itself adsorbs no As, and the pH dependence of adsorption (which drops off rapidly at higher pH, where both the adsorption sites and the As develop negative charges) is consistent with the suggested mechanism. The adsorption capacity is significantly higher than that found for other metal oxide/clay systems.
3.2.4.4 Adsorption and Release of Bioactives
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 pHpzc, 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 pHpzc 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 |
3.2.5 Conclusion
What this chapter has attempted to show is that highly functional and functioning advanced materials can be produced relatively simply from biomass residues, utilising the inherent functionality and structural properties of the polysaccharide materials. This aids in the important aim of transforming our mindset as a society from a linear economy model (extract – make – use – discard) to a more circular structure, where the discarded elements are treated as a resource to be valorised rather than to be disposed of. Such models are crucial if we are to attain a sustainable society which can support the needs of all of us.
Due to the conversion of the ‘waste’ polysaccharides into a range of tunable high‐surface area, highly mesoporous materials can now be carried out at scale and the understanding of the processes has developed rapidly over the last few years, such that these materials have moved from the lab to production and commercialisation. This chapter has focused on two major areas of activity: adsorption and catalysis. We describe the various adsorption/desorption processes at which the materials excel, partly due to their relatively large pore size (with respect to the more traditional activated carbons) which allows them to effectively adsorb (and desorb) relatively large molecules, which are excluded from micropores. The tunable surface functionality also plays a significant role here, with the evolution of the surface being controlled by thermal treatments. However, it is not only small molecules that are adsorbed in impressive amounts – a wide range of acidic and basic gases are also taken up by the material, including ammonia, sulphur dioxide, hydrogen sulphide, and carbon dioxide, making these materials capable of air purification as well as water treatment.
The catalytic aspects of the materials have also been explored, and in this part of the review, a range of surface functionalisation methodologies are presented. Again, these rely on the various surface functionalities for their success (e.g. bromination of unsaturated functionality to provide anchor points for further functionalisation, attachment of activating groups via hydroxyl functionalities and sulphonation to generate strongly acidic sites) and complex structures can be built up on the surface of the materials, again aided by large enough pores to allow ingress and egress of relatively bulky substrates.
For purposes of space, the chapter does not include the development of nanoparticulate metal – Starbon composites, readily carried out, generally without the need for an additional reductant, making the process particularly
