Trials of clay materials for discoloration and detoxification of water from the olive mil waste

Olive mill waste (OMW) or alperujo is the solid residue with high moisture content produced in the extraction process of oil in two phase system. Its use as soil fertilizer and conditioner improves the soil quality, contributes to carbon sequestration, and facilitates a sustainable solution for the disposal of this waste, which is extensively produced in the Mediterranean basin. Nevertheless, the addition of OMW to soils can also give rise to some adverse effects as: a) colouration of water, due to the high levels of tannins and lignin that may cause ecotoxicological effects, b) increase the mobility of some heavy metals and pesticides and c) phytotoxic effects due to its content in phenolic compounds. All those negative effects are mainly related to the content of soluble organic matter (SOM) of OMW and they may be aggravated when uncontrolled spill happens from the site where it is stored. Two natural cationic clays, SWy and CTI, along with synthetic anionic clay or carbonate-hydrotalcite HTCO3 and its calcined product HT500, were assayed to remove color and toxic components extracted from alperujo as soluble organic matter (SOM). Both cationic clays are expandable lamellar phyllosilicates of smectite type with different purity and cation exchange capacity. The anionic clay (HTCO3) is a layered magnesium-aluminum double hydroxide, also known as LDH, which has a high anion exchange capacity. Its calcination product (HT500) has the ability to adsorb anions from water by the reconstruction of its layered structure or “memory effect”. The purpose of this work was to assess the utility of those natural or synthetic adsorbents to reduce the adverse effects associated with the application of alperujo or OMW to agricultural soils or to remediate the effects produced by uncontrolled spills. Anionic clays had a greater affinity and adsorption capacity to eliminate both the color and the organic components of the SOM from an alperujo water extract, as compared to the smectite clays. The anionic clay HTCO3 showed a higher discoloring capacity than the cationic SWy and CTI clays, but the amounts of organic carbon removed from the water alperujo extracts were similar for the three clays, ranging from 34 to 39%, at the higher sorbent/solution ratio assayed (50:1); that fact suggests that both type of adsorbent remove different organic compounds from alperujo extract. The calcined anionic clay HT500 showed the maximum discoloring power and removed more than 75% of the organic carbon from the water alperujo extract. The UV-VIS spectra of the untreated and sorbent treated water alperujo extracts indicated that cationic clays adsorb different organic components compared to the anionic clay. A bioassay showed that HT500 decreased the phytotoxic effect of alperujo water extract towards Lepidium sativum. The calcined product of the hydrotalcite offers a potential use as possible adsorbent in systems for elimination of the phytotoxicity of alperujo contaminated waters or even as an alternative treatment to composting, for subsequent agricultural application of alperujo.

Funding by P07-AGR-03077, P11-AGR-07400 and AGR-264 of Junta de Andalucía, (FEDER-FSE EU Programs) and Contract RECUPERA 2020 of MICINN-CSIC

Peer Reviewed