Dr. Valdes-Morales, Hector

The influence of the variation of chemical surface properties of activated carbons on the sorption capacity of activated carbons used in hybrid heterogeneous ozonation systems is still under discussion. In this study, the effect of long exposure of activated carbon to ozone and its implication on the removal of emerging organic pollutants from waters is evaluated. A commercial activated carbon (Filtrasorb-400) is used here as a raw material. It is chemically modified by continuous ozone exposure. 2-hydroxybenzothiazole (OHBT) is chosen as a target organic contaminant, representative of emerging micro-pollutants. Results obtained here reveal that extensive exposition of activated carbon surface to ozone weakens adsorbate–adsorbent interactions. Highly exposed activated carbon to ozone increases the concentration of oxygen-containing acidic functional groups, leading to a higher concentration of surface electron-withdrawing groups such as carboxylic acid anhydrides and carboxylic acids and reducing the sorption capacity toward OHBT in the hybrid heterogeneous ozonation system. At pH conditions around the point of zero charge (pHPZC), such sorption reduction could be due to a decrease on dispersive interactions among π-electrons of aromatic ring of OHBT molecules and the π-electron system of carbon graphene layers, coming after extensive exposition of activated carbon surface to ozone. However, at pH >pHPZC low removal of OHBT is obtained because of the appearing of repulsive electrostatic interactions among the ionised form of OHBT molecules and the de-protonated form of oxygen-containing functional groups that appears after long contact with ozone. In addition, a new concept to predict activated carbon performances in a hybrid heterogeneous ozonation process is proposed.

The influence of seven commercial activated carbons (ACs) to promote hydrogen peroxide decomposition and radical generation is assessed during four operating cycles. The amount of generated hydroxyl radicals is estimated from quenching experiments using methanol as a radical scavenger. The change in chemical surface composition of ACs upon contact with hydrogen peroxide after each operating cycle is measured by Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and by following the change in the value of the pH of the point of zero charge (pHPZC). Results reveal that when ACs are exposed to hydrogen peroxide for extended periods, their chemical surface composition is modified, reducing the capacity of these materials to promote hydrogen peroxide decomposition, and in turn decreasing the generation of hydroxyl radicals. Moreover, DRIFTS analyses show that ACs with an appreciable content of basic surface functionalities, such as chromene-type structures, would guarantee a continuous radical generation, reducing the loss of catalytic activity.

The efficiency of hydrogen peroxide in the regeneration of dimethyl sulphide (DMS) exhausted-activated carbons (ACs) is assessed in this study. Moreover, the influence of chemical surface composition is evaluated using six ACs (two commercial ACs and four chemically modified ACs) with different surface features. Chemical surface composition of ACs before and after different adsorption-regeneration cycles is assessed by temperature-programmed desorption coupled with mass spectroscopy (TPD-MS) and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Results reveal that up to a 60% of regeneration efficiency is attained, being more effective in the presence of ACs derived from Filtrasorb 300 AC sample. Experimental findings show that ACs with high content of basic oxygen–containing functional surface groups and mineral fraction are responsible to promote the catalytic decomposition of hydroxide peroxide, leading to a higher formation of hydroxyl radicals and to the observed increase in the regenerative oxidation of adsorbed DMS. However, a decrease in removal efficiency is related to an increase in the amounts of oxygen functionalities mainly in the form of strong acidic surface groups such as carboxylic acid anhydrides and carboxylic acids that shift the reaction mechanism from promoting the initiation of radical chain reactions to termination. Additionally, DRIFTS analyses indicate that after successive adsorption-regeneration cycles the fraction of organic molecules that remains adsorbed limits the access to active surface sites responsible for radical generation, reducing drastically the catalytic activity of ACs.

Ethylene is a plant hormone regulator that stimulates chlorophyll loss and promotes softening and aging, resulting in a deterioration and reduction in the post-harvest life of fruit. Commercial activated carbons have been used as ethylene scavengers during the storage and transportation of a great variety of agricultural commodities. In this work, the effect of the incorporation of copper oxide over activated carbons obtained from baru waste was assessed. Samples were characterized by X-ray diffraction (XRD), N2 adsorption-desorption at −196 °C, field-emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDS), and infrared (IR) spectroscopy. The results showed that the amount of ethylene removed using activated carbon obtained from baru waste and impregnated with copper oxide (1667 μg g−1) was significantly increased in comparison to the raw activated carbon (1111 μg g−1). In addition, carbon impregnated with copper oxide exhibited better adsorption performance at a low ethylene concentration. Activated carbons produced from baru waste are promising candidates to be used as adsorbents in the elimination of ethylene during the storage and transportation of agricultural commodities at a lower cost.