Fenuron sorption on homoionic natural and modified smectites

Optimizing a low added value bentonite as adsorbent material to remove pesticides from water

A local low value bentonite from Southern Spain (Raw Bentonite), previously decarbonated (Bent), was modified to improve its pesticide adsorption capacity with Fe³⁺, hexadecyltrimethylammonium (HDTMA) and the biopolymer chitosan (CH). Adsorption of pesticides on powdered samples showed that Fe³⁺ and HDTMA were appropriate modifiers for this purpose. The modification was optimized by saturation with Fe³⁺ (Bent-Fe) and hexadecyltrimethylammonium (Bent-HDTMA) and the obtained adsorbents were characterized by several physicochemical techniques (X-ray diffraction, thermogravimetric analysis, X-ray fluorescence, physisorption of N₂). Their adsorption capacity to remove three widely used pesticides in Andalusian crops (terbuthylazine, tebuconazole and MCPA) from water was assessed and compared with the commercial organoclay Cloisite® 10A (Clo10). The modified bentonites adsorbed the selected pesticides in a percentage ranging from 30 to 100%, whereas sorption on Clo10 ranged from 30 to 90%. For their possible use as filtering beds, Bent-HDTMA, Bent-Fe and Clo10 were granulated by using three different binders (colophony resin and carnauba and bee waxes) at three different mixing ratios and the water resistance and pesticide adsorption of the granules were measured. Results showed that the granulation process did not alter the pesticide adsorption capacity of the powdered modified bentonites, and both waxes granules (carnauba and bee wax) showed better behavior than resin granules. In this work, we succeeded in the preparation of granulated adsorbents derived from a low cost material with similar behavior against a high purity smectite (precursor of Cloisite® 10A). This raises an alternative for this waste material to be used in filter systems for removing pesticides from contaminated water.

Modeling of fenuron pesticide adsorption on CNTs for mechanistic insight and removal in water

Inexpensive multi-walled carbon nanotubes (MCNTs) were prepared with 10-40 nm particle sizes and 9.0 m²g^-1 surface area. Fenuron pesticide was removed in water using these CNTs with 100.0 µgL^-1 concentration, 60 min contact time, 2.0 g L^-1 dose, 7.0 pH, and 25 °C. 90% removal of fenuron pesticide was achieved. Adsorption data obeyed Tempkin, Freundlich, Langmuir and Dubinin-Radushkevich models. The standard free energies values of fenuron pesticide adsorption were -11.89, -11.59, -11.55 kJ mol^-1. The values of enthalpy and entropy were -9.12 kJmol^-1 and -26.61 × 10^-3 kJ mol^-1 K. The negative values of free energy showed speedy adsorption of fenuron pesticide on CNTs. The supramolecular mechanism of fenuron adsorption onto CNTs was fixed by simulation studies and the binding energy and binding affinity of fenuron with CNTs were - 6.5 kcal mol^-1 and 5.85 × 10⁴ M^-1, respectively. There were one π-σ, seven π-π stacked, one π-π T-shaped, and three π-alkyl type of hydrophobic interactions between fenuron and carbon nanotube. These results clearly indicated the physical nature of the adsorption. The method is speedy, cost-effective, efficient and repeatable. Therefore, the established adsorption method is appropriate for adsorption of fenuron pesticide in waters.

Leaching of hazardous substances from a composite construction product--an experimental and modelling approach for fibre-cement sheets

The leaching behaviour of a commercial fibre-cement sheet (FCS) product has been investigated. A static pH dependency test and a dynamic surface leaching test have been performed at lab scale. These tests allowed the development of a chemical-transport model capable to predict the release of major and trace elements over the entire pH range, in function of time. FCS exhibits a cement-type leaching behaviour with respect to the mineral species. Potentially hazardous species are released in significant quantities when compared to their total content. These are mainly heavy metals commonly encountered in cement matrixes and boron (probably added as biocide). Organic compounds considered as global dissolved carbon are released in significant concentrations, originating probably from the partial degradation of the organic fibres. The pesticide terbutryn (probably added during the preservative treatment of the organic fibres) was systematically identified in the leachates. The simulation of an upscaled runoff scenario allowed the evaluation of the cumulative release over long periods and the distribution of the released quantities in time, in function of the local exposure conditions. After 10 years of exposure the release reaches significant fractions of the species' total content - going from 4% for Cu to near 100% for B.

