35 ORIGINAL RESEARCH ARTICLE Sedimentation of nanoparticle titanium dioxide in the presence of am- monium Duo Li1,2, Nan Xu1,2* 1 School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, Jiangsu, China 2 Jiangsu Key Laboratory of Environmental Functional Materials, Suzhou 215009, Jiangsu, China. E-mail: nanxu@ mail.usts.edu.cn ABSTRACT Due to its physicochemical properties, nanoparticles titanium dioxide (nTiO2) is being put into mass production and widespread applications, which inevitably results in their increasing exposure to the water body. After it entering the water body, the chemical properties of nTiO2 can be influenced by ion compositions, ion strength and pH, which affects their ecological risk. Excess of ammonium (NH4+) fertilizer has contaminated soil and water environments. In this paper, the Zeta potentials and hydrodynamic radius of nTiO2 were studied in NH4+ solution compared to those in Na+ solution. In addition, the sedimentation rate of nTiO2 was also investigated. The experiment results show that high pH inhibits the sedimentation of nTiO2. Moreover, NH4+ increases the stability of nTiO2 more than Na+ at the same IS, which was attributed the more negative Zeta potentials and the smaller hydraulic radius. Our results provide a theoretical basis for evaluating the ecological risk of nTiO2 in aqueous solution containing NH4+. Keywords: Ammonium; Chloride; Nanoparticles Titanium Dioxide (nTiO2); Sedimentation; Zeta Potential Characterization and Application of Nanomaterials (2021) Volume 4 Issue 2 doi:10.24294/can.v4i2.1333 ARTICLE INFO Received: 13 June 2021 Accepted: 5 August 2021 Available online: 12 August 2021 COPYRIGHT Copyright © 2021 Duo Li, 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/ 1. Introduction Nanomaterials have small size effects, surface effects, quantum size effects, and macroscopic quantum tunneling effects, so that nano- materials are widely used in medical[1], chemical[2], environmental[3], microelectronics[4] and other industries. Nanotitanium oxide (nTiO2), as common nanomaterial, is heavily produced and widely used due to low production cost, good chemical stability and strong photocatalytic capacity[5]. In the production and consumption of nTiO2, some nTiO2 cannot be released into the natural environment[6,7]. After entering the environment, they will resemble other environmental pollutants and participate in the circulation of the biosphere, where complex migra- tion and transformation processes occur, which will produce ecological effects. For example, Yamamoto et al. found that nTiO2 increases the generation of reactive oxygen species in skin cells[8], which may dam- age DNA or mutation of the gene and eventually develop to cellular cancer[9,10]. It was found that the nanoparticles affect the properties of the particles themselves. Particle size, surface potential, ion species, ion concentration, and pH values all affect the aggregation and settling of nanoparticles in water, hence their reactivity and biotoxicity. There- fore, studying the sedimentation capacity of nTiO2 in water bodies can 36 provide important theoretical support for assessing the potential ecological risk of nTiO2. At present, the global economy is developing rapidly, and science and technology are becoming more and more developed. In agricultural produc- tion, a large number of pesticides and fertilizers containing ammonia nitrogen are being used, and the phenomenon of ammonia nitrogen pollution in lakes, groundwater and rivers has become more and more serious. Therefore, preventing and controlling ammonia nitrogen pollution is an urgent task in Chi- nese agricultural science, and it is of great practical significance to study the harm of ammonia nitrogen pollution. For example, the fixation of phosphorus in soil was studied by Li Yanan et al. in the case of synthetic calcium magnesium carbonate, and high pH favored fixation of phosphorus by the material[11]. At the same time, Na+ is one of the most widely dis- tributed and abundant ions in groundwater, which is inevitably considered when studying the relevant water environment. It has been shown that nTiO2 can be used to remove ammonia nitrogen from sewage[12,13]. After entering the water environment, nTiO2 causes aggre- gation and settlement under van der Waals force and electrostatic attraction, which may lead to short stay in water, but also reduce the specific surface area of the particles, affect nTiO2 reactivity, reduce photocat- alytic performance, and ultimately affect the ability of nTiO2 to remove ammonia nitrogen. So far, most reports have been on the effect of nTiO2 migration. For example, Liu Cheng et al. studied the effect of phosphate on nTiO2 migration in soil[14], and Xu Xiaoting et al. studied the effect of phosphate and humic acid on nTiO2 migration[15]. However, to date, no report has been reported on the impact of ammo- nia nitrogen on nTiO2 settlement performance, so it is very important to study the aggregation and set- tlement capacity of nTiO2 in water bodies containing ammonia nitrogen. In view of this, the authors mainly studied the mechanism of NH4+ on nTiO2 particle settlement, studied the settlement curve of pH values, calculated the settlement rate through Zeta potential and nanop- ularity, and then explained the influence and mecha- nism of NH4+ on the settlement rate of nTiO2 particles in water bodies. 