M.C. Hermosín

Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity

Hydrotalcite-like compounds (HT) and their calcined product (HT500) remove diverse anionic pollutants from waters by two mechanisms: anionic exchange for HT and reconstruction for HT500, and both can be affected by structural characteristics of these sorbents such as degree of layer substitution, interlayer anion and crystallinity. This work deals with assessing the effect of crystallinity and interlayer anion of the sorbent on the adsorption of two anionic organic contaminants in water: 2,4,6-trinitrophenol (TNP) and dodecylbenzylsulfonate (DBS). [Mg3Al(OH)8]2CO3·nH2O (HTCO3) and Mg3Al(OH)8Cl·nH2O (HTCl) were prepared by the coprecipitation method and subjected to hydrothermal treatment in air at 80°C and 130°C for 24 h. The adsorption of TNP and DBS was followed by adsorption isotherms, quantitatively described by application of the Langmuir equation. The adsorption of TNP and DBS on HT by anionic exchange is dramatically affected by the interlayer anion. On the calculated anion exchange capacity (AEC) basis, HTCO3 adsorbed 23% of TNP and 0% of DBS, whereas HTCl adsorbed up to 100% of both anionic contaminants. The specific affinity of carbonate anion for HT layer strongly complicates its displacement. These exchange reactions were scarcely modified by the crystallinity, more important in the case of carbonate-samples. Inversely, the adsorption of TNP and DBS by reconstruction on HT500 was affected by the crystallinity in all cases; however, HT500 derived from HTCO3 samples adsorbed better than those derived from HTCl samples. The adsorption of TNP and DBS on HT500 (from the untreated samples) was about 50% in both sorbents, but TNP adsorption increased up to 77% on HTCO3500 and only up to 52% on HTCl500 (from 130°C treated samples), and DBS adsorption increased up to 100% on HTCO3500 and 70% on HTCl500. A crystallinity increase of HT seems to render probably a better ordered HT500, favouring the memory effect.

Hydrotalcite as sorbent for trinitrophenol: sorption capacity and mechanism

The aim of this work was to evaluate the sorbent power of hydrotalcite compound [Mg3Al(OH)8]2CO3·xH2O (HT) and its calcined product Mg3AlO4(OH) (HT500) for 2,4,6-trinitrophenol (TNP) from water solution. The adsorption behaviour of TNP was investigated at diverse solid/solution ratios, pH and TNP concentration by batch equilibration technique. The L and H type adsorption isotherms obtained in those sorbents respectively indicated a chemisorption process which was irreversible and fitted the Langmuir equation model well. Sorption capacity and energy were found to be very high for HT500. The X-ray diffraction and IR spectroscopy techniques applied to TNP-HT and TNP-HT500 products indicate that anionic TNP is adsorbed by anion exchange in the interlayer of HT to 20% of the anion exchange capacity (AEC) and, by reconstruction of the layered structure on HT500, to 40% of the AEC. The results suggested the potential use of HT500 as a filter for TNP, being also easily recyclable.

Hydrotalcite-like compounds as potential sorbents of phenols from water

Abstract Hydrotalcite-like compounds, being anti-types of clay minerals, are potential sorbents for polar and anionic molecules. This work summarizes the experimental results of adsorption—desorption of two phenols (trichlorophenol, TCP, and trinitrophenol, TNP) on an hydrotalcite (HT) and its calcined product (HT500). Both phenols are adsorbed on HT by anion exchange, but TNP adsorption was much higher than that of TCP due to their different pKa. The adsorption of both phenols on HT500 occurs by the reconstruction of the layered structure, TNP being also adsorbed at more than TCP. These adsorption mechanisms were confirmed by X-ray diffraction, IR spectroscopy and TG-DTA analysis of the HT-TCP and HT-TNP products. Results presented here indicate HT500 as potential sorbent for phenols from waters, and that its recyclability is possible.

Organosmectites as sorbent and carrier of the herbicide bentazone

Sorption isotherms of bentazone on diverse organoclays (OCls) were measured at aqueous concentrations from 25 to 1500 microM. Sorption on organophilic OCls, OCls with large quaternary alkylammonium groups, i.e. hexadecyltrimethylammonium (AHDT) and dioctadecyldimethylammonium (ADOD), were compared to that on sorptive OCls, which have large primary alkylammonium (octadecylammonium, AC18) and small quaternary alkylammonium (phenyltrimethylammonium, PTA) groups. The organophilic OCls showed much higher sorption (AHDT Kd = 682-1789; ADOD Kd = 838-1728) than the sorptive OCls (AC18 Kd = 38-40; PTA Kd = 0), suggesting hydrophobic bentazone-organocation interactions as the main mechanism responsible for sorption. The incubation of the two organophilic OCls with soil contaminated with bentazone showed that availability of the herbicide decreased almost instantaneously from 124 microg g(-1) to 1 microg g(-1). Bentazone sorbed on the OCl, and thus immobilized, was partially extractable with CaCl2/methanol solution suggesting its potential bioavailability. Two OCls, with high and low sorptions, were assayed as herbicide carriers by preparing bentazone-OCl complexes and monitoring their herbicide release in water and soil-water suspensions. These bentazone-OCl complexes released 20-80% of their bentazone content, depending of the sorbent capacity and the bentazone-OCl interaction. Results of this work show that by varying the sorptive properties of OCls, they can be used to immobilize pesticide in a contaminated soil and to protect soil and water by using OCl as pesticide carriers in slow release formulations.

