- Email: [email protected]
Application of freeze concentration for fluoride removal from water solution
Journal of Water Process Engineering 19 (2017) 260–266
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Application of freeze concentration for fluoride removal from water solution Yahui Yang a b
a,b
, Yudong Lu
a,b,⁎
a,b
, Jinyan Guo
, Xiaozhou Zhang
MARK
a,b
College of Environmental Science and Engineering, Chang’an University, Xi’an 710054, Shaanxi, China Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education, Chang’an University, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluoride removal Freeze concentration Deionized water Salinity
A study on fluoride removal by freeze concentration was conducted using deionized and added dissolved substances to the water samples. Fluoride removal characteristics were evaluated in a batch system with reference to changes in freezing temperature, initial concentration, freezing rate and total dissolved solids (TDS). Fluoride removal was found to be freeze temperature-dependent and the optimal temperature range was −15 °C to approximately −20 °C. Fluoride removal rate ranged from 50 to 75% for added dissolved substances to water and from 75 to 85% for deionized water spiked with fluoride, which gives a more satisfactory result. The decline of salinity was consistent with the decline of fluoride concentrations. However, from the perspective of fluoride removal rate, TDS played an inverse role in the fluoride removal efficiency. The removal efficiency by freeze concentration was finally validated using the high fluoride groundwater samples collected from 5 monitoring wells in the field, which showed a good correlation. Our results show that freeze concentration is possible to develop as a feasible technology for fluoride removal from aqueous environment in remote and small areas.
1. Introduction
A substantial and growing researches have been conducted on the removing excessive fluoride from drinking water. The fluoride removal technique based on the mode of action includes adsorption, ion-exchange, precipitation, membrane separation process, electrochemical process, and electrodialysis [3,9–12] and so on. However, the major limitations for these methods are high investment in infrastructure and equipment, secondary pollution, unpleasant taste of treated water and bad maneuverability in the treatment, particularly in rural areas of developing countries. Therefore, such problems limit the further application and promotion of fluoride removal technique. Nowadays an increasing number of people are beginning to concern about freeze concentration, which is an economically feasible, little pollution, free of corrosion and scaling technique. This technique can be used for decontaminating water during the formation of ice crystals, especially in the regions where natural cool energy is available. It has been reported that it is effective to remove various organic and inorganic contaminants from industrial wastewater/liquid waste [13–15]. Three basic concentration techniques are available: suspension crystallization, film freeze concentration and freeze-thaw method (also known as block freeze concentration). The last one of them includes freezing, thawing and separation steps, which was discussed in our study [16]. Although the theory of impurity separation by freeze concentration has been studied [2,17–19], more work is needed to be explored the fluoride separation rules on the basis of the theory of crystallization kinetics.
Fluoride removal from contaminated drinking water has been the focus of hydrochemistry and hygiene studies in recent years, since fluorosis is one of the significant issues worldwide and poses a major health risk to people and pets [1,2]. Fluoride in minute quantity is an essential component and intimately related with human life activity, tooth and skeleton constitution metabolism [3]. However, excess ingestion of fluoride is responsible for dental fluorosis or crippling fluorosis. Data from World Health Organization (WHO) indicate that, the desirable and permissible limit range of fluoride content of drinking water is 0.5–1.0 mg/L [4]. Endemic fluorosis occurring on account of consumption of groundwater with high-concentration fluoride has become a worldwide problem, particularly in the United States, Africa, Asia, the Middle East and China [5,6]. For example in China, the endemic fluorosis is mainly distributed in the northwestern, northern and northeastern China, most of which are arid and semi-arid areas [7]. Recent research on fluoride of groundwater in northwestern arid region is mainly concentrated on the Junggar Basin, the Tarim Basin, Heihe River Basin [8] and the Ningxia region, however little research in the Ulan Buh Desert. Concerning the toxic effects of fluoride on human health, it is imperative to find out a cost-effective and feasible method for the removing excessive fluoride from drinking water. Accordingly, sustainable management of that water resources requires proper evaluation of fluoride removal efficiency. ⁎
Corresponding author at: College of Environmental Science and Engineering, Chang’an University, Xi’an 710054, Shaanxi, China. E-mail address: [email protected] (Y. Lu).
http://dx.doi.org/10.1016/j.jwpe.2017.05.009 Received 27 November 2016; Received in revised form 7 May 2017; Accepted 23 May 2017 2214-7144/ © 2017 Published by Elsevier Ltd.
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Fig. 1. The location of the study area.
of up to 4 mg/L. Studies have identified the fluoride present in the phreatic aquifer as being sourced from aeolian sandy soil and fluorine minerals from the surrounding mountainous sedimentary and magmatic rocks. The hydrochemical types of phreatic groundwater are ClNa and Cl·SO4-Na primarily.
Batch experiments with deionized and added dissolved substances water samples were performed to investigate fluoride separation capacity by freeze concentration in the artificial refrigeration system. The influence of several significant experimental parameters were subsequently examined, including initial fluoride concentration, freezing temperature, freezing rate and the presence of other salinities on fluoride removal efficiency. The regularity of desalination by freezethaw method was also experimentally assessed in our study. Furthermore, the results were validated using field data for fluoride removal and desalination efficiency. It is hoped that the results of this study will allow for a better understanding of the performance of freeze concentration for fluoride removal in similar semi-arid basin globally; a topic of vital importance to water supply and human health.
3. Materials and methods 3.1. Materials Experiment reagents were selected including sodium fluoride, sodium chloride, calcium chloride, magnesium sulfate, sodium bicarbonate, sodium citrate and hydrochloride. Laboratory instruments mainly contain deionized water, temperature controlled refrigeration freezer -FYL-YS-128 (Beijing Fu Italian Electric Co., Ltd.), fluoride selective electrode, ion meter or pH meter, saturated calomel electrode, magnetic stirrer, volumetric flask, polyethylene beaker, pipette, and conductivity meter DDS-11A. All the instruments were calibrated before use. The desired initial fluoride concentration of 2 mg/L, 3 mg/L, 4 mg/L, 5 mg/ L, 6 mg/L, 8 mg/L, 10 mg/L, 15 mg/L, 20 mg/L (by preparation with deionized water and sodium fluoride) were used in this work. Field water samples were collected in October 2013, from the monitoring wells S5-06, S4-09, S4-24, S4-02 and S2-06 with high fluorine content or high salinity. Sampling, preservation, transportation and analytical protocols were conducted by technical regulations and standards for groundwater samples (HJ493-2009).
