Iron Removal from Drinking Water System using Ecofriendly Synthesized Metal-Organic Frameworks

Urbanization and industrialization had decreased the drinking water quality to a greater extent. Iron contamination is a serious issue in many open well and bore well samples of Kerala, India. Intake of large amount of iron causes serious health issues to humans. There were several methods for the removal of iron from drinking water. But the cost of these treatments is not affordable to common people. Use of eco-friendly materials for the effective removal of iron from water samples is the major concern for this study. Different plant materials which are easily available in Kerala were used in this study for the effective removal of iron. Clitoria ternatea, which is a climber was selected as the adsorbent material. The extracts of this plant materials were incorporated in Magnesium based Metal-Organic-Framework (MOF) and used for water treatment. Among the prepared MOFs, that modified with flower extract of Clitoria ternatea, was found to be more effective at an adsorbent dosage of 40ppm. The adsorption process was analyzed using different isotherms and found suitable for Langmuir adsorption. Hence a mono-layer of adsorption was formed on the adsorbent surface by physical adsorption


INTRODUCTION
Ground water is the major source of drinking water and its quality is greatly affected by both natural and man-made causes.Maintenance of drinking water quality standards is very difficult nowadays due to urbanization and industrialization 1 .Studies showed that some drinking water quality parameters were exceeding the limit in the samples collected from many open and bore wells of Kerala, India 2 .Iron is the major contaminant in these samples 3 .Increased amount of iron in water will increase the bacterial growth and will produce an unpleasant odour.It also affects household fittings and even human health.Excessive intake of iron in the body may leads to cardiovascular diseases, Parkinson's, diabetes mellitus, Alzheimer's, hyperkeratosis, like chronic cholic infection, diarrhoea, cholera 1,4 etc.It also affects the immunity of human beings too.In water, iron is present in two different forms, one is in ferrous iron and the other one is ferric iron.Ferrous iron may dissolve in water and does not affect the color of the water.But it gives a metallic taste to water.On oxidation, ferrous iron changes to ferric iron results in yellow-brown precipitate, which gives water a yellowish brown colour.It was reported by World Health Organisation (WHO) that the amount of iron found in drinking water should be less than 0.3 ppm.
There are several methods for the removal of excess iron from water.These include hydrogenation 5 , oxidation [6][7][8] , bio-remediation 9 etc.Bio-remediation is less harmful to the environment, but its effectiveness decreases as pollution increases.Different materials were synthesized for the removal of iron by altering their physio-chemical properties.Adsorption is the best method which is highly efficient technique and has low cost of operation.Many naturally occurring materials can be used as adsorbent.The selective adsorption of pollutants from water samples by the adsorbent is also a very important factor to determine the efficiency of this process.Despite of these advantageous, adsorption has disadvantage such as disposal problem and reliability for real application in water samples.
Metal-organic frameworks (MOFs) have received important attraction in adsorption field, owing to specific constructive characteristics that are not found in other poriferous materials.The elastic and exceedingly porous structures of MOFs make the diffusion of the guest ions or molecules easy into the bulk structure 10 .The presence of biosorbents have good adsorption capacity for iron removal from water sample.Since biosorbents contain groups like amine, amide, sulfonate, carbony l and phenolic functional groups that can attract metal ions easily 11 .To increase the effectiveness, the microorganism must be immobilized on suitable surface 12 .Natural extracts prepared from different parts of plants like seeds, leaves, bark or sap, roots, and fruits were used for water treatment from ancient times [13][14][15] .
Aim of the study is to synthesize a material which has the properties of low cost, ease of availability in more amount and simple desorption of the adsorbed metal ion and reproducibility of the material.So, a low cost magnesium metal was selected for the synthesis of MOF and the flower and leaf extracts of Clitoria ternatea (which is otherwise called butterfly pea plant) as the biosorbent for its modification.Clitoria ternatea is an attractive climber, grows up to 2-3 m tall, bears blue or white flowers which is locally available in the southern part of Kerala.The extract of Clitoria ternatea leaves were used as reducing agents to synthesize various nano particles 16,17 .

Preparation of plant extract incorporated Mg-MOF
Mg-MOF nano particles were synthesized by incorporating the flower extract of Clitoria ternatea as in the previous part of this study 18 .The synthesis was carried out using a microwave oven of Samsung MS2 3F301E made.The leaf extract incorporated Mg-MOF was prepared from the leaves of Clitoria ternatea by using the same procedure.

Adsorption studies
The water samples were collected from borewell near Sanghumugham, the Coastal area of Thiruvananthapuram, Kerala.It was filtered using Whatman No. 40 filter paper to remove macro contaminants and initial iron concentration was estimated using a spectro-photometer at a wavelength of 565nm 19 .The iron removal efficiency was studied by adding 2, 4, 10, 20, 40 ppm each of pure Mg-MOF, flower extract incorporated Mg-MOF and leaf extract incorporated Mg-MOF.All these samples were shaken well and kept without any disturbance.The absorbance value of each sample is measured after definite interval of time and suitability of different isotherms were also analyzed.

Physio-chemical characteristics of borewell water
The physio-chemical characteristics were measured for borewell water sample and are given in Table 1.

Determination of initial iron concentration of bore well water
Initial iron concentration of collected bore well water sample is determined by using freshly prepared ferrous ammonium sulphate as the standard in spectrophotometer at 565nm.A calibration curve was drawn using ferrous ammonium sulphate(1000 ppm) with different concentration of 2, 4, 6 and 8 ppm.After that unknown concentration of iron in borewell water sample was determined.Absorbance vs concentration was plotted and the Y-intercept was obtained slope and correlation were measured for each sample at 565nm which is shown in Figure1.
The ferrous ion concentration in borewell water sample was 0.4401 ppm from this graph.According to the Indian Standard Drinking Water-Specification value IS: 3025 (Part 53), the normal water should contain the ferrous ion less than 0.3 ppm 20.

