CHAPTER 1: INTRODUCTION
Chemically modi?ed polymeric adsorbents are resins with functional groups such as phenolic hydroxyl, acetyl, sulfonic group or amino group, grafted on hyper cross linked polymers (Alexandratos & Natesan 1999), (Cai et al. 2005). Among these, aminated hyper cross linked polymers display a unique advantage in adsorption of aromatic sulphonates (Pan et al. 2005) phenol derivatives (Pan, Zhang, Wei & Ren 2008), and phenolic acids (Wang, Zhang, Zhao, Xia, & Chen 2005) due to the presence of electrostatic interactions, hydrogen bonding interaction or ?–? interaction between adsorbents and adsorbents.
Polystyrene particles have excellent chemical and physical properties that make them not easy to be degraded and damaged. Therefore polystyrene based adsorbents are usually used repeatedly. Unmodified particles are not suitable to adsorb substances from aqueous solutions because the surface is hydrophobic and lack of selective groups (Wang et al. 2005). However their hydrophobicity and selectivity is increased after modification.
Nitrate is the most widely available contaminant in ground and surface waters (Liu et al 2005). Excess of nitrate in drinking water results from anthropogenic sources, for example, over fertilization in agriculture, cattle discharge, untreated sewage, leakage from septic systems, infiltration of landfill leachate, and industrial waste water (Nuhoglu et al. 2005), (Hell et al. 1998), (Samatya et al. 2006), (Nataraj et. al 2006). Out of these, synthetic fertilizers are the major contributors of nitrate pollution (Rupert 2008).
Nitrate concentration above the maximum permissible limit in drinking water is injurious to human health. Nitrate exposure can lead to several health issues such as increased infant mortality, birth defects, abdominal pain, diarrhoea, vomiting, diabetes, hypertension, respiratory tract infections, and changes in the immune system (Majumdar & Guptar 2002), (Kross et al. 1992), (Fewtrell 2004), (Greener & Shannon 2005), (Ward, et al. 2005).
Numerous technologies are available for removal of nitrate from water. These include reverse osmosis, electro dialysis, biological denitrification, and ion exchange methods. In case of reverse osmosis (RO) water passes through a semipermeable membrane, and nitrate and other ions are rejected because their size is greater than the membrane pore size.
Biological denitrification is widely practiced for the treatment of municipal and industrial wastewater but is less commonly used in drinking water applications (Hu et al. 2001)
Ion exchange is a process in which the target ion gets exchanged with a loosely adsorbed ion on a resin. Ion exchange is also like a reversible chemical process in which ions from an insoluble permanent solid medium (the ion exchanger is usually a resin) are exchanged for ions in a solution (Baes et al. 2002). This process is widely adopted for nitrate removal because of its simplicity, effectiveness, and relatively low cost (Baes et al. 2002). Adsorption has proved to be a relatively cheaper option which is readily applicable (Gupta & Ali 2000).
There are several methods that can be used to determine the concentration of nitrate ions and amongst them ion selective method is the most versatile. The nitrate electrode contains an internal reference solution in contact with a porous plastic organophilic membrane which acts as selective nitrate exchanger (Qingshan et al. 1999). When the membrane is exposed to nitrates present in water, a potential, E is developed across the membrane which is measured against a constant reference electrode potential , E0 .The magnitude of E depends on the concentration of nitrates present (Alexiades, & Mitrakas 1990).
1.1 Justification of the study
Conventional methods (example use of activated carbon) of removing nitrates are expensive. The methods are also ineffective when the ions are present in high concentration (Somdutta et al. 2012). Use amino alkylated polystyrene will be of benefit because the particles have excellent chemical and physical properties that make them not easy to be degraded and damaged. Therefore polystyrene based adsorbents are used repeatedly. The amino alkylated polystyrene has a large adsorption capacity.
1.2 Statement of the problem
Nitrate contamination in groundwater has become an ever increasing and serious environmental threat since 1970s (Jeong, Kim, & Park 2012). The excessive application of fertilizers in agriculture causes the infiltration of large quantities of this ion into underground and surface water (Zhou et al 2007). Nitrate, due to its high water solubility (Thomson 2001), is the most widespread groundwater contaminant in the world, imposing a serious threat to drinking water supplies and promoting eutrophication.
Elevated levels of nitrate in drinking water can cause many health problems such as gastric cancer, which results from the reduction of nitrate to nitrosamines in the stomach (Zheng & Wang 2010). In addition, methemoglobinemia or blue baby syndrome, a serious health risk, occurs when nitrate is converted to nitrite, which then reacts with the haemoglobin to cause blueness of the skin of new born infants (Chatterjee & Woo 2009). After ingestion of plants or water high in NO3? acute poisoning may occur within 30 minutes to 4 hours in cattle. Thus, the problem occurs very quickly and often the cattle are observed to be normal one day and dead the next day (Romano & Zeng 2009).
Ward et al. (2005) reviewed the epidemiologic evidence for the linkages between drinking water NO3? and the risk of specific cancers, adverse reproductive outcomes, and other health outcomes in the context of the current regulatory limit for nitrate in drinking water. Nitrate contaminated water supplies have also been linked to outbreaks of infectious diseases in humans (Lin 1996). Literature survey reveals that NO3? ion also causes diabetes and is a precursor of carcinogen (Kostraba 1992), (Wolfe & Patz 2002).
Conventional method have several disadvantages such as only limited to certain concentrations, generation of large amount of toxic sludge and the capital costs are much too high to be economical. Adsorption based on the interaction between sorbent and adsorbent, offers the advantage such as low operating cost, and high efficiency of removing low concentration ions from water. In this study amino alkylated polystyrene has been chosen to be the adsorbent for the removal of nitrate from water as it is a very cheap option and also reduces pollution by polystyrene.
1.3 Research questions
The following questions will be answered by the end of the research.
What are the adsorption properties of amino alkylated polystyrene?
What are the optimum conditions for the removal of nitrates from water using amino alkylated polystyrene?
Which suitable isotherm best fits the adsorption of nitrate ions onto amino alkylated polystyrene?
The aims of the project were;
To assess the performance of amino alkylated polystyrene in the adsorption of nitrate ions from water.
To determine the adsorption properties of amino alkylated polystyrene.
The objectives of the study were
To prepare amino alkylated polystyrene
To characterise nitrated polystyrene, aminated polystyrene and amino alkylated polystyrene.
To identify the optimum conditions in the removal nitrate ions from water using amino alkylated polystyrene
To determine the isotherm that best fits the adsorption of nitrate ions.
To identify the adsorption properties of amino alkylated polystyrene
The limitations of the study were failure to characterize the fuctionalized polystyrene by SEM, BET, XRD and elemental analysis.
CHAPTER 2: LITERATURE REVIEW
This section outlines the information on research that has been carried out with respect to the research problem at hand. The analytical methods of analysis are also reviewed.
All living organisms require the nutrient nitrogen for the growth and metabolism. Nitrogen is a component of nucleic acids and other cell components. Nitrates are essential nutrients for plants protein synthesis and play a critical role in nitrogen cycle. The findings from (Wang & Li 2004) show that nitrogen mainly contributes in protein and chlorophyll formation.
Nitrate is a well-known contaminant of ground and stream water. It is an important environmental and human health analyte, and thus its detection and quantification are considered to be essential. The heavy utilization of artificial fertilizers and the uncontrolled discharges of raw sewage have been known to cause the penetration of large nitrate quantities into the ground and surface waters (Shrimali & Singh, 2001).
