C. Gómez-Lahoz

Kinetics of the chemical reduction of nitrate by zero-valent iron

The use of reactive barriers is one of the preferred remediation technologies for the remediation of groundwater contamination. An adequate design of these barriers requires the understanding of the kinetics of the reaction between the target contaminant and the solid phase in the barrier. A study of the kinetics between metallic iron and aqueous nitrate is presented in this paper. Published literature regarding this reaction indicates that researchers are far from a consensus about the mechanism of this reaction. This paper presents the results obtained from experiments performed at different constant pH values and iron dosages, together with a mathematical analysis of the kinetic results. We have found that an Eley-Rideal kinetic model yields a good explanation of the relatively complicated dependence between rate of nitrate reduction and the pH value of the solution.

Cobalt(II) removal from water by chemical reduction with sodium borohydride

Abstract The stoichiometry and kinetics of Co 2+ reduction with NaBH 4 in aqueous solution have been studied. A substantial excess of borohydride is needed to achieve complete Co 2+ removal from water, mainly due to borohydride consumption in the BH 4 − /H 2 O reaction unless the pH is sufficiently alkaline. In this case, cobalt precipitation occurs mainly as Co(OH) 2 . Absence of dissolved oxygen in the reactor is required to avoid rapid reoxidation of reduced cobalt. The elemental analysis of the reduced species is consistent with Co 2 B. The ease of sludge reoxidation and the final boron concentration in the effluent resulting from the needed excess of borohydride are serious disadvantages of this technique for Co 2+ removal. A kinetic model has been proposed to describe the behaviour of the system. The model reproduces reasonably well the experimental results, although some discrepancies were observed in the early stages of the process. These can be mainly attributed to mass-transfer limitations.

Biomethanization of mixtures of fruits and vegetables solid wastes and sludge from a municipal wastewater treatment plant

The possible management of Fruit and Vegetable Solid Wastes (FVSWs) through their simultaneous digestion with the primary sludge of Municipal Wastewater Treatment plants is investigated. This alternative allows the recovery of energy and a solid product that can be used as an amendment for soils that generated the residue, while is not expensive. Results indicate that the ratio of FVSWs to sludge and the pH control are the main variables determining the methane production and concentration. NaHCO3 was selected to achieve the pH control. The results for a ratio of 50% sludge together with 10 g NaHCO3/kg of residue are among the best obtained, with a methane yield of about 90 L per kg of volatile solids, and a methane concentration of 40% (v/v) of the biogas. A 50% reduction of the total solids; 21% reduction of the volatile solids (in terms of total solids); and a pH value of the sludge, which is 6.9 indicate that the digested sludge can be a good material for soil amendment.

Electrokinetic Remediation. II. Amphoteric Metals and Enhancement with a Weak Acid

Abstract A One-Dimensional Modl Is Developed For The Electrokinetic Treatment Of Aquifers Contaminated With An Ionic Salt. Electrokinetic Removal Of Amphoteric Metals Such As Zinc And Lead Is Simulated. The Use Of A Weak Acid (Acetic Acid) To Neutralize A Portion Of The Oh− Generated Electrolytically In The Cathode Compartment Is Explored In Connection With The Electrokinetic Removal Of Nonamphoteric Metals Such As Copper And Cadmium.

Heavy metals and disposal alternatives for an anaerobic sewage sludge

Abstract Treatment of wastewater unavoidably results in the generation of huge quantities of sludge in which pollutants concentrate and which must be disposed of. There may be significant levels not only of salts and organic pollutants, but also of heavy metals, which are of particular concern. The presence of these substances causes environmental damage in soil, plants, groundwater and air. The classical disposal alternatives at hand include thermal treatment (incineration), land application and ocean dumping, all of these being objectionable to some extent because of major problems created by the pollutants in the sludge. Knowledge of the heavy metal content of the sludge is a key step in selecting or in ruling out the various disposal alternatives. An anaerobically stabilized sewage sludge from an urban‐residential wastewater treatment facility has been collected and studied in order to identify the most suitable disposal alternative. The sludge has been analyzed for its calorific value and heavy metal...

Electrokinetic Remediation. I. Modeling of Simple Systems

Abstract A One-Dimensional Model Is Developed For Simulating The Electrokinetic Treatment Of Saturated Porous Media Contaminated With An Ionic Salt. Simulations Of Simple, Unenhanced Electrokinetic Treatment For The Removal Of A Nonamphoteric Salt Such As Cadmium Sulfate Exhibit A Severe Drop-Off In Electric Current And In Remediation Rate After About 50–60% Of The Cation Has Been Removed. Simulation Of Electrokinetic Treatment In Which The Oh− Generated In The Cathode Compartment Is Partially Neutralized By The Addition Of Acid Show Rapid And Complete Removal Of The Cation. Partial Neutralization Of H+ In The Anode Compartment By Addition Of Base Results In Immobilization Of The Toxic Metal As The Solid Hydroxide, Although This Should Be A Useful Technique For The Removal Of Arsenate And Selenate.

Biodegradation Phenomena during Soil Vapor Extraction: A High-Speed Nonequilibrium Model

Abstract A mathematical model of the biodegradation processes occurring during remediation of contaminated soils by soil vapor extraction (SVE) is presented. The model includes Monod kinetics for biodegradation and rate-limited mass transfer between the aqueous and the gas phases. These mass transfer limitations appear to explain the major features observed in field results presented in the literature: zero-order kinetics with respect to biomass and contaminant, and a very important increase in the percentage contribution of biodegradation to cleanup with decreasing gas flux. Microcomputer runs with the “exact” algorithm are very time-consuming so faster algorithms have been developed using several approximations. These result in savings of more than 95% in computation time, with errors due to the approximations of less than 1%.

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.

Soil Cleanup by In-Situ Aeration. XVIII. Field-Scale Models with Diffusion from Clay Structures

Abstract Mathematical models for soil vapor extraction (SVE) are developed which model solution of nonaqueous phase liquid (NAPL) and mass transport of volatile organic compounds (VOCs) through low-permeability lumps, lenticular structures, and discontinous layers of clay by means of a distributed diffusion approach. The well configurations modeled are that of a single buried horizontal slotted pipe and that of a single vertical well screened along a short length near its bottom. The models yield high off-gas VOC concentrations initially, followed typically by vary rapid drop-offs to relatively long plateaus, followed in turn by terminal tailing, the length of which is highly variable and determined by the thickness of the low-permeability layers from which diffusion is occurring and by the size of the NAPL droplets (if NAPL is present). The results are in agreement with a previous, simpler model; they indicate that it will be impossible to accurately predict SVE cleanup times from data taken in short-ter...

Soil Clean Up by in-situ Aeration. VI. Effects of Variable Permeabilities

Abstract A mathematical model for in-situ soil vapor stripping (vacuum extraction) is developed and used to examine the effects of a spatially variable pneumatic permeability tensor on the rate of clean-up of a site contaminated with volatile organic compounds. Runs are made with low-permeability clay lenses placed at various locations in the domain of interest; also the effect of soil moisture distribution on the soil gas flow field is examined. The model permits one to carry out a sensitivity analysis of the effects of heterogeneity in the permeability, and to develop strategies for minimizing the damaging effects of domains of low permeability.