Ultrasonic extraction of iron from non-aqueous liquids

Abstract

A novel procedure for the extraction of iron from predominately organic solvents has been described. An ultrasonic probe was used to create a microemulsion with a small quantity of nitric acid such that labile iron could be released into the aqueous vesicles and subsequently quantified after phase separation. The analytical and operational viability of using a simple colorimetric assay based on the coordination of aminothiol ligands (principally homocysteine) was evaluated in terms of signal sensitivity, selectivity and stability. The use of homocysteine provided a linear range for iron(III) from 9 μM to 50 μM with a corresponding limit of detection of 2 μM (based on 3s_b). The effectiveness of the approach was assessed through the recovery of 0.3 ppm iron from a sample of commercial kerosene and the results compared with those obtained through attempting to quantify the iron under passive (ultrasonically silent) conditions.

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1. Introduction

The presence of trace quantities of water within oil can present a significant corrosion hazard and is of particular concern in the chemical and petroleum industries where the elevated temperatures encountered during the processing of non-aqueous feedstocks can substantially exacerbate the process. A bewildering number of strategies have been developed to minimise the degradation that can occur as a result of external environmental factors but moisture contamination within the internal surfaces of pipework, reactor bodies or metallic storage vessels can be a particularly pernicious problem.¹ Dissolved iron resulting from the corrosion of steel casings could potentially be used as a versatile indicator of material fatigue and through being able to monitor changes in the concentration of the metal ion, structural failure of the various containment systems could be predicted. The aim of the present Communication has been to develop a procedurally simple protocol that can enable the detection of labile iron within oil products whilst meeting the detection demands levied by the selective quantification of the ion at the trace level.

The direct determination of iron within oil samples tends to be hampered by the low concentration of the target and the difficulties encountered in handling the non-aqueous fluid.² Extraction of the iron into an aqueous layer is undoubtedly the preferred route though achieving this is a non-trivial task that often requires recourse to elaborate separation and sample digestion procedures. The methodology proposed herein relies upon the ultrasonically induced emulsification of the oil with a small quantity of aqueous acid.³ The latter serves to collect and effectively pre-concentrate the target iron. It was envisaged that the removal of the ultrasound field would result in the separation of the respective layers and thus present a facile method through which the iron could be rendered accessible to analysis.

Through extracting the iron into the aqueous phase it should be possible to utilise simple colorimetric techniques to provide a semi quantitative on-site or field assessment of the iron content. Transferability of the approach would clearly be dependent upon the acquisition of selectivity and to this end we have sought to investigate the applicability of one such protocol. Our approach seeks to exploit the highly specific colorimetric response that can arise through the complexation of iron(III) with aminothiol ligands such as cysteine. It must be acknowledged that the coordination of iron with such ligand systems can be somewhat complicated and involve a number of competing reactions,^4,5 as indicated by Scheme 1. Adaptation of the system for analytical purposes however may be relatively straightforward and holds considerable merit given the selectivity of the response (particularly when compared with other ligand systems such as bipyridyl^6–8 or phenanthroline.^9,10

Scheme 1 Reaction pathway and analytical signal

The analytical characteristics of the assay procedure and the efficacy of coupling it to the ultrasonic extraction protocol have been assessed. The applicability of the technique was evaluated through examining the recovery of iron within a commercial kerosene sample.

2. Experimental details

2.1 Equipment and reagents

All reagents were obtained from Aldrich and were of the highest grade available and used without further purification. All solutions and subsequent dilutions were prepared using deionised water from an Elgastat (Elga, UK) UHQ grade water system with a resistivity of 18 MΩ cm. Spectroscopic measurements were conducted using a UV/Vis UNICAM dual beam spectrometer using disposable polystyrene cuvettes (1 cm path length). The sonoelectrochemical assembly consisted of a 20 kHz transducer (VCX400, Sonics and Materials, USA) with a stepped 3 mm titanium horn tip. A transducer power setting of 15% was applied throughout.

2.2 Assay procedure

An aliquot of the sample (typically 1 mL) was taken and added to 1 mL of sodium borate buffer (0.1 M, pH 10). Homocysteine (200 μL, 0.2 M) and NaOH (50 μL, 2 M) were added to the cuvette, the resulting mixture was then gently agitated and the absorption spectrum recorded. Consecutive aliquots of a standard iron solution (20 μL, 1.05 mM/0.1 M HNO₃) were then added to the cuvette and the absorbance spectra recorded. The amount of iron present within the original sample was evaluated using conventional standard addition plots using the absorbance values monitored at 566 nm.

3. Results and discussion

3.1 Validation of the detection assay

The addition of iron (10 μL, 1.05 mM) to the assay solution in which homocysteine was employed as the ligand (20 mM, pH 10 borate) led to development of a purple colouration (λ_max 566 nm) and is attributed to the bis-homocysteinate ligand system, [FeOH(HomoCys)₂]²⁻.^1,2 The resulting absorption spectra are detailed in Fig. 1A with near identical spectra obtained when cysteine was used as the indicating reagent. A quantitative comparison between the two systems is shown in Fig. 1B with similar molar absorptivities (5.0 × 10³ and 4.8 × 10³ L mol⁻¹ cm⁻¹ for cysteine and homocysteine respectively). Homocysteine was however found to possess a larger dynamic range with the linear region extending from 9 μM to 50 μM iron(III) while cysteine was found to exhibit a plateau after 30 μM. As such, homocysteine was subsequently used throughout the following experiments and was found to possess a limit of detection of 2 μM (based on 3s_b).