The most important environmental problems caused by nitrogen compounds are eutrophication of water supplies and infectious disease (Guo et al. 2001), (Chiban et al. 2012).
In order to protect public health from the adverse effects of high nitrate intake, World Health Organisation (WHO) set the standard as 50 mg/L to regulate the nitrate concentration in drinking water (WHO 2001).
Polymeric adsorbents have attracted increasing attention over as an alternative to activated carbon in industrial effluent treatment mainly due to their favourable physicochemical stability, large adsorption capacity, good selectivity and structural diversity (Zhang et al. 2007). To obtain large adsorption capacity and better selectivity for a specific anion, chemical modification of ordinary polymeric resins is adopted by introducing functional groups onto the matrix of the resin (Huang, Liu, Luo & Xu 2007). In particular introduction of amino and hydroxyl groups as hydrogen bonding acceptors or donors onto the matrices of the resin will develop a series of hydrogen bonding polymeric resins (Pan et al. 2003), (Ming et al. 2006).
2.1 Functionalization of polystyrene
Polystyrene is fuctionalized through various methods of nitration, amination and alkylation to produce polynitrostyrene, polyaminostyrene and amino alkylated polystyrene.
2.1.1 Nitration of polystyrene
There are several methods that can be used nitration of polystyrene and they include direct nitration with a concentrated nitric acid and sulphuric acid mixture, nitration in carbon tetrachloride with acetyl nitrate, nitration in N, N?-dimethyl formamide with a concentrated nitric acid and sulphuric acid mixture and nitration in 3-nitrotoluene with a concentrated nitric acid and sulphuric acid mixture. Philippides et al. (1993) reported that reported that the first three methods give products with low degrees of substitution and lead to a broadening of the molecular weight distribution, but that method last gives poly (4-nitrostyrene) with minimal effect on the breadth of the molecular weight distribution. Philippides et al. (1993) has reported the effect of nitrating medium on the nitration of polystyrene in which polystyrene is nitrated in anhydrous mixture of nitric acid and sulphuric acid. Degree of substitution varies from one to two nitro groups per benzene ring and increases with increasing time, temperature and concentration of nitric acid in the nitrating medium. The effect of polar and nonpolar solvents on nitration are examined by nitrating the polymer in fuming nitric acid or with a mixture of nitric acid and sulphuric acid in presence of dimethyl formamide and carbon tetrachloride.
Dimethyl formamide increases the reaction rate with fuming nitric acid but decreases the nitration rate in nitrating mixtures. Carbon tetrachloride decreases the nitration rate in fuming nitric acid but increases the nitration rate in nitrating mixtures. The results are explained in terms of mechanism of formation Of NO2+ in various nitrating media. Degradation of polymer is less in the presence of organic solvents than in concentrated acids.
Shyaa (2012) indicates that nitration involves formation of a very strong electrophile, the nitronium ion which is linear. This occurs following the interaction of two very strong acids, sulphuric acid and nitric acid. Sulphuric acid is strong and it protonates the nitric acid on the OH group such that a water molecule can leave. Benzene attacks the positively charged atom of the electrophile, where one of the N=O is broken at the same time. This is followed by rapid loss of a proton to generate the aromaticity. The equation 2.1 shows generation of the electrophile NO2+.
HNO3 + H2SO4 ? NO2+ + OSO3H- + H20 equation 2.1
Figure 2.1 Nitration of polystyrene
Concentrated sulphuric acid acts a catalyst. Nitration of polystyrene results in monomer disubstituted product depending on the conditions of the nitration experiment, but the nitration in ortho-position is slow and does not occur in this reaction due to steric hindrance.
2.1.2 Amination of nitrated polystyrene
Several methods different methods for the direct amination of nitrobenzene have been reported (Stern & Cheng 1993). The most useful of these with respect to synthetic utility is vicarious nucleophilic substitution for hydrogen (VNS). This class of reaction has been shown to be useful for the introduction of carbon, oxygen and amine nucleophiles into nitro arenes but demands the positioning of a good leaving group ? to the nucleophilic center which is eliminated during the decomposition of the proposed ? complex intermediate. These methods generate reasonable yields of nitroaninilines.
By contrast a more direct and automatically efficient route for the production of aromatic amines would be via the direct displacement of hydrogen from nitrobenzene using amides as nucleophiles. Aminolysis of the resulting aromatic amide would produce the corresponding nitroaniline.
Amination of polynitrostryrene can also be carried out by reductive methods of the nitro group (Abadie et al. 2006). Metals such as tin and iron are used to reduce the nitro groups. Polyaminostyrene are synthetically important compounds that acts as precursors to the synthesis of many interesting molecule and can be readily synthesized from polynitrostyrene compound via reduction methods. Tin powder in concentrated HCl in ethanol gives a yield of 67% .This process has been considered as effective method for the synthesis of polyaminostyrene. However notable disadvantages to these methods include high reaction temperatures and relatively long reaction times.
Figure 2.2 Amination of nitrated polystyrene
2.1.3 Alkylation of aminated polystyrene
One of the most frequently used procedures for the preparation of tertiary amines is the N-alkylation of primary and secondary amines with alkyl halides in the presence of a base such as KOH potassium, sodium amide, CsOH, (Salvatore 2002) thallium(I) ethoxide, CsF / Celite (Hayat 2001) and Hünig’s base (Moore 2005). Other methods for N-alkylation include the displacement of methanesulfonates, p-toluenesulfonates or p-nitrobenzene sulphonates by amines on solid supports (Olsen 2003).
Tertiary amines on solid supports have also been synthesised by a variety of other protocols (Lober 2004). Some other methods, such as a Mannich-type reaction (Tremblay-
Morin 2004) reductive and catalytic amination, (Sajiki et al. 2004) metal initiated amination of alkenes, alkynes and aryl halides, (Okano et al. 2003) deamination of quaternary hydrazinium halides and reduction of N-tosylamidines, have been devised for alkylation.
Unsymmetrical tertiary amines have been obtained in a single step through the CuCl/B(OMe)3 catalysed reaction of primary amines, alkyl halides and ?-chlorine-substituted allylsilanes. Synthesis of tertiary amines using a palladium-catalysed nucleophilic substitution of benzylic esters and secondary amines has been reported (Kuwano et al. 2003).
Figure 2.3 Alkylation of aminated polystyrene
2.2 Application of functionalized polystyrene
Functionalized polystyrene has many applications in the field of Chemistry. Some of the most significant applications include use as a stationary phase in chromatography and as an adsorbent for removal of organic and inorganic compounds in aqueous media.
2.2.1 Application as stationary phase in chromatography
The majority of stationary phases currently used for separation of ionic compounds are based on organic polymers and silica gel. In contrast to stationary phases prepared on silica gel, organic polymers show higher stability towards extreme pH conditions. The silica-based anion exchangers (Matsushita, Tada, Baba ; Osako 1983), (Vydac 1991) can be operated only in pH range between 2.0–9.5 while polymeric ion exchangers are stable across the entire pH range. Thus, styrene/divinylbenzene (PS/DVB) copolymers (Weiss ; Jensen 2003), (Gawdzik, Matynia ; Osypiuk 1998) polyvinyl and polymethacrylate resin are the most important organic polymers used as materials in the manufacturing process for polymer-based anion exchangers.
Bocian, Kosobucki ; Gawdzik (2011) described the synthesis and properties of the multilayered stationary phases, which contain quaternary amine functional groups for the analysis of anions by ion chromatography. They worked on the separation of an inorganic anions sample (F–, Cl?, NO2? Br?, NO3?, additionally HPO42? and SO42?).
