Biomass digestion and total organic carbon (TOC) analysis
The amount of alkali (Na and K) and alkaline earth metals (Ca
and Mg), present in the raw seaweed, was determined by digesting
the samples in a glass tube at 140 C for 2 h (DK 6 heating digester
VELP Scientifica), after adding 5.0 mL of D.I. water, 4.0 mL of HNO3,
12.0 mL of HCl to approximately 0.5 g of L. hyperborea. The metal
concentrations in the digested samples were determined by AAS,
after filtration through cellulose acetate membrane filters. A blank
digestion was also performed. The TOC content of raw and protonated
L. hyperborea was measured on a total organic carbon analyzer
(Shimadzu TOC-VCSN) with module for solid samples
(Shimadzu model SSM-5000). The protonated alga was obtained
by soaking in 0.2 M HNO3 under constant shaking for 3 h (biomass
concentration of 10 g/L). This procedure was repeated twice. Afterwards,
the biomass was rinsed several times with distilled water
until achieving a pH 4 and then dried in an oven at 45 C.