Effect of adsorbent dosage
The iron removal efficiency of each adsorbent was studied by adding 2, 4, 10, 20, 40 ppm each of pure Mg-MOF, flower extract modified Mg-MOF and leaf extract modified Mg-MOF.Effect of adsorbent dosage on iron removal is shown in Figure 2.  From the figure it is clear that, as the concentration of the adsorbent increases, more and more iron is removed from the water samples.This is mainly due to increase in the surface area of the adsorbent.All the three prepared MOFs were found to be effective in iron removal.But after 40 ppm of adsorbent dosage, the iron concentration in water samples remain almost constant which means most of the adsorbent surface get occupied by the adsorbate.Hence, maximum effectiveness is observed in 40 ppm of the adsorbent dosage.Among the prepared MOFs, that prepared from the flower extract of Clitoria ternatea was found to be most effective in iron removal from water sample.

Study of adsorption and iron removal efficiency
The adsorption efficiency and iron removal efficiency with respect to adsorbent dose was studied for all the three prepared samples using the equations.

Fig. 3a. Adsorption and iron removal efficiency of pure Mg-MOF
W h e r e , C e i s t h e l i q u i d -p h a s e concentration of iron at equilibrium in mg/L, q e is the amount of iron adsorbed at equilibrium in mg/L, Q 0 is the maximum monolayer adsorption capacity in mg/g, and b is Langmuir constant in L/mg or L/mol.Plots of (C e /q e ) vs. C e are drawn for the three prepared compounds and are shown in Figures 4(a  Thus, from the graphs, it is clear that on adding plant extracts, the active adsorption sites of the Mg-MOF increases.Among the prepared MOFs, flower extract added Mg-MOF was the most effective one with adsorption efficiency of 54.55% at an adsorbent dosage of 40 ppm.

Studies on mechanism of adsorption
The adsorption isotherm models explain the interaction between adsorbent and adsorbate and are important in finding the mechanisms of adsorption.The distribution of iron between water and adsorbent surface was studied using Langmuir 21 and Freundlich 22 isotherm models.

Langmuir adsorption isotherm
The Langmuir adsor ption isother m has been successfully applied to many pollutant adsorption processes from aqueous solution 23,24 .The linear form of the Langmuir model is, The values of intercept b, slope Q 0 and the correlation coefficient R 2 were calculated.From these values the equilibrium parameter, R L was calculated using the equation, Where, C 0 is the initial concentration of iron 25,26 .All these values for the prepared compounds were given in Table 2.All the values of R 2 were higher than 0.999 indicating that the adsorption of iron by the three prepared MOFs can be suitably explained by the Langmuir isotherm model.The values of RL indicates whether the isotherm is unfavourable, favourable or irreversible 25 .The R L values were almost equal to one, showing a favourable, linear adsorption.These adsorption processes can be well explained by Langmuir adsorption isotherm and it happened by physisorption forming a mono layer of iron over the surface of the adsorbent.

Freundlich adsorption isotherm
Freundlich adsorption isotherm model is suitable for multi-layer adsorption.The linear form of Freundlich model is 22,27,28 , Where q e is the amount of iron adsorbed at equilibrium in mg/g, K F is the adsorption capacity of the adsorbent in mg•g −1 , n indicates the degree to which an adsorption process is favorable and C e is the liquid-phase concentrations of iron at equilibrium in mg/L.Both K F and n are Freundlich constants.Plots of ln q e vs. ln Ce for the prepared MOF samples in the adsorption study are shown in Figures 5(a In general, as the K F increases the adsorption capacity of the adsorbent increases.If n<1, this means poor adsorption; from one to two means moderately difficult adsorption; and from two to ten good adsorption Table 3.

CONCLUSION
Increase in human population had increased the pollution in water bodies like rivers, lakes and even in wells.Even though majority of the population depends on municipal water supply, wells are the important source of drinking water in many rural and urban areas of Kerala, India.Metal-organic frameworks (MOFs) have the potential in water treatment sector.This is due to its eco-friendly, highly efficient and reliable water treatment nature.In this study, removal of excess iron from bore well samples was carried out using eco-friendly synthesized Mg-MOFs as adsorbent materials.Among the prepared MOFs, that modified with flower extract of Clitoria ternatea, was found to be more effective at an adsorbent dosage of 40 ppm.The adsorption process was analyzed using different isotherms and found suitable for Langmuir adsorption.Hence a mono layer of adsorption was formed on the adsorbent surface by physical adsorption.These studies pave the way for the usage of many naturally available plant materials for the drinking water treatment.

Fig. 1 .Fig. 2 .
Fig. 1.Calibration of ferrous ion using spectrophotometer at 565nm Adsorption efficiency (%) = [(C i -C e ) x 100]/C e Removal efficiency (%) = [(C i -C e ) x100]/C i Where C i and C e are initial and final concentrations of iron in water sample.These are shown in Figures 3(a), 3(b) and 3(c).

Fig. 3b .
Fig. 3b.Adsorption and iron removal efficiency of flower extract incorporated Mg-MOF

Table 3 : Freundlich adsorption parameters
All of the n values obtained from the Freundlich model are less than unity, indicating that poor chemisorption of iron on the MOFs.Its correlation coefficient R 2 are much lower than those for the Langmuir isotherm.It means that the Langmuir isotherm model is the best model to fit with the experimental data.