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Oct 12, 2012 - oleifera/Ultrafiltration Process. Gisele Cristina dos Santos Bazanella & Gabriel Francisco da Silva &. Angélica Marquetotti Salcedo Vieira ...
Water Air Soil Pollut (2012) 223:6083–6093 DOI 10.1007/s11270-012-1342-y

Fluoride Removal from Water Using Combined Moringa oleifera/Ultrafiltration Process Gisele Cristina dos Santos Bazanella & Gabriel Francisco da Silva & Angélica Marquetotti Salcedo Vieira & Rosângela Bergamasco

Received: 17 May 2012 / Accepted: 26 September 2012 / Published online: 12 October 2012 # Springer Science+Business Media Dordrecht 2012

Abstract The occurrence of fluoride in groundwater has been reported in many countries, mainly because the excess fluoride in drinking water can lead to dental or skeletal fluorosis. Fluoride removal by coagulation with Moringa oleifera seeds, followed by separation with membranes, was investigated in this work. Artificially fluoridated water, at a starting fluoride concentration of 10 mgL−1, was submitted to a coagulation process with aqueous extracts of M. oleifera seeds. The coagulation process was followed by ultrafiltration

G. C. dos Santos Bazanella : R. Bergamasco Chemical Engineering Department, State University of Maringá, Av. Colombo, 5790, 87020900 Maringá, Paraná, Brazil G. C. dos Santos Bazanella e-mail: [email protected] R. Bergamasco e-mail: [email protected] G. F. da Silva Department of Chemical Engineering, Federal University of Sergipe, Av. Mal. Rondon, s/n Jardim Rosa Elze, 49100-000 São Cristóvão, Sergipe, Brazil e-mail: [email protected] A. M. S. Vieira (*) Food Engineering Department, State University of Maringá, Av. Colombo, 5790, 87020900 Maringá, Paraná, Brazil e-mail: [email protected]

with membranes at different pressures. The coagulation process with 2.5 gL−1 of M. oleifera promoted a reduction of 90.90 % in the fluoride content of the treated water, making it possible for poor communities to consume this water. It is noteworthy that the combined coagulation/filtration process using raw coagulant showed the highest values of colour and turbidity, which, however, were still below the limits set for drinking water by Brazilian legislation. The advantage of proposing a sequential process using membrane separation is that it removes colour and turbidity, caused by the use of M. oleifera as a coagulant, resulting in water that meets potability standards. Keywords Defluoridation . Fluoride groundwater . Membranes . Moringa oleifera

1 Introduction Water provision in remote communities is a serious problem globally, as a vast number of lives are lost annually due to lack of access to potable water. Then, the level of purity of the water being consumed is very crucial since it has a direct effect on health (Amagloh and Benang 2009). Many countries meet from 70 to 90 % of the demand for public water supply with groundwater. About 70 % of the total water supply in Mexico City is obtained from groundwater (Edmunds et al. 2002). In 1990, 39 % of the water used for public supply in the USA was groundwater (Baird 2002). The

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occurrence and destination of inorganic (mineral origin) trace contaminants in surface, brackish, and groundwater are also of considerable concern regarding public health and the environment. The importance of groundwater as an alternative water supply is recognised due to the cost of abstraction and reduced quality of surface water. Unfortunately, due to various natural and anthropogenic factors, groundwater is becoming polluted (Meenakshi and Maheshwari 2006). Studies reveal that many water sources do not meet drinking water standards, often with components at high concentrations—for example, fluoride, which above the maximum permissible concentration can become a contaminant. The exposure from other sources may originate from industrial production of phosphatic fertilisers and volcanic activities that might result in wide distribution of fluoride in the atmosphere (Mandinic et al. 2010). Fluoride is an essential micronutrient for human beings, serving to strengthen the apatite matrix of skeletal tissues and teeth (Rafique et al. 2009). In regions where the fluoride (F−) content of water is naturally low (1.5 mgL−1) results in dental and skeletal fluorosis, renal and neuronal disorders, along with myopathy (Rafique et al. 2009). Many countries have regions where the water contains more than 1.5 mgL−1 of fluoride due to its natural presence in the Earth’s crust or discharge by agricultural and industrial activities, such as steel, aluminium, glass, and electroplating (Fan et al. 2003). In developing countries such as Morocco, F− concentrations up to 20 mgL−1 have been found in groundwater, while the maximum acceptable concentration is 1.5 mgL−1 (Amor et al. 2001). Rafique et al. (2009) reported fluoride concentrations ranging from 1.13 to 7.85 mgL−1 in groundwater near Nagar Parkar, in Southeastern Pakistan. Another country that has problems with high fluoride concentration in groundwater is China. Guo et al. (2007) reported F− concentrations up to 6.20 mgL−1 in Taiyuan basin, northern China. In Brazil, Forte et al. (2008) found fluoride levels ranging from 0.5 to 3.26 mgL−1 in drinking water samples in Catolé da Rocha, Paraiba State.

