Massachusetts Wastewater Practice Exam

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What is the pH range for the least solubility of chrome in treatment processes?

3-5
5-7
7-9
9-11

The correct answer is: 7-9

The pH range for the least solubility of chrome, specifically chromium hydroxide, is typically between 7 and 9. At this neutral to slightly alkaline range, chromium compounds tend to precipitate, thus minimizing their solubility in water. This characteristic is particularly relevant in wastewater treatment processes, where controlling chrome levels is essential for compliance with environmental regulations and for the protection of aquatic life. In this pH range, chromium exists primarily in the trivalent state (Cr(III)), which is less toxic and more likely to form stable complexes that can be more easily removed from wastewater. Lower pH levels, such as those found in the acidic range, can increase chromium's solubility, allowing more of it to remain in solution and complicating treatment efforts. Conversely, at higher pH levels (above 9), chromium can also become more soluble, which can lead to complications in wastewater management, but the least solubility occurs around the neutral pH levels. Hence, the answer correctly identifies the conditions under which chromium is least soluble in treatment processes.