The absorption reactions of arsenite on Fe （hydro-）oxides are studied. The three absorbent types are Fe（OH）3 gel and two Fe （hydro-）oxides, in which the Fe（OH）3 gel was dried in a microwave oven under vacuum at 80℃. It is found that pH changes from 9.71 to 10.36 in 6 minutes after the Fe （OH）3 gel was mixed with NaAsO2 solution, as the arsenite replaces the OH- in goethite and Fe（OH）3. At the 40th minute after the start of the reaction, pH decreases, which is most probably because that the monodentate surface complex of absorbed arsenite has changed into mononuclear-bidentate complex and released proton. The decline in pH values indicates not the end of the absorption but a change in the reaction type. Temperature and dissolved gas has little effect on these two types of reactions. The total absorption of arsenite increases after the absorbent is irradiated with ultrasound, which also lead to difficulty in separating the solids from solution. The absorption capacity for arsenite of Fe（OH）3 gel dried in a microwave oven under vacuum is 53.18% and 17.22% respectively better than that of Fe （OH）3 gel and gel dried at 80℃. The possible reasons are that the water molecules in the gel vibrates with high frequency under the effect of microwave irradiation, thereby producing higher porosity and improved surface activity.