Graphene oxide biopolymer aerogels for the removal of lead from drinking water using a novel nano-enhanced ion exchange cascade

Abstract

Potable water in developing countries often contains levels of toxic metals that exceed the recommended international limits, with impacts on human health. The aim of the present study was to develop a low cost aerogel synthesised from graphene oxide (GO) cross-linked with alginate to remove Pb2+ from potable water. Aerogels were made by a sol-gel of the composite materials followed by a freeze drying method. The shape of the aerogels were 50 mm diameter disks, 5 mm deep and characterised by an open porous network of 50 to 150 micrometres which are mechanically robust upon hydration. Firstly, the study was conducted using a batch adsorption method from a starting concentration 0.48 mM (100 mg/l) of Pb2+ in ultrapure water over 240 min, n = 4 with controls. A second series of experiments compared the adsorption of different competing ions at different valencies (Na+, Ca2+, Cu2+, La3+) in an equivalent media. A third series of experiments explored Pb2+ desorption from the aerogel at low pH and in highly acidic conditions. This simple filter system, based on a batch adsorption methodology expresses a high affinity for Pb2+ resulting in an ultra-high mean maximum adsorption capacity of 504 mg/g of Pb2+ within 240 mins at pH 5. The aerogel can also adsorb other toxic metal salts such as La3+ and Cu2+ with a capacity of 146 and 193 mg/g respectively. Furthermore, the aerogel structure can be acid washed removing 98% of the Pb2+ from the structure within three minutes. Overall, the data shows that GO alginate aerogels are highly effective at removing Pb2+ from water and the primary mechanism involved is ion exchange, although other phenomenon such as proton tunnelling may be a contributing factor to the ultra-high efficiency of the aerogel for Pb2+ remediation.