Sorption of fluoride using chemically modified Moringa oleifera leaves

Sorption of fluoride using chemically modified Moringa oleifera leaves Contamination of drinking water due to fluoride is a severe health hazard problem. Excess of fluoride (> 1.5 mg/L) in drink- ing water is harmful to human health. Various treatment technologies for removing fluoride from groundwater have been investigated. The present study showed that the leaves of Moringa oleifera, a herbal plant is an effective adsorbent for the removal of fluoride from aqueous solution. Acid treated Moringa oleifera leaves powder showed good adsorption capacity than alkali treated Moringa oleifera leaves powder. Batch sorptive defluoridation was conducted under the variable experi - mental condition such as pH, contact time, adsorbent dose and initial fluoride ion concentration. Maximum defluoridation was achieved at pH 1. The percentage of fluoride removal increases with adsorbent dose. The equilibrium sorption data were fitted into Langmuir, Freundlich and Temkin isotherms. Of the three adsorption isotherms, the R value of Langmuir isotherm model was the highest. The maximum monolayer coverage (Q ) from Langmuir isotherm model was determined max to be 1.1441 mg/g, the separation factor indicating a favorable sorption experiment is 0.035. It was also discovered that the adsorption did not conform to the Freundlich adsorption isotherm. The heat of sorption process was estimated from Temkin Isotherm model to be − 0.042 J/mol which vividly proved that the adsorption experiment followed a physical process. Keywords Adsorption · Fluoride · Freundlich isotherm · Langmuir isotherm · Moringa oleifera · Temkin isotherm Introduction of it is beneficial for human health for preventing dental car - ries, it is very harmful when present in excess. Maximum per- Water is most abundant and is an essential component of our missible limit of fluoride in drinking water has been set as life supporting system. Near about 97% of earth’s surface is 1.5 mg/L by many regulatory authorities such as WHO, US covered by water. But from the last few decades, these water EPA, CPCB and so forth (Bashir et al. 2015; Bell and Ludwig resources are getting polluted by various natural and anthro- 1970; Kanaujia et al. 2015; Singh 2017). The high fluoride pogenic contaminants such as heavy metals, fluoride, arsenic, levels in drinking water and its impacts on human health have lead and mercury (Abu Bakar et al. 2016; Bashir et al. 2012). increased the importance of defluoridation studies (Chidam - Among all the contaminants, fluoride contamination of water baram et al. 2004; Singh 2017). The magnitude of the problem has now become a major issue in most of the parts of the world is sinking in, and efforts are being made towards defluoridation because of its toxic effects. Fluoride is well recognized as an of drinking water, combating the debilating fluorosis and tak - element of public health concern. Fluoride is present univer- ing steps to prevent and control the disease (Karthikeyan and sally in almost every water (higher concentrations are found Ilango 2007). Chemical coagulants like Aluminum sulphate in ground water), earth crust, many minerals, rocks, etc. It is (alum), FeCl are used in the Municipal drinking water treat- also present in most of our everyday needs, viz. toothpastes, ment plant for the purification process. This excess use of an drugs, cosmetics, chewing gums, mouthwashes, and so forth amount of chemical coagulants can affect human health, e.g., (Bashir et al. 2012; Rout et al. 2015). Though a small amount Aluminum has also been indicated to be a causative agent in neurological diseases such as pre-senile dementia (Muyibi and * Shabnam Dan Evison 1995). The conventional method of fluoride removal shabnamjasmine@gmail.com includes: ion-exchange, reverse osmosis, and adsorption (Popat Amit Chattree et al. 1994). Adsorption processes using natural adsorbents or amit.chattree@shiats.edu.in agricultural waste products are becoming the new alternatives for the removal of fluoride from aqueous solution as they are Department of Chemistry, Sam Higginbottom Institute of Agriculture, Science and Technology, Allahabad, India Vol.:(0123456789) 1 3 76 Page 2 of 8 Applied Water Science (2018) 8:76 cheap, simple, sludge-free, regenerable, environment friendly, The fluoride removal studies by adsorption were con- involve small initial cost, and minimal chemical use (Saka and ducted in 250 mL conical flask using 100 mL of synthetic Sahin 2011). water sample. To these conical flasks acid and alkali treated The use of Moringa oleifera has an added advantage over (activated) adsorbent was added and after giving a required the chemical treatment of water because it is biological and contact time of 150 min the contents of the flask were then has been reported as edible. filtered using Whatman’s filter paper no. 41. The filtrate was used for fluoride ion estimation using the SPADNS method. The fluoride content in the filtrate was determined Experimental methods by UV–visible spectrophotometer. The values of percent fluoride removal by adsorbent were calculated using the Collection and preparation of adsorbent following relation (Bashir et al. 2015). and adsorbate C − C 0 i Removal%= × 100 Moringa oleifera leaves were used as adsorbent and were collected from local trees and were washed with water many where, C is initial fluoride ion concentration, and C is the 0 i times to remove dirt and then sun dried for 3 days. The dried final fluoride ion concentration. sample was then ground to powder using a pestle and mortar, the dried sample powder was then sieved to select the parti- Langmuir isotherm cle size of 350 µm and was then used as an adsorbent. 