J. Cornejo

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.

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.

Removing 2,4-D from water by organo-clays

Abstract Decylammonium-montmorillonite (C10M) and decylammonium-vermiculite (C10V) were assayed as sorbent for the weak acid herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) and were compared with untreated clays and activated carbon. Langmuir and Freundlich sorption parameters were calculated from the sorption isotherms. These indicated an increase of sorption capacity for 2,4-D of clays after decylammonium exchange. The sorption at different pH showed that molecular forms were preferentially adsorbed on C10M whereas anionic forms were on C10V. The C10V sample showed much higher and stronger sorption capacity than C10M. The different behaviour of both organic sorbents was related to the different arrangement of decylammonium cations in the interlayer of both minerals, as a consequence of their different layer charge.

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.

Hydrotalcites as sorbent for 2,4,6-trinitrophenol: influence of the layer composition and interlayer anion

This work evaluates the effects of different variables such as MII/MIII ratio (layer charge), interlayer anion (CO32− or Cl−) and nature of the trivalent cation (Al3+ or Fe3+) in the sorption of 2,4,6-trinitrophenol (TNP) by layered double hydroxides (LDH) [MII1 − xMIIIx(OH)2]An−x/n·mH2O (MII = Mg2+, MIII = Al3+ or Fe3+, A = CO32− or Cl−, x = 0.33 − 0.20) and its calcined products MII1 − xMIIIxO1 + x/2. TNP adsorption by anionic exchange is strongly affected by exchangeable interlayer anion, being dramatically favoured in the case of MgAlCl. TNP adsorption by anionic exchange on the MgAlCO3 system is very low and it is not affected by the Mg/Al ratio while it is in MgAlCl. In the case of the MgFeCl system, the higher polarizing character of Fe3+ makes the exchange reaction less favorable. TNP adsorption by reconstruction of the calcined products decreases in the order MgAlCO3500 > MgAlCl500 > MgFeCl500, and it is favored by the layer charge decrease.

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.

Layered double hydroxides as adsorbents and carriers of the herbicide (4-chloro-2-methylphenoxy)acetic acid (MCPA): systems Mg-Al, Mg-Fe and Mg-Al-Fe.

Hydrotalcite-like compounds [Mg(3)Al(OH)(8)]Cl x 4H(2)O; [Mg(3)Fe(OH)(8)]Cl x 4H(2)O; [Mg(3)Al(0.5)Fe(0.5)(OH)(8)]Cl x 4H(2)O (LDHs) and calcined product of [Mg(3)Al(OH)(8)]Cl x 4H(2)O, Mg(3)AlO(4.5) (HT500), were studied as potential adsorbents of the herbicide MCPA [(4-chloro-2-methylphenoxy)acetic acid] as a function of pH, contact time and pesticide concentration, and also as support for the slow release of this pesticide, with the aim to reduce the hazardous effects that it can pose to the environment. The information obtained in the adsorption study was used for the preparation of LDH-MCPA complexes. The results showed high and rapid adsorption of MCPA on the adsorbents as well as that MCPA formulations based on LDHs and HT500 as pesticide supports displayed controlled release properties and reduced herbicide leaching in soil columns compared to a standard commercial MCPA formulation. Thereby, we conclude that the LDHs employed in this study can be used not only as adsorbents to remove MCPA from aqueous solutions, but also as supports for the slow release of this highly mobile herbicide, thus controlling its immediate availability and leaching.

Organo/LDH nanocomposite as an adsorbent of polycyclic aromatic hydrocarbons in water and soil–water systems

Polycyclic aromatic hydrocarbons (PAHs) are considered as priority pollutants because of their high risk to human health. In this paper, we addressed the issue of using hydrotalcite-based nanocomposites as adsorbents of six low molecular weight PAHs (acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene) to reduce their negative effects on the environment. A nanocomposite (HT-DDS) was prepared by intercalating the organic anion dodecylsulfate (DDS) in a Mg-Al hydrotalcite (HT), and then characterized using several analytical techniques. A Mediterranean soil was selected for being a high-risk scenario of groundwater contamination by leaching of pollutants. The nanocomposite displayed enhanced affinity for the PAHs in water as compared to carbonate-hydrotalcite (HTCO(3)) and its calcined product (HT500), and showed a high irreversibility of the adsorption process (hysteresis coefficient, H<0.15). The results revealed an increase of the pollutants retention in the soil by the addition of the nanocomposite that depended on the nanocomposite application rate and also on the hydrophobicity of each PAH. Accordingly, the use of HT-DDS as an amendment or barrier in contaminated soil is proposed for reducing the mobility of PAHs and, consequently, the adverse effect derived from rapid transport losses of the pollutants to the adjoining environmental compartments.