Speciation of inorganic selenium in wastewater using liquid electrode plasma-optical emission spectrometry combined with supramolecule-equipped solid-phase extraction system

Abstract

A new approach for the rapid analysis of inorganic Se (i-Se: SeIV and SeVI) in an aqueous acidic matrix is proposed. A supramolecule-equipped solid-phase extraction (SPE) system is used for the ion-selective retention and pre-concentration of i-Se as SeIV, followed by analysis using a portable liquid electrode plasma-optical emission spectrometer (LEP-OES). The SeIV in the matrix is analyzed in the preliminary step. The total i-Se (i-SeTot) is transformed to SeIV via an acid-induced reduction treatment in a concurrent step, and the value from the initial step is subtracted from the latter for the quantification of SeVI. Eight different SPE systems, containing diverse supramolecules, were evaluated in terms of selectivity towards SeIV or SeVI, in addition to retention intensity and capacity. Superior performance was observed with AnaLig As-01. AnaLig As-01 was optimized considering implementation at different solution pHs, sample-loading flow rate, eluent-type or eluent-volume, and the influence of competing ions in the matrix. The intrusive factors of LEP-OES sensitivity, such as interfering component ions or low i-Se content in the matrix, were minimized by SPE-assisted pretreatment. The LEP-OES operating variables, e.g., applied voltage, on-time, off-time, and pulse count for the applied voltage, were optimized to obtain a distinct peak in the spectrum for i-Se at λmax, 196.026 nm. The limit of detection (3σ) for i-Se determination using the proposed process was 2.15 µg mL−1. The method was applied to the analysis of i-Se in a certified reference material of wastewater (SPS-WW1). The recoveries (%) by the standard addition method were ≤ 100 for SeIV and ≤ 83 for SeVI. The datasets from the LEP-OES analysis and inductively coupled plasma optical emission spectrometric measurements were also comparable for the same matrix (coefficient of variation: SeIV, 0.9%; SeVI, 3.6%).