40 g of the powder sample was added to 400 mL of 1N HNO for 3 The Langmuir isotherm is valid for monolayer adsorption acid treatment, and 40 g of the powder sample was added onto a surface containing a finite number of identical sites. to 0.5N NaOH for alkali treatment. This modified proce- The model assumes uniform energies of adsorption onto dure is in accordance with the reported work of Parlikar and the surface and no transmigration of adsorbate in the plane Mokashi (2013). The mixture was boiled for about 20 min of the surface (Foo and Hameed 2010). The linear form of on a sand bath. Washing of the powder sample was car- Langmuir isotherm model is described as: ried out using distilled water until the maximum color was removed and clear water was obtained. Finally, it was dried C 1 1 = + c again in an oven at 50 °C for 6 h. q q q e maxK max The preparation of adsorbate was carried out by prepar- ing a standard solution of 2 ppm fluoride. It was prepared by where, dissolving 2 mg of anhydrous sodium fluoride in 1000 mL q = The monolayer adsorption capacity of the adsor- max of distilled water. bent (mg/g). K = The Langmuir adsorption constant (L/mg). Preparation of the reagent C = Equilibrium concentration of the solution in (mg/L). q = Amount adsorbed per unit weight of adsorbent The SPADNS solution, 0.003721 M was prepared by dis- (mg/g). solving 0.4750 g of SPADNS in 250 mL deionized water. Further, the essential characteristics of the Langmuir iso- The Zirconyl chloride acid solution, 0.0375 M was by pre- therm can be described by a separation factor, R , which is pared by dissolving 0.0665 g of ZrOCl .8H O in 25 mL of 2 2 defined by the following equation, deionized water. Then 25 mL of HCl was added, and deion- R = 1∕ 1 + bC . L i ized water was added until the total volume of the solution was 250 mL. The concentration of zirconyl acid-SPADNS where C is the initial concentration of the fluoride solu- reagent was prepared by mixing the two solutions in the ratio tion and b is the Langmuir constant related to the energy −1 of 1:1. This reagent was further diluted. of adsorption (L mg ). R value indicates the adsorption nature to be either unfavorable if R > 1), linear if R = 1, L L Sorption experiment favorable if 0 < R < 1 and irreversible if R = 0 (Foo and L L Hameed 2010). Synthetic fluoride bearing water sample having initial fluo- ride ion concentration of 2 mg/L was used. This fluoride Freundlich isotherm solution was filtered using Whatman’s filter paper no. 41 to this filtrate, SPADNS and zirconyl acid solution of 5 mL It is applicable to both monolayer and multilayer adsorption each was used. The sample was checked for fluoride detec- and is based on assumption that adsorbate adsorbs onto het- tion using spectrophotometer at wavelength 570 nm. erogeneous surfaces of an adsorbent (Bashir et al. 2017; Foo 1 3 Applied Water Science (2018) 8:76 Page 3 of 8 76 and Hameed 2010). The linear form of Freundlich equation equilibrium; and q is the amount of dye adsorbed in mg/g is expressed as: at any time t. The linear form of pseudo-second-order kinetic model 1∕n q = K C rate equation is expressed as: e f where, 2 t∕ qt = 1∕ k q + 1 q t 2 e q = Amount adsorbed per unit weight of adsorbent (mg/g). where k is the rate constant of second-order adsorption (g/ C = Equilibrium concentration of the solution in (mg/L). mg-min) depends on the sorption capacity of the solid phase. K and n are Freundlich Isotherms constants which is a The initial sorption rate, h (mg/g-min), is also calculated measure of adsorption capacity or fundamental ee ff ctiveness with the help of second-order kinetic model by the follow- of the adsorbent. K and n can be determined from the linear ing expression: plot of log q verses log C as shown in the equation. The e e 2 h = k q . adsorption process is said to be favorable when the value of ‘n’ satisfies the condition 1 < n < 10 which indicated favora- bility of adsorption and the degree of heterogeneity, other- wise it is unfavorable (Foo and Hameed 2010). Results and discussions Understanding of adsorption technique is possible with Temkin isotherm knowledge of the optimal conditions, which would herald a better design and modeling process. Thus, the effect of This isotherm contains a factor that explicitly takes into some major parameters such as pH, contact time, dose of account of adsorbent–adsorbate interactions. The model adsorbent and initial concentration of fluoride ions uptake assumes that heat of adsorption (function of temperature) on adsorbent materials were investigated from a kinetic of all molecules in the layer would decrease linearly rather viewpoint. Adsorption studies were performed by batch than logarithmic with coverage. As implied in the equation, technique to obtain the equilibrium data. The experimental its derivation is characterized by a uniform distribution of data from batch experiment were analyzed using adsorp- binding energies (up to some maximum binding energy) was tion isotherm equations (Langmuir, Freundlich, and Tem- carried out by plotting the quantity sorbed q against ln C e e kin), in which linear regression analysis was used to evalu- and the constants were determined from the slope and inter- ate whether the theoretical models have better or worse fit cept (Foo and Hameed 2010). The model is given by the for the experimental data. following equation: q = B ln A + B ln C . e T e A = Temkin isotherm equilibrium binding constant (L/g). Eec ff t of pH on defluoridation b = Temkin isotherm constant. R = universal gas constant (8.314 J/mol/K). The pH of the aqueous solution is a controlling factor in T = Temperature. the adsorption process (Bashir et al. 2015). The experi- B = Constant related to heat of sorption (J/mol). ments were carried out for acid treated and alkali treated Moringa oleifera leaves powder for determining optimum pH. The pH was varied from 1 to 10 for acid treated Mor- Chemical kinetics inga oleifera leaves powder and 2–10 for alkali treated Moringa oleifera leaves powder. The adsorbent having The experimental data are analyzed to observe the appropri- 350 µm size, acid treated as well as alkali treated, was ate kinetic model followed by adsorption process including used to determine optimal pH at which the adsorption mass transfer and chemical reactions (Bashir et al. 2017). was maximum. For these experiments, initial fluoride Pseudo-first-order and pseudo-second-order kinetic models ion concentration was 2 mg/L, with an adsorbent dose of are used to describe the kinetics of adsorption. The inte- 2.5 g/L and contact time of 150 min. As shown in Table 1, grated form of Lagergren first-order kinetic model is repre- the acid treated adsorbent showed the maximum removal sented by the equation given below: efficiency 83% at pH 1. Whereas in case of alkali treated k1 adsorbent the maximum removal efficiency was 85% at pH log q − q = log q − t e t e 2.303 10. Similar observations have been reported by (Parlikar and Mokashi 2013). where k is the rate constant of first-order adsorption (1/min); q is the amount of dye adsorbed in mg/g at 1 3 76 Page 4 of 8 Applied Water Science (2018) 8:76 Table 1 Effect of pH on sorption of fluoride after sometime it approaches a constant value, denoting attainment of equilibrium. Whereas, in case of acid treated S. no pH Acid treated powder Alkali treated powder adsorbent the amount of percentage removal of fluoride Absorbance % Removal Absorbance % Removal ion increased with increase in time, as well as the maxi- (A) (A) mum percentage removal (98%) was obtained at 150 min. 1. 