Abstract
Metaldehyde is a pesticide used extensively to control slugs and snails on farms and gardens and it was frequently detected in drinking water above regulatory limits. Metaldehyde is highly persistent in the environment and its recalcitrance to conventional water treatment processes led to its ban in the UK. This study investigated the potential of graphene oxide sponges, to remove metaldehyde from aqueous solutions and to also inactivate microbial populations when functionalised with metal nanoparticles. The graphene oxide sponges were self-assembled from graphene oxide dispersion in the presence of L-ascorbic acid, and subsequently thermally treated and freeze-dried. Physical characterisations revealed they form a highly interconnected network with pore size below 0.5 μm. Raman analysis revealed that the intensity ratio that expresses the defects in the carbon structure, increased from 1.44 in pristine sponges to 2.02 in metal sponges due to incorporation of metal nanoparticles in the graphene matrix. Spectroscopic investigations into the chemical structure indicated high content of oxygen containing functional groups on the sponge surface which suggest that they are favourable for micropollutant removal.
Pristine graphene oxide sponges removed 95% of initial metaldehyde concentrations of 30 mg/L. Adsorption data were fitted to the Langmuir isotherm model that revealed a maximum adsorption capacity of 24.19 mg/g. A pseudo-second order kinetic model was applied, providing a maximum adsorption rate of 0.021 g/mg min. Competing humic acids had no impact on the adsorption performance and adsorption was favoured for low pH values. Regeneration of the sponges at temperatures below 140°C is a potential solution to reuse the graphene oxide sponges, at low cost and with no loss in the adsorbent material. Zinc oxide and silver oxide enriched sponges were tested for their antiseptic effect on E. coli and S. aureus achieved inactivation rates of 99.13 % and 100% respectively. Leaching tests established the formation of a strong functionalised substrate which was safe for use in water treatment.
In conclusion, this research provided the proof-of-concept evidence that graphene oxide sponges do have the potential to remove metaldehyde from aqueous solutions. Furthermore, zinc oxide and silver oxide enriched sponges can also provide excellent inactivation rates for microbial populations. Taking this forward, the potential to achieve these effects simultaneously merits further investigation.