Rocío López

Analysis of phosphate adsorption onto ferrihydrite using the CD-MUSIC model.

Ferrihydrite nanoparticles may dominate the ion binding properties of the natural oxide fraction present in soil and aquatic systems. A correct description of the adsorption properties of ferrihydrite nanoparticles may be useful for gaining a better insight into the adsorption processes in natural systems and at the same time will be essential for developing surface complexation models able to describe these processes. In the present study, phosphate speciation in ferrihydrite has been analyzed combining the available spectroscopic data and molecular information with modeling calculations. For this purpose, a new data set that analyzes the effect of pH and ionic strength on the phosphate adsorption onto ferrihydrite has been used. Description of the phosphate adsorption process onto ferrihydrite nanoparticles, for the entire pH and ionic strength range, has been made taking into account the presence of protonated and nonprotonated bidentate surface complexes. The presence of monodentate complexes, protonated and nonprotonated, was also analyzed, but no significant improvement in the description of the results was observed. The surface complexation constants obtained with the CD-MUSIC modeling calculations are comparable to the values found in the literature for phosphate surface complexes in goethite particles.

Adsorption of MCPA on goethite and humic acid-coated goethite.

Anionic pesticides are adsorbed on the mineral oxide fraction of the soil surface but considerably less on the organic fraction, so that the presence of organic matter causes a decrease in the amount of pesticide adsorbed, and may affect the mechanism of adsorption. In the present study we investigated the adsorption of the weak acid pesticide MCPA on the surface of goethite and of humic acid-coated goethite, selected as models of the mineral oxide fraction and organic components present in soil systems. Adsorption of the anionic form of the pesticide on goethite fitted an S-type isotherm and the amount adsorbed increased as the ionic strength decreased and the pH of the medium decreased. Application of the charge distribution multi site complexation model (CD-MUSIC model) enabled interpretation of the results, which suggested the formation of inner and outer sphere complexes between the pesticide and the singly-coordinated surface sites of goethite. Less pesticide was adsorbed on the humic acid-coated goethite than on the bare goethite and the pattern fitted an L-type isotherm, which indicates a change in the mechanism of adsorption. Simplified calculations with the CD-MUSIC model enabled interpretation of the results, which suggested that the pesticide molecules form the same type of surface complexes as in the previous case.

Comparison of arsenate, chromate and molybdate binding on schwertmannite: surface adsorption vs anion-exchange.

The presence of iron oxides may play an important role in controlling the mobility and availability of contaminants in soils and waters affected by acid mine drainage. The present study describes the uptake of arsenate, chromate and molybdate from solution by synthetic schwertmannite. Batch experiments were performed at different pH values in order to obtain the adsorption isotherms for the three oxyanions. In addition to the formation of surface complexes between the oxyanions and the iron surface reactive groups, it is also expected that anion exchange will occur between sulphate anions from the schwertmannite structure and the oxyanions present in the solid/solution interface. Comparison of the experimental adsorption results for the different oxyanions showed large differences, not only the amount adsorbed, which was much higher for arsenate, but also in the sulphate exchange with the anions in solution. In case of chromate, the main mechanism of adsorption process is the exchange reaction with the sulphate groups present in the schwertmannite. The observed results suggest a different adsorption mechanisms for each of the three oxyanions, with important implications for the mobility of these anions in acid mine drainage systems.

Effect of pH and ionic strength on the binding of paraquat and MCPA by soil fulvic and humic acids

The effect of pH and ionic strength on the interaction between pesticides (paraquat and MCPA) and humic substances (soil extracted humic acid and fulvic acid) was interpreted with a simple electrostatic model. Potentiometric titrations were carried out, the charge curves for the humic substances were obtained for three values of ionic strength, and the parameters that define proton binding to humic and fulvic acid were calculated by application of the NICA-Donnan model. The binding isotherms were obtained for paraquat-humic acid and paraquat-fulvic acid at three different pH values and two ionic strengths, and the MCPA-fulvic acid binding isotherms for two pH values and two ionic strengths. Binding experiments were carried out by use of a membrane dialysis technique and the concentrations of pesticide were measured by HPLC. The amount of paraquat bound to the humic substances increased with pH, decreased with increasing ionic strength, decreased in the presence of Ca(2+) and was greater for humic acid than for fulvic acid. Much less binding was observed with MCPA than with paraquat, and therefore the isotherms were not well defined. The application of a simple electrostatic model enabled us to conclude that the effect of pH and ionic strength on binding of paraquat to humic substances is due to the effect that these parameters have on the humic substance charge, and the model provided an excellent reproduction of the experimental binding isotherms.

Analysis of copper and calcium-fulvic acid complexation and competition effects

The binding of Cu2+ and Ca2+ to a fulvic acid (FA), simulating naturally occurring conditions, was studied. Furthermore, copper-FA complexation in the presence and absence of calcium was compared. For this, potentiometric titrations were carried out using a solution of FA at a concentration of 100 mgL(-1), and of ionic strength 0.1M. The Ca(2+)-FA complexation reaction was carried out at pH 6.5, 7.5 and 8.5 and the Cu(2+)-FA complexation reaction at pH 5.5 and 6.5, in both the presence and absence of calcium ion. The calcium ion had a significant effect on copper binding at [Ca2+]> or =2.5x10(-3)M. The experimental binding curves were analyzed using non-electrostatic discrete site and continuous distribution models. Competition by the calcium ion mainly affected the maximum binding capacity of the Cu(2+)-FA complexation, whereas the slight effect observed on the binding constant appears to indicate that calcium only competes for specific copper sites, thus modifying the binding sites distribution function. It was also found that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on FA at the pH values studied.

