There seems to be two parts ...

There seems to be two parts to this question. Most have answered the first.

It is possible to design a treatment wetland to handle most effluent contaminants from a landfill given the proper design and enough landscape to accommodate the design. It will protect the surface waters, but may only be soaking up the contaminants where they will reside until some other type of disturbance to the treatment wetland. Some persistent chemicals (e.g. trace elements)  can never be treated, but merely accumulated either in the soils or biota of the wetland.

But the original question asks another question most have disregarded in their answer.  Jose points out that “ we are facing problem of Landfill leachates since rainy season” That indicates a time frame for the use of wetlands.  Here is a real problem with any design.  The wetlands must be able to mature substantially prior to their ability to impact the chemical composition of any leachate in any substantive or significant way.

So while wetlands can be designed to help purify landfill leachate, their utility really depends on their being part of the original design of the landfill, and their construction should have begun during the landfill construction, and certainly prior to the acceptance of the fill.