1 0.020 83 Similar results were reported by  Kosari and Sepehrian 2. 2 0.023 81 0.055 54 (2017). 3. 4 0.031 74 0.040 67 4. 6 0.044 63 0.030 75 Eec ff t of adsorbent dose on defluoridation 5. 8 0.057 53 0.021 82 6. 10 0.059 51 0.018 85 The response of adsorbent dose on the removal of fluoride is presented in Table  3. In case of alkali treated powder, the observations reveal that an increase in the adsorption Eec ff t of contact time on defluoridation occurs with the corresponding increase in the amount of adsorbent. The increase in the removal efficiency with simul- The adsorbent dose of 2.5 g/L was taken and kept constant taneous increase in adsorbent dose is due to the increase in quantity, and hence more active sites were available for the throughout the experimental work. The contact time was varied from 0.5 to 2.5 h for acid treated as well as alkali adsorption of fluoride. The results showed that the alkali treated Moringa oleifera leaves powder was efficient for 95% treated Moringa oleifera leaves powder of particle size 350 µm. The experimental study was carried out to deter- removal of fluoride ions at the lowest dose of 100 mg/L and 98% at a maximum dose of 400 mg/L, respectively, at room mine optimal contact time using acid and alkali treated adsorbents with same particle size. The pH was 8 and dose temperature. This finding is supported by (Tembhurkar and Dongre 2009). was 2.5 g/L for the study. As depicted in Table 2 in case of alkali treated  adsorbent it is found that the removal fluoride ion increased with increase in contact time but Table 2 Effect of contact time S. no Time (min) Acid treated powder Alkali treated powder on defluoridation Absorbance (A) % Removal Absorbance (A) % Removal 1. 30 0.015 87 0.015 87 2. 60 0.013 89 0.011 90 3. 90 0.012 90 0.010 91 4. 120 0.006 95 0.006 95 5. 150 0.002 98 0.005 95 Table 3 Effect of adsorbent S. no Adsorbent dose Acid treated powder Alkali treated powder dose on defluoridation (mg) Absorbance (A) % Removal Absorbance (A) % Removal 1. 100 0.003 97 0.006 95 2. 200 0.006 95 0.004 96 3. 300 0.008 93 0.003 97 4. 400 0.010 91 0.002 98 Table 4 Effect of fluoride ion S. no Fluoride ion con- Acid treated powder Alkali treated powder concentration on defluoridation centration (mg) Absorbance (A) % Removal Absorbance (A) % Removal 1. 0.5 0.017 86 0.023 81 2. 1 0.010 91 0.020 83 3. 1.5 0.007 94 0.013 89 4. 2 0.002 98 0.009 92 1 3 Applied Water Science (2018) 8:76 Page 5 of 8 76 fluoride on alkali treated Moringa oleifera leaves is situated Eec ff t of initial fluoride ion concentration in the range of 1–10 indicating favorable adsorption process. It is well defined from the Table  5 and so is shown in Studies on the effect of initial fluoride concentration were conducted by varying it from 0.5 to 2 mg/L keeping adsor- Fig. 3a and b, that Temkin isotherm data fitted quite well. The values of R are positioned within 0.983–0.986, which bent dose of 2.5 g/L, pH of 8, and contact time of 150 min. The response of different fluoride ion concentration on gave a close fit to the adsorption of fluoride on acid and alkali treated Moringa oleifera leaves powder. Furthermore, the removal of fluoride is presented in Table  4. For alkali as well as acid treated powder, the observations reveal that with it can also be observed in Table 5, that the heat of adsorption of fluoride on Moringa oleifera leaves was restricted within the increase in fluoride ion concentration the % removal of fluoride also increases. With increase in initial concentration − 59,979 to 26,241 kJ/mol. In the present study, the value of R is greater than 0–1 for of fluoride the driving force for transport of fluoride from the bulk to the surface of adsorbent increases, which results acid and alkali treated adsorbent indicating that Langmuir isotherm is favorable (Bashir et al. 2015; Foo and Hameed more adsorption of fluoride per unit mass of adsorbent. This finding is supported by (Dwivedi et al. 2014). 2010). While the value of n is greater than 10 for acid treated adsorbent indicating unfavorable adsorption process whereas Adsorption isotherms for alkali treated adsorbent it is favorable. Through the adsorption equilibrium study employing In this study, adsorption isotherm study was carried out on Langmuir, Freundlich and Temkin isotherms models, the three isotherm models: Langmuir, Freundlich and Tem- kin. The applicability of isotherm models to the adsorption 0.3 study done was compared by observing the correlation coef- (a) 0.25 ficients, R value. Adsorption isotherm helps in determining 0.2 the feasibility of acid and alkali treated Moringa oleifera leaves powder for treating fluoride ion in water. Langmuir, 0.15 Freundlich and Temkin isotherms were plotted to provide y = 0.8745x + 0.0056 0.1 R² = 0.9716 deep insight to the adsorption of fluoride ion on Moringa 0.05 oleifera leaves powder. From Table 5, it is clear that acid and alkali treated adsor- 00.1 0.20.3 0.4 bent responded good behavior with all three types of iso- therm models but Langmuir’s isotherm fitted best with them C (mg/L) which is also well explained and depicted in Fig. 1a and b 0.6 (b) by the good regression coefficient of 0.97 and 0.88, respec- 0.5 y = 1.2061x tively. The Langmuir’s model described the monolayer sorp- R² = 0.8869 0.4 tion nature of the adsorbent. Table 5 shows adsorption of fluoride on the adsorbents 0.3 having a range of values of linear regression coefficient, R 0.2 (0.885–0.993) as illustrated in Fig. 2a and b, demonstrating 0.1 that the experimental data fitted well with the Freundlich isotherm equation. Moreover, it was reported that the Fre- 00.1 0.20.3 0.4 undlich isotherm constant can be used to explore the favora- C (mg/L) bility of adsorption process (Foo and Hameed 2010). As observed in Table  5, the value of n for the adsorption of Fig. 1 a Langmuir Isotherm for effect of initial fluoride ion concen- fluoride on acid treated Moringa oleifera leaves is situated tration on defluoridation (Acid treated Moringa oleifera leaves pow - outside the range of 1–10 indicating unfavorable adsorp- der). b Langmuir Isotherm for effect of initial fluoride ion concentra- tion process. Whereas, the value