Characterization of the residue from double alginate extraction from sargassum filipendula seaweed

Residue of alginate extraction from Sargassum filipendula brown seaweed shows high potential to be applied as bioadsorbent for the removal of heavy metals from contaminated wastewater, since it contains some constituents of the raw algae. Consequently the waste of the extraction process could be also employed as bioadsorbent, valorising the residue from alginate extraction and making the metal sorption more economically attractive. In front of this, the alginate extraction was carried out twice to verify if after the double extraction the residue presents the same constituents and the same potential to remove toxic metals. Furthermore, the double extraction was used in order to verify if the alginate extraction yield increases and if it preserves the characteristics of the residue as bioadsorbent. The yield (w/w) of alginate and residue in the first extraction was 24 ± 5.00 % and 52 ± 2.51 %, respectively. With the second extraction, the yield obtained represented 11 ± 1.88 % of alginate and 76 ± 3.84 % of residue. The high amount of residue encourages its use as biosorbent adding value to a waste of the process. The SEM-EDX results indicate that the materials' surface is mainly composed by Si, Na, Mg, Al, S, K, Ca and Fe, being that the presence and reduction of some elements concentration is due the diatoms shells. The spectra obtained from FTIR of first and second residues were similar, indicating that the main functional groups involved in adsorption phenomenon are not totally removed. It suggests that even after extracting more alginate it is possible to use the residue as bioadsorbent.