2. Study area The study area located in the southwest edge of Ulan Buh Desert, Alashan league in Inner Mongolia, is an intra-continental rift basin spreading in the NE–SW direction. It is bounded by longitudes 105°15′ − 106°40′ E and latitudes 39°10′ ∼40°30′ N, covering an area of 9200 km2 (Fig. 1). According to climate statistics between 1955 and 2010 from weather station of Jilantai, the average annual precipitation is roughly 108.1 mm, and the mean evaporation rate is about 1734.2 mm per year. Extensive evaporation and prolonged rock–water interaction cause the enrichment of fluoride in groundwater at Jilantai [20]. Previous survey data have shown that shallow groundwater containing excess fluoride is common in the Jilantai Basin, reaching levels 261
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
3.2. Experimental procedure 3.2.1. Deionized water spiked with fluoride To investigate the freeze concentration process under controlled conditions, a laboratory experiment was conducted using deionized water. The controlled freeze concentration experiment was designed to understand the fluoride separation capacity and the factors affecting fluoride removal during freeze concentration. The experiment adopted a progressive crystallization method for freeze concentration that was accomplished with samples of 500 ml in the freezer. The freezer temperature fluctuates a range of ± 0.5 °C. The freezer was rapid precooled to keep at the set temperature during the freezing operation. The test samples of various concentration were frozen at a series of different temperatures: −5 °C, −10 °C, −15 °C, −20 °C, −25 °C and −30 °C. Samples were allowed to freeze progressively for 2 h, 4 h, 6 h, 8 h, 10 h respectively, during which time ice crystal grew inwards from circumference zone after freezing procedure [21]. The ice was then separated from the unfrozen liquid. This step was repeated at different time intervals. To remove fluoride from the ice surface, the surface was rinsed with deionized water three times. Subsequently the ice was melted at room temperature and analyzed for fluoride content by a fluoride selective electrode. In this method, electrical conductivity meter was used to measure the conductivity of the remaining melted ice, and then fluoride concentration was obtained based on the calibration curve (Fig. 2). Furthermore, samples were stirred with an electromagnetic stirrer during the freezing process. The TDS was evaluated by the conductivity method [22]. In order to know the freeze concentration during freezing step, the remaining melted ice was collected to calculate the freezing rate, which was expressed as: S = [(V0 − Vi)/V0]*100%, where V0 was the volume of original water and Vi was the glacial melt water volume. The performance of fluoride removal was expressed by fluoride removal rate: R = [(C0 − Ci)/C0]* 100%, where, C0 and Ci were the initial fluoride concentration and fluoride concentration in the melted ices, respectively. Consequently the higher fluoride removal rate is, the better the efficiency of freeze concentration will be.
Fig. 3. Relationship between freezing temperature and fluoride removal rate.
frozen at −10 °C, −15 °C, −20 °C for 2 h, 4 h, 6 h, respectively. Then the freezing rate, fluoride concentration in the melted ices and TDS in the melted ices were measured. 4. Results and discussion 4.1. Deionized water spiked with fluoride 4.1.1. The effect of freezing temperature on fluoride removal Multiple freezing temperatures have been studied for fluoride removal from aqueous solution (ranging from −5 °C to −30 °C). Fig. 3 shows the effect of freezing temperature on fluoride removal and the shifts in the fluoride removal rate after freeze concentration. It was shown that the freezing temperature significantly influenced the fluoride removal rate (R) of the high fluoride water, which decreased dramatically below −20 °C. The incredible decrease in the amount of fluoride removal below −20 °C is probably due to the growth of dendrites of ice crystals and the transfer of heat away from the freezing front [23], thus causing solutes easily to be trapped between the dendrites. Considering the slow velocity of icing-formation of the aqueous solution at −5 °C, the freezing temperature should be lower than −5 °C and the temperature of crystallization point [24]. Given the above situation, it can be considered that the optimal freezing temperature of fluoride removal should be controlled between −15 °C and −20 °C. To validate the fluoride removal regularities within this temperature range and to ensure the stability and reliability of engineering applications, the influence of initial fluoride concentration on fluoride removal by frozen at the freezing temperature of −20 °C was also investigated. When evaluating the impact of the initial fluoride concentration upon fluoride removal, a virtually equal separation was observed below 10 mg/L of fluoride, unless the freezing rate was more than 70% (Fig. 4). In other words, the fluoride removal efficiency by freeze concentration is relatively stable in the scenario where the initial fluoride concentration was below 10 mg/L and the freezing rate was
3.2.2. Deionized water spiked with salinity A series of twenty orthogonal experiments designed by Minitab software was adopted. The factors influencing the fluoride removal including freezing temperature, initial fluoride concentration, Ca2+, Mg2+, NaHCO3, NaCl and frozen time were considered. The freezethaw process was the same as that used for the deionized water spiked with fluoride experiment. 3.2.3. Validation of the removal efficiency with field water samples The removal efficiency by freeze concentration was validated using the field water samples. The samples of various concentration were
Fig. 4. Relationship between the freezing rate and fluoride concentration in the melted ices at the freezing temperature of −20 °C.
Fig. 2. The calibration curve of fluoride concentration.
262
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Fig. 5. Relationship between the freezing rate and fluoride removal rate at the freezing temperature of −5 °C to −30 °C. Fig. 7. Relationship between fluoride concentration in the initial feed solution and the melted ices, the horizontal dashed line is 1.0 mg/L of fluoride in the water.
less than 70%.
ice branches gradually forming. Besides, an increased viscosity and a decline of the diffusive coefficient appeared, which resulted in more solute being adhered to the surface and ineffectively expelled from frozen domain [26]. The responses at the freezing temperature of −25 °C and −30 °C probably are largely due to the condition where all the water was frozen. As shown in Fig. 7, an approximate linear relationship was found between fluoride concentration in the initial feed solution and the melted ice, except at −30 °C. This indicates that the initial fluoride concentration determines the fluoride concentration in the melted ices. Moreover, fluoride concentration in the melted ices did not meet the standards of drinking water (1.0 mg/L) with once freezing and thawing cycle, when the initial fluoride concentration was greater than 3 mg/L. In this case, two or more cycles should be considered to remove fluoride. Progressive freeze concentration is imperative after pretreatment to decrease the initial concentration, which has been widely accepted by other researchers [27–29].
