Introduction

A serious problem with many types of industrial effluent is the concentration of heavy metals within the waste stream.  These contaminants, which may include chromium, copper, cadmium, zinc, nickel, etc., have been linked to various health disorders from Alzheimer’s disease to cancer and, as a result, must be removed from the waste stream.  In most regions and municipalities, the limits on these substances are below 1 mg/L.  For example, the monthly average concentration limit on the effluent from metal mining industries in Ontario is just 0.3 mg/L (Ontario Regulation 306/17).

The most common method of control, and still one of the most cost-effective approaches, involves the PRECIPITATION of these substances as a metal hydroxide.  These precipitated particles are then allowed to settle to the bottom of a gravity settler or encouraged to float to the surface of a flotation clarifier, and removed as a sludge for disposal at a secure landfill facility.

A major complication of the hydroxide precipitation process, however, is that the dissociated metal ions exhibit a pH-sensitive and metal-specific solubility profile.  Typically, if the solubility of a metal hydroxide is plotted against pH, the resulting solubility profile will be gaussian with rapidly increasing solubility levels at high or low pH values.  Also, the exact shape of this profile will vary significantly from one metal species to another.  As a result, the optimum control point for the wastewater treatment operation is that pH value which defines the vertex of the gaussian curve, ie the point of minimum solubility for any given metal species, or that pH value which is the best compromise level of minimum solubility for the range of metals in the solution.

Purpose

The purpose of this experiment is:

1. To define the relationship between pH and solubility for copper to be removed from a solution as copper hydroxide precipitate.
2. To identify the lowest level of solubility for copper hydroxide.

Equipment

Figure 1 displays the equipment used in the E030 lab with its primary functions pointed-out.

Figure 1: Copper precipitation apparatus with major components highlighted.

The major components of the experimental setup are:

• A Continuously-Stirred-Reactor (tank) with a mixer and a sampling/draining port
• A pH indicator/probe
• A pH Controller unit with two pumps attached to it
• Containers with H₂SO₄ and NaOH reagents (their strength is not critical).

Procedure

The video describing the equipment and the experimental procedure can be seen here.

Based on the video, use the activity below to verify that you know the basic elements of the pump.

Activity 1: Drag the “frequency” and “stroke” elements to the correct position on the pump

Copper Addition, Precipitation, and Sampling

1. Prepare a 50 L batch of an effluent stream (i.e. a tank), at a contamination level of 50 mg/L of copper. Note that we are not using pure copper.  Calculate the mass of cupric sulfate pentahydrate (CuSO₄ ⋅ 5H₂O) required for raising the water’s copper concentration to 50 mg/L.
   Check your calculation by using the multiple-choice questions below
   

2. Turn on the tank mixer.
3. Adjust the tank pH to between 3 and 4 using the controller.  To set the pH limits:
   1. For the upper setpoint, click on Menu → Control → Channel 2 Parameter Set 1 → Upper Setpoint → Set the pH using the up/down arrows → Press OK.
   2. For the lower setpoint, move up to Lower setpoint → Set the pH using the up/down arrows → Press OK → Press Esc to go to the main screen.
4. Start the controller (if not on already) and wait for the pH to reach the desired range.
5. After equilibrium is reached, wait 3 minutes and then take a sample using the sampling port at the bottom of the tank. To take the sample (and all subsequent samples), fill the sampling container completely, pour the solution back into the tank and then fill it again and keep the sample. Also record the pH at the sampling instant.
6. Adjust the pH to about 5.  To do that, set the upper and lower setpoints of the controller to values close to 5 (e.g. 4.7 and 5.3).
7. Repeat step 5 to take a sample.
8. Adjust pH to about 6. 0 then 7.0, 8.0, 8.5, 9.0, 9.5. Repeat sampling for each pH.
9. Analyze the samples for metals concentration with the HACH spectrophotometer, using the instructions provided with the instrument. Samples for copper analysis will need to be diluted 10 times. Be sure to not carry any precipitate into the sample cell. This can be done with the use of a filtering syringe.
10. Neutralize and dispose of the effluent.
11. Return the equipment to the original conditions.

Sample Analysis

The sample analysis can start as soon as 4 samples have been collected.  The purpose of the analysis is to determine the concentration of copper remaining dissolved in the solution.

1. Insert four samples to the centrifuge and centrifuge them as instructed in the lab video.  This is done to separate the solids from the solution.
2. Because the maximum level of dissolved copper that the spectrophotometer can read is 5 mg/L and our original solution has approximately 50 mg/L of copper, we need to dilute the samples (if we do not, the readings will be outside the range of the spectrophotometer).  Dilute all sampled by a factor of 10, following the procedure described in the lab video.
3. Add the necessary reagent and wait 5 minutes, as instructed in the lab video.
4. Insert the diluted samples in the spectrophotometer after you first zero the equipment using tap water.
5. Record all your readings from the spectrophotometer.

Clean-up

1. Clean-up any spills.
2. Wash all containers and return to their original positions.
3. Ask your lab instructor about whether you should empty the tank.

Report

1. Prepare a plot of dissolved copper concentration vs pH and comment on the shape of the curve.
2. Identify the pH value for optimum copper removal.
3. Comment on the effect of pH on the removal efficiency of copper as a hydroxide complex.