| Chromium species are of great interest in the metropolitan-industrial areas of southern Lake Michigan; anthropogenic sources generate a concentration of total chromium in surface water that ranges from 0.1 to >100 μg/L. Because chromium toxicity is dependent upon chemical speciation—Cr(III) is considered an essential trace nutrient for humans while Cr(VI) is a known carcinogen—accurate quantification of hexavalent chromium in surface water is essential to assessing the level of toxicity.; The research described herein details the development and optimization of a membrane sampling sequential injection system for the determination of hexavalent chromium in high-particulate matrices and the subsequent design, fabrication, optimization, and validation of an in situ monitoring design based on this approach.; A bench-top method for determining hexavalent chromium in high particulate-containing surface waters by sequential injection (SI) was developed. For utility in particulate-laden environmental waters, the relative performance of two membrane-based methods for sampling was compared. The first membrane approach was based on a commercial design known as the “supported capillary membrane sampler (SCMS)” that uses tubular membranes; the second approach was based on a conventional parallel-plate dialyzer (PPD) design that uses planar membranes. The membranes were evaluated using the colorimetric method for the determination of hexavalent chromium by complexation with 1,5-diphenylcarbazide (DPC) at 540 nm. Thin-walled (∼200 μm) microporous (pore size ∼0.2 μm) polypropylene membranes were equilibrated with DPC during each sampling period. Formation of the DPC-Cr(VI) complex allowed for efficient membrane transport; without the membrane, Cr(VI) transport decreased by ∼90%. Factors optimized included reagent concentration, sampling time, flow rate, and spectrophotometric conditions. Optimal conditions for the reagents were 2.00 mM DPC and 0.100 M nitric acid, and sampling times of 600 s and 900 s sampling times for the planar and tubular designs, respectively. The planar (PPD) design increased the sensitivity relative to the tubular (SCMS) design by ∼225%. The PPD-SI method was applied to the determination of dissolved Cr(VI) in high particulate-containing surface water samples. (Abstract shortened by UMI.)... |
