55 Characterization and Application of Nanomaterials (2021) Volume 4 Issue 1 doi:10.24294/can.v4i1.1328 Original Research Article Influence of flow rate on the transport of nTiO2 and phosphate and its modeling Gang Feng1,2, Nan Xu1,2*, Zuling Li1,2, Yuhe Cao1,2, Keqing Sun1,2 1School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, China. E-mail: nanxu@mail.usts.edu.cn 2Jiangsu Key Laboratory of Environmental Functional Material, Suzhou 215009, China ABSTRACT We studied Zeta potentials of nanoparticles titanium dioxides (nTiO2) in different concentration of NaNO3 and phosphate (P) solutions. In addition, the effect of flow rate on the transport of nTiO2 in P was investigated at pH = 6.5. Experimental results show that the Zeta potential of nTiO2 is compressed with the increasing ion concentration (IC) of NaNO3 at pH = 6.5. The negative charge increases with the augment of P. Therefore, the high P and low NaNO3 induce the stabilization of nTiO2 aggregates. The transport experiments suggest that the rapid flow rate is favorable for the transportability of nTiO2 and soluble phosphate. The breakthrough transport curves (BTCs) of nTiO2 in sand columns can be fitted well with two-site kinetic attachment model. The modeling results suggest that the values of first-order at- tachment rate coefficients (k2) and detachment rate coefficients (k2d) on site 2 and first-order attachment rate coefficients (k1) on site 1 are responsible to the attaching efficiency of nTiO2 on sands and their transportability. Keywords: nTiO2; Zeta Potential; Transport; Phosphate ARTICLE INFO Received: 14 January 2021 Accepted: 28 February 2021 Available online: 6 March 2021 COPYRIGHT Copyright © 2021 Gang Feng, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ Nano titanium dioxide (nTiO2) is one of the mass-produced metal oxide nano materials. Due to the ultra-high photocatalytic ability of nano materials, it has been more and more used in various fields and commercial products[1–5]. In mass production and wide application, some nTiO2 cannot be avoided to be released into natural water and soil environment. A large amount of evidence shows that after arti- ficial nTiO2 enters the water body, it has adverse effects on aquatic organisms, including microorganisms, algae, invertebrates and fish[6,7]. Therefore, the study of the relationship between nanoparticles attached to soil saturated particles and nanoparticles in soil particle saturated po- rous media has become a hot spot in the study of the environmental be- havior of nanoparticles. At present, the artificial nano materials studied internationally are mainly industrial fullerene nC60, carbon nanotubes and silica nanoparticles. The mobility of these nano materials in satu- rated porous media is different. The flow rate of solution, ion concen- tration and surface potential of nanoparticles will affect their migration behavior[8,9]. Therefore, it is necessary to explore the changes of surface properties of nTiO2 under different environmental conditions and its migration in the natural world. In agricultural production and people’s life and production, a large number of phosphorus containing substanc- es are used, which makes phosphorus containing substances enter into 56 soil and water. nTiO2 particles have certain adsorp- tion on phosphate, which changes its surface proper- ties, and finally changes the migration properties of nTiO2 in soil[10,11]. Therefore, this paper will explore the changes of surface Zeta potential of nTiO2 under different environmental conditions and the effects of different environmental water flow velocities on its migration in phosphate environment. 1. Materials and methods 1.1 Preparation of the nTiO2 phosphate sus- pension All chemicals used in the experiment are analyt- ical pure and purchased from Sinopharmgroup. 20 nm nTiO2 was purchased from Shanghai Gaoquan Chemical Co., Ltd., 1 g·L–1 TiO2 was weighed and dissolved in 0.1 mM phosphate solution (NaH2PO4) and 10 mM NaNO3 electrolyte solution, and ul- trasonic was used with an ultrasonic cleaner (KQ 2200B, ultrasonic instruments Co., Ltd., Hunshan, China) for 30 mins for migration experiment. 