Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculations

doi:10.1016/S0169-1317(01)00068-0

Applied Clay Science

Volume 20, Issue 6, February 2002, Pages 273-282

https://doi.org/10.1016/S0169-1317(01)00068-0 Get rights and content

Abstract

Adsorption of the divalent organic cations paraquat (PQ), diquat (DQ) and methyl green (MG) on sepiolite was determined experimentally and investigated with an adsorption model. The largest amounts of DQ, PQ and MG adsorbed were between 100% and 140% of the cation exchange capacity (CEC) of sepiolite. In previous experiments with monovalent organic cations (dyes), the largest amounts of dyes adsorbed were about 400% of the CEC of sepiolite. Consequently, it was proposed that most of this adsorption was to neutral sites of the clay. The large differences between the adsorption of these divalent organic cations and the monovalent dyes may indicate that there is almost no interaction between DQ, PQ and MG and the neutral sites of sepiolite. This assumption was confirmed by infrared (IR) spectroscopy measurements, that did not show changes in the peaks arising from the vibrations of external SiOH groups of the clay when the divalent organic cations were added. Adsorption results were compared with calculations of an adsorption model that combines the Gouy–Chapman solution and specific binding in a closed system. The model considers cation adsorption on neutral sites of the clay, in addition to adsorption to mono- or divalent negatively charged sites, forming neutral or charged complexes. The model could adequately simulate the adsorption of the divalent organic cations DQ and PQ when added alone, and could yield good fit for the competitive adsorption experiment between the monovalent dye methylene blue and DQ. In competitive adsorption experiments, when total cationic charges exceeded the CEC, monovalent organic cations were preferentially adsorbed on the clay at the expense of the divalent cations.

Introduction

Sepiolite is a clay mineral with a unit cell formula Si₁₂O₃₀Mg₈(OH,F)₄(H₂O)₄·8H₂O Brauner and Preisinger, 1956, Santaren et al., 1990, and a general structure formed by an alternation of blocks and tunnels that grow up in the fibre direction (c-axis). Each block is constructed by two tetrahedral silica sheets enclosing a central magnesia sheet. In some aspects sepiolite is similar to other 2:1 trioctahedral silicates, such as talc, but it has discontinuities and inversions of the silica sheets that give rise to structural tunnels. In the inner blocks, all corners of silica tetrahedra are connected to adjacent blocks, but in outer blocks some of the corners are Si atoms bound to hydroxyls (SiOH). These silanol groups at the “external surface” of the silicate (Alrichs et al., 1975), are usually accessible to organic species, acting as neutral adsorption sites (denoted as N). In addition to that, some isomorphic substitutions in the tetrahedral sheet of the lattice of the mineral, such as Al³⁺ instead of Si⁴⁺, form negatively charged adsorption sites (P sites). Such sites are occupied by exchangeable cations that compensate for the electrical charge.

These characteristics of sepiolite make it a powerful sorbent (Alvarez, 1984) for neutral organic molecules and organic cations.

In this work we present results for the adsorption to sepiolite of three divalent organic cations: methyl green (MG), diquat (DQ) and paraquat (PQ) (Fig. 1).

Methyl green is basic triphenylmethane-type dicationic dye, usually used for staining solutions in medicine and biology (Green, 1990). Preadsorbing it on clay minerals was also proposed as means to stabilize pesticides against photodecomposition Margulies et al., 1985, Margulies et al., 1987. Previous studies Rytwo et al., 1998a, Rytwo et al., 2000 addressed the issue of MG adsorption on montmorillonite and sepiolite by elucidating the existence of pH-dependent transformations of MG between mono- and di-valent states, which may result in two patterns of adsorption.

Diquat and paraquat are divalent contact herbicides widely used with several crops. However, in soils, strong interactions between DQ and PQ and certain clays drastically reduce their herbicidal activity (Calderbank, 1968). The environmental impact of these herbicides is highly influenced by their interactions with the clay components of the soil Basile et al., 1990, Moyer and Lindwall, 1985.

The main aim of the current study has been to visualize the pattern of adsorption of these divalent organic cations on sepiolite. The adsorption was measured experimentally, and the results were compared with calculated adsorption amounts evaluated by a computer model that considers cation adsorption on neutral and charged sites of sepiolite (Rytwo et al., 1998b).

Section snippets

Materials

The clay used was Yunclillos sepiolite (<200 mesh), provided by Tolsa (Madrid, Spain) and with a 99% content of pure mineral. The chemical composition of the clay is described in Ruiz-Hitzky and Casal (1985). The surface area (N₂, BET) is 340 m² g⁻¹ and the cation exchange capacity (CEC) is near 0.14 mol_c kg⁻¹. Analyzes performed to the clay in previous studies (Rytwo et al., 1998b), showed that Na⁺ contributed to the CEC 0.03 mol_c kg⁻¹, and the rest of the CEC is contributed by Mg²⁺. The

Results and discussion

Fig. 3 shows the amounts of DQ, PQ and MG adsorbed on sepiolite after 7 days, as a function of total divalent cation added amount. This is not a classic isotherm representation, but it helps to emphasize that when the added amounts were lower than the CEC of the sepiolite (0.14 mol_c kg⁻¹), all cations were completely adsorbed. Similar results were observed for the adsorption of these cations on smectites (Tomlinson et al., 1969, Narine and Guy, 1981, Margulies and Rozen, 1986, indicating a very

Concluding remarks

Our results show that when the added concentrations of MG, DQ and PQ are below the CEC of the clay, less than 5% of the added concentration remained in solution. Such behavior corresponds to high-affinity (H-type) adsorption isotherms, and demonstrates that the affinity of these divalent organic cations for sepiolite is very strong.

From FTIR spectra of the silanol groups, we deduced that unlike neutral organic compounds and monovalent organic cations, DQ and PQ do not bind to the neutral

Acknowledgements

This research was supported in part by a Grant G-0405-95 from G.I.F., the German–Israeli Foundation for Scientific Research and Development, by a grant for Strategic Research in Environmental and Water Quality #1317 from Israel's Ministry of Science, and by a fellowship from the Sacta-Rashi Foundation. We are grateful to Prof. Shlomo Nir for reading the manuscript and for his helpful comments and suggestions.

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Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculations

Abstract

Introduction

Section snippets

Materials

Results and discussion

Concluding remarks

Acknowledgements

J. Mol. Struct.

J. Catal.

J. Colloid Interface Sci.

J. Colloid Interface. Sci.

Structural hydroxyls in sepiolites

Clays Clay Miner.

Sepiolite: properties and uses

Surfactant adsorption and rheological behavior of surface modified sepiolite

Adsorption of methylene blue on sepiolite gels: spectroscopic and rheological studies

Clay Miner.

Sorption of pesticides by soils of Campania [Italy]

Agrochimica

Structure and origin of sepiolite

Miner. Petr. Mitt.