of n for the adsorption of tion on defluoridation (Alkali treated Moringa oleifera leaves powder) Table 5 Langmuir, Freundlich and Temkin equation values for adsorption of fluoride with respect to initial fluoride ion concentration Adsorbents Langmuir Freundlich Temkin 2 2 2 Q (mg/g) R K (L/mg) R K (mg/g) Value of ‘n’ R B (J/mol) A (L/g) R max L L f T −7 Acid treated 1.1441 0.003589 138.79 0.971 0.6546 17.543 0.885 − 0.042 1.951 × 10 0.897 Alkali treated 0.8340 0.000833 0.002 0.884 0.5727 7.2463 0.983 − 0.096 0.0029 0.986 1 3 C /q (mg/g) e e C /q (mg/g) e e 76 Page 6 of 8 Applied Water Science (2018) 8:76 0 0 -1 -0.8 -0.6 -0.4 -0.2 0 -2 -1.5 -1 -0.5 0 (a) (a) -0.05 -0.04 -0.1 -0.08 y = -0.1382x -0.2427 -0.15 -0.12 y = -0.057x -0.1842 R² = 0.9837 R² = 0.8852 -0.2 -0.16 log C -0.2 ln C (mg/L) -1 -0.8 -0.6 -0.4 -0.2 0 0.76 -0.05 (b) (b) 0.74 -0.1 y = -0.0963x + 0.5598 0.72 R² = 0.9869 0.7 -0.15 0.68 y = -0.1382x -0.2427 -0.2 0.66 R² = 0.9837 log C 0.64 -2.5 -2 -1.5 -1 -0.5 0 ln C (mg/L) Fig. 2 a Freundlich Isotherm for effect of initial fluoride ion concen- e tration on defluoridation (Acid treated Moringa oleifera leaves pow - der). b Freundlich Isotherm for effect of initial fluoride ion concentra- Fig. 3 a Temkin Isotherm for effect of initial fluoride ion concentra- tion on defluoridation (Alkali treated Moringa oleifera leaves powder) tion on defluoridation (Acid treated Moringa oleifera leaves powder). b Temkin Isotherm for effect of initial fluoride ion concentration on defluoridation (Alkali treated Moringa oleifera leaves powder) maximum adsorption capacity was determined where the adsorption of fluoride with acid treated Moringa oleifera leaves powder was 1.1441 mg/g and with alkali treated These positive charges on heteroatoms are responsible for Moringa oleifera leaves powder was 0.8340  mg/g. It is the adsorption of fluoride ions from aqueous solution. concluded by referring to the data obtained that the acid + + treated Moringa oleifera leaves powder was more efficient S– OH + H S–OH (protonation) [+ F (Electrostatic attraction)] in removing fluoride than alkali treated Moringa oleifera leaves powder. + + S– C=O + H S–C=OH (protonation) [+ F (Electrostatic attraction)] + + Mechanism of adsorption S– HN–R + H S –H N –R (protonation) 2 2 2 [+ F (Electrostatic attraction)] The acid treated adsorbent contains the functional groups such as –OH, C=O and secondary amine group that is involved in adsorption process (Abu Bakar et al. 2016; Bello Electrostatic physisorption between ions and dipole (sec- et  al. 2015). Whereas, in case of alkali treated adsorbent ond strongest among the physical bonds). Here, Electrostatic functional groups that are involved in the process of adsorp- attraction between positive poles of partial polar bonds of tion are –OH and C=O (Bello et al. 2015). Protonation of OH, NH and CO groups of adsorbent and anionic fluoride heteroatoms took place due to lone pairs of electrons. Thus, ion took place. heteroatoms such as O and N developed the positive charge. 1 3 log q log q e log q (mg/g) q (mg/g) e Applied Water Science (2018) 8:76 Page 7 of 8 76 Table 6 Kinetics of pseudo-first-order and pseudo second order of the two adsorbents studied Adsorbents Pseudo first order Pseudo second order Q (exp) (mg/g) 2 2 Q (mg/g) K (L/min) R Q (mg/g) K (g/mg-min) R H e(cal) 1 e(cal) 2 Acid treated 4.89 0.88 0.835 1.23 0.093 0.996 0.142 1.20 Alkali treated 1.65 1.70 0.867 1.27 0.135 0.999 0.219 1.20 Open Access This article is distributed under the terms of the Crea- tive Commons Attribution 4.0 International License (http://creat iveco –δ +δ -– δ +δ - SO HS + F O H …….Fmmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- tion, and reproduction in any medium, provided you give appropriate –δ +δ –δ +δ - credit to the original author(s) and the source, provide a link to the SN HS + F- NH …. F Creative Commons license, and indicate if changes were made. +δ - +δ - SC + F SC …..F || || –δ –δ O O References Abu Bakar A, Koay Y, Ching Y, Abdullah L, Choong T, Alkhatib M et al (2016) Removal of fluoride using quaternized palm kernel shell as adsorbents: equilibrium isotherms and kinetics stud- Chemical kinetics ies. Bioresources 11:4485–4511. https: //doi.org/10.15376/ biore s.11.2.4485-4511 Bashir MT, Bashir A, Rasheed M (2012) Fluorides in the groundwa- A larger adsorption rate constant k usually represents a ter of Punjab. Pak J Med Health Sci 6:132–135 quicker adsorption rate. Larger the k values slower the rate Bashir MT, Salmiaton A, Idris A, Harun R, Nourouzi MM (2015) of adsorption. The correlation coefficients of pseudo first Fluoride removal by chemical modification of palm kernel order are less than those of pseudo-second-order reaction shell-based adsorbent: a novel agricultural waste utilization approach. 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Sorption of fluoride using chemically modified Moringa oleifera leaves

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Abstract

Contamination of drinking water due to fluoride is a severe health hazard problem. Excess of fluoride (> 1.5 mg/L) in drink- ing water is harmful to human health. Various treatment technologies for removing fluoride from groundwater have been investigated. The present study showed that the leaves of Moringa oleifera, a herbal plant is an effective adsorbent for the removal of fluoride from aqueous solution. Acid treated Moringa oleifera leaves powder showed good adsorption capacity than alkali treated Moringa oleifera leaves powder. Batch sorptive defluoridation was conducted under the variable experi - mental condition such as pH, contact time, adsorbent dose and initial fluoride ion concentration. Maximum defluoridation was achieved at pH 1. The percentage of fluoride removal increases with adsorbent dose. The equilibrium sorption data were fitted into Langmuir, Freundlich and Temkin isotherms. Of the three adsorption isotherms, the R value of Langmuir isotherm model was the highest. The maximum monolayer coverage (Q ) from Langmuir isotherm model was determined max to be 1.1441 mg/g, the separation factor indicating a favorable sorption experiment is 0.035. It was also discovered that the adsorption did not conform to the Freundlich adsorption isotherm. The heat of sorption process was estimated from Temkin Isotherm model to be − 0.042 J/mol which vividly proved that the adsorption experiment followed a physical process. Keywords Adsorption · Fluoride · Freundlich isotherm · Langmuir isotherm · Moringa oleifera · Temkin isotherm Introduction of it is beneficial for human health for preventing dental car - ries, it is very harmful when present in excess. Maximum per- Water is most abundant and is an essential component of our missible limit of fluoride in drinking water has been set as life supporting system. Near about 97% of earth’s surface is 1.5 mg/L by many regulatory authorities such as WHO, US covered by water. But from the last few decades, these water EPA, CPCB and so forth (Bashir et al. 2015; Bell and Ludwig resources are getting polluted by various natural and anthro- 1970; Kanaujia et al. 2015; Singh 2017). The high fluoride pogenic contaminants such as heavy metals, fluoride, arsenic, levels in drinking water and its impacts on human health have lead and mercury (Abu Bakar et al. 2016; Bashir et al. 2012). increased the importance of defluoridation studies (Chidam - Among all the contaminants, fluoride contamination of water baram et al. 2004; Singh 2017). The magnitude of the problem has now become a major issue in most of the parts of the world is sinking in, and efforts are being made towards defluoridation because of its toxic effects. Fluoride is well recognized as an of drinking water, combating the debilating fluorosis and tak - element of public health concern. Fluoride is present univer- ing steps to prevent and control the disease (Karthikeyan and sally in almost every water (higher concentrations are found Ilango 2007). Chemical coagulants like Aluminum sulphate in ground water), earth crust, many minerals, rocks, etc. It is (alum), FeCl are used in the Municipal drinking water treat- also present in most of our everyday needs, viz. toothpastes, ment plant for the purification process. This excess use of an drugs, cosmetics, chewing gums, mouthwashes, and so forth amount of chemical coagulants can affect human health, e.g., (Bashir et al. 2012; Rout et al. 2015). Though a small amount Aluminum has also been indicated to be a causative agent in neurological diseases such as pre-senile dementia (Muyibi and * Shabnam Dan Evison 1995). The conventional method of fluoride removal shabnamjasmine@gmail.com includes: ion-exchange, reverse osmosis, and adsorption (Popat Amit Chattree et al. 1994). Adsorption processes using natural adsorbents or amit.chattree@shiats.edu.in agricultural waste products are becoming the new alternatives for the removal of fluoride from aqueous solution as they are Department of Chemistry, Sam Higginbottom Institute of Agriculture, Science and Technology, Allahabad, India Vol.:(0123456789) 1 3 76 Page 2 of 8 Applied Water Science (2018) 8:76 cheap, simple, sludge-free, regenerable, environment friendly, The fluoride removal studies by adsorption were con- involve small initial cost, and minimal chemical use (Saka and ducted in 250 mL conical flask using 100 mL of synthetic Sahin 2011). water sample. To these conical flasks acid and alkali treated The use of Moringa oleifera has an added advantage over (activated) adsorbent was added and after giving a required the chemical treatment of water because it is biological and contact time of 150 min the contents of the flask were then has been reported as edible. filtered using Whatman’s filter paper no. 41. The filtrate was used for fluoride ion estimation using the SPADNS method. The fluoride content in the filtrate was determined Experimental methods by UV–visible spectrophotometer. The values of percent fluoride removal by adsorbent were calculated using the Collection and preparation of adsorbent following relation (Bashir et al. 2015). and adsorbate C − C 0 i Removal%= × 100 Moringa oleifera leaves were used as adsorbent and were collected from local trees and were washed with water many where, C is initial fluoride ion concentration, and C is the 0 i times to remove dirt and then sun dried for 3 days. The dried final fluoride ion concentration. sample was then ground to powder using a pestle and mortar, the dried sample powder was then sieved to select the parti- Langmuir isotherm cle size of 350 µm and was then used as an adsorbent. 40 g of the powder sample was added to 400 mL of 1N HNO for 3 The Langmuir isotherm is valid for monolayer adsorption acid treatment, and 40 g of the powder sample was added onto a surface containing a finite number of identical sites. to 0.5N NaOH for alkali treatment. This modified proce- The model assumes uniform energies of adsorption onto dure is in accordance with the reported work of Parlikar and the surface and no transmigration of adsorbate in the plane Mokashi (2013). The mixture was boiled for about 20 min of the surface (Foo and Hameed 2010). The linear form of on a sand bath. Washing of the powder sample was car- Langmuir isotherm model is described as: ried out using distilled water until the maximum color was removed and clear water was obtained. Finally, it was dried C 1 1 = + c again in an oven at 50 °C for 6 h. q q q e maxK max The preparation of adsorbate was carried out by prepar- ing a standard solution of 2 ppm fluoride. It was prepared by where, dissolving 2 mg of anhydrous sodium fluoride in 1000 mL q = The monolayer adsorption capacity of the adsor- max of distilled water. bent (mg/g). K = The Langmuir adsorption constant (L/mg). Preparation of the reagent C = Equilibrium concentration of the solution in (mg/L). q = Amount adsorbed per unit weight of adsorbent The SPADNS solution, 0.003721 M was prepared by dis- (mg/g). solving 0.4750 g of SPADNS in 250 mL deionized water. Further, the essential characteristics of the Langmuir iso- The Zirconyl chloride acid solution, 0.0375 M was by pre- therm can be described by a separation factor, R , which is pared by dissolving 0.0665 g of ZrOCl .8H O in 25 mL of 2 2 defined by the following equation, deionized water. Then 25 mL of HCl was added, and deion- R = 1∕ 1 + bC . L i ized water was added until the total volume of the solution was 250 mL. The concentration of zirconyl acid-SPADNS where C is the initial concentration of the fluoride solu- reagent was prepared by mixing the two solutions in the ratio tion and b is the Langmuir constant related to the energy −1 of 1:1. This reagent was further diluted. of adsorption (L mg ). R value indicates the adsorption nature to be either unfavorable if R > 1), linear if R = 1, L L Sorption experiment favorable if 0 < R < 1 and irreversible if R = 0 (Foo and L L Hameed 2010). Synthetic fluoride bearing water sample having initial fluo- ride ion concentration of 2 mg/L was used. This fluoride