The occurrence of heavy metal ions as emerging contaminants in the aquatic environment is of increasing interest,so that modern sensitive techniques have been employed for their identification and determination. It can induce acute and chronic toxic effects both on human and animals through contaminated foods and feeds. The determination of particular elements is usually preceded by their separation from the major components of the sample and from other elements. Separation methods often enable a particular element be determined in a sample containing no interfering elements [1]. The use of synthetic inorganic ion-exchangers as stationary phases in thin layer chromatography (TLC) has led to a new era in separation science [2, 3, 4],leading to the development of fast and selective separation methods [5, 6, 7, 8]. These ion exchangers have many advantages over the traditional stationary phases used in TLC [9]. They show very selective behavior,since the retention factor (Rf) values depend on the combined partition-adsorption-ion exchange process in addition to the nature of the mobile phase.
Amongst the inorganic ion exchangers,tetravalent metal acid salts are widely studied and have attracted interest due to their excellent thermal stability,chemical resistivity and unique selectivity towards certain metal ions [10, 11]. In continuation to our work on testing new supports in TLC [12, 13, 14, 15],we have studied the chromatographic behavior of toxic metal ions on titanium phosphate as the cation exchanger. Some important separations are summarized in this paper. The effects of pH of the solvent system on the Rf values in the presence of complex-forming anions have been studied.
A Camag automatic TLC coater was used to prepare plates. The development was performed in a twin-trough chamber (Camag) for 20 cm×20 cm plates. All chemicals and reagents were of analytical grade (Merck or BDH).
Five hundred milliliters of 0.1 mol/L solution of titanium (IV) chloride in 20% (v/v) hydrochloric acid and 500 mL of 0.1 mol/L sodium dihydrogen phosphate were simultaneously added dropwise with constant stirring to 3 L of demineralized water in a flask [16]. The rate of addition of the reactants was so adjusted that the contents of the flask were just neutral to methyl red indicator. After complete precipitation,the white gel obtained was stirred further for 2 h. This was then kept in contact with the mother liquor overnight for aging,and the formed gel was washed several times with demineralized water by decantation until the supernatant was free from chloride,titanium and phosphate ions. The supernatant liquid was completely removed and a slurry prepared by mixing the gel (75 mL) with silica gel G60 (15 g) as a binder in a 500 mL conical flask with a Teflon stopper. Shake the flask vigorously for 3 min. The slurry was then poured immediately into the automatic TLC plate coater to coat seven 20 cm×20 cm glass plates with a 300 μ m layer thickness. The plates were dried in an oven at 70 ℃ for 2 h and then stored at room temperature inside an incubator.
The metal ions studied were Mg2+,Al3+,V5+,Cr3+,Cr6+,Mn2+,Fe3+,Co2+,Ni2+,Cu2+,Zn2+,As3+,Se4+,Sr2+,Zr4+,Mo6+,Ag+,Cd2+,Sn2+,Sb3+,Ba2+,La3+,Ce4+,W6+,Au3+,Hg2+,Tl+,Pb2+,Bi3+,UO2+2.
The 0.05 mol/L solutions of chlorides,nitrates or sulphates of most of the metals were prepared in 1% (v/v) solution of the corresponding acids.
The metal ion solutions were applied to each plate as circular spots,by means of fine glass capillaries. The spots were completely dried and the plates were developed in ascending mode to a distance of 12.5 cm from the starting point. When the development was completed the plates were dried in an air oven and the ions were detected with appropriate reagents.
Saturated solution of sodium sulfide in water was used to detect antimony,mercury,silver,thallium,lead,cadmium,bismuth and arsenic. Nickel and cobalt were detected with 10 g/L dimethylglyoxime in ethanol. Iron,copper and uranium were detected with 30 g/L potassium ferrocyanide in water. Saturated solution of sodium rhodizonate in water was used to detect barium and strontium. Zinc,manganese and chromium were detected by means of 10 g/L diphenylcarbazide in ethanol. A mixture of 50 g/L stannous chloride in 4 mol/L hydrochloric acid was used to detect selenium,molybdenum,gold and tungsten. Lanthanum,magnesium,cerium and zirconium were detected by means of 1 g/L alizarine in ethanol. 1% (v/v) hydrogen peroxide was used to detect vanadium. Aluminium was detected with 10 g/L aluminon in ethanol [17, 18]. The development times for the separation are recorded in the last columns of Table 1 and Table 2.
During the last decades,increasing attention has been focused on pollution of natural environment by heavy metals,which are not biodegradable. The abatement of pollution of surface and drinking water and soil is an important task requiring selective separation methods for removal of heavy metals. The role of the adsorbent is critical in most separation techniques [19].
Titanium phosphate,an inorganic ion-exchanger for the class of tetravalent metal acid salts has been synthesized by sol-gel method [16]. This cation exchanger possesses structural hydroxyl groups,in which the H+of -OH is located at the exchangeable sites. This ion-exchanger can be prepared as both amorphous and crystalline solids. The amorphous materials can be obtained in granular form,since they are prepared by precipitation method which does not exhibit preparative reproducibility [20]. Materials prepared by sol-gel method,on the other hand,exhibit high homogeneity and purity,low temperature processing,structural control of materials formed and materials with improved and desired properties [21].
