Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 72

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 for 75% of these sites adsorbing 1 As unit. The Starbon 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.