Influence of biochar amendments on the sorption-desorption of aminocyclopyrachlor, bentazone and pyraclostrobin pesticides to an agricultural soil.

The many advantageous properties of biochar have led to the recent interest in the use of this carbonaceous material as a soil amendment. However, there are limited studies dealing with the effect of biochar on the behavior of pesticides applied to crops. The objective of this work was to determine the effect of various biochars on the sorption-desorption of the herbicides aminocyclopyrachlor (6-amino-5-chloro-2-cyclopropyl-4-pyrimidinacarboxylic acid) and bentazone (3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) and the fungicide pyraclostrobin (methyl 2-[1-(4-chlorophenyl) pyrazol-3-yloxymethil]-N-methoxycarbanilate) to a silt loam soil. Aminocyclopyrachlor and bentazone were almost completely sorbed by the soils amended with the biochars produced from wood pellets. However, lower sorption of the herbicides was observed in the soils amended with the biochar made from macadamia nut shells as compared to the unamended soil, which was attributed to the competition between dissolved organic carbon (DOC) from the biochar and the herbicides for sorption sites. Our results showed that pyraclostrobin is highly sorbed to soil, and the addition of biochars to soil did not further increase its sorption. Thus, addition of biochars to increase the retention of low mobility pesticides in soil appears to not be necessary. On the other hand, biochars with high surface areas and low DOC contents can increase the sorption of highly mobile pesticides in soil.

Montmorillonite–chitosan bionanocomposites as adsorbents of the herbicide clopyralid in aqueous solution and soil/water suspensions

Montmorillonite (SWy-2)-chitosan bionanocomposites (SW-CH) were prepared following different methodologies, characterized, and assayed as adsorbents of the herbicide clopyralid (3,6-dichloropyridine-2-carboxylic acid) in aqueous solution and soil/water suspensions, to assess the potential of the materials to prevent and remediate soil and water contamination by anionic pesticides. The SW-CH bionanocomposites were good adsorbents for the herbicide at pH levels where both the anionic form of the herbicide (pK(a)=2.3) and the cationic form of CH (pK(a)=6.3) predominated. The performance of the SW-CH bionanocomposites as adsorbents of clopyralid depended on the amount and arrangement of chitosan in the samples. Clopyralid adsorption was rapid and mostly linear up to herbicide concentrations as high as 0.5mM. High salt concentrations (0.1M NaCl) promoted desorption of the adsorbed pesticide from SW-CH, strongly suggesting that adsorption of clopyralid occurred primarily through an ion exchange mechanism on positively charged CH sites at the montmorillonite surface. Amendment of an acidic soil (pH=4.5) with SW-CH at rates of 5% and 10% led to a significant increase in clopyralid adsorption, whereas this effect was negligible when SW-CH was added to an alkaline soil (pH=8.0), reflecting the absence of positively charged sites in SW-CH at high pH values. Montmorillonite-CH bionanocomposites can be useful as adsorbents for the removal and/or immobilization of anionic pesticides in soil and water under mild acidic conditions.

Sorption of two polar herbicides in soils and soil clays suspensions

Abstract Adsorption of the polar herbicides thiazafluron (1,3-dimethyl-1-(5-trifluromethyl-1,3,4-thia-diazol-2-yl)urea) and metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) in the aqueous suspension of five soils of southern Spain, their respective clay fractions (with diverse organic carbon content and clay mineralogy) and model pure clay minerals has been monitored as an integrated study to assess the role of the diverse colloidal soil components and their solid/solution ratio, as relevant to the transport of contaminants by particulate matter in water. Adsorption isotherms obtained were analysed and fitted to the logarithmic form of the Freundlich equation and adsorption coefficients Kf calculated. Thiazafluron adsorbs on soils, soil clays and model mineral sorbents to a higher extent than the herbicide metamitron due to their different molecular structure. The sorption of both herbicides in clay fractions increases with decreasing solid/solution ratio. The highest Kf value at high solid/solution ratio for both herbicides is found in a saline soil with its clay fraction predominantly composed of an altered illite mineral which behaves as a montmorillonite. Thiazafluron and metamitron also show the highest adsorption capacity (at low solid/solution ratio) on a predominantly montmorillonitic clay fraction of low cation exchange capacity (CEC), whereas low adsorption is found on a montmorillonitic clay fraction of high CEC. The negative influence of the clay CEC is confirmed in adsorption studies on pure clay minerals suspensions. The sorption of both herbicides by soil clays after removing organic matter (OM), shows that contribution of the colloidal OM is very low for thiazafluon, although rather important for metamitron. The influence of the different nature of the OM associated to the clay fractions of diverse soils is suggested. The mineral components of the soil clays, especially expandable layer silicates such as montmorillonite and a type of altered illite, are revealed to be responsible for the adsorption and hence the transport of these polar herbicides by waters in contact with soils or fine-size soil separates. Not only the relative amounts of the organic and inorganic components are important, but also the surface properties and the accessibility of the functional active groups of the herbicide molecule to those surfaces.

