Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by coprecipitation in synthetic and natural groundwater

Abstract

Competitive effects of phosphate, silicate, sulfate, and carbonate on As(III) and As(V) removal at pH ∼7.2 have been investigated to test the feasibility of Fe(II) aq and hydroxylapatite crystals as inexpensive and potentially efficient agents for remediation of contaminated well-water, using Bangladesh as a type study. Arsenic(III) removal ∼50–55% is achieved, when Fe(II) aq oxidizes to Fe(III) and precipitates as Fe(OH) 3 at 25 °C and 3 h reaction time, in the presence of all the oxyanion. Similar results were obtained for well-water samples from two sites in Bangladesh. Heating at 95 °C for 24 h results in 70% As(III) uptake due to precipitation of magnesian calcite. A two-step process, Fe(II) oxidation and Fe(OH) 3 precipitation at 25 °C for 2 h, followed by magnesian calcite precipitation at 95 °C for 3 h, yields ∼65% arsenic removal while reducing the expensive heating period. In the absence of silicate, up to 70% As(III) uptake occurs at 25 °C. In all cases, As(III) was oxidized to As(V) in solution by dissolved oxygen and the reaction rate was probably promoted by intermediates formed during Fe(II) oxidation. Iron-catalyzed oxidation of As(III) by oxygen and hydrogen peroxide is pH-dependent with formation of oxidants in the Fenton reaction. Buffering pH at near-neutral values by dissolved carbonate and hydroxylapatite seeds is important for faster Fe(II) oxidation kinetics ensuring rapid coprecipitation of As as As(V) in the ferric phases.