Monitoring Of Sulfide And Heavy Metals Speciation
MONITORING OF SULFIDE AND HEAVY METALS SPECIATION
N SEA WATER USING CHEMILUMINESCENT,
DIFFUSE-REFLECTANCE SPECTROSCOPIC
AND VISUAL-TEST METHODS
O.A. Zaporozhets, L.S. Ivanko, T.Ye. Keda, R.P. Linnik,
O.Yu. Nadzhafova, L.Ye. Tsukalo, V.V. Sukhan
Taras Shevchenko National University, Kyiv, Ukraine
Sulfide. As is known, the sulfides of metals have toxic effect toward aquatic organisms, and the excess of H2S in water bodies causes decrease of oxygen concentration. Some researchers consider that the sulfides can have an important role in influencing the speciation of dissolved heavy metals in natural waters, competing even with such active complexing ligands, as dissolved organic substances. Being reducers, sulfides in natural waters should be oxidized completely by the dissolved oxygen. However, rate of this oxidation is small and free sulfide ions can persist for many hours or even days after receipt in a reservoir, depending on рН of aquatic medium and some other factors. The standard methods are characterized by low sensitivity and are not always suitable for determination of free sulfides and investigation of binding process of metals by sulfide ions. Therefore, the development of new methods of sulfide determination with a low detection limit is an important task.
The quenching of the chemiluminescence by various organic and inorganic substances is a well known phenomenon that has often been used for acquiring qualitative and quantitative information on the effect of these substances on the environmental fate of metal ions. We investigated influence of sulfides at low concentration on the chemiluminescent reaction between 4-diethylaminophtalhydrazide (4-DEAPH) and hydrogen peroxide at the presence of cobalt (II), copper (II) and vanadium (IV) ions.
The sulfide determination was based on the quenching of the chemiluminescence in catalytic reaction 4-DEAPH–Co(Cu)-H2O2 as result of metal ions binding in CoS and CuS. It was established that the detection limits of chemiluminescent sulfide determination was 3·10-4mol/l(Co) and 2·10-8mol/l(Cu).
However, humic acids prevented detection of sulfides with use of these systems. Therefore, the application of these reactions for sulfide determination in fresh surface water was limited. It is known [3] that the chemiluminescence of 4-DEAPH was increased in the presence of vanadium (IV). Sulfide reduces of V (V) to V (IV) in acidic medium. A chemiluminescence method for sulfide determination on the base of system 4-DEAPH–H2O2–V (IV) was developed. The detection limit was 2·10-8 mol/l. These reactions are usefulness for detection of sulfide traces in natural water.
Heavy metals speciation. The speciation of heavy metals (HMs) in natural waters and their overall concentration significantly effects water quality. Toxicity and bioavailability of HMs depends not only on their total content in water, but also on concentration and ratio of the coexisting forms. Free (hydrated) metal ions are regarded as the most toxic form. Binding HMs in complexes with inorganic, and particularly with dissolved organic matter, more often than not decreases or completely suppresses their toxicity. In this connection the need of development methodology of investigation of HM speciation is arisen. The developed methodology envisages complex study of different HM forms including their phase distribution, concentration of toxic and nontoxic forms, binding degree in complexes with dissolved organic matter (DOM) of surface water, chemical nature, molecular weight and charge sign of complex compounds etc.
Gel-penetration chromatography was used for the investigation of molecular weight distribution of HM complex compounds. The neutral gel Molselect G–75 (Hungary) was applied for these purposes. Chemical nature of metal complexes was studied after their separation on the columns with cellulose ion exchangers DEAE and CM (Austria). Concentration of HM free ions and total content of their dissolved forms (after photochemical destruction) was determined by chemiluminescent methods. UV-irradiation of the water samples (pH~1) was carried out in the quartz glasses by DRT–1000 mercury lamp during 3.5–4.0 hours. These methods were realized in the scheme of investigation of the cobalt coexisting forms in the Dnieper reservoirs.
Labile forms of heavy metals. Attention has recently been focused on the determination of toxic metals, Pb in particularly, in environmental objects. The reagents adsorption on the silica matrixes are known as effective method of modified sorbents obtained. The methods using sorbents loaded with chromophorous reagents permit to combine the recovery of metal ions from dilute solutions with their visual or spectroscopic detection on the sorbent surface. Moreover, the using of specific reagents as a modifying agent allows for the increase in selectivity and sensitivity of determination. Xylenol Orange (XO) is known to be one of the effective reagent for Pb(II) spectrophotometric determination. As far as we know, no silica with XO and its complexes adsorbed has been proposed for heavy metals determination. In the present work we have studied the adsorption of XO and its complexes from aqueous and organic solution onto nonfunctionalized and functionalized silica surface and established the XO immobilized usefulness in the sorption-spectroscopic and visual test determination of Pb(II) in water. XO was found to be well adsorbed onto the surface of nonmodified SG from aqueous solution at pH10 ppb. Visual test scale for Pb(II) determination at one third of the maximum admissible concentration (MAC) level was prepared.
