Study On Simultaneous Stabilization And Its Mechanisms Of Multiple Heavy Metals In Arsenic Slag By Thermal Desorption Slag

China has rich reserved arsenic resources and is the largest producer of arsenic trioxide all over the world.A large amount of arsenic slag generated during arsenic smelting and production process,which has caused multiple heavy metals pollution of surrounding site due to its open-air disposal.Stabilization has attracted extensive attention in the field of heavy metal remediation.The key to this technology is the research of stabilization material.Previous studies have focused on exploring the pollution characteristics and stabilization of As in arsenic slag.There is no report on the development and application of synchronous stabilization materials for multiple heavy metals in arsenic slag.Therefore,developing material for the simultaneous stabilization of multiple heavy metals in arsenic slag is of great significance to its harmless treatment.As a kind of solid waste produced from thermal desorption of oil sludge contaminated soil,the thermal desorption slag is of large discharge and usually contains clay minerals,carbon residues and so on,which is a potential material for synchronous stabilization of multiple heavy metals in arsenic slag.But no relevant studies have been reported.From this,in view of multiple heavy metals pollution of abandoned arsenic slag from Guangxi Province,the thermal desorption slag was used as raw material to study its simultaneous stabilization performance and mechanisms of multiple heavy metals in arsenic slag.This research provides theoretical basis and technical support for the simultaneous stabilization of heavy metals in arsenic slag by thermal desorption slag and achieves waste control by waste.The arsenic slag used was acidic with a pH value of 3.36 and its main pollutants included As,Zn,Cu,Cd,with the leaching concentration of 0.69 mg·L-1、85.33 mg·L-1、41.62 mg·L-1、2.58 mg·L-1 respectively.And the pH value of leachate was 3.65.The dominant heavy metal carrier minerals were arsenic-bearing Jarosite,Scorodite,Arthurite and Carminite,Franklinite and Marmatite and so on.The thermal desorption slag was mainly composed of quartz,calcite,portlandite and muscovite,with a pH value of 12.06.The thermal desorption slag exhibited perfect adsorption capacity for As,Zn,Cu,Cd in solution.The maximum theoretical adsorption capacity of thermal desorption slag for As,Zn,Cu,Cd were 86.59 mg·g-1.249.07 mg·g-1.339.88 mg·g-1.322.86 mg·g-1,respectively.The As was stabilized via formation of Ca-As precipitation,and the Zn,Cu,Cd were stabilized through precipitating as hydroxides/carbonates/silicates combining with ion exchange.The thermal desorption slag showed synchronous stabilization effect on As,Zn,Cu and Cd in arsenic slag.Under the conditions that the dosage of thermal desorption slag was 5%,the reaction time was 1 day and the initial pH value of leaching solution was 6,the leaching concentrations of As,Zn,Cu and Cd in the arsenic slag were 0.08 mg·L-1,0.62 mg·L-1,0.0010 mg·L-1 and 0.0326 mg·L-1,respectively.The pH value of the leachate after reaction was 6.85.But the leaching concentrations of As,Zn,Cu,Cd cannot synchronously meet the environmental quality standards for surface water-IV category and the As could be released under thermal desorption slag treatment.By comparing the three modification methods involving FeSO4 impregnation,pyrolysis and FeSO4 impregnation-pyrolysis,it was found that the FeSO4 impregnation-pyrolysis modification could significantly increase the synchronous stabilization ability of thermal desorption slag for As、Zn、Cu、Cd in arsenic slag.The optimal modified material(FPOP)was prepared at Fe/thermal desorption slag of 10%,pyrolysis temperature of 900℃ and pyrolysis time of 60 min.When the dosage of FPOP was 5%,the reaction time was 1 day and the initial pH value of leaching solution was 6,the leaching concentrations of As,Zn,Cu,Cd in arsenic slag were 0.0513 mg·L-1,0.1032 mg·L-1,0.0018 mg·L-1,0.0016 mg·L-1,respectively.The pH value of leachate after reaction was 7.37,which synchronously achieved the environmental quality standards for surface water-Ⅳ category.The FeSO4 impregnation-pyrolysis modification process mainly included the dissolution of calcium hydroxide,the formation of gypsum and ferrous silicate,the dehydration of gypsum,the reduction decomposition of carbon and calcium sulfate,the decomposition of calcium carbonate and the solid reaction at high temperature.The prepared FPOP was a Ca-Fe-Si-S composite,which was mainly composed of calcium sulfide,ferrous sulfide,calcium silicate,calcium sulfosilicate,grossularite and andradite,and also contained amorphous phase and oxygen-containing functional groups.The FPOP could promote the transformation of As、Zn、Cu、Cd in arsenic slag into stable forms through complexation,chemical precipitation and ion/anion exchange,and also exhibited good long-term stability for heavy metals.Under the simulated field conditions of temperature 20℃,humidity 78%and water content 20%,the pH value of arsenic slag increased to 6.73 after incubation with 5%FPOP for 90 days.Meanwhile,the extracted As,Zn,Cu,Cd decreased to 0.0630 mg·L-1,0.2530 mg·L-1,0.0125 mg·L-1,0.0036 mg·L-1 and the pH value of leachate was 6.88,synchronously achieving the environmental quality standards for surface water-Ⅳ category.The results showed that FPOP was feasible for simultaneous stabilization of multiple heavy metals in arsenic slag under field situation.The findings of this study provide an efficient and feasible stabilization material for simultaneous reduction of the leaching toxicity of multiple heavy metals in arsenic slag and similar wastes as well as develop a new way for the resource utilization of thermal dsorption slag.