Valorisation of olive mill wastewater by phenolic compounds adsorption: Development and application of a procedure for adsorbent selection

A procedure for the selection of the optimal adsorbent for phenolic compounds (PC) recovery from PC-rich wastes and wastewaters was innovatively proposed and applied to compare 4 neutral resins (Amberlite XAD16N, Optipore SD-2, Amberlite FPX66, Amberlite XAD761) and 1 ion-exchange resin (Amberlite IRA958 Cl) for PC recovery from a Tunisian olive mill wastewater (OMW). In the initial batch isotherm tests a neutral resin (XAD16N) performed best thanks to its high PC sorption capacity (81 mgPC/gdry resin) and PC content in the sorbed product (0.19 gPC/gvolatile solids). Also ion-exchange resin IRA958, used in OH form in this work, resulted interesting thanks to its satisfactory performances and very low cost (8 €/L). These two pre-selected resins were further compared by means of continuous-flow adsorption/desorption tests conducted in a 1-m packed column. The results indicate that if a low (20%) breakpoint is selected, XAD16N leads to a PC-richer sorbed product (0.14 gPC/gvolatile solids) and a higher operating capacity (0.30) than IRA958. Conversely, if a very high (90%) breakpoint is selected, the two resins produce similar desorbed products in terms of both PC content (0.19–0.21 gPC/gvolatile solids) and antioxidant capacity (4.6–4.9 gascorbic acid equivalent/gPC). Resin-specific dynamic desorption procedures led to very high PC desorption yields (87–95%). The identification of the actual PCs present in the final desorbed product indicated for XAD16N a higher capacity to preserve the integrity of the PC mixture of the studied OMW. OMW microfiltration (0.2 μm pore-size) led to a 99.8% suspended solid removal – thus protecting the packed column from potential clogging – with a very low PC loss.