CHAPTER-3. Experimental Methods

Transcription

1 CHAPTER-3 Experimental Methods

2 EXPERIMENTAL METHODS For present study, Analysis of Trace Elements in Various Industrial Effluents in and Around Indore, six industries located in different areas of Indore were selected. The industries taken into consideration were as follows- 1. Rajratan Global Wire Ltd., Pithampur Rajratan Global Wire is one of the leading manufacturer of high carbon steel wire, specializing in automotive tyre bead wire, high quality spring, rope wire and prestressed concrete wires. Prestressed concrete wires are used in construction of bridges, buildings, dams, airport hangars, railway sleepers, etc. 2. Mahindra Two Wheelers Ltd., Pithampur Mahindra has state of art manufacturing facility at Pithampur, Indore. The plant products and processes are ISO certified from DNV. Mahindra two wheelers plant has a total capacity to assemble about units p.a. on three shift basis. 3. Bridgestone India Pvt. Ltd., Pithampur Bridgestone is one of the leading manufacturers of tyres, tubes and flaps for passenger cars, buses and trucks. It is official tyre supplier for Formula-1. 43

3 4. MP Beer Products Pvt. Ltd., Indore One of largest manufacturers and distributors of beer in M.P. 5. IPCA Laboratories Ltd., Indore IPCA is a fully integrated Indian Pharmaceutical company manufacturing over 350 formulations and 80 APIs for various therapeutic segments. 6. Ruchi Soya Industries Ltd., Indore Ruchi soya is the largest manufacturer and marketer of edible oil and soya products. Ruchi soya is highest exporter of soya meal, lecithin and other food ingredients from India. The industrial waste water effluent samples were collected randomly six times in the year in the months of January, March, May, July, September and November for three years from different industries around Indore. All chemicals and reagents of certified reference material (CRM) were used for analysis. SAMPLING AND SAMPLE PRESERVATION Sampling was done as per standard methods from various industrial units. Serious errors may be introduced during sampling and storage because of contamination from sampling device, failure to remove residues of previous samples from sample container and loss of metals by absorption on and/or precipitation in sample container caused by failure to acidify the sample properly. 44

4 SAMPLE CONTAINERS The best sample containers are made of quartz or TFE. Because these containers are expensive, the preferred sample container is made of polypropylene or linear polyethylene. Borosilicate glass containers also may be used but avoid soft glass containers for samples containing metals in the microgram-per-litre range. We used plastic cans of 2 litre capacity for sampling of effluents. Samples were collected from effluent release point or a little away from it. Thoroughly clean sample containers with a metal free non-ionic detergent solution, rinse with tap water, soak in acid and then rinse with metal free water. DIGESTION FOR METALS To reduce interference by organic matter and to convert metals associated with particulates to a form (usually the free metal) that can be determined by atomic absorption spectrometry or inductively-coupled plasma spectroscopy digestion for metals is necessary. Heavy metals readily form complexes with organic constituents; therefore it is necessary to destroy them by digestion with strong acids. Digestion destroys the organic matter, removes interfering ions and brings metallic compounds in suspension to solution. To ensure removal of organic impurities and prevent interference during analysis, each of 50 ml volume sample was digested using10 ml concentrated HNO 3 in a 250 ml conical flask placed on a fume cupboard. The samples were covered properly with aluminum foil to avoid spillage and heated on a hot plate until the solution was reduced to 10 ml. This was 45

5 allowed to cool and made up to mark with distilled water before filtering into a 50ml standard flask, labeled and ready for analysis. The blank constituted 5% HNO 3. Standard solutions of Certified Reference Material of different metals were used as supplied. Solution of 1000ppm strength served as the stock solution. Subsequently lower concentrations in the range 2-10ppm were prepared from the stock by serial dilution. Standard solutions of varied concentrations of metal were used. Instrumentation: AAS instrument- Avanta M (GBC Scientific Equipment Pvt. Ltd.) was used. The AAS at IIT Roorkee caters to the need of various users from all over the country since It is used to determine the concentration of metal 46

