Spectrophotometric Method for Rapid Determination of As(V) and As(III) in Industrial Wastewater
Arsenic is a toxic metalloid predominantly present in water as As(V) and As(III), whose speciation governs toxicity and mobility. Conventional speciation methods (HPLC-ICP-MS, IC-HG-AFS) offer ultralow detection limits but suffer from high cost, long analysis times, and non-portability, hindering on-site rapid monitoring. This study presents a sulfide-based spectrophotometric method exploiting the quantitative reaction between As(V) and S2− to form monothioarsenate (H3AsO3S) with a characteristic absorption at 233 nm. Under optimized conditions (H+ concentration 1 mol·L−1, Na2S dosage 5 mmol·L−1, reaction time 3 min, N2 purging 2 min), As(V) is directly quantified. Total arsenic is determined after complete oxidation of As(III) to As(V) using NaClO (10 mmol·L−1, pH 12, 5 min), and As(III) is obtained by difference. The method exhibits linearity over 0.5–50 mg·L−1 (A = 0.0209c + 0.0627, R² = 0.999), a detection limit of 0.17 mg·L−1, spike recoveries of 101.9%–104.1%, and relative standard deviation of 1.06%. Validation against real industrial wastewater samples showed relative deviations <10% compared with HPLC-ICP-MS and IC-HG-AFS. Total analysis time is within 15 min. The method is simple, cost-effective, and suitable for field monitoring.