Chemical Reduction of Nitrate by Zero-Valent Iron: Shrinking-Core versus Surface Kinetics Models.

Zero valent iron (ZVI) is being used in permeable reactive barriers (PRB) for the removal of oxidant contaminants, from nitrate to chlorinated organics. A sound design of these barriers requires a good understanding of kinetics. Here we present a study of the kinetics of nitrate reduction under relatively low values of pH, from 2 to 4.5. We use a particle size of 0.42 mm, which is within the recommended size for PRBs (0.2 mm to 2.0 mm). In order to avoid possible mass-transfer limitations, a well-stirred reactor coupled with a fluidized bed reactor was used. The experiments were performed at constant pH values using a pH controller that allows to accurately track the amount of acid added. Since the reduction of H + to H 2 by the oxidation of ZVI will always be present for these pH values, blank experiments (without nitrate) were performed and the rate of this H + reduction obtained. This rate of reduction was studied using three kinetic models: a regular empirical one, the Shrinking-Core Model (SCM), and the Surface Kinetics Model (SKM). The best performance was obtained from the SKM model. Therefore, this model was also used to study the results for the nitrate reduction, also with satisfactory results. In both cases, some assumptions are introduced to maintain a moderate number of fitting parameters.

Modeling of electrokinetic remediation combining local chemical equilibrium and chemical reaction kinetics.

A mathematical model for reactive-transport processes in porous media is presented. The modeled system includes diffusion, electromigration and electroosmosis as the most relevant transport mechanisms and water electrolysis at the electrodes, aqueous species complexation, precipitation and dissolution as the chemical reactions taken place during the treatment time. The model is based on the local chemical equilibrium for most of the reversible chemical reactions occurring in the process. As a novel enhancement of previous models, the local chemical equilibrium reactive-transport model is combined with the solution of the transient equations for the kinetics of those chemical reactions that have representative rates in the same order than the transport mechanisms. The model is validated by comparison of simulation and experimental results for an acid-enhanced electrokinetic treatment of a real Pb-contaminated calcareous soil. The kinetics of the main pH buffering process, the calcite dissolution, was defined by a simplified empirical kinetic law. Results show that the evaluation of kinetic rate entails a significant improvement of the model prediction capability.

Anaerobic co-digestion of municipal sewage sludge and fruit/vegetable waste: effect of different mixtures on digester stability and methane yield.

The anaerobic co-digestion of fruit and vegetable waste (FVW) and municipal sewage sludge (MSS) is investigated under mesophilic conditions. This was done at a constant hydraulic retention time (HRT) similar to that typically used at waste water treatment plant digesters, 20 days. The effects on digester performance of the FVW:MSS ratio and the organic loading rate (OLR) were examined. Initially the digester was fed with MSS from wastewater treatment plants (WWTP) with an average OLR of 1.03 kgVS (m3 d)-1. The co-digestion of MSS and FVW was performed at various ratios of FVW in the mixture, while increasing the OLRs, from 1.03 to 4.78 kgVS (m3 d)-1. The experimental specific methane production (SMP) was 0.303 m3 (kgVS)-1 for MSS and 0.403 m3 (kgVS)-1 for FVW as single substrate. This value varied for co-digestion with a maximum of 0.445 m3 (kgVS)-1 for a FVW:MSS ratio of 40:60. Alkalinity and pH values remain relatively constant regardless the different FVW:MSS ratios fed. As this ratio increases, the removal of the volatile solids (VS) increased from 38.7% to 82%. The average methane content of digester biogas was about 62-64%.

Modeling of electrokinetic remediation of Cd- and Pb-contaminated kaolinite.

A physiscochemical model is presented for the reactive-transport of chemical scpecies through a contaminated soil during an acid-enhanced electrokinetic remediation treatment. Numerical simulations in the specific case of the removal of cadmium and lead from spiked kaolinite, compared with experimental results from the literature. The reactive-transport based on the local chemical equilibrium assumption, including a surface complexation to model the adsorption of cations (metals and protons). Comparison of simulation results show different beavior of the target metals, as cadmium is mainly retaind by surface interaction while lead is retained by precipitation of a solid phase.

