ATR-FTIR sensor development for continuous on-line monitoring of chlorinated aliphatic hydrocarbons in a fixed-bed bioreactor

This article describes the continuous on-line monitoring of a dechlorinatio n process by a novel attenuated total reflection-fourier transform infrared (ATR-FTIR) sensor. This optical sensor was developed to measure noninvasiv ely part-per-million (ppm) concentrations of trichloroethylene (TCE), tetra chloroethylene (PCE), and carbon tetrachloride (CT) in the aqueous effluent of a fixed-bed dechlorinating bioreactor, without any prior sample prepara tion. The sensor was based on an ATR internal reflection element (IRE) coat ed with an extracting hydrophobic polymer, which prevented water molecules from interacting with the infrared (IR) radiation. The selective diffusion of chlorinated compound molecules from aqueous solution into the polymer ma de possible their detection by the IR beam. With the exclusion of water the detection limits were lowered, and measurements in the low ppm level becam e possible. The best extracting polymer was polyisobutylene (PIB) in the fo rm of a 5.8-mu m thick film, which afforded a detection limit of 2, 3, and 2.5 mg/L (ppm) for TCE, PCE, and CT, respectively. Values of the enrichment factors between the polymer coating and the water matrix of these chloro-o rganics were determined experimentally and were compared individually with predictions obtained from the slopes of absorbance/concentration curves for the three analytes. Before coupling the ATR-FTIR sensor to the dechlorinat ing bioreactor, preliminary spectra of the chlorinated compounds were acqui red on a laboratory scale configuration in stop-flow and flow-through close d-loop modes. In this way, it was possible to study the behavior and direct response of the optical sensor to any arbitrary concentration change of th e analytes. Subsequently, the bioreactor was monitored with the infrared se nsor coupled permanently to it. The sensor tracked the progression of the a nalytes' spectra over time without perturbing the dechlorinating process. T o calibrate the ATR-FTIR sensor, a total of 13 standard mixtures of TCE, PC E and CT at concentrations ranging from 0 to 60 ppm were selected according to a closed symmetrical experimental design derived from a 3(2) full-facto rial design. The above range of concentrations chosen for calibration refle cted typical values during normal bioreactor operation. Several partial lea st squares (PLS) calibration models were generated to resolve overlapping a bsorption bands. The standard error of prediction (SEP) ranged between 0.6 and 1 ppm, with a relative standard error of prediction (RSEP) between 3 an d 6% for the three analytes. The accuracy of this ATR-FTIR sensor was check ed against gas chromatography (GC) measurements of the chlorocompounds in t he bioreactor effluents. The results demonstrate the efficiency of this new sensor for routine continuous on-line monitoring of the dechlorinating bio reactor. This strategy is promising for bioprocess control and optimization . (C) 2000 John Wiley & Sons, Inc.