Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 73

Shannon et al. [39] investigated the adsorption and desorption of bioactive molecules from alginic‐acid‐derived Starbon compared to activated carbon. The set of molecules studied included two plant growth promoters and two plant growth inhibitors, with a view to developing controlled release technology for horticultural applications (Figure 3.13).

Figure 3.13 Four bioactive molecules studied for adsorption/desorption behaviour. (a) Gibberellic acid, (b) indole‐3‐acetic acid, (c) kinetin, (d) abietic acid. The first three are growth promoters, and the fourth is an inhibitor.

Alginic‐acid‐derived materials carbonised at 300, 500, and 800 °C were investigated alongside an activated carbon. The authors studied the physicochemical nature of the four sorbents using pH drift and Boehm titrations to determine surface functionality and acidity/basicity. They found that pH_pzc, the pH at which the surface has net zero charge, was 7.9 for the activated carbon, but started at 6.1 for the A300 material, with the value shifting significantly to higher values for the A500 and A800 materials (8.7 and 9.2, respectively). Combining the adsorption results with Boehm titration indicated that the changes in pH_pzc are predominantly due to the partial loss of carboxylic functionality in the alginic acid structure at 300 °C, a process that is complete by 500 °C, where more basic, oxygen‐based, functionality predominates. As discussed in Section 3.2.4.1, the alginate materials contain significant inorganic content, which will increase by a factor of 2 from 300 to 800 °C – this is likely to increase the basic character of the materials as a function of temperature. The other major differences between the sorbents were in porosity, with the activated carbon being 85% microporous and the alginate materials predominantly mesoporous (66–93%), albeit with some microporosity developing at higher temperatures.

Adsorption capacities are given in Table 3.5. It can be seen that all four adsorbates are taken up by all of the adsorbents without dramatic differences in capacity. Adsorption of the four molecules followed second‐order kinetics, and it appeared that surface area was an important factor, and that diffusion limitations were important in systems with high microporosity.

Where the Starbons really shine is in their ability to desorb the adsorbed materials. While activated carbon adsorbs all four materials well, it does not desorb them – the highest desorption in aqueous systems was for indole acetic acid, and that was as low as 1.2% of the total adsorbed. For the Starbons, the situation is much more variable, but up to 46% release was observed. This perhaps indicates that for the essentially microporous activated carbons, once desorption has taken place, desorption out of the micropores is extremely difficult, whereas, for the mixed micro‐/mesoporous systems, there may be a proportion of the material adsorbed in larger, more open pores where diffusion (out) is easier. Alternatively, it may be that the micropores are shallower and ‘decorate’ the walls of the mesopores in the case of Starbons, but in activated carbons, the pores are longer and thus retain material better. Whatever the reason, the Starbon materials offer excellent potential for adsorption and release of key bioactive molecules.

Table 3.5 Adsorption capacities for various alginic‐acid‐derived Starbons and an activated carbon.

Source: Data from Shannon et al. [39].

Bioactive	Adsorption capacity (mg g−1)
AC	A300	A500	A800
Gibberelic acid (A)	72	98	76	118
Indole‐acetic acid (B)	210	115	150	157
Kinetin (C)	205	120	125	121
Abietic acid (D)	314	282	239	370