Removal of fluoride by nature diatomite from high-fluorine water: An appropriate pretreatment for nanofiltration process
Introduction
Nowadays, due to the increasing contamination-hazardous in water bodies, safety of drinking water has been a critical issue. Fluorine is the most reactive non-metallic element and an essential trace element of promoting the growth of bones and teeth [1], [2]. According to the standard of World Health Organization, it is considered to be beneficial in drinking water at the level of about 0.7 mg L− 1, but extremely harmful if it exceeds 1.5 mg L− 1 [3], [4]. Fluoride, as a contaminant in water bodies, could be generated from not only natural geological sources but also industries that use fluoride-containing compounds as raw materials [5]. With the increase of industrial activity, water with high fluoride content has become a great concern.
Endemic fluorosis has been a considerable worldwide health problem, and millions of people are affected by high fluoride drinking water [6], [7], [8]. There are more than 20 developed and developing nations suffering the problem of endemic fluorosis, for example, Algeria, Libya, Egypt, U.S.A., Turkey, China, Australia, Japan, Canada, India, etc. [9], [10]. What's more, fluoride concentration in some place of these countries is up to more than 30 mg L− 1 [4]. Chronic uptakes of high amounts of fluoride can cause cumulative toxic symptom [11], [12], such as dental fluorosis in children, or even skeletal fluorosis in both children and adults. Thus, it is absolutely essential to control the fluoride amount to the safe limitation.
The only way to circumvent this problem is defluorination. Currently, nanofiltration (NF) technology is widely used for the production of drinking water [13], [14], [15], [16], [17], [18], which significantly provides more opportunities and higher possibilities than the RO [19], [20], [21], [22]. However, for some nanofiltration membranes, the salt rejection decreases with the increase of ion concentration in NF process. For example, G. Bargeman et al. [23] reported the reduction of the NaCl rejection with the increase of NaCl concentration in NF process. Yang Yu et al. [24] reported that the arsenic rejection of the NF membrane decreased as the ion concentration increased. And as reported by Ramesh R. Sharma et al. [25], the reflection coefficient of NaCl and MgSO4 decreased with increasing ion concentration. According to the literature survey, the fluoride concentration of the feed solution will severely influence the quality of NF product water. The permeate water could not meet the safe limitation when the feed fluoride concentration is too high. So it is necessary to find an appropriate pretreatment technology for the good functioning NF system to defluorinate.
Diatomite or diatomaceous earth, which consists of amorphous silica (SiO2·nH2O) essentially derived from the skeletons of aquatic plants called diatoms, is a pale-colored, soft, light-weight sedimentary rock. Owing to its physical and chemical properties (such as high porosity, large surface area, high permeability, low density, small particle size, thermal resistance, and chemical stability) [26], [27], diatomite has been used as adsorbent for different contamination removals from different water bodies. For example, it has been used to adsorb radiocobalt, thorium, antimony, lead and dyes from various aqueous solutions [28], [29], [30], [31], [32], [33]. The surface of the silica contains many silanol groups as shown in Fig. 1, and the silanol group is very active, which can react with many contaminants by the formation of hydrogen bond, including fluoride [30]. The diatomite is very rich and distributes widely all over the world. Moreover, the diatomite after adsorption process can also be used as building material [34]. So it is viable and economical for nature low-cost diatomite to be used as the pretreatment adsorbent material for NF system to defluorinate high fluoride water. We reported here, for the first time, that the diatomite was used as pretreatment adsorbent in NF defluorination system.
This study focused on the feasibility of diatomite to be used as the pretreatment adsorbent for NF system to adsorb fluoride from aqueous solution. In this study, the characterization of diatomite was investigated by SEM and XRD and the feasibility was studied by XPS and FT–IR. The influences of diatomite dosage, pH, ionic strength and anions concentration were researched by batches of adsorption experiments. The results demonstrated that the presence of HA enhanced the adsorption of fluoride at pH < 5, while reduced fluoride adsorption at pH > 5. The adsorption process fitted the Langmuir isotherm and the adsorption kinetics fitted the pseudo-second-order rate equation. In addition, the combination of adsorption process and NF treatment was analyzed, which confirmed that the nature diatomite can effectively ensure the permeate water of NF2A system to meet the standard. In a word, the nature diatomite was viable and effective to be used as the pretreatment adsorbent for NF system to defluorinate high fluoride water.
Section snippets
Materials
Nature diatomite was purchased from Huali company (Shengzhou county, Zhejiang province, China) and used as received. The predominant mineralogical components of nature diatomite are SiO2 (72%), Al2O3 (19%), Fe2O3 (5.3%), MgO (2.1%), other oxide (1.6%), which were obtained by X-ray diffraction analyzer (XRD) and X-ray photoelectron spectroscopy (XPS). Other chemicals in this work came from analytical reagent grade and were used directly without any further purification.
The NF membrane module
Performances of the NF treatment
The NF experiments were conducted with inflow water of different fluoride concentrations (2, 4, 6, 8, 10 mg L− 1), as it was one of the most important influences to the quality of NF product water. The experiments were performed at 293 K, pH 7 and a pressure of 1.0 MPa. As shown in Fig. 3, the fluoride concentration in the permeate water increased in the range of feed fluoride concentration, and it was higher than 1.5 mg L− 1 when the feed fluoride concentration was higher than 4.5 mg L− 1. The results
Conclusion
This study focused on the feasibility of using diatomite as the adsorbent material for NF system to adsorb fluoride from aqueous solution. In the NF experiments, the permeate fluoride concentration could not meet the limitation when the initial fluoride concentration was higher than 4.5 mg L− 1. So the nature diatomite was used to adsorb the fluoride. The characterization of the diatomite was investigated by SEM, XRD, XPS and FT–IR. The results demonstrate that it is feasible for diatomite to
Acknowledgments
The authors gratefully acknowledge financial support from National High Technology Research and Development Program of China (No. 2012AA03A602), Special Scientific Research Fund of Public Welfare Profession (No. 201405035) and Shandong Science and Technology Agency (No. 2014BAK13B02).
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