Contributions of adsorption, bioreduction and desorption to uranium immobilization by extracellular polymeric substances

Hexavalent uranium(U(VI))can be immobilized by various microbes.The role of extracellular polymeric substances(EPS)in U(VI)immobilization has not been quantified.This work provides a model framework to quantify the contributions of three processes involved in EPS-mediated U(VI)immobilization:adsorption,bioreduction and desorption.Loosely associated EPS was extracted from a pure bacterial strain,Klebsiella sp.J1,and then exposed to H_(2) and O_(2)(no bioreduction control)to immobilize U(VI)in batch experiments.U(VI)immobilization was faster when exposed to H_(2) than O_(2) and stabilized at 94%for H_(2) and 85%for O_(2),respectively.The non-equilibrium data from the H_(2) experiments were best simulated by a kinetic model consisting of pseudo-second-order adsorption(ka=2.87×10^(−3) g EPS·(mg U)^(−1)·min^(−1)),first-order bioreduction(kb=0.112 min−1)and first-order desorption(kd=7.00×10^(−3) min^(−1))and fitted the experimental data with R^(2) of 0.999.While adsorption was dominant in the first minute of the experiments with H_(2),bioreduction was dominant from the second minute to the 50th min.After 50 min,adsorption was negligible,and bioreduction was balanced by desorption.This work also provides the first set of equilibrium data for U(VI)adsorption by EPS alone.The equilibrium experiments with O_(2) were well simulated by both the Langmuir isotherm and the Freundlich isotherm,suggesting multiple mechanisms involved in the interactions between U(VI)and EPS.The thermodynamic study indicated that the adsorption of U(VI)onto EPS was endothermic,spontaneous and favorable at higher temperatures.

Bioreduction of nitrate in groundwater using a pilot-scale hydrogen-based membrane biofilm reactor

A long-term pilot-scale H_(2)-based membrane biofilm reactor(MBfR)was tested for removal of nitrate from actual groundwater.A key feature of this secondgeneration pilot MBfR is that it employed lower cost polyester hollow fibers and still achieved high loading rate.The steady-state maximum nitrate surface loading at which the effluent nitrate and nitrite concentrations were below the Maximum Contaminant Level(MCL)was at least 5.9 g·N·(m^(2)·d)^(–1),which corresponds to a maximum volumetric loading of at least 7.7 kg·N·(m^(3)·d)^(–1).The steady-state maximum nitrate surface area loading was higher than the highest nitrate surface loading reported in the firstgeneration MBfRs using composite fibers(2.6 g·N·(m^(2)·d)^(–1)).This work also evaluated the H_(2)-utilization efficiency in MBfR.The measured H_(2)supply rate was only slightly higher than the stoichiometric H_(2)-utilization rate.Thus,H_(2)utilization was controlled by diffusion and was close to 100%efficiency,as long as biofilm accumulated on the polyester-fiber surface and the fibers had no leaks.

Determination of growth kinetics of microorganisms linked with 1,4-dioxane degradation in a consortium based on two improved methods

The conventional method for determining growth kinetics of microbial consortia relies on the total biomass concentration.This may be inaccurate for substrates that are uncommon in nature and can only be degraded by a small portion of the microbial community.1,4-dioxane,an emerging contaminant,is an example of such substrates.In this work,we evaluated an improved method for determining the growth kinetics of a 1,4-dioxane-degrading microbial consortium.In the improved method,we considered only bacterial taxa whose concentration increase correlated to 1,4-dioxane concentration decrease in duplicate microcosm tests.Using PEST(Parameter Estimation),a modelindependent parameter estimator,the kinetic constants were estimated by fitting the Monod kineticsbased simulation results to the experimental data that consisted of the concentrations of 1,4-dioxane and the considered bacterial taxa.The estimated kinetic constants were evaluated by comparing the simulation results with experimental results from another set of microcosm tests.The evaluation was quantified by the sum of squared relative residual,which was four orders of magnitude lower for the improved method than the conventional method.By further dividing the considered bacterial taxa into oligotrophs and copiotrophs,the sum of squared relative residual further decreased.