Speciation Analysis Of Iron In Coastal Waters With Electroanalytical Methods

Iron is one of the essential micro-nutrient elements for all organisms in marine ecosystems.Trace iron in seawaters has been considered to be a key factor of phytoplankton growth limitation.Besides,iron's speciation has a close relationship with its uptake of microorganisms,thus iron plays an important role in marine biogeochemistry.Therefore,accurate and fast analysis of trace iron and its speciation in time is now a difficult and hot spot in marine environmental sciences.To date,many analytical methods have been developed for determination of trace iron in seawaters,such as atomic absorption spectrometry,inductively coupled plasma-mass spectrometry,spectrophotometry,and so on.However,these methods are relatively not only cumbersome and expensive,but also require complicated operation and high cost of testing and maintenance,which can not meet the growing demands of the fast and real-time detection of iron.Most importantly,these methods can only determine total iron.As a low cost,simple,fast,portable,high sensitivity and in situ analytical technique,electrochemical method has shown great prospects in rapid analysis of trace iron and its speciation in seawater.Working electrode material is the core component of electrochemical sensor,directly determining the analysis of detection performance.In this paper,a series of electrochemical methods based on different working electrodes have been developed for trace iron and iron speciation analysis.In addition,the electrode process of Fe^3+/2+ and Fe???-2,2'-bipyridyl complex have been systematically studied.The main contents are as follows:1.Determination of Fe???at reduced graphene oxide/methylene blue/gold nanoparticles modified glassy carbon electrode.The reduced graphene oxide and methylene blue were chosen as the materials to help gold nanoparticles to grow in situ and self catalysis.Taking the advantage of gold nanoparticles' fast electron transfer rate,the modified electrode showed a very good reduction signal of Fe???.This is an adsorption controlled and quasi-reversible electrode process.The response to Fe???is in the linear range of 0.3¹00 ?M with a detection limit of 15 nM.This method has been successfully applied to determine total dissolved iron in real coastal seawaters and river waters.2.Determination of Fe???at titanium carbide nanoparticles/Nafion modified glassy carbon electrode.Titanium carbide nanoparticles possess promising electrochemical and catalytic properties which can accelerate electron transfer rate.Nafion,as a typical cation-exchange polymer,can easily form a thin film on solid electrode and facilitate the preconcentration of Fe???.Based on the synergistic effect of titanium carbide nanoparticles and Nafion,with the catalytic amplifying effect of H₂O₂,an excellent cathodic signal of Fe???can be obtained.The linear range of this sensor is from 0.07 to 70 ?M and the detection limit is 7.2 nM.This method has been successfully used to sensitively determine total dissolved iron in real coastal waters.3.Adsorptive cathodic stripping voltammetric determination of Fe???at tin-bismuth alloy electrode.The novel home-made tin-bismuth alloy electrode has the similar electrochemical properties with mercury electrode and is better than bismuth film electrode.This environmental-friendly tin-bismuth alloy electrode has great potentials for stripping analysis of iron.In this system,tin-bismuth alloy electrode was employed as working electrode,and a ligand of Fe???called1-?2-piridylazo?-2-naphthol?PAN?was introduced to improve the selectivity and sensitivity.Under an accumulation potential of-0.3 V and an accumulation time of60 s,a linear range of 1-900 nM and a detection limit of 0.2 nM can be obtained based on adsorptive cathodic stripping voltammetry.This method has been applied to determine total dissolved iron in coastal waters.4.Determination of Fe???at titanium carbide nanoparticles-Nafion/platinum nanoflowers modified glassy carbon electrode.Cubic structure titanium carbide nanoparticles were used as the growth template of three-dimensional platinum nanoflowers.Nafion was used to help titanium carbide nanoparticles to be better attached onto the electrode surface and to slow down the crystal rate of platinumnanoflowers during the electrodeposition which resulted in more active surface sites.Taking the advantage of synergistic effect of titanium carbide nanoparticles and Nafion as well as the catalytic amplifying effect of platinum nanoflowers,an excellent anodic stripping responses to the oxidation of Fe???-2,2'-bipyridyl complex can be obtained under an accumulation potential of-0.1 V and an accumulation time of 60 s.The linear range of this sensor is from 1 nM to 6 ?M with the lowest detectable concentration of 0.1 nM.This method has been successfully applied for sensitive determination of Fe???in coastal waters.5.Theoretical study of Fe^3+/2+ electrode process.The heterogeneous electron transfer kinetics?k0 values?and thermodynamic properties associated with Fe^3+/2+process in different electrolytes(HCl,bis?trifluoromethanesulfonyl?imide?HNTf2?,HClO₄,and silicotungstic acid(H4[?-SiW₁₂O₄₀]))and at different electrodes?glassy carbon and boron doped diamond electrodes?have been systematically studied based on direct current?DC?cyclic voltammetry and large amplitude Fourier transformed alternating current voltammetry?FTACV?.The diffusion coefficient?D?values for Fe^3+/2+ are dependent on the radius of the electrolyte anion.The k0 values are found to be smaller at boron doped diamond electrode?BDD?than glassy carbon electrode?GCE?,and influenced by the ion-pairing effect.Based on the reversible potential,HNTf2 is suggested to be a more innocent electrolyte than commonly used HClO₄ to obtain the true k0 values.FTACV with a favorable signal to background ratio,was used to determine k0 values associated with the Fe^3+/2+ process in electrolyte of H₄[?-SiW₁₂O₄₀] and HNTf2 at GCE and probe the influence of the heterogeneity of the BDD surface on k0 values in H₄[?-SiW₁₂O₄₀].6.The study of the electrode process of Fe???-2,2'-bipyridyl.The electrode transfer process of Fe???-2,2'-bipyridyl?Bp?complex at different electrodes?glassy carbon,boron doped diamond,platinum and gold electrodes?have been studied by direct current?DC?cyclic voltammetry and large amplitude Fourier transformed alternating current voltammetry?FTACV?.This is a one-electron,diffusion-controlled process at all electrodes.The slower electron transfer kinetics?k0 value?at BDD,relative to the other electrode materials,are discussed in terms ofthe density of electronic states.Moreover,the electrochemical evidence for2,2'-bipyridyl can be used to separate Fe???and Fe???,thus to determine Fe???in solutions containing Fe???that has been reported.7.Determination of different iron speciation in coastal waters.The concentrations of total reactive iron,reactive Fe???,reactive Fe???,total dissolved iron and iron complexed with natural organic matters in Yantai coastal waters can be obtained based on catalytic adsorptive cathodic stripping voltammetry and suitable pretreatments.2,3-dihydroxynaphthalene?DHN?was chosen as the complexing ligand to Fe???.2,2'-bipyridyl was added during the sampling to mask Fe???in order to determine reactive Fe???,and KBrO₃ was utilized to catalytically improve the reduction current of Fe???-DHN.This method has been successfully used for 5days'?every second day?determination of iron speciation in Yantai coastal waters.Besides,the concentration and distribution of total dissolved iron and iron speciation in Sishili Bay in February 2017 and March 2017 have also been investigated in this study.