4.1.2. The effect of the freezing rate on fluoride removal With an increased freezing rate of up to 30%, a minor increase was also observed in the fluoride removal rate (Fig. 5). However, a decline in fluoride removal rate was noticed when the freezing rate was higher than 30%, especially after 60%. The rate was averaged ignoring the different initial fluoride concentration. This observation is consistent with other compounds as well [25]. At the beginning of freezing, the insulation effect of the ice layer is poor, and the latent heat that is released is faster than that is being removed. The solute in the aqueous solution will promote heterogeneous nucleation, thereby accelerating the nucleation process and the instantaneous growth rate of ice crystals. And then the initial freezing point is based on Raoult’s Law [24]. It leads to more fluoride entrained by ice crystals. Therefore, for water samples adopted in this experiment, the reasonable choice for the freezing rate is 30–60% for a practical application. 4.1.3. The effect of initial solution concentration on fluoride removal Based on the characteristics of fluoride removal rate fluctuation and fluoride concentration in the melted ices, it can be concluded that the fluoride removal efficiency is most responsive to initial solution concentration than other factors. In Fig. 6, it can be seen that the fluoride removal rate values decreased with the increase of the sample concentration below the freezing temperature of −20 °C, whereas those freezing temperature above −20 °C were not influenced or showed an attenuated response, with rates commonly within a narrow range of 75–85%. It is noteworthy that the effect of fluoride removal decreased as the initial concentration was over 8 mg/L, which the theory of crystallization kinetics can explain. When the initial fluoride concentration increased, the solute remains even more concentrated near the ice-water interface, the result being a depression of the freezing point of the liquid [17]. As a consequence, the solid/liquid interface became more labile and ice crystallise as dendrites with more advanced
4.2. Deionized water spiked with salinity 4.2.1. Fluoride removal In reality, fluoride contaminated drinking water contains several other substances that may affect the purification process. This study assessed fluoride purification behavior in the presence of some dissolved substances of sodium bicarbonate, calcium chloride, magnesium sulfate and sodium chloride. Through regression analysis of fluoride removal efficiency, the freezing temperature, initial fluoride concentration and frozen time have a positive correlation with fluoride concentration in the melted ices, the regression equation is as follows: Cice = 0.010 + 0.354Csolution − 0.148 t − 0.0636 T + 0.0153Ssolution where Cice is fluoride concentration in the melted ices, Csolution is initial fluoride concentration, t is frozen time, T is freezing temperature, S is TDS in the initial feed solution. A residual analysis is required to verify the fit of the above regression equation and analyze the correlation of related parameters in the equation (Table 1). It can be seen from the data that the value of R-Sq (adjust) reached 88.9% (> 70%), so the matching effect of the regression equation was preferable. In addition, the P value of TDS was greater than 0.05, which had no obvious correlation with fluoride concentration in the melted ices. However, from the perspective of fluoride removal rate (50–75%), TDS played an inverse role in the fluoride removal efficiency (Fig. 8), which may relate to the hydration free energy. 4.2.2. TDS removal The purification of water containing different amount of dissolved
Fig. 6. Relationship between fluoride removal rate and initial fluoride concentration.
263
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Table 1 The residual analysis of the regression equation of TDS on fluoride removal. independent variable
coefficient
standard error of the coefficient
Constant 0.0095 0.4071 F0.35367 0.03095 frozen time −0.14813 0.04916 freezing temperature −0.06356 0.01560 TDS 0.01525 0.05949 S = 0.389890 R-Sq = 91.3% R-Sq(adjust) = 88.9%
T
P
0.02 11.43 −3.01 −4.08 0.26
0.982 0.000 0.009 0.001 0.801
substances proved to be relatively successful, even though TDS with higher initial concentrations was more difficult to purify (Fig. 9). In addition, at the same freezing temperature, the TDS removal rate values decreased with an increase in initial concentration and an initial TDS concentration greater than 1 g/L (Fig. 10). The main factors influencing the TDS removal were freezing temperature, concentration of sodium bicarbonate, concentration of sodium chloride and frozen time. The regression equation is as follows: S
ice
Fig. 9. Effect of initial TDS concentration on TDS removal at the freezing temperature of −5 °C to −20 °C.
= −0.218 − 0.0669 t − 0.0383T + 0.565Csolution
where Sice is TDS in the melted ices, Csolution is TDS in the initial feed solution, t is frozen time, T is freezing temperature. Through residual analysis, the value of R-Sq (adjust) reached by 91.5%, so the results obtained was satisfactory. 4.3. Validation of the removal efficiency with field water samples
Fig. 10. Relationship between initial TDS concentration and TDS removal rate at the freezing temperature of −5 °C to −20 °C.
The removal efficiency by freeze concentration was validated using groundwater samples collected from 5 monitoring wells in the field. The groundwater samples exhibited high fluoride or TDS values, which respectively ranged from 1.05 mg/L to 4.16 mg/L and from 835.538 mg/L to 7842.85 mg/L before freeze concentration; from 0.72 mg/L to 3.57 mg/L and from 277.2 mg/L to 4790.2 mg/L after freeze concentration. The removal rates of fluoride and TDS that were calculated using the field water samples were relatively small. When analyzing removal rate using the field water samples and comparing it to the deionized water samples, the differences in the water quality of these two cases need to be emphasized: the laboratory experiment was conducted with deionized water which contains no suspended matter, while the field water samples obviously carried much more impurities and microorganisms, which in turn may reduce the removal efficiency.
4.3.1. Fluoride removal Fig. 11 shows the fluoride removal rate variations with different initial fluoride concentration. The figure shows that the fluctuation pattern in fluoride removal rate was very similar to that of the deionized water spiked with salinity when the initial fluoride concentration was smaller than 5 mg/L. However, there was one major difference: the maximal fluoride removal rate rise using the deionized water samples reached about 86%, whereas using the field water samples the fluoride removal rate rise was only 45%. This observed change is mainly due to high-TDS condition (e.g. when the initial fluoride concentration is 1.73 mg/L, the TDS is 7842.85 mg/L). Under this circumstance, we have developed a universal life-type
Fig. 8. Effect of initial fluoride concentration on fluoride removal rate using deionized water spiked with fluoride and salinity when applying freeze concentration at temperature of −15 °C and −20 °C.
264
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
freezing process have a great impact upon the fluoride removal efficiency. The freeze concentration process seems to be more sensitive to the freezing temperature, especially below −20 °C. The removal of fluoride increased as the freezing rate increased up to 20% and then decreased with an increasing freezing rate at a certain temperature. The optimum freezing rate was 30% − 60%, which was suitable for the purpose of separation. Solutions of low initial concentration were concentrated with little hindrance, which is affected by the trapping and rejection of soluble particles and the amount of dissolved substance. Meanwhile, the process procured a fluoride removal efficiency as high as 85% and 75% for deionized water spiked with fluoride and salinity respectively. The removal efficiency by freeze concentration was finally validated using the field water samples, which appeared to be very similar to experimental simulation water, even though the fluctuation range was relatively large. And 45% fluoride removal efficiency was found for real water samples. This is an indication of the negative effects of impurities and microorganisms. In addition, this study provided an important demonstration to fluoride removal and desalination from the aqueous phase. With the significant reduction in TDS, it could be used as a pretreatment method for other methods of desalination. The preconditioning of the aqueous solution needs to be considered for future improvements of this technology. These potential applications require further investigation and validation but can be of great value based on results obtained in this study.
Fig. 11. Fluoride removal rate variation with different initial fluoride concentration at the freezing temperature of −10 °C to −20 °C with field water samples.
