Remediation of PAH-Contaminated Soils: Experimental Analysis and Modeling of Hydrodynamics and Mass Transfer in a Soil-Slurry Bioreactor

Extended Abstract Polycyclic Aromatic Hydrocarbon (PAHs)-contaminated soils are a great environmental and public health concern nowadays. Their remediation is an important field of research as several remediation techniques have been developed with the purpose of removing PAHs from soil. However, further researches are necessary to develop environmental friendly biotechnologies that allows public and private sectors to implement efficient and adaptable treatments for contaminated soils. Aerobic soil-slurry bioreactor technology has emerged as one of these technologies with high potential as an effective and feasible treatment technic for PAH-contaminated soils. For this treatment, soil is excavated, conditioned and loaded into an aerated aqueous bioreactor. Then, mechanical and/or pneumatic mixing maintains aerobic conditions and homogeneity. Furthermore, air supply and mixing represent the most energy intensive units Although, extensive research has been done on this topic, mechanisms involved in the removal of PAHs from soil are still not completely understood. In addition to the biological processes involved, important mass transfer mechanisms need to be considered (oxygen gas-liquid mass transfer, adsorption-desorption, volatilization of PAH, etc.). In general, even for volatile PAHs, volatilization is not considered in the studies whereas, in some conditions (high aeration rate), it can be a major mechanism of "PAH removal". The soil composition and concentration in the reactor should influence strongly the fluid viscosity, which is a key parameter governing the hydrodynamics and thus the mass transfer phenomena. Therefore, the aeration and mixing optimization requires a fine understanding of how different operational parameters influence the mixing and mass transfer mechanisms involved in the removal of PAHs from soil In this study, the influence of soil content (composition and concentrations) and operating conditions (air superficial velocity, stirring rate, etc.) on the mixing (rheology, etc.) and mass transfer phenomena (gas-liquid, adsorption-desorption) is addressed. Experiments are performed in a glass standard bioreactor designed to control hydrodynamic conditions and temperature. Air is injected from the bottom through a porous glass sparger. Mechanical agitation is performed by a marine propeller connected to a motor. Hydrodynamic parameters are monitored in order to study their influence on the process and, particularly on the oxygen and PAH transfer phenomena. Rheological behavior of soil/water matrix has been measured with a capillary rheometer The oxygen transfer tests showed that for a given air superficial velocity and stirring rate, the oxygen transfer coefficient in soil/water matrix is reduced in comparison with clean water results. This decrease depends on the soil composition and was more pronounced with an increase in the soil content. Moreover, the soil/water matrix could be assimilated to a non-Newtonian fluid with shear-thinning behavior (mainly pronounced for high soil content). The impact

Numerical study of nano-biofilm stagnation flow from a nonlinear stretching/shrinking surface with variable nanofluid and bioconvection transport properties

A mathematical model is developed for stagnation point flow toward a stretching or shrinking sheet of liquid nano-biofilm containing spherical nano-particles and bioconvecting gyrotactic micro-organisms. Variable transport properties of the liquid (viscosity, thermal conductivity, nano-particle species diffusivity) and micro-organisms (species diffusivity) are considered. Buongiorno’s two-component nanoscale model is deployed and spherical nanoparticles in a dilute nanofluid considered. Using a similarity transformation, the nonlinear systems of partial differential equations is converted into nonlinear ordinary differential equations. These resulting equations are solved numerically using a central space finite difference method in the CodeBlocks Fortran platform. Graphical plots for the distribution of reduced skin friction coefficient, reduced Nusselt number, reduced Sherwood number and the reduced local density of the motile microorganisms as well as the velocity, temperature, nanoparticle volume fraction and the density of motile microorganisms are presented for the influence of wall velocity power-law index (m), viscosity parameter (c2), thermal conductivity parameter (c4), nano-particle mass diffusivity (c6), micro-organism species diffusivity (c8), thermophoresis parameter (Nt), Brownian motion parameter (Nb), Lewis number (Le), bioconvection Schmidt number (Sc), bioconvection constant (σ) and bioconvection Péclet number (Pe). Validation of the solutions via comparison related to previous simpler models is included. Further verification of the general model is conducted with the Adomian decomposition method (ADM). Extensive interpretation of the physics is included. Skin friction is elevated with viscosity parameter (c2) whereas it is suppressed with greater Lewis number and thermophoresis parameter. Temperatures are elevated with increasing thermal conductivity parameter (c4) whereas Nusselt numbers are reduced. Nano-particle volume fraction (concentration) is enhanced with increasing nano-particle mass diffusivity parameter (c6) whereas it is markedly reduced with greater Lewis number (Le) and Brownian motion parameter (Nb). With increasing stretching/shrinking velocity power-law exponent (m), skin friction is decreased whereas Nusselt number and Sherwood number are both elevated. Motile microorganism density is boosted strongly with increasing micro-organism diffusivity parameter (c8) and Brownian motion parameter (Nb) but reduced considerably with greater bioconvection Schmidt number (Sc) and bioconvection Péclet number (Pe). The simulations find applications in deposition processes in nano-bio-coating manufacturing processes