Freundlich isotherm solution was filtered using Whatman’s filter paper no. 41 to this filtrate, SPADNS and zirconyl acid solution of 5 mL It is applicable to both monolayer and multilayer adsorption each was used. The sample was checked for fluoride detec- and is based on assumption that adsorbate adsorbs onto het- tion using spectrophotometer at wavelength 570 nm. erogeneous surfaces of an adsorbent (Bashir et al. 2017; Foo 1 3 Applied Water Science (2018) 8:76 Page 3 of 8 76 and Hameed 2010). The linear form of Freundlich equation equilibrium; and q is the amount of dye adsorbed in mg/g is expressed as: at any time t. The linear form of pseudo-second-order kinetic model 1∕n q = K C rate equation is expressed as: e f where, 2 t∕ qt = 1∕ k q + 1 q t 2 e q = Amount adsorbed per unit weight of adsorbent (mg/g). where k is the rate constant of second-order adsorption (g/ C = Equilibrium concentration of the solution in (mg/L). mg-min) depends on the sorption capacity of the solid phase. K and n are Freundlich Isotherms constants which is a The initial sorption rate, h (mg/g-min), is also calculated measure of adsorption capacity or fundamental ee ff ctiveness with the help of second-order kinetic model by the follow- of the adsorbent. K and n can be determined from the linear ing expression: plot of log q verses log C as shown in the equation. The e e 2 h = k q . adsorption process is said to be favorable when the value of ‘n’ satisfies the condition 1 < n < 10 which indicated favora- bility of adsorption and the degree of heterogeneity, other- wise it is unfavorable (Foo and Hameed 2010). Results and discussions Understanding of adsorption technique is possible with Temkin isotherm knowledge of the optimal conditions, which would herald a better design and modeling process. Thus, the effect of This isotherm contains a factor that explicitly takes into some major parameters such as pH, contact time, dose of account of adsorbent–adsorbate interactions. The model adsorbent and initial concentration of fluoride ions uptake assumes that heat of adsorption (function of temperature) on adsorbent materials were investigated from a kinetic of all molecules in the layer would decrease linearly rather viewpoint. Adsorption studies were performed by batch than logarithmic with coverage. As implied in the equation, technique to obtain the equilibrium data. The experimental its derivation is characterized by a uniform distribution of data from batch experiment were analyzed using adsorp- binding energies (up to some maximum binding energy) was tion isotherm equations (Langmuir, Freundlich, and Tem- carried out by plotting the quantity sorbed q against ln C e e kin), in which linear regression analysis was used to evalu- and the constants were determined from the slope and inter- ate whether the theoretical models have better or worse fit cept (Foo and Hameed 2010). The model is given by the for the experimental data. following equation: q = B ln A + B ln C . e T e A = Temkin isotherm equilibrium binding constant (L/g). Eec ff t of pH on defluoridation b = Temkin isotherm constant. R = universal gas constant (8.314 J/mol/K). The pH of the aqueous solution is a controlling factor in T = Temperature. the adsorption process (Bashir et al. 2015). The experi- B = Constant related to heat of sorption (J/mol). ments were carried out for acid treated and alkali treated Moringa oleifera leaves powder for determining optimum pH. The pH was varied from 1 to 10 for acid treated Mor- Chemical kinetics inga oleifera leaves powder and 2–10 for alkali treated Moringa oleifera leaves powder. The adsorbent having The experimental data are analyzed to observe the appropri- 350 µm size, acid treated as well as alkali treated, was ate kinetic model followed by adsorption process including used to determine optimal pH at which the adsorption mass transfer and chemical reactions (Bashir et al. 2017). was maximum. For these experiments, initial fluoride Pseudo-first-order and pseudo-second-order kinetic models ion concentration was 2 mg/L, with an adsorbent dose of are used to describe the kinetics of adsorption. The inte- 2.5 g/L and contact time of 150 min. As shown in Table 1, grated form of Lagergren first-order kinetic model is repre- the acid treated adsorbent showed the maximum removal sented by the equation given below: efficiency 83% at pH 1. Whereas in case of alkali treated k1 adsorbent the maximum removal efficiency was 85% at pH log q − q = log q − t e t e 2.303 10. Similar observations have been reported by (Parlikar and Mokashi 2013). where k is the rate constant of first-order adsorption (1/min); q is the amount of dye adsorbed in mg/g at 1 3 76 Page 4 of 8 Applied Water Science (2018) 8:76 Table 1 Effect of pH on sorption of fluoride after sometime it approaches a constant value, denoting attainment of equilibrium. Whereas, in case of acid treated S. no pH Acid treated powder Alkali treated powder adsorbent the amount of percentage removal of fluoride Absorbance % Removal Absorbance % Removal ion increased with increase in time, as well as the maxi- (A) (A) mum percentage removal (98%) was obtained at 150 min. 1. 1 0.020 83 Similar results were reported by  Kosari and Sepehrian 2. 2 0.023 81 0.055 54 (2017). 3. 4 0.031 74 0.040 67 4. 6 0.044 63 0.030 75 Eec ff t of adsorbent dose on defluoridation 5. 8 0.057 53 0.021 82 6. 10 0.059 51 0.018 85 The response of adsorbent dose on the removal of fluoride is presented in Table  3. In case of alkali treated powder, the observations reveal that an increase in the adsorption Eec ff t of contact time on defluoridation occurs with the corresponding increase in the amount of adsorbent. The increase in the removal efficiency with simul- The adsorbent dose of 2.5 g/L was taken and kept constant taneous increase in adsorbent dose is due to the increase in quantity, and hence more active sites were available for the throughout the experimental work. The contact time was varied from 0.5 to 2.5 h for acid treated as well as alkali adsorption of fluoride. The results showed that the alkali treated Moringa oleifera leaves powder was efficient for 95% treated Moringa oleifera leaves powder of particle size 350 µm. The experimental study was carried out to deter- removal of fluoride ions at the lowest dose of 100 mg/L and 98% at a maximum dose of 400 mg/L, respectively, at room mine optimal contact time using acid and alkali treated adsorbents with same particle size. The pH was 8 and dose temperature. This finding is supported by (Tembhurkar and Dongre 2009). was 2.5 g/L for the study. As depicted in Table 2 in case of alkali treated  adsorbent it is found that the removal fluoride ion increased with increase in contact time but Table 2 Effect of contact time S. no Time (min) Acid treated powder Alkali treated powder on defluoridation Absorbance (A) % Removal Absorbance (A) % Removal 1. 