To eliminate hydrogen sulfide (H2S), the pH of the water should be lowered to _______ or less before beginning aeration.

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Multiple Choice

To eliminate hydrogen sulfide (H2S), the pH of the water should be lowered to _______ or less before beginning aeration.

Explanation:
To remove hydrogen sulfide efficiently with aeration, you want the sulfide to be in the molecular, volatile form (H2S) rather than as sulfide ions (HS− or S2−). The balance between H2S and its ions depends on pH: lowering the pH shifts the equilibrium toward H2S, whereas higher pH shifts it toward HS− and S2−, which are not easily stripped by air. The pH around 6.0 is where the majority of sulfide exists as H2S, making aeration much more effective at off-gassing the gas. Therefore lowering the water to about 6.0 or lower maximizes removal. If the pH is near neutral or alkaline, most sulfide is in ionic form and aeration removes it much less effectively.

To remove hydrogen sulfide efficiently with aeration, you want the sulfide to be in the molecular, volatile form (H2S) rather than as sulfide ions (HS− or S2−). The balance between H2S and its ions depends on pH: lowering the pH shifts the equilibrium toward H2S, whereas higher pH shifts it toward HS− and S2−, which are not easily stripped by air. The pH around 6.0 is where the majority of sulfide exists as H2S, making aeration much more effective at off-gassing the gas. Therefore lowering the water to about 6.0 or lower maximizes removal. If the pH is near neutral or alkaline, most sulfide is in ionic form and aeration removes it much less effectively.

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