Colloidal mobilization and fate of trace heavy metals in semi-saturated artificial soil (OECD) irrigated with treated wastewater

The mobility of selected heavy metals in trace concentrations was investigated in a standard OECD soil irrigated with the effluent of a real municipal wastewater treatment plant. While Cd, Cu and Ni accumulation-migration patterns were mainly influenced by the mobility of colloids generated from soil organic and inorganic matter, Zn mobility was more influenced by the wastewater content of dissolved organic matter and by its salinity. Metal accumulation caused by interaction with colloids resulted in contamination peaks both in different zones of the soil column and in the leaching solution. The release of metals in the leachate was correlated to the contemporary release of silicates from kaolinite and dissolved organic matter, identified through UV absorbance and chemical oxygen demand monitoring. The hypothesized colloidal mobilization was confirmed by spectroscopic studies. The highly heterogeneous complexes of organic and inorganic molecules responsible for metal transport through soil appeared to be structured in highly stable micellar aggregates

Colloidal mobilization and fate of trace heavy metals in semi-saturated artificial soil (OECD) irrigated with treated wastewater

The mobility of selected heavy metals in trace concentrations was investigated in a standard OECD soil irrigated with the effluent of a real municipal wastewater treatment plant. While Cd, Cu and Ni accumulation-migration patterns were mainly influenced by the mobility of colloids generated from soil organic and inorganic matter, Zn mobility was more influenced by the wastewater content of dissolved organic matter and by its salinity. Metal accumulation caused by interaction with colloids resulted in contamination peaks both in different zones of the soil column and in the leaching solution. The release of metals in the leachate was correlated to the contemporary release of silicates from kaolinite and dissolved organic matter, identified through UV absorbance and chemical oxygen demand monitoring. The hypothesized colloidal mobilization was confirmed by spectroscopic studies. The highly heterogeneous complexes of organic and inorganic molecules responsible for metal transport through soil appeared to be structured in highly stable micellar aggregates

Anodic oxidation of surfactants and organic compounds entrapped in micelles – Selective degradation mechanisms and soil washing solution reuse -

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Functional potential of sewage sludge digestate microbes to degrade aliphatic hydrocarbons during bioremediation of a petroleum hydrocarbons contaminated soil

Sewage sludge digestate is a valuable organic waste which can be used as fertilizer in soil bioremediation. Sewage sludge digestate is not only a good source of nutrients but is also rich in bacteria carrying alkB genes, which are involved in aliphatic hydrocarbons metabolism. Increase of alkB genes ratio in polluted soils has been observed to improve bioremediation efficiency. In this study, for the first time, the genetic potential of indigenous microorganisms of digestate to degrade petroleum products was assessed. The objectives were to study petroleum hydrocarbons (PHCs) removal together with shifts in soil taxa and changes in the concentration of alkB genes after digestate application. Initial alkB genes concentration in contaminated soils and digestate was 1.5% and 4.5%, respectively. During soil incubation with digestate, alkB genes percentage increased up to 11.5% and after the addition of bacteria immobilized onto biochar this value increased up to 60%. Application of digestate positively affected soil respiration and bacterial density, which was concomitant with enhanced PHCs degradation. Incubation of soil amended with digestate resulted in 74% PHCs decrease in 2 months, while extra addition of bacteria immobilized onto biochar increased this value up to 95%. The use of digestate affected the microbial community profiles by increasing initial bacterial density and diversity, including taxa containing recognized PHCs degraders. This study reveals the great potential of digestate as a soil amendment which additionally improves the abundance of alkB genes in petroleum contaminated soils

Removal of hydrophobic organic pollutants from soil washing/flushing solutions: A critical review.

International audienceThe release of hydrophobic organoxenobiotics such as polycyclic aromatic hydrocarbons, petroleum hydrocarbons or polychlorobiphenyls results in long-term contamination of soils and groundwaters. This constitutes a common concern as these compounds have high potential toxicological impact. Therefore, the development of cost-effective processes with high pollutant removal efficiency is a major challenge for researchers and soil remediation companies. Soil washing (SW) and soil flushing (SF) processes enhanced by the use of extracting agents (surfactants, biosurfactants, cyclodextrins etc.) are conceivable and efficient approaches. However, this generates high strength effluents containing large amount of extracting agent. For the treatment of these SW/SF solutions, the goal is to remove target pollutants and to recover extracting agents for further SW/SF steps. Heterogeneous photocatalysis, technologies based on Fenton reaction chemistry (including homogeneous photocatalysis such as photo-Fenton), ozonation, electrochemical processes and biological treatments have been investigated. Main advantages and drawbacks as well as target pollutant removal mechanisms are reviewed and compared. Promising integrated treatments, particularly the use of a selective adsorption step of target pollutants and the combination of advanced oxidation processes with biological treatments, are also discussed

Combination of anodic oxidation and biological treatment for the removal of phenanthrene and Tween 80 from soil washing solution

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