Synthesis Of Ferric Hydroxide And Layered Double Oxides (LDO) And Their Absorption Of Arsenic Anion In Solution

Despite its wide useage, arsenic is also a poisonous element, especially when existing in the form of As(III). Compared with other methods, ferric hydroxide and layered double oxides (LDO) (calcined by layered double hydroxides (LDH)) were chosen as two ideal absorbents.The adsorption capacity of ferric hydroxide on As(III) was studied. The three absorbent types were Fe(OH)₃ gel and two goethites in which the Fe(OH)₃ gel had been dried in a microwave oven under vacuum and at 80℃, whose adsorption capacity on As(III) in solution were 60.70%, 71.15% and 46.45% respectively. The adsorption capacity of the ferric hydroxide dried in a microwave oven under vacuum was better than the other two absorbents. The possible reasons were that the water molecules in the gel vibrated with high frequency under the effect of microwave irradiation, thereby producing higher porosity and improved surface activity. The total absorption of aresenite increased after the absorbent had been irradiated with ultrasound, which also led to difficulty in separating the solids from solution. Temperature and dissolved gas had little effect on the absorption reaction. The absorption capability of goethite and Fe(OH)₃ gel on As(III) was almost independent from dissolved gas.Mg-Al-LDH, Mg-Fe-LDH and Zn-Fe-LDH were produced by co-precipitation method, thereinto the synthesis of Zn-Fe-LDH was reported for the first time. The three LDHs were used as precursor. Being calcined at 550℃, 380℃and 170℃for two hours, they were transformed into Mg-Al-LDO, Mg-Fe-LDO and Zn-Fe-LDO respectively. Both of them have strong tendency of obtaining anion and water from ambient to recover their primary crystal structure of LDH. The adsorption capacities of Mg-Al-LDO and Mg-Fe-LDO were as follows: 114.9 mg/L (decocted), 199.0 mg/L (decocted); while the adsorption capacities of Zn-Fe-LDO were 77.8 mg/L (room temperature) and ferric hydrate were 128.3 mg/L (dried in a microwave oven under vacuum, without being irradiated with ultrasound, at room temperature). By comparing, he adsorption ability of Mg-Fe-LDO was the highest.The adsorption of Mg-Al-LDO and Mg-Fe-LDO on As(III) was interfered by dissolved CO₂. The deleterious effect of dissolved CO₂ could be reduced by increasing the temperature of the solution, which would increase the adsorption capacity of LDO for As(III), especially to Mg-Fe-LDO. But the adsorption capacity of Zn-Fe-LDO for As(III) was reduced by heating, the reason for which is under research.The As(III) solution being treated with Mg-Al-LDO and Mg-Fe-LDO was alkalescent, because MgO in the solution was alkalescent oxide, which was dissolved slightly. The pH was. increased to 10.5-11.4, because of the balance of the LDO containing magnesium on dissolving-despoiting, which was beyond the standard of drinking water, which predicating the LDO containing magnesium is not fitful to deal with drinking water. Only the As(III) solution being treated with Zn-Fe-LDO was neutral. Therefore, ferric hydroxide and Zn-Fe-LDO were more applicable for cleaning drinking water.