Applied techniques to deal with high concentrations of heavy metals in soils include removal and controlled storage of contaminated soil, hydrological isolation of polluted sites and permanent pumping plus purification of groundwater from below polluted sites.

What is biochar? (Special?) charcoal for absorption of cations of heavy metals? Or nano-materials?

Heavy metals are naturally found in earth and human activities have concentrated them after consumption, so I think it is possible to dilute them back in soil only in high concentration they bio-accumulates or  magnified along food chain. 

What if we use wastewater for irrigation. If the wastewater has heavy metals

Waste water for irrigation and heavy metals in soil- two different aspects. In case of waste water with heavy metals: plants will uptake them and depending on concentration toxicities will be the final result. Plants may die and edible portion will be toxic for human and animal consumption. Treating waste water for irrigation purposes is essential- the cheapest one is to use biological techniques; such as growing water hyacinth in ponded water and then use for irrigation purposes. Use of microbes, e.g. arsenic oxidizers/reducers, also important to make heavy metals unavailable for plant's uptake.

Contaminated soils with heavy metals: trap crops could be used. If it is in small area- physical isolation can be made.

Physico chemical charecteristic of waste water need to be assessed before it is being used for irrigation.and if it contains heavymetal it should be used for irrigation  only after its removal

their fate does not change very much, its only that they may become adsorbed on "biochar"-organic matter. But they are still present and environmentally relevant.

I feel , a soil contaminated with heavy metals treated with biochar , will strengthen the  adsorption of heavy metals further  , simply because of much higher surface offered by biochar particles , without any expectation to divert those heavy metals into soil solution, quite unlike the typical ternary system created in a soil treated with organic matter   to initially coat the soil particles and later filter the heavy metals through filtering role of  organic matter  coated soil particles ( Especially the sand particles) as a result of consistent organic manuring .

Biochars are organic substances, be it wood, plant residues, leaves etc that are expose to controlled high temperature, mostly above 400C . These substance have high surface area that enable heavy metals to be adsorbed to it greatly. so, when applied to soil, they readily get metal particles adsorbed and render them immobile in soil, thereby reducing their environmental-relevance. But application of biochars to metal contaminated soil does not reduce the metal level of the soil. As per wastewater treatment, biochars can help to reduce the level of metals by the same process of adsorption as waste water flow through/on the biochar..

Biochar is just like other organic amendments that increase surface area and charge density (CEC) of soil, however, its addition to soil would not decrease the total concentration of metals in soil, rather it would temporarily decrease the bio-available levels of certain (not all) heavy metals in soil (solution). Sometimes biochar may add heavy metals to soil because of presence of heavy metals in feedstock from which biochar was produced. Further, the heavy metals trapped in biochar may released into soil solution and this would be hidden hazard, even metals are not being added to soil from all other possible sources. So utmost care is needed when selecting and applying  biochar to metal contaminated soils. There is another aspect that warrants immediate attention that almost all types of biochars had been shown to increase EC and pH of the soil, hence could increase the mobility of metals and/or metalloids like Cd (sensitive to salinity) and As (sensitive to pH and redox conditions).

Soil pH and EC have been mostly reported , if we apply biochars in alkaline soils , but works fantastically as a soil conditioner with regard to improving nutrient pool and other physical properties . Here our major concern is whether or not , biochar will be able to scavenge the heavy metals out of the soil exchange complex, if I have understood the question rightly. 

Dear Dr. Anoop Kumar thanks for your comments and concerns. How can we segregate the soil exchange complex from biochar exchange complex in mixture of biochar and soil. Further both biochar and soil would work as solid phase and both will contribute to scavenge metals from soil solution. However, adding biochar to soil will increase its CEC hence affinity for metallic cations. Biochar addition to soil will definitely improve its capacity to retain/fix/adsorb metals on its surface/pore/functional groups. However, very limited information is available regarding how long this would work. As we can not separate the biochar loaded/saturated with heavy metals from soil system, so it would increase the level of heavy metal in soils amended with biochar. Now the problem is how to remove these metals from such a soil?

Thanks again for your response . In a binary or trinary  adsorption complex system , organic manures subjected to further decomposition releasing humic /fulvic acid  , thereby , facilitating further to coat the soil particles and heavy metals further  are adsorbed on those coated particles , which upon microbial decomposition , will release the adsorbed metals to divert into solution phase of the soil for onward leaching offsite. While , this is not the scenario in case of biochar having recalcitrant carbon with its exeptional residence time   in soil ,  will act as a neutral molecule having no charge at all , since biochar have six carbon rings without any o- or oh -groups at their peripherals , just like an additive as soil conditioner  , least prone to any microbial decomposition . Ewald ahs very rightly opined in this regard .    

Dear Dr Annop thanks for the comments. There are many publications who show that depending upon type of feedstock biochar may be decomposed but not as easy as other organic compounds. Irrespective of its decomposition, how much biochar would be needed to hold heavy metals in highly contaminated soils. Further under a multi metal natural contaminated soils not all the metals will bind with biochar-soil mixture with a similar binding force. Not only biochar type (cec,porosity etc) but concentration and type of metals will determine the strength of adsorption. So what to do with least absorbed metals and also biochar is saturated with non essential and essential elements in soil.

Thanks again for the response.  Why biochar has so much of promise with regard to soil healith and consequently the possibility of carbon sequestration .For the simple reason , it has maximum recalcitrant form of carbon whose resilience time in soil comes out to be 75-100 years cpmpared to 3-5 years with any of the conventionally use dorganic manures according to studies conducted at CSIRO , Australia ( Personal communication with Johannes Baila). And , thereby , carbon will not be available to microbes for their proliferation as it happens in case of any microbe using any easily available organic manures. Hence , heavy metals which need to be filtered out,  need to have such a form of carbon which is easily assimilable to microbes , unless you have taht form of acrbon , it will be no longer so effective on so short term basis.. You can please refer to our excellent discussion in context to  my one question  entiled  DO YOUFEEL,  BIOCHAR COULD BE A PROMISING ALTERNATIVE TO ORGANIC MANURE  tossed up on RG . You will love reading the discussion with Johannes Baila and other learned colleagues.