Study On The Transportation Of Colloidal Mg?OH?₂ In Aquifer And Its Stabilization Performance For Heavy Metals

In recent years,heavy metal pollution in groundwater is accelerating annually.How to stabilize the heavy metals in groundwater efficiently has become a focus of environmental problems.In-situ reaction zone?IRZ?can transform heavy metals into non toxic or less toxic forms by chemical reactions such as oxidation,reduction,adsorption,precipitation and organic complexation.IRZ technology has been extensively used in groundwater remediation in recent years due to the advantages of cost-effectiveness,simple operation and the ability to remediate deep groundwater pollution.To make sure an IRZ functions well,IRZ technology requires the following characteristics of the remediation reagents:firstly,the injected remediation reagents have a high capacity to treat pollutants;secondly,the remediation reagents have a satisfied migration performance in the underground environment.As a low solubility alkali,Mg?OH?₂ is capable to limit the drastic pH rise of groundwater and to provide a long-term source of alkalinity after injected to groundwater.Therefore,Mg?OH?₂ is an excellent remediation reagent for heavy metal polluted groundwater,which has broad application in the control and remediate of soil and groundwater contamination.However,Mg?OH?₂ is easy to aggregate and with a positive surface charge.Resulting in a poor stability and low migration performance of its suspension,which limits its application in groundwater remediation.In this paper,colloidal Mg?OH?₂ was prepared by modify Mg?OH?₂ using surfactants.After modification the stability and transport performance of Mg?OH?₂were significantly enhanced.The sustained release OH^-and stabilize heavy metal ions characteristics of colloidal Mg?OH?₂ were investigated.The effects of influencing factors?including injection velocity,porous media size,ion strength of groundwater and initial concentration of Mg?OH?₂?on the transport and distribution of colloidal Mg?OH?₂ in aquifer were studied by one-dimensional simulation column experiments.Aiming at the heterogeneous characteristics of Mg?OH?₂ particles,a one-dimensional migration model of colloidal Mg?OH?₂ in aquifer was established by combining the existing colloid migration models with the calculus method.The effects of pollutant types and concentrations on the migration properties of colloidal Mg?OH?₂ in aquifers were investigated.The migration and distribution of colloidal Mg?OH?₂ in aquifers polluted by high concentrations of heavy metals were enhanced by increasing viscosity.Finally,the stabilization effect of heavy metals by Mg?OH?₂ was investigated,and the interaction mechanism of heavy metals in Mg?OH?₂ in-situ reactive zone was studied.Furthermore,the stabilization effect of Mg?OH?₂ on Cd can be enhanced by blending with MgCO₃.The results are as follows:?1?Effects of surfactant addition on the stability of colloidal Mg?OH?₂?1?High concentration of SDS could significantly enhance the stability of Mg?OH?₂.With the usage of SDS in modification of Mg?OH?₂,the utilization rate of SDS molecule was very low,and most of SDS did not distribute on the surface of Mg?OH?₂ particles,which has contributed to modification.?2?Tween-80 could obviously decrease the critical micelle concentration of SDS?from 300 mg/L to 10 mg/L?,resulting in SDS easy aggregation around Mg?OH?₂particles surface,and significant increased the utilization rate of SDS.Therefore,Tween-80/SDS composite surfactant had a satisfying modification effect on Mg?OH?₂.The modification effect of composite surfactant was remarkable when Tween-80:SDS exceeded 1:1,and the stability and dispersion of colloidal Mg?OH?₂ were obviously enhanced.?2?Stabilization effect of heavy metal ions by colloidal Mg?OH?₂ and its OH^-release characteristics?1?Multi-heavy metals were precipitated successively by colloidal Mg?OH?₂.Results suggested that Cr?III?was the first to be precipitated,followed by Pb?II?,and Cd?II?was the last.Precipitation reactions of three heavy metals were followed first order reaction kinetics,R² was greater than 0.95,and the reaction rate constant of Cr?III?,Pb?II?,and Cd?II?were 0.047,0.02 and 0.0016 min^-1 respectively.?2?When Mg?OH?₂ injected into the simulated column,it could both retain in porous media and exhibit certain mobility in aquifer,which greatly increased the pH buffer capacity of porous media.Mg?OH?₂ on the surface of the media could slowly release OH^-,when groundwater flowed through the reactive zone,thus effluent pH kept steady around 9.Therfore,Mg?OH?₂ could provide a long-term source of alkalinity which makes it has a broad application in groundwater remediation.?3?Transport properties and model simulation of colloidal Mg?OH?₂ in aquifer?1?The transport properties of colloidal Mg?OH?₂ were affected by flow velocity,porous media size and ionic strength.Results indicated that higher velocity,larger porous media size and lower ionic strength will improve the migration of colloidal Mg?OH?₂ in porous media.However,experiments results illustrated that C/C₀ was independent with the initial colloidal Mg?OH?₂ concentration.?2?Since the heterogeneous properties of colloidal Mg?OH?