GRAPHENE OXIDE BIOPOLYMER AEROGELS: A NOVEL SOLUTION TO TOXIC METAL POLLUTION IN WATER

Abstract

Clean drinking water is a fundamental human right, but still in the 21st century millions of people are forced to drink water contaminated with toxic metals above recommended safe limits. This work proposes to remediate toxic lead (Pb) from potable water using a robust low-cost aerogel fabricated from graphene oxide (GO) and seaweed (calcium alginate) by batch adsorption using simple agitation. Firstly, GO was synthesised using the improved Hummers’ method (IHM) and characterised with TEM, Raman and FTIR. The GO alginate aerogel composites were made by a sol-gel method, freeze dried and crosslinked in CaCl2 and further characterised using SEM and FTIR. The first set of experiments studied Pb2+ adsorption at a starting concentration 0.48 mM (100mg/l) over a range of pH values, at different temperatures and included metal digest recovery analysis. A second study investigated competing ions at different valencies (La3+, Na+, Cu2+, Ca2+), varying the starting concentrations (100 - 10 mg/l) and identified a desorption mechanism for metal recovery in low pH conditions. A third set of experiments compared the adsorption capacity of the aerogel in natural and ultrapure waters for all competing ions, correlating observed kinetics to theory, in an attempt to illuminate the pronounced efficacy for Pb2+ adsorption. The maximum mean adsorption capacity for Pb2+ was 504 mg/g within 240 minutes in ultrapure water outperforming all competing ions, irrespective of the starting concentrations and water media variations. The GO alginate aerogels are robust with an ultrahigh adsorption capacity for Pb2+ in water, the data indicates this is only partially explained through ion exchange and this work proposes a possible quantum tunnelling effect due to the collective charge density of the Pb2+ within the confines of graphene oxide layers resulting in an internal proton cascade.