2.2.2 Application as an adsorbent for removal of organic and inorganic compounds from aqueous media
Zhang et al. (2008) did a comparative investigation for uptake of dissolved organic matter (DOM). They did their investigation on refractory dissolved organic matter (DOM) from land?ll leachate treatment plant with high dissolved organic carbon (DOC) content. An aminated polymeric adsorbent NDA-8 with tertiary amino groups was synthesized, which exhibited high adsorption capacity to the DOM (raw water after coagulation). In their findings resin NDA-8 performed better in the uptake of the DOM than resin DAX-8 and A100 which are commercial polystyrene resins.
Electrostatic attraction was considered as the decisive interaction between the adsorbent and adsorbate. Special attention was paid to the correlation between porous structure and adsorption capacity. The mesopore of NDA-8 played a crucial role during uptake of the DOM. In general, resin in chloride form performed a higher removal rate of DOC. According to the column adsorption test, total adsorption capacity of NDA-8 was calculated to 52.28 mg DOC/mL wet resin. 0.2 mol/L sodium hydroxide solution could regenerate the adsorbent e?ciently.
Zhang et al. (2009) worked on removal of aromatic sulphonates from aqueous solution by aminated polymeric sorbents. They worked on sorption of aromatic sulphonates onto two aminated polystyrene sorbents with different pore structures M-101 and D-301 was investigated for optimization of their potential in application in chemical waste water treatment.
Sodium benzene BS, sodium 2 naphthalene sulphonate 2- NS and disodium 2,6 naphthalene disulphonate 2,6 NDS were selected as reference solutes and sodium disulphate was used as a competitive inorganic salt. Sorption selectivity of both sorbents was dependent upon the concentration levels of aromatic sulphonates in solution coexisting with sodium sulphate at high levels. Their findings showed that both sorbents exhibited different characters. D-301 presented more favourable sorption for the solutes at relatively high levels (higher than 5 mM 0.7 mM for 2-NS and 0.05 mM for 2.6 NS, while M-101emoved aromatic sulfonates more completely when the solute concentration was kept at relatively low levels.
2.3 Characterization of functionalized polystyrene adsorbent.
The functionalized polystyrene is characterized using various techniques which include FTIR, XRD, BET and SEM.
FTIR is one of the most widely used tools for the detection of functional groups in pure compounds and mixtures and for compound comparison. Infrared study is related to the vibrational motion of atoms or molecules.
This study is mainly used for structure elucidation in organic and inorganic compounds. These compounds absorb electromagnetic energy in the infrared region of the spectrum. IR radiation does not have sufficient energy to cause the excitation of electrons. However, it causes atoms or group of atoms to vibrate faster about the bonds, which connect them. The compounds absorb energy from a particular region since the vibrations are quantized. The position of a particular absorption band is specified by a particular wave number (Tourintio et al, 1998).
Shyaa (2012) worked on synthesis, characterization and thermal study of polyimides derived from polystyrene. Infrared (FTIR) spectra and thermo gravimetric analysis (TGA) were used to characterize polymers. The chemical structure of poly(4nitro styrene) was analysed by FTIR analysis which confirmed the nitration of polystyrene.
There finding were as the vibration band at 3107 cm-1 attributed for aromatic C-H stretching, the band at 2854 cm-1 for aliphatic C-H stretching, 1597, 1518 cm-1 for asymmetric (ArNO2) N=O stretching, 1390 cm-1 for symmetric stretching N=O, 1329 cm-1 for C-N stretching. The polyaminostyrene was analysed by FT-IR, there were two bands, the asymmetrical N- H stretch and symmetrical N-H stretch, located at 3323, 3215 cm-1. The N-H bending vibration for primary amines was observed in the region 1618, 1583 cm-1. The C-N stretching vibration for aromatic amines was observed in the region 1315 -1273 cm-1.
Sun et al. (2014) determined the microstructure and mechanical properties of aminated polystyrene spheres / epoxy polymer blends. In their research polystyrene spheres were chosen as soft fillers to toughen epoxy polymer. In order to weaken the aggregation of polystyrene spheres in epoxy matrix caused by phase separation, amination treatment was firstly done on them. They reported that FTIR was employed to distinguish the difference between native polystyrene spheres and aminated polystyrene spheres.
The FTIR test displayed a spectral profile characterized by the presence of specific bands related to polystyrene. Their findings were, at 3024cm-1 (-CH aromatic) and 2847 cm-1 (-CH2), 1600 cm-1 (-C-C) aromatic, 1492-1451 cm-1 (-C6H5 in plane), 1200-1100 cm-1 (=C-H aromatic, out of plane) 900-600cm-1 (=C-H aromatic in plane). Compared with the FTIR spectra of polystyrene sphere, new peaks at 3420 cm-1, 3323 cm-1, 3215 cm-1 and 1613 cm-1 were observed on that of aminated polystyrene spheres. There were typical peaks of N-H stretching and bending modes in primary amines (Covolan et al. 2000). The disappeared peak 1830 cm –1 indicated that the amine substitution reaction occurs on the para orienting of the polystyrene.
Figure 2.4 Comparison of FTIR spectrum for aminated polystyrene and polystyrene (Covolan et al. 2000)
X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analysed material is finely ground, homogenized, and average bulk composition is determined. X-ray powder diffraction is most widely used for the identification of unknown crystalline materials. Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology. Other applications include, characterization of crystalline materials, identification of fine grained minerals such as clays and mixed layer clays that are difficult to determine and optically determination of unit cell dimensions measurement of sample purity (Khondker1 ; Lakhani 2015).
XRD technique offers the following advantages its powerful and rapid (; 20 minutes) technique for identification of an unknown mineral, it provides an unambiguous mineral determination, minimal sample preparation is required, and data interpretation is relatively straight forward
Limitations of the technique include the following there must be access to a standard reference file of inorganic compounds (d-spacings, hkls) requires tenths of a gram of material which must be ground into a powder, for mixed materials, detection limit is approximately 2% of sample, for unit cell determinations, indexing of patterns for non-isometric crystal systems is complicated, peak overlay may occur and worsens for high angle reflections and homogeneous and single phase material is best for identification of an unknown (Khondker1 ; Lakhani 2015).
BET analysis provides precise specific surface area evaluation of materials by nitrogen multilayer adsorption measured as a function of relative pressure using a fully automated analyser. The technique encompasses external area and pore area evaluations to determine the total specific surface area in m2/g yielding important information in studying the effects of surface porosity and particle size in many applications (Sun et al. 2014).
Scanning Electron Microscopy (SEM is used very effectively in microanalysis of solid inorganic materials. Scanning electron microscopy is performed at high magnifications, generates high-resolution images and precisely measures very small features and objects. The signals generated during SEM analysis produce a two-dimensional image and reveal information about the sample including external morphology (texture) and chemical composition (when used with EDS).The EDS component of the system is applied in conjunction with SEM analysis to determine elements in or on the surface of the sample for qualitative information to measure elemental composition for semi-quantitative results (Li et al. 2004).
2.4 Comparison of analytical methods for nitrate determination.
There are several methods that can be used to determine the concentration of nitrate. The methods include spectroscopic methods, HPLC and IC methods and Ion Selective Electrodes (ISE) and electrophoresis methods.
2.4.1 Spectroscopic methods
Most of the recent work concerning nitrate determination has embraced the classical reagents. Several reported spectrophotometric methods involve the use of common reactions, such as a reduction reaction followed by diazotization (Horita 1997), (Wang et al. 1998) and nitration reactions.
