Use of The Electrodialysis Process For Fluoride Ion And Salt Removal From Multi-Constituent Aqueous Solutions

S t r e s z c z e n i e Fluor jest powszechnie występującym pierwiastkiem chemicznym. Zgodnie z wytycznymi WHO zawartość fluorków w wodzie do picia nie może być większa niż 1.5 mg/dm3. Nadmiar fluoru prowadzi do licznych problemów zdrowotnych (Alzheimer, problemy neurologiczne, fluoroza zębów lub szkieletu). Fluorki mogą być usunięte z roztworów wodnych z wykorzystaniem różnych metod (adsorpcja, strącanie, wymiana jonowa czy procesy membranowe). Celem pracy była ocena efektywności elektrodializy w usuwaniu jonów fluorkowych w obecności substancji organicznych. W trakcie doświadczeń zostały wykorzystane roztwory zawierające fluorki (5, 10, 100 i 200 mg F-/dm3), sól mineralną (0.5 g NaCl/dm3) oraz substancje organiczne (5, 10 i 15 mg/dm3 kwasów humusowych). Doświadczenia zostały przeprowadzone z wykorzystaniem instalacji PCCell BED-1 System. Gęstość prądu wynosiła 1.72 mA/cm2. Wykazano, że w procesie elektrodializy fluorki są skutecznie usuwane pod warunkiem, że ich początkowe stężenie nie przekracza 10 mg F-/dm3. Wpływ substancji organicznych na przebieg i efektywność procesu zależał od stężenia fluorków w oczyszczanym roztworze. K e y w o r d s : Water Treatment; Electrodialysis; Fluoride. 4/2016 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 107 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 4/2016 M . G r z e g o r z e k , K . M a j e w s k a N o w a k 108 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 4/2016 and anthropogenic. Fluorine is used in glass, fertilizers, semiconductors, phosphates, ceramics, the production of cosmetics and in the food industry. The production of pesticides also generates waste containing a large amount of fluorine in organic and inorganic form. Fluorine commonly occurs in the Earth’s crust (625 mg/kg). Fluorine is present in minerals like fluorspar, cryolite, sellaite, basalt, topaz, granite, syenite and fluoroapatite or hydroxyapatite. As the result of mineral dissolution, fluoride is released to the environment. The dissolution process can be affected by many factors such as residence time, groundwater age, well depth or rock chemistry. Fluorine also migrates to the natural environment as a result of volcanic activity [3-7]. In small amounts fluoride has a beneficial influence on health – it protects teeth against decay and also has a positive effect on bone mineralization [6]. It is important to maintain the permissible level of fluoride content in drinking water. Excessive concentrations lead to neurological disorders, cancer, gastrointestinal problems, Alzheimer’s disease and fluorosis (skeleton or dental). Fluorine also causes lesions of the thyroid, liver and endocrine glands [2, 4, 8, 9]. Fish, water plants and algae are also very vulnerable to fluoride toxic action [8]. Excessive concentrations of Fions in natural water have been observed in China, India, Brazil, Argentina, Chile, Pakistan and in northeastern regions of Africa. For example, the fluoride content in lakes in the East African Rift Valley is about 2800 mg/dm3. In Kenya, Fconcentration is around 180 mg/dm3, in China – 10 mg/dm3 and in Tanzania – 250 mg/dm3 [4]. Fcontent in wastewater from phosphoric acid production can achieve a value of 3000 mg/dm3 [10]. There are many methods that allow fluoride to be removed from aqueous solutions: chemical precipitation, ion exchange, adsorption and membrane techniques (reverse osmosis, nanofiltration) [2, 4, 8, 11]. Fluoride removal efficiency is affected by many factors: pH, type of adsorbent, membrane characteristics, temperature or coexisting ions. Most of the conventional methods have disadvantages such as the generation of toxic sludge, the requirement of pretreatment, high costs of waste disposal. For example, after the ion-exchange process reagents used for resin regeneration will be released to the environment [6, 12]. Dev Brahman et al. [13] applied biosorption for simultaneous fluoride and arsenic removal. As an adsorbent, sawdust of the plant Tecomella undulate was used. Tests were conducted on the real water samples from different sites in Mithi (Pakistan). Fluoride concentration decreased from 42.5 to 12 mg F-/dm3, which is unacceptable for drinking water. Nasr et al. [14] also used adsorption for fluoride removal from water solutions. Calcite was applied as an adsorbent under the presence of acetic acid. It has been proved that a dosing of 0.1 mol/dm3 of acetic acid brought about a significant improvement of fluoride removal (from 17.4 to 30.4% without and with acetic acid respectively, with an initial fluoride content of 5 mg F-/dm3). The observed beneficial effect of acetic acid on process efficiency was attributed to the increase of the surface area available for adsorption on calcite particles. However, a further increase of acetic acid concentration (to 0.4 mol/dm3) only resulted in a 38.2% reduction of