1.2 Test of the Zeta potential on the nTiO2 particle surface Accurately weigh 0.01 g of TiO2 particles into a 100 mL beaker and prepare suspensions under different conditions. The different conditions are electrolyte NaNO3 concentration (0.1–5 mM) and phosphate solution (0.1–5 mM), in which the pH value of all suspensions is adjusted to pH = 6.50 with diluted HCl and NaOH. Then, the suspension of nTiO2 particles with adjusted pH value was placed on the ultrasonic instrument for 30 mins. After ultra- sonic treatment, the suspension was placed on a stir- rer and stirred for 30s. Finally, the Zeta potential of nTiO2 particles was tested with Marvin Nano-ZS90. All samples were tested three times, and the average value was taken as the final Zeta potential value. 1.3 Migration experiment of quartz sand col- umn Referring to Fang, et al., column leaching ex- periment was used to study the migration behavior of nTiO2 [2]. A chromatographic column with a length of 17.5 cm and an inner diameter of 25 mm was se- lected. The chromatographic column was filled with quartz sand and saturated with deionized water for 12 h. Inject 200 ml (10 mM NaNO3) of background solution into the saturated chromatographic column with a peristaltic pump, and collect 10ml of efflu- ent every 10 mins with an automatic sample partial collector (BS-100A, Huxi, Shanghai). Then, about 5 pore volumes (PV) of the nTiO2 suspension were injected into the chromatographic column, and 20 tubes of effluent were collected with an automatic partial collector. After the suspension is injected, continue to inject 5 PV NaNO3 background solution, collect the effluent and wait for test. 1.4 Analysis and determination of the titani- um and phosphorus concentrations Analysis and determination of titanium (TI): take 2 mL of nTiO2 suspension and put it into a 25 mL beaker, add 1–2 mL of sulfuric acid ammonium sulfate digestion solution to the beaker, place it on a heating plate and heat it at 220 ℃ for 1–1.5 h. After digestion, transfer the solution to a 50 mL volumet- ric flask for constant volume, then transferring 5 mL into a 50 mL volumetric flask, and successively adding 8 mL (Vhydrochloric acid:Vdeionized water = 5:1) dilute hydrochloric acid, 2 ml (10 g·L–1) ascorbic acid and 10 mL diantipyrylmethane hydrochloride solution with constant volume. The concentration of Ti was measured with an ultraviolet spectrophotometer (TU-1901, Shimadzu, Japan) at a wavelength of 390 nm. Dilute 1000 mg·L–1 titanium standard stock solution (matrix is 0.15 mol·L–1 HNO3) into a series of standard solutions (concentration gradient is 1–5 mg·L–1), and then obtain the standard curve and mea- sure the concentration of titanium. Analysis and determination of total phosphorus: determine the phosphorus concentration by molyb- denum blue chromogenic method, put the solution to be measured into a 50 mL volumetric flask to vol- ume, and successively add a drop of phenolphthalein, a drop of 1 mol·L–1 NaOH solution (shake well), a drop of 1 mol·L–1 sulfuric acid solution (shake well to colorless), 1 mL (100 g·L–1) anti chemical acid and 2 mL molybdate to volume[12]. After 20 mins of 57 color development, the absorbance of phosphorus (P) was measured with an ultraviolet spectrophotome- ter at the wavelength of 700 nm. In addition, dilute the phosphorus standard stock solution into a series of standard solutions with a concentration gradient of 1–5 mg·L–1, measure the absorbance at the same wavelength, then obtain the standard curve and de- termine the phosphorus concentration. Analysis and determination of dissolved phos- phorus: take 5 mL of nTiO2 suspension into 7 mL high-speed centrifuge tube, place the centrifuge tube in ultra-high-speed centrifuge (GL-21M, Thermo Fisher Technology Company), and centrifuge at 4 ℃ and 15,000 r·min–1 for 1 h. Pass the supernatant over 0.22 μM porous filter membrane, test the concentra- tion of dissolved phosphorus according to the above method of testing total phosphorus. 