While this may not directly answer your question, I have been doing some reading for a work-related application of biochar on saline affected soils for mining rehabilitation and found this interesting article;
'Results showed that the biochar-amended columns discharged efflux 24 to 40 days earlier than that of the control without biochar (CK). Biochar addition saved 56 to 62 days for the EC values of the efflux to be reduced to 5 dS m−1.'
I don't have access to the full study, so can't really make any comment beyond my interpretation of the summary; biochar may provide a mechanism for increased leaching rates of sodium salts.
Heavy metals are a tricky one, it not so much as what can be done for their presence/removal, yet more a measure of their plant availability and mobility; which is largely driven by pH, modifying which ionic form is available and water solubility.
Biochar may have direct mechanisms for sorbing the charged ions to its surface, not so much changing their ionic form as chemically binding it in a bond stronger than osmotic pressure (I'm guessing, can't be bothered trying to back it up with science
) locking it up beyond plant availability.
Another benefit of biochar may be in its liming effect (depending on biochar); generally raises the pH of the soil, so if the soil is currently acidic it could prove to change bio-availability.
'According to relative heavy metals availability decrement, liming resulted in the strongest effect in extremely acid soils with the highest initial concentrations of available Zn, Pb, Cr and Cd. On the other side, the weakest relative liming effect on heavy metals availability decrement as recorded in moderately acid soils with the lowest initial concentrations of available heavy metals. Considering impact of initial humus content in soil, higher relative liming efficiency of heavy metals availability decrement was determined in soils with higher soil organic matter content and with lower initial concentrations of available heavy metals. '
I actually recall Dr. Elaine Ingham
speaking about heavy metal concentrations and the soil food web
, and I have read about it in Peter McCoy book Radical Mycology; fungi tend to lock up and bind heavy metals to their biomass, and can actively concentrate heavy metals in their fruiting bodies. So an interesting option could be to make swales, fill with lots of wood
chips, and innoculate with some oyster mushroom/other species appropriate to your climate, and then harvest the mushrooms for incineration (wouldn't go eating them)/hazardous waste disposal...
Mycoremediation is a pretty new field of study, so it is an exciting area, which also makes it a bit experimental in nature - I would highly recommend Radical Mycology and read up about the bioremediation options. Also read up on Paul Stammets, has some great articles on bioremediation of hydrocarbon contamination.
Anyways that doesn't really help with your saline problem, but it sounds like biochar may assist in the leaching of salts through the profile, and chemically binding heavy metals; could probably just leave it in the soil, as it would likely have some benefit and the heavy metals may be immobilised. Don't quote me on that.
Not an expert, just a tinkerer.