The development of new solid-phase reagents on the base of high dispersed silicas modified as tool of analytical sampling and detection of various compounds in natural objects is a subject of growing interest now. Such sorbents are one of the best matrixes for analytical reagents immobilization because of their specific characteristics. There are a various techniques for sorbent modification. Recently, adsorption of reagents from organic solvents has been utilized as effective and simple method for silica surface modification. Quercetin (Qu) and Hematoxylin (Hx) are known to be effective chelating reagents for spectroscopic determination of Sn(IV). In the present work we examine the use of Qu and Hx adsorbed on silica gel as a solid-phase reagents, and establish their usefulness in the quantitative determination of Sn(IV) in the sea water. The study of reagents adsorption from different organic solvents and their mixtures has shown that the optimum is mixture with the ratio of acetone:hexane = 1:4. The silicas modified by Qu and Hx adsorption from acetone-hexane solution were found to be stable for more than two month and suitable for analytical purposes. We have studied the nature of reagent binding with the surface by using of spectroscopic (UV/VIS- and IR-spectroscopy) methods. In order to apply the sorbents modified with Quercetin (Qu-SG) and Hematoxylin (Hx-SG) for Sn(IV) determination in diluted solutions the adsorption of Sn(IV) onto Qu-SG at pH>1.0 in the presents of 1.0 mol·L-1 NaCl was studied. The recovery of Sn(IV) with Qu-SG at pH 2.0 was found to be quick and quantitative. The maximum coefficient of Sn(IV) distribution was found to be 1·106 ml.g-1 under V:m=100 ml·g-1. The equation of calibration graph was F(R)420=(-0.030.05)+ +(2.660.01)·C(mg.L-1), r=0.998. The detection limit was 0.06 mg·L-1. The linearity of calibration graph was observed till 1.2 mg·L-1. The interference from Zn(II), Cd(II), Fe(III), Pb(II), Cu(II), Zr(IV) on to Sn(IV) adsorption and the value of analytical signal in diffusion reflectance spectra was carried out. No interference was observed for such ions (divisible excesses): Zn(105), Cd, Pb(10), Cu(5), Zr(2), Fe (20, in the presence of ascorbic acid). The method has been shown to be appropriate for the Sn(IV) determination in sea water.
Aluminum compounds are widely used in the treatment of tap water. On the opposite, large quantities of Al(III) are well known to create strong threats to the water ecosystem and has been long associated with some diseases such as Alzheimer’s and bone softening. Hence a great deal of attention to the determination of Al(III) has been paid recently. Numerous binary complexes of Al(III) with azo-, xanthene and triphenylmetane reagents have been used for its spectrophotometric determination. Eriochrome cyanine R (ECR) is known to be one of the effective reagents for Al(III) determination. Aluminium reacts with ECR to give a colored complex with high molar extinction coefficient. The sensitivity of the reaction can be increased by its transformation on a dextrane type resin and XAD-2 (styrene-divinylbenzene cross-linked copolymer). The application of high dispersed silica as support offers some distinct advantages over the one of the organic polymer matrixes. The aim of this paper was the development of a sensitive and selective method for the spectroscopic determination of aluminum with ECR by selective enrichment of the metal as Al(III)-reagent species on a silica modified with ion associate of ERC or its complexes and higher molecular weight quaternary ammonium salts and measurement of the a diffuse reflectance coefficient of the sorbents. The reagent was obtained by adsorption of Al-ECR complex from aqueous solution on the QAS-SG surface. It was satisfactorily applied to the determination of aluminum in natural water at the levels > 8,0 mg of Al (III) per sample (V=10ml). The interference of metal ions and anions on Al(III) adsorption and the value of analytical signal in diffuse reflectance spectra was studied. It was found that among natural water components only Fe(III) was adsorbed by modified SG at pH 3,0-3,3. No interference was observed for Fe(III) in the presence of ascorbic acid. Therefore, [QAS]+ [ECR]- immobilized on the SG surface in described way ensured a prospective reagent phase for the development of a sensitive, regenerable and reproducible optical Al(III) sensor.
The determination of molybdenum in environmental objects has traditionally been hampered by preconcentration and separation from foreign ions. This has made the determination of this bio-metal exceptionally difficult, particularly in matrices containing high levels of chloride, such as seawater and biological samples. 1,5-Diphenylcarbazone (DFC) is an effective reagent for molybdenum spectrophotometric determination. The DFC adsorption from acetone-hexane (1:4) solutions onto highly dispersed silica gel was studied. The analytical properties of immobilisied reagent depend on the nature of interaction on surface SG/reagent. For this reason, the nature of the DFC immobilization onto SG was spectrophotometric and spectroscopic (UR and diffuse reflectance) studied. The solid-phase reagent on the base of DFC immobilized (DFC-SG) on the silica was obtained.
Molybdenum. It was found that molybdenum interact with DFC-SG at pH 2.8-3.5. The maximal concentration factor was 1000 ml.g-1 at volume of solution 100 ml and mass of sorbent 0.1g. No interference was observed from Zn, Na, K, Mg, Mn, V, W, Hg, Ni and Co under optimal conditions of molybdenum determination. Fe(III) and Cr(III) do not influence of ascorbic acid and EDTA. The adsorption degree of Mo was found to be 50 % at СNaCl 1.0 - 5.0 mol/L. The color scale for molybdenum determination in seawater with detection limit 50 ppb was worked out. The color changing was light-pink/violet. The solid-phase reagent for the sorption-spectrophotometric and visual test determination of 50 and 100 ppb of molybdenum in seawater were proposed.
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