Acknowledgment Authors are thankful to Alashan Society of Entrepreneurs and Ecology (SEE) conservation for financial support. References [1] P.M. Williams, M. Ahmad, B.S. Connolly, D.L. Oatley-Radcliffe, Technology for freeze concentration in the desalination industry, Desalination 356 (2015) 314–327, http://dx.doi.org/10.1016/j.desal.2014.10.023. [2] D.G. Randall, J. Nathoo, A succinct review of the treatment of Reverse Osmosis brines using Freeze Crystallization, J. Water Process Eng. 8 (2015) 186–194, http:// dx.doi.org/10.1016/j.jwpe.2015.10.005. [3] Meenakshi R.C. Maheshwari, Fluoride in drinking water and its removal, J. Hazard. Mater. 137 (2006) 456–463, http://dx.doi.org/10.1016/j.jhazmat.2006.02.024. [4] WHO, Guidelines for Drinking Water Quality vol. 1, (2006). [5] S. Ayoob, A.K. Gupta, Fluoride in drinking water: a review on the status and stress effects, Crit. Rev. Environ. Sci. Technol. 36 (2006) 433–487, http://dx.doi.org/10. 1080/10643380600678112. [6] C. Reimann, K. Bjorvatn, B. Frengstad, Z. Melaku, R. Tekle-Haimanot, U. Siewers, Drinking water quality in the Ethiopian section of the East African Rift Valley I—data and health aspects, Sci. Total Environ. 311 (2003) 65–80, http://dx.doi. org/10.1016/s0048-9697(03)00137-2. [7] G.Y. Sun Dianjun, Progress and prospect of study on endemic fluorosis control in China, Chin. J. Endem. 32 (2013) 119–120, http://dx.doi.org/10.3760/cma.j.issn. 2095-4255.2013.02.001. [8] G. Cheng, The distributing regularity of fluorine and its environmental characteristcs in arid area of northwest China, Sci. Geogr. Sin. 20 (2000) 153–159. [9] E. Ergun, A. Tor, Y. Cengeloglu, I. Kocak, Electrodialytic removal of fluoride from water: effects of process parameters and accompanying anions, Sep. Purif. Technol. 64 (2008) 147–153, http://dx.doi.org/10.1016/j.seppur.2008.09.009. [10] N. Chen, Z. Zhang, C. Feng, M. Li, D. Zhu, R. Chen, N. Sugiura, An excellent fluoride sorption behavior of ceramic adsorbent, J. Hazard. Mater. 183 (2010) 460–465, http://dx.doi.org/10.1016/j.jhazmat.2010.07.046. [11] A. Tor, Removal of fluoride from water using anion-exchange membrane under Donnan dialysis condition, J. Hazard. Mater. 141 (2007) 814–818, http://dx.doi. org/10.1016/j.jhazmat.2006.07.043. [12] M. Mohapatra, S. Anand, B.K. Mishra, D.E. Giles, P. Singh, Review of fluoride removal from drinking water, J. Environ. Manag. 91 (2009) 67–77, http://dx.doi.org/ 10.1016/j.jenvman.2009.08.015. [13] W. Gao, Y. Shao, Freeze concentration for removal of pharmaceutically active compounds in water, Desalination 249 (2009) 398–402, http://dx.doi.org/10. 1016/j.desal.2008.12.065. [14] C. Luo, W. Han, Experimental study on factors affecting the quality of ice crystal during the freezing concentration for the brackish water, Desalination 260 (2010) 231–238, http://dx.doi.org/10.1016/j.desal.2010.04.018. [15] S. Lemmer, R. Klomp, R. Ruemekorf, R. Scholz, Preconcentration of wastewater through the niro freeze concentration process, Chem. Eng. Technol. 24 (2001). [16] F.L. Moreno, C.M. Robles, Z. Sarmiento, Y. Ruiz, J.M. Pardo, Effect of separation
Fig. 12. TDS removal rate variation with different initial TDS concentration at the freezing temperature of −10 °C to approximately −20 °C with field water samples.
automatic fluoride removal technology for drinking water, and designed the corresponding equipment[30]. The equipment is capable of producing low-fluoride clean water for residents in a short period of time. The costs of the fluoride removal process mainly include equipment costs and operating costs, the operating costs are mainly electricity. The cost of working hours is about $ 200. Therefore it is cost effective. 4.3.2. TDS removal The TDS removal rate response to the freeze concentration was mapped (Fig. 12). The freeze concentration caused the TDS removal rate to rise at the start and to reach a maximum value of 71% at −10 °C. Then, the TDS removal rate declined gradually. Up until this stage, the pattern of TDS removal rate change was relatively similar to that observed during the laboratory experiment using the deionized water samples. The whole fluctuation pattern of TDS removal rate was generally similar at −10 °C to approximately −20 °C using the field water samples. This indicates the TDS removal efficiency was directly affected by the TDS in the initial feed solution, which is in accordance with the results observed in Fig. 9. 5. Conclusion This study shows that block freeze concentration seems to have the potential for fluoride removal from water, where the initial feed water impurity concentration, freezing temperature, freezing rate and 265
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
[17] [18] [19]
[20] [21] [22] [23]
[24]
8904(01)00129-7. [25] Xuening Fei, G. Du, X. Liu, J. Wang, Application of freeze-dephlegmation to bromamine acid aqueous solution purification, Chem. Ind. Eng. Prog. (2008) 1074–1079. [26] T. Yu, J. Ma, L. Zhang, Factors affecting ice crystal purity during freeze concentration process for urine treatment, J. Harbin Inst. Technol. (2007) 593–597. [27] F. Melak, G. Du Laing, A. Ambelu, E. Alemayehu, Application of freeze desalination for chromium (VI) removal from water, Desalination 377 (2016) 23–27, http://dx. doi.org/10.1016/j.desal.2015.09.003. [28] O. Lorain, P. Thiebaud, E. Badorc, Y. Aurelle, Potential of freezing in wastewater treatment: soluble pollutant applications, Water Res. 35 (2001) 541–547. [29] K.C. Kang, P. Linga, K. Park, S.-J. Choi, J.D. Lee, Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl−, SO42−), Desalination 353 (2014) 84–90, http://dx.doi.org/10. 1016/j.desal.2014.09.007. [30] X. Li, X. Zhang, Y. Yang, X. Yang, Z. Wang, P. Li, J. Guo, Y. Lu, H. Wang, D. Zhang, H. Liu, A Kind of Indoor General Automatic Dispenser Using Freeze Concentration for Fluoride Removal, (2016) http://d.wanfangdata.com.cn/Patent/ CN201521096081.6/ ..
and thawing mode on block freeze-concentration of coffee brews, Food Bioprod. Process. 91 (2013) 396–402, http://dx.doi.org/10.1016/j.fbp.2013.02.007. R. Halde, Concentration of impurities by progressive freezing, Water Res. 14 (1980) 575–580. C.J. Martel, Influence of dissolved solids on the mechanism of freeze–thaw conditioning, Water Res. 34 (2000) 657–662. D.G. Randall, J. Nathoo, F.E. Genceli-Güner, H.J.M. Kramer, G.J. Witkamp, A.E. Lewis, Determination of the metastable ice zone for a sodium sulphate system, Chem. Eng. Sci. 77 (2012) 184–188, http://dx.doi.org/10.1016/j.ces.2011.12.022. S. Zhao, X. Wang, Z. Huang, Study on formation causes of high fluorine groundwater in Hetao area of Inner Mongolia, Rock Miner. Anal. 26 (2007) 320–324. D.S. Jean, C.P. Chu, D.J. Lee, Effects of electrolyte and curing on freeze/thaw treatment of sludge, Water Res. 34 (2000) 1577–1583. J. Yao, Determination of mineralization degree of groundwater by the conductivity method, Environ. Monit. China 2 (1986) 31–33. N.N. Khusnatdinov, V.F. Petrenko, Fast-growth technique for ice single crystals, J. Cryst. Growth 163 (1996) 420–425, http://dx.doi.org/10.1016/0022-0248(95) 00980-9. M. Akyurt, G. Zaki, B. Habeebullah, Freezing phenomena in ice-water systems, Energy Convers. Manag. 43 (2002) 1773–1789, http://dx.doi.org/10.1016/s0196-
266
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Application of freeze concentration for fluoride removal from water solution Yahui Yang a b
a,b
, Yudong Lu
a,b,⁎
a,b
, Jinyan Guo
, Xiaozhou Zhang
MARK
a,b
College of Environmental Science and Engineering, Chang’an University, Xi’an 710054, Shaanxi, China Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education, Chang’an University, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluoride removal Freeze concentration Deionized water Salinity
A study on fluoride removal by freeze concentration was conducted using deionized and added dissolved substances to the water samples. Fluoride removal characteristics were evaluated in a batch system with reference to changes in freezing temperature, initial concentration, freezing rate and total dissolved solids (TDS). Fluoride removal was found to be freeze temperature-dependent and the optimal temperature range was −15 °C to approximately −20 °C. Fluoride removal rate ranged from 50 to 75% for added dissolved substances to water and from 75 to 85% for deionized water spiked with fluoride, which gives a more satisfactory result. The decline of salinity was consistent with the decline of fluoride concentrations. However, from the perspective of fluoride removal rate, TDS played an inverse role in the fluoride removal efficiency. The removal efficiency by freeze concentration was finally validated using the high fluoride groundwater samples collected from 5 monitoring wells in the field, which showed a good correlation. Our results show that freeze concentration is possible to develop as a feasible technology for fluoride removal from aqueous environment in remote and small areas.