6 elements in a sample. The technique makes use of the fact that neutral or ground state atoms of an element can absorb electromagnetic radiation over a series of very narrow, sharply defined wavelengths. The sample in solution is aspirated as a fine mist into a flame where it is converted into atomic vapour. Most of the atoms remain in the ground state and are therefore capable of absorbing radiation of a suitable wavelength. This discrete radiation is supplied by a hollow cathode lamp which a sharp line source consisting of a cathode is containing the element to be determined along with the tungsten anode. The lines characteristic of the element are emitted by the hollow cathode and pass through the flame where they may be absorbed by the atomic vapour, since only the test element can absorb this radiation, the method becomes specific. SPECIFICATIONS Sensitivity: up to ppb level Channels: Two (Independent or simultaneous) Wavelength range: 180nm to 900nm Available Lamps: Al, As, Bi, Ca, Cd, Cu, Hg, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Zn, Mo, Cr, Sn, Sr, Si, Ba, Special facilities: Zoom lens Optics, Deuterium arc background corrector Probe: Teflon tubing- 1.6mm OD (0.8mm ID) Software: Avanta Software Fuel: Acetylene Oxidants: Air, Nitrous Oxide 47

7 GBC Avanta is a versatile modular AAS for specific requirements. It is not only easier to use but also produces meaningful results faster and with less operator intervention. The Avanta AAS makes use of the latest in atomic absorption spectrometer technology, developed after exhaustive research into the needs of the analytical community. It has power, performance and speed required in today s modern laboratory. GBC scientific equipment is a world leader in the development and manufacture of atomic absorption spectrometers since its inception in GBC quality management system has been accredited to the ISO 9001 quality standard. This certification is assurance that the procedures and processes used to produce the goods and services which GBC provides to comply with the relevant international standards and demonstrates commitment to meet the needs and expectations. In flame atomic absorption spectrometry, a sample is aspirated into a flame and atomized. A light beam is directed through the flame into a monochromator and onto a detector that measures the amount of light absorbed by the atomized element in the flame. For some metals, atomic absorption exhibits superior sensitivity over flame emission. Because each metal has its own characteristic absorption wavelength, a source lamp composed of that element is used; this makes the method relatively free from spectral or radiation interferences. An air-acetylene flame attaining maximum temperature up to 2300 degree centigrade was used for this work. Samples collected and preserved after nitric acid treatment were analysed by flame atomic absorption spectrophotometer installed at Regional Lab, M.P. Pollution Control Board, Indore. 48

8 ATOMIC ABSORPTION SPECTROSCOPY Atomic Absorption Spectrometry (AAS) is a technique for measuring quantities of chemical elements present in environmental samples by measuring the absorbed radiation by the chemical element of interest. This is done by reading the spectra produced when the sample is excited by radiation. The atoms absorb ultraviolet or visible light and make transitions to higher energy levels. Atomic absorption methods measure the amount of energy in the form of photons of light that are absorbed by the sample. A detector measures the wavelengths of light transmitted by the sample and compares them to the wavelengths which originally passed through the sample. A signal processor then integrates the changes in wavelength absorbed which appear in the readout as peaks of energy absorption at discrete wavelengths. The energy required for an electron to leave an atom is known as ionization energy and is specific to each chemical element. When an electron moves from one energy level to another within the atom, a photon is emitted with energy E. Atoms of an element emit a characteristic spectral line. Every atom has its own distinct pattern of wavelengths at which it will absorb energy due to the unique configuration of electrons in its outer shell. This enables the qualitative analysis of a sample. The concentration is calculated based on the Beer-Lambert law. Absorbance is directly proportional to the concentration of the analyte absorbed for the existing set of conditions. The concentration is usually determined from a calibration curve obtained using standards of known concentration. However, applying the Beer-Lambert law directly in AAS is difficult due to variations in atomization efficiency from the sample matrix, nonuniformity of concentration and path length of analyte atoms (in graphite furnace AA). 49

