Voltammetric characteristics of hydrous Fe(III) oxide embedded into Nafion and immobilised onto a screen-printed carbon electrode: binding of arsenate versus phosphate

Abstract

Nafion offers microporous channels of typically 1-4-nm diameter for cation exchange. Recently, it has been shown that these cation exchanger properties can be inverted to allow anion binding by pre-filling the hydrophilic channel structure. Here, prefilling is performed with hydrous iron oxide and sensitivity towards anionic phosphate and arsenate analytes is investigated. After a period of phosphate/arsenate accumulation, the voltammetric response in aqueous 1 M NaNO 3 is obtained based on the Fe(III/ II) redox process. The position of the peak at distinct potentials clearly reveals the presence of either phosphate or arsenate, presumably present in the form of FePO 4 and FeAsO 4. In the presence of mixtures of phosphate and arsenate, a competition of FePO 4 versus FeAsO 4 nucleation (within the Nafion microporous host) is suggested to result in a switch of phosphate detection at higher concentrations (ca. > 500 μM, solubility controlled) towards arsenate detection at lower concentrations (ca. < 500 μM, nucleation controlled). This phenomenon is suggested to be linked to the Ostwald step rule.

Electroanalysis with a single microbead of phosphate binding resin (FerrIX™) mounted in epoxy film

“…The capacitor consistently shows a value of approximately 30 pF (independent of KCl concentration), which is assigned here to the charging of the epoxy film holding the FerrIX™ microbead. The resistor Rp represents the high-frequency resistivity of the microporous FerrIX™ microbead (with contributions from the electrolyte close to the microbead [8]). Perhaps interestingly, the value for Rp is (in part) KCl concentration dependent.…”

“…Perhaps interestingly, the value for Rp is (in part) KCl concentration dependent. There appear to be two components based on (i) a resistor that is inversely proportional to the KCl concentration as would be expected for access diffusion [8] and electrolyte filled pore regions and (ii) a resistor that remains constant associated with smaller pores. Figure 6B shows a plot of Rp −1 data and a fitted simplistic model (the two resistors in series, one electrolyte concentration dependent and one independent) with fixed 5500 Ohm and a concentration dependent resistance of (250/[KCl]) Ω.…”

“…In contrast to these electrically conducting high surface area materials, traditional ion exchanger materials with high surface area and ion binding capacity have been developed based on polymer or polymer-inorganic hybrids [6]. The use of hydrous iron oxide as electrosorption material (particularly for arsenate and phosphate) in conjunction with high surface area carbons [7] and with ion exchanger membranes [8] has been explored.…”