Mechanism Investigation Of Colloid And NH₄⁺ Co-Transportation In Groundwater

As a widely existing substance in aquifers,colloids have an influence on the migration of pollutants in groundwater.Nitrogen is a common contaminant with diverse sources in groundwater.Previous studies on ammonia nitrogen in porous media generally focused on the biological transformation between three forms of nitrogen,and the research on the interaction mechanism between colloids and ammonia nitrogen in porous media aquifers was relatively lacking.Therefore,this study selected NH4+as the research object,selected organic humic acid and inorganic colloid silica colloidal as a typical colloid,which are widely existed in groundwater.The effects of different hydrochemical and hydrodynamic conditions on the co-migration of NH₄⁺and colloids were studied through the indoor static adsorption experiment and dynamic migration experiment,which further supplemented the current cognition of the synergistic migration of pollutants and colloids.Research variable including the type of medium and colloid,colloid concentration,pH,IS,cation valence state and flow velocity.The main conclusions of this study as follows:1.Physical and chemical properties of"NH₄⁺-colloid"suspensionWith the increase of colloidal concentration,the hydrodynamic diameter of particles in suspension also increases.Compared with the hydrodynamic diameter under the condition of normal colloid concentration and double colloid concentration,the particle size of"NH₄⁺-colloidal silicon"increased by 18.76%,and that of"NH₄⁺-colloidal humic acid"increased by 11.27%.When IS of suspension increased due to the addition of NaCl or CaCl₂,the hydrodynamic diameters tend to increase.Compared with the influence of the addition of two different cations,adding CaCl₂affects the hydrodynamic diameter more obvious.In the"NH₄⁺-colloidal silicon"system,compared with the neutral condition,the increase and decrease of pH will increase the hydrodynamic diameter of the system,and under the alkaline condition,its particle size is the largest,reaching 390.8nm on average,which is approximately equal to twice the particle size under the neutral condition.In the"NH₄⁺-colloidal humic acid"system,compared with the neutral condition,the particle size increased by 118.55%under acidic condition and decreased by 21.60%under alkaline condition.With the increase of the colloid concentration,the Zeta potential of both"NH₄⁺-colloidal silicon"and"NH₄⁺-colloidal humic acid"systems showed an increasing trend,and both of them were in a stable state.The increase of IS will reduce the Zeta potential of the suspension.For the addition of different cation valence states,the presence of NaCl is always greater than the Zeta potential of the suspension in the presence of CaCl₂,so the suspension is more stable.The influence of acid-base change on the systems of"NH₄⁺-silica colloid"and"NH₄⁺-humic acid colloid"has the same trend.In the acidic case,the Zeta potential of suspension is the lowest,while in the neutral case,the Zeta potential is the highest.Moreover,under any acid-base condition,the stability of"NH₄⁺-colloidal humic acid"system is stronger than that of"NH₄⁺-colloidal silicon"system.2.Adsorption mechanism of"NH₄⁺-colloid"In the adsorption kinetics experiment,the order of adsorption amount on the three media was sand>quartz sand>glass beads,which was mainly caused by the surface properties of different porous media such as surface roughness,TOC and specific surface area.When the colloidal silicon concentration changes from 0mg/L to 20mg/L,the adsorption capacity of NH₄⁺on the porous media is significantly reduced.When the colloid concentration changed from 20mg/L to 40mg/L,the adsorption increased slightly.When the concentration of colloidal humic acid changed from 0mg/L to40mg/L,the adsorption capacity of NH₄⁺on the porous medium also increased with the continuous increase of the colloid concentration.In the presence of colloidal silicon,the increase of IS still inhibits the adsorption of NH₄⁺on the porous medium,and the adsorption amount reduced compared with that in the absence of colloid.After the addition of colloidal humic acid,as it can also provide adsorption sites for other particles,the competitive adsorption relationship between NH₄⁺,calcium ions and sodium ions was alleviated,resulting in a small difference in the adsorption amount of NH₄⁺at the end of the experiment under different