Passivation performance and mechanism of coal gangue with different pollution levels based on CaO and calcite
Coal gangue represents a substantial bulk solid waste,which is the main by-product of coal mining and processing,accounting for 10％to 25％of the total mining output.Its current aggregate exceeds 6 billion tons,with an annual growth rate of 800 million tons.As a quintessential form of industrial waste,the extensive accumulation of coal gangue results in significant environmental concerns.Coal gangue harbors a multitude of pollutants with heavy metal pollution notably pronounced.Upon exposure to atmospheric,aquatic,and soil environments,heavy metals in coal gangue can dissolve,migrate,and transform into more hazardous forms,resulting in environmental pollution and potential health risks to humans.Particularly for communities residing near coal mines,prolonged exposure to heavy metals in coal gangue markedly elevates their health risks.Consequently,the detoxification of coal gangue prior to its resourceful utilization holds paramount importance.Typically,two approaches are employed to diminish the bioavailable forms of heavy metals in coal gangue.One strategy involves the extraction of heavy metals through the application of leaching agents.Although this method is effective,this approach may lead to potential secondary pollution.Alternatively,heavy metals can be passivated.Passivation effectively reduces the bioavailable state of heavy metals,rendering them more stable and less prone to migration.Passivation stands out as an efficacious approach for heavy metal pollution treatment,notable for its large-scale applicability,straightforward procedure,and significant remediation results.This research utilized CaO and calcite for the passivation of coal gangue contaminated at different levels.Experimental results revealed that both CaO and calcite had decent passivation performances on heavy metals in coal gangue,effectively reducing moderate or high level pollution of coal gangue to a slight or clean level of pollution.The passivation effect of calcite was slightly better than that of CaO,with the average passivation efficiency of CaO on Pb2+,Zn2+and Cu2+of 37.4％,37.8％and 25.5％,and the average passivation efficiency of calcite on Pb2+,Zn2+and Cu2+of 53.8％,43.5％and 34.4％,respectively.Passivation processes using CaO and calcite convert the active heavy metals in coal gangue into residual form.Characterization was conducted on coal gangue pre-and post-passivation.SEM analysis revealed that post-passivation coal gangue had a denser structure,more surface grooves,and a significant number of small particles adhering to its surface.The passivating agents successfully adhered to the coal gangue surface or formed precipitates post-passivation.XRD results revealed insoluble substances containing heavy metals in the post-passivation coal gangue.The formed precipitates within the system likely interact with other groups to form various insoluble minerals,contributing to the passivation effect.Visual MINTEQ software was utilized to simulate the passivation process,modeling the behavior of heavy metal ions under varying pH levels and temperatures.Simulation findings indicated that CaO formed insoluble hydroxide precipitates by supplying OH-ions and elevating pH levels,subsequently binding with heavy metal ions.Furthermore,calcite,in addition to increasing the system's pH,also contributed CO32-,which leaded to the formation of insoluble carbonates that binded with heavy metal ions in the coal gangue.Under the post-passivation,the coal gangue exhibited favorable results in simulated acid rain leaching experiments.In the CaO-treated system,heavy metal ions were undetected post-leaching due to the rise in overall pH of the coal gangue following CaO passivation,which neutralized the effects of simulated acid rain,thereby inhibiting heavy metal ion leaching.Post-passivation with calcite,there was a marked reduction in leaching,demonstrating that passivation effectively impedes the mobility of heavy metals.The utilization of CaO and calcite for passivation is demonstrated to be effective in immobilizing heavy metals in coal gangue,offering significant implications for detoxifying coal gangue and treating industrial heavy metal pollution.
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