We will see
some water features especially in liquid state.
To explain what happens in contamination and decontamination of water is
important to know the features of water. Among others, pure water has the
following:
•
Colorless, odorless and tasteless.
• Density:
1 g/cm3 at 4 ° C
• Melting
point: 0 ° C
• Boiling
point: 100 ° C.
•
Dielectric constant: 78.3
• pH = 7
• Very low
electrical conductivity and high resistivity.
Liquid
water is colorless, odorless and
tasteless. Actually, water is considered colorless, but analyzed with a
spectrophotometer a slight blue-green coloration is detected. Although the blue
of sea is caused by reflections of the sky, this property of water contributes
to blue color of sea, lakes and large bodies of water.
Other
properties can be understood from two characteristics of the water molecule. One
is, as we saw in the initial post of this blog, that has a dipolar character
and other feature is the dissociation of water molecules (H2O) into protons (H
+) and hydroxyl (OH-).
Both the melting point and boiling point are outliers with respect to the same series of
hydrides oxygen due to its more pronounced dipolar character allowing links
with hydrogen bridges. As a consequence of water’s melting point and boiling
point higher than the rest of hydrides series, it can be found on Earth in
three states (solid, liquid and vapor).
The high dielectric constant of water makes that
the water has great ability to dissolve polar substances
Thus, the
negative parts of the water molecule (Oxygen side) interact with positively
charged substances and the positive part of the dipole (Hidrogen side) of the
water molecule interacts with negatively charged substances.
In the
following graph is represented as water dissolves salt (sodium chloride) and solvating
ions of Na + and Cl- to form a solution.
In pure
water, there is a very small part that is dissociated into protons (H +) and
hydroxyl (OH-). This amount of dissociated water molecules are related
with pH. Pure water has a pH of 7 which is considered neutral pH.
Water conductivity is very low due to the low
dissociation presented. Pure water practically does not conduct electricity. However,
the water that we find in nature always has other dissolved substances which
make its conductivity high and therefore, we consider that water is a good
conductor of electricity.
These are
some of the characteristics of pure water. Nevertheless, water in nature hasn´t
got the features described above on the same values that pure water, but the
inclusion of other substances due to water ability of dissolving, transporting,
reacting to form structures or compounds with other substances, it happens that
some parameter, several or all are different.
Water in
nature contains minerals and organics substances from plants, microorganisms,
animals.
Although
natural waters have different characteristics to pure water, tend to have
constant characteristics that vary with the seasons, rainfall, etc. But we can
say that they have an identity and characteristic parameters.
Wastewater,
however is water that has been used by humans in any of its activities and has
very different components, quantity or quality than original water had. This wastewater
with these abnormal characteristics adversely affects the receiving environment
and can cause serious effect on it (surface water, soil, air, groundwater,
oceans) and living beings in general.
The wastewater treatment consists on
removing the abnormal components of wastewater and obtaining water with natural
features. When
pollutants including on waste water are more closely included in the water or
are smaller, is harder to debug, it require more investment and cost and is
more difficult to control the systems of treatment.
Treatment
of water require physical, chemical and biologic methods depending on the type
of contamination containing wastewater and the degree of purification required.