Aging effects on the mobility of Pb in soil: Influence on the energy requirements in electroremediation.

This paper studies the possible differences in the behavior of lead as a contaminant in soil samples when it is present as "naturally-aged" for decades after the contamination, and when it has been spiked in the laboratory. This behavior differences are stablished mainly in two ways: as changes in the fractionation analysis obtained after a sequential extraction procedure (SEP) and as changes in the efficiency of the acid-enhanced electroremediation (EKR) technique. Additionally, aging effects have been studied for almost five years. In the case of the lead-spiked soil the influence of storage conditions on contaminant behavior have also been explored: 1) samples stored in capped containers at constant moisture conditions, and 2) samples in containers open to the atmosphere, with periods of water flooding and drying. Lab-spiked and the "naturally-aged" contaminants show very different behavior with respect not only to SEP analysis but also to EKR experiments. The soil spiked with a soluble lead salt presents a higher percent in the more mobile fractions. Regarding storage conditions, some changes were observed in the lead distribution along the vertical soil profile for samples stored in uncapped containers. The EKR results were also in agreement with those from fractionation analysis. Energy requirements for the remediation were estimated by a mathematical model with important differences obtained for the different soil samples. Results are indicating that it will be very unreliable to draw estimations for the "naturally-aged" soils from contaminant-spiked samples.

Production of biogas from co-digestion of livestock and agricultural residues: A case study.

This study was undertaken to determine the possible changes in the digester yield and performance for the anaerobic co-digestion under mesophilic conditions of strawberry residues (SRs) together with pig manure (PM). The first part of this paper deals with the digestion of SR as a single substrate. For organic loading rates (OLRs) of 4.4 (g L-1 d-1) or less, the experimental specific biogas and methane productions are 0.588 and 0.231 L g-1, respectively. When higher OLRs (5.5 g L-1 d-1) are used the digester fails due to acidification. In the second part, the co-digestion of both residues is explored using a wide variety of SR:PM ratios and OLRs of 5.5 g L-1 d-1 with good stability. Therefore, it is demonstrated that co-digestion allows the improvement of the treatment capacity as compared with SR as a single residue. The methane and biogas productions increase as the SR:PM ratio increases. It may be concluded that, when a digester works with a certain OLR, the performance for co-digestion is always better than for single substrates because the presence of PM provides a better stability and the presence of SR improves the biogas and methane production.

Effects of the buffering capacity of the soil on the mobilization of heavy metals. Equilibrium and kinetics.

Understanding the possible pH-buffering processes is of maximum importance for risk assessment and remediation feasibility studies of heavy-metal contaminated soils. This paper presents the results about the effect of the buffering capacity of a polluted soil, rich in carbonates, on the pH and on the leaching evolution of its main contaminant (lead) when a weak acid (acetic acid) or a strong one (nitric acid) are slowly added. In both cases, the behavior of lead dissolution could be predicted using available (scientifically verified freeware) models assuming equilibrium between the solid and the aqueous phase. However, the experimental results indicate that the dissolution of calcium and magnesium carbonates is kinetically controlled. These kinetic limitations affect the overall behavior, and should be considered to understand also the response of the metals under local equilibrium. The well-known BCR sequential extraction procedure was used before- and after-treatment, to fractionate the lead concentration in the soil according to its mobility. The BCR results were also in agreement with the predictions of the equilibrium model. This agreement allows new insights about the information that could be derived from the BCR fractionation analysis.

Effect of slow desorption on the kinetics of biodegradation of polycyclic aromatic hydrocarbons.

