Graphene as a rational interface for enhanced adsorption of microcystin-LR from water.

Cyanotoxins such as microcystin-LR (MC-LR) represent a global environmental threat to ecosystems and drinking water supplies. The study investigated the direct use of graphene as a rational interface for removal of MC-LR via interactions with the aromatic ring of the ADDA1 chain of MC-LR and the sp2 hybridized carbon network of graphene. Intra-particle diffusion model fit indicated the high mesoporosity of graphene provided significant enhancements to both adsorption capacities and kinetics when benchmarked against microporous granular activated carbon (GAC). Graphene showed superior MC-LR adsorption capacity of 75.4 mg/g (Freundlich model) compared to 0.982 mg/g (Langmuir model) for GAC. Sorption kinetic studies showed graphene adsorbs 99% of MC-LR in 30 min, compared to zero removal for GAC after 24 hr using the same MC-LR concentration. Density functional theory (DFT), calculations showed that postulated π-based interactions align well with the NMR-based experimental work used to probe primary interactions between graphene and MC-LR adduct. This study proved that π-interactions between the aromatic ring on MC-LR and graphene sp2 orbitals are a dominant interaction. With rapid kinetics and adsorption capacities much higher than GAC, it is anticipated that graphene will offer a novel molecular approach for removal of toxins and emerging contaminants with aromatic systems.

Graphene-Mediated removal of Microcystin-LR in chitosan/graphene composites for treatment of harmful algal blooms.

Water quality can be severely impacted by algal blooms alone, yet cyanotoxins, such as microcystin (MC), are potent underlying hazards produced by various species of cyanobacteria. Currently there is a need for environmentally compatible and economically viable media to address large scale application for HAB impacted waters. This study evaluated the interactions of chitosan/graphene (CSG) composites with three different species of cyanobacteria: Anabaena sp, Synechocystis sp, and Microcystis aeruginosa for both removal of algal optical density and toxins. Although results suggest that CSG has an algae dependent removal of density with a range of 40-90% removal, graphene/CSG is highly effective at MC toxin removal, removing >94% of MC-LR produced by Microcystis aeruginosa. Characterization by SEM and XRD revealed that 750 m2/g surface area graphene, imparts graphene morphology and functionality into the chitosan matrix surface, potentially enabling π-π interactions between graphene and the aromatic ring of microcystin. This proposed π-π removal mechanism of microcystin via the CSG chitosan biopolymer substrate offers a promising sustainable and selective media suitable for deployable treatment of HAB impacted waters.

Effect of UV-light exposure duration, light source, and aging on nitroguanidine (NQ) degradation product profile and toxicity.

Previous studies have reported increased aquatic toxicity of UV-degraded nitroguanidine (NQ), but many details underlying the dynamics of NQ degradation and toxicity remain unknown. These data gaps represent critical barriers to assessing the environmental relevance of laboratory-generated UV-degradation results and extrapolation to environmental risk. In the present study, the toxicity of NQ increased with increasing proportional degradation of the parent compound. Specifically, while the LC50 of undegraded NQ was 1485 mg/L, the toxicity at the lowest degradation level examined (7% parent compound degraded) increased by nearly two-orders of magnitude (LC50 = 17.3 mg/L) and increased by nearly three-orders of magnitude (LC50 = 6.23 mg/L) in the highest percent NQ degradation (90%) treatment. Similar LC50 values between immediately tested and aged (8-13 days) NQ degradation products suggested the degradation product(s) causing the toxicity were stable, although concentrations of nitrite and nitrate increased after aging. Finally, experiments where NQ was degraded in natural sunlight confirmed increased toxicity in environmentally relevant D. pulex exposures; however, the two-order of magnitude increase in toxicity (LC50 = 21.3 mg/L) at 53% degradation was less than NQ degraded by a laboratory photoreactor by a similar percentage (46% degraded). Identification of principal toxic agents in the UV-degraded NQ product mixture remains a critical data gap. Mass balance calculations were generated for our experimental results and literature values revealing difficulty in accounting for all NQ degradation products. Products with suspected high potency in D. pulex were identified which require further testing including: nitrosoguanidine, nitrosourea, and hydroxylamine. SYNOPSIS: The toxicity of NQ increased with increasing UV-degradation where toxicity-inducing degradation products were stable over 1-2 weeks; increased toxicity was validated from natural-sunlight degradation of NQ, however toxicity was lower than UV-photoreactor degraded NQ; and the identity of specific toxic UV-degradation products remains elusive where carefully-designed mass-balance experiments and toxicity testing are needed to provide definitive identification.