for the heterogeneous surface energy. Physisorption was also indicated by the isotherms (via heterogeneity factor and adsorption energy values). Perhaps, unsurprisingly, for systems where mesoporosity is important, multilayer adsorption is evident, with all systems going well beyond surface coverage. What is surprising is that, despite the similar capacities of the two materials, the monolayer adsorption capacities are similar for all phenols for S800, but vary significantly for the same molecules in the case of A800. Similarly, the thermodynamic parameters for the adsorption of S800 and A800 are very different, suggesting a quite different mode of adsorption within the group of phenolics, with some being entropy‐driven (approximating to release of waters of hydration at the surface/around the phenol) and others enthalpic (stabilising interactions between surface and adsorbate). A positive contribution to the adsorption via entropy seems more prevalent in the alginic acid series of adsorbents.
The same group, in collaboration with a group at Menoufia University in Egypt, also examined the potential of Starbon materials in decontaminating grey water from laundrettes. This is the post‐wash waters that are expelled from washing machines and which are contaminated with suspended solids, minerals such as potassium and phosphate, as well as detergent molecules and aggregates. Re‐use of the very substantial quantities of grey water would go a long way to reducing the water requirements of large conurbations.
The first paper [37] focused on a combination of Fentons reagent (Fe(III) and hydrogen peroxide) with Starbon‐300 to clean up grey water by a combination of oxidation and adsorption. Chemical oxygen demand (COD) reduction was used as a determinant of success. A series of operating parameters were investigated for each of the two components separately, and then the optimum combination of the two was investigated. For the Fenton’s reagent and hydrogen peroxide system, optimal iron and hydrogen peroxide concentrations were determined beyond which efficiency dropped significantly. The influence of pH was also investigated, and a definite trend was seen with lower pH values giving significantly better results. This is attributed to the role of pH in guiding the relatively complex Fenton’s chemistry to the highest production of hydroxyl radicals, the most active species. For adsorption on Starbon, similarly acidic conditions were optimal for adsorption of materials and reduction in COD.
A separate series of experiments using dual treatment (Fentons reagent and Starbon together) led to a different set of optimum conditions, indicating that there is a significant interaction between the two systems, or that the products of Fenton’s treatment adsorb under different conditions than the untreated components. This allowed a significant improvement of performance with around half the Fenton’s reagent (half the Fe concentration and half the hydrogen peroxide). Importantly too, neutral pH (pH 7.4) was found to be ideal, closer to real conditions and avoiding acidification. Up to 93% reduction in COD was demonstrated.
The second paper focused purely on adsorbency of a range of Starbon materials [38]. In this case, alginic acid‐based materials were used, with pyrolysis temperatures from 300 to 800 °C, whereas the earlier paper used corn starch‐derived materials. A direct comparison of adsorbency between the A300 used in the 2019 paper and the corn starch‐derived S300 used in the original paper indicates contrasting results in terms of pH dependence (Figure 3.12). This may possibly indicate that the surface chemistry of the S300 and A300 materials is significantly different – perhaps not surprising as there are significant amounts of acidic groups remaining in the A300 material [39] which should not be present in the starch‐derived materials.
What can also be seen is that increasing the pyrolysis temperature of the alginic acid material gives a dramatic increase in adsorbency, with A800 not only being very effective at neutral and slightly alkaline pH values but also losing very little adsorbency over the entire pH range evaluated. This is likely related to the much lower oxygen content and hydrophobicity of the material, meaning that its surface charge is much less affected by pH, but also that hydrophobic effects may be important in adsorbing at least some of the components of the grey water. It was found that the adsorption capacity for A800 was 0.92 g g−1, outperforming the other adsorbents tested (and also silica gel and Norit‐activated carbon) in terms of capacity, and also reaching maximal adsorbency in the fastest time (c. 10 minutes cf. 60 minutes for the other adsorbents tested). The high capacity and rapid adsorption make the A800 material a very promising material for laundry waste clean‐up, giving potential to the reuse of grey laundry water in domestic situations.
Figure 3.12 Comparison of pH‐dependent adsorption behaviour (via COD reduction) of A300 and S300 and of A450 and A800.
Source: Data from Shannon et al. [39].
3.2.4.3 Metal Recovery
Muñoz Garcia et al. [40] have shown that Starbon is an excellent adsorbent for precious metals, in particular, gold. Stirring S800 with an aqueous solution of a range of metals designed to mimic a model waste stream from a platinum group metals mine – Au(III), Pt(II), Pd(II), Ni(II), Cu(II), Ir(II), and Zn(II) – under mildly acidic (HCl) conditions gave remarkable results. Gold (99% adsorption), followed by Pd (>90%) and Pt (>80%) were adsorbed very well, whereas iridium (31%) followed by the others (<10%) were less well adsorbed. This selectivity was also observed even when the solution contained far higher concentrations of the poor adsorbers. Adsorption ability appeared to follow the reduction potential of the metals, with gold being the most easily reduced. This fits with the XPS data that confirmed that the predominant species adsorbed was the M(0) oxidation state, as well as demonstrating an increase in oxidised forms of carbon on the Starbon material. The highest capacity observed for gold was an astonishing 3.8 g Au/g Starbon, well in excess of other adsorbents [41]. Selectivity was explained on the basis of reduction potentials, and data from previous work (albeit under different conditions of solvent and precursor (chloride ions can play a major role in metal speciation)) indicate that the reductive adsorption of other metals with similar reduction potentials is also relatively facile on a Starbon surface [42].
The recovery of arsenic (As) from wastewater has been demonstrated by Baikousi et al. [43] using iron oxide/hydroxide‐modified Starbon. They conducted an in‐depth study of the surface of the Starbon‐S700 material before and after iron adsorption, providing details on the nature of surface sites, and a mechanistic understanding of the As uptake. Potentiometric titration of the S700 shows the presence of two sets of functional groups, one (considered to be carboxylic acids) with a pKa of 2.75. This is slightly stronger than most carboxylic acids, but is in line with carboxylic acid strength, as measured on other carbonaceous surfaces. The second set of functionalities had a pKa of 10.3, and were considered to be phenolic in nature. Such an analysis is consistent with that of Shannon et al. [39] who carried out similar analyses on alginic‐acid‐derived materials (see Section 3.2.4.4), suggesting a reasonable degree of similarity in surface functionality between the two material classes.
Loading iron onto the material involved the adsorption of FeCl3 from ethanol, evaporation, and reduction with aqueous sodium borohydride. This gave nanoparticles of iron on the surface of the Starbon. This led to additional protonatable functionalities at pKa = 3.8 and pKa = 7.3, attributed to carboxylic groups and Fe–OH groups, respectively. The latter are thought to be formed by aerial oxidation of the surface of the Fe nanoparticles.
Adsorption of As from water was then carried out at pH 7, at which the As is present as the neutral HAsO3 species. Maximum loading was found to be 26.8 mg As/g material at pH 7. This is attributed to adsorption solely on the FeOH sites, and accounts
