Mechanisms Of Mineral Transformation In Stabilization Of Realgar Tailings Induced By Microwave

In China,large amounts of realgar mine tailings have been abondoned in the realgar mining area due to our world’s largest reserves of realgar ore,which are threatening ecological environment and human health.Stabilization technologies can effectively improve the stability of As in tailings,reducing their environmental risk.Efficient transformation of metastable As species to stable Fe-As minerals would be the desired pathway,which could be enhanced by microwave irradiation due to its capacity for reduction of reaction activation energy and promotion of reaction rate.In this study,microwave technologies were applied to the stabilization of realgar tailings,which came from Yunnan province,China.The transformation and mineralization mechanisms of As-containing minerals in realgar tailings under microwave irradiation were investigated in detail.Main research conclusions are as follows:（1）The total content of As in realgar tailings was 12.6%,and As（Ⅱ）,As（Ⅲ）and As（Ⅴ）accounted for 32.2%,17.4%and 50.4%,respectively.The leaching concentration of As is192 mg/L,mainly in the form of As（Ⅴ）（about 95.3%）,which exceeded the limit of As leaching concentration（5 mg/L）of the―Identification standards for hazardous wastes‖（GB5085.3-2007）,thus,realgar tailings were identified as a hazardous waste.As-containing minerals in Realgar tailings mainly include realgar(As^ⅡS)and pharmacolite（CaHAs^VO₄?2H₂O）.The high leachability of As in realgar tailings could be attributied to the dissolution of pharmacolite and realgar.（2）The efficient stablization of As in realgar tailings was achieved by the joint action of microwave irradiation and ferrous sulfate heptahydrate（FeSO₄）addition.The As leaching concentration of realgar tailings was decreased from 192 mg/L to 2.07 mg/L through 20 wt%FeSO₄ addition and 560 W microwave irradiation for 10 min,which met―Standard for pollution control on the security landfill site for hazardous wastes‖（GB18598-2001,As:2.5 mg/L）.Because of the heating effect of microwave treatment,realgar tailings were dried quickly and could be directly buried in the landfill,which was beneficial for reducing maintenance time and space cost.（3）Microwave irradiation can accelerate the release of As from realgar tailings.As leaching concentration of realgar tailings increased from 192 mg/L to 472 mg/L after microwave irradiation（560 W,45 min）,which increased by 146%,and the pH remained about 7.5.As（Ⅴ）leaching concentration was 273 mg/L,an increase of 50%,and As（Ⅲ）leaching concentration was also significantly increased to 199 mg/L.In aerobic conditions,As（Ⅱ）of realgar tailings decreased from 32.2%to 27.4%,while As（Ⅲ）increased from17.4%to 23.3%,and As（Ⅴ）remained unchanged（50.4%）.About 13.3%of S₂（-Ⅱ）is oxidized to SO₄^2-.Ca-As minerals are dissolved with the release of As（Ⅴ）and Ca²⁺into solution under neutral conditions,and the released Ca²⁺reacts with SO₄^2-to form CaSO₄.（4）The mechanism of microwave-induced stabilization of realgar tailings by FeSO₄is the transformation from metastable As to stable Fe-As minerals.The ions of FeSO₄ have synergistic effects on stabilizing As of realgar tailings with microwave irradiation.Sulfates anions can raise As leaching concentration,showing that sulfate anions can enhance the dissolution of As-contained mineral in realgar tailings.After FeSO₄ treatment with microwave irradiation（560 W,20 min）,the As leaching concentration is less than 2.5mg/L,forming insoluble crystalline As-containing mineral（Symplesite,Fe₃（As^VO₄）₂?8H₂O）.Microwave irradiation significantly reduced the time for Fe-As minerals crystallization and mineralization.