If it is not degraded or effectively extracted from industrial effluent, the synthetic dye used in the textile and cosmetics field can be harmful to the environment, both in terms of production and use. Scientists from the UNESP Institute of Chemistry (IQ/UNESP, Brazil) and SOLEIL have succeeded in improving the material used in the treatment of this polluted effluent. They worked on various multi-functional porous materials, notably layered double hydroxide (LDH) phases also known as synthetic anionic clays which are commonly used to treat this type of effluent.
Various physical, chemical or biological processes can be used to treat industrial effluent. Among them, are the adsorption processes that combine a low cost and large capture capacity (see Figure 1).
Figure 1: Left: aqueous solution containing AB113 dye. Right: after addition of Calc-ZnAl which absorbed the dye and precipitated at the bottom of the vial.
As part of an international collaboration funded by the Brazilian National Research Council (CNPq / Ciência Sem Fronteiras programme), researchers from UNESP Institute of Chemistry (Araraquara Campus (SP), Brazil) and SOLEIL ROCK beamline are interested in the study of various multi-functional porous materials with hierarchical structures, by an operando characterisation approach using time-resolved synchrotron radiation techniques. One of their joint research lines involves synthetic anionic clays of layered double hydroxides (LDH) type commonly used in the treatment of industrial effluent.
Accordingly, the structural properties of LDH during dye capture were studied by synchrotron techniques implemented on ROCK at SOLEIL and on SAXS1 at LNLS (National Synchrotron Light Laboratory, Campinas, Brazil).
Dye capture by LDH
LDH materials consist of brucite-type layers positively charged due to the partial substitution of divalent metal cations in octahedral coordination (Fig. 2 (a)) by trivalent cations. The presence of anions in the interlayer ensures the electrical neutrality of the structure and offers a capacity for exchange with other anions, notably the anionic dye "Acid Blue 113" (AB113) used in the textile industry (Fig. 2(b)).
The teams studied the LDH matrix capture capacity of AB113 dye composed of divalent Zn and trivalent Al cations with the anion carbonate (CO32-) in the interlayer space, LDH-ZnAl-CO3, and by the calcined matrix Calc-ZnAl, obtained by thermal decomposition of LDH-ZnAl-CO3 at moderate temperature (T ≈ 450°C).
Figure 2: (a) Diagram of the structure of layered double hydroxides (LDH). (b) Acid Blue 113 Anionic Dye Formula (AB113)
Positive effects of calcination...
The Calc-ZnAl matrix proves to be 6 times more effective than the parent matrix in the capture of AB113. To understand this behaviour, a detailed structural study using Wide Angle X-ray scattering (WAXS) on the SAXS1 beamline of the LNLS and X-ray absorption spectra at the Zn K-edge in quick EXAFS mode (Quick-EXAFS) on the SOLEIL’s ROCK beamline was undertaken during the thermal decomposition of the LDH and the contact of the parent LDH and calcined LDH with the dye.
The significant enhancement of dye capture by calcined LDH is based on a unique LDH property called memory effect which leads to the regeneration of the LDH layered structure when the calcined LDH is brought into contact with water or a saline solution. Both teams have finely analysed the mechanisms underlying this memory effect through the coupling of synchrotron techniques.
In the study of temperature decomposition, the WAXS and Quick-EXAFS techniques show destabilisation of the LDH structure during water loss, dehydroxylation of the layers and decomposition of the anion, leading to the destruction of the layered structure and oxide formation, notably ZnO.
Decryption of the LDH "memory effect"
For the study of the regeneration of the LDH layered structure, the results obtained by WAXS and quick-EXAFS helped explain the adsorption effects of large anions such as AB113 (Fig. 2b)) during regeneration and provided experimental evidence of the mechanisms, often invoked in the literature but unproven, leading to the memory effect of this type of material. In agreement with transmission electron microscopy (TEM; Figure 3) also carried out during the regeneration of LDH, the recovery of LDH is driven by an aggregative nucleation and growth mechanism, which is limited in the development of the stacking of the layers caused by the AB113 anion steric hindrance. The material reconstructed will be a mixture of nanocrystalline agglomerate layers and aggregates of exfoliated LDH layers. The dye is then preferentially adsorbed onto the external surface of the layers or layer aggregates. Due to the size limitations imposed by the dye upon LDH recovery, the external surface of the calcined LDH is much larger than the one of the parent LDH.
Figure 3: Electronic Microscopy images of LDH-ZnAl-CO3 (left) and Calc-ZnAl (right)
Consequently, this explains the different adsorption capacities observed for both matrices.
The ROCK beamline is supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d'Avenir” Program (reference: ANR-10-EQPX-45).