Page 94 - Soil and water contamination, 2nd edition

Solid phase constituents 81

fullerenes are also called buckyballs. Carbon-based nanomaterials have a variety of potential
applications, including improved films and coatings, stronger and lighter materials, and
electronic and medical applications. Metal-based nanomaterials include quantum dots,
nanosilver, nanogold, and metal oxides such as titanium dioxide and zinc oxide. A quantum
dot is a closely packed semiconductor crystal comprising hundreds or thousands of atoms
with varying optical properties that depend on the size of the quantum dots. Examples of
the application fields of metal-based nanomaterials are catalysts, solar cells, batteries, fuel
cells, sunscreens, coatings, and nanopharmaceutical agents for chemotherapy. Dendrimers
are nanosized polymers whose surface has numerous chain ends and some contain interior
cavities. These forms can be tailored to carry out specific chemical functions. Current and
potential applications can be found in catalysis and drug delivery. Nanoparticles can be
combined with other nanoparticles or with larger materials to form composites, in order to
enhance mechanical, thermal and electrical properties.
Despite their potential beneficial uses, nanoparticles also present potential hazards to
human health and the environment. Most of these are associated with their large surface to
volume ratio, which can make the particles very reactive. In addition, because they are so
small, nanomaterials can pass through biological membranes: however, their interactions
with biological systems are still poorly understood. The reactive properties of nanomaterials
increase the probability that they could produce unanticipated toxicological effects. Some
nanomaterials, such as carbon fullerenes, carbon nanotubes and nanoparticle metal oxides,
are known to increase the production of free radicals, particularly of reactive oxygen species
(ROS). This is one of the main mechanisms of nanoparticle toxicity to human tissue and cell
cultures. The presence of reactive oxygen species may result in oxidative stress, inflammation,
and consequent damage to proteins, membranes and DNA, which may ultimately result in
cell death.
At present, the health risks and environmental impacts associated with nanomaterials
remain largely unknown. That is why nanomaterials are often classified as so-called emerging
substances of concern (ESOCs). Other types of ESOCs are described and discussed in
Section 9.7.

4.3 ORGANIC COMPONENTS

4.3.1 Composition and formation

Organic matter in soil and water consists of living biomass (plants, animals, bacteria, algae ,
fungi, and viruses), undecayed dead plant and animal tissues (litter or detritus ), and their
organic transformation products. In soils, the living biomass makes up about 10 percent of

the soil organic matter (SOM) on average. However, some scientists do not classify living
biomass as organic matter. As soon as the living biomass dies, the detritus is decomposed
by microorganisms (bacteria, fungi) to form humic substances , which are temporary but
relatively resistant intermediate products remaining after considerable degradation. Humic
substances are amorphous , colloidal, polymeric, dark-brown organic compounds with a high
molecular weight. They are smaller than 2 μm and therefore have a large specific surface
area. In general, humic substances make up 60 to 70 percent of the total soil organic matter.
The term humus is frequently used as a synonym for humic substances. Unfortunately, here
too the terminology is not consistent, since many soil and environmental scientists also
use the term humus as a synonym for soil organic matter, i.e. all organic material in the
soil including humic substances. The process of formation of humic substances is called
humification , whereas mineralisation refers to the complete breakdown to inorganic
compounds, mainly water and carbon dioxide , but also other minerals and nutrients that

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