The well-known spectrophotometric methods for the determination of nitrate are based on the nitration of phenolic compounds, chromophoric acids, 2,4-xylenol 2,6-xylenol ,4-xylenol, phenoldisulfonic acid, brucine and phenol 4-aminoazobenzene (Dayananda ; Revanasiddappa 2007). Some sensitive spectrophotometric methods for determine nitrate utilize extractable ion associates of the nitrate ion with basic dyes, like crystal violet and nile blue.
In comparison with other methods, the method is more sensitive and has a wide range of determination limit. However, a lack of high sensitivity for the detection of trace levels of the analytes could cause results to be unreliable due to sample matrix interferences. Badea et al. (2001) strongly argues that the method is time consuming and require sample pre-treatment thus cannot be applied for rapid tests.
2.4.2 Ion selective methods
The use of Ion Selective Electrodes in environmental analysis offer several advantages over other methods of analysis. The cost of initial setup to make analysis is relatively low. The instrumentation is simple to operate and sample preparation is not complex. The expense is considerably less than other methods, such as Atomic Adsorption Spectrophotometry or Ion Chromatography. ISE determinations are not subject to interferences such as colour in the sample. The ion selective methods are generally fast and can be used for rapid tests. The nitrate electrode has an adequate value of accuracy Perres et al. (2013). The ion selective methods can be used for analysis of nitrate content in water, blood, soil, food, samples, vegetables, and fertilizers.
The ion selective electrodes are subjected to sources of error. The background matrix can affect the accuracy of measurements taken using ISE’s.
Frant (1994) points out that some interferences may be eliminated by reacting the interfering ions prior to analysis. The nitrate electrode responds to numerous interfering anions. Most of the interferants, however, can be rendered harmless by adding suitable reagents. Cyanide, bisulfide, bicarbonate, carbonate, and phosphate are removed by adjusting the solution to pH 4 with boric acid. Chloride, bromide, and iodide are removed by adding silver sulphate solution. Nitrite is also an interferent and can be removed by adding sulphamic acid. The amounts of silver sulfate and sulfamic acid required will vary based on the concentrations of interferants. As a general guide, 1 mL of silver sulfate will eliminate chloride interference in a 50-mL sample containing 35 mg/L of Cl-. 1 mL of sulfamic acid solution will eliminate nitrite interference in a 50 mL sample containing 95 mg/L of NO2-.
Orion Research (1997) points out that the differences in the rates of diffusion of ions based on size can lead to some error. In the example of Sodium Iodide, sodium diffuses across the junction at a given rate. Iodide moves much slower due to its larger size. This difference creates an additional potential resulting in error. To compensate for this type of error it is important that a positive flow of filling solution move through the junction and that the junction not become clogged or fouled.
Frant (1994) points out that the total ionic strength of a sample affects the activity coefficient and that it is important that this factor stay constant. In order accomplish this the addition of an ionic strength adjuster is used. This adjustment is large, compared to the ionic strength of the sample, such that variation between samples becomes small and the potential for error is reduced. It is important that temperature be controlled as variation in this parameter can lead to significant measurement errors. A single degree change in sample temperature can lead to measurement errors greater than 4%. Some samples may require conversion of the analyte to one form by adjusting the pH of the solution. Failure to adjust the pH in these instances can lead to significant measurement errors.
2.4.3 Chromatographic methods
Chou et al. (2003) reports that IC and HPLC methods are generally faster, more accurate and have higher sensitivity than the spectrophotometric methods. Vera at al., (2002) reports that high performance liquid chromatography has the following advantages short time of analysis, minimal sample pre-treatment, long life of the analytical column and stable eluent solution. The main disadvantage of is that the instruments are not always readily available. Rassar (2006) reported that the nitrate value for lettuce and spinach were 2684mg/kg and 1747mg/kg respectively.
2.4.4 Electrophoretic methods
Capillary electrophoresis is a fast and easy technique. Asopuru et al (1999) worked on determination of nitrate content and the recovery for spiked samples was 93-100%. Merusi (2010) also reports that it is cheap, can analyse a wide variety of samples and has a great recovery percentage, 94 %. Electrophoresis is a reliable method and prepare is easy. Chou et al. (2003) argues that it is fast for simultaneous detection of a wide variety of anions.
2.5 Methods nitrate removal from aqueous media
Several purification techniques based on the physicochemical and biological processes are used to remove or reduce the amount of toxic pollutants found in water and wastewater. These include methods of ion exchange, biological treatment, membrane separation and adsorption (Rijn et al. 2006), (Bhatnagar et al. 2011), (Chiban et al. 2011). Among these methods the adsorption appears the most appropriate (Faust et al. 1987).
Activated carbon is generally considered a universal adsorbent for the removal of various aquatic pollutants especially organic pollutants. However, it shows less affinity for anionic pollutants giving low adsorption capacity (Bhatnagar et al. 2011). In addition, its generalized use in the treatment of wastewater is sometimes limited due to its high costs (Chiban et al. 2012), (Wang et al. 2007), which requires significant technological and financial investment.
The search for new materials that would be good adsorbents, easy to prepare and whose prices are low is one of the most promising ways. Therefore, attention has been paid to the development of attractive sorbents for removal of nitrate pollutants from waste water. Different polystyrene based materials such as benzoyl-PS-DVD, 2,4-dicarboxybenzoyl-PS-DVB, acetyl-PS-DVB, toluene-PS-DVD, diethylenetriamine-PS-DVB, sulfonated PS-DVB, have been reported for the removal of nitrates compounds (Alexandratos & Natesan 1999). Aminoalkylated polystyrene is a very attractive option, because it is cheap, widely available and has good mechanical stability for handling purposes.
2.6 Adsorption studies
Recently, the adsorption process has gained interest as a more promising method for the long term as it is seen to be a more effective and economic approach for organic and inorganic wastes. Adsorption is a fundamental process today due to its flexibility in design and simple operation instead of having to perform adsorptions that are perceived as impractical by most conventional techniques. The term “adsorption” refers as a mass transfer process by which a substance is transferred from the liquid phase to the surface of a solid and becomes bound by physical and/or chemical the interactions (Babel & Kurniawan 2003).
The advantages of the adsorption process in removing or minimizing ions rate even at low concentration enhance the application of adsorption as one practical treatment.