30 0.015 87 0.015 87 2. 60 0.013 89 0.011 90 3. 90 0.012 90 0.010 91 4. 120 0.006 95 0.006 95 5. 150 0.002 98 0.005 95 Table 3 Effect of adsorbent S. no Adsorbent dose Acid treated powder Alkali treated powder dose on defluoridation (mg) Absorbance (A) % Removal Absorbance (A) % Removal 1. 100 0.003 97 0.006 95 2. 200 0.006 95 0.004 96 3. 300 0.008 93 0.003 97 4. 400 0.010 91 0.002 98 Table 4 Effect of fluoride ion S. no Fluoride ion con- Acid treated powder Alkali treated powder concentration on defluoridation centration (mg) Absorbance (A) % Removal Absorbance (A) % Removal 1. 0.5 0.017 86 0.023 81 2. 1 0.010 91 0.020 83 3. 1.5 0.007 94 0.013 89 4. 2 0.002 98 0.009 92 1 3 Applied Water Science (2018) 8:76 Page 5 of 8 76 fluoride on alkali treated Moringa oleifera leaves is situated Eec ff t of initial fluoride ion concentration in the range of 1–10 indicating favorable adsorption process. It is well defined from the Table  5 and so is shown in Studies on the effect of initial fluoride concentration were conducted by varying it from 0.5 to 2 mg/L keeping adsor- Fig. 3a and b, that Temkin isotherm data fitted quite well. The values of R are positioned within 0.983–0.986, which bent dose of 2.5 g/L, pH of 8, and contact time of 150 min. The response of different fluoride ion concentration on gave a close fit to the adsorption of fluoride on acid and alkali treated Moringa oleifera leaves powder. Furthermore, the removal of fluoride is presented in Table  4. For alkali as well as acid treated powder, the observations reveal that with it can also be observed in Table 5, that the heat of adsorption of fluoride on Moringa oleifera leaves was restricted within the increase in fluoride ion concentration the % removal of fluoride also increases. With increase in initial concentration − 59,979 to 26,241 kJ/mol. In the present study, the value of R is greater than 0–1 for of fluoride the driving force for transport of fluoride from the bulk to the surface of adsorbent increases, which results acid and alkali treated adsorbent indicating that Langmuir isotherm is favorable (Bashir et al. 2015; Foo and Hameed more adsorption of fluoride per unit mass of adsorbent. This finding is supported by (Dwivedi et al. 2014). 2010). While the value of n is greater than 10 for acid treated adsorbent indicating unfavorable adsorption process whereas Adsorption isotherms for alkali treated adsorbent it is favorable. Through the adsorption equilibrium study employing In this study, adsorption isotherm study was carried out on Langmuir, Freundlich and Temkin isotherms models, the three isotherm models: Langmuir, Freundlich and Tem- kin. The applicability of isotherm models to the adsorption 0.3 study done was compared by observing the correlation coef- (a) 0.25 ficients, R value. Adsorption isotherm helps in determining 0.2 the feasibility of acid and alkali treated Moringa oleifera leaves powder for treating fluoride ion in water. Langmuir, 0.15 Freundlich and Temkin isotherms were plotted to provide y = 0.8745x + 0.0056 0.1 R² = 0.9716 deep insight to the adsorption of fluoride ion on Moringa 0.05 oleifera leaves powder. From Table 5, it is clear that acid and alkali treated adsor- 00.1 0.20.3 0.4 bent responded good behavior with all three types of iso- therm models but Langmuir’s isotherm fitted best with them C (mg/L) which is also well explained and depicted in Fig. 1a and b 0.6 (b) by the good regression coefficient of 0.97 and 0.88, respec- 0.5 y = 1.2061x tively. The Langmuir’s model described the monolayer sorp- R² = 0.8869 0.4 tion nature of the adsorbent. Table 5 shows adsorption of fluoride on the adsorbents 0.3 having a range of values of linear regression coefficient, R 0.2 (0.885–0.993) as illustrated in Fig. 2a and b, demonstrating 0.1 that the experimental data fitted well with the Freundlich isotherm equation. Moreover, it was reported that the Fre- 00.1 0.20.3 0.4 undlich isotherm constant can be used to explore the favora- C (mg/L) bility of adsorption process (Foo and Hameed 2010). As observed in Table  5, the value of n for the adsorption of Fig. 1 a Langmuir Isotherm for effect of initial fluoride ion concen- fluoride on acid treated Moringa oleifera leaves is situated tration on defluoridation (Acid treated Moringa oleifera leaves pow - outside the range of 1–10 indicating unfavorable adsorp- der). b Langmuir Isotherm for effect of initial fluoride ion concentra- tion process. Whereas, the value of n for the adsorption of tion on defluoridation (Alkali treated Moringa oleifera leaves powder) Table 5 Langmuir, Freundlich and Temkin equation values for adsorption of fluoride with respect to initial fluoride ion concentration Adsorbents Langmuir Freundlich Temkin 2 2 2 Q (mg/g) R K (L/mg) R K (mg/g) Value of ‘n’ R B (J/mol) A (L/g) R max L L f T −7 Acid treated 1.1441 0.003589 138.79 0.971 0.6546 17.543 0.885 − 0.042 1.951 × 10 0.897 Alkali treated 0.8340 0.000833 0.002 0.884 0.5727 7.2463 0.983 − 0.096 0.0029 0.986 1 3 C /q (mg/g) e e C /q (mg/g) e e 76 Page 6 of 8 Applied Water Science (2018) 8:76 0 0 -1 -0.8 -0.6 -0.4 -0.2 0 -2 -1.5 -1 -0.5 0 (a) (a) -0.05 -0.04 -0.1 -0.08 y = -0.1382x -0.2427 -0.15 -0.12 y = -0.057x -0.1842 R² = 0.9837 R² = 0.8852 -0.2 -0.16 log C -0.2 ln C (mg/L) -1 -0.8 -0.6 -0.4 -0.2 0 0.76 -0.05 (b) (b) 0.74 -0.1 y = -0.0963x + 0.5598 0.72 R² = 0.9869 0.7 -0.15 0.68 y = -0.1382x -0.2427 -0.2 0.66 R² = 0.9837 log C 0.64 -2.5 -2 -1.5 -1 -0.5 0 ln C (mg/L) Fig. 2 a Freundlich Isotherm for effect of initial fluoride ion concen- e tration on defluoridation (Acid treated Moringa oleifera leaves pow - der). b Freundlich Isotherm for effect of initial fluoride ion concentra- Fig. 3 a Temkin Isotherm for effect of initial fluoride ion concentra- tion on defluoridation (Alkali treated Moringa oleifera leaves powder) tion on defluoridation (Acid treated Moringa oleifera leaves powder). b Temkin Isotherm for effect of initial fluoride ion concentration on defluoridation (Alkali treated Moringa oleifera leaves powder) maximum adsorption capacity was determined where the adsorption of fluoride with acid treated Moringa oleifera leaves