Copper(II)-mediated thermolysis of alginates: a model kinetic study on the influence of metal ions in the thermochemical processing of macroalgae

Abstract

Thermochemical processing methods such as pyrolysis are of growing interest as a means of converting biomass into fuels and commodity chemicals in a sustainable manner. Macroalgae, or seaweed, represent a novel class of feedstock for pyrolysis that, owing to the nature of the environments in which they grow coupled with their biochemistry, naturally possess high metal contents. Although the impact of metals upon the pyrolysis of terrestrial biomass is well documented, their influence on the thermochemical conversion of marine-derived feeds is largely unknown. Furthermore, these effects are inherently difficult to study, owing to the heterogeneous character of natural seaweed samples. The work described in this paper uses copper(II) alginate, together with alginic acid and sodium alginate as model compounds for exploring the effects of metals upon macroalgae thermolysis. A thermogravimetric analysis–Fourier transform infrared spectroscopic study revealed that, unusually, Cu 2+ ions promote the onset of pyrolysis in the alginate polymer, with copper(II) alginate initiating rapid devolatilization at 143°C, 14°C lower than alginic acid and 61°C below the equivalent point for sodium alginate. Moreover, this effect was mirrored in a sample of wild Laminaria digitata that had been doped with Cu 2+ ions prior to pyrolysis, thus validating the use of alginates as model compounds with which to study the thermolysis of macroalgae. These observations indicate the varying impact of different metal species on thermochemical behaviour of seaweeds and offer an insight into the pyrolysis of brown macroalgae used in phytoremediation of metal-containing waste streams.

Macroalgae-Derived Biofuel: A Review of Methods of Energy Extraction from Seaweed Biomass

“…High potassium content caused high char yields and reduced bio-oil yields [67,68], but copper ions have been shown to promote the onset of pyrolysis in alginate polymer and Laminaria digitata [70]. In addition, high inorganic content (e.g., Mg, K and S) has been found to vastly increase the specific surface areas of the char produced from Laminaria digitata (1490 m ) [71].…”

Demineralization of Sargassum spp. Macroalgae Biomass: Selective Hydrothermal Liquefaction Process for Bio-Oil Production

“…The high ash content of macroalgae is due to the presence of inorganic salts and metals. The effect of inorganic content on the thermal conversion of terrestrial biomass has been well studied and documented; however, the effect of ash content on HTL conversion of marine feedstocks such as macroalgae is not well known (Rowbotham et al, 2013). It is expected that the high inorganic content in macroalgae will affect the physicochemical characteristics of HTL bio-oil and its storage through the catalysis of polymerization reactions.…”

Metal Removal by Seaweed Biomass

“…Alginates ibers are able to adopt an ordered conformation in solution through dimerization of homopolymeric regions of guluronic acid, in the presence of calcium or other divalent cations, as they are illed with carboxylic groups and other electronegative oxygen atoms. This description is known as the model of "egg box" [25]. The carboxyl groups are the most abundant functional group in brown algae, determined by the percentage of quantiiable sites by titration, reaching about 70%.…”