The natural sulfur cycle involves many sulfur reducing and sulfur-oxidizing bacteria. For example, S²‾ is oxidized to elemental sulfur and to [SO₄]^2–, and the reverse processes convert mobile [SO₄]²⁻ to immobilized S²⁻. 

(a) At the end of the sulfur reduction sequence, HS⁻ may be produced instead of S²⁻. What influences this outcome, and what other product is possible? 

(b) Sulfur-reducing bacteria can be applied to the removal of mobile heavy metals (e.g. Pb, Cd, Hg) from the environment. Explain why the metals are termed ‘mobile’, and describe the chemical processes and equilibria that lead to the immobilization of the metals.