Sorption of the ionizable pesticide imazamox by organo-clays and organohydrotalcites

The ability of two montmorillonites, SWy-2 and SAz-1, one hydrotalcite, HT [(Mg 3 Al(OH) 8 ) 2 .CO 3 .4H 2 O], and some of their organoderivatives (organoclays and organohydrotalcites) to sorb the ionizable pesticide imazamox (2-[4,5-dihydro-4-methyl-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid) was determined at different pH levels to determine their potential use as sorbent materials for ionizable organic pollutants. At the pH of the sorbents (pH 6-7), the anionic form of imazamox predominated and the calcined product of HT (HT500) was found to be the best sorbent for imazamox anion. Negligible sorption of imazamox anion was measured on the pure clays, on hydrotalcite and organohydrotalcites, whereas sorption on organoclays was moderate and highly irreversible. Decreasing the pH led to a great increase in the sorption capacity of the organoclays, due to formation of molecular imazamox, which had a great affinity for the interlayer organic phase of the organoclays. In contrast, extensive dissolution of the hydroxide structure of the hydrotalcites at low pH limited their use as sorbent material in acidic conditions.

Trichlorophenol adsorption on layered double hydroxide: a potential sorbent

Abstract The objective of this work was to assess the adsorption mechanism of trichlorophenol (TCP) on a layered double hydroxide, the hydrotalcite compound [Mg3Al(OH)8]2CO3.xH2O), (HT), and its calcined product Mg3AIO4(OH), (HT500), as exploring their potential use as phenol sorbents. TCP adsorption on HT and HT500 was measured by batch equilibration technique at diverse solid/solution ratios, pH, contact times and TCP concentrations. X‐ray diffraction and Fourier Transform‐Infrared (FT‐IR) spectroscopy were applied to the adsorption products to uscertain TCP‐HT interactions. The results showed that TCP adsorption on HT is very low and seems to occur as anion exchange reaction only on the external surface sites at the entrance of the interlayer spaces. TCP adsorption on the calcined compound HT500 is high and irreversible and occurs by the reconstruction of the layered structure. X‐ray powder diffraction and FT‐IR spectroscopy showed that TCP adsorbed at the interlayer spaces as phenolate form that besid...

Environmental behavior of the enantiomers of the chiral fungicide metalaxyl in Mediterranean agricultural soils

Improving the existing knowledge on the enantioselectivity of processes affecting chiral pesticide enantiomers in the environment is necessary to maximize the efficacy and minimize the environmental impact caused by the use of pesticides with chiral properties. In this work, the enantioselectivity of the sorption, degradation, and leaching processes of the chiral fungicide metalaxyl in three slightly alkaline, agricultural soils from southern Spain was studied. Batch sorption experiments indicated that the sorption of racemic-metalaxyl on soils, their clay (<2 μm) fractions, and a number of model sorbents simulating naturally-occurring soil colloidal particles was non-enantioselective; the S-enantiomer was sorbed to the same extent as the R-enantiomer on all soil materials. Soil incubation experiments revealed that the R-enantiomer of metalaxyl was degraded faster than the S-enantiomer in all three soils, but the extent and enantioselectivity of metalaxyl degradation was soil-dependent, occurring more slowly and with less enantioselectivity in the fine-textured soil (soil 1) than in the coarse-textured soils (soils 2 and 3). For soils 2 and 3, S- and R-metalaxyl dissipation data were very well described by single first-order kinetics, whereas for soil 1 dissipation data were better fitted by two coupled first-order equations. It is suggested that sorption and entrapment of metalaxyl enantiomers in the abundant small-size pores of soil 1 (i.e., pore radius<100 nm) could have resulted in a fraction of the fungicide of reduced bioavailability, and consequently, protected from enantioselective degradation. Metalaxyl leaching through soil columns was also enantioselective; the concentration of S-metalaxyl in all leachates collected was greater than that of R-metalaxyl. Despite being non-enantioselective, sorption influenced the enantioselectivity of metalaxyl leaching, as it determined the residence time of the fungicide within the soil column, and consequently, the extent and enantioselectivity of its degradation during leaching.