1. Introduction
A substantial and growing researches have been conducted on the removing excessive fluoride from drinking water. The fluoride removal technique based on the mode of action includes adsorption, ion-exchange, precipitation, membrane separation process, electrochemical process, and electrodialysis [3,9–12] and so on. However, the major limitations for these methods are high investment in infrastructure and equipment, secondary pollution, unpleasant taste of treated water and bad maneuverability in the treatment, particularly in rural areas of developing countries. Therefore, such problems limit the further application and promotion of fluoride removal technique. Nowadays an increasing number of people are beginning to concern about freeze concentration, which is an economically feasible, little pollution, free of corrosion and scaling technique. This technique can be used for decontaminating water during the formation of ice crystals, especially in the regions where natural cool energy is available. It has been reported that it is effective to remove various organic and inorganic contaminants from industrial wastewater/liquid waste [13–15]. Three basic concentration techniques are available: suspension crystallization, film freeze concentration and freeze-thaw method (also known as block freeze concentration). The last one of them includes freezing, thawing and separation steps, which was discussed in our study [16]. Although the theory of impurity separation by freeze concentration has been studied [2,17–19], more work is needed to be explored the fluoride separation rules on the basis of the theory of crystallization kinetics.
Fluoride removal from contaminated drinking water has been the focus of hydrochemistry and hygiene studies in recent years, since fluorosis is one of the significant issues worldwide and poses a major health risk to people and pets [1,2]. Fluoride in minute quantity is an essential component and intimately related with human life activity, tooth and skeleton constitution metabolism [3]. However, excess ingestion of fluoride is responsible for dental fluorosis or crippling fluorosis. Data from World Health Organization (WHO) indicate that, the desirable and permissible limit range of fluoride content of drinking water is 0.5–1.0 mg/L [4]. Endemic fluorosis occurring on account of consumption of groundwater with high-concentration fluoride has become a worldwide problem, particularly in the United States, Africa, Asia, the Middle East and China [5,6]. For example in China, the endemic fluorosis is mainly distributed in the northwestern, northern and northeastern China, most of which are arid and semi-arid areas [7]. Recent research on fluoride of groundwater in northwestern arid region is mainly concentrated on the Junggar Basin, the Tarim Basin, Heihe River Basin [8] and the Ningxia region, however little research in the Ulan Buh Desert. Concerning the toxic effects of fluoride on human health, it is imperative to find out a cost-effective and feasible method for the removing excessive fluoride from drinking water. Accordingly, sustainable management of that water resources requires proper evaluation of fluoride removal efficiency. ⁎
Corresponding author at: College of Environmental Science and Engineering, Chang’an University, Xi’an 710054, Shaanxi, China. E-mail address: [email protected] (Y. Lu).
http://dx.doi.org/10.1016/j.jwpe.2017.05.009 Received 27 November 2016; Received in revised form 7 May 2017; Accepted 23 May 2017 2214-7144/ © 2017 Published by Elsevier Ltd.
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Fig. 1. The location of the study area.
of up to 4 mg/L. Studies have identified the fluoride present in the phreatic aquifer as being sourced from aeolian sandy soil and fluorine minerals from the surrounding mountainous sedimentary and magmatic rocks. The hydrochemical types of phreatic groundwater are ClNa and Cl·SO4-Na primarily.
Batch experiments with deionized and added dissolved substances water samples were performed to investigate fluoride separation capacity by freeze concentration in the artificial refrigeration system. The influence of several significant experimental parameters were subsequently examined, including initial fluoride concentration, freezing temperature, freezing rate and the presence of other salinities on fluoride removal efficiency. The regularity of desalination by freezethaw method was also experimentally assessed in our study. Furthermore, the results were validated using field data for fluoride removal and desalination efficiency. It is hoped that the results of this study will allow for a better understanding of the performance of freeze concentration for fluoride removal in similar semi-arid basin globally; a topic of vital importance to water supply and human health.
3. Materials and methods 3.1. Materials Experiment reagents were selected including sodium fluoride, sodium chloride, calcium chloride, magnesium sulfate, sodium bicarbonate, sodium citrate and hydrochloride. Laboratory instruments mainly contain deionized water, temperature controlled refrigeration freezer -FYL-YS-128 (Beijing Fu Italian Electric Co., Ltd.), fluoride selective electrode, ion meter or pH meter, saturated calomel electrode, magnetic stirrer, volumetric flask, polyethylene beaker, pipette, and conductivity meter DDS-11A. All the instruments were calibrated before use. The desired initial fluoride concentration of 2 mg/L, 3 mg/L, 4 mg/L, 5 mg/ L, 6 mg/L, 8 mg/L, 10 mg/L, 15 mg/L, 20 mg/L (by preparation with deionized water and sodium fluoride) were used in this work. Field water samples were collected in October 2013, from the monitoring wells S5-06, S4-09, S4-24, S4-02 and S2-06 with high fluorine content or high salinity. Sampling, preservation, transportation and analytical protocols were conducted by technical regulations and standards for groundwater samples (HJ493-2009).
