Arsenic trioxide as2o3

The reagent-grade chemicals including arsenic trioxide (As₂O₃), humic acid (HA), salicylic acid (SA) and sodium arsenate dibasic heptahydrate (Na₂HAsO₄.7H₂O) were purchased from Sigma Aldrich (St. Louis, MO, USA). The other chemicals, including nitric acid (HNO₃), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium dihydrogen phosphate (NaH₂PO₄.H₂O), magnesium sulfate (MgSO₄.7H₂O) and ferric chloride hexahydrate (FeCl₃.6H₂O), were procured from local suppliers. The pure water from Millipore water purification system (Milli-Q, Millipore Co., Bedford, MA, USA) was used to prepare stock solutions and synthetic test samples. Before use, all glassware was washed with 15% HNO₃ followed by rinsing with pure water to avoid contamination. The stock solutions of As(III) and As(V) at 100 mg/L were prepared separately by dissolving As₂O₃ and Na₂HAsO₄.7H₂O in 1 M NaOH solution and pure water, respectively. The stock solutions of hydrophobic HA and hydrophilic SA were prepared by dissolving 500 mg chemical in 100 mL pure water. The detailed procedure for HA and SA solution preparation can be found elsewhere [58 (link)]. The stock solution of coagulant (100 mM) was prepared using FeCl₃.6H₂O. The working solutions of trivalent and pentavalent As were prepared via sequential dilution from each stock solution for each experimental run.

Screen-printed electrodes were purchased from CH Instruments, Inc. (TE100, Bee Cave, TX, USA). The SPE pattern included 3 mm diameter carbon working electrode, carbon counter electrode, and Ag/AgCl reference electrode. Hydrogen tetrachloroaurate(iii) trihydrate (HAuCl₄·3H₂O), sodium citrate, hydrochloric acid (37%), and sodium hydroxide, were supplied by ACROS Organics. Arsenic trioxide (As₂O₃) was purchased from Sigma Aldrich. All other reagents were obtained either from Sigma Aldrich or Fisher Scientific with the highest grade available and were used without further purification. All solutions were prepared using deionized (DI) water with a resistivity of 18.2 MΩ cm at room temperature (Ultrapure Water System, Millipore, and Billerica, MA, USA). A 20 mM primary stock solution of As(iii) was prepared by dissolving As₂O₃ (solubility in water at 25 °C is 20 g L⁻¹) in DI water. To produce a standard calibration curve for As(iii), different concentrations (0.001 mM, 0.01 mM, and 0.1 mM) of As(iii) solutions were prepared by diluting the primary stock solution using DI water. Tap water samples were from our laboratory at the University of Wisconsin-Madison, WI, which did not contain any visible sediments so samples were not filtered prior to use.