Aggregation and surface properties of iron oxide nanoparticles: Influence of ph and natural organic matter

Abstract

The interactions between unpurified manufactured nanoparticles (NPs; iron oxide NPs, approximately 7 nm) and standard Suwannee River humic acid (SRHA) were investigated under a range of environmentally relevant conditions. At low pH, approximately 35% of the total iron was in the dissolved phase (< 1 kDa), present from the initial synthesis, whereas at pH more than 4, this concentration was negligible because of the formation of new particles via hydrolysis. Dynamic light scattering results indicated that extensive aggregation of NPs began at approximately pH 5 to 6 and reached a maximum at approximately pH 8.5, whereas with added SRHA, aggregation was shifted to lower pH values of 4 to 5 and was affected by SRHA concentration. Aggregation could be explained mainly by charge neutralization. Further, more detailed investigations by flow field-flow fractionation and transmission-electron microscopy were performed under a more restricted set of conditions (pH 2-6) to examine the aggregation process. Results indicated the formation of SRHA surface coating on iron oxide NPs of approximately 1 nm and the increase in thickness of this coating with the increase of SRHA concentration. Iron oxide NPs were shown to form increasingly large aggregates with increases in both pH (from 2 to 6) and SRHA concentration (from 0 to 25 mg/L). The structure and aggregation mechanism of these aggregates were found to be both pH and SRHA concentration dependent, with open, porous aggregates in the absence of SRHA and compact aggregates in the presence of SRHA.

Potential scenarios for nanomaterial release and subsequent alteration in the environment

“…Because natural organic matter (NOM) is a ubiquitous constituent of natural waters, these changes in the surface properties of PW-ENM and ET-ENM by NOM should be significant, for example, because they affect agglomeration of particles [1,41,42].…”

Heteroaggregation of nanoparticles with biocolloids and geocolloids

“…Naturally occurring colloids are ubiquitous in natural surface water and are likely to affect the aggregation and sedimentation of NPs [39]. Natural colloids include [40][41][42][43]: (1) compact inorganic colloids; (2) large and rigid biopolymers (0.1-1 μm); and (3) soil-derived fulvic compounds (few nanometers) or their equivalent in pelagic waters, aquagenic refractory organic matter.…”

Toxicity of ZnO Nanoparticles, ZnO Bulk, and ZnCl2 on Earthworms in a Spiked Natural Soil and Toxicological Effects of Leachates on Aquatic Organisms

“…The main differences between NPs and their bulk counterparts relate to their high surface-to-volume ratio and the consequent changes in physicochemical, optical, reactive, and electrical properties. In addition, the environment in which NPs are present determines their behavior, reactivity, and potential toxicity (Baalousha et al 2008;Bian et al 2011). Information regarding the impact of ZnO-NPs on human health as the result of occupational or public exposure is already available, but data concerning the potential impact of these NPs in the environment is still scarce despite the rapid increase of peer-reviewed articles (Maurer-Jones et al 2013;Kahru and Dubourguier 2010).…”