Several clay-organic complexes were synthesized by placing quaternary ammonium cations on smectite by cation exchange. They were then examined for their ability to adsorb phenol and several of its chlorinated congeners. The organic cations used were: hexadecylpyridinium (HDPY+), hexadecyltri-methyl ammonium (HDTMA+), trimethylphenyl ammonium (TMPA), and tetramethylammonium (TMA+). The complexes containing long-chain alkyl (hexadecyl) groups were the most hydrophobic and adsorbed the phenols from water in proportion to their hydrophobicities, which increase with chlorine addition (phenol < chlorophenol < dichloropohenol < trichlorophenol). With n-hexane as the solvent, different adsorption was found which depended on the type and degree of solvent interactions with the compound and the clay-organic complex. Thus, the amount of adsorption of these phenols on clay-organic complexes was dependent on the relative energies of adsorbent-adsorbate and adsorbate-solvent interactions.

Several clay organic complexes were prepared by placing organic cations on the exchange sites of smectite clays and studied as sorbents for pentachlorophenol (PCP). The organic cations used ranged from very hydrophobic in nature (e.g., dioctadecyldimethyl+ (DODMA+)- and hexadecyltrimethyl+ (HDTMA+)-ammonium) to those having minimal hydrophobic properties, such as tetramethylammonium+ (TMA+). In general, the more hydrophobic the cation on the smectite the greater the uptake of PCP from water. For the very hydrophobic clays (DODMA+- and HDTMA+-smectite) the uptake of PCP was via non-polar interactions between the alkyl (e.g., -C18) groups on the organic cation and PCP. In a mechanistic sense, this interaction appeared to be similar to a partitioning process between water and the organic phase of the clay-organic complex. The organic phases of DODMA+-smectite were about 10 times more effective than the organic matter of natural sediments for removing PCP from water. For those organo-clays containing small organic cations (e.g., TMA+), the organic phase consisted of separate small organic moieties, such as the methyl group. This phase did not act as an effective partitioning medium despite a significant carbon content, and very little PCP was taken up. Results from this study suggest the possibility of treating soils and subsurface materials with large hydrophobic organic cations to enhance the sorptive properties of these natural materials.

A high-charge smectite from Arizona [cation-exchange capacity (CEC) = 120 meq/100 g] and a low-charge smectite from Wyoming (CEC = 90 meq/100 g) were used to prepare homoionic tetra-methylammonium (TMA)-clay complexes. The adsorption of benzene, toluene, and o-xylene as vapors by the dry TMA-clays and as solutes from water by the wet TMA-clays was studied. The adsorption of the organic vapors by the dry TMA-smectite samples was strong and apparently consisted of interactions with both the aluminosilicate mineral surfaces and the TMA exchange ions in the interlayers. In the adsorption of organic vapors, the closer packing of TMA ions in the dry high-charge TMA-smectite, compared with the dry low-charge TMA-smectite, resulted in a somewhat higher degree of shape-selective adsorption of benzene, toluene, and xylene. In the presence of water, the adsorption capacities of both samples for the aromatic compounds were significantly reduced, although the uptake of benzene from water by the low-charge TMA-smectite was still substantial. This lower sorption capacity was accompanied by increased shape-selectivity for the aromatic compounds. The reduction in uptake and increased selectivity was much more pronounced for the water-saturated, high-charge TMA-smectite than for the low-charge TMA-smectite. Hydration of the TMA exchange ions and/or the mineral surfaces apparently reduced the accessibility of the aromatic molecules to interlamellar regions. The resulting water-induced sieving effect was greater for the high-charge TMA-smectite due to the higher density of exchanged TMA-ions. The low-charge Wyoming TMA-smectite was a highly effective adsorbent for removing benzene from water and may be useful for purifying benzene-contaminated water.

