Mechanism And Model Investigation Of Natural Colloid And Fe Co-transportation In Groundwater Under Managed Aquifer Recharge Condition

The Managed aquifer recharge(MAR)technology is more and more mature.However,the mechanism of co-transport of colloids and trace elements is not clear.In previous studies the effect of colloids on trace elements often is ignored.Therefore finding out the co-transportation mechanism of colloids and polluting trace element Fe is important to ensure the safety of groundwater,especially in the region of drink groundwater.The inorganic silica colloids and organic humic acid(HA)colloids were selected as representative natural colloid,and Fe was selected as representative trace element in this study.This study was conducted to compare the transport,deposition and retain of Fe under different colloid types and concentration(silica colloids and HA colloids),different porous media(glass beed,quartz sand and sand),different ionic strength(IS),different ion valence and different flow rate conditions in saturated columns.Finally,comparing experiment result Comsol Multiphysics was used to quantify simulate and prediction the transportation of Fe.The main conclusion of this study as follows:(1)The physical and chemical properties of colloids suspension contains FeThe addition of silica colloids can combine a small amount Fe and form “Fe-silica colloids”.The addition of HA colloids can combine all Fe and form “Fe-HA colloids”.The size of “Fe-natural colloids” increased with increasing of IS.The influence of Ca2+ is stronger than Na+ on the size of collloids.In addition,different colloids types have different influence on the zeta potential of “Fe-natural colloids”.The zeta potential of “Fe-silica colloids”slight reduced with increasing of silica colloids concentration.The zeta potential of “Fe-HA colloids” become more negative with increasing of HA colloids concentration.The zeta potential of above suspension between ±30mV,and unstable.In the condition of Na+,the zeta potential of “Fe-silica colloids” decreasing with IS increased,and unstable.In the condition of Ca2+,the zeta potential of silica colloids increasing with IS increased.The negative of zeta potential of “Fe-HA colloids” decreasing with IS increased.The influence ability on suspension from big to small is HA colloids,Fe and silica colloids.Due to the changeable of size and zeta potential of colloids,the transportation of colloids is complicated.(2)The adsorption of “Fe-natural colloids”In “Fe-silica colloids”,the adding of silica colloids hold effective adsorption site of porous media and decreasing the adsorption of Fe on porous media.Comparing the condition without silica colloids,silica colloids can reduce 3% of Fe adsorb.The adsorption ability of glass bead and silica sand is equal,and the adsorption ability of sand is strongest.In the conditions with and without silica colloids,the adsorption ability of porous increasing with IS increased.In “Fe-HA colloids”,HA colloids mechanism is different from silica colloids,the adding of HA colloids increasing effective adsorption site of porous media and increasing the adsorption of Fe on porous media.In three porous media,the adsorption ability of sand is strongest(87%).The adsorption ability become stronger when added the HA colloids in three porous media.(3)The co-transportation of “Fe-natural colloids”In “Fe-silica colloids”,the adding of silica colloids hold effective adsorption site of porous media and increasing the transport of Fe on porous media.In conditions with and without silica colloids,the transport ability of Fe from fast to slow as follows: glass bead,silica sand and sand.The improvation can reach 25% of C/C0 max in silica sand is strongest.The transport ability of Fe reduce with IS increasing and flow rate decreasing.In the condition of IS=0.05M(CaCl2),effluent contain “Fe-silica colloids”,suggest that “Fe-silica colloids” can transport in porous media.In “Fe-HA colloids”,the adding of HA colloids increase effective adsorption site of porous media and decreasing the transport of Fe on porous media.The reducing can reach 70% of C/C0 max in silica sand is strongest.In sand,the retain is most obvious and k can reach 10-2s-1.Sand has strongest ability of block the transport of Fe.The transport ability of Fe improve with IS and flow rate increasing.The transport ability of Fe improve with IS and flow rate increased.In the condition of high IS,especially divalent cations,divalent cations can hold the adsorption site of HA and decreasing the adsorption of Fe and increasing the transport of Fe.In the condition of IS=0.05M(CaCl2),“Fe-HA colloids” retain in porous media,effluent contain only Fe that not linked HA.(4)Numerical simulation of Fe transportationThe simulation results of breakthrough curve is close to experimental breakthrough curve results with and without colloid.The Fe and “Fe-silica colloids” breakthrough k is smaller(<1.5×10-3s-1).The“Fe-HA colloids” breakthrough k is bigger,in sand the k can reach 4×10-2s-1.