Abstract
Bio-derived lipophilic polydentate chelators have been synthesized and tested for their chelating ability using a range of metal salts of Cu, Co, Ni, Fe, and Cr. These novel molecules were produced by the Michael addition reaction of 14,16-hentriacontanedione, isolated from wheat straw wax, with methyl acrylate or bio-derived dimethyl itaconate via microwave heating. The Michael adducts could either be used directly as esters or be hydrolysed to their acid form. Critically, the creation of additional binding sites via the carboxylate moieties leads to an enhanced metal uptake over both a non-renewable commercially available lipophilic β-diketone (dibenzoylmethane) and the unmodified hentriacontane-14,16-dione, for the chelation of Fe(iii), Cr(iii) and Ni(ii). The modified β-diketone containing a single carboxylic acid functionality was able to extract 167 mg L−1 of Fe(iii) from an FeCl3 solution with no pH adjustment. In comparison, no chelation was observed with dibenzoylmethane, while unmodified hentriacontane-14,16-dione was able to extract 81 mg L−1. The modified chelators containing one and two ester carboxylates extracted 255 and 305 mg L−1 Cr(iii) from a solution of CrCl3 at pH 5–6, 238 mg L−1 was extracted by the unmodified β-diketone whilst no extraction was observed using dibenzoylmethane. This suggest some minor contribution or positive effect to chelation due to neighbouring ester groups. The chelator containing two carboxylic acid groups (tetra-dentate when combined with the diketone) was the most proficient in this study for removal of Ni from an NiCl2 solution (140 mg L−1). It was also found that at higher pH almost quantitative extraction was achieved using the polydentate chelators.
Highlights
Concerns over the security of critical element supplies that are vital to the chemical industry have created the need to nd new sources of these elements
Lipophilic chelators are of signi cant interest as they allow for application of a biphasic metal recovery system (Fig. 1), and some lipophilic petroleumderived chelators do exist, including dibenzoylmethane, 1
Methyl acrylate (MA), methyl methacrylate (MMA), dimethyl itaconate (DMI) and trimethyl aconitate (TMA) were included as Michael acceptors, but the low yields obtained by MMA and TMA meant they were not included for the remainder of this investigation
Summary
Concerns over the security of critical element supplies that are vital to the chemical industry have created the need to nd new sources of these elements. The development of bio-derived lipophilic chelators from agricultural wastes has the potential to contribute to a holistic metal recovery system as part of a circular economy. One such chelator, hentriacontane-14,16-dione (HTD), 2, a major component of wheat straw wax, has recently been reported as an effective bio-derived chelating agent (Fig. 2).[5 2] is a lipophilic bdiketone that can be extracted from the raw biomass using green solvents such as supercritical carbon dioxide.[6] As it is composed only of carbon, hydrogen and oxygen, the release of nitrogen or phosphorous into the environment can be avoided, and its high log P(o/w) means efficient extraction from the aqueous phase can be achieved.[5] In addition, being extracted from a waste agricultural residues such as wheat straw wax means an abundant supply is ensured.[5]
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