In-line membrane separation method for sulfide monitoring in wastewaters exploiting multisyringe flow injection analysis

Abstract

A fully automated multisyringe flow injection analysis (MSFIA) system coupled to gas-diffusion (GD) separation is proposed for the spectrophotometric determination of sulfide in urban wastewaters containing suspended solids. No preliminary batch sample treatment is required. The method carried out in a multicommutated system is based on analyte release as hydrogen sulfide from the donor channel of the GD module into a stagnant recipient solution composed of N, N-dimethyl- p-phenylene diamine (DMPD) and Fe(III). The methylene blue dye generated in-line via oxidative coupling is subsequently delivered downstream to a miniaturized flow-through light-emitting diode (LED)-based optical fiber plug-in spectrophotometer. The effect of various experimental variables on gas transfer across the membrane and concentration enhancement in the receiver solution via iterative flow reversals are discussed. Under the optimized chemical and hydrodynamic variables, the GD–MSFIA method features coefficients of variation better than 0.8% ( n=10) at the 10 mg l −1 level, a linear working range of 0.5–20 mg l −1 sulfide, a 3 σ blank-detection limit of 0.03 mg l −1 and an injection throughput of 13 h −1 for a sample volume of 700 μl and a forward flow mode. The analytical performance of the proposed GD–MSFIA system is also critically compared with that of in-line dialysis separation schemes using passive semi-permeable hydrophilic membranes. Accuracy was assessed by using the batch APHA–AWWA–WPFC standard method as an external reference.