Which of the following would not be effective at lowering the pH of wastewater?

To lower the pH of wastewater effectively, the substance used must be capable of donating protons (H+) to the solution, thus increasing the acidity. Sulfuric acid, carbon dioxide, and hydrochloric acid are all acids that can effectively dissociate in water to produce hydrogen ions, leading to a decrease in pH.

Sulfuric acid is a strong acid that completely dissociates in solution, providing a substantial increase in H+ concentration. Hydrochloric acid, another strong acid, functions similarly and readily lowers pH. Carbon dioxide can contribute to the lowering of pH in a different manner, as it can dissolve in water to form carbonic acid, which also releases protons.

Conversely, calcium oxide is a basic compound, commonly known as quicklime. When it is added to water, it reacts to form calcium hydroxide, which increases the pH by introducing hydroxide ions (OH-) into the solution. Therefore, calcium oxide would not only fail to lower the pH but would actually increase it, making it ineffective for the purpose of lowering wastewater pH. This is why calcium oxide is the correct choice in this context.