fluoride content, which did not comply with WHO standards (< 1.5 mg F-/dm3). Membrane processes, including dialytic techniques, are characterized by high separation efficiency and can be potentially attractive methods for removing undesirable ionic pollutants or concentrating valuable compounds [9, 11, 15]. The driving force in these techniques involves concentration gradient (dialysis, DD) or electrical potential gradient (electrodialysis, ED). Due to the rather slow kinetics of the DD process, electrodialysis seems to be more beneficial for natural water treatment. During the ED process ions migrate in the constant electric field. They are transported through the ion-exchange membranes. Cations migrate to the cathode and are able to pass through the cation-exchange membranes but they cannot be transported through the anion-exchange membranes. The opposite situation occurs in the case of anions. As a result, two streams are formed – diluate and concentrate (brine) [15, 16]. ED is known as a simple and environmentally friendly technique. A lack of waste solids and a low demand for chemicals are crucial advantages [17]. Boubakri et al. [11] conducted experiments with the use of Donnan dialysis (DD) for fluoride removal. During the research, strongly basic anion-exchange AM3 membranes (Tokuyuama Soda) were applied. The initial fluoride concentration was equal to 5, 10 and 15 mg F-/dm3. The lowest removal efficiency (34.14%) was observed when Fconcentration amounted to 15 mg F-/dm3 and therefore the final water quality after the DD process was above the standard for drinking water. In the case of lower fluoride content (10 mg F-/dm3), the removal efficiency reached 56.48% and the final water quality also exceeded WHO guidelines. USE OF THE ELECTRODIALYSIS PROCESS FOR FLUORIDE ION AND SALT REMOVAL FROM MULTI-CONSTITUENT AQUEOUS SOLUTIONS E N V I R O N M E N T e 4/2016 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 109 Although there are undeniable advantages of the ED method in the treatment of aqueous solutions containing ionic compounds, only some research has been performed on fluoride ion removal. Moreover, most of the reported studies deal with the impact of mineral species on fluoride separation [9,18,19]. Ergun et al. [9] used electrodialysis (ED) for fluoride removal from natural water containing 20.6 mg F-/dm3. The strong-basic anion-exchange membranes (SB-6407, Gelman Science) were used during the ED experiments. By applying a current density of 8.48 mA/cm2, it was possible to diminish Fconcentration to 0.8 mg F-/dm3 (a value significantly lower than WHO guidelines – 1.5 mg F-/dm3). The authors also reported that chloride and sulphate ions significantly deteriorated the rate of fluoride transport through anionexchange membranes. On the contrary, Kabay et al. [18] proved that sulphates had no impact on fluoride removal in the course of the ED process and only chlorides influenced Fseparation efficiency. A study [19] conducted in a pilot ED installation revealed a satisfactory removal of fluoride ions from solutions containing less than 10 mg F-/dm3 – the reduction of Fions reached 99.6%. The increased initial concentration of fluorides, i.e. 20-50 mg F-/dm3 arisen to moderate Fion removal (79.22-95.95%) and diluates did not meet WHO standards. Taking into account the above reported results on fluoride removal by ED it can be concluded that there are still some gaps in recognizing the mechanism of Fion separation, especially from multi-component systems involving not only mineral species, but organic substances as well. Organic matter (OM) is a common constituent of natural water. Humic substances are the main constituents of natural organic matter, which are typically present in surface water in an amount of 0.1-20 mg/dm3 [20]. Dissolved organic substances (e.g. humic substances) can interact with water contaminants (also with fluoride ions) and form soluble or insoluble complexes [21]. On the other hand, humic substances (especially humic acid) are known as a serious foulant in electromembrane processes [22]. Consequently, humic acid can affect both inorganic ion transport through the ion exchange membranes and also membrane resistance. Due to the health concern of fluoride content in drinking water, it is reasonable to evaluate the impact of OM on fluoride removal by ED. The aim of this paper was to evaluate the influence of organic matter on fluoride removal efficiency by batch electrodialysis. Aqueous salt solutions containing variable amounts of fluoride and humic acid were subjected to treatment by electrodialysis. The energy needed for ion transport through ion-exchange membranes was also estimated. 2. MATERIALS AND METHODS In the course of the ED experiments the PC-Cell BED-1 System (PCCell GmbH, Germany) equipped with classic ion-exchange membranes (PC-SA and PC-SK, PCA GmbH, Germany) was used. The membrane stack contained 11 cation-exchange membranes and 10 anion-exchan