Transport Behavior Of Ferrihydrite Colloids In Saturated Porous Media

A good mastery of the rule of colloid transport is important for the understanding of the fate of pollutants in natural environment and the restoration of contaminated environment.It has been found that besides surface properties of colloidal particle and factors of solution chemistry,the physicochemical properties of biofilms on collector surface also impact the transport of colloidal particles.However,little information is availabe concerning the effects of biofilm's metabolic activity towards colloidal particles on their transport and deposition behaviors.As an iron oxide mineral widely distributed in water and soil system,ferrihydrite often exists in the form of colloidal particles and plays an important role in the iron geochemical circle,the migration of pollutants and the transformation of other minerals.It has been shown that Shewanella and other dissimilatory metal reducing bacteria can use ferrihydrite and other iron mineral particles as electron receptors for anaerobic respiration.Using saturated quartz sand column as model porous meida,the impact of Shewanella oneidensis MR-1 biofilm coating and its extracellular electron transfer activity on the transport of ferrihydrite in saturated porous media.After the investigating the breakthrough and retention of ferrihydrite colloids in bare sand and MR-1 biofilm-coated sand columns,it was found that the ferrihydrite colloids showed similar responses to changes of different solution chemical factors.However,the deposition of ferrihydrite colloids was more significant in the presence of biofilm.In bare sand column,when the p H value of influent was increased from 5.0 to 8.5,the mass percentage of effluent colloids increased from 35.3%to 44.2%.It was presumed that the surface potential of ferrihydrite partilces changed from positive to negative,resulting in gradually increasing electrostatic repulsion between colloids and porous medium and enhancing the mobility of colloidal ferrihydrite.When the ionic strength(i.e.Na Cl concentration)in the influent suspension was enhanced from 0.1 to 10 m M,the effluent colloids mass percentage decreased from 45.1%to 21.8%.In this process,as the ionic strength increased,the double electric layer of on colloid surface was constantly compressed,which caused the decrease of static repulsion between colloid particles,and made colloids easier to aggregate and deposit on the surface of the porous media.After replacement of Na⁺with Ca²⁺,the effluent colloids percentage decreased from 35.9%to 14.4%,indicating the more significant effects of Ca²⁺on colloids deposition.The presence of low concentration(3mg C/L)of humic acid led to the heterogeneous distribution of charges on colloidal surface and promote the deposition of ferrihydrite particles.The deposition rate coefficient k_dof ferrihydrite colloids increased from 1.4 to 1.6 h^-1.In the presence of higher concentration(10and 30 mg C/L)of humic acid that completely covered the surface of ferrihydrite particles,the colloidal surface became negatively charged and therefore the electrostatic repulsion between the colloids and the porous medium increased.The transport capacity of the ferrihydrite colloids was improved and the corresponding k_dvalue was reduced to 0.9 and 0.3h^-1,respectively.The transports of ferrihydrite particles in biofilm-coated columns were inhibited in comparison to those in bare sand columns under different hydrochemical conditions.The involvement of extracellular electron transfer(EET)of biofilm in ferrihydrite colloids transport was further investigated.It was found that with the incrase of glucose(electron donor)concentration from 0 to 10 m M,the mass fraction of retained ferrihydrite particles increased from 80.4%to 91.2%in biofilm-coated sands,while the colloid transport was barely affected in the absence of biofilm.Transport of ferrihydrite particles in sand columns coated with biofilms of two mutant strains without c-type cytochrome(?omc A/?mtr C and?cym A)was also conducted and compared with that in sand column coated with wild-type biofilm.It was shown that the wild-type biofilm had the strongest retention and adsorption effects on ferrihydrite colloids and confirmed the important influence of EET on transport of ferrihydrite particles.Moreover,the long-term release experiment results also showed that the EET plays an important role in maintaining the long retention and promoting the mineral phase transformation of ferrihydrite colloids in sand column.