Amine-modified halloysite nanotube embedded PEI cryogels as adsorbent nanoarchitectonics for recovery of valuable phenolic compounds from olive mill wastewater

Here, natural tubular clays, halloysite nanotubes (Hal), were modified with ethylenediamine (EDA), triethylenetetramine (TETA), pentaethylenehexamine (PEHA), and polyethyleneimine (PEI) to enhance their adsorption capabilities for phenolic compound for the treatment of olive mill wastewater (OMWW). Among the modified Hal adsorbents, Hall-PEHA adsorbent demonstrated a higher surface charge of +15.9 ± 1.9 mV and a larger surface area of 24.8 m 2 /g. The hydroxytyrosol, tyrosol, and oleuropein adsorption from their 1000 mg/L 10 mL single aqueous solutions were determined as 696 ± 44, 536 ± 36 and 91 ± 22 mg/g, respectively. Furthermore, it was determined that 90 ± 8%, 45 ± 0%, and 35 ± 3% of hydroxytyrosol, tyrosol, and oleuropein phenolic compounds were recovered from their 10 mL of 1000 mg/L aqueous mixture solution. On the other hand, the Hal-PEI adsorbents were able to adsorb 7.1 ± 1.2, 3.3 ± 0.5 and 0.5 ± 0.05 mg/g of hydroxytyrosol, tyrosol, and oleuropein compounds, respectively, from the actual OMWW. These values correspond to approximately 85 ± 8%, 71 ± 10%, and 70 ± 7% of the total amount of these compounds present in the OMWW. The PEI cryogel composite system, where Hal-PEHA was embedded into PEI cryogels (Hal-PEHA@PEI), exhibited improved adsorption amount for hydroxytyrosol, tyrosol, and oleuropein compounds from the actual OMWW compared to Hal-PEHA alone. The Hal-PEHA@PEI composite system adsorption amount was determined as 7.6 ± 1.6, 3.4 ± 0.2 and 0.7 ± 0.02 mg/g of hydroxytyrosol, tyrosol, and oleuropein compounds, respectively. These values correspond to approximately 90 ± 3%, 73 ± 3%, and 92 ± 3% of the total amount of these compounds present in the OMWW.