Transport And Mechanisms Of Typical Functional Microorganism FA1 In Saturated Porus Media

With rapid population growth,robust economy development and a continuous improvement of living conditions,the groundwater contamination has become a global problem due to the strong human disturbance.Among various remediation technologies,bioremediation has gained more attention and been recognized as an environment friendly and promising way for the remediation of groundwater contaminations.Microorganisms with specialized functions are usually involved when in situ bioremediation technique or natural attenuation is employed.Nevertheless,a high remediation efficiency is often difficult to achieve partially due to the insufficient distribution of functional microorganisms.It is therefore of great importance to advance current knowledge of fate and transport of functional microorganisms in porous media,and thus to improve the effectiveness of the subsurface bioremediation technologies.However,related studies were limited.In this study,laboratory experiments(batch experiments,column experiments)and mathematical modeling were conducted to systematically explore the transport behaviors of functional bacteria Herbaspirillum chlorophenolicum FA1 in porous media.The effect of various physical(particle size,flow rate,temperature),chemical(media type,ion type,ionic strength,pH,humic acid),biological(microbial concentration,extracellular polymeric substance)factors,as well as the coexistence the heavy metals were investigated.The influences caused by various elements and the mechanisms involved were clarified to provide information and guidelines for field application of functional bacteria for groundwater remediation.The main findings are as followings:(1)Experimental data showed that grain size,solution pH,IS,and HA concentration all played important roles in the retention and transport of the functional bacterium FA1 in saturated porous media.Increase of gain size,solution pH,and HA concentration as well as decrease of IS all enhanced the transport and reduced the retention of FA1 in saturated porous media under various physicochemical conditions.The shape of retention profiles(RPs)was hyper-exponential.The amounts of retained bacteria in the media also varied with the experimental conditions with opposite trends to that of effluent.Both experimental BTCs and RPs were simulated by a mathematical model that accounted for deposition kinetics to better interpret the effects of physicochemical conditions on FA1 deposition dynamics.(2)Laboratory saturated columns packed with granular limestone grains were used to explore the retention and remobilization of functional bacteria FA1 under various physicochemical conditions.The unique surface properties of limestone and FA1 caused some unexpected phenomena.Initial solution pH showed little influence due to the strong buffering ability of limestone.Solution IS,cation type,temperature and surface biological property all affected FA1 retention in the columns.Increasing of IS can have much stronger effect in reducing bacterial mobility in quartz sand porous media,and IS effect was temperature dependent.Ca2+was more effective in reducing the bacterial mobility than Na-over the IS investigated.Lower temperatures,EPS treatment,increasing FA1 concentration promoted FA1 mobility.Experimental BTCs were well simulated by a mathematical model.(3)Perturbations of solution IS caused very slight release of previously retained bacteria in some columns with NaCl as the background electrolyte,while increase in flow rate caused no release at all.When CaCl2 was the background,bacterial remobilization only occurred following both cation exchange and IS reduction.DLVO incorporating surface roughness were calculated to assist with interpretation of interaction mechanisms.All the experimental evidences suggest the importance of cation bridging,cation exchange,surface roughness,and hydrophobic interaction in controlling the bacteria transport in saturated limestone porous media.(4)For the first time,biosorption of lead(?)using a PAHs-degrading bacterium,Herbaspirillum chloropheholicum FA1,was investigated as a function of initial lead concentration,biomass dosage,pH,and temperature in batch conditions.Results showed that FA1 was highly resistant to lead and grew well even at lead concentration of 200 mg L-1.The kinetic and isotherm data were well described by commonly used models,such as the pseudo-second-order and Langmuir models.Adsorption thermodynamics was spontaneous and endothermic,and FA1 exhibited a high sorption capacity of 151.52 mg g-1.SEM-EDS and FTIR analysis was conducted to further explore the interaction mechanisms.Compared to Pb2+,the adsorption ability of FAlfor Ca2+was much smaller.(5)Batch and column experiments were conducted to explore the cotransport behaviors of functional bacteria(FA1)and heavy metals(Pb2+/Cd2+)in saturated sand media under different conditions.The sorption capacity of heavy metals on FA1 was much greater than that of the sand,while both FA1 and sand showed stronger affinity to Pb2+than Cd2+.The surface properties,especially zeta potential,of the bacteria and sand were altered by metal adsorption.As a result,the co-existence of Pb2+ decreased the transport of FA1 more significantly than that of Cd2+,and the influence was more significant with higher heavy metal concentration.On the other hand,the co-existence of FA1 inhibited the mobility of Pb2+and Cd2+in most scenarios,except when the cotransport concentration of Pb2+ was 5 mg L-1,and the inhibition was more pronounced for Pb2+ than Cd2+.Increase in metal concentrations decreased the FA1-associated Pb21/Cd2+ in effluents due to the remarkable decrease in FA1 mobility,and free soluble Pb2+/Cd2+became the major migration species.In addition,due to stronger attractive forces and affinity between Pb2+and FA1,nearly all presorbed-Pb2+by sand was remobilized by FA1 and transported mainly in FA 1-associated form other than soluble Pb2+.Findings from this study indicated that the cotransport of biocolloids and heavy metals are highly sensitive to the ion type and concentration,and evaluation of their transport in subsurface should be carefully carried out to avoid inaccurate estimations.