Sorptive properties of organoclays may be greatly influenced by the physicochemical properties of organic sorbates. Hexadecyltrimethylammonium(HDTMA) clays were prepared using a high-charge smectite (HDTMA-SAz-1), a low-charge smectite (HDTMA-SWy-2), and an illite (HDTMA-ILL). The resultant organoclays were used to sorb aqueous phase 1,2-dichlorobenzene (o-DCB), 1,3-dichlorobenzene (m-DCB), and 1,4-dichlorobenzene (p-DCB). Sorptive characteristics of these compounds were determined by their molecular polarities (o-DCB > m-DCB > p-DCB) and the HDTMA-clay interlayer distance. HDTMA-ILL was used for comparison to HDTMA-SAz-1 and HDTMA-SWy-2. All dichlorobenzene isomers were directly intercalated in the interlayers of HDTMA-SAz-1, causing interlayer expansion. o-DCB and m-DCB were not intercalated in the interlayers of HDTMA-SWy-2 at low concentrations, but intercalation occurred at higher concentrations, which caused interlayer expansion. The concentration needed to produce interlayer expansion depended on the solute molecular polarity, hence a higher concentration of m-DCB than o-DCB was required. p-DCB was sorbed primarily by the HDTMA phase on the external surfaces of HDTMA-SWy-2. In the presence of chlorobenzene (CB), p-DCB sorption by HDTMA-SWy-2 is greatly enhanced, owing to the interlayer expansion by CB and a cosolvent effect. Sorption of o-DCB resulted from both direct solvation-type interactions with HDTMA and partitioning into HDTMA. Such sorption results in double-sigmoid isotherms. m-DCB weakly solvates the HDTMA and partitions into the HDTMA, displaying either a double-sigmoid or a type-III isotherm depending on clay type. p-DCB lacks ability to solvate HDTMA and partitions into HDTMA as its sole mechanism, producing type-III isotherms. HDTMA-clays are potentially effective for treating dichlorobenzene-contaminated wastewater.

Adsorption studies of aromatic hydrocarbons of various molecular sizes on organo-clays in aqueous solution were carried out for characterizing the surface heterogeneity of organo-clays. Benzene, toluene, p-xylene, ethylbenzene and n-propylbenzene adsorption by a smectite with 5 different exchange degrees of trimethylphenylammonium (TMPA) cations for Ca2+ was measured. The Langmuir isotherm equation did not adequately describe the experimental data, especially for small molecules, whereas the Dubinin-Radushkevich (DR) equation combined with a gamma-type adsorption energy distribution function described all experimental data well, suggesting the surface and structural heterogeneity of TMPA-smectites. The calculated adsorption energy distributions indicated that the apparent heterogeneity depends on the molecular size of adsorbates. Small adsorbate molecules such as benzene explore a highly heterogeneous surface of TMPA-smectites while large molecules such as n-propylbenzene detect a relatively homogeneous surface. The surface fractal dimension was dependent on the extent of TMPA exchange for Ca2+. When TMPA content is less than 75% of the cation exchange capacity (CEC) of the smectite, the heterogeneity decreases as TMPA content increases; it increases with TMPA content thereafter. These results are related to the size distribution of micropores in TMPA-smectites, which are defined by the 2 semi-infinite aluminosilicate sheets and the interlayer cations. The micropore size distributions and, hence, heterogeneity are created in part by the inhomogeneity of the charge density of clay surfaces and the tendency for cation segregation in these systems.

Montmorillonites saturated with small quaternary alkylammonium ions such as tetramethyl-ammonium (TMA) or trimethylphenylammonium (TMPA) are excellent sorbents for aromatic pollutants. In some cases, water inhibits arene sorption, but the inhibition mechanism is not understood completely. The objectives of this study were to determine whether arenes interact with adsorbed TMA and TMPA ions and/or with siloxane surfaces, and how water affects these interactions. We reacted benzene and ethylbenzene vapors with normal- and reduced-charge TMA- and TMPA-montmorillonite films at several relative humidities, and obtained infrared spectra of the resulting sorbate-clay complexes. Arene sorption caused the methyl asymmetric deformation vibrations adsorbed TMA and TMPA to shift to lower wave-number, whereas water sorption caused shifts to higher wavenumber. In the absence of water, benzene and ethylbenzene adsorbed on the siloxane surface as well as interacted directly with TMA and TMPA ions. The proportion of TMA and TMPA ions that interacted with benzene and ethylbenzene was greater for reduced-charge than normal-charge montmorillonite. Comparison of the HOH deformation and cation methyl asymmetric deformation vibrations indicated that both benzene and ethylbenzene inhibited water sorption substantially, and that water more readily displaced benzene and ethylbenzene from TMA and TMPA ions than from siloxane surfaces. Water inhibited arene sorption mainly by hydrating exchangeable cations, thereby obscuring siloxane surfaces adjacent to adsorbed TMA and TMPA ions and decreasing the average pore dimensions. These results indicate that in the presence of bulk water, arene adsorption likely occurs primarily on the siloxane surface.

