 Solution

Neutron-rich material ejected dynamically or in remnant-disk outflows can undergo the r-process: successive neutron captures, followed by beta decays, make heavy nuclei. The yield depends on the amount ejected, its entropy, expansion and electron fraction; not all outflow is equally neutron rich. Radioactive products can power a kilonova. A neutron-star–black-hole event contributes if matter escapes capture. An isolated binary black hole merger has no analogous reservoir of neutron-rich baryonic material, so it contributes essentially no such nucleosynthesis.