Rafael A. García-Delgado

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...

Influence of soil carbonates in lead fixation

Abstract Despite the progressive decrease in emissions coming from a variety of sources, lead continues to be one of the toxic metals more often found as environmental pollutants. Of particular concern is lead migration through the soil which may result in contamination of water supplies through the leaching caused by water infiltration. Carbonate minerals are frequently found in soils and those of heavy metals are usually insoluble so carbonates in the soil could act as a sink for heavy metals for as long as the environmental conditions guarantee their stability. The influence of soil composition on the fixation of lead from aqueous solutions of this metal has been studied. Two clayey soils with different carbonate content have been used in the tests. The kinetics and equilibrium of retention of lead in the soils under different conditions of pH and salinity have been determined using batch and fixed‐bed column experiments. Carbonate precipitation and ion exchange were the main processes responsible for ...

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 theretention phenomena may be represented by a Langmuir isotherm foran aqueous metal concentration up to 100 mg L-1. First order kinetics with respect to the solid phase yield good predictabilityfor both batch and column experiments. Kinetics and thermodynamics of lead retention predominate over those ofcadmium. As a consequence, lead is preferentially retainedand can even displace sorbed cadmium. In the event of anspill involving both metals, cadmium would move further inthe soil and its aqueous concentration downstream could beeven higher than that of the influent solution, increasingpotential 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.

Column study of the influence of air humidity on the retention of hydrocarbons on soil

Inverse gas chromatography (IGC) is used for the analysis of the influence of air relative humidity on the retention of hexane, benzene, toluene and p-xylene in a sandy soil under experimental conditions similar to those typical of soil vapor extraction (SVE). The advantages of IGC over other techniques, are (a) an efficient use of lab resources, (b) a high sensitivity to low partitioning coefficients and (c) a closer reproduction of field conditions. In our procedure, experiments with only two samples of different mass are necessary to establish if linear isotherms can be used to describe the retention of the contaminants. This approach gives information necessary for analyzing the feasibility and design of remediation technologies with a laboratory effort significantly smaller than the adsorption/desorption cycle for isotherm determination. The retention coefficients of the contaminants decrease as humidity increases in a similar fashion for all of them, probably because the reduction in the number of the adsorption sites available for the organic compounds due to the presence of water is quite similar for all the contaminants studied. These retention coefficients may be related to those obtained for dry air conditions for all the contaminants through (R - 1)RH% = A(R - 1)dry air(B), where the parameter B is found to remain approximately constant (0.90), while the parameter A decreases linearly with the relative moisture.

Lead mobilization from a clayey soil in relation to carbonate content

Abstract The efficiency of removing lead from two contaminated clayey soils of different carbonate content was studied in the laboratory. Water and aqueous solutions of sodium chloride, acetic, hydrochloric and nitric acids and ethylenediaminetetraacetic acid disodium salt (EDTA‐Na2) were examined for their potential extractive capabilities in batch and column tests. Pb concentrations in the soil ranged from 13, 000 to 49, 000 mg/kg mainly distributed in the carbonate and the exchangeable fractions of the soil. The effects of varying the soil/solution ratio and the concentrations of the extraction agents have been studied. Results show that removal of Pb from the exchangeable fraction is readily achieved when little or no carbonates are present. Mobilization of the Pb in the carbonate fraction of the soil is only feasible with acid solutions or concentrated EDTA‐Na2 solutions.

A model with mass transport limitations for pump and treat remediation of soils polluted with NAPL

A model is presented for the description of the pump and treat (or flushing) remediation of the saturated zone with non-aqueous phase liquid (NAPL) present as droplets. Sensitivity analysis shows that the most important variables are the NAPL droplet size and the distance through which the dissolved organic compound must diffuse to reach the advecting aqueous phase. The time needed to achieve complete remediation for different initial contaminant concentrations in soil depends more on the NAPL droplet radius and the size of the stagnant boundary layer than on the initial contaminant mass itself. Location of wells and flux rates are of little significance over the time needed for completion as long as all the water that flows through the contaminated region is captured in the recovery well.

Two‐dimensional groundwater transport of reactive solutes with competitive adsorption

Chemical contamination of groundwater is typically associated with multicomponent solutions, the migration of which is affected by preferential adsorption and solution reactions. Transport models should, therefore, account for advection and dispersion and for the reactions in the solution and on the solid phase. The two-dimensional model METLI has been developed for the personal computer (PC) simulation of groundwater transport with competitive adsorption of a three-species system consisting of a metal, a ligand, and their dominant complex at a given pH. The model structure, which employs spatial and process splitting algorithms, is described. One- and two-dimensional application examples are used to demonstrate how chemical reactivity and affinity for adsorption of a species, adsorption capacity of soil, and the number of the adsorbing species affect pollutant plumes in groundwater. The model executes rapidly on PCs using grids of several hundred nodes.

A COLUMN STUDY OF SOIL CONTAMINATION BY LEAD: INFLUENCE OF pH AND CARBONATE CONTENT. I. EXPERIMENTAL RESULTS

The influence of soil carbonate content on the fate of lead in soil was studied in a lab-scale column under different pH values of the contaminant solution. Results indicated that retention of this toxic heavy metal (up to 38% weight at pH = 5) occurred which was proportional to the total carbonate content. A decrease in the pH of the aqueous solution entering the column resulted in a decrease of the retention of lead in the soil. Furthermore, the concentration of lead in the effluent was increased dramatically. Concentrations of lead about 2.5 times higher than in the contaminant solution were measured at pH = 3. Knowledge of these phenomena is important for risk assessment and remediation feasibility studies.

A column study of soil contamination by lead: influence of pH and carbonate content. II. Mathematical model.

A mathematical model is used for the interpretation of the results from earlier experimental studies in lab-scale columns on the contamination of a carbonatic soil with lead. Local equilibrium conditions suffice to reproduce the experimental curves for every pH value of the influent contaminant solution and carbonate content of the soils essayed, but heterogeneous contact between the aqueous and solid phase should be included. This heterogeneous contact is responsible for the important tailing effects observed, and is difficult to estimate even for the lab conditions. Then, important uncertainties should be accepted both for risk assessment and in situ remediation feasibility studies.