Organoclays were prepared by exchanging Ca2+ in a Ca2+-saturated smectite partially or fully with trimethylphenylammonium (TMPA) cations. The mechanistic function of these organoclays as adsorbents for neutral organic compounds in aqueous solution was examined. TMPA cations were found to take a random distribution on the surfaces of mixed Ca/TMPA-smectites. The presence of TMPA, and its random distribution, resulted in water associated with the clay surfaces being held more weakly. Apparently, the interspersing of TMPA and Ca2+ ions prohibits the formation of a stable network of water molecules around Ca2+. Water molecules associated with the siloxane surface in mixed Ca/TMPA-clays are removed during the adsorption of neutral organic compounds from bulk water, leaving only ∼11 strongly held water molecules around each Ca2+, as opposed to ∼58 water molecules in homoionic Ca2+-smectite. These results demonstrate that the amount of water associated with the clay surfaces and interlayers depends on the nature of the exchange cation(s), and not on the amount of available siloxane surface area by itself. We conclude that in TMPA-smectites the TMPA cations function as nonhydrated pillars, and sorption of organic solutes occurs predominantly on the adjacent siloxane surfaces, which are hydrophobic in nature. The water molecules around Ca2+ in mixed Ca/TMPA-smectites obscures some of the siloxane surfaces. This diminishes sorption capacity, in an amount roughly equivalent to the fraction of the CEC occupied by Ca2+, because organic solutes cannot displace the waters of hydration of Ca2+.

Tetramethylphosphonium-smectite (TMP-clay) and tetramethylammonium-smectite (TMA-clay), were prepared and characterized as adsorbents for a series of aromatic and chlorinated hydrocarbons. The sorption of benzene, alkylbenzenes, and carbon tetrachloride as vapors and as solutes from water was studied to evaluate the effect of water on adsorption efficiency. Adsorption of organic vapors depended on the N2 BET surface area. TMA-clay was a slightly better adsorbent than TMP-clay, due to its somewhat higher surface area. The Langumir isotherms obtained indicated that adsorption occurred predominantly in the interlayer micropores, apparently on mineral surfaces between onium ions. Adsorption efficiency of both organo-clays decreased, compared to vapor sorptions, in presence of water. Lower sorption was apparently due to shrinkage of the interlayer pore or cavity sizes by hydration of interlayer TMA and TMP cations. Although sorption efficiencies of both organo-clays was reduced in presence of bulk water, the extent of reduction was much less for TMP-clay. Thus, TMP-clay was a better adsorbent than TMA-clay in presence of water, despite its lower surface area, in direct contrast to vapor sorption. The Langumir isotherms indicated interlayer sorption of benzene, alkylbenzenes and carbon tetrachloride from water by TMP-clay. The absence of Langumir isotherms for toluene, ethylbenzene and p-xylene uptake from water by TMA-clay indicated that these bulkier solutes were not adsorbed in the interlayers. These results indicate that hydration of TMA cations causes shrinkage of the interlayer pores to dimensions that exclude these solutes. The lower degree of hydration of TMP cations enables TMP-clay to maintain interlayer pores large enough to accommodate the bulkier alkylbenzenes.

The size of quaternary ${\text{NH}}_4^{+}$ cations defines their packing configurations in the interlayers and the basal spacings of organoclays, and hence strongly influences the sorptive properties of organoclays. A series of organoclays (TAA-SACs) was prepared from a smectite (SAC) fully exchanged with symmetrical tetraalkylammonium (TAA) cations of progressively increasing sizes with the carbon number of single alkyl group from 1 to 6. X-ray diffraction analysis indicated the packing configurations of monolayer, monolayer-to-bilayer transition, bilayer, and bilayer-to-trilayer transition in the interlayers of SAC. Calculations of the interionic distances between TAA ions support such packing configurations. Sorption of benzene by TAA-SACs displayed a high-low-high uptake trend and progressively weaker sorptive interactions as the size of TAA ions increased. Both the siloxane surfaces and TAA ions contributed to the overall sorption, with their relative contributions dependent on the TAA interionic distances and the basal spacings of TAA-SACs. High benzene sorption by small TAA-SAC (tetramethylammonium(TMA)-SAC) was attributed to the strong interactions between the siloxane surfaces and benzene molecules. With large TAAs, high sorption was due to the effective solute partitioning. Compared to benzene sorption, TCE sorption by small TAA-SACs (TMA-SAC and tetraethylammonium(TEA)-SAC) was less effective and displayed an abnormal trend, due largely to the lack of the siloxane surface-TCE interactions and to the stronger hydration of TMA as compared to TEA ions. The results provide strong evidence to support the use of either small or large quaternary ${\text{NH}}_4^{+}$ cations in preparation of organoclays as effective sorbents for removing organic contaminants from water.

