in the single TiO 2 system, the CEC and average pore size of the Zeo increased to 6.15 cmol kg −1 . The increased CEC was due to the -OH adsorbed on TiO 2 , providing more negative charge sites on the Zeo. After the adsorption was completed in the Cd + TiO 2 and TC + TiO 2 composite systems, the CEC of the Zeo decreased to 5.36 cmol kg −1 and 4.86 cmol kg −1 , respectively. The changing trend of the CEC was Zeo-Cd < Zeo-Cd + TiO 2 < Zeo-TiO 2 , indicating that Cd 2+ in the Cd + TiO 2 composite system consumed a part of negative charges provided by TiO 2 adsorbed on Zeo. The CEC of Zeo-TC + TiO 2 was greater than that of Zeo-TC and smaller than that of Zeo-TiO 2 , indicating that the negative charges provided by TiO 2 adsorbed on the Zeo were consumed by TC (Thiagarajan et al. 2019).

S BET and average pore width
The N 2 adsorption-desorption isotherm and pore size distribution of the Zeo before and after the adsorption are shown in Fig. 2g and h. The type-II N 2 adsorption-desorption isotherm confirmed that the adsorption of nitrogen on the Zeo surface was a free reversible adsorption process on macroporous solids. The calculation results of S BET (specific surface area) and average pore width are listed in Table 2. The S BET and average pore width of the Zeo were 2.16 m 2 g −1 and 22.83 nm, respectively.
After the adsorption of Cd 2+ in the single Cd system, the S BET of the Zeo decreased to 2.08 m 2 g −1 ; however, the pore width increased to 24.14 nm. Furthermore, the micropore structures on the Zeo were filled by Cd 2+ , and the same phenomenon is more obvious after the adsorption of TC (S BET decreased to 1.52 m 2 g −1 , while the average pore width increased to 26.81 nm). These observations are consistent with the results of SEM analysis.
After the adsorption of TiO 2 in a single TiO 2 , the average pore width of the Zeo increased to 25.47 nm and S BET decreased to 1.96 m 2 g −1 . The decreased S BET and increased pore width confirmed the adsorbed TiO 2 block part of the micropore structure on Zeo.
After the adsorption was completed in the Cd + TiO 2 and TC + TiO 2 composite systems, S BET of the Zeo decreased to 2.06 m 2 g −1 and 1.64 m 2 g −1 , respectively; however, the average pore width increased to 24.04 nm and 24.84 nm.
The changing trends of S BET and pore width in the Cd + TiO 2 composite system were smaller than the trend of the single TiO 2 system. Combined with the phenomenon observed in SEM images (TiO 2 aggregate particle size reduction), these results confirmed that the TiO 2 adsorption was inhibited by Cd 2+ in the Cd + TiO 2 composite system. The S BET of Zeo-TC + TiO 2 was smaller than Zeo-Cd + TiO 2 , indicating that the effect of organic-phase TC on the specific surface area was greater than that of inorganic Cd 2+ .