2.6.1 Factors that affect adsorption
The effectiveness of the adsorption process is mainly influenced by the nature of solution in which the contaminants are dispersed, the molecular size, the polarity of the contaminant and also the type of adsorbent used. Adsorption also exists due to the attractive interactions between a surface and the species adsorbed at certain molecular level (Monser & Adhoum 2002)
Adsorption can be categorized into two; physical adsorption and chemisorption. Physical adsorption is a phenomenon resulting from intermolecular forces of attraction between molecules of the adsorbent and adsorbate. Meanwhile, chemisorption is a result of the chemical interaction between the solid and the adsorbed substance. It is an irreversible phenomenon and is also called activated adsorption (Yadla, Sridevei & Chandana, 2012)
In terms of the temperature high physical adsorption occurs at a temperature as close to the critical temperature of a given gas while chemical adsorption occurs at temperatures as close to the critical temperature. Under certain conditions, both processes can occur simultaneously or alternately (Dabrowsky 2001)
Various factors influence the adsorption capacity of potential adsorbents during the adsorption process. Previous researches have assumed that the efficiency of any adsorbent is strongly influenced by the physicochemical characteristics of the solutions such as pH, temperature, initial concentration, contact time and also adsorbent dose (Cheremisinoff & Morresi 1978)
A large portion of adsorption studies has been compiled to investigate the relationship of these parameters. pH is the most important factors than others as pH controls the distribution of charge on the adsorbent surface between the adsorbate ion. However, in most sorption studies point of zero charge (pHzpc) should be considered to be compared with pH as (pHzph) determines the limitations of the adsorbent’s pH. pHzph is the charge at the solid surface of adsorbent determined by protonation and deprotonation of adsorbate ions. The surface charge density of surface depends on the ions which react directly with the adsorbent surface. For instance, when the pH value of solution is higher than pHzph, the surface charge of the adsorbent will be negatively charged and vice versa (Haghseresht, Nouri ; Lu 2003) otherwise, the increase of pH within a certain limit can increase the adsorption rate (Kumar ; Bandyopadhyay 2006) but further increase in pH can decrease the adsorption rate as ions in a particular range tends to be unaffected by pH (Lata, Garg ; Gupta 2007)
Depending on the types of adsorbent used, temperature can affect the adsorption capacity of adsorbent. Temperature can change the adsorption equilibrium depending on the exorthermic or endothermic nature of a process. Enthalpy, entropy and gibbs free energy are such parameters that need to be determined before the spontaneity of the process can be inferred (Auta ; Hameed, 2011)
Contact time can affect adsorption capacity. The adsorption depends on the interaction of functional groups between solution and surface of adsorbent. Adsorption can be assumed to be complete when equilibrium is achieved between the solute of the solution and the adsorbent. However specific time is required is needed to maintain the equilibrium interactions to ensure that the adsorption process is complete (Kumar ; Kirthika, 2009).
The adsorbent dose is another factor which influences the adsorption process. Based on assumption, when the adsorbent doses increase, the rate also increases. However, the adsorption rate can decrease if the adsorbent due to the presence of more occupied active sites when the concentration gradient of the adsorbate is kept constant (Ahmaruzzaman ; Sharma 2005).
Initial concentration of adsorbate can alter the adsorbate removal efficiency through a combination of factors such as the availability of specific surface functional groups and the ability of functional groups to bind the adsorbate. The initial concentration influences the adsorption rate based on the availability of the specific surface functional groups and the ability of the surface functional groups to bind ions (especially at high concentrations) concentration of solution can provide an important driving force to overcome the mass transfer resistance of adsorbate between the aqueous and solid phases (Arief, Trilestari, Sunarso, Indraswati ; Ismadji 2008)
2.6.2 Adsorption Isotherm
Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among solid and liquid phases, and are important data to understand the mechanism of the adsorption. Sorption isotherm is used describe the mechanism of how ardsobate ions interact on the surface of adsorbent. There are several isotherm equations available to analyse the experimental sorption equilibrium parameters, but the well known adsorption isotherm models used for single solute systems are Langmuir (Langmuir 1918) and Freundlich isotherms (Freundlich 1906).
Both isotherm models are found to be more suitable to describe the relationship between qc (quantity adsorbed at equilibrium, mg/g) and Cc (concentration of adsorbate ions in adsorbates remained in the bulky solution at the equilibrium, mg/L).
Langmuir isotherm model is an empirical model assuming that adsorption can only occur at a finite number of definite localized sites, and the adsorbed layer is one molecule in thickness or monolayer adsorption (Langmuir 1918). This isotherm was based on different assumptions one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. It is based on four assumptions: the surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent, adsorbed molecules do not interact, all adsorption occurs through the same mechanism, at the maximum adsorption, only a monolayer is formed and molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent (Febrianto et al. 2009)
The following linearised equation represents the Langumuir isotherm
1q?= 1qmaxKLCe+1qmax (Chen et al. 2008).Where qe is the amount of nitrate adsorbed on adsorbent (mg/g), Ce is the residual nitrate concentration at equilibrium in the solution (mg/l), qmax is the maximum adsorption capacity of the adsorbent (mg/g), KL is the Langmuir adsorption constant (l/mg)
This isotherm is suitable for representing chemisorptions on a set of distinct localized adsorption sites. A high KL value indicates a high affinity for the binding of nitrate ions. A plot of 1q? againist 1 C? gives the constant KL and qe from the slope and the intercept respectively.
Freundlich isotherm model interprets the adsorption on heterogeneous surfaces with interactions occurring between the adsorbed molecules and is not restricted to the formation of a monolayer. This isotherm is commonly used to describe the adsorption of organic and inorganic compounds on a wide variety of adsorbents (Febrianto et al. 2009). The isotherm is summarized by the equation.
Log (qe) = log (Kf)+ 1n log (Ce) (Febrianto et al. 2009)
Where, n is an affinity constant, Kf is the adsorption capacity,
A plot of log (qe) againist log (Ce) gives the constants 1n and Kf from the slope and intercept respectively. The Freundlich is a more widely used isotherm but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir isotherm. The Freundlich is more suitable for a highly heterogeneous surface. The isotherm suggests that sorption energy exponentially decreases on completion of the sorption centers on the adsorbent (Chen et al., 2008). Langmuir and Freundlich isotherm models are frequently used for describing the short term mono component adsorption of ions by different materials (Aksu ; Kabasakal 2004), (Liu 2006).
Temkin isotherm is represented by the following equation
Qe = B??Kt + B??C (Lawal et al. 2010)
Where Kt is the Temkin isotherm constant
A plot of qe against ??C gives the constant B and Kt from the slope and the intercept respectively. The isotherm take into account adsorbent-adsorbate interactions. The heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions. Adsorption is characterized with a uniform distribution of energies up to a maximum binding energy.
CHAPTER 3: METHODOLOGY
This chapter addresses chemical reagents and equipment used, sampling and sample preparation, functionalization of polystyrene to give nitropolystyrene, aminated polystyrene, and amino alkylatedpolystyrene, preparation of stock and buffer solution, adsorption studies and experiments, characterization of functionalized polystyrene by FTIR.
3.1 Chemicals and chemical reagents
Chemicals used in the various experiments were of analytical reagent grade and were used without further purification. All the chemicals were purchased from Skylabs. Concentrated sulphuric acid (98 %) and nitric acid (55 %) were used for nitration of polystyrene. Chloroform and methanol were used for the purification of polystyrene. 10 % Sodium hydrogen carbonate was used for removal of excess acid in nitrated polystyrene. The 10 % NaHCO3 was made by weighing 10 g of NaHCO3, dissolving it in water and diluting it to the mark with distilled water in a 100 ml volumetric flask. Hydrochloric acid (37%), absolute ethanol and granular tin was used for reduction of nitrated polystyrene. 10% NaOH made by weighing 10 g of sodium hydroxide, dissolving it in water and diluting to the mark with distilled water in a 100 ml volumetric flask. Sodium hydrogen carbonate, dichloromethane, sodium dodecyl sulphate, ethylacetate and hexane were used for alkylation of aminated polystyrene. Potassium nitrate was used to prepare the nitrate stock solution. Potassium sulphate, silver sulphate and sulphuric acid was used for preparation of the Ionic Strength Adjustor (ISA) buffer.
A Thermo Fischer Scientific Nicolet iS5 MIR FTIR spectrophotometer equipped with an ATR module and omnic software was used to determine functionalization of the adsorbent. A nitrate ion selective electrode (Hannah instruments) was used to determine the concentration of nitrate ions before and after adsorption. An analytical balance (aE Adam PW 254) was used to weigh samples. An oven (Heraeus D6450 Hanau) was used to dry samples. A Stuart Scientific laboratory shaker, model SF1 was used for adsorption studies.