It is clear from the results in Table 1 and 2 that titanium phosphate is a good adsorbent and is very useful as a stationary phase in planar chromatography. One inorganic ion often can be easily separated from several other ions. A few such separations are given in Table 1. As pointed in the third column in this table,some of the ions interfere in the separation because of the same Rf values or Rf close to the appropriate ion. Therefore they are considered as interfering ions and can’t be separated from the appropriate ion. Some simple aqueous systems have been used to achieve many ternary and binary separations of ions on titanium phosphate layers. Some important separations of analytical interest are given in Table 2.
As thin layer prepared from titanium phosphate gel alone deteriorates during chromatographic development,we have used mixed titanium phosphate-silica gel (as binder) layers that are suitable and stable. Silica gel is known to act as a weak acidic cation exchanger [22]. RT (Rf of the rear of the spot) and RL (Rf of the front of the spot) values measured for many ions on pure silica gel plates differed considerably from those on mixed beds developed with the same mobile phases (Fig. 1). Thus the mixed layers of titanium phosphate with silica gel show high adsorption capacity and increased resolution and results in many rapid and complete separations of inorganic ions with simple aqueous mobile phases. The surface adsorption is not a monolayer with single site. Two or more sites with different affinities may be involved in metal ion sorption [23]. Since the sorption has been carried out in aqueous medium,the most important factor responsible for selectivity was the size of the hydrated ionic radius. Smaller the size of the cation,more heavily it was hydrated and therefore hindered ion-exchange. Many important ternary and binary separations achieved (Table 2) were based on selective hydrolytic adsorption and the ion-exchange process on the mixed bed was carried out not only through electrostatic exchange but also through varying sorption of anionic,neutral and cationic complexes.
Tartaric acid,widely used in baking powder,medicine and printing technology [24] depending on pH,forms a variety of complex species with metal ions which can be very useful in ion-exchange separations [25]. Change in the pH of the solution is also known to change ion-exchange behavior of inorganic ion-exchanger [26]. The variation of the Rf values of the metal ion studied here was due to the formation of new species in solution at different pH and their interactions with the ion-exchanger. This wide variation in Rf values,however,did not afford new possibilities for improvement of specific separations.
Fig. 2 shows that,irrespective of the pH of the mobile phase,cations such as Ag+,Tl+and Pb2+were strongly adsorbed at the point of application (origin) because of the strong interactions with the adsorbent,whereas Au3+and Hg2+formed negatively-charged complexes,which were not easily adsorbed and had high Rf values. For most other cations,however,Rf values decreased with increasing pH of the mobile phase. This might be because of the weak dissociation of tartaric acid in acidic media and strong interaction of H+ ions of mobile phase in acidic pH which occupied the adsorbent sites of exchanger,causing high Rf values. With increasing pH,tartaric acid dissociated,acting as a complex-forming agent,and resulted in gradual formation of cationic and neutral complexes [27, 28],which interacted with the adsorbent and resulted in a decrease in their Rf values.
The quality of separation depended to a large extent on the mobile phase,and selectivity was achieved by varying the composition of the mobile phase. Chromatographic separation of the metal ion with hydrochloric acid and acetone in various volume ratios as mobile phases has resulted in widely different behavior (Rf values) (Fig. 3). However,this wide variation in Rf values provided new possibilities for improvement of specific separations. Fig. 3 shows the plot of Rf values versus various volume ratios of 10% (v/v) HCl and acetone as mobile phase. Most of the metal ions showed a decreasing trend of Rf values with increasing HCl content,and an increasing pattern with decreasing amount of acetone,except Ag+,Tl+,Pb2+ and W6+ which recorded a low Rf pattern throughout and Au3+ and Hg2+ which had high Rf values. This might be due to the fact that at first most of the cations easily formed negatively-charged chloride complexes [29, 30],which were weakly solvated by acetone as a mild solvent and were not adsorbed on the separating bed resulting in enhancement of their Rf values. After that as the amount of hydrochloric acid content gradually increased with decreasing acetone by breaking the anionic and neutral complexes the cations were adsorbed on ion-exchanger plates resulting in decreasing Rf values. Further increase in HCl content of mobile phase increased the H+ ions which in competition with metal ions occupied the exchange sites of the plate and resulted in increasing Rf values. Zr4+ and W6+ did not form chloride complexes and were adsorbed easily while Ag+,Tl+ and Pb2+ were precipitated on separating bed and did not move. However Au3+ and Hg2+ formed negatively-charged complexes which were not adsorbed on the cation exchanger bed resulting in their high Rf values.
The present study shows that the inorganic ion exchanger titanium phosphate exhibits high sorption capacity and various selectivity of titanium phosphate towards these metal ions and results in achieving various specific separations. Therefore titanium phosphate ion-exchanger is more suitable as a stationary phase in thin layer chromatography. Simple and rapid method has been developed for separation of metal ions which facilitate the quantitative analysis of numerous ions by scanning densitometry of real samples. Further quantitative work is under progress.
2014, Vol. 32 Issue (12): 1373-1379
PDF (668 KB)
2014, Vol. 32 Issue (12): 1373-1379
PDF (668 KB)