Effect of organic matter and pH on the adsorption of metalaxyl and penconazole by soils

Soil organic matter (SOM) is considered to be the primary adsorbent of non-ionic pesticides, and it is therefore thought to determine the concentration of such pesticides in the soil solution and how they are transported throughout the medium. It is generally assumed that the sorption capacity of different soils is the same per unit mass of SOM; however, the reactivity also depends on the SOM composition and the pH of the medium. We carried out experiments to study the effects of pH and ionic strength on the adsorption of the non-ionic fungicides metalaxyl and penconazole on four soils containing different amounts of organic carbon. The adsorption isotherms fitted a Freundlich equation. For pH>5, partitioning of the fungicides between the solid phase and the soil solution did not vary with the pH, while at lower pH, the fraction adsorbed on the solid phase increased as the pH decreased. The response was related to the effect of pH on the ionization of the carboxylic groups of the SOM and therefore to the hydrophilic nature of the SOM. Analysis of the charge effect on the partitioning of both fungicides revealed a common response in all four soils. Adsorption appears to be related to the magnitude of the charge developed at the SOM due to ionization of the carboxylic acid groups.

Adsorption of paraquat on soil organic matter: Effect of exchangeable cations and dissolved organic carbon

Herbicides that interact with soil organic matter do so with both the solid and the dissolved fractions, so that the distribution of herbicide between the soil solution and solid phases is determined by competitive effects. In the present study, adsorption experiments were carried out with the cationic herbicide paraquat and untreated and acid-washed samples of a peat soil, at different values of pH and ionic strength. Less herbicide was adsorbed onto the untreated peat than onto the acid-washed peat; the difference was due to the presence of exchangeable cations, as demonstrated in experiments carried out by adding Ca(2+) to suspensions of acid-washed peat. The results were interpreted by an electrostatic model and the fitting parameters indicated that the adsorption constants were the same for both samples of peat, although the number of binding sites available was different. Simultaneous resolution of the adsorption equilibrium of paraquat for the soil organic matter (SOM) and of the binding equilibrium between paraquat and dissolved organic matter (DOM) enabled the distribution of paraquat between the solid and solution phases to be determined. The increased solubility of the SOM with increasing pH led to a decrease in the fraction of paraquat retained on the peat surface above pH 5.5, which favors the mobility of the herbicide in the soil.

Effect of fulvic acid concentration on modeling electrostatic and heterogeneity effects in proton binding reactions

Abstract Potentiometric titrations of two soil extracted fulvic acids (FA) were carried out at various FA concentrations ( 25 mg l −1 FA ] mg l −1 ) and ionic strengths ( 0.005 M KNO 3 ] M ). Experimental data have been analyzed by means of the master curve approach which includes an electrostatic spherical double layer model, and Langmuir–Freundlich isotherm was used for fitting the data. For one of the samples, the chemical heterogeneity of the fulvic acid has been described by means of a unique master curve for all the concentrations studied. For the other sample, two master curves were necessary to analyze the chemical heterogeneity: one for the lowest concentration and other for the higher concentrations; however, the calculated intrinsic proton binding parameters for both master curves are not quite different. The concentration of carboxylic sites obtained from the Ca-acetate titration method matched the maximum charge corresponding to the acid carboxylic sites of the Langmuir–Freundlich isotherm quite well.

Analysis of the effect of concentration on the acid-base properties of soil fulvic acid. Conformational changes

Abstract Potentiometric titrations of a sample of soil extracted fulvic acid (FA) were carried out using different concentrations (20 mg l−1

Copper adsorption on humic acid coated gibbsite: comparison with single sorbent systems

Environmental context. Adsorption processes control the mobility and bioavailability of nutrients and contaminants in soils, sediments and aquatic systems. Natural organic matter and aluminium oxides are important reactive materials present in natural systems and their mutual interaction may alter the surface properties of both materials, playing an important role on the fate of different contaminants, such as copper, in the environment. The present study illustrates the importance of these interactions, showing that the presence of natural organic matter has a synergic effect on the copper adsorption on the aluminium oxide surface. Abstract. Copper adsorption processes on aluminium oxides may significantly control the mobility and transport of copper ions in soils and surface waters. The binding of protons and copper to humic acid (HA) and to gibbsite as single sorbent systems was investigated and the results then used to test the validity of the Linear Additivity Model (LAM) for describing copper binding to gibbsite/HA systems. More copper was adsorbed in the gibbsite/HA/Cu2+ ternary system, at pH 4 and 6 and ionic strength 0.1 M, than in the corresponding binary systems. Although copper adsorption on gibbsite at pH 4 is rather small, the enhancement in sorption was noteworthy, and can be attributed to the formation of ternary complexes and changes in the electrostatic potentials at the mineral surface or at the HA as a result of their mutual interaction. The LAM predicted satisfactorily the experimental results at pH 6, whereas it underestimated the copper binding at pH 4.