Research On Transport Of Graphite Carbon Nitride And Its Co-transport With Lead In Saturated Porous Media

In recent years,graphite carbon nitride（g-C₃N₄）was considered to be the most stable chemical allotrope in the environment.It was one of the most promising materials at the moment and was widely used in the fields of hydrogen production and organic dye adsorption and degradation.The conjugatedπregion on the g-C₃N₄ plane could adsorb aromatic pollutants.In addition,the amino groups on the surface of g-C₃N₄ and the negative charges derived from the six nitrogen lone pairs of ordered tri-s-triazine unit made it capable of adsorbing various heavy metals.Therefore,g-C₃N₄ could adsorb pollutants and carried them to migrate after entering the environment,thereby causing greater harm to the ecological environment.In this paper,column experiments were used to study the migration behavior of g-C₃N₄ in montmorillonite and humic acid modified media under different factors（ionic strength（IS）,flow rate,p H,multivalent cations and multivalent anions）.The co-migration behavior of g-C₃N₄ and lead(Pb²⁺)in montmorillonite and humic acid modified media and the mutual release ability of Pb²⁺and g-C₃N₄ in the medium were explored.The effect of physical and chemical conditions on the migration of g-C₃N₄ in saturated porous media was clarified.The migration behavior of g-C₃N₄ in montmorillonite and humic acid modified media was clarified,and the interaction between g-C₃N₄ and Pb²⁺in saturated porous media was revealed.The main conclusions of this research were as follows:（1）Under different IS,the blocking effect of g-C₃N₄ was greater with the increase of IS.In distilled water,g-C₃N₄ exhibited super mobility,and g-C₃N₄ was obviously blocked when IS=0.0001;most of the g-C₃N₄ particles stayed in the column under 0.01M.The transport of g-C₃N₄ in porous media was less affected by the flow rate.In an alkaline environment,the migration ability of g-C₃N₄ was much stronger than that of acidic environment.Under different cation valences,Al³⁺had the greatest hindrance to g-C₃N₄,followed by divalent cations Ca²⁺and Mg²⁺,and K⁺had the weakest inhibitory effect on the migration of g-C₃N₄.The increase of ionic strength was not conducive to the migration of g-C₃N₄ in the saturated medium,while the flow rate of the solution and the alkaline environment promoted the migration of g-C₃N₄.Cations could play an obstructive effect on the migration of g-C₃N₄,and the higher the ion valence,the stronger the obstructive effect.（2）With the increase of IS,the mobility of g-C₃N₄ in porous media decreased in montmorillonite modified quartz sand and humic acid modified quartz sand.The flow rate could significantly enhance the migration of g-C₃N₄ compared to the quartz sand column.Under different monovalent anions,g-C₃N₄ had the strongest migration ability in the presence of HCO₃^-,and the most retarded effect in the presence of Cl^-.The migration behavior of g-C₃N₄ showed similar laws in the three media.With divalent anions,the retardation effect of g-C₃N₄ was significantly greater than that of monovalent anions.Among the three media,CO₃^2-could promote the migration of g-C₃N₄ more than SO₄^2-.Comparing the three media,it could be found that the mobility of g-C₃N₄ in pure quartz sand was lower than that in montmorillonite and humic acid modified media,and the mobility in montmorillonite modified media was the strongest.（3）The cooperative migration experiment of g-C₃N₄ and Pb²⁺ions showed that the presence of Pb²⁺significantly reduced the fluidity of g-C₃N₄ when g-C₃N₄ and Pb²⁺coexisted in the porous media.It had little effect on the mobility of Pb²⁺when g-C₃N₄ was present in pore water.The breakthrough curve of g-C₃N₄ in Pb²⁺contaminated sand column showed that g-C₃N₄ could promote the release of Pb²⁺in quartz sand and humic acid modified medium.The montmorillonite modified medium had a strong adsorption capacity for Pb²⁺so that the carbon nitride was almost completely retained in the sand column and could not be released.The migration of Pb²⁺in the sand column contaminated by g-C₃N₄would release part of the g-C₃N₄ that was previously trapped in the sand column.The recovery rates of g-C₃N₄ in the quartz sand,montmorillonite modified quartz sand and humic acid modified quartz sand were9%,10.6%and 9%,indicating that Pb²⁺could also effectively release g-C₃N₄ trapped in the medium.Among the three sand columns,g-C₃N₄ could release Pb²⁺in the modified medium of quartz sand and humic acid,while Pb²⁺could release g-C₃N₄ in the three media;the presence of Pb²⁺significantly reduced the mobility of g-C₃N₄.