Sorption and transformation of 1-naphthol by a K-smectite (K-SWy-2) were studied using batch sorption isotherms, Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The sorbents included three preparations of the reference smectite clay (SWy-2): (1) whole clay containing naturally occurring carbonate impurities, (2) SWy-2 with the removal of carbonate impurities, and (3) the carbonate-free SWy-2 fraction amended with calcite. For the whole clay and carbonate-free clay amended with calcite, >80% of added 1-naphthol disappeared from aqueous solution within 24 h, corresponding to a sorbed concentration of ≥2 mg/g of clay. In contrast, only 35% of the added 1-naphthol disappeared from solution in the carbonate-free clay after 24 h of exposure. For the clays from the three preparations in this study, <1% of sorbed 1-naphthol could be recovered by methanol extraction from the clays. The XRD data suggested that 1-naphthol was intercalated in the smectite, but was not conclusive because the 1-naphthol sorption range (1.5–2.8 mg/g of clay) in this study had relatively minor effects on the XRD patterns. The FTIR spectra of sorbed 1-naphthol-clay complexes demonstrated structural Fe3+ reduction. The spectra also showed evidence of the transformation of 1-naphthol. It suggests that reduction of structural Fe3+ may be coupled to oxidation/polymerization of 1-naphthol. Further transformation of oxidized 1-naphthol, such as by oxidative coupling reactions, is implicated by formation of a dark gray color on the clay and the inability to extract sorbed 1-naphthol.

The influence of clay preparation procedure on sorption and hydrolysis of carbaryl (1-naphthyl, A-methyl carbamate) by the reference smectite SWy-2 was examined. For research purposes, reference clays are sometimes used without purification, or more commonly, the <2 μm size fraction is obtained by gravity sedimentation or low-speed centrifugation. We determined that these common methods did not remove all the inorganic carbonate impurities present in SWy-2, and that these impurities caused alkaline conditions in aqueous clay suspensions leading to the alkaline hydrolysis of carbaryl to 1-naphthol. The hydrolytic activity of homoionic K-SWy-2 disappeared once carbonates were eliminated. Two methods were evaluated for preparing K-SWy-2 devoid of inorganic carbonates. In Method A, inorganic carbonates were first removed by incremental additions of a 0.5 M sodium acetate buffer (pH 5.0) until the clay suspension reached pH 6.8, followed by low-speed centrifugation to obtain the <2 μm size fraction; in Method B, the order of these steps was reversed. Carbaryl hydrolysis was used as a probe to determine the effectiveness of the two methods in terms of the removal of carbonate accessory minerals. Homoionic K-SWy-2 obtained by Methods A and B produced near neutral pH when suspended in water and hydrolysis of carbaryl in these suspensions was not evident. In this regard, both clay preparation methods were acceptable. However, there were procedural advantages with Method B, which is therefore recommended for the partial purification of reference clays, as detailed in this paper.

The adsorption of two dinitrophenol herbicides, 4,6-dinitro-o-cresol (DNOC) and 4,6-dinitro-o-sec-butyl phenol (dinoseb), by two reference smectite clays (SWy-2 and SAz-1) was evaluated using a combination of sorption isotherms, Fourier transformation infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and molecular dynamic simulations. Clays were subject to saturation with various cations, and charge reduction. The DNOC adsorption decreased with increasing pH indicating that DNOC was primarily adsorbed as the neutral species. The FTIR spectra of DNOC-clay films showed that DNOC molecules are oriented parallel to the clay surface. Interlayer cations have a strong effect on adsorption depending largely on their hydration energies. Weakly hydrated cations, e.g. K+ and Cs+, resulted in greater sorption compared to more strongly hydrated cations such as Na+ or Ca2+. Lower hydration favors direct interactions of exchangeable cations with -NO2 groups of DNOC and manifests optimal interlayer spacings for adsorption. In the presence of sorbed DNOC, an interlayer spacing for K-SWy-2 of between 12 and 12.5 Å was maintained regardless of the presence of water. This d-spacing allowed DNOC molecules to interact simultaneously with the opposing clay layers thus minimizing contact of DNOC with water. The charge density of clays also affected sorption by controlling the size of adsorption domains. Accordingly, DNOC adsorption by low-charge clay (K-SWy-2) was much higher than by high-charge clay (K-SAz-1) and Li-charge reduction greatly enhanced dinoseb adsorption by K-SAz-1. Steric constraints were also evident from the observation that adsorption of DNOC, which contains a methyl substituent, was much greater than dinoseb, which contains a bulkier isobutyl group. Adsorption of DNOC by K-SAz-1 was not affected in the presence of dinoseb, whereas dinoseb adsorption was greatly reduced in the presence of DNOC.