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One method that can help solve the problem is to seek an alternative low-fluoride drinking water source. If this is unattainable, defluoridation becomes obligatory (Amor et al. 2001). Several technologies have been suggested to remove the excess F− from groundwater. Among the methods being investigated are: addition of chemicals to cause precipitation (Hu et al. 2005); ion exchange (Castel et al. 2000); adsorption (Fan et al. 2003); membrane processes (Tahaikt et al. 2007); electrocoagulation, electroflotation, and electrochemical oxidation (Khatibikamal et al. 2010). Flocculation/coagulation is a promising treatment when environmentally friendly agents, such as Moringa oleifera, are used. Water extract of M. oleifera seeds is effective in flocculating organic matter and removing microorganisms from raw water. It is currently used to treat drinking water, where it has shown its efficiency as a coagulating agent (Kalogo et al. 2001). The use of M. oleifera seeds as coagulant has some advantages, such as the formation of a biodegradable sludge whose volume is lower when compared with chemical coagulants. Additionally, according to some authors such as Ndabigengesere et al. (1995) and Nkurunziza et al. (2009) the change in water pH is minor, regardless of the added amount of Moringa solution. This is another advantage of Moringa as a coagulating agent, that is, its addition does not significantly affect the pH of the water. The processes of fluoride removal using precipitation or coagulation by chemical compounds have the disadvantage of using large quantities of chemicals to achieve the desired levels of defluoridation. The supplementary difficulty of eliminating excess chemicals leads to the incompatibility of such methods with the preservation of the quality and originality of natural waters (Castel et al. 2000). Membrane separation processes offer the advantage of producing high quality drinking water (Velizarov et al. 2004). Ultrafiltration (UF) systems are becoming an attractive alternative for groundwater and surface water clarification, particularly concerning the removal of particles, colloidal species, and microorganisms (Xia et al. 2007). Moreover, the use of a coagulant prior to UF may improve water quality and reduce membrane fouling. Therefore, the present research aimed to investigate the efficiency of fluoride removal from water with an excess of this ion by a combined coagulation/ filtration process. Coagulation with aqueous extracts of

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M. oleifera seeds was applied followed by UF membrane separation.

initial fluoride concentrations (CiF−): 3, 5, 7, and 10 mgL−1. Final Ef concentrations in the jars were: 0.25, 0.5, 1.25, and 2.5 gL−1.

2 Methods

2.4 Membranes

2.1 Preparation of Fluoridated Water

Al2O3/ZrO2 ceramic membranes (TAMI, France) were used in the filtration process. The physical characteristics of the membranes are presented in Table 1. In order to better present the results, the membranes were classified into types mono 1, mono 2, and multi 1, according to pore diameter and number of channels. In the filtration experiments using water with F− concentration of 10 mgL−1, two different temperatures (25 and 35 °C) and four pressures (1, 2, 3, and 4 bar) were evaluated.

A stock solution with F− concentration of 1 gL−1 was prepared using reagent grade sodium fluorosilicate (Na2SiF6) and reverse osmosis water. The 10 mgL−1 solution was prepared from the stock solution, simulating groundwater with excess F−. Fluoride concentration was determined, at all stages of the process, using the colorimetric SPADNS method, according to the Standard Methods (APHA 1995). This method is based on the reaction between fluoride and zirconium dye. The fluoride dissociates a part of the dye, forming a colourless anionic complex (ZrF62−). The amount of fluoride is inversely proportional to the colour of the solution, that is, colour becomes progressively lighter as fluoride concentration increases. Fluoride concentration is measured spectrophotometrically at 570 nm. The reaction is instantaneous, and follows Beer’s law in the concentration range 0.0–1.4 mg F− L−1 (Bellack and Schouboe 1958). The pH of the water being treated was 5.5.

2.5 Combined Coagulation/Membrane Filtration Process

Raw (Eb) and filtered (Ef) coagulants of M. oleifera seeds were prepared with different concentrations (5, 10, 25, and 50 gL−1), according to the methodology previously described by Madrona et al. (2010). Eb was prepared by adding seed pulp (previously peeled) to 1 L of reverse osmosis water under stirring for 30 min. After being submitted to vacuum filtration using quantitative JP 42 blue band filter paper, Eb was called Ef.

Experiments consisted in adding the fluoride solution to the Eb or Ef M. oleifera coagulant in the reservoir of the micro/ultrafiltration module in order to have a final M. oleifera concentration of 2.5 gL−1. Runs started when the coagulation process with recirculation of the suspension for 5 min was performed. Then, the suspension was allowed to percolate through the membrane system as shown in Fig. 1. UF tests with membranes were carried out in a micro/ultrafiltration unit (NETZSCH), using the principle of cross-flow filtration. The filtration module was made of stainless steel. The system was equipped with manometers at the inlet and outlet to control the transmembrane pressure, and connected to a thermostatic bath for temperature control of the solution contained in the feed tank. The output of permeate was collected by opening the valve and the concentrate was returned to the feed tank by the hose.