IS conditions.In the presence of colloidal silicon,the adsorption capacity of NH₄⁺when pH=7 is nearly twice that of the adsorption capacity when pH=9,and the adsorption capacity is in the middle when pH=5.After the addition of colloidal humic acid,the adsorption capacity of NH₄⁺was significantly increased at pH=7,which was twice as much as that of the other two under this condition.In addition,the adsorption capacity of NH₄⁺at pH=5 and pH=9was not significantly different.In the adsorption thermodynamics experiment,with the increase of the adsorption concentration in the solution,the adsorption capacity of NH4+on the medium also increases,and the increase of this adsorption capacity is most obvious in the sand medium,followed by glass beads,and the least is the sand medium.The experimental results were fitted with the Langmuir,Freundlich and Henry isothermal adsorption models.In the three models,the Freundlich isothermal adsorption model could better fit the adsorption behavior of NH₄⁺in three different media,and the correlation coefficient R² was greater than 0.96.However,for a single medium,the Langmuir isothermal adsorption model has the highest fitting degree,indicating that there are many adsorption sites on the surface of the sand medium,and its adsorption for NH₄⁺is mainly single-layer adsorption.3.Migration mechanism of"NH₄⁺-colloid"In the presence of colloidal silicon,the shape of the penetration curve in the three media was similar to that in the absence of colloid,but the amount of NH₄⁺in outflow was increased compared with that in the absence of colloid.Finally,the retention of NH₄⁺in quartz sand,glass beads and sand was 1.27%,18.15%and 8.92%,respectively,indicating that NH₄⁺was most likely to migrate in quartz sand.In the presence of colloidal humic acid,the migration of NH₄⁺in the medium was reduced compared with that in the absence of colloid.The final results showed that NH₄⁺was most likely to migrate in glass beads,but most difficult to migrate in sand medium.When the concentration of colloidal silicon was 40mg/L,it inhibited the migration of NH₄⁺in the sand medium.In contrast to the concentration of 20mg/L,the migration of NH₄⁺in the sand medium was significantly affected.The increase of colloidal concentration leads to the aggregation of particles in the suspension and the particle size increases.As a result,it is easy to cause the blocking of medium pores during migration and finally leads to the relatively slow and delayed release in the leaching stage?stage 2?.The higher the concentration of colloidal humic acid,the more obvious the inhibitory effect was.In the absence of colloid,the hysteresis of NH₄⁺in the sand medium decreases with the increase of IS,and the increase of IS promotes the migration of NH₄⁺in the sand medium,and the effect of divalent cation is more significant than that of univalent cation.In the presence of colloidal silicon,the overall trend of the penetration curve was similar to that in the absence of colloid,but the value of the penetration curve was increased compared with that in the absence of colloid.In the presence of colloidal humic acid,the increase of IS still plays a role in promoting the migration of NH₄⁺in sand medium.However,in the context of the inhibitory effect of humic acid colloid on the migration of NH₄⁺,it is not as obvious as the above two conditions.Whether the colloid exists or not,compared with low velocity,high flow rate of further promote the transport of NH₄⁺in sand,this is mainly because with the increase of flow velocity,the shear stress on the surface of the medium caused by water flow also increase,the material adsorbed on the surface of the medium is more likely to be shear and carried away by it,thus leading to an increase in concentration in influent.Under acidic conditions,colloidal silicon still promoted the migration of NH₄⁺in sand medium,while colloidal humic acid had the opposite inhibitory effect.On the contrary,under alkaline conditions,humic acid colloid promoted NH₄⁺migration,while silica colloid inhibited its migration.In the longitudinal comparison of NH₄⁺migration under the conditions of pH=5,7 and 9,the peak area of the penetration curve under acidic and alkaline conditions was significantly earlier than that under neutral conditions,mainly because the electronegativity of"NH₄⁺-colloidal silicon"and"NH₄⁺-colloidal humic acid"systems were reduced compared with that under neutral conditions.