1.5 A two-point kinetic model A two-point kinetic adsorption model (TSKAM) was chosen with the equation[13,14]. (1) Among them, θ is the porosity of the quartz sand column, and C represents the concentration of nTiO2 particles in the solution, ρb represents the unit weight of quartz sand, x represents the spatial vertical coor- dinate axis, and D represents the hydraulic disper- sion coefficient, ν represents the water flow velocity, S1 and S2 represent nTiO2 sites l and 2, respectively. The core of TSKAM model is to divide the sites on the quartz sand surface conducive to the adsorption of TiO2 particles into site 1 and site 2. The nTiO2 particles retained at site 2 are controlled by convec- tion dispersion, and the mass conservation equation is the first-order kinetic adsorption and desorption equation. (2) k2 and k2d are the adsorption and desorption rates at site 2, respectively, and the adsorption of the nTiO2 particles at site 2 belongs to the reversible adsorption. The mass conservation equation on site 1 is: (3) k1 is the adsorption rate at the colloidal site 1. Adsorption at site 1 is irreversible adsorption. ψx is a function related to the depth of the filled column[13]. (4) Where, dc is the average particle size of quartz sand and x0 is the distance on the coordinate axis. At this distance, the reten of nTiO2 particles is related to the column depth. β is an empirical coefficient that controls the shape of the spatial nTiO2 curve. The smaller the values of site 2 adsorption efficiency (k2), analytical efficiency (k2d) and site 1 adsorption effi- ciency (k1) are, the less the reten nTiO2 of particles on quartz sand and the higher their mobility will be. The penetration curve of nTiO2 particles is simulated by HYDRUS-1d software to obtain parameters k1, S1, k2 and k2d [15]. 1.6 Migration parameters The nTiO2 particle mass recovery can be ob- tained by performing area integration of its migration curves. (5) In formula, Q is the pore flow velocity (mL·min–1), C0 and C are the inflow and outflow TiO2 concentration (mg·L–1), t is time (min) and t0 is pulse duration (min). The probability of nTiO2 particles adsorbing on the quartz sand surface is called the adsorption effi- ciency (α). (6) In formula, L is the length of the column. θ is 58 the porosity of the filled column. dc is the diameter of sand and η0 is the theoretical single medium contact efficiency. Net bed penetration coefficient: (7) Particulate deposition rate coefficient: (8) In formula, vp is the flow velocity and k is the coefficient of time and distance correlation. The maximum migration distance of the nTiO2 particles was defined as the distance at which the nTiO2 particles move when 99.9% of the nTiO2 par- ticles are trapped, that is, (9) The results of the various migration parameters for the above formula are shown in Table 1. Table 1. Physical and computational parameters of nTiO2 particles and quartz sand columns in the migration experiments Number NaNO3 Concen- trated/mM P con- centrat- ed/mM Flow speed/ ml· min–1 Outflow ratio/% Single medi- um contact efficiency Adsorption efficiency Adsorption efficiency/ cm–1 Sedimen- tation rate coefficient/h–1 Maximum migration distance/cm 1 10 0.1 0.5 3.9 35.9 3.56 × 10-4 0.19 2.705 37.3 2 10 0.1 1 12 35.9 2.32 × 10-4 0.12 3.536 57 3 10 0.1 2.5 38 35.9 1.06 × 10-4 0.06 4.034 124.9 2. Results and discussion 2.1 Effect of different concentrations of NaNO3 electrolytes on the Zeta potential on the nTiO2 surface When pH = 6.5, the change of surface Zeta po- tential of nTiO2 in different concentrations of NaNO3 is shown in Figure 1(a). At different electrolyte concentrations, the Zeta potential on the surface of TiO2 particles is negative, which indicates that the surface of TiO2 particles is negatively charged at different concentrations of NaNO3. With the increas- ing concentration of NaNO3 in the solution, the Zeta potential on the surface of nTiO2 particles decreases (the absolute value decreases, that is, the negative is getting smaller and smaller). When the NaNO3 con- centration increased from 0.1 mM to 5 mM, the cor- responding Zeta potential changed from –19 mV to –6.09 mV. This is mainly because with the increas- ing solubility of NaNO3 in the solution, the charge shielding effect and electrostatic double layer on the surface of nTiO2 particles are compressed, and the net negative charge on the surface of TiO2 particles decreases[16,17]. As a result, the Zeta potential on the Figure 1. Zeta potentials of nTiO2 particles with different NaNO3 concentrations (a) and different phosphorus concentrations (b) in the 10 mM NaNO3 background solution (pH = 6.5). 