Fig. 1 (A) Absorbance spectra detailing the responses obtained to iron (10 μM aliquots) in the presence of excess homocysteine (0.02 M). (B) A plot of absorbance against concentration of iron added for three coordinating systems: Cysteine pH 10 (◇), homocysteine pH 8 (●) and homocysteine pH 10 (∇).

A major limitation of the approach emerged on assessing the stability of the analytical signal over time. The intensity of the colouration was found to decrease upon standing and can be attributed to the reduction of the ferric centre to the ferrous state through the conversion of the aminothiol ligand to the disulfide form (R–SS–R). The kinetics of the reaction have been studied previously^1,2 and as such are not discussed in any depth here. The adaptation of the approach to provide a viable analytical protocol can however be realised despite the apparent poor stability. Gentle agitation of the solution was found to return the purple colouration to the level of that originally observed. This is highlighted in Fig. 2 where the two experiments are compared. An assay was performed on 52 μM Fe in which the solution within the cuvette was agitated vigorously by manual mixing immediately prior to the commencement of each absorbance measurement. This procedure was performed over a period of 30 min at 5 min intervals with no deterioration in the signal observed. This was contrasted by the second experiment in which the contents of the cuvette were left under quiescent conditions and the absorbance again measured at 5 min intervals. The intensity of the purple colouration was found to decrease markedly. However, mixing the contents of cuvette after a period of 2 h returned the signal to that observed in the initial measurement.

Fig. 2 Influence of agitation on the homocysteine assay response to 52 μM iron over a period of 2 h.

3.2 Appraisal of selectivity

The selectivity of the assay procedure for iron is vital both to ensure accuracy and indeed the transferability of the approach. The influence of other metal ions (Cu(II), Mn(II), Cr(III), Ni(II), Pb(II), Cd(II), Co(II), Zn(II)) on the colorimetric response were assessed under the conditions of the assay at 1.5 mM (representing an excess of more than 100 times that of the iron). The purple colouration was absent in each case and the addition of the metal ions was found to induce no appreciable effect on the absorbance. A transient brown colouration was however observed in a few occasions and was attributed to the hydrolysis products under the alkaline conditions of the assay. The presence of any residual Fe(II) would not affect the quantification due to the homocysteine complexing specifically to Fe(III) (Scheme 1).

3.3 Ultrasonic extraction of iron from non-aqueous solvents

Two model systems (decane and decyl alcohol) were selected to probe the efficacy of using ultrasound as a means of facilitating the extractive pre-concentration of iron from a predominately non-aqueous environment. The solvents were used individually and seeded with water (0.6% v/v) containing a non-complex ferric salt (sulfate). An aliquot of the wet solvent (typically 40 cm³ of liquid providing 16.8 μM (0.3 ppm Fe)) was placed within a boiling tube and 6 mL of 0.1 M HNO₃ added. The ultrasound probe was inserted to a depth of 4 cm within the organic layer (avoiding disruption of the aqueous layer) and the ultrasonic field initiated for two periods of 5 min with a ‘silent’ equilibration interval of 5 min between each insonation. The use of an ultrasound probe rather than a conventional ultrasound bath was found to significantly improve the rate of emulsification with the contents of the tube becoming turbid within a few seconds of commencing the probe-based sonication process. Relinquishing the ultrasound field led to the passive separation of the two layers over a period of several hours. The aqueous layer was subsequently extracted and the assay procedure, detailed in the previous section, used to determine the iron content. Absorption spectra detailing the standard addition determination of the liberated iron are detailed in Fig. 3. Typical recoveries for an initial 16.8 μM (0.3 ppm) Fe sample were 102% (%RSD < 5%, n = 3). The pre-concentration factor was 6.7 providing a final aqueous layer concentration of 123.2 μM (2.2 ppm) Fe. The recovery experiments were repeated without the imposition of the ultrasound field with the exchange of iron from oil to water allowed to progress under a passive regime whereby the solution was mixed initially after the addition, left to stand for 5 min and then once again agitated prior to the absorbance measurement. The typical recovery of iron under such conditions was 21% and clearly serves to highlight the efficacy of employing ultrasonic extraction.

Fig. 3 (A) Absorbance spectra detailing the standard addition determination of 2.2 ppm iron (0.3 ppm before preconcentration) within kerosene. (B) Corresponding standard addition plot.

3.4 Recovery of trace iron within kerosene

The applicability of the approach to an authentic processing sample was assessed with kerosene. The ultrasonic emulsification and subsequent iron analysis were carried out using an analogous procedure to that detailed in the previous section for decane and decyl alcohol. Absorption spectra detailing the standard addition determination of iron after extraction are detailed in Fig. 3. The recovery of 16.8 μM (0.3 ppm) Fe from 40 mL of kerosene was found to be 105% (RSD = 5%, n = 3).

4. Conclusion

The use of sono-emulsification allied to a simple colorimetric analysis allows the ready quantification of iron in organic media such as kerosene: for routine samples prior calibration would obviate the need for the standard addition procedure. The ready extension of the methodology to other metals is envisaged.

5. Acknowledgements

The authors thank Windsor Scientific Ltd for assisting the support of this project.

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