2. Study area The study area located in the southwest edge of Ulan Buh Desert, Alashan league in Inner Mongolia, is an intra-continental rift basin spreading in the NE–SW direction. It is bounded by longitudes 105°15′ − 106°40′ E and latitudes 39°10′ ∼40°30′ N, covering an area of 9200 km2 (Fig. 1). According to climate statistics between 1955 and 2010 from weather station of Jilantai, the average annual precipitation is roughly 108.1 mm, and the mean evaporation rate is about 1734.2 mm per year. Extensive evaporation and prolonged rock–water interaction cause the enrichment of fluoride in groundwater at Jilantai [20]. Previous survey data have shown that shallow groundwater containing excess fluoride is common in the Jilantai Basin, reaching levels 261
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
3.2. Experimental procedure 3.2.1. Deionized water spiked with fluoride To investigate the freeze concentration process under controlled conditions, a laboratory experiment was conducted using deionized water. The controlled freeze concentration experiment was designed to understand the fluoride separation capacity and the factors affecting fluoride removal during freeze concentration. The experiment adopted a progressive crystallization method for freeze concentration that was accomplished with samples of 500 ml in the freezer. The freezer temperature fluctuates a range of ± 0.5 °C. The freezer was rapid precooled to keep at the set temperature during the freezing operation. The test samples of various concentration were frozen at a series of different temperatures: −5 °C, −10 °C, −15 °C, −20 °C, −25 °C and −30 °C. Samples were allowed to freeze progressively for 2 h, 4 h, 6 h, 8 h, 10 h respectively, during which time ice crystal grew inwards from circumference zone after freezing procedure [21]. The ice was then separated from the unfrozen liquid. This step was repeated at different time intervals. To remove fluoride from the ice surface, the surface was rinsed with deionized water three times. Subsequently the ice was melted at room temperature and analyzed for fluoride content by a fluoride selective electrode. In this method, electrical conductivity meter was used to measure the conductivity of the remaining melted ice, and then fluoride concentration was obtained based on the calibration curve (Fig. 2). Furthermore, samples were stirred with an electromagnetic stirrer during the freezing process. The TDS was evaluated by the conductivity method [22]. In order to know the freeze concentration during freezing step, the remaining melted ice was collected to calculate the freezing rate, which was expressed as: S = [(V0 − Vi)/V0]*100%, where V0 was the volume of original water and Vi was the glacial melt water volume. The performance of fluoride removal was expressed by fluoride removal rate: R = [(C0 − Ci)/C0]* 100%, where, C0 and Ci were the initial fluoride concentration and fluoride concentration in the melted ices, respectively. Consequently the higher fluoride removal rate is, the better the efficiency of freeze concentration will be.
Fig. 3. Relationship between freezing temperature and fluoride removal rate.
frozen at −10 °C, −15 °C, −20 °C for 2 h, 4 h, 6 h, respectively. Then the freezing rate, fluoride concentration in the melted ices and TDS in the melted ices were measured. 4. Results and discussion 4.1. Deionized water spiked with fluoride 4.1.1. The effect of freezing temperature on fluoride removal Multiple freezing temperatures have been studied for fluoride removal from aqueous solution (ranging from −5 °C to −30 °C). Fig. 3 shows the effect of freezing temperature on fluoride removal and the shifts in the fluoride removal rate after freeze concentration. It was shown that the freezing temperature significantly influenced the fluoride removal rate (R) of the high fluoride water, which decreased dramatically below −20 °C. The incredible decrease in the amount of fluoride removal below −20 °C is probably due to the growth of dendrites of ice crystals and the transfer of heat away from the freezing front [23], thus causing solutes easily to be trapped between the dendrites. Considering the slow velocity of icing-formation of the aqueous solution at −5 °C, the freezing temperature should be lower than −5 °C and the temperature of crystallization point [24]. Given the above situation, it can be considered that the optimal freezing temperature of fluoride removal should be controlled between −15 °C and −20 °C. To validate the fluoride removal regularities within this temperature range and to ensure the stability and reliability of engineering applications, the influence of initial fluoride concentration on fluoride removal by frozen at the freezing temperature of −20 °C was also investigated. When evaluating the impact of the initial fluoride concentration upon fluoride removal, a virtually equal separation was observed below 10 mg/L of fluoride, unless the freezing rate was more than 70% (Fig. 4). In other words, the fluoride removal efficiency by freeze concentration is relatively stable in the scenario where the initial fluoride concentration was below 10 mg/L and the freezing rate was
3.2.2. Deionized water spiked with salinity A series of twenty orthogonal experiments designed by Minitab software was adopted. The factors influencing the fluoride removal including freezing temperature, initial fluoride concentration, Ca2+, Mg2+, NaHCO3, NaCl and frozen time were considered. The freezethaw process was the same as that used for the deionized water spiked with fluoride experiment. 3.2.3. Validation of the removal efficiency with field water samples The removal efficiency by freeze concentration was validated using the field water samples. The samples of various concentration were
Fig. 4. Relationship between the freezing rate and fluoride concentration in the melted ices at the freezing temperature of −20 °C.
Fig. 2. The calibration curve of fluoride concentration.
262
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Fig. 5. Relationship between the freezing rate and fluoride removal rate at the freezing temperature of −5 °C to −30 °C. Fig. 7. Relationship between fluoride concentration in the initial feed solution and the melted ices, the horizontal dashed line is 1.0 mg/L of fluoride in the water.
less than 70%.
ice branches gradually forming. Besides, an increased viscosity and a decline of the diffusive coefficient appeared, which resulted in more solute being adhered to the surface and ineffectively expelled from frozen domain [26]. The responses at the freezing temperature of −25 °C and −30 °C probably are largely due to the condition where all the water was frozen. As shown in Fig. 7, an approximate linear relationship was found between fluoride concentration in the initial feed solution and the melted ice, except at −30 °C. This indicates that the initial fluoride concentration determines the fluoride concentration in the melted ices. Moreover, fluoride concentration in the melted ices did not meet the standards of drinking water (1.0 mg/L) with once freezing and thawing cycle, when the initial fluoride concentration was greater than 3 mg/L. In this case, two or more cycles should be considered to remove fluoride. Progressive freeze concentration is imperative after pretreatment to decrease the initial concentration, which has been widely accepted by other researchers [27–29].
4.1.2. The effect of the freezing rate on fluoride removal With an increased freezing rate of up to 30%, a minor increase was also observed in the fluoride removal rate (Fig. 5). However, a decline in fluoride removal rate was noticed when the freezing rate was higher than 30%, especially after 60%. The rate was averaged ignoring the different initial fluoride concentration. This observation is consistent with other compounds as well [25]. At the beginning of freezing, the insulation effect of the ice layer is poor, and the latent heat that is released is faster than that is being removed. The solute in the aqueous solution will promote heterogeneous nucleation, thereby accelerating the nucleation process and the instantaneous growth rate of ice crystals. And then the initial freezing point is based on Raoult’s Law [24]. It leads to more fluoride entrained by ice crystals. Therefore, for water samples adopted in this experiment, the reasonable choice for the freezing rate is 30–60% for a practical application. 4.1.3. The effect of initial solution concentration on fluoride removal Based on the characteristics of fluoride removal rate fluctuation and fluoride concentration in the melted ices, it can be concluded that the fluoride removal efficiency is most responsive to initial solution concentration than other factors. In Fig. 6, it can be seen that the fluoride removal rate values decreased with the increase of the sample concentration below the freezing temperature of −20 °C, whereas those freezing temperature above −20 °C were not influenced or showed an attenuated response, with rates commonly within a narrow range of 75–85%. It is noteworthy that the effect of fluoride removal decreased as the initial concentration was over 8 mg/L, which the theory of crystallization kinetics can explain. When the initial fluoride concentration increased, the solute remains even more concentrated near the ice-water interface, the result being a depression of the freezing point of the liquid [17]. As a consequence, the solid/liquid interface became more labile and ice crystallise as dendrites with more advanced
4.2. Deionized water spiked with salinity 4.2.1. Fluoride removal In reality, fluoride contaminated drinking water contains several other substances that may affect the purification process. This study assessed fluoride purification behavior in the presence of some dissolved substances of sodium bicarbonate, calcium chloride, magnesium sulfate and sodium chloride. Through regression analysis of fluoride removal efficiency, the freezing temperature, initial fluoride concentration and frozen time have a positive correlation with fluoride concentration in the melted ices, the regression equation is as follows: Cice = 0.010 + 0.354Csolution − 0.148 t − 0.0636 T + 0.0153Ssolution where Cice is fluoride concentration in the melted ices, Csolution is initial fluoride concentration, t is frozen time, T is freezing temperature, S is TDS in the initial feed solution. A residual analysis is required to verify the fit of the above regression equation and analyze the correlation of related parameters in the equation (Table 1). It can be seen from the data that the value of R-Sq (adjust) reached 88.9% (> 70%), so the matching effect of the regression equation was preferable. In addition, the P value of TDS was greater than 0.05, which had no obvious correlation with fluoride concentration in the melted ices. However, from the perspective of fluoride removal rate (50–75%), TDS played an inverse role in the fluoride removal efficiency (Fig. 8), which may relate to the hydration free energy. 4.2.2. TDS removal The purification of water containing different amount of dissolved
Fig. 6. Relationship between fluoride removal rate and initial fluoride concentration.
