Beatrice Graziania, Luca Rivoiraa, Marta Gallob, Maria Ricciardic, Antonio Protoc, Barbara Onidab, Maria Concetta Bruzzonitia

aDipartimento di Chimica, Università di Torino, 10125 Torino, Italy;
bPolitecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy
cUniversità degli Studi di Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy

Global olive oil production has currently reached 2,73 thousand tons, mainly located in the Mediterranean area. The major ecological and environmental issue of this activity is the production of high volumes of Olive mill wastewater (OMW). In particular, the high content of reducing sugars determines high values of Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD), exceeding the maximum concentrations established for discharge by the current normative. In addition, the presence of polyphenols reduces the performance of the degradation process of organic compounds promoted by microorganisms. Hence, the presence of the above-mentioned compounds makes it difficult to treat OMW by traditional biological oxidation methods. Based on these premises, this work aims to develop, characterize and compare different microporous sorbents and their behaviour in removing reducing sugar, such as Glucose, and polyphenols (tyrosol and hydroxytyrosol, selected as representative compounds) from OMW. A SBA-15 adsorbent was properly modified with (3-aminopropyl)-triethoxysilane (APTES) to specifically promote interactions with -OH moieties of glucose and polyphenols. Its absorption capacity was tested alongside traditional sorbents such as activated carbons and commercial primary and secondary amine (PSA). Several physicochemical techniques were used to characterize the modified SBA15, namely thermogravimetric analysis and IR spectroscopy were used to characterize the surface, while X-ray diffraction and nitrogen gas adsorption were used to investigate morphology and pore size. To what concern removal tests, high-performance anion exchange chromatography (HPAEC) with pulsed amperometry detection (PAD) was used for sugar determination, while Folin-Ciocalteu colorimetric reaction was used to detect polyphenols. The adsorption properties were studied at various pH conditions and analyte concentrations. The results obtained showed that activated carbon, due to its high surface area, seems to be the most efficient material in removing sugars and polyphenols. However, it lacks selectivity and its adsorption, which is completely irreversible, requires longer contact times than the modified SBA-15. The commercial PSA showed interactions only towards sugars (and limited to acidic pH). In contrast, the SBA-15 functionalized material showed promising adsorption performances in the first hours (shorter contact times than activated carbons) with reversible adsorption, thus suggesting that retained polyphenols could be further desorbed for their recovery, in a circular economy perspective. The SBA-15 material was successfully tested in a real OMW sample.