| Heavy metal pollution is a key factor in environmental degradation, which is seriously endangering human health. Heavy metal elements are widely distributed, and the toxicity depends on their chemical forms. In this respect, sensitive and accurate determination of heavy metals in the environment is of great significance.The existing techniques for detection of heavy metals, e.g., molecular spectrometry, atomic spectrometry, mass spectrometry, etc. are frequently limited in applications due to sample matrix interferences and the insufficient sensitivity of instrumentations. Moreover, elemental speciation analysis has been an urgent demand with increase of requirements for more information from complex matrix. As a result, preliminary species separation and preconcentration are often inevitable before determination by the afore-mentioned techniques. Solid phase extraction hyphenated with atomic absorption spectrometry is an important approach to develop separation/preconcentration and speciation procedures for trace and ultra-trace heavy metals. Since the adsorbents greatly influence the sensitivity and selectivity, the development of novel adsorbents with high performance is a hotspot in this field.Carbon nanotubes have excellent adsorption performance due to its unique structure and have been applied in seperation/preconcentration of various trace/ultra-trace analytes. However, in practical applications, carbon nanotubes tend to aggregate and thus dramatically deteriorate its adsorption ability. In addition, the bare carbon nanotubes generally exhibit non-specific adsorption for a wide range of targets, which restricts its application in highly sensitive and selective analysis. Its application range can be broadened via surface modification or functionalization. The unique local structures of iron phosphate have the potential for the adsorption of various species. However, its applications in seperation/preconcentration of trace/ultra-trace analytes requires further exploitations.The thesis aims at achieving selective separation/preconcentration and speciation of trace/ultra-trace heavy metals in environmental samples using functionalized carbon nanotubes and iron phosphate as solid phase adsorbents with detection by electrothermal atomic absorption spectrometry. The main contents are described in the following:Chapter1introduces the toxicity, detection techniques and separation/preconcentration approaches of heavy metals, highlights the application of solid phase extraction in this area. On the other hand, it also summarizes the updated progress of the applications of carbon nanotubes and iron phosphate.In chapter2, multi-walled carbon nanotubes (MWNTs) are noncovalently wrapped by a cationic polyelectrolyte poly(diallydimethylammoniumchloride)(PDDA) via π-π interactions. A novel procedure based on flow injection technique for on-line separation/preconcentration of trace Cr(Ⅵ) using the PDDA-MWNTs composites as adsorbent with detection by electrothermal atomic absorption spectrometry is developed. A32%improvement on the adsorption efficiency of Cr(Ⅵ) by the PDDA-MWNTs composites at pH6is achieved with respect to that obtained by the bare MWNTs. The retained Cr(VI) is afterwards recovered by using80μL of ammonium nitrate solution (0.1mol L-1) as stripping reagent producing an elution efficiency of81%. The presence of a certain amount of Cr(Ⅲ) within a Cr(Ⅲ)/Cr(Ⅵ) concentration ratio of6/1causes no interfering effect for the adsorption of Cr(Ⅵ) by the PDDA-MWNTs. A sample loading volume of1000μL creates an enrichment factor of8.6, along with a detection limit of0.016μg L-1(3σ, n=7) and a RSD of3.9%(0.5μg L-1, n=11) within a linear calibration range of0.05-1.5μg L-1. The procedure is validated by analyzing a certified reference material of GBW08608(Trace Elements in Water), in addition to spiking recovery within95-105%in a series of water samples.In chapter3, iron phosphate (FePO4) is for the first time used as a novel sorbent for highly selective adsorption of Cr(Ⅲ) in the presence of Cr(Ⅵ). At pH5.9,100%of Cr(Ⅲ)(<5μg L-1) is selectively retained on the surface of FePO4, while the adsorption of Cr(Ⅵ) is less than2%, i.e., Cr(Ⅲ) is adsorbed with an ca.62/1selectivity over Cr(Ⅵ). The adsorption of Cr(Ⅲ) fits Langmuir model, corresponding to a maximum adsorption capacity of8.12mg g". The retained Cr(Ⅲ) could be readily recovered by200μL of aqueous mixture of0.1%H2O2and0.05mol L-1NH3as stripping reagent, giving rise to a recovery of96.5%. The chromium in the eluate is quantified with detection by electrothermal atomic absorption spectrometry. A novel procedure is developed for separation/preconcentration of Cr(Ⅲ) and chromium speciation in environmental waters. A sample volume of2000μL creates an enrichment factor of8.7, along with a detection limit of0.02μg L-1(3a, n=9) and a RSD of2.5%(0.5μg L-1, n=11) within a linear calibration range of0.05-2.5μg L-1. Total chromium is determined after reduction of Cr(VI) to Cr(III) by hydroxylamine hydrochloride as a reducing reagent. The content of Cr(VI) is achieved by difference. The procedure is validated by analyzing two certified reference materials of GBW08608(Trace Elements in Water) and GBW09101(human hair), in addition to spiking recovery within94-102%for chromium speciation in a series of environmental waters.In chapter4, MWNTs are functionalized by iron phosphate to improve the selectivity and sorption capacity of carbon nanotubes. At pH6, the iron phosphate coated carbon nanotubes offer a much improved sorption capacity of32.68mg g-1for cadmium over6.72mg g-1by bare carbon nanotubes after oxidation. A novel procedure based on flow injection technique for on-line separation/preconcentration of trace/ultra-trace cadmium using the composite as adsorbent with detection by electrothermal atomic absorption spectrometry is developed. The functionalized MWNTs provide high selectivity to cadmium against complex sample matrix components, i.e., the tolerant limit for coexisting species is5-100fold improved. A100%sorption of Cd2+is achieved at pH6, and50μL of aqueous mixture of0.002mol L-1H3PO4and0.1mol L-1NH4NO3gives rise to a recovery of77%. With a sample volume of1000μL, an enhancement factor of31.2is obtained, along with a detection limit of1.3ng L-1(3a, n=11) and a RSD of2.2%(0.1μg L-1, n=11) within a linear calibration range of0.003-0.2μg L-1. The procedure is validated by two certified reference materials, i.e., GBW08608(Trace Elements in Water) and GBW07404(soil), in addition to spiking recovery within97-105%in a series of environmental waters. |
PDF Full Text Request