| Natural siderite and synthetic siderite have been used as adsorbents for As removal in this study. Effects of contact time, initial As concentration, As species, temperature, pH, co-existing anions on As adsorption were intensively investigated.Results show that it takes 72.0 h to reach equilibrium of As adsorption on nature siderite, which is much longer than 3.0 h on synthetic siderite. Adsorption Kinetics of nature siderite agrees with Lagergren pseudo-second order model as well as synthetic siderite. Langmuir isotherm gives a better description of adsorption data of As(III) and As(V) on natural siderite, indicating that the physics adsorption controls the whole process.The maximum adsorption capacities of As(III) and As(V) on synthetic siderite are up to 14.47 and 30.77 mg·g-1 at 45 oС, respectively, which are much higher than natural siderite (0.84 and 0.70 mg·g-1). Adsorption isotherm can be better described by Langmuir isotherm model for As(III) adsorption at 45 oС, indicating that the coverage of the adsorption sites is in the form of monolayer, although Freundlich isotherm yields a better fit to the experimental data at 25 and 35 oС. The adsorption data for As(V) better fits Langmuir isotherm at low temperatures (i.e., 15 and 25 oС), while Freundlich isotherm at relatively high temperatures (35-45 oС). Arsenic adsorption on the synthetic siderite is spontaneous and endothermic in nature. The optimum pH for As(III) and As(V) removal by synthetic siderite is broad, ranging from 3.0 to 10.0. The presence of nitrate, sulfate, phosphate or silicate does not have adverse effect on As(III) adsorption. But the As(V) adsorption is impeded by the presence of SiO32?, followed by PO43? and NO3?. Removal of both As(III) and As(V) is independent on background electrolyte concentration. The formation of Fe hydroxide minerals (such as lepidocrocite and goethite) followed by As coprecipitation and adsorption is shown to be the main mechanism of As removal by the synthetic siderite.
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