2. Materials and methods 2.1 Experimental reagents nTiO2 was purchased from Shanghai Gaoquan Chemical Co., Ltd. All other reagents were analytical reagents (AR). 2.2 Aggregation experiment of nTiO2 Accurately measure 1 g of TiO2, transfer it to a 100 ml beaker, add 100 ml NH4Cl solution and NaCl solution with different concentrations (1, 5 and 10 mmol·L–1) respectively, adjust the pH value to 6.0 and 8.0 with HCl and NaOH respectively, place the TiO2 suspension in the ultrasonic instrument for ul- trasonic for 30 min, and then stand for interval sam- pling, sampling 2 ml each time. The measurement method of nTiO2 was measured by diantipyrylmeth- ane method, and the absorbance was measured at 390 nm by Shimadzu uv-2450 UV spectrophotom- eter[16]. Dilute 1000 mg·L–1 titanium standard solu- tion to 1–5 mg·L–1 for measurement, and obtain the standard curve of nTiO2 concentration (as shown in Figure 1). Figure 1. Curve for nTiO2 concentration was determined by UV spectrophotometry. 2.3 The Zeta potential and hydraulical diam- eter measurements of nTiO2 Accurately weigh 0.01 g of TiO2, transfer it to a 100 ml beaker, add 100 ml of NH4Cl solution and NaCl solution with different concentrations (1, 5 and 37 10 mmol·L–1) respectively, and adjust the pH to 6.0 and 8.0 with HCl and NaOH respectively. Referring to the research method of Fang et al.[17], in order to promote the better dispersion of nTiO2 in the solu- tion, the suspension was placed in the ultrasonic instrument for ultrasonic for 10 min, and then each sample was sampled three times. The zeta potential and hydraulic diameter were measured by Malvin Nano-ZS90, and finally the average value was taken. 2.4 Settlement efficiency model The sedimentation data were analyzed using the empirical model of Quik et al. and Velzeboer et al.[18,19]: (1) in which t is the settlement time, Ct is the colloidal concentration (g·L–1) at the time t, Cres is the resid- ual concentration (g·L–1), C0 is the initial concentra- tion (g·L–1), Vs is the sedimentation rate (mm·min–1), h is the height of the water surface distance sampling point, aggregate Vs. According to equation (1), as- suming not in each solution, the nTiO2 samples were dissolved in ionic states. 3. Results and discussion 3.1 Zeta potential and hydroechanical diame- ter of nTiO2 in NaCl solution In Figure 2, it studied the Zeta potential (a) and hydroechanical diameter (b) of nTiO2 at pH = 6.0 and 8.0, in NaCl solution. In Figures 2(a) and 2(b), the Zeta potential be- comes slightly less negative with Na+ concentration, with particle size (hydroechanical diameter) larger and prone to reunion. When the pH value continues to increase to 8.0, the Zeta potential of nTiO2 in the water environment becomes more negative and the hydraulical diameter decreases. For specific pH val- ues, both Zeta potentials become less negative with increasing Na+ concentration, mainly because the charge shielding effect and electrostatic bielectric layer on the nTiO2 surface are compressed resulting in reduced net negative charge and reduced Zeta po- tention the particle surface[20,21]. 3.2 Zeta potential and hydroechanical diame- ter of nTiO2 in NH4Cl solution In Figure 3, it studied The Zeta potential (a) and hydroechanical diameter (b) of nTiO2 at pH = 6.0, were studied in NH4Cl solution 8.0. In Figure 3(a), the Zeta potentials of nTiO2 at pH = 6.0 and 8.0 are all negative. At the same NH4Cl concentration, the Zeta potential at pH = 6.0 is less negative than that at pH = 8.0; in Figure 3(b), at the same NH4Cl concentration, the particle size of nTiO2 decreases with increasing pH values. The possible mechanism leading to this phenomenon is that, Zeta, with less than a negative potential. This results in a smaller electrostatic repulsion between the nTiO2 particles, making it easier to reunite between nTiO2 particles and then leading to a larger particle size. In Figure 3(a), the Zeta potential of nTiO2 becomes less negative with increasing ion concentrations for spe- Figure 2. Zeta potentials (a) and Hydraulical diameter (b) of nTiO2 in different concentrations of NaCl solution under pH= 6.0 and 8.0. 