Gallium arsenates have been widely used in the manufacturing of communication and LED optical electronics products. Hydrogen arsenate gas is one of the compounds that chemical vapor deposition (CVD) process used. Wastewater containing soluble arsenic is generated due to waste gas wash by scrubbers. Currently, the conventional precipitation method is able to treat wastewater containing arsenic. However, it generates a large amount of hazardous sludge, which cannot be disposed of easily and economically; and a wastewater containing arsenic in excess of 10 mg/L, which is much higher than the regulated effluent concentration of 0.5 mg/L. It is, therefore, readily apparent that the development of an environmentally friendly method of treating the arsenic-containing wastewater is urgently called for.
U.S. Pat. No. 4,861,493 discloses a process for the removal of metals, in particular heavy metals, from the wastewater in the form of their sulfides by mixing the wastewater with a water-soluble sulfide. According to the invention the metal-containing wastewater is thoroughly mixed with the water-soluble sulfide at a suitable pH in a reactor of the fluidized bed type provided with an appropriate bed material, on which the metal sulfide crystallizes out, whereby the thus obtained bed material covered with crystalline metal sulfide is removed from the reactor and new bed material is added to the reactor from time to time. Usually as water-soluble sulfide, an alkali metal sulfide, alkali metal hydrogen sulfide, ammonium sulfide or ferrous sulfide is used, whereas the use of sodium sulfide, sodium hydrogen sulfide, potassium sulfide or potassium hydrogen sulfide is preferred. According to this prior art process, the following metals: Ni, Sr, Zn, Cu, Fe, Ag, Pb, Cd, Hg, Co, Mn, Te, Sn, In, Bi or Sb may be removed. However, only Hg was removed from water at a pH value of 4–10 as shown in the examples disclosed in this prior art.
U.S. Pat. No. 5,348,662 discloses a process of removing heavy metals (arsenic, tin and lead) from aqueous solutions (groundwater) by precipitation of a salt thereof, wherein an oxidizing agent (ozone, hydrogen peroxide, sulfuric acid, nitric acid or hydrochloric acid) is optionally used to increase the valence of said metal, and a precipitation-enhancing agent (calcium sulfate, arsenic trioxide, calcium arsenate or cupric oxide) is added to maximize particle size of the precipitate and to facilitate its separation from said solution. This prior art process will generate sludge with water content of 60–80%, which is not only bulky but also difficult to be resourced due to various contaminations contained therein.