The absorption reactions of goethite and its precursors onarsenite were 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 the vacuum condition and at 80℃. It was found that, due to the replacement of OH- by arsenite in goethite and Fe(OH)3, pH changed from 9.71 to 10.36 6 minutes after the Fe(OH)3 gel was mixed with NaAsO2 solution. At 40 minutes after the start of the reaction, pH decreased, which was caused by the condensation reaction between the arsenite and the absorbent. The decline in pH values did not indicate the end of the absorption, but was an indication of a change in the reaction type. Temperatures and dissolved gas had little effect on these two types of reactions. The total absorption of aresenite increased after the absorbent had been irradiated with ultrasound, which also caused difficulty in separating the solids from the solution. The absorption capacity for arsenite of Fe(OH)3 gel dried in a microwave oven under vacuum was 53.18% and 17.22% better than that of Fe(OH)3 gel and gel dried at 80℃, respectively. The reasons for such phenomena might be that the water molecules in the gel vibrated with high frequency under the action of microwave irradiation, thereby producing higher porosity and improving surface activity.