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In this project anionic clays or layered double hydroxides (LDHs) of magnesium and aluminium (LDH-MgAl) and of zinc and aluminium (LDH-ZnAl) were synthesized. Subsamples of these materials were treated at 450 °C obtaining LDH-MgAl450 and LDH-ZnAl450. Moreover, other two sorbents were obtained: a fine grained poorly crystalline iron-aluminium oxide (HyAlFe) and a iron-aluminium-montmorillonite (HyAlFeMt) complex. Olive mill waste waters (OMWW) were treated by methanol obtaining two components: a dark precipitate containing the high molecular fraction, named polymerin, and a supernatant, named OMWW-S, containing the monomeric organic component and inorganic anions and cations. Moreover, from OMWW-S a phenolic component, named OMWW-E was extracted by ethyl acetate. Other two sorbents named LDH-MgAlPol and LDH-ZnAlPol were prepared by sorption of polymerin onto LDH-MgAl and LDH-ZnAl. Finally polymerin was also complexed with HyAlFe obtaining HyAlFePol.
Results of sorption isotherms evidenced that the best materials for removal of phenols were the calcined materials LDH-MgAl450 and LDH-ZnAl450. Phytotoxic bioassays indicated that OMWW-S after three cycle of cyclic sorption onto LDH-ZnAl450 decreased its phytotoxicity on seeds and seedlings of tomato. Chemical analysis conducted after every cycle showed that phenolic content decreased after the third decontamination cycle of about 87%. We also conducted another phytotoxic bioassay on Lepidium sativum seeds using LDH-MgAl450 as sorbent and OMWW-E as sorbate. Results indicated that germination index increased to 94% and that the content of phenols decreased by 96% after the third decontamination cycle.
Moreover, we also studied the behaviour of nanomaterials for retention of pesticides with the aim to develop slow-release formulations of these chemicals. Sorption experiments were carried out using the fungicide metalaxyl-M. The results indicated that metalaxyl-M had greater affinity for LDH-MgAl450. Desorption studies showed the only presence in solution of the metalaxyl acid, the active form of the pesticide. Experiments of coating indicated that the complex LDH-MgAl450-metalaxyl-M protected the seeds of tomato against the fungi Phitium ultimum. In fact, germination index, fresh and dry weight, and seedling height showed values very similar to the control and in all the cases higher than those observed when the seeds were treated with the pathogen fungi.
In conclusion this study has developed a novel method to recycle OMWW for land use. The partitioning of OMWW in two components, namely polymerin and OMWW-S, and the sorption of the phenolic component and anions onto the LDHs resulted in two residues with low phytotoxicity behaviour. This treatment appeared to be more convenient than diluting OMWW before spraying it onto soil or treating it microbiologically. Moreover, sorption onto LDHs calcined materials proved to be very effective since it decreased the phytotoxic risk linked to the intrinsic compositional variability of OMWW. Thus, OMWW-S could represent a low cost source of irrigation water with limited phytotoxic effects for soil where the water is scarce.
The preparation of formulate LDH-metalaxyl-M can be an alternative to the commercial formulates actually used to protect the culture from pathogen fungi. In fact, the strong linkage between the pesticide and the LDHs present three advantage: i) protect the seeds from pathogenic fungi; ii) increase the time which the pesticide is active because the LDHs protect it from the degradation and iii) reduce the pollution from pesticide of water and soil for their accidental release from farmyards and inputs of washings. In fact, after the treatment the seeds can be simply washed to remove the formulate; this latter successively can be recovered from solution by a simple process of centrifugation. Finally, the preparation of LDHs and LDHs calcined materials represent a simple and low cost preparation procedure.