FTIR analysis
The changes in the surface functional groups before and after the adsorption on the adsorption material are of great significance to explain the adsorption mechanism. The changes in the surface functional groups before and after the adsorption on the Zeo surface were investigated using FTIR, and the results are shown in Fig. 3. The peaks at 3427 cm −1 and 1621 cm −1 in the Zeo spectrum were O-H vibration peaks. Furthermore, the peak at 992 cm −1 was the vibration peak of Si (Al)-O in the Zeo structure (Bare et al. 2016). The peaks at 458 cm −1 and 755 cm −1 corresponded to Mg-O/Ca-O structural vibration peaks in the Zeo (Chen et al. 2020b).
After the Cd 2+ adsorption in the single Cd system, the -OH vibration peaks on the Zeo surface were shifted to 3425 cm −1 and 1619 cm −1 , respectively. The shape of the hydroxyl peaks had not changed, confirming Cd 2+ combined with -OH through electrostatic attraction (Chen et al. 2020c). After the TC adsorption in the single TC system, the -OH peaks on the Zeo surface were shifted to 3430 cm −1 and 1630 cm −1 , respectively. The shift in the -OH peak after the TC adsorption in the single TC system confirmed the interaction between TC and -OH on the Zeo (Chang et al. 2009). After the TiO 2 adsorption in the single TiO 2 system, the -OH peaks on the Zeo shifted to 3421 cm −1 and 1608 cm −1 , respectively, and the shape of -OH peaks broadened, confirming that the adsorbed TiO 2 provides the Zeo with additional hydroxyl groups (Chen et al. 2020b;Zhao et al. 2012).
After the adsorption of Cd 2+ and TiO 2 in the Cd + TiO 2 composite systems, the -OH peaks of Zeo-Cd + TiO 2 shifted to 3430 cm −1 and 1610 cm −1 , respectively. Studies confirmed that the -OH peak at 3427 cm −1 mainly occurs from the water molecule adsorbed on the surface (Jalilvand et al. 2020), while the peak at 1621 cm −1 was the -OH peak on the surface of the material (Wang et al. , 2020. The -OH peak at 3427 cm −1 moved to a higher wavenumber in the Cd + TiO 2 composite system, which was different from those in the single Cd and single TiO 2 systems. These results were due to the Cd 2+ combined with the adsorbed TiO 2 through electrostatic attraction. After the adsorption of TC and TiO 2 in the TC + TiO 2 composite systems, the -OH peaks of Zeo-TC + TiO 2 moved to 3431 cm −1 and 1612 cm −1 , respectively. The shift of -OH peak at 1621 cm −1 in the TC + TiO 2 composite systems was greater than that of the single TC systems, confirming that more -OH was consumed on the Zeo in the composite systems. This was because TC and TiO 2 in the composite systems can be adsorbed on the Zeo surface through hydrogen bonding, increasing the shift of the -OH peak on the Zeo surface. The -OH peak at 3427 cm −1 moved to a higher wavenumber in the TC + TiO 2 composite system, confirming the TC combined with the adsorbed TiO 2 through hydrogen bonding. However, the Si(Al)-O and Mg-O/Ca-O peaks in the Zeo structure did not shift in the single and composite systems, indicating that there was no ion exchange on the Zeo surface. This was different from the research on Na-zeolites. Studies have confirmed that Na + ion exchange is one of the main ways for zeolites to adsorb heavy metal pollutants (Chen et al. 2020b;Fibikar et al. 2010).