3.3 Sampling and sample preparation
Polystyrene waste was collected from a food outlet. The waste was washed with water and dried in an oven. The dried polystyrene was ground into a fine powder. The ground polystyrene was purified by dissolving in chloroform then precipitated into methanol, filtered and dried. The purified polystyrene was ready for functionalization.
3.4 Functionalization of polystyrene
The purified polystyrene was nitrated using concentrated sulphuric acid and nitric acid. The nitrated polystyrene was reduced using concentrated hydrochloric acid and tin. The aminated polystyrene was finally alkylated with dichloromethane. The functionalization is shown in the reaction scheme in figure 3.1.
Step 1 Nitration
Step 2 Amination
Step 3 Alkylation
Figure 3.1 Reaction scheme showing functionalization of polystyrene
3.4.1 Preparation of nitrated polystyrene
The procedure was adopted from (Shyaa, 2012) with minor adjustments. 10 g of purified polystyrene powdered was added slowly in a portion wise to a nitrating mixture of concentrated 10 ml HNO3 and 25 ml of H2SO4 in a 250 ml of 3 neck round bottom flask. The initial temperature during addition of polystyrene was kept below 5 °C. The mixture was vigorously stirred by means of a magnetic stirrer, and the time of addition was limited 10-15 minutes. The mixture was stirred under constant speed at 60 °C for 1 hour. The polymer turned orange, when the mixture became homogeneous. At the end of the reaction, a yellow coloured viscous liquid was obtained, it was poured over ice cold water and stirred, after sometime a yellow coloured solid precipitate was obtained which was thoroughly washed several times with warm water and 10 % NaHCO3 till it was free from acid. The sample was then dried in vacuum oven at 60 °C for 12 hours.
3.4.2 Preparation of aminated polystyrene
The method was adopted from (Abadie et al. 2006) with minor adjustments. 10 g of polynitrostyrene was added in 250 ml three neck round flask, and 60 ml of 37 % HCl, and 50 ml of absolute ethanol, then 40 g of granular tin (Sn) was added and the mixture refluxed for 9 hours. The solid polystyrene amine hydrochloride was collected and washed with water, then the polystyrene amine hydrochloride was heated with 100 ml of 10 % sodium hydroxide. The resulting solid was collected and washed with water.
3.4.3 Alkylation of aminated polystyrene
The method for alkylation of aminated polystyrene was adopted from (Chingakham et al. 2007). 10 g aminopolystyrene, 20g sodium hydrogen carbonate, and 6g sodium dodecyl sulphate were taken up in water (100 mL) and heated at 80 °C for 5 minutes. Then dichloromethane 35ml was added to the reaction mixture and heated for a period of 1 h. The reaction mixture was cooled and the alkylated product filtered and dried. The crude reaction mixture was recrystallized from a mixture of ethyl acetate and hexane to yield the product.
3.5 Adsorption studies
Adsorption of nitrate on AAP was conducted in batch experiments. The stock solution of nitrate was prepared by dissolving KNO3 in deionized water. Adsorption was optimised in terms of pH, contact time, initial concentration, adsorbent dosage and temperature. The pH value was adjusted using 0.1 M sodium hydroxide and 0.1M HCl. The method of (Zheng ;Wang 2010) was adopted in batch experiments. In all batch adsorption AAP was added onto nitrate solution at a known concentration, temperature and pH value. After a desired period of adsorption the mixture was filtered and the concentration of nitrate was measured using nitrate ion selective electrode.
Adsorption efficiency and adsorption capacity was calculated as shown in equation 3.5.1 and 3.5.2 respectively.
The concentration of nitrate removed (Cads) from aqueous solutions was calculated by difference of initial concentration, C0, and the concentration at time t Ct
(Cads = C0 – Ct).
The removal percentage of nitrate from aqueous solutions Pr (%) on adsorbent is given by equation:
% removal = C?-CtC? ×100 Equation 3.1
The amount of nitrate adsorbed on adsorbent at instant t (Qt (mg/g)) is calculated by the following equation:
Qt = C?-CtVm Equation 3.2
Where, C0 is the initial concentration of the nitrate in the feed solution (mg/L), Ct is the concentration of nitrate in solution at a given time t (mg/L), V is the total volume of the feed solution (ml), m is the weight of the adsorbent (g).
At adsorption equilibrium, nitrate concentration in the feed Ct becomes the equilibrium concentration Ce and adsorbed amount Qt becomes Qmax.
3.5.1 Preparation of stock solution
The method was adopted from Perez et al. (2013). A stock solution of 1000 ppm was prepared by dissolving 1.63 g Potassium nitrate in deionised water in a 1L volumetric flask and making up to the mark. A 10 ml aliquot of the standard solution was pipetted into a 100 ml volumetric flask and topped to the mark with deionised water to give a 100 ppm solution. A 10 ppm solution was prepared by pipetting 10.0 ml of the 100 ppm solution into a 100.0 ml volumetric flask and topped with deionised water. An aliquot of 10.0 ml of the 10 ppm solution was pipetted into a 100.0 ml volumetric flask to obtain the 1 ppm solution and topped to the mark. The potential in mV for the standard solutions was determined using the nitrate ion electrode. The potential for the standard solution in mV was plotted against log concentration to obtain a calibration curve.
3.5.2 Preparation of ISA Buffer
The method was adopted from Arlo et al. (2015) who are in agreement with Parrab et al. (2013).
An Ionic Strength Adjustor (ISA) was be prepared by dissolving 10.5 g Potassium Sulphate, 3.1g Silver Sulphate and 25 ml of 0.1 M Sulphuric acid in 1.0 L of deionised water. Its purpose was to maintain constant ionic strength of the solution. The ISAB removes interferences due to chloride, bicarbonate, nitrite, sulphide, bromide and cyanide ions.
3.5.3 Determination of nitrates concentration
A volume of 50 ml of ISA buffer was added to each 50ml sample/standard. The nitrate ISE tip was dipped into each solution. The potential was measured and recorded after the reading has stabilized. The pH range was maintained at between 2 – 11 and the temperature was at 25 °C throughout the analysis period. The electrode was rinsed by a jet of deionised water and blotted dry with tissue paper before making a new reading.
3.5.4 Effect of initial pH
Six portions of AAP (1 g) each was added into six 250 ml conical flasks containing 100 ml of 100 ppm nitrate solution with initial pH value of 2, 4, 6, 8, 10, and 12 respectively then the flasks were put into a shaker at rotary rate of 180 rpm. After adsorption for 3 hours, the samples were filtrated to separate the adsorbent, and the supernatant was chosen for determining the concentration of nitrate using nitrate selective electrode. All experiments had three replicates.
3.5.5Effect of contact time
Nitrate solution (100 ml) with an initial concentration of 100 ppm was added to a 250 ml conical flask. The shaker was set at the speed of 180 rpm. 1 g of dried AAP equilibrated to pH of 2.0 was added to the flask at 25 °C. The concentration of nitrate in the solution at different times 30 minutes, 1 hour, 1hour 30 minutes, 2 hours, 2 hours 30 minutes, 3 hours and 3 hours 30 minutes, 4 hours and 4 hours 30 minutes was measured using nitrate selective electrode and the adsorbed amount was calculated. All experiments had three replicates.
