Biopolymeric-Hydrothermal Carbon Beads for Decontamination of Polluted Waters ^†

In the field of water depollution, considerable attention in past years was given to bio-based and biodegradable compounds from renewable resources, generically termed “biomass” [1]. Biomass represents the sum of raw materials from which one can produce bio-substances like alginate (from brown algae), chitosan (from shellfish), starch, cellulose, and also hydrothermal carbon (HTC). Sodium alginate is an extract from the walls of brown algae cells, used in the food industry and for body care products as an emulsifier, due to its viscosity [2]. HTC can be produced from lignocellulosic biomass by hydrothermal conversion in water, catalytic or non-catalytic, under thermally generated pressure in a hermetic reactor at temperatures ranging from 140 to 280 °C and different reaction times.

A 2% alginate solution was prepared by dissolving the alginate in water at 90 °C, under vigorous stirring. HTC was produced by hydrothermal treatment of lignocellulosic biomass from corn stalks at 200 °C, for 20 h. Then, 3% HTC (Figure 1b) was added to the alginate solution and homogenized for 1 h, and the obtained mixture was loaded in an automatic syringe and dripped with 180 mL/h flow in a CaCl₂ bath for ionotropic reticulation. The beads of alginate (Figure 1a) and of alginate with HTC (Figure 1c) were washed to remove Cl⁻ ions and dried afterwards in an oven at 60 °C for 24 h. The beads were characterized by means of SEM-EDS, XRD, zeta potential, and batch adsorption experiments of Mn²⁺, evaluated with Atomic Adsorption Spectroscopy (AAS).

For comparison, simple alginate beads were also prepared and tested in the adsorption of metal ions from synthetic water solutions. A quantity of 0.5 g beads were added in batch adsorption experiments to 40 mL aqueous solution of 1 mg/L Mn(NO₃)₂. Alginate-only beads presented a maximum adsorption efficiency of approximately 76% after 60 min, while the alginate/HTC beads presented a maximum adsorption efficiency of approximately 83% after 45 min.

By adding hydrothermal carbon to the alginate beads, a more efficient adsorbent was obtained for the removal of metal ions from polluted waters, Mn²⁺ for this case, with an increased adsorption capacity, while maintaining the biodegradability, renewability, and low cost of bioadsorbents.