Organic ligands containing the thiol (-SH) metal-chelating functionality were either grafted to the external surface silanol groups of sepiolite or introduced in the interlayers of montmorillonite, and the resulting functionalized clays were characterized and assayed as adsorbents for Hg(II), Pb(II), and Zn(II) ions from solution. Sepiolite was functionalized by covalently grafting 3-mercaptopro-pyltrimethoxysilane (MPS) to the surface tSi-OH groups of the clay, whereas montmorillonite was functionalized by replacement of the interlayer inorganic cation (Na + ) by 2-mercaptoethylammonium (MEA) cations. These clayorganic ligand systems were selected to minimize the congestion of the internal porosity of the clays, which has recently been shown to be the main obstacle to heavy metal adsorption by functionalized clays. Infrared spectroscopy and elemental analyses demonstrated the presence of the organic ligands in the modified clays. X-ray diffraction analysis indicated the organic cations (MEA) occupied the interlayers of montmorillonite. N 2 specific surface area measurements suggested that much of the surface area of montmorillonite and sepiolite remained accessible upon functionalization and that the organic ligand kept the montmorillonite interlayers open. The functionalized clays adsorbed most of the Hg(II) ions present in solution up to saturation and were also good adsorbents of Pb(II) at low metal ion concentrations (i.e., <0.02 mM). They were, however, less effective toward Pb(II) and Zn(II) at high metal ion concentrations. The presence of NaNO 3 or Ca(NO 3 ) 2 as background electrolytes at concentrations ranging from 0.001 to 0.1 M did not alter the great adsorption capacity of functionalized sepiolite for Hg(II). The results show that clay functionalization can be optimized by matching clay structure with a suitable reactive (i.e., fibrous clay with a graftable ligand or expandable clay with an exchangeable cationic ligand) and minimizing the gallery volume taken up by the organic ligand, thus improving the performance of the functionalized clay as adsorbent of heavy metals from solution.

The risk of ground water contamination resulting from rapid leaching of highly soluble pesticides can be minimized through the application of the pesticide adsorbed on a matrix or carrier, which limits the amount of pesticide immediately available for undesirable losses. The use of natural materials for this purpose is of special interest in terms of economy and sustainability. In this work the adsorption of the herbicide hexazinone by two montmorillonites saturated with various inorganic and organic cations was determined and the ability of the two clays displaying the highest adsorption capacities [Fe(3+)-saturated Wyoming montmorillonite, (Fe-SW) and hexadecyltrimethylammonium-saturated Arizona montmorillonite (HDTMA-SA)] to act as carriers for slow release of hexazinone and to reduce herbicide leaching losses was evaluated. Hexazinone formulations based on Fe-SW and HDTMA-SA displayed slow release properties in water and soil/water suspensions, reduced herbicide leaching in soil columns, and maintained herbicidal activity, as compared with the currently available commercial hexazinone formulation (wettable powder). Loosely bound hexazinone-HDTMASA formulations, which led to the slowest breakthrough of hexazinone in soil columns along with the greatest amounts of herbicide released from the clay particles, displayed the most interesting characteristics for their use as slow release formulations and to prevent ground water contamination.

It has been assessed the influence of four organic amendments (OA) consisting of two commercial humic amendments (liquid LF and solid SF) from olive-mill wastes, a solid urban waste (SUW), and a sewage sludge (SS) on the sorption properties and leaching potential of simazine and 2,4-D. A sandy soil (TR) and a sandy-clay soil with a relatively high montmorillonite content (A) were treated with the diverse OA. Dissolved organic matter (DOM) was extracted from the amendments, the soils, and the amended soils and studied by fluorescence spectroscopy. A humification index (HIX) was calculated from the fluorescence data. Sorption was determined with the batch technique. Spectroscopical studies revealed that the DOM of the LF differs from the other OA by having a very low ability to absorb and to fluoresce and by its very low HIX values, which indicates that the LF contains large amounts of nonhumified material and consists of small molecules. On the other hand, the SF amendment contains the highest amounts of highly humified material and a large number of carboxylic groups. Amended soils sorbed simazine and 2,4-D to a greater extent than the untreated soils, except in the case of simazine sorption in the LF amended soil A, which had a lower simazine sorption than the original soil. The small molecules of DOM in the LF compete with simazine for montmorillonite sorption sites in soil A. This is not the case for 2,4-D, since this herbicide does not sorb on montmorillonite. In the case of the soil TR, with a lower montmorillonite content, there is no competition between simazine and the LF molecules for sorption sites. Soils amended with the highly humified SF were the best sorbents for simazine but not for 2,4-D, which can be attributed to repulsion between negatively charged 2,4-D molecules and COOgroups, which are more abundant in SF.

Binary and ternary model particles containing montmorillonite, ferrihydrite, and humic acid (HA) were used to determine the changes in the sorption behavior of the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) upon the interassociation of these three major soil constituents. On single sorbents, 2,4-D sorption was high with S-type isotherms on ferrihydrite, moderate with L-type isotherms on HA, and zero on montmorillonite. In binary sorbents, ferrihydrite and humic acid coatings on montmorillonite provided sorption sites for 2,4-D, although the exclusion of the anionic form from highly charged clay surfaces partially obscured the role of Fe and HA as sorbents of 2,4-D. Sorption of 2,4-D on ferrihydrite-HA binary particles was not very different fromthat on pure ferrihydrite because most of the ferrihydrite surface area was from micropores being not accessible to large humic macromolecules and hence remained available for herbicide sorption. In ternary sorbents HA coatings reduced the sorption of 2,4-D by the montmorillonite-ferrihydrite binary complex, indicating that HA blocked many of the sorption sites provided by the Fe coatings. This work shows that the amount and nature of the surface that remains available after the interassociation of single soil constituents is a critical parameter in determining the sorptive behavior of the resultant aggregate for ionizable contaminants such as 2,4-D. Thus, the use of calculated sorption parameters such as K oc , K ow , K mineral , or K Fe , for modeling contaminant behavior in aquatic or soil environments may result in serious deviation from the reality.

A Mg/Al layered double hydroxide (LDH) was intercalated with the anionic herbicides 2,4-D, MCPA, and picloram by using three different methodologies: (i) direct synthesis (DS), (ii) regeneration (RE), and (iii) ion exchange (IE). The resulting complexes were characterized and assayed by batch release and column leaching tests, aiming at the controlled release of these herbicides. All the tested LDHherbicide complexes displayed similar slow herbicide release properties in water, although the IE method seemed to result in complexes with a greater fraction of herbicide in a readily available form. Apparently, the LDH-herbicide complexes released most of the active ingredient present in the complexes at the end of the batch release experiment. This was attributed to the replacement of the intercalated herbicide by carbonate and hydroxyl anions from the aqueous solution. Compared to the free herbicides, the application of the three LDH-herbicide complexes (RE) to soil columns resulted in reduction in the maximum herbicide concentration in leachates and led to the retardation of herbicide leaching through the soil. All LDH-herbicide complexes presented an herbicidal efficacy similar to that of the free (technical) herbicides. Our results indicated the potential applicability of LDHs as supports for the preparation of slow release formulations of acid herbicides such as 2,4-D, MCPA, or picloram.