9 The chemical methods used are based on matter interactions, i.e. chemical reactions. For a long period of time these methods were essentially empirical, involving, in most cases, great experimental skills. In analytical chemistry, AAS is a technique used mostly for determining the concentration of a particular metal element within a sample. AAS can be used to analyze the concentration of over 62 different metals in a solution. ATOMIC ABSORPTION SPECTROSCOPY (AAS) is an analytical technique that measures the concentrations of elements. It makes use of the absorption of light by these elements in order to measure their concentration. Atomic absorption spectroscopy quantifies ground state atoms in the gaseous state. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analytic concentration is determined from the amount of absorption. Atomic absorption is a very common technique for detecting metals and metalloids in environmental samples. Atomic Absorption Spectrometer: Atomic absorption spectrometers have 4 principal components 1. A light source (usually a hollow cathode lamp) 2. An atom cell (atomizer) 3. A monochromator 4. A detector and read out device. 50

10 1. Light Source The light source is usually a hollow cathode lamp of the element that is being measured. It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas (neon or argon). Each element has its own unique lamp which must be used for that analysis. Atoms of different elements absorb characteristic wavelengths of light. Analyzing a sample to see if it contains a particular element means using light from that element. Reactions in the hollow-cathode lamp Ionization of filler gas: Ar + e Ar e Sputtering of cathode atoms: M(s) + Ar + Excitation of metal atoms: M (g) + Ar + M (g) + Ar M*(g) + Ar Light emission: M*(g) M (g) + hν Light emitted by hollow-cathode lamp has the same wavelength as the light absorbed by the analyte element. Different lamp required for each element (some are multi-element) Hollow-cathode lamps are discharge lamps that produce narrow emission from atomic species. Atomic absorption and emission linewidths are inherently narrow. Due to low pressure and low temperature in the lamp, lines are even narrower than those of analyte atoms. 51

11 2. Atomizer: Elements to be analyzed need to be in atomic state. Atomization is separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperatures in a flame or graphite furnace. The role of the atom cell is to primarily dissolve a liquid sample and then the solid particles are vaporized into ground state form. In this form atoms will be available to absorb radiation emitted from the light source and thus generate a measurable signal proportional to concentration. There are two types of atomization: Flame and Graphite furnace atomization. 3. Monochromator: This is a very important part in an AA spectrometer. It is used to separate out all of the thousands of lines. Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light which is absorbed by the sample and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. 4. Detector and Readout Device: The light selected by the monochromator is directed onto a detector that is typically a detector that is photomultiplier tube whose function is to convert the light signal into an electrical signal proportional to the light intensity. The processing of electrical signal is fulfilled by a signal 52

12 amplifier for readout or further fed into a data station for printout by the requested format. APPLICATIONS OF ATOMIC ABSORPTION SPECTROSCOPY water analysis (e.g. Ca, Mg, Fe, Si, Al, Ba content) food analysis analysis of animal feedstuffs (e.g. Mn, Fe, Cu, Cr, Se, Zn) analysis of additives in lubricating oils and greases (Ba, Ca, Na, Li, Zn, Mg) analysis of soils clinical analysis (blood samples: whole blood, plasma, serum; Ca, Mg, Li, Na, K, Fe). Atomic absorption spectroscopy is based on the same principle as the flame test used in qualitative analysis. When an alkali metal salt or a calcium, strontium or barium salt is heated strongly in the Bunsen flame, a characteristic flame colour is observed: Na Li Ca Sr Ba yellow crimson brick red crimson green In the flame, the ions are reduced to gaseous metal atoms. The high temperature of the flame excites a valence electron to a higher-energy 53

13 orbital. The atom then emits energy in the form of (visible) light as the electron falls back into the lower energy orbital (ground state). The ground state atom absorbs light of the same characteristic wavelengths as it emits when returning from the excited state to the ground state. The intensity of the absorbed light is proportional to the concentration of the element in the flame. Atomic absorption spectroscopy and atomic emission spectroscopy are used to determine the concentration of an element in solution. Atomic absorption spectroscopy absorbance = -log (I t /I 0 ) I t = transmitted radiation I 0 = incident radiation Atomic emission spectroscopy transmission = -log (I 0 / I t ) I 0 = intensity of radiation that reaches the detector in the absence of sample I t = intensity of radiation that reaches the detector in the presence of sample The concentration of an absorbing species in a sample is determined by applying Lambert-Beer s Law. 54