Nitroaromatic compounds (NACs) are components of munitions commonly found as soil contaminants at military training sites and elsewhere. These compounds pose possible threats to human health and ecological systems. Recent studies indicate that these compounds are strongly retained by smectite clays. The adsorption mechanisms are not fully reconciled, but it is known that the type of exchangeable cation strongly affects NAC affinity for smectites. This study examined the sorption of 1,3-dinitrobenzene, 2,4-dinitrotoluene and naphthalene from water by a smectite clay (SWy-2) saturated with ammonium, tetramethylammonium (TMA), trimethylphenylammonium (TMPA) and hexadecyltrimethylammonium (HDTMA). In all cases, we observed greater sorption of 2,4-dinitrotoluene compared with 1,3-dinitrobenzene. The sorption isotherms for 2,4-dinitrotoluene and 1,3-dinitrobenzene displayed a concave-downward curve for NH4-SWy-2 and TMA-SWy-2, whereas the isotherms for sorption of HDTMA-SWy-2 and TMPA-SWy-2 were essentially linear. The magnitude of sorption followed the order: NH4-SWy-2 > TMA-SWy-2 > TMPA-SWy-2 > HDTMA-SWy-2 for both compounds. The greater affinity of NACs for NH4- and TMA-SWy-2 is due in part to complex formation between the exchangeable cation and −NO2 groups. These clays also provide near optimal interlayer distances that approximate the molecular thickness of NACs hence promoting the simultaneous interaction of the planar aromatic rings with opposing siloxane surfaces and solute dehydration. Both processes are energetically favorable. In HDTMA-SWy-2, sorption of all solutes is via a partition-dominated process. Solute competition (diminished uptake of one solute in the presence of a second) was observed for TMA-SWy-2 but not HDTMA-SWy-2. This is consistent with an adsorptive mechanism for TMA-SWy-2 and a partitioning mechanism for HDTMA-SWy-2. This study demonstrates that the dominant molecular mechanism of NAC sorption by smectite changes fundamentally from complexation between −NO2 groups and the exchangeable cation (viz. NH4 and TMA) to partitioning for a systematic series of ammonium and quaternary ammonium cations in which the locus of positive charge (the central N atom) is progressively shielded by organic moieties of increasing size.

The U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS) has been a leader in weed science research covering topics ranging from the development and use of integrated weed management (IWM) tactics to basic mechanistic studies, including biotic resistance of desirable plant communities and herbicide resistance. ARS weed scientists have worked in agricultural and natural ecosystems, including agronomic and horticultural crops, pastures, forests, wild lands, aquatic habitats, wetlands, and riparian areas. Through strong partnerships with academia, state agencies, private industry, and numerous federal programs, ARS weed scientists have made contributions to discoveries in the newest fields of robotics and genetics, as well as the traditional and fundamental subjects of weed–crop competition and physiology and integration of weed control tactics and practices. Weed science at ARS is often overshadowed by other research topics; thus, few are aware of the long history of ARS weed science and its important contributions. This review is the result of a symposium held at the Weed Science Society of America’s 62nd Annual Meeting in 2022 that included 10 separate presentations in a virtual Weed Science Webinar Series. The overarching themes of management tactics (IWM, biological control, and automation), basic mechanisms (competition, invasive plant genetics, and herbicide resistance), and ecosystem impacts (invasive plant spread, climate change, conservation, and restoration) represent core ARS weed science research that is dynamic and efficacious and has been a significant component of the agency’s national and international efforts. This review highlights current studies and future directions that exemplify the science and collaborative relationships both within and outside ARS. Given the constraints of weeds and invasive plants on all aspects of food, feed, and fiber systems, there is an acknowledged need to face new challenges, including agriculture and natural resources sustainability, economic resilience and reliability, and societal health and well-being.