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.

Keywords: alginate; biofuel; copper; pyrolysis; seaweed; thermochemistry.

Figure 1.

Schematic of the mode of uptake of various mono- and di-valent metallic ions found in seawater by α-l-guluronic acid and β-d-mannuronic acid, which polymerize to form alginic acid (and the corresponding alginate salts) in large quantities in brown macroalgae [36]. (Online version in colour.)

Figure 2.

The affinity of alginic acid in L. digitata for various divalent cations [47].

Figure 3.

The ‘egg box’ model of divalent cation bonding in alginic acid [46]. (Online version in colour.)

Figure 4.

L. digitata collection site: Marsden Bay, South Shields, UK.

Figure 5.

The (a) DTG, (b) TGA and (c) degree of conversion profiles for the thermolysis of H-Alg, Na-Alg and Cu-Alg over the temperature range 25–1000°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. Dashed line, Na; dotted line, Cu; solid line, H.

Figure 6.

Comparison of TGA (dashed line) and DSC (solid line) curves for (a) H-Alg, (b) Na-Alg, and (c) Cu-Alg over the temperature range 50–400°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂.

Figure 7.

The (a) DTG and (b) TGA profiles for the thermolysis of Na₂CO₃, CuCO₃ and CaCO₃ over the temperature range 25–1000°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. Dashed line, Ca; solid line, Cu; dotted line, Na.

Figure 8.

Plot of ln(β/T²) versus 1/T (with T in kelvin) at the points of maximum volatilization of H-Alg, Cu-Alg and Na-Alg, pyrolysed at β = 5, 10, 20, 30 and 40°C min⁻¹.

Figure 9.

The DTG profiles for the main pyrolysis region of (a) H-Alg, (b) Cu-Alg and (c) Na-Alg at heating rates (β) of 5, 10, 20, 30 and 40°C min⁻¹ under N₂.

Figure 10.

TGA-FTIR profile for the thermolysis of (a) Na-Alg, (b) Cu-Alg and (c) H-Alg over the temperature range 25–1000°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂.

Figure 11.

(a) Plot of absorbance at 2360 cm⁻¹ versus temperature for the thermolysis of Na-Alg, Cu-Alg and H-Alg over the temperature range 150–300°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. (b) TGA profile for the thermolysis of Na-Alg, Cu-Alg and H-Alg over the temperature range 150–300°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. (c) Plot of absorbance at 2360 cm⁻¹ versus temperature for the thermolysis of Na-Alg, Cu-Alg and H-Alg over the temperature range 25–1000°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. Dashed line, Na; dotted line, Cu; solid line, H.

Figure 12.

The (a) DTG and (b) TGA profiles for the thermolysis of raw L. digitata and L. digitata treated in a solution of Cu²⁺ ions in the range 25–800°C with a heating rate (β) of 10°C min⁻¹ under N₂. Solid line, copper(II)-doped L. digitata; dashed line, L. digitata.

Figure 13.

(a) Comparison of DSC curves for copper(II) alginate (solid line) and copper(II)-doped L. digitata (dotted line) over the temperature range 50–400°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂. (b) Comparison of DSC curves for unadulterated L. digitata (solid line) and copper(II)-doped L. digitata (dotted line) over the temperature range 50–400°C obtained with a heating rate (β) of 10°C min⁻¹ under N₂.