Natural inorganic nanoparticles – formation, fate, and toxicity in the environment

Abstract

The synthesis, stability, and toxicity of engineered metal nanoparticles (ENPs) have been extensively studied during the past two decades. In contrast, research on the formation, fate, and ecological effects of naturally-occurring nanoparticles (NNPs) has become a focus of attention only recently. The natural existence of metal nanoparticles and their oxides/sulfides in waters, wastewaters, ore deposits, mining regions, and hydrothermal vents, as exemplified by the formation of nanoparticles containing silver and gold (AgNPs and AuNPs), Fe, Mn, pyrite (FeS2), Ag2S, CuS, CdS, and ZnS, is dictated largely by environmental conditions (temperature, pH, oxic/anoxic, light, and concentration and characteristics of natural organic matter (NOM)). Examples include the formation of nanoparticles containing pyrite, Cu and Zn-containing pyrite, and iron in hydrothermal vent black smoker emissions. Metal sulfide nanoparticles can be formed directly from their precursor ions or indirectly by sulfide ion-assisted transformation of the corresponding metal oxides under anaerobic conditions. This tutorial focuses on the formation mechanisms, fate, and toxicity of natural metal nanoparticles. Natural waters containing Ag(I) and Au(III) ions in the presence of NOM generate AgNPs and AuNPs under thermal, non-thermal, and photochemical conditions. These processes are significantly accelerated by existing redox species of iron (Fe(II)/Fe(III)). NOM, metal-NOM complexes, and reactive oxygen species (ROS) such as O2˙(-), ˙OH, and H2O2 are largely responsible for the natural occurrence of nanoparticles. AgNPs and AuNPs emanating from Ag(I)/Au(III)-NOM reactions are stable for several months, thus indicating their potential to be transported over long distances from their point of origin. However, endogenous cations present in natural waters can destabilize the nanoparticles, with divalent cations (e.g., Ca(2+), Mg(2+)) being more influential than their monovalent equivalents (e.g., Na(+), K(+)). The toxicity of NNPs may differ from that of ENPs because of differences in the coatings on the nanoparticle surfaces. An example of this phenomenon is presented and is briefly discussed.

Functionalisation of Colloidal Transition Metal Sulphides Nanocrystals: A Fascinating and Challenging Playground for the Chemist

“…In reference [161], a comprehensive table is reported, collecting many of the wet chemistry routes used to produce, inter alia, different sulphides and other chalcogenides. Transition metal sulphides such as Ag 2 S, CuS, CdS, and ZnS, FeS 2 , which can be formed either directly from their precursors ions or indirectly by sulphide ion-assisted conversion of the corresponding metal oxide, typically under anaerobic conditions, have been the focus of recently appeared review [163].…”

“…A further distinction can be carried out accordingly to the chemical nature of the surface to be derivatised, which can be noble metal NPs [9,141,163,269,318, or metal oxides, the most commonly functionalised colloids, but also of other composition (halogenides, chalcogenides, phosphates, carbonates etc. ).…”

Green Synthesis: Nanoparticles and Nanofibres Based on Tree Gums for Environmental Applications

“…Nanomaterials are emerging contaminants in water and show significant toxicity to living systems. Further studies are still in progress on the fate, transport, and transformation of nanoparticles and nanocomposite bioavailability and exposure of humans and other living species [5,7,177]. Electrospinning polymers, with various functionalities and combinations of different types of natural and synthetic polymeric materials and their potential applications for the removal of toxic heavy metals from water, have been comprehensively reported [18].…”

“…However, many of the materials and processes currently used for the synthesis of nanoparticles are dependent on non-renewable resources and also generate hazardous wastes. The potential environmental and health impacts of engineered nanoparticles have been comprehensively studied and reported in the literature [5][6][7][8]. Green nanotechnology, the combination of nanotechnology and the principles and practices of green chemistry, may hold the key to building an environmentally sustainable society in the near future.…”

Redox Properties of Iron-Based Materials in Water Treatment Technologies: An Overview of Laboratory Versus Field Experiences

“…The iron based materials proved to be promising material due to the availability of iron-bearing precursors and versatility of Fe in its various oxidation states while Fe0/II is a reducing agent [3], FeIV/V/VI is a strong oxidant [4]. Furthermore, the non-toxic iron oxyhydroxides are the final reaction products of Fe-based (nano) materials [5], [6]; these minerals show the similar properties as their naturally occurring analogues [7]. However, the kinetics and reaction mechanisms of contamination removal by the (nano)materials differ under laboratory and real environmental conditions.…”