3.5.6 Effect of temperature
The adsorption isotherms were measured at 20, 25, 30, 35, and 40 °C respectively. For each temperature 1 g of AAP was weighed into 250ml conical flask containing 100 ml of 100 ppm nitrate solution. The flasks were placed in a shaker at present temperature. After adsorption for 3 hour the samples were filtered to separate the adsorbent, and the supernatant was used for determining the concentration of nitrate using nitrate ion selective electrode. All experiments had three replicates.
3.5.7 Effect of dosage of amino alkylated polystyrene
A series of varied amount of AAP 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 g were added to conical flasks containing 100 ml of 100 ppm nitrate solution. After adsorption for 3 hours the samples were filtered to separate the adsorbent, and the supernatant was used for determining the concentration of nitrate using nitrate ion selective electrode. All experiments had three replicates.
3.5.8 Effect of dosage aminated polystyrene
A series of varied amount of aminated polystyrene 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 g were added to conical flasks containing 100 ml of 100 ppm nitrate solution. After adsorption for 3 hours the samples were filtered to separate the adsorbent, and the supernatant was used for determining the concentration of nitrate using nitrate ion selective electrode. All experiments had three replicates.
3.5.9 Effect of initial concentration
A series of varied concentration of nitrate 10, 20, 40, 50, 60, 80, and100 ppm (100 ml each)were added to six conical flasks containing 0.1 g nitrate solution. The samples were shaken on a shaker with a speed of 180 rpm for 3 hours under room temperature. The samples were filtered to separate the adsorbent, and the supernatant was used for determining the concentration of nitrate using nitrate ion selective electrode. All experiments had three replicates.
CHAPTER 4: RESULTS AND DISCUSSION
This chapter looks at the results gathered in this research. Use is made of graphs for purpose of direct comparison between the measured characteristics.
4.1 Adsorption studies
This section will address the results on factors that affect adsorption of nitrate ions on AAP and AP which are adsorbent dose, initial concentration, temperature, contact time and pH. The experimental data was analyzed using Freundlich, Langmuir and Temkin isotherm models to determine the parameters related to the adsorption process.
4.2 Results on factors that affect adsorption of nitrate ions onto AAP and AP
Figure 4.1 Calibration curve for nitrate standards
4.2.1 Effect of AAP dosage on adsorption of nitrate ions
For studying the effect of mass of adsorbent on nitrate adsorption onto AAP, experiments were performed with varying amounts of adsorbent, ranging from 0.1 to 2 g (figure 4.2). It is evident from (figure 4.2) that the removal of nitrate increased from 83 % (0.1 g) to 96 % (1 g). This could be as a result of more adsorption sites. This study revealed that the adsorption capacity of AAP increased with increase in adsorbent dose up to a particular region and then reaches an equilibrium level at higher doses. However it is observed that after dosage of 1 g, there was no significant change in percentage removal of nitrate. This may be due to the overlapping of adsorption sites at higher dosage. There was a small difference in percentage removal of nitrate ions between between successive adsorbent dosage, this could be due to electrostatic interactions, interaction between binding sites and also availability of solute. 1 g in 100 ml of nitrate solution was considered as an optimum dosage. These findings were in line with those of (Chatterjee ; Woo 2009) who worked on the removal of nitrate from aqueous solutions by chitosan hydrogel beads.
Figure 4.2 Effect of dosage on the adsorption of NO3 – ions by AAP, (C0 = 100 mg/L, Temperature- 289 K, pH 2, agitation 180 rpm)
4.2.2 Effect of AP dosage on adsorption of nitrate ions
The experiments for effect of dosage on adsorption of nitrate ions with aminated polystyrene were performed with varying amounts of adsorbent, ranging from 0.1 to 2 g (Figure 4.3). The removal of nitrate increased from 75 % (0.1 g) to 95 % (1 g). This could be because of greater availability of adsorption sites. This revealed that the adsorption capacity of AAP increased with increase in adsorbent dose up to a particular region and then reaches an equilibrium level at higher doses. However it is observed that after dosage of 1 g, there was no significant change in percentage removal of nitrate. This may be due to the overlapping of adsorption sites at higher dosage.
The small difference in percentage removal of nitrate ions between successive adsorbent dosage can be attributed to electrostatic attractions, interactions between binding sites and also availability of solute. 1 g in 100ml of nitrate solution was considered as an optimum dosage. These findings were in line with those of (Chatterjee ; Woo 2009) who worked on the removal of nitrate from aqueous solutions by chitosan hydrogel beads.
Fig 4.3 Effect of dosage on the adsorption of NO3 – ions by AP, (C0 = 100 mg/L, Temperature – 289K, pH 2, agitation 180 rpm)
4.2.3 Effect of initial concentration on adsorption of nitrate ions
The initial concentration of nitrate solution was varied from 10 mg/l to 100mg/l with optimum adsorbent dose (1 g). The result obtained is presented in graphical form as percentage removal versus initial nitrate concentration (figure 4.4). It is evident from the graph that the percentage removal of nitrate decreased from 96 % to 81 % for initial nitrate concentration of 10 mg/l to 100 mg/l. It is due to the limitation of adsorption sites on the AAP adsorbent surface (Moussavi ; Khosravi 2011), (Vimonses, Lei, Jin, Chow, ; Saint 2009)).
The above phenomem can be explained as follows, with increase in the initial nitrate concentration the amount of adsorbate species in the solution increases, but the amount of adsorbent remains constant and hence the percentage removal decreases with increase in initial nitrate concentration. The higher uptake of nitrate at low concentration may be attributed to the availability of more adsorption sites sites on for lesser number of adsorbate species (Islam ; Patel 2011). The optimum initial concentration was considered to be 10 mg/L for 1g of AAP.
Figure 4.4 Effect of initial concentration on the adsorption of NO3 – ions by AAP, ( dosage 0.1g Temperature- 289K, pH 2, agitation 180 rpm)
4.2.4 Effect of pH on adsorption of nitrate ions.
The initial pH values were adjusted by adding 0.1 M of HCl or NaOH. It is evident from figure (4.5) that the percentage removal of nitrate decreased from 98 % to 89 % for increase in pH from 2 to 12. The variation of adsorption with pH can be explained by the electrostatic interaction between the adsorbent and adsorbate. With increase of pH values, the surfaces gradually decrease in the extent of positive charging and become negative. Thus, the high adsorption capacity at low pH is mainly due to the strong electrostatic interaction between the positively charged sites of adsorbent (presence of H+) and the anions. However, the lower sorption of the nitrates ions at alkaline pH could be attributed to the abundance of OH? ions which will compete with the pollutant for the same sorption sites (Elmoubarki, 2015). The optimum pH for adsorption of nitrate ions on AAP was considered to be 2.
lefttopFigure 4.5 Effect of pH on the adsorption of NO3 – ions by AAP, (C0 = 100 mg/L, Temperature- 289K, dosage 1 g, agitation 180 rpm)
4.2.5 Effect of contact time on adsorption of nitrate ions.
The removal of nitrate versus time is illustrated in Figure 4.6. The effect of contact time on the adsorption of NO3- ions was performed within time varying 30-270 minutes for initial concentration 100 mg/L. It was noted that the amount adsorbed of NO3- ions on AAP surface increased with contact time. This could be attributed to the fact that adsorption depends on the interaction of functional groups between solution and surface of adsorbent. More time means more interactions and the higher the percentage removal of the ions (Kumar ; Kirthika, 2009). This result was similar to the findings of (Bhatnagar ; Sillanpaa 2011). The optimum time for adsorption of nitrates on AAP was considered to be 180 minutes.
