The determination of Arsenic in soil by ICP-OES by Alexander Edward Sibiri Whaley Submitted in partial fulfilment of the requirements for the degree MAGISTER SCIENTIAE in the Faculty of Science University of Pretoria Pretor ia January 2 000 © University of Pretoria SYNOPSIS Arsenic has always played a major role in the environment and human life in general. From its earliest uses, in ancient times, as a poison to its most recent use, in medicine, as an anti-leukemia agent, this metal has fascinated mankind. This fascination has already yielded several surveys on its toxicity, concentration, source and specie. However, some approaches have not been fully explored. One of these is its occurrence in Phosphate bearing rocks and subsequent possible contamination offertilizers derived from these rocks. To this end, a new variation ofa speciation mechanism used, solvent extraction followed by ion-exchange, has been developed. Although this method has mainly been used in connection with marine samples, in this dissertation, it has been applied to solid samples. The levels of arsenic were then determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Phosphate bearing rocks, commercial fertilizers and soils were digested in HCI and speciated out as As (III), As(V), Mono­ Methyl Arsenic acid (MMAA) and Dimethyl Arsenic acid (DMAA). The effect of different organic phases were examined as well as two ion-exchange resins. Phosphoric acid, being an intermediate stage in the manufacture of diverse fertilizers, was also investigated. Due to the close relationship between arsenic and phosphorus, some steps in the method had to be reversed in order to yield better results. The resulting samples were analysed at two wavelengths, ie 188.979nm en 193.696nm. Although the values obtained at the two wavelengths differ by between O.Smg/kg and 1.Smg/kg, a simple t-test proves that the values are reconcilable. A micro­ concentric nebuliser was studied regarding its aerosol particle size, with a )l-LASER-particle­ analyser, and the detection limit ofarsenic. The second part ofthis study could not be completed due to the loss of the ICP. The following four nebulisers were compared: Micro-Concentric Nebuliser (MCN), Meinhard Nebuliser, V-groove Nebuliser and a Cross-flow Nebuliser. Due to their similar design, the MCN and Meinhard nebulisers have similar characteristic, the main differences being in their operating characteristics, ie regarding particle size and background intensity. As the inside diameter of the MCN is only O.2)lm special glassware had to be developed to insure that the MCN does not become blocked. SAMEVATTING Arseen het nog a1tyd 'n be1angrike ro1 in die natuur en mens like 1ewe oor die a1gemeen gespeel. Hierdie metaal het die mens van die begin af bet ower, van die vroegste gebruik, in die verre verlede, as gifstof tot die hedendaagse gebruik in die mediese wereld as anti­ leukemie middel. Hierdie betowering het reeds gelei tot vele studies na die giftigheid, konsentrasie, bronne en chemiese vorme van arseen. Sekere benaderings is egter nie ten volle bestudeer nie. Een van die, is die voorkoms van arseen in fosfaatbevattende rots en die gevolglike moontlike besoedeling van kunsmis wat vanuit die rotse vervaardig word. Vir hierdie doel is 'n nuwe variasie van 'n bestaande metode om arseen in sy verskillende chemiese vorme te verkry, gebruik; naamlik oplosmiddel-ekstraksie gevolg deur ioon­ uitrui1ing in 'n gepaste kolom. A1hoewel die metode meestal in die ontleding van seemonsters gebruik word, word die metode tydens hierdie verhandeling op vastetoestand-monsters toegepas. Die arseenvlakke is met behulp van Induktief Gekoppe1de Plasma Optiese Emissie Spektroskopie (IGP-OES) bepaal. Fosfaat­ bevattende rots, bedryfskunsmis en grondmonsters is in Hel opgelos en geskei in die volgende chemiese vorme: As(III), As(IV), monometie1arseensuur (MMAS) en dimetie1arseensuur (DMAS). Die effek van verskillende organiese fases, sowel as twee ioon-uitruilingsharse is ondersoek. Fosforsuur, as tussenproduk in die vervaardiging van verskeie kunsmisstowwe, is ook ondersoek. Weens die nabye verwantskap tussen arseen en fosfor moes verskeie stappe in die gebruikte metode omgeruil word, om beter resu1tate te lewer. Die verkrygde monsters is by twee golflengtes, 188.979nm en 193.696nm, ontleed. Alhoewel daar 'n verskil van 0.5 - 1.5mglkg in die waardes is wat by die verskillende golflengtes verkry word, toon 'n eenvoudige t-toets aan dat die waardes versoenbaar is. 