XPS analysis
The surface functional groups and composition of elements of the Zeo before and after the adsorption were analyzed using an X-ray photoelectron spectrometer, and the XPS surveys are shown in Fig. 4a. The main components of the Zeo were C, O, Si, Mg, and Ca; the peaks of Cd 3d (405.9 eV), N 1 s (399.4 eV), and Ti 2p (458.4 eV) appeared in the XPS surveys, respectively, after the adsorption of Cd 2+ , TC, and TiO 2 in single and composite systems. Figure 4a shows the C 1s spectra of the Zeo before and after the adsorption, and the high-resolution C 1s spectra yielded the following three peaks (Chen et al. 2020a;Song et al. 2020 After the Cd 2+ adsorption in the single Cd system, the relative distribution of C-OH and O-C=O decreased to 74.95% and 5.84%, respectively; however, the binding energy did not change. These results confirmed that Cd 2+ was adsorbed on the Zeo through electrostatic attraction with the -OH. After the Cd 2+ adsorption in the single Cd system, the binding energies of Si-O and H-O increased to 531.4 eV and 532.3 eV, respectively. The increased binding energy confirmed the combination of Cd 2+ and Si-O at the edge of the Zeo through electrostatic attractions (Deng et al. 2017).
After the TC adsorption in the single TC system, the relative distribution of C-OH decreased to 52.12%, confirming the electrostatic attraction between TC and -OH. Moreover, the relative distributions of C=O and O-C=O increased to 37.14% and 10.74%, respectively, while the binding energies remained unchanged. This was due to the organic-phase TC adsorbed on the Zeo through the hydrogen bonding between C=O/O-C=O and TC, and the adsorbed TC provides a large number of C=O and O-C=O functional groups for the Zeo surface (Wang et al. 2019). The binding energy of O-H decreased to 531.1 eV (10.32%), indicating that TC combined with H-O by hydrogen bonding, the -OH on the Zeo surface were the electron donor (Wang et al. 2019).
After the TiO 2 adsorption in the single TiO 2 system, the relative distributions of C=O and O-C=O decreased to 13.71% and 7.13%, respectively, owing to the hydrogen bonding between -OH (TiO 2 ) and C=O/O-C=O (Zeo). Moreover, the binding energy of C=O increased to 285.9 eV, while that of O-C=O decreased to 288.7 eV, indicating that C=O is an electron donor and O-C=O is an electron acceptor. The increased relative distribution of C-OH (79.15%, 284.8 eV) may be due to the appearance of Ti-OH bonds on the surface of Zeo (Wang et al. 2019). New peaks of O (III) Ti-O, 530.3 eV, 10.84% appeared in the O 1s spectrum.
After the adsorption in the Cd+TiO 2 composite systems, the C 1s spectra were decomposed into three peaks (Abbas et al. 2020;Song et al. 2020): C (I) C-OH, 284.8 eV, 71.59%; C (II) C=O, 285.9 eV, 20.66%; and C (III) O-C=O, 288.7 eV, 7.75%. The increased relative contribution of O-C=O compared to that in the single Cd system confirmed that the electrostatic attraction between Cd 2+ and the Zeo was suppressed in the Cd+TiO 2 composite system. The relative contribution of C-OH in the composite system decreased compared to that in the single TiO 2 system. These observations indicate that the Cd 2+ adsorption in the composite system consumes a part of -OH provided by adsorbed TiO 2 , which was consistent with the results of CEC. Moreover, these results confirmed the presence of the Cd-TiO 2 combination in the composite system.
In After the adsorption in the TC + TiO 2 composite systems, the C 1 s spectra were decomposed into three peaks (Abbas et al. 2020;Song et al. 2020 The Cd 3d, N 1s, and Ti 2p spectra are shown in Fig. 4b. Compared with the single Cd system, the relative contribution of Cd 2+ bonding with surface functional groups (Chen et al. 2020a) (412 eV) decreased in the Cd+TiO 2 composite system. The relative contribution of -NH 2 + (402 eV) decreased in the TC+TiO 2 composite system compared to that in the single TC system. Compared to the single TiO 2 system, the binding energies of Ti 2p in the Cd+TiO 2 and TC+TiO 2 composite systems increased while the relative contributions decreased. These results confirmed that there were Cd-TiO 2 and TC-TiO 2 combinations in the composite system, and TiO 2 was the electron donor.