2.3 Coagulation Assays with Filtered M. oleifera Coagulant (Ef)

Table 1 Characteristics of the membranes used in the process

2.2 M. oleifera Coagulant

Coagulation assays were performed in jar test equipment (Nova Ética, model 218 LDB), using rapid mixing gradient of 133 s−1 and rapid mixing time of 5 min (Silva et al. 2006). Samples for determination of colour and turbidity were collected immediately, without allowing sedimentation. In each jar, 10 mL of Ef were added to 200 mL of fluoridated water with different

Type

Area (m2) Nominal cutoff values

Mono-channel 0.005

4 kDa (≈0.002 μm) 0.005 μm

Multi-channel 0.0132

5 kDa (≈0.0025 μm)

Denomination

Mono 1 Mono 2 Multi 1

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Fig. 1 Schematic diagram of the micro/ultrafiltration experimental unit; 1 thermostatic bath, 2 feed tank, 3 pump, 4 manometers, 5 membrane filtration module, 6 flowmeter (rotameter), 7 permeate, 8 concentrated

Filtration assays were initially carried out with reverse osmosis water. Permeate samples were collected at predetermined times during a fixed time interval. Permeate flux ( fpermeate) was calculated from Eq. 1, where m is the mass of collected water, ρ is water density at 25 °C, Δt is the time interval during which the sample was collected, and Am is the filtering area of the membrane. f permeate ¼

m ρ$tAm

ð1Þ

Later, tests were performed with fluoridated water, previously coagulated with extracts of M. oleifera seeds (Eb and Ef). Permeate samples were collected at intervals of 0.5 min at the beginning of filtration. These intervals were gradually increased up to approximately 180 min. At the end of the process, a rapid membrane rinsing was performed with pure water. Reverse osmosis water flux was measured again and denominated ff, which was then compared with the clean membrane flux, fi, by calculating the percentage of fouling (%fouling) according to Eq. 2.     ff % fouling ¼ 1   100 ð2Þ fi Stabilised flux (fS) was determined for all membranes and pressures. To clean the membranes, sodium hydroxide and citric acid solutions (1 %) were used at about 65 °C. 2.6 Colour and Turbidity Apparent colour and turbidity were determined using a spectrophotometer HACH DR 2010, according to the procedure recommended by the Standard Methods (APHA 1995).

Each experiment was conducted in triplicate to ensure the reproducibility of results. Every data represents the average of three independent experiments along with standard deviations at 95 % confidence. Statistical analyses were performed using the statistical functions of Microsoft Excel version Office Xp (Microsoft Corporation, USA). 2.7 Evaluation of the Mechanism of Action of M. oleifera In order to verify the mechanism of action of the M. oleifera coagulant, scanning electron microscopy (Leo 1450 VP) was used to analyse the extract, the water with excess fluoride, and the water after treatment with filtrated coagulant. The material was previously submitted to lyophilization at −50 °C for 24 h in a Christ Alpha 1-4 LD freeze dryer under vacuum. Prior to scanning electron microscopy (SEM) analyses, small portions of each sample were placed on a metal stub, divided into small carbon tapes and coated with gold.

3 Results and Discussions 3.1 Coagulation Assays Table 2 presents the results of final fluoride concentration (CfF−) and the respective percentage of fluoride removal (RF−), after coagulation of fluoridated water using filtrated coagulant of M. oleifera seeds. It is observed that for the initial fluoride concentrations of 3 and 5 mgL−1 the coagulant concentration had little influence on the percentage of fluoride removal, when compared with initial fluoride concentrations of 7 and 10 mgL−1. Higher initial concentrations (CiF−) required higher concentrations of M. oleifera coagulant.

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Table 2 Final fluoride concentration and the respective percentage of fluoride removal after coagulation treatment with different concentrations of filtrated M. oleifera coagulant CiF− (mgL−1)

Filtrated M. oleifera coagulant concentration (gL−1) 0.25

0.50

1.25

2.50

CfF− (mgL−1)

RF− (%)

CfF− (mgL−1)

RF− (%)

CfF− (mgL−1)

RF− (%)

CfF− (mgL−1)

RF− (%)

3

1.74

42.00

1.58

47.33

1.61

46.33

1.03

65.67

5

2.60

48.00

2.23

55.40

1.55

69.00

1.02

79.60

7

6.99

0.14

6.20

11.43

6.03

13.86

1.02

85.43

10

9.99

0.10

8.60

14.00

8.63

13.70

0.91

90.90

The highest values of fluoride removal and therefore the highest process efficiencies were achieved using 2.50 gL−1 of M. oleifera coagulant, reaching up to 90.90 % removal with maximum CfF− of 1.03 mgL−1, which meets Brazilian legislation (