Analysis Of Interaction Mechanism Between Heavy Metals And Layered Double Hydroxides In Salt Lake And Chromite Ore Processing Residue

Heavy metal pollution and resources loss are worldwide environmental problems.Heavy metals not only endanger the ecological environment,but also threaten human health.As a rare resource that can be recycled and reused,it has not been efficiently recycled for a long time.How to achieve the comprehensive utilization of resources and reduce environmental hazards has great theoretical research value.In recent years,layered double hydroxides?LDHs?,which is a kind of two-dimensional anionic clay two-dimensional nanomaterials,have been widely studied in the fields of energy catalysis and environmental remediation.Among them,heavy metal pollution control is one of the hottest research fields.LDHs have an anion-exchange capability featured with lamellar structure.Also,it is easy for preparation and of low-cost.However,the current method of enhancing the interaction between LDHs and heavy metals is generally based on surface complexation or interlayer anion exchange.Theses two forces are generally enhanced by surface or interlayer modification,while the intereaction mechanisms are still not clear.Therefore,in order to further explore the microscopic mechanism of the interaction between LDHs and heavy metals,this work chose the uranium extraction in saline lake and the treatment of chromite ore processing residue?COPR?as the research systems.The staged dominant forces as well as the phase-change behavior during the interaction were studied.The whole research is divided into the following three aspects:?1?Magnesium binary layered double hydroxides?MgAl-LDH?and its Fe-induced ternary LDH?MgAlFe-LDH?were synthesized for the extraction of simulated concentrations of U?VI?in the saline lake brine system.Batch experiments have shown that both LDHs have strong affinity towards uranium.MgAl-LDH yielded of stronger affinity in lower U?VI?concentrations?0.2 mg/L to 5 mg/L?,while MgAlFe-LDH was at higher U?VI?concentrations?5 mg/L to 30 mg/L?.The coexisting cations have little effect on the extraction rate.When the initial concentration of Ca²⁺reached 5000 mg L^-1,the extraction rate of MgAl-LDH is still over90%while for MgAlFe-LDH is above 70%.The pH analysis,XRD,FT-IR and XPS studies concluded that the main mechanisms are surface complexation and interlayer carbonate coprecipitation.?2?Six kinds of the Cr?VI?-host phases in COPR were synthesized to form a simulated COPR system.The results of adsorption experiments showed that LDHs had a larger adsorption capacity of Cr?VI?compared with other phases.Among them,hydrocalumite?CaAl-LDH?has the highest adsorption capacity.During the adsorption process,the pH was stable at around 12,which showed a buffered property and it is consistent with the actual COPR.Combined with the acid washing experiment,when the concentration of sulfuric acid concentration is 3.8mol/kg,the elution rate of the simulated COPR which contains hydrocalumite is 18%lower than that without hydrocalumite.Thus,it proves that the hydrocalumite is a key Cr?VI?-host phase in COPR.?3?The batch adsorption experiments were used to analyze the interaction between hydrocalumite and Cr?VI?under varied initial concentration.Combined with X-ray Absorption Spectroscopy?XAFS?,X-ray powder diffractometer?XRD?and Scanning Electron Microscope?SEM?,the microscopic mechanism and phase change of interaction between hydrocalumite and chromium are further explored.Our studies have shown that the interaction was divided into four stages:surface complexation stage?3-15 mg/L?,transition stage?15-35 mg/L?,ion-exchange stage?35-100 mg/L?and deposition stage?>100 mg/L?.The results showed that the binding force was stronger in the first three stages.Among them,the transition stage is functioned by both surface complexation and ion-exchange,which results in the strong affinity.At the same time,through the study of the interaction of Cr?VI?with hydrocalumite over time,the mechanism was further confirmed.Thus,the strong staged affinity of hydrocalumite to Cr?VI?may be the key factors for the sustained release of COPR.