Mechanism of the interaction between Cd 2+ /TC, TiO 2 , and Zeo
Combining the above adsorption experiment results and characterization analyses, the proposed interaction mechanisms of Cd 2+ /TC and TiO 2 on the Zeo surface in the composite system are illustrated in Fig. 5. The adsorption of Cd 2+ on the Zeo surface was a physical adsorption process with spontaneous endothermic entropy increase. On the other hand, the adsorption of TC and TiO 2 on the Zeo surface was a physical-chemical adsorption process with a spontaneous increase in heat entropy. Cd 2+ and TiO 2 or TC and TiO 2 in the composite system were mutually antagonistic.
Compared to the single Cd and single TiO 2 systems, the adsorption of Cd 2+ and TiO 2 in the Cd + TiO 2 composite system decreased by 3.43% and 9.90%, respectively. Furthermore, the adsorption affinity of Cd 2+ and TiO 2 increased. The antagonism between Cd 2+ and TiO 2 in the Cd + TiO 2 composite system was because the electrostatic attraction (-OH and Si-O) sites for Cd 2+ were occupied by TiO 2 . The adsorption of TiO 2 on the Zeo surface blocked a part of the micropore structure, the specific surface area of the Zeo decreased, and the Cd 2+ adsorption also decreased. TiO 2 on the Zeo surface provided more negative charges for Cd 2+ . Cd 2+ and TiO 2 were combined through electrostatic attraction and co-adsorbed on Zeo; therefore, the inhibition of TiO 2 to Cd 2+ was less than that to TC. The decreased TiO 2 adsorption in the Cd + TiO 2 composite system was due to Fig. 4 XPS survey spectra and high-resolution (C 1 s, O 1 s, Cd 3d, N 1 s, and Ti 2p) spectra of Zeo before and after adsorption in single systems and composite systems ◂ Fig. 5 Mechanism of antagonism in the Cd + TiO 2 and TC + TiO 2 composite systems the fact that the hydrogen bonding between TiO 2 (-OH) and the Zeo (Si-O and H-O) inhibited by Cd 2+ in the Cd + TiO 2 composite system. The electrostatic attraction between Cd 2+ and TiO 2 increased the adsorption affinity of TiO 2 in the Cd + TiO 2 composite system.
Compared to the single TC and single TiO 2 systems, the TC and TiO 2 adsorption in the TC + TiO 2 composite system decreased by 14.39% and 45.47%, respectively. On the other hand, the TC adsorption affinity decreased, and the TiO 2 adsorption affinity increased. The antagonism between TC and TiO 2 in the TC + TiO 2 composite system was because of TiO 2 occupying the C = O and -OH hydrogen bonding sites on Zeo, thereby decreasing the number of TC adsorbed on the Zeo by hydrogen bonding. TiO 2 in the composite system also inhibited the electrostatic attraction between TC and -OH on Zeo. The adsorbed TiO 2 also inhibited the pore filling of TC on Zeo, decreasing the TC adsorption. Because TiO 2 nanoparticles were hydrophilic, the hydrophilicity of the Zeo increased by the adsorbed TiO 2 on Zeo, resulting in weakened adsorption affinity between TC and Zeo. Similar to Cd 2+ , TC-TiO 2 combinations were also observed on the Zeo. In the TC + TiO 2 composite system, the hydrogen bonding between TiO 2 and the Zeo was inhibited by TC. Because TC was an organic-phase macromolecule, the pore filling effect was stronger than that of Cd 2+ . Therefore, the inhibition of TC to TiO 2 in the TC + TiO 2 composite system was more obvious.

Conclusion
In this study, Cd 2+ , TC, and TiO 2 were used to simulate the coexisting pollutants and nanoparticles in the environment. Moreover, the Zeo was used to simulate the clay minerals in the environment. According to our obtained results, the presence of TiO 2 in the environment will increase the mobility of Cd and TC on a clay surface, and this effect is more significant for organic pollutants TC. On the other hand, pollutants in the environment will also reduce the adsorption of TiO 2 on clay, resulting in enhanced mobility of TiO 2 . Compared to inorganic pollutants Cd, TC has a more significant effect on TiO 2 . SEM, FTIR, and XPS results confirmed that TiO 2 present in the composite system inhibited the pore filling, hydrogen bonding, and electrostatic attraction between pollutions and Zeo, thus decreasing the adsorption of Cd 2+ and TC on Zeo. These results provide a theoretical basis for further exploring the environmental fate of nanoparticles and pollutants in the actual environment.
Author contribution All authors contributed to the study conception and design. Meng Zhaofu were involved in planning and supervised the work; Wang Teng processed the experimental data, performed the analysis, drafted the manuscript, and designed the figures. Liu Lin and Li Wenbin manufactured the samples and characterized them with spectroscopy. All authors discussed the results and commented on the manuscript.
Funding This work was supported by the National Natural Science Foundation of China (no. 41271244), Fundamental Research Program of Shanxi Province (202103021223377), and Colleges and Universities Science and Technology Innovation Program of Shanxi Province (2021L525).

Data availability
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Ethics approval This study did not involve any human subjects and/ or animals.

Competing interests
The authors declare no competing interests.