Francisco García-Herruzo

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.

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 ...

Biogas production from pear residues using sludge from a wastewater treatment plant digester. Influence of the feed delivery procedure

Clear economic advantages may be obtained from the management of seasonal fruit wastes by codigestion at existing facilities which are working throughout the year with other residues. We have explored the biomethanization of pear residues in a 5L stirred reactor loaded with sludge from the anaerobic digester of a municipal wastewater treatment plant. Different organic loading rates (OLRs) of fruit waste were tested with two delivery procedures: a discontinuous one (fed once a day) and a pseudocontinuous one. For both procedures, as the OLR increases the pH of the digester drops to acidic values and large OLRs may cause the reactor failure. Nevertheless, the pseudocontinuous delivery allows the treatment of more residue, (10.5 versus 6.0 g of volatile solids per litre of reactor and day), maintaining the specific biogas production (0.44 L of biogas per gram of volatile solids), with some improvement in methane concentration (44% vs 39%).

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.

Acid enhanced electrokinetic remediation of a contaminated soil: Strong vs. weak acid

The most typical enhancement for electrokinetic remediation (EKR) involves the neutralization of the alkaline front generated at the cathode by the addition of an acid to the catholyte. Although the use of strong and weak acids can be found in the literature, there is still a clear lack of reliable comparisons between them. This paper presents a comparison of the results obtained for a real lead contaminated soil treated by EKR enhanced with nitric or with acetic acid. The divergences that are usually observed in those experimental results obtained when using constant voltage drop are avoided here by the use of EKR with constant current intensity and the disposition in series of the soil lab columns. Important differences are observed in the behavior of the system when the acid was changed. The time needed to achieve the same metal recovery is more than 3 times longer for the nitric acid. A standard sequential extraction procedure was used to fractionate the lead concentration in the soil according to its mobility in a before- and after-treatment way. Regardless of these important differences observed in the remediation time, the fractionation results after the experiments were almost the same for the two acid enhanced EKRs.

Copper Removal from Water by Chemical Reduction with Sodium Borohydride

Abstract The use of NaBH4 as a chemical reductant to precipitate copper from water has been studied. Initial Cu2+ concentrations of 25 and 40 mg/L have been checked, and complete removal can be achieved. When solid NaBH4 is fed, BH4 − to Cu2+ molar ratios higher than 2 were needed. This can be substantially improved by using NaBH4 as a stabilized alkaline solution. A kinetic model is proposed which gives information about the relative rates of the two competitive reactions involved (Cu2+ and water reduction with BH4 0) and allows the operating conditions leading to minimum BH4 − consumption to be established.

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.