263
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
Table 1 The residual analysis of the regression equation of TDS on fluoride removal. independent variable
coefficient
standard error of the coefficient
Constant 0.0095 0.4071 F0.35367 0.03095 frozen time −0.14813 0.04916 freezing temperature −0.06356 0.01560 TDS 0.01525 0.05949 S = 0.389890 R-Sq = 91.3% R-Sq(adjust) = 88.9%
T
P
0.02 11.43 −3.01 −4.08 0.26
0.982 0.000 0.009 0.001 0.801
substances proved to be relatively successful, even though TDS with higher initial concentrations was more difficult to purify (Fig. 9). In addition, at the same freezing temperature, the TDS removal rate values decreased with an increase in initial concentration and an initial TDS concentration greater than 1 g/L (Fig. 10). The main factors influencing the TDS removal were freezing temperature, concentration of sodium bicarbonate, concentration of sodium chloride and frozen time. The regression equation is as follows: S
ice
Fig. 9. Effect of initial TDS concentration on TDS removal at the freezing temperature of −5 °C to −20 °C.
= −0.218 − 0.0669 t − 0.0383T + 0.565Csolution
where Sice is TDS in the melted ices, Csolution is TDS in the initial feed solution, t is frozen time, T is freezing temperature. Through residual analysis, the value of R-Sq (adjust) reached by 91.5%, so the results obtained was satisfactory. 4.3. Validation of the removal efficiency with field water samples
Fig. 10. Relationship between initial TDS concentration and TDS removal rate at the freezing temperature of −5 °C to −20 °C.
The removal efficiency by freeze concentration was validated using groundwater samples collected from 5 monitoring wells in the field. The groundwater samples exhibited high fluoride or TDS values, which respectively ranged from 1.05 mg/L to 4.16 mg/L and from 835.538 mg/L to 7842.85 mg/L before freeze concentration; from 0.72 mg/L to 3.57 mg/L and from 277.2 mg/L to 4790.2 mg/L after freeze concentration. The removal rates of fluoride and TDS that were calculated using the field water samples were relatively small. When analyzing removal rate using the field water samples and comparing it to the deionized water samples, the differences in the water quality of these two cases need to be emphasized: the laboratory experiment was conducted with deionized water which contains no suspended matter, while the field water samples obviously carried much more impurities and microorganisms, which in turn may reduce the removal efficiency.
4.3.1. Fluoride removal Fig. 11 shows the fluoride removal rate variations with different initial fluoride concentration. The figure shows that the fluctuation pattern in fluoride removal rate was very similar to that of the deionized water spiked with salinity when the initial fluoride concentration was smaller than 5 mg/L. However, there was one major difference: the maximal fluoride removal rate rise using the deionized water samples reached about 86%, whereas using the field water samples the fluoride removal rate rise was only 45%. This observed change is mainly due to high-TDS condition (e.g. when the initial fluoride concentration is 1.73 mg/L, the TDS is 7842.85 mg/L). Under this circumstance, we have developed a universal life-type
Fig. 8. Effect of initial fluoride concentration on fluoride removal rate using deionized water spiked with fluoride and salinity when applying freeze concentration at temperature of −15 °C and −20 °C.
264
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
freezing process have a great impact upon the fluoride removal efficiency. The freeze concentration process seems to be more sensitive to the freezing temperature, especially below −20 °C. The removal of fluoride increased as the freezing rate increased up to 20% and then decreased with an increasing freezing rate at a certain temperature. The optimum freezing rate was 30% − 60%, which was suitable for the purpose of separation. Solutions of low initial concentration were concentrated with little hindrance, which is affected by the trapping and rejection of soluble particles and the amount of dissolved substance. Meanwhile, the process procured a fluoride removal efficiency as high as 85% and 75% for deionized water spiked with fluoride and salinity respectively. The removal efficiency by freeze concentration was finally validated using the field water samples, which appeared to be very similar to experimental simulation water, even though the fluctuation range was relatively large. And 45% fluoride removal efficiency was found for real water samples. This is an indication of the negative effects of impurities and microorganisms. In addition, this study provided an important demonstration to fluoride removal and desalination from the aqueous phase. With the significant reduction in TDS, it could be used as a pretreatment method for other methods of desalination. The preconditioning of the aqueous solution needs to be considered for future improvements of this technology. These potential applications require further investigation and validation but can be of great value based on results obtained in this study.
Fig. 11. Fluoride removal rate variation with different initial fluoride concentration at the freezing temperature of −10 °C to −20 °C with field water samples.
