Are there benefits to re-circulating hazardous waste landfill leachate?

Generally as leachate is re-circulated, regardless of whether it comes from municipal solid waste or from hazardous waste, as long as there’s biodegradation occurring you should see continued reduction of the biological and chemical oxygen demand. If the hazardous waste leachate happens to be high in organics, you should see some benefit from recirculation.  Because I don’t know the specific chemistry of the particular application you are referring to, it’s a good idea to consult with an expert in landfill chemistry who can tell you what kind of reduction you might see, but generally speaking, there should be some benefit to it.