Comparative sorption and leaching study of the herbicides fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) in a soil amended with biochars and other sorbents

Biochar, the solid residual remaining after the thermochemical transformation of biomass for carbon sequestration, has been proposed to be used as a soil amendment, because of its agronomic benefits. The effect of amending soil with six biochars made from different feedstocks on the sorption and leaching of fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) was compared to the effect of other sorbents: an activated carbon, a Ca-rich Arizona montmorillonite modified with hexadecyltrimethylammonium organic cation (SA-HDTMA), and an agricultural organic residue from olive oil production (OOW). Soil was amended at 2% (w/w), and studies were performed following a batch equilibration procedure. Sorption of both herbicides increased in all amended soils, but decreased in soil amended with a biochar produced from macadamia nut shells made with fast pyrolysis. Lower leaching of the herbicides was observed in the soils amended with the biochars with higher surface areas BC5 and BC6 and the organoclay (OCl). Despite the increase in herbicide sorption in soils amended with two hardwood biochars (BC1 and BC3) and OOW, leaching of fluometuron and MCPA was enhanced with the addition of these amendments as compared to the unamended soil. The increased leaching is due to some amendments' soluble organic compounds, which compete or associate with herbicide molecules, enhancing their soil mobility. Thus, the results indicate that not all biochar amendments will increase sorption and decrease leaching of fluometuron and MCPA. Furthermore, the amount and composition of the organic carbon (OC) content of the amendment, especially the soluble part (DOC), can play an important role in the sorption and leaching of these herbicides.

Study on adsorption of glyphosate (N-phosphonomethyl glycine) pesticide on MgAl-layered double hydroxides in aqueous solution

MgAl-layered double hydroxides with different interlayer anions (nitrate, carbonate and chloride) were evaluated for their abilities to adsorb the organic pesticide glyphosate (N-phosphonomethyl glycine, Gly). The adsorption isotherms of Gly on layered double hydroxides (LDHs) nitrate were described by the Langmuir equation at lower equilibrium concentration of Gly (C(e)<1.0 mmol/L), and the Gly adsorption capacity on LDHs increased with the layer charge density, i.e. the structural Al3+/Mg2+ ratio. Gly adsorption on LDHs nitrate generally occurred through two processes, external surface adsorption and interlayer anion exchange. The adsorption amount on LDHs at C(e)=1.0 mmol/L decreased in the order of interlayer anions: Cl(-)>NO3(-)>CO3(2-).

Hydrolysis of carbaryl by carbonate impurities in reference clay SWy-2

The influence of clay preparation methods on the sorption and hydrolysis of carbaryl (1-naphthyl, N-methyl carbamate) by K+-saturated reference smectite SWy-2 was studied. Four methods were utilized: (1) The reference (or specimen) clay used as received was K+-saturated (hereafter referred to as whole clay). (2) High-speed centrifugation (3295g) of whole clay resulted in a pellet with three discrete bands. The upper, light-colored, low-density band was obtained by manual separation (light fraction). The high-density, dark-colored material comprising the lower band (heavy fraction) was also obtained manually. (3) SWy-2 was subjected to overnight gravity sedimentation to obtain the <2 microm particles (clay-sed.) and then K+-saturated. (4) SWy-2 was subjected to low-speed centrifugation (58-60g) to separate the <2 microm particle size (clay-cent.) and then K+-saturated. Each preparation of mineral fractions manifested significantly different abilities to hydrolyze carbaryl to 1-naphthol, decreasing in the order whole clay > heavy fraction >> clay-sed. > light clay > clay-cent. The extent of 1-naphthol disappearance from solution, accompanied by a progressive darkening of the clay, followed the order whole clay > heavy fraction >>> light clay > clay-sed. > clay-cent. Using ring labeled [14C]carbaryl, approximately 61 and 15% of the total 14C activity added to the whole clay and light fraction, respectively, remained unextractable. X-ray diffraction of the heavy fraction revealed several peaks corresponding to minor impurities, including calcite and dolomite. Aqueous slurries of whole clay, light fraction, clay-sed., and heavy fraction were alkaline, whereas the pH of slurried clay-cent. was neutral. It was concluded that dissolution of inorganic carbonate impurities in SWy-2 caused alkaline conditions in the slurries leading to the hydrolysis of carbaryl. Dissolution of carbonates with sodium acetate buffer eliminated hydrolytic activity associated with SWy-2. None of the four preparation methods reliably removed inorganic carbonates. The use of commercial or reference smectites in surface chemistry studies should be accompanied by a treatment with acetate buffer to remove carbonate impurities.