59 surface of nTiO2 particles is reduced, so the disper- sion stability of nTiO2 particle suspension is also reduced. 2.2 Effect of different phosphorus concentra- tions on the Zeta potential on the nTiO2 sur- face As shown in Figure 1(b), when pH = 6.5 and background solution NaNO3 is 10 mM, the Zeta potential on the surface of nTiO2 particles increases with the increase of P concentration. For example, when the P concentration is 0.1 mM, its surface Zeta potential is –28.6 mV, while when the P concentra- tion is increased to 5 mM, its surface Zeta potential increases to –32.43 mV. The results show that be- cause phosphate is adsorbed on the surface of nTiO2 particles, the charge density on the surface of nTiO2 particles is improved through the deprotonation of surface carboxyl groups[18]. Therefore, the electro- static repulsion between the nTiO2 particles and the nTiO2 particles adsorbing P is strengthened, which eventually leads to the improvement of the disper- sion stability of the nTiO2 particle suspension[19]. 2.3 Effect of water flow velocity on the migra- tion of nTiO2 particles suspended in a phos- phate solution The effects of different water flow velocities (0.5–2.5 mL·min–1) on the migration of TiO2 parti- cles and P in quartz sand column were investigated when the suspension pH was 6.5, the phosphate con- centration was 0.1 mM and the background solution NaNO3 concentration was 10 mM. The flow velocity selected in this group of experiments is within the range of groundwater flow velocity. Figure 2 shows the penetration curve of nTiO2 particles at different water velocities. With the increase of water flow velocity, the outflow ratio (C/C0) of nTiO2 particles increases continuously. When the water flow veloci- ty increases from 0.5 mL·min–1 to 2.5 mL·min–1, the outflow ratio of nTiO2 particles increases from 3.9% to about 38.0%, which is similar to the migration law of nano-hydroxyapatite in quartz sand column under different water flow velocities[21]. The above phe- nomenon is mainly because with the increase of wa- ter flow velocity in the quartz sand column, the total sites on the quartz sand surface that can be adsorbed by nTiO2 particles in the quartz sand column also decrease. When the water flow velocity is very high, the total sites on the quartz sand surface that can be adsorbed by nTiO2 particles decrease sharply due to the action of hydraulic shear force. The retention of nTiO2 particles in quartz sand column also decreas- es. In addition, when the water flow velocity is very low, it is difficult to provide enough kinetic energy for nTiO2 particles to penetrate the quartz sand col- umn in the quartz sand column with small porosity, and a large number of nTiO2 particles are retained in the quartz sand column[21–23]. In addition, it can be seen from Table 1 that when the water flow rate increases from 0.5 mL· min–1 to 2.5 mL·min–1, the adsorption efficiency of nTiO2 particles on the surface of quartz sand reduc- es from 3.56 × 10–4 to 1.06 × 10–4, the retention of nTiO2 particles in quartz sand column is reduced and the migration ability is continuously improved. Although the deposition rate coefficient increased from 2.705 h–1 to 7.699 h–1, due to the increase of water flow velocity, the continuous improvement of hydraulic shear force improves continuously, the active collision increases continuously during migra- tion, the retention of nTiO2 particles on the surface of quartz sand decreased constantly, and more nTiO2 particles penetrated the quartz sand column. In ad- dition, the maximum migration distance of nTiO2 particles is also increasing with the increase of water flow velocity, and the maximum migration distance is greater than the height of the column by 17.5 cm, which shows that nTiO2 particles can smoothly pene- trate the quartz sand column under these three differ- ent water flow velocities. Therefore, the increase of water flow velocity promotes the migration of nTiO2 particles in the quartz sand column. As for the effect of water velocity on the migration of nTiO2 parti- cles