The bioavailability to bacteria of 14C-labeled polycyclic aromatic hydrocarbons (PAHs) sorbed onto lake sediments was assessed using a mathematical model and three experimental series. The experiments were performed under similar conditions and included: (1) abiotic desorption of PAHs from sediments by Tenax extraction, (2) mineralization of dissolved PAHs with no sediment present, and (3) mineralization of PAHs sorbed onto sediments. Results obtained from the first two series were used to obtain the parameter values for the model, and the experimental results of the third series were compared to model results. We found that microorganisms were able to promote desorption of the more-labile fractions, but were unable to increase the desorption rate of the slow- and very slow-desorbing fractions. Also, our model predictions indicate that, after very long contact times, and in the concurrence of biodegradation, sorbed PAHs remain not under equilibrium conditions, but rather in a steady state. The net rates of PAH desorption from the three sediment domains considered (fast, slow, and very slow) become similar, and the ratio between the aqueous and the sediment concentration remains constant with time.

Two-dimensional model for soil electrokinetic remediation of heavy metals. Application to a copper spiked kaolin.

A two-dimensional numerical model has been developed to simulate the electrokinetic remediation of soils contaminated with heavy metals and has been validated using laboratory experiments performed with a copper spiked kaolin. The model divides the soil into compartments in a Cartesian grid and a non-conductivity barrier encloses the considered area. Basically, it consists in two main parts clearly distinguishable. The first part describes the electromigration phenomenon in the soil, which is represented by a set of electric resistors, following the Cartesian grid and using Kirchoff's laws of electricity to calculate the voltage drop distribution in the considered area. The second part describes the chemical equilibrium process between the heavy metal and the soil, assuming local equilibrium conditions within the compartments. A good agreement was obtained between the lab scale experimental assays and the model predictions. The model has also been used to examine the effect of the electrolyte neutralization within the scope of the acid-enhanced electrokinetic method. These simulations have foreseen problems related with the system evolution, which would not arise under one-dimensional geometries and are due to the changes of the potential distribution in the two-dimensional arrangement where some kind of short circuit arises, ultimately leading to a decrease of the system efficiency.

Competitive retention of lead and cadmium on an agricultural soil.

Lead and cadmium contamination of an agricultural soil has been studied using batch and column experiments. Thermodynamics of the retention phenomena may be represented by a Langmuir isotherm for an aqueous metal concentration up to 100 mg L(-1). First order kinetics with respect to the solid phase yield good predictability for both batch and column experiments. Kinetics and thermodynamics of lead retention predominate over those of cadmium. As a consequence, lead is preferentially retained and can even displace sorbed cadmium. In the event of an spill involving both metals, cadmium would move further in the soil and its aqueous concentration downstream could be even higher than that of the influent solution, increasing potential risks. A two-region model has been used to fit all the experimental results. Satisfactory predictions for column experiments are obtained with parameters which are consistent with those obtained for the batch experiments, for which sorption is described by a Langmuir isotherm including competitive retention.

Biosurfactant- and biodegradation-enhanced partitioning of polycyclic aromatic hydrocarbons from nonaqueous-phase liquids.

A study was conducted on the effect of two different biological factors, microbial surfactants and biodegradation, on the kinetics of partitioning of polycyclic aromatic hydrocarbons (PAHs) from nonaqueous-phase liquids (NAPLs). The effect of rhamnolipid biosurfactants on partitioning into the aqueous phase of naphthalene, fluorene, phenanthrene, and pyrene, initially dissolved in di-2-ethylhexyl phthalate (DEHP) or 2,2,4,4,6,8,8-heptamethylnonane (HMN), was determined in multiple-solute experiments. Biosurfactants at a concentration above the CMC enhanced the partitioning rate of fluorene, phenanthrene, and pyrene but were ineffective with naphthalene. Enhancement of partitioning was also observed in the presence of suspended humic acid-clay complexes, which simulated the solids often present in the subsurface. Biosurfactants sorbed to the complexes modified PAH partitioning between the NAPL and these solids, increasing the fraction of solid-phase PAH. Biodegradation-driven partitioning was estimated in mineralization experiments with phenanthrene initially present in HMN and three representative soil bacterial strains, differing in their potential adherence to the NAPL. In the three cases, the rates of mineralization were very similar and significantly higher than the abiotic rate of partitioning. Our study suggests that in NAPL-polluted sites, partitioning of PAH may be efficiently enhanced by in situ treatments involving the use of biosurfactants and biodegradation.