Recovery of Copper in Effluent from Advanced Placement Chemistry Labs

Authors: James Dorn, Sophie Fuller, Allison Glazer, Patrick Hsu, Bethany Lane, Grace Martin, Anders Niemi, Danica Pietrzak, Megan Smith, Tom Talasek*

^*Faculty advisor, Basis High School, Flagstaff, AZ

Peer Reviewer: Hyunjin Lee

Professional Reviewer: Brian Hamilton

Summary

Solutions containing copper are commonly generated in high school chemistry laboratories. Currently, these wastes must be collected and submitted to a hazardous waste facility for treatment, often at a considerable expense. This paper describes a method for extracting copper from solutions using equipment commonly found at most high schools.

Hypothesis

There should be a cost-effective method for treating effluent from small laboratories. The principles of electroplating and electrolytic cells are well-known. It should be feasible to reduce the concentration of copper using an electrolytic cell. The purpose of this research is to demonstrate the feasibility of this approach in a high school laboratory setting using readily available resources.

Introduction

The prevailing copper guidelines by the US Environmental Protection Agency are 1.3mg Cu L^-1 (2.0 µM) in public water sources; a value that many experts agree is too high and may have undesired effects. Professionals in the field recommend a maximum concentration of 1.0 mg?L^-1 (1.6 µM) copper¹ to ensure no detrimental effects in public drinking water. 

Even in small doses, copper exposure has been linked to varying gastrointestinal symptoms such as nausea and abdominal pain. By randomly selecting dosages (0, 2, 4, 6 or 8 times the recommended limit of copper) for 200 participants, this study established a correlation and possibly causation of effects of high copper concentration in drinking water. The most common side effect after the 5-week trial was nausea, often within only 15 minutes after digestion.² 

In another study, significant injuries were observed on the kidney, spleen, and liver of mice as their copper concentration was increased. Increased amyloid beta protein in the brain was also seen, increasing risk for Alzheimer’s disease.³ 

The Journal of Industrial and Engineering Chemistry⁴ examined that copper reduction is prevalent in industrial settings. Electrochemical removal methods such as electrocoagulation and electrodialysis have been used to reduce copper concentration. Other techniques such as absorption, cementation, membrane filtration, and photocatalysis have also been used to treat the effluent from industrial applications. These techniques are both too capital-intensive and impractical in a small-scale high school lab scenario. 

High school laboratories produce copper solutions that are typically either collected in a container for commercial treatment and disposal as a hazardous waste, or simply poured down the drain. One example of copper waste generation in a high school laboratory can be found in a lab textbook published by the College Board, the administrators of Advance Placement (AP) courses. This publication includes a lab “How Can Color Be Used to Determine the Mass Percent of Copper in Brass?” in their publication AP Chemistry Guided-Inquiry Experiments: Applying the Science Practices.^5-7 Over 300,000 students took AP chemistry in 2018⁸. The published lab procedure creates approximately 1.5 grams of copper effluent per AP student. As a result, it is estimated that over 380 kilograms of copper effluent could be eliminated every year. While only about 14% of public and private high schools in the US offer AP chemistry^8-9, it is likely that many non-AP chemistry classes generate copper effluent. While this amount pales in comparison to the annual production of copper of 18 million metric tons¹⁰, large-scale processes already handle this issue as mentioned above.

Described here is an electrochemical method for capture of copper from laboratory-generated solutions in a high school environment. The method uses a simple power supply used in a typical high school physics lab, in addition to a stir plate and ring stand from a high school chemistry lab, and low cost electrodes. In addition to reducing copper effluent and teaching environmental responsibility to high school students, it also teaches the basic principles of an electrolytic cell.

Materials and Methods

• A photograph of the setup is shown in Figure 1. 
• A square of copper sheet was cut into a rectangular shape to serve as a cathode. 
• A neck was fashioned into the sheet, preserving at least a 3×3 inch square surface area available to place in the solution being treated. 
• The copper electrode was attached to a ring-stand using a Vernier nonmetallic electrode support and secured with a tightly wrapped rubber band. 
• The anode was a graphite electrode 12 inches long and 1 inch in diameter attached to a second ring-stand with a clamp.
• A beaker was placed on a stir plate with a Teflon stir disk inside to maintain constant agitation. The size of the beaker can be varied based on the volume of liquid to be processed. 
• The ring stands were arranged in a manner that allowed the electrodes to be situated so that the voltage drop was minimized in the cell, with at least the bottom two inches reaching into the solution. 
• A Vernier SpectroVis Plus spectrometer with LoggerPro was used to determine copper concentration. Colorimetric determination of copper was conducted at a wavelength of 693.4nm. A background absorbance was also measured at 500 nm. 
• Current was determined from the digital readout of the power supply. 
• Temperature was measured with a digital thermometer to determine if the reaction generated heat. 
• In order to quantitate copper, a calibration curve was constructed using various concentrations of Cu(NO₃)₂. The calibration curve is shown in Figure 2.
• The recovery method was tested with both pure copper and brass dissolved in nitric acid. 
• A TEKPower DC regulated power supply was used to provide the power to the corresponding electrodes. Several voltages were evaluated, but based on results described below, the optimum voltage was found to be 6V. The power supply was used in constant voltage mode, and the current was allowed to fluctuate. 
• Graphite particle concentration was determined gravimetrically by extracting particulate matter by gravity filtration, drying, and comparison of particulate mass by difference. The reason for this determination is discussed in the results section.

Results

Copper recovery for both the pure copper and brass were identical within measurement uncertainty. This was the expected result, since zinc complexes do not absorb in the visible range of light. Since the measurements were made in this band of radiation, variations in the zinc concentration would have no effect on the results.

A raw recovery of 103.8% was observed. Also, it was observed that the solution would darken substantially near the end of the run. It was hypothesized that the recovery above 100% might be related to electro-etching of the graphite electrode due to the oxidation of water at the anode, causing light scattering across the visible spectrum. This was supported by suspended particles in the solution that caused the darkening. To further support this, the mass of the graphite electrode was measured before and after a recovery trial. The amount suspended particulate matter was assessed by gravity filtration and gravimetric analysis, and was determined to match the weight loss from the graphite electrode within the measurement capability of available equipment.

An extended run was conducted to determine if the graphite electro-etching could be eliminated. The resulting data from this test is shown in Figure 3 and 4. The appearance of particulate matter began to appear as the copper was exhausted from the solution. An increase in absorbance at wavelengths outside the copper absorption band was also observed as shown in Figure 5. When the particulate matter was removed, and the absorbance spectra measured, the absorbance in the range of 500 nm dropped to zero, and the absorbance at 693 nm corresponded with the difference between the absorbance at 693 and 500 nm. This allowed correction for the scattering caused by suspended graphite by subtracting the absorbance at 500 nm from the absorbance at 693 nm.

A lower detection limit (LDL) of 0.026 M (1.6 g L^-1) was determined by using two times the RMS noise of the spectrophotometer. While still above the recommended levels for drinking water, it can demonstrate a substantial reduction of copper in the lab effluent.

Conclusions

A novel method of removing copper from laboratory effluent has been demonstrated here. The method has been shown to quantitatively remove copper within the limits of available instrumentation. In the future, more sensitive analytical techniques may demonstrate even lower levels of copper remaining in lab effluent. It is also feasible that this method could be applied to other laboratory sources of copper. Finally, there is the potential this technique could be applied to the removal of other metals.