Rapid screening of inorganic arsenic in groundwater on-site by a portable three-channel colorimeter

Rapid screening of inorganic arsenic (iAs) in groundwater used for drinking by hundreds of millions of mostly rural residents worldwide is crucial for health protection. Most commercial field test kits are based on the Gutzeit reaction that uses mercury-based reagents for color development, an environmental concern that increasingly limits its utilization. This study further improves the molybdenum blue (MB) colorimetric method to allow for faster screening with more stable reagents. More importantly, a portable three-channel colorimeter is developed for screening iAs relative to the WHO drinking water guideline (10 µg/L). Adding the reducing reagents in sequence not only prolongs the storage time to >7 days, but also accelerates the color development time to 6 min in conjunction with lowering the H2SO4 concentration in chromogenic reagents. The optimal pH ranges from 1.2 to 1.3 and is achieved by acidifying groundwater to 1 % (v/v) HCl. With detection limits of 3.7 µg/L for inorganic arsenate (iAs(V)) and 3.8 µg/L for inorganic arsenite (iAs(III)), testing groundwater with ∼10 µg/L of As has a precision <20 %. The method works well for a range of phosphate concentrations of 48–950 µg/L (0.5–10 µmol/L). Concentrations of total_iAs (6–300 µg/L), iAs(V) (6–230 µg/L) and iAs(III) (0–170 µg/L) for 14 groundwater samples from Yinchuan Plain, Pearl River Delta, and Jianghan Plain, are in excellent agreements (linear regression slope: 0.969–1.029) with the benchmark methods. The improved chemistry here lays the foundation for the MB colorimetric method to become a commercially viable screening tool, with further engineering and design improvement of the colorimeter.