Table 2.

Steps of sequential chemical extractions. The extraction scheme mainly followed Poulton and Canfield (2005) and Keon et al. (2001). The target Fe phases were listed specifically for this study (not universally), based on sediment Fe mineralogy determined; the target As phases were listed accordingly, and were all As(V), based on sediment As speciation determined.

Step	Extractant and time	Target Fe phase	Target As phase
1	1 mol L⁻¹ magnesium chloride, pH 7, 2 h, one repetition	Exchangeable Fe	Loosely bound As
2 ^a	1 mol L⁻¹ hydroxylamine-hydrochloride in 25% v/v acetic acid, 48 h, one repetition	Amorphous Fe oxides (ferrihydrite)	Ferrihydrite associated As
3 ^a	50 g L⁻¹ sodium dithionite, pH 4.8 with acetic acid/sodium citrate, 2 h, one repetition	Crystalline Fe oxides (goethite and hematite)	Goethite and hematite associated As
4	1 mol L⁻¹ sodium phosphate, pH 5, 16 h & 24 h, one repetition for each time period	--	Magnetite adsorbed As
5 ^a	0.2 mol L⁻¹ ammonium oxalate/0.17 mol L⁻¹ oxalic acid, 6.5 h, one repetition	Recalcitrant Fe oxides (magnetite)	Magnetite co-precipitated As

Note : the step was followed by a wash step with 1 mol L⁻¹ magnesium chloride.