Figure 4.6 Effect of contact time on the adsorption of NO3 – ions by AAP, (C0 = 100 mg/L, Temperature- 289K, pH 2, agitation 180 rpm, dosage 1g )
4.2.6 Effect of temperature on adsorption of nitrate ions.
The study of the effect of temperature on the adsorption of NO3- ions was performed at an initial concentration of 100 mg/l and at different temperatures (from 20 to 45 °C). Figure 4.7 shows the percentage removal of nitrate ions in terms of temperature. The percentage removal of NO3- ions by AAP increased significantly from (20 % to 97 %) with temperature. This adsorption is endothermic. The optimum temperature at equilibrium contact time for nitrate ions adsorption on AAP was obtained as 40 ?. The increasing sorption capacity of the sorbent with temperature is due to the enlargement of pores and the activation of the sorbent surface (Han 2006).
Figure 4.7 Effect of temperature on the adsorption of NO3 – ions by AAP, (C0 = 100 mg/L, pH 2, agitation 180 rpm, dosage I g)
4.3 Results on adsorption equilibrium studies
4.3.1 Langmuir Isotherm
The Langmuir isotherm is based on the assumptions that adsorption takes place at specific homogeneous sites within the adsorbent, there is no significant interaction among adsorbed species, and the adsorbent is saturated after the formation of one layer of adsorbate on the surface of adsorbent (Hoda, Bayram ; Ayranci 2006). The Qm and KL values were calculated from linear plots of 1/Qe versus 1/Ce (Figure 4.8). The Langmuir isotherms parameters are given in Table 4.1. The isotherm data of AAP fits well the Langmuir equation with a correlation coefficient, R2 = 0.9828 and shows excellent linearity (Figure4.8). This means that the adsorption sites on AAP are equivalent and the surface is uniform. Adsorption of the nitrate ions to AAP cannot occur beyond monolayer coverage and the ability of the nitrate ions to adsorb onto AAP is independent of neighbouring sites therefore there is no interaction between the adsorbed ions.
Fig 4.8 Langmuir isotherm linear plot for the adsorption of nitrate ions
Table 4.1 Langmuir parameters for adsorption of nitrate ions onto AAP
2.42 66.2mg/g 0.9828
4.3.2 Freundlich Isotherm
This model states that reactions take place in several sorption sites and as the amount of solute adsorbed rises, the binding surface energy decreases exponentially which means multilayer sorption. A plot of ln(Qe) versus ln(Ce) for the studied samples is shown in Figure 4.9. The plot gives constants 1/n and Kf from the slope and the intercept respectively. The adsorption parameters for this study are given in table 4.2.The value 1/n gives an indication on the validity of the adsorption of adsorbent-adsorbate system. A value 1/n between 0 and 1 indicates a favourable adsorption (Tsai, Chang, Lai, ; Lo 2005). In addition to that, this also indicates that the adsorption capacity increases, and further, adsorption sites appear. When 1/n ; 1, the adsorption is not favourable, the adsorption connections become weak and the adsorption capacity decreases. The value of 1/n was less than one (Table 4.2), this revealed favourable adsorption conditions. The correlation coefficient value was 0.9806 which is less than the Langmuir value. Therefore, adsorption does not fit well to Freundlich isotherm . These findings were in line with those of (Gammoudi ; Srasra 2012) who worked on nitrate sorption by organosmectites.
Table 4.2 Freundlich parameters for adsorption of nitrate ions onto AAP
KF N 1/n R2
18.373 1.862891 0.5368 0.9806
Fig 4.9 Freundlich isotherm linear plot for the adsorption of nitrate ions
4.3.3 Temkin isotherm
A plot of qe againist ?n C? is shown in figure 4.10. The plot gives constants B and KT from the slope and the intercept respectively. The data did not show a good fit to the Temkin isotherm model compared to the Langmuir isotherm, based on the fact that the R2 value was 0.933. The same results were obtained by (Liao et al. 2010) using carbonate hydroxyapatite extracted from egg shell as an as an adsorbent. The adsorption parameters for this study are given in table 4.3.
Table 4.3.Temkin parameters for adsorption of nitrate ions on AAP
B KT R2
25.27 1.4586 0.9333
Fig 4.10 Temkin isotherm linear plot for the adsorption of nitrate ions.
4.4 Results for characterization of functionalized polystyrene by FTIR
4.4.1 Results for characterization of polystyrene by FTIR
The chemical structure of polystyrene was analyzed by FT-IR analysis in figure 4.11.The FTIR, spectra displayed a spectral profile characterized by presence of specific peaks at 3023 cm-1, (-CH aromatic) and 2918 and 2840 cm-1 (-CH2), 1600 cm-1 (-C-C) aromatic, 1491 cm-1 (C6H5 in plane), 1451 cm-1 (-C6H5 in plane), 1100 cm-1 (=CH aromatic, out of plane) 694 cm-1 (-CH- aromatic. These results are in line with the findings of (Covolan et al. 2000).
Figure 4.11 FTIR spectrum of pure polystyrene
4.4.2 Results for characterization of nitrated polystyrene by FTIR
The chemical structure of polynitrostyrene was analyzed by FT-IR analysis which confirmed the nitration of polystyrene as shown in figure 4.12. The new peaks at 1518 cm-1 for asymmetric (C6H5NO2) N=O stretching, 1342 cm-1 for symmetric stretching N=O, and 1200 cm-1 for C-N stretching. These results were in line with those of (Shyaa 2012).
Figure 4.12 FTIR spectrum for nitrated polystyrene
4.4.3 Results for characterization of aminated polystyrene by FTIR
The polyaminostyrene FTIR spectrum is shown figure 4.13. There is a broad band at 3340 cm-1 which is typical of the N-H stretching in primary amines. The N-H bending vibration for primary amines is observed at 1602 cm-1.
Figure 4.13 FTIR spectrum for aminated polystyrene
4.4.4 Results for characterization of amino alkylated polystyrene
The FTIR for amino alkylated polystyrene is shown in figure 4.14. The peak at 1056 cm-1 is responsible for the C-N (N-CH3) stretch in amino alkylated polystyrene. The peak around 3000 cm-1 is for the –CH3 group. Tetiary amines do not show any bond in the region 3300 -3000 cm-1 since they do not have an N-H (Xiong ; Yao 2009).
Figure 4.14 FTIR spectrum of amino alkylated polystyrene
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
This study investigated the adsorption characteristics and suitability of AAP as potential adsorbent for the removal of nitrate from aqueous solutions using batch technique. The results showed that this clay could be used as potential sorbent and it was highly effective as low cost adsorbent for the removal of nitrates ions from aqueous solutions. The batch study parameters, pH of solution, mass of adsorbent, initial solution concentration, contact time and temperature were found to be effective on the adsorption processes. The adsorption equilibrium was attained within 3 hours.. The percentage removal was found to decrease with increase in pH. The increase in adsorbent dosage increased the percent removal of nitrate due to the increase in adsorbent surface area in adsorbent dosage. The equilibrium data fitted well the Langmuir isotherm equation and this adsorbent showed large uptake capacity of nitrate Q max,exp = 81 mg/g). AAP adsorbent proved to be a highly efficient adsorbent for remediation of nitrate contaminated water owing to its exceptional uptake capacity as well as high selectivity for this anionic contaminant.
The recommendations for this study are
Various anions beside nitrate ions should also be analysed to maximise the use of the AAP adsorbent.
Studies on application of the beads on real environmental samples are required
Use of the adsorbent as a anion preconcetration method is also required
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