Question

The concentration of cadmium in the top \(15 \mathrm{~cm}\) of soil in a field (often called the plow layer) can be estimated by taking a representative sample, dissolving the soil and analysing it by atomic absorption spectroscopy with electrothermal atomization. The concentration is found to be \(0.78 \mathrm{ppm}\). Suppose that dewatered (solid) sewage sludge containing \(22 \mathrm{ppm}\) cadmium is added at the rate (mass per area) of 3 tha \(^{-1}\). Assuming sludge is well mixed within the plow layer, calculate the new average concentration of cadmium within this part of the soil. The bulk density of the soil is \(1.1 \mathrm{gmL}^{-1}\).

Short answer

The new average concentration of cadmium in the plow layer is 0.82 ppm.

Step-by-step solution

Calculate the Mass of Soil in the Plow Layer

The volume of the plow layer is determined by the field area, soil depth, and the bulk density. If we assume an area of 1 hectare (10,000 square meters) for simplicity, the volume of the plow layer (15 cm deep) is \(10,000 \times 0.15 = 1,500\) cubic meters. Since the bulk density of soil is \(1.1 \text{ g/cm}^3\), convert this to \(1,500,000\) liters (since 1 cubic meter = 1000 liters), and find the mass: \(1,500,000 \, \text{L} \times 1.1 \text{ g/cm}^3 = 1,650,000\) kg.

Calculate the Mass of Cadmium from Soil

Start with the existing concentration of cadmium in the soil, which is 0.78 ppm. This indicates 0.78 grams of cadmium per 1,000,000 grams of soil. Thus, in 1,650,000 kg (or 1,650,000,000 grams) of soil, the mass of cadmium is \(1,650,000,000 \times 0.78 / 1,000,000 = 1287\) grams.

Calculate the Mass of Cadmium from Sewage Sludge

The sewage sludge added to the soil has a concentration of 22 ppm, and is applied at a rate of 3 tonnes per hectare. This is \(3,000\) kg per hectare. Therefore, the cadmium in the sludge is \(3000 \times 22 / 1,000,000 = 66\) grams.

Calculate New Total Mass of Cadmium

Add the cadmium from both sources: \(1287 \text{ grams} + 66 \text{ grams} = 1353\) grams.

Calculate the New Concentration of Cadmium

Finally, find the new concentration of cadmium by dividing the total cadmium mass by the total mass of the soil (which hasn't changed): \( \frac{1353}{1,650,000,000} \times 1,000,000 = 0.82\) ppm. Thus, the new average cadmium concentration is \(0.82\) ppm.

Key concepts

Sewage Sludge Application

Sewage sludge is a byproduct of wastewater treatment processes. It contains various nutrients and organic materials that can enhance soil fertility. Applying sewage sludge to agricultural land has become a common practice for improving soil quality and crop yield. However, it is essential to manage the application carefully due to potential contaminants like heavy metals, which in this context is cadmium.
When sludge is applied, the rate is usually measured in tonnes per hectare (tha^-1). It is important to ensure that the sludge is thoroughly mixed within the soil layer to achieve uniform distribution of nutrients and any contaminants. By considering the calculated tonnes per hectare, it allows for an accurate understanding of how much cadmium is introduced into the soil. This step is crucial to predict and monitor changes in soil composition and ensure environmental safety.

• Enhances soil fertility
• Contains organic materials and nutrients
• Potentially carries heavy metal contaminants like cadmium

Atomic Absorption Spectroscopy

Atomic Absorption Spectroscopy (AAS) is a powerful analytical technique used to measure the concentration of specific elements, such as cadmium, in soil samples. This method involves dissolving a soil sample and heating it to a high temperature in a special furnace (electrothermal atomization). The atoms absorb light at specific wavelengths, and the amount of absorbed light indicates the concentration of the element in the sample.
AAS is particularly useful for detecting trace elements in environmental samples due to its high sensitivity and precision. When applied to soil analysis, it helps in determining pollutant levels with significant accuracy.

• Accurate measurement of trace elements
• Involves dissolution and heating of samples
• Sensitive to small concentrations of elements

Soil Bulk Density

Soil bulk density is a property that reflects the mass of soil per unit volume. It is often expressed in grams per milliliter (g/mL) or kilograms per cubic meter (kg/m³). Bulk density is crucial for determining how much mass of soil is in a given volume, which is necessary for calculating the concentration of contaminants such as cadmium.
In the exercise, a bulk density of 1.1 g/mL means that for every cubic meter of soil, its mass is approximately 1100 kg. This value helps in estimating the total weight of the soil relevant to the applied area, allowing for precise calculations of changes in contaminant concentration due to external interventions like sludge application.

• Reflects soil mass per volume
• Helps calculate the concentration of soil contaminants
• Expressed in g/mL or kg/m³

Environmental Chemistry Problem Solving

Environmental chemistry involves analyzing and solving challenges concerning chemical processes in the environment. This field requires considering how substances disperse, the impact of contaminants, and their interaction with natural ecosystems. By applying problem-solving techniques, scientists can estimate and mitigate pollution issues in soil and water.
The given exercise illustrates a typical environmental chemistry problem: assessing the impact of cadmium from both soil and sewage sludge on soil health. The solution provided steps to calculate the cadmium concentration before and after sludge application. Such calculations equip environmental chemists and policy-makers with the data needed to develop solutions to maintain ecological balance and ensure agricultural productivity.

• Analyzes chemical processes in nature
• Estimates pollutant impact and dispersion
• Assists in developing ecological balance strategies