2.3. Chemical Analysis. Prior to microcosm incubation, the levels of multiple chemical elements, including Fe, Mn, K, Na, Ca, As, Cd, Mg, Cr, Cu, Ni, Zn, P, and S, in the sediments were analyzed by X-ray fluorescence (XRF) (SPECTRO XEPOS, Kleve, Germany). For aqueous phase analyses, about 2 mL of sample was drawn from each microcosm at the fourth month in an anaerobic chamber. Samples were passed through a 0.45 μm filter for arsenic species analysis using high-performance liquid chromatography/inductive coupled plasma/ mass spectrometry (HPLC-ICP-MS) (Agilent 1260 Infinity Quaternary LC System; Agilent 7700x, Santa Clara, CA, USA). A column containing a mixture of 2.0 mM PBS/0.2 mM EDTA (pH6.0) solution was used as the mobile phase. Parameters for ICP-MS included RF power, 1400 W; plasma gas, 15 L/min; auxiliary gas, 1.0 L/min; carrier gas, 1.1 L/min; and sampling depth, 7.5 mm. Condition settings for LC included mobile phase flow rate, 1.0 mL/min; column temperature ambient; injection volume, 50 μL; run time, 10 min; and number of injections, 1. Typical detection limits for four species, As(III), As(V), MMAA (monomethylarsonic acid), and DMAA (dimethylarsinic acid), are around 0.1 ppb. However, the detection limits regarding actual samples may be somewhat higher due to the sample preparation steps and interferences from matrix. The filtered aqueous samples were also analyzed for multiple metal elements by inductively coupled plasma/atomic emission spectroscopy (ICP-AES) (SPECTRO CIROS 120).