in saturated quartz sand column, the two-point dynamic model can well simulate the penetration curve of nTiO2 particles in quartz sand column. As shown in Table 2, the water flow velocity is 0.5–2.5 mL·min–1, and the simulated R2 are 0.983, 0.996 and 0.990 respectively, indicating that the model has 60 high fitting degree. With the increase of water flow velocity, the adsorption efficiency of site 1 (k1), site 2 (k2) and the first-order desorption rate (k1d) on site 1 decrease, which indicates that the adsorption of nTiO2 particles on the surface of quartz sand is less, resulting in the increase of their migration, which is more conducive to their migration. Figure 2. Penetration curve of nTiO2 particles at different water flow speeds. 2.4 Effect of water flow velocity on phosphate migration As shown in Figure 3, the outflow ratio of to- tal P increases with the increase of water flow rate. When the water flow rate was 0.5 mL·min–1, the out- flow ratio of total P was 38.0%; When the flow rate increases to 1 mL·min–1, the outflow ratio of total P is 50%. Continue to increase the flow rate to 2.5 mL·min–1, and the outflow ratio of total P increases to 67.9%. This is mainly because with the increase of water flow velocity in the quartz sand column, the ability of nTiO2 particles to penetrate the quartz sand column increases, so that the P adsorbed on the nTiO2 particles also migrate out of the quartz sand column. After digestion, the measured total P con- centration also increases. However, with the increase of water flow velocity, the outflow ratio of dissolved P does not change significantly, as shown in Figure 4. At this time, the outflow ratio of dissolved P is basi- cally maintained at about 21.5%. There is no obvious change in the outflow ratio of dissolved P, mainly because the change of water flow velocity will not affect the adsorption capacity of nTiO2 particles for P. Therefore, no matter how the water flow velocity changes, the concentration of dissolved P in the solu- tion will not change. In addition, the dissolved P is obtained by subtracting the total phosphate from the bound P of nTiO2 particles. Therefore, when the ve- locity is changed and the concentration of dissolved P remains unchanged, the water flow rate is at 0.5 mL·min–1, the dissolved P in the effluent is the main, and only a small part of the bound P with nTiO2 particles exists. When the water flow rate increases, most of the P in the effluent exists in the form of bound P with nTiO2 particles, and only a small part exists in the form of dissolved P. Figure 3. Penetration curves of total phosphorus suspended in (0.1 mM) phosphate solution at different flow rates. Table 2. Simulation parameters of two-point kinetic adsorption model under different experimental conditions Number Site 2 adsorption efficiency Site 2 resolution efficiency Maximum value of reten- tion at point 1 Site 1 adsorption efficiency Person mean square correction factor 1 14.470 6.271 97.15 0.230 0.983 2 14.160 6.028 45.67 1.152 0.996 3 3.138 1.224 33.50 0.067 0.990 61 Figure 4. Penetration curves of dissolved phosphorus suspended in (0.1 mM) phosphate solution by different flow velocities. 3. Conclusion (1) When pH = 6.5, with the increase of elec- trolyte NaNO3 concentration, the Zeta negative po- tential on the surface of nTiO2 particles decreased gradually; (2) The surface negative charge of nTiO2 increases with the increase of P concentration, and its dispersion stability also improves constantly; (3) High flow velocity promotes the mobility of nTiO2, and the migration of soluble phosphate also increas- es continuously; (4) The two-point kinetic adsorption model can well simulate the migration and penetra- tion curve of nano materials in quartz sand column. The model results show that with the increase of flow rate, the adsorption efficiency of site 2 (k2), ana- lytical efficiency (k2d) and site 1 adsorption efficiency (k1) decrease, and the adsorption efficiency of nTiO2 particles on quartz sand decreases, so that more nTiO2 particles penetrate the quartz sand column. Conflict of interest The authors declare that they have no conflict of interest. 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