Valorisation of Olive Pomace for the Production of Bio-Composite Adsorbents Applied in as Removal from Drinking Waters

Abstract

Arsenic is a toxic metalloid representing a serious threat to human health, reaching a concentration in drinking water above the limit of 10 µg/L in many regions of the world. Although adsorption technologies are available today to remove arsenic from water, the employed adsorbents are expensive, which severely hinders the possibility of water treatment in marginal and rural areas.
In this study, a two-stage process is investigated in which an adsorbent for the removal of arsenic from water is produced by hydrothermal carbonization (HTC) of olive pomace followed by iron precipitation. In the first part of the study, the kinetics of solid mass variation during the HTC process were analyzed to derive indications about the mechanisms driving the thermochemical conversion of olive pomace to hydrochar. It was thus verified that a satisfactory hydrochar yield could be attained after 30 min through the polymerization of hydrolysis products released during the early stages of HTC. Adsorption isotherms were determined for the Fe-hydrochar and the Fe-biochar produced by iron precipitation onto the hydrochar and the pyrolyzed olive pomace (biochar). Fe-hydrochar showed higher adsorption capacity (q_max=8.7 mg/g) compared to the Fe-biochar (q_max= 5.3 mg/g). Fe-hydrochar was finally tested in a fixed-bed adsorption column for As removal, evidencing the ability to maintain the arsenic concentration below the 10 µg/L limit when employed in the configuration conventionally adopted for water treatment. However, in this configuration, the apparent adsorption capacity was reduced, indicating the need for an optimization of the fixed bed-column design.