From the Ontario Ministry of Ag. and Food handout
Walnut Toxicity
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Excellent soil drainage will also help reduce toxicity problems, even among sensitive plant species. In well-drained soil, toxic reactions only occur when direct contact is made between walnut roots and roots of sensitive species. In poorly drained soil direct contact between roots is not necessary to cause toxic reactions since juglone moves through the soil water. It has been suggested that plants having shallow root systems are more tolerant of juglone than deep-rooted species. Tolerance to juglone by shallow-rooted species may also be attributed to better drainage of soil water in upper soil levels.
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Although I've read some articles which claim that Juglone is poorly soluble in water, the consensus appears to be that it is water-soluble.
So once the tree is removed it should be possible to flush the soil with periodic flooding. Amending the soil after removal would help too.
Bear in mind that the Black Walnuts roots will continue to release Juglone as they decompose.
I'm wondering pineapple if the seedling has some parents nearby who might be causing a greater problem?