Chemical Oxygen Demand(COD)Removal From Bio-treated Coking Wastewater By A Reduced Clay Mineral

Coking wastewater is produced from coal gas purification and chemical refining in coking plants,which usually laden with various organic and inorganic pollutants.Most dissolved organic matter in coking wastewater is refractory with the characteristics of persistent organic pollutants(POPs),which is toxic and has an inhibitory impact on microorganisms.Depending on treatment methods such as anaerobic-anoxic-oxic technology and sequential batch reactors,the effluent quality of coking wastewater,especially the chemical oxygen demand(COD),usually does not meet the requirement for discharging wastewater into streams(for China,COD<80 mg/L),which can induce environmental and ecological consequences.Therefore,it has been focused on how to achieve the standard discharge of coking wastewater through advanced technologies for recent decades.Fe-bearing clay minerals are ubiquitous in natural environments,which consist of soils and sediments.Due to their high specific surface area and high adsorption capacity,these fine-grained minerals have been widely utilized in environmental remediation.Moreover,clay minerals can be used as catalysts for heterogeneous Fenton reaction.Once exposed to oxygen,many reduced Fe-bearing clay minerals can produce reactive oxygen species(ROS),which can oxidize organic pollutants.Here we present an environmental friendly method for removing COD from bio-treated coking wastewater by using chemically reduced nontronite(rNAu-2).A slightly acidic pH 6.5 in a phosphate buffer favored OH production and COD removal(from 199.10±0.90 to 80.4±1.97 mg/L)by rNAu-2.The amount of COD removal was positively correlated with the amount of·OH production.Oxygen is the decisive factor for rNAu-2 to remove COD from bio-treated coking wastewater.NAu-2 removed a fraction of COD by adsorption,and the existence of oxygen did not affect the removal amount of COD.In contrast to NAu-2,once exposed to oxygen,rNAu-2 was able to produce large amounts of·OH which could oxidize organic components,thus significantly decreased COD in bio-treated coking wastewater.Re-oxidized rNAu-2 removed a negligible amount of COD.To elucidate the mechanisms of COD removal by rNAu-2,we applied FT-ICR-MS to compare changes of molecular formulas of DOM after rNAu-2-treatment.Related to DOM degraded by rNAu-2,the DOM produced in rNAu-2-treated wastewater exhibited a higher O/C ratio.Relative to initial wastewater,rNAu-2 treatment increased the proportions of those compounds contained more oxygen atoms,suggesting that the rNAu-2 treatment of bio-treated coking wastewater was an oxidation process.Excitation-emission matrix(EEM)fluorescence spectroscopy showed that significant removals of all fluorescent components in coking wastewater,with the aromatic protein-like compounds being removed the most and the humic-like substances the least,suggesting that aromatic protein-like substances were more vulnerable to·OH while humic-like components were more resistant to oxidation than other components.The redox cycling of structural Fe(II)in nontronite was sustainable.In three cycles,nontronite exhibited similar COD removal efficiencies,suggesting that nontronite could be recycled for continuously removing COD from bio-treated coking wastewater.