Study On Thallium Removal By Persulfate Using Iron Based Heterogeneous Catalyst

Thallium?Tl?is a toxic trace element.In China,there are abundant Tl-bearing mineral resources.However,over-exploitation of Tl-bearing mineral has resulted in large amounts of Tl into water environment,which poses a potential threat to human health.Meanwhile,researches on Tl pollution and removal techniques are relatively limited compared to other toxic metal?loid?s,due to the lack of surveillance on Tl contamination for a long time.Among various Tl treatment methods towards water,advanced oxidation process is a highly efficient and novel approach due to its fast reaction,outstanding removal performance and simple operation.Sodium persulfate?Na₂S₂O₈,i.e.PS?is a cheap and convenient oxidant,which can be activated by catalyst to generate strong oxidants,viz.sulfate free radical?SO₄^-·?and hydroxyl radical?·OH?.These radicals can oxidize Tl?I?into Tl?III?,and then precipitate in alkaline environment,thus improving removal efficiency.Ferrous ion(Fe²⁺)is one of the most common catalysits to activate PS.It is much more feasible and energy-saving than other physical activation.However,excessive levels of Fe²⁺may compete with SO₄^-·,leading to poor oxidation and removal efficiency.To solve this problem,two heterogeneous catalysts were utilized in this study.One catalyst was zero-valent iron?Fe⁰?,the other was biochar?BC?.Biochar may be capable of interacting with various ferric salts to form heterogeneous catalysis to further activate PS.In this study,Fe⁰ and iron-based BC were prepared respectively to activate PS,and to explore different influencing factors on Tl removal,in order to shed insights into the Tl removal mechanism.In Fe⁰-PS system,various influencing factors such as initial Tl concentration,catalyst and oxidant dosage,reaction time,initial and coagulation p H,ionic strength and coexisting organic matter were comprehensively investigated.Results showed that when the initial Tl concentration was 10 mg/L with PS concentration of 6 m M,the maximum removal rate of Tl reached above 99%?dosage:0.1 g/L,reaction time:30 min and coagulation p H:10?.However,the presence of HA,NO₃^-and Cl^-could reduce the Tl removal.Results of scanning electron microscopy X-ray energy spectrometer?SEM-EDS?,X-ray photoelectron spectroscopy?XPS?,and electron spin resonance?ESR?found that Fe⁰-PS system removed Tl via the oxidation of radicals?·OH and SO₄^-·?and the adsorption of iron hydroxide precipitate.The recycle experiments of Fe⁰ showed that it had stable and excellent catalytic ability within four runs.Ficus microcarpa aerial root was calcined into BC.Iron-based biochar Fe?NO₃?₃@BC and Fe Cl₃@BC were derived from hydrothermal and calcination process.In this study,various influencing factors were also investigated.Results indicated that Fe Cl₃@BC could remove Tl up to 96.5%,with the initial Tl concentration at 10 mg/L,both the initial p H and the coagulation p H of 10,reaction time for 30 min and the concentration of PS at 6 m M.However,similarly,the presence of HA and NO₃^-might reduce the removal efficiency of Tl.The adsorption process could be fitted well with the pseudo-first-order kinetic model.The isothermal adsorption of Fe Cl₃@BC was well fitted by Freundlich equation,while Fe?NO₃?₃@BC was better described with Langmuir equation.The findings of SEM-EDS,XPS,FTIR and ESR analysis showed that radicals?·OH and SO₄^-·?oxidation and biochar adsorption played a significant role in iron-based BC-PS system.The regeneration experiments indicated that Fe?NO₃?₃@BC and Fe Cl₃@BC biochar have a good regenerative capability.