38 cific pH values, consistent with the phenomenon in Na+. The increase in the hydrotomechanical diameter in Figure 3(b) is consistent with the less negative of the Zeta potential in Figure 3(a). Compared to Figures 3 and 2, the Zeta potential of nTiO2 is more negative and smaller particle size in NH4Cl elec- trolyte solution at the same pH values and the same ion concentration. For example, under the condition of pH = 6.0, the Zeta potential and hydroechanical diameter of nTiO2 in 1 mmol·L–1 NaCl solution are –19.3 mV, 554 nm, and the corresponding value be- comes -20.6 mV, 472 nm. When the electrolyte solu- tion is 1 mmol·L–1 NH4Cl, the corresponding values are changed to –20.6 mV, 472 nm. 3.3 Settlement of nTiO2 in the NaCl solution Figure 4 is the settlement curve of nTiO2 in NaCl solution in pH = 6.0 and 8.0. The abscissa represents the settling time, and the ordinate represents the ratio of the concentra- tion of nTio2 (c) in the sampled liquid to the initial concentration of nTio2 (C0) under the corresponding settling time. The smaller the ratio, the lower the concentration of nTiO2 in the representative sample, and the more nTiO2 settled in the corresponding solution. Figure 4(a) shows that under the condition of pH = 6.0, although the concentration of NaCl solution has no obvious effect on the sedimentation performance of nTiO2, on the whole, the higher the ion concentration, the faster the sedimentation rate and the worse the suspension stability. The increase of ion concentration is conducive to the sedimenta- tion of particles, but this effect is not very obvious. According to the sedimentation rate (Vs) simulated and calculated according to formula (1), it is found that with the increase of ion concentration, Vs only increases from 0.507 mm·min–1 to 0.534 mm·min–1. In Figure 4(b), under the condition of pH = 8.0, low NaCl concentration (1 mmol·L–1) has obvious sedimentation effect on nTiO2, which significantly enhances the suspension stability of nTiO2 in water environment. Comparing Figure 4(a) and Figure 4(b), under the condition of 1 mmol·L–1 NaCl, pH Figure 3. Zeta potentials (a) and Hydraulical diameter (b) of nTiO2 in different concentrations of NH4Cl solution under pH = 6.0, and 8.0. Figure 4. Settlement curves of nTiO2 in different concentrations of NaCl solution pH = 6.0 (a) and 8.0 (b). 39 value is an important factor affecting the sedimenta- tion performance of nTiO2 in water environment; at higher ion concentration, the inhibition effect of high pH value on sedimentation is not obvious. 3.4 Settlement of nTiO2 in the NH4Cl solution As shown in Figure 5(a), nTiO2 settles at pH = 6.0 with increasing NH4+ concentration, indicating that nTiO2 sedimentation performance is affected by NH4+ concentration, and higher sedimentation rate and worse suspension stability, mainly due to the charge shielding effect of the nTiO2 surface and electrostatic double electric layer compression the- ory, and enhanced electrostatic repulsion between particles[19,20].When the pH value increased to 8.0, the suspension stability of nTiO2 still weakened with increasing NH4+ concentration. The data in Table 1 explain this well: it does increase with ion concen- tration. This suggests that NH4+ concentration is an important factor influencing the settling properties of nTiO2. By comparing Figures 5(a) and 5(b), we also found that when the pH increases at the same NH4+ concentration, the smaller the settlement rate, the better the suspension stability. This is consistent with previous Zeta potential and hydroechanical di- ameter results: the Zeta potential of nTiO2 is more negative and smaller hydraulical diameter at high pH. Moreover, the data in Table 1 indicate values at pH = 6.0 higher than those at pH = 8.0, indicating that increased high pH improves the suspension sta- bility of nTiO2 in the water environment. Comparing Figures 4(a) and 5(a), it is not difficult to find that NH4+ inhibited the aggregate deposition of nTiO2 compared to Na+ under the same ionic strength at pH = 6.0. If settlement equilibrium is reached, the value of C/C0 is 0.029 in 10 mmol·L–1 NaCl solution, and the ratio is 0.011 in NaCl solution. This phenomenon still applies when pH increases to 8.0. The Zeta po- tential and hydraulical diameters in Figures 2 and 3 also show that a more negative Zeta potential and a smaller hydraulical diameter will make the nanopar- ticles difficult to settle and enhance stability. It can be seen that NH4+ is conducive to improve the sus- pension stability of nTiO2 in the water environment. Figure 5. Settlement curve of nTiO2 at pH = 6.0 (a) and 8.0 (b) in different concentrations of NH4Cl solution. Table 1. Settlement rates of nTiO2 (10 g·L–1) in different electrolyte solutions PH of solution Concentration of NH4Cl /mmol·L–1 Settlement rate Vs / mm·min–1 PH of solution Concentration of NaCl /mmol·L–1 Settlement rate Vs / mm·min–1 6.0 1 0.570 6.0 1 0.507 5 0.688 5 0.524 10 0.782 10 0.534 0.4598.0 1 0.142 8.0 1 0.214 5 0.196 5 0.459 10 0.363 10 0.466 40 4. 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