Sorption and detoxification of toxic compounds by a bifunctional organoclay

Organoclays are excellent sorbents for nonionic contaminants and therefore may have many environmental applications. A major limitation on the use of organoclays is that the contaminant merely changes its location from one environmental compartment to another while still remaining intact. In this study, a new type of organoclay, termed a bifunctional organoclay, has been prepared. It is able not only to sorb organophosphate pesticides, but also to catalyze their hydrolysis, and thereby detoxify them. The bifunctional organoclay prepared in this study is based on sodium montmorillonite, in which the inorganic counter ions are replaced by N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium (DDMAEA). The detoxifying capacity of this organoclay for two organophosphate pesticides, methyl parathion [O,O-dimethyl O-(p-nitrophenyl) thionophosphate] and tetrachlorvinphos [2-chloro-1-(2,4,5-trichlorophenyl)ethenyl dimethyl phosphate], was demonstrated. It was shown that although the sorption of these pesticides on the bifunctional organoclay is very similar to that on N-decyl-N,N,N-trimethyl ammonium (DTMA) organoclay (the corresponding nonbifunctional organoclay), the hydrolysis of these pesticides is substantially enhanced only by the bifunctional organoclay. The half-life for the hydrolysis of the investigated pesticides in the presence of the bifunctional organoclay is about 12 times less than for their spontaneous hydrolysis, and the enhancement is even more pronounced relative to the hydrolysis of these pesticides in the presence of the DTMA organoclay (which actually inhibits their hydrolysis). Based on kinetic measurements, the pK(a) of the ethylamino group of the bifunctional organoclay was estimated to be around 9.0. It is postulated that the catalytic effect of the bifunctional organoclay can be attributed to a nucleophilic attack of the unprotonated ethylamino group of the organoclay on the organophosphate ester.

Organoclays for controlled release of the herbicide fenuron

Organoclays were assayed as matrices in which to associate herbicides, with the aim of decreasing product losses that could give rise to water contamination from agricultural activities. Fenuron was selected as model of a very mobile and highly water-soluble herbicide. Two different organoclays of high (A-HDT) and low (H-C18) reversible fenuron sorption were selected. Herbicide-organoclay complexes were prepared from the two organoclays and with two different fenuron contents (20 and 40 g AI kg-1) and two different mixing times, so as to form a series of weak and strong complexes. The release of fenuron from those complexes into water and water/soil suspensions gave values of T50 (time to release 50% of the fenuron content) ranging from 0.3 min to 2400 h. The total fenuron released in these closed systems ranged from 48 to 80% of the fenuron in the complex. The organoclay type (high or low sorptivity) had the greatest influence on fenuron release, followed by the strong or weak complex, suggesting that herbicide-organoclay interactions are the main factors controlling release. Soil column leaching experiments showed fenuron-organoclay complexes to be effective in reducing the peak herbicide concentration in the leachate to a half (6 microns) or a quarter (3 microns) of that obtained from the free technical compound (12 microns). Herbicide lost through leaching was reduced from 78% for the free technical fenuron to 50-30%, depending on the organoclay used as carrier and the strength of the complex. Bioassay with ryegrass showed that the weak fenuron/H-C18 complex (40 g AI kg-1) gave the same herbicidal activity as technical fenuron. The potential suitability of low-sorptive organoclays for conferring slow-release properties on the fenuron complex has been demonstrated.