Problem 78 - Chemistry The Science in Context

Question

Wastewater Treatment Show with appropriate net ionic equations how \(\mathrm{Cr}^{3+}\) and \(\mathrm{Cd}^{2+}\) can be removed from wastewater by treatment with solutions of sodium hydroxide.

Step-by-Step Solution

Answer

Based on the step-by-step solution, explain how chromium (Cr) and cadmium (Cd) ions can be removed from wastewater using sodium hydroxide (NaOH) treatment. Chromium (Cr) and cadmium (Cd) ions can be removed from wastewater using sodium hydroxide (NaOH) treatment by reacting with the hydroxide ions (OH-) from NaOH to form insoluble solid precipitates, chromium hydroxide (Cr(OH)3) and cadmium hydroxide (Cd(OH)2), respectively. The net ionic equations for these reactions are: Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s) for chromium (III) Cd²⁺(aq) + 2OH⁻(aq) → Cd(OH)₂(s) for cadmium (II) The formation of solid precipitates allows for the easy removal of chromium and cadmium ions from the wastewater, thus treating it effectively.

Step 1: Write the balanced chemical equations

First, we need to write the balanced chemical equations for the reactions between chromium (III) and hydroxide ions, and cadmium (II) and hydroxide ions. Chromium (III) + Hydroxide ions: \(\mathrm{Cr}^{3+} (aq) + 3\mathrm{OH}^- (aq) \rightarrow \mathrm{Cr(OH)_3} (s)\) Cadmium (II) + Hydroxide ions: \(\mathrm{Cd}^{2+} (aq) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{Cd(OH)_2} (s)\) In these equations, (aq) denotes "aqueous" meaning dissolved in water, and (s) denotes "solid" meaning a precipitate. Now that we have the balanced chemical equations, we can determine the net ionic equations.

Step 2: Write the net ionic equations

The net ionic equations will only include the ions that are involved in the chemical reaction to form the solid precipitates. The sodium ions from sodium hydroxide are not directly involved in the reaction, so they will be removed from the equations. Net ionic equation for Chromium (III): \(\mathrm{Cr}^{3+} (aq) + 3\mathrm{OH}^- (aq) \rightarrow \mathrm{Cr(OH)_3} (s)\) Net ionic equation for Cadmium (II): \(\mathrm{Cd}^{2+} (aq) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{Cd(OH)_2} (s)\) These net ionic equations show that both chromium (III) and cadmium (II) ions in the wastewater react with hydroxide ions from sodium hydroxide to form solid precipitates, which can then be removed from the water.

Key Concepts

Net Ionic Equations

In chemistry, net ionic equations simplify chemical reactions by highlighting the ions taking part in the reaction, while ignoring those that do not undergo any change. They help to focus on the substances involved in the formation of precipitates, gases, or pure liquids. This can be especially helpful when dealing with complex reactions in solutions.
For example, when a chromium (III) ion (\(\mathrm{Cr}^{3+}\)) reacts with hydroxide ions (\(\mathrm{OH}^-\)) from sodium hydroxide in water, the net ionic equation is:

• \(\mathrm{Cr}^{3+} (aq) + 3\mathrm{OH}^- (aq) \rightarrow \mathrm{Cr(OH)_3} (s)\)

For cadmium (II) ions \((\mathrm{Cd}^{2+}\)) the equation is:

• \(\mathrm{Cd}^{2+} (aq) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{Cd(OH)_2} (s)\)

In both reactions, the sodium ions from sodium hydroxide do not participate and are not shown in the net ionic equations. This makes it clearer to see which ions are responsible for forming the precipitates that can be removed during wastewater treatment.

Chromium Removal

Chromium, specifically in its trivalent form \((\mathrm{Cr}^{3+})\), is a common contaminant in industrial wastewater. It needs to be effectively removed due to its potential health hazards and environmental impact.
Through a precipitation reaction, chromium (III) ions can be removed from the water by adding sodium hydroxide. When hydroxide ions \((\mathrm{OH}^- )\) are added, they combine with chromium (III) ions to form chromium (III) hydroxide \((\mathrm{Cr(OH)_3})\), a solid precipitate.
This precipitate is insoluble in water and can thus be filtered out or settled as a sludge, making it an efficient method for extracting chromium from the wastewater.
The removal process is essential for meeting environmental standards and ensuring water is safe for discharge or reuse.

Cadmium Removal

Cadmium \((\mathrm{Cd}^{2+})\) is another hazardous metal often found in wastewater that requires careful removal. Its presence is harmful to both human health and aquatic life.
In wastewater treatment, cadmium ions react with hydroxide ions from sodium hydroxide, similarly to chromium, forming cadmium hydroxide \((\mathrm{Cd(OH)_2})\).
This reaction leads to the formation of a solid precipitate that can be easily separated from the water. The precipitate of cadmium hydroxide is beneficial because it allows for safe disposal of cadmium-containing waste.
Regular treatment of wastewater for cadmium removal is imperative for preventing toxic buildup in the environment and reducing contamination risks.

Precipitation Reactions

Precipitation reactions are a fundamental aspect of removing heavy metals from wastewater. These reactions occur when two soluble substances react and form an insoluble solid known as a precipitate.
In the context of wastewater treatment, precipitation reactions help separate contaminants from the water stream.
A common example is the reaction of metal ions like chromium and cadmium with hydroxide ions to form metal hydroxides as solid precipitates.

• They offer a simple and cost-effective method for treating wastewater.
• Utilize chemicals, such as sodium hydroxide, to transform soluble metal ions into insoluble compounds.
• These reactions help comply with environmental regulations by lowering the concentration of harmful substances in water before it's released back into the environment.

Understanding how precipitation reactions work is key to designing effective wastewater treatment systems that aid in environmental protection.