Acknowledgment Authors are thankful to Alashan Society of Entrepreneurs and Ecology (SEE) conservation for financial support. References [1] P.M. Williams, M. Ahmad, B.S. Connolly, D.L. Oatley-Radcliffe, Technology for freeze concentration in the desalination industry, Desalination 356 (2015) 314–327, http://dx.doi.org/10.1016/j.desal.2014.10.023. [2] D.G. Randall, J. Nathoo, A succinct review of the treatment of Reverse Osmosis brines using Freeze Crystallization, J. Water Process Eng. 8 (2015) 186–194, http:// dx.doi.org/10.1016/j.jwpe.2015.10.005. [3] Meenakshi R.C. Maheshwari, Fluoride in drinking water and its removal, J. Hazard. Mater. 137 (2006) 456–463, http://dx.doi.org/10.1016/j.jhazmat.2006.02.024. [4] WHO, Guidelines for Drinking Water Quality vol. 1, (2006). [5] S. Ayoob, A.K. Gupta, Fluoride in drinking water: a review on the status and stress effects, Crit. Rev. Environ. Sci. Technol. 36 (2006) 433–487, http://dx.doi.org/10. 1080/10643380600678112. [6] C. Reimann, K. Bjorvatn, B. Frengstad, Z. Melaku, R. Tekle-Haimanot, U. Siewers, Drinking water quality in the Ethiopian section of the East African Rift Valley I—data and health aspects, Sci. Total Environ. 311 (2003) 65–80, http://dx.doi. org/10.1016/s0048-9697(03)00137-2. [7] G.Y. Sun Dianjun, Progress and prospect of study on endemic fluorosis control in China, Chin. J. Endem. 32 (2013) 119–120, http://dx.doi.org/10.3760/cma.j.issn. 2095-4255.2013.02.001. [8] G. Cheng, The distributing regularity of fluorine and its environmental characteristcs in arid area of northwest China, Sci. Geogr. Sin. 20 (2000) 153–159. [9] E. Ergun, A. Tor, Y. Cengeloglu, I. Kocak, Electrodialytic removal of fluoride from water: effects of process parameters and accompanying anions, Sep. Purif. Technol. 64 (2008) 147–153, http://dx.doi.org/10.1016/j.seppur.2008.09.009. [10] N. Chen, Z. Zhang, C. Feng, M. Li, D. Zhu, R. Chen, N. Sugiura, An excellent fluoride sorption behavior of ceramic adsorbent, J. Hazard. Mater. 183 (2010) 460–465, http://dx.doi.org/10.1016/j.jhazmat.2010.07.046. [11] A. Tor, Removal of fluoride from water using anion-exchange membrane under Donnan dialysis condition, J. Hazard. Mater. 141 (2007) 814–818, http://dx.doi. org/10.1016/j.jhazmat.2006.07.043. [12] M. Mohapatra, S. Anand, B.K. Mishra, D.E. Giles, P. Singh, Review of fluoride removal from drinking water, J. Environ. Manag. 91 (2009) 67–77, http://dx.doi.org/ 10.1016/j.jenvman.2009.08.015. [13] W. Gao, Y. Shao, Freeze concentration for removal of pharmaceutically active compounds in water, Desalination 249 (2009) 398–402, http://dx.doi.org/10. 1016/j.desal.2008.12.065. [14] C. Luo, W. Han, Experimental study on factors affecting the quality of ice crystal during the freezing concentration for the brackish water, Desalination 260 (2010) 231–238, http://dx.doi.org/10.1016/j.desal.2010.04.018. [15] S. Lemmer, R. Klomp, R. Ruemekorf, R. Scholz, Preconcentration of wastewater through the niro freeze concentration process, Chem. Eng. Technol. 24 (2001). [16] F.L. Moreno, C.M. Robles, Z. Sarmiento, Y. Ruiz, J.M. Pardo, Effect of separation
Fig. 12. TDS removal rate variation with different initial TDS concentration at the freezing temperature of −10 °C to approximately −20 °C with field water samples.
automatic fluoride removal technology for drinking water, and designed the corresponding equipment[30]. The equipment is capable of producing low-fluoride clean water for residents in a short period of time. The costs of the fluoride removal process mainly include equipment costs and operating costs, the operating costs are mainly electricity. The cost of working hours is about $ 200. Therefore it is cost effective. 4.3.2. TDS removal The TDS removal rate response to the freeze concentration was mapped (Fig. 12). The freeze concentration caused the TDS removal rate to rise at the start and to reach a maximum value of 71% at −10 °C. Then, the TDS removal rate declined gradually. Up until this stage, the pattern of TDS removal rate change was relatively similar to that observed during the laboratory experiment using the deionized water samples. The whole fluctuation pattern of TDS removal rate was generally similar at −10 °C to approximately −20 °C using the field water samples. This indicates the TDS removal efficiency was directly affected by the TDS in the initial feed solution, which is in accordance with the results observed in Fig. 9. 5. Conclusion This study shows that block freeze concentration seems to have the potential for fluoride removal from water, where the initial feed water impurity concentration, freezing temperature, freezing rate and 265
Journal of Water Process Engineering 19 (2017) 260–266
Y. Yang et al.
[17] [18] [19]
[20] [21] [22] [23]
[24]
8904(01)00129-7. [25] Xuening Fei, G. Du, X. Liu, J. Wang, Application of freeze-dephlegmation to bromamine acid aqueous solution purification, Chem. Ind. Eng. Prog. (2008) 1074–1079. [26] T. Yu, J. Ma, L. Zhang, Factors affecting ice crystal purity during freeze concentration process for urine treatment, J. Harbin Inst. Technol. (2007) 593–597. [27] F. Melak, G. Du Laing, A. Ambelu, E. Alemayehu, Application of freeze desalination for chromium (VI) removal from water, Desalination 377 (2016) 23–27, http://dx. doi.org/10.1016/j.desal.2015.09.003. [28] O. Lorain, P. Thiebaud, E. Badorc, Y. Aurelle, Potential of freezing in wastewater treatment: soluble pollutant applications, Water Res. 35 (2001) 541–547. [29] K.C. Kang, P. Linga, K. Park, S.-J. Choi, J.D. Lee, Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl−, SO42−), Desalination 353 (2014) 84–90, http://dx.doi.org/10. 1016/j.desal.2014.09.007. [30] X. Li, X. Zhang, Y. Yang, X. Yang, Z. Wang, P. Li, J. Guo, Y. Lu, H. Wang, D. Zhang, H. Liu, A Kind of Indoor General Automatic Dispenser Using Freeze Concentration for Fluoride Removal, (2016) http://d.wanfangdata.com.cn/Patent/ CN201521096081.6/ ..
and thawing mode on block freeze-concentration of coffee brews, Food Bioprod. Process. 91 (2013) 396–402, http://dx.doi.org/10.1016/j.fbp.2013.02.007. R. Halde, Concentration of impurities by progressive freezing, Water Res. 14 (1980) 575–580. C.J. Martel, Influence of dissolved solids on the mechanism of freeze–thaw conditioning, Water Res. 34 (2000) 657–662. D.G. Randall, J. Nathoo, F.E. Genceli-Güner, H.J.M. Kramer, G.J. Witkamp, A.E. Lewis, Determination of the metastable ice zone for a sodium sulphate system, Chem. Eng. Sci. 77 (2012) 184–188, http://dx.doi.org/10.1016/j.ces.2011.12.022. S. Zhao, X. Wang, Z. Huang, Study on formation causes of high fluorine groundwater in Hetao area of Inner Mongolia, Rock Miner. Anal. 26 (2007) 320–324. D.S. Jean, C.P. Chu, D.J. Lee, Effects of electrolyte and curing on freeze/thaw treatment of sludge, Water Res. 34 (2000) 1577–1583. J. Yao, Determination of mineralization degree of groundwater by the conductivity method, Environ. Monit. China 2 (1986) 31–33. N.N. Khusnatdinov, V.F. Petrenko, Fast-growth technique for ice single crystals, J. Cryst. Growth 163 (1996) 420–425, http://dx.doi.org/10.1016/0022-0248(95) 00980-9. M. Akyurt, G. Zaki, B. Habeebullah, Freezing phenomena in ice-water systems, Energy Convers. Manag. 43 (2002) 1773–1789, http://dx.doi.org/10.1016/s0196-
266