₂ in particle size,colloidal Mg?OH?₂ was divided into several units according to particle size distribution.In each cell,the size of colloidal Mg?OH?₂ was homogeneous and could be simulated by existing models.The migration of colloidal Mg?OH?₂ is the sum of each unit.Therefore,homogeneous colloid migration model was applied to simulate the migration of heterogeneous colloid by combined with calculus method,and the heterogeneous colloid migration model in aquifer was established.The model can not only simulate the concentration variations of colloidal Mg?OH?₂ in the process of migration,but also simulate and calculate the migration and distribution of colloidal Mg?OH?₂ in aquifer under different flow velocity,porous media size and ionic strength.?3?Calculation results showed that the collision efficiency?and the attachment efficiency?are affected by velocity,particle size of porous media and ionic strength.The larger velocity can reduce?and?,and promote the transport of colloidal Mg?OH?₂.The increase of particle size of porous media can significantly reduce?but lead to the increase of?.The increase of ionic strength has no effect on?but can significantly increase?and decrease the migration of colloidal Mg?OH?₂ in aquifer.?4?Migration properties of colloidal Mg?OH?₂ in polluted aquifer?1?The migration properties of colloidal Mg?OH?₂ in aquifer was decreased with the increased of heavy metal concentration.The increase of contamination concentration can lead to the decrease of Zeta potential on the media surface and the increase of ionic strength of groundwater,meanwhile,it will reduce the electrostatic repulsion force between colloidal Mg?OH?₂ and porous media,thus increase the deposition of colloidal Mg?OH?₂ on the media,resulting in the weakness of migration.?2?Addition of xanthan gum could significant increase the viscosity of the fluid.Although the injection pressure increased during the migration of Mg?OH?₂,the distribution of Mg?OH?₂ in simulated column was more uniform,and the stabilization of heavy metals in polluted aquifer was obviously improved.?5?In-situ remediation effect of heavy metal pollution by Mg?OH?₂?1?One dimensional simulated column experimental results showed that the migration and distribution of the multi-heavy metals exhibited zoning characteristics due to the sequential precipitation of Mg?OH?₂ to multi-heavy metal ions,and different heavy metals were distributed in different positions of the simulated column.?2?Mg?OH?₂ had an excellent stabilization effect on Cr and Pb,and no exchangeable fraction has been detected.The main forms of Cr and Pb were carbonate and Fe/Mn oxidized fractions.As to Cd,the immobilizing effect was relatively poor since exchangeable species still occupied a considerable proportion.?6?MgCO₃/Mg?OH?₂ in-situ remediation of Cd contamination?1?MgCO₃ and Mg?OH?₂ could form complementary,MgCO₃ could effectively precipitate Cd in aquifer,Mg?OH?₂ could provide and maintain the pH required for stabilization.The stabilization effect of Cd is the best at the ratio of 1:1?MgCO₃/Mg?OH?₂?.?2?The stabilization effect of MgCO₃/Mg?OH?₂ composite remediation reagent on Cd in aquifer was obviously better than that of Mg?OH?₂.MgCO₃ could effectively enhance the remediation of Cd pollution by Mg?OH?₂ in groundwater.The innovations of this paper are:?1?Synthesis of an in-situ remediation reagent for groundwater with OH^-slow release and excellent migration properties:colloidal Mg?OH?₂Traditional water soluble alkalis?NaOH,Na₂CO₃,NaHCO₃,etc.?for pH adjustment are easy to migrate with groundwater after being injected into aquifers,without long term pH buffer capacity.Unlike the soluble hydroxides,Mg?OH?₂ with a lower aqueous solubility,slowly dissolves and releases OH^-over time.However,the stability of Mg?OH?₂ suspension is very poor,which is easy to agglomerate and precipitation.In this paper,colloidal Mg?OH?₂ with high stability and mobility were modified by surfactants.The slow release properties of Mg?OH?₂ were applied to the in-situ remediation of groundwater contaminated by heavy metals.?2?Transport model of colloidal Mg?OH?₂?heterogeneous colloids?in aquifer has been developed.Most of the existing colloid migration models are used to simulate the migration and distribution of homogeneous colloids in aquifers,while colloidal Mg?OH?₂ has the property of heterogeneous particles.In this paper,according to the properties of colloidal Mg?OH?₂,colloidal Mg?OH?₂ was divided into several units by calculus.In each unit,the particle size of colloidal Mg?OH?₂ was uniform?which can be simulated according to the existing migration model?,and the migration results of colloidal Mg?OH?₂ was the sum of each unit.Thus,the migration model of heterogeneous colloidal Mg?OH?₂ in aquifer was established.?3?Improving the distribution uniformity of colloidal Mg?OH?₂ in aquifer by viscosity modificationIn view of the disadvangtages of poor migration and uneven distribution of colloidal Mg?OH?₂ in the aquifer polluted by high concentration of heavy metals.In this paper,it was proposed that proper increase of viscosity can decrease deposition of Mg?OH?₂ particles and significantly enhance the uniformity of colloidal Mg?OH?₂distribution and stabilization effect of heavy metals.