Ozone as oxidation agent in cyclic activation of biochar

Abstract

Granular activated carbons were produced from grape seed biochar by cyclic activation with ozone. In each cycle, char was first oxidized by exposure to ozone and then subjected to high temperature in inert atmosphere to desorb oxygen groups formed. The study assessed the influence of operating conditions in the development of porosity, from a starting biochar with narrow microporosity (SBET: 47 m 2 g-1 , SDA: 505 m 2 g-1) prepared by flash pyrolysis of grape seed at 800ºC. The variables studied were the number of cycles applied and the oxidation and desorption temperatures (250-275 and 850-950 ºC, respectively). High oxidation temperatures led to higher burn-off, which was also found to increase with the number of activation cycles. The burn-off needed to achieve a high surface area was lower than in conventional physical activation. After 7-9 activation cycles, activated carbons with SBET higher than 1200 m 2 g-1 and SDA above 1500 m 2 g-1 were obtained. The use of ozone resulted in mainly microporous activated carbons (0.37-0.52 cm 3 g-1) with very low contribution of mesopores (<0.04-0.07 cm 3 g-1). The mean micropore size increased with the number of activation cycles due to pore widening, while mesopore mean size decreased along the cycles. The activated carbons showed a unique granular morphology with a hollow core and a porous shell, which is maintained even after 10 activation cycles. Figure 5. Micro and mesopore volume development with number of activation cycles.

Preparation and Modification of Biochar Materials and their Application in Soil Remediation

“…In addition to the above three modification methods, modification methods such as low-temperature plasma [72,73], organic matter grafting [74], and ozone oxidation [32] have also been studied. Low-temperature plasma modification means that plasmas generated by glow, microwave, and corona were collided with C=C on the surface of biochar, and plasmas were oxidized to the oxygen-containing functional group, and enhanced the polarity of biochar [75].…”

“…The reaction atmosphere studied by scholars is dominated by inert gas, such as N 2 , Ar, which mainly act to isolate oxygen. Besides, the atmosphere of CO 2 , H 2 O, NH 3 , O 3 [32] is also used to prepare biochar, which is known as physical activation, also called gas activation. The gases selectively decompose the non-structural components of the biochar surface, open its internal pores, and increase the specific surface area and pore volume [11].…”

Production and utilization of biochar: A review

“…Physical activation, also called gas activation, uses gas, such as steam [67][68][69][70][71][72][73], CO 2 [74][75][76][77][78], and ozone [79,80], to activate biochar at temperatures above 700 . Physical activation can be divided into two steps.…”

Application of biochar-based catalysts in biomass upgrading: a review

“…24 Consequently, the specic surface area of the biochar can be signicantly improved, and a well-developed microporous structure with low contribution of mesopores is observed on the physically activated biochar. 24,46,48 Normally, the porosity, pore size distribution, and specic surface area of the activated biochars vary signicantly depending on the biomass type, activating gas, and reaction conditions. 10 For example, Lima et al 49 examined the effects of steam activation on the surface area and porosity of various biochars obtained from the fast-pyrolysis of several substrates, and further investigated their adsorptive properties for metal ion uptake.…”