'n Mikro-konsentriese newelaar is ondersoek ten opsigte van die sproei­ deeltjiegrootte, met behulp van 'n ).!-LASER-deeltjiegrootte-ontleder en ook ten opsigte van die bepaalbaarheid van arseen. Die deel is egter nie voltooi nie, weens verlies aan die IGP. Tydens die deel van die studie is vier newelaars, die Mikro-Konsentriese Newelaar (MKN) , Meinhard Newelaar, V-Groef Newelaar en 'n Kruis-Vloei Newelaar vergelyk. Weens hulle soortgelyke ontwerp is die eienskappe van die MKN en Meinhard newelaars ook soortgelyk, met die grootste verskille in hul bedryfseienskappe, naamlik ten opsigte van deeltjiegrootte en agtergrondsterkte. Spesiale glasware moes ontwikkel word, om te voorkom dat die MKN verstop word, aangesien die binne deursnee van die MKN slegs O.2).!ffi is. Table of C ontents 1 Introduction 1.1 ICP as an analytical method 1.2 Arsenic in the envirorunent 3 1.2.1 Types of arsenic found in soils and relative toxicity 3 1.2.2 Properties of arsenic useful in analysis 7 1.2.3 Soil conditions 8 1.2.3.1 Type of soil and parent rock type 8 1.2.3.2 pH and Eh of soil 8 1.2.3.3 Presence of other metals 9 1.2.3.4 Depth factors 11 1.2.3.5 Other considerations 11 1.2.4 Extraction methods 12 1.2.5 Speciation methods l3 1.2.6 Hydride Generation 15 1.3 References 17 2 Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) 18 2.1 The theory ofICP-AES 18 2.1.1 Sample introduction 20 2.1.2 Plasma formation 22 2.1.3 RF Generator 23 2.1.4 Detection of emission 24 2.2 Interferences in ICP-AES 24 2.2.1 Matrix effects 25 2.2.2 Spectral Overlap 26 2.2.2.1 Direct overlap 26 2.2.2.2 Wing overlap 27 2.2.2.3 Continuum radiation 28 2.2.3 Stray light 29 2.3 References 30 3 Ion Exchange 31 3.1 Introduction 31 3.2 Ion exchange resins 32 3.2.1 Anion exchange resins 33 3.2.2 Cation exchange resins 34 3.2.3 Chelating ion exchange resins 34 3.2.4 Capacity 35 3.2.5 Selectivity 35 3.2.5.1 Selectivity coefficient 35 3.2.5.2 Factors influencing selectivity 36 3.2.6 Distribution coefficient 37 3.2.7 Kinetics of ion exchange 38 3.3 References 41 4 Comparison of a Micro-concentric nebulizer with a Meinhard nebulizer 42 4.1 Introduction to nebulizers 42 4.2 Theory of nebulizers 42 4.2.1 Pneumatic nebulizers 42 4.2.1.1 Concentric nebulizers 43 4.2.1.2 Cross-flow nebulizers 44 4.2.1.3 Babington-type nebulizers 45 4.2.2 Ultrasonic nebulizer 45 4.3 Experimental for the particle size distribution 47 4.4 Discussion of limits of detection 65 4.5 References 66 5 Extraction and speciation of arsenic 68 5.1 Introduction 68 5.2 Experimental 1: preparation and calibration of resins 69 5.3 Experimental 2: calibration of organic solvents 70 5.4 Experimental 3: extraction of soil samples 70 5.5 Experimental 4: inclusion ofMMAA 71 5.6 Experimental 5: extraction of phosphoric rocks and fertilizers 73 5.7 Experimental 6: new method for analysing phosphorus bearing rocks 76 6 General conclusions 80 7 Appendix CHAPTERl INTRODUCTION 1.1 ICP as an analytical method Inductively Coupled Plasma Atomic Emission spectrometry (ICP-AES) is a technique used predominantly for the quantitative multi-element analysis ofmost types ofsamples, ego biological and geological materials. It is an ideal technique due to its high dynamic range which allows for the analysis of both major and trace elements from a single sample (i.e. from gil to J.lgli range) .. Major elements can easily be analysed with high accuracy and precision through internal standardisation and proper calibration procedures. As with many other analytical techniques, trace elements, however, can be more problematic. These problems can be related to the background enhancement and spectral line overlap from concomitant