Removal Of Arsenic And Fluoride By Siderite Synthesized At High Temperature And High Pressure

Synthetic siderite have been used as adsorbents for As and F removal in thisstudy. Effects of synthetic conditions on As adsorption were intensively investigated.Moreover, the adsorption efficiency and mechanism of As and F by using the optimalsynthetic siderite have been discussed in this study.Siderite was synthesized in a kettle at high temperature and high pressure. Thesiderite synthesized at 15 MPa and 200℃for 24 h was the best for As adsorption. Itwas assigned to the optimum siderite and used in the following studies.Kinetic results showed that Lagergren pseudo-second order model betterdescribed As and F adsorption on the optimal synthetic siderite. Adsorption ofAs（Ⅲ） and As（Ⅴ） on the optimal synthetic siderite agreeed with Langmuir isotherm,while the adsorption data for F better fits Freundlich isotherm. The reactions of Asand F adsorption were spontaneous endothermic reactions, and the maximumadsorption capacities of As（Ⅲ）, As（Ⅴ） and F on the optimal synthetic siderite were upto 14.14, 10.19 and 2.559 mg·g^-1at 55℃and 45℃, respectively.The influence of solution pH on As（Ⅲ）, As（Ⅴ） and F adsorption wasinsignificant. However, As（Ⅲ） and As（Ⅴ） adsorption was negatively affected by thepresence of SiO₃^2- and PO₄^3- , with the greater inhibition effects for the higherconcentrations.It was shown that Fe hydroxide minerals producing from siderite surfacehydrolysis and oxidation contribute to high As adsorption capacity under aerobiccondition. During adsorption, As（Ⅲ） were oxidized to As（Ⅴ）. However, oxidations ofsiderite and As（Ⅲ） were much less in the anaerobic condition than those in oxicconditions. The removal capacity of the optimal synthetic siderite in the presence ofoxygen was greater than adsorption in the absence of oxygen.In the coexistence of F-, As adsorption was not affected. Besides, As had nosignificant effect on F-adsorption. Due to higher As adsorption capacity, removal ofF- should be paid more concern. The investigation provided an empirical equationbetween F-initial concentration and the dose of the optimal synthetic siderite to meet F and As drinking water guidline for the waters with initial As concentrations lessthan 5.0 mg/L.