Flagella, palmella and cyst Haematococcus lacustris microalgae cells decorated on graphene oxide and graphene nanoplatelets-activated carbon as novel adsorbents for the removal of lead from water

Industrialization has led to generation of large quantities of waste which constitutes various toxic heavy metals such as lead (Pb). In this work, novel bio-nanostructured graphene-based microalgae nanohybrid adsorbents, using three different cell types of Haematococcus lacustris (i.e., flagella (flg-C), palmella (Pal-C) and cyst (Cyst-C)) to introduce more functional moieties and enhance the surface properties of the nanohybrids. The nanostructured graphene oxide-activated carbon modified with algae cells (GO-AC@algae) and graphene nanoplatelets-activated carbon modified with algae cells (GNPs-AC@algae) nanohybrids were characterized and used for the removal of Pb ions. The GO-AC@algae nanohybrids demonstrated a high lead removal efficiency of over 98.0%, whereas the GNPs-AC@algae nanohybrids achieved more than 85.0%. Among the GO-AC@algae nanohybrids, the nanohybrid with cyst cell (GO-AC@Cyst-C) shown remarkable efficacy as an adsorbent for the removal of Pb²⁺ ions from aqueous solutions due to its high specific area, abundance of oxygen-nitrogen-based functional moieties, hydrophilicity, and pore structure. Chemisorption was found to be a beneficial process for both GO-AC@algae and GNPs-AC@algae samples, where Pb²⁺ was adsorbed in a single layer onto the uniform material surface. Among the various adsorbents, GO-AC@Cyst-C achieved the highest monolayer adsorption capacity of 25.58 mg/g according to the Langmuir model, making it the most effective nanoadsorbents. Kinetic studies revealed that the sorption mechanism of GO-AC@algae were better described by the second-order kinetic model. Meanwhile, the first-order kinetic model was found to be suited for GNPs-AC@algae samples. The nanohybrids could be employed as greener adsorbents at industrial scale for wastewater treatment without incurring significant costs.