elements. Physical interference can occur during the nebulization process that will affect accuracy and precision. Here again, internal standardisation can be used to rectify the sample transport variation. Sensitive measurements of refractory elements such as Band P are also catered for by this technique through the high temperature of the ICP. There are two systems available for ICP-AES, simultaneous and sequential. Simultaneous systems have a higher sample throughput as more elements can be analysed for a time period. This is because the wavelengths at which the elements are analysed are preselected in the manufacturing process. A direct result of this however is the lack of control over the analysis. Sequential systems have the advantage of flexibility of wavelength selection. This in effect, allows for variations in analyte -1­ concentrations and matrix types and the removal of interfering peaks by the selection of a different wavelength, with the major drawback of much longer analysis time. As geological materials are generally very complex in composition, several sources of interference can generally be expected in ICP-AES. These can be eliminated by using a high resolution spectrometer, on-line background compensation, predetermined interference coefficients or matrix matched standards. Several methods for background compensation exist, many mathematical, which usually involve scanning a spectral segment on either or both sides ofpeak positions. Then, the spectrum for each interferent is measured and removed mathematically from the analyte signal. Any productive spectrometer should, in theory, fulfil these capabilities: a high degree of resolution to achieve good separation from nearby spectral lines, thereby reducing inter-element interference exhibit high sensitivity through excellent light-gathering and least stray light be stable, both mechanically and thermally, ensuring both the precision and repeatability of the analysis and ensuring that the correct wavelength has been selected. low detection limits to measure trace elements plus major elements. As ICP-AES is a very similar spectroscopic tool to flame and graphite furnace atomic emission spectroscopy, the advantages and disadvantages of this technique can best be described though a brief comparison of the three. Although atomic absorption is a stronger phenomenon than atomic emission, where typically only 1 *10.6 photons emitted by atoms reaches the detector, ICP-AES offers a tremendous narrowing of the detection -2­ limits gap. Species are highly excited due to the high temperatures ofthe plasma. This translates into extensively populated excited states yielding a simultaneous, intense emission from many lines. Flame and graphite furnaces' emissions are much less intense as the nonnal operating temperatures ofthese techniques are characteristically between 2,000 and 3,500 K. More popularly, flame and graphite furnaces operate as absorption spectrometers, as absorption is a stronger phenomenon than emission, through the absorption of light by excited states with low energies less than 3 eV. The low detection limits achieved by ICP-AES are predominantly due to the large emission signals with respect to the noise of the background of some elements (Ilgll sometimes). These detection limits are obviously sample dependant. Therefore, they are easily degraded by difficult matrices, increased background, and by spectral overlaps. 1.2 ARSENIC IN THE ENVIRONMENT 1.2.1 Types of arsenic found in soils and relative toxicity: Although several fonns ofarsenic can be found in soils, there are four major groups that are important. These different fonns ofarsenic can be separated into two major groups, inorganic arsenic in the oxidation states As(III) and As(V) and organo-arsenicals, mono­ methyl (MMA)- and dimethyl-arsenic (DMA) acids. These last two arsenic fonns are mainly found in an aquatic environment and are thought to be lOO times less toxic than the inorganic fonns. These methylated fonns are the result ofbiological transfonnation of the inorganic species [1,2]. -3­