14 Applying Lambert-Beer s law in atomic absorption spectroscopy is difficult due to variations in the atomization from the sample matrix and non-uniformity of concentration and path length of analyte atoms. Concentration measurements are usually determined from a calibration curve generated with standards of known concentration. Advantages of Flame Atomic Absorption Spectroscopy: inexpensive (equipment, day-to-day running) high sample throughput easy to use high precision Disadvantages of Flame Atomic Absorption Spectroscopy: only solutions can be analysed relatively large sample quantities required (1-2 ml) less sensitivity (compared to graphite furnace) problems with refractory elements 55

15 Advantages of Graphite furnace atomic absorption spectroscopy: Solutions, slurries and solid samples can be analysed. much more efficient atomization greater sensitivity smaller quantities of sample (typically 5-50µL) provides a reducing environment for easily oxidized elements Disadvantages of Graphite furnace atomic absorption spectroscopy: expensive low precision low sample throughput requires high level of operator skill Sensitivity of Atomic Absorption Spectroscopy: high sensitivity for most elements flame atomization: concentrations at the ppm level electro-thermal atomization (graphite furnace): Concentrations at the ppb level 1 ppm = 10-6 g/g or 1µg/g If we assume that the density of the analyte solution is approximately 1.0, then 1 ppm = 1µg/g = 1µg/mL 1 ppm Fe = 1 x 10-6 g Fe/mL = 1.79 x 10-5 mol/l Sensitivity = concentration of an element which will reduce the transmission by 1%. 56

16 Atomic Absorption Spectrometry (AAS) is an analytical method for quantification of over 70 different elements in solution or directly in solid samples. Procedure depends on atomization of elements by different atomization techniques like flame (FAAS), electro thermal (ETAAS), hydride or cold vapor. Each atomization technique has its advantages and limitations or drawbacks. Two types of flame are used in FAAS: (i) (ii) Air/acetylene flame, Nitrous oxide/acetylene flame. Flame type depends on thermal stability of the analyte and its possible compounds formed with flame concomitants. Temperature formed in airacetylene flame is around 2300 C whereas acetylene-nitrous oxide (dinitrogen oxide) flame is around 3000 C 103. Generally with air/ acetylene flame antimony, bismuth, cadmium, calcium, cesium, chromium, cobalt, copper, gold, iridium, iron, lead, lithium, magnesium, manganese, nickel, palladium, platinum, potassium, rhodium, ruthenium, silver, sodium, strontium, thallium, tin and zinc can be determined. On the other hand for refractory elements such as aluminum, barium, molybdenum, osmium, rhenium, silicon, thorium, titanium and vanadium, nitrous oxide/acetylene flame should be used. But some elements like vanadium, zirconium, molybdenum and boron have lower sensitivity in the determination by FAAS because the temperature is insufficient to break down compounds of these elements. Samples should be in solution form, or digested to be detected by FAAS. Typical detection limits are around ppm range and sample analysis took seconds per element 104. Generally, hollow cathode lamps such as source, flame or graphite furnace as an atomizer and grating as a wavelength selector and photomultiplier as 57

17 a detector are used. Mahmoud et al 105 determined Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb by FAAS after enrichment with chemically modified silica gel N-(1-carboxy-6-hydroxy) benzylidenepropylamine (SiGCHBPA). Afkhami et al 106 determined Cd in water samples after cloud point extraction in Triton X-114 without adding chelating agents. Mohamed et al 107 determined chromium species based on the catalytic effect of Cr (III) and/or Cr (VI) on the oxidation of 2-amino-5- methylphenol (AMP) with H 2 O 2 by FAAS. Mahmoud et al 108 did pre-concentration of Pb (II) by newly modified three alumina physically loaded-dithizone adsorbents determined by FAAS. Casella et al 109 prepared a minicolumn packed with a styrenedivinylbenzene resin functionalized with (S)-2-[hydroxy-bis-(4-vinylphenyl)-methyl]- pyrrolidine-1-carboxylic acid ethyl ester to determine Cu in water samples. Carletto et al used 8-hydroxyquinoline-chitosan chelating resin in an automated on-line preconcentration system for determination of Zn (II) by FAAS. Gunduz et al 110 did pre-concentration of Cu and Cd using TiO 2 core-au shell nanoparticles modified with 11-mercaptoundecanoic acid and analyzed their slurry. ETAAS is basically same as FAAS; the only difference is flame is replaced by graphite tube which can be heated up to 3000 C for atomization. Since sample is atomized in a much smaller volume, the atoms density will be higher, its detection limit is much more than FAAS, around ppb range. Graphite furnace program typically consists of four stages; drying for evaporation of solvent; pyrolysis for removal of matrix constituents; atomization for generation of free gaseous atoms of the 58

