The net negative charge on a clay particle arises due to a phenomenon called ion exchange. Clay minerals have a layered structure consisting of stacked sheets of atoms or molecules. These sheets have a slight negative charge due to the presence of certain elements like aluminum or silicon, which have a tendency to give up electrons.
When clay particles come into contact with water, ions present in the water (such as sodium, potassium, calcium, etc.) can interact with the charged surfaces of the clay minerals. These ions can be either positively charged (cations) or negatively charged (anions).
In the case of clay particles, the cations present in the water can be adsorbed onto the surface of the clay, replacing some of the existing cations in the clay's structure. This process is called cation exchange. As a result, the clay particle gains a net negative charge because the replaced cations had positive charges.
The specific mechanisms and factors influencing the ion exchange process in clay particles can be complex and depend on various factors such as the composition of the clay minerals, the pH of the surrounding environment, and the types and concentrations of ions in the water. Additionally, the surface of clay particles can also attract and retain other negatively charged species, such as organic molecules or dissolved ions.
It's important to note that the net negative charge on clay particles can vary depending on the environmental conditions and the composition of the clay itself. However, the presence of a net negative charge is a common characteristic of many clay minerals.