powder was 1.1441 mg/g and with alkali treated These positive charges on heteroatoms are responsible for Moringa oleifera leaves powder was 0.8340  mg/g. It is the adsorption of fluoride ions from aqueous solution. concluded by referring to the data obtained that the acid + + treated Moringa oleifera leaves powder was more efficient S– OH + H S–OH (protonation) [+ F (Electrostatic attraction)] in removing fluoride than alkali treated Moringa oleifera leaves powder. + + S– C=O + H S–C=OH (protonation) [+ F (Electrostatic attraction)] + + Mechanism of adsorption S– HN–R + H S –H N –R (protonation) 2 2 2 [+ F (Electrostatic attraction)] The acid treated adsorbent contains the functional groups such as –OH, C=O and secondary amine group that is involved in adsorption process (Abu Bakar et al. 2016; Bello Electrostatic physisorption between ions and dipole (sec- et  al. 2015). Whereas, in case of alkali treated adsorbent ond strongest among the physical bonds). Here, Electrostatic functional groups that are involved in the process of adsorp- attraction between positive poles of partial polar bonds of tion are –OH and C=O (Bello et al. 2015). Protonation of OH, NH and CO groups of adsorbent and anionic fluoride heteroatoms took place due to lone pairs of electrons. Thus, ion took place. heteroatoms such as O and N developed the positive charge. 1 3 log q log q e log q (mg/g) q (mg/g) e Applied Water Science (2018) 8:76 Page 7 of 8 76 Table 6 Kinetics of pseudo-first-order and pseudo second order of the two adsorbents studied Adsorbents Pseudo first order Pseudo second order Q (exp) (mg/g) 2 2 Q (mg/g) K (L/min) R Q (mg/g) K (g/mg-min) R H e(cal) 1 e(cal) 2 Acid treated 4.89 0.88 0.835 1.23 0.093 0.996 0.142 1.20 Alkali treated 1.65 1.70 0.867 1.27 0.135 0.999 0.219 1.20 Open Access This article is distributed under the terms of the Crea- tive Commons Attribution 4.0 International License (http://creat iveco –δ +δ -– δ +δ - SO HS + F O H …….Fmmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- tion, and reproduction in any medium, provided you give appropriate –δ +δ –δ +δ - credit to the original author(s) and the source, provide a link to the SN HS + F- NH …. F Creative Commons license, and indicate if changes were made. +δ - +δ - SC + F SC …..F || || –δ –δ O O References Abu Bakar A, Koay Y, Ching Y, Abdullah L, Choong T, Alkhatib M et al (2016) Removal of fluoride using quaternized palm kernel shell as adsorbents: equilibrium isotherms and kinetics stud- Chemical kinetics ies. Bioresources 11:4485–4511. https: //doi.org/10.15376/ biore s.11.2.4485-4511 Bashir MT, Bashir A, Rasheed M (2012) Fluorides in the groundwa- A larger adsorption rate constant k usually represents a ter of Punjab. Pak J Med Health Sci 6:132–135 quicker adsorption rate. Larger the k values slower the rate Bashir MT, Salmiaton A, Idris A, Harun R, Nourouzi MM (2015) of adsorption. The correlation coefficients of pseudo first Fluoride removal by chemical modification of palm kernel order are less than those of pseudo-second-order reaction shell-based adsorbent: a novel agricultural waste utilization approach. Asian J Microbiol Biotech Environ Sci 17:533–542 (Tables 6). This indicates that kinetic data for adsorption of Bashir MT, Salmiaton A, Idris A, Harun R (2017) Kinetic and fluoride by the acid and alkali treated adsorbents prepared thermodynamic study of nitrate adsorption from aqueous solu- from Moringa oleifera leaves are better fitted to pseudo-sec- tion by lignocellulose-based anion resins. Desalin Water Treat ond-order rate equation than to pseudo first-order rate equa- 62:449–456 Bell MC, Ludwig TG (1970) The supply of fluoride to man: inges- tion. The values of the pseudo first-order rate constant (k ) tion from water. Fluorides and Human Health. World Health for both the adsorbent are not close to each other compared Organization, Geneva, p 59 (WHO Monograph Series) to those of pseudo-second-order rate constant (k ). Moreo- Bello SO, Adegoke AK, Akinyunni O (2015) Preparation and char- ver, the calculated values of qe for both the adsorbents from acterization of a novel adsorbent from Moringa oleifera leaf. Appl Water Sci 7:1295–1305. https ://doi.or g/10.1007/s1320 second-order kinetic equation are found to be agreed well 1-015-0345-4 with the experimental values of q . Therefore, it is concluded Chidambaram S, Ramanathan A, Vasudevan S (2004) Fluoride that the adsorption of fluoride by these adsorbents follows removal studies in water using natural materials: technical note. the pseudo-second-order reaction kinetics. This result is in Water SA 29:339–343. https://doi.or g/10.4314/wsa.v29i3.4936 Dwivedi S, Mondal P, Majumder CB (2014) Removal of fluoride agreement with the results obtained for the defluoridation using citrus limettain batch reactor: kinetics and equilibrium using banana peel and coffee husk (Getachew et al. 2014). studies. 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The process of adsorption also fol- https ://doi.org/10.9734/irjpa c/2015/17965 Karthikeyan G, Ilango SS (2007) Fluoride sorption using Moringa lows pseudo-second-order rate equation in both cases. It is indica based activated carbon. Iran J Environ Health Sci Eng concluded by referring to the data obtained that the acid 6:21–28 treated Moringa oleifera leaves powder was more efficient Kosari M, Sepehrian H (2017) Fluoride ions removal using yttrium in removing fluoride than alkali treated Moringa oleifera alginate biocomposite from an aqueous solution. Int J Eng. https ://doi.org/10.5829/idosi .ije.2017.30.01a.01 leaves powder. 1 3 76 Page 8 of 8 Applied Water Science (2018) 8:76 Muyibi S, Evison L (1995) Moringa oleifera seeds for softening hard- skins. Coloration Technol 127:246–255. https ://doi.org/10.111 water. 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J-EN 90:18–23 https ://doi.org/10.1016/0923-1137(94)90107 -4 Rout T, Verma R, Dennis R, Banerjee S (2015) Study the removal Publisher’s Note Springer Nature remains neutral with regard to of fluoride from aqueous medium by using nano-compos- jurisdictional claims in published maps and institutional affiliations. ites. J Encapsul Ads Sci 05:38–52. https ://doi.or g/10.4236/ jeas.2015.51004 Saka C, Sahin Ö (2011) Removal of methylene blue from aqueous solutions by using cold plasma- and formaldehyde-treated onion 1 3

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Applied Water ScienceSpringer Journals

Published: May 7, 2018

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