18 analyte; cleaning for removal of residuals in high temperature. Generally samples are liquids, but there are some commercial solid sampling instruments also. Analyze took 3-4 minutes per element. 50 and more elements can be analyzed by GFAAS. Burguera et al 111 determined beryllium in natural and waste waters using on-line flow-injection pre-concentration by precipitation dissolution for electro thermal atomic absorption spectrometry. They used a precipitation method quantitatively with NH 4 OH-NH 4 Cl and collected in a knotted tube of Tygon without using a filter, then the precipitate was dissolved with nitric acid injected to graphite furnace. Baysal et al 112 accomplished to pre-concentrate Pb by cobalt/ pyrrolidinedithio- carbamate complex [Co(PDC) 2 ]. For this purpose, lead was co precipitated first with cobalt/pyrrolidinedithiocarbamate complex formed using ammonium pyrrolidinedithiocarbamate (APDC) as a chelating agent and cobalt as a carrier element. The supernatant was then separated and the slurry of the precipitate prepared in Triton X-100 was directly analyzed. Hydride generation atomic absorption spectrometry is a technique for some metalloid elements such as arsenic, antimony, selenium as well as tin, bismuth and lead which are introduced to instrument in gas phase. Hydride is generated mostly by adding sodium borohydride to the sample in acidic media in a generator chamber. The volatile hydride of the analyte generated is transferred to the atomizer by inert gas where it is atomized. The oxidation state of the metalloid is very important so before introducing to the hydrid system, specific metalloid oxidation state should be produced. This method lowers limit of detection (LOD) times

19 Coelho et al 114 presented a simple procedure developed for the direct determi-nation of As (III) and As(V) in water samples by flow injection hydride generation atomic absorption spectrometry (FI HG AAS) without pre-reduction of As(V). Carbon and Madec determined antimony in sea water samples by continuous flow injection hydride generation atomic absorption spectrometry. Antimony was determined by graphite furnace atomic absorption spectrometry. Yersel et al 115 developed a separation method with a synthetic zeolite (morde-nite) developed in order to eliminate the gas phase interference of Sb(III) on As(III) during quartz furnace hydride generation atomic absorption spectrometric determination. Anthemidis et al determined arsenic (III) and total arsenic in water by using an on-line sequential insertion system and hydride generation atomic absorption spectrometry. Erdogan et al determined inorganic arsenic species by hydride generation atomic absorption spectrometry in water samples after preconcentration/separation on nano ZrO 2 /B 2 O 3 by solid phase extraction. Korkmaz et al developed novel silica trap for lead determination by hydride generation atomic absorption spectrometry. The device consists of 7.0cm silica tubing which is externally heated to a desired temperature. The lead hydride vapor is generated by a conventional hydride-generation flow system. The rap is placed between the gas liquid separator and silica T-tube; the device traps analyte pieces at 500 C and releases them when heated further to 750 C. The presence of hydrogen as is required for revolatilization; O 2 gas must also be present. Cold vapour atomization technique is used for the determination of mercury which is the only element to have enough vapour pressure at room temperature. Method is based on converting mercury into Hg +2, followed by reduction of Hg +2 with tin(ii)chloride or borohydride. 60

20 Then produced elemental mercury swept into a long-pass absorption tube along with inert gas. Absorbance of this gas at nm determines the concentration. Besides inorganic mercury compounds, organic mercury compounds are problematic as they cannot be reduced to the element by sodiumtetrahydroborate and particularly not by stannous chloride. So it is advised to apply an appropriate ingestion method prior to the actual determination. Other methods are: Inductively Coupled Plasma Optical Emission Spectrometry Inductively Coupled Plasma Mass Spectrometry Laser Induced Breakdown Spectroscopy (LIBS) Anodic Stripping Voltammeter (ASV) 61