New insights into the efficient removal of emerging contaminants by biochars and hydrochars derived from olive oil wastes

Highlights: • Synthesis and characterization of low-cost chars from olive oil wastes • Unmodified biochars and hydrochars as adsorbents of emerging contaminants (ECs) • Importance of char's pH and surface chemistry for removing ECs from water • Hydrochar and active carbon (AC) showed high adsorption capacities for triclosan. • Hydrochars from pitted and reprocesses olive mill waste as an alternative to AC. The removal of emerging contaminants (ECs) for water source reclamation, minimizing energy and chemical use, is an environmental concern worldwide. In this study, we used the technologically cleaner pyrolysis and hydrothermal carbonization (HTC) processes to convert olive oil production wastes into chars in order to simultaneously remove triclosan (TCS), ibuprofen (IBP) and diclofenac (DCF) from water. The chars prepared from olive stone (S), olive tree pruning (P) and pitted and reprocessed wet olive mill waste (H), as well as commercial biochars and a commercial active carbon (CAC), were characterized using different techniques and assayed as adsorbents. Pyrolysis temperatures had only a slight effect on the adsorption capacity of chars. The pseudo second-order reaction kinetic and the Freundlich equation provided the best fit for experimental data. The pH values of char suspensions were negatively correlated with their maximum adsorption capacities. The hydrochars synthetized at the lowest temperatures (≤ 240 °C), which had an acidic pH and were rich in oxygenated functional groups, recorded the highest adsorption rates (64% for DCF, 43% for IBP) and especially for TCS, with a rate of 98%, despite of a low surface area of 7.5 m²/ g. This study demonstrates for the first time that unmodified hydrochars from pitted and reprocessed wet olive mill waste are inexpensive, sustainable and environmentally friendly adsorbents which can be used to remove ECs and other similar compounds in water treatments.