Question

A particular lake sediment sample is made up of \(23 \%\) organic matter \(77 \%\) mineral fraction \(42 \%\) clay minerals \(24 \%\) silt \(11 \%\) sand The clay minerals are \(90 \%\) kaolinite and \(10 \%\) halloysite. Calculate an approximate cationexchange capacity \(\left(\mathrm{cmol}(+) \mathrm{kg}^{-1}\right)\).

Short answer

The approximate cation exchange capacity is 1.554 cmol(+)/kg.

Step-by-step solution

Calculate Cation Exchange Capacity for Kaolinite

Kaolinite has a cation exchange capacity (CEC) of approximately 3 cmol(+)/kg. Since clay minerals make up 42% of the sample, and 90% of those are kaolinite, we calculate the CEC contribution from kaolinite as follows:\[ \text{CEC for Kaolinite} = 42\% \times 90\% \times 3 \text{ cmol(+)/kg} \] Converting percentages to decimals, we have:\[ = 0.42 \times 0.9 \times 3 \text{ cmol(+)/kg} \]\[ = 1.134 \text{ cmol(+)/kg} \]

Calculate Cation Exchange Capacity for Halloysite

Halloysite has a cation exchange capacity (CEC) of approximately 10 cmol(+)/kg. Since clay minerals make up 42% of the sample, and 10% of those are halloysite, we calculate the CEC contribution from halloysite as follows:\[ \text{CEC for Halloysite} = 42\% \times 10\% \times 10 \text{ cmol(+)/kg} \]Converting percentages to decimals, we have:\[ = 0.42 \times 0.1 \times 10 \text{ cmol(+)/kg} \]\[ = 0.42 \text{ cmol(+)/kg} \]

Sum Cation Exchange Capacities

Add the CEC values obtained from both kaolinite and halloysite to find the total CEC of the clay minerals in the sample.\[ \text{Total CEC} = 1.134 \text{ cmol(+)/kg} + 0.42 \text{ cmol(+)/kg} \]\[ = 1.554 \text{ cmol(+)/kg} \]

Key concepts

Clay Minerals

Clay minerals are essential components of soils and sediments known for their ability to retain water and nutrients. These minerals are usually tiny particles, often less than 2 micrometers in size. They have unique properties due to their layered structure, which allows them to interact with cations, enhancing the soil's fertility and water retention capabilities.
Clays have a high surface area, which provides them with a significant cation exchange capacity (CEC). This means that they can hold and exchange positively charged ions (cations) like calcium, magnesium, and potassium, crucial for plant nutrition.
When we talk about clay minerals in this context, we're often referring to multiple types, each with its properties and behaviors, such as kaolinite and halloysite.

Kaolinite

Kaolinite is a common type of clay mineral, making up a significant portion of many soils and sediments. This mineral is recognized for its low cation exchange capacity, around 3 cmol(+)/kg, which limits its ability to hold onto nutrients compared to other clay minerals. However, its stability under various pH conditions makes it an important component in many environmental settings.
Kaolinite has relatively large crystals with a simple, repeating structure. The simplicity of its structure results in fewer sites for ion exchange, contributing to its lower CEC. Despite this, kaolinite is widespread due to its formation from the weathering of feldspar in hot and moist climates.
Due to its unique properties, kaolinite is often used in ceramics and as a white pigment, but its primary role in soils is associated with providing a stable environment for plant growth.

Halloysite

Halloysite is another important clay mineral, similar to kaolinite but with some differences in structure and properties. It has a cation exchange capacity of around 10 cmol(+)/kg, which makes it somewhat more effective at holding cations than kaolinite.
Halloysite often occurs in the form of hollow, tubular structures, which can absorb water more efficiently compared to other clay minerals. These tubes are shaped due to the rolling of layers that can curl, leading to its distinctive morphology.
In soils and sediments, halloysite contributes to nutrient retention and soil structure, but it is less stable than kaolinite under intense weathering conditions. Despite that, it plays a crucial role in certain soil types, especially those formed in tropical and volcanic regions.

Lake Sediment

Lake sediments are composed of various materials deposited over time, including organic matter, mineral particles, and sometimes clay minerals. These sediments accumulate at the bottom of lakes and reflect the environmental conditions of the area.
The composition of lake sediments can tell us a lot about the past climate, erosion rates, and biological activity. In the case study, 42% of the lake sediment is made up of clay minerals, with specific proportions of kaolinite and halloysite.
Understanding the content of these sediments helps in assessing the sediment's role in nutrient cycling and pollution dynamics, as clay minerals influence the sediment's cation exchange capacity and, consequently, its interaction with pollutants and nutrients in the water body.

Mineral Fraction

The mineral fraction in a sediment or soil sample refers to the inorganic components, excluding organic matter. This fraction plays a dominant role in determining the physical and chemical properties of the sediment.
In the given sample, the mineral fraction makes up 77% of the total composition, influencing the sediment's texture and reactivity. Within this fraction, clays like kaolinite and halloysite are crucial because they provide a substrate for chemical reactions, particularly affecting the cation exchange capacity (CEC).
Other minerals in this fraction can affect color, hardness, and more specific reactions, such as those forming iron oxides or carbonates, contributing to the landscape's development and the environmental quality of aquatic systems.